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 a substance containing silicon and carbon and a compound containing a first element. The electrolyte contains an imide salt containing the first element and an imide anion. The first element is one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al and Zn.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 also 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. Generally, graphite is used as the negative electrode active material, but a higher-capacity negative electrode active material is required. Therefore, negative electrode active materials containing silicon (Si), which have a much larger theoretical capacity than the theoretical capacity of graphite (372 mAh / g), have attracted attention.

[0004] The negative electrode active material containing Si is accompanied by a large volume expansion during charging. When the negative electrode active material expands in volume, the negative electrode active material may break. The volume expansion of the negative electrode active material is one of the causes of the deterioration of the cycle characteristics of the battery.

[0005] The electrolyte is one of the factors that affect the cycle characteristics of the battery. For example, Patent Documents 1 to 3 describe electrolytes containing specific lithium salts and ether compounds.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] Further improvement in cycle characteristics is required.

[0008] 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

[0009] In order to solve the above problems, the following means are provided.

[0010] (1) The lithium ion secondary battery according to the first aspect has 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 a substance containing silicon and carbon and a compound containing a first element. The electrolytic solution contains an imide salt containing the first element and an imide anion, and the first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn.

[0011] (2) In the lithium ion secondary battery according to the above aspect, when the total of silicon and carbon in the substance is 100 wt%, the weight ratio of silicon may be 30 wt% or more and 70 wt% or less, and the weight ratio of carbon may be 30 wt% or more and 70 wt% or less.

[0012] (3) In the lithium ion secondary battery according to the above aspect, the molar concentration ratio of the imide salt containing the first element 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] (4) In the lithium ion secondary battery according to the above aspect, the compound containing the first element may be any one or more selected from the group consisting of fluorides, oxides, silicides, silicate oxides, and phosphates.

[0014] (5) In the lithium-ion secondary battery according to the above aspect, the negative electrode active material may have a core and a coating layer covering the core. Further, the coating layer may contain a compound containing the first element.

[0015] (6) In the lithium-ion secondary battery according to the above aspect, the median diameter (D50) of the negative electrode active material may be 1 μm or more and 10 μm or less.

Advantages of the Invention

[0016] The lithium-ion secondary battery according to the above aspect is excellent in cycle characteristics.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. The drawings used in the following description may show the characteristic parts enlarged for the sake of easy understanding of the characteristics, 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 appropriately modified and implemented without changing the gist thereof.

[0019] “Lithium-ion secondary battery” FIG. 1 is a schematic diagram of the 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 and 62. The non-aqueous electrolyte is housed inside the exterior body 50.

[0020] (Power generation element) The power generation element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

[0021] [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.

[0022] [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, or stainless steel. 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.

[0023] [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 necessary.

[0024] The basis weight of the positive electrode active material layer 24 is, for example, 15 mg / cm 2 or more, preferably 20 mg / cm 2 or more. The basis weight of the positive electrode active material layer 24 is 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, if the basis weight is too large, it becomes difficult for the electrolyte to impregnate into the positive electrode active material layer 24.

[0025] The positive electrode active material includes an electrode active material capable of reversibly proceeding with the intercalation and deintercalation of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

[0026] 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 also be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene.

[0027] The positive electrode active material may be a lithium-free material. The lithium-free material is, for example, FeF3, a conjugated polymer containing an organic conductive substance, a Chevrel phase compound, a transition metal chalcogenide, a vanadium oxide, a niobium oxide, etc. The lithium-free material may use only one of the materials, or may be used in combination of multiple 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 chemically or electrochemically pre-doped with lithium.

[0028] The conductive additive enhances the electron conductivity between the cathode active materials. The conductive additive is, for example, carbon powder, carbon nanotubes, carbon materials, metal fine powder, a mixture of carbon materials and metal fine powder, or a conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, etc.

[0029] The binder binds the active materials together. Known binders can be used. The binder is, for example, a fluororesin. The fluororesin is, for example, 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.

[0030] 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), etc., which are vinylidene fluoride-based fluororubbers. The binder may also be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc.

[0031] The binder may also be an electronically conductive conductive polymer or an ionically conductive conductive polymer. Examples of the electronically conductive conductive polymer include polyacetylene, polythiophene, polyaniline, and the like. Examples of the ionically conductive 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.

[0032] 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 auxiliary agent 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.

[0033] <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.

[0034] [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.

[0035] [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 auxiliary agent as needed. The negative electrode active material contains a substance containing silicon and carbon and a compound containing a first element.

[0036] The substance containing silicon and carbon is, for example, a composite of silicon and carbon. The substance containing silicon and carbon may consist only of silicon and carbon, for example. The substance containing silicon and carbon may also be a compound of silicon and carbon, for example. The compound of silicon and carbon is, for example, silicon carbide. In the said substance, silicon and carbon may exist in a single state respectively, may exist in a combined state (for example, silicon carbide), or may be a mixture of a single substance and a compound. The negative electrode active material is, for example, amorphous. When the negative electrode active material is amorphous, the desorption of silicon fine particles is less likely to occur during charge and discharge.

[0037] In the substance containing silicon and carbon, 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. In the substance containing silicon and carbon, the weight ratio of carbon is, for example, the difference obtained by subtracting the weight ratio of silicon from the whole. In the substance containing silicon and carbon, the weight ratio of carbon is, for example, 30 wt% or more and 70 wt% or less, preferably 30 wt% or more and 50 wt% or less. That is, when the total of silicon and carbon in the substance containing silicon and carbon is 100%, the weight ratio of silicon is, for example, 30 wt% or more and 70 wt% or less, and the weight ratio of carbon is, for example, 30 wt% or more and 70 wt% or less.

[0038] If the weight ratio of silicon is 30 wt% or more, the discharge specific capacity of the negative electrode active material can be increased to 1000 mAh / g or more. Also, if the weight ratio of silicon is 70 wt% or less, the formation of a large number of voids in the negative electrode active material due to volume expansion can be suppressed. The voids in the negative electrode active material inhibit the conduction of electrons and ions and can cause a decrease in the cycle characteristics of the lithium-ion secondary battery.

[0039] The compound containing the first element is one or more selected from the group consisting of, for example, fluoride, oxide, silicide, and silicate. The first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn. The first element is not limited to one type of element and may be a plurality of types of element groups. For example, when the first element is Mg, the compound containing the first element is, for example, MgF2, MgO, Mg2Si, Li x Mg y Si, and Mg2SiO4.

[0040] 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 deinsertion reaction of Li uniform and suppresses the deterioration of the negative electrode active material 35.

[0041] The core 36 is a material containing silicon and carbon. The core 36 is, for example, a composite of silicon and carbon. The coating layer 37 contains, for example, a material containing silicon and carbon and a compound containing the first element. The coating layer 37 may contain a carbon material, metal fine powder, a mixture of a carbon material and a metal fine powder, a conductive oxide, an inorganic compound, etc. The carbon material is, for example, carbon powder, carbon nanotube, etc. The metal fine powder is, for example, copper, cobalt, iron, etc. The inorganic material is, for example, ceramics, and is an oxide, carbide, nitride, boride, etc.

[0042] 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 the first element and the material contained in the electrolytic solution. For example, when the first element is Mg, the reaction product of the first element and the material contained in the electrolytic solution is, for example, Mg3(PO4)2.

[0043] The median diameter (D50) of the negative electrode active material is, for example, 1 μm or more and 10 μm or less. 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 electrolytic solution and the negative electrode active material, and the easier it is for the electrolytic solution 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 portions to occur on the surface of the negative electrode active material. Potential non-uniformity can cause redox decomposition of the electrolytic solution. 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.

[0044] 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 negative electrode potential can cause a decrease in cycle characteristics. The higher the negative electrode potential, the more lithium can be trapped, but the electrolytic solution and the coating layer 37 are more likely to be oxidatively decomposed. The lower the negative electrode potential, although the expansion and contraction of the negative electrode active material 35 can be suppressed, the electrolytic solution and the coating layer 37 are more likely to be reductively decomposed. 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.

[0045] 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.

[0046] 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.

[0047] The contents of the negative electrode active material, conductive assistant, and 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. 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.

[0048] <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.

[0049] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a single layer or a laminate of a polyolefin film. The separator 10 may be a stretched film of a mixture such as polyethylene or polypropylene. The separator 10 may be a fibrous non-woven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The 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. The separator 10 may be an inorganic-coated separator. The inorganic-coated separator is obtained by coating the surface of the above film with a mixture of a resin such as PVDF or CMC and an inorganic substance such as alumina or silica. The inorganic-coated separator is excellent in heat resistance and suppresses the precipitation of transition metals eluted from the positive electrode onto the surface of the negative electrode.

[0050] <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, and an imide salt. The imide salt may be one type or two or more types.

[0051] The solvent is not particularly limited as long as it is a solvent generally used in lithium-ion secondary batteries. The solvent includes, 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 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.

[0052] 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 product of the fluorinated organic solvent and the first element contained in the electrolyte 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.

[0053] The electrolyte is, for example, a lithium salt. Examples of the electrolyte include 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.

[0054] The imide salt contains a first element and an imide anion. The first element is as described above. The first element contained in the electrolyte 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 electrolyte and the first element contained in the negative electrode active material layer 34 may be different elements. The imide salt is not limited to one type and may be contained in a plurality of types in the electrolyte.

[0055] Imide anions include, 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 - . The imide anion may be used alone or in combination of two or more kinds.

[0056] The molar concentration ratio of the imide salt containing the first element 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 imide salt of the first element is sufficiently contained in the electrolyte, the decomposition of the electrolyte is suppressed during charge and discharge of the lithium ion secondary battery 100. Also, when the imide salt of the first element 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.

[0057] The molar concentration ratio of the first element and lithium in the electrolyte can be measured, for example, by gas chromatography, ICP (inductively coupled plasma) emission analysis, ICP mass spectrometry, etc.

[0058] <Outer package> The outer package 50 seals the power generation element 40 and the non-aqueous electrolyte therein. The outer package 50 prevents leakage of the non-aqueous electrolyte to the outside and intrusion of moisture or the like from the outside into the lithium ion secondary battery 100.

[0059] The outer package 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 outer package 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0060] As the metal foil 52, for example, aluminum foil can be used. As the resin layer 54, a polymer film such as polypropylene can be utilized. 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.

[0061] <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.

[0062] "Method for manufacturing a lithium - ion secondary battery" The lithium - ion secondary battery 100 is manufactured by respectively preparing the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50, and assembling them. Hereinafter, an example of the manufacturing method of the lithium - ion secondary battery 100 will be described.

[0063] The negative electrode 30 is manufactured, for example, by sequentially performing a compounding step, a slurry preparation step, an electrode coating step, a drying step, and a rolling step.

[0064] The compounding step is a step of mixing a substance containing silicon and carbon and a compound containing a first element while applying a shearing force. The substance containing silicon and carbon is prepared in advance. For example, a composite of silicon and carbon is produced by mixing silicon and carbon while applying a shearing force. Also, for example, a compound of silicon and carbon is produced by reacting silicon and carbon.

[0065] When a shearing force is applied to and the substance containing silicon and carbon and the compound containing the first element are mixed, the surface of the negative electrode active material is coated with the compound containing the first element. Further, the particle size of the negative electrode active material can be adjusted according to the degree of the mixing. Further, the produced negative electrode active material may be sieved to make the particle sizes uniform.

[0066] The slurry preparation step is a step of mixing the composite negative electrode active material, a binder, and a solvent to prepare a slurry. In the slurry preparation step, a conductive auxiliary agent 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% in terms of mass ratio. These mass ratios are adjusted so as to be 100 wt% in total.

[0067] The electrode coating step is a step 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.

[0068] The drying step is a step 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. By drying the slurry, a negative electrode active material layer 34 is formed on the negative electrode current collector 32.

[0069] The rolling step is performed as necessary. The rolling step is a step 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 step is performed, for example, with a roll press device or the like.

[0070] The positive electrode 20 can be manufactured by the same procedure as the negative electrode 30 except that the composite step is not performed. As the separator 10 and the exterior body 50, commercially available ones can be used.

[0071] The electrolytic solution can be prepared, for example, by adding and mixing an imide salt containing the first element and an imide anion to a mixture of a lithium salt and a solvent.

[0072] Next, the separator 10 is positioned between the produced 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, the positive electrode 20, negative electrode 30, and one end side of the separator 10 are wound around an axis.

[0073] Finally, the power generation element 40 is encapsulated in an exterior body 50. The non-aqueous electrolyte is injected into the exterior body 50. By performing decompression, heating, etc. after injecting the non-aqueous electrolyte, the non-aqueous electrolyte 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 electrolyte into the exterior body 50, the power generation element 40 may be impregnated with the electrolyte.

[0074] The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics. The reason for this will be described below.

[0075] When the negative electrode active material expands in volume, silicon fine particles may desorb from the negative electrode active material. These silicon fine particles cause an increase in the viscosity of the electrolyte and clogging of the separator 10. When the electrolyte becomes highly viscous, it becomes difficult for the electrolyte to impregnate the negative electrode active material layer 34. The increase in the viscosity of the electrolyte causes a decrease in the cycle characteristics of the lithium ion secondary battery. Also, when the separator 10 becomes clogged, the movement of Li ions is inhibited. The clogging of the separator 10 causes a decrease in the cycle characteristics of the lithium ion secondary battery.

[0076] In the lithium ion secondary battery 100 according to the first embodiment, a compound containing a first element is present in the negative electrode active material. The compound containing the first element reacts with the desorbed silicon fine particles and incorporates the silicon fine particles. Therefore, the lithium ion secondary battery 100 according to the first embodiment can prevent the silicon fine particles from flowing out into the electrolyte, and a decrease in cycle characteristics is suppressed.

[0077] In addition, in the lithium-ion secondary battery 100 according to the first embodiment, an imide salt containing a first element is present in the electrolytic solution. Therefore, even when the first element in the negative electrode active material is depleted due to the reaction between the silicon fine particles and the compound containing the first element, the first element can be supplied from the electrolytic solution to the negative electrode active material. Therefore, the lithium-ion secondary battery 100 can maintain a high capacity retention rate even when the number of cycles increases.

[0078] In addition, when the silicon fine particles flow out into the electrolytic solution, they pass through the vicinity of the surface of the negative electrode active material. Therefore, when the coating layer 37 of the negative electrode active material contains the first element, the effect of suppressing the deterioration of the cycle characteristics of the lithium-ion secondary battery 100 is high.

[0079] 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

[0080] 「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.

[0081] Li x CoO2 was used. Acetylene black was used as the conductive assistant. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 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 And 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.

[0082] Using Hosokawa Micron's mechanofusion inclined at 10 degrees, silicon and carbon were compounded to produce a substance containing silicon and carbon. In the substance containing silicon and carbon, the weight ratio of silicon was 60 wt% and the weight ratio of carbon was 40 wt%. Next, using the same apparatus as above, a substance containing silicon and carbon was compounded with magnesium oxide and LiF to produce a negative electrode active material. The rotational speed of the apparatus during compounding was 2500 rpm. Magnesium oxide is an oxide containing a first element. The median diameter of the negative electrode active material after the compounding treatment was 5 μm. The negative electrode active material had a coating layer containing Mg formed on its surface.

[0083] 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 obtained by the above compounding 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 a negative electrode slurry. The loading amount of the negative electrode active material in the dried negative electrode active material layer was 3.0 mg / cm 2 as specified. 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 pressurized by roll pressing and then heat baked at 300 °C or higher for 5 hours under a nitrogen atmosphere.

[0084] 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 was added to this solution as an imide salt to the electrolyte to prepare an electrolyte. The concentration of the imide salt of the first element with respect to the Li salt concentration was adjusted to 10% (0.1 mol / L).

[0085] (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 nickel negative electrode lead was attached to the negative electrode of the laminate. An aluminum positive electrode lead was attached to the positive electrode of the laminate. The positive electrode lead and the negative electrode lead were welded by 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 while being depressurized by a vacuum sealer to fabricate a lithium-ion secondary battery. Note that two lithium-ion secondary batteries were fabricated, one for electrolytic solution composition analysis and one for charge-discharge characteristic evaluation.

[0086] The electrolytic solution was sampled from the lithium-ion secondary battery for electrolytic solution composition analysis. Next, the composition of the sampled electrolytic solution was analyzed using ICP emission. As a result, it was confirmed that the content of Mg in the electrolytic solution was the same as the content at the time of electrolytic solution preparation.

[0087] (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).

[0088] Charging was performed at a constant current charge rate of 1.0C (the current value at which charging ends in 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.4V, and discharging was performed at a constant current discharge rate of 1.0C until the battery voltage reached 3.0V. The discharge capacity after charge-discharge was detected, and the battery capacity Q1 before the cycle test was obtained.

[0089] 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 4.4 V with constant current charging at a charging rate of 1.0C, and then discharged until the battery voltage reached 3.0 V with constant current discharging at a discharging rate of 1.0C. The above charge and discharge were counted as one cycle, and charge and discharge were performed 300 cycles. Thereafter, the discharge capacity after 300 cycles of charge and discharge was detected, and the battery capacity Q2 after 300 cycles was obtained.

[0090] 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 77%.

[0091] "Examples 2 to 8" Examples 2 to 8 are different from Example 1 in that the mixing ratio of silicon and carbon was changed when producing a substance containing silicon and carbon. That is, Examples 2 to 8 are different from Example 1 in that the weight ratio of silicon and carbon in the negative electrode active material is different. The capacity retention rate E was obtained in the same manner as in Example 1 under other conditions. The results are summarized in Table 1.

[0092] "Examples 9 to 16" Examples 9 to 16 are different from Example 1 in that at least one of the imide salt added to the electrolyte and the first element contained in the negative electrode active material was changed. The capacity retention rate E was obtained in the same manner as in Example 1 under other conditions. The results are summarized in Table 1.

[0093] "Examples 17 to 21" Examples 17 to 21 are different from Example 1 in that the Li salt concentration and the imide salt concentration of the first element in the electrolyte were changed. In all of Examples 17 to 21, the imide salt concentration of the first element with respect to the Li salt concentration in the electrolyte was adjusted to be 10%. The capacity retention rate E was obtained in the same manner as in Example 1 under other conditions. The results are summarized in Table 1.

[0094] "Examples 22 to 25" Examples 22 to 25 are different from Example 1 in that the Li salt concentration in the electrolyte solution was fixed and the imide salt concentration of the first element was 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.

[0095] "Examples 26 to 29" Examples 26 to 29 are different from Example 1 in that the imide salt added to the electrolyte solution and the first element contained in the negative electrode active material were changed. In Examples 26 to 29, 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 determined in the same manner as in Example 1. The results are summarized in Table 2.

[0096] "Examples 30 to 34" Examples 30 to 34 are different from Example 1 in that the particle size of the negative electrode active material was 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.

[0097] "Comparative Example 1" Comparative Example 1 is different from Example 1 in that magnesium oxide (a compound containing the first element) was not added when preparing the negative electrode active material and no imide salt was added to the electrolyte 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.

[0098] "Comparative Example 2" Comparative Example 2 is different from Example 1 in that no imide salt was added to the electrolyte 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.

[0099] "Comparative Example 3" Comparative Example 3 is different from Example 1 in that magnesium oxide (a compound containing the first element) was not added when preparing the negative electrode active material. 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.

[0100] Examples 1 to 34 all had a higher capacity retention rate compared to Comparative Examples 1 to 3. That is, the lithium-ion secondary batteries according to Examples 1 to 34, in which the negative electrode active material contains a compound containing a first element and the electrolytic solution contains an imide salt of the first element, were excellent in cycle characteristics.

[0101]

Table 1

[0102]

Table 2

Explanation of Signs

[0103] 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 comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolytic solution, wherein the negative electrode has a negative electrode active material containing a substance including silicon and carbon, an oxide containing a first element, and LiF, the electrolytic solution contains an imide salt containing the first element and an imide anion, the first element is any one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn, and the first element contained in the negative electrode active material is the same as the first element contained in the electrolytic solution.

2. The lithium-ion secondary battery according to Claim 1, wherein when the total of silicon and carbon in the substance is 100 wt%, the weight ratio of silicon is 30 wt% or more and 70 wt% or less, and the weight ratio of carbon is 30 wt% or more and 70 wt% or less.

3. The lithium-ion secondary battery according to Claim 1 or 2, wherein the molar concentration ratio of the imide salt containing the first element in the electrolytic solution is 5% or more and 20% or less with respect to the molar concentration ratio of the lithium 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 an oxide containing the first element. 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 the median diameter (D50) of the negative electrode active material is 1 μm or more and 10 μm or less.

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