Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery excellent in cycle characteristics.SOLUTION: The lithium ion secondary battery includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte. The negative electrode comprises a negative electrode active material which contains silicon or a silicon compound, a first compound containing a first element, and a second compound containing a second element. The electrolyte contains a first imide salt containing the first element and an imide anion, and a second imide salt containing the second element and an imide anion. The first element is an element forming a divalent or trivalent cation in the electrolyte. The second element is an element forming a monovalent cation in the electrolyte.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Lithium ion secondary batteries are widely used as power sources for mobile devices such as mobile phones and notebook computers, and for hybrid cars and the like.

[0003] The capacity of a lithium ion secondary battery mainly depends on the active material of the electrode. Graphite is generally used as the negative electrode active material, but a higher capacity negative electrode active material is required. Therefore, a negative electrode active material containing silicon (Si), which has a much larger theoretical capacity than the theoretical capacity of graphite (372 mAh / g), has attracted attention.

[0004] For example, Patent Documents 1 to 3 describe that a negative electrode active material containing silicon can be used in a lithium ion secondary battery. Patent Documents 1 to 3 also describe that the cycle characteristics of a lithium ion secondary battery can be improved by using an electrolytic solution containing a specific lithium salt and an ether compound.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] Further improvement of cycle characteristics is required.

[0007] The present disclosure has been made in view of the above problems, and an object thereof is to provide a lithium-ion secondary battery having excellent cycle characteristics.

Means for Solving the Problems

[0008] In order to solve the above problems, the following means are provided.

[0009] (1) The lithium-ion secondary battery according to the first aspect includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolytic solution. The negative electrode has a negative electrode active material containing silicon or a silicon compound, a first compound containing a first element, and a second compound containing a second element. The electrolytic solution contains a first imide salt containing the first element and an imide anion, and a second imide salt containing the second element and an imide anion. The first element is one or more elements that form a divalent or trivalent cation in the electrolytic solution, and the second element is one or more elements that form a monovalent cation in the electrolytic solution.

[0010] (2) In the lithium-ion secondary battery according to the above aspect, the first element may be any one or more elements selected from the group consisting of Mg, Ca, Cs, Al, and Zn.

[0011] (3) In the lithium-ion secondary battery according to the above aspect, the second element may be any one or more elements selected from the group consisting of K and Na.

[0012] (4) In the lithium-ion secondary battery according to the above aspect, the molar concentration ratio of the first imide salt in the electrolytic solution may be 5% or more and 20% or less with respect to the molar concentration ratio of the lithium salt in the electrolytic solution.

[0013] (5) In the lithium-ion secondary battery according to the above aspect, the molar concentration ratio of the second imide salt in the electrolytic solution may be 10% or more and 50% or less with respect to the molar concentration ratio of the first imide salt.

[0014] (6) In the lithium-ion secondary battery according to the above aspect, the first compound is any one or more selected from the group consisting of fluoride, oxide, silicide, silicate, and phosphate oxide, and the second compound may be any one or more selected from the group consisting of fluoride, oxide, silicide, silicate, and phosphate oxide.

[0015] (7) In the lithium-ion secondary battery according to the above aspect, the negative electrode active material has a core and a coating layer covering the core, and the coating layer may contain the first compound and the second compound.

[0016] (8) In the lithium-ion secondary battery according to the above aspect, the median diameter (D50) of the negative electrode active material may be 500 nm or more and 10 μm or less.

[0017] (9) In the lithium-ion secondary battery according to the above aspect, the discharge specific capacity of the negative electrode active material may be 1000 mAh / g or more and 2500 mAh / g or less.

Advantages of the Invention

[0018] The lithium-ion secondary battery according to the above aspect has excellent cycle characteristics.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0020] Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. The drawings used in the following description may show, for the sake of clarity, parts that are characteristic in an enlarged manner, and the dimensional ratios of each component may be different from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be implemented with appropriate modifications within the scope of not changing the gist thereof.

[0021] "Lithium-ion secondary battery" FIG. 1 is a schematic diagram of a lithium-ion secondary battery according to the first embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generation element 40. The power generation element 40 is connected to the outside by a pair of connected terminals 60, 62. The non-aqueous electrolyte is housed inside the exterior body 50.

[0022] (Power generation element) The power generation element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

[0023] [Positive electrode] The positive electrode 20 has, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[0024] [Positive electrode current collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate such as aluminum, copper, nickel, titanium, stainless steel, etc. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0025] [Positive electrode active material layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive auxiliary agent and a binder as required.

[0026] The basis weight of the positive electrode active material layer 24 is, for example, 15 mg / cm2 or more, preferably 20 mg / cm 2 or more. The basis weight of the positive electrode active material layer 24 is, for example, preferably 35 mg / cm 2 or less. The basis weight means the mass of the positive electrode active material layer 24 supported on the surface of the positive electrode current collector 22 per unit area. When the basis weight is large, the amount of the positive electrode active material per unit area increases, and the capacity of the battery increases. On the other hand, when the basis weight is too large, it becomes difficult for the electrolytic solution to impregnate into the positive electrode active material layer 24.

[0027] The positive electrode active material includes an electrode active material capable of reversibly proceeding with the occlusion and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

[0028] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 compound (in the general formula, x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound (LiV2O5), olivine type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material may be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0029] The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF3, conjugated polymers containing organic conductive substances, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. The lithium-free material may be used alone or in combination of a plurality of materials. When the positive electrode active material is a lithium-free material, for example, discharging is first performed. Lithium is inserted into the positive electrode active material by discharging. In addition, the lithium-free material of the positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0030] The conductive assistant enhances the electron conductivity between the positive electrode active materials. Examples of the conductive assistant include carbon powder, carbon nanotubes, carbon materials, metal fine powder, a mixture of carbon materials and metal fine powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, Ketjen black, etc. Examples of the metal fine powder include powders of copper, nickel, stainless steel, iron, etc.

[0031] The binder binds the active materials to each other. A known binder can be used. Examples of the binder include fluororesins. Examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc.

[0032] In addition to the above, the binder may be, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), or other vinylidene fluoride-based fluororubbers. The binder may also be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, or the like.

[0033] The binder may also be an electronically conductive polymer or an ionically conductive polymer. Examples of the electronically conductive polymer include polyacetylene, polythiophene, polyaniline, and the like. Examples of the ionically conductive polymer include a composite material of a polyether-based polymer compound and a lithium salt. Examples of the polyether-based polymer compound include polyethylene oxide, polypropylene oxide, and the like. Examples of the lithium salt include LiFSI, LiTFSI, LiBETI, LiClO4, LiBF4, LiPF6, and the like.

[0034] The constituent ratio of the positive electrode active material in the positive electrode active material layer 24 is, for example, 80% or more and 98% or less by mass ratio. The constituent ratio of the conductive assistant in the positive electrode active material layer 24 is, for example, 1.0% or more and 10% or less by mass ratio. The constituent ratio of the binder in the positive electrode active material layer 24 is, for example, 1.0% or more and 10% or less by mass ratio.

[0035] <Negative electrode> The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0036] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The same material as the positive electrode current collector 22 can be used for the negative electrode current collector 32.

[0037] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. Further, the negative electrode active material layer 34 may contain a conductive assistant as needed. The negative electrode active material contains silicon or a silicon compound, a first compound containing a first element, and a second compound containing a second element.

[0038] The silicon or silicon compound may be crystalline or amorphous. The silicon is elemental silicon. The silicon compound is a compound containing silicon element. The silicon compound is, for example, a silicon alloy, silicon oxide, silicon carbide, or a composite of silicon and other substances (e.g., carbon). The silicon compound may be a mixture of these.

[0039] The silicon alloy is represented by, for example, XnSi. X is a cation. X is, for example, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Ti, Na, K. X is preferably one or more elements that can be a cation of divalent or higher. Elements that can be a cation of divalent or higher are, for example, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Ti. n satisfies 0≦n≦0.5.

[0040] Silicon oxide is represented by SiO x where x satisfies 0≦x≦2, preferably 0.8≦x≦2. The silicon oxide may consist only of SiO2, may consist only of SiO, or may be a mixture of SiO and SiO2. Also, a part of oxygen in the silicon oxide may be deficient. The silicon oxide may be crystalline or amorphous.

[0041] The composite of silicon and other substances is obtained by subjecting silicon and other substances to a composite treatment. For example, when silicon and carbon are mixed while applying a shearing force thereto, a composite of silicon and carbon can be obtained. In the composite, the weight ratio of silicon is, for example, 30 wt% or more and 70 wt% or less, preferably 50 wt% or more and 70 wt% or less.

[0042] The first compound is a compound containing a first element. The first compound is, for example, one or more selected from the group consisting of fluorides, oxides, silicides, and silicate oxides. The first element is one or more elements that form divalent or trivalent cations in the electrolyte. The first element is, for example, any one or more elements selected from the group consisting of Mg, Ca, Cs, Al, and Zn. The first element is not limited to a single element, and may be a plurality of elements that satisfy the above conditions. For example, when the first element is Mg, the first compound is, for example, MgF2, MgO, Mg2Si, Li x Mg y Si, and Mg2SiO4.

[0043] The second compound is a compound containing a second element. The second compound is, for example, one or more selected from the group consisting of fluorides, oxides, silicides, and silicate oxides. The second element is one or more elements that form monovalent cations in the electrolyte. The second element is, for example, one or more elements of K and Na. The second element is not limited to either K or Na, and may be both K and Na. For example, when the second element is Na, the first compound is, for example, NaF, Na2O.

[0044] FIG. 2 is a schematic diagram of the negative electrode active material 35 according to the first embodiment. The negative electrode active material 35 may have a core 36 and a coating layer 37. The coating layer 37 covers at least a part of the core 36. The coating layer 37 may cover the entire surface of the core 36. The coating layer 37 makes the insertion and extraction reaction of Li uniform and suppresses the deterioration of the negative electrode active material 35.

[0045] The core 36 contains silicon or a silicon compound. The core 36 is, for example, silicon or a silicon compound. The coating layer 37 contains, for example, silicon or a silicon compound, a first compound, and a second compound. The coating layer 37 may contain a carbon material, metal fine powder, a mixture of a carbon material and metal fine powder, a conductive oxide, an inorganic compound, etc. The carbon material is, for example, carbon powder, carbon nanotubes, etc. The metal fine powder is, for example, copper, cobalt, iron, etc. The inorganic material is, for example, ceramics, such as oxides, carbides, nitrides, borides, etc.

[0046] The coating layer 37 may have a first layer 37A and a second layer 37B. The second layer 37B is the outermost surface layer of the coating layer 37. The second layer 37B may contain a reaction product of a first element or a second element and a material contained in the electrolyte. For example, when the first element is Mg, the reaction product of the first element and the material contained in the electrolyte is, for example, Mg3(PO4)2. For example, when the second element is Na, the reaction product of the second element and the material contained in the electrolyte is, for example, Na3PO4.

[0047] The median diameter (D50) of the negative electrode active material is, for example, 500 nm or more and 10 μm or less. When the particle size of the negative electrode active material is sufficiently small, aggregation of the negative electrode active material in the negative electrode active material layer 34 can be suppressed. Also, if the median diameter of the negative electrode active material is within a predetermined range, aggregation of the first compound and the second compound in the negative electrode active material can be suppressed. By these compounds not aggregating, damage to the negative electrode active material due to volume expansion can be suppressed.

[0048] Also, the smaller the particle size of the negative electrode active material, the larger the specific surface area of the negative electrode active material. The larger the specific surface area of the negative electrode active material, the higher the contact frequency between the electrolyte and the negative electrode active material, and the easier it is for the electrolyte to decompose. On the other hand, the larger the particle size of the negative electrode active material, the less likely it is for locally low-potential or high-potential locations to occur on the surface of the negative electrode active material. Potential non-uniformity can cause redox decomposition of the electrolyte. If the median diameter of the negative electrode active material is within the above range, the cycle characteristics of the lithium-ion secondary battery 100 are improved.

[0049] The discharge specific capacity of the negative electrode active material is, for example, 1000 mAh / g or more and 2500 mAh / g or less. The discharge specific capacity of the negative electrode active material can be adjusted by changing the thickness of the positive electrode active material layer 24 of the positive electrode 20. The larger the discharge specific capacity of the negative electrode active material, the lower the potential of the negative electrode, and the smaller the discharge specific capacity, the higher the potential of the negative electrode. The increase or decrease in the potential of the negative electrode can cause a decrease in cycle characteristics. The higher the potential of the negative electrode, the more lithium can be trapped, but the electrolyte and the coating layer 37 are more likely to undergo oxidative decomposition. The lower the potential of the negative electrode, the more the expansion and contraction of the negative electrode active material 35 can be suppressed, but the electrolyte and the coating layer 37 are more likely to undergo reductive decomposition. If the discharge specific capacity of the negative electrode active material is within the above range, it has a high capacity and excellent cycle characteristics.

[0050] The conductive assistant enhances the electron conductivity between the negative electrode active materials. The same conductive assistant as that used for the positive electrode 20 can be used.

[0051] The binder binds the negative electrode active materials to each other and binds the negative electrode active material and the negative electrode current collector 32. The same binder as that used for the positive electrode 20 can be used.

[0052] The contents of the negative electrode active material, the conductive assistant, and the binder in the negative electrode active material layer 34 are not particularly limited. The constituent ratio of the negative electrode active material in the negative electrode active material layer 34 is, for example, 70% or more and 100% or less by mass ratio. Also, the constituent ratio of the conductive assistant in the negative electrode active material layer 34 is, for example, 0% or more and 10% or less by mass ratio, and the constituent ratio of the binder in the negative electrode active material layer 34 is, for example, 0% or more and 20% or less by mass ratio.

[0053] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 spreads in the plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0054] Separator 10 has, for example, a porous structure with electrical insulation. Separator 10 is, for example, a single layer or a laminate of a polyolefin film. Separator 10 may also be a stretched film of a mixture such as polyethylene or polypropylene. Separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. Separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. Separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is obtained by applying a mixture of a resin such as PVDF or CMC and an inorganic substance such as alumina or silica on the surface of the above film. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.

[0055] <Electrolyte solution> The electrolyte solution is enclosed in the exterior body 50 and impregnates the power generation element 40. The non-aqueous electrolyte solution has, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent. The electrolyte solution contains, for example, a solvent, an electrolyte, a first imide salt, and a second imide salt. Each of the first imide salt and the second imide salt may be one type or two or more types.

[0056] The solvent is not particularly limited as long as it is a solvent generally used in lithium-ion secondary batteries. The solvent contains, for example, any one of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may contain these mixed in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), etc. Examples of the chain carbonate compound include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of the cyclic ester compound include γ-butyrolactone, etc. Examples of the chain ester compound include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0057] The solvent may contain a fluorinated organic solvent. The fluorinated organic solvent forms a good film on the surface of the negative electrode active material. When the decomposition products of the fluorinated organic solvent and the first element and the second element contained in the electrolytic solution are contained in the film, Li ions are easily transported in the film. As a result, an increase in the resistance of the lithium ion secondary battery 100 is suppressed.

[0058] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, etc. The lithium salt may be used alone or in combination of two or more. From the viewpoint of ionization degree, it is preferable that the electrolyte contains LiPF6.

[0059] The first imide salt contains a first element and an imide anion. The first element is as described above. The first element contained in the electrolytic solution and the first element contained in the negative electrode active material layer 34 are, for example, the same element. The first element contained in the electrolytic solution and the first element contained in the negative electrode active material layer 34 may be different elements. The first imide salt is not limited to one type and may be contained in a plurality of types in the electrolytic solution.

[0060] The second imide salt contains a second element and an imide anion. The second element is as described above. The second element contained in the electrolytic solution and the second element contained in the negative electrode active material layer 34 are, for example, the same element. The second element contained in the electrolytic solution and the second element contained in the negative electrode active material layer 34 may be different elements. The second imide salt is not limited to one type and may be contained in a plurality of types in the electrolytic solution.

[0061] The imide anion is, for example, (SO2F)2N - (FSI - : bis(fluorosulfonyl)imide anion), (SO2CF3)2N - (TFSI -: Bis(trifluoromethanesulfonyl)imide anion, (SO2C2F5)2N - (BETI - : Bis(pentafluoroethanesulfonyl)imide anion, (SO2F)(SO2CF3)N - , (SO2CF3)(SO2C2F5)N - It is. The imide anion may be used alone or in combination of two or more kinds.

[0062] The molar concentration ratio of the first imide salt in the electrolyte is, for example, 5% or more and 20% or less with respect to the molar concentration ratio of the lithium salt in the electrolyte. When the first imide salt is sufficiently contained in the electrolyte, the decomposition of the electrolyte is suppressed during charge and discharge of the lithium ion secondary battery 100. On the other hand, when the first imide salt is excessively contained in the electrolyte, the viscosity of the electrolyte increases, and the impregnation property of the electrolyte into the positive electrode 20 and the negative electrode 30 of the electrolyte decreases.

[0063] The molar concentration ratio of the second imide salt in the electrolyte is, for example, 10% or more and 50% or less with respect to the molar concentration ratio of the first imide salt in the electrolyte. Also, the molar concentration ratio of the second imide salt in the electrolyte is, for example, 1% or more and 5% or less with respect to the molar concentration ratio of the lithium salt in the electrolyte. When the second imide salt is sufficiently contained in the electrolyte, the decomposition of the electrolyte is suppressed during charge and discharge of the lithium ion secondary battery 100. On the other hand, when the second imide salt is excessively contained in the electrolyte, the viscosity of the electrolyte increases, and the impregnation property of the electrolyte into the positive electrode 20 and the negative electrode 30 of the electrolyte decreases.

[0064] The molar concentration ratio of the first element and lithium in the electrolyte can be measured by, for example, gas chromatography, ICP (inductively coupled plasma) emission analysis, ICP mass spectrometry, etc.

[0065] <Outer package> The outer package 50 seals the power generation element 40 and the non-aqueous electrolyte therein. The outer package 50 suppresses the leakage of the non-aqueous electrolyte to the outside and the intrusion of moisture or the like from the outside into the lithium ion secondary battery 100.

[0066] The exterior body 50 has, for example, as shown in FIG. 1, a metal foil 52 and resin layers 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated from both sides with a polymer film (resin layer 54).

[0067] As the metal foil 52, for example, an aluminum foil can be used. For the resin layer 54, a polymer film such as polypropylene can be used. The material constituting the resin layer 54 may be different between the inner side and the outer side. For example, as the outer material, a polymer with a high melting point, such as polyethylene terephthalate (PET), polyamide (PA), etc., can be used, and as the material of the inner polymer film, polyethylene (PE), polypropylene (PP), etc. can be used.

[0068] <Terminal> The terminals 60 and 62 are respectively connected to the positive electrode 20 and the negative electrode 30. The terminal 60 connected to the positive electrode 20 is the positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is the negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection with the outside. The terminals 60 and 62 are formed of a conductive material such as aluminum, nickel, copper, etc. The connection method can be welding or screwing. The terminals 60 and 62 are preferably protected with insulating tape to prevent short circuit.

[0069] "Method for manufacturing a lithium-ion secondary battery" The lithium-ion secondary battery 100 is manufactured by preparing the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50 respectively and assembling them. Hereinafter, an example of the manufacturing method of the lithium-ion secondary battery 100 will be described.

[0070] The negative electrode 30 is manufactured, for example, by sequentially performing a compounding process, a slurry preparation process, an electrode coating process, a drying process, and a rolling process.

[0071] The compounding process is a process of mixing silicon or a silicon compound with a first compound and a second compound while applying a shearing force. When the compounding process is performed, the surface of the silicon or silicon compound is coated with the first compound and the second compound. Also, the particle size of the negative electrode active material can be adjusted according to the degree of the mixing. Further, the negative electrode active material after production may be sieved to make the particle sizes uniform.

[0072] The slurry preparation process is a process of mixing the compounded negative electrode active material, a binder, and a solvent to prepare a slurry. In the slurry preparation process, a conductive assistant may be added as necessary. The solvent is, for example, water, N-methyl-2-pyrrolidone, or the like. The composition ratio of the negative electrode active material, the conductive material, and the binder is preferably 70 wt% to 100 wt%: 0 wt% to 10 wt%: 0 wt% to 20 wt% by mass ratio. These mass ratios are adjusted so as to be 100 wt% in total.

[0073] The electrode coating process is a process of coating the slurry on the surface of the negative electrode current collector 32. The coating method of the slurry is not particularly limited. For example, the slit die coating method, the doctor blade method can be used as the coating method of the slurry.

[0074] The drying process is a process of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere of 80°C to 150°C. When the slurry dries, a negative electrode active material layer 34 is formed on the negative electrode current collector 32.

[0075] The rolling process is performed as necessary. The rolling process is a process of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling process is performed, for example, with a roll press device or the like.

[0076] The positive electrode 20 can be produced in the same procedure as the negative electrode 30 except that the compounding process is not performed. As the separator 10 and the exterior body 50, commercially available ones can be used.

[0077] The electrolytic solution can be prepared, for example, by adding and mixing a first imide salt and a second imide salt to a mixture of a lithium salt and a solvent.

[0078] Next, the separator 10 is positioned between the prepared positive electrode 20 and negative electrode 30, and these are laminated to produce a power generation element 40. When the power generation element 40 is a wound body, these are wound around one end side of the positive electrode 20, negative electrode 30, and separator 10 as an axis.

[0079] Finally, the power generation element 40 is enclosed in an exterior body 50. The non-aqueous electrolytic solution is injected into the exterior body 50. By performing decompression, heating, etc. after injecting the non-aqueous electrolytic solution, the non-aqueous electrolytic solution impregnates the power generation element 40. By applying heat, etc. to seal the exterior body 50, a lithium-ion secondary battery 100 is obtained. Note that instead of injecting the electrolytic solution into the exterior body 50, the power generation element 40 may be impregnated with the electrolytic solution.

[0080] The lithium-ion secondary battery 100 according to the first embodiment is excellent in safety. The reason for this will be described below.

[0081] When the negative electrode active material expands in volume, the negative electrode active material may be damaged. When the negative electrode active material is damaged, the active surface is exposed to the electrolytic solution. When the active surface and the solvent come into contact, the solvent may decompose, and the cycle characteristics of the lithium-ion secondary battery may deteriorate.

[0082] The lithium-ion secondary battery 100 according to the first embodiment includes a first compound and a second compound as negative electrode active materials. Generally, when a plurality of compounds exist in a crystal, the crystals of the respective compounds are less likely to coarsen. Therefore, when the first compound and the second compound coexist in the negative electrode active material, these compounds are less likely to form coarse crystals. When the crystals in the negative electrode active material are refined, local hardening of the negative electrode active material is suppressed, and breakage of the negative electrode active material is suppressed. Further, when the negative electrode active material is refined, hardening of the negative electrode coating film obtained by drying the slurry containing the negative electrode active material is suppressed, and breakage of the negative electrode coating film is suppressed. If breakage of the negative electrode active material and the negative electrode coating film is suppressed, the active surface formed during charge and discharge of the lithium-ion secondary battery is reduced. Therefore, in the lithium-ion secondary battery 100 according to the first embodiment, decomposition of the solvent due to the active surface is suppressed, and the cycle characteristics are excellent.

[0083] Further, the first compound and the second compound are polarized in the negative electrode active material during the charge and discharge reaction. The polarized compound attracts lithium ions. When lithium ions are attracted near the negative electrode active material, a solvent having a shared electron pair moves relatively away from the negative electrode active material compared to the electrolyte, the first imide salt, and the second imide salt. The probability of contact between the active surface and the solvent is lower than the probability of contact between the active surface and the electrolyte (lithium salt) or the imide salt. The active surface reacts with the electrolyte (lithium salt) or the imide salt rather than the solvent to reform the SEI film. That is, in the lithium-ion secondary battery 100 according to the first embodiment, decomposition of the solvent due to reaction with the active surface is suppressed, and the cycle characteristics are excellent.

[0084] When the first imide salt and the second imide salt are present in the electrolytic solution, the first element and the second element are incorporated into the negative electrode active material during the charge and discharge reaction of the lithium-ion secondary battery 100. Even when an active surface is generated due to expansion and contraction of the negative electrode active material, the first element and the second element are supplied from the electrolytic solution, so that the first element and the second element function as an anchor for the negative electrode active material. As a result, the strength of the negative electrode active material increases, and the negative electrode active material can be prevented from collapsing.

[0085] In addition, the reaction between the active surface described above and the electrolyte (lithium salt) or imide salt occurs on the surface of the negative electrode active material. Therefore, when the first element and the second element are included in the coating layer 37 of the negative electrode active material, the effect of suppressing the deterioration of the cycle characteristics of the lithium ion secondary battery 100 is high.

[0086] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, each configuration and their combinations in each embodiment are examples, and additions, omissions, substitutions, and other changes of the configuration are possible without departing from the spirit of the present invention.

Example

[0087] 「Example 1」 A positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive assistant, a binder, and a solvent.

[0088] As the positive electrode active material, Li x CoO2 was used. As the conductive assistant, acetylene black was used. As the binder, polyvinylidene fluoride (PVDF) was used. As the solvent, N-methyl-2-pyrrolidone was used. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive assistant, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. The loading amount of the positive electrode active material in the dried positive electrode active material layer was 25 mg / cm 2 . The solvent was removed from the positive electrode slurry in a drying furnace to form a positive electrode active material layer. The positive electrode active material layer was pressed by a roll press to produce a positive electrode.

[0089] Crystalline silicon, magnesium oxide, and sodium oxide were compounded using a Hosokawa Micron Mechanofusion inclined at 10 degrees to produce a negative electrode active material. The rotation speed of the apparatus during the compounding was 2500 rpm. Magnesium oxide is a first compound containing a first element. Sodium oxide is a second compound containing a second element. The median diameter of the negative electrode active material after the compounding treatment was 3 μm. The negative electrode active material had a coating layer containing Mg and Na formed on its surface.

[0090] Next, a negative electrode slurry was applied to one side of a copper foil with a thickness of 10 μm. The negative electrode slurry was prepared by mixing a negative electrode active material, a conductive assistant, a binder, and a solvent. The negative electrode active material was the one subjected to the above composite treatment. The conductive assistant used was carbon black. The binder used was a polyimide resin. The solvent used was N-methyl-2-pyrrolidone. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive assistant, and 5 parts by mass of the binder were mixed in N-methyl-2-pyrrolidone to prepare the negative electrode slurry. The loading amount of the negative electrode active material in the dried negative electrode active material layer was 2.5 mg / cm 2 was used. The solvent was removed from the negative electrode slurry in a drying furnace to form a negative electrode active material layer. The negative electrode active material layer was pressed by roll pressing and then heat-baked at 300 °C or higher for 5 hours in a nitrogen atmosphere.

[0091] Next, a solvent was prepared by mixing in a volume ratio of fluoroethylene carbonate (FEC): ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC) = 5:5:20:70. LiPF6 was dissolved in this mixed solvent to a concentration of 1 mol / L. Then, Mg(TFSI)2 as the first imide salt and NaTFSI as the second imide salt were added to this solution to prepare an electrolyte solution. The concentration of the first imide salt with respect to the Li salt concentration was adjusted to 10% (0.1 mol / L), and the concentration of the second imide salt with respect to the Li salt concentration was adjusted to 3% (0.03 mol / L). The concentration of the second imide salt with respect to the first imide salt concentration was 30%.

[0092] (Fabrication of Lithium-Ion Secondary Battery for Evaluation) The fabricated negative electrode and positive electrode were laminated via a separator (porous polyethylene sheet) such that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. A negative electrode lead made of nickel was attached to the negative electrode of the laminate. A positive electrode lead made of aluminum was attached to the positive electrode of the laminate. The positive electrode lead and the negative electrode lead were welded using an ultrasonic welder. This laminate was inserted into an exterior body of an aluminum laminate film, and a heat seal was performed except for one location around it to form a closed portion. And finally, after injecting the above electrolytic solution into the exterior body, the remaining one location was heat-sealed under reduced pressure using a vacuum sealer to fabricate a lithium-ion secondary battery. Note that two lithium-ion secondary batteries were fabricated, one for analyzing the electrolytic solution composition and the other for evaluating charge-discharge characteristics.

[0093] The electrolytic solution was collected from the lithium-ion secondary battery for analyzing the electrolytic solution composition. Next, the composition of the collected electrolytic solution was analyzed using ICP emission. As a result, it was confirmed that the contents of Mg and Na in the electrolytic solution were the same as those at the time of preparing the electrolytic solution.

[0094] (Measurement of capacity retention rate after 300 cycles) The cycle characteristics of the lithium-ion secondary battery for evaluation were measured. The cycle characteristics were measured using a secondary battery charge-discharge test device (manufactured by Hokuto Denko Corporation).

[0095] Charging was performed at a constant current charging rate of 1.0C (the current value at which charging is completed in 1 hour when performing constant current charging at 25°C) until the battery voltage reached 4.4V, and discharging was performed at a constant current discharging rate of 1.0C until the battery voltage reached 3.0V. The discharge capacity after charging and discharging was detected, and the battery capacity Q1 before the cycle test was obtained.

[0096] The battery for which the battery capacity Q1 was obtained above was charged again using a secondary battery charge-discharge test device until the battery voltage reached 4.4V with constant current charging at a charging rate of 1.0C, and then discharged until the battery voltage reached 3.0V with constant current discharging at a discharging rate of 1.0C. The above charge and discharge were counted as one cycle, and 300 cycles of charge and discharge were performed. After that, the discharge capacity after 300 cycles of charge and discharge was detected, and the battery capacity Q2 after 300 cycles was obtained.

[0097] From the capacities Q1 and Q2 obtained above, the capacity retention rate E after 300 cycles was obtained. The capacity retention rate E is obtained by E = Q2 / Q1 × 100. The capacity retention rate E of Example 1 was 88%.

[0098] "Examples 2 and 3" Examples 2 and 3 are different from Example 1 in that when preparing the negative electrode active material, silicon oxide or a composite of silicon and carbon was used instead of crystalline silicon. Under other conditions, the capacity retention rate E was obtained in the same manner as in Example 1. The results are summarized in Table 1.

[0099] "Examples 4 to 10" Examples 4 to 10 are different from Example 1 in that the imide salt added to the electrolyte and the first element contained in the negative electrode active material were changed. In Examples 8 to 10, both the first element contained in the imide salt and the first element contained in the negative electrode active material were two or more types. Under other conditions, the capacity retention rate E was obtained in the same manner as in Example 1. The results are summarized in Table 1.

[0100] "Examples 11 and 12" Examples 11 and 12 are different from Example 1 in that the imide salt added to the electrolyte and the second element contained in the negative electrode active material were changed. In Example 12, both the second element contained in the imide salt and the second element contained in the negative electrode active material were two types. Under other conditions, the capacity retention rate E was obtained in the same manner as in Example 1. The results are summarized in Table 1.

[0101] "Examples 13 to 17" Examples 13 to 17 are different from Example 1 in that the Li salt concentration, the first imide salt concentration, and the second imide salt concentration in the electrolytic solution are changed. In Examples 13 to 17, the first imide salt concentration with respect to the Li salt concentration in the electrolytic solution was adjusted to be 10%, and the second imide salt concentration with respect to the first imide salt concentration in the electrolytic solution was adjusted to be 30%. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 1.

[0102] "Examples 18 to 23" Examples 18 to 23 are different from Example 1 in that the first imide salt concentration and the second imide salt concentration are changed. In Examples 18 to 23, the second imide salt concentration with respect to the first imide salt concentration in the electrolytic solution was adjusted to be 30%. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 1.

[0103] "Examples 24 to 30" Examples 24 to 30 are different from Example 1 in that the second imide salt concentration in the electrolytic solution is changed. In Examples 24 to 30, the first imide salt concentration with respect to the Li salt concentration in the electrolytic solution was adjusted to be 10%. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0104] "Examples 31 to 38" Examples 31 to 38 are different from Example 1 in that the particle size of the negative electrode active material is changed. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0105] "Examples 39 to 43" Examples 39 to 43 are different from Example 1 in that the discharge specific capacity of the negative electrode active material is changed. The discharge specific capacity of the negative electrode active material was adjusted by changing the thickness of the positive electrode active material layer. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0106] "Comparative Example 1" Comparative Example 1 is different from Example 1 in that magnesium oxide (the first compound) and sodium azide (the second compound) were not added when preparing the negative electrode active material, and the first imide salt and the second imide salt were not added to the electrolytic solution. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0107] "Comparative Example 2" Comparative Example 2 is different from Example 1 in that sodium azide (the second compound) was not added when preparing the negative electrode active material, and the second imide salt was not added to the electrolytic solution. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0108] "Comparative Example 3" Comparative Example 3 is different from Example 1 in that magnesium azide (the first compound) was not added when preparing the negative electrode active material, and the first imide salt was not added to the electrolytic solution. Under other conditions, the capacity retention rate E was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0109] In all of Examples 1 to 43, the capacity retention rate was higher than that in Comparative Examples 1 to 3. That is, the lithium ion secondary batteries according to Examples 1 to 43, in which the negative electrode active material contains the first compound and the second compound and the electrolytic solution contains the first imide salt and the second imide salt, had excellent cycle characteristics.

[0110]

Table 1

[0111]

Table 2

Explanation of Symbols

[0112] 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 35 Negative electrode active material 36 Core 37 Coating layer 37A First layer 37B Second layer 40 Power generation element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 Terminals 100 Lithium-ion secondary battery

Claims

1. A lithium-ion secondary battery having a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolytic solution, wherein the negative electrode has a negative electrode active material containing silicon or a silicon compound, a first oxide containing a first element, and a second oxide containing a second element, the electrolytic solution contains a first imide salt containing the first element and an imide anion, and a second imide salt containing the second element and the imide anion, the first element is any one or more elements selected from the group consisting of Mg, Ca, Cs, Al, and Zn, the second element is any one or more elements selected from the group consisting of K and Na, the first element contained in the first oxide is the same as the first element contained in the first imide salt, and the second element contained in the second oxide is the same as the second element contained in the second imide salt.

2. The lithium-ion secondary battery according to Claim 1, wherein a molar concentration ratio of the first imide salt in the electrolytic solution is 5% or more and 20% or less with respect to a molar concentration ratio of a lithium salt in the electrolytic solution.

3. The lithium-ion secondary battery according to Claim 1 or 2, wherein a molar concentration ratio of the second imide salt in the electrolytic solution is 10% or more and 50% or less with respect to the molar concentration ratio of the first imide salt in the electrolytic solution.

4. The negative electrode active material has a core and a coating layer covering the core, and the coating layer contains the first oxide and the second oxide. The lithium-ion secondary battery according to any one of Claims 1 to 3.

5. The lithium-ion secondary battery according to any one of Claims 1 to 4, wherein a median diameter (D50) of the negative electrode active material is 500 nm or more and 10 μm or less.

6. The lithium-ion secondary battery according to any one of Claims 1 to 5, wherein a discharge specific capacity of the negative electrode active material is 1000 mAh / g or more and 2500 mAh / g or less.

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