Electrolyte additive for natural graphite anode

Abstract

The disclosure relates to a cell comprising natural graphite and an electrolyte additive. Natural graphite has higher surface functionality and thus reactivity with electrolyte than artificial graphite. This means that anode active material comprising natural graphite will typically degrade faster than the synthetic variant. It has surprisingly been found than the addition of a specific group of electrolyte additives improves the stability of a cell comprising large amounts of natural graphite as anode active material.

Description

[0001] ELECTROLYTE ADDITIVE FOR NATURAL GRAPHITE ANODE

[0002] TECHNICAL FIELD

[0003] The disclosure relates to the use of an electrolyte additive to enhance the stability of an electrochemical cell comprising natural graphite. The disclosure also relates to an electrochemical cell comprising natural graphite and said electrolyte additive.

[0004] BACKGROUND

[0005] Graphite is widely used as a negative electrode material in lithium-ion batteries. It is chosen for its layered structure, which allows lithium ions to intercalate or insert between the layers during charging. This process is reversible, allowing the ions to be released during discharging, providing electrical energy.

[0006] In general, high-end applications such as electric vehicles and portable electronics often use artificial graphite due to its superior performance, while lower-cost applications may use natural graphite to save on costs.

[0007] Artificial graphite is made from petroleum coke or pitch through a high-temperature graphitization process. On the other hand, natural graphite is mined and then refined to achieve the necessary purity for use in batteries.

[0008] As natural graphite is mined, its performance can vary due to natural variations in the material. One reason for the poor performance of natural graphite may be attributed to poor Solid-Electrolyte Interphase (SEI) formation.

[0009] The SEI layer forms on the surface of the graphite when the graphite reacts with components of the electrolyte during charging and discharging. Natural graphite has higher surface functionality and thus reactivity with electrolyte than artificial graphite. Therefore, SEI formation is particularly important to consider for batteries comprising natural graphite-based anodes.

[0010] There is therefore a need for an electrolyte that enhances the stability of cells comprising natural graphite-based anodes. SUMMARY

[0011] An object of the present disclosure is to provide an electrolyte additive that enhances stability of an electrochemical cell comprising a natural graphite anode active material.

[0012] According to a first aspect of the disclosure, there is provided a cell comprising a cathode; an anode comprising natural graphite; an electrolyte comprising an electrolyte additive of Formula I wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl.

[0013] The anode may comprise at least 30 wt% natural graphite based on the total anode active material weight.

[0014] The anode may additionally comprise artificial graphite.

[0015] The anode may additionally comprise a silicon-based material.

[0016] The anode may additionally comprise a silicon-based material which comprises elemental silicon or a silicon alloy.

[0017] The anode may additionally comprise a silicon-based material which comprises SiOx and / or a M-SiOx material, wherein 0.5<x< 1.5, and M is one or more metals, optionally M is selected from Al, B, Ca, Cu, Fe, K, Li, Mg, Na, Ni, Sn, Ti, Zn, Zr, Ca, V, Cr, Nb, Mo, W or any combination thereof.

[0018] The electrolyte may comprise from 0.06 to 6 wt% electrolyte additive having Formula I.

[0019] The electrolyte may comprise the additive in Formula I, wherein R = methyl.

[0020] The electrolyte may additionally comprise vinylene carbonate (VC). The anode may comprise a silicon-based material, such as one or more chosen from the group of elemental silicon, a silicon alloy, SiOx and a M-SiOxmaterial, wherein M is one or more metals except silicon.

[0021] According to a second aspect of the disclosure, there is provided a use of an electrolyte additive having Formula I wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl to reduce the growth of DCIR during cycling and / or improve cyclability in a cell comprising natural graphite as an anode active material.

[0022] FIGURES SUMMARY

[0023] Figure 1A: Shows the DCIR for the pouch-cell of Example 1 during cycling at 45 °C.

[0024] Figure IB: Shows the capacity retention for the pouch-cell of Example 1 during cycling at 45 °C.

[0025] Figure 2A: Shows the DCIR for the pouch-cell of Example 1 during 4 weeks of storage at 60 °C.

[0026] Figure 2B: Shows the capacity retention for the pouch-cell of Example 1 during 4 weeks of storage at 60 °C.

[0027] DETAILED DESCRIPTION

[0028] The disclosure relates to electrolyte additives that have been found to mitigate the issues that arise when using natural graphite. In particular, the electrolyte additives improve the stability of a cell comprising a graphite active material.

[0029] Without wishing to be bound by theory, it is thought that the electrolyte additive decomposes on the surface of the anode to form a thin and dense film. This film serves a dual purpose: it impedes the continuous decomposition of the electrolyte during further charge / discharge cycles while maintaining a low impedance interface on the anode surface facilitating efficient discharge properties.

[0030] The electrolyte additive is found in the electrolyte of a cell. Each cell comprises a cathode and an anode.

[0031] CATHODE

[0032] The cathode may also be referred to as the positive electrode.

[0033] The cathode comprises a cathode active layer deposited on a current collector, which connects the cathode active layer with the remainder of the cell.

[0034] The cathode active layer comprises cathode active material, binder, and optionally conductive additives.

[0035] The term "cathode active material" (CAM) is to be understood as an electrochemical species which can be oxidised 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 (or otherwise bind) ions (such as lithium ions) during cell charge and discharge cycles.

[0036] The cathode active material may comprise a suitable material for use as a cathode active material in the cathode of a cell, for example a lithium-ion cell.

[0037] The cathode active material may comprise an intercalation material, such as a lithium intercalation material, for example a lithium metal oxide which may include lithium and a transition metal.

[0038] The cathode active material may comprise any one or a mixture of two or more of lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel manganese oxide (LNMO), lithium nickel cobalt oxide, lithium nickel manganese cobalt (NMC) oxide, lithium iron phosphate (LFP), lithium iron manganese phosphate (LFMP) and lithium nickel cobalt aluminium oxide (NCA).

[0039] Preferably, the cathode active material is a transition metal complex such as layered lithium metal oxide (LiMO?) cathode materials, wherein the metal is typically nickel. Even more preferably, the cathode active material is a lithium nickel manganese cobalt oxide (NMC), such as a high nickel lithium nickel manganese cobalt oxide (high Ni NMC).

[0040] In some embodiments, the cathode active material comprises lithium nickel cobalt manganese oxides (NMC) LibNii x-y-zCoxMnyAzO? wherein 0<x+y+z< 1, 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.

[0041] In some embodiments, the NMC cathode materials are high in nickel. As such, the cathode active material typically comprises lithium nickel cobalt manganese oxides (NMC) represented by the formula LibNii-x-y-zCoxMnyAzCh, wherein 0<x+y+z<0.4, preferably 0<x+y+z<0.25, 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.

[0042] The ratio of the nickel of the high-nickel NMC material may range from 33 mol% to 98 mol%, in terms of the non-lithium metals. Preferably, the ratio may range from 60 mol% to 95 mol%, even more preferably, the ratio may range from 80 mol% to 95 mol%, in terms of the non-lithium metals.

[0043] In preferred embodiments, the NMC cathode materials is defined as LibNii x-yCoxMnyAzO2, wherein 0<x+y<0.4, preferably 0<x+y<0.25, 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.

[0044] In an embodiment, the cathode active layer comprises cathode active material in an amount of from about 60-99.9 wt%, for example from about 70-99.9 wt%, from about SO- 99.8 wt%, from about 90-99.6 wt%, or from about 92-99 wt% based on the combined weight of cathode active material, binder and optional conductive additives.

[0045] Preferably the cathode active layer comprises from about 94-99 wt% cathode active material based on the combined weight of cathode active material, binder and optional conductive additives, even more preferably the cathode active layer comprises about 98 wt% cathode active material based on the combined weight of cathode active material, binder and optional conductive additives.

[0046] ANODE

[0047] The anode may be referred to as the negative electrode.

[0048] The anode comprises an anode active layer deposited on a current collector, which connects the anode active layer with the remainder of the cell.

[0049] The anode active layer comprises anode active material, binder, and optionally conductive additives.

[0050] The role of the anode active material is to reversibly bind ions (such as lithium ions) during cell charge and discharge cycles. The mechanism of binding will vary and may be by intercalation / deintercalation (for instance with graphite), alloying / dealloying (for instance with silicon), or by plating / stripping (for instance with metallic lithium), or combinations thereof.

[0051] In the context of the disclosure "anode active material" refers to any material that is suitable for use as the negative electrochemically active material in a battery, and suitable for use in a cell.

[0052] The anode of this disclosure comprises graphite.

[0053] Graphite exists as natural graphite and as artificial graphite.

[0054] Artificial graphite may also be referred to as synthetic graphite.

[0055] Natural graphite occurs naturally in mineral deposits and is extracted from the ground, for instance through open-pit and underground mining.

[0056] Synthetic graphite is a material consisting of graphitic carbon which has been obtained by graphitizing non-graphitic carbon such as petroleum coke and coal tar at temperatures above 2,500 °C. Natural graphite has a higher crystalline structure and offers better electrical and thermal conductivity than synthetic material. However, synthetic graphite possesses superior physical properties and typically better performance properties when used in a cell. On the other hand, natural graphite offers a more cost-effective and environmentally friendly solution for various industrial applications. Therefore, it is desirable to include higher amounts of natural graphite in anodes.

[0057] Without wishing to be bound by theory, it is believed that one reason for artificial graphite's typically superior performance in cells is due to the different surface properties resulting from its higher purity. The lower carbon content of natural graphite results in a higher surface functionality and thus increased reactivity towards the electrolyte compared to artificial graphite. The higher surface functionality often manifests as heteroatoms such as oxygen, nitrogen and sulphur being present at the edge of the graphite planes. Although only present in small quantities, these can be highly reactive leading to increased reactivity during initial cycling and SEI formation. Additionally, natural graphite suffers from increased rate of continual SEI formation during ageing, which is induced by swelling and cracking of SEI during cycling. The higher reactivity of the graphite edge portions in natural graphite during SEI formation can lead to graphite-SEI structures that have a reduced tendency to undergo lithium intercalation during cycling, when compared to comparable graphite-SEI structures formed from artificial graphite.

[0058] Therefore, SEI formation is particularly important to consider for cells comprising natural graphite-based anodes.

[0059] The present disclosure uses an electrolyte additive which allows for high amounts of natural graphite in the anode. In particular, the electrolyte additive allows for favourable SEI formation for the specific surface chemistry of natural graphite.

[0060] The anode of this disclosure comprises natural graphite.

[0061] The anode active material comprises at least 20 wt% natural graphite based on the combined weight of all anode active materials, such as at least 25 wt%, for example at least 30 wt%, such as at least 35 wt%, for example at least 40 wt%, such as at least 45 wt%, for example at least 50 wt%, such as at least 55 wt%, for example at least 60 wt%, such as at least 65 wt%, for example at least 70 wt%, such as at least 75 wt%, for example at least 80 wt%. The amount of natural graphite compared to artificial graphite may be determined by any suitable means. For instance, X-ray CT may be used to resolve artificial from natural graphite, and the respective densities may be used to determine the wt% in the layer.

[0062] Preferably, the anode active material comprises at least 40 wt % natural graphite based on the combined weight of all anode active materials, more preferably at least 50 wt% natural graphite.

[0063] The natural graphite may have any morphology known to the skilled person. For example, the natural graphite may be flake graphite, amorphous graphite, or crystalline vein graphite.

[0064] Preferably, the natural graphite is amorphous graphite. Amorphous graphite is usually formed by contact metamorphism between an anthracite coal seam and a metamorphism agent (e.g. tectonic stress). The resultant material contains crystalline regions and tends to have a higher ash content as a result of the metamorphism agent depositing agent depositing other organic materials as it interacts with the anthracite coal. The macrocrystalline arrangement found in amorphous graphite means that it is less prone to cracking and delamination compared to for instance flake graphite, wherein all the crystalline layers are aligned.

[0065] Optionally, the natural graphite may be coated with an amorphous carbon (i.e. non- graphitic, non-crystalline), for instance pitch coated.

[0066] Pitch is a residue that results from the heat treatment and subsequent distillation of coal tar or petroleum fractions. After coating on natural graphite, it may subsequently be processed, for instance to improve surface morphology of the particles.

[0067] Coating with amorphous carbon such as pitch improves the electrochemical properties of the natural graphite.

[0068] Preferably, the natural graphite is pitch coated.

[0069] Even more preferably, the natural graphite is pitch coated amorphous graphite. The anode of this disclosure typically further comprises artificial graphite.

[0070] In an embodiment, the anode comprises natural graphite and artificial graphite.

[0071] The anode may comprise additional anode active materials. For example, the anode may comprise a silicon-based anode active material. That means that the anode may comprise both natural graphite and a silicon-based anode active material. Silicon-based materials enable an increase in specific cell capacity, as compared to graphite materials. Addition of a silicon-based material to the anode active material would therefore result in an anode with a higher specific capacity. Silicon-based material, akin to the natural graphite, may cause an increased rate of continual SEI formation during ageing, which is induced by swelling of the silicon-based material and resulting cracking of SEI during cycling.

[0072] Without wishing to be bound by theory, natural graphite and silicon-based materials, such as silicon or silicon oxide, have similar redox potentials with regards to lithium insertion / removal with these potentials being lower than the reduction potential of the electrolyte additives. Therefore, the electrolyte additives, and in particular the additive according to formula I, described herein in combination with natural graphite are also beneficial for anodes comprising silicon-based materials and natural graphite. Such combination of natural graphite and silicon-based materials in the anode and the herein described electrolyte additive therefore enables a cell with high specific capacity and reduced DCIR increase during cycling and storage, especially during higher temperatures, such as 45 °C or 60 °C, due to the reduced SEI growth.

[0073] In an embodiment the anode active material comprises at least 5 wt% silicon-based material based on the combined weight of all anode active materials, such as at least 10 wt%, for example at least 20 wt%, such as at least 30 wt%, for example at least 40 wt%, such as at least 45 wt%, for example at least 50 wt%, such as at least 55 wt%, for example at least 60 wt%, such as at least 65 wt%, for example at least 70 wt%, such as at least 75 wt%, for example at least 80 wt%.

[0074] In an embodiment, the silicon-based material comprises, or essentially consists of, elemental silicon or a silicon alloy. The silicon alloy is an alloy between silicon and one or more elements, except silicon itself. The alloying elements may be one or more chosen from the list of Li, Mg, Ca, Ti, V, Fe, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Mo, In, Sn, B, C, and P. Elemental silicon or silicon alloys have very high specific capacities with regards to lithium insertion / removal, enabling cells with increased specific capacities. However, such materials also suffer from large particle swelling during lithium insertion. The electrolyte additives of this disclosure enable a more stable SEI formation for the combination of elemental silicon / silicon alloy and natural graphite.

[0075] In an embodiment, the silicon-based material comprises, or essentially consists of, SiOx and / or a M-SiOxmaterial, wherein 0.5<x< 1.5, and M is one or more metals except silicon. M-SiOx denotes a metal-doped, also called 'metallized', silicon oxide material, where the dopant is not silicon. Furthermore, M may be selected from Al, B, Ca, Cu, Fe, K, Li, Mg, Na, Ni, Sn, Ti, Zn, Zr, Ca, V, Cr, Nb, Mo, W or any combination thereof.

[0076] SiOx and a M-SiOx materials have higher specific capacities than graphite, such as natural graphite. However, such silicon oxide materials suffer from considerable coulombic inefficiencies during the first charge / discharge cycles. The more stable SEI layer provided by this disclosure is therefore seen as beneficial for the combination of SiOx / M-SiOx materials and natural graphite.

[0077] In an embodiment, the anode active layer comprises from about 60-99.9 wt% anode 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% anode active material based on the combined weight of anode active material, binder and optional conductive additives. Preferably the anode active layer comprises from about 96-99.5 wt% anode active material, even more preferably the anode active layer comprises about 98 wt% anode active material based on the combined weight of anode active material, binder and optional conductive additives.

[0078] In an embodiment, the silicon-based material is included in a carbon composite material, wherein the silicon-based material and a carbonaceous material form a composite material, such as a composite material in where the silicon-based material is at least in part embedded in a carbonaceous material matrix.

[0079] BINDER

[0080] The binder adhesively connects all the electrode materials for long-term charge / discharge cycling.

[0081] Suitable binders are well known in the art and may be water-insoluble or water-soluble. 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.

[0082] 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.

[0083] The cathode 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 based on the combined weight of cathode active material, binder and optional conductive additives. Preferably the cathode 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 based on the combined weight of cathode active material, binder and optional conductive additives. Most preferably the cathode comprises about 1 wt% binder based on the combined weight of cathode active material, binder and optional conductive additives.

[0084] The anode 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 based on the combined weight of anode active material, binder and optional conductive additives. Preferably the anode 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 based on the combined weight of anode active material, binder and optional conductive additives. Most preferably the anode comprises about 1 wt% binder based on the combined weight of anode active material, binder and optional conductive additives.

[0085] CONDUCTIVE ADDITIVES The role of the optional conductive additive is to improve the electronic properties of the cathode and to provide an electrical connection between the particles of cathode active material in the cathode.

[0086] 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.

[0087] Preferred conductive additives are selected from carbon black, graphite, carbon nanotubes, or mixtures thereof.

[0088] In an embodiment, the cathode comprises from 0-10 wt% conductive additive, for example from about 0.01-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt% conductive additive based on the combined weight of cathode active material, binder and optional conductive additives. 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 based on the combined weight of cathode active material, binder and optional conductive additives. Most preferably the cathode active layer comprises about 1 wt% conductive additive based on the combined weight of cathode active material, binder and optional conductive additives.

[0089] In an embodiment, the anode may comprise from 0-10 wt% conductive additive, for example from about 0.01-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt% conductive additive based on the combined weight of anode active material, binder and optional conductive additives. Preferably the anode active layer comprises from about 0.1 to 3 wt% conductive additive based on the combined weight of anode active material, binder and optional conductive additives.

[0090] Typically, as the anode active material comprises graphite, no conductive additive is needed. Other carbon-based anode active materials such as hard carbon often require a conductive additive to be included. However, conductive additive may be added in small amounts as set out above to improve the conductive properties of the anode. For instance, even though the bulk conductivity of the graphite materials used in the anode may be high, adding conductive additive, even graphitic conductive additive, with relatively smaller particle morphologies can improve connectivity between the anode storage materials. CURRENT COLLECTOR

[0091] The current collector bridges the electrons from active materials toward the external devices.

[0092] The current collector will typically be a metal foil. Typically, current collectors are made from aluminium or copper. Preferably, the current collector is based on aluminium.

[0093] CELL HOUSING

[0094] The anode and the cathode may be incorporated into the cell as an electrode assembly, for instance in combination with a separator as a roll or folded stack. The electrode assembly may be enclosed in a cell housing, with the current collectors of the anode and cathode being electrically connected to the end terminals, for instance via a current collecting plate at the end of the roll or stack.

[0095] ELECTROLYTE

[0096] The electrolyte facilitates the transport of lithium ions between the cathode and the anode. Typically, the electrolyte is added to the cell prior to the cell housing being sealed.

[0097] Any electrolyte known to the skilled person may be used.

[0098] Preferably, the electrolyte comprises a lithium containing compounds such as LiPFe, and / or LiBF4. Even more preferably, the electrolyte comprises LiPFe.

[0099] Suitably, the lithium containing compound is present in an amount from 0.1 to 2 wt%, preferably from 0.5 to 1.5 wt%.

[0100] The electrolyte additionally comprises a solvent or a mixture of solvents comprising carbonates such as ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate; ethers such as dimethoxyethane, 1,4- dioxane, tetra hydrofuran, 2-methyl Tetrahydrofuran, and 2- trifluoromethyltetrahydrofuran; phosphates such as triphenyl phosphate, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, dimethyl methyl phosphonate, and bis(2,2,2-trifluoroethyl) methyl phosphate; carboxylic esters such as methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and propyl propionate; borate esters such as trimethyl borate, triethyl borate, tripropyl borate, tributyl borate, tris(2,2,2-trifluoroethyl) borate, tris (Hexafluoroisopropyl) borate, triphenyl borate, tris (trimethylsilyl) borate, tris (triethylsilyl) borate, and tris (pentafluorophenyl) borate).

[0101] Preferably, the mixture of solvents in the electrolyte comprises at least ethylene carbonate combined with at least one of diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and dimethoxy ethane.

[0102] More preferably, the mixture of solvents in the electrolyte comprises ethylene carbonate combined with at least one of diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate.

[0103] Preferred solvent mixtures for the electrolyte include ethylene carbonate and ethyl methyl carbonate; ethylene carbonate and dimethyl carbonate; ethylene carbonate and diethyl carbonate; ethylene carbonate, ethyl methyl carbonate and diethyl carbonate; ethylene carbonate, diethyl carbonate and dimethyl carbonate; and ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate.

[0104] Preferred ratios of solvents in the electrolyte include (in wt%):

[0105] 30-70 EC : 30-70 EMC;

[0106] 30-70 EC : 30-70 DMC;

[0107] 30-70 EC : 30-70 DEC;

[0108] 20-60 EC : 20-60 EMC : 20-60 DEC;

[0109] 20-60 EC : 20-60 DEC : 20-60 DMC; and

[0110] 20-60 EC : 20-60 EMC : 20-60 DMC.

[0111] Typically, the weight ratio between the fluorine containing compounds (when present) and the combined solvents is from 0.1 : 100 to 2.5 : 100, for example from 0.5 : 100 to 2 : 100, such as from 0.75 : 100 to 1.5 : 100.

[0112] Additionally, the electrolyte comprises electrolyte additive as described below.

[0113] ELECTROLYTE ADDITIVE The function of the electrolyte additive is to form a protective surface coating on the surface of the anode.

[0114] The electrolyte additive of the disclosure is tailored for natural graphite anodes. Anodes comprising natural graphite are known for their inferior properties compared to artificial graphite, in particular worse cyclability and increased Direct Current Internal Resistance (DCIR) growth at higher temperature, during both cycling and storage. By including the electrolyte additive of the disclosure, improved capacity retention and decreased DCIR growth in high temperature cycle life and high temperature storage are obtained.

[0115] The electrolyte comprises an electrolyte additive having Formula I: wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl.

[0116] R may be branched or linear.

[0117] Exemplary Ci-Ce-alkyl includes methyl, ethyl, propyl, butyl, pentyl, hexyl, isobutyl, secbutyl, tert-butyl, isopentyl, neopentyl, isohexyl, 3-methylpentyl, 2-methylpentyl. This list is non-exhaustive and other isomers than those listed may be applicable as well.

[0118] Exemplary Ci-Ce-alkenyl include vinyl, allyl, 1-butenyl, 1-pentenyl, 1-hexenyl, isoperopenyl, 2-butenyl, 2-methyl-2-propenyl, 2-methyl-l-butenyl, 2-methyl-l-butenyl, 3-methyl-l-pentenyl, 2-methyl-l-pentenyl. This list is non-exhaustive and other isomers than those listed may be applicable as well.

[0119] Exemplary Ci-Ce-alkenyl include ethynyl, propynyl, 1-butynyl, 2-butynyl, 1-pentynyl, 2- pentynyl, 3-pentynyl, 1-hexynyl, 2-hexynyl, 3-hexynyl, 4-hexynyl, isopropylacetylene, isobutylacetylene, isopentylacetylene, cyclopropylacetylene, cyclobutylacetylene, cyclopentylacetylene, cyclohexylacetylene, vinylacetylene, propargyl. This list is non- exhaustive and other isomers than those listed may be applicable as well. Preferably, R is Ci-Ce-alkyl, and more preferably methyl. When R is methyl, the electrolyte additive of Formula I may be referred to as a-angelica lactone (also known as 4-Hydroxy- 3-pentenoic acid y-lactone or 5-Methyl-2(3 / 7)-furanone).

[0120] It has surprisingly been found that the addition of an electrolyte additive of Formula I to the electrolyte improves storage stability, improves capacity retention and decreases DCIR growth in high temperature cycle life.

[0121] By high temperature is meant a temperature above 40 °C, for example above 45 °C, such as above 50 °C, for example above 55 °C, such as above 60 °C. Exemplary high temperatures include 45 °C and 60 °C.

[0122] Without wishing to be bound by theory, it is believed that the electrolyte additive of Formula I undergoes a ring opening reaction on the anode surface prior to the oxidation of organic solvents. This allows for the formation of decomposition products on the anode surfaces, which subsequently generate a thin and dense film. This film impedes the continuous decomposition of the electrolyte while maintaining a low impedance interface on the surface. Additionally, it is believed that the oligomeric residues in the electrolyte additive improves the flexibility of the formed SEI layer, so that subsequent swelling and volume changes of natural graphite are more efficiently accommodated.

[0123] In an embodiment, the electrolyte comprises from 0.01-10 wt% electrolyte additive having Formula I, for example from about 0.01-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt%. Preferably the electrolyte comprises from about 0.1 to 3 wt%, even more preferably 0.2 wt% to 2.5 wt%, such as from 0.5 wt% to 2 wt% electrolyte additive having Formula I. Most preferably the electrolyte comprises about 1 wt% electrolyte additive having Formula I.

[0124] Typically, the weight ratio between the electrolyte additive having Formula I and the combined solvents in the electrolyte is from 0.1 : 100 to 2.5 : 100, for example from 0.5

[0125] : 100 to 2 : 100, such as from 0.75 : 100 to 1.5 : 100.

[0126] More than one electrolyte additive may be present. For example, the electrolyte may additionally comprise vinylene carbonate (VC), poly(VC), vinylethylene carbonate (VEC), propargylmethyl carbonate (PMC), vinyl acetate (VA), ethylene sulphite (ES), l,3,2-dioxathiolane-2,2-dioxide (DTD), and / or 2-vinyl pyridine.

[0127] Preferably, the electrolyte may additionally comprise vinylene carbonate (VC) and / or vinylethylene carbonate (VEC).

[0128] More preferably, the electrolyte may additionally comprise vinylene carbonate (VC).

[0129] In an embodiment, the electrolyte comprises an electrolyte additive of Formula I and vinylene carbonate.

[0130] Vinylene carbonate, when present, may be included in an amount from 0.01 to 5 wt%, such as from 0.1 to 2 wt%, or preferably from 0.1 to 1 wt%.

[0131] The weight ratio between the electrolyte additive of Formula I and vinylene carbonate may be from 5: 1 to 1:5, for example from 4: 1 to 1 :4, such as from 3: 1 to 1:2, for example from 2: 1 to 1 : 1. Preferably, the electrolyte additive comprises more electrolyte additive of Formula I than vinylene carbonate.

[0132] The present disclosure further relates to the use of said electrolyte additive to reduce DCIR, particularly during high temperature storage or during high temperature cycling, in a cell comprising natural graphite as an anode active material. For example the cell may comprise at least 20 wt% natural graphite based on the combined weight of all anode active materials, such as at least 25 wt%, for example at least 30 wt%, such as at least 35 wt%, for example at least 40 wt%, such as at least 45 wt%, for example at least 50 wt%, such as at least 55 wt%, for example at least 60 wt%, such as at least 65 wt%, for example at least 70 wt%, such as at least 75 wt%, for example at least 80 wt%.

[0133] The use of said electrolyte additive to reduce DCIR in a cell comprising natural graphite as an anode active material is particularly beneficial at elevated temperatures. For example, at temperatures ranging from 30 °C to 80 °C, such as from 40 °C to 70 °C, for example from 45 °C to 60 °C.

[0134] The use of said electrolyte additive to reduce DCIR in a cell may further comprise an anode comprising a silicon-based material, such as one or more chosen from the group of elemental silicon, a silicon alloy, SiOx and a M-SiOxmaterial, wherein M is one or more metals except silicon. By combining the use of the electrolyte additive in question and an anode with both natural graphite and a silicon-based material, an increase in specific capacity can be achieved.

[0135] The present disclosure further relates to the use of said electrolyte additive to improve capacity retention, particularly during high temperature storage or during high temperature cycling high temperature cycling, in a cell comprising natural graphite as an anode active material. For example the cell may comprise at least 20 wt% natural graphite based on the combined weight of all anode active materials, such as at least 25 wt%, for example at least 30 wt%, such as at least 35 wt%, for example at least 40 wt%, such as at least 45 wt%, for example at least 50 wt%, such as at least 55 wt%, for example at least 60 wt%, such as at least 65 wt%, for example at least 70 wt%, such as at least 75 wt%, for example at least 80 wt%.

[0136] The use of said electrolyte additive to improve cyclability in a cell comprising natural graphite as an anode active material is particularly beneficial at elevated temperatures (i.e. high temperatures). For example, at temperatures ranging from 30 °C to 80 °C, such as from 40 °C to 70 °C, for example from 45 °C to 60 °C.

[0137] The present disclosure further relates to the use of said electrolyte additive to improve storage stability, particularly during high temperature storage or during high temperature cycling, in a cell comprising natural graphite as an anode active material. For example the cell may comprise at least 20 wt% natural graphite based on the combined weight of all anode active materials, such as at least 25 wt%, for example at least 30 wt%, such as at least 35 wt%, for example at least 40 wt%, such as at least 45 wt%, for example at least 50 wt%, such as at least 55 wt%, for example at least 60 wt%, such as at least 65 wt%, for example at least 70 wt%, such as at least 75 wt%, for example at least 80 wt%.

[0138] The use of said electrolyte additive to improve storage stability in a cell comprising natural graphite as an anode active material is particularly beneficial at elevated temperatures. For example, at temperatures ranging from 30 °C to 80 °C, such as from 40 °C to 70 °C, for example from 45 °C to 60 °C. The use of said electrolyte additive to improve storage stability may further comprise an anode comprising a silicon-based material, such as one or more chosen from the group of elemental silicon, a silicon alloy, SiOx and a M-SiOxmaterial, wherein M is one or more metals except silicon. By combining the use of the electrolyte additive in question and an anode with both natural graphite and a silicon-based material, an increase in specific capacity can be achieved.

[0139] The present disclosure therefore relates to an electrolyte for an electrochemical cell, said electrolyte comprising: ethylene carbonate in combination with at least one of diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate; a lithium containing compound selected from LiPFe, or UBF4; optionally vinylene carbonate; and an electrolyte additive having Formula I: wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl.

[0140] The electrolyte of the disclosure may contain any of the components in the amounts, ratios and preferred combinations as set out above.

[0141] CELL

[0142] The present disclosure also relates to cells comprising the electrolyte additive. Such cells typically comprise an anode, a cathode, 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 cell comprises an electrolyte comprising the electrolyte additive of the disclosure to facilitate the transport of lithium ions between the 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 cathode.

[0143] These cells may be combined to form a battery (i.e. an array of cells). It has surprisingly been found that a cell or a battery comprising an electrolyte additive of Formula I in electrolyte improves capacity retention and decreases DCIR growth in high temperature cycle life.

[0144] The cell of the disclosure comprises a cathode; an anode comprising natural graphite; an electrolyte comprising an electrolyte additive of Formula I wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl.

[0145] The disclosure also relates to an electrical device comprising a cell of the disclosure.

[0146] Suitable electrical devices include an electric vehicle, such as a car, bus, truck or the like, particularly a car.

[0147] EXAMPLES

[0148] Example 1

[0149] A multilayer pouch cell was prepared using an electrolyte additive of Formula I. The electrolyte additive of Formula I used in this example is a-angelica lactone. An NMC cathode active material was used as cathode active material. An anode active material consisting of 50% natural graphite and 50% artificial graphite was used as anode active material.

[0150] A ceramic-coated separator was used to separate the anode and cathode.

[0151] The electrolyte consists of LiPFe, ethylene carbonate (EC), ethyl methyl carbonate (EMC), vinylene carbonate (VC), and a-angelica lactone. Additionally, a comparative multilayer pouch cell was prepared, wherein no a-angelica lactone was present. The formed pouch cells were cycled at 45°C to evaluate the cycling stability. The rate was 1 C for both charging and discharging.

[0152] The discharge capacity retention of the cells prepared as described above is seen in Figure IB. Discharge capacity retention, which is used to evaluate cycling stability, is the ratio of discharge capacity to initial discharge capacity. A high value indicates high cycling stability. The pouch cell comprising a-angelica lactone displayed better discharge capacity retention after cycling compared to the pouch cell lacking a-angelica lactone.

[0153] The direct current internal resistance (DCIR) was measured at 50% state of charge (SOC). A low value indicates that a large current can be delivered with minimal voltage drop. The DCIR results of the pouch cell comprising a-angelica lactone were better compared to the pouch cell lacking a-angelica lactone. That means that the addition of a-angelica lactone decreases DCIR growth. This is evident from Figure 1A.

[0154] The storage stability at 60 °C at 100% state of charge (SOC) was evaluated for each of the pouch cells.

[0155] The direct current internal resistance (DCIR) during storage at 60°C was measured. The DCIR results of the pouch cell comprising a-angelica lactone were better compared to the pouch cell lacking a-angelica lactone. That means that the addition of a-angelica lactone decreases DCIR growth. This is evident from Figure 2A.

[0156] The capacity retention during storage at 60°C was measured. The pouch cell comprising a-angelica lactone displayed better capacity retention during storage at 60°C compared to the pouch cell lacking a-angelica lactone, see Figure IB.

[0157] This example highlights that using a-angelica lactone as an electrolyte additive in a cell comprising natural graphite increase storage stability and cycling stability.

[0158] 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. All embodiments of the invention and particular features mentioned herein may be taken in isolation or in combination with any other embodiments and / or particular features mentioned herein (hence describing more particular embodiments and particular features as disclosed herein) without departing from the disclosure of the invention.

[0159] As used herein, the term "comprises" will take its usual meaning in the art, namely indicating that the component includes but is not limited to the relevant features (i.e. including, among other things). As such, the term "comprises" will include references to the component consisting essentially of the relevant substance(s).

[0160] Wherever the word 'about' is employed herein in the context of amounts, for example absolute amounts, weights, volumes, sizes, diameters etc., or relative amounts (e.g. percentages) of individual constituents in a composition or a component of a composition (including concentrations and ratios), timeframes, and parameters such as temperatures etc., it will be appreciated that such variables are approximate and as such may vary by ±10%, for example ±5% and preferably ±2% (e.g. ±1%) from the actual numbers specified herein. This is the case even if such numbers are presented as percentages in the first place (for example 'about 10%' may mean ±10% about the number 10, which is anything between 9% and 11%).'

Claims

CLAIMS1. A cell comprising a cathode; an anode comprising natural graphite; an electrolyte comprising an electrolyte additive of Formula Iwherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl.

2. The cell of claim 1, wherein the anode comprises at least 30 wt% natural graphite based on the total anode active material weight.

3. The cell of any of the preceding claims, wherein the anode additionally comprises artificial graphite.

4. The cell of any of the preceding claims, wherein the anode additionally comprises a silicon-based material.

5. The cell according to claim 4, wherein the silicon-based material comprises elemental silicon or a silicon alloy.

6. The cell according to any one of claims 4-5, wherein the silicon-based material comprises SiOx and / or a M-SiOxmaterial, wherein 0.5<x< 1.5, and M is one or more metals except silicon.

7. The cell of any of the preceding claims, wherein the electrolyte comprises from 0.06 to 6 wt% electrolyte additive having Formula I.

8. The cell of any of the preceding claims, wherein R = methyl in Formula I.

9. The cell of any of the preceding claims, wherein the electrolyte additionally comprises vinylene carbonate (VC).

10. Use of an electrolyte additive having Formula IFormula I wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl to reduce the growth of DCIR during cycling and / or improve cyclability and / or improve storage stability in a cell comprising natural graphite as an anode active material.

11. The use of claim 10, wherein the reduced DCIR growth and / or improved cyclability and / or improved storage stability is seen at elevated temperatures ranging from 30 °C to 80 °C.

12. The use of any of claims 10 or 11, wherein the cell comprises at least 30 wt% natural graphite based on the combined weight of all anode active materials.

13. The use of any one of claims 10-12, wherein the cell comprises an anode comprising a silicon-based material, such as one or more chosen from the group of elemental silicon, a silicon alloy, SiOx and a M-SiOxmaterial, wherein M is one or more metals except silicon.

14. An electrolyte for an electrochemical cell, said electrolyte comprising: ethylene carbonate in combination with at least one of diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate; a lithium containing compound selected from LiPFe, or UBF4; optionally vinylene carbonate; and an electrolyte additive having Formula I:Formula I wherein R = H, Ci-Ce-alkyl, Ci-Ce-alkenyl, or Ci-Ce-alkynyl.

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