Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode current collector, and a positive electrode mixture layer supported on the positive electrode current collector. The negative electrode includes a negative electrode current collector, and a negative electrode mixture layer supported on the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material capable of intercalation and deintercalation of lithium ions, and negative electrode-side carbon nanotubes. The positive electrode mixture layer contains a positive electrode active material capable of intercalation and deintercalation of lithium ions, and positive electrode-side carbon nanotubes. The expansion rate of a negative electrode during charging is at least 10%.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, are used as power sources for electronic devices such as mobile terminals, and as power sources for vehicles such as electric vehicles. Graphite is generally used as the negative electrode active material for non-aqueous electrolyte secondary batteries.

[0003] In recent years, the use of negative electrode active materials with a higher capacity density (capacity per unit mass) than graphite has been investigated for non-aqueous electrolyte secondary batteries. For example, silicon-containing materials have been investigated as such negative electrode active materials. Furthermore, in order to increase the amount of electrode active material per unit area of ​​the electrode, it has also been investigated to increase the active material density by compressing the electrode under high pressure.

[0004] On the other hand, Patent Document 1 (Examples 2 and 6) discloses a nonaqueous electrolyte secondary battery in which artificial graphite is used as the negative electrode active material and vapor grown carbon fiber (VGCF) is contained in each of the positive electrode mixture layer and the negative electrode mixture layer.

[0005] JP 2009-43715 A

[0006] Electrodes with high active material density are susceptible to expansion and contraction during charging and discharging. For example, the conductive path may be damaged, resulting in loss of conductivity, or the fluidity or circulation of the non-aqueous electrolyte may be impaired, resulting in a loss of battery durability. A negative electrode with a high degree of expansion and contraction may push out excessive non-aqueous electrolyte during charging and absorb excessive non-aqueous electrolyte during discharging, resulting in a decrease in the non-aqueous electrolyte in the positive electrode. As a result, the capacity and cycle characteristics tend to decrease with repeated charging and discharging.

[0007] VGCF has a relatively high rigidity, and electrodes containing VGCF are less susceptible to expansion and contraction during charge and discharge. However, electrodes containing VGCF are difficult to compress, making it difficult to highly increase the active material density. Excessive compression of an electrode containing VGCF can cause the VGCF to break, disrupting the conductive path.

[0008] In view of the above, one aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector, the negative electrode includes a negative electrode current collector and a negative electrode mixture layer supported on the negative electrode current collector, the negative electrode mixture layer includes a negative electrode active material capable of absorbing and desorbing lithium ions and a negative electrode-side carbon nanotubes, and the positive electrode mixture layer includes a positive electrode active material capable of absorbing and desorbing lithium ions and a positive electrode-side carbon nanotubes, and the expansion rate of the negative electrode during charge is 10% or more.

[0009] According to the present disclosure, it is possible to simultaneously achieve high capacity and improved cycle characteristics for a non-aqueous electrolyte secondary battery.

[0010] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] 1 is a partially cutaway schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0012] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0013] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer supported on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material capable of absorbing and desorbing lithium ions and a negative electrode-side carbon nanotubes. The positive electrode mixture layer includes a positive electrode active material capable of absorbing and desorbing lithium ions and a positive electrode-side carbon nanotubes. Hereinafter, carbon nanotubes will also be referred to as "CNTs." The expansion rate of the negative electrode during charging is 10% or more. The negative electrode includes a negative electrode active material (hereinafter also referred to as a first negative electrode active material) having a capacity density (capacity per unit mass) greater than that of a carbon material (graphite).

[0015] When the expansion rate of the negative electrode during charging becomes 10% or more, the capacity density of the negative electrode increases, but generally, the cycle characteristics tend to deteriorate. In contrast, by incorporating CNTs into the negative electrode mixture layer and the positive electrode mixture layer, it is possible to achieve high capacity while maintaining cycle characteristics. This is thought to be because the CNTs significantly increase the conductivity of the positive electrode and negative electrode, and also improve the fluidity or liquid circulation of the non-aqueous electrolyte (electrolyte solution). Furthermore, the effect of improving the fluidity or liquid circulation of the non-aqueous electrolyte by CNTs is thought to be significantly achieved by increasing the conductivity of the positive electrode and negative electrode to a certain level or more.

[0016] The following factors are presumed to be factors that contribute to the improvement in the fluidity (or liquid circulation) of the non-aqueous electrolyte by CNTs. CNTs have a high aspect ratio and can impart conductivity without destroying the voids in the mixture layer. Furthermore, CNTs easily follow the expansion and contraction of the active material (particularly the first negative electrode active material) during charge and discharge and are less likely to break, so that the fluidity and conductive paths of the non-aqueous electrolyte can be maintained in good condition even when the negative electrode expands and contracts.

[0017] Therefore, in the negative electrode mixture layer containing CNTs, excessive extrusion of nonaqueous electrolyte out of the negative electrode mixture layer due to expansion of the first negative electrode active material during charge is suppressed. Furthermore, excessive absorption of nonaqueous electrolyte into the negative electrode mixture layer due to contraction of the first negative electrode active material during discharge is suppressed. In the positive electrode mixture layer containing CNTs, excessive extrusion of nonaqueous electrolyte out of the positive electrode mixture layer due to the positive electrode mixture layer being pressed by the expanded negative electrode mixture layer during charge is suppressed. Furthermore, a decrease in the amount of nonaqueous electrolyte in the positive electrode mixture layer due to excessive absorption of nonaqueous electrolyte into the negative electrode mixture layer during discharge is suppressed.

[0018] Furthermore, CNTs have low rigidity, and a mixture layer containing CNTs is easily compressed. By compressing the mixture layer, the active material density can be increased, which facilitates high capacity. The density of the positive electrode mixture layer containing CNTs is, for example, 3.3 g / cm ３ or more (or 3.5 g / cm ３ or more).

[0019] From the above, it is possible to simultaneously achieve high capacity and improved cycle characteristics for non-aqueous electrolyte secondary batteries.

[0020] The expansion rate of the negative electrode containing the first negative electrode active material during charging may be 15% or more, or may be 18% or more. The first negative electrode active material has a larger capacity density and a larger degree of expansion and contraction than carbon materials (such as graphite) described below. Examples of the first negative electrode active material include active materials containing Si (Si-containing materials). For example, the negative electrode active material may be SiO ｘ When the negative electrode active material contains graphite without the first negative electrode active material, the expansion rate of the negative electrode during charging is 18%. On the other hand, when the negative electrode active material does not contain the first negative electrode active material but contains graphite, the expansion rate is 7%.

[0021] The expansion rate (%) of the negative electrode during charging is a value calculated by the following formula: Expansion rate of the negative electrode during expansion = {(thickness of the negative electrode during charging - thickness of the negative electrode during discharging) / thickness of the negative electrode during discharging} x 100

[0022] The negative electrode thickness during charging is determined by disassembling a fully charged battery, removing the negative electrode, measuring the thickness at any 10 points on the negative electrode, and calculating the average value. The negative electrode thickness during discharge is determined by disassembling a fully discharged battery, removing the negative electrode, and performing the same procedure as above. Note that a fully charged battery refers to a battery with a state of charge (SOC) of 90% or more. A fully discharged battery refers to a battery before the first charge or a battery with a depth of discharge (DOD) of 90% or more.

[0023] (Carbon Nanotubes) CNTs are carbon materials with nanometer diameters that have a cylindrical structure in which a sheet (graphene) of a six-membered ring network formed by carbon atoms is rolled up, and have excellent electrical conductivity. When the number of graphene layers constituting the cylindrical structure is one, they are called single-walled CNTs (SWCNTs: single-walled carbon nanotubes). When the number of layers is multiple, they are called multi-walled CNTs (MWCNTs: multi-walled carbon nanotubes). CNTs can be identified, for example, by SEM images of the cross section of the composite layer. Examples of methods for analyzing CNTs include Raman spectroscopy and thermogravimetric analysis.

[0024] From the viewpoint of improving cycle characteristics, the average diameter of the CNTs on the positive electrode side and the negative electrode side may be 20 nm or less, preferably 5 nm or less, and may be 1 nm or more and 20 nm or less, or 1 nm or more and 5 nm or less. When the average diameter of the CNTs is 5 nm or less, the CNTs are more likely to be interposed between the active material (e.g., Si-containing material) and the surrounding active material, and the CNTs are more likely to form contact points with the active material (e.g., Si-containing material) and the surrounding active material. Furthermore, the effects of adding CNTs are more likely to be obtained even in small amounts. When the average diameter of the CNTs is 1 nm or more, the strength of the CNTs is sufficiently ensured, and a conductive path is more likely to be formed between the active material (e.g., Si-containing material) and the surrounding active material, and the conductive path is more likely to be maintained during charge and discharge.

[0025] When the average diameter of the CNTs is 5 nm or less, the CNTs may contain a large number of SWCNTs. For example, when the average diameter of the CNTs is 5 nm or less, the proportion (number ratio) of SWCNTs in the CNTs contained in the composite layer is 1 / 2 or more.

[0026] The proportion of SWCNTs in the CNTs contained in the mixture layer is determined by the following method. An electron microscope is used to obtain an image of the surface or cross section of the mixture layer or an image of the CNTs. The electron microscope may be a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Using the image, N0 CNTs (e.g., approximately 50 to 200) are randomly selected and observed to determine the number N1 of SWCNTs, and the ratio N1 / N0 of the number N1 of SWCNTs to the total number N0 of the selected CNTs is calculated.

[0027] From the viewpoint of ensuring a conductive path between the active material (e.g., a Si-containing material) and the surrounding active materials, the average length of the CNTs on the positive electrode side and the negative electrode side is preferably 0.5 μm or more, may be 1 μm or more, or may be 0.5 μm or more (or 1 μm or more) and 100 μm or less, and more preferably 0.5 μm or more (or 1 μm or more) and 20 μm or less.

[0028] The average length and average diameter of the CNTs are determined by obtaining an image of the surface or cross section of the composite layer or an image of the CNTs using an SEM or TEM, randomly selecting a number of CNTs (e.g., approximately 50 to 200) using the image, measuring the lengths and diameters, and averaging the measurements. The length of the CNTs refers to the length when they are linear. CNTs with different average diameters and / or lengths may be used on the positive electrode side and the negative electrode side.

[0029] CNTs easily form contacts with active materials (e.g., Si-containing materials) and surrounding active materials, and can efficiently form conductive paths even with small amounts of CNT. The amount of CNT can be reduced to increase the amount of active material, which is advantageous in terms of increasing capacity. Furthermore, the amount of CNT contained in the electrode slurry during electrode fabrication can be reduced. When the amount of CNT in the electrode slurry is small, CNT aggregation is suppressed, improving battery reliability and making it easier to achieve stable cycle characteristics.

[0030] From the viewpoint of fully obtaining the effect of the CNTs and increasing the amount of negative electrode active material, the content of CNTs in the negative electrode mixture layer may be 1.0 parts by mass or less, 0.5 parts by mass or less, or 0.1 parts by mass or less, relative to 100 parts by mass of the negative electrode active material (the total of the first negative electrode active material and the second negative electrode active material). Furthermore, the content of CNTs in the negative electrode mixture layer may be 0.001 parts by mass or more, or 0.01 parts by mass or more, relative to 100 parts by mass of the negative electrode active material (the total of the first negative electrode active material and the second negative electrode active material). The range of the CNT content in the negative electrode mixture layer may be a range that arbitrarily combines the above upper and lower limits.

[0031] From the viewpoint of sufficiently obtaining the effect of the CNTs and increasing the amount of the positive electrode active material, the content of the CNTs in the positive electrode mixture layer may be 1.0 part by mass or less, 0.5 parts by mass or less, or 0.1 parts by mass or less, relative to 100 parts by mass of the positive electrode active material.

[0032] From the viewpoint of fully obtaining the effect of the CNTs and reducing the resistance of the positive electrode mixture layer, the content of the CNTs in the positive electrode mixture layer may be 0.01 parts by mass or more, or may be 0.05 parts by mass or more, relative to 100 parts by mass of the positive electrode active material. In this case, the resistance of the positive electrode mixture layer can be reduced to 30 Ω cm or less (or 20 Ω cm or less). The range of the CNT content in the positive electrode mixture layer may be a range that arbitrarily combines the above upper and lower limits.

[0033] The resistance of the positive electrode mixture layer is measured using a positive electrode taken out by disassembling a battery (in which lithium ions are absorbed in the positive electrode active material) in an initial fully discharged state (discharge depth of 90% or more). The measurement may also be performed using the positive electrode used in manufacturing the battery. The measurement is performed in a 25°C environment using an electrode resistance measurement system (manufactured by Hioki E.E. Corporation, product name "RM2610").

[0034] (First Negative Electrode Active Material) The first negative electrode active material preferably contains a Si-containing material. The Si-containing material preferably contains a composite material including a lithium ion conductive phase (matrix) and a fine silicon phase dispersed in the lithium ion conductive phase. The lithium ion conductive phase is preferably a composite material containing SiO ２ It is preferable that the composite material contains at least one selected from the group consisting of a silicon phase, a silicate phase, and a carbon phase. The lithium ion conductive phase can form an amorphous phase. The lithium ion conductive phase relieves stress caused by the expansion and contraction of the silicon phase during charge and discharge, so the composite material is advantageous in improving cycle characteristics. Examples of Si-containing materials include SiO ２ Examples of such composite materials include a composite material in which a silicon phase is dispersed within a phase, a composite material in which a silicon phase is dispersed within a silicate phase, and a composite material in which a silicon phase is dispersed within a carbon phase.

[0035] SiO ２ The phase is an amorphous phase containing 95% or more by mass of silicon dioxide. ２ Composite materials with silicon particles dispersed within the phase are SiO ｘ where x is, for example, 0.5≦x<2, and may be 0.8≦x≦1.6. ｘ is, for example, obtained by heat treating silicon monoxide to produce SiO ２ It is obtained by separating the Si phase into a Si phase and a fine Si phase.

[0036] The silicate phase preferably contains at least one of an alkali metal element (a Group 1 element other than hydrogen in the long periodic table) and a Group 2 element in the long periodic table. The alkali metal element includes lithium (Li), potassium (K), sodium (Na), etc. The Group 2 element includes magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase has the formula: Li ２ｙ SiO ２＋ｙ The composite material may have a composition expressed as (0<y<2), where y may be 1 / 2 or 1. A composite material having silicon particles dispersed in a silicate phase can be obtained, for example, by pulverizing a mixture of silicate and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere.

[0037] The average particle size of the fine particulate silicon phase (before the first charge) dispersed within the silicate phase may be 50 nm or more and 500 nm or less, or 100 nm or more and 400 nm or less. The average particle size of the silicon phase is obtained by calculating the average value of the maximum diameters of 100 randomly selected silicon phases using an SEM image of the cross section of the composite material. The content of the silicon phase dispersed within the silicate phase may be 30 mass% or more and 95 mass% or less, or 35 mass% or more and 75 mass% or less, based on the entire composite material.

[0038] The carbon phase contains, for example, amorphous carbon with low crystallinity. The amorphous carbon may be, for example, easily graphitized carbon (hard carbon) or hardly graphitized carbon (soft carbon). A composite material in which silicon particles are dispersed in the carbon phase can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere. For example, sugars such as carboxymethyl cellulose (CMC) or water-soluble resins such as polyvinylpyrrolidone are used as the carbon source.

[0039] The composition of the Si-containing material can be determined, for example, by obtaining a backscattered electron image of the cross section of the negative electrode mixture layer using a field emission scanning electron microscope (FE-SEM), observing the particles of the Si-containing material, and performing elemental analysis on the observed particles of the Si-containing material. For example, an electron probe microanalyzer (EPMA) or the like is used for the elemental analysis. The composition of the lithium ion conductive phase can also be determined by the above analysis.

[0040] The Si-containing material is, for example, a particulate material. The average particle size (D50) of the Si-containing material is, for example, 1 μm or more and 25 μm or less, preferably 4 μm or more and 15 μm or less. Within the above range, good battery performance is likely to be obtained. In this specification, the average particle size (D50) refers to the particle size (volume average particle size) at which the volume integrated value is 50% in the particle size distribution measured by the laser diffraction scattering method. For example, an "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

[0041] From the viewpoint of improving electrical conductivity, at least a portion of the particle surface of the Si-containing material may be coated with a conductive layer. The conductive layer contains a conductive material such as conductive carbon. The amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the total of the Si-containing material particles and the conductive layer. The Si-containing material particles having a conductive layer on their surface can be obtained, for example, by mixing coal pitch or the like with the Si-containing material particles and heat treating the mixture in an inert atmosphere.

[0042] (Second Negative Electrode Active Material) The negative electrode mixture layer may include, in addition to the first negative electrode active material, a second negative electrode active material having an expansion rate of less than 10% during charging. As the second negative electrode active material, a carbon material that electrochemically absorbs and releases lithium ions is used. The carbon material expands and contracts less during charging and discharging than the Si-containing material. By using the Si-containing material and the carbon material in combination, the contact state between the negative electrode active material particles and between the negative electrode mixture layer and the negative electrode current collector can be better maintained during repeated charging and discharging. In other words, the cycle characteristics can be improved while providing the high capacity of the Si-containing material to the negative electrode.

[0043] Examples of the carbon material used for the second negative electrode active material include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). One type of carbon material may be used alone, or two or more types may be used in combination.

[0044] Among these, graphite is preferred as the carbon material because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, graphitizable carbon, or non-graphitizable carbon.

[0045] Graphite is a carbon material with a developed graphite-type crystal structure. The interplanar spacing d002 of the (002) plane of graphite measured by X-ray diffraction may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite may be, for example, 5 nm or more, or 5 nm or more and 200 nm or less. The crystallite size Lc(002) is measured, for example, by the Scherrer method. When the interplanar spacing d002 of the (002) plane of graphite and the crystallite size Lc(002) are within the above ranges, a high capacity is easily obtained.

[0046] The first negative electrode active material and the second negative electrode active material may be made of a Si-containing material and a carbon material, respectively. In this case, from the viewpoint of increasing capacity and improving cycle characteristics, the proportion of the Si-containing material in the total of the Si-containing material and the carbon material is preferably 2% by mass or more, more preferably 2% by mass or more and 30% by mass or less, and even more preferably 3% by mass or more and 25% by mass or less.

[0047] The nonaqueous electrolyte secondary battery will be described in detail below.

[0048] (Negative Electrode) The negative electrode comprises a negative electrode current collector and a negative electrode mixture layer carried on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying it. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0049] The negative electrode mixture contains, as essential components, a negative electrode active material and CNTs. The negative electrode mixture may contain, as optional components, a conductive agent other than CNTs, a binder, and the like.

[0050] Examples of conductive agents other than CNT include carbons such as acetylene black, metals such as aluminum, etc. The conductive agents may be used alone or in combination of two or more.

[0051] Examples of the binder include resin materials. Examples of the resin material used for the binder include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; acrylic resins such as polyacrylic acid; polyamide resins such as aramid resin; and polyimide resins such as polyimide and polyamideimide. Rubber-like materials such as styrene-butadiene copolymer rubber (SBR) may also be used as the binder.

[0052] Examples of the binder include carboxymethyl cellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), etc. One type of binder may be used alone, or two or more types may be used in combination.

[0053] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0054] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but may be 1 to 50 μm or 5 to 20 μm.

[0055] (Positive Electrode) The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium such as NMP, to the surface of the positive electrode current collector and drying it. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains a positive electrode active material and CNT as essential components, and may contain a binder, a conductive agent other than CNT, etc. as optional components.

[0056] As the positive electrode active material, for example, a lithium-containing transition metal oxide is used. Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al.

[0057] Among lithium-containing transition metal oxides, composite oxides containing Co, Ni and / or Mn as transition metal elements, which may contain Al as an optional component, and which have a layered rock-salt type crystal structure are preferred in terms of obtaining high capacity.

[0058] Examples of lithium-containing transition metal oxides include Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2-b M b O4. Here, a = 0 to 1.2, b = 0 to 0.9, and c = 2.0 to 2.3. M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0059] The positive electrode active material is Li a Ni b M 1-bO2 (where 0<a≦1.2, 0.3≦b≦1, and M is at least one selected from the group consisting of Mn, Co, and Al). From the viewpoint of increasing capacity, it is more preferable that 0.85≦b≦1 is satisfied. From the viewpoint of stability of the crystal structure, the positive electrode active material is preferably Li a Ni b Co c Al d It may also be a composite oxide represented by O2 (0<a≦1.2, 0.85≦b<1, 0<c≦0.15, 0<d≦0.1, b+c+d=1).

[0060] The binder may be the resin material exemplified for the negative electrode. The conductive agent may be the same as the material exemplified for the negative electrode. Graphite such as natural graphite or artificial graphite may also be used as the conductive agent.

[0061] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0062] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0063] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0064] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonylimide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The lithium salts may be used alone or in combination of two or more.

[0065] The non-aqueous electrolyte may contain a phosphate and / or a fluorinated carbonate as an additive. Examples of the phosphate include lithium difluorophosphate. Examples of the fluorinated carbonate include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC). By including an additive in the non-aqueous electrolyte, a coating containing components derived from the additive is formed on the surface of the positive electrode active material, thereby suppressing deterioration of the positive electrode active material due to side reactions between the positive electrode active material and the non-aqueous electrolyte. Furthermore, the coating has low resistance, suppressing an increase in the resistance of the positive electrode mixture layer. As a result, the cycle characteristics are further improved. The content of the additive in the non-aqueous electrolyte is, for example, 2 mass% or less of the total non-aqueous electrolyte.

[0066] (Separator) It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. As the separator, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used. As the material of the separator, polyolefin such as polypropylene or polyethylene is preferred.

[0067] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0068] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery as an example of the nonaqueous electrolyte secondary battery according to the present disclosure will be described with reference to Fig. 1. Fig. 1 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away.

[0069] The battery includes a bottomed prismatic battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0070] One end of a negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. A non-aqueous electrolyte injection hole provided in the sealing plate 5 is closed with a seal 8.

[0071] Examples Examples of the present disclosure will be specifically described below, but the present disclosure is not limited to the following examples.

[0072] Examples 1 to 4 Preparation of Negative Electrode An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture was a mixture of a negative electrode active material, a negative electrode conductive agent, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR). The amount of CMC-Na added was 1 part by mass per 100 parts by mass of the negative electrode active material. The amount of SBR added was 1.5 parts by mass per 100 parts by mass of the negative electrode active material.

[0073] The negative electrode active material was a mixture of Si-containing material and graphite (average particle size (D50) 25 μm). The mass ratio of the Si-containing material to the graphite in the negative electrode active material was set to the value shown in Table 1.

[0074] The Si-containing material includes SiO 2 whose surface is coated with a conductive layer containing conductive carbon. ｘ The conductive layer was coated with SiO particles (x=1, average particle size (D50) 5 μm). ｘ The amount was 5 parts by mass per 100 parts by mass of the total of the particles and the conductive layer.

[0075] The negative electrode conductive agent used was CNT1 (average diameter 10 nm, average length 0.5 μm) or CNT2 (average diameter 3 nm, average length 10 μm). For CNT1, the ratio (number ratio) of MWCNTs to the total CNTs was 9 / 10 or more. For CNT2, the ratio (number ratio) of SWCNTs to the total CNTs was 9 / 10 or more.

[0076] The amount of the negative electrode conductive agent added (amount per 100 parts by mass of the negative electrode active material) was set to the value shown in Table 1.

[0077] Next, the negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then the copper foil was rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm ) on both sides of the copper foil. ３ ) was formed to obtain a negative electrode.

[0078] [Preparation of Positive Electrode] An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture and mixed to obtain a positive electrode slurry. The positive electrode mixture contained a lithium-containing composite oxide (LiNi) ０．８ Co ０．１８ Al ０．０２ O ２A mixture of the positive electrode active material (aluminum fluoride), a positive electrode conductive agent, and polyvinylidene fluoride (PVDF) was used. The amount of PVDF added was 2.5 parts by mass per 100 parts by mass of the positive electrode active material.

[0079] The positive electrode conductive agent used was CNT1 or CNT2. The amount of the positive electrode conductive agent added (amount per 100 parts by mass of the positive electrode active material) was set to the value shown in Table 1.

[0080] Next, the positive electrode slurry was applied to the surface of the aluminum foil, the coating was dried, and then the aluminum foil was rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm ) on both sides of the aluminum foil. ３ ) was formed to obtain a positive electrode.

[0081] [Preparation of non-aqueous electrolyte] LiPF 6 was dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 3:7). ６ was dissolved in a solution at a concentration of 1.0 mol / L to obtain a non-aqueous electrolyte.

[0082] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to produce a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer, and the non-aqueous electrolyte was injected. The exterior body was then sealed to produce a non-aqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, portions of the positive electrode lead and the negative electrode lead were exposed to the outside from the exterior body. In Table 1, A1 to A4 refer to the non-aqueous electrolyte secondary batteries of Examples 1 to 4.

[0083] Example 5 Lithium difluorophosphate and fluoroethylene carbonate (FEC) were further added as additives to the non-aqueous electrolyte. The content of lithium difluorophosphate was 1 mass% with respect to the total non-aqueous electrolyte. The content of FEC was 5 mass% with respect to the total non-aqueous electrolyte. Battery A5 of Example 5 was fabricated in the same manner as Battery A4 of Example 4, except for the above.

[0084] Comparative Example 1: Graphite was used as the negative electrode active material, and no Si-containing material was used. CNT1 was not added as the negative electrode conductive agent. 0.8 parts by mass of carbon black was added as the positive electrode conductive agent instead of CNT1 per 100 parts by mass of the positive electrode active material. Other than the above, Battery B1 of Comparative Example 1 was fabricated in the same manner as Battery A1 of Example 1.

[0085] Comparative Example 2 Battery B2 of Comparative Example 2 was fabricated in the same manner as Battery A2 of Example 2, except that graphite was used as the negative electrode active material and no Si-containing material was used.

[0086] Comparative Example 3: No CNT1 was added as the negative electrode conductive agent. Carbon black was added as the positive electrode conductive agent instead of CNT1 in an amount of 0.8 parts by mass per 100 parts by mass of the positive electrode active material. Battery B3 of Comparative Example 3 was fabricated in the same manner as Battery A1 of Example 1, except for the above.

[0087] Comparative Example 4 Battery B4 of Comparative Example 4 was produced in the same manner as Battery A2 of Example 2, except that carbon black was added as the positive electrode conductive agent instead of CNT1 in an amount of 0.8 parts by mass per 100 parts by mass of the positive electrode active material.

[0088] Comparative Example 5 Battery B5 of Comparative Example 5 was produced in the same manner as battery A2 of Example 2, except that CNT1 was not added as the negative electrode conductive agent.

[0089] The following evaluations were performed on each of the batteries prepared above. [Evaluation 1: Initial capacity] (Charging) The batteries were charged at a constant current of 0.2 C (1000 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C (100 mA).

[0090] (Discharge) The battery was discharged at a constant current of 0.5 C (2500 mA) until the voltage reached 2.5 V. The discharge time was measured to determine the discharge capacity (initial capacity). The rest time between charge and discharge was 10 minutes. The charge and discharge were performed in an environment of 25°C.

[0091] [Evaluation 2: Cycle Capacity Retention Rate] (Charging) Constant current charging was performed at a current of 0.5 C (2500 mA) until the voltage reached 4.2 V, and then constant voltage charging was performed at a voltage of 4.2 V until the current reached 0.02 C (100 mA).

[0092] (Discharge) Constant current discharge was performed at a current of 1.0 C (5000 mA) until the voltage reached 2.5 V. The rest time between charge and discharge was 20 minutes. Charging and discharging were performed in an environment of 25°C.

[0093] Charge and discharge were repeated under the above conditions, and the ratio (percentage) of the discharge capacity at the 400th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate.

[0094] The evaluation results are shown in Table 1. Table 1 also shows the resistance of the positive electrode mixture layer obtained by the above-described method using the positive electrodes prepared above. Furthermore, the expansion rate of the negative electrode during charging was 18% for the negative electrodes used in A1 to A4 and B3 to B5 that contained a Si-containing material as the negative electrode active material, and 7% for the negative electrodes used in B1 and B2 that contained only graphite as the negative electrode active material.

[0095]

[0096] In samples B1 and B2, which used only graphite as the negative electrode active material, the initial capacity decreased. Furthermore, in sample B2, the addition of CNTs to the positive and negative electrodes hardly improved the capacity retention rate compared to sample B1. In sample B3, which used graphite and a Si-containing material as the negative electrode active material, the initial capacity improved, but the capacity retention rate decreased significantly because no CNTs were used in the positive and negative electrodes.

[0097] In B4, which used a Si-containing material, the same amount of CNT as in A2 was added to the negative electrode, but no CNT was added to the positive electrode, so almost no improvement in capacity retention was achieved compared to B3. In B5, which used a Si-containing material, the same amount of CNT as in A2 was added to the positive electrode, but no CNT was added to the negative electrode, so almost no improvement in capacity retention was achieved compared to B3.

[0098] In A2, which used a Si-containing material, CNTs were added to both the negative and positive electrodes, and therefore the capacity retention rate was significantly improved compared to B3.

[0099] Although A1 and A2 exhibited high initial capacity and high capacity retention, A2, which contained a larger amount of CNT than A1, exhibited a significant improvement in capacity retention. In A2, the resistance of the positive electrode mixture layer was 30 Ω cm or less.

[0100] A high capacity retention rate was obtained in all of A2 to A4. Comparing A2, in which CNT1 was added to the positive and negative electrodes, A3, in which CNT2 was added to the positive electrode and CNT1 to the negative electrode, and A4, in which CNT2 was added to the positive and negative electrodes, the relationship of capacity retention rate A2 < A3 < A4 was shown. When CNT2 was used, the effect of improving the capacity retention rate was more pronounced. The effect of adding CNT2 was more pronounced even when the amount added was small.

[0101] A5, which had the same configuration as A4 except that an additive was added to the non-aqueous electrolyte, had a higher capacity retention rate than A4.

[0102] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0103] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0104] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, the negative electrode includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, the negative electrode active material layer contains a negative electrode active material capable of occluding and releasing lithium ions and a negative electrode side carbon nanotube, the positive electrode active material layer contains a positive electrode active material capable of occluding and releasing lithium ions and a positive electrode side carbon nanotube, and the expansion rate of the negative electrode during charging is 10% or more.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material contains a Si-containing material.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the resistance of the positive electrode active material layer is 30 Ω·cm or less.

4. The density of the positive electrode mixture layer is 3.3 g / cm ３ or more. The non-aqueous electrolyte secondary battery according to claim 1.

5. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the positive electrode side carbon nanotube has an average diameter of 20 nm or less and an average length of 0.5 μm or more.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode side carbon nanotube has an average diameter of 20 nm or less and an average length of 0.5 μm or more.

7. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the content of the positive electrode side carbon nanotube in the positive electrode is 1 part by mass or less with respect to 100 parts by mass of the positive electrode active material.

8. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the content of the negative electrode side carbon nanotube in the negative electrode is 1 part by mass or less with respect to 100 parts by mass of the negative electrode active material.

9. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the non-aqueous electrolyte contains fluoroethylene carbonate.