Title of Invention	A LITHIUM SECONDARY BATTERY
Abstract	A lithium secondary battery is disclosed. The lithium secondary battery comprises: (i) a cathode (C) in which cathode active materials with a particle diameter of between 5 µm and 30 µm are loaded in an amount of between 10 mg/cm2 and 30 mg/cm2, (ii) an anode (A) in which anode active materials are loaded in an amount of between 4.4 mg/cm2 and 21 mg/cm2, (iii) a separator, and (iv) an electrolyte, wherein the battery has a weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode of between 0.44 and 0.70, and has a ratio (A/C) of thickness of the cathode (C) to that of the anode (A) of between 0.7 and 1.4, and shows a charge cut-off voltage of between 4.35V and 4.6V.

Title of Invention

A LITHIUM SECONDARY BATTERY

Abstract

A lithium secondary battery is disclosed. The lithium secondary battery comprises: (i) a cathode (C) in which cathode active materials with a particle diameter of between 5 µm and 30 µm are loaded in an amount of between 10 mg/cm2 and 30 mg/cm2, (ii) an anode (A) in which anode active materials are loaded in an amount of between 4.4 mg/cm2 and 21 mg/cm2, (iii) a separator, and (iv) an electrolyte, wherein the battery has a weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode of between 0.44 and 0.70, and has a ratio (A/C) of thickness of the cathode (C) to that of the anode (A) of between 0.7 and 1.4, and shows a charge cut-off voltage of between 4.35V and 4.6V.

Full Text	Technical Field The present invention relates to a lithium secondary battery having a charge-cutoff voltage of 4.35V or higher. More particularly, the present invention, relates to a lithium secondary battery, which has a charge cut-off voltage of between 4.35V and 4.6V, high capacity, high output and improved safety and is provided with capacity balance suitable for a high- voltage battery by controlling the weight ratio (A/C) of both electrode active materials, i.e., weight ratio of anode active material (A) to cathode active material (C) per unit area of each electrode. Background Art Recently, as electronic devices become smaller and lighter, batteries used therein as power sources are increasingly required to have a compact size and light weight. As rechargeable batteries with a compact size, light weight and high capacity, lithium secondary batteries such as secondary lithium ion batteries have been put to practical use and widely used in portable electronic and communication devices such as compact camcorders, portable phones, notebook PCs, etc. A lithium secondary battery comprises a cathode, anode and an electrolyte. Lithium secondary batteries are classified into liquid electrolyte lithium secondary batteries using an electrolyte comprising a liquid organic solvent and lithium polymer batteries using an electrolyte comprising a polymer. Although lithium having high electronegativity and high capacity per unit mass has been used as electrode active material for a lithium secondary battery, there is a problem in that lithium cannot ensure the stability of a battery by itself. Therefore, many attempts have been made to develop batteries using a material capable of lithium ion intercalation/deintercalation as electrode active material. Cathode active materials that are currently used in lithium secondary batteries include lithium- containing transition metal composite oxides such as LiCo02, LiNi02, LiMn204, LiMn02 and LiFe02. Particularly, LiCo02 providing excellent electroconductivity, high voltage and excellent electrode characteristics is a typical example of commercially available cathode active materials. As anode active materials, carbonaceous materials capable of intercalation/deintercalation of lithium ions in an electrolyte are used. Additionally, polyethylene-based porous polymers are used as separators. A lithium secondary battery formed by using a cathode, anode and an electrolyte as described above permits repeated charge/discharge cycles, because lithium ions deintercalated from the cathode active material upon the first charge cycle serve to transfer energies while they reciprocate between both electrodes (for example, they are intercalated into carbon particles forming the anode active material and then deintercalated upon a discharge cycle). In order to provide such lithium secondary batteries having high capacity, output and voltage, it is necessary to increase the theoretically available capacity of the cathode active material in a battery. To satisfy this, it is required that the charge-cutoff voltage of a battery is increased. Conventional batteries having a charge-cutoff voltage of 4.2V using LiCo02 among the above-described cathode active materials, utilize only about 55% of the theoretically available capacity of LiCo02 by intercalation/deintercalation processes. Therefore, selection of the anode active material in such batteries is limited so as to be conformed to the capacity of lithium ions to be deintercalated from the cathode. When such batteries are overcharged to a voltage of 4.35V or higher, the anode has no sites into which an excessive amount of lithium ions deintercalated from the cathode are intercalated. Therefore, lithium dendrite growth occurs, resulting in problems of rapid exothermic reactions and poor safety of the batteries. Additionally, side reactions between the cathode and electrolyte may occur to cause degradation of the cathode surface and oxidation of electrolyte. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a graph showing variations in discharge capacity of the secondary lithium ion battery having a charge-cutoff voltage of 4.35V, obtained from Example 2; FIG. 2 is a graph showing variations in discharge capacity of the secondary lithium ion battery having a charge-cutoff voltage of 4.2V, obtained from Comparative - Example 1; FIG. 3 is a graph showing the results of the overcharge test for the secondary lithium ion battery having a charge-cutoff voltage of 4.35V, obtained from Example 2; FIG. 4 is a graph showing the results of the overcharge test for the secondary lithium ion battery having a charge-cutoff voltage of 4.2V, obtained from Comparative Example 1; FIG. 5 is a graph showing high-temperature (45°C) cycle characteristics of each of the lithium secondary battery having a charge-cutoff voltage of 4.35V and using no additive for electrolyte according to Example 1, the lithium secondary battery having a charge-cutoff voltage of 4.35V and using cyclohexylbenzene (CHB) as additive for electrolyte according to Comparative Example 2 and the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 4-fluorotoluene (para-FT) as additive for electrolyte according to Comparative Example 3; FIG. 6 is a graph showing high-temperature (45°C) cycle characteristics of the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 3- fluorotoluene (3-FT) as additive for electrolyte according to Example 5; FIG. 7 is a graph showing the results of the hot box test for the lithium secondary battery having a charge-cutoff voltage of 4.35V and using CHB as additive for electrolyte according to Comparative Example 2; FIG. 8 is a graph showing the results of the hot box test for the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 4-fluorotoluene (para-FT) as additive for electrolyte according to Comparative Example 3; FIG. 9 is a graph showing the results of the hot box test for the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 3-fluorotoluene (3-FT) as additive for electrolyte according to Example 5; FIG. 10 is a graph showing the results of the high-temperature storage test (30 cycles: 80°C/3 hr + 25°C/7 hr) for each of the lithium secondary battery having a charge-cutoff voltage of 4.35V and using CHB as additive for electrolyte according to Comparative Example 2, the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 4-fluorotoluene (para-FT) as additive for electrolyte according to Comparative Example 3 and the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 3- fluorotoluene (3-FT) as additive for electrolyte according to Example 5; and FIG. 11 is a graph showing the results of the high-temperature/short-term storage test (90°C/4 hr) for each of the lithium secondary battery having a charge- cutoff voltage of 4.35V and using no additive for electrolyte according to Example 1, the lithium secondary battery having a charge-cutoff voltage of 4.35V and using 3-fluorotoluene (3-FT) as additive for electrolyte according to Example 5 and the lithium secondary battery having a charge-cutoff voltage of 4.35V and using CHB as additive for electrolyte according to Comparative Example 2. Disclosure of the Invention Therefore, the present invention has been made in view of the above-mentioned problems occurring in manufacturing a high-capacity battery having charge- cutoff voltages over 4.35V. We have found that when the weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode is controlled to an optimized condition, it is possible to ensure a plurality of sites into which an excessive amount of lithium ions deintercalated from a cathode can be intercalated. We have also found that it is possible to reduce side reactions between a cathode and electrolyte by controlling the particle diameter (particle size) of a cathode active material, and thus to improve the safety of a high-voltage battery. Therefore, it is an object of the present invention to provide a high-capacity lithium secondary battery that has a charge-cutoff voltage of between 4.35V and 4.6V and is stable even under overcharge conditions. According to an aspect of the present invention, there is provided a lithium secondary battery comprising a cathode (C) , an anode (A), a separator and an electrolyte, wherein the battery has a weight ratio (A/C) of anode active material to cathode active material per unit area of each electrode of between 0.44 and 0.70, and shows a charge cut-off voltage of between 4.35V and 4.6V. Hereinafter, the present invention will be explained in more detail. According to the present invention, the high- voltage lithium secondary battery showing charge-cutoff voltages over 4.35V, for example a high-output lithium secondary battery showing a charge-cutoff voltage of between 4.35V and 4.6V is characterized in that whose capacity balance is satisfied by controlling the weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode. The present invention characterized by the above- mentioned weight ratio provides the following effects. (1) The high-voltage battery having a charge-cutoff voltage of 4.35V or higher according to the present invention can show improved safety as well as higher capacity, voltage and output compared to conventional batteries having a charge-cutoff voltage of 4.2V. Japanese Laid-Open Patent No. 2001-68168 discloses a high-voltage battery having a charge cut-off voltage of 4.35V or higher, wherein the battery uses a cathode active material doped with transition metals or non- transition metals such as Ge, Ti, Zr, Y and Si so as to show such high voltage. When the battery is charged to a voltage higher than 4.35V, a great amount of lithium ions are deintercalated from the cathode. However, the anode has no sites into which such excessive amount of lithium ions can be intercalated, resulting in a rapid drop in battery safety. On the contrary, the lithium secondary battery according to the present invention is designed so that capacity balance can be satisfied by the presence of multiple anode sites, into which an excessive amount of lithium ions deintercalated from the cathode while the battery is charged to a voltage of 4.35V or higher, obtained by controlling the weight ratio (A/C) of anode active material (A.) to cathode active material (C) per unit area of each electrode. Therefore, the lithium secondary battery according to the present invention not only can provide high capacity and high output but also can solve the safety-related problem occurring in the high-voltage battery according to the prior art. (2) Additionally, the lithium secondary battery according to the present invention can prevent side reactions between the cathode active material and electrolyte, which may occur under overcharge conditions (over 4.35V), by controlling the particle diameter (size) of cathode active material, and thus prevent a drop in battery safety. In other words, as the specific surface area of a cathode active material increases, side reactions between a cathode active material and electrolyte increase. Therefore, the lithium secondary battery according to the present invention uses a cathode active material with a particle size greater than that of a currently used cathode active material so as to reduce the specific surface area of the cathode active material. Additionally, in order to prevent loss in reaction kinetics in the battery caused by the use of the cathode active material having such a large particle diameter, it is possible to control the loading amount of each electrode active material per unit area in the cathode and anode, and thus to realize improvement in battery safety. (3) Further, the lithium secondary battery according to the present invention can significantly increase the available capacity and average discharge voltage of a battery, even when using a lithium cobalt- based cathode active material such as LiCo02 that provides only about 55% of its theoretically available capacity by intercalation/deintercalation processes in a conventional battery having a charge-cutoff voltage of 4.2V. In fact, the following experimental examples show that although the lithium secondary battery according to the present invention uses LiCo02 in the same manner as a conventional battery, the battery provides an available capacity of LiCo02 increased by at least 14% (see, Table 1) - According to the present invention, the range of charge-cutoff voltages of the lithium secondary battery may be controlled in order to provide a high voltage and output of 4.35V or higher. Otherwise, the cathode active material used in the battery may be doped or substituted with another element, or may be. surface-treated with a chemically stable substance. More particularly, the lithium secondary battery according to the present invention has a charge-cutoff voltage of 4.35V or higher, preferably of between 4.35V and 4.6V. When the battery has a charge-cutoff voltage of lower than 4.35V, it is substantially the same as a conventional 4.2V battery and does not show an increase in the available capacity of a cathode active material so that a high-capacity battery cannot be designed and obtained. Additionally, when the battery has a charge- cutoff voltage of higher than 4.6V, the cathode active material used in the battery may experience a rapid change in structure due to the presence of the H13 phase generated in the cathode active material. In this case, there are problems in that transition metal is dissolved out of a lithium transition metal composite oxide used as cathode active material and oxygen loss may occur. Further, as the charge-cutoff voltage increases, reactivity between the cathode and electrolyte also increases, resulting in problems including explosion of the battery. The anode active material that may be used in the high-voltage lithium secondary battery having charge- cutoff voltages over 4.35V according to the present invention includes conventional anode active materials known to one skilled in the art (for example, materials capable of lithium ion intercalation/ deintercalation). There is no particular limitation in selection of the anode active material. Non-limiting examples of the anode active material include lithium alloys, carbonaceous materials, inorganic oxides, inorganic chalcogenides, nitrides, metal complexes or organic polymer compounds. Particularly preferred are amorphous or crystalline carbonaceous materials. The cathode active material that may be used in the high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V according to the present invention includes conventional cathode active materials known to one skilled in the art (for example, lithium-containing composite oxides having at least one element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, transition metals and rare earth elements) . There is no particular limitation in selection of the cathode active material. Non- limiting examples of the cathode active material include various types of lithium transition metal composite oxides (for example, lithium manganese composite oxides such as LiMn204; lithium nickel oxides such as LiNi02; ' lithium cobalt oxides such as LiCoC^; lithium iron oxides; the above-described oxides in which manganese, nickel, cobalt or iron is partially doped or substituted with other transition metals or non-transition metals (for example, Al, Mg, Zr, Fe, Zn, Ga, Si, Ge or combinations thereof); lithium-containing vanadium oxides; and chalcogenides (for example, manganese dioxide, titanium disulfide, molybdenum disulfide, etc.). As cathode active material, lithium cobalt composite oxides optionally doped with Al, Mg, Zr, Fe, Zn, Ga, Sn, Si and/or Ge are preferable and LiCo02 is more preferable. Even if LiCo02 is used as cathode active material in the same manner as conventional batteries, the lithium secondary battery according to the present invention can provide an increase in available capacity of the cathode active material and thus can be a high- voltage battery due to a suitable design in electrodes. In the high-voltage battery having a charge-cutoff voltage of 4.35V or higher according to the present invention, the weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode ranges suitably from 0.44 to 0.70 and more preferably from 0.5 to 0.64. When the weight ratio is less than 0.44, the battery is substantially the same as a conventional 4.2V-battery. Therefore, when the battery is overcharged to 4.35V or higher, the capacity balance may be broken to cause dendrite growth on the surface of anode, resulting in short-circuit in the battery and a rapid drop in the battery capacity. When the weight ratio is greater than 0.64, an excessive amount of lithium sites exists undesirably in the anode, resulting in a drop in energy density per unit volume/mass of the battery. According to the present invention, such controlled weight ratio of anode active material to cathode active material per unit area of each electrode can be obtained preferably by using LiCo02, LiNiMnCo02 or LiNiMn02 having a capacity similar to that of LiCo02, etc., as cathode active material and using graphite as anode active material. When high-capacity cathode materials such as Ni-containing materials and/or high- capacity anode materials such as Si are used, it is possible to design and manufacture an optimized lithium secondary battery having high capacity, high output and improved safety through recalculation of the weight ratio considering a different capacity. However, the scope of the present invention is not limited to the above-mentioned cathode active materials and anode active materials. The cathode active materials used in the lithium secondary battery according to the present invention (for example, LiCo02) have a problem in that they are deteriorated in terms of thermal properties when being -charged to 4.35V or higher. - To prevent- the -problem,—it is possible to control the specific surface area of the cathode active material. As the particle size of the cathode active material increases (in other words, as the specific surface area of the cathode active material decreases), reactivity between the cathode active material and electrolyte may decrease, resulting in improvement in thermal stability. For this reason, it is preferable to use a cathode active material having a particle diameter larger than that of a currently used cathode active material. Therefore, the cathode active material used in the battery according to the present invention preferably has a particle diameter (particle size) of between 5 and 30 pm- When the cathode active material has a particle diameter of less than' 5 jj&, side reactions between the cathode and electrolyte increase to cause the problem of poor safety of the battery. When the cathode active material has a particle diameter of greater than 30 [Mr reaction kinetics may be slow in the battery. Additionally, in order to prevent the degradation of reaction kinetics in the whole battery, caused by the use of a cathode active material having a particle diameter greater than that of a currently used cathode active material, it is possible to control the loading amount of cathode active material and anode active material per unit area of each electrode. It is preferable that the loading amount of cathode active material per unit area of cathode ranges from 10 to 30 mg/cm2. When the loading amount of cathode active material is less than 10 mg/cm2, the battery may be degraded in terms of. capacity and efficiency. When the loading amount of cathode active material is greater than 30 mg/cm2, thickness of the cathode increases, resulting in degradation of reaction kinetics in the battery. Additionally, it is preferable that the loading amount of anode active material per unit area of anode ranges from 4.4 to 21 mg/cm2. When the loading amount of anode active material is less than 4.4 mg/cm2, capacity balance cannot be maintained, thereby causing degradation in battery safety. When the loading amount of anode active material is greater than 21 mg/cm2, an excessive amount of lithium sites is present undesirably in the anode, resulting in a drop in energy density per unit volume/mass of the battery. The electrode used in the battery according to the present invention can be manufactured by a conventional process known to one skilled in the art. In one embodiment, slurry for each electrode is applied onto a current collector formed of metal foil, followed by rolling and drying. Slurry for each electrode, i.e., slurry for a cathode and anode may be obtained by mixing the above- described cathode active material/anode active material with a binder and dispersion medium. Each of the slurry for a cathode and anode preferably contains a small amount of conductive agent. There is no particular limitation in the conductive agent, as long as the conductive agent is an electroconductive material that experiences no chemical change in the battery using the same. Particular examples of the conductive agent that may be used include carbon black such as acetylene black, ketchen black, furnace black or thermal black; natural graphite, artificial graphite and conductive carbon fiber, etc., carbon black, graphite powder or carbon fiber being preferred. The binder that may be used includes thermoplastic resins, thermosetting resins or combinations thereof. Among such resins, polyvinylidene difluoride (PVdF), styrene butadiene rubber (SBR) or polytetrafluoroethylene (PTFE) is preferable, PVdF being more preferable. The dispersion medium that may be used includes aqueous dispersion media or organic dispersion media such as N-methyl-2-pyrollidone. In both electrodes of the lithium secondary battery according to the present invention, the ratio (A/C) of the thickness of cathode (C) to that of anode (A) suitably ranges from 0.7 to 1.4, preferably from 0.8 to 1.2. When the thickness ratio is less than 0.7, loss of energy density per unit volume of the battery may occur. When the thickness ratio is greater than 1.4, reaction kinetics may be slow in the whole battery. The high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V or higher according to the present invention includes a cathode (C) , an anode (A) , a separator interposed between both electrodes and an electrolyte, wherein the cathode(C) and anode(A) are obtained by controlling the weight ratio (A/C) of anode active material to cathode active material per unit area of each electrode to 0.44-0.70. The high-voltage lithium secondary battery having a charge-cutoff voltage of 4.35V or higher is also characterized by using an electrolyte that further comprises a compound having a_ reaction potential of 4.7V or higher in addition to a currently used electrolyte for batteries. Due to the presence of the above characteristic electrolyte, it is possible to improve the safety and high-temperature storage characteristics of a high- voltage lithium secondary battery having a charge-cutoff voltage of 4.35V or higher. (1) When cyclohexylbenzene (CHB) or biphenyl (BP), currently used as additives for electrolyte in conventional batteries having a charge-cutoff voltage of 4.2V or higher, are used in order to improve the safety and high-temperature storage characteristics of a high- voltage lithium secondary battery having a charge-cutoff voltage of 4.35V or higher, cycle characteristics of the battery at room temperature and high temperature are degraded rapidly. Additionally, because a large amount of the above additives are decomposed under high- temperature storage conditions, a very thick insulator film is formed on a cathode to prevent lithium ions from moving in the battery, so that recovery capacity of the battery cannot be obtained. On the contrary, the battery according to the present invention uses fluorotoluene (FT) compounds having a reaction potential of 4.7V or higher (for example, 2-fluorotoluene (2-FT) and/or 3-fluorotoluene (3-FT)) as additives for electrolyte. Because such additives have high reaction potentials and experience little change in reaction potentials during repeated cycles, it is possible to prevent degradation of battery quality caused by decomposition of an additive at a voltage of between 4.35V and 4.6V and a rapid change in reaction potentials, and to improve high-temperature storage characteristics of a battery. (2) When such additives for electrolyte are used, it is possible to reduce a contact surface where side reactions between a cathode and electrolyte may occur in case of the battery containing only conventional electrolyte, and thus to improve battery safety. There is no particular limitation in the additive that may be added to the electrolyte of the high-voltage lithium secondary battery having a charge-cutoff voltage of 4.35V or higher, as long as the additive is a compound having a reaction potential of 4.7V or higher. Preferably, the additive is a fluorotoluene (FT) compound. Among fluorotoluene compounds, 2-fluorotoluene (2-FT) and/or 3-fluorotoluene (3-FT) are more preferable, because they have high reaction potentials and experience little change in reaction potentials during repeated cycles. Because 2-fluorotoluene and/or 3-fluorotoluene are physically stable and have such a high boiling point as to prevent thermal decomposition as well as a high reaction potential of 4.7V or higher (the reaction potential being higher than the reaction potential of CHB or BP by about 0.1V), they can improve high- temperature storage characteristics and safety of a battery using an electrolyte comprising them as additives, contrary to conventional additives such as CHP and BP. Additionally, because they experience little change in reaction potentials during repeated cycles, as compared to conventional fluorotoluene compounds, they can prevent degradation in cycle characteristics of a high-voltage battery. In fact, when a fluorotoluene compound other than 2-fluorotoluene and 3-fluorotoluene, or 4-fluorotolune (4-FT) having a reaction potential similar to that of CHB is used, a battery having a charge-cutoff voltage of 4.35V or higher shows significant degradation in cycle characteristics during repeated cycles due to a reaction of a cathode active material with a fluorine atom substituted in the para-position. Therefore, it is not possible to improve the safety and high-temperature storage characteristics of a battery. Preferably, the compound having a reaction potential of 4.7V or higher (for example, 2-FT and/or 3- FT) is added to an electrolyte in an amount of between 0.1 and 10 wt% based on 100 wt% of the total weight of electrolyte. When the compound is used in an amount of less than 0.1 wt%, it is not possible to improve the safety and quality of a battery significantly. When the compound is used in an amount of greater than 10 wt%, there are problems in that viscosity of the electrolyte decreases and the additive causes an exothermic reaction to emit heat excessively. The high-voltage battery having a voltage of 4.35V or higher according to the present invention can be manufactured by a conventional process known to one skilled in the art. In one embodiment, a cathode and anode are provided with a separator interposed between both electrodes and an electrolyte is introduced, wherein the cathode (C) and anode (A) are obtained by controlling the weight ratio (A/C) of anode active material to cathode active material per unit area of each electrode to 0.44-0.70. The electrolyte that may be used in the present invention includes a.salt represented by the formula of A+B~, wherein A+ represents an alkali metal cation selected from the group consisting of Li+, Na+, K+ and combinations thereof, and B" represents an anion selected from the group consisting of PF6~, BF4", CI", Br", I", C104", AsF6~, CH3C02", CF3SO3-, N(CF3S02)2", C(CF2S02)3" and combinations thereof, the salt being dissolved or dissociated in an organic solvent selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone (y-butyrolactone) and mixtures thereof. However, the electrolyte that may be used in the present invention is not limited to the above examples. Particularly, when an electrolyte comprising a compound having a reaction potential of 4.7V or higher (for example, 2- fluorotoluene and/or 3-fluorotoluene) is used, it is possible to improve high-temperature storage characteristics and safety with no degradation in cycle characteristics of the high-voltage battery. Although there is no particular limitation in the separator that may be used in the present invention, porous separators may be used. Particular examples of porous separators include polypropylene-based, polyethylene-based and polyolefin-based porous separators. There is no particular limitation in the shape of the lithium secondary battery according to the present invention. The lithium secondary battery may be a cylindrical, prismatic, pouch-type or a coin-type battery. Additionally, according to another aspect of the present invention, there is provided a lithium secondary battery that includes a cathode, an anode, a separator and an electrolyte, wherein the battery has a charge- cutoff voltage of between 4.35V and 4.6V, and the electrolyte comprises a compound having a reaction potential of 4.7V or higher. In the lithium secondary battery, the compound having a reaction potential of 4.7V or higher is the same as defined above. Best Mode for Carrying Out the Invention Reference will now be made in detail to the preferred embodiments of the present invention. It is to be understood that the following examples are illustrative only and the present invention is not limited thereto. [Examples 1-5. Manufacture of batteries having charge-cutoff voltage over 4.35V] Example 1. Lithium secondary battery having charge-cutoff voltage of 4.35V (1) (Manufacture of cathode) 95 wt% of LiCo02 having a particle diameter of 10 IMr 2.5 wt% of a conductive agent and 2.5 wt% of a binder were mixed to form slurry. The slurry was applied uniformly on both surfaces of aluminum foil having a thickness of 15 ^m, followed by rolling, to provide a cathode having an active material weight of 19.44 mg/cm2. The finished cathode had a thickness of 128 p^. (Manufacture of anode) To 95.3 _wt% of graphite, 4.0 wt% of a binder and_ 0.7 wt% of a conductive agent were added and mixed to form slurry. The slurry was applied uniformly on both surfaces of copper foil having a thickness of 10 j^m, followed by rolling, to provide an anode having an active material weight of 9.56 mg/cm2. The weight ratio (A/C) of the anode active material to cathode active material per unit area of each electrode was 0.49, and the finished anode had a thickness of 130 y&. (Preparation of electrolyte) To a solution containing ethylene carbonate and dimethyl carbonate in a volume ratio of 1:2 (EC:DMC), 1M LiPF6 was dissolved to provide an electrolyte. (Manufacture of battery) The cathode and anode obtained as described above were used to provide a coin-type battery and prismatic battery. The manufacturing process of each battery was performed in a dry room or glove box in order to prevent the materials from contacting with the air. Example 2. Lithium secondary battery having charge-cutoff voltage of 4.35V (2) Example 1 was repeated to provide a lithium secondary battery, except that a cathode (C) having an active material weight of 22 mg/cm2 and an anode having an active material weight of 11 mg/cm2 were used to adjust the weight ratio (A/C) of the anode active material to cathode active material per unit area of each electrode to 0.50. Example 3. Lithium secondary battery having charge-cutoff voltage of 4.4V Example 1 was repeated to provide a lithium secondary battery having a charge-cutoff voltage of 4.4V, except that a cathode (CJ having an active material weight of 22 mg/cm2 and an anode having an active material weight of 11.66 mg/cm2 were used to adjust the weight ratio (A/C) of the anode active material to, cathode active material per unit area of each electrode to 0.53. Example 4. Lithium secondary battery having charge-cutoff voltage of 4.5V Example 1 was repeated to provide a lithium secondary battery having a charge-cutoff voltage of 3 4.5V, except that a cathode (C) having an active material weight of 22 mg/cm2 and an anode having an active material weight of 12.57 mg/cm2 were used to adjust the weight ratio (A/C) of the anode active material to cathode active material per unit area of each electrode to 0.57. Example 5. Lithium secondary battery having charge-cutoff voltage of 4.35V Example 1 was repeated to provide a lithium secondary battery, except that 3 wt% of 3-fluorotoluene (3-FT) was added to 100 wt% of the electrolyte containing 1M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio= 1:2 (EC:DMC)). [Comparative Examples 1-4] Comparative Example 1. Manufacture of lithium secondary battery having charge-cutoff voltage of 4.2V Example 1 was repeated to provide a lithium secondary battery, except that a cathode (C) having an active material weight of 22 mg/cm2 and an anode having an active material weight of 9.68 mg/cm2 were used to adjust the weight ratio (A/C) of the anode active material to cathode active material per unit area of each electrode to 0.44, as described in the above Table ' 1. _ Comparative Example 2 _ Example 1 was repeated to provide a lithium secondary battery, except that 3 wt% of cyclohexyl benzene (CHB) was added to the electrolyte. Comparative Example 3 Example 1 was repeated to provide a lithium secondary battery, except that 3 wt% of 4-fluorotoluene (para-FT) was added to the electrolyte instead of 3- fluorotoluene. Comparative Example 4 Example 1 was repeated to provide a lithium secondary battery, except that the weight ratio (A/C) of the anode active material to cathode active material per unit area of each electrode was adjusted to 0.44 and 3 wt% of cyclohexyl benzene (CHB) was added to the electrolyte. Experimental Example 1. Evaluation for high- voltage battery having charge-cutoff voltages over 4.35V vs. battery having charge-cutoff voltage of 4.2V 1-1. Evaluation of charge/discharge capacity The following experiment was carried out to compare the charge/discharge capacity of the lithium secondary battery having a charge-cutoff voltage of 4.35V or higher according to the present invention with that of the lithium secondary battery having a charge- cutoff voltage of 4.2V. The batteries according to Examples 2-4 were used as samples for batteries having charge-cutoff voltages over 4.35V and the battery according to Comparative Example 1 was used as control (4.2V-battery) . The battery according to Example 2 was tested in a charge/discharge voltage range of between 3V and 4.35V, the battery according to Example 3 was tested in a range of between 3V and 4.4V, the battery according to Example 4 was tested in a range of between 3V and 4.5V, and the battery according to Comparative Example 1 was tested in a range of between 3V and 4.2V, each battery being subjected to cycling under 1C charge/lC discharge conditions. The test were performed at room temperature (25°C/45°C) - After the experiment, the 4.2V battery according to Comparative Example 1 showed an initial charge capacity and discharge capacity of 155.0 mAh/g and 14 9.4 mAh/g, respectively. The battery had an energy density per unit volume of battery of 380.0 Wh/kg (see, FIG. 2 and Table 2). On the contrary, the 4.35V-battery according to Example 2 showed an initial charge capacity and discharge capacity of 179.7 mAh/g and 171.3 mAh/g, respectively, and had an energy density per unit volume of battery of 439.2 Wh/kg, resulting in improvements in terms of discharge capacity and energy density per unit volume of battery by 14.6% and 15.6%, respectively (see, FIG. 1 and Table 2). Additionally, the 4. 4V-battery and 4.5V-battery according to Examples 3 and 4 showed an increase in discharge capacity of 20% and 30%, respectively, compared to the 4.2V-battery according to Comparative example 1 as control. Further, the batteries according to Examples 3 and 4 showed an increase in energy density per unit volume of 22.3% and 33.4%, respectively (see, Table 2). As can be seen from the above results, even if the lithium secondary according to the present invention uses the same cathode active material (LiCo02) that is used in a conventional battery, it increases the available capacity of LiCo02 by at least 14% and improves the energy density per unit volume significantly through the modification in the electrode design. 1-2. Evaluation for safety The following overcharge test was performed for the lithium secondary battery having charge-cutoff voltages over 4.35V according to the present invention and the battery having a charge-cutoff voltage of 4.2V. The battery according to Example 2 was used as sample for a battery having a charge-cutoff voltage of 4.35V or higher and the battery according to Comparative Example 1 was used as control (4.2V-battery). Each battery was subjected to the overcharge test under an overcharge voltage of 5.0V with an electric current of 2A at room temperature (25°C). After the experiment, the temperature of 4.2V- battery according to Comparative Example 1 increased to 200 °C after the lapse of 1 hour and exploded due to short-circuit in the battery (see, FIG. 4) . This indicates that when the conventional 4.2V-battery was overcharged to 5.0V, reactivity between the cathode and electrolyte increases to cause the decomposition of the cathode surface and oxidation of the electrolyte, and lithium dendrite growth occurs due to the lack of anode sites, into which an excessive amount of lithium ions deintercalated from the cathode upon overcharge is intercalated, resulting in a significant drop in electrochemical stability of the battery. On the contrary, when the battery having a charge- cutoff voltage of 4.35V according to the present invention was overcharged to 5.0V, the battery temperature increased to 40D. However, the temperature was stabilized with time (see, FIG. 3) . This indicates that the battery according to the present invention has a large amount of anode sites, into which an excessive amount of lithium ions deintercalated from the cathode upon overcharge can be intercalated, and shows a significant decrease in side reactions between the cathode and electrolyte due to an increased reactivity between them caused by overcharge. As can be seen from the foregoing, the lithium secondary battery according to the present invention has significantly improved overcharge safety, because it has a controlled weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode, contrary to the conventional 4.2V battery. Experimental Example 2. Evaluation for cycle characteristics of high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V The high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V according to the present invention was evaluated for cycle characteristics as follows. The lithium secondary battery using no additive for electrolyte according to Example 1 and the lithium secondary battery using 3-fluorotoluene (3-FT) as additive for electrolyte according to Example 5 were used as samples for batteries having charge-cutoff voltages over 4.35V. As controls, the battery using CHB as additive for electrolyte according to Comparative Example 2 and the battery using 4-fluorotoluene (4-FT) as additive for electrolyte according to Comparative Example 3 were used. Each battery was tested in a charge/discharge voltage range of between 3.0V and 4.35V and was subjected to cycling under a charge/discharge current of 1C (= 880 mA) . At the zone of 4.35V constant voltage, the voltage was maintained at 4.35V until the current dropped to 50 mA and the test was performed at 45°C- After the experiment, the lithium secondary battery using the electrolyte containing CHB as additive showed significant degradation in cycle characteristics under high temperature conditions, as compared to the lithium secondary battery using no additive for electrolyte according to Example 1 and the lithium secondary battery using the electrolyte containing 3- fluorotoluene (3-FT) as additive according to Example 5 (see, FIG. 5). This indicates that because CHB having a reaction potential of less than 4.7V experiences electropolymerization to form a coating layer, charge transfer reaction of the cathode active material is inhibited and resistance is increased at the cathode, resulting in degradation in cycle characteristics of the battery. Additionally, the battery using 4-fluorotoluene having a reaction potential similar to that of CHB according to Comparative Example 3 showed a rapid drop in cycle characteristics, because the cathode active material may react with the fluorine atom present at the para-position of 4-FT during cycles under 4.35V (see, FIG. 5). On the contrary, the lithium secondary battery using 3-fluorotoluene (3-FT) having a reaction potential of higher than 4.7V as additive for electrolyte according to Example 5 did not show any significant change in high-temperature cycle characteristics, as can be seen from FIG. 5 (see, FIG. 6) . Therefore, it can be seen that the high-voltage lithium secondary battery using a compound having a reaction potential higher than 4.7 V (for example, 3- fluorotoluene (3-FT)) as additive for electrolyte according to the present invention can prevent degradation in high-temperature cycle characteristics, contrary to a 4.2V-battery using CHB as additive for electrolyte. Experimental Example 3. Evaluation for safety of high-voltage lithium secondary having charge-cutoff voltages over 4.35V The following hot box test was performed in order to evaluate the safety of the high-voltage lithium secondary having charge-cutoff voltages over 4.35V according to the present invention. The high-voltage lithium secondary battery using 3-fluorotoluene as additive for electrolyte according to Example 5 was used as sample. As controls, the lithium secondary batteries using CHB and 4-fluorotoluene (4-FT) as additives for electrolyte according to Comparative Examples 2 and 3, respectively, were used. Each battery was charged to 4.4V under 1C (=880 mA) for 2.5 hours and then maintained under the constant voltage condition. Then, each battery was introduced into an oven capable of convection, warmed from room temperature to a high temperature of 15 0°C at a rate of 5°C/min., and exposed to such high-temperature condition for 1 hour. Additionally, each battery was checked for explosion. After the experiment, the batteries using CHB and 4-FT as additive for electrolyte, respectively, according to Comparative Example 2 and Comparative Example 3 exploded with time (see, FIGs. 7 and 8). On the contrary, the lithium secondary battery using 3- fluorotoluene as additive for electrolyte according to Example 5 showed a stable state even at a high temperature of 150°C (see, FIG. 9) . Experimental Example 4. Evaluation for high- temperature storage characteristics of high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V The high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V was evaluated in the following high-temperature storage tests. 4-1. Long-term high-temperature storage test The lithium secondary battery using 3- fluorotoluene as additive for electrolyte was used as sample. As controls, the batteries using CHB and 4-FT as additives for electrolyte, respectively, according to Comparative Example 2 and Comparative Example 3 were used. Each battery was charged at a charging current of IC to 4.35V and discharged at IC to 3V to determine the initial discharge capacity. Next, each battery was recharged to 4.35V and was subjected to repeated 30 cycles of 3-hour storage at 80°C/7-hour storage at 25°C. During such cycles, the thickness of each battery was measured. Then, each battery was discharged at IC to determine the residual capacity of each battery. After measuring the residual capacity, each battery was subjected to three charge/discharge cycles and measured for the recovery capacity. In order to ensure reproducibility, the above-described procedure was repeated 4 times. After the experiment, the battery comprising CHB according to Comparative Example 2 showed a significant swelling phenomenon before the fifth charge/discharge cycle (see, FIG. 10). Additionally, the battery using 4- fluorotoluene whose reaction potential is similar to that of CHB also showed a significant swelling phenomenon after approximately 10 charge/discharge cycles (see, FIG. 11) . On the contrary, the battery using 3-fluorotoluene according to Example 5 showed a significant drop in the battery swelling phenomenon (see, FIG. 10). 4-2. Short-term high-temperature storage test The lithium secondary battery using no additive for electrolyte according to Example 1 and the lithium secondary battery using 3-fluorotoluene as additive for electrolyte according to Example 5 were used as samples. As controls, the batteries using CHB and 4-FT as additives for electrolyte, respectively, according to Comparative Example 2 and Comparative Example 3 were used. Each battery was charged at a charging current of IC to 4.35V and discharged at IC to 3V to determine the initial discharge capacity. Next, each battery was recharged to 4.35V and was stored at 90°C for 4 hours, during which the thickness of each battery was measured. Then, each battery was discharged at IC to determine the residual capacity of each battery. After measuring the residual capacity, each battery was subjected to' three charge/discharge cycles and measured for the recovery capacity. After the storage at 90°C for 4 hours, the battery having a charge-cutoff voltage of 4.35V or higher according to Comparative Example 2 showed a significant increase in its thickness, particularly compared to the battery using no additive for electrolyte according to Example 1 (see, FIG. 11). This indicates that the electrolyte is decomposed due to the increase in reactivity between the cathode and electrolyte to form a thick insulator film, resulting in an increase in the battery thickness. Therefore, it can be seen that a conventional additive (for example, CHB) for a 4.2V battery is . not suitable for a high-voltage battery having a charge-cutoff voltage of 4.35V or higher. On the contrary, the high-voltage lithium secondary battery having charge-cutoff voltages over 4.35V and using 3-fluorotoluene as additive for electrolyte according to Example 5 did not show a swelling phenomenon even after the storage at 90°C. This indicates that the battery shows little degradation in the battery quality (see, FIG. 11). Therefore, it can be seen that a f luorotoluene compound having a reaction potential of 4.7V or higher (for example, 2-fluotoluene and 3-fluorotoluene) is suitable for an additive for electrolyte in the high- voltage battery having a charge-cutoff voltage of 4.35V or higher according to the present invention. Industrial Applicability As can be seen from the foregoing, the high- voltage lithium secondary battery according to the present invention satisfies capacity balance by controlling the weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode. By doing so, it is possible to increase the available capacity of cathode active material significantly by at least 14%, as compared to the available capacity of cathode active material in a conventional battery of merely about 50%. Therefore, the battery according to the present invention can solve the problems occurring in 4.2V-batteries according to the prior art upon overcharge, and thus can provide a high- voltage lithium secondary battery having excellent safety and long service life. Further, when a fluorotoluene compound having a reaction potential of 4.7V or higher is used as additive for electrolyte in a high-voltage battery having a charge-cutoff voltage of 4.35V or higher, it is possible to improve the safety and high-temperature storage characteristics of the battery with no degradation in cycle characteristics. While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings. On the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims. WE CLAIM: 1. A lithium secondary battery comprising: (i) a cathode (C) in which cathode active materials with a particle diameter of between 5 µm and 30 µm are loaded in an amount of between 10 mg/cm and 30 mg/cm2, (ii) an anode (A) in which anode active materials are loaded in an amount of between 4.4 mg/cm2 and 21 mg/cm2, (iii) a separator, and (iv) an electrolyte, wherein the battery has a weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode of between 0.44 and 0.70, and has a ratio (A/C) of thickness of the cathode (C) to that of the anode (A) of between 0.7 and 1.4, and shows a charge cut-off voltage of between 4.35V and 4.6V. 2. The lithium secondary battery as claimed in claim 1, wherein the cathode (C) is obtained from a cathode active material capable of lithium intercalation /deintercalation, the cathode active material being doped with at least one metal selected from the group consisting of Al, Mg, Zr, Fe, Zn, Ga, Sn, Si and Ge. 3. The lithium secondary battery as claimed in claim 1, wherein the cathode active material is a lithium-containing composite oxide having at least one element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, transition metals and rare earth elements. 4. The lituum secondary battery as claimed in claim 1, wherein the electrolyte further comprises a compound having a reaction potential of 4.7V or higher. 5. The lithium secondary battery as claimed in claim 4, wherein the compound having a reaction potential of 4.7V or higher is at least one fluorotoluene compound selected from the group consisting of 2-fluorotoluene and 3-fluorotoluene. 6. The lithium secondary battery as claimed in claim 4, wherein the compound having a reaction potential of 4.7V or higher is used in an amount of between 0.1 and 10 wt% based on 100 wt% of the electrolyte. A lithium secondary battery is disclosed. The lithium secondary battery comprises: (i) a cathode (C) in which cathode active materials with a particle diameter of between 5 µm and 30 µm are loaded in an amount of between 10 mg/cm2 and 30 mg/cm2, (ii) an anode (A) in which anode active materials are loaded in an amount of between 4.4 mg/cm2 and 21 mg/cm2, (iii) a separator, and (iv) an electrolyte, wherein the battery has a weight ratio (A/C) of anode active material (A) to cathode active material (C) per unit area of each electrode of between 0.44 and 0.70, and has a ratio (A/C) of thickness of the cathode (C) to that of the anode (A) of between 0.7 and 1.4, and shows a charge cut-off voltage of between 4.35V and 4.6V.

Full Text

Technical Field
The present invention relates to a lithium
secondary battery having a charge-cutoff voltage of
4.35V or higher. More particularly, the present
invention, relates to a lithium secondary battery, which
has a charge cut-off voltage of between 4.35V and 4.6V,
high capacity, high output and improved safety and is
provided with capacity balance suitable for a high-
voltage battery by controlling the weight ratio (A/C) of
both electrode active materials, i.e., weight ratio of
anode active material (A) to cathode active material (C)
per unit area of each electrode.
Background Art
Recently, as electronic devices become smaller and
lighter, batteries used therein as power sources are
increasingly required to have a compact size and light
weight. As rechargeable batteries with a compact size,
light weight and high capacity, lithium secondary
batteries such as secondary lithium ion batteries have
been put to practical use and widely used in portable
electronic and communication devices such as compact
camcorders, portable phones, notebook PCs, etc.
A lithium secondary battery comprises a cathode,
anode and an electrolyte. Lithium secondary batteries
are classified into liquid electrolyte lithium secondary
batteries using an electrolyte comprising a liquid
organic solvent and lithium polymer batteries using an
electrolyte comprising a polymer.

Although lithium having high electronegativity and
high capacity per unit mass has been used as electrode
active material for a lithium secondary battery, there
is a problem in that lithium cannot ensure the stability
of a battery by itself. Therefore, many attempts have
been made to develop batteries using a material capable
of lithium ion intercalation/deintercalation as
electrode active material.
Cathode active materials that are currently used
in lithium secondary batteries include lithium-
containing transition metal composite oxides such as
LiCo02, LiNi02, LiMn204, LiMn02 and LiFe02. Particularly,
LiCo02 providing excellent electroconductivity, high
voltage and excellent electrode characteristics is a
typical example of commercially available cathode active
materials. As anode active materials, carbonaceous
materials capable of intercalation/deintercalation of
lithium ions in an electrolyte are used. Additionally,
polyethylene-based porous polymers are used as
separators. A lithium secondary battery formed by using
a cathode, anode and an electrolyte as described above
permits repeated charge/discharge cycles, because
lithium ions deintercalated from the cathode active
material upon the first charge cycle serve to transfer
energies while they reciprocate between both electrodes
(for example, they are intercalated into carbon
particles forming the anode active material and then
deintercalated upon a discharge cycle).
In order to provide such lithium secondary
batteries having high capacity, output and voltage, it
is necessary to increase the theoretically available
capacity of the cathode active material in a battery. To

satisfy this, it is required that the charge-cutoff
voltage of a battery is increased. Conventional
batteries having a charge-cutoff voltage of 4.2V using
LiCo02 among the above-described cathode active
materials, utilize only about 55% of the theoretically
available capacity of LiCo02 by
intercalation/deintercalation processes. Therefore,
selection of the anode active material in such batteries
is limited so as to be conformed to the capacity of
lithium ions to be deintercalated from the cathode. When
such batteries are overcharged to a voltage of 4.35V or
higher, the anode has no sites into which an excessive
amount of lithium ions deintercalated from the cathode
are intercalated. Therefore, lithium dendrite growth
occurs, resulting in problems of rapid exothermic
reactions and poor safety of the batteries.
Additionally, side reactions between the cathode and
electrolyte may occur to cause degradation of the
cathode surface and oxidation of electrolyte.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The foregoing and other objects, features and
advantages of the present invention will become more
apparent from the following detailed description when
taken in conjunction with the accompanying drawings in
which:
FIG. 1 is a graph showing variations in discharge
capacity of the secondary lithium ion battery having a
charge-cutoff voltage of 4.35V, obtained from Example 2;
FIG. 2 is a graph showing variations in discharge
capacity of the secondary lithium ion battery having a
charge-cutoff voltage of 4.2V, obtained from Comparative -

Example 1;
FIG. 3 is a graph showing the results of the
overcharge test for the secondary lithium ion battery
having a charge-cutoff voltage of 4.35V, obtained from
Example 2;
FIG. 4 is a graph showing the results of the
overcharge test for the secondary lithium ion battery
having a charge-cutoff voltage of 4.2V, obtained from
Comparative Example 1;
FIG. 5 is a graph showing high-temperature (45°C)
cycle characteristics of each of the lithium secondary
battery having a charge-cutoff voltage of 4.35V and
using no additive for electrolyte according to Example
1, the lithium secondary battery having a charge-cutoff
voltage of 4.35V and using cyclohexylbenzene (CHB) as
additive for electrolyte according to Comparative
Example 2 and the lithium secondary battery having a
charge-cutoff voltage of 4.35V and using 4-fluorotoluene
(para-FT) as additive for electrolyte according to
Comparative Example 3;
FIG. 6 is a graph showing high-temperature (45°C)
cycle characteristics of the lithium secondary battery
having a charge-cutoff voltage of 4.35V and using 3-
fluorotoluene (3-FT) as additive for electrolyte
according to Example 5;
FIG. 7 is a graph showing the results of the hot
box test for the lithium secondary battery having a
charge-cutoff voltage of 4.35V and using CHB as additive
for electrolyte according to Comparative Example 2;
FIG. 8 is a graph showing the results of the hot
box test for the lithium secondary battery having a
charge-cutoff voltage of 4.35V and using 4-fluorotoluene

(para-FT) as additive for electrolyte according to
Comparative Example 3;
FIG. 9 is a graph showing the results of the hot
box test for the lithium secondary battery having a
charge-cutoff voltage of 4.35V and using 3-fluorotoluene
(3-FT) as additive for electrolyte according to Example
5;
FIG. 10 is a graph showing the results of the
high-temperature storage test (30 cycles: 80°C/3 hr +
25°C/7 hr) for each of the lithium secondary battery
having a charge-cutoff voltage of 4.35V and using CHB as
additive for electrolyte according to Comparative
Example 2, the lithium secondary battery having a
charge-cutoff voltage of 4.35V and using 4-fluorotoluene
(para-FT) as additive for electrolyte according to
Comparative Example 3 and the lithium secondary battery
having a charge-cutoff voltage of 4.35V and using 3-
fluorotoluene (3-FT) as additive for electrolyte
according to Example 5; and
FIG. 11 is a graph showing the results of the
high-temperature/short-term storage test (90°C/4 hr) for
each of the lithium secondary battery having a charge-
cutoff voltage of 4.35V and using no additive for
electrolyte according to Example 1, the lithium
secondary battery having a charge-cutoff voltage of
4.35V and using 3-fluorotoluene (3-FT) as additive for
electrolyte according to Example 5 and the lithium
secondary battery having a charge-cutoff voltage of
4.35V and using CHB as additive for electrolyte
according to Comparative Example 2.

Disclosure of the Invention
Therefore, the present invention has been made in
view of the above-mentioned problems occurring in
manufacturing a high-capacity battery having charge-
cutoff voltages over 4.35V. We have found that when the
weight ratio (A/C) of anode active material (A) to
cathode active material (C) per unit area of each
electrode is controlled to an optimized condition, it is
possible to ensure a plurality of sites into which an
excessive amount of lithium ions deintercalated from a
cathode can be intercalated. We have also found that it
is possible to reduce side reactions between a cathode
and electrolyte by controlling the particle diameter
(particle size) of a cathode active material, and thus
to improve the safety of a high-voltage battery.
Therefore, it is an object of the present
invention to provide a high-capacity lithium secondary
battery that has a charge-cutoff voltage of between
4.35V and 4.6V and is stable even under overcharge
conditions.
According to an aspect of the present invention,
there is provided a lithium secondary battery comprising
a cathode (C) , an anode (A), a separator and an
electrolyte, wherein the battery has a weight ratio
(A/C) of anode active material to cathode active
material per unit area of each electrode of between 0.44
and 0.70, and shows a charge cut-off voltage of between
4.35V and 4.6V.
Hereinafter, the present invention will be
explained in more detail.
According to the present invention, the high-
voltage lithium secondary battery showing charge-cutoff

voltages over 4.35V, for example a high-output lithium
secondary battery showing a charge-cutoff voltage of
between 4.35V and 4.6V is characterized in that whose
capacity balance is satisfied by controlling the weight
ratio (A/C) of anode active material (A) to cathode
active material (C) per unit area of each electrode.
The present invention characterized by the above-
mentioned weight ratio provides the following effects.
(1) The high-voltage battery having a charge-cutoff
voltage of 4.35V or higher according to the present
invention can show improved safety as well as higher
capacity, voltage and output compared to conventional
batteries having a charge-cutoff voltage of 4.2V.
Japanese Laid-Open Patent No. 2001-68168 discloses
a high-voltage battery having a charge cut-off voltage
of 4.35V or higher, wherein the battery uses a cathode
active material doped with transition metals or non-
transition metals such as Ge, Ti, Zr, Y and Si so as to
show such high voltage. When the battery is charged to a
voltage higher than 4.35V, a great amount of lithium
ions are deintercalated from the cathode. However, the
anode has no sites into which such excessive amount of
lithium ions can be intercalated, resulting in a rapid
drop in battery safety.
On the contrary, the lithium secondary battery
according to the present invention is designed so that
capacity balance can be satisfied by the presence of
multiple anode sites, into which an excessive amount of
lithium ions deintercalated from the cathode while the
battery is charged to a voltage of 4.35V or higher,
obtained by controlling the weight ratio (A/C) of anode
active material (A.) to cathode active material (C) per

unit area of each electrode. Therefore, the lithium
secondary battery according to the present invention not
only can provide high capacity and high output but also
can solve the safety-related problem occurring in the
high-voltage battery according to the prior art.
(2) Additionally, the lithium secondary battery
according to the present invention can prevent side
reactions between the cathode active material and
electrolyte, which may occur under overcharge conditions
(over 4.35V), by controlling the particle diameter
(size) of cathode active material, and thus prevent a
drop in battery safety.
In other words, as the specific surface area of a
cathode active material increases, side reactions
between a cathode active material and electrolyte
increase. Therefore, the lithium secondary battery
according to the present invention uses a cathode active
material with a particle size greater than that of a
currently used cathode active material so as to reduce
the specific surface area of the cathode active
material. Additionally, in order to prevent loss in
reaction kinetics in the battery caused by the use of
the cathode active material having such a large particle
diameter, it is possible to control the loading amount
of each electrode active material per unit area in the
cathode and anode, and thus to realize improvement in
battery safety.
(3) Further, the lithium secondary battery
according to the present invention can significantly
increase the available capacity and average discharge
voltage of a battery, even when using a lithium cobalt-
based cathode active material such as LiCo02 that

provides only about 55% of its theoretically available
capacity by intercalation/deintercalation processes in a
conventional battery having a charge-cutoff voltage of
4.2V. In fact, the following experimental examples show
that although the lithium secondary battery according to
the present invention uses LiCo02 in the same manner as a
conventional battery, the battery provides an available
capacity of LiCo02 increased by at least 14% (see, Table
1) -
According to the present invention, the range of
charge-cutoff voltages of the lithium secondary battery
may be controlled in order to provide a high voltage and
output of 4.35V or higher. Otherwise, the cathode active
material used in the battery may be doped or substituted
with another element, or may be. surface-treated with a
chemically stable substance.
More particularly, the lithium secondary battery
according to the present invention has a charge-cutoff
voltage of 4.35V or higher, preferably of between 4.35V
and 4.6V. When the battery has a charge-cutoff voltage
of lower than 4.35V, it is substantially the same as a
conventional 4.2V battery and does not show an increase
in the available capacity of a cathode active material
so that a high-capacity battery cannot be designed and
obtained. Additionally, when the battery has a charge-
cutoff voltage of higher than 4.6V, the cathode active
material used in the battery may experience a rapid
change in structure due to the presence of the H13 phase
generated in the cathode active material. In this case,
there are problems in that transition metal is dissolved
out of a lithium transition metal composite oxide used
as cathode active material and oxygen loss may occur.

Further, as the charge-cutoff voltage increases,
reactivity between the cathode and electrolyte also
increases, resulting in problems including explosion of
the battery.
The anode active material that may be used in the
high-voltage lithium secondary battery having charge-
cutoff voltages over 4.35V according to the present
invention includes conventional anode active materials
known to one skilled in the art (for example, materials
capable of lithium ion intercalation/ deintercalation).
There is no particular limitation in selection of the
anode active material. Non-limiting examples of the
anode active material include lithium alloys,
carbonaceous materials, inorganic oxides, inorganic
chalcogenides, nitrides, metal complexes or organic
polymer compounds. Particularly preferred are amorphous
or crystalline carbonaceous materials.
The cathode active material that may be used in
the high-voltage lithium secondary battery having
charge-cutoff voltages over 4.35V according to the
present invention includes conventional cathode active
materials known to one skilled in the art (for example,
lithium-containing composite oxides having at least one
element selected from the group consisting of alkali
metals, alkaline earth metals, Group 13 elements, Group
14 elements, Group 15 elements, transition metals and
rare earth elements) . There is no particular limitation
in selection of the cathode active material. Non-
limiting examples of the cathode active material include
various types of lithium transition metal composite
oxides (for example, lithium manganese composite oxides
such as LiMn204; lithium nickel oxides such as LiNi02;
'

lithium cobalt oxides such as LiCoC^; lithium iron
oxides; the above-described oxides in which manganese,
nickel, cobalt or iron is partially doped or substituted
with other transition metals or non-transition metals
(for example, Al, Mg, Zr, Fe, Zn, Ga, Si, Ge or
combinations thereof); lithium-containing vanadium
oxides; and chalcogenides (for example, manganese
dioxide, titanium disulfide, molybdenum disulfide,
etc.).
As cathode active material, lithium cobalt
composite oxides optionally doped with Al, Mg, Zr, Fe,
Zn, Ga, Sn, Si and/or Ge are preferable and LiCo02 is
more preferable. Even if LiCo02 is used as cathode active
material in the same manner as conventional batteries,
the lithium secondary battery according to the present
invention can provide an increase in available capacity
of the cathode active material and thus can be a high-
voltage battery due to a suitable design in electrodes.
In the high-voltage battery having a charge-cutoff
voltage of 4.35V or higher according to the present
invention, the weight ratio (A/C) of anode active
material (A) to cathode active material (C) per unit
area of each electrode ranges suitably from 0.44 to 0.70
and more preferably from 0.5 to 0.64. When the weight
ratio is less than 0.44, the battery is substantially
the same as a conventional 4.2V-battery. Therefore, when
the battery is overcharged to 4.35V or higher, the
capacity balance may be broken to cause dendrite growth
on the surface of anode, resulting in short-circuit in
the battery and a rapid drop in the battery capacity.
When the weight ratio is greater than 0.64, an excessive
amount of lithium sites exists undesirably in the anode,

resulting in a drop in energy density per unit
volume/mass of the battery.
According to the present invention, such
controlled weight ratio of anode active material to
cathode active material per unit area of each electrode
can be obtained preferably by using LiCo02, LiNiMnCo02 or
LiNiMn02 having a capacity similar to that of LiCo02,
etc., as cathode active material and using graphite as
anode active material. When high-capacity cathode
materials such as Ni-containing materials and/or high-
capacity anode materials such as Si are used, it is
possible to design and manufacture an optimized lithium
secondary battery having high capacity, high output and
improved safety through recalculation of the weight
ratio considering a different capacity. However, the
scope of the present invention is not limited to the
above-mentioned cathode active materials and anode
active materials.
The cathode active materials used in the lithium
secondary battery according to the present invention
(for example, LiCo02) have a problem in that they are
deteriorated in terms of thermal properties when being
-charged to 4.35V or higher. - To prevent- the -problem,—it
is possible to control the specific surface area of the
cathode active material.
As the particle size of the cathode active
material increases (in other words, as the specific
surface area of the cathode active material decreases),
reactivity between the cathode active material and
electrolyte may decrease, resulting in improvement in
thermal stability. For this reason, it is preferable to
use a cathode active material having a particle diameter

larger than that of a currently used cathode active
material. Therefore, the cathode active material used in
the battery according to the present invention
preferably has a particle diameter (particle size) of
between 5 and 30 pm- When the cathode active material has
a particle diameter of less than' 5 jj&, side reactions
between the cathode and electrolyte increase to cause
the problem of poor safety of the battery. When the
cathode active material has a particle diameter of
greater than 30 [Mr reaction kinetics may be slow in the
battery.
Additionally, in order to prevent the degradation
of reaction kinetics in the whole battery, caused by the
use of a cathode active material having a particle
diameter greater than that of a currently used cathode
active material, it is possible to control the loading
amount of cathode active material and anode active
material per unit area of each electrode.
It is preferable that the loading amount of
cathode active material per unit area of cathode ranges
from 10 to 30 mg/cm2. When the loading amount of cathode
active material is less than 10 mg/cm2, the battery may
be degraded in terms of. capacity and efficiency. When
the loading amount of cathode active material is greater
than 30 mg/cm2, thickness of the cathode increases,
resulting in degradation of reaction kinetics in the
battery. Additionally, it is preferable that the loading
amount of anode active material per unit area of anode
ranges from 4.4 to 21 mg/cm2. When the loading amount of
anode active material is less than 4.4 mg/cm2, capacity
balance cannot be maintained, thereby causing
degradation in battery safety. When the loading amount

of anode active material is greater than 21 mg/cm2, an
excessive amount of lithium sites is present undesirably
in the anode, resulting in a drop in energy density per
unit volume/mass of the battery.
The electrode used in the battery according to the
present invention can be manufactured by a conventional
process known to one skilled in the art. In one
embodiment, slurry for each electrode is applied onto a
current collector formed of metal foil, followed by
rolling and drying.
Slurry for each electrode, i.e., slurry for a
cathode and anode may be obtained by mixing the above-
described cathode active material/anode active material
with a binder and dispersion medium. Each of the slurry
for a cathode and anode preferably contains a small
amount of conductive agent.
There is no particular limitation in the
conductive agent, as long as the conductive agent is an
electroconductive material that experiences no chemical
change in the battery using the same. Particular
examples of the conductive agent that may be used
include carbon black such as acetylene black, ketchen
black, furnace black or thermal black; natural graphite,
artificial graphite and conductive carbon fiber, etc.,
carbon black, graphite powder or carbon fiber being
preferred.
The binder that may be used includes thermoplastic
resins, thermosetting resins or combinations thereof.
Among such resins, polyvinylidene difluoride (PVdF),
styrene butadiene rubber (SBR) or
polytetrafluoroethylene (PTFE) is preferable, PVdF being
more preferable.

The dispersion medium that may be used includes
aqueous dispersion media or organic dispersion media
such as N-methyl-2-pyrollidone.
In both electrodes of the lithium secondary
battery according to the present invention, the ratio
(A/C) of the thickness of cathode (C) to that of anode
(A) suitably ranges from 0.7 to 1.4, preferably from 0.8
to 1.2. When the thickness ratio is less than 0.7, loss
of energy density per unit volume of the battery may
occur. When the thickness ratio is greater than 1.4,
reaction kinetics may be slow in the whole battery.
The high-voltage lithium secondary battery having
charge-cutoff voltages over 4.35V or higher according to
the present invention includes a cathode (C) , an anode
(A) , a separator interposed between both electrodes and
an electrolyte, wherein the cathode(C) and anode(A) are
obtained by controlling the weight ratio (A/C) of anode
active material to cathode active material per unit area
of each electrode to 0.44-0.70.
The high-voltage lithium secondary battery having
a charge-cutoff voltage of 4.35V or higher is also
characterized by using an electrolyte that further
comprises a compound having a_ reaction potential of 4.7V
or higher in addition to a currently used electrolyte
for batteries.
Due to the presence of the above characteristic
electrolyte, it is possible to improve the safety and
high-temperature storage characteristics of a high-
voltage lithium secondary battery having a charge-cutoff
voltage of 4.35V or higher.
(1) When cyclohexylbenzene (CHB) or biphenyl (BP),
currently used as additives for electrolyte in

conventional batteries having a charge-cutoff voltage of
4.2V or higher, are used in order to improve the safety
and high-temperature storage characteristics of a high-
voltage lithium secondary battery having a charge-cutoff
voltage of 4.35V or higher, cycle characteristics of the
battery at room temperature and high temperature are
degraded rapidly. Additionally, because a large amount
of the above additives are decomposed under high-
temperature storage conditions, a very thick insulator
film is formed on a cathode to prevent lithium ions from
moving in the battery, so that recovery capacity of the
battery cannot be obtained.
On the contrary, the battery according to the
present invention uses fluorotoluene (FT) compounds
having a reaction potential of 4.7V or higher (for
example, 2-fluorotoluene (2-FT) and/or 3-fluorotoluene
(3-FT)) as additives for electrolyte. Because such
additives have high reaction potentials and experience
little change in reaction potentials during repeated
cycles, it is possible to prevent degradation of battery
quality caused by decomposition of an additive at a
voltage of between 4.35V and 4.6V and a rapid change in
reaction potentials, and to improve high-temperature
storage characteristics of a battery.
(2) When such additives for electrolyte are used,
it is possible to reduce a contact surface where side
reactions between a cathode and electrolyte may occur in
case of the battery containing only conventional
electrolyte, and thus to improve battery safety.
There is no particular limitation in the additive
that may be added to the electrolyte of the high-voltage
lithium secondary battery having a charge-cutoff voltage

of 4.35V or higher, as long as the additive is a
compound having a reaction potential of 4.7V or higher.
Preferably, the additive is a fluorotoluene (FT)
compound. Among fluorotoluene compounds, 2-fluorotoluene
(2-FT) and/or 3-fluorotoluene (3-FT) are more
preferable, because they have high reaction potentials
and experience little change in reaction potentials
during repeated cycles.
Because 2-fluorotoluene and/or 3-fluorotoluene are
physically stable and have such a high boiling point as
to prevent thermal decomposition as well as a high
reaction potential of 4.7V or higher (the reaction
potential being higher than the reaction potential of
CHB or BP by about 0.1V), they can improve high-
temperature storage characteristics and safety of a
battery using an electrolyte comprising them as
additives, contrary to conventional additives such as
CHP and BP. Additionally, because they experience little
change in reaction potentials during repeated cycles, as
compared to conventional fluorotoluene compounds, they
can prevent degradation in cycle characteristics of a
high-voltage battery.
In fact, when a fluorotoluene compound other than
2-fluorotoluene and 3-fluorotoluene, or 4-fluorotolune
(4-FT) having a reaction potential similar to that of
CHB is used, a battery having a charge-cutoff voltage of
4.35V or higher shows significant degradation in cycle
characteristics during repeated cycles due to a reaction
of a cathode active material with a fluorine atom
substituted in the para-position. Therefore, it is not
possible to improve the safety and high-temperature
storage characteristics of a battery.

Preferably, the compound having a reaction
potential of 4.7V or higher (for example, 2-FT and/or 3-
FT) is added to an electrolyte in an amount of between
0.1 and 10 wt% based on 100 wt% of the total weight of
electrolyte. When the compound is used in an amount of
less than 0.1 wt%, it is not possible to improve the
safety and quality of a battery significantly. When the
compound is used in an amount of greater than 10 wt%,
there are problems in that viscosity of the electrolyte
decreases and the additive causes an exothermic reaction
to emit heat excessively.
The high-voltage battery having a voltage of 4.35V
or higher according to the present invention can be
manufactured by a conventional process known to one
skilled in the art. In one embodiment, a cathode and
anode are provided with a separator interposed between
both electrodes and an electrolyte is introduced,
wherein the cathode (C) and anode (A) are obtained by
controlling the weight ratio (A/C) of anode active
material to cathode active material per unit area of
each electrode to 0.44-0.70.
The electrolyte that may be used in the present
invention includes a.salt represented by the formula of
A+B~, wherein A+ represents an alkali metal cation
selected from the group consisting of Li+, Na+, K+ and
combinations thereof, and B" represents an anion selected
from the group consisting of PF6~, BF4", CI", Br", I",
C104", AsF6~, CH3C02", CF3SO3-, N(CF3S02)2", C(CF2S02)3" and
combinations thereof, the salt being dissolved or
dissociated in an organic solvent selected from the
group consisting of propylene carbonate (PC), ethylene
carbonate (EC), diethyl carbonate (DEC), dimethyl

carbonate (DMC), dipropyl carbonate (DPC), dimethyl
sulfoxide, acetonitrile, dimethoxyethane,
diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone
(NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone
(y-butyrolactone) and mixtures thereof. However, the
electrolyte that may be used in the present invention is
not limited to the above examples. Particularly, when an
electrolyte comprising a compound having a reaction
potential of 4.7V or higher (for example, 2-
fluorotoluene and/or 3-fluorotoluene) is used, it is
possible to improve high-temperature storage
characteristics and safety with no degradation in cycle
characteristics of the high-voltage battery.
Although there is no particular limitation in the
separator that may be used in the present invention,
porous separators may be used. Particular examples of
porous separators include polypropylene-based,
polyethylene-based and polyolefin-based porous
separators.
There is no particular limitation in the shape of
the lithium secondary battery according to the present
invention. The lithium secondary battery may be a
cylindrical, prismatic, pouch-type or a coin-type
battery.
Additionally, according to another aspect of the
present invention, there is provided a lithium secondary
battery that includes a cathode, an anode, a separator
and an electrolyte, wherein the battery has a charge-
cutoff voltage of between 4.35V and 4.6V, and the
electrolyte comprises a compound having a reaction
potential of 4.7V or higher.
In the lithium secondary battery, the compound

having a reaction potential of 4.7V or higher is the
same as defined above.
Best Mode for Carrying Out the Invention
Reference will now be made in detail to the
preferred embodiments of the present invention. It is to
be understood that the following examples are
illustrative only and the present invention is not
limited thereto.
[Examples 1-5. Manufacture of batteries having
charge-cutoff voltage over 4.35V]
Example 1. Lithium secondary battery having
charge-cutoff voltage of 4.35V (1)
(Manufacture of cathode)
95 wt% of LiCo02 having a particle diameter of 10
IMr 2.5 wt% of a conductive agent and 2.5 wt% of a binder
were mixed to form slurry. The slurry was applied
uniformly on both surfaces of aluminum foil having a
thickness of 15 ^m, followed by rolling, to provide a
cathode having an active material weight of 19.44 mg/cm2.
The finished cathode had a thickness of 128 p^.
(Manufacture of anode)
To 95.3 _wt% of graphite, 4.0 wt% of a binder and_
0.7 wt% of a conductive agent were added and mixed to
form slurry. The slurry was applied uniformly on both
surfaces of copper foil having a thickness of 10 j^m,
followed by rolling, to provide an anode having an
active material weight of 9.56 mg/cm2. The weight ratio
(A/C) of the anode active material to cathode active
material per unit area of each electrode was 0.49, and
the finished anode had a thickness of 130 y&.
(Preparation of electrolyte)

To a solution containing ethylene carbonate and
dimethyl carbonate in a volume ratio of 1:2 (EC:DMC), 1M
LiPF6 was dissolved to provide an electrolyte.
(Manufacture of battery)
The cathode and anode obtained as described above
were used to provide a coin-type battery and prismatic
battery. The manufacturing process of each battery was
performed in a dry room or glove box in order to prevent
the materials from contacting with the air.
Example 2. Lithium secondary battery having
charge-cutoff voltage of 4.35V (2)
Example 1 was repeated to provide a lithium
secondary battery, except that a cathode (C) having an
active material weight of 22 mg/cm2 and an anode having
an active material weight of 11 mg/cm2 were used to
adjust the weight ratio (A/C) of the anode active
material to cathode active material per unit area of
each electrode to 0.50.
Example 3. Lithium secondary battery having
charge-cutoff voltage of 4.4V
Example 1 was repeated to provide a lithium
secondary battery having a charge-cutoff voltage of
4.4V, except that a cathode (CJ having an active
material weight of 22 mg/cm2 and an anode having an
active material weight of 11.66 mg/cm2 were used to
adjust the weight ratio (A/C) of the anode active
material to, cathode active material per unit area of
each electrode to 0.53.
Example 4. Lithium secondary battery having
charge-cutoff voltage of 4.5V
Example 1 was repeated to provide a lithium
secondary battery having a charge-cutoff voltage of
3

4.5V, except that a cathode (C) having an active
material weight of 22 mg/cm2 and an anode having an
active material weight of 12.57 mg/cm2 were used to
adjust the weight ratio (A/C) of the anode active
material to cathode active material per unit area of
each electrode to 0.57.
Example 5. Lithium secondary battery having
charge-cutoff voltage of 4.35V
Example 1 was repeated to provide a lithium
secondary battery, except that 3 wt% of 3-fluorotoluene
(3-FT) was added to 100 wt% of the electrolyte
containing 1M LiPF6 dissolved in a mixed solvent of
ethylene carbonate and dimethyl carbonate (volume ratio=
1:2 (EC:DMC)).

[Comparative Examples 1-4]
Comparative Example 1. Manufacture of lithium
secondary battery having charge-cutoff voltage of 4.2V
Example 1 was repeated to provide a lithium
secondary battery, except that a cathode (C) having an
active material weight of 22 mg/cm2 and an anode having
an active material weight of 9.68 mg/cm2 were used to
adjust the weight ratio (A/C) of the anode active
material to cathode active material per unit area of
each electrode to 0.44, as described in the above Table
' 1.
_ Comparative Example 2 _
Example 1 was repeated to provide a lithium
secondary battery, except that 3 wt% of cyclohexyl
benzene (CHB) was added to the electrolyte.
Comparative Example 3
Example 1 was repeated to provide a lithium
secondary battery, except that 3 wt% of 4-fluorotoluene
(para-FT) was added to the electrolyte instead of 3-
fluorotoluene.

Comparative Example 4
Example 1 was repeated to provide a lithium
secondary battery, except that the weight ratio (A/C) of
the anode active material to cathode active material per
unit area of each electrode was adjusted to 0.44 and 3
wt% of cyclohexyl benzene (CHB) was added to the
electrolyte.
Experimental Example 1. Evaluation for high-
voltage battery having charge-cutoff voltages over 4.35V
vs. battery having charge-cutoff voltage of 4.2V
1-1. Evaluation of charge/discharge capacity
The following experiment was carried out to
compare the charge/discharge capacity of the lithium
secondary battery having a charge-cutoff voltage of
4.35V or higher according to the present invention with
that of the lithium secondary battery having a charge-
cutoff voltage of 4.2V.
The batteries according to Examples 2-4 were used
as samples for batteries having charge-cutoff voltages
over 4.35V and the battery according to Comparative
Example 1 was used as control (4.2V-battery) .
The battery according to Example 2 was tested in a
charge/discharge voltage range of between 3V and 4.35V,
the battery according to Example 3 was tested in a range
of between 3V and 4.4V, the battery according to Example
4 was tested in a range of between 3V and 4.5V, and the
battery according to Comparative Example 1 was tested in
a range of between 3V and 4.2V, each battery being
subjected to cycling under 1C charge/lC discharge
conditions. The test were performed at room temperature
(25°C/45°C) -
After the experiment, the 4.2V battery according

to Comparative Example 1 showed an initial charge
capacity and discharge capacity of 155.0 mAh/g and 14 9.4
mAh/g, respectively. The battery had an energy density
per unit volume of battery of 380.0 Wh/kg (see, FIG. 2
and Table 2). On the contrary, the 4.35V-battery
according to Example 2 showed an initial charge capacity
and discharge capacity of 179.7 mAh/g and 171.3 mAh/g,
respectively, and had an energy density per unit volume
of battery of 439.2 Wh/kg, resulting in improvements in
terms of discharge capacity and energy density per unit
volume of battery by 14.6% and 15.6%, respectively (see,
FIG. 1 and Table 2). Additionally, the 4. 4V-battery and
4.5V-battery according to Examples 3 and 4 showed an
increase in discharge capacity of 20% and 30%,
respectively, compared to the 4.2V-battery according to
Comparative example 1 as control. Further, the batteries
according to Examples 3 and 4 showed an increase in
energy density per unit volume of 22.3% and 33.4%,
respectively (see, Table 2).
As can be seen from the above results, even if the
lithium secondary according to the present invention
uses the same cathode active material (LiCo02) that is
used in a conventional battery, it increases the
available capacity of LiCo02 by at least 14% and improves
the energy density per unit volume significantly through
the modification in the electrode design.

1-2. Evaluation for safety
The following overcharge test was performed for
the lithium secondary battery having charge-cutoff
voltages over 4.35V according to the present invention
and the battery having a charge-cutoff voltage of 4.2V.
The battery according to Example 2 was used as
sample for a battery having a charge-cutoff voltage of
4.35V or higher and the battery according to Comparative

Example 1 was used as control (4.2V-battery). Each
battery was subjected to the overcharge test under an
overcharge voltage of 5.0V with an electric current of
2A at room temperature (25°C).
After the experiment, the temperature of 4.2V-
battery according to Comparative Example 1 increased to
200 °C after the lapse of 1 hour and exploded due to
short-circuit in the battery (see, FIG. 4) . This
indicates that when the conventional 4.2V-battery was
overcharged to 5.0V, reactivity between the cathode and
electrolyte increases to cause the decomposition of the
cathode surface and oxidation of the electrolyte, and
lithium dendrite growth occurs due to the lack of anode
sites, into which an excessive amount of lithium ions
deintercalated from the cathode upon overcharge is
intercalated, resulting in a significant drop in
electrochemical stability of the battery.
On the contrary, when the battery having a charge-
cutoff voltage of 4.35V according to the present
invention was overcharged to 5.0V, the battery
temperature increased to 40D. However, the temperature
was stabilized with time (see, FIG. 3) . This indicates
that the battery according to the present invention has
a large amount of anode sites, into which an excessive
amount of lithium ions deintercalated from the cathode
upon overcharge can be intercalated, and shows a
significant decrease in side reactions between the
cathode and electrolyte due to an increased reactivity
between them caused by overcharge.
As can be seen from the foregoing, the lithium
secondary battery according to the present invention has
significantly improved overcharge safety, because it has

a controlled weight ratio (A/C) of anode active material
(A) to cathode active material (C) per unit area of each
electrode, contrary to the conventional 4.2V battery.
Experimental Example 2. Evaluation for cycle
characteristics of high-voltage lithium secondary
battery having charge-cutoff voltages over 4.35V
The high-voltage lithium secondary battery having
charge-cutoff voltages over 4.35V according to the
present invention was evaluated for cycle
characteristics as follows.
The lithium secondary battery using no additive
for electrolyte according to Example 1 and the lithium
secondary battery using 3-fluorotoluene (3-FT) as
additive for electrolyte according to Example 5 were
used as samples for batteries having charge-cutoff
voltages over 4.35V. As controls, the battery using CHB
as additive for electrolyte according to Comparative
Example 2 and the battery using 4-fluorotoluene (4-FT)
as additive for electrolyte according to Comparative
Example 3 were used.
Each battery was tested in a charge/discharge
voltage range of between 3.0V and 4.35V and was
subjected to cycling under a charge/discharge current of
1C (= 880 mA) . At the zone of 4.35V constant voltage,
the voltage was maintained at 4.35V until the current
dropped to 50 mA and the test was performed at 45°C-
After the experiment, the lithium secondary
battery using the electrolyte containing CHB as additive
showed significant degradation in cycle characteristics
under high temperature conditions, as compared to the
lithium secondary battery using no additive for
electrolyte according to Example 1 and the lithium

secondary battery using the electrolyte containing 3-
fluorotoluene (3-FT) as additive according to Example 5
(see, FIG. 5). This indicates that because CHB having a
reaction potential of less than 4.7V experiences
electropolymerization to form a coating layer, charge
transfer reaction of the cathode active material is
inhibited and resistance is increased at the cathode,
resulting in degradation in cycle characteristics of the
battery. Additionally, the battery using 4-fluorotoluene
having a reaction potential similar to that of CHB
according to Comparative Example 3 showed a rapid drop
in cycle characteristics, because the cathode active
material may react with the fluorine atom present at the
para-position of 4-FT during cycles under 4.35V (see,
FIG. 5).
On the contrary, the lithium secondary battery
using 3-fluorotoluene (3-FT) having a reaction potential
of higher than 4.7V as additive for electrolyte
according to Example 5 did not show any significant
change in high-temperature cycle characteristics, as can
be seen from FIG. 5 (see, FIG. 6) .
Therefore, it can be seen that the high-voltage
lithium secondary battery using a compound having a
reaction potential higher than 4.7 V (for example, 3-
fluorotoluene (3-FT)) as additive for electrolyte
according to the present invention can prevent
degradation in high-temperature cycle characteristics,
contrary to a 4.2V-battery using CHB as additive for
electrolyte.
Experimental Example 3. Evaluation for safety of
high-voltage lithium secondary having charge-cutoff
voltages over 4.35V

The following hot box test was performed in order
to evaluate the safety of the high-voltage lithium
secondary having charge-cutoff voltages over 4.35V
according to the present invention.
The high-voltage lithium secondary battery using
3-fluorotoluene as additive for electrolyte according to
Example 5 was used as sample. As controls, the lithium
secondary batteries using CHB and 4-fluorotoluene (4-FT)
as additives for electrolyte according to Comparative
Examples 2 and 3, respectively, were used.
Each battery was charged to 4.4V under 1C (=880
mA) for 2.5 hours and then maintained under the constant
voltage condition. Then, each battery was introduced
into an oven capable of convection, warmed from room
temperature to a high temperature of 15 0°C at a rate of
5°C/min., and exposed to such high-temperature condition
for 1 hour. Additionally, each battery was checked for
explosion.
After the experiment, the batteries using CHB and
4-FT as additive for electrolyte, respectively,
according to Comparative Example 2 and Comparative
Example 3 exploded with time (see, FIGs. 7 and 8). On
the contrary, the lithium secondary battery using 3-
fluorotoluene as additive for electrolyte according to
Example 5 showed a stable state even at a high
temperature of 150°C (see, FIG. 9) .
Experimental Example 4. Evaluation for high-
temperature storage characteristics of high-voltage
lithium secondary battery having charge-cutoff voltages
over 4.35V
The high-voltage lithium secondary battery having
charge-cutoff voltages over 4.35V was evaluated in the

following high-temperature storage tests.
4-1. Long-term high-temperature storage test
The lithium secondary battery using 3-
fluorotoluene as additive for electrolyte was used as
sample. As controls, the batteries using CHB and 4-FT as
additives for electrolyte, respectively, according to
Comparative Example 2 and Comparative Example 3 were
used.
Each battery was charged at a charging current of
IC to 4.35V and discharged at IC to 3V to determine the
initial discharge capacity. Next, each battery was
recharged to 4.35V and was subjected to repeated 30
cycles of 3-hour storage at 80°C/7-hour storage at 25°C.
During such cycles, the thickness of each battery was
measured. Then, each battery was discharged at IC to
determine the residual capacity of each battery. After
measuring the residual capacity, each battery was
subjected to three charge/discharge cycles and measured
for the recovery capacity. In order to ensure
reproducibility, the above-described procedure was
repeated 4 times.
After the experiment, the battery comprising CHB
according to Comparative Example 2 showed a significant
swelling phenomenon before the fifth charge/discharge
cycle (see, FIG. 10). Additionally, the battery using 4-
fluorotoluene whose reaction potential is similar to
that of CHB also showed a significant swelling
phenomenon after approximately 10 charge/discharge
cycles (see, FIG. 11) . On the contrary, the battery
using 3-fluorotoluene according to Example 5 showed a
significant drop in the battery swelling phenomenon
(see, FIG. 10).

4-2. Short-term high-temperature storage test
The lithium secondary battery using no additive
for electrolyte according to Example 1 and the lithium
secondary battery using 3-fluorotoluene as additive for
electrolyte according to Example 5 were used as samples.
As controls, the batteries using CHB and 4-FT as
additives for electrolyte, respectively, according to
Comparative Example 2 and Comparative Example 3 were
used.
Each battery was charged at a charging current of
IC to 4.35V and discharged at IC to 3V to determine the
initial discharge capacity. Next, each battery was
recharged to 4.35V and was stored at 90°C for 4 hours,
during which the thickness of each battery was measured.
Then, each battery was discharged at IC to determine the
residual capacity of each battery. After measuring the
residual capacity, each battery was subjected to' three
charge/discharge cycles and measured for the recovery
capacity.
After the storage at 90°C for 4 hours, the battery
having a charge-cutoff voltage of 4.35V or higher
according to Comparative Example 2 showed a significant
increase in its thickness, particularly compared to the
battery using no additive for electrolyte according to
Example 1 (see, FIG. 11). This indicates that the
electrolyte is decomposed due to the increase in
reactivity between the cathode and electrolyte to form a
thick insulator film, resulting in an increase in the
battery thickness. Therefore, it can be seen that a
conventional additive (for example, CHB) for a 4.2V
battery is . not suitable for a high-voltage battery
having a charge-cutoff voltage of 4.35V or higher.

On the contrary, the high-voltage lithium
secondary battery having charge-cutoff voltages over
4.35V and using 3-fluorotoluene as additive for
electrolyte according to Example 5 did not show a
swelling phenomenon even after the storage at 90°C. This
indicates that the battery shows little degradation in
the battery quality (see, FIG. 11).
Therefore, it can be seen that a f luorotoluene
compound having a reaction potential of 4.7V or higher
(for example, 2-fluotoluene and 3-fluorotoluene) is
suitable for an additive for electrolyte in the high-
voltage battery having a charge-cutoff voltage of 4.35V
or higher according to the present invention.
Industrial Applicability
As can be seen from the foregoing, the high-
voltage lithium secondary battery according to the
present invention satisfies capacity balance by
controlling the weight ratio (A/C) of anode active
material (A) to cathode active material (C) per unit
area of each electrode. By doing so, it is possible to
increase the available capacity of cathode active
material significantly by at least 14%, as compared to
the available capacity of cathode active material in a
conventional battery of merely about 50%. Therefore, the
battery according to the present invention can solve the
problems occurring in 4.2V-batteries according to the
prior art upon overcharge, and thus can provide a high-
voltage lithium secondary battery having excellent
safety and long service life.
Further, when a fluorotoluene compound having a
reaction potential of 4.7V or higher is used as additive

for electrolyte in a high-voltage battery having a
charge-cutoff voltage of 4.35V or higher, it is possible
to improve the safety and high-temperature storage
characteristics of the battery with no degradation in
cycle characteristics.
While this invention has been described in
connection with what is presently considered to be the
most practical and preferred embodiment, it is to be
understood that the invention is not limited to the
disclosed embodiment and the drawings. On the contrary,
it is intended to cover various modifications and
variations within the spirit and scope of the appended
claims.

WE CLAIM:
1. A lithium secondary battery comprising:
(i) a cathode (C) in which cathode active materials with a particle diameter of
between 5 µm and 30 µm are loaded in an amount of between 10 mg/cm and 30
mg/cm2,
(ii) an anode (A) in which anode active materials are loaded in an amount of
between 4.4 mg/cm2 and 21 mg/cm2,
(iii) a separator, and
(iv) an electrolyte, wherein the battery has a weight ratio (A/C) of anode active
material (A) to cathode active material (C) per unit area of each electrode of between
0.44 and 0.70, and has a ratio (A/C) of thickness of the cathode (C) to that of the anode
(A) of between 0.7 and 1.4, and shows a charge cut-off voltage of between 4.35V and
4.6V.
2. The lithium secondary battery as claimed in claim 1, wherein the cathode
(C) is obtained from a cathode active material capable of lithium intercalation
/deintercalation, the cathode active material being doped with at least one metal
selected from the group consisting of Al, Mg, Zr, Fe, Zn, Ga, Sn, Si and Ge.
3. The lithium secondary battery as claimed in claim 1, wherein the cathode
active material is a lithium-containing composite oxide having at least one element
selected from the group consisting of alkali metals, alkaline earth metals, Group 13
elements, Group 14 elements, Group 15 elements, transition metals and rare earth
elements.

4. The lituum secondary battery as claimed in claim 1, wherein the
electrolyte further comprises a compound having a reaction potential of 4.7V or higher.
5. The lithium secondary battery as claimed in claim 4, wherein the
compound having a reaction potential of 4.7V or higher is at least one fluorotoluene
compound selected from the group consisting of 2-fluorotoluene and 3-fluorotoluene.

6. The lithium secondary battery as claimed in claim 4, wherein the
compound having a reaction potential of 4.7V or higher is used in an amount of
between 0.1 and 10 wt% based on 100 wt% of the electrolyte.

A lithium secondary battery is disclosed. The lithium secondary battery
comprises: (i) a cathode (C) in which cathode active materials with a particle diameter
of between 5 µm and 30 µm are loaded in an amount of between 10 mg/cm2 and 30
mg/cm2, (ii) an anode (A) in which anode active materials are loaded in an amount of
between 4.4 mg/cm2 and 21 mg/cm2, (iii) a separator, and (iv) an electrolyte, wherein
the battery has a weight ratio (A/C) of anode active material (A) to cathode active
material (C) per unit area of each electrode of between 0.44 and 0.70, and has a ratio
(A/C) of thickness of the cathode (C) to that of the anode (A) of between 0.7 and 1.4,
and shows a charge cut-off voltage of between 4.35V and 4.6V.

Documents:

03647-kolnp-2006 abstract.pdf

03647-kolnp-2006 claims.pdf

03647-kolnp-2006 correspondence others.pdf

03647-kolnp-2006 description(complete).pdf

03647-kolnp-2006 drawings.pdf

03647-kolnp-2006 form-1.pdf

03647-kolnp-2006 form-3.pdf

03647-kolnp-2006 form-5.pdf

03647-kolnp-2006 general power of authority.pdf

03647-kolnp-2006 international publication.pdf

03647-kolnp-2006 international search authority report.pdf

3647-KOLNP-2006-ABSTRACT 1.1.pdf

3647-KOLNP-2006-ASSIGNMENT.pdf

3647-KOLNP-2006-CLAIMS.pdf

3647-KOLNP-2006-CORRESPONDENCE.pdf

3647-KOLNP-2006-CORRESPONDENCE1.1.pdf

3647-KOLNP-2006-DESCRIPTION (COMPLETE) 1.1.pdf

3647-KOLNP-2006-DRAWINGS 1.1.pdf

3647-KOLNP-2006-EXAMINATION REPORT REPLY RECIEVED.pdf

3647-KOLNP-2006-EXAMINATION REPORT.pdf

3647-KOLNP-2006-FORM 1 1.1.pdf

3647-KOLNP-2006-FORM 18.1.pdf

3647-kolnp-2006-form 18.pdf

3647-KOLNP-2006-FORM 3 1.1.pdf

3647-KOLNP-2006-FORM 3.pdf

3647-KOLNP-2006-FORM 5.pdf

3647-KOLNP-2006-FORM-27.pdf

3647-KOLNP-2006-GPA.pdf

3647-KOLNP-2006-GRANTED-ABSTRACT.pdf

3647-KOLNP-2006-GRANTED-CLAIMS.pdf

3647-KOLNP-2006-GRANTED-DESCRIPTION (COMPLETE).pdf

3647-KOLNP-2006-GRANTED-DRAWINGS.pdf

3647-KOLNP-2006-GRANTED-FORM 1.pdf

3647-KOLNP-2006-GRANTED-FORM 2.pdf

3647-KOLNP-2006-GRANTED-SPECIFICATION.pdf

3647-KOLNP-2006-OTHERS 1.1.pdf

3647-KOLNP-2006-OTHERS.pdf

3647-KOLNP-2006-PETITION UNDER RULE 137.pdf

3647-KOLNP-2006-REPLY TO EXAMINATION REPORT.pdf

abstract-03647-kolnp-2006.jpg

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Patent Number

249910

Indian Patent Application Number

3647/KOLNP/2006

PG Journal Number

47/2011

Publication Date

25-Nov-2011

Grant Date

23-Nov-2011

Date of Filing

05-Dec-2006

Name of Patentee

LG CHEM, LTD.

Applicant Address

20, YOIDO-DONG YOUNGDUNGPO-GU, SEOUL 150-721, REPUBLIC OF KOREA

Inventors:

#	Inventor's Name	Inventor's Address
1	KIM DONG MYUNG	103-1401, HWANGSIL APARTMENT WOLPYEONG-DONG, SEO-GU, DAEJEON 302-280, REPUBLIC OF KOREA
2	KIM YONG JEONG	111-1108,SEJONG APARTMENT, JEONMIN-DONG, YUSEONG-GU, DAEJEON 305-390 REPUBLIC OF KOREA
3	CHO BENJAMIN	513-802 ,SAMSUNG 5-CHA APARTMENT, PUNGDEOKCHEON 2-DONG, YONGIN-SI, GYEONGGI-DO 449-785, REPUBLIC OF KOREA
4	JEONG JUN YONG	107-408 HANBAT GARDEN APARTMENT, SANSUNG-DONG, CHUNG-GU, DAEJEON 301-756, REPUBLIC OF KOREA
5	JEONG DAE JUNE	557-9,GWAEBEOP-DONG, SASANG-GU, BUSAN 617-060, REPUBLIC OF KOREA
6	BAE JOON SUNG	LG CHEMICAL DEVELOPMENT CORP. MUNJI-DONG, YUSEONG-GU, DAEJEON 305-380, REPUBLIC OF KOREA
7	YOON JONG MOON	102-802,NEULPUREUN APARTMENT, YONGDU-DONG,JUNG-GU, DAEJEON 301-110 REPUBLIC OF KOREA

PCT International Classification Number

H01M 10/24

PCT International Application Number

PCT/KR2005/001556

PCT International Filing date

2005-05-27

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10-2004-0038374	2004-05-28	Republic of Korea
2	10-2004-0116386	2004-12-30	Republic of Korea