Lithium secondary battery with high energy density and safety

Abstract

The present invention relates to a lithium secondary battery comprising a) a positive electrode comprising a nickel-rich lithium transition metal oxide as a positive electroactive material, b) a negative electrode, and c) a liquid electrolyte comprising at least one lithium salt dissolved in a solvent mixture comprising at least one organic carbonate and from 0.1 to 50.0% by volume (vol%) of at least one fluorinated acyclic di-ether, vol% being based on the total volume of the solvent mixture. The present invention also relates to use of the liquid electrolyte according to the present invention in improving safety for a lithium secondary battery and to an electronic device, a transportation device or a telecommunication device comprising a lithium secondary battery of the present invention.

Description

LITHIUM SECONDARY BATTERY WITH HIGH ENERGY DENSITY AND SAFETYCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to European patent application No.23219528.9 filed on December 21 , 2023, the whole content of this application being incorporated herein by reference for all purposes.TECHNICAL FIELD

[0002] The present invention relates to a lithium secondary battery comprising a) a positive electrode comprising a nickel-rich lithium transition metal oxide as a positive electroactive material, b) a negative electrode, and c) a liquid electrolyte comprising at least one lithium salt dissolved in a solvent mixture comprising at least one organic carbonate and from 0.1 to 50.0% by volume (vol%) of at least one fluorinated acyclic di-ether, vol% being based on the total volume of the solvent mixture. The present invention also relates to use of the liquid electrolyte according to the present invention in improving safety for a lithium secondary battery and to an electronic device, a transportation device or a telecommunication device comprising a lithium secondary battery of the present invention.BACKGROUND OF THE INVENTION

[0003] Lithium-ion batteries have retained 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.

[0004] In parallel, higher energy density has been continuously required pursuant to the development of high power applications such as electrical vehicles (EVs), hybrid electrical vehicles, grid energy storages, etc. and these requirements keep increasing according to the strong demand from the industry.

[0005] The energy density is defined by capacity multiplied by voltage. Accordingly, the energy density can be increased by increasing capacity and / or voltage. Voltage of a lithium secondary battery is decided by the potential difference between the positive electrode and the negative electrode, among which thepositive electroactive materials, serving as the main provider of lithium ions, substantially determines the energy capacity of a lithium secondary battery.

[0006] LiCoO2 (LCO, hereinafter) has been substantially used as a positive electroactive material for a lithium secondary battery, thanks to its high capacity and high density, as well as relatively easy processing for the manufacture. However, LCO also has drawbacks, mainly due to the scarcity of Co resources and its high cost. Accordingly, it becomes necessary to find an alternative, preferably one with higher energy capacity to meet the ever increasing demand from the industry.

[0007] Among various positive electroactive materials such as lithium metal oxide compounds with layered structures, notably ternary positive electroactive materials, such as LiNixCoyMni-x-yO2 (NCM, hereinafter) and LiNixCoyAli-x-yO2 (NCA, hereinafter); those with spinel structure such as LiMn2O4 and Lii+xMn2- yO4; and those with olivine-type structures such as LiFePCM, the positive electroactive materials having high content of Ni and hence providing desired specific capacity have been highlighted as promising candidates to satisfy the needs. Nonetheless, this also comes with a downside, i.e. the adverse impact on the thermal stability and long-term cycling performance. This is because the residual lithiums accumulate on the surface of a Ni-rich compound over time when exposed to air, which becomes detrimental to the electrode making as well as cyclability. Subsequently, a safety issue arises.

[0008] In addition, increasing the cut-off voltage has been tried to improve the energy density. In this regard, at the cut-off voltage higher than about 4.2 V, the electrolyte system becomes often deteriorated because the components of the electrolyte, such as a solvent, a conducting 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, while the battery which can be operated at higher voltage (for instance, up to 5.0 V) is desired in the art. Accordingly, the development of liquid electrolytes suitable for the high- voltage batteries and / or the components for such a liquid electrolyte has been consistently required in the art.

[0009] However, notably at such a high voltage, the positive electroactive materials 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 and safety. This is because the high voltage accelerates the gas formation such as O2, CO, and CO2 that eventually brings about safety concerns, including but not limited to large volume change, microcracks accompanying surface modification of the positive electrode, etc.

[0010] In response to the above diverse challenges, C. Liao et. al. proposed several approaches such as element doping, surface coating, single-crystal fabrication, structural design change, and use of multifunctional electrolyte additives in “Challenges and modification strategies of Ni-rich cathode materials operating at high voltage (Nanomaterials, 2022, 12, 1888)”. In parallel, the decrease of the electrical conductivity was observed as the content of Ni increases such that use of well-dispersed carbon nanotubes (CNTs) as a conductive agent in a cathode of LiNiO.8Coo.1Mno.1O2 (NMC811 , hereinafter), while controlling the solid content within the cathode, was proposed by J. H. Choi et. al. in “Improved electrochemical performance using well-dispersed carbon nanotubes as conductive additive in the Ni-rich positive electrode of lithium-ion batteries (Electrochemistry Communications 146 (2023) 107419)”.

[0011] WO2020 / 168764A (Ningde Amperex Technology Limited) discloses an electrochemical device comprising an anode comprising an anode current collector, an insulation layer and a skeleton layer; a cathode, and an electrolyte comprising a sulfone compound, a phosphorus compound and a fluoroether compound, wherein the skeleton layer is preferably selected from porous carbon, carbon nanotubes, carbon fibers and hollow carbon spheres. Preferably, WO’764A discloses that the skeleton structure may accommodate deposited lithium metal without significant changes in the thickness of the anode, so that the problem of interface stripping caused by thickness shrinkage can be substantially reduced, while the current density per unit area is significantly lowered thereby inhibiting the formation of lithium dendrites. In addition, the insulation layer on an edge portion of the anode current collectorenables to reduce uneven current distribution, thereby reducing the risk of short circuit and thus significantly improving the safety performance of the batteries.

[0012] In the meantime, organic carbonates have been conventionally used as liquid electrolytes for lithium secondary batteries, for instance acyclic carbonates, such as ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, and cyclic carbonates, such as ethylene carbonate or propylene carbonate. However, these organic carbonates relatively easily decompose at certain voltage, e.g. above 4.3V. Typically, driving the electrodes to higher / extreme voltage or exposing the cells to higher temperatures accelerates undesired reactions between liquid electrolytes and highly reactive electrodes that may result in reduced cycle life and capacity reduction. In a worst case scenario, a thermal runaway may occur, which accompanies fire / flame, followed by cell rupture / explosion and eventually by cell disintegration. Notably, such safety concerns are mainly because of the use of organic carbonates having relatively low boiling point and high flammability.

[0013] Accordingly, various approaches have been made to overcome the limitations of commonly used liquid electrolytes based on the organic carbonates, 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 comprising a positive electroactive material having high content of Ni, in particular at normal voltage, which nonetheless may exhibit high energy density and high thermal safety performance.SUMMARY OF THE INVENTION

[0014] A first object of the present invention is to a lithium secondary battery comprising a) a positive electrode comprising a nickel-rich lithium transition metal oxide as a positive electroactive material, b) a negative electrode, and c) a liquid electrolyte comprising at least one lithium salt dissolved in a solvent mixture comprising at least one organic carbonate and from 0.1 to 50.0% by volume (vol%) of at least one fluorinated acyclic di-ether, vol% being based on the total volume of the solvent mixture.

[0015] Another object of the present invention is use of the liquid electrolyte according to the present invention in improving safety for a lithium secondary battery.

[0016] The other object of the present invention is an electronic device, a transportation device, or a telecommunication device comprising a lithium secondary battery of the present invention.

[0017] It was surprisingly found by the inventors that a liquid electrolyte comprising at least one lithium salt dissolved in a solvent mixture comprising from 0.1 to 50.0 vol% of at least one fluorinated acyclic di-ether, vol% being based on the total volume of the solvent mixture according to the present invention, may deliver a particularly advantageous combination of properties, when used in a lithium secondary battery comprising a positive electroactive material having high content of Ni at normal voltage.DETAILED DESCRIPTION OF THE INVENTION

[0018] 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, and 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.

[0019] 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.

[0020] The present invention provides a lithium secondary battery comprising: a) a positive electrode comprising, as a positive electroactive material, a lithium transition metal oxide represented by the following formula (I)LiNii-x-y-zMnxCoyAlzO2 (I) wherein x>0; y>0; z>0; 0<x+y+z<0.50; and Ni, Mn, Co, and / or Al are optionally doped with at least one metal selected from the group consisting of Sc, Ti, V, Cr, Fe, Cu, Zn, Mg, Al, Sn, B, Ga, Sr, Ca, In, Si, Zr, La, P, Nb, and Ge, respectively; b) a negative electrode; and c) a liquid electrolyte comprising at least one lithium salt dissolved in a solvent mixture comprising at least one organic carbonate and at least one fluorinated acyclic di-ether represented by the formula (II)R1-O-R2-O-R3(II) wherein each R1and R3is a fluorinated straight-chain alkyl group, respectively; R2is a straight-chain alkyl group, optionally fluorinated; and the sum of carbon atoms in R1, R2and R3is from 4 to 10, preferably from 5 to 8, more preferably 6; and wherein the fluorinated acyclic di-ether is in an amount of from 0.1 to 50.0% by volume (vol%), based on the total volume of the solvent mixture.

[0021] 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, while the term “negative electrode” is intended to denote, in particular, the electrode of an electrochemical cell, where oxidation occurs during discharging.

[0022] In the present invention, the term “electroactive material” is intended to denote a material that is able to incorporate or insert into its structure and substantially release therefrom lithium ions during the charging and discharging phases in a battery.

[0023] An electrode in a lithium secondary battery 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 acell. A bipolar electrode is an electrode that functions as an anode of one cell and as an cathode of another cell. When a cell is being charged, the anode becomes the positive electrode and the cathode becomes the negative electrode, while when a cell is being discharged, the anode becomes the negative electrode and the cathode becomes the positive electrode.

[0024] 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 to achieve the maximum useful capacity of a battery. The cut-off voltage is different from one battery to the other and highly dependent on the type of batteries.

[0025] 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.

[0026] In the present invention, a positive electrode comprises, as a positive electroactive material, a lithium transition metal oxide represented by the following formula (I)LiNii-x-y-zMnxCoyAlzO2 (I) wherein x>0; y>0; z>0; 0<x+y+z<0.50; and Ni, Mn, Co, and / or Al are optionally doped with at least one metal selected from the group consisting of Sc, Ti, V, Cr, Fe, Cu, Zn, Mg, Al, Sn, B, Ga, Sr, Ca, In, Si, Zr, La, P, Nb, and Ge, respectively.

[0027] The positive electrode according to the present invention corresponds to a Ni-rich or high-Ni positive electrode comprising a positive electroactive material having high content of Ni. In the present invention, “Ni-rich” or “high- Ni” is intended to denote that the content of Ni within a positive electroactive material is high, wherein x+y+z is at most 0.50 in LiNii-x-y-zMnxCoyAlzO2.

[0028] In a particular embodiment, 0.05<x+y+z<0.40.

[0029] In a more particular embodiment, 0.08<x+y+z<0.25.

[0030] In another more particular embodiment, 0.10<x+y+z<0.20.

[0031] In a particular embodiment, the positive electroactive material is selected from the group consisting of LiNio.8Mno.1Coo.1O2 and LiNi0.9Mn0.05Co0.05O2.

[0032] In another particular embodiment, the positive electroactive material is selected from the group consisting of LiNi0.92Mn0.04Co0.03AI0.01 O2, LiNi0.9Mn0.045Co0.045AI0.01O2, LiNi0.89Co0.08AI0.03O2, LiNi0.88Co0.09AI0.03O2, and LiNio .815C00.15AI0.035O2.

[0033] In some embodiments, b) the negative electrode comprises, as a negative electroactive material, graphite, metallic lithium or a composite material of at least a carbon material and at least a silicon material.

[0034] In a particular embodiment, b) the negative electrode comprises metallic lithium as a negative electroactive material.

[0035] In another particular embodiment, b) the negative electrode comprises, as a negative electroactive material, a composite material of at least a carbon material and at least a silicon material, selected from the group consisting of Si / C, SiOa / C, and Si / SiOa / C, with 0<a<2.

[0036] The carbon material should be able to absorb and desorb lithium ions. Non- limitative examples of the carbon material include graphite, amorphous carbon, diamond-like carbon, or a complex thereof. Carbon materials typically commercialized for a negative electroactive material are mesocarbon microbead (MCMB), mesophase-pitch-based carbon fiber (MCF) and massive artificial graphite (MAG). In one embodiment, carbon materials comprise between 2.0 and 99.0 wt%, more preferably between 2.0 and 97.0 wt%, wt% being based on the total weight of the negative electroactive material. In the other embodiment, carbon materials comprise between 2.0 and 30.0 wt%, or between 30.0 and 50.0 wt%, or between 50.0 and 97.0 wt%, wt% being based on the total weight of the negative electroactive material.

[0037] The silicon material should be able to absorb and desorb lithium ion and / or should be able to alloy with lithium. The silicon material may be metallic silicon (“Si”) or silicon oxide (“SiOa”, 0<a<2), or mixtures thereof. Metallic silicon may preferably consist of between 3.0 and 90.0 wt%, more preferably between 3.0 and 50.0 wt%, based on the total weight of the negative electroactive material. Metallic silicon may consist of either between 3.0 and 20.0 wt%, or between 20.0 and 50.0 wt%, or between 50.0 and 90.0 wt% of the negative electroactive material. Silicon oxide, i.e. SiOa (with 0<a<2), may preferably consist of between 3.0 and 90.0 wt%, more preferably between 3.0 and 50.0wt%, based on the total weight of the negative electroactive material. Silicon oxide may consist of between 3.0 and 40.0 wt%, or between 40.0 and 70.0 wt%, or between 70.0 and 90.0 wt%, based on the total weight of the negative electroactive material.

[0038] A composite material of at least a carbon material and at least a silicon material, i.e. silicon carbon composite (SiC), may preferably consist of between 3.0 and 90.0 wt%, more preferably between 3.0 and 50.0 wt%, based on the total weight of the negative electroactive material.

[0039] Methods to make such composite materials are based on mixing the individual ingredients (e.g. C and Si and / or SiOa, or a precursor for the intended matrix material) while preparing an electrode-forming composition, or by a separate composite manufacturing step that is then carried out via dry milling / mixing of at least a carbon material and at least a silicon material (which may be followed by a firing step), or via wet milling / mixing of at least a carbon material and at least a silicon material (followed by removal of the liquid medium and possibly by a firing step).

[0040] In one embodiment, a) the positive electrode and / or b) the negative electrode comprise as a conductive agent, at least one carbon material having a specific surface area of 100 to 1200 m2 / g, preferably 300 to 1000 m2 / g, measured pursuant to the method ISO9277.

[0041] Advantageously, a conductive agent is a carbon material having a specific surface area of at least 200 m2 / g, at least 250 m2 / g, at least 300 m2 / g, at least 400 m2 / g, or at least 500 m2 / g, and / or at most 1100 m2 / g, at most 1000 m2 / g, at most 900 m2 / g, or at most 800 m2 / g, measured pursuant to the method ISO9277.

[0042] Specific surface area (SSA) is a physical property that may be used to determine the type / property of a solid and is defined as the total surface area of a material per unit mass (m2 / kg or m2 / g). SSA of a carbon material is determined by the most commonly used adsorption-based method, i.e. BET (Brunauer-Emmett-Teller) adsorption method as stipulated in ISO9277.

[0043] In another embodiment, a) the positive electrode and / or b) the negative electrode comprise as a conductive agent, at least one carbon material having a ratio (ID / IG ratio) between the intensity of the D-band (ID) and the intensity ofthe G-band (IG) of 0.01 or more, characterized by Raman spectroscopy defined in the ISO / TS 80004-13.

[0044] In a particular embodiment, the ID / IG ratio is from 0.01 to 2.0.

[0045] In a more particular embodiment, the ID / IG ratio is from 0.01 to 1 .0.

[0046] In a preferred embodiment, a) the positive electrode and / or b) the negative electrode comprise as a conductive agent, at least one carbon material having a specific surface area of 100 to 1200 m2 / g, preferably 300 to 1000 m2 / g, measured pursuant to the method ISO9277 and having a ratio (ID / IG) between the intensity of the D-band (ID) and the intensity of the G-band (IG) of 0.01 or more, characterized by Raman spectroscopy defined in the ISO / TS 80004-13.

[0047] Raman spectroscopy is a spectroscopic technique typically used to determine vibrational modes of molecules and hence to provide chemical and structural fingerprints by which molecules can be identified. When light is scattered by molecules, the oscillating electromagnetic field of a photon induces a polarization of the molecular electron cloud which leaves the molecule in a higher energy state with the energy of the photon transferred to the molecule. Such a so-called virtual state of the molecule is unstable and the photons are re-emited almost immediately, as scattered light.

[0048] In particular, the G-band refers to the primary mode in carbon materials, for instance graphene, which represents the planar configuration of sp2-bonded carbon that constitutes graphene, in other words, graphite-type lattice vibrations. D-band is also known as a disorder band or a defect band, which represents the presence of defects in graphitic structure resulting from out-of- plane vibration. Notably, the intensity of the D-band (ID) is directly proportional to the level of defects in the structure of carbon materials.

[0049] In the present invention, the term “conductive agent” is intended to denote, in particular, a material which is used to ensure electrodes that have good charging and discharging performance, and to provide additional electrical conductivity.

[0050] Non-limitative examples of the conductive agents are carbonaceous materials and metals in powders or in fibers, for instance carbon nanotubes (CNT), vapor-grown carbon fibers (VGCF), graphite, graphene, graphene oxide, graphite fibers and the like.

[0051] In some embodiments, the conductive agent is at least one selected from the group consisting of carbon nanotube (CNT), graphene, and graphene oxide.

[0052] In a particular embodiment, the conductive agent is at least one selected from the group consisting of mono-layer graphene, multi-layer graphene, mono- layer graphene oxide and multi-layer graphene oxide.

[0053] In another particular embodiment, the conductive agent is CNT.

[0054] In one preferred embodiment, the conductive agent is single-walled CNT (SWCNT).

[0055] In the other preferred embodiment, the conductive agent is multi-walled CNT (MWCNT).

[0056] In some embodiments, the conductive agent is included in an electrode in an amount of from 0.01 to 2.0 wt%, preferably from 0.1 to 1.5 wt%, wt% being based on the total weight of the electrode.

[0057] In a particular embodiment, the conductive agent is included in an electrode in an amount of from 0.5 to 1.0 wt%, wt% being based on the total weight of the electrode.

[0058] In the present invention, c) the liquid electrolyte comprises at least one lithium salt dissolved in a solvent mixture comprising at least one fluorinated acyclic di-ether represented by the formula (II)R1-O-R2-O-R3(II) wherein each R1and R3is a fluorinated straight-chain alkyl group, respectively; R2is a straight-chain alkyl group, optionally fluorinated; and the sume of carbon atoms in R1, R2and R3is from 4 to 10, preferably from 5 to 8, more preferably 6; and wherein the fluorinated acyclic di-ether is in an amount of from 0.1 to 50.0% by volume (vol%), vol% being based on the total volume of the solvent mixture.

[0059] In the present invention, the term "alkyl" is intended to denote saturated hydrocarbons having one or more carbon atoms, including straight-chain alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, cyclic alkyl groups (or "cycloalkyl" or "alicyclic" or "carbocyclic" groups), such as cyclopropyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl, branched-chain alkyl groups, such as isopropyl, tert-butyl, sec-butyl, and isobutyl, and alkyl-substituted alkyl groups, such as alkylsubstituted cycloalkyl groups and cycloalkyl-substituted alkyl groups.

[0060] In the present invention, the term “fluorinated acyclic di-ether” is intended to denote an acyclic diether compound, wherein at least one hydrogen atom is replaced by fluorine. One, two, three or a higher number of hydrogen atoms may be replaced by fluorine.

[0061] In one embodiment, the molar ratio F / H in the fluorinated acyclic di-ether is from 0.16 to 13.0, preferably from 0.4 to 6.0, more preferably 1.3 to 6.0.

[0062] In a particular embodiment, the molar ratio F / H in the fluorinated acyclic diether is from 1 .3 to 4.0.

[0063] In another particular embodiment, the molar ratio F / H in the fluorinated acyclic di-ether is from 1 .3 to 2.5.

[0064] In one embodiment, the boiling point of the fluorinated acyclic di-ether is at least 80°C, preferably from 80°C to 180°C, and more preferably from 120°C to 160°C.

[0065] In the present invention, the term “boiling point” is intended to denote the temperature at which the vapour pressure of a liquid substance equals to the pressure surrounding the liquid and the liquid changes its physical status into a vapour. The boiling point of a liquid substance varies depending on the surrounding environmental pressure and the boiling point according to the invention corresponds to the boiling point when the liquid is at atmospheric pressure, also known as the atmospheric boiling point.

[0066] In the present invention, the fluorinated acyclic di-ether is in an amount of from 0.05 to 50.0 vol%, vol% being based on the total volume of the liquid electrolyte.

[0067] In one embodiment, the fluorinated acyclic di-ether is in an amount of from 0.05 to 45.0 vol%, and preferably from 0.5 to 40.0 vol%, vol% being based on the total volume of the liquid electrolyte.

[0068] In another embodiment, the fluorinated acyclic di-ether is in an amount of from 10.0 to 45.0 vol%, preferably from 20.0 to 40.0 vol%, vol% being based on the total volume of the liquid electrolyte.

[0069] In a preferred embodiment, the fluorinated acyclic di-ether contains 6 carbon atoms.

[0070] Non-lim itative examples of suitable fluorinated acyclic di-ether according to the present invention include, notably, the followings: CF3CH2-O-CF2CHF-O-CF3, CHF2CH2-O-CF2CF2-O-CF3, CF3CF2-O- CHFCHF-O-CHF2, CHF2CF2-O-CHFCHF-O-CF3, CF3CHF-O-CHFCF2-O- CHF2, CF3CHF-O-CF2CHF-O-CHF2, CH3CF2-O-CF2-O-CF2CF3, CFH2CHF-O- CF2-O-CF2CF3, CF3CF2-O-CHF-O-CHFCHF2, CF3CF2-O-CHF-O-CHFCHF2, CF3CH2-O-CF2CF2-O-CF3, CHF2CHF-O-CF2CF2-O-CF3, CH2FCF2-O-CF2CF2- 0-CF3, CF3CF2-O-CHFCHF-O-CF3, CF3CF2-O-CF2CH2-O-CF3, CF3CF2-O- CH2CF2-O-CF3, CF3CF2-O-CF2CFH-O-CHF2, CF3CHF-O-CHFCF2-O-CF3, CF3CHF-O-CF2CHF-O-CF3, CHF2CF2-O-CF2CHF-O-CF3, CHF2CF2-O- CHFCF2-O-CF3, CHF2CF2-O-CF2CF2-O-CHF2, CF3CHF-O-CF2CF2-O-CHF2, CF3CF2-O-CF2-O-CHFCF3, CF2HCF2-O-CF2-O-CF2CF3, CF3CHF-O-CF2-O- CF2CF3, CF3CF2-O-CHF-O-CF2CF3, CF3CF2-O-CF2-O-CF2CHF2, CF2HCF2-O- CF2CH2-O-CF2CF2H, CF3CF2-O-CH2CH2-O-CF2CF3, CF2HCF2-O-CHFCHF- O-CF2CF2H, CF3CF2-O-CHFCH2-O-CF2CF2H, CF2HCF2-O-CHFCHF-O- CF2CF2H, CF3CF2-O-CH2CHF-O-CF2CF2H, CF3-O-CHFCF2CH2-O- CF2CF2H, CF2HCF2-O-CF2CF2-O-CF2CF2H, CF3CF2-O-CF2CHF-O- CF2CF2H, CF3CF2-O-CHFCF2-O-CF2CF2H, CF3CF2-O-CF2CH2-O-CF2CF3, CF3CF2-O-CHFCHF-O-CF2CF3, CF3CF2-O-CHFCHF-O-CF2CF3, and mixtures thereof.

[0071] The fluorinated acyclic di-ether is advantageously at least one selected from the group consisting of CFH2CH2-O-CH2CH2-O-CH2CFH2, CF2HCH2-O- CH2CH2-O-CH2CF2H, CF2HCF2-O-CH2CH2-O-CF2CF2H, CF2HCF2-O- CHFCHF-O-CF2CF2H, CF2HCF2-O-CF2CHF-O-CF2CF2H, CF2HCF2-O- CF2CF2-O-CF2CF2H, and mixtures thereof.

[0072] In a more preferred embodiment, the fluorinated acyclic di-ether is CHF2CF2- O-CH2CH2-O-CF2CF2H.

[0073] In another embodiment, the fluorinated acycic di-ether contains 7 carbon atoms.

[0074] In the other embodiment, the fluorinated acyclic di-ether contains 8 carbon atoms.

[0075] In a particular embodiment, the liquid electrolyte according to the present invention comprises neither a non-fluorinated ether nor a fluorinated monoether.

[0076] In the present invention, the term “non-fluorinated ether” is intended to denote an ether compound, wherein no fluorine atom is present.

[0077] In the present invention, the term “fluorinated mono-ether” is intended to denote a mono-ether compound, wherein at least one hydrogen atom is replaced by fluorine. One, two, three or a higher number of hydrogen atoms may be replaced by fluorine.

[0078] In the present invention, the solvent mixture comprises at least one organic carbonate.

[0079] In the present invention, the organic carbonate may be partially or fully fluorinated.

[0080] Non-limiting examples of the organic carbonate include, notably, 4- fluoroethylene carbonate (4-fluoro-1 ,3-dioxolan-2-one), 4,5-difluoro-1 ,3- dioxolan-2-one, 4,5-difluoro-4-methyl-1 ,3-dioxolan-2-one, 4,5-difluoro-4,5- dimethyl-1 ,3-dioxolan-2-one, 4,4-difluoro-1 ,3-dioxolan-2-one, 4,4, 5-trifluoro- 1 ,3-dioxolan-2-one, 4-fluoromethyl-1 ,3-dioxolan-2-one, tetrafluoroethylene carbonate, 4-(2,2-difluoroethoxy)ethylene carbonate, 4-(2,2,2- trifluoroethyoxy)ethylene carbonate, methyl-2,2-difluoroethyl carbonate, methyl-2,2,2-trifluoroethyl carbonate, methyl-2,2,3,3-tetrafluoropropyl carbonate, ethyl-2,2-difluoroethyl carbonate, ethyl-2,2,2-trifluoroethyl carbonate, ethylene carbonate (1 ,3-dioxolan-2-one), propylene carbonate, butylene carbonate, trimethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, dimethylvinylene carbonate, ethyl propyl carbonate, cyclohexene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, or combinations thereof.

[0081] In some embodiments, the organic carbonate is at least one selected from the group consisting of 4-fluoroethylene carbonate, 4,5-difluoro-1 ,3-dioxolan-2- one, 4,5-difluoro-4-methyl-1 ,3-dioxolan-2-one, 4,5-difluoro-4,5-dimethyl-1 ,3- dioxolan-2-one, 4,4-difluoro-1 ,3-dioxolan-2-one, 4,4, 5-trifluoro-1 ,3-dioxolan- 2-one, 4-fluoromethyl-1 ,3-dioxolan-2-one, tetrafluoroethylene carbonate, 4-(2,2-difluoroethoxy)ethylene carbonate, 4-(2,2,2-trifluoroethoxy)ethylene carbonate, and mixtures thereof.

[0082] In some embodiments, the organic carbonate is at least one selected from the group consisting of methyl 2,2,2-trifluoroethyl carbonate, methyl 2,2- difluoroethyl carbonate, methyl 2,2,3,3-tetrafluoropropyl carbonate, ethyl 2,2,- difluoroethyl carbonate, ethyl 2,2,2-trifluoroethyl carbonate, and mixtures thereof.

[0083] In other embodiments, the organic carbonate is at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, trimethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, dimethylvinylene carbonate, ethyl propyl carbonate, cyclohexene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and mixtures thereof.

[0084] In one particular embodiment, the organic carbonate is ethylene carbonate.

[0085] In another particular embodiment, the organic carbonate is a mixture of ethylene carbonate and dimethyl carbonate.

[0086] In the other particular embodiment, the organic carbonate is a mixture of propylene carbonate and ethyl methyl carbonate.

[0087] In one embodiment, the solvent mixture further comprises at least one fluorinated acyclic carboxylic acid ester represented by the formula (III)R4-C(O)O-R5(III) wherein R4and R5represent an alkyl group respectively; the sum of carbon atoms in R4and R5is from 2 to 7; R4does not contain fluorine; and R5contains fluorine.

[0088] In a particular embodiment, R4contains neither a CH2F- group nor a -CHF- group.

[0089] In a preferred embodiment, the number of carbon atom in R4in the formula (III) is 1.

[0090] In another preferred embodiment, the number of carbon atom in R4in the formula (III) is 2.

[0091] 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 combinations thereof.

[0092] In a particular embodiment, the fluorinated acyclic carboxylic acid ester is 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 combinations thereof.

[0093] In a more particular embodiment, the fluorinated acyclic carboxylic acid ester is CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate).

[0094] In another more particular embodiment, the fluorinated acyclic carboxylic acid ester is CH3CH2-C(O)O-CH2CF2H (2,2-difluoroethyl propionate).

[0095] In one particular embodiment, the the fluorinated acyclic carboxylic acid ester is in an amount of from 5.0 to 30.0 vol%, vol% being based on the total volume of the solvent mixture.

[0096] In another particular embodiment, the the fluorinated acyclic carboxylic acid ester is in an amount of from 10.0 to 20.0 vol%, vol% being based on the total volume of the solvent mixture.

[0097] The term “lithium salt” is hereby intended to denote a substance which needs to be dissolved in a solvent to ensure ionic conduction.

[0098] In a lithium secondary battery, a liquid electrolyte consists mainly of lithium salts dissolved 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 aconductive 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, l_i2O, etc.

[0099] 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 the battery performance.

[0100] 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.

[0101] In one embodiment, the lithium salt according to the present invention is selected from the group consisting of lithium hexafluorophosphate (LiPFe), lithium perchlorate (LiCIC ), lithium hexafluoroarsenate (LiAsFe), lithium hexafluoroantimonate (LiSbFe), lithium hexafluorotantalate (LiTaFe), lithium tetrachloroaluminate (LiAICk), lithium trifluoromethane sulfonate (LiCFsSOs), lithium b / s(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), LiN(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) andLiC(SO2CPF2PSO2)(SO2CqF2q+i) wherein p=1 -10 and q=1 -10, and combinations thereof.

[0102] In one particular embodiment, the lithium salt is lithium b / s(trifluoromethane sulfonyl)imide (LiN(CF3SO2)2) (LiTFSI).

[0103] In another particular embodiment, the lithium salt is LiFSI.

[0104] In the other particular embodiment, the lithium salt is LiPFe.

[0105] 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 commonly used unit for molarity is the number of moles per liter, having theunit of mol / L. A solution with a concentration of 1 mol / L is indicated as 1 molar and designated as 1 M.

[0106] In one embodiment, a molar concentration (M) of a lithium salt in the liquid electrolyte according to the present invention is from 1 M to 4 M, preferably from 1 M to 2 M.

[0107] According to one embodiment, c) the liquid electrolyte further comprises at least one additive. The additive may contain a solid electrolyte interface (SEI)- forming additive. SEI-forming additive promotes the formation of the SEI layer on the surface of the electrodes by reacting in advance of the solvents on the surface of the electrodes. For the SEI layer, the main components hence comprise the decomposed products of liquid electrolytes and salts, lithium alkyl carbonate, lithium alkyl oxide and other salt moieties such as Li F in case of LiPFe-based electrolytes.

[0108] The term “SEI”, as used herein, refers to a solid electrolyte interphase layer formed on the active material of an electrode. A lithium secondary battery is assembled in an uncharged state and must be activated (a process called formation) for use. During the first few charging events (battery formation) of a lithium secondary battery, components of the electrolyte are reduced or otherwise decomposed or incorporated onto the surface of the negative electroactive material and oxidized or otherwise decomposed or incorporated onto the surface of the positive electroactive material, electrochemically forming a solid-electrolyte interphase on the active materials. These layers, which are electrically insulating but ionically conducting, help preventing the decomposition of the electrolyte and can extend the cycle life and improve the performance of the battery. On the negative electrode, the SEI can suppress the reductive decomposition of the electrolyte; on the positive electrode, the SEI can suppress the oxidation of the electrolyte components.

[0109] In one embodiment, the film-forming additive stabilizes the SEI layer at the surface of a positive electrode by preventing the structural change of the positive electrode, notably under high voltage, e.g. exceeding 4.3 V.

[0110] This is because the reduction potential of the film-forming additive is higher than that of the liquid electrolyte when a reaction occurs at the surface of a negative electrode, and the oxidation potential of the film-forming additive islower than that of the liquid electrolyte when the reaction occurs at the positive electrode.

[0111] In a particular embodiment, the additive according to the present invention is selected from the group consisting of sulfur compounds such as 1 ,3,2- dioxathiolane-2,2-dioxide, 1 ,3,2-dioxathiane-2,2-dioxide, 1 ,3-propanesultone, ethylene sulphite and prop-1 -ene-1 ,3-sultone; sulfone derivatives such as dimethyl sulfone, tetramethylene sulfone (also known as sulfolane), ethyl methyl sulfone and isopropyl methyl sulfone; nitrile derivatives such as succinonitrile, adiponitrile, and glutaronitirle; and lithium nitrate (LiNOs); boron derivatives salt such as lithium difluorooxalato borate (LiDFOB), lithium b / s(oxalato)borate (LiB(C2O4)2; LiBOB), lithium fluoromalonato (difluoro)borate (LiB(O2CCHFCO2)2; LiFMDFB), lithium b / s(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)]; lithium tris(oxalato) phosphate [LiP(C2O4)3], lithium f / 7s(difluoromalonato) phosphate [LiP(O2CCF2CO2)3], lithium difluorophosphate (LiPO2F2), tris(trimethylsilyl)phosphate, triphenyl phosphine, ethyl diphenylphosphinite, triethyl phosphite, f / 7s(2,2,2-trifluoroethyl) phosphite; vinyl acetate, biphenyl benzene, isopropyl benzene, hexafluorobenzene, maleic anhydride, cesium b / s(trifluoromethanesulfonyl)imide (CsTFSI), cesium fluoride (CsF), cetyl trimethylammonium chloride, and mixtures thereof.

[0112] In the present invention, the total amount of 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%, with respect to the total weight of the liquid electrolyte.

[0113] The total amount of 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% with respect to the total weight of the liquid electrolyte.

[0114] In a particular embodiment, the total amount of the additive accounts for at least 1.0 wt% of the liquid electrolyte.

[0115] In another particular embodiment, the total amount of the additive is from 1 .0 to 2.0 wt% wirh respect to the total weight of the liquid electrolyte.

[0116] In the present invention, the additive is different from the lithium salt.

[0117] The present invention also relates to use of the liquid electrolyte according to the present invention in improving safety for a lithium secondary battery.

[0118] The lithium secondary battery according to the present invention advantageously complies with thermal safety performance, measured pursuant to the standards IEC62660, IEC62619 and / or IEC62133-2, respectively.

[0119] The international electrotechnical commission (IEC) addresses international standards for all electrical, electronic and related technologies.

[0120] First, IEC62660 specifies test procedures and acceptance criteria for safety performance of secondary lithium ion batteries used for propulsion of EVs including battery electric vehicles and hybrid electric vehicles. This standard determines the basic safety performance of cells used in a battery pack and system under intended use and reasonably foreseeable misuse or incidene, during the normal operation of the EVs, based on the premise that the cells are properly used in a battery pack and system within the limits for voltage, current and temperature as specified by the cell manufacturer. IEC62660, however, does not cover the evaluation of the safety of cells during transport and storage.

[0121] In addition, IEC62619 specifies the safety requirements for secondary lithium batteries for use in industrial applications including stationary applications, and is particularly important for exporting batteries used in stationary applications, uninterruptible power supplies (UPS), telecommunications, forklift trucks, automated guided vehicle (AGV), railway vehicles, marine vehicles, etc.

[0122] Lastly, IEC62133-2 is for the safety of rechargeable lithium ion batteries, commonly used in a wide range of consumer electronics and other applications. In particular, IEC62133-2 specifies requirements and tests for the safe operation of portable sealed secondary lithium batteries containing non-acid electrolyte, under intended use and reasonably foreseeable misuse in portable electronic devices, including cell phones, laptops, tablets and other devices. IEC62133-2 covers various aspects of battery safety, including electrical, mechanical and chemical safety, particularly those issues such asovercharging, short-circuit and thermal runaway that may potentially cause safety hazards if not properly managed.

[0123] Compliance with those standards helps to ensure that lithium secondary batteries are safe and reliable for use in a wide range of applications.

[0124] In this regard, the test results according to IEC62660, IEC62619 and IEC62133-2 may be reported as defined by EUCAR, i.e. European Council for Automotive R&D of the major European passenger car and commercial vehicle manufacturers. EUCAR facilitates and coordinates pre-com petitive R&D projects and its members participate in a wide range of collaborative European R&D programs.

[0125] Automotive requirements widely differ per manufacturer due to a large variety of vehicle sizes and applications within the transportation sector and hence different requirements need to be considered in context of specific transportation applications.

[0126] In this regad, Hazard Level from 0 to 7, adopted and modified by EUCAR, provides detailed description and classification criteria / effect, commonly known as “EUCAR hazard level” as described in Table 1 below.Table 1

[0127] Notably, EUCAR hazard level enables to assess the level of danger associated with batteries in a more specified / objective manner. In this regard, most of the automotive companies require a hazard level of 4 or less as a minimum requirement, which corresponds to the imperative demand not to have fire nor flame, even under serious accidental conditions.

[0128] EUCAR hazard level has been also referred to by many automotive manufacturers, not limited to European players. For instance, Sandia National Laboratories, which is one of three National Nuclear Security Administration R&D laboratories in the United States and is operated for the Department of Energy by Sandia Corporation, refer to EUCAR hazard level in evaluating safety performance of the cells and provide the detailed test protocol of SAND 2005-3123.

[0129] In one embodiment, the lithium secondary battery of the present invention shows neither fire nor explosion after the temperature increases from about (25.0 ± 5)°C to (85.0 ± 5.0)°C, with the increase speed of (5.0 ± 2.0)°C / min according to the test protocol of IEC62619.

[0130] In the other embodiment, the lithium secondary battery of the present invention shows neither fire nor explosion for 10 minutes after the temperature increases from about (25.0±5)°C to (130.0±2.0)°C, with the increase speed of(5.0±2.0)°C / min according to the test protocol of IEC62133-2.

[0131] The lithium secondary battery as disclosed herein can be unsed 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 transporation device including a motor vehicle, an electric vehicle, an airplane, an urban air mobility (UAM), etc.

[0132] 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.

[0133] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0134] 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.

[0135] EXAMPLES

[0136] Raw Materials :- BP160: a fluorinated acyclic di-ether of CeFsHeC (CF2HCF2-O-CH2CH2-O- CF2CF2H), synthesized within Solvay;- TTE: 1 , 1 ,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, commercially available from SynQyest;- EC: ethylene carbonate, commercially available from Soulbrain;- EMC: ethyl methyl carbonate, commercially available from Soulbrain;- FEC: fluoroethylene carbonate, commercially available from Soulbrain;- LiPOF: lithium difluorophosphate (UPO2F2), commercially available from Soulbrain;- Li salt: lithium hexafluorophosphate (LiPFe), commercially available from Soulbrain;- Anode electroactive material: graphite BTR918-2, commercially available from BTR New Material Group;- SBR (styrene butadiene rubber): BM-451 B, commercially available from Zeon Corporation;- Carboxymethyl cellulose (CMC): Sunrose® MAC350HC, commercially available from Nippon Paper;- Polyvinilidene difluoride (PVDF): SOLEF®5130, commercially available from Solvay Specialty Polymers Italy;- CB: carbon black having a specific surface area of about 62 m2 / g, measured pursuant to the method ISO9277, commercially available from Imerys;- CNT: carbon nanotubes having a specific surface area of 250~300 m2 / g, measured pursuant to the method ISO9277, commercially available from Advanced NanoProducts Co. Ltd.

[0137] Formulation of the liquid electrolyte:

[0138] The liquid electrolyte (EL1 ) was prepared by dissolving 1 M of LiPFe in a solvent mixture of EC, EMC, and FEC in 24.5:64.5:3.5 (in vol%) within a reactor, followed by introducing 0.5 wt% of LiPOF and by mixing under stirring within a glove box. After the solution became transparent. 7.5 vol% of BP160 was added to the solution. The vol% was relative to the total volume of the solvent mixture and the wt% was relative to the total weight of the liquid electrolyte.

[0139] The liquid electrolyte (EL2) was prepared in the same manner as EL1 , except that the volume ratio of EC / EMC / FEC / BP160 was set as 26.5:46.0:3.5:24.0.

[0140] The liquid electrolyte (EL3) was prepared similar to EL1 , except that BP160 was not included and the volume ratio of EC / EMC / FEC was set as 23.5:73.0:3.5.

[0141] The liquid electrolyte (EL4) was prepared similar to EL1 , except that TTE was incorporated insated of BP160 and the volume ratio of EC / EMC / FEC / TTE was set as 27.0:46.0:4.0:23.0.

[0142] The contituents of the liquid electrolytes (EL1 -EL4) are summarized in Table 1 below:Table 1dissolved** wt% with respect to the total weight of the liquid electrolyte.

[0143] Pouch cell preparation:

[0144] Positive electrodes were prepared with LiNio.8CoO.1Mno.1O2, i.e. NCM811 (commercially available from Cosmo AM&T Co., ltd.) as a cathode electroactive material; PVDF as a binder, and either CNT or CB as a conductive aid.

[0145] With CNT, the weight ratio of NCM811 / CNT / PVDF was set as 97.6:0.9:1.5 (positive electrode 1 ). With CB, the weight ratio of NCM811 / CB / PVDF was set as 96.5:2.0:1.5 (positive electrode 2).

[0146] A negative electrode was prepared with graphite, CB, SBR and CMC in a weight ratio of 97.2:0.6: 1 .2: 1 .0.

[0147] Pouch cells were made by assembling either positive electrode 1 or positive electrode 2 and the negative electrode as above prepared. Subsequently, the liquid electrolytes as prepared above (EL1-EL4) were injected into the dry pouch cells comprising either positive electrode 1 or positive electrode 2, and the negative electrode in combinations as described in Table 2 below.Table 2

[0148] Ageing

[0149] After injection, the pouch cells (E1 -E2 & CE1 -CE4) were kept respectively in a container under vacuum for better wettability, subsequently sealed using a vacuum sealer, and then kept for additional 24 hours at room temperature (1stageing).

[0150] The pouch cells were charged to 30% charging level [state of charge (SOC) 30%] after the 1stageing and then the cells were kept at room temperature for additional 24 hours (2ndageing).

[0151] Gases generated in the pouch cells during the formation were removed by opening the cells, followed by re-sealing. The cells were then cycled between 3.0 and 4.3V at 45°C and were charged / discharged at a rate of 1 .00 / 1 .00 to evaluate the cycling performances.

[0152] Results

[0153] Cycling performance (at 45°C)

[0154] The cycle retention of the cells according to the present invention (E1 -E2) and the cells for comparison (CE1 -CE4) was measured at high temperature (45°C). The results are indicated in Table 3 below.Table 3

[0155] It was clearly demonstrated that E1-E2 according to the present invention can exhibit betrer cycling performance, i.e. outstanding cycling retention in comparison to the comparative ones (CE1 -CE4).

Claims

1 . A lithium secondary battery comprising a) a positive electrode comprising, as a positive electroactive material, a lithium transition metal oxide represented by the following formula (I)LiNii-x-y-zMnxCoyAlzO2 (I) wherein x>0; y>0; z>0; 0<x+y+z<0.50; and Ni, Mn, Co, and / or Al are optionally doped with at least one metal selected from the group consisting of Sc, Ti, V, Cr, Fe, Cu, Zn, Mg, Al, Sn, B, Ga, Sr, Ca, In, Si, Zr, La, P, Nb, and Ge, respectively; b) a negative electrode; and c) a liquid electrolyte comprising at least one lithium salt dissolved in a solvent mixture comprising at least one organic carbonate and at least one fluorinated acyclic di-ether represented by the formula (II)R1-O-R2-O-R3(II) wherein each R1and R3is a fluorinated straight-chain alkyl group, respectively; R2is a straight-chain alkyl group, optionally fluorinated; and the sum of carbon atoms in R1, R2and R3is from 4 to 10, preferably from 5 to 8, more preferably 6; wherein the fluorinated acyclic di-ether is in an amount of from 0.1 to 50.0% by volume (vol%), vol% being based on the total volume of the solvent mixture; and wherein a) the positive electrode and / or b) the negative electrode comprise, as a conductive agent, at least one carbon material having a specific surface area of from 100 to 1200 m2 / g, measured pursuant to the method ISO9277 and / or having a ratio (ID / IG ratio) between the intensity of the D-band (ID) and the intensity of the G- band (IG) of 0.01 or more, characterized by Raman spectroscopy defined in the ISO / TS 80004-13.

2. The lithium secondary battery according to claim 1 , wherein 0.05<x+y+z<0.40, preferably 0.08<x+y+z<0.25.

3. The lithium secondary battery according to claim 1 or 2, wherein the carbon material has a specific surface area of at least 200 m2 / g, at least 250 m2 / g, at least 300 m2 / g, at least 400 m2 / g, or at least 500 m2 / g, and / or at most 1100 m2 / g, at most 1000 m2 / g, at most 900 m2 / g, or at most 800 m2 / g, measured pursuant to the method ISO9277.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein the conductive agent is included in a positive electrode or a negative electrode in an amount of from 0.01 to 2.0% by weight (wt%), preferably from 0.01 to 1 .5 wt%, wt% being based on the total weight of the positive electrode or the negative electrode.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein the conductive agent is at least one selected from the group consisting of carbon nanotube (CNT), graphene, and graphene oxide.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein the molar ratio F / H in the fluorinated acyclic di-ether is from 0.16 to 13.0, preferably from 0.4 to 6.0, more preferably from 1 .3 to 6.0.

7. The lithium secondary battery according to any one of claims 1 to 6, wherein the boiling point of the fluorinated acyclic di-ether is at least 80°C, preferably from 80°C to 180°C, and more preferably from 100°C to 160°C.

8. The lithium secondary battery according to any one of claims 1 to 7, wherein the fluorinated acyclic di-ether is at least one selected from the group consisting of CFH2CH2-O-CH2CH2-O-CH2CFH2, CF2HCH2-O-CH2CH2-O-CH2CF2H, CF2HCF2-O-CH2CH2-O-CF2CF2H, CF2HCF2-O-CHFCHF-O-CF2CF2H, CF2HCF2-O- CF2CHF-O-CF2CF2H, CF2HCF2-O-CF2CF2-O-CF2CF2H, and mixtures thereof.

9. The lithium secondary battery according to any one of claims 1 to 8, wherein the organic carbonate is at least one selected from the group consisting of 4-fluoroethylene carbonate (4-fluoro-1 ,3-dioxolan-2-one), 4,5-difluoro-1 ,3-dioxolan-2- one, 4,5-difluoro-4-methyl-1 ,3-dioxolan-2-one, 4,5-difluoro-4,5-dimethyl-1 ,3-dioxolan-2-one, 4,4-difluoro-1 ,3-dioxolan-2-one, 4,4,5-trifluoro-1 ,3-dioxolan-2-one, 4- fluoromethyl-1 ,3-dioxolan-2-one, tetrafluoroethylene carbonate, 4-(2,2- difluoroethoxy)ethylene carbonate, 4-(2,2,2-trifluoroethoxy)ethylene carbonate, methyl 2,2,2-trifluoroethyl carbonate, methyl 2,2-difluoroethyl carbonate, methyl2.2.3.3-tetrafluoropropyl carbonate, ethyl 2, 2, -difluoroethyl carbonate, ethyl 2,2,2- trifluoroethyl carbonate, ethylene carbonate (1 ,3-dioxolan-2-one), propylene carbonate, butylene carbonate, trimethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, dimethylvinylene carbonate, ethyl propyl carbonate, cyclohexene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and mixtures thereof.

10. The lithium secondary battery according to any one of claims 1 to 9, wherein the solvent mixture further comprises a fluorinated acyclic carboxylic acid ester represented by the formula (III):R4-C(O)O-R5(III) wherein R4and R5represent an alkyl group respectively; the sum of carbon atoms in R4and R5is from 2 to 7; R4does not contain fluorine; R5contains fluorine; and R5contains neither a CH2F- group nor a -CHF- group.11 . The lithium secondary battey according to any one of claims 1 to 10, wherein the solvent mixture further comprises at least one non-fluorinated carboxylic acid ester, preferably alkyl propionates.

12. The lithium secondary battery according to any one of claims 1 to 11 , wherein c) the liquid electrolyte further comprises at least one additive selected from the group consisting of sulfur compounds such as 1 ,3,2-dioxathiolane-2,2-dioxide,1 ,3,2-dioxathiane-2,2-dioxide, 1 ,3-propanesultone, ethylene sulphite and prop-1 -ene-1.3-sultone; sulfone derivatives such as dimethyl sulfone, tetramethylene sulfone, ethyl methyl sulfone and isopropyl methyl sulfone; nitrile derivatives such as succinonitrile, adiponitrile, and glutaronitirle; lithium nitrate (LiNOs); boron derivatives salt such as lithium difluorooxalato borate (LiDFOB), lithium b / s(oxalato)borate (LiB(C2O4)2; LiBOB), lithium fluoromalonato (difluoro)borate (LiB(O2CCHFCO2)2; LiFMDFB), lithium b / s(malonato)borate [LiB(O2CCH2CO2)2], lithiumb / s(difluoromalonato) borate [LiB(O2CCF2CO2)2], lithium (malonatooxalato) borate [LiB(C2O4)(O2CCH2CO2)], lithium (difluoromalonatooxalato) borate [LiB(C2O4)(O2CCF2CO2)]; lithium tris(oxalato) phosphate [LiP(C2O4)3], lithium f / 7s(difluoromalonato) phosphate [LiP(O2CCF2CO2)3], lithium difluorophosphate (LiPO2F2), fr / s(trimethylsilyl)phosphate, triphenyl phosphine, ethyl diphenylphosphinite, triethyl phosphite, f / 7s(2,2,2-trifluoroethyl) phosphite; vinyl acetate, biphenyl benzene, isopropyl benzene, hexafluorobenzene, maleic anhydride, cesium b / s(trifluoromethanesulfonyl)imide (CsTFSI), cesium fluoride (CsF), cetyl trimethylammonium chloride, and mixtures thereof.

13. The lithium secondary battery according to any of claims 1 to 12, wherein b) the negative electrode comprises, as a negative electroactive material, graphite, metallic lithium or a composite material of at least a carbon material and a silicon material, selected from the group consisting of Si / C, SiOa / C, and Si / SiOa / C, with 0<a<2.

14. Use of the liquid electrolyte defined in any of claims 1 to 13 in improving safety for a lithium secondary battery, advantageously complying with thermal safety performance measured pursuant to the standards IEC62660, IEC62619 and / or IEC62133-2, respectively.

15. An electronic device, a transportation device or a telecommunication device comprising a lithium secondary battery according to any one of claims 1 to 13.

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