Electrolyte composition

Abstract

An electrolyte composition for a lithium-ion battery, the composition comprising: (a) 18-35 wt% of lithium salt; (b) 1-25 wt% of solvent additive; and (c) 45-80 wt% of solvent. The total amount of (a)

Description

B ACKGROUND Lithium-ion secondary batteries are the leading battery technology currently used in applications from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. Manufacturers of lithium-ion secondary batteries aim to balance good cell performance of the electrolyte with safety and cost-effectiveness. SUMMARY In a first aspect, the present invention provides an electrolyte composition for a lithium-ion battery, the composition comprising: (a) 18-35 wt% of lithium salt; (b) 1-25 wt% of solvent additive; and (c) 45-80 wt% of solvent; wherein the total amount of (a), (b) and (c) is less than or equal to 100 wt% of the electrolyte composition, and wherein: the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI) and lithium difluoro(oxalato)borate (LiDFOB), the solvent additive comprises one or more fluorinated and / or unsaturated carbonate compounds; and the solvent comprises a cyclic carbonate. The identification of new and useful lithium-ion battery electrolyte compositions is not straightforward. The electrolyte compositions of the present invention may contain quantities of components not typically observed in standard electrolyte compositions, thereby potentially increasing the ability to tune battery performance properties. In some embodiments, the present electrolyte compositions may achieve higher power performance and / or higher cycle life. Furthermore, the present electrolyte compositions do not require the presence of LiPFe. In some embodiments, the present electrolyte compositions may have only a small amount of LiPFe compared to commercial electrolyte compositions. LiPFg is commonly used as a lithium source in commercial lithium-ion batteries, but in the presence of water and heat may undergo a thermal reaction to form POF3 and HF which may pose a health and safety hazard. Avoidance of water is possible during manufacture, but at higher cost. Therefore, electrolyte compositions herein may, in some embodiments, display improved safety and reduced financial cost. In general, the electrolyte compositions of the present invention contain a cyclic carbonate. In some embodiments, the lithium salt comprises 65-85 wt% LiFSI, 0.1-25 wt% of LiTDI, and 0.1-25 wt% LiDFOB, based on the total amount of lithium salt, wherein the total amount of lithium salt is 100 wt%. Accordingly, the electrolyte compositions herein may contain higher amounts of LiFSI compared to conventional electrolyte compositions. In some embodiments, this may improve battery performance characteristics without compromising safety. In some embodiments, LiFSI may be present at higher wt% amounts than LiTDI for ionic conductivity properties of the electrolyte solution, which in turn may improve performance at higher rate of discharge. Furthermore, the presence of LiDFOB in the electrolyte alongside LiFSI and LiTDI may improve cell performance relative to an electrolyte lacking LiDFOB. In some embodiments, the use of LiDFOB may improve cycle life properties such as capacity retention during repeated cell cycling. In some embodiments, such improvements may be found at either higher or lower discharge currents, such as either at 10 mA or 80 mA. In some embodiments, the lithium salt further comprises LiPFe. This may further improve battery performance characteristics. Additionally, since LiPFe may be absent, or may be present in the present compositions at a lower proportion than in commercial electrolyte compositions, the safety profile of such embodiments may be improved. In some embodiments, the lithium salt further comprises lithium bis(oxalato) borate (LiBOB). Electrolyte compositions comprising LiBOB may show improved rate performance properties. In some embodiments, electrolyte compositions comprising LiBOB may show improved cycle life capacity retention properties. In some embodiments, the lithium salt consists of LiFSI, LiTDI, and LiDFOB. Such electrolyte compositions may have improved battery performance characteristics. The solvent comprises a cyclic carbonate. In some embodiments, the solvent further comprises a linear carbonate. In some embodiments, the solvent further comprises a linear carbonate that comprises one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl-methyl carbonate (EMC). In some embodiments, the linear carbonate comprises one or both of DMC and EMC. In some embodiments, the linear carbonate comprises one or both of DMC and DEC. In some embodiments, the linear carbonate comprises DMC. In some embodiments, the inclusion of a linear carbonate alongside a cyclic carbonate allows for an increased amount of cyclic carbonate such as propylene carbonate (PC) to be included in the electrolyte composition. In some embodiments, the solvent comprises a cyclic carbonate that includes ethylene carbonate (EC) and PC. In some embodiments, the cyclic carbonate comprises one or more of ethylene carbonate (EC) and propylene carbonate (PC). In some embodiments, the cyclic carbonate consists of one or more of ethylene carbonate (EC) and propylene carbonate (PC). In some embodiments, the cyclic carbonate consists of EC. In some embodiments, the cyclic carbonate consists of a mixture of EC and PC. In some embodiments, the solvent consists of EC, or consists of a mixture of EC and PC. In some embodiments, the solvent comprises 60-100 wt% EC and 0-40 wt% PC, based on the total amount of solvent, wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent comprises 70-100 wt% EC and 0-30 wt% PC, based on the total amount of solvent, wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent consists of 60-100 wt% EC and 0-40 wt% PC, based on the total amount of solvent. In some embodiments, the solvent consists of 70-100 wt% EC and 0-30 wt% PC, based on the total amount of solvent. Accordingly, in some embodiments, the present electrolyte compositions permit higher levels of PC compared to standard electrolyte compositions. In some embodiments, the choice of weight ratio of EC: PC may be used to target particular cell properties, such as ionic conductivity, resistance against corrosion, operational temperature, and cycle life. In some embodiments, the solvent additive comprises one or more fluorinated carbonate compounds. In some embodiments, the solvent additive comprises one or more unsaturated carbonate compounds. In some embodiments, the solvent additive comprises one or more fluorinated cyclic carbonate compounds. In some embodiments, the solvent additive comprises one or more unsaturated cyclic carbonate compounds. In some embodiments, the solvent additive comprises one or more fluorinated and / or unsaturated 5-membered heterocyclic carbonate compounds. In some embodiments, the solvent additive comprises one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and trifluoro-propylene carbonate (TFPC). In some embodiments, the solvent additive comprises 3-60 wt% FEC and 5-85 wt% VC, based on the total amount of solvent additive, wherein the total amount of solvent additive is 100 wt%. In some embodiments, the solvent additive comprises 3-60 wt% FEC and 15-85 wt% VC, based on the total amount of solvent additive, wherein the total amount of solvent additive is 100 wt%. In some embodiments, the solvent additive comprises 15-60 wt% FEC and 40-85 wt% VC, based on the total amount of solvent additive, wherein the total amount of solvent additive is 100 wt%. In some embodiments, the solvent additive consists of FEC and VC. In some embodiments, the solvent additive consists of 15-60 wt% FEC and 40-85 wt% VC, based on the total amount of solvent additive, wherein the total amount of solvent additive is 100 wt%. Such solvent additives may show particularly good battery performance characteristics. In some embodiments, the solvent additive further comprises an organosilicon (OS) or succinonitrile (SN) in an amount of up to 5 wt%, based on the total amount of the electrolyte composition, wherein the total amount of the electrolyte composition is 100 wt%. In some embodiments, the electrolyte composition may contain up to 15,000 ppm of water, on a mass basis, based on the total amount of the electrolyte composition, wherein the total amount of the electrolyte composition is 100 wt%. Such electrolyte compositions may advantageously reduce the need to keep moisture to a minimum when preparing electrolyte compositions. In turn, this may improve cost effectiveness of the compositions and / or may reduce costs of production. In particular embodiments, such electrolyte compositions do not contain LiPFb, from the viewpoint of improving safety. In some embodiments, the electrolyte composition is selected from the group consisting of compositions 1 to 3 listed in Table 1 below. Table 1 LiFSI (wt%) LiTDI (wt%) LiDFOB (wt%) FEC (wt%) (wt%) EC (wt%) PC (wt%) 1 16.21 3.20 3.12 2.11 4.09 49.89 21.38 2 16.21 3.20 3.12 2.11 4.09 71.28 0.00 3 20.88 3.20 3.12 1.98 3.84 66.98 0.00 4 17.77 3.20 3.12 2.06 4.01 69.84 0.00 5 19.32 3.20 3.12 2.02 3.93 68.41 0.00 In a second aspect, the present invention provides an electrochemical cell comprising the electrolyte composition of the first aspect. In some embodiments, the electrochemical cell of the second aspect is a lithium-ion electrochemical secondary cell. In some embodiments, the electrochemical cell of the second aspect comprises, in addition to the electrolyte composition of the first aspect, a cathode and an anode. In some embodiments, the cathode comprises an electroactive material. In some embodiments, the cathode electroactive material includes a layered oxide material. In some embodiments, the cathode comprises a nickel manganese cobalt oxide (NMC) active material. In some embodiments, the anode comprises an electroactive material. In some embodiments, the anode electroactive material comprises graphite and optionally one or more silicon-containing materials. In some embodiments, the silicon-containing material may be a silicon-based material such as silica or a silicate. In a third aspect, the present invention provides an electrochemical energy storage device comprising an electrochemical cell according to the second aspect. In a fourth aspect, the present invention provides the use of an electrolyte composition according to the first aspect as an electrolyte in a lithium-ion battery. In some embodiments, the lithium-ion battery has an operating voltage of between 2.5 V and 4.8V vs Li° / Li+. In a fifth aspect, the present invention provides the use of LiDFOB or LiPF6 to improve the performance of a lithium-ion cell having an electrolyte composition comprising: (a) 18-35 wt% of lithium salt; (b) 1-25 wt% of solvent additive; and (c) 45-80 wt% of solvent; wherein the total amount of (a), (b) and (c) is less than or equal to 100 wt% of the electrolyte composition, and wherein: the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI) and lithium difluoro(oxalato)borate (LiDFOB), the solvent additive comprises one or more fluorinated and / or unsaturated carbonate compounds; and the solvent comprises a cyclic carbonate. In some embodiments, the electrolyte composition used in the fifth aspect is an electrolyte composition according to the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a chart of capacity retention (%) at 200 mA discharge current at 30°C of compositions 1, 2, 3, 4, 5 and 6 of Table 2. Figure 2 shows a chart of capacity retention (%) against current (mA) at 30°C with two different discharge currents (200 mA and -400 mA) of compositions 1-5 of Table 2. Figure 3 shows a chart of capacity retention (%) against current (mA) at 45°C with two different discharge currents (5C and 10C) of compositions 1, 2, 4 and 5 of Table 2. Figure 4 shows a chart of capacity retention (%) against cycle number for compositions 1 (inventive) and 6 (comparative) of Table 2. DETAILED DESCRIPTION It is to be noted that any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the optional features of any one aspect may be combined, either singly or in combination, with any other aspect of the invention unless the context demands otherwise. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Methods and materials are described herein for use in the present disclosure’ other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. Methods described herein usually employ ambient temperature of a typical laboratory, which is typically between 20°C and 30°C, such as between 22°C and 26°C, such as around 23°C -25°C, at atmospheric pressure, unless a different condition is defined herein or is more usually employed in the art e.g. for a particular apparatus. Where embodiments discussed herein use the term “comprises” or the like, corresponding embodiments using the term “consists of’ should be considered explicitly disclosed. The present invention generally describes electrolyte compositions suitable for lithium-ion batteries. The electrolyte compositions of the present invention generally comprise (a) 18-35 wt% of lithium salt, (b) 1-25 wt% of solvent additive, and (c) 45-80 wt% of solvent. The electrolyte composition has a total of 100 wt%, so that the total amount of (a), (b) and (c) should be up to 100 wt%. In embodiments where there are components in addition to (a), (b), and (c), the total amount of (a), (b) and (c) will be less than 100 wt%. In some embodiments, the electrolyte compositions of the present invention consist of (a), (b), and (c). In those embodiments, the total amount of (a), (b) and (c) will be 100 wt%. The electrolyte compositions of the present invention contain 18-35 wt% of lithium salt based on the total amount of the electrolyte composition. In some embodiments, the electrolyte composition contains at least 19 wt%, such as at least 20 wt%, at least 21 wt%, at least 22 wt% or at least 23 wt% of lithium salt based on the total amount of the electrolyte composition. In some embodiments, the electrolyte composition contains up to 34wt%, such as up to 32 wt%, up to 30 wt%, or up to 28 wt% of lithium salt based on the total amount of the electrolyte composition. In some embodiments, these end-points can be combined to form any suitable range, for example 18-34 wt%, 19-31 wt%, 20-30 wt%, 21-32 wt%, 22-35 wt%, or 23-28 wt% of lithium salt based on the total amount of the electrolyte composition. The total amount of the electrolyte composition is 100 wt%. The lithium salt comprises LiFSI, LiTDI and LiDFOB. In some embodiments, the lithium salt can comprise LiFSI, LiTDI, LiDFOB and LiPFe. In some embodiments, the lithium salt comprises LiFSI, LiTDI, LiDFOB and one or both of LiPFe, and LiBOB. In some embodiments, one or more other lithium salts may be included. In some embodiments, the lithium salt consists of LiFSI, LiTDI and LiDFOB, and optionally one or more of LiPFe and LiBOB. In some embodiments, the lithium salt consists of LiFSI, LiTDI and LiDFOB, and optionally LiPFe. In some embodiments, the lithium salt consists of LiFSI, LiTDI and LiDFOB. In some embodiments, the lithium salt consists of LiFSI, LiTDI, LiDFOB and LiPFe. In some embodiments, the lithium salt consists of LiFSI, LiTDI, LiDFOB and LiBOB. In some embodiments, the lithium salt consists of LiFSI, LiTDI, LiDFOB, LiPFe and LiBOB. In some embodiments, the lithium salt comprises 65-85 wt% of LiFSI, 0.1-25 wt% of LiTDI and 0.1-25 wt% LiDFOB, based on the total amount of lithium salt. The total amount of lithium salt is 100 wt%, so that where the sum of the amounts of LiFSI, LiTDI and LiDFOB is less than 100 wt%, there may be other lithium salts present in the lithium salt component (a). In some embodiments, the sum of the amounts of LiFSI, LiTDI and LiDFOB is 100 wt%, and in such cases the lithium salt consists of LiFSI, LiTDI and LiDFOB. In some embodiments, the lithium salt consists of 65-85 wt% of LiFSI, 0.1-25 wt% of LiTDI and 0.1-25 wt% LiDFOB, based on the total amount of lithium salt. In some embodiments, the lithium salt comprises at least 65 wt%, such as at least 66 wt% or at least 68 wt% or at least 70 wt% or at least 71 wt% of LiFSI, based on the total amount of lithium salt. In some embodiments, the lithium salt comprises up to 85 wt%, such as up to 84 wt%, up to 83 wt%, up to 82 wt%, up to 81 wt%, up to 80 wt%, or up to 77 wt% of LiFSI based on the total amount of lithium salt. In some embodiments, these end-points can be combined to form any suitable range, for example 65-85 wt%, 65-84 wt%, 65-83 wt%, 66-82 wt%, 70-80 wt%, or 71-77 wt% of LiFSI based on the total amount of lithium salt. In some embodiments, the lithium salt comprises at least 0.1 wt%, such as at least 0.5 wt%, at least 1 wt%, at least 2 wt%, at least 3 wt%, at least 4 wt%, at least 5 wt%, at least 10 wt% or at least 11 wt% of LiTDI, based on the total amount of lithium salt. In some embodiments, the lithium salt comprises up to 25 wt%, such as up to 20 wt% or up to 18 wt% or up to 15 wt% LiTDI based on the total amount of lithium salt. In some embodiments, these end-points can be combined to form any suitable range, for example 0.1-20 wt%, 0.5-15 wt%, 1-20 wt%, 2-15 wt%, 3-20 wt%, 4-18 wt%, 5-20 wt%, 5-15 wt% or 10-15 wt% LiTDI based on the total amount of lithium salt. In some embodiments, the lithium salt comprises LiDFOB in an amount of at least 0.1 wt%, such as at least 0.5 wt%, at least 1 wt%, at least 2 wt%, at least 3 wt%, at least 4 wt%, at least 5 wt%, at least 10 wt% or at least 11 wt% based on the total amount of lithium salt. In some embodiments, the lithium salt comprises up to 25 wt%, such as up to 20 wt% or up to 18 wt% or up to 15 wt% LiDFOB, based on the total amount of lithium salt. In some embodiments, these end-points can be combined to form any suitable range, for example 0.1-20 wt%, 0.5-15 wt%, 1-20 wt%, 2-15 wt%, 3-20 wt%, 4-18 wt%, 5-20 wt%, 5-15 wt% or 10-15 wt% LiDFOB based on the total amount of lithium salt. In some embodiments, the lithium salt comprises LiPFe. The LiPFe may be present in any suitable amount, such as at least 0.1 wt%, at least 0.5 wt% or at least 1 wt% based on the total amount of lithium salt. In some embodiments, the lithium salt comprises up to 10 wt% LiPFe, such as up to 8 wt%, up to 7 wt%, up to 5 wt% or up to 4 wt% based on the total amount of lithium salt. In some embodiments, these end-points can be combined to form any suitable range, for example 0.1-10 wt%, 0.1-8 wt%, 0.5-8 wt%, 0.5-7 wt% or 1-4 wt% LiPFe based on the total amount of lithium salt. Accordingly, in some embodiments of the present invention, LiDFOB and / or LiPFe may be used to improve the performance characteristics of electrolyte compositions as described herein that otherwise do not have LiDFOB and / or LiPFe. In some embodiments, the use of LiDFOB may improve cycle life energy retention and / or cycle life capacity retention properties. In some embodiments, the use of LiDFOB and / or LiPFe may improve rate performance properties. Additionally, since LiPFe may be present in the present compositions at a lower proportion than in commercial electrolyte compositions, the safety profile of such embodiments may be improved and thus the performance characteristic that is improved may be longevity, for example. In some embodiments, the lithium salt comprises LiBOB. The LiBOB may be present in any suitable amount, such as at least 0.1 wt%, at least 0.5 wt% or at least 1 wt% based on the total amount of lithium salt. In some embodiments, the lithium salt comprises up to 5 wt% LiBOB, such as up to 4 wt%, up to 3 wt%, or up to 2 wt% based on the total amount of lithium salt. In some embodiments, these end-points can be combined to form any suitable range, for example 0.1-5 wt%, 0.1-4 wt%, 0.5-4 wt%, 0.5-3 wt% or 1.0-2 wt% LiBOB based on the total amount of lithium salt. The electrolyte compositions of the present invention contain 1-25 wt% of solvent additive based on the total amount of the electrolyte composition, the total amount of the electrolyte composition being 100 wt%. In some embodiments, the electrolyte composition contains at least 1 wt%, at least 1.1 wt%, at least 1.2 wt%, at least 1.3 wt%, at least 1.4 wt%, at least 1.5 wt%, at least 2.0 wt%, at least 2.5 wt% or at least 5.0 wt% of solvent additive based on the total amount of the electrolyte composition. In some embodiments, the electrolyte composition contains up to 25 wt%, up to 20 wt%, up to 15 wt% or up to 10 wt% of solvent additive based on the total amount of the electrolyte composition. In some embodiments, these end-points can be combined to form any suitable range, for example 1-25 wt%, 1.1-15 wt%, 1.1-10 wt%, 1.3-20 wt%, 1.5-10 wt%, 2.0 to 10wt%, 2.5 to 10 wt% or 5.0 to 10 wt% of solvent additive based on the total amount of the electrolyte composition. In some embodiments, the solvent additive comprises one or more fluorinated carbonate compounds. In some embodiments, the solvent additive comprises one or more unsaturated carbonate compounds. In some embodiments, the solvent additive comprises one or more fluorinated cyclic carbonate compounds. In some embodiments, the solvent additive comprises one or more unsaturated cyclic carbonate compounds. In some embodiments, the solvent additive comprises one or more fluorinated and / or unsaturated 5-membered heterocyclic carbonate compounds. In some embodiments, the solvent additive comprises one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and trifluoro-propylene carbonate (TFPC). In general, the compound(s) present within the solvent additive may have a higher reactivity or lower stability than the compound(s) present within the solvent, so in some embodiments components of the solvent additive may be present in the electrolyte composition in a lower amount than components of the solvent. In some embodiments, the solvent additive also comprises one or more of an OS and SN. In some embodiments, the solvent additive consists of one or more of FEC, VC and TFPC, and optionally one or more of an OS and SN. In some embodiments, the solvent additive consists of FEC and VC, and optionally one of an OS and SN. In some embodiments, the solvent additive consists of FEC, VC and TFPC, and optionally one or more of an OS and SN. In some embodiments, the solvent additive consists of FEC and VC. In some embodiments, the solvent additive consists of FEC, VC and TFPC. In some embodiments, the solvent additive comprises 1-60 wt% FEC, 5-85 wt% VC, and 0-95 wt% TFPC, based on the total amount of solvent additive. The total amount of solvent additive is 100 wt%, so that where the sum of the amounts of FEC, VC and TFPC is less than 100 wt%, there may be other solvent additives present in the solvent additive component (b). In some embodiments, the sum of the amounts of FEC, VC and TFPC is 100 wt%, and in such cases the solvent additive consists of FEC, VC and / or TFPC. In some embodiments, the lithium salt consists of 1-60 wt% FEC, 5-85 wt% VC, and 0-95 wt% TFPC based on the total amount of solvent additive. In some embodiments, the solvent additive consists of 1-60 wt% FEC and 5-85 wt% VC, for example 30-40 wt% FEC and 60-70 wt% VC, based on the total amount of solvent additive. In some embodiments, the solvent additive comprises at least 1 wt%, such as at least 3 wt%, at least 5 wt%, at least 10 wt%, at least 12 wt%, at least 15 wt%, at least 20 wt% or at least 30 wt% of FEC based on the total amount of solvent additive. In some embodiments, the solvent additive comprises up to 60 wt% of FEC, such as up to up to 55 wt%, up to 50 wt%, up to 45 wt% or up to 40 wt% of FEC based on the total amount of solvent additive. In some embodiments, these end-points can be combined to form any suitable range, for example 15-60 wt%, 5-55 wt%, 10-55 wt%, 15-50 wt%, 12-45 wt%, 15-40 wt%, 20-40 wt% or 30-40 wt% of FEC based on the total amount of solvent additive. In some embodiments, the solvent additive comprises at least 5 wt% VC, such as at least 10 wt%, at least 15 wt%, at least 20 wt%, at least 30, at least 40 wt%, at least 50 wt%, at least 55 wt%, at least 60 wt% or at least 65 wt% of VC based on the total amount of solvent additive. In some embodiments, the solvent additive comprises up to 85 wt%, such as up to 84 wt%, up to 83 wt%, up to 82 wt%, up to 81 wt%, up to 75 wt%, up to 70 wt% or up to 68 wt% VC based on the total amount of solvent additive. In some embodiments, these end-points can be combined to form any suitable range, for example 40-85 wt%, 5-82 wt%, 10-84 wt%, 20-83 wt%, 40-81 wt%, 50-80 wt%, 60-80 wt% or 60-70 wt% VC based on the total amount of solvent additive. In some embodiments, the solvent additive comprises TFPC. In some embodiments, TFPC is present in an amount of up to 95 wt% based on the total amount of solvent additive. In some embodiments, the solvent additive comprises at least 10 wt%, at least 15 wt%, at least 20 wt% or at least 25 wt% of TFPC based on the total amount of solvent additive. In some embodiments, the solvent additive comprises up to 95 wt% TFPC, such as up to 94 wt%, up to 93 wt%, up to 92 wt%, up to 91 wt% or up to 90 wt% TFPC based on the total amount of solvent additive. In some embodiments, these end-points can be combined to form any suitable range, for example 0-90 wt%, 10-95 wt%, 15-91 wt%, 20-93 wt%, or 25-90 wt% TFPC based on the total amount of solvent additive. In some embodiments, the solvent additive comprises an organosilicon (OS). In some embodiments, the OS is a compound comprising silicon, oxygen and an organic group. In some embodiments, the OS is a compound comprising silicon, oxygen, carbon and hydrogen. The OS may be present in any suitable amount, such as at least 0.1 wt%, at least 0.2 wt%, at least 0.3 wt%, or at least 0.4 wt% based on the total amount of the electrolyte composition, wherein the total amount of the electrolyte composition is 100 wt%. In some embodiments, the solvent additive comprises up to 5 wt% OS, such as up to 4.5 wt%, up to 4 wt%, up to 3.5 wt% or up to 3 wt% based on the total amount of the electrolyte composition. In some embodiments, these end-points can be combined to form any suitable range, for example 0.1-5 wt%, 0.2-4.5 wt%, 0.2-4 wt%, 0.3-3.5 wt% or 0.4-3 wt% based on the total amount of the electrolyte composition. In some embodiments, the OS may be present in an amount of up to 40 wt%, such as up to 35 wt%, up to 30 wt%, or up to 25 wt% based on the total amount of the solvent additive where the total amount of solvent additive is 100 wt%. In some embodiments, the solvent additive comprises at least 1 wt% OS, such as at least 2 wt%, at least 3 wt%, at least 4 wt%, or at least 5 wt% based on the total amount of the solvent additive. In some embodiments, these end-points can be combined to form any suitable range, for example 1-40 wt%, 2-35 wt%, 3-40 wt%, 4-35 wt% or 5-25 wt% based on the total amount of the solvent additive. In some embodiments, the solvent additive comprises succinonitrile (SN). The SN may be present in any suitable amount, such as at least 0.1 wt%, at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt% or at least 0.5 wt% based on the total amount of the electrolyte composition (wherein the total amount of electrolyte composition is 100 wt%). In some embodiments, the solvent additive comprises up to 5 wt% SN, such as up to 4.5 wt%, up to 4 wt%, up to 3.5 wt% or up to 3 wt% based on the total amount of the electrolyte composition. In some embodiments, these end-points can be combined to form any suitable range, for example 0.1-5 wt%, 0.2-4.5 wt%, 0.2-4 wt%, 0.3-3.5 wt% or 0.5-3 wt% based on the total amount of the electrolyte composition. In some embodiments, the SN may be present in an amount of up to 40 wt%, such as up to 39 wt%, up to 38 wt%, up to 37 wt%, up to 36 wt%, or up to 35 wt% based on the total amount of the solvent additive where the total amount of solvent additive is 100 wt%. In some embodiments, the solvent additive comprises at least 10 wt% SN, such as at least 15 wt%, at least 20 wt%, or at least 25 wt% SN based on the total amount of the solvent additive. In some embodiments, these end-points can be combined to form any suitable range, for example 10-40 wt%, 15-35 wt%, 20-38 wt%, 15-40 wt% or 20-35 wt% based on the total amount of the solvent additive. The electrolyte compositions of the present invention contain 45-80 wt% of solvent based on the total amount of the electrolyte composition. In some embodiments, the electrolyte composition contains at least 48 wt%, at least 50 wt%, at least 55 wt%, at least 60 wt% or at least 65 wt% of solvent based on the total amount of the electrolyte composition. In some embodiments, the electrolyte composition contains up to 80 wt%, such as up to 79 wt%, up to 78 wt%, up to 77 wt%, up to 76 wt%, or up to 75 wt% of solvent based on the total amount of the electrolyte composition. In some embodiments, these end-points can be combined to form any suitable range, for example 48-80 wt%, 50-78 wt%, 60-77 wt%, 55-76 wt%, 60-75 wt% or 65-75 wt% of solvent based on the total amount of the electrolyte composition. In some embodiments, solvent is used herein to refer to a component capable of having the lithium salt dissolved therein. The solvent comprises a cyclic carbonate. In some embodiments, the solvent comprises a cyclic carbonate that comprises one or more of ethylene carbonate, propylene carbonate, and butylene carbonate. In some embodiments, the solvent comprises a cyclic carbonate that comprises one or both of ethylene carbonate (EC) and propylene carbonate (PC). In some embodiments, the inclusion of a linear carbonate in the solvent allows for an increased amount of PC to be included in the electrolyte composition. The cyclic carbonate may be a non-fluorinated compound. The cyclic carbonate may be a saturated (i.e. non-alkene containing) compound. The cyclic carbonate may be a nonfluorinated saturated compound. The cyclic carbonate may be a non-fluorinated saturated carbonate compound comprising a heterocyclic ring. The cyclic carbonate may be a nonfluorinated saturated carbonate compound comprising a 5-membered heterocyclic ring. In some embodiments, the solvent consists of ethylene carbonate (EC). In some embodiments, the solvent consists of a mixture of ethylene carbonate (EC) and propylene carbonate (PC). In some embodiments, the solvent comprises a linear carbonate that comprises one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl-methyl carbonate (EMC). In some embodiments, the solvent comprises a linear carbonate that comprises one or both of DMC and EMC. In some embodiments, the solvent comprises a linear carbonate that comprises one or both of DMC and DEC. In some embodiments, the linear carbonate comprises DMC. In some embodiments, the solvent comprises a cyclic carbonate that includes EC and PC, and a linear carbonate that includes DMC. In some embodiments, the solvent comprises at least 30 wt%, such as at least 31 wt%, at least 32 wt%, at least 33 wt%, at least 34 wt% or at least 35 wt%, of cyclic carbonate, based on the total amount of solvent. In some embodiments, the solvent comprises up to 100 wt%, such as up to 95 wt%, up to 92 wt%, up to 90 wt%, up to 88 wt%, or up to 85 wt%, of cyclic carbonate, based on the total amount of solvent. In some embodiments, these end-points can be combined to form any suitable range, for example 30-100 wt%, 31-90 wt%, 32-92 wt%, 33-88 wt%, 34-92 wt%, or 30-85 wt% of cyclic carbonate, based on the total amount of solvent. In some embodiments, the solvent comprises at least 5 wt%, such as at least 7 wt%, at least 10 wt%, at least 12 wt%, at least 15 wt%, at least 18 wt% or at least 20 wt% of linear carbonate, based on the total amount of solvent. In some embodiments, the solvent comprises up to 65 wt%, such as up to 59 wt%, up to 58 wt%, up to 57 wt, up to 56 wt% or up to 55 wt% of linear carbonate, based on the total amount of solvent. In some embodiments, these end-points can be combined to form any suitable range, for example 5-65 wt%, 10-65 wt%, 12-65 wt%, 18-58 wt%, 20-55 wt%, or 20-65 wt% of linear carbonate, based on the total amount of solvent. In some embodiments, the solvent comprises 30-100 wt% cyclic carbonate and 0-65 wt% linear carbonate, based on the total amount of solvent and wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent comprises 30-95 wt% cyclic carbonate and 5-65 wt% linear carbonate, based on the total amount of solvent and wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent comprises 30-85 wt% cyclic carbonate and 15-70 wt% linear carbonate. In some embodiments, the solvent comprises 35-80 wt% cyclic carbonate and 20-65 wt% linear carbonate. In some embodiments, the solvent consists of 30-95 wt% cyclic carbonate and 5-65 wt% linear carbonate, based on the total amount of solvent and wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent consists of 30-85 wt% cyclic carbonate and 15-70 wt% linear carbonate. In some embodiments, the solvent consists of 35-80 wt% cyclic carbonate and 20-65 wt% linear carbonate. In some embodiments, the solvent comprises 60-100 wt% EC and 0-40 wt% PC, based on the total amount of solvent, wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent comprises 65-100 wt% EC and 0-35 wt% PC, based on the total amount of solvent, wherein the total amount of solvent is 100 wt%. In some embodiments, the solvent consists of EC and PC. In some embodiments, the solvent consists of 60-100 wt% EC and 0-40 wt% PC, based on the total amount of solvent. In some embodiments, the solvent consists of 70-100 wt% EC and 0-30 wt% PC, based on the total amount of solvent. Accordingly, in some embodiments, the present electrolyte compositions permit higher levels of PC compared to standard electrolyte compositions. In some embodiments, the choice of weight ratio of EC: PC: may be used to target particular cell properties, such as ionic conductivity, resistance against corrosion, operational temperature, and cycle life. In some embodiments, the solvent comprises ethylene carbonate (EC) and propylene carbonate (PC). In some embodiments, the solvent consists of at least one of EC and PC as cyclic carbonate, and at least one of DMC, EMC and DEC as linear carbonate. In some embodiments, the solvent consists of one or both of EC and PC as cyclic carbonate, and one or more of DMC, EMC and DEC as linear carbonate. In some embodiments, the solvent consists of one or both of EC and PC as cyclic carbonate, and one or two of DMC, EMC and DEC as linear carbonate. In some embodiments, the solvent consists of EC and PC as cyclic carbonate, and one or more of DMC, EMC and DEC as linear carbonate. In some embodiments, the solvent consists of EC and PC as cyclic carbonate, and DMC as linear carbonate. In some embodiments, the solvent comprises 0-70 wt% EC and 0-70 wt% PC as cyclic carbonate, based on the total amount of solvent, wherein at least one of EC and PC is present. In some embodiments, the solvent comprises at least 25 wt% EC, such as at least 28 wt%, at least 30 wt%, at least 33 wt%, at least 35 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 65 wt% or at least 70 wt%,EC based on the total amount of solvent. In some embodiments, the solvent comprises up to 100 wt% EC, such as up to 99 wt%, up to 95 wt%, up to 90 wt%, up to 85 wt%, up to 80 wt% or up to 75 wt% EC based on the total amount of solvent. In some embodiments, these end-points can be combined to form any suitable range, for example 20-100 wt%, 35-100 wt%, 40-100 wt%, 50-100 wt% or 70-100 wt% EC based on the total amount of solvent. In some embodiments, the solvent comprises at least 0 wt% PC, such as at least 5 wt%, at least 10 wt% at least 15 wt%, at least 20 wt%, at least 25 wt% or at least 30 wt% PC based on the total amount of solvent. In some embodiments, the solvent comprises up to 60 wt% PC, such as up to 50 wt%, up to 45 wt%, up to 40 wt%, up to 35 wt%, or up to 30 wt% based on the total amount of solvent. In some embodiments, these end-points can be combined to form any suitable range, for example 0-60 wt%, 0-50 wt%, 0-40 wt%, 0-35 wt%, or 0-30 wt% PC based on the total amount of solvent. In some embodiments, the solvent comprises at least 5 wt% DMC, such as at least 7 wt%, at least 10 wt%, at least 12 wt%, at least 14 wt%, at least 16 wt% or at least 18 wt% DMC based on the total amount of solvent. In some embodiments, the solvent comprises up to 65 wt% DMC, such as up to 63 wt%, up to 60 wt%, up to 58 wt, or up to 55 wt% DMC based on the total amount of solvent. In some embodiments, these end-points can be combined to form any suitable range, for example 5-65 wt%, 10-60 wt%, 12-58 wt%, 7-63 wt% or 18-55 wt% DMC based on the total amount of solvent. In some embodiments, the solvent comprises 60-100 wt% EC and 0-40 wt% PC based on the total amount of solvent. The total amount of solvent is 100 wt%, so that where the sum of the amounts of EC and PC is less than 100 wt%, there may be other solvents present in the solvent component (c). In some embodiments, the sum of the amounts of EC and PC is 100 wt%, and in such cases the solvent consists of EC and PC. In some embodiments, the solvent consists of 60-100 wt% EC and 0-40 wt% PC based on the total amount of solvent. In some embodiments, the electrolyte compositions may also comprise water. In some embodiments, the amount of water is up to 15,000 ppm, on a mass basis, based on the total weight of the electrolyte composition. Accordingly, the water may not function as a solvent or a solvent additive in the present compositions. In some embodiments, the water may be introduced during manufacture. Nevertheless, in some embodiments, the present compositions are more tolerant of the presence of water than other, standard electrolyte compositions for which water must be at a very low level, such as the order of 20 ppm. In some embodiments, the compositions comprising up to 15,000 ppm water do not comprise LiPF6. In some embodiments, the water may be present in an amount of at least 100 ppm, at least 150 ppm or at least 200 ppm, on a mass basis, based on the total amount of the electrolyte composition. In some embodiments, water may be present in an amount of up to 15,000 ppm, such as up to, up to 10,000 ppm, up to 5,000 ppm, up to 3,000 ppm, up to 2,500 or up to 2,000 ppm, on a mass basis, based on the total amount of the electrolyte composition. In some embodiments, these end-points may be combined to provide any suitable range, such as 100-15,000 ppm, 150-10,00 ppm, 200-5,000 ppm or 200-2,000 ppm, on a mass basis, based on the total amount of the electrolyte composition. In some embodiments, the electrolyte compositions comprising water may be used in electrochemical cells comprising a cathode including a layered oxide material, and an anode comprising graphite. In some embodiments, the anode comprises an electroactive material including graphite and a silicon-containing material, which may be a silicon-based material such as silica or a silicate. In some embodiments, the electrolyte composition comprises: (a) 18-35 wt% of lithium salt; (b) 1-25 wt% of solvent additive; and (c) 45-80 wt% of solvent; wherein the total amount of (a), (b) and (c) is less than or equal to 100 wt% of the electrolyte composition, and wherein: the lithium salt consists of lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI) and lithium difluoro(oxalato)borate (LiDFOB), the solvent additive consists of one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and trifluoro-propylene carbonate (TFPC); and the solvent consists of ethylene carbonate (EC) or a mixture of ethylene carbonate (EC) and propylene carbonate (PC). In some embodiments, the electrolyte compositions of the present invention comprise or consist of the compositions outlined in Table 1 herein. The electrolyte compositions of the present invention may be made using any suitable method. Appropriate methods will be known to the skilled person. The kind of electrochemical cell with which the electrolyte compositions of the present invention may be used is not particularly limited. The electrolyte compositions of the present invention may be used in an electrochemical cell, particularly an electrochemical cell which is a lithium-ion electrochemical secondary cell. In some embodiments, the electrochemical cell comprises, in addition to the electrolyte composition of the present invention, a cathode and an anode. In some embodiments, the cathode comprises an electroactive material. In some embodiments, the cathode includes a layered oxide. In some embodiments, the layered oxide may be a nickel manganese cobalt oxide (NMC). In some embodiments, the anode comprises graphite. In some embodiments, the anode comprises graphite and one or more silicon-containing materials. In some embodiments, the silicon-containing material is a silicon-based material, such as silica or a silicate. Such anodes and cathodes, combined with the electrolyte compositions herein, may show particularly useful performance characteristics. In some embodiments, an operational voltage of the electrochemical energy storage device is at or below 4.8V, such as up to 4.7V, up to 4.6V, or up to 4.5V. In some embodiments, an operational voltage of the electrochemical energy storage device is at or above 2.5 V, such as at least 2.6V, at least 2.7V or at least 2.8V. Any of these end-points can be combined to form a range, such as between 2.5 and 4.8V, between 2.5 and 4.7V, between 2.7 and 4.6V or between 2.8 and 4.5V. These operational voltages are all vs Li° / Li+. The electrochemical cells comprising the electrolyte compositions of the present invention may be used as part of an electrochemical energy storage device. In some embodiments, the electrolyte compositions of the present invention are used as an electrolyte in a lithium-ion battery. Particularly useful voltage ranges for operation of the lithium-ion battery are between 2.5 V and 4.8V vs Li° / Li+, but any of the ranges disclosed above for the electrochemical energy storage device may be applied. In some embodiments, the electrolyte compositions described herein may be used as an electrolyte in a lithium-ion battery. EXAMPLES Electrolyte compositions were prepared and used as part of electrochemical cells having the formulations set out in Table 2. The electrochemical cell having the electrolyte compositions in those formulations were tested for rate performance and cycle life properties. In each case, the electrochemical cell set up was the same. Compositions 1 to 5 are examples according to claim 1, while Compositions 6 to 8 are comparative examples. All cells were assembled in a dry room with dewpoint below -40°C. All cells were electrochemically formed at 30°C with charge currents of C / 20 and C / 10 and discharge currents of C / 10. Subsequently the cells were subjected to multiple cycles, with different discharge currents in each cycle but the charge current was kept the same. The cells were cycled between 2.5 and 4.2V, at 30 or 45°C. Cycle life tests were carried on with the same discharge and charge current in all cycles, except for one in every 20 cycles in which the discharge current was higher than the other discharge cycles. The electrolyte compositions were tested with graphite and silicon / silica-based anodes, and cathodes comprising NMC. Table 2 Composition No. LiFSI (wt%) LiTDI (wt%) LiDFOB (wt%) FEC (wt%) VC (wt%) EC (wt%) PC (wt%) 1 16.21 3.20 3.12 2.11 4.09 49.89 21.38 2 16.21 3.20 3.12 2.11 4.09 71.28 0.00 3 20.88 3.20 3.12 1.98 3.84 66.98 0.00 4 17.77 3.20 3.12 2.06 4.01 69.84 0.00 5 19.32 3.20 3.12 2.02 3.93 68.41 0.00 6 12.50 3.20 0.00 2.21 4.49 58.20 19.40 7 14.0 1.60 0.00 2.24 4.56 58.20 19.40 8 14.8 0.80 0.00 2.24 4.56 58.20 19.40 Figure 1 shows a chart of capacity retention (%) for the compositions of Table 2 at 30°C. The data was obtained from cycling at a discharge current of 200 mA. It can be seen from Figure 1 that the capacity retention was higher for compositions according to the present invention i.e. electrolyte compositions comprising LiDFOB as part of the solvent. Accordingly, the results support that compositions containing the claimed mixture of components have an improvement in the capacity retention, in particular at higher discharge currents such as 200 mA. Figure 2 shows a chart of capacity retention (%) against current (mA) for the five inventive compositions from Table 2: compositions 1-5, cycled at 30 °C. The data set taken at 200 mA is on the left of the chart, and the data set taken at 400 mA is on the right of the chart. These data show that, like the compositions shown in Figure 1, electrolyte compositions comprising the claimed mixture of components show good capacity retention, even at higher discharge currents such as 400 mA. Figure 3 shows a chart of capacity retention (%) against current (mA) for four inventive compositions from Table 2: compositions 1, 2, 4 and 5, cycled at 45 °C. The data set taken at 200 mA is on the left of the chart, and the data set taken at 400 mA is on the right of the chart. These data show that, like the compositions shown in Figure 1, electrolyte compositions comprising the claimed mixture of components show good capacity retention, even at higher discharge currents such as 400 mA. Figure 4 shows the capacity retention over 250 cycles for compositions 1 and 6. The capacity of the cell using composition 6 fades more quickly with cycling, especially from 100 cycles onwards where the capacity of the cell using composition 6 begins to fall significantly faster than the cell using composition 1. This demonstrates the improved cyclability of cells employing the claimed electrolyte composition.

Claims

1. An electrolyte composition for a lithium-ion battery, the composition comprising:(a) 18-35 wt% of lithium salt;(b) 1-25 wt% of solvent additive; and(c) 45-80 wt% of solvent;wherein the total amount of (a), (b) and (c) is less than or equal to 100 wt% of the electrolyte composition, and wherein:the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI) and lithium difluoro(oxalato)borate (LiDFOB),the solvent additive comprises one or more fluorinated and / or unsaturated carbonate compounds; andthe solvent comprises a cyclic carbonate.

2. An electrolyte composition according to claim 1, wherein the lithium salt comprises 65-85 wt% LiFSI, 0.1-25 wt% of LiTDI, and 0.1-25 wt% LiDFOB, based on the total amount of lithium salt and wherein the total amount of lithium salt is 100 wt%.

3. An electrolyte composition according to claim 1 or 2, wherein the lithium salt further comprises LiPFe, lithium bis(oxalato) borate (LiBOB), or a mixture thereof.

4. An electrolyte composition according to any one of the preceding claims, wherein the lithium salt consists of LiFSI, LiTDI, and LiDFOB.

5. An electrolyte composition according to any one of the preceding claims, wherein the solvent consists of one or more cyclic carbonates.

6. An electrolyte composition according to any one of the preceding claims, wherein the cyclic carbonate comprises one or more of ethylene carbonate (EC) and propylene carbonate (PC).

7. An electrolyte composition according to any one of the preceding claims, wherein the solvent consists of ethylene carbonate (EC), or consists of a mixture of ethylene carbonate (EC) and propylene carbonate (PC).

8. An electrolyte composition according to any one of the preceding claims, wherein the solvent comprises 60-100 wt% EC, and 0-40 wt% PC, based on the total amount of solvent and wherein the total amount of solvent is 100 wt%.

9. An electrolyte composition according to any one of the preceding claims, wherein the solvent additive comprises one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and trifluoro-propylene carbonate (TFPC).

10. An electrolyte composition according to any one of the preceding claims, wherein the solvent additive comprises 15-60 wt% FEC and 40-85 wt% VC, based on the total amount of solvent additive and wherein the total amount of solvent additive is 100 wt%.

11. An electrolyte composition according to any one of the preceding claims, wherein the solvent additive comprises or consists of FEC and VC.

12. An electrolyte composition according to any one of the preceding claims, wherein the solvent additive further comprises an organosilicon or succinonitrile in an amount of up to 5 wt%, based on the total amount of the electrolyte composition and wherein the total amount of the electrolyte composition is 100 wt%.

13. An electrolyte composition according to any one of the preceding claims, wherein the electrolyte composition comprises up to 15,000 ppm of water, on a mass basis, based on the total weight of the composition.

14. An electrolyte composition according to claim 12 or claim 13, wherein the electrolyte composition does not contain LiPFe.

15. An electrolyte composition according to any one of the preceding claims, selected from the group consisting of:Composition No. LiFSI (wt%) LiTDI (wt%) LiDFOB (wt%) 1717 / ^ Jr tLv. (wt%) VC (wt%) EC (wt%) PC (wt%) 1 16.21 3.20 3.12 2.11 4.09 49.89 21.38 2 16.21 3.20 3.12 2.11 4.09 71.28 0.00 3 20.88 3.20 3.12 1.98 3.84 66.98 0.00 4 17.77 3.20 3.12 2.06 4.01 69.84 0.00 5 19.32 3.20 3.12 2.02 3.93 68.41 0.0016. An electrochemical cell comprising an electrolyte composition according to any one of claims 1 to 15.

17. An electrochemical cell according to claim 16, which is a lithium-ion electrochemical secondary cell.

18. An electrochemical cell according to claim 16 or claim 17, further comprising a cathode optionally comprising a layered oxide, and an anode optionally comprising graphite and one or more forms of silicon, silicate or silica.

19. An electrochemical energy storage device comprising an electrochemical cell according to any one of claims 16 to 18.

20. Use of an electrolyte composition according to any one of claims 1 to 15 as an electrolyte in a lithium-ion battery.

21. Use according to claim 20, wherein the lithium-ion battery has an operating voltage of between 2.5 V and 4.8 V vs Li° / Li+.

22. Use of LiDFOB or LiPFe to improve the performance of a lithium-ion cell having an electrolyte composition comprising:(a) 18-35 wt% of lithium salt;(b) 1-25 wt% of solvent additive; and(c) 45-80 wt% of solvent; andwherein the total amount of (a), (b) and (c) is less than or equal to 100 wt% of the electrolyte 5 composition, and wherein:the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI) and lithium difluoro(oxalato)borate (LiDFOB),the solvent additive comprises one or more fluorinated and / or unsaturated carbonate 10 compounds; andthe solvent comprises a cyclic carbonate.

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