Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery excellent in safety.SOLUTION: The lithium ion secondary battery includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte. The negative electrode comprises a negative electrode active material which contains silicon oxide 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 widely used as power sources for mobile devices such as mobile phones and notebook computers, and for hybrid cars and the like.

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

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

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] The negative electrode active material containing silicon can store a large amount of energy with a high capacity. However, if the stored energy gets out of control, there is a risk of thermal runaway of the active material. Therefore, even when using a high-capacity silicon-based negative electrode active material, the development of a highly safe lithium-ion secondary battery is required.

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

Means for Solving the Problems

[0008] To solve the above problems, the following means are provided.

[0009] (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 silicon oxide 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.

[0010] (2) 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.

[0011] (3) 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 fluoride, oxide, silicide, silicate, and phosphate.

[0012] (4) 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.

[0013] (5) In the lithium-ion secondary battery according to the above aspect, the silicon oxide is SiO x represented by, and x may satisfy 0.8 ≦ x ≦ 2.

[0014] (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

[0015] The lithium-ion secondary battery according to the above aspect is excellent in safety.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0017] Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. The drawings used in the following description may show, for the sake of clarity, parts that are characteristic by being enlarged for convenience, and the dimensional ratios and the like 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 changed and implemented without changing the gist thereof.

[0018] “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.

[0019] (Power Generation Element) The power generation element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

[0020] <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.

[0021] [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.

[0022] [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 assistant and a binder as necessary.

[0023] 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 electrolytic solution to impregnate into the positive electrode active material layer 24.

[0024] The positive electrode active material includes an electrode active material capable of reversibly proceeding with the occlusion and release of lithium ions, the desorption and insertion (intercalation) of lithium ions, or the doping and dedoping of lithium ions and counter anions.

[0025] 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, or polyacene.

[0026] 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 a plurality. When the positive electrode active material is a lithium-free material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharge. In addition, the lithium-free material of the positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] The binder may also be an electronically conductive polymer or an ionically conductive polymer. Examples of the electronically conductive polymer include polyacetylene, polythiophene, polyaniline, etc. Examples of the ionically conductive polymer include a composite material of a polyether-based polymer compound and a lithium salt. Examples of the polyether-based polymer compound include polyethylene oxide, polypropylene oxide, etc. Examples of the lithium salt include LiFSI, LiTFSI, LiBETI, LiClO4, LiBF4, LiPF6, etc.

[0031] 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.

[0032] <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.

[0033] [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.

[0034] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer 34 may also contain a conductive auxiliary agent as needed. The negative electrode active material contains silicon oxide and a compound containing a first element.

[0035] Silicon oxide is represented by SiO x where x satisfies 0.8 ≤ x ≤ 2. The silicon oxide may consist only of SiO2, may consist only of SiO, or may be a mixture of SiO and SiO2. Also, a part of the oxygen in the silicon oxide may be deficient. The silicon oxide may be crystalline or amorphous.

[0036] The compound containing the first element is one or more selected from the group consisting of, for example, fluorides, oxides, silicides, and silicate oxides. 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.

[0037] 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 / desorption reaction of Li uniform and suppresses the deterioration of the negative electrode active material 35.

[0038] The core 36 contains silicon oxide. The core 36 is, for example, silicon oxide. The coating layer 37 contains, for example, a compound containing silicon oxide and the first element. The coating layer 37 may contain a carbon material, metal fine powder, a mixture of a carbon material and metal fine powder, a conductive oxide, an inorganic compound, etc. The carbon material is, for example, carbon powder, carbon 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.

[0039] 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 electrolyte. For example, when the first element is Mg, the reaction product of the first element and the material contained in the electrolyte is, for example, Mg3(PO4)2.

[0040] The median diameter (D50) of the negative electrode active material is, for example, 1 μm or more and 10 μm or less. If the particle size of the negative electrode active material is within this range, the coating layer of the negative electrode active material is less likely to peel off due to volume expansion, and the first element is efficiently supplied from the electrolytic solution to the negative electrode active material. As a result, a resistance layer (details will be described later) generated by the reaction between the first element and oxygen can be stably maintained.

[0041] 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. If the negative electrode potential is low, the stability of the lithium-ion secondary battery 100 is improved.

[0042] 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.

[0043] 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.

[0044] The contents of the negative electrode active material, the conductive assistant, and the binder in the negative electrode active material layer 34 are not particularly limited. The constituent ratio of the negative electrode active material in the negative electrode active material layer 34 is, for example, 70% or more and 100% or less by mass ratio. 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.

[0045] <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.

[0046] Separator 10 has, for example, a porous structure with electrical insulation. Separator 10 is, for example, a single layer or a laminate of a polyolefin film. Separator 10 may also be a stretched film of a mixture such as polyethylene or polypropylene. Separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. Separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. Separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is obtained by 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 has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0047] <Electrolyte solution> The electrolyte solution is enclosed within 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.

[0048] The solvent is not particularly limited as long as it is a solvent generally used in lithium-ion secondary batteries. The solvent contains, for example, any one of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may contain these mixed in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), etc. Examples of the chain carbonate compound include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of the cyclic ester compound include γ-butyrolactone, etc. Examples of the chain ester compound include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0049] 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 electrolytic solution are contained in the film, Li ions are easily transported in the film. As a result, an increase in the resistance of the lithium ion secondary battery 100 is suppressed.

[0050] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, etc. The lithium salt may be used alone or in combination of two or more. From the viewpoint of ionization degree, it is preferable that the electrolyte contains LiPF6.

[0051] The imide salt contains a first element and an imide anion. The first element is as described above. The first element contained in the electrolytic solution and the first element contained in the negative electrode active material layer 34 are, for example, the same element. The first element contained in the electrolytic solution and the first element contained in the negative electrode active material layer 34 may be different elements. The imide salt is not limited to one type and may be contained in a plurality of types in the electrolytic solution.

[0052] The imide anion is, for example, (SO2F)2N - (FSI - : bis(fluorosulfonyl)imide anion), (SO2CF3)2N - (TFSI - : bis(trifluoromethanesulfonyl)imide anion), (SO2C2F5)2N - (BETI - : bis(pentafluoroethanesulfonyl)imide anion), (SO2F)(SO2CF3)N - , (SO2CF3)(SO2C2F5)N - and so on. The imide anion may be used alone or in combination of two or more.

[0053] The molar concentration ratio of the imide salt containing the first element in the electrolytic solution is, for example, 5% or more and 20% or less with respect to the molar concentration ratio of the lithium salt in the electrolytic solution. When the imide salt of the first element is sufficiently contained in the electrolytic solution, it is possible to avoid depletion of the first element even when the first element continues to react with oxygen. Also, when the imide salt of the first element is excessively contained in the electrolytic solution, the viscosity of the electrolytic solution increases, and the impregnation property of the electrolytic solution into the positive electrode 20 and the negative electrode 30 decreases.

[0054] The molar concentration ratio of the first element and lithium in the electrolytic solution can be measured, for example, by gas chromatography, ICP (inductively coupled plasma) emission analysis, ICP mass spectrometry, or the like.

[0055] <Outer package> The outer package 50 seals the power generation element 40 and the non-aqueous electrolytic solution inside it. The outer package 50 suppresses leakage of the non-aqueous electrolytic solution to the outside and intrusion of moisture or the like from the outside into the lithium ion secondary battery 100.

[0056] As shown in FIG. 1, for example, the outer package 50 has 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 from both sides with a polymer film (resin layer 54).

[0057] As the metal foil 52, for example, an aluminum foil can be used. As the resin layer 54, a polymer film such as polypropylene can be used. The material constituting the resin layer 54 may be different between the inner side and the outer side. For example, as the outer material, a polymer with a high melting point, such as polyethylene terephthalate (PET), polyamide (PA), etc. can be used, and as the material of the inner polymer film, polyethylene (PE), polypropylene (PP), etc. can be used.

[0058] <Terminal> 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. Terminals 60 and 62 are responsible for the electrical connection with the outside. Terminals 60 and 62 are formed of a conductive material such as aluminum, nickel, or copper. The connection method can be welding or screwing. In order to prevent short circuits, it is preferable to protect terminals 60 and 62 with insulating tape.

[0059] "Method for manufacturing a lithium-ion secondary battery" The lithium-ion secondary battery 100 is manufactured by respectively preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolyte, and an exterior body 50, and assembling these components. Hereinafter, an example of the manufacturing method of the lithium-ion secondary battery 100 will be described.

[0060] The negative electrode 30 is manufactured, for example, by sequentially performing a compounding process, a slurry preparation process, an electrode coating process, a drying process, and a rolling process.

[0061] The compounding process is a process of mixing silicon oxide and a compound containing a first element while applying a shearing force. When the compounding process is performed, the surface of the silicon oxide is coated with the compound containing the first element. Also, the particle size of the negative electrode active material can be adjusted according to the degree of the mixing. Further, the manufactured negative electrode active material may be sieved to make the particle sizes uniform.

[0062] The slurry preparation process is a process of mixing the compounded negative electrode active material, a binder, and a solvent to form a slurry. In the slurry preparation process, 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% - 100 wt%: 0 wt% - 10 wt%: 0 wt% - 20 wt% by mass ratio. These mass ratios are adjusted so that the total is 100 wt%.

[0063] The electrode coating process is a process of coating a 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 or the doctor blade method can be used as the coating method of the slurry.

[0064] The drying process is a process of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere of 80°C to 150°C. By drying the slurry, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.

[0065] The rolling process is performed as necessary. The rolling process is a process of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling process is performed, for example, with a roll press device or the like.

[0066] The positive electrode 20 can be manufactured by the same procedure as the negative electrode 30, except that the composite process is not performed. As the separator 10 and the exterior body 50, commercially available products can be used.

[0067] The electrolytic solution can be prepared, for example, by adding an imide salt containing a first element and an imide anion to a mixture of a lithium salt and a solvent and mixing them.

[0068] Next, these are laminated so that the separator 10 is positioned between the manufactured positive electrode 20 and negative electrode 30 to manufacture the power generation element 40. When the power generation element 40 is a wound body, they are wound around one end side of the positive electrode 20, negative electrode 30, and separator 10 as an axis.

[0069] Finally, the power generation element 40 is enclosed in the exterior body 50. The non-aqueous electrolytic solution is injected into the exterior body 50. By performing decompression, heating, etc. after injecting the non-aqueous electrolytic solution, the non-aqueous electrolytic solution is impregnated into the power generation element 40. By applying heat, etc. to seal the exterior body 50, the lithium ion secondary battery 100 is obtained. Note that instead of injecting the electrolytic solution into the exterior body 50, the power generation element 40 may be impregnated with the electrolytic solution.

[0070] The lithium-ion secondary battery 100 according to the first embodiment is excellent in safety. The reasons are explained below.

[0071] The negative electrode active material containing silicon can store a large amount of energy with high capacity. On the other hand, when an internal short circuit or the like occurs and the stored energy becomes uncontrollable, the lithium-ion secondary battery may experience thermal runaway. The thermal runaway of the lithium-ion secondary battery has an adverse effect on surrounding elements and may lead to ignition in the worst case.

[0072] In the lithium-ion secondary battery 100 according to the first embodiment, a compound containing a first element exists in the negative electrode active material. The first element contained in the negative electrode active material reacts with oxygen contained in silicon oxide by using heat generated in the process leading to thermal runaway or the like. When the first element and oxygen react, an oxide containing the first element is generated. The oxide has a high resistance and forms a resistance layer. The resistance layer inhibits internal short circuits. The thermal runaway of the lithium-ion secondary battery occurs because the heat dissipation of the heat generated by the internal short circuit cannot keep up. The lithium-ion secondary battery 100 according to the first embodiment can inhibit continuous internal short circuits by the resistance layer and suppress thermal runaway.

[0073] Also, in the lithium-ion secondary battery 100 according to the first embodiment, an imide salt containing the first element exists in the electrolytic solution. Therefore, even when the first element in the negative electrode active material is depleted due to the reaction between oxygen 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 continuously form a resistance layer and has high stability.

[0074] Also, when the first element is contained in the coating layer 37 of the negative electrode active material, a resistance layer is formed within the coating layer 37, which is the outermost layer of the negative electrode active material, and internal short circuits can be efficiently prevented. Therefore, when the first element is contained in the coating layer 37 of the negative electrode active material, the safety of the lithium-ion secondary battery 100 is further enhanced.

[0075] 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

[0076] 「Example 1」 The positive electrode slurry was applied to one surface of an aluminum foil with 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.

[0077] 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 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.

[0078] Silicon oxide, magnesium oxide, and LiF were compounded using a Hosokawa Micron Mechanofusion inclined at 10 degrees to produce a negative electrode active material. The rotation speed of the apparatus during 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 3 μm. The negative electrode active material had a coating layer containing Mg formed on its surface.

[0079] Next, the 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-described composite treatment. The conductive assistant used was carbon black. The binder used was a polyimide resin. The solvent used was N-methyl-2-pyrrolidone. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive assistant, and 5 parts by mass of the binder were mixed in N-methyl-2-pyrrolidone to prepare the negative electrode slurry. The loading amount of the negative electrode active material in the dried negative electrode active material layer was 3.0 mg / cm 2 was used. The solvent was removed from the negative electrode slurry in a drying furnace to form the negative electrode active material layer. After the negative electrode active material layer was pressed by roll pressing, it was thermally baked at 300 °C or higher for 5 hours in a nitrogen atmosphere.

[0080] 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 the electrolyte. The concentration of the imide salt of the first element with respect to the Li salt concentration was adjusted to be 10% (0.1 mol / L).

[0081] (Fabrication of Lithium-Ion Secondary Battery for Evaluation) The fabricated negative electrode and positive electrode were laminated through a separator (porous polyethylene sheet) such that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. A negative electrode lead made of nickel was attached to the negative electrode of the laminate. A positive electrode lead made of aluminum was attached to the positive electrode of the laminate. The positive electrode lead and the negative electrode lead were welded 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 under reduced pressure by a vacuum sealer to fabricate a lithium ion secondary battery. Ten cells of the same sample were fabricated for the lithium ion secondary battery.

[0082] Also, the electrolytic solution was collected from the lithium ion secondary battery. Next, the composition of the collected 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.

[0083] (Nail penetration test) First, the fabricated lithium ion secondary battery was charged. The charging was performed by constant current charging at a charging rate of 1.0C (the current value at which charging is completed in 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.4V. Then, a nail with a diameter of 2.5 mm was stabbed into the charged battery at a speed of 150 mm / s to perform a nail penetration test.

[0084] The nail penetration test was performed for each of the 10 cells, and the ratio of the cells in which ignition did not occur was determined. The pass probability E of the nail penetration test was determined by E = "number of cells that did not catch fire" / 10 × 100.

[0085] "Examples 2 to 9" Examples 2 to 9 are different from Example 1 in that at least one of the imide salt added to the electrolytic solution and the first element contained in the negative electrode active material was changed. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 1.

[0086] "Examples 10 to 14" Examples 10 to 14 are different from Example 1 in that the Li salt concentration and the imide salt concentration of the first element in the electrolytic solution are changed. In each of Examples 10 to 14, the imide salt concentration of the first element with respect to the Li salt concentration in the electrolytic solution was adjusted to 10%. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 1.

[0087] "Examples 15 to 20" Examples 15 to 20 are different from Example 1 in that the Li salt concentration in the electrolytic solution is fixed and the imide salt concentration of the first element is changed. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 1.

[0088] "Examples 21 to 24" Examples 21 to 24 are different from Example 1 in that the imide salt added to the electrolytic solution and the first element contained in the negative electrode active material are changed. In Examples 21 to 24, 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. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0089] "Examples 25 to 31" Examples 25 to 31 are different from Example 1 in that the particle size of the negative electrode active material is changed. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0090] "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 electrolytic solution. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0091] "Comparative Example 2" Comparative Example 2 is different from Example 1 in that no imide salt was added to the electrolytic solution. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0092] "Comparative Example 3" Comparative Example 3 is different from Example 1 in that magnesium oxide (a compound containing a first element) was not added when preparing the negative electrode active material. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0093] "Comparative Example 4" Comparative Example 4 is different from Example 1 in that silicon was used instead of silicon oxide when preparing the negative electrode active material. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0094] "Comparative Example 5" Comparative Example 5 is different from Example 1 in that a composite of silicon and carbon was used instead of silicon oxide when preparing the negative electrode active material. For other conditions, the pass rate E of the nail penetration test was determined in the same manner as in Example 1. The results are summarized in Table 2.

[0095] In Examples 1 to 31, the pass rate of the nail penetration test was higher than that of Comparative Examples 1 to 3. That is, the lithium-ion secondary batteries according to Examples 1 to 31, 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 safety.

[0096]

Table 1

[0097]

Table 2

Explanation of Signs

[0098] 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 silicon oxide, 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 a 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 a molar concentration ratio of a lithium salt in the electrolytic solution.

3. 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 Claim 1 or 2.

4. The silicon oxide is SiO ｘ represented by, where x satisfies 0.8 ≤ x ≤ 2, the lithium ion secondary battery according to any one of claims 1 to 3.

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

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