Liquid electrolyte for lithium secondary battery

Abstract

This present invention relates to a liquid electrolyte for a lithium secondary battery comprising at least one lithium salt dissolved in a solvent mixture, the solvent mixture comprising a) from 1.0 to 15.0% by weight (wt%) of at least one cyclic carbonate, devoid of ethylene carbonate and propylene carbonate; b) at least one fluorinated acyclic ester compound; and c) at least one phosphorous- containing chemical compound, the wt% being based on the total weight of the solvent mixture. The present invention also relates to a lithium secondary battery comprising the liquid electrolyte, and to use of the liquid electrolyte in improving cycling performance at high temperature of a lithium secondary battery.

Description

SPOP 2025 / 008- WO-PCTLIQUID ELECTROLYTE FOR LITHIUM SECONDARY BATTERYCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to European patent application No. 25315086.6 filed on March 18, 2025, the whole content of this application being incorporated herein by reference for all purposes.TECHNICAL FIELD

[0001] The invention relates to a liquid electrolyte for a lithium secondary battery comprising at least one lithium salt dissolved in a solvent mixture, the solvent mixture comprising a) from 1.0 to 15.0% by weight (wt%) of at least one cyclic carbonate, devoid of ethylene carbonate and propylene carbonate; b) at least one fluorinated acyclic ester compound; and c) at least one phosphorous- containing chemical compound, the wt% being based on the total weight of the solvent mixture. The present invention also relates to a lithium secondary battery comprising the liquid electrolyte, and to use of the liquid electrolyte in improving cycling performance at high temperature of a lithium secondary battery.TECHNICAL BACKGROUND

[0002] Lithium-ion batteries have retained a dominant position in the market of rechargeable energy storage devices for decades, thanks to many benefits such as light-weight, reasonable energy density and good cycle life.

[0003] A liquid electrolyte for a lithium-ion battery, notably a lithium secondary battery includes at least one lithium salt, a non-aqueous solvent and optionally additive(s). The non-aqueous solvent in a lithium secondary battery typically incorporates organic carbonates, in particular cyclic carbonates, acyclic carbonates, or mixtures thereof. The presence of an additive, if any, can enhance the performance and safety of the batteries, and hence an optimal solvent must be able to dissolve the lithium salt as well as the additives. The solvent must also be stable under various conditions in an active battery system, for instance under high voltage and / or high temperature environments.

[0004] In this regard, however, while a lithium-ion battery is in the status of charging under high temperature, side reactions may occur between the liquid electrolyte and electrodes, resulting in a decrease of battery capacity.SPOP 2025 / 008- WO-PCT

[0005] Ethylene carbonate (EC) and propylene carbonate (PC) have been used as main elements for a liquid electrolyte separately or in combination, mainly thanks to their high dielectric constant which enables more dissociation of lithium salts, resulting in the improvement of ionic conductivity to ensure an excellent cycling performance. In practice, EC has been used as an indispensable solvent component in a liquid electrolyte. In this regard, however, EC having melting point of about 35-37°C may suffer a series of side reactions such as ring-opening at the cathode interphase, especially at high voltage, e.g. exceeding 4.3V, forming an organic-rich unstable interphase. PC creates effective solvation around lithium ions, thereby creating a conductive electrolyte, whereas PC also has drawbacks that are obstacles to be eligible for a solvent in a liquid electrolyte, particularly in view of the destructive effect of PC-Li+solvation structures on graphite exfoliation. Cheng et. al. discloses that the poor compatibility of PC with graphite is probably because of PC’s ability to co-intercalate into graphite materials which is supposed to occur around their crystal boundaries in “New insight into the interaction between propylene carbonated-based electrolytes and graphite anode material for lithium-ion batteries (The Journal of Physical Chemistry C, 2007, 111, 12, 4740-4748)”. This is also reported that the presence of a methyl group in the PC molecule is the main wirepuller, which makes decomposition products of PC aggregated and thus incompetent in inhibiting the subsequent cointercalation. It is believed that the terminal group in PC could modify the rigidity of the conformation and then the energy barrier of the ring-opening reaction.

[0006] In parallel, increasing the cut-off voltage has been also tried to improve energy density. For instance, at the cut-off voltage higher than about 4.2 V, the electrolyte system is often deteriorated because the components of the electrolyte, such as a solvent, an electrolyte salt, and an additive, especially a film-forming additive which is believed to form a protective layer (often called “solid electrolyte interphase (SEI)”) on a surface of the electrode(s) on initial charging, cannot endure such high voltage. Meanwhile, the battery which can be operated at higher voltage (for instance, up to 5.0 V) is desired in the art, and accordingly the development of a liquid electrolyte suitable for the high-SPOP 2025 / 008- WO-PCTvoltage batteries, and / or the components for such a liquid electrolyte has been consistently required in the art. In terms of the cut-off voltage, such high- voltage batteries have a charge cut-off voltage of at least 4.3 V.

[0007] Nonetheless, at such a high voltage, the positive electroactive materials, notably those having high content of Ni, rapidly decrease its capacity because of the side reactions occurring between the positive electroactive materials and the liquid electrolyte and subsequently encounter collateral difficulties in view of thermal stability. This is because the high voltage accelerates the gas formation such as O2, CO, and CO2that eventually results in the safety concerns, including but not limited to large volume change, microcracks accompanying surface modification of the positive electrode, etc.

[0008] In response to the above challenges, CN112635830B (Huizhou Eve Energy Co. Ltd.) discloses an EC-free lithium-ion battery electrolyte comprising 1- propene-1,3-sultone and any one or a combination of at least two of propylene sulfite, tris(trimethylsilane) phosphite, or succinonitrile as positive electrode additives and vinylene carbonate and vinyl sulfate as negative electrode additives.

[0009] Likewise, various approaches have been made to overcome the limitations of commonly used liquid electrolytes based on the organic carbonates, notably EC and PC, which becomes more crucial in case the positive electroactive material has high Ni content, and there still exist outstanding needs for a liquid electrolyte optimal for a lithium secondary battery, which may exhibit high cycle performance and less volume change at high temperature.SUMMARY OF THE INVENTION

[0010] The first object of the present invention is a liquid electrolyte for a lithium secondary battery comprising at least one lithium salt dissolved in a solvent mixture, the solvent mixture comprising:a) from 1.0 to 15.0% by weight (wt%) of at least one cyclic carbonate, devoid of ethylene carbonate and propylene carbonate;b) at least one fluorinated acyclic ester compound; andc) at least one phosphorous-containing chemical compound,the wt% being based on the total weight of the solvent mixture.

[0011] The invention also relates to a lithium secondary battery comprising the liquid electrolyte according to the present invention, and to use of the liquidSPOP 2025 / 008- WO-PCTelectrolyte in improving cycling performance at high temperature of a lithium secondary battery.DETAILED DESCRIPTION OF THE INVENTION

[0012] In the present application, unless otherwise indicated, any specific embodiment or technical feature relating to a subject-matter is applicable to and interchangeable with another embodiment or technical feature also relating to the same subject-matter and disclosed elsewhere in the application.

[0013] Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. In the context of the present invention, the term ‘percent by weight’ (wt%) indicates the content of a specific component in a mixture, calculated as the ratio between the weight of the component and the total weight of the mixture. As used herein, the term ‘percent by volume’ (vol%) indicates the content of a specific component in a mixture, calculated as the ratio between the volume of the component and the total volume of the mixture.

[0014] In the present invention, the term “cut-off voltage” is intended to denote a prescribed lower-limit voltage at which the discharging is considered complete. The cut-off voltage is usually chosen so that the maximum useful capacity of the battery is achieved. The cut-off voltage is different from one battery to the other and highly dependent on the type of batteries.

[0015] In the present invention, the term “normal voltage” is intended to denote the voltage in the range of from 2.5 to 4.3 V, while the term “high voltage” is intended to denote the voltage exceeding 4.3 V.

[0016] It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. Accordingly, various changes and modifications described herein will be apparent to those skilled in the art. Moreover, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.SPOP 2025 / 008- WO-PCT

[0017] The present invention relates to a liquid electrolyte for a lithium secondary battery comprising at least one lithium salt dissolved in a solvent mixture, the solvent mixture comprising:a) from 1.0 to 15.0 wt% of at least one cyclic carbonate, devoid of ethylene carbonate (EC) and propylene carbonate (PC);b) at least one fluorinated acyclic ester compound; andc) at least one phosphorous-containing chemical compound,the wt% being based on the total weight of the solvent mixture.

[0018] In one embodiment, the solvent mixture comprises at least 1.0 wt%, preferably at least 2.0 wt%, more preferably at least 3.0 wt%, and / or at most 15.0 wt%, preferably at most 12.0 wt%, more preferably at most 10.0 wt% of a) at least one cyclic carbonate devoid of EC and PC, the wt% being based on the total weight of the solvent mixture.

[0019] In some embodiments, the solvent mixture comprises from 1.0 to 12.0 wt%, preferably from 1.0 to 10.0 wt% of a) the cyclic carbonate devoid of EC and PC, the wt% being based on the total weight of the solvent mixture.

[0020] In a particular embodiment, a) the cyclic carbonate is at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, 4- fluoroethylene carbonate, and mixtures thereof.

[0021] In another particular embodiment, a) the cyclic carbonate comprises vinylene carbonate and 4-fluoroethylene carbonate.

[0022] In the other particular embodiment, a) the cyclic carbonate comprises 4- fluoroetylene carbonate only.

[0023] In the present invention, the term “devoid of ethylene carbonate (EC) and propylene carbonate (PC)” means that the liquid electrolyte contains EC and PC less than 0.01 wt% based on the total weight of the liquid electrolyte.

[0024] In a preferred embodiment, the liquid electrolyte according to the present invention does not comprise EC and PC.

[0025] In one embodiment, b) at least one fluorinated acyclic ester compound is represented by formula (I):R1-C(O)O-R2(I)wherein R1and R2represent an alkyl group respectively; the sum of carbon atoms in R1and R2is from 2 to 7; R1does not contain fluorine, and R2contains fluorine.SPOP 2025 / 008- WO-PCT

[0026] In one embodiment, R2contains neither a CH2F- group nor a -CHF- group.

[0027] In a preferred embodiment, the number of carbon atom in R1in the formula (I) is 1.

[0028] In another preferred embodiment, the number of carbon atom in R1in the formula (I) is 2.

[0029] Non-limitative examples of suitable fluorinated acyclic carboxylic acid ester according to the present invention include, notably, the followings:CH3-C(O)O-CH2CF2H, CH3-C(O)O-CF2CF3, CH3-C(O)O-CH2CF3, CH3-C(O)O-CF2CF2CF3, (CH3)2CH-C(O)O-CF3, CH3CH2-C(O)O-CF2H, CH3CH2- C(O)O-CF2CH3, CH3-C(O)O-CH(CF3)CH3, CH3CH2-C(O)O-CH2CF2H, CH3-C(O)O-CH2CH2CF2H, CH3-C(O)O-CH2CF2CF2H, CH3CH2-C(O)O-CH2CH2CF2H, CH3CH2-C(O)O-CH2CH2CF2H, CH3-C(O)O-CF2CF2H, CH3-C(O)O-CF2CF2CF2CF2H, CH3CH2-C(O)O-CH2CF2H, CH3CH2CH2-C(O)O-CH2CF3, CH3-C(O)O-CH2CH2CF2CF3, (CH3)2CH-C(O)O-CH2CF2H, CH3CH2CH2-C(O)O-CF2H, (CH3)2CH-C(O)O-CF2H, CH3-C(O)O-CH2CF2H, CH3-C(O)O-CH2CF3, CH3CH2-C(O)O-CH2CH2CF3, CH3CH2-C(O)O-CH2CF3, CH3-C(O)O-CF2CH3, CH3-C(O)O-CH2CF2CH3, CH3CH2-C(O)O-CF2CF2H, and mixtures thereof.

[0030] In a particular embodiment, the fluorinated acyclic carboxylic acid ester is at least one selected from the group consisting of CH3-C(O)O-CH2CF2H, CH3-C(O)O-CF2CF2H, CH3-C(O)O-CH2CF3, CH3-C(O)O-CH2CH2CF2H, CH3-C(O)O-CH2CF2CF2H, CH3-C(O)O-CH2CH2CF2CF3, CH3CH2-C(O)O-CH2CF2H, CH3CH2-C(O)O-CH2CF3, CH3CH2-C(O)O-CH2CH2CF2H, CH3CH2-C(O)O-CH2CH2CF3, CH3-C(O)O-CF2CH3, CH3-C(O)O-CH2CF2CH3, CH3CH2-C(O)O-CF2CF2H, and mixtures thereof.

[0031] In a more particular embodiment, the fluorinated acyclic carboxylic acid ester is CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate).

[0032] In another more particular embodiment, the fluorinated acyclic carboxylic acid ester is CH3CH2-C(O)O-CH2CF2H (2,2-difluoroethyl propionate).

[0033] In the present invention, the solvent mixture may comprise at least 10.0 wt%, preferably at least 20.0 wt%, more preferably at least 50.0 wt%, and / or at most 99.0 wt%, preferably at most 98.0 wt%, more preferably at most 90.0 wt% ofSPOP 2025 / 008- WO-PCTb) the fluorinated acyclic ester compound, the wt% being based on the total weight of the solvent mixture.

[0034] In a particular embodiment, the solvent mixture comprises from 10.0 to 99.0 wt% of b) the fluorinated acyclic ester compound, the wt% being based on the total weight of the solvent mixture.

[0035] In another particular embodiment, the solvent mixture comprises from 20.0 to 90.0 wt% of b) the fluorinated acyclic ester compound, the wt% being based on the total weight of the solvent mixture.

[0036] In some embodiments, the liquid electrolyte comprises a non-fluorinated acyclic ester compound.

[0037] In a particular embodiment, the non-fluorinated acyclic ester compound is at least one selected from the group consisting of ethyl propionate, propyl propionate, ethyl acetate, isobutyl acetate, vinyl acetate, and mixtures thereof.

[0038] In the present invention, c) the phosphorous-containing chemical compound is at least one selected from the group consisting of lithium monofluorophosphate, lithium difluorophosphate (LiPO2F2), lithium trifluoromethane phosphate, lithium tetrafluoro phosphate, lithium difluoro b / s(oxalato) phosphate, lithium tetrafluoro (oxalato) phosphate, lithium tris(oxalato) phosphate [LiP(C2O4)3], lithium tris(difluoromalonato) phosphate [LiP(O2CCF2CO2)3], triphenyl phosphine, tris(trimethylsilyl)phosphate, ethyl diphenylphosphinite, triethyl phosphite, tris(2,2,2-trifluoroethyl) phosphite, cyclic fluorinated phosphazene compounds such as ethoxy (pentafluoro) cyclotriphosphazene, hexafluoro cyclotriphosphazene, pentafluoro (phenoxy) cyclotriphosphazene, and mixtures thereof.

[0039] In a particular embodiment, c) the phosphorous-containing chemical compound is selected from the group consisting of lithium monofluorophosphate, lithium difluorophosphate (LiPO2F2), and lithium trifluoromethane phosphate.

[0040] In a more particular embodiment, c) the phosphorous-containing chemical compound is lithium difluorophosphate (LiPO2F2).

[0041] In the present invention, the solvent mixture may comprise at least 0.1 wt%, preferably at least 0.2 wt%, more preferably at least 0.5 wt%, and / or at most 5.0 wt%, preferably at most 3.0 wt%, more preferably at most 2.0 wt% of c)SPOP 2025 / 008- WO-PCTthe phosphorous-containing chemical compound, the wt% being based on the total weight of the solvent mixture.

[0042] In one embodiment, the solvent mixture comprises from 0.1 to 5.0 wt% of c) the phosphorous-containing chemical compound, the wt% being based on the total weight of the solvent mixture.

[0043] In another embodiment, the solvent mixture comprises from 0.5 to 2.0 wt% of c) the phosphorous-containing chemical compound, the wt% being based on the total weight of the solvent mixture.

[0044] In the present invention, c) at least one phosphorous-containing chemical compound differs from a lithium salt.

[0045] The term “lithium salt” is hereby intended to denote a substance which needs to be dissolved in a solvent to ensure ionic conduction.

[0046] In a lithium secondary battery, a liquid electrolyte consists mainly of lithium salts in a non-aqueous organic solvent, where lithium ions (i.e. Li+cations) are used as charge carriers such that the liquid electrolyte acts as a conductive pathway for the movement of cations, i.e. Li+cations passing from the cathode to anode during the charge. The dissolution of a lithium salt is through solvent- Li+interactions, i.e. the dissociation of Li+cation-(counter)anion interaction is critical. Accordingly, many simple lithium salts are excluded from electrolyte usage because of their strong cation-anion interactions resulting in high lattice energies and thus poor solubility in relevant aprotic solvents, e.g. LiCI, LiF, Li2O, etc.

[0047] The Li+cation conductivity originates from both the total ionic conductivity and the cation transference number. Given that the cation transference number in a non-aqueous organic solvent is low, e.g. usually smaller than 0.5, the ionic conductivity plays a critical role in battery performance.

[0048] In a nutshell, a liquid electrolyte where at least one lithium salt is dissolved in at least one non-aqueous organic solvent plays a pivotal role as one of the major components of a conventional lithium secondary battery.

[0049] Non-limitative examples of the lithium salt according to the present invention include, notably, a lithium ion complex such as lithium hexafluorophosphate (LiPFe), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsFe), lithium hexafluoroantimonate (LiSbFe), lithium hexafluorotantalate (LiTaFe), lithium tetrachloroaluminate (LiAICU), lithium tetrafluoroborate (LiBF4), lithiumSPOP 2025 / 008- WO-PCTchloroborate (Li2B10Cl10), lithium fluoroborate (Li2B10F10), Li2B12FxH12-xwherein x=0-12, LiPFx(RF)6-xand LiBFy(RF)4-ywherein RF represents perfluorinated Ci- 020 alkyl groups or perfluorinated aromatic groups, x=0-5 and y=0-3, lithium bis(oxalato)borate [LiB(C2O4)2], lithium bis(difluoromalonato) borate [LiB(O2CCF2CO2)2], lithium difluorooxalato borate, and lithium fluoromalonato (difluoro)borate, LiPF2[O2C(CX2)nCO2]2, LiPF4[O2C(CX2)nCO2] wherein X is selected from the group consisting of H, F, Cl, C1-C4 alkyl groups and fluorinated alkyl groups, and n=0-4, lithium trifluoromethane sulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), LiN(SO2CmF2m+1)(SO2CnF2n+1) and LiC(SO2CkF2k+1)(SO2CmF2m+1)(SO2CnF2n+1) wherein k=1-10, m=1-10 and n=1-10, LiN(SO2CpF2pSO2) and LiC(SO2CpF2pSO2)(SO2CqF2q+1) wherein p=1-10 and q=1-10, or combinations thereof.

[0050] In one embodiment, the lithium salt according to the present invention is selected from the group consisting of lithium hexafluorophosphate (LiPFe), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsFe), lithium hexafluoroantimonate (LiSbFe), lithium hexafluorotantalate (LiTaFe), lithium tetrachloroaluminate (LiAICU), lithium tetrafluoroborate (LiBF4), lithium chloroborate (Li2B10Cl10), lithium fluoroborate (Li2B10F10), Li2B12FxH12-xwherein x=0-12, lithium trifluoromethane sulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium b / s(trifluoromethanesulfonyl) imide LiN(CF3SO2)2 (LiTFSI), and combinations thereof.

[0051] In one particular embodiment, the lithium salt is LiTFSI.

[0052] In another particular embodiment, the lithium salt is LiFSI.

[0053] In the other particular embodiment, the lithium salt is LiPFe.

[0054] In some embodiments, the lithium salt is the mixture of LiPFe and LiFSI.

[0055] According to one embodiment, the solvent mixture further comprises d) at least one additive.

[0056] In the present invention, d) the additive differs from c) the phosphorous- containing chemical compound.

[0057] In the present invention, d) the additive differs from the lithium salt.

[0058] In a particular embodiment, d) the additive is at least one selected from the group consisting of sulfone derivatives such as dimethyl sulfone,SPOP 2025 / 008- WO-PCTtetramethylene sulfone (also known as sulfolane), ethyl methyl sulfone and isopropyl methyl sulfone; ethylene sulfate; 1,3-propane sultone, 1-propene- 1,3-sultone; nitrile derivatives such as succinonitrile, adiponitrile, and glutaronitirle; and lithium nitrate (LiNOs); boron derivatives salt such as lithium difluoro oxalato borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium fluoromalonato (difluoro)borate (LiB(O2CCHFCO2)2; LiFMDFB), lithium bis(malonato)borate [LiB(O2CCH2CO2)2], lithium bis(difluoromalonato) borate [LiB(O2CCF2CO2)2], lithium (malonatooxalato) borate [LiB(C2O4)(O2CCH2CO2)], lithium (difluoromalonatooxalato) borate [LiB(C2O4)(O2CCF2CO2)]; biphenyl benzene, isopropyl benzene, hexafluorobenzene, tris(pentafluorophenyl) borane, maleic anhydride, cesium bis(trifluoromethanesulfonyl)imide (CsTFSI), cesium fluoride (CsF), 1,3,6- hexanetricarbonitrile (HTCN), and mixtures thereof

[0059] In the present invention, the total amount of d) the additive may be from 0 to 10.0 wt%, preferably from 0 to 8.0 wt%, and more preferably from 0 to 5.0 wt%, the wt% being based on the total weight of the liquid electrolyte.

[0060] The total amount of d) the additive, if contained in the liquid electrolyte of the present invention, is from 0.05 to 5.0 wt%, preferably from 0.05 to 3.0 wt%, and more preferably from 0.05 to 2.0 wt%, the wt% being based on the total weight of the liquid electrolyte.

[0061] In a preferred embodiment, the total amount of d) the additive accounts for at least 0.5 wt% of the liquid electrolyte.

[0062] In the present invention, the solvent mixture may further comprise e) at least one acyclic carbonate.

[0063] In some embodiments, e) the acyclic carbonate is at least one selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and mixtures thereof.

[0064] As used herein, the molar concentration or the molarity is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of the amount of substance per unit volume of solution. The most used unit for molarity is the number of moles per liter, having the unit of mol / L. A solution with a concentration of 1.0 mol / L is indicated as 1 molar and designated as 1.0 M.SPOP 2025 / 008- WO-PCT

[0065] In one embodiment, a molar concentration (M) of a lithium salt in the liquid electrolyte according to the present invention is from 1.0 M to 4.0 M, preferably from 1.0 M to 2.0 M.

[0066] A second object of the present invention is a lithium secondary battery comprising the liquid electrolyte according to the present invention.

[0067] The lithium secondary battery as disclosed herein can be used in a variety of applications. For instance, the lithium secondary battery can be used for grid storage or as a power source in various electronically powered or assisted devices (electronic devices) such as a computer, a camera, a power tool, etc., a telecommunication device or a transportation device including a motor vehicle, an electric vehicle (EV), an urban air mobility (UAM), an airplane, etc. The present invention also relates to an electronic device, a transportation device or a telecommunication device comprising a lithium secondary battery according to the present invention.

[0068] In one embodiment, a lithium secondary battery comprises a liquid electrolyte according to the present invention, a positive electrode, a negative electrode, and a separator that is positioned between the positive electrode and the negative electrode.

[0069] In the present invention, the term “separator” is intended to denote, in particular, an ionically permeable membrane placed between a positive electrode and a negative electrode. Its function is to be permeable to the lithium ions while blocking electrons and assuring the physical separation between the electrodes. That is, the separator refers to a monolayer or multilayer in a polymeric, nonwoven cellulose or ceramic material / film, which electrically and physically separates the electrodes having opposite polarities in an electrochemical device and is permeable to ions flowing between them.

[0070] An electrode in an electrochemical cell is referred to as either an anode or cathode. The anode is defined as the electrode where electrons leave the cell and oxidation occurs, and the cathode as the electrode where electrons enter the cell and reduction occurs. Each electrode may become either an anode or a cathode depending on the direction of electric current through a cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell. When a cell is being charged, the anode becomes the positive electrode and the cathode becomes the negative electrode, whileSPOP 2025 / 008- WO-PCTwhen a cell is being discharged, the anode becomes the negative electrode, and the cathode becomes the positive electrode.

[0071] In the present invention, the term “negative electrode” is intended to denote, in particular, the electrode of an electrochemical cell, where oxidation occurs during discharging.

[0072] In the present invention, the term “positive electrode” is intended to denote, in particular, the electrode of an electrochemical cell, where reduction occurs during discharging.

[0073] A third object of the present invention relates to use of the liquid electrolyte according to the present invention in improving cycling performance at high temperature (typically 45 °C ) of a lithium secondary battery. The liquid electrolyte according to the present invention also contributes to decreasing thickness growth of a lithium secondary battery (typically 60°C, after 4 weeks) as demonstrated in the following experimental section, that can be interpreted as having less formation of gas such as CO, O2, CO2, etc.

[0074] In one embodiment, the liquid electrolyte according to the present invention exhibits at least 90.0% of capacity retention at 100 cycles at 45°C. In the other embodiment, the liquid electrolyte according to the present invention exhibits at least 80.0% of capacity retention at 300 cycles at 45°C.

[0075] In one embodiment, the liquid electrolyte according to the present invention exhibits at most 65.0% of thickness change after 2 weeks at 60°C. Advantageously, the liquid electrolyte according to the present invention exhibits at most 50.0% of thickness change after 2 weeks at 60°C.

[0076] In the other embodiment, the liquid electrolyte according to the present invention exhibits at most 80.0% of thickness change after 4 weeks at 60°C. Advantageously, the liquid electrolyte according to the present invention exhibits at most 70.0% of thickness change after 4 weeks at 60°C.

[0077] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of theSPOP 2025 / 008- WO-PCTpresent application to the extent that it may render a term unclear, the present description shall take precedence.

[0078] The invention will be now explained in more detail with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.EXPERIMENTAL SECTION

[0079] Raw MaterialsDFEA: a fluorinated acyclic ester of CH3-C(O)O-CH2CF2H, synthesized within Syensqo;FEC: 4-fluoroethylene carbonate, commercially available from Soulbrain, EC: ethylene carbonate, commercially available from Soulbrain;PC: propylene carbonate, commercially available from Soulbrain;VC: vinylene carbonate, commercially available from Soulbrain;EMC: ethyl methyl carbonate, commercially available from Dongwha;DEC: diethyl carbonate, commercially available from Dongwha;LiPFe: lithium hexafluorophosphate, commercially available from Soulbrain; LiFSI: lithium b / s(fluorosulfonyl)imide, commercially available from Soulbrain; UPO2F2: lithium difluorophosphate, commercially available from Soulbrain.

[0080] Preparation of liquid electrolytes

[0081] The liquid electrolytes consisting of the components as disclosed in Table I below were prepared by simple mixing. All required components were added to one bottle and mixed using magnetic stirrer, until the solution became transparent.

[0082] Injections of liquid electrolytes to dry cells & Ageing for wetting

[0083] The liquid electrolytes were injected into 1Ah pouch cells (NCA / artificial graphite + 10% SiOx dry cells, commercially available from LiFun). The cells were left in a vacuum container for wetting the liquid electrolyte to the electrodes in the dry cells. After releasing vacuum, the cells were left for sufficient time for wetting. Subsequently, the cells were sealed by vacuum sealing machine and were left at room temperature for an additional 1 day (1stageing).Table 1SPOP 2025 / 008- WO-PCT(in wt%) Li salt(s) EC PC FEC VC EMC DEC DFEA UPO2F2* E1 0.8M LiPF6& 0 0 4 0 0 0 96 0.5 E2 0.4M LiFSI 0 0 4 0 48 0 48 0.5 E3 0 0 2 0 0 0 98 0.51M LPF6E4 0 0 0 2 0 0 98 0.5 CE1 16 0 4 0 40 40 0 0.5 CE2 0.8M LiPF6& 16 0 4 0 0 0 80 0.5 CE3 0.4M LiFSI 0 0 4 0 96 0 0 0.5 CE4 10 0 4 0 38 0 48 0.5 CE5 5 0 4 0 0 0 91 0.5 CE6 2.5 2.5 4 0 0 0 91 0.51M LPF6CE7 0 5 4 0 0 0 91 0.5 CE8 5 0 0 0 0 0 95 0.5*LiPO2F2was added to the solvent mixture and was dissolved in an amount of 0.5 wt%, the wt% being based on the total weight of solvent mixture.

[0084] Activation of cells and measurement of initial cell performance

[0085] 1- Formation (Activation of lithium-ion cells): Aged pouch cells were charged to 30% charging level (SOC30%) and then the pouch cells were left at room temperature for one day (2ndageing).

[0086] 2-Degassing: Gas generated during the formation within the pouch cell was removed by opening the cell, which was re-sealed.

[0087] 3- Measurement of cycling performance: The cycling ability of each pouch cell was evaluated. Then, each cell was subjected to a repetition of cycles of charging and discharging. One cycle consisted of a charging phase at a charging current of C, followed by a discharging phase at a discharge current of C.

[0088] Performance Measurement of the pouch cells

[0089] 1 -Cycle performance

[0090] The pouch cells were tested at 45°C up to 300 cycles under conditions as detailed below:Charging: 1C / 4.2V / 0.05C at constant current and constant voltage (CC-CV)SPOP 2025 / 008- WO-PCTDischarging: 1 C / 3. OV (CC)

[0091] The capacity retention (%) was measured after 100 cycles and after 300 cycles, respectively. The results were recorded in Table 2 below.

[0092] 2-Storage test at 60°C (after 4 weeks)

[0093] Cells were charged to 4.2V and were transferred to thermal chamber for storage test. Thickness of the cells was measured on a weekly basis for four (4) weeks and recorded in Table 2 below.Table 2Capacity retention (%) Thickness change (%)100 cycles at 45°C 300 cycles at 45°C 2 weeks at 60°C 4 weeks at 60°C E1 94.1 88.5 8.0 35.0E2 90.8 80.6 61.0 78.0E3 94.7 89.1 20.0 26.0E4 92.5 84.4 47.0 66.0 CE1 85.8 75.4 97.0 141.0 CE2 87.9 78.0 78.0 120.0 CE3 89.9 77.7 41.0 99.0 CE4 86.1 76.0 82.0 103.0 CE5 87.9 77.9 83.0 128.0 CE6 90.4 80.0 69.0 113.0 CE7 92.0 83.1 68.0 101.0 CE8 76.4 65.1 11.0 13.0

[0094] The liquid electrolytes according to the present invention (E1-E4) showed good balance between capacity retention and thickness change, i.e. good capacity retention at high temperature (45°C) and the thickness change at 60°C, which clearly demonstrated that the liquid electrolytes according to the present invention contributed in decreasing the gas generation within the cellsSPOP 2025 / 008- WO-PCTin comparison to the comparative examples, while maintaining good capacity retention at high temperature.

Claims

SPOP 2025 / 008- WO-PCTCLAIMS1. A liquid electrolyte for a lithium secondary battery comprising at least one lithium salt dissolved in a solvent mixture, the solvent mixture comprising:a) from 1.0 to 15.0% by weight (wt%) of at least one cyclic carbonate, devoid of ethylene carbonate and propylene carbonate;b) at least one fluorinated acyclic ester compound; andc) at least one phosphorous-containing chemical compound, selected from the group consisting of lithium monofluorophosphate, lithium difluorophosphate (LiPO2F2), lithium trifluoromethane phosphate, lithium tetrafluoro phosphate, lithium difluoro b / s(oxalato) phosphate, lithium tetrafluoro (oxalato) phosphate, lithium tns(oxalato) phosphate [LiP(C2O4)3], lithium tris(difluoromalonato) phosphate [LiP(O2CCF2CO2)3], triphenyl phosphine, tr / s(trimethylsilyl) phosphate, ethyl diphenylphosphinite, triethyl phosphite, tris(2,2,2- trifluoroethyl) phosphite, cyclic fluorinated phosphazene compounds such as ethoxy (pentafluoro) cyclotriphosphazene, hexafluoro cyclotriphosphazene, and pentafluoro (phenoxy) cyclotriphosphazene, and mixtures thereof;the wt% being based on the total weight of the solvent mixture.

2. The liquid electrolyte according to claim 1, wherein the solvent mixture comprises from 1.0 to 10.0 wt% of a) the cyclic carbonate, the wt% being based on the total weight of the solvent mixture.

3. The liquid electrolyte according to claim 1 or 2, wherein a) the cyclic carbonate is at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, 4-fluoroethylene carbonate, and mixtures thereof.

4. The liquid electrolyte according to any one of the preceding claims, wherein b) the fluorinated acyclic ester compound is represented by formula (I):R1-C(O)O-R2(I)SPOP 2025 / 008- WO-PCTwherein R1and R2represent an alkyl group respectively; the sum of carbon atoms in R1and R2is from 2 to 7; R1does not contain fluorine, and R2contains fluorine.

5. The liquid electrolyte according to claim 4, wherein the fluorinated acyclic ester compound is at least one selected from the group consisting of CH3-C(O)O-CH2CF2H, CH3-C(O)O-CF2H, CH3-C(O)O-CF2CF3, CH3-C(O)O-CH2CF3, CH3-C(O)O-CF3, CH3-C(O)O-CF2CF2CF3, (CH3)2CH-C(O)O-CF3, CH3CH2-C(O)O-CF2H, CH3CH2-C(O)O-CF3, CH3CH2-C(O)O-CF2CH3, CH3-C(O)O-CH(CF3)CH3, CH3CH2-C(O)O-CH2CF2H, CH3-C(O)O-CH2CH2CF2H, CH3-C(O)O-CH2CF2CF2H, CH3CH2-C(O)O-CH2CH2CF2H, CH3-C(O)O-CF2CF2H, CH3-C(O)O-CF2CF2CF2CF2H, CH3CH2CH2-C(O)O-CH2CF3, CH3-C(O)O-CH2CH2CH2CF2CF3, (CH3)2CH-C(O)O-CH2CF2H, CH3CH2CH2-C(O)O-CF2H, (CH3)2CH-C(O)O-CF2H, CH3CH2-C(O)O-CH2CF3, CH3-C(O)O-CF2CH3, CH3-C(O)O-CH2CF2CH3, CH3CH2-C(O)O-CF2CF2H, and mixtures thereof.

6. The liquid electrolyte according to claim 4 or 5, wherein b) the acyclic ester compound further comprises a non-fluorinated acyclic ester compound.

7. The liquid electrolyte according to claim 6, wherein the non-fluorinated acyclic ester compound is at least one selected from the group consisting of ethyl propionate, propyl propionate, ethyl acetate, isobutyl acetate, vinyl acetate, and mixtures thereof.

8. The liquid electrolyte according to any one of the preceding claims, wherein the solvent mixture comprises from 10.0 to 99.0 wt%, preferably from 20.0 to 90.0 wt% of b) the acyclic ester compound, the wt% being based on the total weight of the solvent mixture.

9. The liquid electrolyte according to any one of the preceding claims, wherein the solvent mixture comprises from 0.1 to 5.0 wt%, preferably from 0.5 to 2.0 wt% of c) the phosphorous-containing chemical compound, the wt% being based on the total weight of the solvent mixture.SPOP 2025 / 008- WO-PCT10. The liquid electrolyte according to any one of the preceding claims, wherein the lithium salt is at least one selected from the group consisting of lithium hexafluorophosphate (LiPFe), lithium perchlorate (l_iCIC>4), lithium hexafluoroarsenate (LiAsFe), lithium hexafluoroantimonate (LiSbFe), lithium hexafl uorotantalate (LiTaFe), lithium tetrachloroaluminate (LiAIC ), lithium tetrafluoroborate (LiBF4), lithium chloroborate (Li2B10Cl10), lithium fluoroborate (Li2B10F10), Li2B12FxH12-xwherein x=0-12, UPFX(RF)6-X and LiBFy(RF)4-ywherein RF represents perfluorinated C1-C20 alkyl groups or perfluorinated aromatic groups, x=0-5 and y=0-3, LiBF2[O2C(CX2)nCO2], LiPF2[O2C(CX2)nCO2]2, LiPF4[O2C(CX2)nCO2], wherein X is selected from the group consisting of H, F, Cl, C1-C4 alkyl groups and fluorinated alkyl groups, and n=0-4, lithium trifluoromethane sulfonate (LiCF3SO3), lithium b / s(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), UN(SO2CmF2m+i)(SO2CnF2n+i) and LiC(SO2CkF2k+i)(SO2CmF2m+i)(SO2CnF2n+i) wherein k=1 -10, m=1-10 and n=1-10, LiN(SO2CPF2pSO2) and LiC(SO2CpF2pSO2)(SO2CqF2q+i) wherein p=1-10 and q=1 -10, and mixtures thereof.

11. The liquid electrolyte according to any one of the preceding claims, wherein the solvent mixture further comprises d) at least one additive selected from the group consisting of sulfone derivatives such as dimethyl sulfone, tetramethylene sulfone (also known as sulfolane), ethyl methyl sulfone and isopropyl methyl sulfone; ethylene sulfate; 1,3-propane sultone, 1-propene-1,3-sultone; nitrile derivatives such as succinonitrile, adiponitrile, and glutaronitirle; and lithium nitrate (LiNOs); boron derivatives salt such as lithium difluoro oxalato borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium fluoromalonato (difluoro)borate (LiB(O2CCHFCO2)2; LiFMDFB), lithium bis(malonato)borate [LiB(O2CCH2CO2)2], lithium b / s(difluoromalonato) borate [LiB(O2CCF2CO2)2], lithium (malonatooxalato) borate [LiB(C2O4)(O2CCH2CO2)], lithium (difluoromalonatooxalato) borate [LiB(C2O4)(O2CCF2CO2)], biphenyl benzene, isopropyl benzene, hexafluorobenzene, tris(pentafluorophenyl) borane, maleic anhydride, cesium bis(trifluoromethanesulfonyl)imide (CsTFSI), cesium fluoride (CsF), 1,3,6-hexanetricarbonitrile (HTCN), and mixtures thereof.SPOP 2025 / 008- WO-PCT12. The liquid electrolyte according to any one of the preceding claims, wherein the solvent mixture further comprises e) at least one acyclic carbonate selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and mixtures thereof.

13. A lithium secondary battery comprising a liquid electrolyte according to any one of the preceding claims.

14. Use of the liquid electrolyte defined in any one of claims 1 to 12 in improving cycling performance at high temperature of a lithium secondary battery.

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