Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery that has excellent cycle characteristics. The present invention relates to a lithium secondary battery that includes: a negative electrode active material that contains Si; and an electrolyte solution. The average particle diameter of the negative electrode active material is 5.0–30 μm. The electrolyte solution includes LiFSI, LiPF6, and LiPO2F2 as lithium salts. When the mole fraction of LiFSI is x, the mole fraction of LiPF6 is y, the mole fraction of LiPO2F2 is z, and x+y+z=1, 0.05≤x≤0.95, 0.05≤y≤0.90, and 0.005≤z≤0.35. The lithium ion concentration of the electrolyte solution is 0.7–3.0 M.

Description

Lithium secondary battery

[0001] The present invention relates to a lithium secondary battery.

[0002] BACKGROUND ART Conventionally, technology for converting natural energy such as solar or wind power into electrical energy has been attracting attention. Accordingly, various secondary batteries have been developed as electricity storage devices that are highly safe and capable of storing a large amount of electrical energy.

[0003] Among these, lithium secondary batteries, which are charged and discharged by the movement of lithium ions between a positive electrode and a negative electrode, are known to exhibit high voltage and high energy density. A typical lithium secondary battery is a lithium-ion secondary battery (LIB), which has active materials capable of retaining lithium elements in the positive electrode and the negative electrode, and is charged and discharged by the exchange of lithium ions between the positive electrode active material and the negative electrode active material.

[0004] Furthermore, for the purpose of achieving high energy density and improving productivity, lithium secondary batteries (LMB: Lithium-metal battery) that use lithium metal as the negative electrode active material instead of a material that can insert lithium ions, such as a carbon material, and lithium secondary batteries (AFB: Anode-free battery) that use a negative electrode made of a negative electrode current collector that does not have a negative electrode active material such as a carbon material or lithium metal have been developed.

[0005] On the other hand, lithium secondary batteries, which can achieve high voltages, have the problem of reduced cycle characteristics due to the high reactivity between the battery components and the electrolyte. Specifically, repeated charging and discharging causes side reactions between the negative electrode material, positive electrode material, and the electrolyte, resulting in a decrease in battery capacity. Therefore, there is a need for methods to prevent such a decrease in battery capacity.

[0006] For example, Patent Document 1 discloses an electrolyte solution containing an electrolyte salt and an organic solvent for the purpose of improving the warm storage performance, high-temperature cycle performance, and power performance of a lithium ion battery, wherein the electrolyte salt contains a lithium salt, the organic solvent contains a cyclic ether, and where W1 is the mass fraction of the lithium salt relative to the electrolyte solution and W2 is the mass fraction of the cyclic ether relative to the electrolyte solution, the relationship 0.2≦W1 / W2≦1.06 is satisfied.

[0007] Furthermore, Patent Document 2 discloses an electrolyte solution containing an electrolyte salt and an organic solvent, with the aim of achieving low internal resistance, high-temperature storage performance, and high-temperature cycle performance in a battery, wherein the electrolyte salt contains lithium bis(fluorosulfonyl)imide, the mass % content of the lithium bis(fluorosulfonyl)imide in the electrolyte solution is 4.5% to 11%, the organic solvent contains ethylene carbonate, and the electrolyte solution satisfies 0.9≦WLiFSI / (16.77%−WEC)≦2.9 (where WLiFSI is the mass % content of lithium bis(fluorosulfonyl)imide in the electrolyte solution, and WEC is the mass % content of ethylene carbonate in the electrolyte solution).

[0008] Special table 2023-537443 publication Special table 2023-520319 publication

[0009] However, in a lithium secondary battery containing an active material containing Si, repeated charge and discharge causes Si to easily expand and contract, resulting in a more significant decrease in capacity and cycle life.

[0010] The present invention has been made in view of the above circumstances, and has as its object to provide a lithium secondary battery with improved cycle characteristics.

[0011] A lithium secondary battery according to one embodiment of the present invention is a lithium secondary battery including a negative electrode active material containing Si and an electrolyte, wherein the negative electrode active material has an average particle size of 5.0 μm or more and 30 μm or less, and the electrolyte contains, as lithium salts, LiFSI and LiPF ６ and LiPO ２ F ２and wherein the mole fraction of LiFSI is x, and the LiPF ６ The mole fraction of LiPO is y. ２ F ２ where z is the molar fraction of x and x+y+z=1, the ranges of 0.05≦x≦0.95, 0.05≦y≦0.90, and 0.005≦z≦0.35 are satisfied, and the concentration of lithium ions in the electrolyte solution is 0.7 M or more and 3.0 M or less.

[0012] According to the present invention, a lithium secondary battery having excellent cycle characteristics can be provided.

[0013] 1 is a schematic diagram showing an example of a lithium secondary battery of the present invention.

[0014] Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail, but the present invention is not limited to the following embodiment. The present invention can be modified in various ways without departing from the gist of the present invention. In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right are based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0015] 1. Lithium Secondary Battery The basic configuration of a lithium secondary battery according to one embodiment of the present invention will be described with reference to Figure 1. As shown in Figure 1, the lithium secondary battery according to one embodiment of the present invention includes, for example, a plurality of negative electrodes 10, a plurality of separators 20, and a plurality of positive electrodes 30, and the negative electrodes 10 and the positive electrodes 30 are spaced apart via the separators 20. Each component will be described in detail below.

[0016] The negative electrode 10 typically includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode 10 of this embodiment includes the negative electrode current collector and a negative electrode active material layer containing a Si-containing negative electrode active material (hereinafter also simply referred to as "Si active material").

[0017] The negative electrode active material is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, in the negative electrode 10. The negative electrode active material of this embodiment includes a Si active material, and may include other negative electrode active materials that do not contain Si, as necessary. Si has a high theoretical Li insertion capacity per weight, and by including a Si active material, a high-capacity lithium secondary battery can be obtained.

[0018] The Si active material is an active material containing the Si element, for example, Si, Li ｙ -SiO ｘ , SiC, SiO ｘ As the Si active material, Li ｙ -SiO ｘ or SiC, more preferably Li ｙ -SiO ｘ By including such a Si active material, the battery tends to have excellent cycle characteristics and a high capacity. ｙ -SiO ｘ is SiO ｘ Compared to the above, the irreversible capacity at the initial charge / discharge is small, and the energy density can be increased, which is preferable.

[0019] In this embodiment, the average particle size of the Si active material is 5.0 μm or more and 30 μm or less, preferably 7.0 μm or more and 20 μm or less, and 8.0 μm or more and 15 μm or less. When the average particle size of the Si active material is 5.0 μm or more, side reactions with the electrolyte components tend to be suppressed and an increase in electrical resistance tends to be suppressed, and when it is 30 μm or less, volume change during charging and discharging of the charge-discharge battery tends to be suppressed. In this embodiment, the particle size distribution is measured using a laser diffraction particle size distribution analyzer "MT-3000 manufactured by Microtrackbell Corporation."

[0020] The content (amount used) of the Si active material is preferably 1% by mass or more and 90% by mass or less, 3% by mass or more and 80% by mass or less, or 5% by mass or more and 70% by mass or less, relative to the total amount of the negative electrode active material. When the amount of the Si active material used is within the above range, a balance between energy density and cycle life tends to be achieved. From the same viewpoint, the content (amount used) of the Si active material per 100 parts by mass of the negative electrode active material composition excluding the solvent component is preferably 2 parts by mass or more and 90 parts by mass or less, 5 parts by mass or more and 80 parts by mass or less, or 7 parts by mass or more and 70 parts by mass or less.

[0021] Other negative electrode active materials that do not contain Si element include, for example, carbon-based active materials such as graphite, graphene, hard carbon, and carbon nanotubes; metal oxide-based active materials such as titanium oxide compounds and cobalt oxide compounds; and metals such as germanium, tin, lead, aluminum, and gallium, and metal / alloy-based active materials pre-doped with lithium.From the viewpoint of more effectively and reliably achieving the present invention, it is preferable to use a carbon-based active material together with a Si active material, and among carbon-based active materials, graphite is preferred.

[0022] The content of the other negative electrode active material is preferably 10% by mass or more and 99% by mass or less, 30% by mass or more and 97% by mass or less, or 80% by mass or more and 95% by mass or less, relative to the total amount of the negative electrode active material. When the content of the other negative electrode active material is within the above range, a balance between energy density and cycle life tends to be achieved. From the same viewpoint, the content (amount used) of the other negative electrode active material per 100 parts by mass of the negative electrode active material composition excluding the solvent component is preferably 10 parts by mass or more and 95 parts by mass or less, 30 parts by mass or more and 90 parts by mass or less, or 50 parts by mass or more and 85 parts by mass or less.

[0023] The negative electrode active material layer may be formed by mixing the negative electrode active material with a solvent, additives, binder, conductive aid, etc., and then applying the mixture to one or both surfaces of the negative electrode current collector and pressing it. The solvent is not particularly limited, and for example, water is used. The additive is not particularly limited, and for example, a binder such as carboxymethyl cellulose (CMC) or styrene-butadiene rubber (SBR) may be used, and a conductive aid such as carbon black, carbon nanofiber (VGCF), single-walled carbon nanotubes (SWCNT), or multi-walled carbon nanotubes (MWCNT) may be used.

[0024] When forming the negative electrode active material layer, the negative electrode active material composition is applied to one or both surfaces of the negative electrode current collector. The amount of the negative electrode active material composition applied is preferably 1 mg / cm2 in terms of basis weight on one surface. ２ 30mg / cm or more ２ is less than 2 mg / cm ２ 25mg / cm or more ２ is less than or equal to 3 mg / cm ２ 20mg / cm or more ２ By setting the coating amount of the negative electrode active material composition within the above range, the cycle characteristics of the battery tend to be further improved.

[0025] The thickness of the negative electrode active material layer is preferably 20 μm to 200 μm, 30 μm to 175 μm, or 40 μm to 150 μm. By setting the thickness of the negative electrode active material layer within the above range, the cycle characteristics of the battery tend to be further improved.

[0026] Examples of the negative electrode current collector include a metal foil and a current collector film having a metal layer on both sides of a resin layer. Examples of the metal foil include copper foil. In this embodiment, a current collector film having a metal layer on both sides of a resin layer is preferred from the viewpoints of ease of processing and high energy density. A current collector film for a negative electrode is preferably a film having a metallic copper layer on both sides of a resin layer.

[0027] In the negative electrode current collector film, the resin layer preferably contains at least one selected from the group consisting of polyethylene terephthalate (PET), polypropylene, polyamide, acrylic resin, polycarbonate, polyethylene, polyvinyl chloride, and polystyrene, and more preferably consists of polyethylene terephthalate or polypropylene. By using such a resin layer, the cycle characteristics of the battery tend to be further improved.

[0028] In the negative electrode current collector film, the resin layer is preferably in the form of a film (sheet), and its thickness is preferably 2.0 μm to 20 μm, 3.0 μm to 10 μm, or 4.0 μm to 7.0 μm. By setting the thickness of the resin layer within the above range, the cycle characteristics of the battery tend to be further improved.

[0029] In the negative electrode current collector film, the metal layer is preferably a copper (Cu) layer, and the copper layer has a Cu element content of 99% by mass or more, 99.9% by mass or more, 99.99% by mass or more, or 99.9999% by mass or more. The thickness of each metal layer or copper layer is preferably 0.2 μm to 10 μm, 0.3 μm to 5.0 μm, 0.5 μm to 3.0 μm, or 0.8 μm to 2.0 μm. By keeping the thickness of the metal layer or copper layer within the above range, the cycle characteristics of the battery tend to be further improved.

[0030] 1.2. Electrolyte The electrolyte of this embodiment contains LiFSI and LiPF as lithium salts. ６ and LiPO ２ F ２ and wherein the mole fraction of LiFSI is x, and the LiPF ６ The mole fraction of LiPO is y. ２ F ２ When the mole fraction of LiFSI is z and x + y + z = 1, the ranges of 0.05≦x≦0.95, 0.05≦y≦0.90, and 0.005≦z≦0.35 are satisfied, and the concentration of lithium ions in the electrolyte is 0.7 M or more and 3.0 M or less. ２ F) ２ means.

[0031] It has been known that the use of a negative electrode active material containing Si results in a high Li insertion capacity per weight, making it easy to produce a high-capacity lithium secondary battery. However, Si is a material that undergoes significant volume expansion with repeated charge and discharge of the battery, which increases the electrical resistance and shortens the battery's lifespan. As a result of extensive research into this issue, the inventors have found that by using the above-mentioned type and concentration of salt in the electrolyte, the effects of Si volume expansion can be reduced, resulting in a lithium secondary battery with a long lifespan.

[0032] The reason for this is not entirely clear, but is presumed to be as follows: LiPF, a lithium salt that is commonly used in lithium-ion secondary batteries, ６ exhibits excellent performance as an electrolyte, but when the temperature rises due to repeated charge and discharge, side reactions with Si become active, which tends to inhibit the function of the electrolyte. ２ F ２ When LiFSI, LiPF are simultaneously contained, it is believed that the side reactions of the electrolyte are suppressed even when the temperature rises, while the dissociation of ions can be maintained. ６ , LiPO ２ F ２ By ensuring that the ratio of and the concentration of lithium ions in the electrolyte satisfy the above ranges, it is possible to suppress not only side reactions with Si but also side reactions with the material of the positive electrode 30, which is more suitable for maintaining the cycle characteristics and battery capacity of the battery. As described above, the lithium secondary battery of this embodiment has excellent cycle characteristics as a result of the synergistic action of the above-mentioned configuration. However, the reasons for this are not limited to those mentioned above.

[0033] 1.2.1. Lithium Salt The electrolyte solution of this embodiment contains LiFSI and LiPF as lithium salts. ６ and LiPO ２ F ２ and may contain other salts as needed. Examples of other salts include LiI, LiCl, LiBr, LiF, and LiBF. ４ , LiAsF６ , LiSO ３ CF ３ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ CF ３ CF ３ ) ２ , LiBF ２ (C ２ O ４ ), LiB(O ２ C ２ H ４ ) ２ , LiB(O ２ C ２ H ４ ) F ２ , LiB(OCOCF ３ ) ４ , LiNO ３ , Li ２ SO ４ or salts of metals such as Na, K, Ca, and Mg.

[0034] In the electrolyte solution of this embodiment, the mole fraction of LiFSI is x, and LiPF ６ is the mole fraction of LiPO ２ F ２ When the mole fraction of LiPF is z and x + y + z = 1, the ranges of 0.05≦x≦0.95, 0.05≦y≦0.90, and 0.005≦z≦0.35 are satisfied. The mole fraction x of LiFSI is preferably 0.06≦x≦0.90, 0.10≦x≦0.80, or 0.30≦x≦0.70. ６ The mole fraction y of LiPO is preferably 0.06≦y≦0.70, 0.10≦y≦0.50, or 0.20≦y≦0.40. ２ F ２ The mole fraction z of LiFSI, LiPF is preferably 0.008≦z≦0.30, 0.01≦z≦0.25, or 0.03≦z≦0.18. ６ , and LiPO ２ F ２ When the molar fraction of is within the above range, side reactions with the negative electrode material and the positive electrode material tend to be suppressed, and the battery tends to have even better cycle characteristics.

[0035] The lithium ion concentration in the electrolyte solution is 0.7 M or more and 3.0 M or less, preferably 0.8 M or more and 2.5 M or less, 1.0 M or more and 2.0 M or less, or 1.3 M or more and 1.7 M or less. By setting the lithium ion concentration within the above range, the battery tends to have even more excellent cycle characteristics.

[0036] The concentration of LiFSI in the electrolyte is preferably 0.1 M to 2.0 M, 0.3 M to 1.5 M, or 0.5 M to 1.2 M. By setting the LiFSI concentration within the above range, the battery tends to have even better cycle characteristics.

[0037] LiPF in electrolyte ６ The concentration of LiPF is preferably 0.1 M or more and 2.0 M or less, 0.2 M or more and 1.0 M or less, or 0.3 M or more and 0.8 M or less. ６ By setting the concentration within the above range, the battery tends to have even better cycle characteristics.

[0038] LiPO in the electrolyte ２ F ２ The concentration of LiPO is preferably 0.01 M or more and 1.0 M or less, 0.03 M or more and 0.8 M or less, or 0.05 M or more and 0.4 M or less. ２ F ２ By setting the concentration within the above range, the battery tends to have even better cycle characteristics.

[0039] In the electrolyte, LiFSI, LiPF ６ , and LiPO ２ F ２ The concentration of the other salt other than the above is preferably 1.0 M or less, 0.8 M or less, or 0.6 M or less. By setting the concentration of the other salt within the above range, the cycle characteristics of the battery tend to be further improved. The concentration of the other salt in the electrolyte may be, for example, 0 M or more, 0.0001 M or more, 0.001 M or more, or 0.005 M or more.

[0040] 1.2.2. Solvent The solvent for the electrolyte solution is not particularly limited as long as it dissolves the lithium salt of this embodiment. Examples of such solvents include linear or cyclic carbonate compounds, ether compounds, and linear or cyclic fluorine compounds in which one or more hydrogen atoms in the carbon skeleton are substituted with fluorine atoms. The solvent for the electrolyte solution may be used alone or in combination of two or more.

[0041] The solvent for the electrolyte solution preferably contains at least one of a chain carbonate compound, a cyclic carbonate compound, a chain fluorine compound, and a cyclic fluorine compound, more preferably contains at least one of a chain carbonate compound, a cyclic carbonate compound, and a cyclic fluorine compound, even more preferably contains a chain carbonate compound, a cyclic carbonate compound, and a cyclic fluorine compound, and even more preferably contains a chain carbonate compound, a cyclic carbonate compound, a chain fluorine compound, and a cyclic fluorine compound. By using a solvent with the above-mentioned configuration, the effect of improving cycle characteristics according to the present invention tends to be more pronounced.

[0042] The chain carbonate compound is a carbonate compound in which one or more hydrogen atoms in the carbon skeleton are not substituted with fluorine atoms and which does not have a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, or a heterocyclic ring. Examples of the chain carbonate compound include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, and chloroethylene carbonate. Preferably, the compound contains dimethyl carbonate or ethylene carbonate, and more preferably, dimethyl carbonate and ethylene carbonate. By containing a chain carbonate compound, the effect of improving cycle characteristics according to the present invention tends to be more pronounced.

[0043] The cyclic carbonate compound is a carbonate compound in which one or more hydrogen atoms in the carbon skeleton are not substituted with fluorine atoms and which has a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, or a heterocyclic ring. Examples of the cyclic carbonate compound include ethylene carbonate and propylene carbonate. Among these, it is preferable to contain ethylene carbonate. By containing a cyclic carbonate compound, the effect of improving the cycle characteristics according to the present invention tends to be more remarkable.

[0044] A chain fluorine compound is a compound that does not have a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, or a heterocyclic ring, and in which one or more hydrogen atoms in the carbon skeleton are substituted with fluorine atoms. Examples of chain fluorine compounds include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, methyl perfluoroisobutyl ether, ethyl nonafluoroisobutyl ether, 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethoxy-2,2,3,3-tetrafluoropropoxymethane, methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, and propyl-1,1,2,2-tetrafluoroethyl ether. Among these, a chain fluorine compound having an ether group is preferred, and more preferably contains at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, methyl perfluoroisobutyl ether, ethyl nonafluoroisobutyl ether, and 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane, and even more preferably contains 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, or ethyl nonafluoroisobutyl ether. By containing the above chain fluorine compound, the effect of improving cycle characteristics according to the present invention tends to be even more remarkable.

[0045] A cyclic fluorine compound is a compound having a cyclic structure such as an aromatic ring, an alicyclic ring, a homocyclic ring, or a heterocyclic ring, in which one or more hydrogen atoms in the carbon skeleton are substituted with fluorine atoms. Examples of cyclic fluorine compounds include fluoroethylene carbonate, 1,1,2,2,3,3,4-heptafluorocyclopentane, hexadecafluoro(1,3-dimethylcyclohexane), tetradecafluoromethylcyclohexane, fluorocyclohexane, fluorocyclopentane, and octadecafluorodecahydronaphthalene. Among these, fluoroethylene carbonate is preferred. By including the above cyclic fluorine compound, the effect of improving cycle characteristics according to the present invention tends to be more pronounced.

[0046] The electrolyte solution of this embodiment contains a chain fluorine compound having at least one monovalent group represented by the following formulas (A) to (D), and the content of the chain fluorine compound is preferably 0.5% by mass to 30% by mass, more preferably 1% by mass to 25% by mass, and even more preferably 3% by mass to 20% by mass, relative to the total amount of components in the electrolyte solution other than the chain fluorine compound. By containing such a chain fluorine compound, the cycle characteristics of the lithium secondary battery tend to be further improved. Note that the wavy lines in the formulas represent bonding sites in the monovalent group.

[0047] The electrolyte solution preferably contains a chain fluorine compound having the above formula (A), (B), or (C), and more preferably contains a chain fluorine compound having the above formula (A) or (B). By containing a chain fluorine compound having such a structure, the cycle characteristics of the lithium secondary battery tend to be further improved.

[0048] The electrolyte solution of this embodiment preferably contains a chain or cyclic fluorinated carbonate. By containing a chain or cyclic fluorinated carbonate, the effect of improving the cycle characteristics according to the present invention tends to be more pronounced. From the same viewpoint, it is preferable that the electrolyte solution contains fluoroethylene carbonate as the chain or cyclic fluorinated carbonate.

[0049] The ether compound is a non-fluorinated compound having an ether group, such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1,2-dimethoxypropane (DMP), diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dimethoxypropane, 1,4-dimethoxybutane, 1,1-dimethoxyethane, 2,2-dimethoxypropane, 1,2-diethoxybutane, and 2,3-diethoxybutane.

[0050] The content of the carbonate compound is preferably 30% by mass or more and 100% by mass or less, 35% by mass or more and 95% by mass or less, 45% by mass or more and 90% by mass or less, or 55% by mass or more and 85% by mass or less, relative to the total amount of the electrolyte solution. By setting the content of the carbonate compound within the above ranges, the effect of improving the cycle characteristics according to the present invention tends to be more remarkable. Note that the carbonate compound includes both fluorinated and non-fluorinated carbonate compounds.

[0051] The content of the chain carbonate compound is preferably 10% by volume or more and 60% by volume or less, 20% by volume or more and 50% by volume or less, or 25% by volume or more and 45% by volume or less, based on the total amount of the chain carbonate compound and the cyclic carbonate compound. By setting the content of the chain carbonate compound within the above ranges, the effect of improving the cycle characteristics according to the present invention tends to become more remarkable. Note that the chain carbonate compound includes both fluorinated chain carbonate compounds and non-fluorinated chain carbonate compounds.

[0052] The content of the cyclic carbonate compound is preferably 5% by volume or more and 60% by volume or less, 10% by volume or more and 50% by volume or less, or 20% by volume or more and 40% by volume or less, based on the total amount of the chain carbonate compound and the cyclic carbonate compound. By setting the content of the cyclic carbonate compound within the above range, the effect of improving the cycle characteristics according to the present invention tends to be more remarkable. Note that the cyclic carbonate compound includes both fluorinated and non-fluorinated cyclic carbonate compounds.

[0053] The content of the chain fluorine compound is preferably 0 to 50% by mass, 0 to 40% by mass, 1 to 30% by mass, or 3 to 25% by mass, relative to the total amount of the chain carbonate compound and the cyclic carbonate compound. By setting the content of the chain fluorine compound within the above range, the effect of improving the cycle characteristics according to the present invention tends to become more pronounced.

[0054] The content of the cyclic fluorine compound is preferably from 0 to 30% by mass, from 1 to 20% by mass, or from 3 to 10% by mass, relative to the total amount of the chain carbonate compound and the cyclic carbonate compound. By setting the content of the cyclic fluorine compound within the above range, the effect of improving the cycle characteristics according to the present invention tends to become more pronounced.

[0055] The content of the fluorinated carbonate is preferably from 0 to 30% by mass, from 1 to 20% by mass, or from 3 to 10% by mass, relative to the total amount of the chain carbonate compound and the cyclic carbonate compound. By setting the content of the fluorinated carbonate within the above range, the effect of improving the cycle characteristics according to the present invention tends to become more pronounced.

[0056] 1.3. Separator The separator 20 of this embodiment is not particularly limited as long as it has the function of physically and / or electrically isolating the positive electrode 30 and the negative electrode 10 and the function of ensuring ionic conductivity of lithium ions. Examples of such a separator include an insulating porous material, a polymer electrolyte, a gel electrolyte, and an inorganic solid electrolyte. Typically, the separator 20 includes at least one material selected from the group consisting of an insulating porous material, a polymer electrolyte, and a gel electrolyte. Furthermore, as the separator 20, one type of material may be used alone, or two or more types of materials may be used in combination.

[0057] The separator 20 is preferably made of an insulating porous material, a polymer electrolyte, or a gel electrolyte, either singly or in combination. When an insulating porous material is used alone as the separator 20, the lithium secondary battery must further include an electrolytic solution. Examples of the polymer electrolyte include, but are not limited to, solid polymer electrolytes primarily containing a polymer and an electrolyte, and semi-solid polymer electrolytes primarily containing a polymer, an electrolyte, and a plasticizer. Examples of the gel electrolyte include, but are not limited to, those primarily containing a polymer and a liquid electrolyte (i.e., a solvent and an electrolyte).

[0058] Polymers that may be contained in polymer electrolytes and gel electrolytes include, but are not limited to, polymers containing functional groups containing oxygen atoms such as ethers and esters, halogen groups, and polar groups such as cyano groups. Specific examples include resins having ethylene oxide units in the main chain and / or side chains such as polyethylene oxide (PEO), resins having propylene oxide units in the main chain and / or side chains such as polypropylene oxide (PPO), acrylic resins, vinyl resins, ester resins, nylon resins, polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polysiloxane, polyphosphazene, polymethyl methacrylate, polyamide, polyimide, aramid, and polytetrafluoroethylene. The above-mentioned resins may be used alone or in combination of two or more.

[0059] Examples of electrolytes contained in the polymer electrolyte and gel electrolyte include salts of Li, Na, K, Ca, and Mg. Typically, in this embodiment, the polymer electrolyte and gel electrolyte contain a lithium salt. The lithium salt is not particularly limited, but may be, for example, any salt that can be contained in the above-mentioned electrolytic solution. Such salts or lithium salts may be used alone or in combination of two or more.

[0060] The compounding ratio of the polymer to the lithium salt in the polymer electrolyte and the gel electrolyte may be determined by the ratio of the polar group of the polymer to the lithium atom of the lithium salt. For example, when the polymer contains oxygen atoms, the compounding ratio may be determined by the ratio ([Li] / [O]) of the number of oxygen atoms of the polymer to the number of lithium atoms of the lithium salt. In the polymer electrolyte and the gel electrolyte, the compounding ratio of the polymer to the lithium salt can be adjusted so that the ratio ([Li] / [O]) is, for example, 0.02 to 0.20, 0.03 to 0.15, or 0.04 to 0.12.

[0061] The solvent contained in the gel electrolyte is not particularly limited, and for example, the solvents that can be contained in the above-mentioned electrolyte solution can be used alone or in combination of two or more. Examples of preferred solvents are the same as those in the above-mentioned electrolyte solution. The plasticizer contained in the semi-solid polymer electrolyte is not particularly limited, and examples thereof include the same components as the solvents that can be contained in the gel electrolyte and various oligomers.

[0062] When the separator 20 includes an insulating porous member, the pores of the member are filled with an ion-conductive substance, thereby causing the member to exhibit ion conductivity. Thus, in this embodiment, the pores are filled with, for example, the electrolytic solution of this embodiment or a gel electrolyte containing the electrolytic solution of this embodiment.

[0063] The material constituting the insulating porous member is not particularly limited, and examples thereof include insulating polymer materials, specifically polyethylene (PE) and polypropylene (PP). That is, the separator 20 may be a porous polyethylene (PE) film, a porous polypropylene (PP) film, or a laminated structure thereof.

[0064] 1.4. Positive Electrode The positive electrode 30 of this embodiment includes a positive electrode current collector and a positive electrode active material layer. The average thickness of the positive electrode 30 is not particularly limited, but is, for example, 20 μm to 100 μm, 30 μm to 80 μm, or 40 μm to 70 μm. However, the average thickness of the positive electrode 30 can be adjusted appropriately depending on the desired battery capacity.

[0065] The positive electrode current collector of this embodiment may have a positive electrode current collector film including a resin layer containing polyethylene terephthalate and metal layers provided on both sides of the resin layer, or may have a metal layer without a resin layer. When a positive electrode current collector film is provided, the metal layers are formed by bonding the metal layers to both surfaces of the resin layer by vapor deposition, sputtering, electrolytic plating, or with an adhesive.

[0066] The resin layer of the positive electrode current collector is an insulator and prevents electrical conduction between the metal layers provided on both sides of the resin layer. The resin constituting the resin layer is not particularly limited, but may be, for example, a sheet-like (film-like) or fibrous resin. The resin may include at least one selected from the group consisting of polyethylene terephthalate (PET), polypropylene, polyamide, acrylic resin, polycarbonate, polyethylene, polyvinyl chloride, and polystyrene. The above resins may be used alone or in combination of two or more.

[0067] In addition to the resins described above, the resin layer of the positive electrode current collector may contain other additives as appropriate depending on the desired physical properties. The additives are not particularly limited, but examples thereof include colorants, flame retardants, surfactants, etc.

[0068] The thickness of the resin layer of the positive electrode current collector is, for example, 2 μm or more and 15 μm or less, 3 μm or more and 12 μm or less, or 4 μm or more and 10 μm or less.

[0069] The metal layer of the positive electrode current collector is in physical and / or electrical contact with the positive electrode active material layer and functions to donate and receive electrons to and from the positive electrode active material layer. The metal layer of the positive electrode current collector is composed of a conductor such as a metal that does not react with lithium in a battery. The metal constituting the metal layer of the positive electrode current collector is not particularly limited, but is at least one selected from the group consisting of aluminum, titanium, stainless steel, nickel, and alloys thereof. From the viewpoint of more effectively and reliably achieving the effects of the present invention, aluminum or an aluminum alloy is preferred, and aluminum is particularly preferred. The metal may be used alone or in combination of two or more. In this specification, the term "metal that does not react with lithium" refers to a metal that does not react with lithium ions or lithium metal to form an alloy under the operating conditions of a lithium secondary battery.

[0070] When the positive electrode current collector has a positive electrode current collector film, the thickness of the metal layer is not particularly limited, but is, for example, 0.1 μm to 4.0 μm, 0.2 μm to 3.0 μm, 0.3 μm to 2.5 μm, or 0.4 μm to 2.0 μm. When the positive electrode current collector has a metal layer without a resin layer, the thickness of the metal layer is not particularly limited, but is, for example, 4.0 μm to 20.0 μm, 6.0 μm to 17.5 μm, or 8.0 μm to 15.0 μm.

[0071] The positive electrode active material is a material that causes an electrode reaction, i.e., an oxidation reaction and a reduction reaction, in the positive electrode 30. The positive electrode active material of the present embodiment is not particularly limited, but is, for example, contained in a positive electrode active material composition that includes a binder, a conductive additive, a sacrificial positive electrode agent, and other additives, and the positive electrode active material composition is applied to at least one or both surfaces of a positive electrode current collector and press-molded, thereby disposing a positive electrode active material layer on at least one or both surfaces of the positive electrode current collector.

[0072] The method for disposing the positive electrode active material layer on the positive electrode current collector is not limited to press molding, and examples thereof include a method in which a thermosetting compound is added to a positive electrode active material composition and the composition is heated to harden it, a method in which a photocurable compound is added to a positive electrode active material composition and the composition is hardened by irradiating it with light, and a method in which the positive electrode active material composition is a two-component hardening composition and the two components are mixed to harden it.

[0073] The positive electrode active material layer of this embodiment is a compound represented by the general formula: Li ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.5≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is one or more elements selected from the group consisting of Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B).

[0074] Preferably, the positive electrode active material layer is a compound represented by the general formula: Li ｚ Ni ｘ Co ｙ M １－ｘ－ｙ O ２＋α (wherein 0.7≦x≦1.0, 0≦y≦0.35, 0.9≦z≦1.3, −0.2≦α≦0.15, and M is one or more elements selected from the group consisting of Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B). By including the above-mentioned compound with a high nickel ratio as the positive electrode active material, the energy density of the lithium secondary battery tends to be further improved. Furthermore, as the nickel ratio increases, a redox shuttle reaction is more likely to occur, but by including an additive, which will be described in detail below, in the electrolyte, the reaction is suppressed, and the electrolyte tends to have excellent performance stability at high temperatures.

[0075] The positive electrode active material may include other positive electrode active materials in addition to the above-mentioned compounds. Specifically, the other positive electrode active material of this embodiment may be a host material of lithium element (typically, lithium ion). Such other positive electrode active materials are not particularly limited, but include, for example, metal oxides and metal phosphates. The metal oxides are not particularly limited, but include, for example, cobalt oxide-based compounds, manganese oxide-based compounds, and nickel oxide-based compounds. The metal phosphates are not particularly limited, but include, for example, iron phosphate-based compounds and cobalt phosphate-based compounds. Typical other positive electrode active materials include LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiCoO ２ , LiNi ｘ Mn ｙ O(x+y=1), LiNiO ２ , LiMn ２ O ４ , LiFePO , LiCoPO , LiFeOF , LiNiOF , and TiS ２ The other positive electrode active materials may be used alone or in combination of two or more.

[0076] The positive electrode active material composition may contain a binder, which makes it easier for the positive electrode active material layer to be bound to the positive electrode current collector and improves flexibility after the positive electrode active material layer is disposed on the positive electrode current collector.

[0077] The positive electrode binder of this embodiment is not particularly limited, but examples thereof include polyvinylidene fluoride; modified polyvinylidene fluoride obtained by introducing functional groups such as hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, phenyl groups, and methyl groups into polyvinylidene fluoride; polytetrafluoroethylene; modified polytetrafluoroethylene obtained by introducing functional groups such as hydroxyl groups, amino groups, carbonyl groups, carboxyl groups, phenyl groups, and methyl groups into polytetrafluoroethylene; block copolymers, random copolymers, and graft copolymers having tetrafluoroethylene as a constituent unit; styrene butadiene rubber; carboxymethyl cellulose; acrylic resins; polyimide resins, etc. The binder may be used alone or in combination of two or more.

[0078] The positive electrode active material composition of this embodiment may contain a sacrificial positive electrode agent. The sacrificial positive electrode agent of this embodiment is a lithium-containing compound that undergoes an oxidation reaction and does not substantially undergo a reduction reaction in the charge / discharge potential range of the positive electrode active material. The sacrificial positive electrode agent is not particularly limited, but examples thereof include Li ２ O ２ Lithium oxides such as Li ３ Lithium nitrides such as N; Li ２ S-P ２ S ５ , Li ２ S-LiCl, Li ２ S-LiBr and Li ２ Lithium sulfide-based solid solutions such as S-LiI; Li １＋ｘ (Ti １－ｙ Fe ｙ ) １－ｘ O ２ (0<x≦0.25, 0.4<y≦0.9), Li ２－ｘ Ti １－ｚ Fe ｚ O ３－ｙ (0≦x<2, 0≦y≦1, 0.05≦z≦0.95), Li ５ FeO ４ The sacrificial positive electrode agent may be used alone or in combination of two or more.

[0079] 2. Manufacturing Method of Lithium Secondary Battery There are no particular limitations on the manufacturing method of a lithium secondary battery, as long as it is a method that can manufacture a lithium secondary battery having the above-mentioned battery configuration. For example, the following method can be mentioned.

[0080] The negative electrode 10, separator 20, and positive electrode 30 described above are prepared. The components and reagents used for the components may be produced by conventionally known methods, or commercially available products may be used. The positive electrode 30, separator 20, and negative electrode 10 thus prepared are stacked in this order, with the positive electrode 30 and the separator 20 facing each other, to obtain a laminate. The resulting laminate is enclosed in a sealed container together with the electrolyte solution of this embodiment, to obtain a lithium secondary battery. The sealed container is not particularly limited, but examples thereof include laminate films.

[0081] 1 , a plurality of positive electrodes 30 and negative electrodes 10 may be alternately stacked with a separator 20 sandwiched between the positive electrode 30 and negative electrode 10, which tends to further improve battery performance such as energy density. As a stacking method, for example, the negative electrode 10 and the positive electrode 30 may be wrapped so that they do not contact each other and face opposite sides of the separator 20, and the separator 20 may be stacked without cutting. Such stacking is preferable from the viewpoints of preventing short circuits and improving productivity.

[0082] Although not shown, the negative electrode 10 of this embodiment has a negative electrode active material layer containing a Si active material. Therefore, the negative electrode active material layer is obtained by applying a negative electrode active material composition, which is a mixture of the above-described negative electrode active material, a binder, an additive, a conductive aid, a solvent, etc., to one or both sides of a negative electrode current collector, followed by press molding. The resulting molded body is punched to a predetermined size to obtain the negative electrode 10 of this embodiment.

[0083] The shape of the battery of the lithium secondary battery of this embodiment is not particularly limited, and may be, for example, a sheet type, a laminated sheet type, a thin shape, a cylindrical shape with a bottom, a prismatic shape with a bottom, etc. From the viewpoint of more effectively and reliably achieving the effects of this embodiment, the sheet type, laminated sheet type, or thin shape is preferred.

[0084] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0085] 1. Fabrication of Lithium Secondary Batteries Lithium ion secondary batteries of Examples 1 to 15 and Comparative Examples 1 to 11 were fabricated as follows.

[0086] 1.1. Preparation of Negative Electrode A negative electrode current collector was prepared by vapor-depositing a 1.0 μm copper layer on both sides of a 6.0 μm-thick polyethylene terephthalate (PET) film. Then, a negative electrode active material composition was prepared by mixing 18 parts by mass of a Si-containing active material having the particle size shown in Table 1 as the negative electrode active material, 75 parts by mass of graphite as another negative electrode active material, 2 parts by mass of carbon black as a conductive additive, and 1 part by mass of carboxymethyl cellulose (CMC) and 4 parts by mass of styrene-butadiene rubber (SBR) as binders with 100 parts by mass of water as a solvent. This composition was applied to both sides of the negative electrode current collector so that the basis weight was 6.2 mg / cm. ２ The negative electrode 10 was then coated and pressed to form a sheet having a size of 4.4 cm x 4.4 cm, and a metal sheet for the negative electrode (copper foil having a thickness of 4.0 μm) was attached to each end of the negative electrode 10 by ultrasonic welding.

[0087] 1.2. Preparation of separator Polyvinylidene fluoride (PVDF) and Al ２ O ３ A polyethylene microporous film sheet (thickness: 15 μm, 4.8 cm×4.8 cm) whose surface was coated with the mixture was prepared as separator 20 .

[0088] 1.3 Preparation of Positive Electrode A current collector film was used as the positive electrode current collector. The current collector film was a 6.0 μm thick PET film resin layer with a 1.0 μm thick aluminum metal layer deposited on both sides. The positive electrode active material was LiNi in N-methyl-pyrrolidone (NMP) as the solvent. ０．８ Co ０．１５ Al ０．０５ O ２A positive electrode active material composition was prepared by mixing 96 parts by mass of the above-mentioned cellulose acetate copolymer, 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder. This positive electrode active material composition was applied to both sides of a positive electrode current collector in a manner that the basis weight was 15 mg / cm. ２ The positive electrode active material layer was formed on both sides of the positive electrode current collector by applying the coating solution and pressing it to obtain a molded body. The molded body was cut out to a predetermined size (4 cm x 4 cm) to obtain the positive electrode 30.

[0089] 1.4. Preparation of Electrolyte Solution A solvent solution was prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35, respectively. Furthermore, 5% by mass (wt%) of fluoroethylene carbonate (FEC) and the amount (wt%) of hydrofluoroether (HFE) listed in Table 1 were added to the total amount of the solvent solution. The hydrofluoroether (HFE) compounds used were 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE1), 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane (HFE2), and ethyl nonafluoroisobutyl ether (HFE3).

[0090] The above solution was added with lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF ６ ) and lithium difluorophosphate (LiPO ２ F ２ ) and a lithium salt was dissolved therein so as to achieve the molar fraction and lithium ion concentration shown in Table 1. In Comparative Example 8, the lithium salt was not completely dissolved, and therefore a lithium secondary battery was not produced.

[0091] 1.5. Battery Assembly The negative electrode 10, separator 20, and positive electrode 30 obtained as described above were stacked multiple times in the order shown in Figure 1 to obtain a laminate. The lead portions of the stacked positive and negative electrodes were then ultrasonically welded to a 100 μm Al terminal for the positive electrode and a 100 μm Ni terminal for the negative electrode, and the resulting battery was then inserted into a laminate exterior. The resulting electrolyte was then injected into the exterior and sealed, yielding lithium-ion secondary batteries for each of the Examples and Comparative Examples.

[0092] 2. Evaluation of Lithium Secondary Battery [Capacity Retention Rate] The lithium secondary battery obtained above was initially charged at a current of 23 mA in a 45°C environment until the voltage reached 4.2 V, and then initially discharged at a current of 23 mA until the voltage reached 2.7 V. Next, in a 45°C environment, the battery was CC charged at a current of 460 mA until the voltage reached 4.2 V, and then CC discharged at a current of 460 mA until the voltage reached 2.7 V (first cycle). This cycle was repeated 299 times, for a total of 300 cycles. For the above cycles of the Examples and Comparative Examples, the capacity retention rate was calculated by dividing the capacity determined from the CC discharge at the 300th cycle by the capacity determined from the CC discharge at the first cycle. The results are shown in Table 1.

[0093]

[0094] As shown in Table 1, a lithium secondary battery includes a negative electrode active material containing Si and an electrolyte solution, wherein the average particle size of the negative electrode active material is 5.0 μm or more and 30 μm or less, and the electrolyte solution contains, as lithium salts, LiFSI and LiPF ６ and LiPO ２ F ２ and wherein the mole fraction of LiFSI is x, and LiPF ６ is the mole fraction of LiPO ２ F ２where the molar fraction of x is z and x+y+z=1, Examples 1 to 15 satisfy the ranges of 0.05≦x≦0.95, 0.05≦y≦0.90, and 0.005≦z≦0.35, and the lithium ion concentration in the electrolyte solution is 0.7 M or more and 3.0 M or less. These Examples have a higher capacity retention rate than Comparative Examples 1 to 11, which do not have such a concentration. Therefore, it has been confirmed that these lithium secondary batteries have excellent cycle characteristics.

[0095] <Supplementary Note> Embodiments of the present disclosure include the following aspects: [1] A lithium secondary battery including a negative electrode active material containing Si and an electrolyte solution, wherein the negative electrode active material has an average particle size of 5.0 μm or more and 30 μm or less, and the electrolyte solution contains, as lithium salts, LiFSI and LiPF ６ and LiPO ２ F ２ and wherein the mole fraction of LiFSI is x, and LiPF ６ is the mole fraction of LiPO ２ F ２ and x + y + z = 1, the ranges of 0.05≦x≦0.95, 0.05≦y≦0.90, and 0.005≦z≦0.35 are satisfied, and the concentration of lithium ions in the electrolyte solution is 0.7 M or more and 3.0 M or less. [2] The lithium secondary battery according to [1], wherein the electrolyte solution contains a chain fluorine compound having at least one monovalent group represented by formulas (A) to (D), and the content of the chain fluorine compound is 0.5 mass % or more and 30 mass % or less with respect to the total amount of components in the electrolyte solution other than the chain fluorine compound. [3] The negative electrode active material is Li ｙ -SiO ｘor SiC. [4] The lithium secondary battery according to any one of [1] to [3], wherein the lithium ion concentration is 1.0 M or more and 2.5 M or less. [5] The lithium secondary battery according to any one of [1] to [4], wherein the negative electrode active material has an average particle size of 7.0 μm or more and 20 μm or less. [6] The lithium secondary battery according to any one of [1] to [5], wherein the electrolyte solution contains a fluorinated carbonate. [7] The lithium secondary battery according to any one of [1] to [6], wherein the electrolyte solution contains a chain fluorine compound A having the formula (A) or the formula (B).

[0096] The lithium secondary battery according to the present invention has excellent cycle characteristics and is therefore industrially applicable as an electricity storage device for a variety of uses.

[0097] 10... negative electrode, 20... separator, 30... positive electrode

Claims

1. A lithium secondary battery comprising a negative electrode active material containing Si and an electrolytic solution, wherein an average particle diameter of the negative electrode active material is 5.0 μm or more and 30 μm or less, and in the electrolytic solution, as a lithium salt, LiFSI, LiPF ６ and LiPO ２ F ２ are included, where a molar fraction of the LiFSI is x, a molar fraction of the LiPF ６ is y, and a molar fraction of the LiPO ２ F ２ is z, and when x + y + z = 1, a range of 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.90, and 0.005 ≤ z ≤ 0.35 is satisfied, and in the electrolytic solution, a concentration of lithium ions is 0.7 M or more and 3.0 M or less. A lithium secondary battery.

2. The lithium secondary battery according to claim 1, wherein the electrolytic solution contains a chain fluorine compound having at least one of monovalent groups represented by the following formula (A) to the following formula (D), and the content of the chain fluorine compound is 0.5% by mass or more and 30% by mass or less with respect to the total amount of components other than the chain fluorine compound in the electrolytic solution. (Note that the wavy line in the formula represents the bonding site in the monovalent group.) (Note that the wavy line in the formula represents the bonding site in the monovalent group.) 3. The negative electrode active material is Li ｙ -SiO ｘ or SiC, and the lithium secondary battery according to claim 1.

4. The lithium secondary battery according to claim 1, wherein the concentration of the lithium ions is 1.0 M or more and 2.0 M or less.

5. The lithium secondary battery according to claim 1, wherein the average particle diameter of the negative electrode active material is 7.0 μm or more and 20 μm or less.

6. The lithium secondary battery according to claim 1, wherein the electrolytic solution contains fluorinated carbonate.

7. The lithium secondary battery according to claim 1, wherein the electrolytic solution contains a chain fluorine compound A having the formula (A) or the formula (B).

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