Electrolyte for secondary battery

Abstract

The disclosure provides a non-aqueous electrolytes providing improved long-term storage-stability at elevated temperature In particular, the electrolyte is suitable for use in cells wherein the cathode active material has a high nickel content.

Description

[0001] ELECTROLYTE FOR SECONDARY BATTERY

[0002] FIELD OF THE INVENTION

[0003] The disclosure relates to a composition for an electrolyte, and to the use of said electrolyte to improve the long-term storage stability of an electrochemical cell at elevated temperature.

[0004] BACKGROUND

[0005] Rechargeable or 'secondary' batteries find widespread use as electrical power supplies and energy storage systems. For example, in automobiles, battery packs formed of a plurality of battery modules, wherein each battery module includes a plurality of electrochemical cells, are provided as a means of effective storage and utilization of electric power. Typically, secondary batteries such as lithium-ion batteries, comprise a non-aqueous, liquid electrolyte.

[0006] Nonaqueous electrolyte solutions are ion conductors comprising a solvent or blends of solvents and a dissolved salt or several dissolved salts. For example, non-aqueous electrolytes for lithium-ion batteries are typically a combination of organic solvents like ether, ester, and carbonates that can solvate lithium salts like LiPFe, LiTFSI, LiFSI.

[0007] Such organic components often have poor temperature stability, leading to reduced battery performance in hot and / or cold environments. This is particularly problematic in electric vehicle applications wherein batteries are often positioned close to, or adjacent to, heatgenerating components such as an engine.

[0008] In addition, poor temperature stability is also associated with accelerated electrolyte decomposition during storage.

[0009] Electrochemical cells for batteries are often formed of laminated electrode structures to increase the maximum capacity per volume. During storage and use at elevated temperature e.g., 60°C, side reactions between the electrolyte and electrochemically active material leads to electrolyte decomposition. Volatile compounds are formed during electrolyte decomposition which generates gas. Gas generated in the electrolyte causes expansion and deformation of batteries leading to eventual breakdown of the laminated structure. Side reactions between electrolyte and electrochemically active material are particularly problematic in 'NMC' cells (cells wherein the cathode active material is a composite oxide of nickel, manganese and cobalt), wherein the nickel content is high e.g. above 80 mol% based on the total amount of non-lithium metals in the cathode active material.

[0010] There is therefore a need for non-aqueous electrolytes providing improved long-term storage-stability at elevated temperature. In particular, there is a need for such an electrolyte, suitable for use in batteries wherein the cathode active material has a high nickel content.

[0011] SUMMARY

[0012] The object of the present disclosure is to provide a non-aqueous electrolyte that provides increased storage stability at elevated temperatures.

[0013] The improved storage stability of the electrolyte results in a reduction in gas generation during storage, protecting the battery from damage.

[0014] According to a first aspect of the disclosure is an electrochemical cell comprising a nonaqueous electrolyte and a cathode active material, wherein the cathode active material comprises Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material; and the non-aqueous electrolyte comprises 0.1-5 wt% lithium bis(oxalato)borate (LiBOB) and 0.1-5 wt% lithium (difluorophosphonyl)fluorosulfonylimide (LiDFPFSI).

[0015] According to a second aspect of the disclosure is use of a non-aqueous electrolyte for improving the storage stability of an electrochemical cell when stored, for instance as measured in a cell (preferably comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material) under storage in a charged state at 60 °C for 4 weeks, and / or reducing gas generation in a cell comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material, for instance as measured in a cell under storage in a charged state at 60 °C for 4 weeks, wherein the electrolyte comprises 0.1-5 wt% LiBOB and 0.1-5 wt% LiDFPFSI. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1. Comparison of storage time at 60°C vs thickness of pouch cells comprising different non-aqueous electrolytes. Same amount of electrolyte has been used for each, however different additives has been used. An electrolyte comprising lwt% LiBOB and lwt% LiDFPFSI is marked with circles, an electrolyte comprising lwt% VC and lwt% LiD- FPFSI is marked with triangles, while an electrolyte comprising lwt% LiBOB, lwt% VC and lwt% LiDFPFSI is marked with squares.

[0017] Figure 2. Comparison of storage time at 60°C vs DCIR increase for pouch cells comprising different non-aqueous electrolytes. Same amount of electrolyte has been used for each, however different additives has been used. An electrolyte comprising lwt% LiBOB and lwt% LiDFPFSI is marked with circles, while an electrolyte comprising lwt% VC and lwt% LiDFPFSI is marked with triangles.

[0018] DETAILED DESCRIPTION

[0019] The disclosure relates to a composition for a non-aqueous electrolyte comprising LiBOB and LiDFPFSI, which aims to improve the long-term storage-stability.

[0020] An electrolyte facilitates the transport of lithium-ions between the cathode and the anode in an electrochemical cell.

[0021] The non-aqueous electrolyte solution of the present disclosure comprises two or more additives and one or more non-aqueous solvents.

[0022] The type of the non-aqueous solvent used in the non-aqueous electrolyte solution of the present invention is not particularly limited, and any non-aqueous solvent can be used. For example, the one or more non-aqueous solvents comprise one or more solvents from the group comprising cyclic carbonates, chain carbonates, cyclic esters, chain esters, cyclic ethers, chain ethers, sulfone compounds, sulfoxide compounds, and ionic liquids.

[0023] Preferably, the non-aqueous solvent comprises one or more cyclic and / or chain carbonates such as selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and mixtures thereof. Preferably, the non-aqueous solvent is selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixtures thereof.

[0024] For example, the non-aqueous solvent may be a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0025] In an embodiment, where the non-aqueous electrolyte comprises EC, EMC and DMC, the concentration of EC may be from 1 wt% to 50 wt%, such as from 5 wt% to 40 wt%, for example from 10 wt% to 30 wt%, ideally from 15 wt% to 25 wt%, based on the total amount of non-aqueous electrolyte.

[0026] In an embodiment, where the non-aqueous electrolyte comprises EC, EMC and DMC, the concentration of EMC may be from 1 wt% to 60 wt%, such as from 10 wt% to 50 wt%, for example from 20 wt% to 40 wt%, ideally from 25 wt% to 35 wt%, based on the total amount of non-aqueous electrolyte.

[0027] In an embodiment, where the non-aqueous electrolyte comprises EC, EMC and DMC, the concentration of DMC may be from 1 wt% to 60 wt%, such as from 10 wt% to 50 wt%, for example from 20 wt% to 40 wt%, ideally from 30 wt% to 35 wt%, based on the total amount of non-aqueous electrolyte.

[0028] For example, the non-aqueous electrolyte comprises from 1 wt% to 50 wt% EC, from 1 wt% to 60 wt% EMC, and from 1 wt% to 60 wt% DMC, such as from 5 wt% to 40 wt% EC, from 10 wt% to 50 wt% EMC, and from 10 wt% to 50 wt% DMC, for example from 10 wt% to 30 wt% EC, from 20 wt% to 40 wt% EMC, and from 20 wt% to 40 wt% DMC, ideally from 15 wt% to 25 wt% EC, from 25 wt% to 38 wt% EMC and from 30 wt% to 35 wt% DMC, based on the total amount of non-aqueous electrolyte.

[0029] In an embodiment, where the non-aqueous electrolyte comprises EC, EMC and DMC, the weight ratio between EC and EMC may be from 2: 1 to 1:4, for example from 1 : 1 to 1:3. Preferably the weight ratio between EC and EMC is about 2:3.

[0030] In an embodiment, where the non-aqueous electrolyte comprises EC, EMC and DMC, the weight ratio between EC and DMC may be from 2: 1 to 1 :4, for example from 1: 1 to 1:3. Preferably the weight ratio between EC and EMC is about 2:3. In an embodiment, where the non-aqueous electrolyte comprises EC, EMC and DMC, the weight ratio between DMC and EMC may be from 2: 1 to 1:2. Preferably the weight ratio between DMC and EMC is about 1: 1.

[0031] Often, the non-aqueous electrolyte comprises a lithium salt (i.e. an addition lithium salt beyond LiBOB and LiDFPFSI), such as LiFSI (lithium bis(fluorosulfonyl)imide) or LiPFe, preferably LiPFe.

[0032] For example, the non-aqueous electrolyte may comprise from 1 wt% to 30 wt% LiPFe, such as from 5 wt% to 25 wt%, for example from 10 wt% to 20 wt%, such as from 12 wt% to 16 wt% LiPFe, based on the total amount of non-aqueous electrolyte.

[0033] In a preferred embodiment, the non-aqueous electrolyte comprises (or consists of) EC, EMC, DMC, LiPFe, LiBOB and LiDFPFSI, for instance in any combination of any of the preferred ranges for these components as set out above.

[0034] In a preferred embodiment, the non-aqueous electrolyte comprises (or consists of): from 10 wt% to 30 wt% EC, from 20 wt% to 40 wt% EMC, from 20 wt% to 40 wt% DMC, from from 10 wt% to 20 wt% LiPFe, from 0.2 wt% to 3 wt% LiBOB, and from 0.2 wt% to 3 wt% LiDFPFSI.

[0035] Such an electrolyte composition desirably comprises less than 0.005 wt% VC, and preferably does not contain VC.

[0036] In a preferred embodiment, the non-aqueous electrolyte comprises (or consists of): from 15 wt% to 25 wt% EC, from 25 wt% to 35 wt% EMC, from 25 wt% to 38 wt% DMC, from 12 wt% to 16 wt% LiPFe, from 0.5 wt% to 1.5 wt% LiBOB, and from 0.5 wt% to 1.5 wt% LiDFPFSI.

[0037] Such an electrolyte composition desirably comprises less than 0.005 wt% VC, and preferably does not contain VC. Typically, the detrimental side reactions occur at the interface between the electrode surfaces and the electrolyte. For example, the detrimental side reactions include reactions that release gases resulting in swelling of the battery. By adding specific additives, a stable solid electrolyte interphase (SEI) layer may form on the electrodes which may impede detrimental side reactions such as gas generating reactions.

[0038] Therefore, forming a robust SEI is the key to the suppression of harmful side reactions including those generating gas. In turn, the suppression of side reactions will improve the overall storage stability of the battery.

[0039] In the context of the present disclosure, a "solid electrolyte interphase layer" or "SEI layer" is used to describe the layer that forms on each of the electrodes upon exposure to the electrolyte during the initial charge / discharge cycle.

[0040] The additives of the present disclosure comprises lithium bis(oxalato)borate (LiB(C2O4)2, or commonly referred to as LiBOB) and lithium (difluorophosphonyl)fluorosulfonylimide (LiN(FSO2)(POF2), or commonly referred to as LiDFPFSI).

[0041] The combination of LiBOB and LiDFPFSI has been found to be particularly useful in preventing gas generation resulting in improved battery long term stability, particularly stability at elevated temperatures. Further, the combination has been found to impede the increase of internal resistance which typically occurs upon prolonged storage.

[0042] Based on the total amount of non-aqueous electrolyte (i.e. the total amount of the nonaqueous solvent(s) and the additives in the electrolyte), the lower limit of the content of each of the two or more additives for a non-aqueous electrolyte solution in the electrolyte solution is 0.1 weight % or more, preferably 0.2 weight % or more, more preferably from 0.3 wt% or more, and further preferably 0.5 weight % or more, and the upper limit is preferably 5 weight % or less, more preferably 3 weight % or less, more preferably 2 weight % or less and most preferably 1.5 weight % or less.

[0043] In an embodiment of the disclosure, the non-aqueous electrolyte comprises 0.1-5 wt% LiBOB and 0.1-5 wt% LiDFPFSI.

[0044] Preferably, the non-aqueous electrolyte comprises from 0.2 wt% to 3 wt% LiBOB and from 0.2 wt% to 3 wt% LiDFPFSI, more preferably from 0.3 wt% to 2 wt% LiBOB and from 0.3 wt% to 2 wt% LiDFPFSI, and even more preferably from 0.5 wt% to 1.5 wt% LiBOB and from 0.5 wt% to 1.5 wt% LiDFPFSI. The weight ratio between LiBOB and LiDFPFSI may be from 1 :50 to 50: 1, for example the weight ratio may be from 1 : 10 to 10: 1, for instance from 1 :5 to 5: 1, or more preferably from 1:3 to 3: 1. Preferably, the weight ratio between LiBOB and LiDFPFSI is between 1 :2 and 2: 1, for example 1: 1.

[0045] LiBOB is a commonly used electrolyte additives for Li-ion batteries (LIBs) due to its numerous advantages such as thermal stability, good solubility in organic solvents, high conductivity, and low cost as well as providing safer operations with superior electrochemical performance compared to conventional electrolyte combinations.

[0046] A structure of LiBOB is shown below:

[0047] LiDFPFSI is thought to participate in the formation of a beneficial SEI layer. Although the mechanism is not clear, it is believed that the film has a high lithium conductivity.

[0048] The structure of LiDFPFSI is shown below:

[0049] Although the mechanism of the action of improving the cell properties by the embodiments of the present disclosure is not clear, it is conceived that the combination of LiBOB and LiDFPFSI is partially decomposed at the interface between the electrodes and the electrolyte solution to form a beneficial SEI lay on each of the electrodes. It is conceived that this layer inhibits the direct contact between the non-aqueous solvent and the active material to prevent the detrimental side reactions, for example to inhibit gas formation, so as to prevent the resulting deterioration of the battery performance. Often, vinylene carbonate (VC) is seen as an additive in electrolytes. However, it has been found that the addition of VC to an electrolyte of the present disclosure, typically, results in inferior results.

[0050] Without wishing to be bound by theory, it is believed that VC impedes the formation of a protective yet conductive SEI layer resulting in inferior properties upon long term storage at elevated temperatures.

[0051] Typically, the non-aqueous electrolyte of the disclosure contains less than 0.1 weight % VC, preferably less than 0.05 weight % VC, such as less that 0.01 wt% VC, more preferably less than 0.005 wt% VC and ideally less than 0.001 wt% VC. When VC is present in such a small amount, it is not expected to affect the storage stability.

[0052] In an embodiment of the disclosure the electrolyte does not contain VC (vinylene carbonate).

[0053] The use of the non-aqueous electrolyte of the disclosure is found to improve the storage stability of an electrochemical cell, for instance when stored in a charged state at 60°C for at least 4 weeks.

[0054] Cathode active material

[0055] In the context of the disclosure "cathode active material" refers to any material that is suitable for use as the positive electrochemically active material in a cathode, and suitable for use in a cell (i.e. an electrochemical cell).

[0056] The term "electrochemically active material" is to be understood as an electrochemical species which can be oxidized and reduced in a system which enables a cell to produce electric energy during discharge. The role of the cathode active material is to reversibly intercalate ions (such as lithium ions) during cell charge and discharge cycles.

[0057] The cathode active material of the disclosure is an intercalation material, wherein the intercalation metal is lithium.

[0058] The electrolyte of the disclosure has been found to reduce gas generation for cathode active materials under long term storage, particularly storage at 60°C. The electrolyte finds particular use with cathode active materials having high amounts of nickel, i.e. at 80wt% or more nickel content. In the cell of the disclosure, the cathode active material comprises 80 mol% or more Ni, based on the total amount of non-lithium metals in the cathode active material, typically 85 mol% or more, preferably 88 mol% or more and ideally 90 mol% or more nickel, based on the total amount of non-lithium metals in the cathode active material.

[0059] Likewise, the cathode active material comprises 98 mol% or less Ni, based on the total amount of non-lithium metals in the cathode active material, typically 95 mol% or less, or even 92 mol% or less nickel, based on the total amount of non-lithium metals in the cathode active material.

[0060] For instance, the cathode active material comprises from 80 mol% to 98 mol% of nickel, preferably from 80 mol% to 95 mol% nickel, more preferably from 85 to 95 mol% nickel, such as from 88 mol% to 92 mol% nickel, based on the total amount of non-lithium metals in the cathode active material.

[0061] Preferably the cathode active material is a transition metal complex such as nickel manganese cobalt oxide (NMC) material. Even more preferably, the cathode active material is an NMC material intercalated with lithium or an "Li-NMC" material.

[0062] Exemplary cathode active materials include nickel-cobalt-manganese (NMC) composite oxides and lithium NMC (Li-NMC) composite oxides or lithium nickel cobalt manganese (NMC) oxides (LiNiixyCoxMnyO? (0<x+y<0.2)).

[0063] In some embodiments, the cathode active material comprises lithium nickel cobalt manganese oxides (NMC) (LibNii x-yCOxMnyAzCL (0<x+y+z<0.2)), where A is an element other than Li, Ni, Co, Mn or O and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0064] In preferred embodiments, the NMC cathode materials is defined as LibNii x-yCOxMnyAzCh, wherein 0<x+y+z<0.2, preferably 0<x+y+z<0.15, more preferably 0<x+y+z<0.12, and wherein 0<z<0.05, preferably 0.002<z<0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< l.l. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr. High nickel NMC are particularly useful given their improved capacity compared to those having a lower Ni-content. The caveat is that increasing the Ni content in NMC cathodes compromises their chemical and structural stability, causing the batteries to deteriorate much faster than their lower Ni content counterparts. In particular, Hi Ni-NMC cathodes tend to evolve considerably more gas during storing than their lower Ni content counterparts.

[0065] The non-aqueous electrolyte of the disclosure provides particularly advantageous results when used in a cell comprising a high-nickel NMC cathode material. For example, the nonaqueous electrolyte of the disclosure results in decreased gas formation.

[0066] The cathode active layer is formed from the cathode active material, together with a binder and optional conductive additive.

[0067] Suitable binders are well known in the art and may be water-insoluble or water-soluble.

[0068] Examples of suitable water-insoluble binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an ethylene oxide-containing polymer, an ethylene propylene copolymer, polystyrene, polyvinylpyrrolidone, polyurethane, polytetrafluoro ethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0069] Examples of suitable water-soluble binders include a rubber binder or a polymer resin binder. The rubber binder may be selected from a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an acrylonitrile-butadiene rubber, an acrylic rubber, a butyl rubber, a fluororubber, and a combination thereof. The polymer resin binder may be selected from polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, an ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, a polyester resin, an acrylic resin, a phenol resin, an epoxy resin, polyvinyl alcohol, and a combination thereof.

[0070] The cathode active layer may comprise, by weight, 0.01-10 wt% binder, for example from about 0.02-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt% binder. Preferably the cathode active layer comprises from about 0.1 to 3 wt% binder, even more preferably 0.2 wt% to 2 wt% binder, such as from 0.5 wt% to 1.5 wt% binder. Most preferably the cathode active layer comprises about 1 wt% binder. Suitable conductive additives include acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitch-based carbon, coke powders, carbon nanotubes or metallic powders. These conductive additives may be used alone or in combination.

[0071] Preferred conductive additives are selected from carbon black, graphite, carbon nanotubes, or mixtures thereof.

[0072] In an embodiment, the cathode active layer may comprise 0.01-10 wt% conductive additive, for example from about 0.02-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt% conductive additive. Preferably the cathode active layer comprises from about 0.1 to 3 wt% conductive additive, even more preferably 0.2 wt% to 2 wt% conductive additive, such as from 0.5 wt% to 1.5 wt% conductive additive. Most preferably the cathode active layer comprises about 1 wt% conductive additive.

[0073] In the cathode active layer, the cathode active material is typically present in an amount of, by weight, from about 60-99.9 wt% active material, for example from about 70-99.9 wt%, from about 80-99.8 wt%, from about 90-99.6 wt%, or from about 95-99.5 wt% active material. Preferably the cathode active layer comprises from about 96-99.5 wt% active material, even more preferably the cathode active layer comprises about 98 wt% active material.

[0074] The cathode active layer is typically formed by forming a slurry of cathode active material, binder and optional conductive agent; and depositing the slurry on a current collector. Any suitable dispersing medium may be used for the slurry, with N-methylpyrrolidone being typical.

[0075] In the context of this disclosure, "storage stability" is understood as the stability of a cell during storage; that is not during use (or charge / discharge). For example, the storage stability may be evaluated using a fully charged cell.

[0076] In some embodiments, the storage stability is evaluated at elevated temperatures, for example at a temperatures of 40 °C, preferably at 50 °C, or more preferably at 60 °C.

[0077] It will be understood that elevated temperatures increase the stress on the cell, and essentially accelerates the formation of conditions and components detrimental to cell performance. The increasing temperatures (namely 40 °C, 50 °C and 60 °C) essentially reflect increasingly stressful conditions. A cell showing stability over a certain period of time at 40 °C may not show stability over the same period of time at a higher temperature. Conversely, a cell showing stability for a certain period of at a higher temperature can be assumed to be stable for at least the same period of time at a lower elevated temperature (that is a lower temperature that is still above room temperature).

[0078] Thus, elevated temperatures typically accelerate electrolyte decomposition. This is particularly problematic in electric vehicle applications wherein batteries are often positioned close to, or adjacent to, heat-generating components such as an engine. Therefore, storage stability at elevated temperatures is particularly important.

[0079] The storage stability of a lithium-ion cell can be accessed using a variety of methods.

[0080] As mentioned above, a typical cause of low storage stability within cells is unwanted gas generating reactions. Gas generation causes swelling of a cell, which involves an increase in cell thickness - a property which is easy to measure. The swelling ultimately results in delamination of the layers within the cell, higher internal resistance and poorer cell performance.

[0081] Therefore, in some embodiments "improved storage stability" refers to reduced gas generation compared to (otherwise identical) electrochemical cells not comprising the nonaqueous electrolyte of the disclosure, for instance as measured in a cell (preferably comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material) under storage in a charged state at 60 °C for 4 weeks.

[0082] One method of measuring the storage stability of a cell is to measure the change in thickness of the cell, for instance as measured in a cell (preferably comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material) under storage in a charged state at 60 °C for 4 weeks. An increase of thickness is indicative of gas evolution within the cell, which necessarily stems from undesirable side reactions.

[0083] As such, a relatively smaller increase of thickness of a cell is indicative of improved storage stability compared to a different cell having a relatively larger increase of thickness, for instance as measured in a cell (preferably comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material) under storage in a charged state at 60 °C for 4 weeks. The thickness may be measured using any measuring tool known to the skilled person. For example, a ruler or a calliper may be used.

[0084] Another means of assessing the storage stability of a lithium-ion cell is to determine the internal resistance.

[0085] In a lithium-ion cell, internal resistance refers to the resistance that the cell encounters as it delivers current. This resistance is caused by a number of factors, including the resistance of the materials used in the cell's electrodes, the resistance of the electrolyte, and the resistance of the current collectors and connectors.

[0086] In general, a low internal resistance is indicative of good cell performance. However, typically the internal resistance increases during the life of a cell.

[0087] The increase in internal resistance may be the result of an undesirable (or poor performing) SEI layer being formed. Therefore, it is believed that the selection of electrolyte additives is highly important, as they are typically involved in the formation of the SEI layer.

[0088] In an example of the present disclosure, direct current internal resistance (DCIR) is measured to access the stability of the cell upon prolonged storage at elevated temperatures (e.g. 60 °C). More specifically, the increase in DCIR is being evaluated, as a substantial increase in DCIR corresponds to poor storage stability.

[0089] Therefore, in some embodiments "improved storage stability" refers to a smaller DCIR increase compared to (otherwise identical) electrochemical cells not comprising the nonaqueous electrolyte of the disclosure, for instance as measured in a cell (preferably comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material) under storage in a charged state at 60 °C for 4 weeks.

[0090] In the context of the disclosure, "internal resistance" refers to the direct current internal resistance (DCIR) value of a cell.

[0091] The terms "internal resistance" and "DCIR value" may be used interchangeably throughout the disclosure. The DCIR value of an electrode assembly may be determined via the method described in US8,482,254B2, however any suitable method for determining the DCIR may be used, as long as the same procedure is used for to two half cells to be compared.

[0092] The DCIR value can be determined according to the following equation :

[0093] (Voc— VTX) / ILOAD where ILOAD is the current rate,

[0094] Voc is the open circuit potential (OCP) of the cell (also referred to as the open circuit voltage (OCV)) and

[0095] VTX is the voltage after applying the load for a time of x seconds.

[0096] Typically, DCIR measurements are the 0.1s-30s DCIR, where the load is applied for 0.1s to 30s. The load may be applied at a state of charge (SOC) of anywhere between 0-100%. When DCIR measurements are used to evaluate the storage stability, the load is typically high. For example, the load applied may be above 50% SOC, such as above 75% SOC, for example above 90% SOC, such as above 95% SOC. Preferably, the load is applied at 96% SOC. The current to apply the load may be between a rate of 0.5C to 3C, normalized to the active material capacity, wherein typically a 2C current is preferred.

[0097] In some embodiments, the internal resistance of a cell comprising the electrolyte of the disclosure has a maximum increase of 15% after 90 days of storage of the charged cell at 60 °C.

[0098] To compare the internal resistance of electrolytes comprising different additives, different cells may be prepared comprising the electrolytes comprising different additives. Any suitable cell may be used, for example prismatic cell. Apart from the electrolyte additives, the cells should be the same.

[0099] Electrochemical cell

[0100] The present disclosure also relates to electrochemical cells comprising the non-aqueous electrolyte of the disclosure, for example a secondary lithium-ion cell. Such cells typically comprise a cathode, an anode, a separator disposed between the anode and cathode, said cathode, anode and separator forming an electrode assembly, the cell further comprising a housing for the electrode assembly. The non-aqueous electrolyte facilitates the transport of lithium ions between the composite cathode and anode. The housing is typically sealed to ensure the electrolyte is retained within the housing. Said housing usually includes terminals in electrical contact with the anode and composite cathode. These cells may be combined to form a battery system (i.e. an array of cells).

[0101] The disclosure also relates to an electrical device comprising a cell of the disclosure. For instance, the disclosure relates to a vehicle comprising a cell (or battery system) of the disclosure. The vehicle is preferably an electric vehicle, such as a car, truck, bus, scooter, motorbike, bicycle or the like, preferably a car, truck or bus.

[0102] Unless otherwise indicated, 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 invention pertains.

[0103] EXAMPLES

[0104] Example 1

[0105] A pouch cell battery was prepared using electrolytes comprising different additives. Slurry compositions were prepared using either water or NMP as dispersing media. A Ni88-NMC material was used as cathode active material. Polyvinylidene fluoride (PVDF) was used as binder for the cathode. Conductive carbon was used as conductive additive for the cathode.

[0106] The anode used in the cell was graphite. Styrene butadiene rubber (SBR) was used as a binder for the anode. A ceramic-coated separator was used to separate the anode and cathode. Stainless steel was used for the positive and negative casing elements of the cell.

[0107] Electrolytes comprising three different combinations of electrolyte additives were used:

[0108] ® 1 wt% LiBOB and 1 wt% LiDFPFSI;

[0109] S 1 wt% LiBOB, 1 wt% VC and 1 wt% LiDFPFSI; and

[0110] • 1 wt% VC and 1 wt% LiDFPFSI;

[0111] • wherein the symbols correspond to the symbols found in figure 1.

[0112] Further each electrolyte comprises 14 wt% LiPFe, 20 wt% EC, 31 wt% EMC, and 33 wt% DMC. Each of the three cells were charged to 100% SOC and left in an oven set to 60°C. After 0 weeks, 1 week, 2 weeks, 3 weeks and 4 weeks the thickness of each cell was measured. After each measurement the cells were again charged to 100% SOC. The results are shown in figure 1.

[0113] As is evident, the cell with the electrolyte comprising 1% LiBOB and 1% LiDFPFSI results in markedly lower increase in thickness as compared to the other cells. This implies that this combination is particularly good at preventing gas formation.

[0114] Without wishing to be bound by theory, it is believed that the combination of LiBOB and LiDFPFSI forms a particularly protective SEI layer which mitigates unwanted gas forming reactions, while VC reacts with the cathode in an undesirable manner.

[0115] Example 2

[0116] A prismatic cell battery was prepared using electrolytes comprising different additives. Slurry compositions were prepared using either water or NMP as dispersing media. A Ni88- NMC material was used as cathode active material. Polyvinylidene fluoride (PVDF) was used as binder for the cathode. Conductive carbon was used as conductive additive for the cathode.

[0117] The anode used in the cell was graphite. Styrene butadiene rubber (SBR) was used as a binder for the anode. A ceramic-coated separator was used to separate the anode and cathode. Stainless steel was used for the positive and negative casing elements of the cell.

[0118] Electrolytes comprising two different combinations of electrolyte additives were used:

[0119] ® 1 wt% LiBOB and 1 wt% LiDFPFSI; and

[0120] A 1wt% VC and 1 wt% LiDFPFSI; wherein the symbols correspond to the symbols found in figure 2.

[0121] Further each electrolyte comprises 14 wt% LiPFe, 20 wt% EC, 31 wt% EMC, and 33 wt% DMC.

[0122] Each of the three cells were charged to 100% SOC left in an oven set to 60°C. After 0 days, 30 days, 60 days and 90 days, the DCIR was measured at SOC96%. After each measurement the cells were again charged to 100% SOC. The results are shown in figure 2. As is evident, the cell with the electrolyte comprising 1% LiBOB and 1% LiDFPFSI results in markedly lower DCIR increase as compared to the cell not comprising the electrolyte additives of the disclosure.

[0123] Without wishing to be bound by theory, it is believed that the combination of LiBOB and LiDFPFSI forms a conductive SEI layer which impedes the usual DCIR increase, while VC reacts with the cathode in an undesirable manner.

Claims

CLAIMS1. An electrochemical cell comprising a non-aqueous electrolyte and a cathode active material, wherein the cathode active material comprises Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material; and the non-aqueous electrolyte comprises 0.1-5wt% lithium bis(oxalato)borate (LiBOB) and 0.1-5wt% lithium (difluorophosphonyl)fluorosulfonylimide (LiDFPFSI).

2. The electrochemical cell of claim 1, wherein the cathode active material comprises from 80 mol% to 95 mol% of nickel, based on the total amount of non-lithium metals in the cathode active material.

3. The electrochemical cell of claims 1-2, wherein the non-aqueous electrolyte comprises a maximum of 0.01 wt% VC (vinylene carbonate).

4. The electrochemical cell of claims 1-3, wherein the non-aqueous electrolyte does not contain VC (vinylene carbonate).

5. The electrochemical cell of claims 1-4, wherein the non-aqueous electrolyte comprises one or more cyclic and / or chain carbonates such as selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and mixtures thereof.

6. The electrochemical cell of claims 1-5, wherein the non-aqueous electrolyte comprises ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

7. The electrochemical cell of claim 6, wherein the non-aqueous electrolyte comprises a solvent mixture comprising 10 wt% to 30 wt% EC, 20 wt% to 40 wt% EMC, and 20 wt% to 40 wt% DMC.

8. The electrochemical cell of claims 1-7, wherein the non-aqueous electrolyte comprises from 10 wt% to 20 wt% LiPFe.

9. The electrochemical cell of claims 1-8, wherein the weight ratio of LiBOB and LiDFPFSI is from 1:2 to 2: 1.

10. The electrochemical cell of claims 1-9, wherein the internal resistance of the cell has a maximum increase of 15% after 90 days of storage of the charged cell at 60 °C.

11. Use of a non-aqueous electrolyte for improving the storage stability of an electrochemical cell, for instance as measured in a cell (preferably comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material) under storage in a charged state at 60 °C for 4 weeks, wherein the electrolyte comprises 0.1-5wt% LiBOB and 0.1-5wt% LiDFPFSI.

12. Use of a non-aqueous electrolyte for reducing gas generation in a cell comprising a cathode active material comprising Ni at 80 mol% or more based on the total amount of non-lithium metals in the cathode active material, for instance as measured in a cell under storage in a charged state at 60 °C for 4 weeks, wherein the electrolyte comprises 0.1-5 wt% LiBOB and 0.1-5 wt% LiDFPFSI.

13. The use of claim 11 or claim 12, wherein the weight ratio LiBOB to LiDFPFSI is between 1:2 and 2: 1.

14. The use of any one of claims 11-13, wherein the non-aqueous electrolyte comprises a maximum of 0.001 mass % VC (vinylene carbonate).

15. The use of any one of claims 8-11, wherein the non-aqueous electrolyte does not contain VC (vinylene carbonate).

16. The use of any one of claims 11-15, wherein the non-aqueous electrolyte comprises one or more cyclic and / or chain carbonates such as selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and mixtures thereof.

17. The use of any one of claims 11-16, wherein the non-aqueous electrolyte comprises ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

18. The use of any one of claims 11-17, wherein the electrolyte additionally comprises from 1 wt% to 30 wt% lithium-fluor-salt such as LiPFe or LiFSI.

19. The use of any one of claims 11-18, wherein the electrochemical cell comprises a cathode active material comprises Ni at 85 mol% or more based on the total amount of non-lithium metals in the cathode active material.

20. The use of any one of claim 11-19, wherein improving the storage stability refers to reduced gas generation compared to electrochemical cells not comprising the non-aqueous electrolyte as defined in claim 11-18.

21. The use of any of claims 11-19, wherein improving the storage stability refers to a smaller DCIR increase compared to electrochemical cells not comprising the non-aqueous electrolyte as defined in claim 11-18.

22. The use of any one of claims 11-19, wherein improving the storage stability refers to a maximum DCIR increase of 15% after 90 days of storage at 60 °C.

23. A vehicle comprising the cell of any one of claims 1-10.

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