Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery comprising a cathode comprising a lithium-manganese-rich layered oxide as a cathode electroactive material and a liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester. The present invention also relates to the use of a liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester in a lithium secondary battery to improve cycle performance, the lithium secondary battery comprising a cathode comprising the lithium-manganese-rich layered oxide according to the present invention as a cathode electroactive material.

Description

Technical Field

[0001] Cross - reference to related applications This application claims priority based on European Patent Application No. 21217533.5 filed on December 23, 2021, and the entire contents of this application are incorporated herein by reference for all purposes.

[0002] The present invention relates to a lithium secondary battery including a cathode containing a lithium - manganese - rich layered oxide as a cathode electroactive material and a liquid electrolyte containing at least one fluorinated acyclic carboxylic acid ester. The present invention also relates to the use of a liquid electrolyte containing at least one fluorinated acyclic carboxylic acid ester in a lithium secondary battery for improving cycle performance, wherein the lithium secondary battery includes a cathode containing the lithium - manganese - rich layered oxide according to the present invention as a cathode electroactive material.

Background Art

[0003] Lithium secondary batteries have maintained a dominant position in the market for rechargeable energy storage devices due to many advantages such as being lightweight, having a moderate energy density, and having a good cycle life.

[0004] Historically, lithium secondary batteries have attracted great attention since it was first demonstrated by Goodenough et al. (Materials Research Bulletin 1980, Vol. 15, pp. 783 - 789) that Li x CoO2(0 < x ≦ 1) has a relatively high energy density and good cycle stability, and was later commercialized as a cathode electroactive material by Sony Corporation in the early 1990s. This discovery changed the paradigm of lithium secondary batteries.

[0005] The search for novel high-performance and low-cost cathode electroactive materials has always been a challenging theme in this field. The development of electrochemical energy storage technologies is important not only for the advancement of various applications ranging from home appliances to electric vehicles (EVs), but also for the efficient and controllable utilization of natural resources.

[0006] Therefore, several approaches have been pursued to develop new cathode electroactive materials with higher energy density, with particular focus always on cathode electroactive materials, as they are more expensive than anode electroactive materials.

[0007] For this purpose, various layered oxides with the formula LiMO2 (M=Co, Mn, and / or Ni) have been investigated, as well as spinel LiMn2O4 and olivine LiFePO4. However, it was found that spinel LiMn2O4 and olivine LiFePO4 have low energy densities, limiting their applications, especially in large-scale EV and energy storage fields.

[0008] LiCoO2 was subsequently found to have drawbacks such as low real capacity and the relatively high cost of Co, and layered LiNiO2 was proposed as an alternative. However, LiNiO2 proved to be difficult to fabricate due to its poor thermal stability and Li / Ni disorder resulting from Li / Ni exchange at octahedral sites, especially as the Ni content increased to higher values, which adversely affected Li diffusivity, cycling stability, first cycle efficiency, and overall electrode performance. Therefore, an alternative solution was needed, despite LiNiO2's low cost and high rechargeable capacity compared to LiCoO2.

[0009] Therefore, other layered oxides, such as LiNiO2 and LiMnO2, which are solid solutions, 0.5 Mn 0.5 Binary oxides such as O2 and LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3Ternary oxides such as LiCoO2 have been further investigated. 1 / 3 Ni 1 / 3 Mn 1 / 3 O2 exhibited the best electrochemical performance with high reversible capacity, making it a promising cathode electroactive material for high-power lithium secondary batteries.

[0010] Nevertheless, the specific energy density of commercialized lithium-ion batteries is still unable to meet the ever-increasing practical demands in high-power applications such as EVs, hybrid electric vehicles (HEVs), and grid energy storage.

[0011] As one of the diverse research efforts to find alternatives to meet such ever-increasing demands, Jiang et al. proposed a Li- and Mn-rich layered oxide cathode electroactive material represented as x[Li2MnO3]·(1-x)[LiMO2] (M=Co, Ni, Mn, Fe, Cr, etc.) in Molecular Systems Desigh&Engineering, 2018, Vol.3, pp.748-803 ("Li- and Mn-rich layered oxide cathode materials for lithium-ion batteries: a review from fundamentals to research progress and applications"). It is composed of two phases, namely the trigonal LiMO2 phase (space group R3m) and the monoclinic Li2MnO3 phase (space group C2 / m). This Li2MnO3 phase can be converted to the active LiMnO2 phase after the first cycle, thus enhancing the electrochemical capacity of the cathode. In short, there exists a synergistic effect between the two phases, which contributes to the higher electrochemical performance of the cathode electroactive material. In addition, the LMRO cathode electroactive material is also economically competitive and environmentally compatible. However, in order to be commercialized, some problems and challenges still need to be overcome, such as unclear crystal structure, unclear reaction mechanism, fast voltage drop, and poor rate capability.

[0012] Therefore, there is a continuous need for lithium secondary batteries having improved cycle performance including stability, reliability, low cost, etc., which can be met by using novel cathode electroactive materials, novel anode electroactive materials, liquid electrolyte formulations compatible with newly designed electroactive materials as introduced above, and the like. SUMMARY OF THE INVENTION

[0013] The present invention relates to a) a cathode comprising a lithium transition metal oxide represented by the following formula (I) Li [(2x+2) / (x+2)] Mn [2x / (x+2)] M [(2-2x) / (x+2)] O2 (I) (where 0 < x < 1, M is a combination of elements represented by the general formula (II) (Ni a Mn b Co c ) 1-d M’ d (II) and a + b + c = 1, 0 ≤ c ≤ 0.1, 0 ≤ d ≤ 0.1, and M’ includes 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), as a cathode electroactive material; and b) a liquid electrolyte containing at least one fluorinated acyclic carboxylic acid ester represented by the formula (III) R 1 -C(O)O-R 2 (III) (where R 1 and R 2 each represent an alkyl group, the total number of carbon atoms in R 1 and R 2 is 2 to 7, R 1 does not contain fluorine, and R 2 contains fluorine), and relates to a lithium secondary battery comprising the same.

[0014] The cathode electroactive material according to the present invention corresponds to a lithium-manganese-rich layered transition metal oxide.

[0015] The present invention also relates to the use of a liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester in a lithium secondary battery to improve the cycling performance, the lithium secondary battery comprising a cathode comprising the lithium-manganese-rich layered transition metal oxide according to the present invention as a cathode electroactive material.

[0016] The inventors have surprisingly found that the above-mentioned technical problems can be solved by using a specific combination of a cathode comprising a lithium-manganese-rich layered transition metal oxide according to the invention and a liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] definition Throughout this specification, unless the context requires otherwise, the terms "comprise" or "include" or variations such as "comprises," "comprising," "includes," "including" will be understood to imply the inclusion of a stated element or method step or group of elements or method steps, but not the exclusion of any other element or method step or group of elements or method steps. According to a preferred embodiment, the terms "comprise" and "comprises" and variations thereof mean "consisting only of".

[0018] As used herein, the singular forms "a", "an" and "the" include plural embodiments unless the context clearly indicates otherwise. The term "and / or" includes the meaning "and", "or" and also all other possible combinations of the elements associated with this term.

[0019] The term "~" should be understood to be inclusive.

[0020] The term "alkyl" is intended to mean 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, and the like; 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 alkyl-substituted cycloalkyl groups and cycloalkyl-substituted alkyl groups.

[0021] The term "aliphatic group" includes organic moieties characterized by straight or branched chains, typically having from 1 to 18 carbon atoms. In complex structures, the chains may be branched, bridged or cross-linked. Aliphatic groups include alkyl, alkenyl and alkynyl groups.

[0022] In the present invention, the term "cut-off voltage" is intended to mean a predetermined lower voltage limit below which the discharge is considered complete. The cut-off voltage is usually selected so that the maximum usable capacity of the battery is achieved. The cut-off voltage varies from battery to battery and is highly dependent on the type of battery, e.g., type of cathode or anode.

[0023] In the present invention, the term "anode" is intended to mean in particular the electrode of an electrochemical cell where oxidation takes place during discharge.

[0024] In the present invention, the term "cathode" is intended to mean in particular the electrode of an electrochemical cell where reduction takes place during discharge.

[0025] In the present invention, the type of "current collector" depends on whether the electrode provided thereby is a cathode or an anode. When the electrode of the present invention is a cathode, the current collector typically comprises, and is preferably composed of, at least one metal selected from the group consisting of aluminum (Al), nickel (Ni), titanium (Ti) and alloys thereof, preferably Al. When the electrode of the present invention is an anode, the current collector typically comprises, and is preferably composed of, at least one metal selected from the group consisting of lithium (Li), sodium (Na), zinc (Zn), magnesium (Mg), copper (Cu) and alloys thereof, preferably Cu.

[0026] In the present invention, the term "electroactive material" is intended to mean an electroactive material capable of incorporating or inserting lithium ions into its structure and subsequently releasing them therefrom during the charging and discharging phases of the battery. The nature of the electroactive material will depend on whether it is used to form the cathode or the anode. Thus, the electroactive material can be selected from cathode electroactive materials and anode electroactive materials.

[0027] As used herein, the terminology for organic groups is “(C n ~C m )" (where n and m are each integers) indicates that the group may contain from n carbon atoms to m carbon atoms per group.

[0028] Ratios, concentrations, amounts, and other numerical data may, in this specification, be presented in a range format. Such a range format is used merely for convenience and brevity and is to be flexibly interpreted as encompassing not only the numerical values explicitly recited as the limits of the range but also all the individual numerical values or sub-ranges subsumed within that range as if each numerical value and sub-range were explicitly recited. For example, a temperature range of about 120°C to about 150°C is to be interpreted as encompassing not only the explicitly recited limits of about 120°C to about 150°C but also sub-ranges such as 125°C to 145°C, 130°C to 150°C, etc., and individual amounts such as decimal points within the specified range, e.g., 122.2°C, 140.6°C, and 141.3°C.

[0029] Unless otherwise specified, in the context of the present invention, the amount of a component in a composition is expressed as the ratio between the volume of the component multiplied by 100 and the total volume of the composition, i.e., volume % (vol%) or the ratio between the weight of the component multiplied by 100 and the total weight of the composition, i.e., weight % (wt%). It is to be understood that both the foregoing summary and the following detailed description are exemplary and are intended to provide further explanation of the claimed invention. Accordingly, various modifications and variations described herein will be apparent to those skilled in the art. Further, descriptions of well-known functions and structures may be omitted for clarity and brevity.

[0030] The present invention relates to a) the following formula (I) Li [(2x+2) / (x+2)] Mn [2x / (x+2)] M [(2-2x) / (x+2)] O2 (I) (wherein 0 < x < 1 and M is of the general formula (II) (Ni a Mn b Co c ) 1-d M’ d (II) a+b+c=1, 0≦c≦0.1, and 0≦d≦0.1; and M' includes 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. A cathode comprising as a cathode electroactive material a lithium transition metal oxide represented by the formula: b) Formula (III) R 1 -C(O)OR 2 (III) (In the formula, R 1 and R 2 Each represents an alkyl group, and R 1 and R 2 The total number of carbon atoms in R is 2 to 7. 1 does not contain fluorine, R 2 contains fluorine) A liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester represented by the formula: The present invention relates to a lithium secondary battery comprising:

[0031] In a preferred embodiment, 0.1≦x≦0.7.

[0032] In a more preferred embodiment, 0.3≦x≦0.55.

[0033] The cathode electroactive material according to the present invention corresponds to a lithium-manganese-rich layered oxide.

[0034] In one embodiment, the cathode electroactive material according to the present invention is Co-free, i.e., c is zero.

[0035] In another embodiment, the cathode electroactive material comprises more manganese than nickel, and a*(1-d)<[x / (1-x)]+b(1-d).

[0036] Non-limiting examples of suitable cathode electroactive materials according to the present invention include, among others: Li1.09 Mn 0.91 O2 (same as 0.2[Li2MnO3] 0.8[LiMnO2]), Li 1.25 Mn 0.625 Ni 0.125 O2, Li 1.15 Mn 0.56 Ni 0.29 O2 and Li 1.17 Mn 0.54 Ni 0.28 O2.

[0037] In a preferred embodiment, the cathode electroactive material is Li 1.15 Mn 0.56 Ni 0.29 O2 and Li 1.17 Mn 0.54 Ni 0.28 O2.

[0038] In one embodiment, R 2 does not include a -CH2F- group or a -CHF- group.

[0039] In one embodiment, R in formula (III) 1 The number of carbon atoms in is 1, 2, 3, 4, or 5. In a preferred embodiment, R 1 The number of carbon atoms in is 1.

[0040] Non-limiting examples of suitable fluorinated acyclic carboxylic acid esters according to the present invention include, among others: 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-C F3, 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-CH2CH2C F2H, 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, and mixtures thereof.

[0041] In a preferred embodiment, the fluorinated acyclic carboxylic acid ester is CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate).

[0042] In one embodiment, the fluorinated acyclic carboxylic acid ester is in an amount of 20 to 90 weight percent (wt %), preferably 40 to 80 wt %, and more preferably 50 to 75 wt %, based on the total weight of the liquid electrolyte.

[0043] In one embodiment, the liquid electrolyte b) is a compound represented by formula (IV): R 3 -OC(O)OR 4 (IV) (In the formula, R 3 and R 4 Each represents an alkyl group; R 3 and R 4 The total number of carbon atoms in R is 2 to 7; 3 and / or R 4 At least one hydrogen atom in the The present invention further includes fluorinated acyclic carbonates represented by the formula:

[0044] In certain embodiments, R 3 and R 4 does not contain a -CH2F- group or a -CHF- group.

[0045] In another particular embodiment, R 3 and R 4 each independently represents a linear or branched alkyl group having 2 to 7 carbon atoms, and at least two hydrogen atoms are replaced by fluorine. That is, R 3 At least two hydrogen atoms in R are replaced by fluorine atoms, or 4 At least two hydrogen atoms in R are replaced by fluorine atoms, or 3 At least two hydrogen atoms and R 4 At least two hydrogens in are replaced by fluorine.

[0046] Non-limiting examples of suitable fluorinated acyclic carbonates according to the present invention include, inter alia: CH3-OC(O)O-CH2CF2H (methyl 2,2-difluoroethyl carbonate), CH3-OC(O)O-CH2CF3 (methyl 2,2,2-trifluoroethyl carbonate), CH3-OC(O)O-CH2CF2CF2H (methyl 2,2,3,3-tetrafluoropropyl carbonate), CF2HCH2-OC(O)O-CH2CF3 (2,2-difluoroethyl 2,2,2-trifluoroethyl carbonate), CH3CH2-OC(O)O-CH2CF2H (ethyl 2,2-difluoroethyl carbonate), CF3CH2-OC(O)O-CH2CH3 (ethyl 2,2,2-trifluoroethyl carbonate), and mixtures thereof.

[0047] In a preferred embodiment, the fluorinated acyclic carbonate is CH3-OC(O)O-CH2CF3 (methyl 2,2,2-trifluoroethyl carbonate).

[0048] In another embodiment, b) the liquid electrolyte is a compound represented by the formula (V): R5 -OR 6 -OR 7 (V) (In the formula, R 5 and R 7 each represents a fluorinated linear alkyl group; R 6 represents an optionally fluorinated linear alkyl group; R 5 , R 6 , and R 7 The total number of carbon atoms in is 5 to 8, preferably 6. The compound further comprises at least one fluorinated acyclic diether represented by the formula:

[0049] In one embodiment, the boiling point of the fluorinated acyclic diether is at least 80°C, preferably from 80°C to 160°C, more preferably from 120°C to 160°C.

[0050] In another embodiment, the molar ratio F / H in the fluorinated acyclic diether is from 1.3 to 13.0, preferably from 2.5 to 6.0.

[0051] In a preferred embodiment, the fluorinated acyclic diether contains 6 carbon atoms.

[0052] In a more preferred embodiment, the fluorinated acyclic diether is CHF2CF2-O-CH2CH2-O-CF2CF2H.

[0053] Non-limiting examples of suitable fluorinated acyclic diethers according to the present invention include, inter alia: CF3CH2-O-CF2CHF-O-CF 3、 CHF2CH2-O-CF2CF2-O-CF 3、 CF3CF2-O-CHFCHF-O-CHF 2、 CHF2CF2-O-CHFCHF-O-CF 3、 CF3CHF-O-CHFCF2-O-CHF 2、 CF3CHF-O-CF2CHF-O-CHF 2、 CH3CF2-O-CF2-O-CF2CF 3、CFH2CHF-O-CF2-O-CF2CF 3、 CF3CF2-O-CHF-O-CHFCHF 2、 CF3CH2-O-CF2CF2-O-CF 3、 CHF2CHF-O-CF2CF2-O-CF 3、 CH2FCF2-O-CF2CF2-O-CF 3、 CF3CF2-O-CHFCHF-O-CF 3、 CF3CF2-O-CF2CH2-O-CF 3、 CF3CF2-O-CH2CF2-O-CF 3、 CF3CF2-O-CF2CFH-O-CHF 2、 CF3CHF-O-CHFCF2-O-CF 3、 CF3CHF-O-CF2CHF-O-CF 3、 CHF2CF2-O-CF2CHF-O-CF 3、 CHF2CF2-O-CHFCF2-O-CF 3、 CHF2CF2-O-CF2CF2-O-CHF 2、 CF3CHF-O-CF2CF2-O-CHF 2、 CF3CF2-O-CF2-O-CHFCF3、CF2HCF2-O-CF2-O-CF2CF 3、 CF3CHF-O-CF2-O-CF2CF 3、 CF3CF2-O-CHF-O-CF2CF 3、 CF3CF2-O-CF2-O-CF2CHF 2、 CF2HCF2-O-CF2CH2-O-CF2CF2H、CF3CF2-O-CH2CH2-O-CF2CF 3、 CF2HCF2-O-CHFCHF-O-CF2CF2H、CF3CF2-O-CHFCH2-O-CF2CF2H、CF3CF2-O-CH2CHF-O-CF2CF2H、CF3-O-CHFCF2CH2-O-CF2CF2H、CF2HCF2-O-CF2CF2-O-CF2CF2H、CF3CF2-O-CF2CHF-O-CF2CF2H、CF3CF2-O-CHFCHF-O-CF2CF3、CF3CF2-O-CHFCHF-O-CF2CF3、

[0054] In another embodiment, the fluorinated acyclic diether contains 7 carbon atoms.

[0055] In another embodiment, the fluorinated acyclic diether contains 8 carbon atoms.

[0056] In a particular embodiment, b) the liquid electrolyte according to the present invention does not comprise a non-fluorinated ether or a fluorinated monoether.

[0057] In the present invention, the term "non-fluorinated ether" is intended to mean an ether compound in which no fluorine atoms are present.

[0058] In the present invention, the term "fluorinated monoether" is intended to mean a monoether compound in which at least one hydrogen atom is replaced by fluorine. One, two, three or more hydrogen atoms may be replaced by fluorine.

[0059] The liquid electrolyte according to the present invention does not include fluorinated cyclic carboxylic acid esters, such as fluorinated lactones containing a 1-oxacycloalkan-2-one structure.

[0060] In one embodiment, b) the liquid electrolyte further comprises at least one organic carbonate.

[0061] In the present invention, organic carbonates include fluorinated cyclic carbonates, non-fluorinated cyclic carbonates, and non-fluorinated acyclic carbonates.

[0062] Non-limiting examples of organic carbonates according to the invention include in particular: 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, ethylene carbonate, propylene carbonate, butylene carbonate, trimethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, dimethylvinylene carbonate, ethyl propyl carbonate, cyclohexene carbonate, bisphenol A, B, and F carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and mixtures thereof.

[0063] In certain preferred embodiments, the organic carbonate is a mixture of fluoroethylene carbonate, propylene carbonate, and ethylene carbonate.

[0064] In another particular embodiment, the organic carbonate is a mixture of fluoroethylene carbonate and propylene carbonate.

[0065] In the present invention, the total amount of the at least one organic carbonate is 0 to 80% by weight, preferably 0 to 60% by weight, and more preferably 0 to 50% by weight, based on the total weight of the liquid electrolyte.

[0066] When at least one organic carbonate is contained in the liquid electrolyte of the present invention, the total amount thereof is 10 to 80% by weight, preferably 20 to 60% by weight, more preferably 25 to 50% by weight, based on the total weight of the liquid electrolyte.

[0067] In one embodiment, b) the liquid electrolyte further comprises at least one lithium salt.

[0068] Non-limiting examples of lithium salts according to the present invention include, inter alia, the following: Lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluoroantimonate (LiSbF6), lithium hexafluorotantalate (LiTaF6), lithium tetrachloroaluminate (LiAlCl4), lithium tetrafluoroborate (LiBF4), lithium chloroborate (Li2B 10 Cl 10 ), lithium fluoroborate (Li2B 10 F 10 ), Li2B 12 F x H 12-x (wherein x=0 to 12), LiPF x (R F ) 6-x and LiBF y (R F ) 4-y (In the formula, R F Perfluorinated C1-C 20 represents an alkyl group or a perfluorinated aromatic group, x=0-5, and y=0-3), LiBF2[O2C(CX2) n CO2], LiPF2[O2C(CX2) n CO2]2, LiPF4[O2C(CX2) n CO2] (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 trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (Li(FSO2)2N (LiFSI), LiN(SO2C m F 2m+1 )(SO2C n F 2n+1 ) and LiC(SO2C k F 2k+1 )(SO2C m F 2m+1 )(SO2C n F2n+1 ) (wherein k = 1 to 10, m = 1 to 10, and n = 1 to 10), LiN(SO2C p F 2p SO2) and LiC(SO2C p F 2p SO2)(SO2C q F 2q+1 ) (wherein p=1-10 and q=1-10) and mixtures thereof.

[0069] In one embodiment, the lithium salt is lithium bis(trifluorosulfonyl)imide (LiN(CF3SO2)2; LiTFSI).

[0070] In another embodiment, the lithium salt is LiPF6.

[0071] In another embodiment, the lithium salt is LiFSI.

[0072] In one embodiment, the molar concentration (M) of the lithium salt in the liquid electrolyte according to the present invention is 1M to 8M, preferably 1M to 4M, and more preferably 1M to 2M.

[0073] Lithium salts according to the present invention do not include lithium salts having a nitrogen atom on a heterocyclic ring such as imidazole, for example lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI).

[0074] According to one embodiment, the liquid electrolyte according to the present invention further comprises at least one film-forming additive, which promotes the formation of a solid electrolyte interface (SEI) layer at the negative electrode surface by pre-reacting the solvent on the electrode surface. For the SEI layer, the main components therefore comprise the electrolyte solvent and the decomposition products of the salt, which may include Li2CO3, lithium alkyl carbonate, lithium alkyl oxide, and other salt moieties, such as LiF for LiPF6-based electrolytes. According to another embodiment, the film-forming additive stabilizes the cathode electrolyte interface (CEI) layer at the positive electrode surface by preventing structural changes of the positive electrode, especially under high voltage. Usually, the reduction potential of the film-forming additive is higher than the reduction potential of the solvent when the reaction occurs at the negative electrode surface, and the oxidation potential of the film-forming additive is lower than the oxidation potential of the solvent when the reaction occurs at the positive electrode side.

[0075] In the present invention, the film-forming additive is different from the lithium salt.

[0076] In a particular embodiment, the film-forming additive according to the present invention is 1,3,2-dioxathiolane-2,2-dioxide, 1,3,2-dioxathiolane-4-ethynyl-2,2-dioxide, 1,3,2-dioxathiolane-4-ethenyl-2,2-dioxide, 1,3,2-dioxathiolane-4,5-diethenyl-2,2-dioxide, 1,3,2-dioxathiolane-4-methyl-2,2-dioxide, 1,3,2-dioxathiolane-4,5-dimethyl-2,2-dioxide, 1,3,2-dioxathiane-2,2-dioxide, 1,3,2-dioxathiolane-4-methyl-2,2-dioxide, 1,3,2-dioxathiolane-4,5-dimethyl ... Thiane-4-ethynyl-2,2-dioxide, 1,3,2-dioxathiane-5-ethynyl-2,2-dioxide, 1,3,2-dioxathiane-4-ethenyl-2,2-dioxide, 1,3,2-dioxathiane-5-ethenyl-2,2-dioxide, 1,3,2-dioxathiane-4,5-diethenyl-2,2-dioxide, 1,3,2-dioxathiane-4,6-diethenyl-2,2-dioxide, 1,3,2-dioxathiane-4,5,6-triethenyl-2,2-dioxide, 1,3,2-dioxathiane-4-methyl-2,2-dioxide , 1,3,2-dioxathiane-5-methyl-2,2-dioxide, 1,3,2-dioxathiane-4,5-dimethyl-2,2-dioxide, dioxathiane-4,6-dimethyl-2,2-dioxide, dioxathiane-4,5,6-trimethyl-2,2-dioxide; 1,3-propane sultone, 1-fluoro-1,3-propane sultone, 2-fluoro-1,3-propane sultone, 3-fluoro-1,3-propane sultone, 1,4-butane sultone, 3-fluoro-1,4-butane sultone, 4-fluoro-1,4-butane sultone, 5-fluoro sulfur compounds including fluoro-1,4-butane sultone, 6-fluoro-1,4-butane sultone, preferably 1,3,2-dioxathiolane-2,2-dioxide, 1,3,2-dioxathiane-2,2-dioxide, 1,2-oxathiolane-2,2-dioxide (1,3-propane sultone), 1,3,2-dioxathiolane-2-oxide (ethylene sulfite) and prop-1-ene-1,3-sultone, dimethyl sulfone, tetramethylene sulfone (also known as sulfolane), ethyl methyl sulfone and isopropyl methyl sulfone;Nitrile derivatives including succinonitrile, adiponitrile, and glutaronitrile; lithium nitrate (LiNO3); lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalato)borate (LiB(C2O4)2; 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)], lithium tris(oxalato)phosphate [LiP(C2O4)3], lithium tris(difluoromalonato)phosphate [LiP(O2CCF2CO2)3], lithium difluorophosphate (LiPO2F2), vinyl acetate, biphenylbenzene, isopropylbenzene, hexafluorobenzene, Orobenzene, tris(trimethylsilyl)phosphate, triphenylphosphine, ethyl diphenylphosphinite, triethyl phosphite, tris(2,2,2-trifluoroethyl)phosphite, maleic anhydride, vinylene carbonate, vinyl ethylene carbonate, cesium bis(trifluoromethanesulfonyl)imide (CsTFSI), cesium hexafluorophosphate (CsPF6), cesium fluoride (CsF), trimethylboroxine (TMB), tributyl borate (TBB), 2-(2,2,3 ,3,3-pentafluoropropoxy)-1,3,2-dioxaphospholane (PFPOEPi), 2-(2,2,3,3,3-pentafluoropropoxy)-4-(trifluoromethyl)-1,3,2-dioxaphospholane (PFPOEPi-1CF3), lithium hexafluorophosphate (LiPF6), silver nitrate (AgNO3), silver hexafluorophosphate (AgPF6), tris(trimethylsilyl)phosphine (TMSP), 1,6-divinylperfluorohexane, and mixtures thereof;

[0077] In a preferred embodiment, the film-forming additive is a sulfur compound including 1,3,2-dioxathiolane-2,2-dioxide, 1,3,2-dioxathiane-2,2-dioxide, 1,3-propane sultone, ethylene sulfite, and prop-1-ene-1,3-sultone; sulfone derivatives including dimethyl sulfone, tetramethylene sulfone (also known as sulfolane), ethyl methyl sulfone, and isopropyl methyl sulfone; succinonitrile, adiponitrile, and nitrile derivatives including glutaronitrile; and lithium nitrate (LiNO3); lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalato)borate (LiB(C2O4)2; 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)]; lithium tris(oxalato)phosphate [LiP(C2O4)3], lithium tris(difluoromalonato)phosphate [LiP(O2CCF2CO2)3], lithium difluorophosphate (LiPO 2F2), vinyl acetate, biphenylbenzene, isopropylbenzene, hexafluorobenzene, tris(trimethylsilyl)phosphate, triphenylphosphine, ethyldiphenylphosphinite, triethylphosphite, tris(2,2,2-trifluoroethyl)phosphite, maleic anhydride, cesium bis(trifluoromethanesulfonyl)imide (CsTFSI), cesium fluoride (CsF), and mixtures thereof.

[0078] In a more preferred embodiment, the film forming additive according to the present invention is LiBOB.

[0079] In another more preferred embodiment, the film forming additive according to the present invention is LiDFOB.

[0080] In a particular embodiment, the film-forming additive according to the present invention is an ionic liquid.

[0081] The term "ionic liquid" as used herein refers to compounds that contain positively charged cations and negatively charged anions and that are in the liquid state at temperatures below 100° C. at atmospheric pressure. Whereas ordinary liquids such as water are composed primarily of electrically neutral molecules, ionic liquids are composed primarily of ions and short-lived ion pairs. As used herein, the term "ionic liquid" refers to compounds that do not contain solvent.

[0082] Non-limiting examples of ionic liquids according to the invention include N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI), N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI), and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), among others.

[0083] In the present invention, the total amount of the film-forming additives may be 0 to 10% by weight, preferably 0 to 8% by weight, and more preferably 0 to 5% by weight, based on the total weight of the liquid electrolyte.

[0084] When the liquid electrolyte of the present invention contains a film-forming additive, the total amount thereof is 0.05 to 5.0% by weight, preferably 0.05 to 3.0% by weight, based on the total weight of the liquid electrolyte.

[0085] In a more preferred embodiment, the total amount of film-forming additives comprises at least 1.0% by weight of the liquid electrolyte.

[0086] According to one embodiment, the liquid electrolyte according to the present invention further comprises at least one HF scavenger. HF generated by hydrolysis of lithium salts, such as LiPF6, can dissolve transition metal components at the interface between the cathode and the electrolyte, adversely affecting the stability of the SEI layer protecting the electrode during repeated cycling, promoting the leaching of SEI components, and promoting the decomposition of the electrolyte at the reactive electrode. This ultimately leads to poor cycle life of the cathode electroactive material.

[0087] In particular, for Si-based anodes, the generation of HF has a negative impact on the reliability of the SEI layer, making the presence of an HF scavenger more important for achieving high-performance batteries.

[0088] In a preferred embodiment, the HF scavenger is a nitrile compound, such as adiponitrile (AN), succinonitrile (SN), hexanetricyanide (1,3,6-HTCN), and the like.

[0089] When forming an anode for a lithium secondary battery, the anode electroactive material is not particularly limited, - graphitic carbon capable of intercalating lithium, typically present in the form of lithium-hosting powders, flakes, fibers or spheres (e.g., mesocarbon microbeads) or the like; - Lithium metal; - lithium alloy compositions such as those described in particular in US Pat. No. 6,203,944 (3M Innovative Properties Co.) and WO 2000 / 03444 (Minnesota Mining & Manufacturing Co.); - Generally, the formula Li4Ti5O 12 Lithium titanates represented by the formula: (These compounds have mobile ions, i.e., Li + generally considered to be a "zero-strain" intercalation material that has a low level of physical expansion when absorbed); - Lithium-silicon alloys, commonly known as lithium silicides, with high Li / Si ratios, especially those of the formula Li 4.4Lithium silicide; - Lithium 4.4 - Germanium-containing lithium-germanium alloy containing a crystalline phase of Ge; - Silicon; and - Silicon-carbon composite material; may be included.

[0090] In one embodiment, the anode contains silicon or a silicon-carbon composite material as the anode electroactive material.

[0091] The present invention relates to the use of a liquid electrolyte containing at least one fluorinated acyclic carboxylic acid ester represented by formula (III) R 1 -C(O)O-R 2 (III) (wherein R 1 and R 2 each represent an alkyl group, the total number of carbon atoms in R 1 and R 2 is 2 to 7, R 1 does not contain fluorine, and R 2 contains fluorine) in a lithium secondary battery for improving cycle performance, wherein the secondary battery is of formula (I) Li [(2x+2) / (x+2)] Mn [2x / (x+2)] M [(2-2x) / (x+2)] O2(I) (where 0 < x < 1, M is of general formula (II) (Ni a Mn b Co c ) 1-d M’ d (II) is a combination of elements represented by, a + b + c = 1, 0 ≦ c ≦ 0.1, 0 ≦ d ≦ 0.1, and M’ contains 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) relates to the use, wherein the lithium transition metal oxide represented by is included as the cathode electroactive material.

[0092] In a preferred embodiment, 0.1≦x≦0.7.

[0093] In a more preferred embodiment, 0.3≦x≦0.55.

[0094] In one embodiment, the liquid electrolyte is - 20 to 90% by weight, preferably 40 to 80% by weight, more preferably 50 to 75% by weight of a fluorinated acyclic carboxylic acid ester; and - 10 to 80% by weight, preferably 20 to 60% by weight, more preferably 25 to 50% by weight of at least one organic carbonate; Includes.

[0095] In certain embodiments, the fluorinated acyclic carboxylic acid ester is CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate).

[0096] In another particular embodiment, the organic carbonate is a mixture of fluoroethylene carbonate, propylene carbonate, and ethylene carbonate.

[0097] In one embodiment, the liquid electrolyte comprises a mixture of a fluorinated acyclic carboxylic acid ester, a fluorinated acyclic diether, and an organic carbonate.

[0098] In another embodiment, the liquid electrolyte comprises a mixture of a fluorinated acyclic carboxylic ester, a fluorinated acyclic carbonate, a fluorinated acyclic diether, and an organic carbonate.

[0099] In another embodiment, the liquid electrolyte comprises a mixture of a fluorinated acyclic carboxylic acid ester, a fluorinated acyclic carbonate, and an organic carbonate.

[0100] In a preferred embodiment, the liquid electrolyte for the lithium secondary battery according to the present invention comprises: - CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate) as a fluorinated acyclic carboxylic acid ester; - CH3-OC(O)O-CH2CF3 (methyl 2,2,2-trifluoroethyl carbonate) as a fluorinated acyclic carbonate; - CF2HCF2-O-CH2CH2-O-CF2CF2H as fluorinated acyclic diethers; - mixtures of PC with FEC and / or EC as organic carbonates; - 1M LiPF6 as lithium salt; - AN, SN, and / or 1,3,6-HTCN as HF scavengers; and - LiDFOB as a film-forming additive; Includes.

[0101] In another preferred embodiment, the liquid electrolyte for the lithium secondary battery according to the present invention comprises - CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate) as a fluorinated acyclic carboxylic acid ester; - CF2HCF2-O-CH2CH2-O-CF2CF2H as fluorinated acyclic diethers; - mixtures of PC with FEC and / or EC as organic carbonates; - 1M LiPF6 as lithium salt; - AN, SN, and / or 1,3,6-HTCN as HF scavengers; and - LiDFOB as a film-forming additive; Includes.

[0102] In another preferred embodiment, the liquid electrolyte for the lithium secondary battery according to the present invention comprises - CH3-C(O)O-CH2CF2H (2,2-difluoroethyl acetate) as a fluorinated acyclic carboxylic acid ester; - CH3-OC(O)O-CH2CF3 (methyl 2,2,2-trifluoroethyl carbonate) as a fluorinated acyclic carbonate; - mixtures of PC with FEC and / or EC as organic carbonates; - 1M LiPF6 as lithium salt; - AN, SN, and / or 1,3,6-HTCN as HF scavengers; and - LiDFOB as a film-forming additive; Includes.

[0103] To the extent that the disclosures of any patents, patent applications, and publications incorporated herein by reference conflict with the statements of this application to the extent that a term may be unclear, this statement shall control.

[0104] The present invention will now be described in detail with reference to the following examples, the purpose of which is merely illustrative and is not intended to limit the scope of the invention. EXAMPLES

[0105] raw materials - FEC: 4-fluoro-1,3-dioxolan-2-one, commercially available from Soulbrain; - EMC: ethyl methyl carbonate, commercially available from Enchem; - SA076: 2,2-difluoroethyl acetate, a fluorinated acyclic carboxylic acid ester, i.e. CH3-C(O)O-CH2CF2H, synthesized in Solvay; - SA024: Methyl 2,2,2-trifluoroethyl carbonate, a fluorinated acyclic carbonate, i.e. CH3-OC(O)O-CH2CF3, synthesized in Solvay; - BP160: Fluorinated acyclic diether C6F8H6O2 (CHF2CF2-O-CH2CH2-O-CF2CF2H), synthesized in-house at Solvay; - LiBOB: Lithium bis(oxalato)borate, a film forming additive, available from Enchem; - LiPF6: Li salt of lithium hexafluorophosphate, available from Enchem.

[0106] A / Liquid electrolyte formulation: Liquid electrolytes for the inventive examples E1-E3 and comparative example CE1 were prepared. 1M LiPF6 was used as the Li salt, and 0.5 wt% LiBOB was blended as a film-forming additive based on the total weight of the liquid electrolyte. The components of the liquid solvent are summarized in Table 1 below:

[0107] [Table 1]

[0108] In preparing the liquid electrolyte of E1, SA076 and FEC were first mixed under stirring to form a liquid solvent, then 0.5 wt% LiBOB was introduced into it using a magnetic stirrer until the solution became clear. Finally, 1M LiPF6 was dissolved into the solution.

[0109] The liquid electrolyte of E2 was prepared by mixing 58 wt% SA076, 30 wt% FEC, and 12 wt% SA024 together under stirring to form a liquid solvent, followed by introducing 0.5 wt% LiBOB into it using a magnetic stirrer until the solution became clear. Finally, 1M LiPF6 was dissolved in the solution.

[0110] In preparing the liquid electrolyte of E3, first all the components except BP160 were mixed together under stirring, then 0.5 wt% LiBOB was introduced into it using a magnetic stirrer until the solution became clear, then 1M LiPF6 was dissolved in the solution, and finally BP160 was added to the solution under stirring.

[0111] The liquid electrolyte of CE1 was prepared in a similar manner to E1, except that 70 wt % EMC was used instead of 70 wt % SA076.

[0112] All electrolyte preparation steps were carried out in a glove box.

[0113] B / Coin cell fabrication The cathode, anode, and separator were prepared for the electrolyte test. The cathode electroactive material, namely Li 1.2 Ni 0.2 Mn 0.6 NM13 of O2 was prepared by coprecipitation method. The cathode consisted of NM13, carbon black, and PVDF binder (polyvinylidene difluoride; SOLEF® 5130 available from Solvay Specialty Polymers Italya) in a ratio of 95 / 3 / 2 wt%. The anode consisted of artificial graphite, carbon black, and PVDF binder in a ratio of 90 / 4 / 6 wt%. A polyethylene porous film was used as the separator.

[0114] The electrolyte test was performed by coin cells. All coin parts with CR2032 specifications were commercially available from Wellcos. The components for the coin cell test were a cathode electrode, an anode electrode, and a separator. Each element was first cut into a disk shape. The disk size was Φ15 for the cathode, Φ16 for the anode, and Φ19 for the separator. The elements were then dried overnight under vacuum, i.e., at 100°C for the electrodes and 60°C for the separator. After drying, all components were transferred to a glove box and a coin cell was assembled. The separator was placed between the cathode and anode, and the electrolyte was injected along with the separator. After sealing, the cell was stored at 25°C for 24 hours. The cell was cycled from 2.0 to 4.7 V at 25°C using a PEBC050.1 cycler from WONIKPNE Co.Ltd. to perform the formation of SEI. The cell was charged and discharged at a C / 10 rate for 3 cycles.

[0115] C / Cell evaluation procedure Cycling at 25°C: Cells containing NM13 were maintained at 25±0.1°C and cycled at 0.33C charge / 0.33C discharge from 2.0 to 4.7V.

[0116] D / Result The cycle retention rates of the liquid electrolytes E1 to E3 and CE1 at room temperature (25° C.) are shown in Table 2 below.

[0117] [Table 2]

[0118] The liquid electrolytes E1 to E3 according to the present invention exhibited excellent cycle performance, which was much higher than that of CE1 at 25° C. In particular, CE1, i.e., a liquid electrolyte containing only an organic carbonate without a fluorinated acyclic carboxylate, exhibited a cycle retention rate much inferior to that of E1 to E3.

[0119] In short, it has been clearly demonstrated that the liquid electrolytes E1-E3 according to the present invention provide higher capacity retention than the liquid electrolyte of CE1.

Claims

1. A lithium secondary battery, a) a compound represented by the following formula (I): Li ［（２ｘ＋２）／（ｘ＋２）］ Mn ［２ｘ／（ｘ＋２）］ M ［（２－２ｘ）／（ｘ＋２）］ O ２ (I) (wherein 0<x<1, and M is a group represented by the general formula (II) (N ａ Mn ｂ Co ｃ ) １－ｄ M' ｄ (II) wherein a+b+c=1, 0≦c≦0.1, and 0≦d≦0.1, and M′ includes 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. a cathode comprising as a cathode electroactive material a lithium transition metal oxide represented by b) Formula (III) R １ -C(O)O-2 ２ (999) (In the formula, R １ and R ２ each represents an alkyl group, and R １ and R ２ The total number of carbon atoms in R is 2 to 7, １ does not contain fluorine, and R ２ contains fluorine) a liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester represented by the formula: A lithium secondary battery comprising:

2. 2. The lithium secondary battery according to claim 1, wherein 0.1≦x≦0.7, preferably 0.3≦x≦0.

55.

3. 3. The lithium secondary battery according to claim 1, wherein c is 0.

4. 3. The lithium secondary battery according to claim 1, wherein a*(1-d)<[2x / (x+2)]+b*(1-d).

5. The lithium transition metal oxide is Li １．０９ Mn ０．９１ O ２ , Li １．２５ Mn ０．６２５ Ni ０．１２５ O ２ , Li １．１５ Mn ０．５６ Ni ０．２９ O ２ , and Li １．１７ Mn ０．５４ Ni ０．２８ O ２ The lithium secondary battery according to claim 1 or 2, comprising:

6. R ２ is CH ２ 3. The lithium secondary battery according to claim 1, which contains neither an F- group nor a -CHF- group.

7. where the fluorinated acyclic carboxylic acid ester is CH ３ -C(O)O-CH ２ CF ２ H, CH ３ -C(O)O-CF ２ H, CH ３ -C(O)O-CF ２ CF ３ 、CH ３ -C(O)O-CH ２ CF ３ 、CH ３ -C(O)O-CF ３ 、CH ３ -C(O)O-CF ２ CF ２ CF ３ 、(CH ３ ) ２ CH-C(O)O-CF ３ 、CH ３ CH ２ -C(O)O-CF ２ H, CH ３ CH ２ -C(O)O-CF ３ 、CH ３ CH ２ -C(O)O-CF ２ CH ３ 、CH ３ -C(O)O-CH(CF ３ )CH ３ 、CH<​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​ ２ CF ２ CF ２ H, CH ３ CH ２ CH ２ —C(O)O—CH ２ CF ３ , C.H. ３ —C(O)O—CH ２ CH ２ CH ２ CF ２ CF ３ , (CH ３ ) ２ CH-C(O)O-CH ２ CF ２ H, CH ３ CH ２ CH ２ —C(O)O—CF ２ H, (CH ３ ) ２ CH-C(O)O-CF ２ H, CH ３ CH ２ —C(O)O—CH ２ CF ３ 3. The lithium secondary battery according to claim 1, comprising:

8. 3. The lithium secondary battery according to claim 1, wherein the fluorinated acyclic carboxylic acid ester is present in an amount of 20 to 90 weight % (wt %), preferably 40 to 80 wt %, and more preferably 50 to 75 wt %, based on the total weight of the liquid electrolyte.

9. b) the liquid electrolyte is a compound represented by formula (IV): R ３ -OC(O)O-R ４ (IV) (In the formula, R ３ and R ４ each represents an alkyl group; R ３ and R ４ the total number of carbon atoms in any of R is 2 to 7; ３ and / or R ４ At least one hydrogen atom in The lithium secondary battery according to claim 1 or 2, further comprising a fluorinated acyclic carbonate represented by the formula:

10. b) the liquid electrolyte is a compound represented by the formula (V): R ５ -O-R ６ -O-R ７ (V) (In the formula, R ５ and R ７ each represents a fluorinated linear alkyl group; R ６ represents an optionally fluorinated linear alkyl group; R ５ , R ６ , and R ７ The total number of carbon atoms in is 5 to 8, preferably 6. The lithium secondary battery according to claim 1 or 2, further comprising at least one fluorinated acyclic diether represented by the formula:

11. b) The liquid electrolyte is 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, ethylene carbonate 3. The lithium secondary battery according to claim 1 or 2, further comprising at least one organic carbonate including a fluorinated cyclic carbonate, a non-fluorinated cyclic carbonate, and a non-fluorinated acyclic carbonate selected from the group consisting of butyl ether, propylene carbonate, butylene carbonate, trimethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, dimethyl vinylene carbonate, ethyl propyl carbonate, cyclohexene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and mixtures thereof, in an amount of 10 to 80 wt %, preferably 20 to 60 wt %, and more preferably 25 to 50 wt %, based on the total weight of the liquid electrolyte.

12. b) The liquid electrolyte is lithium hexafluorophosphate (LiPF ６ ), lithium perchlorate (LiClO ４ ), lithium hexafluoroarsenate (LiAsF ６ ), lithium hexafluoroantimonate (LiSbF ６ ), lithium hexafluorotantalate (LiTaF ６ ), lithium tetrachloroaluminate (LiAlCl ４ ), lithium tetrafluoroborate (LiBF ４ ), lithium chloroborate (Li ２ B １０ Cl １０ ), lithium fluoroborate (Li ２ B １０ F １０ ), Li ２ B １２ F ｘ H １２－ｘ (wherein x=0 to 12); LiPF ｘ (R Ｆ ) ６－ｘ and LiBF ｙ (R Ｆ ) ４－ｙ (In the formula, R Ｆ is perfluorinated C １ ~C ２０ represents an alkyl group or a perfluorinated aromatic group, x=0 to 5, and y=0 to 3), LiBF ２ [O ２ C (CX ２ ) ｎ CO ２ ], LiPF ２ [O ２ C (CX ２ ) ｎ CO ２ ] ２ , LiPF ４ [O ２ C (CX ２ ) ｎ CO ２ ] (wherein X is H, F, Cl, C １ ~C ４ alkyl groups and fluorinated alkyl groups, n=0 to 4), lithium trifluoromethanesulfonate (LiCF ３ SO ３ ), lithium bis(fluorosulfonyl)imide Li(FSO ２ ) ２ N(LiFSI), LiN(SO ２ C ｍ F ２ｍ＋１ ) (SO ２ C ｎ F ２ｎ＋１ ) and LiC(SO ２ C ｋ F ２ｋ＋１ ) (SO ２ C ｍ F ２ｍ＋１ ) (SO ２ C ｎ F ２ｎ＋１ ) (wherein k=1 to 10, m=1 to 10, and n=1 to 10), LiN(SO ２ C ｐ F ２ｐ SO ２ ) and LiC(SO ２ C ｐ F ２ｐ SO ２ ) (SO ２ C ｑ F ２ｑ＋１ 3. The lithium secondary battery according to claim 1, further comprising at least one lithium salt selected from the group consisting of:

13. b) the liquid electrolyte is selected from the group consisting of sulfur compounds including 1,3,2-dioxathiolane-2,2-dioxide, 1,3,2-dioxathiane-2,2-dioxide, 1,3-propane sultone, ethylene sulfite, and prop-1-ene-1,3-sultone; sulfone derivatives including dimethyl sulfone, tetramethylene sulfone (also known as sulfolane), ethyl methyl sulfone, and isopropyl methyl sulfone; nitrile derivatives including succinonitrile, adiponitrile, and glutaronitrile; and lithium nitrate (LiNO ３ ); lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalato)borate (LiB(C) ２ O ４ ) ２ ; LiBOB), lithium fluoromalonato(difluoro)borate (LiB(O ２ CCHFCO ２ ) ２ ; LiFMDFB), lithium bis(malonato)borate [LiB(O ２ CCH ２ CO ２ ) ２ ], lithium bis(difluoromalonato)borate [LiB(O ２ CCF ２ CO ２ ) ２ ], lithium (malonatooxalato)borate [LiB(C ２ O ４ ) (O ２ CCH ２ CO ２ )], lithium (difluoromalonatooxalato)borate [LiB(C ２ O ４ ) (O ２ CCF ２ CO ２ ) )]; boron derivative salts containing lithium tris(oxalato)phosphate [LiP(C ２ O ４ ) ３ ], lithium tris(difluoromalonato)phosphate [LiP(O ２ CCF ２ CO ２ ) ３ ], lithium difluorophosphate (LiPO ２ F ２ 3. The lithium secondary battery according to claim 1, further comprising at least one film-forming additive selected from the group consisting of phenyl ether, phenyl ether, phenyl ether (phenyl ether), ...

14. Formula (III) in a lithium secondary battery to improve cycle performance R １ -C(O)O-2 ２ (999) (In the formula, R １ and R ２ each represents an alkyl group, and R １ and R ２ The total number of carbon atoms in R is 2 to 7, １ does not contain fluorine, and R ２ contains fluorine) a liquid electrolyte comprising at least one fluorinated acyclic carboxylic acid ester represented by As a cathode electroactive material, Li ［（２ｘ＋２）／（ｘ＋２）］ Mn ［２ｘ／（ｘ＋２）］ M ［（２－２ｘ）／（ｘ＋２）］ O ２ (I) (wherein 0<x<1, and M is a group represented by the general formula (II) (N ａ Mn ｂ Co ｃ ) １－ｄ M' ｄ (II) a + b + c = 1, 0 ≦ c ≦ 0.1, and 0 ≦ d ≦ 0.1; M' includes 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. Lithium transition metal oxide represented by Use of.

15. The fluorinated acyclic carboxylic acid ester is CH ３ —C(O)O—CH ２ CF ２ 15. Use of the liquid electrolyte according to claim 14, wherein the liquid electrolyte is H.