Electrolyte compositions

Abstract

An electrolyte composition for a lithium ion battery comprises 5-25wt% of lithium salt, 2-10wt% of additive and 65-93wt% of solvent; and wherein (a) the lithium salt comprises 20-100mol% of lithium 2-

Description

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 / DMC / EMC 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, the composition comprising 5-25wt% 20 of lithium salt, 2-10wt% of additive and 65-93wt% of solvent; and wherein (a) the lithium salt comprises 20-50mol% of lithium 2-trifluoromethyl-4,5-dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 50-80mol% lithium bis(fluorosulfonyl)imide; 25 (b) the additive comprises 28-35mol% fluoroethylene carbonate and 60- 72mol% vinylene carbonate; and (c) the solvent comprises 70-90mol% ethylene carbonate and 10-30mol% propylene carbonate. 30 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 18 03 25 techniques which involved elevated temperatures. (LiPFe decomposes at such elevated temperatures. It may also be advantageous to avoid using LiPF6 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 5 (meaning that graphite can be used as the anode material), (b) are stable at high 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. 10 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 15 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 20 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 25 and natural graphite anode at 30°C. The solid line is data for example 1 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 up to 2C is similar. 30 Detailed Description In some cases, the lithium concentration in the electrolyte composition is between about 0.7M and 2.0M. 18 03 25 In some cases, the lithium salt consists of 20-50mol% of lithium 2-trifluoromethyl-4,5-dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 50-80mol% lithium bis(fluorosulfonyl)imide. 5 In some cases, the solvent consists of 70-90mol% ethylene carbonate and 10-30mol% propylene carbonate. In some cases, the electrolyte composition is selected from the group consisting 10 of: a) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 12.5wt% lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt% ethylene carbonate, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; b) 1.6wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 14.0wt% 15 lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt% ethylene carbonate, 4.5wt% vinylene carbonate and 2.3wt% fluoroethylene carbonate; c) 0.8wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 14.8wt% lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt% ethylene carbonate, 4.5wt% vinylene carbonate and 2.3wt% fluoroethylene carbonate; 20 d) 3.2wt% lithium bis(oxalato) borate, 12.5wt% lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt% ethylene carbonate, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate; e) 9.6wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 2.4wt% lithium bis(oxalato) borate, 11.7wt% lithium bis(fluorosulfonyl)imide, 17.5wt% 25 propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; f) 3.2wt% lithium bis(oxalato) borate, 20.3wt% lithium bis(fluorosulfonyl)imide, 17.5wt% propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; 30 g) 1.6wt% lithium bis(oxalato) borate, 21.8wt% lithium bis(fluorosulfonyl)imide, 17.5wt% propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; 18 03 25 h) 0.8wt% lithium bis(oxalato) borate, 22.6wt% lithium bis(fluorosulfonyl)imide, 17.5wt% propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; and i) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 1.6wt% 5 lithium bis(oxalato) borate, 18.7wt% lithium bis(fluorosulfonyl)imide, 17.5wt% propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate. In some such cases, the electrolyte composition is composition a. 10 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 ethylmethylcarbonate in a 1:3 weight ratio. 15 - 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). Several electrolyte compositions are described in table 1 below. These have 20 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. 18 03 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 cycl e effic ienc y 5C rate capacity retention (%) C Comparative data (LiPF6 Benchmark) LiPF6 = 13.4% EC / EMC=1:3 Total = 84.1% VC / FEC = 4 / 1 Total = 2.5% 89.5 39 1 LiTDI / LiFSI+ EC / PC + VC / FEC LiTDI = 3.2% LiFSI = 12.5% EC / PC=3:1 Total = 77.6% VC / FEC=2 :1 Total = 6.7% 89.0 27 2 (Comparative) LiTDI / LiFSI+ EC / PC + VC / FEC LiTDI = 1.6% LiFSI = 14.0% EC / PC=3:1 Total = 77.6% VC / FEC=2 :1 Total = 6.8% 89.1 24 3 (Comparative) LiTDI / LiFSI+ EC / PC + VC / FEC LiTDI = 0.8% LiFSI = 14.8% EC / PC=3:1 Total = 77.6% VC / FEC=2 :1 Total = 6.8% 89.1 23 18 03 25 4 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 5 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 6 (Comparative) 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 7 (Comparative) 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 8 (Comparative) 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 9 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 Table 1 18 03 25 The following notation is used in table 1: LiBOB: lithium bis(oxalato) borate LiFSI: lithium bis(fluorosulfonyl)imide LiTDI: lithium 2-trifluoromethyl-4,5-dicyanoimidazolide 5 LiPF6: lithium hexafluorophorsphate EC: ethylene carbonate PC: propylene carbonate VC: vinylene carbonate FEC: fluoroethylene carbonate 10 Electrochemical evaluations of the electrolytes were carried out with Swagelok (RTM) 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 15 consists of over 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 (RTM) or pouch type cells, respectively. The capacity balance was controlled at about 85-90% 20 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 (RTM) or pouch cells, respectively. 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 25 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 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 30 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 a rate capability test, all the charging was carried out at current of C / 5 while the discharging ranging from C / 10 to IOC. The rate capacities were thus determined, which can be further normalised by dividing the C / 10 capacity from the same test. 5 In addition to the data presented in table 1, the capacity retention of a cells including electrolyte compositions C and 1 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 10 invention. Further embodiments of the invention are envisaged. It is to be understood 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 15 may also be employed without departing from the scope of the invention, which is defined in the accompanying claims. 28 03 25

Claims

1. An electrolyte composition for a lithium ion battery, the composition comprising 5-25wt% of lithium salt, 2-10wt% of additive and 65-93wt% of solvent;5 and wherein(a) the lithium salt comprises 20-50mol% of lithium 2-trifluoromethyl-4,5-dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 50-80mol% lithium bis(fluorosulfonyl)imide;(b) the additive comprises 28-35mol% fluoroethylene carbonate and 60-10 72mol% vinylene carbonate; and(c) the solvent comprises 70-90mol% ethylene carbonate and 10-30mol% propylene carbonate.

2. An electrolyte composition according to claim 1, wherein the lithium 15 concentration in the composition is between about 0.7M and 2.0M.

3. An electrolyte composition according to any preceding claim, wherein the lithium salt consists of 20-50mol% of lithium 2-trifluoromethyl-4,5-dicyanoimidazolide or lithium bis(oxalato) borate or a mixture thereof, and 50-80mol% 20 lithium bis(fluorosulfonyl)imide.

4. An electrolyte composition according to any preceding claim, wherein the solvent consists of 70-90mol% ethylene carbonate and 10-30mol% propylene carbonate.

255. An electrolyte composition according to any preceding claim, selected from the group consisting of:a) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 12.5wt% lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt% ethylene 30 carbonate, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate;28 03 25b) 3.2wt% lithium bis(oxalato) borate, 12.5wt% lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt% ethylene carbonate, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate;c) 9.6wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 2.4wt%5 lithium bis(oxalato) borate, 11.7wt% lithium bis(fluorosulfonyl)imide, 17.5wt% propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate; andd) 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 1.6wt% lithium bis(oxalato) borate, 18.7wt% lithium bis(fluorosulfonyl)imide, 17.5wt% 10 propylene carbonate, 52.7wt% ethylene carbonate, 4.1wt% vinylene carbonate and 2wt% fluoroethylene carbonate.

6. An electrolyte composition according to claim 5, wherein the electrolyte composition consists of 3.2wt% lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, 15 12.5wt% lithium bis(fluorosulfonyl)imide, 19.4wt% propylene carbonate, 58.2wt%ethylene carbonate, 4.5wt% vinylene carbonate and 2.2wt% fluoroethylene carbonate.

7. An extruded battery component comprising an electrolyte composition according to any one of claims 1 to 6.

208. 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 6 to a temperature in excess of 55°C.25 9. A method according to claim 8, wherein the processing step includesextruding a composition according to any one of claims 1 to 6.

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