Electrolyte and lithium-ion secondary battery

Abstract

The present invention provides an electrolyte that can gel even with a low content of acrylic copolymer. [Solution] The electrolyte according to the embodiment comprises an aprotic solvent, a supporting salt, an acrylic copolymer containing a specific structural unit represented by formula (3), and a silyl compound represented by formula (3). In formula (3), R 6 , R 7 and R 8 Each of these independently represents an alkyl group or hydrogen atom having 1 to 4 carbon atoms, A represents P, B, or S, s represents 2 or 3, and t represents an integer from 0 to 2. JPEG0007868241000009.jpg29135

Description

[Technical Field]

[0001] Embodiments of the present invention relate to an electrolyte and a lithium-ion secondary battery using the same. [Background technology]

[0002] In lithium-ion secondary batteries, there is a need to improve battery performance, such as output characteristics and cycle characteristics. Various proposals have been made regarding the electrolyte that constitutes lithium-ion secondary batteries (see Patent Document 1). For example, Patent Document 2 proposes using an electrolyte containing an acrylic copolymer of methyl methacrylate and (3-ethyloxetane-3-yl)methyl methacrylate, and crosslinking the copolymer to create a polymer gel electrolyte in order to improve battery performance (see Patent Documents 1 and 2).

[0003] On the other hand, in lithium-ion secondary batteries, it is known that silyl compounds are added to the electrolyte as components that form a good ion-conductive film on the electrode surface in order to suppress the increase in internal resistance (see Patent Document 3). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Special Publication No. 2024-503376 [Patent Document 2] Japanese Patent Publication No. 2022-054081 [Patent Document 3] International Publication No. 2020 / 116509 [Overview of the project] [Problems that the invention aims to solve]

[0005] As mentioned above, it is known to add silyl compounds to electrolytes, but it is not known to incorporate silyl compounds into electrolytes containing acrylic copolymers that have oxetane rings and gel by crosslinking. On the other hand, while acrylic copolymers are components for gelling electrolytes, it is desirable to have a low content from the viewpoint of ionic conductivity. In other words, it is desirable to be able to gel the electrolyte and form a polymer gel electrolyte while reducing the content of acrylic copolymers to improve battery performance.

[0006] In view of the above, embodiments of the present invention aim to provide an electrolyte that can gel even with a small amount of acrylic copolymer, and a lithium-ion secondary battery using the same. [Means for solving the problem]

[0007] The present invention includes embodiments shown below. [1] An electrolyte comprising an aprotic solvent, a supporting salt, an acrylic copolymer containing a constituent unit represented by the following general formula (1) and a constituent unit represented by the following general formula (2), and a silyl compound represented by the following general formula (3). [ka] (In general formulas (1) and (2), R 1 and R 3 Each of these independently represents a hydrogen atom or a methyl group, R 2 R represents an alkyl group with 1 to 5 carbon atoms. 4 R represents an alkanediyl group with 1 to 5 carbon atoms. 5 (This represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms.) [ka] (In general formula (3), R 6 , R 7 and R 8 Each of the following independently represents an alkyl group or hydrogen atom having 1 to 4 carbon atoms; A represents P, B, or S; s represents 2 or 3; and t represents an integer from 0 to 2. [2] The content of the acrylic copolymer is 1.0% by mass or more and 2.6% by mass or less, and the content of the silyl compound is 0.05% by mass or more and 3.0% by mass or less. The electrolytic solution according to [1]. [3] The silyl compound contains at least one selected from the group consisting of tris(trialkylsilyl) phosphate, tris(trialkylsilyl) phosphite, tris(trialkylsilyl) borate, and bis(trialkylsilyl) sulfate. The electrolytic solution according to [1] or [2]. [4] The aprotic solvent contains at least one selected from the group consisting of cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, and chain ethers. The electrolytic solution according to any one of [1] to [3]. [5] The supporting salt contains at least one selected from the group consisting of LiPF6, LiBF4, LiAsF6, LiSbF6, LiN(SO2F)2, and LiN(CF3SO2)2. The electrolytic solution according to any one of [1] to [4]. [6] A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a polymer gel electrolyte obtained by crosslinking the acrylic copolymer using the electrolytic solution according to any one of [1] to [5]. [Effects of the Invention]

[0008] According to an embodiment of the present invention, an electrolytic solution capable of gelling even when the content of the acrylic copolymer is small can be provided. [Modes for Carrying Out the Invention]

[0009] The electrolytic solution according to the embodiment is a non-aqueous electrolytic solution containing an aprotic solvent, a supporting salt, an acrylic copolymer having an oxetane ring, and a silyl compound. The electrolytic solution is an electrolyte solution in which a supporting salt, an acrylic copolymer, and a silyl compound are dissolved in an aprotic solvent which is a non-aqueous solvent. By crosslinking the acrylic copolymer to form a crosslinked body, a polymer gel electrolyte can be formed.

[0010] [Aprotic solvent] Examples of aprotic solvents include cyclic carbonates such as ethylene carbonate (ethylene carbonate, EC), propylene carbonate (PC), and butylene carbonate (BC); chain carbonates such as diethyl carbonate (diethyl carbonate, DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (ethyl methyl carbonate, EMC), and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate, and ethyl propionate; γ-lactones such as γ-butyrolactone; chain ethers such as 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME); cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran; their fluorine derivatives; dimethyl sulfoxide (DMSO), 1,3-dioxolane, formamide, acetamide, dimethylformamide (DMF), acetonitrile, propylnitrile, nitromethane, triphosphate ester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, fluorinated carboxylic acid esters, and the like. Any one of these aprotic organic solvents may be used, or two or more of them may be used in combination.

[0011] The aprotic solvent preferably contains at least one solvent selected from the group consisting of cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, and chain ethers. The aprotic solvent more preferably contains at least one selected from the group consisting of cyclic carbonates and chain carbonates. In one embodiment, the aprotic solvent may be composed of cyclic carbonates and chain carbonates, and in that case, the volume ratio (cyclic carbonates / chain carbonates) may be 1 / 9 to 6 / 4, or 2 / 8 to 5 / 5.

[0012] In one embodiment, the aprotic solvent may include at least one selected from the group consisting of EC, PC, BC, DEC, DMC, EMC, and DPC, or it may include at least one selected from the group consisting of EC, PC, and BC, and at least one selected from the group consisting of DEC, DMC, EMC, and DPC.

[0013] [Supporting salt] Supporting salts are substances that enhance the conductivity of aprotic solvents. Examples of supporting salts include lithium salts such as LiPF6, LiBF4, LiAsF6, LiSbF6, LiN(SO2F)2, LiAlCl4, LiClO4, LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, lithium carboxylate, lithium chloroborane, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, and LiF. These supporting salts may be used individually or in combination of two or more.

[0014] The supporting salt preferably contains a lithium salt containing fluorine, and more preferably contains at least one substance selected from the group consisting of LiPF6, LiBF4, LiAsF6, LiSbF6, LiN(SO2F)2, and LiN(CF3SO2)2.

[0015] The concentration of the supporting salt in the electrolyte is not particularly limited, but is preferably 0.5 to 2.0 mol / L, more preferably 0.6 to 1.8 mol / L, and may also be 0.8 to 1.6 mol / L.

[0016] [Acrylic copolymer] Acrylic copolymers are copolymers containing constituent units represented by the following general formula (1) (hereinafter referred to as "constituent unit (1)") and constituent units represented by the following general formula (2) (hereinafter referred to as "constituent unit (2)"). Here, a constituent unit refers to an atomic group that makes up a polymer, and is also called a repeating unit. [ka]

[0017] The structural unit (1) is derived from an alkyl (meth)acrylate, and the structural unit (2) is derived from a (meth)acrylate ester having an oxetane ring (a 4-membered cyclic ether structure). Therefore, the acrylic copolymer is a copolymer of an alkyl (meth)acrylate and a (meth)acrylate ester having an oxetane ring. Here, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid.

[0018] In formulas (1) and (2), R 1 and R 3 each independently represent a hydrogen atom or a methyl group, and preferably both are methyl groups. Therefore, the acrylic copolymer is preferably a copolymer of an alkyl methacrylate and a methacrylate ester having an oxetane ring.

[0019] R 2 represents an alkyl group having 1 to 5 carbon atoms, which may be a linear alkyl group or a branched alkyl group. R 2 is preferably an alkyl group having 1 to 3 carbon atoms, and more preferably a methyl group.

[0020] R 4 represents an alkanediyl group having 1 to 5 carbon atoms (that is, a divalent group obtained by removing two hydrogen atoms from an alkane), which may be linear or branched. R 4 is preferably an alkanediyl group having 1 to 3 carbon atoms, and more preferably a methylene group.

[0021] R 5 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and the alkyl group may be linear or branched. R 5 is preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. More preferably, R 5 is a methyl group or an ethyl group.

[0022] The arrangement of constituent units (1) and (2) is not particularly limited. Therefore, the acrylic copolymer may be, for example, a statistical copolymer (including a random copolymer) or a block copolymer.

[0023] The acrylic copolymer preferably consists substantially only of constituent unit (1) and constituent unit (2) as monomer-derived constituent units, but may also contain other monomer-derived constituent units to the extent that it does not impair the effect. Although not particularly limited, the total content of constituent unit (1) and constituent unit (2) is preferably 90 mol% or more, more preferably 95 mol% or more, even more preferably 99 mol% or more, and may be 100 mol%, relative to 100 mol% of the total monomer-derived constituent units.

[0024] In the acrylic copolymer, the degree of polymerization n of constituent unit (1) is not particularly limited and may be, for example, 1000 to 4000. The degree of polymerization m of constituent unit (2) is also not particularly limited and may be, for example, 200 to 700. Here, the degrees of polymerization of constituent units (1) and (2) are calculated from the number-average molecular weight Mn of the acrylic copolymer and the ratio of m / n. The number-average molecular weight Mn was determined as the number-average molecular weight in polystyrene equivalent by gel permeation chromatography (GPC) (Prominence, Shimadzu Corporation) using four linked columns (Shodex GPC columns KF-601, KF-602, KF-603, KF-604, manufactured by Resonaq Corporation) packed with polystyrene gel, after dissolving the acrylic copolymer in tetrahydrofuran (column oven temperature 40°C, THF flow rate 0.6 mL / min, sample concentration 0.1% by mass, sample injection volume 100 μL, differential refractive index detector: Shodex RI-504, manufactured by Resonaq Corporation). The m / n ratio was determined by dissolving the acrylic copolymer in deuterated chloroform and using a nuclear magnetic resonance spectrometer (JEOL). 1 The molar ratio of constituent unit (1) to constituent unit (2) can be determined by performing 1H-NMR measurements.

[0025] In one embodiment, the acrylic copolymer is preferably a copolymer represented by the following formula (4). The copolymer of formula (4) is a copolymer of methyl methacrylate (MMA) and (3-ethyloxetan-3-yl)methyl methacrylate (OXMA). [ka] In equation (4), n satisfies 1000 ≤ n ≤ 4000, and m satisfies 200 ≤ m ≤ 700.

[0026] In formula (4), the constituent unit listed to the left of "-co-" is a constituent unit derived from methyl methacrylate (MMA unit) as an example of constituent unit (1). The constituent unit listed to the right of "-co-" is a constituent unit derived from (3-ethyloxetan-3-yl)methyl methacrylate (OXMA unit) as an example of constituent unit (2). "-co-" means that the arrangement pattern of these constituent units is not specified, and therefore the copolymer of formula (4) may be a statistical copolymer (including a random copolymer) or a block copolymer.

[0027] The acrylic copolymer of formula (4) preferably consists substantially only of MMA units and OXMA units as monomer-derived constituent units, but may also contain other monomer-derived constituent units to the extent that it does not impair the effect. The total content of MMA units and OXMA units is preferably 90 mol% or more, more preferably 95 mol% or more, even more preferably 99 mol% or more, and may be 100 mol%, based on 100 mol% of all monomer-derived constituent units.

[0028] Acrylic copolymers can be obtained, for example, by radical copolymerization of an alkyl (meth)acrylate and a (meth)acrylic acid ester having an oxetane ring.

[0029] The content of the acrylic copolymer is preferably 1.0 to 2.6% by mass of the total electrolyte (i.e., relative to 100% by mass of the electrolyte), more preferably 1.0 to 2.0% by mass, and even more preferably 1.2 to 1.8% by mass. A higher content of acrylic copolymer is preferable for obtaining a stable polymer gel electrolyte, but a lower content of acrylic copolymer is preferable from the viewpoint of battery performance. According to this embodiment, the electrolyte can be gelled even when the content of acrylic copolymer is low, for example, less than 2.0% by mass.

[0030] [Silyl compounds] The electrolyte according to this embodiment contains a silyl compound represented by the following general formula (3). By adding the silyl compound of formula (3) to an electrolyte containing the above-mentioned acrylic copolymer together with a supporting salt in an aprotic solvent, it is thought that ring-opening polymerization occurs not only by trace amounts of acidic substances obtained by hydrolysis of the anionic component of the supporting salt, but also by inorganic acids obtained by the elimination of silyl groups, and further crosslinking occurs through ester condensation reactions between the inorganic acids and the ring-opened oxetanyl groups. As a result, a polymer gel electrolyte can be obtained by gelling the electrolyte while reducing the content of the acrylic copolymer.

[0031] [ka] In equation (3), R 6 , R 7 and R 8 Each of these independently represents an alkyl group or hydrogen atom having 1 to 4 carbon atoms. A represents a phosphorus atom (P), a boron atom (B), or a sulfur atom (S). s represents 2 or 3. t represents an integer from 0 to 2.

[0032] Examples of silyl compounds in formula (3) include silyl phosphate esters (A=P, s=3, t=1), silyl phosphite esters (A=P, s=3, t=0), silyl borate esters (A=B, s=3, t=0), and silyl sulfate esters (A=S, s=2, t=2). Any one of these may be used, or two or more may be used in combination.

[0033] R in equation (3) 6 , R 7 and R 8 Preferably, at least one of the elements is an alkyl group having 1 to 4 carbon atoms, more preferably at least two are alkyl groups having 1 to 4 carbon atoms, and even more preferably all are alkyl groups having 1 to 4 carbon atoms. That is, R 6 , R 7 and R 8 Preferably, each of these independently represents an alkyl group having 1 to 4 carbon atoms. The alkyl group may be linear or branched, and is preferably a methyl group, an ethyl group, an isopropyl group, or a tert-butyl group.

[0034] In one embodiment, the silyl compound of formula (3) preferably includes at least one selected from the group consisting of tris(trialkylsilyl) phosphate, tris(trialkylsilyl) phosphite, tris(trialkylsilyl) borate, and bis(trialkylsilyl) sulfate. More preferably, it includes at least one selected from the group consisting of tris(trialkylsilyl) phosphate and tris(trialkylsilyl) phosphite.

[0035] Specific examples of tris(trialkylsilyl) phosphate include tris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate, and tris(tert-butyldimethylsilyl) phosphate. Specific examples of tris(trialkylsilyl) phosphite include tris(trimethylsilyl) phosphite and tris(triethylsilyl) phosphite. Specific examples of tris(trialkylsilyl) borate include tris(trimethylsilyl) borate and tris(triethylsilyl) borate. Specific examples of bis(trialkylsilyl) sulfate include bis(trimethylsilyl) sulfate and bis(triethylsilyl) sulfate.

[0036] The silyl compound content is preferably 0.05 to 3.0% by mass of the total electrolyte (i.e., relative to 100% by mass of the electrolyte), more preferably 0.1 to 2.0% by mass, and even more preferably 0.2 to 1.0% by mass.

[0037] [Other ingredients] In addition to the components described above, the electrolyte according to this embodiment may also contain additives such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), 1,3-propanesultone, ethylene sulfate, fluoroethylene, anhydride, sulfonic acid ester, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, and t-butylbenzene as appropriate. In one embodiment, the electrolyte may contain at least one selected from the group consisting of VC, FEC, and LiBOB in an amount of 0.1 to 3.0% by mass, or 0.5 to 2.0% by mass, as an additive to improve cycle characteristics.

[0038] [Lithium-ion rechargeable battery] The lithium-ion secondary battery according to this embodiment includes a positive electrode, a negative electrode, and a polymer gel electrolyte obtained by crosslinking an acrylic copolymer using the above-mentioned electrolyte.

[0039] The polymer gel electrolyte comprises an aprotic solvent, a supporting salt, and a crosslinked body obtained by crosslinking the above-mentioned acrylic copolymer. The crosslinked body is obtained by ring-opening polymerization of the oxetanyl groups of the acrylic copolymer using a cationic polymerization initiator. While generally known polymerization initiators can be used as cationic polymerization initiators, it is preferable to use lithium salts and trace amounts of acidic substances obtained by hydrolysis of the anionic components of lithium salts contained in the electrolyte as supporting salts, as this minimizes the impact on the battery's properties.

[0040] Polymer gel electrolytes are electrolytes in which the above-mentioned electrolyte has become gel-like due to crosslinking of acrylic copolymers. The term "gel-like" here includes not only ordinary gels but also states similar to those of electrolytes referred to as gel-like electrolytes in the battery industry, and even states where the liquid has little to no fluidity or has stopped flowing, even if it is not a gel in the strict sense.

[0041] The concentration of the supporting salt in the polymer gel electrolyte is the same as the concentration of the supporting salt in the electrolyte described above. Furthermore, the content of the crosslinked acrylic copolymer in the polymer gel electrolyte is the same as the content of the acrylic copolymer in the electrolyte described above. The content of the silyl compound in the polymer gel electrolyte is the same as the content of the silyl compound in the electrolyte described above.

[0042] The positive electrode contains at least a positive electrode active material, but for example, a positive electrode mixture layer containing the positive electrode active material may be formed on one or both sides of a current collector made of metal such as aluminum foil. The positive electrode mixture layer can be formed by applying and drying a positive electrode mixture-containing paste to the current collector, and then compressing and molding it. The positive electrode mixture-containing paste can be obtained by dispersing and kneading the positive electrode active material together with a conductive additive such as carbon black or graphite, and a binder such as polyvinylidene fluoride (PVDF) in a dispersion medium such as N-methyl-2-pyrrolidone (NMP).

[0043] The positive electrode active material is not particularly limited, and for example, lithium-containing composite oxides such as LiCoO2, LiNiO2, and LiMn2O4 can be used. Furthermore, lithium-containing composite oxides in which the transition metal portion of these lithium-containing composite oxides is replaced with other elements can also be used. These can be used individually or in combination of two or more.

[0044] The negative electrode contains at least a negative electrode active material, but for example, a negative electrode mixture layer containing the negative electrode active material can be formed on one or both sides of a current collector made of metal such as copper foil. The negative electrode mixture layer can be formed, for example, by applying and drying a negative electrode mixture-containing paste to the current collector, and then compressing and molding it. The negative electrode mixture-containing paste can be obtained by dispersing and kneading the negative electrode active material together with a binder such as styrene-butadiene rubber in a dispersion medium such as water. The negative electrode mixture-containing paste may further contain a thickener such as carboxymethylcellulose salt or a conductive additive such as carbon black. The negative electrode mixture layer may also be formed by methods such as vapor deposition, CVD, or sputtering.

[0045] The negative electrode active material is not particularly limited, and one or more substances can be used, selected from lithium metal or lithium alloy, as well as materials capable of intercalating and releasing lithium. Specific examples of materials capable of intercalating and releasing lithium include carbon materials and oxides.

[0046] Lithium alloys are composed of lithium and metals that can form alloys with lithium. Specifically, examples include binary or trinary or more alloys of lithium with metals such as Al, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, and La.

[0047] Examples of carbon materials include graphite, amorphous carbon, diamond-like carbon, carbon nanotubes, and composite oxides thereof. Among these, graphite or amorphous carbon are preferred.

[0048] Examples of oxides that can be used include silicon oxide, tin oxide, indium oxide, zinc oxide, lithium oxide, and composites thereof. Among these, silicon oxide is preferred because it is stable and does not react with other compounds.

[0049] The method for manufacturing lithium-ion secondary batteries is not particularly limited and can be manufactured by known methods. For example, a positive electrode and a negative electrode can be stacked with a separator in between to form a laminate. Alternatively, the positive electrode and negative electrode can be flattened and wound with a separator in between, and then molded to form a wound body. Then, the laminate or wound body can be inserted into an outer casing such as a can or laminate material, and an electrolyte can be injected into the casing and cured (gelled) to obtain a lithium-ion secondary battery.

[0050] The curing process involves, for example, charging and heating after injecting the electrolyte. This allows the acrylic copolymer to be crosslinked using trace amounts of acidic substances obtained from the hydrolysis of lithium salts and their anionic components in the electrolyte, thereby forming a polymer gel electrolyte.

[0051] As separators, nonwoven fabrics, polyolefin microporous membranes, and other materials commonly used in lithium-ion secondary batteries can be used. Note that separators are not essential; the polymer gel electrolyte can also function as a separator.

[0052] In this embodiment, by adding the silyl compound to an electrolyte solution containing an acrylic copolymer together with a supporting salt in an aprotic solvent, it is possible to gel the electrolyte solution and obtain a polymer gel electrolyte while reducing the content of the acrylic copolymer. In other words, it is possible to form a polymer gel electrolyte by gelling the electrolyte solution while reducing the amount of acrylic copolymer in order to improve battery performance. [Examples]

[0053] The present invention will be described in more detail below with reference to examples, but it is not limited to the following examples.

[0054] (Gelling properties evaluation) Six mL of electrolyte was placed in a 20 mL screw-cap tube, sealed, and heated in a 60°C oven for 20 hours. Afterward, the screw-cap tube was tilted on its side, and its gelling ability was evaluated according to the following criteria. A score of 2 or higher indicates good gelling ability. 4. Even when tilted on its side, it remains at the bottom of the screw tube, resulting in virtually no shift in the liquid level. 3: Even when tilted on its side, it remains at the bottom of the screw tube, but the liquid level shifts slightly. 2: Even when tilted on its side, it remains at the bottom of the screw tube, but the liquid level shifts and tilts. 1: Although a gel-like substance is observed, it flows to the side of the screw tube when the tube is tilted on its side. 0: No gel-like substance is observed, and it flows to the side of the screw tube when tilted on its side.

[0055] [Synthesis Example 1 (Synthesis of Acrylic Copolymers)] In a thoroughly dried 3000 mL separable flask, 132.0 g of methyl methacrylate, 44.0 g of (3-ethyl-3-oxetanyl)methyl methacrylate, and 704.0 g of ethylene carbonate (EC) were added and stirred for 90 minutes at 70°C with nitrogen bubbling. Then, a solution of 2.61 g of diethyl carbonate (DEC) in which 0.29 g of 2,2'-azobis(isobutyronitrile) (AIBN) was dissolved was added to initiate the reaction. After 3 and 6 hours of further reaction, a solution of 0.79 g of DEC in which 0.09 g of AIBN was added, and heating and stirring were continued for a total of 9 hours. Finally, the solution was diluted with 28.8 g of EC and 1277.4 g of DEC. By drying this solution through a molecular sieve, the acrylic copolymer represented by formula (4) above was obtained as an 8% by mass solution.

[0056] [Example 1] In a glove box filled with dry nitrogen gas, an 8% by mass solution of the acrylic copolymer obtained in Synthesis Example 1 was mixed and dissolved in predetermined amounts of tris(trimethylsilyl) phosphate (TMSPa), lithium hexafluorophosphate (LiPF6), EC, and DEC to prepare a solution with a solvent composition of EC / DEC = 3 / 7 (volume ratio), a LiPF6 concentration of 1 mol / L, an acrylic copolymer concentration of 1.6% by mass, and a TMSPa concentration of 0.2% by mass, thereby obtaining the electrolyte of Example 1.

[0057] [Examples 2-12] The concentrations of the acrylic copolymer and TMSPa were changed as shown in Table 1 below, and the electrolytes for Examples 2 to 12 were obtained in the same manner as in Example 1.

[0058] [Comparative Example 1] The electrolyte for Comparative Example 1 was obtained by omitting the addition of TMSPa and otherwise proceeding in the same manner as in Example 1.

[0059] [Comparative Examples 2-4] Instead of TMSPa, comparative example 2 used vinylene carbonate (VC), comparative example 3 used fluoroethylene carbonate (FEC), and comparative example 4 used lithium bis(oxalate)borate (LiBOB), each added to a concentration of 1.0% by mass. The electrolytes for comparative examples 2 to 4 were obtained in the same manner as in Example 1.

[0060] [Examples 13-15] TMSPa was added to a concentration of 0.2% by mass, and in Example 13, VC was added to a concentration of 1.0% by mass, in Example 14, FEC was added to a concentration of 1.0% by mass, and in Example 15, LiBOB was added to a concentration of 1.0% by mass. Otherwise, the electrolytes for Examples 13 to 15 were obtained in the same manner as in Example 1.

[0061] [Synthesis Example 2 (Synthesis of Acrylic Copolymers)] In a thoroughly dried 3000 mL separable flask, 132.0 g of methyl methacrylate, 44.0 g of (3-ethyl-3-oxetanyl)methyl methacrylate, and 4.0 g of EC70 were added. The mixture was stirred at 70°C for 90 minutes while bubbling with nitrogen, and then a solution of 2.61 g of EMC in which 0.29 g of AIBN was dissolved was added to initiate the reaction. After 3 and 6 hours of further reaction, a solution of 0.79 g of EMC in which 0.09 g of AIBN was added, and heating and stirring were continued for a total of 9 hours. The mixture was then diluted with 22.7 g of EC and 3.9 g of EMC129. By drying this solution through a molecular sieve, an acrylic copolymer represented by formula (4) was obtained as an 8% by mass solution.

[0062] [Example 16] In a glove box filled with dry nitrogen gas, an 8% by mass solution of the acrylic copolymer obtained in Synthesis Example 2 was mixed and dissolved in predetermined amounts of TMSPa, LiPF6, EC, and EMC to prepare a solution with a solvent composition of EC / EMC = 3 / 7 (volume ratio), a LiPF6 concentration of 1 mol / L, an acrylic copolymer concentration of 1.6% by mass, and a TMSPa concentration of 0.2% by mass, thereby obtaining the electrolyte of Example 16. [Comparative Example 5] The electrolyte for Comparative Example 5 was obtained by omitting the addition of TMSPa and otherwise proceeding in the same manner as in Example 16.

[0063] [Comparative Example 6] The EC / DEC volume ratio was changed to 4 / 6, and the rest of the procedure was the same as in Comparative Example 1 to obtain the electrolyte of Comparative Example 6.

[0064] [Example 17] The EC / DEC volume ratio was changed to 4 / 6, and the rest of the procedure was the same as in Example 1 to obtain the electrolyte of Example 17.

[0065] [Examples 18-21] Instead of TMSPa, tris(trimethylsilyl) phosphite (TMSPi) was added, and the concentration of TMSPi was as shown in Table 1 below. For Examples 18 and 19, the procedure was the same as in Example 1, and for Examples 20 and 21, the procedure was the same as in Example 16 to obtain the electrolytes of Examples 18 to 21.

[0066] The gelling properties of the electrolytes obtained in the examples and comparative examples were evaluated. The results are shown in Table 1 below.

[0067] [Table 1]

[0068] As shown in Table 1, in Comparative Examples 1-6, where the acrylic copolymer content was low and no silyl compound was included with the acrylic copolymer, the electrolyte could not be gelled. In contrast, in Examples 1-21, where a silyl compound was included with the acrylic copolymer, the electrolyte could be gelled despite the low acrylic copolymer content. Since a low acrylic copolymer content in the electrolyte is advantageous for improving ion conductivity and thus battery performance, it can be seen that the electrolytes in the examples can be gelled while improving battery performance.

[0069] Furthermore, the various numerical ranges described in this specification can be any combination of their upper and lower limits, and all such combinations are described herein as preferred numerical ranges. Also, the description of a numerical range as "X~Y" means X or greater and Y or less.

[0070] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their omissions, substitutions, and modifications are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

Claims

1. The material comprises an aprotic solvent, a supporting salt, an acrylic copolymer containing a constituent unit represented by the following general formula (1) and a constituent unit represented by the following general formula (2), and a silyl compound represented by the following general formula (3). 【Chemistry 1】 In general formulas (1) and (2), R １ and R ３ Each of these independently represents a hydrogen atom or a methyl group, R ２ R represents an alkyl group having 1 to 5 carbon atoms. ４ R represents an alkanediyl group with 1 to 5 carbon atoms. ５ represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. 【Chemistry 2】 In general formula (3), R ６ , R ７ and R ８ Each of these independently represents an alkyl group or hydrogen atom having 1 to 4 carbon atoms, A represents P, B, or S, s represents 2 or 3, and t represents an integer from 0 to 2. The content of the acrylic copolymer is 1.0% by mass or more and 2.6% by mass or less, and the content of the silyl compound is 0.05% by mass or more and 3.0% by mass or less. Electrolyte.

2. The electrolyte according to claim 1, wherein the silyl compound comprises at least one selected from the group consisting of tris(trialkylsilyl) phosphate, tris(trialkylsilyl) phosphite, tris(trialkylsilyl) borate, and bis(trialkylsilyl) sulfate.

3. The electrolyte according to claim 1, wherein the aprotic solvent comprises at least one selected from the group consisting of cyclic carbonates, linear carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, and linear ethers.

4. The supporting salt is LiPF ６ , LiBF ４ , LiAsF ６ , LiSbF ６ , LiN(SO ２ F) ２ , and LiN(CF ３ SO ２ ) ２ The electrolytic solution according to claim 1, comprising at least one selected from the group consisting of

5. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and a polymer gel electrolyte obtained by crosslinking the acrylic copolymer using the electrolyte described in any one of claims 1 to 4.

Images