Title of Invention

"AN AQUEOUS COMBUSTION PROCESS OF MAKING NANOSTRUCTURED LI4TI5O12."

Abstract

This invention relates to an aqueous combustion process of making nanostructured Li4TisOi2 (90-110 nm). This invention particularly relates to an economic process for the preparation of nanostructured Li4Ti5O12, the anode material for lithium ion battery at a low synthesis temperature (145-155°C) by alanine assisted combustion synthesis using aqueous solutions of the precursor salts. After heating the as-synthesized powder at 800°C, the single phase nanostructured Li4Ti5O12 product poqdwe is obtained that can be used as an anode material for lithium ion batteries.

Full Text

FIELD OF INVENTION The present invention relates to an aqueous combustion process of making nanostructured Li4TisOi2. The present invention particularly relates to L-alanine assisted aqueous combustion synthesis for preparation of nano-structured Li4TisOi2 anode material for lithium-ion batteries. BACKGROUND OF THE INVENTION Rechargeable lithium ion battery technology has become increasingly important in recent years because it provides lightweight, compact, high energy density batteries for powering appliances in the rapidly growing electronic industries. These batteries are also of interest for their possible application as power sources for electric vehicles. State-of-the-art rechargeable lithium batteries are known as lithium-ion' batteries because during the charge-discharge process lithium ions are shuttled between two host structures with a concomitant reduction and oxidation of host electrodes. The typical commercial lithium-ion cell is a 3.5V LixC6/Li-xCoO2 cell, in which lithium ion is extracted from a layered LiCoO2 structure (cathode) during charge and inserted into a carbonaceous structure, typically graphite or a pyrolyzed carbon (anode). Initially, metallic lithium was used as anode but when the cell is subjected to repeated charge-discharge cycles, lithium is partly divided into fine pieces and the dendrite growth causes the anode to be deformed resulting in a short cycle life. In order to overcome this problem, use of carbonaceous materials, alloys and metal oxides as anode have been investigated. Lithium ions are intercalated into the carbonaceous materials at potential values as low as the lithium deposition potential. Most common electrolytes are not stable at this potential value and a passivation film is formed during charging which consumes lithium from the cathode inducing decomposition of the electrolyte. Furthermore, there is no clear end-of-charge indicator in the voltage profile that can signal the release of oxygen from the cathode (LiCoO2). This can result in the structural damage to the positive electrode. Alloys, when used as an anode, suffer from a large volume expansion/contraction during charging and discharging eventually leading to a complete breakdown of the structure after a few cycles. Transition metal oxides are more attractive than carbonaceous or alloys as anode. Particularly, Li4Ti5O12 is an attractive alternate anode material to replace carbon. Chemical and electrochemical studies have shown that various titanium oxides can incorporate lithium in different ratio. Out of these, spinel Li4Ti5O12 (Fd3m) exhibits the best behavior with a theoretical capacity of 175mAh/g. The electrochemical processes are: Insertion/extraction of Li into/from LUTisOu is known to proceed with no significant change in lattice dimensions and thus, it is considered as a zero-strain insertion compound. The excellent cycling performance and long cycle life of the Li4TisOi2 make it a candidate as an anode especially for high energy applications such as all solid state lithium-ion batteries and hybrid supercapacitors. Nano-structured materials are being actively pursued for use in a wide range of applications, including electrochemical energy storage and generation, chemical sensors, optoelectronics, semiconductors, wear and scratch resistant coatings and heat transfer devices. Researchers have recently found that by using nano-structured electrodes, the rate capability can be improved drastically. Limitation of rate capabilities is caused mainly by the slow solid-state diffusion of Li+ ion within the conventional micron-sized electrode materials. Nano-structured electrodes mitigate the problem of slow diffusion because the distance that Li* ion must diffuse in the solid state is limited to the radius of the nano-particle resulting in an improved lithium intercalation kinetics. This behavior allows for higher discharge rate and minimizes structural distortion at the surface of the electrode material, which is related to the capacity fade. Over the past decade, a number of techniques have been developed for the production of Li4TisOi2 anode powders - they are mainly divided into two categories: 1) dry methods (solid state synthesis) and 2) wet chemistry methods (all in non-aqueous medium). The conventional solid state process is tedious, requires prolonged heat treatment and has several disadvantages such as inhomogeneity, irregular morphology, larger particle size, poor control of stoichiometry etc. On the other hand, synthesis of LUTijOu by wet chemistry methods such as sol-gel, micro-emulsion, co-precipitation etc. necessitates a non-aqueous medium. This is due the solubility problem of titanium alkoxides in aqueous medium. For the same reason, no combustion synthesis of Li4Ti5O12 has been reported till date as per best of our knowledge. Low temperature combustion process has several advantages, such as single step process, relatively lower calcination temperature, smaller particle size, relatively high surface area and better stoichiometry of the product powder. Reference may be made to U.S. Pat.No. 6,221,531 Bl by J. T. Vaughey, M. M. Thackerey, A. J. Kahaian, A. N. Jansen and C. -H. Chen wherein they disclosed a process for the preparation of doped lithium-titanium-oxide by solid state reaction of LiOH-H2O, TiO2 (anatase) and Mg(OH)2 or Mg(N03)2 in the required stoichiometric amounts. Materials were intimately mixed and calcined at 1000°C for 6 hours under a helium atmosphere containing 3% Hydrogen. However, the major drawback of the patented process is that the calcination is to be carried out under a controlled atmosphere that makes the process tedious and expensive. Above all, when calcined at a high temperature of 1000°C, there is a chance of lithium evaporation resulting in lithium deficient lithium titanate spinel. Reference may again be made to U.S. Pat. No. 6,827,921 by Amit Singhal and Ganesh Skandan wherein they utilized nanoparticles of TiO2, a lithium salt such as, lithium nitrate, lithium iodide, lithium hydroxide, lithium chloride and lithium carbonate, and an organic solvent with a boiling point in the range of 70-230°C. The process was carried out at pressure in the range of 0.5 to 10 atmospheres. An organic salt of Lithium was mixed with TiO2 nano particles (~ 20-25 nm) in an organic solvent such as, hexanol. The solution was heated to a high enough temperature (70-230°C) under a controlled pressure to facilitate the diffusion of Li ions into nanoparticles. After completion of reaction, the powder is heat treated in O2 at a relatively low temperature to get the nanocrystalline LuTisOu. However, the major drawback of the above mentioned process is that the process is extremely complicated and requires precise control of both temperature and pressure for successful synthesis of nanostructured LUTisO^. Moreover, the process uses a non-aqueous medium i.e. hexanol, a costly and hazardous solvent Refernce may also be made to the work of Ladislav Kavan and Michael Gratzel, Electrochemical Solid State Letters 5(2002) A39-A42, entitled "Facile synthesis of nanocrystalline LUTisOu spinel exhibiting fast Li insertion" wherein the synthesis of nanostructured LUTisOn has been described by a sol-gel route. Lithium ethoxide and t Ti(TV) alkoxides were used as the starting reagents. In the process, 50 mL of the solution of Li and Ti alkoxides was hydrolyzed in 300 mL of water and the produced slurry was concentrated on a rotary evaporator (40°C, 20 mbar) to a concentration of 10-20 wt %. Polyethylene glycol (molecular weight 20,000) was added, in the proportion of 50-100 wt % of LUTisOu and the mixture was stirred overnight. The resulting viscous liquid was deposited on a sheet of conducting glass and was annealed at 500°C for 30 min to get desired product phase. The major drawback of the process is that it is complicated involving several steps like hydrolysis of alkoxide salts, mixing of slurry with polyethylene glycol in rotary evaporator, stirring overnight etc that makes the process a lengthy one. Moreover, the process requires a precise control of pressure inside the processing equipment to get the appropriate slurry of active materials Reference may again be made to the work of Y-J Hao, Q-Y Lai, J-Z Lu, H-Li Wang, Y-D Chen and X-Y Ji, Journal of Power Sources, 158 (2006) 1358-1364, entitled " Synthesis and characterization of spinel LUTisOrc anode material by oxalic acid assisted sol gel process" wherein, oxalic acid was used as a chelating agent. Stoichiometric amount of LiCO3 and Ti(OC4H9)4 were dissolved in ethanol. An ethanolic solution of oxalic acid was added to this with constant stirring to get a sol. Continuous stirring of the sol for several hours resulted in a gel which was then fired at 500°C to get the as-synthesized powder. In addition to the fact that the process has to be carried out in a nonaqueous medium, the processing involves multiple stages and is a major drawback of the process. The combustion synthesis is of particular interest because it offers several attractive advantages over the other wet-chemical/soft chemical routes-like sol-gel, co- Precipitation, inner-gel etc. The major advantages of combustion synthesis are: ultrafine homogeneous phase pure product powder, simplicity of experimental set up, relatively short processing time and cost effectiveness. However, the entire prior art reports on the synthesis of nanocrystalline Li4TisOi2 either by sol-gel in non-aquous medium or by solid-state synthesis process. On the contrary, use of an aqueous soft chemical method to synthesize nanostructured L^TisOu will solve the drawbacks of the prior art processes. OBJECTS OF THE INVENTION Thus the main object of the present invention is to propose an aqueous combustion process of making nano structured Li4TisOi2 using solutions consisting of the corresponding metal salts e.g., lithium nitrate and titanium methoxide together with complexing agents such as L-alanine taken individually or in a mixture to undergo a controlled exothermic and self igniting oxidation reduction reaction within the individual components. Another object of the present invention is to initiate an auto ignition reaction within the reaction system leading to generation of considerable heat, and utilizing the heat evolved during auto ignition to produce a fully decomposed ceramic oxide powder at a relatively lower temperature of only 150°C in a single step. Yet another object of the present invention is to use a novel complexing agent as well as the reducing fuel e.g., L-alanine to ignite the reaction mixture at relatively lower temperature, thus yielding nanostructured powder with a higher surface area than the precursor powder. Still another object of the present invention is to have a clean process with the feasibility of operating it on a continuous basis to produce large-scale nanostructured Yet another object of the present invention is to calcine the precursor powder at temperatures in the range of 750-850°C to convert them to reactive Li4TisOi2. Still another object of the present invention is to synthesize doped Li4T-isOi2 by suitable dopant like AI, Fe, Ni, Cr, Mn, V or other transition metal ions. STATEMENT OF INVENTION According to this invention there is provided an aqueous combustion process of making nanostructured Li4TisOi2 which comprises homogeneous mixing of the saturated aqueous solutions of lithium nitrate and titanium methoxide dissolved in HNOa acid to obtain a mixed salt solution heating the mixed salt solution to a temperature in the range of 145-155°C with constant stirring, adding aqueous solution of L-alanine to the to mixed salt solution in the molar proportion of (4.8-5):l with respect to metal nitrates, with constant stirring till the formation of a clear viscous gel, further heating the viscous gel to a temperature in the range of 145-155°C to initiate a auto-ignition and obtain a solid mass, calcining the solid mass. DETAILED DESCRIPTION OF THE INVENTION In summary, in the process of the present invention, a self-propagating auto ignition takes place, which utilizes a part of the heat evolved during combustion process to synthesize LUTisOi2. The novelty of the present invention is that the synthesis is carried out in and aqueous medium. Yet another novelty of the present invention is the introduction of L-alanine that acts both as a complexing agent as well as a combustion fuel which helps in producing nanostructured powders at a low combustion temperature in the range of 145-155°C, which is significantly lower than those described in the prior-art processes. As the electrochemical reactivity of ceramic powders used for battery materials depends largely on its particle size and surface area, a low combustion temperature is needed for greater reactivity. Our invention on this account thus has great advantage as it produces particles in the nanorange (90-110 nm) with a very high surface area at a low temperature of 145-155°C by cost effective aqueous combustion process. The non-obvious inventive steps presents in the present invented process is the method of preparation i.e., aqueous combustion synthesis and the use of L-alanine as a reducing fuel for preparation of nanostructured Li4Ti5Oi2. Accordingly, the present invention provides an aqueous combustion process of making nanostructured Li4Ti5O12 which comprises homogeneous mixing of the saturated aqueous solutions of lithium nitrate and titanium methoxide dissolved in HNOa acid to obtain a mixed salt solution (where Li:Ti=4:5), heating the mixed salt solution at a temperature in the range of 145-155°C with constant stirring, adding aqueous solution of L-alanine in the hot mixed salt solution in the molar proportion of (4.8-5):l with respect to metal nitrates, with constant stirring till the formation of a clear viscous gel, further heating the viscous gel at a temperature in the range of 145-155°C to initiate a auto-ignition and obtain a solid mass, clacining the solid mass at a temperature in the range of 750-800°C in air. In an embodiment of the present invention, the metal ion such as lithium may be obtained from lithium oxides, lithium carbonate, lithium hydroxide, lithium chloride, lithium acetates and lithium nitrate. In another embodiment of the present invention, the metal ion such as titanium may be obtained from titanium oxide, titanium chloride and titanium alkoxide In yet another embodiment of the present invention, the precursor salts may be dissolved in acidified water In still another embodiment of the present invention, the fuel may be obtained from sources such as various amino acids such as alanine (L or D), glycine, valine, leucine, iso-leucine, polycarboxilic acid like citric acid, oxalic acid or an amide like urea. In another embodiment of the present invention, the fuel (L-alanine) to nitrate ratio may be used in the range of 4.8-5 In yet another embodiment of the present invention, doped L'uTisOn may be obtained by suitable dopant like Al, Fe, Ni, Cr, Mn, V or other transition metal ions. The complete description of all the steps involved in the process is given below: (i) Preparing a saturated solution of Lithium nitrate or acetate in distilled water [solution A] (ii) Dissolving titanium methoxide inHNOsacid [solutions] (Hi) Preparing a saturated solution of L-alanine [alanine: nitrate = (4.8-5):!] in distilled water [solution C] (iv) Mixing of solution A and B in a beaker in stoichiometric amount (4:5 molar ratios) and placing the beaker containing the mixed solution on a hot plate at 145 -155°C with constant stirring. (v) Adding solution C to the hot mixture and stirred till a clear gel is formed (vi) Further heating of the gel when spontaneous burning of the same occurs which results in a blackish ash. (vii) Heating the blackish ash in the temperature range of 750-800°C for 10 hours in air to get Li4Ti5O12, the final product (powder) The aqueous combustion synthesis of nanostructured Li4Ti5O12 is an economic process rather than solid state or organic sol-gel process. Lithium source is nitrate which is also cheap. The introduction of alanine in synthesizing lithium titanium oxide in aqua-combustion process has a great advantage. Alanine having an extra methyl group in branch chain accelerates the combustion process. The branch-chain methyl groups present in alanine evolutes more gaseous species during combustion making the product highly porous. More carbon in the chain enhances the reaction temperature momentarily up to 800°C. Another specialty of alanine is that it can chelate with both monovalent lithium and tetravalent titanium resulting in a highly viscous gel. Presence of alanine as fuel results in a low combustion temperature as well as a high flame temperature forming ~50% LLjTisOtf phase even in the ash form. Thereby, the required calcination (firing) temperature is also lowered. The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention.; Example 1 LiNOs and Ti (OCH3)4HaO ware taken and dissolved in distilled water and HNOa acid respectively and mixed together in stiochiometric ratio (Li:Ti=4:5) to prepare a clear saturated aqueous solution of the two. The whole mixture was stirred with magnetic stirrer on a hot plate. The temperature of the hot plate was maintained at 155°C. A saturated aqueous solution of L- Alanine (Alanine: Nitrate = 5:1) was prepared and mixed with the above mixture. The stirring was continued upto the clear gel formation. After a few minutes a slow reaction began with the evolution small amount of gases and flame and the whole gel turned into a blackish ash. The ash obtained after the combustion was fluffy, light and fragile mass. It was then gently ground followed by the heating at 800°C for 10 hrs. The powder thus obtained was single phase determined by XRD. The average crystallite size was found to be 110 nm. Example2 LiNO3 and Ti(OCH3)4,4H2O ware taken and dissolved in distilled water and HNOj acid respectively and mixed together in stiochiometric ratio (Li:Ti=4:5) to prepare a clear saturated aqueous solution of the two. The whole mixture was stirred with magnetic stirrer on a hot plate. The temperature of the hot plate was maintained at 145°C. A saturated aqueous solution of L-Alanine (Alanine: Nitrate = 5:1) was prepared and mixed with the above mixture. The stirring was continued upto the clear gel formation. After a few minutes a slow reaction began with the evolution of small amount of gases and flame and the whole gel turned into a blackish ash. The ash obtained after the combustion was fluffy, light and fragile mass. It was then gently ground followed by the heating at 800°C for 10 hours. The powder thus obtained was single phase as determined by XRD. The average crystallite size was found to be 90 nm. Examples LiNOs and Ti(OCH3)4,4H2O ware taken and dissolved in distilled water and HNOs acid respectively and mixed together in stiochiometric ratio (Li:Ti=4:5) to prepare a clear saturated aqueous solution of the twoThe whole mixture was stirred with magnetic stirrer on a hot plate. The temperature of the hot plate was maintained at 155°C. A saturated aqueous solution of L-Alanine (Alanine: Nitrate = 5:1) was prepared and mixed with the above mixture. The stirring was continued upto the clear gel formation. After a few minutes a slow reaction began with the evolution of small amount of gases and flame and the whole gel turned into a blackish ash. The ash obtained after the combustion was fluffy, light and fragile mass. It was then gently ground followed by the heating at 750 °C for 10 hours. The powder thus obtained was single phase as determined by XRD. The average crystallite size was found to be 100 nm. Example 4 LiNOs and Ti(OCH3)4,4H2O ware taken and dissolved in distilled water and HNOs acid respectively and mixed together in stiochiometric ratio (Li:Ti=4:5) to prepare a clear saturated aqueous solution of the twoThe whole mixture was stirred with magnetic stirrer on a hot plate. The temperature of the hot plate was maintained at 14S°C. A saturated aqueous solution of L-Alanine (Alanine: Nitrate = 5:1) was prepared and mixed with the above mixture. The stirring was continued upto the clear gel formation. After a few minutes a slow reaction began with the evolution of small amount of gases and flame and the whole gel turned into a blackish ash. The ash obtained after the combustion was fluffy, light and fragile mass. It was then gently ground followed by the heating at 750°C for 10 hours. The powder thus obtained was single phase as determined by XRD. The average crystallite size was found to be 95 run. The main advantages of the present invention are: 1) A cost effective simple combustion process in aqueous medium. 2) Production of nanostructured particles (90-110 nm) of the product powder with a large surface area 3) Use of L-alanine as a novel fuel for the synthesis of LUTisOia introduces large amount of heat (~800°C) during combustion which supply the required temperature for desired phase formation 4) A low synthesis temperature in the range of 145-155° is required for the process. 5) Formation of ~ 50% Li4Ti5O12 phase in as-synthesized form. 6) It may be possible to prepare the desired compositions of doped Li4TisOi2 by this aqueous combustion process 7) It may be possible to scale up the process by spraying the precursor liquid in a hot chamber It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:- WE CLAIM: 1.An aqueous combustion process of making nanostrctured Li4Ti5O12 comprises homogenous mixing of the saturated aqueous solutions of lithium nitrate and titanium methoxide dissolved in HNO3 acid to obtain a mixed salt solution, heating the mixed salt solution to a temperature in the range of 145-155°C with constant stirring, characterized by adding aqueous solution of L-alanine to mixed salt solution in the molar proportion of (4.8-5): 1 with respect to metal nitrates, with constant stirring till the formation of a clear viscous gel, further heating the viscous gel to a temperature in the range of 145-155°C to initiate a auto-ignition and obtain a solid mass, calcining the solid mass at a temperature in the range of 750-800°C. 2.A process as claimed in claim 1, where in Li:Ti ratio is 4:5 in the mixed salt solution. 3.A process as claimed in claim 1, wherein the reaction can be carried out in acidified aqueous medium.

Documents:

2278-del-2007-Abstract-(26-12-2012).pdf

2278-del-2007-abstract.pdf

2278-del-2007-Claims-(09-05-2013).pdf

2278-del-2007-Claims-(26-12-2012).pdf

2278-del-2007-claims.pdf

2278-del-2007-Correspondence Others-(09-05-2013).pdf

2278-del-2007-Correspondence Others-(26-12-2012).pdf

2278-del-2007-correspondence-others.pdf

2278-del-2007-description (complete).pdf

2278-del-2007-form-1.pdf

2278-del-2007-form-2.pdf

2278-del-2007-GPA-(26-12-2012).pdf