Battery system

Abstract

Provided is a battery system having high-temperature resistance and comprising a lithium secondary battery unit and a control part, wherein the lithium secondary battery unit has a positive electrode capable of storing and releasing lithium ions, a negative electrode capable of storing and releasing lithium ions, and an electrolyte disposed between the positive electrode and the negative electrode. The electrolyte contains LiN(SO2F)2 and a non-aqueous solvent that dissolves the LiN(SO2F)2, and the concentration of the LiN(SO2F)2 with respect to the total electrolyte volume is 3 mol / L or more. The control part controls the upper limit of the operating voltage of the lithium secondary battery unit to be 4.15 V.

Description

Battery system

[0001] This invention relates to a battery system. This application claims priority based on Japanese Patent Application No. 2024-207532, filed in Japan on November 28, 2024, the contents of which are incorporated herein by reference.

[0002] Lithium-ion batteries are used as power sources for small electronic devices such as smartphones and laptops. In recent years, lithium-ion batteries have also been increasingly put into practical use in medium and large-scale power sources for applications such as automobiles and power storage.

[0003] Lithium-ion batteries degrade depending on how and where they are used. For example, high temperatures are a major cause of accelerated degradation of lithium-ion batteries. Therefore, research is being conducted to maintain battery performance even in high-temperature environments.

[0004] For example, Patent Document 1 discloses a positive electrode active material for lithium batteries that can provide a lithium secondary battery that exhibits excellent battery characteristics even under high-temperature operating environments.

[0005] Japanese Patent Publication No. 2006-134851

[0006] Lithium-ion batteries that can operate in high-temperature environments have advantages such as eliminating the need for cooling and enabling rapid charging, and further improvements are expected. The present invention has been made in view of the above circumstances and aims to provide a battery system that has high temperature resistance and good cycle characteristics. Here, high temperature resistance means durability against degradation or deterioration when the battery system is operated in a high-temperature environment of 50°C to 70°C.

[0007] In other words, the present invention includes the following embodiments: [1] A battery system having high temperature resistance, comprising a lithium secondary battery unit and a control unit, wherein the lithium secondary battery unit has a positive electrode capable of intercalating and releasing lithium ions, a negative electrode capable of intercalating and releasing lithium ions, and an electrolyte disposed between the positive electrode and the negative electrode, wherein the electrolyte is LiN(SO4). ２ F) ２ and the LiN(SO ２ F)２ It contains a non-aqueous solvent that dissolves the LiN(SO4) in proportion to the total amount of the electrolyte. ２ F) ２ [1] A battery system comprising a lithium secondary battery unit and a control unit, wherein the concentration of is 3 mol / L or more, and the control unit controls the upper limit of the operating voltage of the lithium secondary battery unit to 4.15 V. [2] The battery system according to [1], wherein the electrolyte comprises a surfactant that dissolves in the non-aqueous solvent. [3] The battery system according to [1] or [2], wherein the positive electrode comprises a positive electrode active material, and the positive electrode active material comprises at least Li and Ni. [4] A battery system having high temperature resistance, comprising a lithium secondary battery unit and a control unit, wherein the lithium secondary battery unit comprises a positive electrode capable of intercalating and releasing lithium ions, a negative electrode capable of intercalating and releasing lithium ions, and an electrolyte disposed between the positive electrode and the negative electrode, wherein the electrolyte is LiN(SO4). ２ F) ２ A battery system comprising an ionic liquid, wherein the control unit controls the upper limit of the operating voltage of the lithium secondary battery unit to 4.15V.

[0008] According to the present invention, a battery system with high temperature resistance and good cycle characteristics can be provided. Furthermore, according to the present invention, even when the battery system is operated in a high-temperature environment of 50°C to 70°C, degradation and alteration are less likely to occur, and the battery characteristics can be maintained. In addition, according to the present invention, a battery system that can operate even in a high-temperature environment can be provided, so cooling means for cooling the battery cells can be omitted or simplified. Cooling means include, for example, air cooling using fans or blowers, liquid cooling using refrigerants, and phase change cooling using phase change materials. As a result of omitting or simplifying these cooling means, the number of components and the weight of the entire battery system can be reduced, and the energy density of the battery system can be improved.

[0009] Figure 1 is a schematic diagram showing the battery system according to this embodiment. Figure 2 is a schematic diagram showing the lithium secondary battery unit included in the battery system. Figure 3 is a graph showing the relationship between voltage and capacity of the lithium secondary battery of Reference Example 1. Figure 4 is a graph showing the relationship between capacity and cycle count of the battery system of Reference Example 2. Figure 5 is a graph showing the relationship between capacity and cycle count of the battery systems of Examples 1-2 and Comparative Examples 1-2. Figure 6 is a graph showing the relationship between capacity and cycle count of the battery systems of Examples 2 and 3. Figure 7 is a graph showing the relationship between capacity and cycle count of the battery systems of Examples 3 and 4. Figure 8 is a graph showing the relationship between capacity and cycle count of the battery systems of Examples 3 and 5. Figure 9 is a graph showing the relationship between voltage and capacity of the lithium secondary battery of Reference Example 1 under 50°C conditions. Figure 10 is a graph showing the relationship between voltage and capacity of the lithium secondary battery of Reference Example 2 under 50°C conditions. Figure 11 is a graph showing the relationship between voltage and capacity of the lithium secondary battery of Example 6 under 50°C conditions. Figure 12 is a graph showing the relationship between voltage and capacity of the lithium secondary battery of Example 7 under 50°C conditions. Figure 13 is a graph showing the relationship between capacity and cycle count for Reference Example 1, Reference Example 2, Example 6, and Example 7.

[0010] <Battery System> The battery system of this embodiment will be described with reference to Figure 1. Figure 1 is a schematic diagram showing the battery system according to this embodiment. The battery system 1 of this embodiment comprises a lithium secondary battery unit 10 and a control unit 20 that controls the charging and discharging of the lithium secondary battery unit 10.

[0011] The battery system 1 shown in Figure 1 comprises three lithium secondary battery units 10. The number of lithium secondary battery units 10 in the battery system 1 can be changed as appropriate depending on the design.

[0012] The control unit 20 includes a CMU (Cell Management Unit) 21 connected one-to-one with the lithium secondary battery unit 10, and a BMU (Battery Management Unit) 22 connected to the CMU 21.

[0013] The CMU 21 is connected to the lithium secondary battery unit 10 via a signal line. The CMU 21 receives the voltage measured by a voltage sensor (voltage detection line) (not shown) provided at the terminals of the lithium secondary battery unit 10 as a signal. The CMU 21 then outputs the voltage-related signal to the BMU 22.

[0014] The BMU 22 controls the upper limit of the operating voltage of the lithium secondary battery unit 10 to 4.15V based on a signal regarding the voltage of the lithium secondary battery unit 10. That is, if the BMU 22 is about to exceed 4.15V, it stops charging the lithium secondary battery unit 10. In one embodiment of the present invention, the control unit 20 controls the operating voltage range of the lithium secondary battery unit 10 to, for example, a range of 2.5V to 4.15V.

[0015] As described above, in the battery system 1, the control unit 20 controls the upper limit of the operating voltage of the lithium secondary battery unit 10 to 4.15V. When the operating voltage is 4.15V or less, the transition metal constituting the positive electrode active material contained in the positive electrode is less likely to dissolve when the battery is operating. When the transition metal dissolves, oxygen atoms bound to the transition metal are released, and the released oxygen oxidizes and decomposes the electrolyte. Therefore, by setting the operating voltage to 4.15V or less, the electrolyte is less likely to be oxidized and decomposed. Here, transition metals include, for example, V, Cr, Mn, Fe, Co, Ni, Cu, Al, etc., and the above effect is more likely to occur when Ni is used.

[0016] In this embodiment, as shown in Figure 1, the CMU 21 is connected to each of the three lithium secondary battery units 10, and the BMU 22 is connected to the three CMU 21s, but this is not limited to this configuration. For example, multiple lithium secondary battery units 10 may be connected to a single CMU 21. Also, if the BMU 22 has the functions of the CMU 21, the control unit 20 may consist of only the BMU 22.

[0017] Further, the control unit 20 may be provided in an electronic device or the like that is driven by the power supplied from the lithium secondary battery unit 10. In this case, for example, the replaceable lithium secondary battery unit 10 and the control unit 20 may be separate bodies, and by connecting the lithium secondary battery unit 10 to the electronic device, the electronic device becomes a device equipped with the battery system 1 of the present embodiment.

[0018] The lithium secondary battery unit 10 included in the battery system 1 will be described. FIG. 2 is a schematic diagram showing an example of the lithium secondary battery unit 10. The lithium secondary battery unit 10 includes a positive electrode 2 capable of occluding and releasing lithium ions, a negative electrode 3 capable of occluding and releasing lithium ions, and an electrolytic solution 6 disposed between the positive electrode 2 and the negative electrode 3.

[0019] One aspect of the lithium secondary battery unit 10 has a separator 7 that prevents physical contact between the positive electrode 2 and the negative electrode 3, and a battery exterior body 5 that houses the positive electrode 2, the negative electrode 3, the electrolytic solution 6, and the separator 7.

[0020] (Positive Electrode) The positive electrode 2 has a positive electrode active material layer containing a positive electrode active material, and a positive electrode current collector on which the positive electrode active material layer is formed on one surface. A lead wire 8 is connected to the positive electrode current collector (not shown) that constitutes the positive electrode 2, and is drawn out to the outside of the battery exterior body 5. Examples of the positive electrode active material include a lithium metal composite oxide containing lithium (Li) and at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, and Al. Among them, as the positive electrode active material, a lithium metal composite oxide containing at least Li and Ni is preferable.

[0021] Examples of such lithium metal composite oxides include, for example, LiCoO ２ , LiNiO ２ , Li ０．９８ Ni １．０２ O ２ , LiMn ２ O ４ , Li ２ MnO ３ , LiNi ｘ Mn ｙ Co １－ｘ－ｙ O ２ [0 < x + y < 1], LiNiｘ Co ｙ Al １－ｘ－ｙ O ２ [0<x+y<1], LiCr ０．５ Mn ０．５ O ２ LiFePO ４ Li ２ FeP ２ O ７ LiMnPO ４ LiFeBO ３ Li ３ V ２ (PO ４ ) ３ Li ２ CuO ２ Li ２ FeSiO ４ Li ２ MnSiO ４ These are some examples. In one aspect of the present invention, from the viewpoint of providing a battery system that has high temperature resistance and good cycle characteristics, the positive electrode active material is LiNiO ２ It is preferable that this be the case.

[0022] Positive electrode 2 may contain conductive material or binder as needed.

[0023] (Negative electrode) The negative electrode 3 is a material that reversibly absorbs and releases Li, for example, a carbon-based material, a tin oxide-based material, a silicon oxide-based material, Li ４ Ti ５ O １２ One or more materials selected from the group that form an alloy with Li are included. A lead wire 9 is connected to the negative electrode 3 and is led out to the outside of the battery casing 5.

[0024] (Separator) As the separator 7, for example, materials such as polyethylene, polypropylene or other polyolefin resins, fluororesins, nitrogen-containing aromatic polymers, and having the form of a porous membrane, nonwoven fabric, woven fabric, etc. can be used. Furthermore, two or more of these materials may be used to form the separator, or these materials may be laminated to form the separator.

[0025] (Electrolyte) In one embodiment of the present invention, the electrolyte is LiN(SO4). ２ F) ２and LiN(SO ２ F) ２ It contains a non-aqueous solvent that dissolves the electrolyte. LiN(SO4) ２ F) ２ The concentration of is 3 mol / L or higher, and may be 3.5 mol / L or higher, or 4 mol / L or higher. LiN(SO4) relative to the total amount of electrolyte ２ F) ２ The concentrations are, for example, 6 mol / L or less, 5.5 mol / L or less, and 5.2 mol / L or less.

[0026] LiN (SO ２ F) ２ Among lithium salts, it has high thermal stability and high concentration LiN(SO4) ２ F) ２ Electrolytes containing [specific component] exhibit improved oxidation resistance because the amount of non-aqueous solvent components that are easily oxidized and reduced is reduced. Here, oxidation resistance refers to durability against deterioration and alteration caused by reactions with oxygen. Reduction resistance, on the other hand, refers to durability against deterioration and alteration caused by reduction reactions.

[0027] Examples of non-aqueous solvents included in the electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dibutyl carbonate (DBC), methyl butyl carbonate (MBC), ethyl butyl carbonate (EBC), carbonates such as 4-trifluoromethyl-1,3-dioxolan-2-one and 1,2-di(methoxycarbonyloxy)ethane, carbonates such as 4-trifluoromethyl-1,3-dioxolan-2-one and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2 Ethers such as 3,3-tetrafluoropropyldifluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate, propyl propionate, and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propanesalton, or these organic solvents into which a fluoro group has been further introduced (one or more hydrogen atoms in the organic solvent have been replaced with a fluorine atom) can be used.

[0028] In one embodiment of the present invention, LiN(SO ２ F) ２ The electrolyte, including a non-aqueous solvent, may also contain a surfactant. Adding a surfactant reduces the viscosity of the electrolyte, making it easier to impregnate the electrode and resulting in an electrolyte with high wettability to the separator.

[0029] (Surfactants) The surfactants that may be contained in the electrolyte are not limited as long as they have oxidation resistance and reduction resistance, and it is preferable that they include one or more selected from the group consisting of phosphate ester compounds, sulfate esters, nitrate esters, carboxylic acid esters, and acyclic fluorine ethers. Cationic and anionic surfactants of these types are also applicable.

[0030] (Phosphate ester compounds) The phosphate ester compounds are preferably those represented by the following formula (1).

[0031]

[0032] In formula (1), X １ , X ２ , X ３ Each of these independently represents a hydrogen atom or a monovalent organic group. However, X １ , X ２ and X ３ Except when all of them are hydrogen atoms. １ , X ２ , X ３ Examples of monovalent organic groups include any of the following (a) to (c): (a) A linear or branched alkyl group having 1 to 20 carbon atoms, which may have substituents. (b) An aryl group, which may have substituents. (c) An alkylsilyl group having 1 to 20 carbon atoms.

[0033] X １ , X ２ , X ３ When (a) is as described above, a linear or branched alkyl group having 4 to 20 carbon atoms is preferred, and a linear or branched alkyl group having 6 to 18 carbon atoms is more preferred. The alkyl group in (a) may have substituents, and examples of substituents include a nitro group or a halogen atom. Examples of halogen atoms include F, Cl, or Br.

[0034] X １ , X ２ , X ３ If (b) is the case described above, examples of substituents include alkyl groups having 1 to 5 carbon atoms, nitro groups, or halogen atoms. １ , X ２ , X ３ When (c) is as described above, an alkylsilyl group having 4 to 20 carbon atoms is preferred, and an alkylsilyl group having 6 to 18 carbon atoms is more preferred.

[0035] Examples of compounds represented by formula (1) include dialkyl phosphates or trialkyl phosphates. Dialkyl phosphates include one or more selected from the group consisting of dimethyl phosphate and dioctyl phosphate. Trialkyl phosphates include one or more selected from the group consisting of trimethyl phosphate, triethyl phosphate, tributyl phosphate, and trioctyl phosphate.

[0036] The compound represented by formula (1) may be one or more selected from the group consisting of diethylmethyl phosphate, dibutylmethyl phosphate, diethylpropyl phosphate, and dipropylethyl phosphate.

[0037] The compound represented by formula (1) may be one or more selected from the group consisting of tris(trimethylsilyl) phosphate, trityl phosphate, triphenyl phosphate, tris(4-nitrophenyl) phosphate, tris(2-ethylhexyl) phosphate, trifluoroethyldimethyl phosphate, tris(trifluoromethyl) phosphate, tris(chloroethyl) phosphate, tris(libromoneopentyl) phosphate, tris(dichloropropyl) phosphate, tris(2,6-dimethylphenyl) phosphate, and tris(trifluoroethyl) phosphate.

[0038] Among the above, the compound represented by formula (1) is preferably a trialkyl phosphate.

[0039] When the surfactant is a phosphate ester compound, the content ratio of the phosphate ester compound to the total amount of electrolyte is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and even more preferably 0.8% by mass or more. Furthermore, the content ratio of the phosphate ester compound to the total amount of electrolyte is, for example, 5% by mass or less, 4.5% by mass or less, 4% by mass or less, 3% by mass or less, and 1.5% by mass or less. The above upper and lower limits for the content ratio of the phosphate ester compound to the total amount of electrolyte can be arbitrarily combined. Examples of combinations include 0.5% by mass or more and 5% by mass or less, 0.6% by mass or more and 4.5% by mass or less, 0.8% by mass or more and 4% by mass or less, 0.5% by mass or more and 3% by mass or less, and 0.5% by mass or more and 1.5% by mass or less.

[0040] If the proportion of phosphate ester compounds relative to the total volume of the electrolyte is above the lower limit, the viscosity of the electrolyte can be sufficiently reduced. If the proportion of phosphate ester compounds relative to the total volume of the electrolyte is below the upper limit, the ionic conductivity that is sufficient for use as an electrolyte can be maintained.

[0041] (Sulfuric Acid Esters) Examples of sulfuric acid esters include linear sulfuric acid esters, cyclic sulfuric acid esters, linear sulfite esters, and cyclic sulfite esters. Examples of linear sulfuric acid esters include dialkyl sulfate compounds such as dimethyl sulfate, ethyl methyl sulfate, and diethyl sulfate. Examples of cyclic sulfuric acid esters include alkylene sulfate compounds such as 1,2-ethylene sulfate, 1,2-propylene sulfate, 1,3-propylene sulfate, 1,2-butylene sulfate, 1,3-butylene sulfate, 1,4-butylene sulfate, 1,2-pentylene sulfate, 1,3-pentylene sulfate, 1,4-pentylene sulfate, and 1,5-pentylene sulfate. Examples of linear sulfite esters include dialkyl sulfite compounds such as dimethyl sulfite, ethyl methyl sulfite, and diethyl sulfite. Examples of cyclic sulfite esters include alkylene sulfite compounds such as 1,2-ethylene sulfite, 1,2-propylene sulfite, 1,3-propylene sulfite, 1,2-butylene sulfite, 1,3-butylene sulfite, 1,4-butylene sulfite, 1,2-pentylene sulfite, 1,3-pentylene sulfite, 1,4-pentylene sulfite, and 1,5-pentylene sulfite.

[0042] When the surfactant is a sulfate ester, the content of the sulfate ester relative to the total amount of electrolyte is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and even more preferably 0.8% by mass or more. Furthermore, the content of the sulfate ester relative to the total amount of electrolyte is, for example, 5% by mass or less, 4.5% by mass or less, 4% by mass or less, 3% by mass or less, and 1.5% by mass or less. The above upper and lower limits for the content of the sulfate ester relative to the total amount of electrolyte can be arbitrarily combined. Examples of combinations include 0.5% by mass or more and 5% by mass or less, 0.6% by mass or more and 4.5% by mass or less, 0.8% by mass or more and 4% by mass or less, 0.5% by mass or more and 3% by mass or less, and 0.5% by mass or more and 1.5% by mass or less.

[0043] If the ratio of sulfate ester to the total volume of the electrolyte is above the lower limit, the viscosity of the electrolyte can be sufficiently reduced. If the ratio of sulfate ester to the total volume of the electrolyte is below the upper limit, the ionic conductivity that is sufficient for use as an electrolyte can be maintained.

[0044] (Nitric acid esters) Examples of nitrate esters include methyl nitrate, ethyl nitrate, propyl nitrate, isobutyl nitrate, tert-butyl nitrate, and isoamyl nitrate.

[0045] When the surfactant is a nitrate ester, the content of the nitrate ester relative to the total amount of electrolyte is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and even more preferably 0.8% by mass or more. Furthermore, the content of the nitrate ester relative to the total amount of electrolyte is, for example, 5% by mass or less, 4.5% by mass or less, 4% by mass or less, 3% by mass or less, and 1.5% by mass or less. The above upper and lower limits for the content of the nitrate ester relative to the total amount of electrolyte can be arbitrarily combined. Examples of combinations include 0.5% by mass or more and 5% by mass or less, 0.6% by mass or more and 4.5% by mass or less, 0.8% by mass or more and 4% by mass or less, 0.5% by mass or more and 3% by mass or less, and 0.5% by mass or more and 1.5% by mass or less.

[0046] If the ratio of nitrate ester to the total volume of the electrolyte is above the lower limit, the viscosity of the electrolyte can be sufficiently reduced. If the ratio of nitrate ester to the total volume of the electrolyte is below the upper limit, the ionic conductivity that is sufficient for use as an electrolyte can be maintained.

[0047] (Carboxylic Acid Esters) Carboxylic acid esters are ester compounds having a carboxylic acid bond (-C(=O)-O-). Examples of carboxylic acid esters include linear or cyclic carboxylic acid esters. Examples of linear carboxylic acid esters include acetate esters, propionic acid esters, and butyrate esters. Examples of acetate esters include methyl acetate, ethyl acetate, propyl acetate, and butyl acetate. Examples of propionic acid esters include methyl propionate, ethyl propionate, propyl propionate, and butyl propionate. Examples of butyrate esters include methyl butyrate, methyl isobutyrate, methyl trimethylacetate, and ethyl trimethylacetate.

[0048] Examples of cyclic carboxylic acid esters include gamma-butyrolactone, gamma-valerolactone, gamma-macarolactone, and epsilon-caprolactone.

[0049] When the surfactant is a carboxylic acid ester, the content of the carboxylic acid ester relative to the total amount of electrolyte is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and even more preferably 0.8% by mass or more. Furthermore, the content of the carboxylic acid ester relative to the total amount of electrolyte is, for example, 5% by mass or less, 4.5% by mass or less, 4% by mass or less, 3% by mass or less, and 1.5% by mass or less. The above upper and lower limits for the content of the carboxylic acid ester relative to the total amount of electrolyte can be arbitrarily combined. Examples of combinations include 0.5% by mass or more and 5% by mass or less, 0.6% by mass or more and 4.5% by mass or less, 0.8% by mass or more and 4% by mass or less, 0.5% by mass or more and 3% by mass or less, and 0.5% by mass or more and 1.5% by mass or less.

[0050] If the proportion of carboxylic acid ester relative to the total volume of the electrolyte is above the lower limit, the viscosity of the electrolyte can be sufficiently reduced. If the proportion of carboxylic acid ester relative to the total volume of the electrolyte is below the upper limit, the ionic conductivity that is sufficient for use as an electrolyte can be maintained.

[0051] (Acyclic fluorinated ether) Acyclic fluorinated ethers include compounds represented by the following formula (2).

[0052]

[0053] In formula (2), R ３１ is a halogen atom or a halogenated alkyl group having 1 to 12 carbon atoms, and R ３２ is a hydrogen atom, a halogen atom, an alkyl group having 1 to 12 carbon atoms, or a halogenated alkyl group having 1 to 12 carbon atoms.

[0054] Examples of the compound represented by formula (2) include, for example, C ２ F ５ OC ２ F ５ , C ３ F ７ OC ３ F ７ , C ４ F ９ OC ４ F ９ , C ６ F １３ OC ６ F １３ , C ２ F ５ OCH ３ , C ３ F ７ OCH ３ , C ４ F ９ OCH ３ , C ６ F １３ OCH ３ , C ２ F ５ OCH ５ , C ３ F ７ OCH ５ , C ４ F ９ OC ２ H ５ , C ２ F ５ CF(OCH ３ )C ３ F ７ , CF ３ CH ２ OCF ２ CF ２ H, CHF ２CF ２ OCH ２ CF ３ CHF ２ CF ２ CH ２ OCF ２ CF ２ H, CF ３ CF ２ CH ２ OCF ２ CHF ２ CF ３ CH ２ OCF ２ CHFCF ３ , and C ３ HF ６ CH (CH ３ ) OC ３ HF ６ Examples include the following, and it is preferable to select one or more from among them.

[0055] When the surfactant is an acyclic fluorine ether, the content of the acyclic fluorine ether relative to the total amount of electrolyte is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and even more preferably 0.8% by mass or more. Furthermore, the content of the acyclic fluorine ether relative to the total amount of electrolyte is, for example, 5% by mass or less, 4.5% by mass or less, or 4% by mass or less. The above upper and lower limits for the content of the acyclic fluorine ether relative to the total amount of electrolyte can be arbitrarily combined. Examples of combinations include 0.5% by mass or more and 5% by mass or less, 0.6% by mass or more and 4.5% by mass or less, and 0.8% by mass or more and 4% by mass or less.

[0056] If the proportion of acyclic fluorine ether relative to the total volume of the electrolyte is above the lower limit, the viscosity of the electrolyte can be sufficiently reduced. If the proportion of acyclic fluorine ether relative to the total volume of the electrolyte is below the upper limit, the ionic conductivity that is sufficient for use as an electrolyte can be maintained.

[0057] In one embodiment of the present invention, the electrolyte is LiN(SO4). ２ F) ２The electrolyte may also contain an ionic liquid. Examples of suitable ionic liquids include N-methyl-N-propylpyrrolidinium-bisfluorosulfonylamide (P13FSA), ethylmethylimidazole-bisfluoromethylsulfonylamide, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-bisfluorosulfonylamide, and ethylmethylimidazole-bistrifluorosulfonylamide.

[0058] If the electrolyte contains an ionic liquid, LiN(SO ２ F) ２ The ratio of the ionic liquid to the ionic liquid is, for example, the molar ratio (LiN(SO) ２ F) ２ It is preferable that the ionic liquid is in the range of 0.9 to 2.10.

[0059] According to the battery system of this embodiment described above, by operating under conditions where the electrolyte is less susceptible to oxidative decomposition (upper limit of operating voltage of 4.15V), and by using an electrolyte with improved oxidation resistance, it is possible to provide a battery system with improved high-temperature resistance and good cycle characteristics.

[0060] <Reference Example 1> LiNiO as the positive electrode active material ２ Using a mortar and pestle, acetylene black (manufactured by Denka Co., Ltd.) was weighed as the conductive material and PVDF (manufactured by Kureha Corporation) as the binder, in a composition of positive electrode active material:conductive material:binder = 94:2:4 (mass ratio). Then, the positive electrode active material and acetylene black were thoroughly mixed in an agate mortar, N-methyl-2-pyrrolidone (NMP: manufactured by Tokyo Chemical Industry Co., Ltd.) was added to this mixture, and then PVDF was added and mixed further in the agate mortar until homogeneous to obtain a positive electrode mixture paste.

[0061] The obtained positive electrode mixture paste was coated onto a 10 μm thick aluminum foil current collector to a thickness of 200 μm using an applicator. The current collector coated with the positive electrode mixture paste was placed in a dryer and dried while removing NMP to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 1.5 cm using an electrode punching machine, and then pressed together with a hand press to obtain a positive electrode containing the positive electrode active material.

[0062] A positive electrode was placed with aluminum foil facing downwards in the recess of the lower part of a coin cell (manufactured by Hosen Co., Ltd.). This was then combined with electrolyte 11, a polyolefin porous film (20 μm thick) as a separator, and lithium metal foil (manufactured by Honjo Metal Co., Ltd., 100 μm thick) as a negative electrode to fabricate the lithium secondary battery of Reference Example 1. The battery was assembled in a glove box under an argon atmosphere. The electrolyte 11 was LiPF ６ Mix the non-aqueous solvent with the total volume of the electrolyte and calculate the LiPF ６ The electrolyte had a concentration of 1 mol / L. For the non-aqueous solvent of electrolyte 11, a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 ratio was used.

[0063] In the lithium secondary battery of Reference Example 1, charge-discharge tests were performed at room temperature, with a current density of 50 mA / g and a voltage range of 2.5 to 4.5 V, from the 1st to the 30th cycle, and the charge-discharge capacity was measured. The results are shown in Figure 3.

[0064] As shown in Figure 3, LiPF4 is used as the lithium salt in the electrolyte. ６ Under the conditions of Reference Example 1, it was confirmed that the lithium secondary battery deteriorates easily when charged and discharged up to 4.5V. This is because LiPF ６ It is thought that the electrolyte oxidized and decomposed when charging and discharging up to 4.5V using this method.

[0065] <Reference Example 2> Using the lithium secondary battery from Reference Example 1 as a lithium secondary battery unit, and controlling the operating voltage of the lithium secondary battery unit to 2.5 to 4.15 V by a control unit, a charge-discharge test was performed at room temperature with a current density of 50 mA / g, and the charge-discharge capacity was measured. The results are shown in Figure 4.

[0066] As shown in Figure 4, controlling the operating voltage to 4.15V confirmed that the lithium secondary battery was less prone to degradation, but the initial capacity was less than 150mAh / g.

[0067] In the lithium secondary battery of Reference Example 1, charge-discharge tests were performed from the 1st to the 10th cycle under conditions of 50°C, with a current density of 50 mA / g and a voltage in the range of 2.5 to 4.15 V, and the charge-discharge capacity was measured. The results are shown in Figure 9.

[0068] As shown in Figure 9, LiPF4 is used as the lithium salt in the electrolyte. ６ Under the conditions of Reference Example 1, and further under the condition of 50°C, it was confirmed that the lithium secondary battery deteriorates easily when charged and discharged down to 4.15V. This is because LiPF ６ It is thought that the electrolyte oxidized and decomposed when charging and discharging up to 4.5V using this method.

[0069] <Reference Example 2> In the lithium secondary battery of Reference Example 1, the electrolyte 11 is LiPF ６ Mix the non-aqueous solvent with the total volume of the electrolyte and calculate the LiPF ６ An electrolyte solution 12 with a concentration of 3.3 mol / L was used, and charge-discharge tests were performed from the 1st to the 10th cycle under conditions of 50°C, with a current density of 50 mA / g and a voltage in the range of 2.5 to 4.5 V, and the charge-discharge capacity was measured. The results are shown in Figure 10.

[0070] As shown in Figure 10, LiPF4 is used as the lithium salt in the electrolyte. ６ Under the conditions of Reference Example 2, and further under the condition of 50°C, it was confirmed that the lithium secondary battery deteriorates easily when charged and discharged down to 4.15V. This is because LiPF ６ It is thought that the electrolyte oxidized and decomposed when charging and discharging up to 4.5V using this method.

[0071] <Example 1> LiNiO as the positive electrode active material ２ Using a mortar and pestle, acetylene black (manufactured by Denka Co., Ltd.) was weighed as the conductive material and PVDF (manufactured by Kureha Corporation) as the binder, in a composition of positive electrode active material:conductive material:binder = 94:2:4 (mass ratio). Then, the positive electrode active material and acetylene black were thoroughly mixed in an agate mortar, N-methyl-2-pyrrolidone (NMP: manufactured by Tokyo Chemical Industry Co., Ltd.) was added to this mixture, and then PVDF was added and mixed further in the agate mortar until homogeneous to obtain a positive electrode mixture paste.

[0072] The obtained positive electrode mixture paste was coated onto a 10 μm thick aluminum foil current collector to a thickness of 200 μm using an applicator. The current collector coated with the positive electrode mixture paste was placed in a dryer and dried while removing NMP to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 1.5 cm using an electrode punching machine, and then pressed together with a hand press to obtain a positive electrode containing the positive electrode active material.

[0073] The positive electrode described above was placed with aluminum foil facing downwards in the recess of the lower part of a coin cell (manufactured by Hosen Co., Ltd.), and these were combined with electrolyte 1, a polyolefin porous film (thickness 20 μm) as a separator, and lithium metal foil (manufactured by Honjo Metal Co., Ltd., thickness 100 μm) as a negative electrode to fabricate the lithium secondary battery unit 1 of Example 1. The battery was assembled in a glove box under an argon atmosphere. Electrolyte 1 was LiN(SO4). ２ F) ２ Mix with dimethyl carbonate and add LiN(SO4) to the total amount of electrolyte. ２ F) ２ The electrolyte has a concentration of 5.5 mol / L.

[0074] A battery system in which the operating voltage of lithium secondary battery unit 1 was controlled by a control unit to a voltage of 2.5 to 4.15 V was used, and a charge-discharge test was performed at a temperature of 50°C with a current density of 50 mA / g, and the charge-discharge capacity was measured. The results are shown in Figure 5.

[0075] <Example 2> Using a battery system in which the operating voltage of the lithium secondary battery unit 1 was controlled by the control unit to a voltage of 2.5 to 4.15 V, a charge-discharge test was performed at a temperature of 28°C with a current density of 50 mA / g, and the charge-discharge capacity was measured. The results are shown in Figure 5.

[0076] <Comparative Example 1> A charge-discharge test was performed in the same manner as in Example 1, except that electrolyte 1 was changed to electrolyte 11, and the charge-discharge capacity was measured. The results are shown in Figure 5.

[0077] <Comparative Example 2> A charge-discharge test was performed in the same manner as in Example 2, except that electrolyte 1 was changed to electrolyte 11, and the charge-discharge capacity was measured. The results are shown in Figure 5.

[0078] As shown in Figure 5, Example 1 had an initial capacity exceeding 150 mAh / g and maintained its discharge capacity up to 700 cycles even under operating conditions of 50°C. Example 2 had a smaller initial capacity than Example 1, but also maintained its discharge capacity up to 700 cycles. Under high-temperature conditions, Li ions move more easily, increasing the amount of Li ions used for discharge. For this reason, it is thought that Example 1, which was operated at 50°C, had a higher initial capacity than Example 2.

[0079] From the results of Examples 1 and 2, it was confirmed that by operating with an upper limit of 4.15V of operating voltage and using an electrolyte with improved oxidation resistance, good cycle characteristics can be obtained under temperature conditions of 28°C or 50°C.

[0080] As shown in Figure 5, LiPF is used as the lithium salt. ６ Comparative Example 1, which used an electrolyte containing [the specified substance], showed a significant decrease in discharge capacity around 100 cycles under operating conditions of 50°C. This is because LiPF [the specified substance] under high temperature conditions. ６ It is presumed that this was due to oxidative decomposition of the electrolyte containing [the substance]. In Comparative Example 2, the discharge capacity decreased around 200 cycles. This is thought to be because the lithium metal foil of the negative electrode deteriorated along with the oxidative decomposition of the electrolyte.

[0081] <Example 3> A charge-discharge test was performed in the same manner as in Example 1, except that the operating environment of the battery system was changed to a temperature of 70°C, and the charge-discharge capacity was measured. The results are shown in Figure 6. For comparison, the results of Example 2 are also shown in Figure 6.

[0082] As shown in Figure 6, by operating at an upper limit of 4.15V and using an electrolyte with improved oxidation resistance, it was confirmed that the device exhibits good cycle characteristics under a temperature of 70°C.

[0083] <Example 4> Cathode active material is LiNi ０．８２ Co ０．１５ Al ０．０３ O ２ Except for changing the operating environment of the battery system to a temperature of 70°C, a charge-discharge test was performed in the same manner as in Example 1, and the charge-discharge capacity was measured. The results are shown in Figure 7. For comparison, the results of Example 3 are also shown in Figure 7.

[0084] As shown in Figure 7, LiNi ０．８２ Co ０．１５ Al ０．０３ O ２ When changed to this, it exhibited good cycle characteristics up to about 20 cycles under a temperature of 70°C. Comparing Example 3 and Example 4, LiNiO was used as the positive electrode active material. ２ Example 3, which used [the specified method], exhibited better cycle characteristics.

[0085] <Example 5> A charge-discharge test was performed in the same manner as in Example 1, except that electrolyte 1 was changed to electrolyte 2 and the operating environment of the battery system was changed to a temperature of 70°C, and the charge-discharge capacity was measured. The results are shown in Figure 8. For comparison, the results of Example 3 are also shown in Figure 8. Electrolyte 2 is LiFSA(LiN(SO)). ２ F) ２ A mixed solvent of P13FSA in a 1:1 ratio was used.

[0086] As shown in Figure 8, it was confirmed that good cycle characteristics were also observed when using electrolyte 2.

[0087] <Example 6> A charge-discharge test was performed in the same manner as in Example 1, except that electrolyte 1 was changed to electrolyte 3 and the operating environment of the battery system was changed to a temperature of 50°C, and the charge-discharge capacity was measured. The results are shown in Figure 11. Electrolyte 3 is LiFSA(LiN(SO4). ２ F) ２ Mix ) and a non-aqueous solvent, and add LiFSA (LiN(SO)) to the total volume of the electrolyte. ２ F) ２ The electrolyte solution had a concentration of 1.5 mol / L. For the non-aqueous solvent of electrolyte solution 3, a mixed solvent of ethylene carbonate and dimethyl carbonate in a 1:1 ratio was used.

[0088] As shown in Figure 11, when using electrolyte 3, it was confirmed that the decrease in discharge capacity from the 1st to the 30th cycle was small, indicating that capacity degradation was less likely.

[0089] <Example 7> A charge-discharge test was performed in the same manner as in Example 1, except that electrolyte 1 was changed to electrolyte 4 and the operating environment of the battery system was changed to a temperature of 50°C, and the charge-discharge capacity was measured. The results are shown in Figure 12. Electrolyte 4 is LiFSA(LiN(SO4)). ２ F) ２ Mix ) and a non-aqueous solvent, and add LiFSA (LiN(SO)) to the total volume of the electrolyte. ２ F) ２ The electrolyte solution had a concentration of 5.5 mol / L. Dimethyl carbonate was used as the non-aqueous solvent for electrolyte solution 4.

[0090] As shown in Figure 12, when using electrolyte 4, it was confirmed that the decrease in discharge capacity from the 1st to the 30th cycle was small, indicating that capacity degradation was less likely.

[0091] Figure 13 shows Reference Example 1 (LiPF ６ :1 mol / L. EC / DMC), Reference Example 2 (LiPF ６ :3.3mol / L. EC / DMC), Example 6 (LiPF ６ :1.5mol / L. EC / DMC), Example 7 (LiPF ６ : 5.5 mol / L. (DMC) Shows the relationship between capacity and cycle count.

[0092] As shown in Figure 13, Example 7 maintained its discharge capacity up to 150 cycles even under operating conditions of 50°C.

[0093] 1: Battery system, 10: Lithium secondary battery unit, 20: Control unit, 21: CMU, 22: BMU, 2: Positive electrode, 3: Negative electrode, 5: Battery casing, 6: Electrolyte, 7: Separator

Claims

1. A battery system comprising a lithium secondary battery unit and a control unit, wherein the lithium secondary battery unit has a positive electrode capable of intercalating and releasing lithium ions, a negative electrode capable of intercalating and releasing lithium ions, and an electrolyte disposed between the positive electrode and the negative electrode, wherein the electrolyte is LiN(SO4). ２ F) ２ and the LiN(SO ２ F) ２ It contains a non-aqueous solvent that dissolves the LiN(SO4) in proportion to the total amount of the electrolyte. ２ F) ２ The concentration of the solution is 3 mol / L or higher, and the control unit controls the upper limit of the operating voltage of the lithium secondary battery unit to 4.15V, in a battery system.

2. The battery system according to claim 1, wherein the electrolyte contains a surfactant that dissolves in the non-aqueous solvent.

3. The battery system according to claim 1 or 2, wherein the positive electrode includes a positive electrode active material, and the positive electrode active material includes at least Li and Ni.

4. A battery system comprising a lithium secondary battery unit and a control unit, wherein the lithium secondary battery unit has a positive electrode capable of intercalating and releasing lithium ions, a negative electrode capable of intercalating and releasing lithium ions, and an electrolyte disposed between the positive electrode and the negative electrode, wherein the electrolyte is LiN(SO4). ２ F) ２ A battery system comprising an ionic liquid, wherein the control unit controls the upper limit of the operating voltage of the lithium secondary battery unit to 4.15V.

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