lithium-ion secondary batteries

Abstract

An object of the present invention is to provide a lithium-ion secondary battery having excellent cycle characteristics. The lithium-ion secondary battery comprises: a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode comprises a negative electrode active material comprising a substance containing silicon and carbon and a compound containing a first element, and the electrolyte comprises an imide salt comprising the first element and an imide anion, wherein the first element is at least one element selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn.

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 a power source for mobile devices such as mobile phones and laptop computers, and hybrid vehicles.

[0003] The capacity of lithium-ion secondary batteries depends primarily on the active material in the electrodes. Graphite is commonly used as the negative electrode active material, but higher capacity is desired. Consequently, negative electrode active materials containing silicon (Si) are attracting attention, as their theoretical capacity far exceeds that of graphite (372 mAh / g).

[0004] Negative electrode active materials containing Si undergo significant volume expansion during charging. This volume expansion can sometimes damage the negative electrode active material. This volume expansion can also degrade the battery's cycle performance.

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

[0006] [Prior art literature]

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2009-176534

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2014-110235

[0009] Patent Document 3: Japanese Patent Application No. 2015-534254 Summary of the Invention

[0010] [Technical problem to be solved by the invention]

[0011] There is a demand for further improvement in cycle characteristics.

[0012] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a lithium-ion secondary battery having excellent cycle characteristics.

[0013] [Means used to solve technical problems]

[0014] In order to solve the above technical problems, the following means are provided.

[0015] (1) A lithium-ion secondary battery according to a first embodiment comprises a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprising: a substance containing silicon and carbon and a compound containing a first element, the electrolyte comprising an imide salt, the imide salt comprising the first element and an imide anion, the first element being one or more elements selected from the group consisting of K, Na, Mg, Ca, Cs, Al, and Zn.

[0016] (2) In the lithium-ion secondary battery according to the above embodiment, when the total of silicon and carbon in the above 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.

[0017] (3) In the lithium ion secondary battery according to the above embodiment, the molar concentration ratio of the imide salt containing the first element in the electrolyte solution may be 5% or more and 20% or less relative to the molar concentration ratio of the lithium salt in the electrolyte solution.

[0018] (4) In the lithium ion secondary battery according to the above embodiment, the compound containing the first element may be at least one selected from the group consisting of fluorides, oxides, silicides, silicon oxides, and phosphorus oxides.

[0019] (5) In the lithium-ion secondary battery according to the above embodiment, the negative electrode active material may include a core and a coating layer covering the core. The coating layer may also include the compound containing the first element.

[0020] (6) In the lithium ion secondary battery according to the above embodiment, the negative electrode active material may have a median particle size (D50) of 1 μm or more and 10 μm or less.

[0021] [Effects of the Invention]

[0022] The lithium ion secondary battery according to the above embodiment has excellent cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the lithium ion secondary battery according to the first embodiment.

[0024] Figure 2 It is a schematic diagram of the negative electrode active material according to the first embodiment. DETAILED DESCRIPTION

[0025] The following embodiments will be described in detail with reference to the accompanying drawings as appropriate. In the drawings used in the following description, for ease of understanding, portions of the components are sometimes enlarged for convenience, and the dimensional ratios of the various components may differ from actual dimensions. The materials, dimensions, and other materials illustrated in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications can be made within the scope of the present invention.

[0026] [Lithium-ion secondary battery]

[0027] Figure 1 It is a schematic diagram of the lithium ion secondary battery according to the first embodiment. Figure 1 The lithium-ion secondary battery 100 shown includes a power generation element 40, an outer casing 50, and a non-aqueous electrolyte (not shown). The outer casing 50 covers the periphery of the power generation element 40. The power generation element 40 is connected to the outside via a pair of terminals 60 and 62. The non-aqueous electrolyte is contained within the outer casing 50.

[0028] (Power generation element)

[0029] The power generating element 40 includes a positive electrode 20 , a negative electrode 30 , and a separator 10 .

[0030] <Positive electrode>

[0031] The positive electrode 20 includes, 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 .

[0032] [Positive electrode current collector]

[0033] The positive electrode current collector 22 is, for example, a conductive plate. It can be a thin metal plate such as aluminum, copper, nickel, titanium, or stainless steel. Lightweight aluminum is suitable for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, not less than 10 μm and not more than 30 μm.

[0034] [Positive Electrode Active Material Layer]

[0035] The positive electrode active material layer 24 contains, for example, a positive electrode active material and may contain a conductive additive and a binder as needed.

[0036] The eye density of the positive electrode active material layer 24 is, for example, 15 mg / cm 2 Above, preferably 20 mg / cm 2 The visual weight of the positive electrode active material layer 24 is preferably 35 mg / cm 2Below. "Visual measurement" refers to the mass of the positive electrode active material layer 24 carried per unit area on the surface of the positive electrode current collector 22. A higher visual measurement increases the amount of positive electrode active material per unit area, increasing the battery capacity. On the other hand, a lower visual measurement makes it difficult for the electrolyte to penetrate into the positive electrode active material layer 24.

[0037] The positive electrode active material includes an electrode active material capable of reversibly occluding and releasing lithium ions, desorbing and inserting (intercalating) lithium ions, or doping and dedoping lithium ions and counter anions.

[0038] The positive electrode active material is, for example, a composite metal oxide. Composite metal oxides include lithium cobalt oxide (Li CoO 2 ), lithium nickel oxide (Li Ni O 2 ), lithium manganese oxide (Li Mn O 2 ), lithium manganese spinel (LiMn 2 O 4 ), and LiNi x Co y Mn z M a O2 compounds (wherein, x+y+z+a=1, 0≤x<1, 0≤y<1, 0≤z<1, 0≤a<1, and M represents one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV2O5), olivine-type LiMPO4 (wherein, M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or represents VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9<x+y+z<1.1). The positive electrode active material can also be an organic substance. For example, the positive electrode active material can be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0039] The positive electrode active material may be a material that does not contain lithium. Examples of materials that do not contain lithium include FeF3, conjugated polymers containing organic conductive substances, Chevrel's phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, and the like. For materials that do not contain lithium, any one material may be used alone, or a combination of multiple materials may be used. In the case where the positive electrode active material is a material that does not contain lithium, for example, discharge is initially performed. Lithium is inserted into the positive electrode active material by discharge. In addition, the positive electrode active material may be pre-doped with lithium chemically or electrochemically to the material that does not contain lithium.

[0040] Conductive additives improve the electron conductivity of the positive electrode active material. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include powders of copper, nickel, stainless steel, and iron.

[0041] The binder binds the active substances to each other. About the binder, known binders can be used. The binder is, for example, a fluororesin. Examples of fluororesins 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) and polyvinyl fluoride (PVF).

[0042] In addition to the above-mentioned substances, the binder may be, for example, vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), vinylidene fluoride-chlorotrifluoroethylene fluororubber (VDF-CTFE fluororubber), etc. Furthermore, the binder may be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc.

[0043] Alternatively, the binder may be an electronically conductive or ionically conductive polymer. Examples of electronically conductive polymers include polyacetylene, polythiophene, and polyaniline. Examples of ionically conductive polymers include composites of polyether polymers and lithium salts. Examples of polyether polymers include polyethylene oxide and polypropylene oxide. Examples of lithium salts include LiFSI, LiTFSI, LiBETI, LiClO4, LiBF4, and LiPF6.

[0044] The composition ratio of the positive electrode active material in the positive electrode active material layer 24 is, for example, 80% to 98% by mass. Furthermore, the composition ratio of the conductive additive in the positive electrode active material layer 24 is, for example, 1.0% to 10% by mass. Furthermore, the composition ratio of the binder in the positive electrode active material layer 24 is, for example, 1.0% to 10% by mass.

[0045] <Negative electrode>

[0046] The negative electrode 30 includes, 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 .

[0047] [Negative electrode current collector]

[0048] The negative electrode current collector 32 is, for example, a conductive plate material. As the negative electrode current collector 32 , the same current collector as the positive electrode current collector 22 can be used.

[0049] [Negative Electrode Active Material Layer]

[0050] The negative electrode active material layer 34 contains a negative electrode active material and a binder. Furthermore, the negative electrode active material layer 34 may contain a conductive additive as needed. The negative electrode active material includes a substance containing silicon and carbon and a compound containing the first element.

[0051] The substance containing silicon and carbon is, for example, a composite of silicon and carbon. The substance containing silicon and carbon may be, for example, composed only of silicon and carbon. The substance containing silicon and carbon may be, for example, a compound of silicon and carbon. An example of a compound of silicon and carbon is silicon carbide. In this substance, silicon and carbon may exist in a simple substance state, in a compound state (for example, silicon carbide), or as a mixture of simple substances and compounds. The negative electrode active material is, for example, amorphous. If the negative electrode active material is amorphous, it is difficult for silicon particles to separate during charge and discharge.

[0052] In a 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 a 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 total. In a 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, in a substance containing silicon and carbon, when the total of silicon and carbon is set to 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.

[0053] If the silicon content is 30 wt% or more, the negative electrode active material can achieve a high specific discharge capacity of 1000 mAh / g or more. Furthermore, if the silicon content is 70 wt% or less, the formation of multiple voids within the negative electrode active material due to volume expansion can be suppressed. Voids within the negative electrode active material hinder the conduction of electrons and ions, which can reduce the cycling performance of lithium-ion secondary batteries.

[0054] The compound containing the first element is, for example, one or more selected from fluorides, oxides, silicides, and silicon oxides. The first element is any one or more selected from K, Na, Mg, Ca, Cs, Al, and Zn. The first element is not limited to one element, but may also be a group of multiple elements. 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, Mg2SiO4.

[0055] Figure 2 This is a schematic diagram of the negative electrode active material 35 according to the first embodiment. The negative electrode active material 35 may include a core 36 and a coating layer 37. The coating layer 37 covers at least a portion of the core 36. The coating layer 37 may also cover the entire surface of the core 36. The coating layer 37 makes the Li insertion and extraction reactions uniform, thereby suppressing degradation of the negative electrode active material 35.

[0056] Core 36 is a material containing silicon and carbon. For example, core 36 is a composite of silicon and carbon. Coating 37 may include, for example, a material containing silicon and carbon and a compound containing the first element. Coating 37 may include, for example, a carbon material, metal powder, a mixture of carbon and metal powders, a conductive oxide, an inorganic compound, or the like. Examples of carbon materials include carbon powder and carbon nanotubes. Examples of metal powder include copper, cobalt, and iron. Examples of inorganic materials include ceramics, oxides, carbides, nitrides, and borides.

[0057] The coating layer 37 may also include 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 also contain a reactant of the first element and a material contained in the electrolyte. For example, when the first element is Mg, the reactant of the first element and the material contained in the electrolyte is, for example, Mg3(PO4)2.

[0058] The median particle size (D50) of the negative electrode active material is, for example, not less than 1 μm and not more than 10 μm. 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 greater the frequency of contact 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 easier it is for local areas of low potential or high potential to be generated on the surface of the negative electrode active material. The uneven potential may become the cause of redox decomposition of the electrolyte. If the median particle size of the negative electrode active material is within the above range, the cycle characteristics of the lithium ion secondary battery 100 are improved.

[0059] The discharge specific capacity of the negative electrode active material is, for example, not less than 1000 mAh / g and not more than 2500 mAh / g. 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 greater the discharge specific capacity of the negative electrode active material, the lower the potential of the negative electrode; the smaller the discharge specific capacity of the negative electrode active material, the higher the potential of the negative electrode. The high potential or low potential of the negative electrode becomes a cause of reduced cycle characteristics. The higher the negative electrode potential, the greater the amount of lithium that can be captured, but the electrolyte and the coating layer 37 are more likely to be oxidatively decomposed. The lower the negative electrode potential, 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, high capacity can be achieved and excellent cycle characteristics can be achieved.

[0060] The conductive additive improves the electron conductivity between the negative electrode active materials. As the conductive additive, the same material as that used for the positive electrode 20 can be used.

[0061] The binder binds the negative electrode active materials to each other and binds the negative electrode active materials to the negative electrode current collector 32. As the binder, the same binder as that used in the positive electrode 20 can be used.

[0062] The contents of the negative electrode active material, conductive additive, and binder in the negative electrode active material layer 34 are not particularly limited. The composition ratio of the negative electrode active material in the negative electrode active material layer 34 is, for example, 70% to 100% by mass. The composition ratio of the conductive additive in the negative electrode active material layer 34 is, for example, 0% to 10% by mass, and the composition ratio of the binder in the negative electrode active material layer 34 is, for example, 0% to 20% by mass.

[0063] <Diaphragm>

[0064] The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 from the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 extends in the plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0065] The diaphragm 10 has, for example, an electrically insulating porous structure. The diaphragm 10 is, for example, a single layer or a laminate of a polyolefin film. The diaphragm 10 may also be a stretched film of a mixture of polyethylene, polypropylene, etc. The diaphragm 10 may be a fiber non-woven fabric composed of at least one constituent material selected from cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The diaphragm 10 may also be, for example, a solid electrolyte. Examples of solid electrolytes include polymer solid electrolytes, oxide solid electrolytes, and sulfide solid electrolytes. The diaphragm 10 may also be an inorganic coating diaphragm. The inorganic coating diaphragm is formed by coating the surface of the above-mentioned membrane with a mixture of resins such as PVDF and CMC and inorganic substances such as alumina and silica. The inorganic coating diaphragm has excellent heat resistance and inhibits the precipitation of transition metals dissolved from the positive electrode to the surface of the negative electrode.

[0066] <Electrolyte>

[0067] The electrolyte solution is enclosed within the outer casing 50 and permeates the power generating element 40. The non-aqueous electrolyte solution, for example, comprises a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent. The electrolyte solution, for example, comprises a solvent, an electrolyte, and an imide salt. The imide salt may be one or more.

[0068] Regarding the solvent, there is no particular limitation as long as it is a solvent commonly 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. As a solvent, they can also be mixed and contained in any proportion. Examples of cyclic carbonate compounds include ethylene carbonate (EC) and propylene carbonate (PC). Examples of chain carbonate compounds include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of cyclic ester compounds include γ-butyrolactone. Examples of chain ester compounds include propyl propionate, ethyl propionate, and ethyl acetate.

[0069] The solvent may contain a fluorinated organic solvent. Fluorinated organic solvents form a good coating on the surface of the negative electrode active material. When decomposition products of the fluorinated organic solvent and the first element contained in the electrolyte are contained in the coating, lithium ions are easily transported within the coating. As a result, an increase in the resistance of the lithium-ion secondary battery 100 is suppressed.

[0070] 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, and LiBOB. A single lithium salt may be used alone, or two or more may be used in combination. From the perspective of ionization, the electrolyte preferably includes LiPF6.

[0071] The imide salt includes a first element and an imide anion. The first element is the element described above. The first element contained in the electrolyte and the first element contained in the negative electrode active material layer 34 may be, 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 also be different elements. The imide salt is not limited to a single type; the electrolyte may contain multiple imide salts.

[0072] The imide anion is, for example, (SO2F)2N - (FSI - : bis(fluorosulfonyl imide) anion), (SO2CF3)2N - (TFSI - : bis(trifluoromethanesulfonyl imide) anion), (SO2C2F5)2N - (BETI - : bis(pentafluoroethylsulfonylimide) anion), (SO2F)(SO2CF3)N - 、(SO2CF3)(SO2C2F5)N - As for the imide anion, one type may be used alone, or two or more types may be used in combination.

[0073] The molar concentration ratio of the imide salt containing the first element in the electrolyte relative to the molar concentration ratio of the lithium salt in the electrolyte is, for example, 5% or more and 20% or less. If the electrolyte contains a sufficient amount of the imide salt of the first element, decomposition of the electrolyte is suppressed during charge and discharge of the lithium-ion secondary battery 100. Furthermore, if the imide salt of the first element is excessively contained in the electrolyte, the viscosity of the electrolyte increases, reducing the permeability of the electrolyte to the positive electrode 20 and the negative electrode 30.

[0074] The molar concentration ratio of the first element and lithium in the electrolyte solution can be measured by, for example, gas chromatography, ICP (high-frequency inductively coupled plasma) emission spectrometry, ICP mass spectrometry, or the like.

[0075] <Exterior body>

[0076] The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte therein. The outer casing 50 prevents leakage of the non-aqueous electrolyte to the outside and entry of moisture or the like from the outside into the lithium ion secondary battery 100.

[0077] like Figure 1 As shown, the exterior body 50 includes, for example, a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film in which a polymer film (resin layer 54) is applied to both sides of the metal foil 52.

[0078] As metal foil 52, aluminum foil can be used, for example. As resin layer 54, a polymer film such as polypropylene can be used. The material constituting resin layer 54 may be different on the inner and outer sides. For example, the outer side material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), while the inner polymer film material may be polyethylene (PE) or polypropylene (PP).

[0079] <Terminal>

[0080] Terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. Terminal 60 connected to the positive electrode 20 is the positive electrode terminal, and terminal 62 connected to the negative electrode 30 is the negative electrode terminal. Terminals 60 and 62 serve as external electrical connections. Terminals 60 and 62 are formed from conductive materials such as aluminum, nickel, and copper. Connections can be made by welding or screwing. To prevent short circuits, terminals 60 and 62 are preferably protected with insulating tape.

[0081] [Method for manufacturing lithium-ion secondary battery]

[0082] The negative electrode 30, the positive electrode 20, the separator 10, the electrolyte solution, and the outer casing 50 are prepared separately and assembled to produce the lithium ion secondary battery 100. An example of a method for producing the lithium ion secondary battery 100 will be described below.

[0083] For example, the negative electrode 30 is produced by sequentially performing a composite step, a slurry preparation step, an electrode coating step, a drying step, and a rolling step.

[0084] The composite process involves mixing a substance containing silicon and carbon with a compound containing the first element while applying shear forces. A substance containing silicon and carbon is prepared in advance. For example, a silicon-carbon composite can be produced by mixing silicon and carbon while applying shear forces. Alternatively, a silicon-carbon compound can be produced by reacting silicon with carbon.

[0085] When the material containing silicon and carbon and the compound containing the first element are mixed by applying shear force, the surface of the negative electrode active material is coated with the compound containing the first element. In addition, the particle size of the negative electrode active material can be adjusted by the degree of mixing. In addition, the negative electrode active material can be sieved after preparation to make the particle size uniform.

[0086] The slurry preparation process involves mixing the composited negative electrode active material, a binder, and a solvent to create a slurry. A conductive additive may be added as needed during the slurry preparation process. Examples of solvents include water and N-methyl-2-pyrrolidone. The mass ratio of the negative electrode active material, conductive material, and binder is preferably 70% to 100% by weight: 0% to 10% by weight: 0% to 20% by weight. The mass ratio is adjusted to a total of 100% by weight.

[0087] The electrode coating step is a step of coating the slurry on the surface of the negative electrode current collector 32. The slurry coating method is not particularly limited. For example, a slot die coating method or a doctor blade method can be used as the slurry coating method.

[0088] 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 at 80° C. to 150° C. The drying of the slurry forms the negative electrode active material layer 34 on the negative electrode current collector 32 .

[0089] The rolling step is performed as needed. 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 using, for example, a roll press.

[0090] The positive electrode 20 can be produced by the same steps as those of the negative electrode 30, except that the composite forming step is not performed. Commercially available products can be used for the separator 10 and the outer casing 50.

[0091] The electrolyte solution can be prepared, for example, by adding an imide salt containing the first element and an imide anion to a mixture of a lithium salt and a solvent, and mixing the mixture.

[0092] Next, the positive electrode 20 and negative electrode 30 are stacked with the separator 10 positioned between them to form the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, negative electrode 30, and separator 10 are wound around one end thereof.

[0093] Finally, the power-generating element 40 is enclosed in the outer casing 50. A non-aqueous electrolyte is injected into the outer casing 50. After the non-aqueous electrolyte is injected, the non-aqueous electrolyte is permeated into the power-generating element 40 by reducing pressure, heating, or the like. The outer casing 50 is sealed by heating, etc., to obtain the lithium-ion secondary battery 100. It should be noted that the power-generating element 40 can be immersed in the electrolyte instead of injecting the electrolyte into the outer casing 50.

[0094] The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics. The reasons for this will be described below.

[0095] When the negative electrode active material expands in volume, silicon particles may detach from the negative electrode active material. These silicon particles increase the viscosity of the electrolyte and cause clogging of the separator 10. If the electrolyte becomes highly viscous, it becomes difficult for the electrolyte to penetrate into the negative electrode active material layer 34. The increased viscosity of the electrolyte reduces the cycle characteristics of the lithium-ion secondary battery. In addition, if the separator 10 is clogged, the movement of lithium ions is hindered. Clogging of the separator 10 reduces the cycle characteristics of the lithium-ion secondary battery.

[0096] In the lithium-ion secondary battery 100 according to the first embodiment, a compound containing the first element is present in the negative electrode active material. The compound containing the first element reacts with the detached silicon particles and absorbs them. Therefore, the lithium-ion secondary battery 100 according to the first embodiment can prevent the silicon particles from flowing into the electrolyte, thereby suppressing a decrease in cycle performance.

[0097] Furthermore, in the lithium-ion secondary battery 100 according to the first embodiment, an imide salt containing the first element is present in the electrolyte. Therefore, even if the first element in the negative electrode active material is depleted due to the reaction between the silicon particles and the compound containing the first element, the first element can be supplied from the electrolyte to the negative electrode active material. Consequently, the lithium-ion secondary battery 100 can maintain a high capacity retention rate even as the number of cycles increases.

[0098] Furthermore, the silicon fine particles pass near the surface of the negative electrode active material when flowing into the electrolyte. Therefore, if the coating layer 37 of the negative electrode active material contains the first element, the effect of suppressing the degradation of the cycle characteristics of the lithium ion secondary battery 100 is high.

[0099] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the various structures in the embodiments and their combinations are merely examples, and additions, omissions, replacements, and other changes to the structures may be made without departing from the spirit of the present invention.

[0100] [Example]

[0101] Example 1

[0102] The positive electrode slurry is applied to one side of a 15 μm thick aluminum foil. The positive electrode slurry is prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0103] Lithium is used as the positive electrode active material xCoO2. Acetylene black was used as a conductive aid. Polyvinylidene fluoride (PVDF) was used as a binder. N-methyl-2-pyrrolidone was used as a solvent. 97 parts by mass of a positive electrode active material, 1 part by mass of a conductive aid, 2 parts by mass of a binder, and 70 parts by mass of a solvent were mixed to prepare a positive electrode slurry. The positive electrode active material loading in the dried positive electrode active material layer was 25 mg / cm 2 In a drying oven, the solvent is removed from the positive electrode slurry to form a positive electrode active material layer. The positive electrode active material layer is pressed using a roller press to form a positive electrode.

[0104] Using a mechanical fusion machine manufactured by Hosokawa Micron tilted at 10 degrees, silicon and carbon are compounded to produce a substance containing silicon and carbon. In the substance containing silicon and carbon, the weight ratio of silicon is 60wt%, and the weight ratio of carbon is 40wt%. Next, using the same device as above, the substance containing silicon and carbon, magnesium oxide and LiF are compounded to produce a negative electrode active material. The rotation speed of the device during compounding is set to 2500rpm. Magnesium oxide is an oxide containing the first element. The median particle size of the negative electrode active material after the compounding treatment is 5μm. A coating layer containing Mg is formed on the surface of the negative electrode active material.

[0105] Next, a negative electrode slurry is applied on one side of a copper foil having a thickness of 10 μm. The negative electrode slurry is prepared by mixing a negative electrode active material, a conductive aid, a binder and a solvent. The negative electrode active material is the material after the above-mentioned composite treatment. Carbon black is used as a conductive aid. Polyimide resin is used as a binder. N-methyl-2-pyrrolidone is used as a solvent. 90 parts by mass of a negative electrode active material, 5 parts by mass of a conductive aid and 5 parts by mass of a binder are mixed with N-methyl-2-pyrrolidone to prepare a negative electrode slurry. The loading amount of the negative electrode active material in the negative electrode active material layer after drying is 3.0 mg / cm 2 The solvent was removed from the negative electrode slurry in a drying oven to form a negative electrode active material layer. The negative electrode active material layer was pressed using a roller press and then calcined at a temperature of 300°C or higher for 5 hours in a nitrogen atmosphere.

[0106] Next, a solvent was prepared by mixing fluoroethylene carbonate (FEC): ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC) in a volume ratio of 5:5:20:70. LiPF6 was dissolved in this mixed solvent to a concentration of 1 mol / L. Mg(TFSI)2, an imide salt, was then added to this solution to prepare an electrolyte. The concentration of the imide salt of the first element was adjusted to 10% (0.1 mol / L) relative to the concentration of the lithium salt.

[0107] (Fabrication of Lithium-ion Secondary Battery for Evaluation)

[0108] The produced negative electrode and positive electrode are stacked with the positive electrode active material layer and the negative electrode active material layer facing each other through a separator (porous polyethylene sheet) to obtain a laminate. A nickel negative electrode lead is installed on the negative electrode of the laminate. An aluminum positive electrode lead is installed on the positive electrode of the laminate. The positive electrode lead and the negative electrode lead are welded by an ultrasonic welding machine. The laminate is inserted into the outer casing of the aluminum laminate film and heat-sealed except for one place around it to form a closed portion. Finally, after injecting the above-mentioned electrolyte into the outer casing, the remaining one place is decompressed by a vacuum sealing machine while being sealed by heat sealing to produce a lithium ion secondary battery. It should be noted that two lithium ion secondary batteries were produced, one for electrolyte composition analysis and the other for charge and discharge characteristic evaluation.

[0109] Electrolyte was collected from a lithium-ion secondary battery for electrolyte composition analysis. The composition of the collected electrolyte was then analyzed using ICP luminescence. The results confirmed that the Mg content in the electrolyte was the same as that at the time of electrolyte preparation.

[0110] (Determination of capacity retention after 300 cycles)

[0111] The cycle characteristics of the evaluation lithium-ion secondary battery were measured using a secondary battery charge and discharge tester (manufactured by Hokuto Denko Co., Ltd.).

[0112] The battery was charged at a constant current rate of 1.0C (the current value that completes charging after one hour at 25°C) until the battery voltage reached 4.4V. The battery was then discharged at a constant current rate of 1.0C until the battery voltage reached 3.0V. The discharge capacity after the charge and discharge was measured to determine the battery capacity Q1 before the cycle test.

[0113] The battery for which the battery capacity Q1 was calculated was again charged using the secondary battery charge and discharge test apparatus at a constant current of 1.0C until the battery voltage reached 4.4V. The battery was then discharged at a constant current of 1.0C until the battery voltage reached 3.0V. This charge and discharge cycle was counted as one cycle, and 300 cycles of charge and discharge were performed. The discharge capacity after 300 cycles was measured, and the battery capacity Q2 after 300 cycles was calculated.

[0114] The capacity retention rate E after 300 cycles was determined from the capacities Q1 and Q2 determined above. The capacity retention rate E was determined by E=Q2 / Q1×100. The capacity retention rate E of Example 1 was 77%.

[0115] Examples 2 to 8

[0116] Examples 2-8 differ from Example 1 in that the silicon-carbon mixture ratio was varied when preparing the material containing silicon and carbon. Specifically, Examples 2-8 differ from Example 1 in the weight ratio of silicon to carbon in the negative electrode active material. All other conditions were the same as in Example 1, and the capacity retention factor E was calculated. The results are summarized in Table 1.

[0117] Examples 9 to 16

[0118] Examples 9 to 16 differ 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 other conditions were the same as in Example 1, and the capacity retention rate E was calculated. The results are summarized in Table 1.

[0119] Examples 17 to 21

[0120] Examples 17 to 21 differ from Example 1 in that the lithium salt concentration and the imide salt concentration of the first element in the electrolyte were varied. In all examples 17 to 21, the imide salt concentration of the first element was adjusted to 10% relative to the lithium salt concentration in the electrolyte. Other conditions were the same as in Example 1, and the capacity retention rate E was calculated. The results are summarized in Table 1.

[0121] Examples 22 to 25

[0122] Examples 22 to 25 differ from Example 1 in that the lithium salt concentration in the electrolyte was kept constant and the imide salt concentration of the first element was varied. The other conditions were the same as in Example 1, and the capacity retention rate E was determined. The results are summarized in Table 2.

[0123] Examples 26 to 29

[0124] Examples 26 to 29 differ from Example 1 in that the imide salt added to the electrolyte and the first element contained in the negative electrode active material were varied. In Examples 26 to 29, the first element contained in the imide salt and the first element contained in the negative electrode active material were both two or more elements. Other conditions were the same as in Example 1, and the capacity retention rate E was calculated. The results are summarized in Table 2.

[0125] Examples 30 to 34

[0126] Examples 30 to 34 differ from Example 1 in that the particle size of the negative electrode active material was changed. Other conditions were the same as in Example 1, and the capacity retention rate E was determined. The results are summarized in Table 2.

[0127] Comparative Example 1

[0128] Comparative Example 1 differs from Example 1 in that magnesium oxide (a compound containing the first element) is not added to the negative electrode active material, and no imide salt is added to the electrolyte. All other conditions were the same as in Example 1, and the capacity retention factor E was calculated. The results are summarized in Table 2.

[0129] Comparative Example 2

[0130] Comparative Example 2 differs from Example 1 in that no imide salt was added to the electrolyte. Other conditions were the same as in Example 1, and the capacity retention rate E was determined. The results are summarized in Table 2.

[0131] Comparative Example 3

[0132] Comparative Example 3 differs from Example 1 in that magnesium oxide (a compound containing the first element) is not added when preparing the negative electrode active material. Other conditions were the same as in Example 1, and the capacity retention rate E was determined. The results are summarized in Table 2.

[0133] The capacity retention rates of Examples 1 to 34 were all higher than those of Comparative Examples 1 to 3. In other words, the lithium ion secondary batteries of Examples 1 to 34 (where the negative electrode active material contained a compound containing the first element and the electrolyte contained an imide salt of the first element) exhibited excellent cycle characteristics.

[0134]

[0135]

[0136] [Explanation of symbols]

[0137] 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 outer casing; 52 metal foil; 54 resin layer; 60, 62 terminals; 100 lithium-ion secondary battery.

Claims

1. A lithium ion secondary battery, wherein: It has: a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte. The negative electrode has a negative electrode active material, and the negative electrode active material includes: 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 any one or more elements selected from K, Na, Mg, Ca, Cs, Al, and Zn. The compound containing the first element is at least one selected from fluorides, oxides, silicides, silicon oxides, and phosphorus oxides.

2. The lithium ion secondary battery according to claim 1, wherein In the substance, when the total of silicon and carbon 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 electrolyte solution is 5% or more and 20% or less relative to the molar concentration ratio of the lithium salt in the electrolyte solution.

4. The lithium ion secondary battery according to claim 1 or 2, wherein The negative electrode active material has a core and a coating layer covering the core. The coating layer includes the compound containing the first element.

5. The lithium ion secondary battery according to claim 1 or 2, wherein The negative electrode active material has a median particle size D50 of 1 μm or more and 10 μm or less.

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