CN118056305A

CN118056305A - Electrolyte for lithium secondary battery and lithium secondary battery comprising same

Info

Publication number: CN118056305A
Application number: CN202380013850.2A
Authority: CN
Inventors: 黄淳煜; 韩蕙恩; 金东规
Original assignee: LG Energy Solution Ltd
Current assignee: LG Energy Solution Ltd
Priority date: 2022-04-13
Filing date: 2023-04-12
Publication date: 2024-05-17

Abstract

Disclosed are an electrolyte for a lithium secondary battery, which forms a cationic polymer backbone to minimize movement of anions, thereby accelerating cations, and improves additional lithium ion transport capacity and mechanical properties by including an oxide-based ceramic, and a lithium secondary battery including the same. The electrolyte for a lithium secondary battery comprises a polymer electrolyte comprising a hydrocarbon polymer compound containing a cationic functional group; a flame retardant liquid electrolyte; and a crosslinking agent.

Description

Electrolyte for lithium secondary battery and lithium secondary battery comprising same

Technical Field

The present application claims the benefit of priority based on korean patent application No. 10-2022-0045454, filed on 13 at 2022, 4 and korean patent application No. 10-2023-0043320, filed on 3 at 2023, 4.

The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, as follows: the electrolyte for a lithium secondary battery can accelerate cations by forming a cationic polymer backbone to minimize movement of anions, and improve additional lithium ion transporting ability and mechanical properties by including oxide-based ceramics.

Background

In recent years, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries that are small, lightweight, and also have a relatively high capacity has rapidly increased. In particular, lithium secondary batteries are attracting attention as driving power sources for portable devices because of their light weight and high energy density. Accordingly, research and development work is actively being conducted to improve the performance of the lithium secondary battery. In these lithium secondary batteries, electric energy is generated by oxidation and reduction reactions upon intercalation/deintercalation of lithium ions at the positive electrode and the negative electrode in a state in which an organic electrolyte solution or a polymer electrolyte solution is filled between the positive electrode and the negative electrode made of an active material capable of intercalating and deintercalating lithium ions.

As a positive electrode active material of a lithium secondary battery, a lithium cobalt oxide (LiCoO ₂), a lithium nickel oxide (LiNiO ₂), a lithium manganese oxide (LiMnO ₂、LiMn₂O₄, etc.), a lithium iron phosphate compound (LiFePO ₄), a lithium nickel cobalt manganese-based positive electrode active material (or a lithium NCM-based positive electrode active material, or an NCM-based lithium composite transition metal oxide, or a high Ni positive electrode material) capable of realizing a high capacity by substituting a part of nickel (Ni) with cobalt (Co) and manganese (Mn) have been used. However, the above conventional lithium secondary battery causes thermal runaway due to exothermic reaction caused by decomposition reaction of negative electrode SEI (solid electrolyte interface) and reaction between the positive electrode, which becomes unstable with increase of nickel (Ni) content, and an electrolyte solution containing a carbonate-based solvent, and this is a factor greatly threatening stability of the battery. In addition, particularly, lithium Ion Batteries (LIB) using both lithium nickel cobalt manganese-based positive electrode active materials and graphite negative electrodes have a problem of being relatively more susceptible to heat.

Therefore, in this technical field, attempts are being made to change the composition of the positive electrode active material or the composition of the electrolyte, and in particular, efforts are being made to remedy the problems of the existing lithium secondary batteries by applying a polymer compound, an inorganic compound, and the like to the electrolyte. In addition, the polymer electrolyte has superior advantages over the inorganic solid electrolyte in terms of flexibility, light weight, workability, and cost, and thus the application of the polymer electrolyte to a lithium secondary battery tends to be gradually increased.

As described above, although the polymer electrolyte has various advantages, efforts are continuously made to compensate for such problems since it is also clear that it is inferior to other types of electrolytes. In other words, it is pointed out that the polymer electrolyte has weak mechanical properties and low ionic conductivity compared to the inorganic solid electrolyte. In addition, the polymer electrolyte is limited in that it has a low cation (Li ion) transport coefficient (t+), and thus has a large concentration polarization (resistance), as compared to a liquid electrolyte.

Accordingly, there is an urgent need to develop an electrolyte for a lithium secondary battery, which uses a polymer electrolyte having various advantages, but forms a cationic polymer backbone to minimize movement of anions, thereby accelerating cations, and can improve additional lithium ion transport capacity and mechanical properties by including oxide-based ceramics.

Disclosure of Invention

[ Technical problem ]

Accordingly, an object of the present invention is to provide an electrolyte for a lithium secondary battery capable of forming a cationic polymer backbone to minimize movement of anions, thereby accelerating cations, and improving additional lithium ion transport capacity and mechanical properties by including an oxide-based ceramic, and a lithium secondary battery including the same.

Technical scheme

In order to achieve the above object, the present invention provides an electrolyte for a lithium secondary battery, comprising a polymer electrolyte comprising a hydrocarbon polymer compound containing a cationic functional group; a flame retardant liquid electrolyte; and a crosslinking agent.

In addition, the present invention provides a lithium secondary battery including: a positive electrode containing a lithium nickel cobalt manganese-based positive electrode active material; a negative electrode; and an electrolyte for a lithium secondary battery interposed between the positive electrode and the negative electrode.

[ Advantageous effects ]

According to the electrolyte for a lithium secondary battery of the present invention and the lithium secondary battery comprising the same, the present invention has an advantage in that cations are accelerated by forming a cationic polymer backbone to minimize movement of anions; and to improve additional lithium ion transport capacity and mechanical properties by including oxide-based ceramics.

Detailed Description

Hereinafter, the present invention will be described in detail.

The electrolyte for a lithium secondary battery of the present invention comprises: a polymer electrolyte comprising a hydrocarbon polymer compound containing cationic functional groups; a flame retardant liquid electrolyte; and a crosslinking agent.

The lithium secondary battery generates electric energy through oxidation and reduction reactions upon intercalation/deintercalation of lithium ions at the positive electrode and the negative electrode in a state in which an organic electrolyte or a polymer electrolyte is filled between the positive electrode and the negative electrode made of an active material capable of intercalating and deintercalating lithium ions. In addition, in order to improve thermal stability, a positive electrode active material of a lithium nickel cobalt manganese-based active material in which a part of nickel (Ni) is substituted with cobalt (Co) and manganese (Mn) is applied to a battery. However, the conventional lithium secondary battery causes thermal runaway due to exothermic reaction starting from decomposition reaction of negative electrode SEI (solid electrolyte interface) and reaction between the positive electrode, which becomes unstable with increase of nickel (Ni) content, and the electrolyte containing carbonate-based solvent, and thermal runaway is a significant threat to battery stability. In addition, particularly, lithium Ion Batteries (LIB) using both lithium nickel cobalt manganese-based positive electrode active materials and graphite negative electrodes have a problem of being relatively more susceptible to heat.

Therefore, in this technical field, attempts are being made to change the composition of the positive electrode active material or the composition of the electrolyte, and in particular, efforts are being made to remedy the problems of the existing lithium secondary batteries by applying a polymer compound, an inorganic compound, and the like to the electrolyte. In addition, the polymer electrolyte has superior advantages over the inorganic solid electrolyte in terms of flexibility, light weight, workability, and cost, and thus the application of the polymer electrolyte to a lithium secondary battery tends to be gradually increased. As described above, the polymer electrolyte has various advantages, however, since it is also clear that it is inferior to other types of electrolytes, efforts are also constantly being made to compensate for such problems. In other words, it is pointed out that the polymer electrolyte has weak mechanical properties and low ionic conductivity compared to the inorganic solid electrolyte. In addition, the polymer electrolyte is limited in that it has a low cation (Li ion) transport coefficient (t+), and thus has a large concentration polarization (resistance), as compared to a liquid electrolyte.

Accordingly, the applicant of the present invention has discovered an electrolyte for a lithium secondary battery using a polymer electrolyte having various advantages, but forming a cationic polymer backbone to minimize movement of anions, thereby accelerating cations, and can improve additional lithium ion transport capacity and mechanical properties by including oxide-based ceramics, and a lithium secondary battery including the same.

Hereinafter, the electrolyte for a lithium secondary battery of the present invention will be described in more detail. First, the polymer electrolyte included in the electrolyte for a lithium secondary battery of the present invention includes a hydrocarbon polymer compound containing a cationic functional group. Hydrocarbon polymer compounds containing cationic functional groups can accelerate cations by minimizing movement of anions by cations formed in the polymer backbone (i.e., forming positively charged polymer chains). And by this, the cation transfer coefficient can be increased as compared with the case of using a conventional polymer electrolyte containing no cation functional group, and therefore, concentration polarization (resistance) can be reduced.

The hydrocarbon polymer compound containing cationic functional groups contains hydrocarbon structural units having from 6 to 20 carbon atoms, preferably from 8 to 14 carbon atoms, which contain at least one cationic functional group. In addition, the cationic functional group may include at least one cation selected from the group consisting of a nitrogen cation, an oxygen cation, and a sulfur cation, and wherein it preferably substantially contains at least one nitrogen cation.

More specifically, the hydrocarbon polymer compound containing a cationic functional group may include at least one selected from the group consisting of: polydiallyldimethyl ammonium ((C ₈H₁₆N⁺) n), polymethacryloxyethyltrimethyl ammonium ((C ₉H₁₈NO₂ ⁺) n, 1.ltoreq.n.ltoreq.10,000), polyallylamine ([ CH ₂CH(CH₂NH₂) ] n, 1.ltoreq.n.ltoreq.10,000) and sodium poly-4-styrenesulfonate ((C ₈H₇NaO₃ S) n, 1.ltoreq.n.ltoreq.10,000) represented by the following formula 1.

[ 1]

Wherein n is a natural number from 1 to 10,000, preferably from 1 to 100.

Because the anions are bound to the polymer backbone in which the cations are located, movement of the anions is reduced and movement of the anions is minimized by the cations located on the hydrocarbon polymer backbone. In addition, the anion may be any anion that may be contained in the electrolyte, and in particular, it may be an anion of a lithium salt contained in the electrolyte. That is, in other words, the hydrocarbon polymer compound may further contain an anion as a counter ion to the cation contained in the cationic functional group. The anion of the lithium salt contained in the electrolyte may be, for example, an anion of a lithium salt generally known in the art, such as TFSI ^- of LiTFSI, SCN ^- of LiSCN, br ^- of LiBr, and Cl ^- of LiCl. The hydrocarbon polymer compound having a cationic functional group may be in a state in which the above-mentioned anion is bound to a cation as a counter ion.

Thus, for example, if the hydrocarbon polymer compound containing a cationic functional group is polydiallyldimethyl ammonium represented by formula 1, the anion of the lithium salt (TFSI ^-) can be combined with the cation as a counter ion as shown in (polydiallyldimethyl ammonium-bis (trifluoromethanesulfonyl) imine, DADMA-TFSI) of formula 2 below. In addition, as the movement of anions decreases, cations are accelerated, whereby the cation transport coefficient can be increased, and thus concentration polarization (resistance) can be reduced.

[ 2]

Wherein n is a natural number from 1 to 10,000, preferably from 1 to 100.

Meanwhile, the polymer electrolyte included in the electrolyte for a lithium secondary battery of the present invention may include, in addition to the above-mentioned hydrocarbon polymer compound having a cationic functional group, a conventional polymer compound generally used in the art, if necessary.

The content of the polymer electrolyte may be 30 to 60 wt%, preferably 30 to 50 wt%, more preferably 35 to 40 wt%, based on the total weight of the electrolyte for a lithium secondary battery of the present invention. If the content of the polymer electrolyte is less than 30% by weight, there may be a problem in that the effect of the cationic polymer does not appear. If the content of the polymer electrolyte exceeds 60 wt%, there may be a problem in that the weight of the cationic polymer is excessively large and thus ion conduction is rather hindered.

Next, a flame retardant liquid electrolyte included in the electrolyte for a lithium secondary battery of the present invention will be described. The flame retardant liquid electrolyte is used to dissolve the above-mentioned hydrocarbon polymer compound having a cationic functional group and to dissociate and transfer the lithium salt, and should be contained together with the hydrocarbon polymer compound having a cationic functional group. In addition, the flame retardant liquid electrolyte is trapped in the hardened cationic polymer and does not flow or run out, but may also be present in the liquid phase in the final manufactured electrolyte.

More specifically, the flame retardant liquid electrolyte comprises a solvent and a lithium salt. As the solvent contained in the flame-retardant liquid electrolyte, at least one selected from the group consisting of carbonate compounds, phosphate compounds, and ionic liquids can be exemplified. Examples of the carbonate/salt compound may include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methylpropyl carbonate, ethylmethyl carbonate, ethylpropyl carbonate, and methyl carbonate (2, 2-trifluoroethyl carbonate). Examples of the phosphate compound may include trimethyl phosphate, triethyl phosphate, and 2- (2, 2-trifluoroethoxy) -1,3, 2-dioxaphospholane 2-oxide. Examples of ionic liquids may include N-propyl-N-methylpyrrolidinium and 1-butyl-1-methylpyrrolidinium.

In addition, the solvent may further include one or more of an ester, an ether, or a ketone, if necessary. Examples of these may be aprotic organic solvents such as γ -butyrolactone, N-methyl acetate, N-ethyl acetate, N-propyl acetate, dibutyl ether, N-methyl-2-pyrrolidone, 1, 2-dimethoxyethane, tetrahydrofuran derivatives (e.g., 2-methyltetrahydrofuran), dimethyl sulfoxide, formamide, dimethylformamide, dioxolane and derivatives thereof, acetonitrile, nitromethane, methyl formate, methyl acetate, trimethoxymethane, sulfolane, methyl sulfolane, 1, 3-dimethyl-2-imidazolidone, methyl propionate, and ethyl propionate.

The lithium salt contained in the flame-retardant liquid electrolyte may be at least one ：LiFSI、LiTFSI、LiCl、LiBr、LiI、LiClO₄、LiBF₄、LiPF₆、LiB₁₀Cl₁₀、LiCF₃SO₃、LiCF₃CO₂、LiC₄BO₈、LiAsF₆、LiSbF₆、LiAlCl₄、CH₃SO₃Li、CF₃SO₃Li、(C₂F₅SO₂)₂NLi、(CF₃SO₂)₃CLi、 chloroborane lithium selected from the group consisting of lithium, lithium lower aliphatic carboxylate having 4 or less carbon atoms, lithium tetraphenyl borate, and lithium imide. In the electrolyte for a lithium secondary battery of the present invention, the lithium salt may be contained at a concentration of 1.5M to 4.0M, preferably 1.5M to 2.0M. If the concentration of the lithium salt is less than 1.5M or exceeds 4.0M, the degree of contribution to the thermal stability of the battery may not be significant, or there may be no further advantage in improving the thermal stability of the battery. In addition, if the concentration of the lithium salt is less than 1.5M, it may be difficult to ensure ion conductivity suitable for battery operation. If the concentration of the lithium salt exceeds 4.0M, mobility of lithium ions may be lowered due to an increase in viscosity of the electrolyte, or performance of the battery may be lowered due to an increase in decomposition reaction of the lithium salt itself.

The content of the flame retardant liquid electrolyte may be 30 to 50 wt%, preferably 30 to 40 wt%, more preferably 35 to 40 wt%, based on the total weight of the electrolyte for a lithium secondary battery of the present invention. If the content of the flame retardant liquid electrolyte is less than 30 wt%, the ionic conductivity may be too low to be used as an electrolyte. If the content of the flame retardant liquid electrolyte exceeds 50 wt%, there may be a problem that the components in a liquid state rather than a solid state are excessive.

In addition, the crosslinking agent contained in the electrolyte for a lithium secondary battery of the present invention will be described. The crosslinking agent is used to prevent leakage and improve additional mechanical properties, and to gel or cure the electrolyte for a lithium secondary battery of the present invention. The crosslinking agent may include at least one selected from the group consisting of ethoxylated trimethylolpropane triacrylate (ETPTA), trimethylolpropane ethoxytriacrylate, dipentaerythritol penta/hexaacrylate, and tris (2-hydroxyethyl) isocyanurate triacrylate, but is not limited thereto.

The content of the crosslinking agent may be 2 to 30wt%, preferably 10 to 30wt%, more preferably 20 to 30wt%, based on the total weight of the electrolyte for a lithium secondary battery of the present invention. If the content of the crosslinking agent is less than 2% by weight, there may be a problem in that crosslinking cannot be performed normally. If the content of the crosslinking agent exceeds 30% by weight, there may be a problem in that ion conduction is hindered due to an excessive weight of the crosslinking agent.

Meanwhile, the electrolyte for a lithium secondary battery of the present invention may further comprise an active oxide-based ceramic in order to improve additional lithium ion transporting ability and mechanical properties. Examples of the active oxide-based ceramic may include at least one kind ：Li_1+xAl_xGe_2-x(PO₄)₃(LAGP)、Li₇La₃Zr₂O₁₂(LLZO) selected from the group consisting of Li _1+xAl_xTi_2-x(PO₄)₃ (LATP). However, even though not illustrated herein, any active oxide-based ceramic having properties similar to or equivalent to those of LAGP, LLZO, and LATP may be used as the active oxide-based ceramic of the present invention without particular limitation.

In addition, the content of the active oxide-based ceramic may be 15 to 100 parts by weight, preferably 40 to 65 parts by weight, based on 100 parts by weight of the total weight of the electrolyte for a lithium secondary battery including the polymer electrolyte, the flame-retardant liquid electrolyte, and the crosslinking agent.

Next, the lithium secondary battery of the present invention will be described.

The lithium secondary battery includes: a positive electrode containing a lithium nickel cobalt manganese-based positive electrode active material; a negative electrode; and an electrolyte for a lithium secondary battery interposed between the positive electrode and the negative electrode. The positive electrode includes a lithium nickel cobalt manganese-based positive electrode active material (or a lithium NCM-based positive electrode active material, or an NCM-based lithium composite transition metal oxide, or a high Ni positive electrode material), and if it is applied to a battery, a high capacity can be achieved. In addition, the surface of the lithium nickel cobalt manganese-based positive electrode active material may be coated with a metal oxide.

The lithium nickel cobalt manganese-based positive electrode active material may be commercially purchased and used, or prepared and used according to a manufacturing method well known in the art. For example, a lithium nickel cobalt manganese-based positive electrode active material may be prepared by: an ammonium cation-containing complexing agent and an alkaline compound are added to a transition metal solution containing a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material, and co-precipitated to prepare a nickel cobalt manganese precursor, which is then mixed with a lithium raw material and oversintered at a temperature of 980 ℃ or higher.

The nickel-containing raw material may be, for example, nickel-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide, etc., and specifically may be Ni(OH)₂、NiO、NiOOH、NiCO₃·2Ni(OH)₂·4H₂O、NiC₂O₂·2H₂O、Ni(NO₃)₂·6H₂O、NiSO₄、NiSO₄·6H₂O、 fatty acid nickel salt, nickel halide, or a combination thereof, but is not limited thereto. The cobalt-containing feedstock may be, for example, cobalt-containing acetates, nitrates, sulfates, halides, sulfides, hydroxides, oxides, or oxyhydroxides, etc., and specifically Co(OH)₂、CoOOH、Co(OCOCH₃)₂·4H₂O、Co(NO₃)₂·6H₂O、CoSO₄、Co(SO₄)₂·7H₂O or a combination thereof, but is not limited thereto. The manganese-containing feedstock may be, for example, manganese-containing acetates, nitrates, sulfates, halides, sulfides, hydroxides, oxides, oxyhydroxides, or combinations thereof, and specifically may be manganese oxides, such as Mn ₂O₃、MnO₂ and Mn ₃O₄; manganese salts such as MnCO ₃、Mn(NO₃)₂、MnSO₄, manganese acetate, manganese dicarboxylic acid, manganese citrate and manganese salts of fatty acids; manganese oxyhydroxide; manganese chloride; or a combination thereof, but is not limited thereto.

The transition metal solution may be prepared by adding a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material to a solvent (specifically, water, or a mixed solvent of an organic solvent (e.g., alcohol, etc.) and water capable of being uniformly mixed with water), or may be prepared by mixing an aqueous solution of a nickel-containing raw material, an aqueous solution of a cobalt-containing raw material, and a manganese-containing raw material. The ammonium cation-containing complex forming agent may be, for example, NH₄OH、(NH₄)₂SO₄、NH₄NO₃、NH₄Cl、CH₃COONH₄、NH₄CO₃ or a combination thereof, but is not limited thereto. Meanwhile, the complex forming agent containing ammonium cations may be used in the form of an aqueous solution, and in this case, the solvent may be water or a mixture of an organic solvent (specifically, alcohol or the like) and water capable of being uniformly mixed with water.

The basic compound may be, for example, an alkali or alkaline earth metal hydroxide (e.g., naOH, KOH, or Ca (OH) ₂), a hydrate thereof, or a combination thereof. The basic compound may also be used in the form of an aqueous solution, and in this case, the solvent may be water or a mixture of an organic solvent (specifically, alcohol or the like) and water capable of being uniformly mixed with water. The basic compound is added to adjust the pH of the reaction solution, and the amount may be such that the pH of the metal solution is 11 to 13.

Meanwhile, the coprecipitation reaction may be performed at a temperature of 40 to 70 ℃ under an inert atmosphere such as nitrogen or argon. Through the above-described process, particles of nickel cobalt manganese hydroxide are formed and precipitated in the reaction solution. The precipitated nickel cobalt manganese hydroxide particles may be separated and dried according to conventional methods to obtain nickel cobalt manganese precursors. The nickel cobalt manganese precursor may be secondary particles formed by aggregation of primary particles, and the average particle diameter (D ₅₀) of the nickel cobalt manganese precursor secondary particles may be 4 to 8 μm, preferably 4 to 7.5 μm, more preferably 4 to 7 μm.

The lithium raw material may be a sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide, etc. containing lithium, and is not particularly limited as long as it is soluble in water. Specifically, the lithium source may be Li₂CO₃、LiNO₃、LiNO₂、LiOH、LiOH·H₂O、LiH、LiF、LiCl、LiBr、LiI、CH₃COOLi、Li₂O、Li₂SO₄、CH₃COOLi or Li ₃C₆H₅O₇ or the like, and a mixture of any one or two or more of these may be used. The lithium raw material may be mixed such that the molar ratio (Li/M) of lithium (Li) to all metal elements (M) of the nickel cobalt manganese precursor is 1 to 1.5, preferably 1 to 1.1.

The content of the lithium nickel cobalt manganese-based positive electrode active material may be 50 to 95 parts by weight, preferably 60 to 90 parts by weight, based on 100 parts by weight of the positive electrode. If the content of the lithium nickel cobalt manganese based positive electrode active material is less than 50 parts by weight based on 100 parts by weight of the total weight of the positive electrode, the electrochemical characteristics of the battery may be deteriorated due to the positive electrode active material. If the content exceeds 95 parts by weight, it may be difficult to manufacture a highly efficient battery because the content of other components (e.g., binder and conductive material) may be small.

The positive electrode further includes a binder and a conductive material in addition to the positive electrode active material described above. The binder is a component that contributes to the binding between the positive electrode active material and the conductive material and the binding with the current collector, and may be, for example, but not necessarily limited to, at least one selected from the group consisting of: polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF/HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl (meth) acrylate, polyethyl (meth) acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof.

The binder is generally added in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the binder is less than 1 part by weight based on 100 parts by weight of the total weight of the positive electrode, the adhesive strength between the positive electrode active material and the current collector may be insufficient. If the content of the binder exceeds 50 parts by weight based on 100 parts by weight of the total weight of the positive electrode, the adhesive strength is improved, but the content of the positive electrode active material may be correspondingly reduced, thereby reducing the capacity of the battery.

The conductive material contained in the positive electrode is not particularly limited as long as it does not cause side reactions in the internal environment of the lithium secondary battery and has excellent conductivity while not causing chemical changes in the battery. The conductive material may typically be graphite or conductive carbon, and may be, for example, but not limited to, one selected from the group consisting of: graphite, such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, lamp black, and the like; a carbon-based material with a crystal structure of graphene or graphite; conductive fibers such as carbon fibers and metal fibers; a fluorocarbon compound; metal powders, such as aluminum and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; conductive polymers such as polyphenylene derivatives; and mixtures of two or more thereof.

The addition amount of the conductive material is generally 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the conductive material is too low, i.e., if the content is less than 0.5 parts by weight, it is difficult to obtain an effect of improving conductivity, or the electrochemical characteristics of the battery may be deteriorated. If the content of the conductive material exceeds 50 parts by weight, that is, if the content is too much, the amount of the positive electrode active material is relatively small, and thus the capacity and energy density may be reduced. The method of incorporating the conductive material into the positive electrode is not particularly limited, and conventional methods known in the art, such as coating on the positive electrode active material, may be used. Also, if necessary, the addition of a second coating layer having conductivity to the positive electrode material may replace the addition of the conductive material as described above.

In addition, a filler may be optionally added to the positive electrode of the present invention as a component that suppresses expansion of the positive electrode. Such a filler is not particularly limited as long as it can suppress expansion of the electrode without causing chemical changes in the battery, and examples thereof may include olefin-based polymers such as polyethylene and polypropylene; fibrous materials such as glass fibers and carbon fibers.

The positive electrode active material, the binder, the conductive material, and the like are dispersed and mixed in a dispersion medium (solvent) to form a slurry, and the slurry may be coated on a positive electrode current collector, and then dried and rolled to prepare a positive electrode included in the lithium secondary battery of the present invention. The dispersion medium may be, but is not necessarily limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, or mixtures thereof.

The positive electrode current collector may be, but is not necessarily limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO ₂), FTO (F-doped SnO ₂), or an alloy thereof, or aluminum (Al) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag), or the like. The shape of the positive electrode current collector may be in the form of foil, film, sheet, die cut form, porous body, foam, or the like.

The negative electrode may be prepared according to a conventional method known in the art. For example, the anode active material, the conductive material, the binder, and if necessary, the filler, etc. are dispersed and mixed in a dispersion medium (solvent) to form a slurry, and the slurry may be coated on an anode current collector, and then dried and rolled to prepare an anode. As the anode active material, a compound capable of reversibly intercalating and deintercalating lithium may be used. Specific examples of the anode active material may be carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; a metal species capable of alloying with lithium, such as Si, al, sn, pb, sb, zn, bi, in, mg, ga, cd, si alloy, sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium, such as SiO _β(0<β<2)、SnO₂, vanadium oxide, and lithium vanadium oxide; or a composite including the above metal substance and carbonaceous material, such as a si—c composite or a sn—c composite, and any one or a mixture of two or more thereof may be used. In addition, a metallic lithium thin film may be used as the anode active material. In addition, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. As low crystalline carbon, soft carbon and hard carbon are represented, and as high crystalline carbon, amorphous, plate-like, flake-like, spherical or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microsphere, mesophase pitch and high temperature calcined carbon such as petroleum or coal tar pitch-derived coke are represented.

In addition, the binder and the conductive material for the negative electrode may be the same as those described above for the positive electrode. The negative electrode current collector may be, but is not limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), copper (Cu), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO ₂), FTO (F-doped SnO ₂), or an alloy thereof, or copper (Cu) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag), or the like. The shape of the negative electrode current collector may be in the form of foil, film, sheet, die cut form, porous body, foam, or the like.

Meanwhile, in the lithium secondary battery, a solid electrolyte positioned as a film having a layered structure, that is, the electrolyte for a lithium secondary battery of the present invention, may be positioned between the positive electrode and the negative electrode. Therefore, in this case, the solid electrolyte may also function as a separator (i.e., function to electrically insulate the negative electrode and the positive electrode while passing lithium ions). In this case, the solid electrolyte may be contained in the lithium secondary battery by being attached to one surface of the positive electrode or the negative electrode. In addition, the solid electrolyte may be independently disposed between the positive electrode and the negative electrode. In addition, the lithium secondary battery of the present invention may be a semi-solid battery using a combination of a liquid electrolyte and a solid electrolyte, if necessary. In addition, in this case, a separate separator may be additionally included (i.e., one or more of the separator and the solid electrolyte may be disposed between the positive electrode and the negative electrode).

If a separate separator is additionally included, olefin-based polymers such as polyethylene and polypropylene, glass fibers, and the like may be used in the form of sheets, multilayers, microporous films, woven and nonwoven fabrics, and the like, but are not necessarily limited thereto. However, porous polyethylene or porous glass fiber nonwoven fabric (glass filter) may be preferably applied as the separator, and porous glass filter (glass fiber nonwoven fabric) may be more preferably applied as the separator. The separator may be an insulating film having high ion permeability and mechanical strength, and the separator may generally have a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm, but is not limited thereto.

Meanwhile, the lithium secondary battery of the present invention may be manufactured according to a conventional method in the related art. For example, it may be manufactured by inserting a porous separator between a positive electrode and a negative electrode and injecting an electrolyte solution. The lithium secondary battery of the present invention is applied to a single battery used as a power source of a small-sized device, and can also be particularly suitably used as a power source of a medium-sized and large-sized device, i.e., a unit cell of a battery module. In this respect, the present invention also provides a battery module including two or more lithium secondary batteries electrically connected (in series or parallel). Of course, the number of lithium secondary batteries included in the battery module may be variously adjusted in consideration of the use and capacity of the battery module.

Further, the present invention provides a battery pack in which battery modules are electrically connected according to conventional techniques in the art. The battery module and the battery pack may be used as a power source for a medium-sized and large-sized device of any one or more of the following: an electric tool; electric vehicles, including Electric Vehicles (EVs), hybrid Electric Vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); an electric truck; an electric commercial vehicle; or a power storage system, but is not necessarily limited thereto.

Hereinafter, preferred embodiments of the present invention will be described to facilitate understanding of the present invention. It will be apparent, however, to one skilled in the art that the following embodiments are merely examples of the present invention and that various changes and modifications may be made within the scope and spirit of the invention and that such changes and modifications are within the scope of the appended claims.

Example 1 production of lithium Secondary Battery

Preparation of electrolyte

Polydiallyl dimethyl ammonium-bis (trifluoromethanesulfonyl) imide (DADMA-TFSI, polymer electrolyte) was dissolved in a flame retardant liquid electrolyte (SCE 05, solvent: EC/emc=1:9 (v/v), lithium salt: 1M LiFSI and 1M LiPF ₆), and then trimethylolpropane triacrylate (ETPTA, crosslinker) was added thereto and mixed, and then cured with a UV curing lamp for about 40 seconds, to obtain a thin film electrolyte for lithium secondary batteries. Meanwhile, the polymer electrolyte, the flame-retardant liquid electrolyte, and the crosslinking agent were used in a weight ratio of 1:1:0.8 (about 35.7 wt.%: 28.6 wt.%).

Manufacturing of positive electrode

First, niSO ₄、CoSO₄ and MnSO ₄ were mixed in water in an amount such that the molar ratio of nickel to cobalt to manganese was 80:10:10 in a 40L batch reactor set to 50 ℃ to prepare a precursor forming solution having a concentration of 2.4M. 13 liters of deionized water was placed in a coprecipitation reactor (capacity 40L) and then nitrogen was purged into the reactor at a rate of 25 liters/min to remove dissolved oxygen from the water and form a non-oxidizing atmosphere within the reactor. Thereafter, 83g of 25% aqueous NaOH solution was added, followed by stirring at a speed of 700rpm at a temperature of 50℃to maintain pH 11.5. Thereafter, the precursor forming solution was added at a rate of 1.9L/hr, and the aqueous NaOH solution and the aqueous NH ₄ OH solution were added together while performing the coprecipitation reaction for 48 hours to form particles of hydroxide (Ni _0.5Co_0.3Mn_0.2(OH)₂) containing nickel cobalt and manganese. The obtained hydroxide particles were separated, washed and dried in an oven at 120 ℃ to prepare nickel cobalt manganese precursor (d50=4.8 μm). Subsequently, the prepared nickel cobalt manganese precursor and lithium source LiOH were placed in a henschel mixer (20L) in such an amount that the Li/M (Ni, co, mn) molar ratio was 1.02, and mixed at 300rpm at the center for 20 minutes. The mixed powder was put into an alumina crucible having a size of 330mm×330mm and fired at 1,010 to 1,030 ℃ for 15 hours in an oxygen atmosphere to prepare a lithium nickel cobalt manganese-based positive electrode active material.

Subsequently, the prepared lithium nickel cobalt manganese-based positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 96.5:1.5:2 and dispersed in an NMP solvent to prepare a slurry, which was then coated on an aluminum foil (Al foil) having a thickness of 25 μm at a uniform thickness by a blade coater, i.e., mattis coater (Labdryer/LTE type coater, WERNER MATHIS AG company), and dried in a vacuum oven at 120 ℃ for 13 hours, to obtain a positive electrode for a lithium secondary battery.

Manufacturing of lithium secondary battery

The negative electrode including graphite as an active material and the prepared positive electrode are positioned to face each other, and then the prepared electrolyte film for a secondary battery is disposed therebetween to manufacture an electrode assembly, and the manufactured electrode assembly is placed inside a case to manufacture a lithium secondary battery.

Example 2 production of lithium Secondary Battery

A lithium secondary battery was fabricated in the same manner as in example 1 above, except that Li ₇La₃Zr₂O₁₂ (LLZO, active oxide-based ceramic) was also added together when the crosslinking agent (trimethylolpropane triacrylate, ETPTA) was added. Meanwhile, the polymer electrolyte, the flame retardant liquid electrolyte, the crosslinking agent and the reactive oxide ceramics are used in a weight ratio of 1:1:0.8:0.5.

Comparative example 1 production of lithium secondary battery

A lithium secondary battery was fabricated in the same manner as in example 1 above, except that the polymer electrolyte was changed from polydiallyldimethyl ammonium-bis (trifluoromethanesulfonyl) imide (DADMA-TFSI) to polyethylene oxide (PEO, mw=about 10,000).

Comparative example 2 production of lithium secondary battery

A lithium secondary battery was fabricated in the same manner as in example 1 above, except that a crosslinking agent (trimethylolpropane triacrylate, ETPTA) was not used.

Comparative example 3 production of lithium secondary battery

A lithium secondary battery was fabricated in the same manner as in example 1 above, except that the weight ratio of the polymer electrolyte, the flame-retardant liquid electrolyte, and the crosslinking agent was changed from 1:1:0.8 (about 35.7 wt.%: 28.6 wt.%) to 1:2.2:1.78 (about 20 wt.%: 44.5 wt.%: 35.5 wt.%).

Comparative example 4 production of lithium secondary battery

A lithium secondary battery was fabricated in the same manner as in example 1 above, except that the weight ratio of the polymer electrolyte, the flame-retardant liquid electrolyte, and the crosslinking agent was changed from 1:1:0.8 (about 35.7 wt.%: 28.6 wt.%) to 1:2.55:0.7 (about 23.5 wt.%: 60 wt.%: 16.5 wt.%).

Experimental example 1 measurement of ion conductivity

The ion conductivities of the electrolytes for lithium secondary batteries prepared in examples 1 and 2 and comparative examples 1 and 2 were measured, and the results are shown in table 1 below. Ion conductivity was measured using an alternating current impedance (AC impedance) measuring instrument IVIUM STAT (Ivium Technologies, netherlands).

Table 1:

	Ion conductivity (lambda S/cm)
		Example 1	2.77×10^-4
Example 2	3.14×10^-4
		Comparative example 1	7.90×10^-5
Comparative example 2	7.70×10^-5
		Comparative example 3	2.04×10^-5
Comparative example 4	2.54×10^-5

As a result of measuring the ion conductivities of the electrolytes for lithium secondary batteries prepared in examples 1 and 2 and comparative examples 1 and 2, respectively, the electrolyte of example 1 (which contains a polymer electrolyte containing a hydrocarbon polymer compound containing a cationic functional group), a flame-retardant liquid electrolyte, and a crosslinking agent, and the electrolyte of example 2 (which contains an active oxide ceramic in addition to those in example 1) exhibited ion conductivities higher than those of the electrolyte of comparative example 1 containing a conventional polymer electrolyte or the electrolyte of comparative example 2 excluding only the crosslinking agent from the electrolyte of example 1, as shown in table 1 above. In addition, comparative example 3 (in which the content of the polymer electrolyte is out of the range of the present invention) and comparative example 4 (in which the content of the flame retardant liquid electrolyte is out of the range of the present invention) also show ion conductivity lower than that of examples 1 and 2. In addition, it was confirmed by comparing example 1 and example 2 that when the active oxide-based ceramic was contained in the electrolyte, the ion conductivity was further increased.

Experimental example 2 measurement of cation transport coefficient

A voltage of 10mV was applied to the lithium secondary batteries prepared in examples 1 and 2 and comparative examples 1 and 2, respectively, to induce polarization at the electrodes, and then a current in a polarized state was measured, and based on the measured current value, a cation transport coefficient (t+) of the liquid electrolyte was measured using equation 1 according to the Bruce-Vincent method. Meanwhile, the measured current value is a current value per second when the current change becomes 1 μa (i.e., dl=1 μa & dt=1 sec).

[ Equation 1]

Where I _ss is the current value flowing in steady state, which is 25.5 μa (measured for up to 6 hours), I ₀ is the current value flowing in polarized state, which is 44.6 μa, Δv is the potential difference applied across the cell, R ₀ is the interface resistance in polarized state, which is 136.06, and R _ss is the interface resistance in steady state, which is 314.74.

In addition, the measurement results of the cation transport coefficients of the lithium secondary batteries prepared in examples 1 and 2 and comparative examples 1 and 2, respectively, are shown in table 2 below.

Table 2:

	cation transport coefficient
		Example 1	0.63
Example 2	0.60
		Comparative example 1	0.18
Comparative example 2	0.19
		Comparative example 3	0.31
Comparative example 4	0.25

As a result of measuring the cation transport coefficients of the lithium secondary batteries manufactured in examples 1 and 2 and comparative examples 1 and 2, respectively, as described above, the electrolyte of example 1 (which contains a polymer electrolyte containing a hydrocarbon polymer compound containing a cationic functional group), a flame-retardant liquid electrolyte, and a crosslinking agent, and the electrolyte of example 2 (which contains an active oxide ceramic in addition to those in example 1) exhibited cation transport coefficients higher than those of the electrolyte of comparative example 1 containing a conventional polymer electrolyte or the electrolyte of comparative example 2 excluding only the crosslinking agent from the electrolyte of example 1, as shown in table 2 above. In addition, comparative example 3 (in which the content of the polymer electrolyte is out of the range of the present invention) and comparative example 4 (in which the content of the flame retardant liquid electrolyte is out of the range of the present invention) also show cation transfer coefficients higher than those of examples 1 and 2. Thus, it was confirmed that there is an advantage of lowering concentration polarization (resistance) as compared with the conventional electrolyte according to the present invention.

Claims

1. An electrolyte for a lithium secondary battery, comprising:

a polymer electrolyte comprising a hydrocarbon polymer compound containing cationic functional groups;

a flame retardant liquid electrolyte; and

A cross-linking agent.

2. The electrolyte for a lithium secondary battery according to claim 1, wherein the hydrocarbon polymer compound contains a hydrocarbon structural unit having 6 to 20 carbon atoms, the hydrocarbon structural unit containing at least one cationic functional group.

3. The electrolyte for a lithium secondary battery according to claim 1, wherein the cationic functional group comprises at least one cation selected from the group consisting of nitrogen cations, oxygen cations, and sulfur cations.

4. The electrolyte for a lithium secondary battery according to claim 1, wherein the hydrocarbon polymer compound containing a cationic functional group comprises at least one selected from the group consisting of: polydiallyl dimethyl ammonium (C ₈H₁₆N⁺)_n, wherein 1.ltoreq.n.ltoreq.10000; polymethacryloyloxyethyl trimethyl ammonium (C ₉H₁₈NO₂ ⁺)_n, wherein 1.ltoreq.n.ltoreq.10000; polyallylamine [ CH ₂CH(CH₂NH₂)]_n, wherein 1.ltoreq.n.ltoreq.10000; and sodium poly-4-styrenesulfonate (C ₈H₇NaO₃S)_n, wherein 1.ltoreq.n.ltoreq.10000).

5. The electrolyte for a lithium secondary battery according to claim 1, wherein the hydrocarbon polymer compound further comprises an anion as a counter ion of a cation contained in the cationic functional group.

6. The electrolyte for a lithium secondary battery according to claim 5, wherein the anion is an anion of a lithium salt contained in the electrolyte for a lithium secondary battery.

7. The electrolyte for a lithium secondary battery according to claim 1, wherein the flame retardant liquid electrolyte comprises: at least one solvent selected from the group consisting of carbonate compounds, phosphate compounds, and ionic liquids; and lithium salts.

8. The electrolyte for a lithium secondary battery according to claim 1, wherein the crosslinking agent is selected from the group consisting of: ethoxylated trimethylolpropane triacrylate, trimethylolpropane ethoxytriacrylate, dipentaerythritol penta/hexaacrylate and tris (2-hydroxyethyl) isocyanurate triacrylate.

9. The electrolyte for a lithium secondary battery according to claim 1, wherein the polymer electrolyte is contained in an amount of 30 to 60 wt%, the flame retardant liquid electrolyte is contained in an amount of 30 to 50 wt%, and the crosslinking agent is contained in an amount of 2 to 30 wt%.

10. The electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte for a lithium secondary battery further comprises an active oxide-based ceramic.

11. The electrolyte for a lithium secondary battery according to claim 10, wherein the active oxide-based ceramic is selected from the group consisting of ：Li_1+xAl_xGe_2-x(PO₄)₃(LAGP)、Li₇La₃Zr₂O₁₂(LLZO) and Li _1+xAl_xTi_2-x(PO₄)₃ (LATP).

12. The electrolyte for a lithium secondary battery according to claim 10, wherein the content of the active oxide-based ceramic is 15 to 100 parts by weight based on 100 parts by weight of the total weight of the electrolyte for a lithium secondary battery including the polymer electrolyte, the flame-retardant liquid electrolyte, and the crosslinking agent.

13. A lithium secondary battery, comprising: a positive electrode comprising a lithium nickel cobalt manganese-based positive electrode active material; a negative electrode; and the electrolyte for a lithium secondary battery according to claim 1, which is provided between the positive electrode and the negative electrode.

14. The lithium secondary battery according to claim 13, wherein the electrolyte for the lithium secondary battery is attached to one side of the positive electrode or the negative electrode in the form of a thin film or is independently disposed between the positive electrode and the negative electrode.

15. The lithium secondary battery according to claim 13, wherein the lithium secondary battery further comprises a separate separator between the positive electrode and the negative electrode.

16. The lithium secondary battery according to claim 13, wherein the lithium secondary battery is a semi-solid battery comprising a liquid electrolyte and a solid electrolyte.