Electrolyte compositions

Abstract

An electrolyte composition for a lithium ion battery includes (a) 5-35wt% of lithium salt; (b) 2-10wt% of additive; and (c) 55-93wt% solvent. The lithium salt comprises (ai) a salt selected from lithi

Description

14 04 25 ELECTROLYTE COMPOSITIONS Technical Field The present invention relates to electrolyte compositions. 5 Background Commercial lithium-ion batteries typically use LiPFe as the lithium salt source and linear carbonates e.g. DEC / DMCZEMC as solvents. However, the salt and solvent components used in most commercial Li-ion batteries cannot be processed at elevated 10 temperatures due to thermal decomposition and / or their volatility. Manufacture of lithium-ion battery components by extrusion is an area of current interest, due to manufacturing costs and throughput rates. Extrusion typically involves processing at elevated temperatures. Other useful processing techniques for battery manufacture which involve elevated temperatures include hot rolling and hot 15 pressing. Summary According to a first aspect of the present invention, there is provided an electrolyte composition for a lithium ion battery including: 20 (a) 5-3 5 wt% of lithium salt (b) 2-10wt% of additive; and (c) 55-93wt% solvent; wherein the lithium salt comprises; (ai) a salt selected from lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 25 lithium difluoro(oxalato)borate, and lithium bis(oxalato) borate; (aii) and optionally, a co-salt selected from lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide; wherein the molar ratio of the salt to co-salt is between 100:0 and 5:95; with the proviso that the composition does not comprise lithium 30 bis(fluorosulfonyl)imide alongside lithium difluoro(oxalato)borate or lithium tetrafluroborate; 14 04 25 and wherein the additive comprises vinylene carbonate and optionally, fluoroethylene carbonate; and wherein the solvent comprises either (ci) ethylene carbonate and 10-30mol% propylene carbonate, or (cii) y-butyrolactone and optionally ethylene 5 carbonate. The identification of new lithium-ion battery electrolyte compositions is not straightforward. The inventors have identified a series of LiPFe-free liquid electrolytes with low volatility even at elevated temperatures, which can thus be used in processing 10 techniques which involved elevated temperatures. (LiPFe decomposes at such elevated temperatures. It may also be advantageous to avoid using LiPFe because it is moisture sensitive, releasing HF on contact with water, and can cause thermal runaway on contact with water). The presently claimed compositions (a) passivate graphite (meaning that graphite can be used as the anode material), (b) are stable at high 15 temperature with a flash point above 100°C, and have a low vapour pressure, and can therefore be extruded (or otherwise processed at elevated temperatures), (c) are stable with respect to common cathode materials, (d) have sufficient ionic conductivity and (e) provide sufficient rate performance. 20 In some particular cases, the invention provides an electrolyte composition for a lithium ion battery including comprising 15-3 5 wt% of lithium salt, 2-10wt% of additive and 55-83wt% of solvent; and wherein (a) the lithium salt comprises 5-100mol% of lithium 2-trifluoromethyl-4,5-25 dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 0-95mol% lithium bis(fluorosulfonyl)imide; (b) the additive comprises 30-90mol% fluoroethylene carbonate and 10-70mol% vinylene carbonate; and (c) the solvent comprises 0-90mol% ethylene carbonate and 10-100mol% 30 y-butyrolactone. 14 04 25 The invention also provides an extruded battery component comprising an electrolyte composition according to the first aspect, and a method of forming a battery component, including a processing step which requires heating of a composition according to the first aspect to a temperature in excess of about 55°C. Suitably, the 5 processing step may require heating of the composition to a temperature in excess of about 60°C, 70°C or 80°C. In some cases, the processing step requiring heating may include extrusion. Further features and advantages of the invention will become apparent from the 10 following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 shows discharge capacity as function of C-rate with high Ni cathode 15 and natural graphite anode at 30°C. The solid line is data for example 15 and the dashed line is the comparative example. The same batch of electrodes and cell format were used i.e., the only difference is the electrolyte. It can be seen that the rate performance for the example 15 composition exceeds the comparative example data. Figure 2 shows discharge capacity as function of C-rate with high Ni cathode 20 and natural graphite anode at 30°C. The data coding is as follows: Squares - example 2 Circles - example 5 Triangles - example C Diamonds - example 16 25 The same batch of electrodes and cell format were used i.e., the only difference is the electrolyte. It can be seen that the rate performance for the example 5 composition is similar to the comparative example data up to 2C, and that the rate performance of treh example 16 composition exceeds the comparative example data. 30 Detailed Description In some cases, the lithium concentration in the composition is between about 0.7M and 2.0M. 14 04 25 In some cases, the composition consists of (a) 5-35wt% of lithium salt; (b) 2-10wt% of additives; and (c) 55-93wt% solvent. In some cases, the lithium salt consists of: 5 (ai) a salt selected from lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, lithium difluoro(oxalato)borate, lithium bis(oxalato) borate and lithium tetrafluroborate; (aii) and optionally, a co-salt selected from lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide; 10 wherein the molar ratio of the salt to co-salt is between 100:0 and 5:95. In some cases, the additive comprises or consists of 30-90mol% fluoroethylene carbonate and 10-70mol% vinylene carbonate. 15 In some cases, the solvent consists of either (ci) 70-90mol% ethylene carbonate and 10-30mol% propylene carbonate, or (cii) 10-100mol% y-butyrolactone and optionally 0-90mol% ethylene carbonate. In some cases, the electrolyte composition comprises 15-3 5wt% of lithium salt, 20 2-10wt% of additive and 55-83wt% of solvent; and wherein (a) the lithium salt comprises 5-100mol% of lithium 2-trifluoromethyl-4,5-dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 0-95mol% lithium bis(fluorosulfonyl)imide; 25 (b) the additive comprises 30-90mol% fluoroethylene carbonate and 10- 70mol% vinylene carbonate; and (c) the solvent comprises 0-90mol% ethylene carbonate and 10-100mol% y-butyrolactone. 30 In some cases, the composition consists of 15-35wt% of lithium salt, 2-10wt% of additive and 55-83wt% of solvent. 14 04 25 In some cases, the lithium salt consists of 5-100mol% of lithium 2-trifluoromethyl-4,5-dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 0-95mol% lithium bis(fluorosulfonyl)imide. 5 In some cases, the additive consists of 30-90mol% fluoroethylene carbonate and 10-70mol% vinylene carbonate. In some cases, the solvent consists of 0-90mol% ethylene carbonate and 10-100mol% y-butyrolactone. 10 In some cases, the electrolyte composition is selected from the group consisting of: a) 6.4wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 1.6wt% lithium bis(oxalato) borate, 15.6wt% lithium bis(fluorosulfonyl)imide, 7wt% ethylene 15 carbonate, 63.3wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; b) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 18.4wt% ethylene carbonate, 55.1wt% y-butyrolactone, 2.1wt% vinylene carbonate and lwt% fluoroethylene carbonate; 20 c) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 17.6wt% ethylene carbonate, 52.8wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; d) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 11.7wt% ethylene carbonate, 58.7wt% y-25 butyrolactone, 4.1 wt% vinylene carbonate and 2wt% fluoroethylene carbonate; e) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 28.1wt% lithium bis(fluorosulfonyl)imide, 15.8wt% ethylene carbonate, 47.4wt% y-butyrolactone, 3.7wt% vinylene carbonate and 1.8wt% fluoroethylene carbonate; f) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 12.5wt% 30 lithium bis(fluorosulfonyl)imide, 19.4wt% ethylene carbonate, 58.2wt% y-butyrolactone, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; 14 04 25 g) 0.8wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 14.8wt% lithium bis(fluorosulfonyl)imide, 19.4wt% ethylene carbonate, 58.2wt% y-butyrolactone, 4.5wt% vinylene carbonate and 2.3wt% fluoroethylene carbonate; h) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 15.6wt% 5 lithium bis(fluorosulfonyl)imide, 18.7wt% ethylene carbonate, 56.0wt% y-butyrolactone, 4.3wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; i) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 16.9wt% ethylene carbonate, 50.6wt% y-butyrolactone, 6.1wt% vinylene carbonate and 3.1wt% fluoroethylene carbonate; 10 j) 1.6wt% lithium bis(oxalato) borate, 14.0wt% lithium bis(fluorosulfonyl)imide, 19.4wt% ethylene carbonate, 58.2wt% y-butyrolactone, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; k) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 70.4wt% y-butyrolactone, 4.1 wt% vinylene carbonate 15 and 2wt% fluoroethylene carbonate; 1) 0.8wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 22.6wt% lithium bis(fluorosulfonyl)imide, 17.6wt% ethylene carbonate, 52.8wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; m) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 15.6wt% 20 lithium bis(fluorosulfonyl)imide, 18.7wt% ethylene carbonate, 56.0wt% y-butyrolactone, 4.3wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; n) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 12.5wt% lithium bis(fluorosulfonyl)imide, 7.8wt% ethylene carbonate, 69.8wt% y-butyrolactone, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; 25 o) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 7wt% ethylene carbonate, 63.4% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; p) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 28.1wt% lithium bis(fluorosulfonyl)imide, 6.3wt% ethylene carbonate, 56.9wt% y-30 butyrolactone, 3.7wt% vinylene carbonate and 1.8wt% fluoroethylene carbonate; and 14 04 25 q) 3.2wt% lithium bis(oxalato) borate, 20.3wt% lithium bis(fluorosulfonyl)imide, 11.7wt% ethylene carbonate, 58.7wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate. 5 In some such cases, the electrolyte composition is composition a or b. The comparative data used in this application relates to the following electrolyte composition, which is known in the art: 1 Molar LiPFe, in a solvent, the solvent comprising ethylene carbonate and 10 ethylmethylcarbonate in a 1:3 weight ratio. An additive component was added to this solution; this comprised vinylene carbonate (2wt%) and fluoroethylene carbonate (0.5wt%, wt% based on total weight of solution including salt+solvent+additive). 15 Several electrolyte compositions are described table 1 below. These have been tested in cells, as described below, to determine the first cycle efficiency and rate capacity at various discharge rates, as illustrated in the figures. Experiment number Electrolyte composition Lithium salt wt% Solvents breakdown (w / w) and total solvent (wt%) Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) C Comparative data (LiPF6 Benchmark) LiPFe = 13.4% EC / EMC=1:3 Total = 84.1% VC / FEC = 4 / 1 Total = 2.5% 89.5 39 1 LiBF4 + EC / PC + VC LiBF4 = 7.8% EC / PC=4:1 Total = 86.6% 5.5wt% VC 90 13 14 04 25 Experiment number Electrolyte composition Lithium salt wt% Solvents breakdow (w / w) total sob (wt%) n and vent Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) 2 (EFl27) BTEP-V6 - LiBF4 / LiTFSI + EC / PC + VC LiBF4 = 1.6% LiTFSI 19.1% EC / PC=3 Total 74.5% 1 4.8wt% VC 85.6 17 3 (EF59) BTEP-V8 - LiBF4 / LiTFSI + EC / PC + VC / FEC LiBF4 = 1.6% LiTFSI 19.1% EC / PC=3 Total 72.9% 1 VC / FEC = 2:1 Total = 6.3wt% 85.6 14 4 (EF64) BEP-V12 - LiBF4 + EC / PC + VC LiBF4 = 7.8% EC / PC=4:1 Total = 81.1% ll.lwt% VC 90.1 19 5 (EFl29) IFEP-VF8- LiTDI / LiFSI+ EC / PC + VC / FEC LiTDI = 3.2% LiFSI = 12.5% EC / PC=3 Total 77.6% 1 VC / FEC=2:1 Total = 6.7% 89.0 27 6 (EF130) IFEP-VF8 - LiTDI / LiFSI+ EC / PC + VC / FEC LiTDI = 1.6% LiFSI = 14.0% EC / PC=3 Total 77.6% 1 VC / FEC=2:1 Total = 6.8% 89.1 24 7 (EF131) IFEP-VF8 - LiTDI / LiFSI+ EC / PC + VC / FEC LiTDI = 0.8% LiFSI = 14.8% EC / PC=3 Total 77.6% 1 VC / FEC=2:1 Total = 6.8% 89.1 23 14 04 25 Experiment number Electrolyte composition Lithium salt wt% Solvents breakdown (w / w) and total solvent (wt%) Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) 8 LiBOB / LiFSI + EC / PC + VC / FEC LiBOB = 3.2% LiFSI = 12.5% EC / PC=3:1 Total = 77.6% VC / FEC=2:1 Total = 6.7% 89.4 25 9 LiTDI / LiBOB / LiFSI + EC / PC + VC / FEC LiTDI = 9.6% LiBOB = 2.4% LiFSI = 11.7% EC / PC=3:1 Total = 70.2% VC / FEC=2:1 Total = 6.1% 84.5 34 10 LiBOB / LiFSI + EC / PC + VC / FEC LiBOB = 3.2% LiFSI = 20.3% EC / PC=3:1 Total = 70.4% VC / FEC=2:1 Total = 6.1% 83.2 35 11 LiBOB / LiFSI + EC / PC + VC / FEC LiBOB = 1.6% LiFSI = 21.8% EC / PC=3:1 Total = 70.4% VC / FEC=2:1 Total = 6.1% 83.7 37 12 LiBOB / LiFSI + EC / PC + VC / FEC LiBOB = 0.8% LiFSI = 22.6% EC / PC=3:1 Total = 70.4% VC / FEC=2:1 Total = 6.1% 83.2 36 14 04 25 Experiment number Electrolyte composition Lithium salt wt% Solvents breakdown (w / w) and total solvent (wt%) Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) 13 LiTDI / LiBOB / LiFSI + EC / PC + VC / FEC LiTDI = 3.2% LiBOB = 1.6% LiFSI = 18.7% EC / PC=3:1 Total = 70.4% VC / FEC=2:1 Total = 6.1% 85.1 38 14 LiTDI / LiBOB / LiFSI + EC / PC + VC / FEC LiTDI = 6.4% LiBOB = 1.6% LiFSI = 15.6% EC / GBL=1:9 Total = 70.3% VC / FEC=2:1 Total = 6.1% 83.8 37 15 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 20.3% EC / GBL=1:3 Total = 73.5% VC / FEC=2:1 Total = 3.1% 89.1 45 16 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 20.3% EC / GBL=1:3 Total = 70.4% VC / FEC=2:1 Total = 6.1% 89.2 49 17 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 20.3% EC / GBL=1:5 Total = 70.4% VC / FEC=2:1 Total = 6.1% 88.4 43 14 04 25 Experiment number Electrolyte composition Lithium salt wt% Solvents breakdown (w / w) and total solvent (wt%) Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) 18 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 28.1% EC / GBL=1:3 Total = 63.2% VC / FEC=2:1 Total = 5.5% 87.7 37 19 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 12.5% EC / GBL=1:3 Total = 77.6% VC / FEC=2:1 Total = 6.7% 87.8 35 20 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 0.8% LiFSI = 14.8% EC / GBL=1:3 Total = 77.6% VC / FEC=2:1 Total = 6.8% 87.7 15 21 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 15.6% EC / GBL=1:3 Total = 74.7% VC / FEC=2:1 Total = 6.5% 87.8 36 22 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 20.3% EC / GBL=1:3 Total = 67.4% VC / FEC=2:1 Total = 9.2% 88.7 44 23 LiBOB / LiFSI + EC / GBL + VC / FEC LiBOB = 1.6% LiFSI = 14.0% EC / GBL=1:3 Total = 77.6% VC / FEC=2:1 Total = 6.7% 88.1 28 14 04 25 Experiment number Electrolyte composition Lithium salt wt% Solvents breakdown (w / w) and total solvent (wt%) Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) 24 LiTDI / LiFSI + GBL + VC / FEC LiTDI = 3.2% LiFSI = 20.3% GBL=70.4% VC / FEC=2:1 Total = 6.1% 88.1 45 25 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 0.8% LiFSI = 22.6% EC / GBL=1:3 Total = 70.4% VC / FEC=2:1 Total = 6.1% 88.1 43 26 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 15.6% EC / GBL=1:3 Total = 74.7% VC / FEC=2:1 Total = 6.5% 88.2 33 27 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 12.5% EC / GBL=1:9 Total = 77.6% VC / FEC=2:1 Total = 6.7% 89.9 29 28 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 20.3% EC / GBL=1:9 Total = 70.4% VC / FEC=2:1 Total = 6.1% 90.6 60 29 LiTDI / LiFSI + EC / GBL + VC / FEC LiTDI = 3.2% LiFSI = 28.1% EC / GBL=1:9 Total = 63.2% VC / FEC=2:1 Total = 5.5% 90.2 55 Experiment number Electrolyte composition Lithium salt wt% Solvents breakdown (w / w) and total solvent (wt%) Additives breakdown (w / w) and total additive (wt%) First cycle efficiency 5C rate capacity retention (%) 30 LiBOB / LiFSI + EC / GBL + VC / FEC LiBOB = 3.2% LiFSI = 12.5% EC / GBL=1:9 Total = 77.6% VC / FEC=2:1 Total = 6.7% 87.0 27 Table 1 14 04 25 The following notation is used in table 1: 5 LiBF4: ILithium tetrafluoroborate LiTFSI: lithium bis(trifluoromethanesulfonyl)imide LiFSI: lithium bis(fluorosulfonyl)imide LiTDI: lithium 2-trifluoromethyl-4,5-dicyanoimidazolide LiPF6: lithium hexafluorophorsphate 10 EC: ethylene carbonate PC: propylene carbonate GBL: y-Butyrolactone VC: vinylene carbonate FEC: fluoroethylene carbonate 15 Electrochemical evaluations of the electrolytes were carried out with Swagelok or pouch type cells. All the cells have one layer of cathode with areal coating weight over 150 g / m2, which consists of over 90wt% a high nickel NMC active materials and one layer of anode with areal coating weight over 100 g / m2, which consists of over 20 90wt% graphite / SiOx mixed active materials. Cell assembly was carried out in a dry-room with Dew point less than -40°C. By design, the nominal capacity was about 3.5 mAh or 40.0 mAh for Swagelok or 14 04 25 pouch type cells, respectively. The capacity balance was controlled at about 85-90% utilisation of the anode. For all the cells, glass fibre separators were used and 70 pl or 1 ml of an electrolyte was added for Swagelok or pouch cells, respectively. 5 All the cells were electrochemically formed at 30°C. A cell was initially charged with a current of C / 20 (a current with which it takes 20 hours to fully charge or discharge the cell) for the first hour and then increased to C / 10 for the rest of charging until the cell voltage reaching the cut-off voltage of 4.2V. Then the cell is discharged at C / 10 until the cut-off voltage of 2.5 V. The cell cycles two more cycles with the same 10 cut-off voltages at C / 10 for both charging and discharging. The first-cycle efficiency was determined by the first cycle charging capacity divided by first cycle discharging capacity and presented as percentage. Once a cell passed this formation step, rate capability was tested at 30°C and 45°C, sequentially. The C-rates were calculated based on cathode nominal capacity (active material weight times its theoretical capacity). In 15 a rate capability test, all the charging was carried out at current of C / 5 while the discharging ranging from C / 10 to 10C. The rate capacities were thus determined, which can be further normalised by dividing the C / 10 capacity from the same test. In addition to the data presented in table 1, the capacity retention of a cells 20 including electrolyte compositions C, 2, 5 and 16 after rate tests at 0.2C was found to be at or around 100%. The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood 25 that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is 30 defined in the accompanying claims.

Claims

CLAIMS 1. An electrolyte composition for a lithium ion battery, the composition including: (a) 5-35wt% of lithium salt (b) 2-10wt% of additive; and (©) 55-93wt% solvent; wherein the lithium salt comprises; (ai) a salt selected from lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, lithium ~~ difluoro(oxalato)borate, lithium bis(oxalato) borate and lithium tetrafluroborate; (aii) and optionally, a co-salt selected from lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide; wherein the molar ratio of the salt to co-salt is between 100:0 and 5:95: with the proviso that the composition does not comprise lithium bis(fluorosulfonyl)imide alongside lithium difluoro(oxalato)borate or lithium tetrafluroborate; and wherein the additive comprises vinylene carbonate and optionally, fluoroethylene carbonate; and wherein the solvent comprises either (ci) ethylene carbonate and 10- 30mol% propylene carbonate, or (cii) y-butyrolactone and optionally ethylene carbonate.

2. An electrolyte composition according to claim 1, wherein the lithium concentration in the composition is between about 0.7M and 2.0M.

3. An electrolyte composition according to claim 1 or claim 2, wherein the composition consists of (a) 5-35wt% of lithium salt; (b) 2-10wt% of additives; and (c) 55-93wt% solvent.

4. An electrolyte composition according to any preceding claim, wherein the lithium salt consists of: (ai) a salt selected from lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, lithium difluoro(oxalato)borate, lithium bis(oxalato) borate and lithium tetrafluroborate; (ail) and optionally, a co-salt selected from lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide; wherein the molar ratio of the salt to co-salt is between 100:0 and 5:

95. S. An electrolyte composition according to any preceding claim, wherein the additive comprises or consists of 30-90mol% fluoroethylene carbonate and 10- 70mol% vinylene carbonate.

6. An electrolyte composition according to any preceding claim, wherein the solvent consists of either (ci) 70-90mol% ethylene carbonate and 10-30mol% propylene carbonate, or (cii) 10-100mol% y-butyrolactone and optionally 0-90mol% ethylene carbonate.

7. An electrolyte composition according to any of claims 1 to 6, comprising 15-35wt% of lithium salt, 2-10wt% of additive and 55-83wt% of solvent; and wherein (a) the lithium salt comprises 5-100mol% of lithium 2-trifluoromethyl-4,5- dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 0-95mol% lithium bis(fluorosulfonyl)imide; (b) the additive comprises 30-90mol% fluoroethylene carbonate and 10- 70mol% vinylene carbonate; and (c) the solvent comprises 0-90mol% ethylene carbonate and 10-100mol% y-butyrolactone.

8. An electrolyte composition according to claim 7, wherein the composition consists of 15-35wt% of lithium salt, 2-10wt% of additive and 55-83wt% of solvent.

9. An electrolyte composition according to claim 7 or claim 8, wherein the lithium salt consists of 5-100mol% of lithium 2-trifluoromethyl-4,5- dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 0-95mol% lithium bis(fluorosulfonyl)imide.

10. An electrolyte composition according to any of claims 7 to 9, wherein the additive consists of 30-90mol% fluoroethylene carbonate and 10-70mol% vinylene carbonate.

11. An electrolyte composition according to any of claims 7 to 10, wherein the solvent consists of 0-90mol% ethylene carbonate and 10-100mol% y-butyrolactone.

12. An electrolyte composition according to any preceding claim, selected from the group consisting of: a) 6.4wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 1.6wt% lithium bis(oxalato) borate, 15.6wt% lithium bis(fluorosulfonyl)imide, 7wt% ethylene carbonate, 63.3wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; b) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 18.4wt% ethylene carbonate, 55.1wt% y- butyrolactone, 2.1wt% vinylene carbonate and 1wt% fluoroethylene carbonate; c) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 17.6wt% ethylene carbonate, 52.8wt% y- butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; d) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 11.7wt% ethylene carbonate, 58.7wt% y- butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; €) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 28.1wt% lithium bis(fluorosulfonyl)imide, 15.8wt% ethylene carbonate, 47.4wt% y- butyrolactone, 3.7wt% vinylene carbonate and 1.8wt% fluoroethylene carbonate; f) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 12.5wt% lithium bis(fluorosulfonyl)imide, 19.4wt% ethylene carbonate, 58.2wt% y- butyrolactone, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; 2) 0.8wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 14.8wt% lithium bis(fluorosulfonyl)imide, 19.4wt% ethylene carbonate, 582wt% y- butyrolactone, 4.5wt% vinylene carbonate and 2.3wt% fluoroethylene carbonate; h) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 15.6wt% lithium bis(fluorosulfonyl)imide, 18.7wt% ethylene carbonate, 56.0wt% y- butyrolactone, 4.3wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; 1) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 16.9wt% ethylene carbonate, 50.6wt% 7- butyrolactone, 6.1wt% vinylene carbonate and 3.1wt% fluoroethylene carbonate; 7) 1.6wt% lithium bis(oxalato) borate, 14.0wt% lithium bis(fluorosulfonyl)imide, 19.4wt% ethylene carbonate, 58.2wt% y-butyrolactone, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; k) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 70.4wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; 1) 0.8wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 22.6wt% lithium bis(fluorosulfonyl)imide, 17.6wt% ethylene carbonate, 52.8wt% y- butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; m) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 15.6wt% lithium bis(fluorosulfonyl)imide, 18.7wt% ethylene carbonate, 56.0wt% y- butyrolactone, 4.3wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; n) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 12.5wt% lithium bis(fluorosulfonyl)imide, 7.8wt% ethylene carbonate, 69.8wt% y- butyrolactone, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; 0) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 7wt% ethylene carbonate, 63.4% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; p) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 28.1wt% lithium bis(fluorosulfonyl)imide, 6.3wt% ethylene carbonate, 56.9wt% y- butyrolactone, 3.7wt% vinylene carbonate and 1.8wt% fluoroethylene carbonate; and q) 32wt% lithium bis(oxalato) borate, 20.3wt% lithium bis(fluorosulfonyl)imide, 11.7wt% ethylene carbonate, 58.7wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate.

13. An electrolyte composition according to claim 12, wherein the electrolyte composition consists of either: a) 6.4wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 1.6wt% lithium bis(oxalato) borate, 15.6wt% lithium bis(fluorosulfonyl)imide, 7wt% ethylene carbonate, 63.3wt% y-butyrolactone, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; or b) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 20.3wt% lithium bis(fluorosulfonyl)imide, 18.4wt% ethylene carbonate, 55.1wt% y- butyrolactone, 2.1wt% vinylene carbonate and 1wt% fluoroethylene carbonate.

14. An extruded battery component comprising an electrolyte composition according to any one of claims 1 to 13.

15. A method of forming a battery component, including a processing step which requires heating of a composition according to any one of claims 1 to 13 to a temperature in excess of about 55°C.

16. A method according to claim 15, wherein the processing step includes extruding a composition according to any one of claims 1 to 13.

Images