Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery comprising: a gel polymer electrolyte which is a polymerization reaction product of a composition containing a lithium salt, an organic solvent, an oligomer, a lithium salt-based additive, and a polymerization initiator; a positive electrode containing a lithium iron phosphate-based composite oxide; a negative electrode containing a negative electrode active material; and a separator interposed between the positive electrode and the negative electrode, wherein the oligomer is a polyvinylidene fluoride-based polymer containing a vinyl group or an acrylate group at an end thereof, the lithium salt-based additive is any one or more selected from the group consisting of lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium 2-trifluoromethyl-4,5-dicyanoimidazolide, and lithium difluorophosphate, and the lithium salt is different from the lithium salt-based additive.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0111029, filed with the Korean Intellectual Property Office on August 23, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery with improved life characteristics and safety. [Background technology]

[0003] Lithium secondary batteries are generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode including a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode including a negative electrode active material capable of storing lithium ions, inserting the electrode assembly into a battery case, injecting an electrolyte serving as a medium for transferring lithium ions, and then sealing the battery case.

[0004] Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, and therefore are applied in various fields such as mobile devices, electronic products, electric vehicles, etc. As the application fields of lithium secondary batteries become more diverse, the required physical properties are also becoming more stringent. In particular, there is a demand for the development of lithium secondary batteries that can be operated stably even under high temperature conditions.

[0005] At high temperatures, lithium salts such as LiPF6 in the electrolyte are converted to PF6 - Thermal decomposition of anions can generate Lewis acids such as PF5, which react with moisture to generate HF. The decomposition products, such as PF5 and HF, as well as unstable structural changes in the positive electrode caused by charging and discharging, can cause the transition metals in the positive electrode material to leach into the electrolyte. In particular, when a battery contains a lithium iron phosphate (LFP) positive electrode, the structural stability of the positive electrode can be improved, but iron leaching can cause electrolyte decomposition and severe degradation of battery performance, so improvements are needed. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to solve the problems of electrolyte decomposition due to iron elution and gas generation at high temperatures in lithium secondary batteries containing an LFP positive electrode, and its fundamental objective is to provide a lithium secondary battery with improved capacity retention and safety. [Means for solving the problem]

[0007] The present invention relates to a gel polymer electrolyte which is a polymerization reaction product of a composition including a lithium salt, an organic solvent, an oligomer, a lithium salt-based additive, and a polymerization initiator; a positive electrode including a lithium iron phosphate composite oxide; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode, The oligomer is a polyvinylidene fluoride polymer containing a vinyl group or an acrylate group at its terminal, the lithium salt additive is at least one selected from the group consisting of lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium 2-trifluoromethyl-4,5-dicyanoimidazolidine, and lithium difluorophosphate; The lithium secondary battery is provided, wherein the lithium salt is different from the lithium salt-based additive. [Effects of the Invention]

[0008] The present invention realizes a lithium secondary battery that has excellent capacity retention and safety at high temperatures by introducing a gel polymer electrolyte containing a specific oligomer and a lithium salt-based additive into a lithium secondary battery that contains an LFP-based positive electrode material. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 10 is a diagram showing a heat propagation simulator used in Experimental Example 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will now be described in more detail.

[0011] As lithium secondary batteries are used in a variety of fields, including electric vehicles, ensuring safety against the risk of explosion is an important issue. The lithium iron phosphate (LFP)-based positive electrode active material in lithium secondary batteries has a high degree of structural stability, meaning that no structural changes occur during the lithium migration process, making it an extremely advantageous material in terms of safety.

[0012] However, in LFP-based positive electrode materials with an olivine structure, lithium ions move by diffusion through one-dimensional channels. Therefore, if iron dissolution occurs due to electrolyte decomposition reactions in a liquid electrolyte system and blocks the lithium migration channels, irreversible capacity loss may occur, resulting in a decrease in battery performance.

[0013] In addition, LFP-based cathode materials are safe because they do not generate excessive amounts of oxygen at high temperatures, which can lead to fire. However, in high-temperature environments, the decomposition of the liquid electrolyte can generate toxic gases, which can cause venting and potentially damage all components of the battery.

[0014] Therefore, the inventors introduced a gel polymer electrolyte instead of a liquid electrolyte into a lithium secondary battery containing an LFP-based cathode material, thereby reducing the driving force of iron ions, suppressing iron elution, and preventing obstruction of the lithium ion migration channels. They also confirmed that the gel polymer electrolyte blocks heat conduction due to convection, thereby blocking heat transfer from the electrolyte to other components.

[0015] In particular, the gel polymer electrolyte according to the present invention is manufactured using a composition containing a lithium salt-based additive, which stabilizes the coatings formed on the positive and negative electrodes and improves high-temperature life characteristics. Specifically, by simultaneously forming coatings on the positive and negative electrodes and stabilizing the surfaces of the active materials, it is possible to prevent iron from leaching from the LFP positive electrode and depositing on the negative electrode. Furthermore, the formation of an SEI layer containing inorganic components not only contributes to strengthening the coating on the negative electrode, but also prevents decomposition of the electrolyte salt by forming a coating on the positive electrode reinforced with BO-based components.

[0016] The lithium secondary battery according to the present invention includes a gel polymer electrolyte that is a polymerization reaction product of a composition containing a lithium salt, an organic solvent, an oligomer, and a polymerization initiator; a positive electrode that contains a lithium iron phosphate-based composite oxide; a negative electrode that contains a negative electrode active material; and a separator interposed between the positive electrode and the negative electrode, wherein the oligomer is a polyvinylidene fluoride-based polymer that contains a vinyl group or an acrylate group at its terminal.

[0017] The description of each component is as follows:

[0018] (1) Gel polymer electrolyte The gel polymer electrolyte of the present invention is a polymerization reaction product of a composition containing a lithium salt, an organic solvent, an oligomer, a lithium salt-based additive, and a polymerization initiator.

[0019] Specifically, the gel polymer electrolyte can be prepared by injecting the composition into a secondary battery and then curing the composition through a thermal polymerization reaction. For example, the gel polymer electrolyte can be formed by in-situ polymerization of the composition inside the secondary battery.

[0020] More specifically, the gel polymer electrolyte is (a) inserting an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode into a battery case; (b) injecting the composition of the present invention into the battery case; (c) wetting and aging the electrode assembly; (d) polymerizing the composition to form a gel polymer electrolyte.

[0021] In this case, the in-situ polymerization reaction in the lithium secondary battery can be carried out by electron beam (E-BEAM), gamma ray, room temperature or high temperature aging process, and in one embodiment of the present invention, it can be carried out by thermal polymerization. In this case, the polymerization time is about 2 minutes to 48 hours, and the thermal polymerization temperature may be 60°C to 100°C, specifically 60°C to 80°C.

[0022] More specifically, in-situ polymerization reaction in a lithium secondary battery is carried out by adding a polymerization initiator and the oligomer to an organic solvent containing a lithium salt, mixing them, and then injecting the mixture into a battery cell. After sealing the inlet of the battery cell, the gel polymer electrolyte of the present invention can be produced by thermal polymerization at about 60°C to 80°C for 1 to 20 hours.

[0023] (a) Oligomer In one embodiment of the present invention, the oligomer is a polyvinylidene fluoride (PVDF)-based polymer having a terminal vinyl group or an acrylate group, which can react with a polymerization initiator to undergo a crosslinking reaction through free radical polymerization to form a gel electrolyte.

[0024] In one embodiment of the present invention, the gel polymer electrolyte comprises a polymer network derived from the oligomer, where the polymer network refers to a form in which the oligomers are bonded in a three-dimensional structure.

[0025] In one embodiment of the present invention, the terminal group may be represented by any one of the following chemical formulas E-1 to E-6.

[0026] [Chemical formula E-1] [ka]

[0027] [Chemical formula E-2] [ka]

[0028] [Chemical formula E-3] [ka]

[0029] [Chemical formula E-4] [ka]

[0030] [Chemical formula E-5] [ka]

[0031] [Chemical formula E-6] [ka]

[0032] In one embodiment of the present invention, the oligomer contains repeating units derived from vinylidene fluoride (VDF), and may consist of VDF-derived repeating units except for the terminal groups. The oligomer may also optionally further contain one or more repeating units selected from the group consisting of repeating units derived from hexafluoropropylene, repeating units derived from chlorotrifluoroethylene, and repeating units derived from tetrafluoroethylene. Because the oligomer is based on a PVDF structure, it contains a large amount of F, which has the advantage of excellent thermal stability.

[0033] In one embodiment of the present invention, the weight average molecular weight (Mw) of the oligomer may be adjusted by the number of repeating units and may be about 1,000 g / mol to 15,000 g / mol, specifically 3,000 g / mol to 12,000 g / mol, and more specifically 5,000 g / mol to 10,000 g / mol.

[0034] The weight-average molecular weight was measured by gel permeation chromatography (GPC). Specifically, a Waters Styragel HR3 / HR4 (THF) column was used, and tetrahydrofuran (THF) (filtered to 0.45 m) was used as the solvent. The measurement was performed at a flow rate of 1.0 mL / min and a sample concentration of 1 mg / mL. 100 μL of sample was injected, and the column temperature was set to 40°C. A Waters RI detector was used, and PS (polystyrene) was used as the standard. Data processing was performed using the Empower3 program.

[0035] In one embodiment of the present invention, the content of the oligomer may be 1 wt % to 5 wt %, preferably 2 wt % to 5 wt %, more preferably 3 wt % to 5 wt %, based on the total weight of the composition.

[0036] When the content of the oligomer is 1 wt % or more, it is preferable because it can provide physical advantages as a gel polymer electrolyte compared to a liquid electrolyte, and when it is 5 wt % or less, it is preferable in terms of uniform gelation, electrolyte resistance, and conductivity.

[0037] (b) Lithium salt The lithium salt is different from the lithium salt-based additive and has Li as a cation. + and as an anion, F - , Cl - , Br - , I - , NO3- , N(CN)2 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO4 - , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , BC4O8 - , BF2C2O4CHF - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - , and SCN - It may include any one selected from the group consisting of:

[0038] Specifically, the lithium salt may be one or more selected from the group consisting of LiPF, LiClO, LiFSI, LiTFSI, LiSOCF, lithium bis(oxalate)borate (LiBOB), lithium difluoro(bisoxalato)phosphate (LiDFBP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiFMDFB), and is preferably LiPF. LiPF is preferred as the lithium salt of the present invention because it is well soluble in carbonate solvents and has high ionic conductivity.

[0039] The lithium salt may be contained in a non-aqueous organic solution containing a lithium salt, an organic solvent, and a lithium salt-based additive at a concentration of 0.5 M to 3.0 M, specifically 1.0 M to 2.0 M. When the concentration of the lithium salt satisfies the above range, the lithium ion transportence number (Li+ transference number) and the degree of dissociation of lithium ions are improved, and the output characteristics of the battery can be improved.

[0040] (c) Organic solvent The organic solvent may be any of various organic solvents commonly used in lithium electrolytes without any limitation, but preferably, the organic solvent may include a cyclic carbonate solvent and a linear carbonate solvent.

[0041] The cyclic carbonate solvent is a highly viscous organic solvent with a high dielectric constant, which allows for efficient dissociation of the lithium salt in the electrolyte, and may be one or more selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate. Among these, ethylene carbonate (EC) may be included in order to ensure high ionic conductivity.

[0042] The linear carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may be one or more selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0043] In the present invention, the volume ratio of the cyclic carbonate solvent to the linear carbonate solvent may be 1:10 to 5:5, specifically 2:8 to 4:6, and more specifically 2:8 to 3:7.

[0044] In addition, in order to prepare an electrolyte solution having high ionic conductivity, the organic solvent may further include a linear ester solvent and / or a cyclic ester solvent, which have a low melting point and high stability at high temperatures, in addition to the cyclic carbonate solvent and / or linear carbonate solvent.

[0045] The linear ester solvent may be one or more selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0046] The cyclic ester solvent may be at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0047] In the total weight of the composition, the remainder excluding other components other than the organic solvent, such as the lithium salt, oligomer, polymerization initiator, and additives, is the organic solvent unless otherwise specified.

[0048] (d) Additives In one embodiment of the present invention, the lithium salt additive is at least one selected from the group consisting of lithium difluoro(oxalate)borate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium 2-trifluoromethyl-4,5-dicyanoimidazolide (LiTDI), and lithium difluorophosphate (LiPO2F2). The lithium salt additive may be preferably LiODFB. The lithium salt additive has the effect of stabilizing the coatings formed on the positive and negative electrodes and improving high-temperature life characteristics. Specifically, LiODFB has a higher reduction potential than LiPF6 and can therefore be reduced first at the negative electrode, rapidly forming an SEI layer of inorganic components.

[0049] In one embodiment of the present invention, the content of the lithium salt-based additive may be 0.5 wt % to 3 wt % based on the total weight of the composition.

[0050] In one embodiment of the present invention, the concentration of the lithium salt-based additive in the non-aqueous organic solution containing the lithium salt, the organic solvent, and the lithium salt-based additive may be 0.5M to 2.0M.

[0051] Meanwhile, the composition may further include an additive that can form a stable coating on the surfaces of the negative electrode and the positive electrode without significantly increasing the initial resistance, or that can act as a complement to suppress decomposition of the solvent and improve the mobility of lithium ions.

[0052] The electrolyte for a lithium secondary battery of the present invention may optionally further contain the following additives, as needed, to prevent electrode collapse due to decomposition of the non-aqueous electrolyte in a high-voltage environment, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effects of suppressing battery expansion at high temperatures.

[0053] For example, the additive may include one or more compounds selected from the group consisting of carbonate-based compounds, halogen-substituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based compounds, borate-based compounds, nitrile-based compounds, amine-based compounds, silane-based compounds, and benzene-based compounds.

[0054] The cyclic carbonate compound may be at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC), and specifically may be vinylene carbonate.

[0055] The halogen-substituted carbonate compound may be fluoroethylene carbonate (FEC).

[0056] The sultone-based compound is a material capable of forming a stable SEI film on the surface of the negative electrode through a reduction reaction, and may be at least one selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and specifically may be 1,3-propane sultone (PS).

[0057] The sulfate-based compound is a material that can be electrically decomposed on the surface of the negative electrode to form a stable SEI film that does not crack even when stored at high temperatures, and may be at least one selected from the group consisting of ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

[0058] The phosphate-based compound may be any one or more selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilylphosphate), tris(trimethylsilylphosphite), tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0059] The borate-based compound may be lithium tetraphenylborate.

[0060] The nitrile compound may be any one or more selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0061] The amine-based compound may be triethanolamine, ethylenediamine, or a mixture thereof, and the silane-based compound may be tetravinylsilane.

[0062] The benzene-based compound may be any one or more selected from the group consisting of monofluorobenzene, difluorobenzene, trifluorobenzene, and tetrafluorobenzene.

[0063] Meanwhile, the content of the additives excluding the lithium salt-based additive may be 0.1 wt % to 10 wt %, preferably 1 wt % to 5 wt %, based on the total weight of the composition.

[0064] (e) Polymerization initiator The composition of the present invention contains a conventional polymerization initiator capable of generating radicals by heat and light.

[0065] The polymerization initiator may be a peroxide-based compound or an azo-based compound, and the peroxide-based compound may be at least one selected from the group consisting of benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, t-butyl peroxy-2-ethyl-hexanoate, cumyl hydroperoxide, and hydrogen peroxide, but is not limited thereto.

[0066] The azo compound may be any one or more selected from the group consisting of 2,2'-azobis(2-cyanobutane), dimethyl 2,2'-azobis(2-methylpropionate), 2,2'-azobis(methylbutyronitrile), 2,2'-azobis(isobutyronitrile) (AIBN; 2,2'-azobis(isobutyronitrile)), and 2,2'-azobisdimethyl-valeronitrile (AMVN; 2,2'-azobisdimethyl-valeronitrile), but is not limited thereto.

[0067] The polymerization initiator can be decomposed by heat, for example, at 30°C to 100°C, specifically 60°C to 80°C, in the secondary battery, or can be decomposed at room temperature (5°C to 30°C) to form radicals.

[0068] The polymerization initiator may be included in an amount of about 10 parts by weight or less, specifically 0.01 to 10 parts by weight, more specifically 0.1 to 5 parts by weight, based on 100 parts by weight of the oligomer. When included in the above range, the gelation reaction can be easily carried out, and gelation can be prevented from occurring during injection of the composition into a battery, and side reactions can be prevented from occurring due to unreacted polymerization initiator remaining after the polymerization reaction.

[0069] In particular, some polymerization initiators may generate nitrogen or oxygen gas during the process of generating radicals due to heat, etc. This gas generation often leads to gas trapping or gas bubbling during the formation of a gel polymer electrolyte. This gas generation causes defects in the gel polymer electrolyte, resulting in a deterioration in the quality of the electrolyte. Therefore, when the polymerization initiator is included within the above range, defects such as the generation of a large amount of gas can be more effectively prevented.

[0070] (2) Positive electrode The positive electrode according to the present invention includes a lithium iron phosphate (LFP)-based composite oxide. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material included in the positive electrode active material layer may include the lithium iron phosphate-based composite oxide. More specifically, the positive electrode active material may be made of the lithium iron phosphate-based composite oxide.

[0071] LFP-based positive electrodes have an olivine structure and offer superior structural stability and long lifespan compared to layered positive electrodes, such as NCM-based positive electrodes, which are at risk of structural collapse. However, LFP-based positive electrodes have high moisture sensitivity and voltage dependence, and are vulnerable to metal ion elution. If these problems can be resolved by incorporating the gel polymer electrolyte of the present invention, a battery with superior stability and lifespan compared to NCM-based positive electrodes can be obtained.

[0072] The positive electrode active material layer may be prepared by coating a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, and a solvent on a positive electrode current collector, followed by drying and rolling.

[0073] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel; aluminum; nickel; titanium; baked carbon; or aluminum or stainless steel whose surface has been surface-treated with carbon, nickel, titanium, silver, or the like may be used.

[0074] In one embodiment of the present invention, the lithium iron phosphate composite oxide may be represented by the following chemical formula 1.

[0075] [Chemical formula 1] Life 1-x M x PO4

[0076] In the above Chemical Formula 1, M is any 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; 0≦x≦1.

[0077] In one embodiment of the present invention, the lithium iron phosphate composite oxide may be LiFePO4.

[0078] In one embodiment of the present invention, the positive electrode active material may further contain other positive electrode active materials in addition to the lithium iron phosphate-based composite oxide. For example, lithium-manganese-based oxides such as LiMnO2 and LiMn2O4; lithium-cobalt-based oxides such as LiCoO2; lithium-nickel-based oxides such as LiNiO2; LiNi 1-Y Mn Y O2(0 < Y < 1), LiMn 2-z Ni z O4(0 < Z < 2) and other lithium-nickel-manganese-based oxides; LiNi 1-Y1 Co Y1 O2(0 < Y1 < 1) and other lithium-nickel-cobalt-based oxides; LiCo 1-Y2 Mn Y2 O2(0 < Y2 < 1), LiMn 2-z1 Co z1 O4(0 < Z1 < 2) and other lithium-manganese-cobalt-based oxides; Li(Ni p Co q Mn r1 )O2(0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1), Li(Ni p1 Co q1 Mn r2 )O4(0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2) and other lithium-nickel-manganese-cobalt-based oxides; and Li(Ni p2 Co q2 Mn r3 M S2 )O2 (M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are the atomic fractions of independent elements, respectively, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1) and other lithium-nickel-cobalt-transition metal (M) oxides, and may further contain any one or more selected from the group consisting of.

[0079] The positive electrode active material may be included in an amount of 80 wt% to 99 wt%, specifically 90 wt% to 99 wt%, based on the total weight of the solid content in the positive electrode slurry. In this case, if the content of the positive electrode active material is 80 wt% or less, the energy density may be reduced, and the capacity may be reduced.

[0080] The binder in the positive electrode slurry is a component that assists in binding the active material and conductive material, etc., and in binding to the current collector, and is typically added in an amount of 1 to 30 wt % based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene termonomer, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0081] The conductive material in the positive electrode slurry is a substance that imparts conductivity without causing any chemical change to the battery, and may be added in an amount of 0.5 to 20% by weight based on the total weight of the solid content in the positive electrode slurry.

[0082] The conductive material in the positive electrode slurry may be any one or more selected from the group consisting of: carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powder such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon fiber and metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0083] The solvent for the positive electrode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a suitable viscosity when the positive electrode active material, and optionally a binder and a conductive material, are contained. For example, the positive electrode slurry containing the positive electrode active material, and optionally a binder and a conductive material may be contained so that the solids concentration in the positive electrode slurry is 10 wt % to 90 wt %, preferably 40 wt % to 85 wt %.

[0084] (3) Negative electrode The negative electrode can be manufactured by coating a negative electrode slurry containing a negative electrode active material, a binder, a conductive material, and a solvent on a negative electrode current collector, followed by drying and rolling.

[0085] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. There are no particular limitations on the negative electrode current collector, so long as it does not cause chemical changes in the battery and has high conductivity. For example, copper; stainless steel; aluminum; nickel; titanium; calcined carbon; copper or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like; or an aluminum-cadmium alloy may be used. Furthermore, as with the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and the negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0086] In addition, the negative electrode active material may include at least one selected from the group consisting of lithium metal, a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of such a metal and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0087] As the carbon material capable of reversibly intercalating / deintercalating the lithium ions, any carbonaceous negative electrode active material generally used in a lithium ion secondary battery can be used without particular limitation. Representative examples thereof include crystalline carbon, amorphous carbon, or these may be used together. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite. Examples of the amorphous carbon include soft carbon (low-temperature calcined carbon) or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0088] As the metal or an alloy of these metals and lithium, 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 these metals and lithium can be used.

[0089] Examples of the metal composite oxide include 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 (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, elements of Group 1, Group 2, Group 3 of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), and one or more selected from the group consisting thereof can be used.

[0090] As the substance capable of doping and undoping lithium, Si, SiO x(0 < x < 2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), etc. may be mentioned, and at least one of these may be mixed with SiO2 and used. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db (dubnium), 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.

[0091] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0092] In the present invention, when the negative electrode active material is graphite, it is advantageous from the viewpoint of high-temperature durability.

[0093] The negative electrode active material may be contained at 80% to 99% by weight based on the total weight of the solid content in the negative electrode slurry.

[0094] The binder in the negative electrode active material is a component that assists in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 1 to 30 wt % based on the total weight of the solids in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC), and various copolymers thereof.

[0095] The conductive material in the negative electrode active material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solid content in the negative electrode slurry. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and may be, for example, one or more selected from the group consisting of carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0096] The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that provides a suitable viscosity when the negative electrode active material, and optionally a binder and a conductive material, are contained. For example, the solvent may be contained so that the solids concentration in the slurry containing the negative electrode active material, and optionally a binder and a conductive material, is 40 wt % to 75 wt %, preferably 40 wt % to 65 wt %.

[0097] (4) Separator The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

[0098] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used as a separator in a lithium secondary battery can be used without any particular limitation. In particular, a separator that has low resistance to ion movement of the electrolyte and excellent electrolyte impregnation capacity is preferred.

[0099] Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric material may also be used, and may be selectively used as a single-layer or multi-layer structure.

[0100] The lithium secondary battery according to the present invention can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0101] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0102] The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0103] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0104] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in a medium- to large-sized battery module containing a large number of battery cells.

[0105] The present invention will be specifically described below with reference to specific examples.

[0106] <Example: Manufacture of lithium secondary battery> Example 1 (Production of Electrolyte Composition) A non-aqueous organic solution was prepared by dissolving LiPF6 at 1.0 M and lithium difluorooxalatoborate (LiODFB) at 1.0 M in a 30:40:30 volumetric ratio organic solvent mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Next, 2.0 wt% of PVDF (weight average molecular weight: 10,000 g / mol) containing groups represented by Formula E-4 at both terminal groups, 0.05 wt% of 2,2'-azobis(isobutyronitrile) (AIBN), and the remainder of the non-aqueous organic solution were added to prepare a total electrolyte composition of 100 wt%.

[0107] (Cathode manufacturing) A cathode active material (LiFePO4), a conductive material (carbon black), and a binder (polyvinylidene fluoride) were mixed in a weight ratio of 97.5:1:1.5 with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a cathode active material slurry (solid concentration 60 wt%). The cathode active material slurry was applied to a 15 μm-thick cathode current collector (Al thin film), dried, and then roll-pressed to prepare a cathode.

[0108] (Manufacturing of negative electrodes) A negative electrode active material slurry (solid concentration 50 wt%) was prepared by adding a negative electrode active material (graphite), a conductive material (carbon black), and a binder (polyvinylidene fluoride) to water as a solvent in a weight ratio of 96:0.5:3.5. The negative electrode active material slurry was applied to an 8 μm-thick negative electrode current collector (Cu thin film), dried, and then roll-pressed to prepare a negative electrode.

[0109] (Secondary battery manufacturing) The cathode and anode prepared by the above method were laminated together with a polyethylene porous film separator to prepare an electrode assembly, which was then placed in a battery case, and 120 mL of the electrolyte composition was poured into the battery case. The battery case was then sealed and aged for 2 days. A thermal polymerization reaction was then carried out at 60°C for 5 hours to prepare a pouch-type lithium secondary battery containing a gel polymer electrolyte.

[0110] Example 2. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the oligomer containing the group represented by the chemical formula E-4 was changed to 3 wt % when preparing the electrolyte composition.

[0111] Example 3 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the oligomer containing the group represented by the chemical formula E-4 was changed to 5 wt % when preparing the electrolyte composition.

[0112] Comparative Example 1 (Production of liquid electrolytes) A liquid electrolyte was prepared by dissolving LiPF6 to a concentration of 1.0 M in an organic solvent consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) mixed in a volume ratio of 30:40:30.

[0113] (Cathode manufacturing) A positive electrode was produced in the same manner as in Example 1.

[0114] (Manufacturing of negative electrodes) A negative electrode was produced in the same manner as in Example 1.

[0115] (Secondary battery manufacturing) The positive electrode and negative electrode prepared by the above method were laminated together with a polyethylene porous film as a separator to prepare an electrode assembly, which was then placed in a battery case, and 120 mL of the liquid electrolyte was poured into the battery case to prepare a pouch-type lithium secondary battery containing the liquid electrolyte.

[0116] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that the liquid electrolyte was prepared by dissolving LiPF and LiODFB to a concentration of 1.0 M in an organic solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:40:30.

[0117] Comparative Example 3. A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that the NCMA positive electrode prepared as follows was used as the positive electrode.

[0118] During the manufacture of the positive electrode, the positive electrode active material (Li[Ni 0.86 Co 0.10 Mn 0.02 Al 0.02]O2), a conductive material (carbon black), and a binder (polyvinylidene fluoride) were added to a solvent, N-methyl-2-pyrrolidone (NMP), in a weight ratio of 97.5:1:1.5 to prepare a cathode active material slurry (solid concentration 60 wt%). A lithium secondary battery was fabricated in the same manner as in Comparative Example 1, except that the cathode active material slurry was applied to a cathode current collector (Al thin film) with a thickness of 5 μm, dried, and then roll-pressed to prepare a cathode.

[0119] Comparative Example 4. A lithium secondary battery was manufactured in the same manner as in Example 3, except that the NCMA positive electrode of Comparative Example 3 was used as the positive electrode.

[0120] Comparative Example 5. A lithium secondary battery was manufactured in the same manner as in Example 3, except that LiODFB was not added during the preparation of the electrolyte composition.

[0121] <Experimental example: Performance evaluation of lithium secondary batteries> Experimental example 1: Evaluation of high-temperature life characteristics - Measurement of capacity retention rate and resistance increase rate The secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 5 were each activated at 0.1 C / C and then degassed.

[0122] Subsequently, the battery was charged at 0.33C CC to 3.6V under constant current-constant voltage (CC-CV) charging conditions at 25°C, then a 0.05C current cut was performed and the battery was discharged at 0.33C to 2.5V under CC conditions.

[0123] Next, the battery was charged at 0.33 C CC to 3.6 V under constant current-constant voltage (CC-CV) charging conditions at 45°C, followed by a 0.05 C current cut and discharge at 0.33 C CC to 2.5 V. This cycle constitutes one charge / discharge. While charging and discharging at a high temperature (45°C), the discharge capacity and resistance were measured at 100 and 200 cycles using a charger / discharger (5 V, 100 A). The measured discharge capacity and resistance were substituted into the following [Equation 1] and [Equation 2], respectively, to calculate the capacity retention and resistance increase rate (%). The results are shown in Table 1 below.

[0124] [Formula 1] Capacity retention rate (%) = (discharge capacity at the relevant cycle / discharge capacity after 1 cycle) x 100

[0125] [Formula 2] Resistance increase rate (%) = {(resistance after charge / discharge in the cycle - resistance after 1 cycle) / resistance after 1 cycle} × 100

[0126] Experimental Example 2: Measurement of the amount of dissolved Fe After the evaluation of the life characteristics (i.e., after 200 cycles), the secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 5 were disassembled, and the amount of Fe deposited on the negative electrode was confirmed by ICP analysis (ICP-OES, Perkin Elmer), and the results are shown in Table 1 below.

[0127] [Table 1]

[0128] The above experimental results confirm that the batteries of Examples 1 to 3, which used a gel polymer electrolyte, had significantly less Fe elution than the batteries of Comparative Examples 1 and 2, which used a liquid electrolyte. As the difference can be seen from the capacity retention rate and resistance increase rate at 100 and 200 cycles, the batteries of Comparative Examples 1 and 2, which had a large amount of Fe elution, developed lithium dendrites due to metallic foreign matter accumulated on the surface of the negative electrode as cycles continued, resulting in a deterioration of the battery's life characteristics. If the dendrites grow with further cycles, they may penetrate the separator and come into contact with the positive electrode, potentially causing a short circuit or fire.

[0129] Furthermore, even when the gel polymer electrolyte according to the present invention is used, when no lithium salt-based additive is contained as in Comparative Example 5, it can be confirmed that the capacity retention rate is lower, the resistance increase rate is higher, and the amount of Fe elution is also larger than those of the batteries of Examples 1 to 3.

[0130] On the other hand, it can be seen from Comparative Examples 3 and 4 that when the positive electrode material is NCMA, even if the same electrolyte as in Example 3 is used, no significant effect is achieved.

[0131] Table 2 below shows the results of Table 1, specifically, to confirm how much the introduction of a gel polymer electrolyte improves high-temperature performance compared to the case of a liquid electrolyte in batteries employing each cathode material.

[0132] For the battery using the LFP positive electrode material, the capacity retention rate, resistance increase rate, and initial resistance value measured in Comparative Example 1 are taken as 100%, and the relative values for Examples 1 to 3 are shown. For the battery using the NCMA positive electrode material, the capacity retention rate, resistance increase rate, and initial resistance value measured in Comparative Example 3 are taken as 100%, and the relative values for Comparative Example 4 are shown.

[0133] [Table 2]

[0134] Referring to the results in Table 2, it can be seen that Examples 1 to 3 achieved an improvement of 1.7% or more in the 100 cycle capacity retention rate and an improvement of 2.5% or more in the 200 cycle capacity retention rate compared to Comparative Example 1. In contrast, it can be seen that the capacity retention rate of Comparative Example 4 showed a difference of less than 0.5% from that of Comparative Example 3.

[0135] It can also be seen that Examples 1 to 3 showed an improvement of about 25% in the resistance increase rate at 100 cycles compared to Comparative Example 1, but Comparative Example 4 showed an improvement of only about 10% compared to Comparative Example 3. In particular, it can be seen that the difference in the improvement range for the resistance increase rate at 200 cycles is significantly large.

[0136] On the other hand, by comparing the initial resistance values measured after one cycle, it was confirmed that the increase in initial resistance due to the introduction of the gel polymer electrolyte in the battery using the LFP positive electrode was approximately 13 to 24%, while the increase in initial resistance due to the introduction of the gel polymer electrolyte in the battery using the NCMA positive electrode reached 37%.

[0137] The above experimental results confirm that in the case of batteries using an LFP positive electrode, suppressing the elution of Fe has a very important effect on improving battery performance, and that this can be achieved by using the gel polymer electrolyte disclosed in the present invention.

[0138] Experimental Example 3: Thermal Propagation Test A heat propagation test was performed on each of the secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1, 3, and 4 using the heat propagation simulator shown in Figure 1. Specifically, two batteries to be tested were prepared and stacked in order as the first and second batteries between first and second Al plates (10 mm thick), and a heating pad was placed so that it locally contacted the first battery. After heating to 400°C at 30°C / min via the heating pad, the items listed in Table 2 below were evaluated.

[0139] Specifically, the amount of gas generated and the vent pressure were measured using a gas collector, and the results were expressed relative to the value measured in Comparative Example 1, which was set to 100%. Regarding the occurrence of ignition, if either the first or second battery ignited, it was marked as Fail, and if not, it was marked as Pass.

[0140] [Table 3]

[0141] The above experimental results show that the batteries of Examples 1 to 3, which use a gel polymer electrolyte, have superior thermal stability compared to the battery of Comparative Example 1, which uses a liquid electrolyte. In particular, as can be seen from Example 1, even if the first battery ignites, it does not affect the second battery, which is extremely advantageous in terms of safety. Furthermore, it can be seen that the amount of gas generated is actually reduced despite the high vent pressure due to the high density of the gel polymer electrolyte.

[0142] On the other hand, from Comparative Examples 3 and 4, it can be seen that although the amount of gas generated by the battery containing the NCMA positive electrode was slightly reduced by introducing the gel polymer electrolyte of the present invention, the occurrence of fire could not be prevented.

Claims

1. a gel polymer electrolyte that is a polymerization reaction product of a composition including a lithium salt, an organic solvent, an oligomer, a lithium salt-based additive, and a polymerization initiator; a positive electrode including a lithium iron phosphate composite oxide; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode, The oligomer is a polyvinylidene fluoride polymer containing a vinyl group or an acrylate group at its terminal, the lithium salt additive is at least one selected from the group consisting of lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium 2-trifluoromethyl-4,5-dicyanoimidazolidine, and lithium difluorophosphate; the lithium salt is different from the lithium salt-based additive; the gel polymer electrolyte comprises a polymer network derived from the oligomer; a non-aqueous organic solution containing the lithium salt, the organic solvent, and the lithium salt additive, the concentration of the lithium salt additive being 0.5 M or more;

2. The lithium secondary battery according to claim 1 , wherein the oligomer contains a repeating unit derived from vinylidene fluoride.

3. 2. The lithium secondary battery according to claim 1, wherein the content of the oligomer is 1% by weight to 5% by weight based on the total weight of the composition.

4. 2. The lithium secondary battery according to claim 1, wherein the concentration of the lithium salt-based additive in the non-aqueous organic solution containing the lithium salt, the organic solvent, and the lithium salt-based additive is 0.5 M to 2.0 M.

5. 2. The lithium secondary battery of claim 1, wherein the lithium iron phosphate composite oxide is represented by the following chemical formula 1: [Chemical formula 1] LiFe １－ｘ M ｘ PO ４ (In the above chemical formula 1, (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; 0≦x<1.)

6. the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, The lithium secondary battery according to claim 1 , wherein the positive electrode active material contained in the positive electrode active material layer includes the lithium iron phosphate composite oxide.

7. 7. The lithium secondary battery according to claim 6, wherein the positive electrode active material comprises the lithium iron phosphate composite oxide.

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