Composition for preparing solid electrolyte, solid electrolyte, and lithium secondary battery using the same

Abstract

The present invention relates to a composition for preparing a solid electrolyte, comprising a polyrotaxane compound, a crosslinking agent, a lithium salt, and an organic solvent, wherein the crosslinking agent includes a compound represented by Formula 1 and a compound represented by Formula 2; a method for preparing a solid electrolyte for a lithium secondary battery, the method including thermally curing the above-mentioned composition for preparing a solid electrolyte; and a solid electrolyte for a lithium secondary battery and a lithium secondary battery including the same, the solid electrolyte including a thermally cured product of the above-mentioned composition for preparing a solid electrolyte.

Description

TECHNICAL FIELD

[0001] This application claims priority to Korean Patent Application No. 10-2021-0016331, filed on February 4, 2021, the disclosure of which is incorporated herein.

[0002] The present application relates to a composition for preparing a solid electrolyte, a solid electrolyte, and a lithium secondary battery using the same. BACKGROUND

[0003] Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, and thus are being applied to various fields including mobile devices, electronic products, electric vehicles, etc. As the application fields of lithium secondary batteries become diversified, the physical property requirement conditions of lithium secondary batteries are also increasing, and in particular, there is a need to develop lithium secondary batteries that can be stably driven in various environments.

[0004] In general, a secondary battery is manufactured by installing an electrode assembly composed of a negative electrode, a positive electrode, and a separator in a cylindrical, prismatic, or pouch-shaped case having a certain space, and injecting an electrolyte into the electrode assembly.

[0005] In general, as an electrolyte of an electrochemical device, a liquid electrolyte prepared by dissolving a salt in a non-aqueous organic solvent has been mainly used. However, such a liquid electrolyte causes degradation of electrode materials, and the possibility of evaporation of the organic solvent is very high, and combustion occurs due to an increase in temperature, and there is a risk of leakage, making it difficult to implement various types of electrochemical devices requiring safety.

[0006] A solid electrolyte has an advantage in that it has higher electrochemical stability than a liquid electrolyte. However, the ionic conductivity of a solid electrolyte at room temperature is significantly lower than that of a liquid electrolyte, and although research is actively being conducted to solve this problem, there are still limitations in that the electrochemical window is reduced when the ionic conductivity is increased, etc. SUMMARY

[0007] [TECHNICAL PROBLEM]

[0008] One aspect of the present application provides a solid electrolyte having increased flexibility through cross-linking of a supramolecular material.

[0009] Another aspect of the present application provides a lithium secondary battery having improved stability and ionic conductivity by including the solid electrolyte.

[0010] [TECHNICAL SOLUTION]

[0011] According to one aspect of the present application, there is provided a composition for preparing a solid electrolyte, the composition comprising a polyrotaxane compound, a crosslinking agent, a lithium salt, and an organic solvent, wherein the crosslinking agent includes a compound represented by the following Formula 1 and a compound represented by the following Formula 2.

[0012] [Formula 1]

[0013] O==C==N—L1—N==C==O

[0014] In the above Formula 1, L1 is a C1 to C10 alkylene group, and

[0015] [Formula 2]

[0016]

[0017] In the above Formula 2, L2 is a C1 to C5 alkylene group.

[0018] According to another aspect of the present application, there is provided a method of preparing a solid electrolyte for a lithium secondary battery, including thermally curing the above composition for preparing a solid electrolyte.

[0019] According to another aspect of the present application, there is provided a solid electrolyte for a lithium secondary battery, including a thermally cured product of the above composition for preparing a solid electrolyte.

[0020] According to another aspect of the present application, there is provided a lithium secondary battery including a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and the above solid electrolyte for a lithium secondary battery interposed between the positive electrode and the negative electrode.

[0021] [Advantageous Effects]

[0022] The present application can provide a solid electrolyte having excellent flexibility, tensile strength, and ionic conductivity.

[0023] Further, the present application can provide a lithium ion battery having improved stability and ionic conductivity by including the above solid electrolyte. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a diagram showing a molecular structure of a polyrotaxane;

[0025] Figure 2 is a diagram showing a molecular structure (a) of a linear polymer pullulan used in Comparative Example 3 herein and a molecular structure (b) of a cyclic polymer PCD used in Comparative Example 4 herein;

[0026] Figure 3 is a schematic diagram showing a crosslinked network formed by a polyrotaxane compound in the solid electrolyte of the present application through a crosslinking agent;

[0027] Figure 4 is a graph showing the change in ionic conductivity of the solid electrolyte prepared in Example 1 of the present application with temperature;

[0028] Figure 5 shows the measurement results of the tensile physical properties of the solid electrolytes prepared in Example 1 and Comparative Examples 1 to 4 of the present application; and

[0029] Figure 6 is a graph showing the CV curve of a battery using the solid electrolyte prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0030] Hereinafter, the present application will be described in detail.

[0031] The most commonly used polymer in solid polymer electrolytes is poly(ethylene oxide) (PEO), and although PEO has the ability to conduct lithium ions despite being a solid, it has low flexibility due to high crystallinity and cannot dissociate a large number of lithium ions due to a low dielectric constant, thus having low ionic conductivity at room temperature, making it difficult to apply PEO to lithium secondary batteries. Therefore, many studies have been conducted to increase ionic conductivity by modification (e.g., blending PEO-based polymers or synthesizing PEO-based polymers as block copolymers).

[0032] However, when ionic conductivity is increased by the above-described methods, there is a problem in that mechanical properties, which are in trade-off relationship with ionic conductivity, are deteriorated.

[0033] Therefore, the present inventors have found that by introducing a polyrotaxane, which is a supramolecular material, it is possible to simultaneously improve ionic conductivity and an electrochemical window. In particular, it has been confirmed that there is a significant effect of improving flexibility when combined with a specific crosslinking agent.

[0034] A polyrotaxane is a polymer chain of linear structure, for example, a structure in which a cyclic ring molecule such as a cyclodextrin ( Figure 1 (a)) is threaded with polyethylene glycol (PEG) ( Figure 1 (b)), and is a supramolecular material in which the cyclic molecule can move and rotate along the polymer chain. Through a crosslinking agent, the polyrotaxane can form a network structure as shown in Figure 3 .

[0035] Specifically, the composition for preparing a solid electrolyte of the present application comprises a polyrotaxane compound, a crosslinking agent, a lithium salt, and an organic solvent, and each component will be described in more detail below.

[0036] Preparation of a composition for the preparation of a solid electrolyte

[0037] (a) Polyrotaxane

[0038] In an embodiment of the present application, a polyrotaxane compound is used as a base polymer.

[0039] In an embodiment of the present application, the polyrotaxane compound contains 10 to 90, preferably 10 to 50, α-cyclodextrin (α-CD) molecules. When containing 90 or less, preferably 50 or less, α-CD molecules, the advantage is that the molecular mobility of the polyrotaxane is more active. The α-CD molecule has a size suitable for forming an inclusion complex with PEG, and has the advantage of convenient synthesis compared to the polyrotaxane preparation method using β-CD and γ-CD. In addition, when containing 90 or less, preferably 50 or less, α-CD molecules, the molecular mobility of the polyrotaxane is more active, and thus, has the advantage of being able to secure high ionic conductivity.

[0040] The α-CD contains 18 hydroxyl groups in the molecule, and since a functional group can be easily introduced through the hydroxyl group, a crosslinking point can be formed at a high density, and thus the crosslinked α-CD can move or rotate along the polymer chain, and thus can have high flexibility and ionic conductivity.

[0041] In an embodiment of the present application, the end group of the polyrotaxane compound can have a structure represented by the following formula E.

[0042] [Formula E]

[0043]

[0044] In addition, the end group of the polyrotaxane compound can be a functional group formed by linking a compound selected from the group consisting of N-benzyloxycarbonyl-L-tyrosine and N-benzyloxycarbonyl-L-phenylalanine, and 1-adamantane carboxylic acid.

[0045] In an embodiment of the present application, the weight average molecular weight of the polyrotaxane compound can be 10,000 g / mol to 100,000 g / mol, preferably 20,000 g / mol to 90,000 g / mol, most preferably 40,000 g / mol to 60,000 g / mol. When included in the above range, there is an effect of maximizing the molecular mobility.

[0046] In the present application, the "weight average molecular weight" can refer to the conversion value of standard polystyrene measured by gel permeation chromatography (GPC), and unless otherwise indicated, the molecular weight can refer to the weight average molecular weight. For example, in the present application, the weight average molecular weight is measured using the 1200 series of Agilent Corporation under GPC conditions, and at this time, the column used can be the PL mixed B column of Agilent Corporation, and the solvent used is DMSO.

[0047] In an embodiment of the present application, the content of the polyrotaxane compound can be 5 to 20% by weight, preferably 8 to 15% by weight, based on the total weight of the composition for preparing a solid electrolyte.

[0048] When the content of the polyrotaxane compound is within the above range, it is preferred because high ionic conductivity can be ensured. Specifically, when less than 5% by weight, there is a problem that the overall uniformity of the surface is reduced due to the small content of the polyrotaxane, making it difficult to form a restorable monolayer film, and when more than 20% by weight, there is a problem that ionic conductivity is reduced due to the increased crystallinity of the polyrotaxane polymer.

[0049] Meanwhile, the composition for preparing a solid electrolyte can further include one or more polymers selected from the group consisting of poly(ethylene oxide), poly(vinylidene fluoride), and cellulose, and the weight average molecular weight of the polymer can be 100,000 to 1,000,000 g / mol. In this case, there is an advantage of enhancing the electrolyte network, thereby increasing the mechanical strength by adding a polymer having a relatively long chain, while reducing the cost by reducing the amount of the polyrotaxane compound which is relatively expensive and requires a multi-step synthesis process. At this time, the content of the above polymer can be 2 to 30% by weight, based on the total weight of the above composition for preparing a solid electrolyte.

[0050] (b) a crosslinking agent

[0051] In an embodiment of the present application, the crosslinking agent includes a compound represented by the following Formula 1 and a compound represented by the following Formula 2, and preferably, the crosslinking agent can consist of the compound represented by the following Formula 1 and the compound represented by the following Formula 2.

[0052] [Formula 1]

[0053] O==C==N—L1—N==C==O

[0054] In the above Formula 1, L1 is a C1 to C10 alkylene group, and

[0055] [Formula 2]

[0056]

[0057] In the above Formula 2, L2 is a C1 to C5 alkylene group.

[0058] In one embodiment of the present application, L1 is a C3 to C8 alkylene group, preferably hexylene.

[0059] In one embodiment of the present application, the compound represented by the above Formula 1 is hexamethylene diisocyanate.

[0060] The compound represented by Formula 1 is the soft segment in the crosslinking agent, and because the alkylene group in the structure can rotate, it can form various isomers, making the compound usable as a flexible crosslinking agent.

[0061] In one embodiment of the invention, L2 is a C1 to C3 alkylene, preferably methylene.

[0062] In one embodiment of the present invention, the compound represented by Formula 2 above is methylene diphenyl-4,4'-diisocyanate.

[0063] The compound represented by Formula 2 can be a hard segment in a crosslinking agent and has a small number of isomer structures, so it can be used as a hard crosslinking agent.

[0064] In one embodiment of the present invention, the weight ratio of the compound represented by Formula 1 to the compound represented by Formula 2 is 1:1 to 10:1, preferably 2:1 to 6:1, and more preferably 3:1 to 5:1.

[0065] When the weight ratio of the compound represented by Formula 1 to the compound represented by Formula 2 is within the above-mentioned range, it is preferred because tensile strength and tensile strain can be adequately ensured. Specifically, when the content of the compound represented by Formula 1 is greater than the above-mentioned range, it is not preferred because the tensile strength, i.e., mechanical strength, decreases, and when the content of the compound represented by Formula 2 is greater than the above-mentioned range, it is not preferred because the tensile strain decreases. When the tensile strength is too high, the ionic conductivity may decrease; therefore, it is important to use soft and hard segments in an appropriate ratio.

[0066] In one embodiment of the invention, the content of the crosslinking agent is 0.5% to 5% by weight, preferably 0.5% to 3% by weight, and more preferably 1% to 2% by weight, based on the total weight of the composition used to prepare the solid electrolyte.

[0067] When the content of the crosslinking agent is within the above range, it is preferred because suitable tensile properties can be ensured. Specifically, when it is less than 0.5% by weight, the crosslinked body may not form properly, and therefore the electrolyte membrane may not be formed, while when it is greater than 5% by weight, there is a problem of reduced tensile strain.

[0068] (c) Lithium salts

[0069] As the lithium salt, any lithium salt commonly used in electrolytes for lithium secondary batteries can be used without limitation, and for example, the lithium salt may contain Li. + As a cation, and including those selected from F - Cl - ,Br - I - NO3 - N(CN)2- BF4 - ClO4 - B 10 Cl 10 - AlCl4 - AlO4 - PF6 - CF3SO3 - CH3CO2 - CF3CO2 - AsF6 - SbF6 - CH3SO3 - (CF3CF2SO2)2N - (CF3SO2)2N - (FSO2)2N - BF2C2O4 - BC4O8 - BF2C2O4CHF - PF4C2O4 - PF2C4O8 - PO2F2 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - C4F9SO3 - CF3CF2SO3 - CF3CF2(CF3)2CO - (CF3SO2)2CH - CF3(CF2)7SO3 - and SCN - as an anion.

[0070] Specifically, the lithium salt can be one or more selected from the group consisting of LiN03, LiPF6, LiC104, LiBF4, LiFSI, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiS03CF3, LiP02F2, lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFBP), lithium tetrafluoro(oxalato)phosphate (LiTFOP), and lithium fluoromalonate(difluoro)borate (LiFMDFB), and preferably one or more selected from the group consisting of LiN03and LiC104, most preferably LiN03. When the lithium salt is LiN03, it is advantageous that lithium can move more actively due to the small molecular weight.

[0071] In one embodiment of the present application, the concentration of the lithium salt in the composition for preparing a solid electrolyte can be 0.05 M to 3.0 M, preferably 1.25 M to 2.5 M, more preferably 1.5 M to 2.0 M.

[0072] When the concentration of the lithium salt is less than 0.05 M, there is a disadvantage of low ionic conductivity, and when it is more than 3.0 M, the salt prevents the formation of a polymer film, and thus also prevents the formation of a crosslinker.

[0073] (d) organic solvent

[0074] As the organic solvent, various organic solvents generally used in lithium electrolytes can be used without limitation. For example, one or more selected from the group consisting of acetone, ethanol, acetonitrile, dimethyl sulfoxide, anhydrous dimethyl sulfoxide, N-methyl-2-pyrrolidone (NMP), γ-butyrolactone (GBL), dimethylformamide (DMF), dimethylacetamide (DMAc), and tetrahydrofuran (THF) can be used, and preferably anhydrous dimethyl sulfoxide capable of easily dissolving the polyrotaxane compound is used, and when the solubility is changed by modification of the polyrotaxane, other organic solvents can be used.

[0075] Unless otherwise specified, the remaining portion other than the content of the other components (e.g., the polyrotaxane compound, the crosslinking agent, and the lithium salt) in the total weight of the composition for preparing a solid electrolyte can be all the organic solvent.

[0076] Solid electrolyte

[0077] The solid electrolyte of the present application can be prepared by a method known in the art, in addition to using the composition for preparing a solid electrolyte according to the embodiments of the present application. For example, the composition for preparing a solid electrolyte can be coated on a Teflon plate, and then a heat treatment process can be performed thereon.

[0078] Specifically, the method of producing a solid electrolyte of the present embodiment includes thermally curing a composition for producing a solid electrolyte. That is, the solid electrolyte for a lithium secondary battery of the present application includes a thermally cured product of the composition for producing a solid electrolyte.

[0079] The thermal curing can be performed at 50°C to 90°C, preferably 60°C to 80°C.

[0080] When the thermal curing temperature is lower than 50°C, there can be a problem that the curing time can be equal to or more than 24 hours, while if it is higher than 90°C, it is not preferable because the polymer can shrink.

[0081] After drying the Teflon plate to which the composition is applied and which has undergone thermal curing under vacuum, the solid film is removed from the Teflon plate to form a solid electrolyte for a lithium secondary battery. Thereafter, a post-treatment process can be performed by performing heat treatment in an oven at 80°C to 120°C.

[0082] In the present embodiment, the content of the polyrotaxane compound can be 50% by weight to 90% by weight, preferably 50% by weight to 60% by weight, based on the total weight of the solid electrolyte. This refers to the content of the solid matter after the solvent has been removed and the solid electrolyte has been formed.

[0083] In the present embodiment, the content of the crosslinking agent can be 5% by weight to 25% by weight, preferably 10% by weight to 20% by weight, based on the total weight of the solid electrolyte. This refers to the content of the solid matter after the solvent has been removed and the solid electrolyte has been formed.

[0084] In the present embodiment, if necessary, the solid electrolyte can further include a binder resin. The binder resin can be introduced for adhesion between the solid electrolyte materials and adhesion between the solid electrolyte layer and the battery elements (e.g., support layers and / or electrodes) stacked on both sides of the solid electrolyte layer. The material of the binder resin is not particularly limited and can be appropriately selected in the range of components used as an adhesive for an electrochemical device.

[0085] In the present embodiment, the thickness of the solid electrolyte can be 300 μm or less, preferably 50 μm to 100 μm. The thickness can be appropriately adjusted within the above range in consideration of the ionic conductivity, the physical strength, the energy density of the battery to which the solid electrolyte is applied. For example, in terms of the ionic conductivity or the energy density, the thickness can be 100 μm or less, 70 μm or less, or 60 μm or less. Meanwhile, in terms of the physical strength, the thickness can be 50 μm or more, 60 μm or more, or 70 μm or more.

[0086] Lithium secondary battery

[0087] Next, a lithium secondary battery of the present application will be described.

[0088] The lithium secondary battery of the present application includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the above-mentioned solid electrolyte for lithium secondary batteries disposed between the positive electrode and the negative electrode.

[0089] The solid electrolyte of the present application is a self-standing solid electrolyte, and can be used as a film-type separator instead of a general separator, but can further include a separator if necessary.

[0090] The solid electrolyte can be prepared in the form of a film, and then disposed on (introduced into) at least one surface of a pre-prepared negative electrode, positive electrode, and separator, or the above-mentioned composition for preparing a solid electrolyte can be directly applied to at least one surface of a pre-prepared negative electrode, positive electrode, and separator, and then introduced by drying and solidification.

[0091] Since the solid electrolyte has been described above in the components of the lithium secondary battery, the description thereof will be omitted, and the other components will be described below.

[0092] (a) Positive electrode

[0093] The positive electrode of the present application contains a positive electrode active material, and can be prepared by coating a positive electrode current collector with a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, a solvent, and the like, and then drying and roll-pressing.

[0094] The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like can be used.

[0095] A lithium transition metal oxide can be used as the positive electrode active material, and can be used without limitation as long as the insertion and de-insertion of lithium ions are easily performed during charge and discharge, but can be, for example, one or more selected from the group consisting of lithium nickel cobalt-based composite oxides, lithium manganese-based composite oxides, and lithium iron phosphate-based composite oxides.

[0096] The lithium nickel cobalt-based composite oxide can be represented by the following Formula 3:

[0097] [Formula 3]

[0098] Li(Ni a Co b Mn c M d )O2

[0099] In the above formula 3, M is W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, or Mo, and a, b, c, and d are each an atomic fraction of an independent element, wherein 0.50≤a≤0.90, 0.05≤b≤0.25, 0.05≤c≤0.25, 0≤d≤0.05, and a+b+c+d=1.

[0100] Preferably, in the above formula 3, M is Al, and a, b, c, and d can be 0.60≤a≤0.90, 0.05≤b≤0.20, 0.05≤c≤0.20, and 0≤d≤0.03, respectively.

[0101] For an NCM positive electrode active material including nickel (Ni), cobalt (Co), and manganese (Mn), the higher the content of Ni, the higher the energy density, but there is a disadvantage in that the reactivity and stability of the positive electrode surface are deteriorated. However, when aluminum (Al) is introduced as M, the disadvantage can be overcome.

[0102] The lithium-manganese-based composite oxide can be LiMnO2, LiMnO3, LiMn2O3, Li2MnO3, Li 1+y1 Mn 2-y1 O4(0≤y1≤0.33), LiMn 2-y2 M y2 O4(wherein M is one or more selected from the group consisting of Ni, Co, Fe, P, S, Zr, Ti, and Al, and 0≤y2≤2), LiMn 2-y3 M y3 O2(wherein M is one or more selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and 0.01≤y3≤0.1), or Li2Mn3MO8(wherein M is one or more selected from the group consisting of Fe, Co, Ni, Cu, and Zn).

[0103] The lithium-iron-phosphate-based composite oxide can be represented by the following formula 4.

[0104] [Formula 4]

[0105] LiFe 1-x M x PO4

[0106] In the above formula 4, M is one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Y, Zn, In, Ru, Sn, Sb, Ti, Te, Nb, Mo, Cr, Zr, W, Ir, and V, and 0≤x<1.

[0107] The content of the positive active material can be 60 to 99% by weight, specifically 70 to 90% by weight, based on the total weight of solids in the positive electrode slurry. At this time, when the content of the positive active material is 60% by weight or less, the energy density is reduced to a lower capacity.

[0108] The binder is a component that contributes to adhesion between the active material and the conductive material and adhesion to the current collector, and the amount of addition thereof can generally be 1 to 30% by weight, based on the total weight of solids in the positive electrode slurry. Examples of the binder can include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene termonomer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

[0109] Further, the conductive material is a material that imparts electrical conductivity without causing chemical changes in the battery, and the amount of addition thereof can be 0.5 to 20% by weight, based on the total weight of solids in the positive electrode slurry.

[0110] Examples of the conductive material can include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal-cracking carbon black; natural graphite, artificial graphite, carbon nanotubes, or graphite powder of graphite having a well-developed crystal structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as fluorocarbon powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or polyphenylene derivatives and the like conductive materials.

[0111] Further, the solvent of the positive electrode slurry can include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and the amount thereof used can be such that a preferred viscosity is obtained when the positive active material, the binder, and the conductive material, etc. are contained. For example, the content of the solvent can be such that the concentration of solids in the positive electrode slurry containing the positive active material, the binder, and the conductive material is 5 to 90% by weight, preferably 5 to 80% by weight.

[0112] (b) Negative electrode

[0113] The negative electrode of the present application contains a negative active material, and can be prepared by coating a negative current collector with a negative electrode slurry containing a negative active material, a binder, a conductive material, and a solvent, etc., and then drying and roll-pressing.

[0114] The negative current collector generally has a thickness of 3 μm to 500 μm. The negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like, and aluminum-cadmium alloy, and the like can be used. In addition, similarly to the positive current collector, the negative current collector can have minute projections and depressions to improve the binding strength with the negative active material, and the negative current collector can be used in various shapes such as a film, a sheet, a foil, a mesh, a porous body, a foam body, and a nonwoven fabric body.

[0115] In addition, the negative active material can include one or more selected from a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of lithium and the metal, a metal composite oxide, a material capable of doping and undoping lithium, lithium metal, and a transition metal oxide.

[0116] As the carbon material capable of reversibly intercalating / deintercalating lithium ions, a carbon-based negative active material generally used in lithium ion secondary batteries can be used without particular limitation, and representative examples thereof can include crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon can include graphite such as irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon can include soft carbon (low-temperature sintered carbon) or hard carbon, mesophase pitch carbide, and baked coke, etc.

[0117] As the metal or the alloy of lithium and the metal, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn, or an alloy of lithium and the metal can be used.

[0118] As the metal composite oxide, a metal composite oxide selected from the group consisting of PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3(0≤x≤1), Li x WO2(0≤x≤1), and Sn x Me 1-x Me' y O z (wherein Me is Mn, Fe, Pb, or Ge; Me' is Al, B, P, Si, a Group I, II, and III element of the periodic table, or a halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8) of the group.

[0119] The material capable of doping and undoping lithium can be Si, SiO x(0 < x < 2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, and combinations thereof, and is not Si), Sn, SnO2, and Sn-Y (where Y is an element selected from the group consisting of alkali metal, alkaline earth metal, Group 13 element, Group 14 element, transition metal, rare earth element, and combinations thereof, and is not Sn), and the like, or at least one of which can be used in mixture with SiO2. The element Y can be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0120] Examples of transition metal oxides include lithium-containing titanium composite oxides (LTO), vanadium oxides, and lithium vanadium oxides, and the like.

[0121] The content of the negative active material can be 80 to 99% by weight, based on the total weight of solids in the negative electrode slurry.

[0122] The binder is a component that contributes to adhesion between the conductive material, the active material, and the current collector, and is generally added in an amount of 1 to 30% by weight, based on the total weight of solids in the negative electrode slurry. Examples of the binder can include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, fluoro rubber, or various copolymers thereof.

[0123] The conductive material is a component that further improves the electrical conductivity of the negative active material, and the conductive material can be added in an amount of 0.5 to 20% by weight, based on the total weight of solids in the negative electrode slurry. The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and is, for example, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal cracking carbon black; natural graphite, artificial graphite, carbon nanotubes, or graphite powder of graphite having a well-developed crystal structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as fluorocarbon powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or polyphenylene derivatives and the like conductive materials.

[0124] The solvent of the negative electrode slurry can include water or an organic solvent such as NMP and alcohol, and the amount thereof can be such that a preferred viscosity can be obtained when the negative electrode active material, the binder, and the conductive material, etc. are contained. For example, the content of the solvent can be such that the concentration of solids in the slurry containing the negative electrode active material, the binder, and the conductive material is 30 to 80% by weight, preferably 40 to 70% by weight.

[0125] (c) a separator

[0126] The lithium secondary battery of the present application can include a separator between the positive electrode and the negative electrode.

[0127] The separator separates the positive electrode and the negative electrode and provides a path for movement of lithium ions. Any separator can be used without particular limitation as long as it is a separator commonly used in lithium secondary batteries.

[0128] Specifically, as the separator, a porous polymer film such as a porous polymer film prepared using a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer; or a laminated structure having two or more layers thereof can be used. In addition, a typical porous nonwoven fabric such as a nonwoven fabric formed of high-melting-point glass fibers or polyethylene terephthalate fibers, etc. can be used. In addition, a coated separator including a ceramic component or a polymeric material can be used to ensure heat resistance or mechanical strength, and can be used in a single layer or a multilayer structure.

[0129] The lithium secondary battery of the present application can be applied to portable devices such as mobile phones, notebook computers, and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0130] Therefore, according to another embodiment of the present application, there are provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

[0131] The battery module or the battery pack can be used as a power source for at least one of large- and medium-sized devices: power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0132] The outer shape of the lithium secondary battery of the present application is not particularly limited, but a cylindrical, prismatic, pouch, or coin shape, etc. using a can can be used.

[0133] The lithium secondary battery of the present application can be used not only as a battery cell used as a power source for small-sized devices, but also preferably as a unit cell in a large- and medium-sized battery module including a plurality of battery cells.

[0134] Hereinafter, the present application will be described with reference to specific examples.

[0135] Example

[0136] Example 1

[0137] (Preparation of a composition for preparing a solid electrolyte)

[0138] In a glove box, 0.3 g of a polyrotaxane (-OH equivalent = 4.5 mmol), a crosslinking agent, and LiNO3 were added to 2 g of anhydrous dimethyl sulfoxide (DMSO) solvent and uniformly mixed to prepare a composition for preparing a solid electrolyte. At this time, as the polyrotaxane, a polyrotaxane having the same structure as shown in (b) of Figure 1

[0139] (Preparation of a solid electrolyte)

[0140] The composition for preparing a solid electrolyte, which was sufficiently mixed in a glove box, was poured into a Teflon tray and cast, placed in a sealed container and sealed, and then cured in an oven at 60°C for 24 hours and in an oven at 70°C for 24 hours.

[0141] The sealed container was opened in an argon atmosphere so as not to be in contact with the atmosphere, and dried under vacuum for 1 hour, the film was recovered in a glove box, punched into a size suitable for performing each of the following experimental examples, and heat-treated in an oven at 100°C.

[0142] Example 2

[0143] A solid electrolyte was prepared in the same manner as in Example 1, except that when the composition was prepared, the concentration of LiNO3 was changed to 1.25 M.

[0144] Example 3

[0145] A solid electrolyte was prepared in the same manner as in Example 1, except that when the composition was prepared, the concentration of LiNO3 was changed to 1.5 M.

[0146] Example 4

[0147] A solid electrolyte was prepared in the same manner as in Example 1, except that when the composition was prepared, the concentration of LiNO3 was changed to 2 M.

[0148] Example 5

[0149] ​A solid electrolyte was prepared in the same manner as in Example 1, except that lithium perchlorate (LiClO4) was used instead of LiNO3 when preparing the composition.

[0150] Example 6

[0151] A solid electrolyte was prepared in the same manner as in Example 1, except that the concentration of LiNO3 was changed to 0.1 M when preparing the composition.

[0152] Example 7

[0153] A solid electrolyte was prepared in the same manner as in Example 1, except that the concentration of LiNO3 was changed to 0.2 M when preparing the composition.

[0154] Example 8

[0155] A solid electrolyte was prepared in the same manner as in Example 1, except that the concentration of LiNO3 was changed to 0.5 M when preparing the composition.

[0156] Example 9

[0157] A solid electrolyte was prepared in the same manner as in Example 1, except that the concentration of LiNO3 was changed to 1 M when preparing the composition.

[0158] Comparative Example 1

[0159] A solid electrolyte was prepared in the same manner as in Example 1, except that, as a crosslinking agent, only hexamethylene diisocyanate (HDI) was used in the composition at a content of 1.3% by weight without methylene diphenyl-4,4'-diisocyanate (MDI), and the concentration of LiNO3 was set to 1.75 M.

[0160] Comparative Example 2

[0161] A solid electrolyte was prepared in the same manner as in Example 1, except that, as a crosslinking agent, only methylene diphenyl-4,4'-diisocyanate (MDI) was used in the composition at a content of 1.3% by weight without hexamethylene diisocyanate (HDI), and the concentration of LiNO3 was set to 1.75 M.

[0162] Comparative Example 3

[0163] A solid electrolyte was prepared in the same manner as in Example 1, except that a linear polymer pullulan (P-2) was used instead of the branched polymer pullulan (P-1). Figure 2(a) structure, Mw = 300000 g / mol) instead of the polyrotaxane; as a crosslinking agent, a mixture of hexamethylene diisocyanate (HDI) and methylene diphenyl-4,4'-diisocyanate (MDI) mixed at a weight ratio of 4:1 was used at a content of 1.3% by weight in the composition, and the concentration of LiNO3 was set to 1.75 M at the time of preparing the composition.

[0164] Comparative Example 4

[0165] A solid electrolyte was prepared in the same manner as in Example 1, except that a cyclic polymer poly(a-cyclodextrin) Figure 2 (a) structure, Mw = 10000 g / mol) instead of the polyrotaxane; as a crosslinking agent, a mixture of hexamethylene diisocyanate (HDI) and methylene diphenyl-4,4'-diisocyanate (MDI) mixed at a weight ratio of 4:1 was used at a content of 1.3% by weight in the composition, and the concentration of LiNO3 was set to 1.75 M at the time of preparing the composition.

[0166] <Experimental Example>

[0167] Experimental Example 1: Evaluation of Ionic Conductivity

[0168] The ionic conductivity of the solid electrolyte prepared in each of Examples 1 to 5 and Comparative Examples 1 to 4 was obtained by measuring the impedance and then using the following Equation 1.

[0169] Specifically, to perform the measurement, a solid electrolyte sample of each of the examples and comparative examples having a width of 1 cm 2 (1 cm x 1 cm) and a thickness of 3 mm was prepared. On both sides of this plate-like sample, a SUS substrate having excellent electronic conductivity was contacted as an ion-blocking electrode, and then an AC voltage was applied to it through the electrodes on both sides of the sample. At this time, as the application conditions, the measurement frequency was set to a range of 0.1 Hz to 1 MHz in amplitude, and the impedance was measured. The resistance of the whole electrolyte was obtained from the intersection (R b ) of the semicircle or straight line of the measured impedance trajectory with the real axis, and the ionic conductivity of each solid electrolyte was calculated from the area and thickness of the sample, and is shown in Table 1 below.

[0170] Further, the change in the ionic conductivity of Example 1 with temperature was measured, and was plotted by Arrhenius, and the result is shown in Figure 4 .

[0171] [Equation 1]

[0172]

[0173] σ: ionic conductivity

[0174] Rb : intersection of impedance trajectory with real axis

[0175] A: sample area

[0176] T: sample thickness

[0177] Table 1

[0178]

[0179] From the results of Examples 1 to 3 of Table 1 above, it can be confirmed that the higher the lithium salt concentration, the higher the ionic conductivity. However, compared to the ionic conductivity of Example 1, the ionic conductivity of Example 4 is reduced, thereby confirming that the effect of improving ionic conductivity is reduced above a certain concentration (2M or more).

[0180] In addition, when Example 1 is compared with Example 5 using a different lithium salt at the same concentration, it can be confirmed that the smaller the molecular weight of the lithium salt, the higher the ionic conductivity.

[0181] In addition, when Example 1 using the same concentration and type of lithium salt is compared with Comparative Examples 1 to 4, it can be confirmed that Example 1 using a polyrotaxane compound as a matrix and using a compound represented by Formula 1 (HDI) and a compound represented by Formula 2 (MDI) together as a crosslinking agent has a higher ionic conductivity than Comparative Examples 1 to 4.

[0182] Specifically, compared to Comparative Example 3 using a linear polymer as a polymer matrix and Comparative Example 4 using a cyclic polymer as a polymer matrix, Example 1 using a polyrotaxane having a necklace structure has a higher ionic conductivity due to the mobility of the molecules.

[0183] In addition, even if a polyrotaxane is used as a polymer matrix, when only HDI or MDI is used as a crosslinking agent as in Comparative Examples 1 and 2, it can be seen that the effect of improving ionic conductivity is reduced.

[0184] In addition, by Figure 4 It can be confirmed that the ionic conductivity of the electrolyte polymer of Example 1 linearly changes with temperature, which means that the solid electrolyte containing a polyrotaxane compound follows the Arrhenius transport model.

[0185] Experimental Example 2: Measurement of Mechanical Physical Properties

[0186] A micro-tensile tester was used to stretch samples of the solid electrolytes prepared in Examples 1 to 3 and 6 to 9 and Comparative Examples 1 to 4 having a size of 5mm x 50mm at a temperature of 25℃ and a humidity of 60% at a rate of 5mm / min to measure the tensile strength, tensile strain, and tensile toughness, and the results of Table 2 below were obtained.

[0187] The results of tensile physical property measurements for Example 1 and Comparative Examples 1 to 4, which used the same concentration of lithium salt, are also shown in... Figure 5 middle.

[0188] Table 2

[0189]

[0190] The results in Table 2 confirm that tensile strength, tensile strain and tensile toughness were high in Examples 1 to 3 and 6 to 9, but not in Comparative Examples 1 to 4.

[0191] Specifically, it can be confirmed that, compared with the tensile strain and tensile toughness of Example 1 (which did not use MDI as a crosslinking agent) and Comparative Example 2 (which did not use HDI as a crosslinking agent), the tensile strain and tensile toughness are significantly reduced. In particular, the significant reduction in tensile strain in Comparative Example 2 may be due to the benzene structure of the MDI crosslinking agent, which has a rigid structure compared to the flexible HDI with a linear structure.

[0192] Furthermore, it can be confirmed that Example 1, using the necklace-structured polyrotaxane compound of the present invention, has improved tensile strength, tensile strain, and tensile toughness compared to Comparative Example 3, which uses linear pullulan. Moreover, when compared to Comparative Example 4, which uses the cyclic polymer PCD, it can be confirmed that the tensile strength is at a similar level, while the tensile strain and toughness are much higher.

[0193] In particular, through Figure 5 The difference in tensile properties under the same lithium salt concentration is readily apparent. It can be seen that (a) Figure shows that both tensile strain and tensile strength are high in Example 1, and (b) Figure shows that the tensile toughness of Example 1 is the highest.

[0194] Furthermore, by comparing Examples 1 to 3 and Examples 6 to 9, it can be confirmed that the lower the lithium salt content, the better the tensile strength and tensile strain.

[0195] Experiment Example 3: Performance Evaluation of Lithium Secondary Batteries

[0196] A positive electrode slurry (solids content: 37.4 wt%) was prepared by adding positive electrode active material (LiFePO4), conductive material (carbon black), and binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 8:1:1. The positive electrode slurry was coated and dried onto an aluminum (Al) film with a thickness of approximately 12 μm, which served as the positive electrode current collector, and then pressed to produce a loading of 6.0 mg / cm³. 2 The above describes the positive electrode. Lithium-ion batteries are used as the negative electrode active material.

[0197] The positive electrode, the solid electrolyte produced in Example 1, and the negative electrode were stacked in this order to produce an electrode assembly, and the assembled electrode assembly was housed in a coin-type battery case to produce a lithium secondary battery.

[0198] The produced lithium secondary battery was charged to a voltage of 4.0 V at a constant current (CC) at 0.1C rate at 60°C, and then discharged at a constant current (CC) at 0.1C rate and cut off at 2.5 V, for 5 times. Thereafter, the constant current (CC) charge and discharge were continuously performed at 0.2C rate in the above-described manner, and then the battery capacity was measured as shown in Figure 6

[0199] As shown in the charge and discharge curves of Figure 6 It can be seen that the battery using the solid electrolyte of Example 1 can be stably charged / discharged, since the voltage difference between the charge and discharge is only 0.09 V. Furthermore, even at the two-fold rate from 0.1C rate to 0.2C rate, the voltage difference of the charge / discharge curves appears to be maintained at 0.09 V. This is a result of the high stability and ionic conductivity of the electrolyte.​

Claims

1. A composition for preparing a solid electrolyte, the composition comprising: Polyrotaxane compounds, crosslinking agents, lithium salts, and organic solvents, among which, The crosslinking agent comprises the compound represented by Formula 1 and the compound represented by Formula 2: [Formula 1] O = C = N - L1 - N = C = O In Formula 1 above, L1 is a C1 to C10 alkylene group, and [Equation 2] In Formula 2 above, L2 is a C1 to C5 alkylene group.

2. The composition of claim 1, wherein, The polyrotaxane compound contains 10 to 90 α-cyclodextrin molecules.

3. The composition of claim 1, wherein, The weight-average molecular weight of the polyrotaxane compound is from 10,000 g / mol to 100,000 g / mol.

4. The composition of claim 1, wherein, Based on the total weight of the composition used to prepare the solid electrolyte, the content of the polyrotaxane compound is from 5% to 20% by weight.

5. The composition of claim 1, wherein, The weight ratio of the compound represented by Formula 1 to the compound represented by Formula 2 is 1:1 to 10:

1.

6. The composition of claim 1, wherein, The crosslinking agent content is from 0.5% to 5% by weight, based on the total weight of the composition used to prepare the solid electrolyte.

7. The composition of claim 1, wherein, The lithium salt is selected from one or more of LiNO3 and LiClO4.

8. The composition of claim 1, wherein, The concentration of lithium salt in the composition used to prepare the solid electrolyte is from 0.05 M to 3.0 M.

9. The composition of claim 1, further comprising one or more polymers selected from the group consisting of polyethylene oxide, polyvinylidene fluoride, and cellulose.

10. A method for preparing a solid electrolyte for lithium secondary batteries, comprising thermosetting the composition for preparing a solid electrolyte according to claim 1.

11. The method of claim 10, wherein, The thermosetting is carried out at a temperature of 50°C to 90°C.

12. A solid electrolyte for lithium secondary batteries, comprising a thermosetting product of the composition for preparing a solid electrolyte as described in claim 1.

13. A lithium secondary battery, comprising: A positive electrode containing positive electrode active material; A negative electrode containing a negative electrode active material; and The solid electrolyte for a lithium secondary battery according to claim 12 is disposed between the positive electrode and the negative electrode.

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