Title of Invention

A CONDUCTING SALT COMPOSITION FOR GALVANIC CELLS AND A PROCESS FOR PREPARING THE SAME

Abstract

The invention relates to conducting salts which contain lithium bis(oxalato)borate (LiBOB) and mixed lithium borate salts of the type of formula (I), wherein the portion of compound (I) in the conducting salt is 0.01 to 20 mole- % and X in formula (I) is a bridge linked with the boron via two oxygen atoms, selected from formula (II), wherein Y1 and Y2 together = O, m = 1, n = 0 and Y3 and Y4 independently represent H or an alkyl group with 1 to 5 C atoms, or Y1, Y2, Y3, Y4 independently represent OR (with R = alkyl group with 1 to 5 C atoms), or H or an alkyl group with 1 to 5 C atoms, and wherein m = 0 or 1, n = 0 or 1, or Y2 and Y3 are members of a 5-or 6-membered aromatic or heteroaromatic ring (with N, O or S as the hetero element) which can be optionally substituted with alkyl, alkoxy, carboxy or nitrile, and if so, Y1 and Y4 are not applicable and n = 0, m = 0 or 1.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10) TITLE “CONDUCTING SALTS FOR GALVANIC CELLS, PRODUCTION AND USE THEREOF” APPLICANT Chemetall GmbH Trakehner Strasse 3 D-60487 Frankfurt Germany Nationality : a German company The following specification particularly describes the nature of this invention and the manner in which it is to be performed 2 Conducting salts for galvanic cells, production and use thereof The invention relates to lithium-borate complex salts, to the production thereof and to the use thereof as electrolytes in galvanic cells, in particular as conducting salts in lithium-ion batteries. Mobile electronic appliances require ever more efficient rechargeable batteries for their independent power supply. Suitable for this purpose, besides nickel/cadmium and nickel/metal-hydride accumulator batteries, are rechargeable lithium batteries, which in comparison with the nickel batteries have a significantly higher energy density. The conventional systems on the market have a terminal voltage of about 3 V; the consequence of this potential is that water-based electrolyte systems cannot be used in lithium batteries. Instead, non-aqueous, mostly organic electrolytes (i.e. solutions of a lithium salt in organic solvents such as carbonates, ethers or esters) find application in liquid systems. In the battery design that is dominant at the present time - lithium-ion batteries with liquid electrolytes - lithium hexafluorophosphate (LiPF6) is used practically exclusively as conducting salt. This salt possesses the necessary prerequisites for use in high-energy cells - i.e. it is IS readily soluble in aprotic solvents, it results in electrolytes having high conductivities, and it exhibits a high degree of electrochemical stability. Oxidative decomposition occurs only at potentials > approx. 4.5V. However, LiPFe has serious disadvantages, which can mainly 0 be attributed to its lack of thermal stability (decomposition above approx. 130 °C) . In addition, corroding and toxic hydrogen fluoride is released in the event of contact with moisture, which, on the one hand, makes handling difficult and, on the other hand, attacks 5 and damages integral parts of the battery, e.g. the cathode. 2 Against this background, intense efforts are being made to develop alternative conducting salts. Above all, lithium salts with perfluorinated organic residues have been tested as such. In this connection it is a question of 5 lithium trifluoromethanesulfonate (‘Li triflate1), lithium imides (lithium bis (perfluoralkylsulfonyl)imides) and also lithium methides (lithium tris(perfluoraikyisuifonyl) methides). All these salts require relatively elaborate production processes, are therefore relatively expensive, 10 and have other drawbacks, such as corrosivity with respect to aluminium, or poor conductivity. Lithium organoborates have been investigated as a further class of compounds for use as conducting salt in rechargeable lithium batteries. However, on account of 15 their low oxidative stability and on account of misgivings as regards safety in connection with the handling of triorganoboranes, they do not come into consideration for commercial systems, A significant advance is represented by the lithium 20 complex salts of the type ABL2 (where A signifies lithium or a quaternary ammonium ion, B signifies boron, and L signifies a bidentate ligand which is bound to the boron atom via oxygen atoms) which are proposed in EP 698 301 for use in galvanic cells. However, the proposed salts, 25 the ligands of which contain at least one aromatic residue, exhibit sufficient electrochemical stability only when the aromatic hydrocarbon is substituted with electron-attracting residues, typically fluorine, or exhibits at least one nitrogen atom in the ring. Such 30 chelate compounds are not commercially available and can only be produced with high costs. The proposed products have therefore been unable to gain acceptance on the market. Quite similar boron compounds are proposed in EP 907 217 35 as constituents in organic electrolyte cells. By way of boron-containing conducting salt, compounds of the general 3 formula LiBXX’ are proposed, wherein the ligands X and X’ may be the same or different and each ligand contains an electron-attracting group containing oxygen, which binds to the boron atom. However, the listed compounds (lithium 5 boron disalicylate and a special imide salt) exhibit the disadvantages already mentioned above. The lithium bis(oxalato)borate (LiBOB) described for the first time in DE 198 29 030 is the first boron-centred complex salt described for use as an electrolyte that uses 10 a dicarboxylic acid (in this case, oxalic acid) as chelate component. The compound is easy to produce, is non-toxic, and is electrochemically stable up to about 4.5V, which makes its use in lithium-ion batteries possible. A disadvantageous aspect, however, is the fact that it can 15 hardly be employed in new battery systems with cell voltages > 3 V. For electrochemical storage batteries of such a type, salts having stabilities > approx. 5 V are required. A further disadvantageous aspect is the fact that lithium bis(oxalato)borate does not admit of any 20 possibilities for structural variation without the basic framework being destroyed. In EP 1 035 612 additives of the formula Li+B-(OR1)m(OR2)p are named, 25 with m and p = 0, 1, 2, 3 or 4, where m + p = 4, and R1 and R2 are the same or different and are optionally linked to one another directly by a single or double bond, in each case, individually or j ointly, have 30 the significance of an aromatic or aliphatic carboxylic or sulfonic acid, or in each case, individually or jointly, have the significance of an aromatic ring from the group comprising phenyl, naphthyl, anthracenyl 35 or phenanthrenyl’ which may be unsubstituted or monosubstituted to tetrasubstituted by A or Hal, or in each case, individually or jointly, have the significance of a heterocyclic aromatic 5 ring from the group comprising pyridyl, pyrazyl or bipyridyl, which may be unsubstituted or monosubstituted to trisubstituted by A or Hal, or in each case, individually or jointly, have 10 the significance of an aromatic hydroxy acid from the group of aromatic hydroxycarboxylic acids or of aromatic hydroxysulfonic acids, which may be unsubstituted or monosubstituted to tetrasubstituted by A or Hal, and 15 Hal = F, Cl or Br, and A = alkyl residue with 1 to 6 C atoms, which may be monohalogenated to trihalogenated. To be mentioned as particularly preferred additives are 20 lithium bis [1,2-benzenediolato(2-)0,0’Jborate(1-), lithium bis[3-fluoro-l,2-benzenediolaco(2-)0, 0’]borate (1-), lithium bis [2,3-naphthalenediolato(2-) 0, 01]borate (1-), lithium bis [2, 2-biphenyldiolato(2-) 0, 0’Jborate(1-), lithium bis [salicylato(2-)0,0’]borate(1-) , lithium bis [2-2 5 olato-benzenesulf onato (2-) 0,. 0’ ] borate (1-) , lithium bis [5-f luoro-2-olato-benzenesulfonato (2-)0/0’]borate(l-), lithium phenolate and lithium-2,2-biphenolate. These are all symmetrical lithium chelatoborates of the Li[BL2] type. Lithium bis (malonato)borate, .which is supposed to exhibit 30 an electrochemical window of :-up to 5 V, has been described by C. Angel1 as an electrochemically particularly stable, simple lithium (chelato)borate compound. The compound considered has the disadvantage that it is practically insoluble in the conventional battery solvents (e.g. only 35 0.08 molar in propylene carbonate), so that it can be dissolved and characterised only in DMSO and similar 5 solvents that are prohibitive for batteries (Wu Xu and C. Austen Angell, Electrochem. Solid-State Lett. 4, E1-E4, 2001) . In DE 101 08 592 mixed boron chelate complexes of the 5 general formula are described, with either X = -C(R1R2)- or -C (R:R2) -C (=0) -, wherein R1, R2 = independently of one another, H, alkyl 10 (with 1 to 5 C atoms), aryl, silyl or a polymer, and one of the alkyl residues R1 or R2 may be linked to a further chelatoborate residue, or X = 1,2-aryl, with up to 2 substituents S in positions 15 3 to 6 wherein S1, S2 = independently of one another, alkyl (with 1 to 5 C atoms), fluorine or polymer, and M+ = Li + , Na\ K\ Rb\ Cs+ or [ (R3R4R5R6) N]+ or 20 H+, with R3, R4, R5, R6 = independently of one another, H or alkyl with preferably 1 to 4 C atoms. A disadvantageous aspect of these compounds is their 2 5 frequently unsatisfactory solubility in organic solvents such as propylene carbonate, for example. Therefore the o 5 1C L5 electrical conductivity of such solutions is, as a rule, lower than that of established lithium salts (such as LiPF or LiBOB, for example). For this reason, liquid electrolytes that exclusively contain one of the mixed boron chelate complex salts disclosed in DE 101 08 592 cannot be employed for powerful high-performance batteries. The synthesis as described in DE 101 08 592 and in DE 101 08 608 is also not free frcm disadvantages: in the course of the production of mixed salts - starting from an oxidic boron raw material, for example boric acid or boron oxide, and two differing complex ligands L1 and L2 in a molar ratio of 1 : 1 : 1 - not only does the desired mixed complex salt arise but also the homo compounds [BL12] and [BL22j. In DE 101 08 608 the following examples are mentioned: Li Parent Li* substanceBoron compound Molar ratio Proportion of salts ace. to [BI^L2]” [BL1,]” complex nB NMR[BL2?J~ Example from DE 10108608 oxalic acid malonic acid boric acid 1:1:1 71 % 17 % 5 oxalic acid lactic acid boric acid 1:1:1 95 % 2 % 3 % 6 oxalic acid salicylic acid boric acid 1:1: 1 77 % 10 % 13 % r 0 5 * DE 101 08 592 The undesirable homo compounds have varying physicochemical properties, especially an electrochemical stability differing from that of the mixed compound; therefore they have to be separated out by recrystallisation or by a similar purification process, which is relatively costly. WO 01/99209 also discloses the production of mixed lithium-borate salts such as lithium (malonato oxalato)borate (Examples 6 and 7). Two possibilities for synthesis are described, which both yield the desired salt as main product, but contaminations by homo complex 7 compounds cannot be avoided (Example 6: 4.5% lithium bis(oxalato)borate). In EP 1 095 942 complex salts of the formula Li+B-(OR1)m(OR2)p 5 are described (with respect to the significance of R1, R2, m and p, see above in connection with EP 1 035 612) . The serve as conducting salts in electrolytes for electrochemical cells. They may also be used in proportions between 1 % and 99 % in combination with other 10 conducting salts. Suitable are conducting salts from the group comprising LiPF6, LiBF4, LiC10F4, LiAsF6, LiCF3SO3/ LiN(CF3SO2)2 or LiC (CF3SO2) 3 and mixtures thereof. These are all fluorinated conducting salts. The object of the present invention is to overcome the 15 disadvantages of the state of the art and to find, in particular, fluorine-free conducting salts that are capable of being produced easily and inexpensively for Lithium-ion batteries, and to demonstrate the synthesis thereof. Moreover, the conducting salts are to be capable 20 of being adapted to the material-specific and application-specific properties and are to have a forming function and an overcharge-protection function. The object is achieved in that salt mixtures containing lithium bis(oxalato)borate (‘LiBOB1) and also mixed 25 lithium-borate salts of the type X u* (i) are employed by way of conducting salt, the proportion of compound (I) in the salt mixture amounting to 0.01 mol.% to 20 mo1.%. X in formula (I) is a bridge which is linked 30 to the boron by two oxygen atoms and which is selected from 8 > X = wherein Y1 and Y2 together signify 0, m = 1, n = 0, and Y3 and Y4 are, independently of one another, H or an alkyl 5 residue with 1 to 5 C atoms, or Y1, Y2, Y3, Y4 are in each case, independently of one another, OR (with R = alkyl residue with 1 to 5 C atoms) or H or an alkyl residue with 1 to 5 C atoms, and where m = 0 or 1, n = 0 or 1, or 10 - Y2 and Y3 are members of a 5-membered or 6-membered aromatic or heteroaromatic ring (with N, 0 or S as heteroelement) which may be optionally substituted with alkyl, alkoxy, carboxy or nitrile, in which case Y1 and Y4 are not applicable and n = 0, m = 0 or 1. 15 These new, fluorine-free mixtures of substances may, for example, be produced in a manner analogous to a production process described in DE 101 08 592. In this process, the 1:1:1:1 stoichiometry (boron compound (e.g. boric acid) / oxalic acid / chelating agent L / lithium 20 compound) has to be departed from in such a manner that at most 20 mol.% of the chelating agent L2, relative to oxalic acid, is employed. The molar ratio of the substances employed (boron compound / mixture of oxalic acid and chelating agent L / lithium compound) is 1 : 2 : 1, the 25 mixture of oxalic acid and chelating agent L2 containing a maximum of 20 mol.% chelating agent L2. In this case L2 is, for example, a dicarboxylie acid (not oxalic acid), hydroxycarboxylic acid or salicylic acid (which may also be maximally disubstituted). Further possibilities for 30 the chelating agent L2 are listed below in connection with the description of compound part X. 9 The conversion is preferably undertaken in such a manner that the raw-material components are suspended in a medium (e.g. toluene, xylene, methylcyclohexane, perfluorinated hydrocarbons with more than 6 C atoms) that is suitable for azeotropic removal of water, and the water is removed azeotropically in known manner. It is also possible to perform the synthesis in aqueous solution. In this case the components are charged into water in arbitrary sequence and are concentrated by evaporation, subject to stirring, preferably at reduced pressure. After removal of the bulk of the water, a solid reaction product forms which, depending upon the specific product properties, is subjected to final drying at temperatures between 100 °C and 180 °C and at reduced pressure (e.g. 10 mbar). Besides water, alcohols and other polar organic solvents are also suitable as reaction media. Lastly, production of the product may also be undertaken without addition of any solvent, i.e. the commercial raw materials are mixed and are then heated by supply of heat, and are dehydrated, under preferably reduced pressure. In the course of implementation of the process a mixture forms that contains at least 80 mol.% LiBOB in addition to at most 20 mol.% of the mixed lithium-borate salt (I). Surprisingly, no detectable quantities of the homo complex compound are present in synthesis mixtures of such a type. The conducting-salt mixture that is obtained has the advantage, in comparison with pure LiBOB, that in the event of overcharge a decomposition reaction sets in at the cathode, which slows down the rise in cell voltage. 10 As a result, dangerous consequent reactions of the cathode material with constituents of the electrolyte can be avoided or lessened. Preferred examples of compound part X are 1,3-dicarboxylic 5 acids formally lessened by two OH groups, such as malonic acid and alkylmalonic acids (malonic acid substituted with an alkyl group with preferably 1 to 5 C atoms). (The 0 atoms binding to the boron are already contained in formula (I); the 1,3-dicarboxylic acids correspond to L2.) 10 Further preferred examples of compound part X are 1,2- or 1,3-hydroxycarboxylic acids formally lessened by two OH groups, such as glycolic acid or lactic acid. (The 1,2-or 1,3-hydroxycarboxylic acids correspond to L2.) Compound part X may also preferably be constituted by saturated C2 15 chains or saturated C3 chains, this being derivable formally from 1,2- or 1,3-diols lessened by two OH groups. (The 1,2- or 1,3-diols correspond to L2.) Further preferred examples of compound part X are 1,2-bisphenols (such as pyrocatechol) or 1, 2-carboxyphenols 20 (such as salicylic acid) or aromatic or heteroaromatic 1,2-dicarboxylic acids (such as phthalic acid or pyridine-2,3-diol), these compounds having been formally lessened by two OH groups. The listed 1,2-bisphenols, 1,2-carboxyphenols or aromatic 1,2-dicarboxylic acids 25 correspond to L2. The subject-matter of the invention will be elucidated in more detail on the basis of the following Examples: Example 1: In a 250 ml round-bottom flask made of glass 23.95 g 30 oxalic acid dihydrate, 6.81 g boric acid and 1.38 g salicylic acid (10 mol.%, relative to boric acid) were suspended in 50 ml water and, subject to stirring, added to 4.06 g lithium carbonate. After the evolution of gas (CO2 from the neutralisation reaction) had flattened out, 11 the suspension was refluxed for 1 hour at an oil-bath temperature of 115 °C. In this process a clear, colourless solution was formed. This solution was totally concentrated by evaporation in a vacuum in a rotary 5 evaporator’at an oil-bath temperature of 125 °C. The solids left behind were precrushed with a nickel spatula under protective-gas atmosphere (argon) and were finely triturated in a porcelain mortar. The powder was then recharged into a glass round-bottom flask and 10 subjected to final drying in a rotary evaporator at 150 °C and, lastly, at 13 mbar. Yield: 17.3 g (88% of the theoretical value; losses due to baked-on deposits in the glass flask) Analysis: lithium 3.60% (nominal: 3.54%) 15 Purity: In the nB NMR spectrum (solvent THF/C6D6) it is not possible for the homo compound lithium bis(salicylato)borate (literature shift 4.0 ppm) to be detected; recognisable only are the signals of the expected products lithium bis(oxalato)borate (7.6 ppm) , 20 abbreviated to LiBOB, and of the mixed salt lithium (salicylato, oxalato)borate (5.6 ppm), abbreviated to LiSOB, see Figure 1. or 1. Claims 1. A conducting salt containing lithium bis(oxalato)borate (LiBOB) and mixed lithium-borate salts of the type 5 wherein the proportion of compound (I) in the conducting salt amounts to 0.01 mol.% to 20 mol.% and X in formula (I) is a bridge which is linked to the boron by two oxygen atoms and which is selected from 10 wherein Y1 and Y2 together signify 0, m = 1, n = 0, and Y3 and Y4 are, independently of one another, H or an alkyl residue with 1 to 5 C atoms, or L5 - Y1, Y2, Y3, Y4 are in each case, independently of one another, OR (with R = alkyl residue with 1 to 5 C atoms), or H or an alkyl residue with 1 to 5 C atoms, and where m = 0 or 1, n = 0 or 1, or Y and Y are members of a 5-membered or 6-membered ‘0 aromatic or heteroaromatic ring (with N, O or S as heteroelement), which may be optionally substituted with alkyl, alkoxy, carboxy or nitrile, in which case Y1 and Y4 are not applicable and n = 0, m = 0 or 1. 2. A conducting salt according to Claim 1, characterised 5 in that compound part X is formed from 1,3- dicarboxylic acids formally lessened by two OH groups. 13 3. A conducting salt according to Claim 2, characterised in that the 1,3-dicarboxylic acid is malonic acid or an allcylmalonic acid. 4. A conducting salt according to Claim 1, characterised 5 in that compound part X is formed from 1,2- or 1,3- hydroxycarboxylic acids formally lessened by two OH groups. 5. A conducting salt according to Claim 4, characterised in that the 1,2-hydroxycarboxylic acid or 1,3- 10 hydroxycarboxylic acid is glycolic acid or lactic acid. 6. A conducting salt according to Claim 1, characterised in that compound part X is formed by saturated C2 chains or saturated C3 chains.. 15 7. A conducting salt according to Claim 1, characterised in that compound part X is formed from 1,2-bisphenols or from 1,2-carboxyphenols (such as salicylic acid) or from aromatic 1,2-dicarboxylic acids (such as phthalic acid) or pyridine-2,3-diol, these compounds having 20 been formally lessened by two OH groups. 8. A conducting salt according to Claim 7, characterised in that the 1,2-bisphenol is pyrocatechol, the 1,2-carboxyphenol is salicylic acid, and the 1,2-dicarboxylic acid is phthalic acid. 25 9. A process for producing conducting salts according to one or more of Claims 1 to 8, characterised in that a suitable boron compound, oxalic acid, a suitable chelating agent L2 and a suitable lithium compound are mixed, the molar ratio of the substances employed 30 (boron compound / mixture of oxalic acid and chelating agent L / lithium compound) being 1:2:1, and the mixture of oxalic acid and chelating agent L2 containing a maximum of 20 mol.% chelating agent L . 14 10. Process according to Claim 9, characterised in that boric acid is employed as boron compound, and a dicarboxylic acid (not oxalic acid) or hydroxycarboxylic acid is employed as chelating 5 agent L2. 11. Process according to Claim 9, characterised in that 1, 3-dicarboxylic acids, for example malonic acid or an alkylmalonic acid, in which case an alkyl group with 1 to 5 C atoms is preferably employed, 1,2- or 1,3- 10 hydroxycarboxylic acids, for example glycolic acid or lactic acid, 1,2- or 1,3-diols, 1,2-bisphenols, for example pyrocatechol, 1,2-carboxyphenols, for example salicylic acid (which may also be maximally disubstituted) or aromatic or heteroaromatic 1,2- 15 dicarboxylic acids, for example phthalic acid or pyridine-2,3-diol, is employed as chelating agent L2. 12. Process according to one of Claims 9 to 11, characterised in that the raw-material components are suspended in a medium suitable for azeotropic removal 20 of water (e.g. toluene, xylene, methylcyclohexane, perfluorinated hydrocarbons with more than 6 C atoms), and the water is removed azeotropically in known manner. 13. Process according to one of Claims 9 to 11, 25 characterised in that it is implemented in aqueous solution, the components being charged into water in arbitrary sequence and being concentrated by evaporation subject to stirring, preferably at reduced pressure. 30 14. Process according to Claim 13, characterised in that alcohols or other polar organic solvents are used instead of water as reaction media. 15. Process according to ore of Claims 9 to 11, characterised in that the raw-material components are 35 mixed without addition of a solvent, are heated by 15 supply of heat, and are dehydrated under preferably reduced pressure. 16. Use of the conducting salts according to one or more of Claims 1 to 8 in galvanic cells. 5 17. Use of the conducting salts according to one or more of Claims 1 to 8 in lithium-ion batteries.

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