JP7698981B2 - Lithium-ion secondary battery - Google Patents

Description

The present invention relates to a lithium-ion secondary battery.

Lithium-ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptops, as well as hybrid cars.

The capacity of a lithium-ion secondary battery depends mainly on the active material of the electrodes. Graphite is generally used as the negative electrode active material, but there is a demand for negative electrode active materials with higher capacity. For this reason, attention is being paid to negative electrode active materials containing silicon (Si), which has a theoretical capacity much larger than that of graphite (372 mAh/g).

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 electrolyte solution containing a specific lithium salt and an ether compound.

JP 2009-176534 A JP 2014-110235 A Special Publication No. 2015-534254

Further improvements in cycle characteristics are required.

This disclosure was made in consideration of the above problems, and aims to provide a lithium-ion secondary battery with excellent cycle characteristics.

To solve the above problems, the following measures are provided:

(1) A lithium ion secondary battery according to a first aspect 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 including silicon or a silicon compound, a first compound including a first element, and a second compound including a second element. The electrolyte includes a first imide salt including the first element and an imide anion, and a second imide salt including the second element and an imide anion. The first element is one or more elements that form a divalent or trivalent cation in the electrolyte, and the second element is one or more elements that form a monovalent cation in the electrolyte.

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

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

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

(5) In the lithium ion secondary battery according to the above aspect, the molar concentration ratio of the second imide salt in the electrolyte may be 10% or more and 50% or less relative to the molar concentration ratio of the first imide salt.

(6) In the lithium ion secondary battery according to the above aspect, the first compound may be any one or more selected from the group consisting of fluorides, oxides, silicides, silicides, and phosphates, and the second compound may be any one or more selected from the group consisting of fluorides, oxides, silicides, silicides, and phosphates.

(7) In the lithium ion secondary battery according to the above aspect, the negative electrode active material may have a core and a coating layer that coats the core, and the coating layer may contain the first compound and the second compound.

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

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

The lithium ion secondary battery according to the above aspect has excellent cycle characteristics.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; FIG. 2 is a schematic diagram of a negative electrode active material according to the first embodiment.

The following describes the embodiments in detail, with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of clarity, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them. Appropriate modifications may be made without departing from the spirit of the invention.

"Lithium-ion secondary battery"
Fig. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. The lithium ion secondary battery 100 shown in Fig. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte is contained within the exterior body 50.

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

<Positive electrode>
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.

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

[Cathode 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.

The basis weight of the positive electrode active material layer 24 is, for example, 15 mg/ ^cm2 or more, and preferably 20 mg/ ^cm2 or more. The basis weight of the positive electrode active material layer 24 is, for example, preferably 35 mg/ ^cm2 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. If the basis weight is large, the amount of positive electrode active material per unit area increases, and the capacity of the battery increases. On the other hand, if the basis weight is too large, it becomes difficult for the electrolyte to impregnate the positive electrode active material layer 24.

The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, remove and insert lithium ions (intercalation), or dope and dedope lithium ions and counter anions.

The positive electrode active material is, for example, a composite metal oxide. Examples of the composite metal oxide include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and a compound of the general formula: LiNi _x Co _y Mn _z M _a O ₂ (wherein x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV ₂ O ₅ ), an olivine-type LiMPO ₄ (wherein M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and a lithium titanate (Li _{4 Ti5O12} ), _{LiNixCoyAlzO2 (} 0.9< _x +y+ _z <1.1). The positive _electrode active material may be an organic material. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

The positive electrode active material may be a lithium-free material. Examples of the lithium-free material include FeF ₃ , conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The lithium-free material may be any one of the materials or a combination of a plurality of materials. When the positive electrode active material is a lithium-free material, for example, discharge is performed first. Lithium is inserted into the positive electrode active material by discharging. In addition, lithium may be pre-doped chemically or electrochemically into the lithium-free positive electrode active material.

The conductive additive enhances the electronic conductivity between the positive electrode active materials. The conductive additive is, for example, carbon powder, carbon nanotubes, carbon material, metal powder, a mixture of carbon material and metal powder, or conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal powder is, for example, copper, nickel, stainless steel, iron, etc.

The binder binds the active materials together. Any known binder 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), polyvinyl fluoride (PVF), etc.

In addition to the above, the binder may be, for example, vinylidene fluoride-based fluororubber such as 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-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber). The binder may also be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc.

The binder may be an electronically conductive polymer or an ionically conductive polymer. The electronically conductive polymer may be, for example, polyacetylene, polythiophene, polyaniline, etc. The ionically conductive polymer may be, for example, a composite material of a polyether-based polymer compound and a lithium salt. The polyether-based polymer compound may be, for example, polyethylene oxide, polypropylene oxide, etc. The lithium salt may be, for example, LiFSI, LiTFSI, LiBETI, _LiClO4 , _LiBF4 , _LiPF6 , etc.

The composition 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. The composition ratio of the conductive assistant in the positive electrode active material layer 24 is, for example, 1.0% or more and 10% or less by mass. The composition 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.

<Negative electrode>
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.

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

[Negative electrode active material layer]
The negative electrode active material layer 34 includes a negative electrode active material and a binder. The negative electrode active material layer 34 may also include a conductive assistant as necessary. The negative electrode active material includes silicon or a silicon compound, a first compound including a first element, and a second compound including a second element.

Silicon or silicon compounds may be crystalline or amorphous. Silicon is elemental silicon. Silicon compounds are compounds that contain elemental silicon. Examples of silicon compounds include silicon alloys, silicon oxides, silicon carbides, and composites of silicon and other substances (e.g., carbon). Silicon compounds may also be mixtures of these.

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

Silicon oxide is represented by _SiOx , where x satisfies 0≦x≦2, and preferably satisfies 0.8≦x≦2. Silicon oxide may be composed of only _SiO2 , may be composed of only SiO, or may be a mixture of SiO and _SiO2 . Silicon oxide may also have a partial oxygen deficiency. Silicon oxide may be crystalline or amorphous.

A composite of silicon and another substance is a product of a composite process of silicon and another substance. For example, when silicon and carbon are mixed while applying shear force, a composite of silicon and carbon is obtained. In the composite, the weight ratio of silicon is, for example, 30 wt% or more and 70 wt% or less, and preferably 50 wt% or more and 70 wt% or less.

The first compound is a compound containing a first element. The first compound is, for example, one or more selected from the group consisting of fluorides, oxides, silicides, and silicates. The first element is one or more elements that form divalent or trivalent cations in the electrolyte. The first element is, for example, one or more elements selected from the group consisting of Mg, Ca, Cs, Al, and Zn. The first element is not limited to one type of element, and may be multiple types of elements that satisfy the above conditions. For example, when the first element is Mg, the first compound is, for example, MgF ₂ , MgO, Mg ₂ Si, Li _x Mg _y Si, and Mg ₂ SiO ₄ .

The second compound is a compound containing a second element. The second compound is, for example, one or more selected from the group consisting of fluorides, oxides, silicides, and silicates. The second element is one or more elements that form monovalent cations in the electrolyte. The second element is, for example, one or more elements of K and Na. The second element is not limited to either K or Na, and may be both K and Na. For example, when the second element is Na, the first compound is, for example, NaF or Na ₂ O.

Figure 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 portion of the core 36. The coating layer 37 may cover the entire surface of the core 36. The coating layer 37 makes the insertion and desorption reaction of Li uniform and suppresses deterioration of the negative electrode active material 35.

The core 36 includes silicon or a silicon compound. The core 36 is, for example, silicon or a silicon compound. The coating layer 37 includes, for example, silicon or a silicon compound, a first compound, and a second compound. The coating layer 37 may include a carbon material, metal powder, a mixture of a carbon material and a metal powder, a conductive oxide, an inorganic compound, or the like. The carbon material is, for example, carbon powder, carbon nanotubes, or the like. The metal powder is, for example, copper, cobalt, iron, or the like. The inorganic material is, for example, ceramics, oxides, carbides, nitrides, borides, or the like.

The coating layer 37 may have a first layer 37A and a second layer 37B. The second layer 37B is the outermost layer of the coating layer 37. The second layer 37B may include a reaction product between the first element or the second element and a material contained in the electrolyte. For example, when the first element is Mg, the reaction product between the first element and a material contained in the electrolyte is, for example, Mg ₃ (PO ₄ ) _2. For example, when the second element is Na, the reaction product between the second element and a material contained in the electrolyte is, for example, Na ₃ PO ₄ .

The median diameter (D50) of the negative electrode active material is, for example, 500 nm or more and 10 μm or less. If the particle size of the negative electrode active material is sufficiently small, the aggregation of the negative electrode active material in the negative electrode active material layer 34 can be suppressed. Furthermore, if the median diameter of the negative electrode active material is within a predetermined range, the aggregation of the first compound and the second compound in the negative electrode active material can be suppressed. By preventing these compounds from aggregating, damage to the negative electrode active material due to volume expansion can be suppressed.

In addition, 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 more frequently the electrolyte comes into contact with the negative electrode active material, and the easier it is for the electrolyte to decompose. On the other hand, the larger the particle size of the negative electrode active material, the less likely it is that localized areas of low or high potential will occur on the surface of the negative electrode active material. Non-uniform potential can cause oxidation-reduction decomposition of the electrolyte. If the median diameter of the negative electrode active material is within the above range, the cycle characteristics of the lithium ion secondary battery 100 will be improved.

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. A high or low negative electrode potential can cause a decrease in cycle characteristics. The higher the negative electrode potential, the more lithium can be trapped, but the more easily the electrolyte and the coating layer 37 are oxidized and 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 more easily the electrolyte and the coating layer 37 are reductively decomposed. If the discharge specific capacity of the negative electrode active material is in the above range, it has a high capacity and excellent cycle characteristics.

The conductive additive enhances electronic conductivity between the negative electrode active materials. The conductive additive can be the same as that used for the positive electrode 20.

The binder binds the negative electrode active materials together and also binds the negative electrode active materials to the negative electrode current collector 32. The binder may be the same as that used for the positive electrode 20.

The contents of the negative electrode active material, conductive assistant, and binder in the negative electrode active material layer 34 are not particularly limited. The composition 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. The composition 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, and the composition ratio of the binder in the negative electrode active material layer 34 is, for example, 0% or more and 20% or less by mass.

<Separator>
The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates 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-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, etc. The 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. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may be an inorganic-coated separator. The inorganic-coated separator is a separator in which a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica is applied to the surface of the above-mentioned film. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode on the negative electrode surface.

<Electrolyte>
The electrolytic solution is sealed in the exterior body 50 and impregnates the power generating element 40. The nonaqueous electrolytic solution includes, for example, a nonaqueous solvent and an electrolyte. The electrolyte is dissolved in the nonaqueous solvent. The electrolytic solution includes, for example, a solvent, an electrolyte, a first imide salt, and a second imide salt. The first imide salt and the second imide salt may each be one type or two or more types.

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

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

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 the degree _of ionization _, it is preferable that the electrolyte contains _LiPF6 .

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

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

Examples of imide anions include (SO ₂ F) ₂ N ^- (FSI ^- : bis(fluorosulfonyl)imide anion), (SO ₂ CF ₃ ) ₂ N ^- (TFSI ^- : bis(trifluoromethanesulfonyl)imide anion), (SO ₂ C ₂ F ₅ ) ₂ N ^- (BETI ^- : bis(pentafluoroethanesulfonyl)imide anion), (SO ₂ F)(SO ₂ CF ₃ )N ^- , and (SO ₂ CF ₃ )(SO ₂ C ₂ F ₅ )N ^- . One type of imide anion may be used alone, or two or more types may be used in combination.

The molar concentration ratio of the first imide salt in the electrolyte is, for example, 5% or more and 20% or less relative to the molar concentration ratio of the lithium salt in the electrolyte. When the electrolyte contains a sufficient amount of the first imide salt, decomposition of the electrolyte is suppressed during charging and discharging of the lithium ion secondary battery 100. When the electrolyte contains an excessive amount of the first imide salt, the viscosity of the electrolyte increases and the impregnation of the electrolyte into the positive electrode 20 and the negative electrode 30 decreases.

The molar concentration ratio of the second imide salt in the electrolyte is, for example, 10% or more and 50% or less relative to the molar concentration ratio of the first imide salt in the electrolyte. The molar concentration ratio of the second imide salt in the electrolyte is, for example, 1% or more and 5% or less relative to the molar concentration ratio of the lithium salt in the electrolyte. When the second imide salt is contained in a sufficient amount in the electrolyte, decomposition of the electrolyte is suppressed during charging and discharging of the lithium ion secondary battery 100. When the second imide salt is contained in an excessive amount in the electrolyte, the viscosity of the electrolyte increases and the impregnation of the electrolyte into the positive electrode 20 and the negative electrode 30 decreases.

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

<Exterior body>
The power generating element 40 and the non-aqueous electrolyte are sealed inside the exterior body 50. The exterior body 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium ion secondary battery 100 from the outside.

As shown in FIG. 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

The metal foil 52 may be, for example, aluminum foil. The resin layer 54 may be a polymer film such as polypropylene. The materials constituting the resin layer 54 may be different on the inside and outside. For example, the material on the outside may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the material for the polymer film on the inside may be polyethylene (PE), polypropylene (PP), etc.

<Terminals>
The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. The terminals 60 and 62 are preferably protected with insulating tape to prevent short circuits.

"Method of manufacturing lithium-ion secondary batteries"
The lithium ion secondary battery 100 is produced by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for producing the lithium ion secondary battery 100 will be described below.

The negative electrode 30 is produced, for example, by carrying out a composite process, a slurry preparation process, an electrode application process, a drying process, and a rolling process in that order.

The compounding process is a process in which silicon or a silicon compound is mixed with a first compound and a second compound while applying a shear force. When the compounding process is performed, the surface of the silicon or silicon compound is covered with the first compound and the second compound. The particle size of the negative electrode active material can be adjusted by the degree of mixing. The negative electrode active material after preparation may also be sieved to make the particle size uniform.

The slurry preparation process is a process in which the composite negative electrode active material, binder, and solvent are mixed to make a slurry. In the slurry preparation process, a conductive assistant may be added as necessary. The solvent is, for example, water, N-methyl-2-pyrrolidone, etc. The composition ratio of the negative electrode active material, conductive material, and binder is preferably 70 wt% to 100 wt%: 0 wt% to 10 wt%: 0 wt% to 20 wt% by mass. These mass ratios are adjusted so that the total is 100 wt%.

The electrode coating process is a process of coating the surface of the negative electrode current collector 32 with a slurry. There are no particular limitations on the method of coating the slurry. For example, the slit die coating method or the doctor blade method can be used as a method of coating the slurry.

The drying process is a process for removing the solvent from the slurry. For example, the negative electrode current collector 32 on which the slurry is applied is dried in an atmosphere at 80°C to 150°C. As the slurry dries, a negative electrode active material layer 34 is formed on the negative electrode current collector 32.

The rolling process is performed as necessary. The rolling process is a process in which pressure is applied 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.

The positive electrode 20 can be produced in the same manner as the negative electrode 30, except that the composite process is not performed. The separator 10 and the exterior body 50 can be commercially available products.

The electrolyte solution can be prepared, for example, by adding a first imide salt and a second imide salt to a mixture of a lithium salt and a solvent, and then mixing the mixture.

Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. If the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

Finally, the power generating element 40 is enclosed in the exterior body 50. The non-aqueous electrolyte is injected into the exterior body 50. After the non-aqueous electrolyte is injected, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generating element 40. The exterior body 50 is sealed by applying heat, etc., to obtain the lithium ion secondary battery 100. Note that instead of injecting the electrolyte into the exterior body 50, the power generating element 40 may be impregnated with the electrolyte.

The lithium ion secondary battery 100 according to the first embodiment is highly safe. The reasons for this are explained below.

When the negative electrode active material expands in volume, it may be damaged. When the negative electrode active material is damaged, the active surface is exposed to the electrolyte. When the active surface comes into contact with the solvent, the solvent may decompose, and the cycle characteristics of the lithium-ion secondary battery may deteriorate.

In the lithium ion secondary battery 100 according to the first embodiment, the negative electrode active material includes a first compound and a second compound. In general, when multiple compounds are present in a crystal, the crystals of each compound are less likely to become coarse. Therefore, when the first compound and the second compound coexist in the negative electrode active material, these compounds are less likely to form coarse crystals. When the crystals in the negative electrode active material are finer, local hardening of the negative electrode active material is suppressed, and damage to the negative electrode active material is suppressed. Furthermore, when the negative electrode active material is finer, hardening of the negative electrode coating film obtained by drying a slurry containing the negative electrode active material is suppressed, and damage to the negative electrode coating film is suppressed. If damage to the negative electrode active material and the negative electrode coating film is suppressed, the active surface formed during charging and discharging of the lithium ion secondary battery is reduced. Therefore, in the lithium ion secondary battery 100 according to the first embodiment, decomposition of the solvent by the active surface is suppressed, and the cycle characteristics are excellent.

The first compound and the second compound are polarized in the negative electrode active material during the charge/discharge reaction. The polarized compounds attract lithium ions. When the lithium ions are attracted to the vicinity of the negative electrode active material, the solvent having the shared electron pair moves relatively farther away from the negative electrode active material than the electrolyte, the first imide salt, and the second imide salt. The probability that the active surface comes into contact with the solvent is lower than the probability that the active surface comes into contact with the electrolyte (lithium salt) or the imide salt. The active surface reacts with the electrolyte (lithium salt) or the imide salt through the solvent, and reforms the SEI coating. That is, the lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics because the decomposition of the solvent due to the reaction with the active surface is suppressed.

Furthermore, when the first imide salt and the second imide salt are present in the electrolyte, the first element and the second element are incorporated into the negative electrode active material during the charge/discharge reaction of the lithium ion secondary battery 100. Even when the negative electrode active material expands and contracts to create an active surface, the first element and the second element are supplied from the electrolyte, and thus function as anchors for the negative electrode active material. As a result, the strength of the negative electrode active material increases, and the negative electrode active material can be prevented from collapsing.

The reaction between the active surface and the electrolyte (lithium salt) or imide salt occurs on the surface of the negative electrode active material. Therefore, when the coating layer 37 of the negative electrode active material contains the first element and the second element, the effect of suppressing the deterioration of the cycle characteristics of the lithium ion secondary battery 100 is high.

The above describes the embodiments of the present invention in detail with reference to the drawings, but each configuration and their combinations in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the spirit of the present invention.

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

The positive electrode active material was Li _x CoO _2. The conductive assistant was acetylene black. The binder was polyvinylidene fluoride (PVDF). The solvent was N-methyl-2-pyrrolidone. 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 amount of the positive electrode active material carried in the positive electrode active material layer after drying was 25 mg/cm ^2. The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer. The positive electrode active material layer was pressed with a roll press to prepare a positive electrode.

Using a Hosokawa Micron Mechanofusion tilted at 10 degrees, crystalline silicon, magnesium oxide, and sodium oxide were combined to produce the negative electrode active material. The rotation speed of the device during the combination was 2500 rpm. Magnesium oxide is the first compound containing the first element. Sodium oxide is the second compound containing the second element. The median diameter of the negative electrode active material after the combination process was 3 μm. A coating layer containing Mg and Na was formed on the surface of the negative electrode active material.

Next, the negative electrode slurry was applied to one side of the copper foil having a thickness of 10 μm. The negative electrode slurry was prepared by mixing the negative electrode active material, the conductive assistant, the binder, and the solvent. The negative electrode active material was the one subjected to the above-mentioned composite treatment. The conductive assistant was carbon black. The binder was polyimide resin. The solvent 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 with N-methyl-2-pyrrolidone to prepare the negative electrode slurry. The amount of the negative electrode active material carried in the negative electrode active material layer after drying was set to 2.5 mg/cm ^2. The solvent was removed from the negative electrode slurry in a drying furnace to prepare the negative electrode active material layer. The negative electrode active material layer was pressed with a roll press and then thermally baked at 300° C. or higher for 5 hours in a nitrogen atmosphere.

Next, fluoroethylene carbonate (FEC): ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC) were mixed in a volume ratio of 5:5:20:70 to prepare a solvent. LiPF ₆ was dissolved in this mixed solvent to a concentration of 1 mol/L. Then, Mg(TFSI) ₂ as the first imide salt and NaTFSI as the second imide salt were added to this solution to prepare an electrolytic solution. The first imide salt concentration relative to the Li salt concentration was adjusted to 10% (0.1 mol/L), and the second imide salt concentration relative to the Li salt concentration was adjusted to 3% (0.03 mol/L). The second imide salt concentration relative to the first imide salt concentration was 30%.

(Preparation of Lithium-Ion Secondary Battery for Evaluation)
The prepared negative electrode and positive electrode were laminated with a separator (porous polyethylene sheet) between them so that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. A nickel negative electrode lead was attached to the negative electrode of the laminate. An aluminum positive electrode lead was attached to the positive electrode of the laminate. The positive electrode lead and the negative electrode lead were welded by an ultrasonic welding machine. This laminate was inserted into an exterior body of an aluminum laminate film and heat-sealed except for one place around the periphery to form a closed portion. Finally, after injecting the above-mentioned electrolyte into the exterior body, the remaining one place was sealed by heat sealing while reducing the pressure with a vacuum sealer to prepare a lithium ion secondary battery. Two lithium ion secondary batteries were prepared, one for electrolyte composition analysis and one for charge/discharge characteristic evaluation.

Electrolyte was collected from the lithium-ion secondary battery for electrolyte composition analysis. The composition of the collected electrolyte was then analyzed using ICP luminescence. As a result, it was confirmed that the Mg and Na contents in the electrolyte were the same as those at the time of electrolyte preparation.

(Measurement of capacity retention rate after 300 cycles)
The cycle characteristics of the evaluation lithium ion secondary battery were measured using a secondary battery charge/discharge tester (manufactured by Hokuto Denko Corporation).

The battery was charged at a constant current charge rate of 1.0 C (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 4.4 V, and then discharged at a constant current discharge rate of 1.0 C until the battery voltage reached 3.0 V. The discharge capacity after charging and discharging was detected to determine the battery capacity _Q1 before the cycle test.

The battery whose battery capacity _Q1 was determined above was charged again using a secondary battery charge/discharge tester at a constant current charge rate of 1.0 C until the battery voltage reached 4.4 V, and discharged at a constant current discharge rate of 1.0 C until the battery voltage reached 3.0 V. The above charge/discharge was counted as one cycle, and 300 charge/discharge cycles were performed. Thereafter, the discharge capacity after the completion of 300 charge/discharge cycles was detected, and the battery capacity _Q2 after 300 cycles was determined.

The capacity retention rate E after 300 cycles was calculated from the capacities _Q1 and _Q2 obtained above. The capacity retention rate E is calculated by E = _Q2 / _Q1 x 100. The capacity retention rate E of Example 1 was 88%.

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

"Examples 4 to 10"
Examples 4 to 10 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 changed. In Examples 8 to 10, the first element contained in the imide salt and the first element contained in the negative electrode active material were both two or more kinds. The capacity retention rate E was determined under the same conditions as in Example 1. The results are summarized in Table 1.

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

"Examples 13 to 17"
Examples 13 to 17 differ from Example 1 in that the Li salt concentration, the first imide salt concentration, and the second imide salt concentration in the electrolytic solution were changed. In Examples 13 to 17, the first imide salt concentration relative to the Li salt concentration in the electrolytic solution was adjusted to 10%, and the second imide salt concentration relative to the first imide salt concentration in the electrolytic solution was adjusted to 30%. The other conditions were the same as in Example 1, and the capacity retention rate E was obtained. The results are shown in Table 1.

"Examples 18 to 23"
Examples 18 to 23 differ from Example 1 in that the first imide salt concentration and the second imide salt concentration were changed. In Examples 18 to 23, the second imide salt concentration relative to the first imide salt concentration in the electrolytic solution was adjusted to 30%. The other conditions were the same as in Example 1, and the capacity retention rate E was determined. The results are shown in Table 1.

"Examples 24 to 30"
Examples 24 to 30 differ from Example 1 in that the concentration of the second imide salt in the electrolytic solution was changed. In Examples 24 to 30, the concentration of the first imide salt relative to the Li salt concentration in the electrolytic solution was adjusted to 10%. The other conditions were the same as in Example 1, and the capacity retention rate E was obtained. The results are summarized in Table 2.

"Examples 31 to 38"
Examples 31 to 38 differ from Example 1 in that the particle size of the negative electrode active material was changed. The other conditions were the same as in Example 1, and the capacity retention rate E was determined. The results are shown in Table 2.

"Examples 39 to 43"
Examples 39 to 43 differ from Example 1 in that the discharge specific capacity of the negative electrode active material was changed. The discharge specific capacity of the negative electrode active material was adjusted by changing the thickness of the positive electrode active material layer. The other conditions were the same as in Example 1, and the capacity retention rate E was obtained. The results are summarized in Table 2.

"Comparative Example 1"
Comparative Example 1 differs from Example 1 in that magnesium oxide (first compound) and sodium sulfate (second compound) were not added when preparing the negative electrode active material, and the first imide salt and the second imide salt were not added to the electrolyte. The other conditions were the same as in Example 1, and the capacity retention rate E was obtained. The results are summarized in Table 2.

"Comparative Example 2"
Comparative Example 2 differs from Example 1 in that sodium sulfate (second compound) was not added when preparing the negative electrode active material, and the second imide salt was not added to the electrolyte. The other conditions were the same as in Example 1, and the capacity retention rate E was obtained. The results are summarized in Table 2.

"Comparative Example 3"
Comparative Example 3 differs from Example 1 in that no magnesium sulfate (first compound) was added when preparing the negative electrode active material, and no first imide salt was added to the electrolyte. The other conditions were the same as in Example 1, and the capacity retention rate E was obtained. The results are summarized in Table 2.

All of Examples 1 to 43 had a higher capacity retention rate than Comparative Examples 1 to 3. That is, the lithium ion secondary batteries according to Examples 1 to 43, in which the negative electrode active material contained the first compound and the second compound and the electrolyte contained the first imide salt and the second imide salt, had excellent cycle characteristics.

REFERENCE SIGNS LIST 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 Covering layer 37A First layer 37B Second layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (6)

A battery comprising 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 including silicon or a silicon compound, a first oxide including a first element, and a second oxide including a second element;
the electrolytic solution includes a first imide salt including the first element and an imide anion, and a second imide salt including the second element and an imide anion,
The first element is at least one element selected from the group consisting of Mg, Ca, Cs, Al, and Zn ,
The second element is at least one element selected from the group consisting of K and Na ,
the first element contained in the first oxide is the same as the first element contained in the first imide salt,
the second element contained in the second oxide is the same as the second element contained in the second imide salt . 2. The lithium ion secondary battery according to claim 1 , wherein a molar concentration ratio of the first imide salt in the electrolytic solution is 5% or more and 20% or less with respect to a molar concentration ratio of the lithium salt in the electrolytic solution. 3 . The lithium ion secondary battery according to claim 1 , wherein a molar concentration ratio of the second imide salt in the electrolytic solution is 10% or more and 50% or less with respect to a molar concentration ratio of the first imide salt in the electrolytic solution. 4 . The negative electrode active material has a core and a coating layer that coats the core,
The lithium ion secondary battery according to claim 1 , wherein the coating layer contains the first oxide and the second oxide . The lithium ion secondary battery according to any one of claims 1 to 4 , wherein the negative electrode active material has a median diameter (D50) of 500 nm or more and 10 µm or less. The lithium ion secondary battery according to any one of claims 1 to 5 , wherein the discharge specific capacity of the negative electrode active material is 1000 mAh/g or more and 2500 mAh/g or less.

Images