Lithium-ion secondary battery

Abstract

This lithium-ion secondary battery comprises: a positive electrode having a positive electrode mixture layer and a positive electrode current collector; a negative electrode having a negative electrode mixture layer and a negative electrode current collector; and a nonaqueous electrolytic solution. The positive electrode mixture layer contains a positive electrode active material represented by formula (1): Li1+xMn1-y-zFeyAzPO4. The nonaqueous electrolytic solution contains a nonaqueous solvent, hydrogen fluoride, and an electrolyte represented by formula (2): LiN(X1SO2)(X2SO2). The content of the hydrogen fluoride with respect to 100 mass% of the total amount of the nonaqueous electrolytic solution is 5-1000 mass ppm.

Description

Lithium-ion rechargeable battery

[0001] This disclosure relates to lithium-ion secondary batteries.

[0002] Various studies have been conducted to improve the performance of lithium-ion secondary batteries. Recently, lithium-ion secondary batteries equipped with a positive electrode containing manganese iron lithium phosphate as the positive electrode active material have been proposed (for example, Patent Document 1). While such lithium-ion secondary batteries are expected to have high safety and cycle life, it is known that they have the problem of reduced battery performance due to metal leaching from the positive electrode into the non-aqueous electrolyte.

[0003] Special table 2024-515150 publication

[0004] Patent Document 1 states that by using a non-aqueous electrolyte containing a sulfonylimide compound such as lithium bis(fluorosulfonyl)imide, the elution of metals (Mn and Fe) from the positive electrode is suppressed, improving high-temperature cycling and high-temperature storage performance. However, there is room for improvement in this suppression effect of the non-aqueous electrolyte.

[0005] This disclosure has been made in view of the above, and its purpose is to improve battery performance by suppressing the increase in DCR during high-temperature storage and high-temperature cycling in a lithium-ion secondary battery having a positive electrode containing manganese iron lithium phosphate.

[0006] In other words, this disclosure provides the following lithium-ion secondary battery [1]: [1] A lithium-ion secondary battery comprising: a positive electrode having a positive electrode mixture layer and a positive electrode current collector; a negative electrode having a negative electrode mixture layer and a negative electrode current collector; and a non-aqueous electrolyte, wherein the positive electrode mixture layer is based on the following formula (1): Li １＋ｘ Mn １－ｙ－ｚ Fe ｙ A ｚ PO ４... (1) [In formula (1), -0.1 < x < 0.1, 0 < y < 1.0, 0 ≦ z < 0.1, 0 < 1 - y - z, and A is at least one atom selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge.] The positive electrode active material represented by is included,... The non-aqueous electrolyte is,... The following formula (2): LiN(X １ SO ２ )(X ２ SO ２ )... (2) [In formula (2), X １ and X ２ are the same or different and represent a fluorine atom, an alkyl group having 碳原子数1 to 6, or a fluoroalkyl group having 碳原子数1 to 6.] An electrolyte represented by, a non-aqueous solvent, and hydrogen fluoride are included, and a lithium ion secondary battery characterized in that the content of hydrogen fluoride with respect to 100% by mass of the total amount of the non-aqueous electrolyte is 5 ppm or more and 1000 ppm or less.

[0007] Further, the present disclosure also provides the following lithium ion secondary batteries [2] to [5]. [2] The positive electrode active material represented by the formula (1) is the following formula (1-1): Li １＋ｘ Mn １－ｙ－ｚ Fe ｙ A ｚ PO ４ ... (1-1) [In formula (1-1), -0.1 < x < 0.1, 0 < y < 1.0, 0 ≦ z < 0.1, 0 < 1 - y - z, and A is at least one atom selected from the group consisting of Al, Mg, Mo, W, Ti, V, Ni, Sn, Nb, and Ge.] The lithium ion secondary battery according to [1], which is a positive electrode active material represented by. [3] The non-aqueous electrolyte is the following formula (3): LiPF ａ (C ｍ F ２ｍ＋１ ) ６－ａ... (3) A lithium-ion secondary battery according to [1] or [2], comprising an electrolyte represented by formula (3) [wherein 0 ≤ a ≤ 6 and 1 ≤ m ≤ 4]. [4] A lithium-ion secondary battery according to any one of [1] to [3], wherein the non-aqueous electrolyte comprises at least one selected from the group consisting of cyclic carbonate solvents, linear carbonate solvents, cyclic ether solvents, linear ether solvents, lactone solvents, ester solvents and nitrile solvents. [5] A lithium-ion secondary battery according to any one of [1] to [4], wherein the non-aqueous electrolyte comprises at least 90% by volume of at least one selected from the group consisting of non-fluorinated saturated cyclic carbonate solvents, non-fluorinated saturated linear carbonate solvents and linear ester solvents in total, based on 100% by volume of the total amount of the non-aqueous solvent.

[0008] According to this disclosure, in a lithium-ion secondary battery having a positive electrode containing manganese iron lithium phosphate, it is possible to suppress the increase in DCR during high-temperature storage and high-temperature cycling, thereby improving battery performance.

[0009] The following describes these embodiments in detail. The following description of preferred embodiments is essentially illustrative and is not intended to limit the present invention, its applications, or its uses. The upper and lower limits of the various numerical values ​​shown below can be combined as appropriate to form a specific numerical range.

[0010] [Lithium-ion secondary battery] The lithium-ion secondary battery according to this embodiment comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0011] <Positive Electrode> The positive electrode has a positive electrode mixture layer and a positive electrode current collector. The positive electrode is formed by having the positive electrode mixture layer on the positive electrode current collector. The positive electrode mixture layer is usually in the form of a sheet.

[0012] (Positive electrode mixture layer) The positive electrode mixture layer contains positive electrode active material, conductive additive, binder, etc.

[0013] [Positive electrode active material] The positive electrode mixture layer is composed of formula (1): [Chemical formula 1] Li １＋ｘ Mn １－ｙ－ｚ Fe ｙ A ｚ PO ４...contains a manganese iron lithium phosphate-based cathode active material represented by (1). In the following description, the cathode active material represented by formula (1) will be referred to as LMFP-based cathode active material (1).

[0014] In formula (1), "1 + x" represents the molar ratio of Li, where -0.1 < x < 0.1. "y" represents the molar ratio of Fe, where 0 < y < 1.0. "z" represents the molar ratio of A, described below, where 0 ≤ z < 0.1. "1 - y - z" represents the molar ratio of Mn, where 0 < 1 - y - z, and more precisely, 0 < 1 - y - z < 1.0. "A" is at least one atom selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge. In formula (1), "x", "y", and "z" can be adjusted as appropriate within the range of the above molar ratios. The molar ratio of Mn may preferably be 0.1 ≤ 1 - y - z < 1.00 or 0.25 ≤ 1 - y - z < 1.00.

[0015] A specific example of an LMFP-based cathode active material (1) is LiMn ０．００５ Fe ０．９９５ PO ４ LiMn ０．１ Fe ０．９ PO ４ LiMn ０．３ Fe ０．７ PO ４ LiMn ０．７ Fe ０．３ PO ４ LiMn ０．８５ Fe ０．１５ PO ４ LiMn ０．７ Fe ０．２９５ V ０．００５ PO ４ LiMn ０．６ Fe ０．３９３ V ０．００４ Co ０．００３ PO ４ LiMn ０．６５ Fe ０．３４１ V ０．００４ Co ０．００５ PO ４ LiMn ０．７ Fe ０．２９３ V ０．００４ Co ０．００５ PO ４ LiMn０．６ Fe ０．３９３ V ０．００４ Mg ０．００３ PO ４ LiMn ０．６ Fe ０．３９３ V ０．００４ Ni ０．００３ PO ４ Examples include the above. The LMFP-based cathode active material (1) may be a commercially available product or one synthesized by a conventionally known method.

[0016] The LMFP-based positive electrode active material (1) is given by formula (1-1): [Chemical Formula 2] Li １＋ｘ Mn １－ｙ－ｚ Fe ｙ A ｚ PO ４ ... Preferably, the cathode active material is a manganese iron lithium phosphate-based material represented by (1-1).

[0017] In equation (1-1), "1 + x" represents the molar ratio of Li, where -0.1 < x < 0.1. "y" represents the molar ratio of Fe, where 0 < y < 1.0. "z" represents the molar ratio of A, described below, where 0 ≤ z < 0.1. "1 - y - z" represents the molar ratio of Mn, where 0 < 1 - y - z, and more precisely, 0 < 1 - y - z < 1.0. "A" is at least one atom selected from the group consisting of Al, Mg, Mo, W, Ti, V, Ni, Sn, Nb, and Ge. The values ​​of "x", "y", and "z" in equation (1-1) can be adjusted as appropriate within the ranges of the above molar ratios. The molar ratio of Mn may preferably be 0.1 ≤ 1 - y - z < 1.00 or 0.25 ≤ 1 - y - z < 1.00.

[0018] The positive electrode mixture layer may, if necessary, contain other positive electrode active materials different from the LMFP-based positive electrode active material (1). The other positive electrode active material is not particularly limited as long as it can be used as a positive electrode active material for lithium-ion secondary batteries, but examples include lithium cobalt oxide; lithium nickel oxide; LiNi １／３ Co １／３ Mn １／３ O ２ (NCM111), LiNi ０．５ Co ０．２ Mn ０．３ O ２(NCM523), LiNi ０．６ Co ０．２ Mn ０．２ O ２ (NCM622), LiNi ０．８ Co ０．１ Mn ０．１ O ２ Ternary cathode active materials having a layered rock salt structure such as (NCM811); LiAPO ４ Phosphate-based cathode active materials having an olivine structure such as LiA PO (A: Ni, Mn, Co); LiNi ｐ Mn １－ｐ O ２ (0.5 ≤ p ≤ 1); Li ２ NiPO ４ Cathode active materials having a fluorinated olivine structure such as LiF etc.; LiFePO ４ Phosphate iron-based cathode active materials having an olivine structure such as etc.; Solid solution materials incorporating multiple transition metals (layered Li ２ MnO ３ and layered LiMO that is electrochemically active ２ (M = transition metals such as Co, Ni) solid solution); LiCo ｘ Mn １－ｑ O ２ (0 ≤ q ≤ 1); Li ２ APO ４ Compounds having a fluorinated olivine structure such as LiF (A: Fe, Mn, Co); LiMn ２．０ O ４ , LiNi ０．５ Mn １．５ O ４ Cathode active materials having a spinel structure such as etc.; Sulfur etc. can be used. Other cathode active materials may be used alone or in combination of two or more kinds.

[0019] The content of positive electrode active material in the positive electrode mixture layer is, for example, 75% to 99% by mass, but may also be 80% to 98% by mass, or 85% to 95% by mass, from the viewpoint of improving the output characteristics and electrical characteristics of the battery. If the positive electrode mixture layer contains only one type of positive electrode active material, the content of positive electrode active material refers to the content of that specific positive electrode active material. If the positive electrode mixture layer contains multiple positive electrode active materials, the content of positive electrode active material refers to the sum of the individual content of all positive electrode active materials contained in the positive electrode mixture layer. The content of conductive additives and binders, etc., described later, is defined similarly.

[0020] [Conductive Additives] Conductive additives mainly consist of conductive carbon. Examples of conductive carbon include carbon black, carbon fiber, and graphite. Conductive additives may be used individually or in combination of two or more types. Among the conductive additives, carbon black is preferred. Examples of carbon black include Ketjen black and acetylene black. The content of the conductive additive in the positive electrode mixture layer is, for example, 1% by mass or more and 20% by mass or less, and may also be 1.5% by mass or more and 10% by mass or less, from the viewpoint of improving the output characteristics and electrical characteristics of the battery.

[0021] [Binding Agents] Examples of binding agents include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber (SBR) and nitrile butadiene rubber; polyamide resins such as polyamide-imide; polyolefin resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; and cellulose resins such as carboxymethylcellulose (CMC). Each binding agent may be used individually, or two or more may be used in combination. Furthermore, the binding agent may be dissolved in the solvent or dispersed in the solvent at the time of use. The binding agent content in the positive electrode mixture layer is, for example, 1% to 20% by mass, or 1.5% to 10% by mass, from the viewpoint of improving the output characteristics and electrical characteristics of the battery.

[0022] [Other Components] The positive electrode mixture layer may contain, as necessary, other components such as polymers such as non-fluorinated polymers like (meth)acrylic polymers, nitrile polymers, and diene polymers, and fluorinated polymers like polytetrafluoroethylene; emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymeric dispersants like styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethylcellulose (CMC), hydroxyethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers; and preservatives. The content of other components in the positive electrode mixture layer may be, for example, 0% by mass or more and 15% by mass or less, or 0% by mass or more and 10% by mass or less.

[0023] (Positive electrode current collector) Examples of metals that can be used for the positive electrode current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. Among these, aluminum is preferred. The shape and dimensions of the positive electrode current collector are not particularly limited.

[0024] <Method for preparing the positive electrode> The positive electrode mixture layer can be prepared from the positive electrode mixture. The positive electrode mixture contains the positive electrode active material, conductive additive, binder, etc., as described above, and a solvent for dispersing these components. The positive electrode mixture can be prepared, for example, by mixing each component and dispersing them using a bead mill, ball mill, agitator, or the like.

[0025] Examples of solvents include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetonitrile, acetone, ethanol, ethyl acetate, and water. Each solvent may be used individually, or two or more may be used in combination. The amount of solvent used is not particularly limited and should be determined appropriately depending on the manufacturing method and the materials used.

[0026] The method for manufacturing the positive electrode is not particularly limited, and examples include (A) applying the positive electrode mixture to the positive electrode current collector and then drying it; (B) immersing the positive electrode current collector in the positive electrode mixture and then drying it; and (C) joining a sheet formed from the positive electrode mixture to the positive electrode current collector, pressing it, and then drying it.

[0027] <Negative Electrode> The negative electrode has a negative electrode mixture layer and a negative electrode current collector. The negative electrode is formed by the negative electrode mixture layer being formed on the negative electrode current collector. The negative electrode mixture layer is usually in the form of a sheet.

[0028] (Negative electrode mixture layer) The negative electrode mixture layer contains negative electrode active material, conductive additive, binder, etc. The preferred content of each component is the same as that of the positive electrode.

[0029] [Negative electrode active material] Specific examples of negative electrode active materials include graphite materials such as artificial graphite and natural graphite; carbon materials such as mesophase calcined bodies made from coal and petroleum pitch, and non-graphitizable carbon; Si, Si alloys, SiO ｘ Si-based anode materials such as Sn alloys; lithium metals; lithium alloys such as lithium-aluminum alloys; composite materials of graphite and Si-based anode materials (hereinafter also referred to as "Si-containing graphite") can be used. Specific examples of Si-containing graphite include those with a mass ratio (Si:C) of Si-based anode material to graphite of 3:97 or more and 50:50 or less. The anode active materials may be used individually or in combination of two or more types.

[0030] (Negative electrode current collector) Examples of metals that can be used for the negative electrode current collector include iron, copper, aluminum, nickel, stainless steel (SUS), titanium, tantalum, gold, and platinum. Of these, copper is preferred. The shape and dimensions of the negative electrode current collector are not particularly limited.

[0031] <Method for manufacturing the negative electrode> The method for manufacturing the negative electrode may be the same as the method for manufacturing the positive electrode.

[0032] <Non-aqueous electrolyte> A non-aqueous electrolyte contains an electrolyte, a non-aqueous solvent, and a predetermined amount of hydrogen fluoride (HF).

[0033] (Electrolyte) The non-aqueous electrolyte is given by formula (2): [Chemical formula 3] LiN(X １ SO ２ ) (X ２ SO ２ ) ... (2) contains an electrolyte represented by (2) (hereinafter also referred to as "sulfonylimide compound (2)").

[0034] In equation (2), X １ and X ２ These represent, either identically or distinctly, a fluorine atom (F), a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group.

[0035] In formula (2), examples of C1-C6 alkyl groups include linear or branched alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, pentyl, and hexyl groups. Among C1-C6 alkyl groups, linear alkyl groups are preferred.

[0036] In formula (2), the carbon-1 to carbon-6 fluoroalkyl group can be any alkyl group in which some or all of the hydrogen atoms are substituted with fluorine atoms. Examples of carbon-1 to carbon-6 fluoroalkyl groups include fluoromethyl, difluoromethyl, trifluoromethyl, fluoroethyl, difluoroethyl, trifluoroethyl, and pentafluoroethyl groups. The fluoroalkyl group may also be a perfluoroalkyl group in which all of the hydrogen atoms of the alkyl group have carbon-1 to carbon-6 are substituted with fluorine atoms.

[0037] In equation (2), X １ and X ２ Preferably, the components are a fluorine atom and a perfluoroalkyl group; more preferably, a fluorine atom, a trifluoromethyl group, and a pentafluoroethyl group; even more preferably, a fluorine atom and a trifluoromethyl group; and particularly preferably, a fluorine atom.

[0038] Examples of sulfonyliimide compounds (2) include lithium bis(fluorosulfonyl)imide (LiN(FSO) ２ ) ２, hereafter also referred to as "LiFSI"), lithium bis(trifluoromethylsulfonyl)imide (LiN(CF ３ SO ２ ) ２ Examples include lithium(fluorosulfonyl)(methylsulfonyl)imide, lithium(fluorosulfonyl)(ethylsulfonyl)imide, lithium(fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium(fluorosulfonyl)(pentafluoroethylsulfonyl)imide, lithium(fluorosulfonyl)(heptafluoropropylsulfonyl)imide, lithiumbis(pentafluoroethylsulfonyl)imide, lithiumbis(heptafluoropropylsulfonyl)imide, etc. Each sulfonylimide compound (2) may be used individually or in combination of two or more types. The sulfonylimide compound (2) may be a commercially available product or one obtained by synthesis using a conventionally known method.

[0039] Non-aqueous electrolytes are used to improve battery performance, such as LiN(FSO). ２ ) ２ and LiN(CF ３ SO ２ ) ２ Preferably, it contains at least one selected from the group consisting of LiN(FSO) ２ ) ２ It is more preferable to include it.

[0040] The non-aqueous electrolyte may, if necessary, contain other electrolytes different from sulfonylimide compound (2). The other electrolytes are not particularly limited as long as they can be used as electrolytes in lithium-ion secondary batteries, but may include the electrolyte represented by formula (3) (hereinafter also referred to as "fluorophosphate compound (3)"), the electrolyte represented by formula (4) (hereinafter also referred to as "fluoroboric acid compound (4)"), and LiAsF ６ LiSbF ６ LiClO ４ , LiSCN, LiAlF ４ CF ３ SO ３ Li, LiC [(CF ３ SO ２ ) ３], LiN (NO ２ ), LiN[(CN) ２ Examples include: [Chemical Formula 4] LiPF ａ (C ｍ F ２ｍ＋１ ) ６－ａ ... (3) In equation (3), 0 ≤ a ≤ 6 and 1 ≤ m ≤ 4. The fluorophosphate compound (3) is LiPF ６ LiPF ３ (CF ３ ) ３ LiPF ３ (C ２ F ５ ) ３ LiPF ３ (C ３ F ７ ) ３ LiPF ３ (C ４ F ９ ) ３ These are some examples. Among the fluorophosphate compounds (3), LiPF ６ This is preferable. [Chemical Formula 5] LiBF ｂ (C ｎ F ２ｎ+１ ) ４－ｂ ... (4) In equation (4), 0 ≤ b ≤ 4 and 1 ≤ n ≤ 4. The fluoroboric acid compound (4) is LiBF ４ LiBF (CF ３ ) ３ LiBF(C ２ F ５ ) ３ LiBF(C ３ F ７ ) ３ These are some examples. Among the fluoroboric acid compounds (4), LiBF ４ It is preferable.

[0041] Among other electrolytes, fluorophosphate compounds (3), fluoroboric acid compounds (4), and LiAsF are selected from the perspective of ionic conductivity and cost. ６ Preferably, fluorophosphate compound (3) is more preferred, LiPF ６ That is even more preferable.

[0042] Therefore, the non-aqueous electrolyte consists of a sulfonylime compound (2), a fluorophosphate compound (3), a fluoroboric acid compound (4), and LiAsF ６ It may also include at least one selected from the group consisting of LiN(FSO ２ ) ２ and LiN(CF ３ SO ２ ) ２ At least one selected from the group consisting of the following, and a fluorophosphate compound (3), a fluoroboric acid compound (4), and LiAsF ６ It can be said that it may include at least one selected from the group consisting of the following.

[0043] Furthermore, the non-aqueous electrolyte may also contain a sulfonylime compound (2) and a fluorophosphate compound (3), such as LiN(FSO). ２ ) ２ and LiN(CF ３ SO ２ ) ２ It can be said that it may include at least one selected from the group consisting of and a fluorophosphate compound (3).

[0044] Furthermore, the non-aqueous electrolyte is a sulfonylime compound (2) and LiPF ６ It may also include LiN(FSO ２ ) ２ and LiN(CF ３ SO ２ ) ２ At least one selected from the group consisting of and LiPF ６ It can be said that it may also contain. Furthermore, the non-aqueous electrolyte is LiN(FSO). ２ ) ２ and LiPF ６ It can be said that it may include and.

[0045] Furthermore, the sulfonylimide compound (2) and other electrolytes may exist in ionic form.

[0046] The content of sulfonylimide compound (2) in the non-aqueous electrolyte is, for example, 0.01 mol / L to 5.0 mol / L, from the viewpoint of improving battery performance. This content may also be 0.05 mol / L to 3.0 mol / L, 0.1 mol / L to 2.0 mol / L, 0.2 mol / L to 1.5 mol / L, or 0.5 mol / L to 1.0 mol / L. By doing so, it is possible to suppress the increase in DCR during high-temperature storage and high-temperature cycling while suppressing the decrease in battery performance due to the increase in electrolyte viscosity. Note that if the non-aqueous electrolyte contains only one type of sulfonylimide compound (2), the content of sulfonylimide compound (2) refers to the content of that sulfonylimide compound (2). If the non-aqueous electrolyte contains multiple sulfonylimide compounds (2), the content of sulfonylimide compounds (2) refers to the sum of the individual contents of all sulfonylimide compounds (2) contained in the non-aqueous electrolyte.

[0047] The content of sulfonilimide compound (2) relative to 100% by mass of the total amount of non-aqueous electrolyte is, for example, 0.1% by mass or more and 80% by mass or less, from the viewpoint of improving battery performance. This content may also be 0.5% by mass or more and 50% by mass or less, 1% by mass or more and 30% by mass or less, 3% by mass or more and 20% by mass or less, or 5% by mass or more and 10% by mass or less. By doing so, it is possible to suppress the increase in DCR during high-temperature storage and high-temperature cycling, while suppressing the decrease in battery performance due to the increase in electrolyte viscosity.

[0048] The content of sulfonylimide compound (2) relative to 100 mol% of the total amount of electrolytes in the non-aqueous electrolyte is, for example, 1 mol% to 100 mol% from the viewpoint of improving battery performance. This content may be 5 mol% to 95 mol%, 15 mol% to 90 mol%, 20 mol% to 85 mol%, 25 mol% to 75 mol%, 30 mol% to 70 mol%, or 35 mol% to 65 mol%. By doing so, it is possible to suppress the increase in DCR during high-temperature storage and high-temperature cycling, while suppressing the decrease in battery performance due to the increase in electrolyte viscosity. Note that the content of sulfonylimide compound (2) relative to 100 mol% of the total amount of sulfonylimide compound (2) and other electrolytes in the non-aqueous electrolyte is 100 mol%, which means that the non-aqueous electrolyte does not contain electrolytes other than sulfonylimide compound (2).

[0049] (Non-aqueous solvent) The non-aqueous electrolyte contains a non-aqueous solvent. Preferably, the non-aqueous electrolyte contains at least one non-aqueous solvent (5) selected from the group consisting of cyclic carbonate solvents, linear carbonate solvents, cyclic ether solvents, linear ether solvents, lactone solvents, ester solvents, and nitrile solvents. The non-aqueous solvents (5) may be used individually or in combination of two or more types.

[0050] Examples of cyclic carbonate solvents include non-fluorinated saturated cyclic carbonate solvents, non-fluorinated unsaturated cyclic carbonate solvents, and fluorinated cyclic carbonate solvents. Specific examples of non-fluorinated saturated cyclic carbonate solvents include ethylene carbonate, propylene carbonate, 2,3-dimethylethylene carbonate, and 1,2-butylene carbonate. ２－６Examples include alkylene carbonates and erythritol carbonate. Specific examples of non-fluorinated unsaturated cyclic carbonate solvents include vinylene carbonate, methylvinylene carbonate, ethylvinylene carbonate, 2-vinylethylene carbonate, and phenylethylene carbonate, which are cyclic carbonates having unsaturated bonds. Specific examples of fluorinated cyclic carbonate solvents include fluoroethylene carbonate, 4,5-difluoroethylene carbonate, and trifluoropropylene carbonate. Among cyclic carbonate solvents, non-fluorinated saturated cyclic carbonate solvents are preferred, and ethylene carbonate is more preferred.

[0051] Examples of linear carbonate solvents include diC １－４ Alkyl carbonate solvent, C １－４ Examples include alkylphenyl carbonate solvents and diaryl carbonate solvents. １－４ Specific examples of alkyl carbonate solvents include non-fluorinated saturated chain carbonate solvents such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. １－４ Specific examples of alkylphenyl carbonate solvents include methylphenyl carbonate. Specific examples of diaryl carbonate solvents include diphenyl carbonate. Among chain-like carbonate solvents, diC １－４ Alkyl carbonate solvents are preferred, non-fluorinated saturated chain carbonate solvents are more preferred, dimethyl carbonate and ethyl methyl carbonate are even more preferred, and ethyl methyl carbonate is particularly preferred.

[0052] Examples of cyclic ether solvents include tetrahydrofuran solvents, tetrahydropyran solvents, dioxane solvents, dioxolane solvents, and crown ethers. Specific examples of tetrahydrofuran solvents include tetrahydrofuran, 2-methyltetrahydrofuran, and 2,6-dimethyltetrahydrofuran. Specific examples of tetrahydropyran solvents include tetrahydropyran. Specific examples of dioxane solvents include 1,4-dioxane. Specific examples of dioxolane solvents include 1,3-dioxolane.

[0053] Examples of linear ether solvents include alkanediol dialkyl ether solvents and polyalkanediol dialkyl ether solvents. Specific examples of alkanediol dialkyl ether solvents include ethylene glycol dimethyl ether and ethylene glycol diethyl ether. Specific examples of polyalkanediol dialkyl ether solvents include triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0054] Specific examples of lactone-based solvents include cyclic ester solvents such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone.

[0055] Examples of ester solvents include linear ester solvents and aromatic carboxylic acid ester solvents. Specific examples of linear ester solvents include ethyl acetate, butyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Specific examples of aromatic carboxylic acid ester solvents include methyl benzoate and ethyl benzoate. Among ester solvents, linear ester solvents are preferred, and ethyl propionate is more preferred.

[0056] Examples of nitrile solvents include aliphatic nitrile solvents and aromatic nitrile solvents. Specific examples of aliphatic nitrile solvents include acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, valeronitrile, butyronitrile, and isobutyronitrile. Specific examples of aromatic nitrile solvents include benzonitrile and tolunitrile.

[0057] Among the non-aqueous solvents (5), cyclic carbonate solvents, linear carbonate solvents, and ester solvents are preferred, non-fluorinated saturated cyclic carbonate solvents, non-fluorinated saturated linear carbonate solvents, and linear ester solvents are more preferred, and ethylene carbonate, ethyl methyl carbonate, and ethyl propionate are even more preferred. Among these, mixed solvents containing non-fluorinated saturated cyclic carbonate solvents and non-fluorinated saturated linear carbonate solvents, as well as mixed solvents containing non-fluorinated saturated cyclic carbonate solvents and linear ester solvents, are preferred, as are mixed solvents containing ethylene carbonate and ethyl methyl carbonate, and mixed solvents containing ethylene carbonate and ethyl propionate. Therefore, the non-aqueous electrolyte preferably contains at least one non-aqueous solvent (5-1) selected from the group consisting of cyclic carbonate solvents, linear carbonate solvents, and ester solvents; more preferably contains at least one non-aqueous solvent (5-2) selected from the group consisting of non-fluorinated saturated cyclic carbonate solvents, non-fluorinated saturated linear carbonate solvents, and linear ester solvents; and even more preferably contains at least one non-aqueous solvent (5-3) selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, and ethyl propionate.

[0058] The non-aqueous electrolyte preferably contains, in a total of 90% by volume or more of at least one non-aqueous solvent (5-2) selected from the group consisting of non-fluorinated saturated cyclic carbonate solvents, non-fluorinated saturated linear carbonate solvents, and linear ester solvents, based on 100% by volume of the total amount of the non-aqueous solvent. This content may be 95% by volume or more, or 100% by volume. Note that a content of 100% by volume of non-aqueous solvent (5-2) relative to 100% by volume of the total amount of the non-aqueous solvent means that the non-aqueous solvent does not contain any non-aqueous solvent other than non-aqueous solvent (5-2).

[0059] The non-aqueous electrolyte may, if necessary, contain other non-aqueous solvents different from the non-aqueous solvent (5). Other non-aqueous solvents are not particularly limited as long as they can be used as non-aqueous solvents for lithium-ion secondary batteries, but examples include phosphate ester solvents, sulfur compound solvents, nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 3-methyl-2-oxazolidinone, etc. Specific examples of phosphate ester solvents include alkyl phosphates such as trimethyl phosphate, ethyldimethyl phosphate, diethylmethyl phosphate, and triethyl phosphate. Specific examples of sulfur compound solvents include sulfone solvents such as dimethyl sulfone, ethylmethyl sulfone, and diethyl sulfone; and sulfolane solvents such as sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane.

[0060] Non-aqueous electrolytes may be used as a medium such as a polymer or polymer gel in addition to the non-aqueous solvent mentioned above. When using a polymer or polymer gel instead of a non-aqueous solvent, the following methods may be employed: a method in which a solution of an electrolyte dissolved in a non-aqueous solvent is dropped onto a polymer that has been formed into a film by a conventionally known method, thereby impregnating and supporting the electrolyte and non-aqueous solvent; a method in which the polymer and electrolyte are melted and mixed at a temperature above the melting point of the polymer, a film is formed, and the non-aqueous solvent is impregnated therein (a gel electrolyte); a method in which a non-aqueous electrolyte, in which the electrolyte is dissolved in a non-aqueous solvent beforehand, is mixed with a polymer, a film is formed by a casting method or a coating method, and the non-aqueous solvent is volatilized; a method in which the polymer and electrolyte are melted and mixed at a temperature above the melting point of the polymer and then molded (an intrinsic polymer electrolyte).

[0061] Polymers that can be used in addition to non-aqueous solvents include polyether polymers such as polyethylene oxide (PEO) and polypropylene oxide, which are homopolymers or copolymers of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), fluorine polymers such as polyvinylidene fluoride (PVdF) and polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof. These polymers may be used individually or in combination of two or more types.

[0062] (Hydrogen Fluoride (HF)) The non-aqueous electrolyte contains a predetermined amount of hydrogen fluoride (HF). The HF content is 5 ppm by mass or more and 1000 ppm by mass or less, based on 100% by mass of the total amount of the non-aqueous electrolyte.

[0063] By adding a predetermined amount of HF to a non-aqueous electrolyte containing sulfonylimide compound (2), it is believed that a film consisting of inorganic components including fluorine (F) is formed on both the positive and negative electrode surfaces. As a result, performance degradation when the battery is stored at high temperatures is suppressed, and the cycle life characteristics when the battery is repeatedly charged and discharged at high temperatures are also improved. Although it is possible to form a film on the electrode surface using a non-aqueous electrolyte containing sulfonylimide compound (2), it is believed that by adding a predetermined amount of HF, film formation proceeds more rapidly, and a film containing more inorganic components is formed. The degree of effect of including HF is particularly noticeable in batteries equipped with an LMFP-type positive electrode active material (1). In other words, by combining the LMFP-type positive electrode active material (1), sulfonylimide compound (2), and a predetermined amount of HF, the effect of suppressing the rise in DCR during high-temperature storage and high-temperature cycling is enhanced.

[0064] The HF content in the non-aqueous electrolyte may be 10 ppm by mass or more and 900 ppm by mass or less. The content may also be 30 ppm by mass or more and 800 ppm by mass or less, 50 ppm by mass or more and 700 ppm by mass or less, 80 ppm by mass or more and 600 ppm by mass or less, 100 ppm by mass or more and 500 ppm by mass or less, or 200 ppm by mass or more and 400 ppm by mass or less. By doing so, it is possible to prevent corrosion of the current collector while suppressing the rise in DCR during high-temperature storage and high-temperature cycling.

[0065] One method for adjusting the HF content is, for example, the method described in the examples below. A LiFSI composition containing HF is prepared by reacting lithium fluoride (LiF) with bis(fluorosulfonyl)imide [HFSI]. Subsequently, the HF content in the LiFSI composition is analyzed. Finally, the LiFSI composition is used as sulfonyliimide compound (2) when preparing a non-aqueous electrolyte. At this time, the amount of LiFSI composition used should be appropriately determined so that the HF content relative to 100% by mass of the total amount of the non-aqueous electrolyte is 5 ppm by mass or more and 1000 ppm by mass or less. The LiFSI composition may be used alone as sulfonyliimide compound (2), or in combination with other sulfonyliimide compounds (2). The HF content can be measured by the method described in the examples.

[0066] (Additives) The non-aqueous electrolyte may further contain additives as needed. Examples of additives include succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride, and other carboxylic acid anhydrides; ethylene sulfite, 1,3-propanesultone, 1,4-butanesultone, 2,4-butanesultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, trimethyleneglycolate Sulfur-containing compounds such as cholsulfate; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, and cycloheptane; carbonate compounds such as vinylene carbonate, fluoroethylene carbonate (FEC), trifluoropropylene carbonate, phenylethylene carbonate, and erythritol carbonate; lithium fluorosulfonate (LiFSO4). ３ ), sodium fluorosulfonate (NaFSO ３ ), potassium fluorosulfonate (KFSO ３), magnesium fluorosulfonate [Mg(FSO ３ ) ２ Fluorosulfonic acid compounds such as ]; Lithium monofluorophosphate (Li ２ PO ３ F) Lithium difluorophosphate (LiPO ２ F ２ Fluorophosphate compounds such as ); fluorooxalate compounds such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium difluorooxalatophosphate (LIDFOP), lithium tetrafluorooxalatophosphate (LITFOP), lithium difluorobis(oxalato)phosphate (LiDFBOP), lithium tris(oxalato)phosphate, and lithium tris(oxalato)phosphate, which are lithium salts having an oxalate skeleton; amide sulfuric acid (sulfamic acid, H ３ NSO ３ ), amide sulfuric acid salts (alkali metal salts, alkaline earth metal salts, ammonium salts, guanidine salts, etc.), amide sulfuric acid derivatives, taurine [2-aminoethanesulfonic acid (aminoethylsulfonic acid), H ２ N-CH ２ -CH ２ -SO ３ Examples include amidosulfate compounds such as H. Each additive may be used individually, or two or more may be used in combination.

[0067] The additive content may be, for example, 0.1% to 10% by mass, 0.2% to 8% by mass, or 0.3% to 5% by mass, based on 100% by mass of the total amount of the non-aqueous electrolyte.

[0068] <Method for preparing a non-aqueous electrolyte> A non-aqueous electrolyte can be prepared by mixing two components, for example, a sulfonylimide compound (2) containing HF and a non-aqueous solvent. Further HF may be added to adjust the HF content. Other electrolytes and additives may be added as needed.

[0069] <Other components of lithium-ion secondary batteries> Lithium-ion secondary batteries may also include separators, battery casing materials, etc.

[0070] (Separator) The separator is positioned to separate the positive electrode and the negative electrode. There are no particular restrictions on the separator, and conventionally known separators can be used in this disclosure. Specific examples of separators include porous sheets made of polymers capable of absorbing and retaining non-aqueous electrolytes (e.g., polyolefin-based microporous separators and cellulose-based separators), nonwoven fabric separators, porous metal bodies, and the like.

[0071] (Battery casing material) A battery element, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte (and a separator), is usually housed in a battery casing material to protect it from external shocks, environmental degradation, etc., during battery use. The material of the battery casing material is not particularly limited, and any conventionally known casing material can be used. The battery casing material may include expanded metal, fuses, overcurrent protection elements such as PTC elements, lead plates, etc., as needed, to prevent pressure rise inside the battery and overcharging / discharging.

[0072] [Method for Manufacturing Lithium-Ion Secondary Batteries] Lithium-ion secondary batteries can be easily manufactured by the following procedure. For example, the positive electrode and the negative electrode are stacked (with a separator in place if necessary) to form a laminate. Next, the laminate is placed in a battery casing. Finally, a non-aqueous electrolyte is poured into the battery casing and sealed.

[0073] [Shape of Lithium-ion Secondary Batteries] The shape of lithium-ion secondary batteries is not particularly limited; any conventionally known battery shape, such as cylindrical, prismatic, laminated, coin-type, or large, can be used. Furthermore, when used as a high-voltage power source (several tens to several hundred volts) for electric vehicles, hybrid electric vehicles, etc., a battery module can be constructed by connecting individual batteries in series.

[0074] [Rated Charging Voltage of Lithium-ion Secondary Batteries] The rated charging voltage of lithium-ion secondary batteries is not particularly limited, but may be 3.6V or higher, preferably 4.0V or higher, more preferably 4.1V or higher, and even more preferably 4.2V or higher. A higher rated charging voltage can increase energy density, but from the viewpoint of battery safety, the rated charging voltage may be 4.6V or lower (for example, 4.5V or lower).

[0075] [Summary] The lithium-ion secondary battery according to this embodiment, configured as described above, comprises: a positive electrode having a positive electrode mixture layer containing an LMFP-based positive electrode active material (1) and a positive electrode current collector; a negative electrode having a negative electrode mixture layer and a negative electrode current collector; and a non-aqueous electrolyte containing a sulfonylimide compound (2), a non-aqueous solvent, and a predetermined amount of HF. By configuring the lithium-ion secondary battery in this way, film formation proceeds rapidly on both the positive and negative electrode surfaces, and an ideal film containing a larger amount of inorganic components such as fluorine (F) is formed. As a result, in a lithium-ion secondary battery using an LMFP-based positive electrode active material (1), it is possible to suppress the rise in DCR during high-temperature storage and high-temperature cycling processes.

[0076] The present disclosure will be described below based on examples. However, the present disclosure is not limited to the following examples, and the following examples can be modified or changed in accordance with the spirit of the present disclosure, and such modifications do not exclude them from the scope of the present disclosure.

[0077] <Example 1 Series> [Non-aqueous electrolyte] (LiFSI composition) 1.17 g (45 mmol) of lithium fluoride (LiF) was added to a PFA (fluoropolymer) reaction vessel. The reaction vessel was cooled with ice, and 9.59 g (53 mmol) of bis(fluorosulfonyl)imide [HFSI] was further added. The mixture in the reaction vessel was then heated to 120°C and the reaction was carried out for 1.5 hours. The reaction solution was deflorated under reduced pressure at 10 hPa and 140°C for 2 hours to obtain a composition containing LiFSI [sulfonylimide compound (2)]. The amount of LiFSI produced in the composition was as follows: １９ This was determined by F-NMR measurement. As a result, the amount of LiFSI produced was 7.30 g. １９ F-NMR] １９ F-NMR was measured using a Varian Unity Plus-400 (internal standard: benzenesulfonyl fluoride, number of cumulative measurements: 16).

[0078] (Analysis of HF content) First, the LiFSI composition was dissolved in super-dehydrated methanol, and the free acid concentration in the LiFSI composition was determined by neutralization titration using NaOH / super-dehydrated methanol solution. Next, 1,3-ditrifluoroethoxy-1,1,3,3-tetramethyldisiloxane was added to the LiFSI composition in an amount 50 times the obtained free acid concentration, and the mixture was stirred at room temperature for 2 hours. After that, the free acid concentration in the LiFSI composition was determined again by neutralization titration. Since this disiloxane compound is known to react selectively and rapidly with HF, the value obtained by subtracting the free acid concentration after addition from the free acid concentration before addition of the disiloxane compound was taken as the HF content in the LiFSI composition. The details of the neutralization titration are as follows: 10 g of the LiFSI composition was added to a polypropylene container dried in a dry air atmosphere with a dew point of -60°C, and 100 mL of super-dehydrated methanol was added. Neutralization titration was performed using a 0.01 M NaOH / super-dehydrated methanol solution with stirring using an automatic titrator (COM-1700A, manufactured by Hiranuma Sangyo Co., Ltd.). The volume of NaOH / super-dehydrated methanol solution required for titration before the addition of the disiloxane compound was A [mL], and the volume of NaOH / super-dehydrated methanol solution required for titration after the addition of the disiloxane compound was B [mL]. The hydrogen fluoride concentration in the LiFSI composition was calculated using the following formula [1]: [Formula 1] HF concentration = (A - B) × 0.01 × 20 × 1000 / 10 ... [1]. The HF content in the LiFSI composition measured in this manner was 4986 ppm by mass.

[0079] (Non-aqueous electrolyte) (Examples) Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (manufactured by Kishida Chemical Co., Ltd., the same applies hereinafter), a prepared LiFSI composition, commercially available LiFSI (manufactured by Nippon Shokubai Co., Ltd., the same applies hereinafter), and LiPF ６ (Manufactured by Kishida Chemical Co., Ltd., the same applies hereinafter) was mixed with the electrolytes shown in Table 1 to obtain the salt composition and HF content. As a result, LiFSI and LiPF were used as electrolytes. ６Non-aqueous electrolytes [non-aqueous solvent EC:EMC = 30:70 (volume ratio)] were obtained for each example, containing a predetermined molar concentration of and HF in an amount of 5 ppm by mass or more and 1000 ppm by mass or less. In these non-aqueous electrolytes, the electrolyte was dissolved, and no undissolved residue was observed visually. The HF content in the non-aqueous electrolyte was measured by the method described above (the same applies hereafter).

[0080] (Comparative Examples) Except for not mixing the above LiFSI composition as an electrolyte, or mixing it to the content shown in Table 1, non-aqueous electrolytes of each comparative example were obtained in the same manner as in each example, without LiFSI or without HF at a concentration of 5 ppm by mass or more.

[0081] [Positive electrode] (LMFP-based positive electrode) LiMn ０．８５ Fe ０．１５ PO ４ A positive electrode slurry was prepared by weighing [LMFP-based positive electrode active material (1), commercially available product], acetylene black [Denka Black (registered trademark) manufactured by Denka Co., Ltd.], and polyvinylidene fluoride resin (PVdF, manufactured by Kureha Corporation, product number: KF Polymer L#7208, solids content: 8% by weight) in a solids content ratio (mass ratio) of 100:9:6 and dispersing them in N-methylpyrrolidone (NMP) as a solvent. Subsequently, the positive electrode slurry was applied to one side of an aluminum foil (positive electrode current collector) (coating weight 20.2 mg / cm²). ２ The mixture was dried on a 130°C hot plate and then dried for 12 hours in a vacuum drying oven set to 130°C. The dried positive electrode was pressed with a roll press. As a result, a positive electrode mixture layer consisting of positive electrode slurry was formed on the positive electrode current collector, with a mixture density of 1.9 g / cm³. ３ We fabricated an LMFP-based cathode.

[0082] (NCM811 positive electrode) LiNi is a ternary positive electrode active material. ０．８ Co ０．１ Mn ０．１ O ２ (NCM811, manufactured by Beijing Dangsheng) was used, and the combined drug density was 13.8 mg / cm³. ２ The NCM811 positive electrode was fabricated using the same method as described above, except for the change made.

[0083] (NCM111 positive electrode) LiNi is a ternary positive electrode active material. １／３ Co １／３ Mn １／３ O ２ The use of (NCM111, manufactured by Yumicore, product code: MX7h) and a combination density of 18.9 mg / cm³ were confirmed. ２ The NCM111 positive electrode was fabricated using the same method as described above, except for the change made.

[0084] (LFP cathode) LiFePO4 is an iron phosphate-based cathode active material. ４ The LFP cathode was fabricated using the same method as for the LMFP cathode, except that a commercially available product was used.

[0085] [Negative electrode] A water-based slurry (negative electrode slurry) was prepared using the following materials in a mass ratio of 85:15:2:1:1: graphite (OMAC-R, manufactured by Osaka Gas Chemical Co., Ltd.): graphite (SFG-15, manufactured by Imerys): carbon fiber (VGCF®, manufactured by Resonac Co., Ltd.): styrene-butadiene rubber (SBR, commercially available): carboxymethylcellulose (CMC, commercially available). Subsequently, the negative electrode slurry was applied to one side of copper foil (negative electrode current collector) (coating weight 8.8 mg / cm²). ２ The resulting material was dried in the same manner as the positive electrode and roll-pressed. As a result, a negative electrode mixture layer consisting of the negative electrode slurry was formed on the negative electrode current collector, with a mixture density of 1.15 g / cm³. ３ The negative electrode was fabricated.

[0086] [Battery Fabrication] (LMFP-type battery) LMFP-type positive electrode with an effective area of ​​12 cm² ２ The material was cut, and the polarity lead was welded using an ultrasonic welding machine. The negative electrode had an effective area of ​​13.44 cm². ２The polarity leads were cut and welded using an ultrasonic welding machine. These positive and negative electrodes were placed opposite each other via a 16 μm thick polyethylene separator, covered with a laminate casing, and three sides of the laminate casing were sealed. Subsequently, 700 μL of electrolyte was injected from the unsealed end, and then vacuum-sealed to produce a 4.2 V, 25 mAh laminate-type lithium-ion battery. The obtained battery was charged using a charge / discharge test apparatus (Asuka Electronics Co., Ltd., part number: ACD-01, hereafter the same) at room temperature (25°C, hereafter the same) at a constant current of 2.5 mA (0.1 C) for 3 hours, and then left for 48 hours. After the period of time, one side was cut open and degassed by vacuum-sealing again. The degassed battery was charged to 4.2 V at 0.1 C with a constant current constant voltage (CCCV) termination at 0.5 mA. Subsequently, constant current (CC) discharge was performed at 5mA (0.2C) down to 2.5V. Next, CCCV charging was performed at 12.5mA (0.5C) to 4.2V with a 0.5mA termination, followed by CC discharge at 0.2C down to 2.5V. After similar charging, CC discharge was performed at 25mA (1C) and 50mA (2C), respectively. The above constituted the battery aging process. After the aging process, the laminated lithium-ion secondary battery was completed.

[0087] (NCM811 Battery) An NCM811 battery was fabricated using the same method as described above, except that an NCM811 positive electrode was used.

[0088] (NCM111 Battery) An NCM111 battery was manufactured using the same method as described above, except that an NCM111 positive electrode was used.

[0089] (LFP battery) LFP positive electrode with an effective area of ​​12 cm² ２ The material was cut, and the polarity lead was welded using an ultrasonic welding machine. The negative electrode had an effective area of ​​13.44 cm². ２The polarity leads were cut and welded using an ultrasonic welding machine. These positive and negative electrodes were placed opposite each other via a 16 μm thick polyethylene separator, covered with a laminate casing, and three sides of the laminate casing were sealed. Subsequently, 700 μL of electrolyte was injected from the unsealed end, and then vacuum-sealed to produce a 3.6 V, 25 mAh laminate-type lithium-ion battery. The resulting battery was subjected to a charge-discharge test using a charge-discharge test apparatus, and charged at room temperature at 2.5 mA (0.1 C) for 3 hours, followed by 48 hours of standing time. After standing time, one side was cut open and the battery was vacuum-sealed again to remove the gas. The degassed battery was charged to 3.6 V at 0.1 C with a constant current constant voltage (CCCV) rate ending at 0.5 mA. Then, it was discharged to 2.0 V at 5 mA (0.2 C) with a constant current (CC) rate. Next, the battery was charged using CCCV at 12.5mA (0.5C) to 3.6V, ending at 0.5mA, and then discharged using CC at 0.2C to 2.0V. After similar charging, it was discharged using CC at 25mA (1C) and 50mA (2C), respectively. This constituted the battery aging process. After the aging process, the laminated lithium-ion secondary battery was completed.

[0090] [Battery Evaluation] For each evaluation battery, the DCR fluctuation (DCR increase rate) was measured before and after high-temperature storage at 70°C, XPS analysis of the electrode surface coating composition after high-temperature storage at 70°C was performed, and the DCR fluctuation (DCR increase rate) was measured before and after high-temperature cycling tests at 55°C.

[0091] <Measurement of DCR increase rate before and after storage at 70°C> (Initial (before storage at 70°C) DCR) Laminated lithium-ion secondary batteries were fully charged (SOC 100%) by performing CCCV charging to 4.2V at 25mA (1C) and ending at 0.5mA. The 25°C DCR (initial DCR) was measured using the fully charged batteries. For DCR measurement, the batteries were left for 30 minutes after full charge, then discharged at 5mA (0.2C) for 10 seconds. Subsequently, the batteries were left for another 30 minutes, then discharged at 1C for 10 seconds. Finally, the batteries were left for another 30 minutes, then discharged at 75mA (3C) for 10 seconds. An I-V line was created from the relationship between each discharge current (horizontal axis) and the difference (ΔV, vertical axis) between the voltage immediately before the start of discharge and the voltage 10 seconds later at each discharge current. The slope of the I-V line was calculated as the battery's DCR. Furthermore, the upper voltage limit for LFP batteries was set to 3.6V.

[0092] (DCR after 70°C storage) After the initial DCR measurement, the batteries were again charged to 4.2V using CCCV at 25mA (1C) and terminated at 0.5mA to reach a fully charged state (SOC 100%). The fully charged batteries were stored in a 70°C constant temperature bath for 28 days. After storage, the batteries were removed from the constant temperature bath and returned to room temperature, and the DCR (DCR after 70°C storage) was measured under the same conditions as the initial DCR. For LFP batteries, the upper voltage limit was set to 3.6V.

[0093] (DCR increase rate before and after 70°C storage) The ratio of "DCR after 70°C storage" to "initial DCR" was calculated as the DCR increase rate before and after 70°C storage [(DCR after 70°C storage / initial DCR) × 100]. Note that a smaller DCR increase rate means that the DCR increase is suppressed and the battery performance is improved (the same applies below).

[0094] <Analysis of Electrode Surface Coating Composition After Storage at 70°C> After storage at 70°C (after DCR measurement after storage at 70°C), the battery was again charged to 4.2V with CCCV at 25mA (1C) and terminated at 0.5mA to reach a fully charged state (SOC 100%). The fully charged battery was disassembled in a glove box under an argon atmosphere, and the positive and negative electrodes were removed. The chemical state of each surface of the positive and negative electrodes was analyzed by XPS analysis. XPS analysis was performed using a Shimadzu AXIS-NOVA photoelectron spectrometer with a monochromatic Al-KαX source (1486.6 eV) operating at 144W (12mA x 12kV) and 10 －６ The scans were performed using a pressure maintained below Pa. Spectra were recorded with a path energy of 160 eV for survey scans and 40 eV for high-resolution scans, with step sizes of 1 eV and 0.1 eV, respectively. The binding energy was calibrated at the strongest C1s peak at 284.8 eV. The F / C ratio after storage at 70°C was defined as the fluorine (F) content divided by the carbon (C) content among all elements of the detected components. For LFP batteries, the upper voltage limit was set at 3.6 V.

[0095] <Measurement of DCR increase rate around 500 cycles at 55°C> (Cycle test) After initial DCR measurement, the battery was CC discharged at 0.2C to 2.5V, and then left to stand in a constant temperature bath at 55°C for 2 hours. Next, the battery that had been left to stand was CCCV charged at 0.5C to 4.2V with a termination of 0.5mA in a 55°C environment, and then CC discharged at 0.2C to 2.5V. Furthermore, the same battery was CCCV charged at 25mA (1C) to 4.2V with a termination of 0.5mA, and then CC discharged at 1C to 2.5V. The above 1C charging and 1C discharging was repeated 500 cycles. For LFP batteries, the upper voltage limit was set to 3.6V and the lower voltage limit to 2.0V.

[0096] (Post-cycle DCR) Using batteries that had undergone 500 cycle tests, the DCR (post-cycle DCR) was measured under the same conditions as the initial DCR.

[0097] (DCR increase rate around 55°C and 500 cycles) The ratio of "post-cycle DCR" to "initial DCR" was calculated as the DCR increase rate around 55°C and 500 cycles [(post-cycle DCR / initial DCR) × 100].

[0098]

[0099] <Consideration of Example 1 Series> - Comparative Examples 2 and 3, which contain LiFSI as the electrolyte, were found to suppress the rise in DCR during high-temperature storage and high-temperature cycling of the battery as the LiFSI content increased, compared to Comparative Example 1, which does not contain LiFSI. - Each example containing a predetermined amount of HF in addition to LiFSI [5 ppm to 1000 ppm by mass as a ratio to the total amount of non-aqueous electrolyte] was found to show a significantly greater degree of suppression of the rise in DCR during high-temperature storage and high-temperature cycling of the battery compared to Comparative Example 1, compared to Comparative Examples 2 and 3, which have an HF content of less than 5 ppm by mass. The reason why this suppression effect is so pronounced is thought to be that, in each example, compared to Comparative Examples 2 and 3, the F / C ratio after storage at 70°C is larger, so a coating containing more inorganic components such as fluorine is rapidly formed on both the positive and negative electrode surfaces. - The above suppression effect is due to LiFSI and LiPF ６ It was found that a similar effect could be obtained if the molar ratio was in the range of 0.2:1.0 to 1.0:0.2. The above suppression effect was found to be higher with increasing HF content compared to, for example, Examples 3 to 7, which had the same LiFSI content of 0.6 M. In Comparative Examples 4 to 13, which had positive electrodes other than LMFP-based positive electrodes, it was found that although the F / C ratio after storage at 70°C increased with the addition of a predetermined amount of HF, the above suppression effect was not obtained or was small. Therefore, it was found that in lithium-ion secondary batteries equipped with LMFP-based positive electrodes, by further adding a predetermined amount of HF to the LiFSI-containing non-aqueous electrolyte, the increase in DCR can be suppressed even when stored at a high temperature of 70°C, and the increase in DCR can also be suppressed even when repeatedly charged and discharged at a high temperature of 55°C.

[0100] <Example 2 Series> [Non-aqueous electrolyte] A non-aqueous electrolyte was obtained in the same manner as in Example 1 Series, except that a mixed solvent with a composition of EC:ethyl propionate (EP) = 30:70 (volume ratio) or a mixed solvent with a composition of EC:EMC = 15:85 (volume ratio) was used.

[0101] [Negative electrode] A water-based slurry (negative electrode slurry) was prepared using Si-containing graphite (BSO-600, manufactured by BTR): acetylene black (Denka Black®, manufactured by Denka Co., Ltd.): carbon fiber (VGCF®, manufactured by Resonac Co., Ltd.): styrene-butadiene rubber (SBR, commercially available): carboxymethylcellulose (CMC, commercially available) in a mass ratio of 100:5:3:1:1. Subsequently, the negative electrode slurry was applied to one side of copper foil (negative electrode current collector) (coating weight 7.2 mg / cm²). ２ The mixture was dried in the same manner as in Example 1 Series and roll-pressed. As a result, a negative electrode mixture layer consisting of negative electrode slurry was formed on the negative electrode current collector, with a mixture density of 1.5 g / cm³. ３ The negative electrode was fabricated.

[0102] [Battery Fabrication] A laminate-type lithium-ion secondary battery was completed and evaluated using the same method as in the Example 1 series, except that the non-aqueous electrolyte and / or negative electrode shown in Table 2 was used.

[0103]

[0104] <Considerations of Example 2 Series> - From a comparison between Example 9 and Comparative Examples 14-15, it was found that even a battery equipped with a negative electrode containing Si-containing graphite can obtain the above-mentioned suppression effect by HF, similar to a battery equipped with a negative electrode containing only graphite (Example 1 Series). - From a comparison between Example 10 and Comparative Examples 16-17, it was found that even a battery equipped with a mixed solvent containing EC and EP can obtain the above-mentioned suppression effect by HF, similar to a battery equipped with a mixed solvent containing EC and EMC (Example 1 Series). - From a comparison between Example 11 and Comparative Examples 18-19, it was found that even a battery equipped with a mixed solvent with a low proportion of EC in the non-aqueous solvent, such as one with an EC:EMC = 15:85 (volume ratio) composition, can obtain the above-mentioned suppression effect by HF, similar to a battery equipped with a mixed solvent with an EC:EMC = 30:70 (volume ratio) composition (Example 1 Series).

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