Electrolyte for lithium secondary battery and lithium secondary battery containing the same

Abstract

Disclosed is an electrolyte for a lithium secondary battery that can improve the thermal stability of a lithium secondary battery including a lithium nickel cobalt manganese-based positive electrode active material by adjusting the types and ratios of salts and solvents contained in the electrolyte, and a lithium secondary battery including the same. The electrolyte for a lithium secondary battery includes a lithium salt including a first lithium salt and a second lithium salt; and a solvent; wherein the first lithium salt includes lithium hexafluorophosphate, and the second lithium salt includes lithium bis(fluorosulfonyl)imide or lithium bis(trifluorosulfonyl)imide.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0011159 filed January 26, 2022 and Korean Patent Application No. 10-2023-0002051 filed January 6, 2023, and all contents disclosed in the documents of the relevant Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a lithium secondary battery that can improve the thermal stability of a lithium secondary battery including a lithium nickel cobalt manganese-based positive electrode active material by adjusting the types and ratios of salts and solvents contained in the electrolyte, and a lithium secondary battery including the same. [Background technology]

[0003] Recently, with the rapid spread of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles, the demand for small, lightweight, and relatively high-capacity secondary batteries is rapidly increasing. Lithium secondary batteries, in particular, are gaining attention as a power source for portable devices due to their light weight and high energy density. As a result, research and development efforts to improve the performance of lithium secondary batteries are actively underway. Lithium secondary batteries generate electrical energy through oxidation and reduction reactions caused by lithium ions intercalating and deintercalating between a positive electrode and a negative electrode, which are made of active materials capable of lithium ion intercalation and deintercalation, while an organic or polymer electrolyte is charged between the positive and negative electrodes.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2, LiMn2O4, etc.), and lithium iron phosphate (LiFePO4) have been used as positive electrode active materials in lithium secondary batteries. Among these, lithium cobalt oxide (LiCoO2) is widely used due to its high operating voltage and excellent capacity characteristics, making it an ideal positive electrode active material for high-voltage applications. However, lithium cobalt oxide (LiCoO2) suffers from poor thermal properties due to the destabilization of its crystal structure caused by delithiation. Its high cost and high price limit its mass use as a power source in fields such as electric vehicles. Lithium nickel oxide (LiNiO2), which has a high reversible capacity of approximately 200 mAh / g and is therefore suitable for the realization of high-capacity batteries, has also been the subject of active research and development. However, its thermal stability is relatively lower than that of lithium cobalt oxide (LiCoO2). If an internal short circuit occurs during charging due to external pressure, the positive electrode active material itself may decompose, resulting in battery explosion and fire.

[0005] Therefore, as a way to improve the low thermal stability of lithium nickel oxide (LiNiO2) while maintaining its excellent reversible capacity, lithium nickel cobalt manganese-based positive electrode active material (or lithium NCM-based positive electrode active material, or NCM-based lithium composite transition metal oxide, or high Ni positive electrode material) was developed, in which part of the nickel (Ni) is replaced with cobalt (Co) and manganese (Mn). When this lithium nickel cobalt manganese-based positive electrode active material is applied to batteries, it has the advantage of being able to achieve high capacity.

[0006] However, the conventional lithium secondary batteries described above can experience thermal runaway due to an exothermic reaction initiated by the decomposition of the negative electrode SEI (solid electrolyte interface) and a reaction between the positive electrode, which becomes more unstable as the nickel (Ni) content increases, and the electrolyte solution, which contains a carbonate-based solvent. This poses a major threat to the stability of the battery. In particular, lithium-ion batteries (LIBs) that use a lithium-nickel-cobalt-manganese positive electrode active material together with a graphite negative electrode are relatively more vulnerable to heat.

[0007] Therefore, for the safe use of lithium secondary batteries using lithium-nickel-cobalt-manganese-based positive electrode active materials that can realize high capacity, a method for improving the thermal stability of the battery is urgently needed. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to provide an electrolyte for a lithium secondary battery, which can improve the thermal stability of a lithium secondary battery including a lithium nickel cobalt manganese-based positive electrode active material by adjusting the types and ratios of salts and solvents contained in the electrolyte, and a lithium secondary battery including the same. [Means for solving the problem]

[0009] To achieve the above object, the present invention provides an electrolyte solution for a lithium secondary battery, comprising: a lithium salt including a first lithium salt and a second lithium salt; and a solvent; wherein the first lithium salt comprises lithium hexafluorophosphate, and the second lithium salt comprises lithium bis(fluorosulfonyl)imide or lithium bis(trifluorosulfonyl)imide.

[0010] The present invention also provides a lithium secondary battery including: a positive electrode including a lithium nickel cobalt manganese-based positive electrode active material; a negative electrode; at least one of a separator and a solid electrolyte interposed between the positive electrode and the negative electrode; and the electrolyte solution for the lithium secondary battery. [Effects of the Invention]

[0011] The electrolyte for a lithium secondary battery and the lithium secondary battery including the same according to the present invention have an advantage that the thermal stability of a lithium secondary battery including a lithium nickel cobalt manganese-based positive electrode active material can be improved by adjusting the types and ratios of salts and solvents included in the electrolyte. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery including an electrolyte solution according to an embodiment of the present invention. [Figure 2] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery including an electrolyte solution according to an embodiment of the present invention. [Figure 3] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery including an electrolyte solution according to an embodiment of the present invention. [Figure 4] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery including an electrolyte solution according to an embodiment of the present invention. [Figure 5] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery including an electrolyte solution according to an embodiment of the present invention. [Figure 6] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery containing a conventional electrolyte solution. [Figure 7] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery containing a conventional electrolyte solution. [Figure 8] 1 is a graph showing experimental results of thermal stability of a lithium secondary battery containing a conventional electrolyte solution. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below.

[0014] The electrolyte for a lithium secondary battery according to the present invention includes a lithium salt including a first lithium salt and a second lithium salt; and a solvent; wherein the first lithium salt includes lithium hexafluorophosphate, and the second lithium salt includes lithium bis(fluorosulfonyl)imide or lithium bis(trifluorosulfonyl)imide.

[0015] Lithium secondary batteries generate electrical energy through oxidation and reduction reactions when lithium ions are inserted into and extracted from the positive and negative electrodes while an organic or polymer electrolyte is charged between them, which are made of active materials that allow for the intercalation and deintercalation of lithium ions. To improve thermal stability, lithium-nickel-cobalt-manganese-based positive electrode active materials (or lithium NCM-based positive electrode active materials, or NCM-based lithium composite transition metal oxides, or high-Ni positive electrode materials) are used in batteries, with some of the nickel (Ni) replaced with cobalt (Co) and manganese (Mn).

[0016] However, conventional lithium secondary batteries can experience thermal runaway due to an exothermic reaction initiated by the decomposition of the negative electrode SEI (solid electrolyte interface) and a reaction between the positive electrode, which becomes more unstable as the nickel (Ni) content increases, and the electrolyte solution, which contains a carbonate-based solvent. This poses a major threat to the stability of the battery. In particular, lithium-ion batteries (LIBs), which use a lithium-nickel-cobalt-manganese positive electrode active material together with a graphite negative electrode, are relatively more vulnerable to heat.

[0017] Therefore, the present applicant has invented an electrolyte for a lithium secondary battery that improves the thermal stability of the battery, and a lithium secondary battery including the same, for safe use of a lithium-nickel-cobalt-manganese-based positive electrode active material that can realize high capacity.

[0018] The lithium salt contained in the electrolyte for a lithium secondary battery is used to increase ionic conductivity, and the most notable feature of the present invention is that it improves the thermal stability of the battery by combining specific lithium salts (i.e., a first lithium salt and a second lithium salt). That is, the lithium salt contained in the electrolyte for a lithium secondary battery according to the present invention includes a first lithium salt and a second lithium salt. The first lithium salt includes lithium hexafluorophosphate (LiPF6), and the second lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluorosulfonyl)imide (LiTFSI). In other words, the lithium salt contained in the electrolyte for a lithium secondary battery according to the present invention includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, or lithium hexafluorophosphate and lithium bis(trifluorosulfonyl)imide.

[0019] Meanwhile, the first lithium salt and the second lithium salt may each include a common lithium salt used in the art. That is, the first lithium salt and the second lithium salt contained in the electrolyte for a lithium secondary battery of the present invention may each be LiCl, LiBr, LiI, LiClO4, LiBF ... 10 Cl 10 , LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (C2F5SO2)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium aliphatic carboxylates having 4 or less carbon atoms, lithium 4-phenylborate, and lithium imide.

[0020] However, for purposes of the present invention, it is preferred that the first lithium salt contains only lithium hexafluorophosphate and the second lithium salt contains only lithium bis(fluorosulfonyl)imide or lithium bis(trifluorosulfonyl)imide.

[0021] The present applicant has also confirmed that the molar ratio of the first lithium salt to the second lithium salt also contributes to the thermal stability of the battery. The molar ratio of the first lithium salt to the second lithium salt is 1:0.4-0.9, preferably 1:0.4-0.8, more preferably 1:0.4-0.7, and most preferably about 1:0.7. If the molar ratio of the first lithium salt to the second lithium salt is outside the range of 1:0.4-0.9, the contribution to the thermal stability of the battery may be negligible or there may be no further benefit in improving the thermal stability of the battery.

[0022] The present applicant has also confirmed that the concentration of the total lithium salt, including the first and second lithium salts, also contributes to the thermal stability of the battery. The concentration of the total lithium salt, including the first and second lithium salts, may be 1.5M to 4.0M, preferably 1.5M to 2.0M. If the concentration of the total lithium salt is less than 1.5M or more than 4.0M, the contribution to the thermal stability of the battery may be negligible or may not be further beneficial in improving the thermal stability of the battery. Furthermore, if the concentration of the lithium salt is less than 1.5M, it may be difficult to ensure ionic conductivity suitable for battery operation. If the concentration exceeds 4.0M, the viscosity of the electrolyte may increase, reducing the mobility of lithium ions or increasing the decomposition reaction of the lithium salt itself, resulting in reduced battery performance. The preferred concentration of the first lithium salt may be 0.5M to 1.3M, and the preferred concentration of the second lithium salt may be 0.7M to 1.5M.

[0023] Next, the solvent contained in the electrolyte for a lithium secondary battery of the present invention will be described. The solvent contained in the electrolyte for a lithium secondary battery also contributes to the thermal stability of the battery and basically contains a carbonate-based compound. Specifically, the solvent contained in the electrolyte for a lithium secondary battery includes a first solvent and a second solvent. Therefore, each of the first solvent and the second solvent basically contains a carbonate-based compound. Examples of such carbonate-based compounds include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and mixtures containing two or more of these.

[0024] Preferably, the first solvent and the second solvent each contain a carbonate-based compound, and each independently contain another carbonate-based compound. More preferably, the first solvent is ethylene carbonate, and the second solvent is dimethyl carbonate or ethyl methyl carbonate. In other words, the most preferred embodiment is that the solvent contained in the electrolyte for a lithium secondary battery of the present invention contains ethylene carbonate and dimethyl carbonate, or ethylene carbonate and ethyl methyl carbonate.

[0025] The applicant has also confirmed that the mixing ratio of the first solvent to the second solvent also contributes to the thermal stability of the battery. The volume ratio of the first solvent to the second solvent may be 5:95 to 35:65, preferably 10:90 to 30:70, and more preferably close to or even equal to 10:90. If the mixing ratio of the first solvent to the second solvent is outside this range, the contribution to the thermal stability of the battery may be negligible or may not provide any further benefit in improving the thermal stability of the battery. As can be seen from the range of the mixing ratio of the first solvent to the second solvent, the present invention is also characterized in that the second solvent is contained in a higher content ratio than the first solvent among the solvents contained in the electrolyte.

[0026] Meanwhile, the solvent contained in the lithium secondary battery electrolyte of the present invention may further include one or more esters, ethers, or ketones, as needed. Examples of these include aprotic organic solvents such as γ-butyrolactone, n-methyl acetate, n-ethyl acetate, n-propyl acetate, phosphate triesters, dibutyl ether, N-methyl-2-pyrrolidinone, 1,2-dimethoxyethane, tetrahydrofuran derivatives such as 2-methyltetrahydrofuran, dimethyl sulfoxide, formamide, dimethylformamide, dioxolane and its derivatives, acetonitrile, nitromethane, methyl formate, methyl acetate, trimethoxymethane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, methyl propionate, and ethyl propionate. However, to achieve the objectives of the present invention, it is preferable to exclude these and include only the first and second solvents.

[0027] The lithium secondary battery electrolyte solution of the present invention may contain, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, glyme compounds, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc., for the purpose of improving charge / discharge characteristics and flame retardancy. If necessary, the electrolyte solution may further contain a halogen-containing solvent, such as carbon tetrachloride or trifluoroethylene, to impart non-flammability, or carbon dioxide to improve high-temperature storage characteristics.

[0028] Next, the lithium secondary battery according to the present invention will be described.

[0029] The lithium secondary battery includes a positive electrode containing a lithium-nickel-cobalt-manganese-based positive electrode active material; a negative electrode; at least one of a separator and a solid electrolyte interposed between the positive electrode and the negative electrode; and the lithium secondary battery electrolyte. The lithium secondary battery electrolyte is the same as described above, and therefore further description thereof will be omitted. The lithium secondary battery of the present invention is characterized by being thermally stable even at high temperatures of 170°C or higher. In other words, the lithium secondary battery of the present invention is characterized by not deforming or exploding at temperatures of 170°C or higher.

[0030] The positive electrode includes a lithium nickel cobalt manganese-based positive electrode active material (or a lithium NCM-based positive electrode active material, or an NCM-based lithium composite transition metal oxide, or a high Ni positive electrode material), and when applied to a battery, it can achieve high capacity. The lithium nickel cobalt manganese-based positive electrode active material may be coated with a metal oxide on its surface.

[0031] The lithium nickel cobalt manganese-based positive electrode active material can be commercially available or can be prepared by a method well known in the art. For example, a nickel-cobalt-manganese precursor can be prepared by adding an ammonium cation-containing complex former and a basic compound to a transition metal solution containing a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material, and then co-precipitating the mixture. The nickel-cobalt-manganese precursor can then be mixed with a lithium raw material and calcined at a temperature of 980°C or higher to prepare the lithium nickel cobalt manganese-based positive electrode active material.

[0032] The nickel-containing source material may be, for example, a nickel-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide, such as, but not limited to, Ni(OH)2, NiO, NiOOH, NiCO3·2Ni(OH)2·4H2O, NiCO2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, a fatty acid nickel salt, a nickel halide, or a combination thereof. The cobalt-containing source material may be, for example, a cobalt-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide, such as, but not limited to, Co(OH)2, CoOOH, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4, Co(SO4)2·7H2O, or a combination thereof. The manganese-containing raw material may be, for example, a manganese-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide, or a combination thereof, and specifically may be, but is not limited to, manganese oxides such as Mn2O3, MnO2, and Mn3O4; manganese salts such as MnCO3, Mn(NO3)2, MnSO4, manganese acetate, manganese dicarboxylate, manganese citrate, and manganese fatty acid salt; manganese oxyhydroxide, manganese chloride, or a combination thereof.

[0033] The transition metal solution may be prepared by adding the nickel-containing raw material, cobalt-containing raw material, and manganese-containing raw material to a solvent, specifically, water or a mixture of an organic solvent (e.g., alcohol) that is uniformly miscible with water, or by mixing an aqueous solution of the nickel-containing raw material, an aqueous solution of the cobalt-containing raw material, and the manganese-containing raw material. The ammonium cation-containing complexing agent may be, but is not limited to, NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, NH4CO3, or a combination thereof. Meanwhile, the ammonium cation-containing complexing agent may be used in the form of an aqueous solution, and the solvent may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water.

[0034] The basic compound may be a hydroxide of an alkali metal or alkaline earth metal, such as NaOH, KOH, or Ca(OH)2, a hydrate thereof, or a combination thereof. The basic compound may also be used in the form of an aqueous solution, in which case the solvent may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water. The basic compound is added to adjust the pH of the reaction solution, and may be added so that the pH of the metal solution is 11 to 13.

[0035] Meanwhile, the coprecipitation reaction can be carried out at a temperature of 40 to 70°C under an inert atmosphere such as nitrogen or argon. Through this process, nickel-cobalt-manganese hydroxide particles are produced and precipitated in the reaction solution. The precipitated nickel-cobalt-manganese hydroxide particles are separated and dried using a conventional method to obtain a nickel-cobalt-manganese precursor. The nickel-cobalt-manganese precursor may be secondary particles formed by agglomeration of primary particles. The average particle size (D50) of the nickel-cobalt-manganese precursor secondary particles may be 4 to 8 μm, preferably 4 to 7.5 μm, and more preferably 4 to 7 μm.

[0036] The lithium source material may be a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide, and is not particularly limited as long as it is soluble in water. Specifically, the lithium source may be Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, LiH, LiF, LiCl, LiBr, LiI, CHClO1, Li2O, Li2SO4, CHClO1, or Li3C6H5O7, and any one or a mixture of two or more of these may be used. The lithium source materials may be mixed so that the molar ratio (Li / M) of lithium (Li) to the total metal elements (M) of the nickel-cobalt-manganese precursor is 1 to 1.5, preferably 1 to 1.1.

[0037] The content of the lithium nickel cobalt manganese-based positive electrode active material may be 50 to 95 parts by weight, preferably 60 to 90 parts by weight, per 100 parts by weight of the positive electrode. If the content of the lithium nickel cobalt manganese-based positive electrode active material is less than 50 parts by weight per 100 parts by weight of the total weight of the positive electrode, the electrochemical characteristics of the battery due to the positive electrode active material may be reduced, and if the content exceeds 95 parts by weight, additional components such as binders and conductive materials may be included in small amounts, making it difficult to efficiently manufacture the battery.

[0038] The positive electrode further includes a binder and a conductive material in addition to the positive electrode active material. The binder is a component that aids in binding the positive electrode active material and the conductive material, etc., and in binding to the current collector. Examples of the binder include, but are not limited to, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF / HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl(meth)acrylate, polyethyl(meth)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof.

[0039] The binder may be used in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, per 100 parts by weight of the total weight of the positive electrode. If the binder content is less than 1 part by weight per 100 parts by weight of the total weight of the positive electrode, the adhesive strength between the positive electrode active material and the current collector may be insufficient. On the other hand, if the binder content exceeds 50 parts by weight per 100 parts by weight of the total weight of the positive electrode, the adhesive strength is improved, but the content of the positive electrode active material is reduced accordingly, which may result in a lower battery capacity.

[0040] The conductive material contained in the positive electrode is not particularly limited as long as it has excellent electrical conductivity without causing side reactions in the internal environment of the lithium secondary battery or chemical changes to the battery. Representative examples include graphite or conductive carbon, and examples thereof include, but are not limited to, graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon-based materials having a graphene or graphite crystal structure; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives.

[0041] The conductive material may be used in an amount of 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight, per 100 parts by weight of the total weight of the positive electrode. If the conductive material content is too low, such as less than 0.5 parts by weight per 100 parts by weight of the total weight of the positive electrode, the improvement in electrical conductivity may be difficult to expect, or the electrochemical characteristics of the battery may be degraded. Furthermore, if the conductive material content is too high, such as more than 50 parts by weight per 100 parts by weight of the total weight of the positive electrode, the amount of positive electrode active material may be relatively reduced, resulting in reduced capacity and energy density. The method for incorporating the conductive material into the positive electrode is not particularly limited, and conventional methods known in the art, such as coating the positive electrode active material, may be used. If necessary, a conductive second coating layer may be added to the positive electrode material instead of adding the conductive material.

[0042] In addition, a filler may be optionally added to the positive electrode of the present invention as a component for suppressing its expansion. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without causing chemical changes in the battery, and examples of such fillers include olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fiber and carbon fiber; etc.

[0043] The cathode active material, binder, and conductive material are dispersed and mixed in a dispersion medium (solvent) to form a slurry, which is then coated on a cathode current collector, dried, and rolled to produce the cathode included in the lithium secondary battery of the present invention. The dispersion medium may be, but is not limited to, NMP (N-methyl-2-pyrrolidone), DMF (dimethyl formamide), DMSO (dimethyl sulfoxide), ethanol, isopropanol, water, or a mixture thereof.

[0044] The positive electrode current collector may be, but is not limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO), FTO (F-doped SnO), alloys thereof, or aluminum (Al) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag). The positive electrode current collector may be in the form of a foil, film, sheet, punched, porous, foam, or the like.

[0045] The negative electrode can be manufactured by a conventional method known in the art. For example, a negative electrode active material, a conductive material, a binder, and optionally a filler can be dispersed and mixed in a dispersion medium (solvent) to form a slurry, which can then be applied to a negative electrode current collector, dried, and rolled to manufacture the negative electrode. The negative electrode active material can be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Sb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of two or more of these materials can be used. A thin film of metallic lithium can also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon can be used as the carbon material. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0046] The binder and conductive material used in the negative electrode are the same as those described above for the positive electrode. The negative electrode current collector may be made of, but is not limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), copper (Cu), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO2), FTO (F-doped SnO2), alloys thereof, or copper (Cu) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag). The negative electrode current collector may be in the form of a foil, film, sheet, punched, porous, foam, or the like.

[0047] The separator is interposed between the positive and negative electrodes to prevent short circuits and provide a path for lithium ions to move between them. The separator can be made of olefin polymers such as polyethylene and polypropylene, glass fiber, or the like, and can be used in the form of a sheet, multilayer, microporous film, woven fabric, or nonwoven fabric, but is not limited to these. However, porous polyethylene or porous glass fiber nonwoven fabric may be preferred, and porous glass filter may be more preferred. The separator may be an insulating thin film with high ion permeability and mechanical strength. The pore diameter of the separator may be typically 0.01 to 10 μm, and the thickness may be typically 5 to 300 μm, but is not limited to these.

[0048] Meanwhile, in the lithium secondary battery, a solid electrolyte may be positioned between the positive electrode and the negative electrode as a layered membrane. Therefore, in this case, the solid electrolyte may also function as a separator (i.e., electrically insulating the negative electrode from the positive electrode while allowing lithium ions to pass through). In this case, the solid electrolyte may be attached to one surface of the positive electrode or the negative electrode and included in the lithium secondary battery. That is, the lithium secondary battery of the present invention may be a semi-solid-state battery that uses both a liquid electrolyte and a solid electrolyte, as needed. In this case, a separate separator may also be included (i.e., at least one of a separator and a solid electrolyte may be interposed between the positive electrode and the negative electrode). The solid electrolyte may include at least one selected from a polymer-based solid electrolyte, a sulfide-based solid electrolyte, and an oxide-based solid electrolyte, and preferably includes only a polymer-based solid electrolyte.

[0049] Here, the polymer solid electrolyte includes a polymer and a lithium salt. The polymer may be, but is not limited to, one or more selected from the group consisting of polypropylene carbonate (PPC), polyacrylonitrile (PAN), and polyvinylpyrrolidone (PVP). The lithium salt may be, for example, LiNO3, LiOH, LiSCN, LiCl, LiBr, LiI, LiClO4, LiBF4, or LiB 10 Cl 10 Examples of the lithium salt include, but are not limited to, one or more selected from the group consisting of LiCF3SO3, LiPF6, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium aliphatic carboxylates having four or less carbon atoms, lithium tetraphenylborate, and lithium imide. The weight ratio of the polymer and lithium salt contained in the polymer solid electrolyte may be, for example, 1:0.5 to 1:3 in molar ratio.

[0050] Meanwhile, the lithium secondary battery of the present invention can be manufactured by a conventional method in the art. For example, it can be manufactured by inserting a porous separator between a positive electrode and a negative electrode and then adding an electrolyte. The lithium secondary battery of the present invention can be applied to a battery cell used as a power source for a small device, and is particularly suitable for use as a unit battery of a battery module that is a power source for a medium- to large-sized device. In this regard, the present invention also provides a battery module including two or more lithium secondary batteries electrically connected (in series or in parallel). Of course, the number of lithium secondary batteries included in the battery module can be variously adjusted taking into account the use and capacity of the battery module.

[0051] The present invention also provides a battery pack in which the battery modules are electrically connected in accordance with conventional techniques in the art. The battery modules and battery pack can be used as a power source for one or more medium- to large-sized devices, including, but not limited to, power tools, electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), electric trucks, electric commercial vehicles, and power storage systems.

[0052] Below, preferred examples are presented to help understand the present invention, but these are merely illustrative of the present invention, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical idea of ​​the present invention, and it is natural that such changes and modifications also fall within the scope of the accompanying claims.

[0053] [Example 1] Lithium secondary battery manufacturing Electrolyte production First, lithium hexafluorophosphate (LiPF6, the first lithium salt) and lithium bis(fluorosulfonyl)imide (LiFSI, the second lithium salt) were dissolved in a molar ratio of 0.7:0.3 in an organic solvent prepared by mixing ethylene carbonate (first solvent) and dimethyl carbonate (second solvent) in a volume ratio (v / v) of 30:70 to prepare an electrolyte for a lithium secondary battery.

[0054] Cathode manufacturing First, a 2.4M precursor formation solution was prepared in a 40L batch reactor set at 50°C by mixing NiSO4, CoSO4, and MnSO4 in water in amounts such that the molar ratio of nickel:cobalt:manganese was 80:10:10. After 13 L of deionized water was added to a coprecipitation reactor (40L capacity), nitrogen gas was purged into the reactor at a rate of 25 L / min to remove dissolved oxygen in the water and create a non-oxidizing atmosphere inside the reactor. Next, 83 g of a 25% NaOH aqueous solution was added, and the mixture was stirred at 700 rpm at 50°C to maintain a pH of 11.5. The precursor formation solution was then added at a rate of 1.9 L / hr, and the NaOH aqueous solution and NH4OH aqueous solution were added together to conduct a coprecipitation reaction for 48 hours to produce a nickel-cobalt-manganese hydroxide (Ni 0.5 Co 0.3 Mn 0.2 (OH)2) particles were formed. The hydroxide particles were separated, washed, and then dried in an oven at 120°C to prepare a nickel-cobalt-manganese precursor (D50 = 4.8 μm). The nickel-cobalt-manganese precursor and lithium source LiOH were then added to a 20 L Hensel mixer so that the Li / M (Ni, Co, Mn) molar ratio was 1.02 and mixed at 300 rpm at the center for 20 minutes. The mixed powder was placed in a 330 mm x 330 mm alumina crucible and calcined in an oxygen atmosphere at 1,010-1,030°C for 15 hours to prepare a lithium-nickel-cobalt-manganese-based positive electrode active material.

[0055] Next, the prepared lithium-nickel-cobalt-manganese-based positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 96.5:1.5:2 and dispersed in an NMP solvent to prepare a slurry. The slurry was then uniformly coated on a 25 μm thick aluminum foil using a blade-type coating machine, a Labdryer / coater type LTE (Werner Mathis AG), and dried in a vacuum oven at 120° C. for 13 hours to prepare a positive electrode for a lithium secondary battery.

[0056] Lithium secondary battery manufacturing An electrode assembly was fabricated by positioning a negative electrode containing graphite as an active material and the fabricated positive electrode facing each other and interposing a porous polyethylene separator therebetween. The electrode assembly was then placed inside a case, and an electrolyte was injected into the case to fabricate a half-cell type lithium secondary battery.

[0057] [Examples 2 to 5, Comparative Examples 1 to 3] Lithium secondary battery manufacturing Lithium secondary batteries corresponding to Examples 2 to 5 and Comparative Examples 1 to 3 were manufactured in the same manner as in Example 1, except that the composition of the electrolyte in the lithium secondary battery of Example 1 was changed as shown in Table 1 below.

[0058] [Table 1]

[0059] [Experimental Example 1] Evaluation of the thermal stability of lithium secondary batteries The lithium secondary batteries prepared in Examples 1 to 5 and Comparative Examples 1 to 3 were each subjected to a thermal stability test by leaving the batteries in an environment of about 100 to 200°C, and providing and maintaining a waiting time of 30 minutes at each temperature within the temperature range (at which the temperature rise rate was 10°C / min). The results are shown in Table 2 below.

[0060] [Table 2]

[0061] 1 to 5 are graphs showing experimental results of the thermal stability of a lithium secondary battery containing an electrolyte according to an embodiment of the present invention, and FIGS. 6 to 8 are graphs showing experimental results of the thermal stability of a lithium secondary battery containing a conventional electrolyte.

[0062] Specifically, Fig. 1 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Example 1, Fig. 2 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Example 2, Fig. 3 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Example 3, Fig. 4 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Example 4, and Fig. 5 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Example 5. Also, Fig. 6 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Comparative Example 1, Fig. 7 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Comparative Example 2, and Fig. 8 corresponds to the experimental results of the thermal stability of the lithium secondary battery prepared in Comparative Example 3.

[0063] As described above, thermal stability experiments were performed on the lithium secondary batteries prepared in Examples 1 to 5 and Comparative Examples 1 to 3. As shown in Table 2 and Figure 6, the lithium secondary battery of Comparative Example 1, in which the electrolyte contained only lithium hexafluorophosphate (LiPF6) as the lithium salt, had a maximum thermally stable temperature of only 160°C. Also, as shown in Table 2 and Figures 7 and 8, the lithium secondary battery of Comparative Example 2, in which the first and second lithium salts were used in equal amounts, had a maximum thermally stable temperature of only 163°C, and the lithium secondary battery of Comparative Example 3, in which the second lithium salt was used in an amount greater than the first lithium salt, also had a maximum thermally stable temperature of only 162°C.

[0064] Meanwhile, the lithium secondary batteries of Examples 1 to 5, in which the electrolyte contained two specific lithium salts (LiPF6 and LiFSI, LiPF6 and LiTFSI) and the second lithium salt was contained in a smaller amount than the first lithium salt, had a thermally stable temperature of at least 170°C, and in some cases (corresponding to Example 3) excellent thermal stability was observed even at temperatures exceeding 180°C. This confirms that the thermal stability of a battery can be improved by including two types of lithium salts in a specific content ratio together with two specific solvents in the electrolyte.

Claims

1. a lithium salt solution containing only a first lithium salt and a second lithium salt; and a solvent; the first lithium salt is lithium hexafluorophosphate and the second lithium salt is lithium bis(fluorosulfonyl)imide or lithium bis(trifluorosulfonyl)imide; the molar ratio of the first lithium salt to the second lithium salt is 1:0.4 to 0.9; the solvent is composed only of a first solvent and a second solvent, the first solvent being ethylene carbonate, the second solvent being dimethyl carbonate, and the mixing ratio of the first solvent to the second solvent being 5:95 to 10:90 by volume; The total lithium salt concentration including the first lithium salt and the second lithium salt is 1.5M to 4.0M. An electrolyte for lithium secondary batteries containing a lithium nickel cobalt manganese-based positive electrode active material.

2. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the molar ratio of the first lithium salt to the second lithium salt is 1:0.4 to 0.

7.

3. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein a total lithium salt concentration including the first lithium salt and the second lithium salt is 1.5M to 2.0M.

4. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the molar ratio of the first lithium salt to the second lithium salt is 1:0.4 to 1:0.

8.

5. A lithium secondary battery comprising: a positive electrode containing a lithium nickel cobalt manganese-based positive electrode active material; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and the electrolyte solution for a lithium secondary battery according to any one of claims 1 to 4.

6. 6. The lithium secondary battery according to claim 5, wherein the lithium secondary battery does not deform or explode under a temperature condition of 170°C or higher.

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