Composition for forming solid electrolyte, solid electrolyte, and lithium secondary battery

Abstract

A composition for forming a solid electrolyte, according to embodiments of the present disclosure, comprises: a liquid electrolyte, which comprises a solvent including a nitrile-based compound having an ether group, and a lithium salt including lithium difluoro(oxalato)borate; and a monomer having a polymerizable functional group. The solid electrolyte according to embodiments of the present disclosure comprises: a nitrile-based compound having an ether group; a lithium salt including lithium difluoro(oxalato)borate; and a polymer.

Description

Composition for forming a solid electrolyte, solid electrolyte, and lithium secondary battery

[0001] The present disclosure provides a composition for forming a solid electrolyte, a solid electrolyte, and a lithium secondary battery.

[0002] Rechargeable batteries are batteries capable of repeated charging and discharging, and with the advancement of the information and communication and display industries, they are widely applied as power sources for portable electronic communication devices such as camcorders, mobile phones, and laptop PCs. Furthermore, recently, battery packs containing rechargeable batteries are also being developed and applied as power sources for eco-friendly vehicles, such as hybrid cars.

[0003] Examples of secondary batteries include lithium-ion batteries, nickel-cadmium batteries, and nickel-hydrogen batteries; among these, active research and development is being conducted on lithium-ion batteries due to their high operating voltage and energy density per unit weight, as well as their advantages in charging speed and weight reduction.

[0004] Currently commercialized lithium-ion batteries primarily use liquid electrolytes, which present safety issues such as leakage, ignition, and explosion due to rapid environmental changes like temperature fluctuations or external shocks. To address this, attempts are underway to solidify the electrolyte to ensure stability and improve energy density.

[0005] All-solid-state batteries may include solid-state electrolytes such as gel polymers, oxides or sulfides, or composite polymers as electrolytes. Accordingly, safety against ignition and explosion caused by external shocks, changes in the external environment, etc., can be increased.

[0006] One objective of the present disclosure is to provide a composition for forming a solid electrolyte that realizes a solid electrolyte having improved ionic conductivity, high voltage stability, and lifespan characteristics.

[0007] One objective of the present disclosure is to provide a solid electrolyte having improved ionic conductivity, high voltage stability, and lifespan characteristics.

[0008] One objective of the present disclosure is to provide a lithium secondary battery comprising the solid electrolyte.

[0009] A composition for forming a solid electrolyte according to the present disclosure comprises a liquid electrolyte and a monomer having a polymerizable functional group. The liquid electrolyte comprises a solvent comprising a nitrile-based compound having an ether group and a lithium salt comprising lithium difluorooxalatoborate.

[0010] According to exemplary embodiments, the nitrile compound may comprise one or two ether groups and one or two nitrile groups.

[0011] According to exemplary embodiments, the nitrile compound may include an alkoxy group having 1 to 5 carbon atoms.

[0012] According to exemplary embodiments, the nitrile compound may comprise at least one selected from the group consisting of methoxypropionitrile, methoxybutyronitrile, and ethoxypropionitrile.

[0013] According to exemplary embodiments, the molar concentration of the lithium salt may be 0.9 M to 2 M.

[0014] According to exemplary embodiments, the monomer having the polymerizable functional group may comprise at least one selected from the group consisting of bisphenol A ethoxylated di(meth)acrylate, ethylene glycol diacrylate, triethylene glycol diacrylate, trimethylolpropane ethoxylated triacrylate, (meth)acrylic acid, carboxyethyl acrylate, cyano(meth)acrylic acid trimethylolpropane trimethacrylate, and pentaerythritol tetraacrylate.

[0015] According to exemplary embodiments, the content of the monomer may be 5 to 30 parts by weight per 100 parts by weight of the sum of the weights of the liquid electrolyte and the monomer.

[0016] According to exemplary embodiments, the liquid electrolyte may further comprise at least one additive selected from the group consisting of unsaturated cyclic carbonate compounds, fluorine-substituted cyclic carbonate compounds, sulfone compounds, cyclic sulfate compounds, fluorine-substituted phosphate compounds, and oxalatophosphate compounds.

[0017] The solid electrolyte according to the present disclosure comprises a nitrile-based compound having an ether group, a lithium salt comprising lithium difluorooxalatoborate, and a polymer.

[0018] According to exemplary embodiments, the nitrile compound may comprise one or two ether groups and one or two nitrile groups.

[0019] According to exemplary embodiments, the nitrile compound may include an alkoxy group having 1 to 5 carbon atoms.

[0020] According to exemplary embodiments, the nitrile compound may comprise at least one selected from the group consisting of methoxypropionitrile, methoxybutyronitrile, and ethoxypropionitrile.

[0021] According to exemplary embodiments, the solid electrolyte may further comprise at least one additive selected from the group consisting of unsaturated cyclic carbonate compounds, fluorine-substituted cyclic carbonate compounds, sulfone compounds, cyclic sulfate compounds, fluorine-substituted phosphate compounds, and oxalatophosphate compounds.

[0022] A lithium secondary battery according to the present disclosure comprises a positive electrode, a negative electrode facing the positive electrode, and a solid electrolyte interposed between the positive electrode and the negative electrode.

[0023] According to exemplary embodiments, the cathode may include graphite as a cathode active material.

[0024] A composition for forming a solid electrolyte according to exemplary embodiments of the present disclosure comprises a solvent having nitrile groups and ether groups, which can realize a solid electrolyte having high ionic conductivity. Accordingly, the charge and discharge rate of the battery can be improved.

[0025] A composition for forming a solid electrolyte according to exemplary embodiments of the present disclosure comprises lithium difluorooxalatoborate to form a stable interface with the anode, thereby enabling the realization of a solid electrolyte with improved high-voltage stability. Accordingly, the battery can have excellent high-voltage driving characteristics.

[0026] In the solid electrolyte according to the exemplary embodiments of the present disclosure, the stability of the cathode, for example, the interface between the cathode and the solid electrolyte and the anode interface can be improved, and accordingly, a battery having improved electrochemical stability and improved rate capability can be realized.

[0027] A lithium secondary battery according to exemplary embodiments of the present disclosure may have improved lifespan characteristics.

[0028] FIG. 1 is a schematic diagram of a cross-section of a secondary battery according to an exemplary embodiment.

[0029] Figure 2 is a graph showing the results of an oxidation stability evaluation performed on the batteries of Example 1 and Comparative Examples 1 and 2.

[0030] Figure 3 is a graph showing the results of evaluating the rate capability characteristics of the batteries of Example 1 and Comparative Examples 1 and 2.

[0031] Figures 4a to 4f are graphs showing the results of XPS analysis performed on the negative electrode surface of the battery of Example 1 and Comparative Example 1.

[0032] A composition for forming a solid electrolyte according to the present disclosure comprises a solvent comprising a nitrile-based compound, a liquid electrolyte comprising a lithium salt, and a monomer having a polymerizable functional group. Additionally, a solid electrolyte according to the present disclosure is formed from the composition for forming a solid electrolyte, and a lithium secondary battery according to the present disclosure comprises the solid electrolyte.

[0033] The present disclosure will be described in detail below. However, this is merely illustrative and the present disclosure is not limited to the specific embodiments described illustratively.

[0034] In this specification, the term "solid electrolyte" may be used as a concept contrasting with liquid electrolyte. For example, the solid electrolyte may include a semi-solid electrolyte, a gel polymer electrolyte, a semi-solid electrolyte, etc.

[0035] In this specification, the term "composition for forming a solid electrolyte" may include a composition for forming a semi-solid electrolyte, a composition for forming a gel polymer electrolyte, a composition for forming a semi-solid electrolyte, etc.

[0036] In this specification, "~-system compound" may mean a compound to which "~-system compound" is attached, and derivatives of the compound.

[0037] According to exemplary embodiments, a composition for forming a solid electrolyte (hereinafter abbreviated as composition) comprises a liquid electrolyte and a monomer.

[0038] The above liquid electrolyte includes a solvent and a lithium salt. The above liquid electrolyte can prevent the hardness of the solid electrolyte from increasing excessively and can serve as a medium for the movement of lithium ions.

[0039] The above solvent includes a nitrile-based compound having an ether group. The ether group can improve the solubility of the lithium salt. In addition, the stability of the interface between the cathode (e.g., a lithium metal cathode) and the solid electrolyte can be improved, and the lifespan characteristics of the battery can be improved.

[0040] In addition, the nitrile groups of nitrile-based compounds are highly polar functional groups that can reduce the volatility of the solvent and improve the ionic conductivity of the solid electrolyte. Since nitrile-based compounds do not decompose or cause side reactions even at high voltages, the high-voltage stability of the battery can be improved.

[0041] The nitrile-based compound may include at least one ether group and at least one nitrile group. For example, the nitrile-based compound may include one or two ether groups and one or two nitrile groups. If the nitrile-based compound includes two ether groups, the ether groups may be connected to alkylene groups.

[0042] According to exemplary embodiments, the nitrile compound may include an alkoxy group having 1 to 5 carbon atoms. For example, the nitrile compound may include a methoxy group or an ethoxy group.

[0043] For example, the above nitrile compound can be represented by the following chemical formula 1.

[0044] [Chemical Formula 1]

[0045]

[0046] In the above chemical formula 1, L1 and L2 may each be a substituted or unsubstituted alkylene group having 1 to 3 carbon atoms, R1 may be a substituted or unsubstituted alkyl group having 1 to 3 carbon atoms, and n may be an integer of 0 or 1 to 2.

[0047] In this specification, "substitution" may mean that any hydrogen is substituted with a substituent. Non-limiting examples of said substituents may include functional groups such as halogens, hydroxyl groups, carboxyl groups, amine groups, amide groups, cyano groups, thiol groups, and sulfonic acid groups.

[0048] For example, in the above chemical formula 1, L1 may be a methylene group, an ethylene group, or a propylene group.

[0049] For example, in the above chemical formula 1, L2 may be a methylene group or an ethylene group.

[0050] For example, in the above chemical formula 1, R1 may be a methyl group or an ethyl group.

[0051] According to exemplary embodiments, the nitrile-based compound may include methoxypropionitrile, methoxybutyronitrile, ethoxypropionitrile, etc.

[0052] According to exemplary embodiments, the Hansen solubility parameter (δ) of the nitrile-based compound T ) is 17.7 MPa 0.5 to 26.1 MPa 0.5 It could be.

[0053] The above Hansen solubility factor is a parameter for the solubility of a specific substance determined by considering the dispersion force, dipole-dipole interaction, and hydrogen bonding force of the specific substance together, and can be expressed by the following Equation 1.

[0054] [Equation 1]

[0055]

[0056] δ in Equation 1 above T is the Hansen solubility parameter (MPa 0.5 ), δ d is the solubility parameter due to dispersion forces (MPa 0.5 ), δp is the solubility parameter due to dipole-dipole interaction (MPa 0.5 ), and δ h is the solubility parameter due to hydrogen bonding force (MPa 0.5 It means ).

[0057] Within the above range, the interaction forces between molecules of nitrile-based compounds can be appropriately maintained. Accordingly, the volatility of the solvent can be reduced, the electrolyte loss of the battery can be reduced, and the lifespan characteristics of the battery can be improved.

[0058] In Equation 1 above, δ d is 15.4 MPa 0.5 to 18.2 MPa 0.5 or 16 MPa 0.5 to 17 MPa 0.5 It could be. δ p is 6.3 MPa 0.5 to 17 MPa 0.5 or 10 MPa 0.5 to 15 MPa 0.5 It could be. δ h is 6.0 MPa 0.5 to 7.9 MPa 0.5 or 6.5 MPa 0.5 to 7.8 MPa 0.5 It could be.

[0059] According to exemplary embodiments, the liquid electrolyte comprises a lithium salt.

[0060] According to exemplary embodiments, the lithium salt may include lithium difluorooxalatoborate. Lithium difluorooxalatoborate can form a stable interface with the anode to improve the high-voltage stability of the electrolyte. Accordingly, the battery can have excellent high-voltage driving characteristics.

[0061] In some embodiments, the lithium salt may be composed of a borate-based lithium salt. For example, the lithium salt may be composed solely of a compound in which a lithium cation and an anion comprising a borate structure are bonded.

[0062] In some embodiments, the lithium salt may consist of lithium difluorooxalatoborate.

[0063] According to exemplary embodiments, the molar concentration of the lithium salt may be 0.9 M to 2 M. According to some embodiments, the molar concentration of the lithium salt may be 0.9 M to 1.8 M or 0.9 M to 1.5 M. Within these ranges, a battery having improved rate capability and lifespan characteristics may be realized.

[0064] According to some embodiments, the molar concentration of lithium difluorooxalatoborate in the lithium salt may be 0.9 M to 2 M, 0.9 M to 1.8 M, 0.9 M to 1.5 M, 0.8 M to 1.8 M, 0.8 M to 1.5 M, 0.7 M to 1.8 M, or 0.7 M to 1.5 M. Within the above range, a battery having further improved rate capability and lifespan characteristics may be realized.

[0065] In some embodiments, the liquid electrolyte may not contain a carbonate-based solvent. For example, the carbonate-based compound may not be included as a solvent that dissolves the lithium salt and constitutes most of the weight of the liquid electrolyte (e.g., 50% by weight or more). In this case, the use of the carbonate-based compound as an additive is not excluded. For example, the carbonate-based compound may be used as an additive in an amount of about 5% by weight or less of the total weight of the liquid electrolyte.

[0066] According to exemplary embodiments, the liquid electrolyte may further comprise at least one additive selected from the group consisting of unsaturated cyclic carbonate compounds, fluorine-substituted cyclic carbonate compounds, sulfone compounds, cyclic sulfate compounds, fluorine-substituted phosphate compounds, and oxalatophosphate compounds.

[0067] The above unsaturated cyclic carbonate compounds may include vinyl ethylene carbonate (VEC), vinylene carbonate (VC), etc.

[0068] The above fluorine-substituted cyclic carbonate compound may include fluoroethylene carbonate (FEC).

[0069] The above sulfone-based compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0070] The above-mentioned cyclic sulfate compounds may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0071] The above fluorine-substituted phosphate-based compound may include lithium difluorophosphate (LiPO2F2), etc.

[0072] The above oxalatophosphate-based compound may include lithium difluorobis(oxalato)phosphate, etc.

[0073] These can be used individually or in combination of two or more types.

[0074] The content of the above additive may be about 0.01% to 5% by weight of the total weight of the liquid electrolyte.

[0075] According to exemplary embodiments, the composition may include an oxide-based inorganic electrolyte. For example, the oxide-based inorganic electrolyte may include perovskite-based (LLTO-based) compounds, garnet-based (LLZO-based) compounds, NASICON-based (LAGP and LATP-based) compounds, etc. These may be used alone or in combination of two or more types.

[0076] The above LLTO-based compound may be an oxide containing lithium, lanthanum, and titanium. The above LLTO-based compound may further include Al, Ga, In, Sc, Zr, Ta, etc.

[0077] The above LLZO-based compound may be an oxide containing lithium, lanthanum, and zirconium. The above LLZO-based compound may further include Al, Ga, In, Sc, Ba, Nb, etc.

[0078] For example, the oxide-based inorganic electrolyte may include a garnet oxide-based inorganic electrolyte. For example, Li5La3Ta2O 12 , Li7La3Zr2O 12 , Li5La3Nb2O 12 , Li6BaLa2Ta2O 12 , Li 6.4 La3Zr 1.4 Ta 0.3 O 12 It may include the back.

[0079] According to exemplary embodiments, the composition may include a monomer having polymerizable functional groups. When energy is applied to the composition, the monomer may form a polymer through a reaction between the polymerizable functional groups, and the polymer may function as a support and matrix for a solid electrolyte.

[0080] According to exemplary embodiments, the polymerizable functional group may include a (meth)acrylate group.

[0081] According to exemplary embodiments, the monomer having the polymerizable functional group may include bisphenol A ethoxylated di(meth)acrylate, ethylene glycol diacrylate, triethylene glycol diacrylate, trimethylolpropane ethoxylated triacrylate, (meth)acrylic acid, carboxyethyl acrylate, cyano(meth)acrylic acid, trimethylolpropane trimethacrylate, pentaerythritol tetraacrylate, etc.

[0082] According to exemplary embodiments, the content of the monomer may be 5 to 30 parts by weight with respect to 100 parts by weight of the sum of the weights of the liquid electrolyte and the monomer. According to some embodiments, the content of the monomer may be 5 to 25 parts by weight or 5 to 20 parts by weight with respect to 100 parts by weight of the sum of the weights of the liquid electrolyte and the monomer.

[0083] Within the above range, the viscosity of the composition and the hardness of the solid electrolyte may not increase excessively, and the shape of the solid electrolyte may be maintained.

[0084] According to exemplary embodiments, a thermal initiator or a photoinitiator may be further included. The thermal initiator or the photoinitiator may initiate a polymerization reaction of the monomer, thereby forming a polymer.

[0085] For example, the above thermal initiator may include persulfate-based initiators, azo-based initiators, hydrogen peroxide, ascorbic acid, peroxide-based initiators, etc.

[0086] For example, the above photoinitiator may include acetophenone-based initiators, benzophenone-based initiators, thioxanthone-based initiators, benzoin-based initiators, triazine-based initiators, etc.

[0087] The content of the thermal initiator or photoinitiator may be 0.1 to 3 parts by weight per 100 parts by weight of the sum of the liquid electrolyte and the monomer.

[0088] The solid electrolyte according to the present disclosure comprises a nitrile-based compound having an ether group, a lithium salt comprising lithium difluorooxalatoborate, and a polymer. The polymer may be a polymer or a copolymer as a polymer of the monomers described above.

[0089] The above nitrile-based compound and lithium salt may be the same as those described above.

[0090] The above polymer may be formed by polymerizing polymerizable functional groups between a plurality of monomers, and the polymer may include repeating units derived from the monomers.

[0091] According to exemplary embodiments, the polymer may include repeating units derived from bisphenol A ethoxylated di(meth)acrylate, ethylene glycol diacrylate, triethylene glycol diacrylate, trimethylolpropane ethoxylated triacrylate, (meth)acrylic acid, carboxyethyl acrylate, cyano(meth)acrylic acid trimethylolpropane trimethacrylate, pentaerythritol tetraacrylate, etc.

[0092] The above solid electrolyte may be formed from the above solid electrolyte forming composition. The above solid electrolyte may be formed by thermal polymerization or photopolymerization of the above solid electrolyte forming composition, and the above solid electrolyte may include the above polymer, which is a polymer of the above monomer.

[0093] For example, the solid electrolyte forming composition can be applied onto a porous film and the monomer can be polymerized by heating (thermal polymerization) or irradiating with light (photopolymerization). Alternatively, the solid electrolyte forming composition can be introduced into a mold of a certain shape and the monomer can be polymerized by heating (thermal polymerization) or irradiating with light (photopolymerization).

[0094] Accordingly, the polymerizable functional groups can polymerize to form a polymer. The polymer may have a three-dimensional network structure intertwined with one another and may include pores, and the liquid electrolyte may be disposed within the pores.

[0095] For example, the porous film may include a plurality of interconnected pores, and the composition for forming a solid electrolyte may be impregnated into the pores. Monomers of the composition for forming a solid electrolyte may polymerize by thermal polymerization to form a polymer, and the polymer may form an interpenetrating polymer network (IPN) or a semi-interpenetrating polymer network (semi-IPN) with the pores of the porous film. The liquid electrolyte may be disposed in the pores of the formed polymer network.

[0096] When thermally polymerizing the above solid electrolyte forming composition, a solid electrolyte can be formed by leaving it at a temperature of about 50°C to 150°C for about 20 to 60 minutes.

[0097] According to exemplary embodiments, the thickness of the solid electrolyte may be about 10 μm to 200 μm. According to some embodiments, the thickness of the solid electrolyte may be about 30 μm to 150 μm. A lithium secondary battery according to the present disclosure comprises a positive electrode, a negative electrode facing the positive electrode, and the solid electrolyte interposed between the positive electrode and the negative electrode.

[0098] FIG. 1 is a schematic diagram of a cross-section of a secondary battery according to an exemplary embodiment.

[0099] Referring to FIG. 1, the secondary battery includes a positive electrode (300), a negative electrode (200) facing the positive electrode (300), and an electrolyte layer (100) disposed between the positive electrode (300) and the negative electrode (200) and containing the solid electrolyte.

[0100] The above positive electrode (300) may include a positive electrode current collector (310) and a positive electrode active material layer (320) disposed on at least one surface of the positive electrode current collector (310).

[0101] The positive current collector (310) may include stainless steel, nickel, aluminum, titanium, or an alloy thereof. The positive current collector (300) may include carbon, nickel, titanium, aluminum or stainless steel surface-treated with silver. The thickness of the positive current collector may be, for example, 10 μm to 50 μm.

[0102] The positive active material layer (320) may include a positive active material. The positive active material may include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0103] According to exemplary embodiments, the positive electrode active material may comprise a lithium-nickel metal oxide. The lithium-nickel metal oxide may further comprise at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0104] In some embodiments, the positive active material or the lithium-nickel metal oxide may comprise a layered structure or a crystalline structure represented by the following chemical formula 2.

[0105] [Chemical Formula 2]

[0106] Li x Ni a M b O 2+z

[0107] In Chemical Formula 2, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1 may be used. As described above, M may include Co, Mn, and / or Al.

[0108] The chemical structure represented by Chemical Formula 3 represents the bonding relationships included within the layered or crystal structure of the positive electrode active material and does not exclude other additional elements. For example, M includes Co and / or Mn, and Co and / or Mn may be provided as the main active element of the positive electrode active material together with Ni. Chemical Formula 3 is provided to express the bonding relationships of the main active elements and should be understood as encompassing the introduction and substitution of additional elements.

[0109] In one embodiment, auxiliary elements may be further included to enhance the chemical stability of the anode active material or the layered structure / crystal structure by adding to the main active element. The auxiliary elements may be incorporated together within the layered structure / crystal structure to form bonds, and in this case, it should be understood that they are also included within the range of the chemical structure represented by Chemical Formula 2.

[0110] The above auxiliary element may include, for example, at least one of Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The above auxiliary element may also act as an auxiliary active element that contributes to the capacity / output activity of the cathode active material together with Co or Mn, such as Al.

[0111] For example, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 2-1.

[0112] [Chemical Formula 2-1]

[0113] Li x Ni a M1 b M2 c O 2+z

[0114] In Chemical Formula 2-1, M1 may include Co, Mn and / or Al. M2 may include the auxiliary element described above. In Chemical Formula 2-1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b+c≤0.4, and -0.5≤z≤0.1.

[0115] The above-described positive active material may further include a coating element or a doping element. For example, elements substantially identical or similar to the auxiliary elements described above may be used as coating elements or doping elements. For example, any of the elements described above may be used alone or in combination of two or more as coating elements or doping elements.

[0116] The coating element or doping element may be present on the surface of the lithium-nickel metal oxide particles or penetrate through the surface of the lithium-nickel metal composite oxide particles and be included within the bonding structure represented by Formula 2 or Formula 2-1.

[0117] The above-mentioned positive electrode active material may include a nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide with an increased nickel content may be used.

[0118] Ni can be provided as a transition metal associated with the output and capacity of a lithium secondary battery. Therefore, by adopting a high-Ni composition in the cathode active material as described above, a high-capacity cathode and a high-capacity lithium secondary battery can be provided.

[0119] However, as the Ni content increases, the long-term storage stability and lifespan stability of the anode or secondary battery may relatively decrease, and side reactions with the electrolyte may also increase. However, according to exemplary embodiments, lifespan stability and capacity retention characteristics can be improved through Mn while maintaining electrical conductivity by including Co.

[0120] The content of Ni in the above NCM-based lithium oxide (e.g., the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) may be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the content of Ni may be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0121] In some embodiments, the positive electrode active material may include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0122] In some embodiments, the positive electrode active material may include, for example, a Mn-rich active material having a chemical structure or crystal structure represented by Formula 3, an LLO (Li rich layered oxide) / OLO (Over Lithiated Oxide) active material, or a Co-less active material.

[0123] [Chemical Formula 3]

[0124] p[Li2MnO3]·(1-p)[Li q JO2]

[0125] Of chemical formula 3, 0 <p<1이고, 0.9≤q≤1.2이며, J는 Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg 및 B 중 적어도 하나의 원소를 포함할 수 있다.

[0126] For example, an anode slurry can be prepared by mixing a solvent with the anode active material. After coating the anode slurry onto an anode current collector (310), an anode active material layer (320) can be prepared by drying and rolling. The coating process may be carried out by methods such as gravure coating, slot die coating, multilayer simultaneous die coating, imprinting, doctor blade coating, dip coating, bar coating, and casting, but is not limited thereto. The anode active material layer may further include a binder and may further include a conductive material, a thickener, etc.

[0127] Non-limiting examples of solvents used in the preparation of the above anode slurry include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

[0128] The above binder may include polyvinylidenefluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (Poly(vinylidene fluoride-co-hexafluoropropylene), polyacrylonitrile, polymethylmethacrylate, acrylonitrile butadiene rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. In one embodiment, a PVDF-based binder may be used as the anode binder.

[0129] The above conductive material may be added to enhance the conductivity of the positive active material layer and / or the mobility of lithium ions or electrons. For example, the conductive material may include, but is not limited to, carbon-based conductive materials such as graphite, carbon black, acetylene black, ketjen black, graphene, carbon nanotubes, vapor-grown carbon fiber (VGCF), carbon fiber, etc., and metal-based conductive materials such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc.

[0130] If necessary, the anode slurry may further include a thickener and / or a dispersant, etc. In one embodiment, the anode slurry may include a thickener such as carboxymethyl cellulose (CMC).

[0131] The above cathode (200) may include a cathode current collector (210) and a cathode active material layer (220) disposed on at least one surface of the cathode current collector (210).

[0132] The negative current collector (210) may include, for example, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, etc. The negative current collector (210) may be, for example, 10 μm to 50 μm, although it is not limited thereto.

[0133] The negative current collector (210) is not an essential component, and the negative electrode (200) may include only the negative active material layer (220) without including the negative current collector (210).

[0134] The negative electrode active material layer (220) may include a negative electrode active material. As the negative electrode active material, a material capable of inserting / alloying / electrodepositing and removing / dealloying / desorbing lithium ions may be used. For example, the negative electrode active material may be a carbon-based material such as crystalline carbon, amorphous carbon, carbon composite, carbon fiber; lithium metal; lithium alloy; silicon (Si)-containing material or tin (Sn)-containing material.

[0135] Examples of the above-mentioned amorphous carbon include hard carbon, soft carbon, coke, mesocarbon, mesocarbon microbeads (MCMB), and mesophase pitch-based carbon fiber (MPCF).

[0136] Examples of the above-mentioned crystalline carbon include graphite-based carbons such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, and graphitized MPCF.

[0137] According to exemplary embodiments, the cathode active material layer (220) of the cathode (200) may include graphite as the cathode active material.

[0138] The above lithium metal may be pure lithium metal or lithium metal with a protective layer formed thereon for inhibiting dendrite growth, etc. In one embodiment, a lithium metal-containing layer deposited or coated on a negative electrode current collector may be used as a negative electrode active material layer. In one embodiment, a lithium thin film layer may be used as a negative electrode active material layer.

[0139] Examples of elements included in the above lithium alloy include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0140] The above silicon-containing material can provide increased capacity characteristics. The above silicon-containing material is Si, SiO x (0 <x<2), 금속 도핑된 SiO x(0 <x<2), 실리콘-탄소 복합체 등을 포함할 수 있다. 상기 금속은 리튬 및 / 또는 마그네슘을 포함할 수 있으며, 금속 도핑된 SiO x (0 <x<2)는 금속 실리케이트를 포함할 수 있다.

[0141] For example, a cathode slurry can be prepared by mixing the cathode active material in a solvent. A cathode active material layer (220) can be prepared by coating / depositing the cathode slurry onto a cathode current collector, followed by drying and rolling. The coating process can be carried out using substantially the same method as the method for preparing the anode active material layer (320). The cathode active material layer (220) may further include a binder and optionally may further include an electrolyte, a conductive material, a thickener, etc.

[0142] In some embodiments, the cathode (200) may include a cathode active material layer in the form of lithium metal formed through a deposition / coating process.

[0143] Examples of solvents for the above-mentioned cathode active material layer include water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, t-butanol, etc.

[0144] The above-described materials that can be used in the manufacture of the anode as the binder, conductive material, and thickener may be used.

[0145] In some embodiments, styrene-butadiene rubber-based binders, carboxymethyl cellulose, polyacrylic acid-based binders, poly(3,4-ethylenedioxythiophene), PEDOT-based binders, etc., may be used as cathode binders.

[0146] In some embodiments, an electrolyte layer (100) may be interposed between the anode (300) and the cathode (200) within the electrode assembly. For example, an electrode cell is defined by the anode (300), the cathode (200), and the electrolyte layer (100), and a plurality of the electrode cells may be stacked to form an electrode assembly. For example, the electrode assembly may be formed through winding, stacking, folding, etc.

[0147] In exemplary embodiments, the electrode cell may be formed by interposing a solid electrolyte formed from the solid electrolyte forming composition between the anode (300) and the cathode (200).

[0148] For example, electrode tabs (positive tabs and negative tabs) may protrude from a positive current collector and a negative current collector, respectively, and extend to one side of the case. The electrode tabs may be fused together with the one side of the case and connected to electrode leads (positive leads and negative leads) that extend or are exposed to the outside of the case.

[0149] For example, pouch-type cases, rectangular cases, cylindrical cases, coin-type cases, etc., may be used.

[0150] In some embodiments, a porous film may be interposed between the anode (300) and the cathode (200) and embedded in the case to manufacture a pre-cell. The composition for forming a solid electrolyte may be injected into the pre-cell, and a polymerization reaction may be carried out to form an electrolyte layer (100) in-situ. For example, the composition for forming a solid electrolyte may be injected into the pre-cell and heated to carry out a thermal polymerization reaction of the monomer.

[0151] In some embodiments, a porous film may be laminated on an anode (300) or a cathode (200), and a composition for forming a solid electrolyte may be applied, and then the cathode (200) or an anode (300) may be laminated to manufacture a preliminary cell. After placing the preliminary cell in a case, heating may proceed with the thermal polymerization reaction of the monomer and form an electrolyte layer (100).

[0152] According to exemplary embodiments, the battery may be an all-solid-state battery. In this specification, "all-solid-state battery" may include a semi-solid-state battery, a semi-solid-state battery, etc.

[0153] The embodiments of the present disclosure described above include the following aspects and can be implemented through at least one of the following aspects.

[0154] A composition for forming a solid electrolyte according to the first aspect of the present disclosure comprises a liquid electrolyte and a monomer having a polymerizable functional group. The liquid electrolyte comprises a solvent comprising a nitrile-based compound having an ether group and a lithium salt comprising lithium difluorooxalatoborate.

[0155] In the first aspect above, according to the second aspect, the nitrile-based compound may include one or two ether groups and one or two nitrile groups.

[0156] In any one of the first and second aspects above, according to the third aspect, the nitrile-based compound may include an alkoxy group having 1 to 5 carbon atoms.

[0157] In any one of the first to third aspects above, according to the fourth aspect, the nitrile-based compound may include at least one selected from the group consisting of methoxypropionitrile, methoxybutyronitrile, and ethoxypropionitrile.

[0158] In any one of the first to fourth aspects above, according to the fifth aspect, the molar concentration of the lithium salt may be 0.9 M to 2 M.

[0159] In any one of the first to fifth aspects above, according to the sixth aspect, the monomer having the polymerizable functional group may comprise at least one selected from the group consisting of bisphenol A ethoxylated di(meth)acrylate, ethylene glycol diacrylate, triethylene glycol diacrylate, trimethylolpropane ethoxylated triacrylate, (meth)acrylic acid, carboxyethyl acrylate, cyano(meth)acrylic acid trimethylolpropane trimethacrylate, and pentaerythritol tetraacrylate.

[0160] In any one of the first to sixth aspects above, according to the seventh aspect, the content of the monomer may be 5 to 30 parts by weight with respect to 100 parts by weight of the sum of the weights of the liquid electrolyte and the monomer.

[0161] In any one of the first to seventh aspects above, according to the eighth aspect, the liquid electrolyte may further include at least one additive selected from the group consisting of an unsaturated cyclic carbonate compound, a fluorine-substituted cyclic carbonate compound, a sulfone compound, a cyclic sulfate compound, a fluorine-substituted phosphate compound, and an oxalatophosphate compound.

[0162] A solid electrolyte according to the ninth aspect of the present disclosure comprises a nitrile-based compound having an ether group, a lithium salt comprising lithium difluorooxalatoborate, and a polymer.

[0163] In the ninth aspect above, according to the tenth aspect, the nitrile-based compound may include one or two ether groups and one or two nitrile groups.

[0164] In any one of the 9th to 10th aspects above, according to the 11th aspect, the nitrile-based compound may include an alkoxy group having 1 to 5 carbon atoms.

[0165] In any one of the 9th to 11th aspects above, according to the 12th aspect, the nitrile-based compound may include at least one selected from the group consisting of methoxypropionitrile, methoxybutyronitrile, and ethoxypropionitrile.

[0166] In any one of the ninth to twelfth aspects above, according to the thirteenth aspect, the solid electrolyte may further include at least one additive selected from the group consisting of an unsaturated cyclic carbonate compound, a fluorine-substituted cyclic carbonate compound, a sulfone compound, a cyclic sulfate compound, a fluorine-substituted phosphate compound, and an oxalatophosphate compound.

[0167] A lithium secondary battery according to the 14th aspect of the present disclosure comprises a positive electrode, a negative electrode facing the positive electrode, and a solid electrolyte interposed between the positive electrode and the negative electrode.

[0168] In the above 14th aspect, according to the 15th aspect, the cathode may include graphite as a cathode active material.

[0169] In the following, embodiments of the present disclosure are further described with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are merely illustrative of the present disclosure and are not intended to limit the appended claims. It is obvious to those skilled in the art that various changes and modifications to the embodiments are possible within the scope and spirit of the present disclosure, and that such variations and modifications fall within the scope of the appended claims.

[0170] Example 1

[0171] (1) Preparation of solid electrolyte

[0172] A composition for forming a solid electrolyte was prepared by mixing 90 parts by weight of a liquid electrolyte containing 1 M lithium difluorooxalatetoborate (LiDFOB) dissolved in methoxypropionitrile (MPN) and 5 wt% fluoroethylene carbonate, 10 parts by weight of bisphenol A ethoxylate diacrylate (BPADA, number average molecular weight = 688), and 2 parts by weight of t-butyl peroxyfibarate (t-BPP, Seki Arkema Co., South Korea) as a thermosetting initiator.

[0173] (2) Lithium secondary battery manufacturing

[0174] LiNi as a positive electrode active material 0.8 Co 0.1 Mn 0.1 An anode slurry was prepared by placing O2, Super P as a conductive material, and polyvinylidene fluoride (PVDF, Kynar Flex® 2851, Arkema) as a binder into a mixer (Thinky mixer) with N-methylpyrrolidone as a solvent in a weight ratio of 95:2.5:2.5 and mixing for 30 minutes using a rotating and orbiting method. After uniformly coating the anode slurry onto aluminum foil, the material was vacuum dried and rolled at approximately 120°C to produce an anode (loading density of approximately 3.0 mAh·cm²). - 2 , the density of the mixture was approximately 3.4 g / cc) was prepared.

[0175] Artificial graphite as the negative electrode active material, Super P as the conductive material, and styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders were placed in a Thicky Mixer with deionized distilled water as the solvent in a weight ratio of 96:1:3 and mixed for 30 minutes using a rotating and orbiting method to prepare a negative electrode slurry. After uniformly coating the above negative electrode slurry onto copper foil, the material was vacuum dried and rolled at approximately 120 °C to produce a negative electrode (loading density is approximately 3.3 mAh·cm²). - 2 , the density of the mixture was approximately 1.7 g / cc) was prepared.

[0176] A polyacrylonitrile-polyvinylidene fluoride (PAN-PVDF) based nonwoven fabric support (porosity of about 70-75%) was laminated onto the cathode in a glove box, the composition for forming a solid electrolyte was applied to the nonwoven fabric support, and then the anode was laminated to assemble a cell. The cell was aged for more than 7 hours and heat-cured in an oven at about 90°C for about 30 minutes to manufacture a battery.

[0177] Examples 2 to 3 and Comparative Examples 1 to 2

[0178] A solid electrolyte and a battery were prepared in the same manner as in Example 1, except that the negative electrode active material, liquid electrolyte, and monomer listed in Table 1 below were used. For reference, in Example 2, lithium metal was used as the negative electrode.

[0179] Cathode Active Material Liquid Electrolyte Monomer Type Content (parts by weight) Type Content (parts by weight) Lithium Salt / Concentration Solvent Example 1 Artificial Graphite LiDFOB / 1MM PN 90BPADA 10 Example 2 LiLiDFOB / 1MM PN 90BPADA 10 Example 3 Artificial Graphite LiDFOB / 0.7M +LiFSI / 0.3MM PN 90BPADA 10 Comparative Example 1 Artificial Graphite LiDFOB / 1MEC / EMC / DMC (Volume ratio = 1:2:2) 90BPADA 10 Comparative Example 2 Artificial Graphite LiPF6 / 1MEC / DEC (Volume ratio = 1:1) 90BPADA 10

[0180] * LiFSI: Lithium bis(fluorosulfonyl)imide

[0181] Due to the low solubility of LiPF6 in MPN, 1M LiPF6 electrolyte cannot be manufactured.

[0182] Experimental Example 1: Evaluation of Ionic Conductivity of Solid Electrolytes

[0183] An experimental cell in the form of a coin cell was fabricated by placing the solid electrolytes of Example 1 and Comparative Examples 1 and 2 between two SUS (stainless steel) plates. The internal resistance of the cell was measured using an electrochemical impedance analyzer (IM6, Zahner Elekrik), and the impedance spectrum was measured with a full width of 20 mV at the open-circuit potential from 1 Hz to 1 MHz.

[0184] The measurement results are shown in Table 2 below.

[0185] Measurement temperature (°C) Ionic conductivity (10 -3 S / cm) Example 1 Comparative Example 1 Comparative Example 2 25 1.00 0.8 23 0.5 24 5 1.3 4 1.1 3 0.6 3 26 0 1.6 6 1.3 8 1.11

[0186] Referring to Table 2, it can be seen that Example 1, which used a nitrile-based compound having an ether group as a solvent, showed the highest ionic conductivity at all temperatures compared to Comparative Examples 1 and 2, which used a carbonate-based solvent.

[0187] Experimental Example 2: Evaluation of Oxidation Stability of Solid Electrolyte

[0188] An experimental cell was fabricated using a stainless steel electrode as the working electrode, lithium-metal foil (Honjo metal, Japan, thickness = 0.3 mm) as the counter electrode and reference electrode, and the solid electrolyte of Example 1 and Comparative Examples 1 and 2 as the electrolyte.

[0189] The electrochemical stability of the experimental cell was evaluated according to linear sweep voltammetry (LSV).

[0190] Measurements were performed on the experimental cell using a potentiostat (VMP3, Bio-logic, Science Instrument, France) from the open circuit potential (OCP) to 5.5 V (vs. Li / Li + Scanning was performed at room temperature at a scan speed of 0.5 mV / sec up to ). The measurement results are shown in Figure 2.

[0191] Referring to Figure 2, in Example 1 and Comparative Example 1 using LiDFOB, the current increased slightly around 4.2 V, remained constant, and then increased sharply around 5.3 V. This suggests that the oxidation current was measured to be high because LiDFOB decomposed starting from 4.2 V. In Comparative Example 2 using LiPF6 as the lithium salt, the oxidation current gradually increased, and a higher oxidation current than in Example 1 and Comparative Example 1 was measured at 4.7 V or higher. This is attributed to the fact that the product formed on the electrode surface due to the decomposition of LiDFOB at low voltages inhibited the decomposition of the electrolyte at high voltages.

[0192] In addition, seeing that the oxidation current in Example 1 was observed to be lower than that in Comparative Example 1, it can be inferred that the product formed by the decomposition of LiDFOB in MPN solvent formed a thinner layer than the product in EC / EMC / DMC solvent, thereby exhibiting more stable oxidation stability.

[0193] Experimental Example 3: Evaluation of Battery Rate Capability Characteristics

[0194] Rate capability characteristics were evaluated for the batteries of the examples and comparative examples.

[0195] The voltage range was such that the lower discharge limit voltage was 3 V and the upper charge limit voltage was 4.2 V to 4.5 V. The discharge capacity was measured by charging and discharging for 5 cycles each at a constant current at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 0.1 C at 25 ℃. A battery test system (WBCS3000Le, Wonatech, Korea) was used as the measurement device.

[0196] The measurement results when the charging upper limit voltage (operating voltage) is 4.4V are shown in Figure 3.

[0197] Referring to FIG. 3, Example 1 showed discharge capacities of 196 mAh / g, 184 mAh / g, 161 mAh / g, 92.8 mAh / g, and 33.8 mAh / g at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, and discharge capacity retention rates of 93.9% (0.2 C), 82.1% (0.5 C), 47.3% (1 C), and 17.2% (2 C) (based on 0.1 C discharge capacity).

[0198] Comparative Example 1 showed discharge capacities of 185 mAh / g, 172 mAh / g, 144 mAh / g, 93.8 mAh / g, and 40.0 mAh / g at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, and discharge capacity retention rates of 93.0% (0.2 C), 77.8% (0.5 C), 50.7% (1 C), and 21.6% (2 C).

[0199] Comparative Example 2 showed discharge capacities of 187 mAh / g, 64.5 mAh / g, 14.2 mAh / g, 6.11 mAh / g, and 1.79 mAh / g at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, with discharge capacity retention rates of 34.5% (0.2 C), 7.59% (0.5 C), 3.27% (1 C), and 0.96% (2 C).

[0200] The battery of Example 1 exhibited superior discharge capacity and discharge capacity retention rate compared to Comparative Example 1 and Comparative Example 2. This indicates that the LiDFOB formed a stable interface layer, and this interface layer does not decompose relatively even at high voltage.

[0201] Experimental Example 4: Evaluation of Battery Life Characteristics

[0202] (1) The batteries of the examples and comparative examples were charged at 0.33 C and discharged at 0.5 C. The voltage range was 3 V to 4.2 V.

[0203] The measurement results are shown in Table 3.

[0204] 1 Cycle 50 Cycles 100 Cycles 150 Cycles 200 Cycles Example 1 Discharge Capacity (mAh / g) 168 149 136 126 120 Capacity Retention Rate (%) 100 88.78 175 71.4 Example 2 Discharge Capacity (mAh / g) 152 22.9 20.9 17.8 16.6 Capacity Retention Rate (%) 100 15.11 3.7 11.7 10.9 Example 3 Discharge Capacity (mAh / g) 153.21 30 121 10296 Capacity Retention Rate (%) 100 84.8 78.98 66.6 62.7 Comparative Example 1 Discharge Capacity (mAh / g) 154 1159 28273 Capacity Retention Rate (%) 100 74.75 9.75 3.24 7.4 Comparative Example 2 Discharge Capacity (mAh / g) 1423129-- Capacity Retention Rate (%) 10021.820.4--

[0205] Referring to Table 3, it can be seen that the batteries of the examples exhibit excellent lifespan characteristics. For reference, Comparative Example 2 stopped evaluation after 100 cycles because the battery performance deteriorated significantly. (2) The discharge capacity and capacity retention rate were measured in the same way as above by raising the charge voltage cut-off to 4.4 V, and the results are shown in Table 4 below.

[0206] 1 Cycle 50 Cycles 100 Cycles 150 Cycles 200 Cycles Example 1 Discharge Capacity (mAh / g) 184 16 114 5 129 112 Capacity Retention Rate (%) 100 87.5 78.8 70.1 60.9 Example 2 Discharge Capacity (mAh / g) 159.4 40.4 26.1 19.4 14.8 Capacity Retention Rate (%) 100 25.3 16.4 12.2 9.3 Example 3 Discharge Capacity (mAh / g) 180.2 128 103 9050 Capacity Retention Rate (%) 100 71 57.2 49.9 27.7 Comparative Example 1 Discharge Capacity (mAh / g) 14 197 72-- Capacity Retention Rate (%) 100 68.8 51.1-- Comparative Example 2 Discharge Capacity (mAh / g) 13420 --- Capacity Retention Rate (%) 10014.9 ---

[0207] Referring to Table 4, it can be seen that the batteries of the examples are very stable compared to the batteries of the comparative examples at high voltage. In Comparative Example 1 and Comparative Example 2, the battery performance deteriorated significantly after 100 cycles and 50 cycles, respectively, so the evaluation was stopped.

[0208] Experimental Example 5: Chemical composition analysis of the electrode / electrolyte interface layer

[0209] XPS analysis was performed to analyze the components of the SEI between the cathode and the electrolyte.

[0210] For the batteries of Example 1 and Comparative Example 1, charge and discharge were performed two cycles at 0.1 C to form the Solid electrolyte interphase (SEI) and Cathode electrolyte interphase (CEI).

[0211] Subsequently, the cell was disassembled in a glove box under an argon atmosphere, and the anode and cathode were washed with 1,2-dimethoxyethane (DME) and dried in a glove box at room temperature for two days. Afterward, XPS analysis was performed on the cathode surface. The analysis results are shown in Figure 4.

[0212] In Figs. 4a, 4b, and 4c, the charging voltage cut-off is 4.2 V, and in Figs. 4d, 4e, and 4f, it is 4.4 V.

[0213] Figures 4a and 4d show the B 1s XPS spectrum, Figures 4b and 4e show the C 1s XPS spectrum, and Figures 4c and 4f show the F 1s XPS spectrum.

[0214] Li2B4O7 is a component capable of forming a stable SEI due to its high dielectric constant and excellent electrochemical properties. In Fig. 4a, a peak corresponding to Li2B4O7 can be observed at 192.5 eV. The peak size of Li2B4O7 in Example 1 was found to be higher than that of Comparative Example 1. Similarly, in Fig. 4d, Example 1 was measured to be higher than Comparative Example 1.

[0215] In Example 1, it can be determined that LiDFOB decomposed into Li2B4O7 to form a stable SEI. Therefore, Example 1 can exhibit excellent rate capability and lifespan characteristics at high voltage.

[0216] In Fig. 4b, peaks of CC (284.8 eV), C-O (286 eV), and C=O (288–290 eV) appeared. The CC peak is the peak observed in graphite or SBR binders. The C-O peak corresponds to the peak of Li2CO3 generated by the decomposition of CMC binders or electrolytes. A larger Li2CO3 peak was observed in Comparative Example 1 than in Example 1. The C=O peak appeared similarly in the Examples and Comparative Examples.

[0217] Li2CO3 is one of the components that enables the stable formation of SEI. Since Li2CO3 is an organic material, it is not as hard as Li2B4O7, an inorganic material, and is relatively easy to break. Accordingly, even though Comparative Example 1 contains more Li2CO3 than Example 1, it can be determined that the SEI of Example 1 is more stable because the content of Li2B4O7 is lower.

[0218] In Fig. 4e, the CC peak in Example 1 is larger than that in Comparative Example 1, while the CO and C=O peaks are lower. The difference in peaks between Example 1 and Comparative Example 1 is greater than that in Fig. 4b, which has a cut-off of 4.2V. This indicates that a thinner SEI was formed in Example 1 compared to Comparative Example 1, resulting in a higher measurement of graphite CC, while a thicker SEI was formed in Comparative Example 1, resulting in a lower CC peak than in Example 1. Additionally, the large appearance of the C-O C=O peak suggests that the SEI of Comparative Example 1 contains a large amount of carbonate components derived from EC / DEC / EMC.

[0219] In Fig. 4c, a peak corresponding to LiF (685 eV) is visible, and the magnitude of the LiF peak was measured to be higher in Example 1 than in Comparative Example 1. At high voltage in Fig. 4f, the Example and Comparative Example appeared to be nearly similar. LiF is also a representative inorganic material that forms a stable SEI, similar to Li2B4O7.

[0220] From this, in Example 1 using MPN and LiDFOB, LiC03 decreased and Li2B4O7 and LiF, which are components forming a stable SEI, increased. On the other hand, in Comparative Example 1 containing a carbonate-based electrolyte, a thick SEI containing a large amount of organic components was formed.

[0221] Through this, it can be seen that Example 1, which uses MPN and LiDFOB together, is composed mostly of hard inorganic components, and these components formed a thin, dense, and stable SEI. Accordingly, it is determined that Example 1 exhibits superior rate capability and lifespan characteristics compared to an electrolyte using a carbonate-based solvent, as well as stable performance even at high voltages.

Claims

1. A liquid electrolyte comprising a solvent comprising a nitrile-based compound having an ether group and a lithium salt comprising lithium difluorooxalatoborate; and A composition for forming a solid electrolyte comprising a monomer having a polymerizable functional group.

2. A composition for forming a solid electrolyte according to claim 1, wherein the nitrile-based compound comprises one or two ether groups and one or two nitrile groups.

3. A composition for forming a solid electrolyte according to claim 1, wherein the nitrile-based compound comprises an alkoxy group having 1 to 5 carbon atoms.

4. A composition for forming a solid electrolyte according to claim 1, wherein the nitrile-based compound comprises at least one selected from the group consisting of methoxypropionitrile, methoxybutyronitrile, and ethoxypropionitrile.

5. A composition for forming a solid electrolyte according to claim 1, wherein the molar concentration of the lithium salt is 0.9 M to 2 M.

6. A composition for forming a solid electrolyte according to claim 1, wherein the monomer having the polymerizable functional group comprises at least one selected from the group consisting of bisphenol A ethoxylated di(meth)acrylate, ethylene glycol diacrylate, triethylene glycol diacrylate, trimethylolpropane ethoxylated triacrylate, (meth)acrylic acid, carboxyethyl acrylate, cyano(meth)acrylic acid, trimethylolpropane trimethacrylate, and pentaerythritol tetraacrylate.

7. A composition for forming a solid electrolyte according to claim 1, wherein the content of the monomer is 5 to 30 parts by weight per 100 parts by weight of the sum of the weights of the liquid electrolyte and the monomer.

8. A composition for forming a solid electrolyte according to claim 1, wherein the liquid electrolyte further comprises at least one additive selected from the group consisting of an unsaturated cyclic carbonate compound, a fluorine-substituted cyclic carbonate compound, a sulfone compound, a cyclic sulfate compound, a fluorine-substituted phosphate compound, and an oxalatophosphate compound.

9. Nitrile-based compounds having ether groups; A lithium salt comprising lithium difluorooxalatoborate; and Solid electrolyte containing a polymer.

10. A solid electrolyte according to claim 9, wherein the nitrile-based compound comprises one or two ether groups and one or two nitrile groups.

11. The solid electrolyte of claim 9, wherein the nitrile-based compound comprises an alkoxy group having 1 to 5 carbon atoms.

12. A solid electrolyte according to claim 9, wherein the nitrile-based compound comprises at least one selected from the group consisting of methoxypropionitrile, methoxybutyronitrile, and ethoxypropionitrile.

13. A solid electrolyte according to claim 9, further comprising at least one additive selected from the group consisting of an unsaturated cyclic carbonate compound, a fluorine-substituted cyclic carbonate compound, a sulfone compound, a cyclic sulfate compound, a fluorine-substituted phosphate compound, and an oxalatophosphate compound.

14. A lithium secondary battery comprising a positive electrode, a negative electrode facing the positive electrode, and a solid electrolyte according to claim 9 interposed between the positive electrode and the negative electrode.

15. A lithium secondary battery according to claim 14, wherein the negative electrode comprises graphite or lithium metal as a negative electrode active material.

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