Non-aqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte secondary battery is provided with a positive electrode, a separator, a negative electrode which faces the positive electrode with the separator being interposed therebetween, and an electrolyte solution. The electrolyte solution contains an acid anhydride; the negative electrode is provided with a negative electrode mixture layer that contains a negative electrode active material and carbon nanotubes; and the negative electrode active material contains a silicon-containing material and a carbonaceous material.

Description

[Technical Field]

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

[0002] A nonaqueous electrolyte secondary battery, typified by a lithium ion secondary battery, comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte. A nonaqueous electrolyte solution is typically used as the nonaqueous electrolyte. The negative electrode comprises a negative electrode mixture containing a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions. The negative electrode active material is a material capable of electrochemically absorbing and desorbing lithium ions. Examples of such materials include carbonaceous materials and silicon-containing materials. Furthermore, carbonaceous materials that do not absorb or desorb lithium ions, such as carbon fibers and carbon nanotubes, may be added to the negative electrode mixture.

[0003] Patent Document 1 proposes the use of a composite electrode agent in a lithium ion secondary battery, the composite electrode agent including particles containing an element capable of absorbing and desorbing lithium ions, carbon particles capable of absorbing and desorbing lithium ions, multi-walled carbon tubes, and carbon nanofibers.

[0004] Patent Document 2 proposes the use of an electrode in a lithium-ion battery, which is manufactured by dry-mixing an active material, carbon fibers having a fiber diameter of 50 nm or more and 300 nm or less, carbon fibers having a fiber diameter of 5 nm or more and 400 nm or less, carbon black, and a binder to obtain a mixture, adding a liquid medium to the mixture and kneading it, and forming the kneaded mixture into a sheet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-146519 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-160590 Summary of the Invention

[0006] Silicon-containing materials undergo large volume changes due to the absorption and desorption of lithium ions, and therefore, when used as a negative electrode active material, the conductive paths between the particles of the negative electrode active material are severed, isolating the particles and reducing cycle performance.

[0007] Combining a silicon-containing negative electrode active material with carbon nanotubes helps ensure electrical conductivity between particles of the negative electrode active material at the beginning of the charge-discharge cycle. On the other hand, the use of carbon nanotubes increases the negative electrode utilization rate, which increases the expansion and contraction of the negative electrode active material, making it more susceptible to cracking. Therefore, when the battery is subjected to long-term charge-discharge cycles at high temperatures, side reactions gradually increase, resulting in significant electrolyte consumption.

[0008] One aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery including a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolyte solution, wherein the electrolyte solution contains an acid anhydride, and the negative electrode includes a negative electrode mixture containing a negative electrode active material and carbon nanotubes, and the negative electrode active material contains a silicon-containing material and a carbonaceous material.

[0009] According to the present disclosure, it is possible to suppress the decrease in electrolyte when a nonaqueous electrolyte secondary battery is repeatedly charged and discharged at high temperatures for a long period of time, and to improve the capacity retention rate. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a partially cutaway perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] The nonaqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and an electrolyte. The negative electrode includes a negative electrode mixture, and the negative electrode mixture includes a negative electrode active material and carbon nanotubes. The negative electrode active material includes a silicon-containing material and a carbonaceous material. Hereinafter, the silicon-containing material may be referred to as an Si-containing material, and the carbon nanotubes may be referred to as CNTs.

[0012] The electrolyte contains an acid anhydride. The acid anhydride reacts rapidly with the negative electrode active material. If cracks occur in the negative electrode active material during charge-discharge cycling, the acid anhydride quickly reacts with the newly formed surface at the crack to form a low-resistance protective film. Rapid protection of the newly formed surface inhibits further side reactions involving decomposition of the electrolyte. Therefore, by including a Si-containing material and CNTs, electrolyte consumption is inhibited even when the negative electrode active material is prone to cracking due to expansion and contraction. As a result, the capacity retention rate can be significantly improved even when performing long-term charge-discharge cycling at high temperatures.

[0013] When an anode mixture containing no CNTs is used, the acid anhydride hardly contributes to suppressing electrolyte loss and does not significantly contribute to improving the capacity retention rate when performing long-term charge-discharge cycling at high temperatures. However, when an electrolyte containing an acid anhydride is combined with an anode mixture containing CNTs, the electrolyte loss is significantly suppressed and a significant improvement in the capacity retention rate is observed when performing long-term charge-discharge cycling at high temperatures.

[0014] In the nonaqueous electrolyte secondary battery according to the present disclosure, the use of CNTs improves the capacity retention rate at the beginning of charge-discharge cycles, and the use of an acid anhydride improves the capacity retention rate even when charge-discharge cycles are performed at high temperatures for a long period of time.

[0015] The content of the acid anhydride in the electrolyte is, for example, 5% by mass or less, and may be 3% by mass or less. When the content of the acid anhydride is within this range, the function required for the electrolyte is not impaired, and the effect of forming a protective film on newly formed surfaces resulting from cracks in the negative electrode active material is maintained for a long period of time. Therefore, when performing charge / discharge cycles at high temperatures for a long period of time, the decrease in the electrolyte can be further suppressed. To further suppress the decrease in the electrolyte, the content of the acid anhydride in the electrolyte is preferably 2% by mass or less.

[0016] In a non-aqueous electrolyte secondary battery, the content of acid anhydride in the electrolyte solution changes during storage or during charge / discharge cycles. Therefore, it is sufficient that the acid anhydride remains in the electrolyte solution collected from the non-aqueous electrolyte secondary battery at a concentration equal to or higher than the detection limit. The content of acid anhydride in the electrolyte solution may be 0.01% by mass or more, 0.1% by mass or more, or 0.5% by mass or more.

[0017] On the other hand, the content of the acid anhydride in the electrolyte solution used to manufacture the non-aqueous electrolyte secondary battery may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more. The content of the acid anhydride in the electrolyte solution used to manufacture the non-aqueous electrolyte secondary battery may be, for example, 5% by mass or less, 3% by mass or less, or 2% by mass or less. These lower and upper limits can be combined arbitrarily.

[0018] The type of acid anhydride is not particularly limited, but an acid anhydride containing a carbon-carbon unsaturated bond is desirable because it can react more quickly with the newly formed surface caused by cracks in the negative electrode active material to form a protective coating.

[0019] From the viewpoint of effectively utilizing as many of the constituent elements of the molecule as possible in forming the protective coating, it is desirable for the acid anhydride molecule to have as simple a structure as possible. Examples of such acid anhydrides include maleic anhydride, succinic anhydride, acetic anhydride, phthalic anhydride, and benzoic anhydride. Among these, maleic anhydride and succinic anhydride are preferred because they have an excellent balance between stability and reactivity and can form a coating with lower resistance. However, the acid anhydrides may be used alone or in combination of two or more.

[0020] The content of each component in the electrolyte solution can be determined, for example, by gas chromatography under the following conditions.

[0021] Measuring device: Shimadzu GC-2010 Plus Column: J&W HP-1 (1 μm x 60 m) Linear speed: 30.0cm / sec Inlet temperature: 270℃ Detector: FID 290℃ (sens.10 1 ) Each component of the nonaqueous electrolyte secondary battery of the present disclosure will be described in more detail below.

[0022] (Negative electrode) The negative electrode includes a negative electrode mixture. The negative electrode may include a negative electrode mixture and a negative electrode current collector that holds the negative electrode mixture. The negative electrode typically includes a layer of negative electrode mixture (hereinafter referred to as a negative electrode mixture layer). The negative electrode mixture includes a negative electrode active material and CNTs. The negative electrode mixture may further include a binder, a thickener, a conductive agent other than CNTs, etc.

[0023] (Negative electrode active material) The negative electrode active material includes a Si-containing material and a carbonaceous material. The carbonaceous material expands and contracts less during charging and discharging than the Si-containing material. The combined use of a Si-containing material and a carbonaceous material can maintain better contact between the negative electrode active material particles and between the negative electrode mixture and the negative electrode current collector during repeated charging and discharging. Therefore, combining a carbonaceous material with a Si-containing material makes it easier to ensure high cycle performance while maintaining the high capacity of the Si-containing material. The negative electrode active material may contain other negative electrode active materials in addition to the Si-containing material and the carbonaceous material, as needed. Examples of other negative electrode active materials include at least one selected from the group consisting of simple Sn, Sn alloys, and Sn compounds such as Sn oxides.

[0024] (Si-containing material) Examples of Si-containing materials include simple Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase (fine Si phase) is dispersed within a lithium ion conductive phase (matrix). Examples of silicon oxides include SiO x For example, x may be 0.5≦x<2, or 0.8≦x≦1.6.

[0025] From the viewpoint of ensuring higher cycle characteristics, the Si-containing material preferably includes the above-mentioned composite material. The lithium ion conductive phase preferably includes at least one selected from the group consisting of an SiO2 phase and a silicate phase. The lithium ion conductive phase may further include a carbon phase. The lithium ion conductive phase may form an amorphous phase. The Si-containing material may include a composite material in which a silicon phase is dispersed in an SiO2 phase, a composite material in which a silicon phase is dispersed in a silicate phase, a composite material in which a silicon phase is dispersed in a carbon phase, or the like.

[0026] The SiO2 phase is an amorphous phase containing 95% or more silicon dioxide by mass. A composite material in which silicon particles are dispersed within the SiO2 phase is called a SiO x where x may be, for example, in the range described above. xIt can be obtained, for example, by heat-treating silicon monoxide and separating it into a SiO2 phase and a fine Si phase by disproportionation reaction. When observing the particle cross-section of SiO x using a transmission electron microscope (TEM), a silicon phase dispersed in the SiO2 phase can be confirmed.

[0027] The silicate phase preferably contains at least one of an alkali metal element (Group 1 element other than hydrogen in the long-period periodic table) and a Group 2 element in the long-period periodic table. The alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. The Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase can have a composition represented by the formula: Li 2y SiO 2+y (0 < y < 2). y may be 1 / 2 or may be 1. A composite material in which silicon particles are dispersed in the silicate phase can be obtained, for example, by pulverizing a mixture of silicate and raw silicon into fine particles while stirring with a ball mill or the like, and then heat-treating the mixture in an inert atmosphere.

[0028] The content of the silicon phase dispersed in the silicate phase may be 30% by mass or more and 95% by mass or less, or may be 35% by mass or more and 75% by mass or less with respect to the whole composite material.

[0029] The carbon phase contains, for example, amorphous carbon with low crystallinity (amorphous carbon). The amorphous carbon may be, for example, easily graphitizable carbon (hard carbon) or hardly graphitizable 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 into fine particles while stirring with a ball mill or the like, and then heat-treating the mixture in an inert atmosphere. As the carbon source, for example, saccharides such as carboxymethyl cellulose (CMC) and water-soluble resins such as polyvinylpyrrolidone are used.

[0030] 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. Elemental analysis can be performed using, for example, an electron probe microanalyzer (EPMA). The composition of the lithium ion conductive phase can also be determined by the above analysis.

[0031] The Si-containing material may be used alone or in combination of two or more.

[0032] 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, and preferably 4 μm or more and 15 μm or less. Within this range, good battery performance is likely to be obtained.

[0033] In this specification, the average particle size (D50) refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by HORIBA Ltd. can be used as the measuring device.

[0034] To improve 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.

[0035] The volume of a Si-containing material changes significantly due to expansion and contraction during charging and discharging. Therefore, as the ratio of the Si-containing material in the negative electrode active material increases, cycle characteristics tend to decrease. On the other hand, according to the present disclosure, since the negative electrode mixture contains a specific amount of CNTs, even when the ratio of the Si-containing material in the negative electrode active material is relatively high, disconnection of the conductive path is suppressed, making it easier to ensure high cycle characteristics. The ratio of the Si-containing material in the negative electrode active material is preferably 4% by mass or more, and may be 5% by mass or more. The ratio of the Si-containing material is preferably 15% by mass or less, and may be 10% by mass or less. These lower and upper limits can be combined arbitrarily.

[0036] (carbonaceous material) Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination.

[0037] Among these, graphite is preferred as the carbonaceous 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.

[0038] Graphite is a carbonaceous material with a developed graphite 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, high capacity is easily obtained.

[0039] The ratio of the carbonaceous material in the negative electrode active material is, for example, 97% by mass or less, or alternatively 96% by mass or less, or 95% by mass or less. The ratio of the carbonaceous material in the negative electrode active material is, for example, 76% by mass or more, or alternatively 80% by mass or more, or alternatively 85% by mass or more, or alternatively 90% by mass or more. These lower and upper limits can be combined in any desired manner.

[0040] The total amount of the Si-containing material and the carbonaceous material in the negative electrode active material is preferably 90% by mass or more, and may be 95% by mass or more or 98% by mass or more. The total amount of the Si-containing material and the carbonaceous material in the negative electrode active material is 100% by mass or less. The negative electrode active material may be composed only of the Si-containing material and the carbonaceous material.

[0041] (CNT) CNTs are carbonaceous materials with nanometer diameters that have a cylindrical structure consisting of a sheet (graphene) of a six-membered ring network formed by carbon atoms. CNTs have excellent electrical conductivity. When the number of graphene layers that make up the cylindrical structure is one, they are called single-walled carbon nanotubes (SWCNTs). When the number of layers is multiple, they are called multi-walled carbon nanotubes (MWCNTs).

[0042] The CNTs preferably include SWCNTs, which not only makes it easier to ensure higher cycle characteristics but also makes it easier to suppress the loss of electrolyte due to acid anhydride.

[0043] The proportion of SWCNTs in the CNTs is, for example, 50% or more, or may be 75% or more, or 90% or more. The proportion of SWCNTs in the CNTs is 100% or less. The proportion of SWCNTs in the CNTs is the ratio of the number of SWCNTs to the total number of CNTs.

[0044] The presence of CNTs in the negative electrode mixture can be confirmed, for example, by an image of a cross section of the negative electrode mixture layer taken with a scanning electron microscope (SEM).

[0045] The proportion of SWCNTs in the CNTs contained in the negative electrode mixture is determined by the following method.

[0046] An image of the cross section of the negative electrode mixture layer or CNTs is obtained using an SEM. In the SEM image, multiple CNTs (e.g., 50 to 200) are randomly selected and observed, the number of SWCNTs is counted, and the ratio of the number of SWCNTs to the total number of selected CNTs is calculated.

[0047] Quantitative analysis of CNTs is performed, for example, by combining Raman spectroscopy and thermogravimetric analysis.

[0048] From the viewpoint of reducing disconnection of the conductive paths during charge and discharge, the average diameter of the CNTs may be, for example, 1 nm or more and 10 nm or less, or 1 nm or more and 5 nm or less.

[0049] From the viewpoint of reducing disconnection of the conductive paths during charge and discharge, the average length of the CNTs may be, for example, 1 μm or more and 100 μm or less, or 5 μm or more and 20 μm or less.

[0050] The average length and diameter of the CNTs can be determined from an image of the cross section of the negative electrode mixture layer or the CNTs using at least one of an SEM and a TEM. More specifically, in the captured image, a number of CNTs (e.g., 50 to 200) are randomly selected, and their lengths and diameters are measured and averaged to determine the average length and diameter. The length of the CNTs refers to the length of the CNTs when they are stretched linearly.

[0051] The CNT content in the negative electrode mixture is, for example, 0.005% by mass or more and 0.1% by mass or less, or may be 0.01% by mass or more and 0.05% by mass or less, or 0.02% by mass or more and 0.05% by mass or less. By setting the CNT content in the negative electrode mixture to 0.005% by mass or more, the conductivity of the negative electrode and the effect of improving the capacity retention rate at the early stage of charge / discharge cycles are greatly improved. In addition, the effect of suppressing the decrease in electrolyte solution by combining the CNT-containing negative electrode mixture with an electrolyte solution containing an acid anhydride is also manifested. On the other hand, by setting the CNT content in the negative electrode mixture to 0.1% by mass or less (even 0.05% by mass or less), the effect of suppressing the decrease in electrolyte solution by the acid anhydride is manifested.

[0052] (others) As the binder, for example, a resin material is used. Examples of binders include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resin), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0053] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include CMC and its modified products, methyl cellulose, etc. Modified CMC also includes salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts. One type of thickener may be used alone, or two or more types may be used in combination.

[0054] Examples of conductive agents other than CNT include conductive fibers and conductive particles other than CNT. Examples of conductive fibers include carbon fibers and metal fibers. Examples of conductive particles include conductive carbon (carbon black, etc.) and metal powder. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0055] The negative electrode current collector is selected depending on the type of nonaqueous electrolyte secondary battery. Examples of the negative electrode current collector include a sheet-like one. Metal foil or the like may also be used as the current collector. Alternatively, a porous current collector may also be used. Examples of porous current collectors include a mesh-like one, a punched sheet, and an expanded metal.

[0056] Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0057] The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0058] The negative electrode can be formed, for example, by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of a negative electrode current collector and drying the coating. The dried coating film may be rolled, if necessary.

[0059] The dispersion medium is not particularly limited, but examples thereof include water, alcohol (e.g., ethanol), ether (e.g., tetrahydrofuran), amide (e.g., dimethylformamide), N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0060] (positive electrode) The positive electrode may include 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 a positive electrode mixture is dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary. The positive electrode mixture contains, as essential components, a positive electrode active material, and may contain, as optional components, a binder, a conductive agent, and the like. As the dispersion medium, for example, those exemplified for the negative electrode can be selected.

[0061] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal is used. Examples of the transition metal include Ni, Co, Mn, etc. Examples of the composite oxide containing lithium and a transition metal include, for example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b1 Ni 1-b1 O2, Li a Co b1 M 1-b1 O c1 、Li a Ni 1-b1 M b1 O c1 、Li a Mn2O4, Li a Mn 2-b1 M b1 O4. Here, a = 0 to 1.2, b1 = 0 to 0.9, c1 = 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 a value indicating the molar ratio of lithium increases or decreases during charge and discharge.

[0062] Among them, Li a Ni b2 M 1-b2 O2 (0 < a ≤ 1.2, 0.3 ≤ b2 ≤ 1, and M is at least one selected from the group consisting of Mn, Co, and Al.) is preferable. From the viewpoint of increasing the capacity, it is more preferable to satisfy 0.85 ≤ b2 ≤ 1. From the viewpoint of the stability of the crystal structure, Li a Nib2 Co c2 Al d O2 (0 < a ≤ 1.2, 0.85 ≤ b2 < 1, 0 < c2 ≤ 0.15, 0 < d ≤ 0.1, b2 + c2 + d = 1) is more preferable.

[0063] As the binder, resin materials exemplified for the negative electrode can be used. As the conductive agent, for example, those exemplified for the negative electrode can be selected. Graphite may be used as the conductive agent.

[0064] The shape and thickness of the positive electrode current collector can be selected respectively from the shapes and ranges described for the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0065] (Electrolyte solution) As the electrolyte solution, it is usually used in a liquid state, but it may also be in a state where its fluidity is restricted by a gelling agent or the like. The electrolyte solution usually contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent, and in addition to these, it contains additives. In this specification, acid anhydrides and sulfur-containing compounds are classified as additives. Chain carboxylic acid esters are classified as non-aqueous solvents.

[0066] <​​Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). 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 (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The electrolyte may contain one non-aqueous solvent or a combination of two or more non-aqueous solvents.

[0067] When an electrolyte solution contains FEC or a chain carboxylic acid ester, side reactions are likely to occur when combined with a negative electrode mixture containing CNT. However, even in such cases, the effect of using an acid anhydride is significant. Therefore, side reactions are suppressed and high cycle performance can be ensured. When the electrolyte solution contains at least MA as an FEC or a chain carboxylic acid ester, this effect becomes even more significant.

[0068] (lithium salts) Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10Examples of suitable lithium salts include lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of suitable phosphate salts include lithium difluorophosphate (LiPOF), lithium difluorobis(oxalato)phosphate (LiDFBOP), and lithium tetrafluoro(oxalato)phosphate. Examples of suitable borates include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB). Examples of suitable imide salts include lithium bisfluorosulfonylimide (LiN(FSO)), lithium bistrifluoromethanesulfonylimide (LiN(CFSO)), lithium trifluoromethanesulfonylnonylimide (LiN(CFSO)(CFSO)), and lithium bispentafluoroethanesulfonylimide (LiN(CFSO)). The electrolyte may contain one type of lithium salt or a combination of two or more types of lithium salts.

[0069] When an electrolyte solution contains lithium bisfluorosulfonylimide (LiFSI), side reactions are likely to occur when combined with a negative electrode mixture containing CNTs. However, even in such cases, the use of an acid anhydride can significantly improve the battery's performance. This suppresses side reactions and ensures high cycle performance.

[0070] The concentration of the lithium salt in the electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0071] (sulfur-containing compounds) The sulfur-containing compound may be at least one selected from the group consisting of sulfate esters, sulfite esters, and sulfonate esters. The sulfate ester has an -OS(=O)2-O- structure. The sulfate ester may be cyclic, linear, or may form a salt. The sulfite ester has an -OS(=O)-O- structure. The sulfite ester may be cyclic, linear, or may form a salt. The sulfonate ester has an -S(=O)2-O- structure. The sulfonate ester may be cyclic, linear, or may form a salt. The electrolyte may contain one sulfur-containing compound or a combination of two or more sulfur-containing compounds.

[0072] As sulfate esters, C 2-4 Alkyl sulfates are preferred, and specific examples include ethylene sulfate, propylene sulfate, trimethylene sulfate, butylene sulfate, vinylene sulfate, ethyl sulfate, and methyl sulfate.

[0073] As sulfite esters, C 2-4 Alkylene sulfites are preferred, and specific examples include ethylene sulfite (ES), propylene sulfite, trimethylene sulfite, butylene sulfite, and vinylene sulfite.

[0074] Sulfonic acid esters include C 3-5 Alkanesultone and C 3-5 At least one selected from the group consisting of alkene sultones is preferred, specifically 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, etc.

[0075] The sulfur-containing compound may have one or more hydrogen atoms of the compounds exemplified above substituted with a substituent. Examples of the substituent include an alkyl group, a hydroxyalkyl group, a hydroxy group, an alkoxy group, and a halogen atom. The number of carbon atoms in the substituent may be 1 to 4 or 1 to 3. Examples of the halogen atom include a chlorine atom and a fluorine atom.

[0076] The content of the sulfur-containing compound in the electrolyte is, for example, 5% by mass or less, or may be 3% by mass or less, or may be 2% by mass or less. When the content of the sulfur-containing compound is in such a range, the effect of suppressing the decrease of the electrolyte is further enhanced. In this case, the viscosity of the electrolyte can be kept low, and the charge / discharge reaction can proceed more uniformly, which is thought to suppress the consumption of the electrolyte as a whole.

[0077] In addition, in a non-aqueous electrolyte secondary battery, the content of sulfur-containing compounds in the electrolyte solution changes during storage or during charge / discharge cycles. Therefore, it is sufficient that the sulfur-containing compounds remain in the electrolyte solution collected from the non-aqueous electrolyte secondary battery at a concentration equal to or higher than the detection limit. The content of the sulfur-containing compounds in the electrolyte solution may be 0.01% by mass or more, 0.1% by mass or more, or 0.5% by mass or more.

[0078] The content of the sulfur-containing compound in the electrolyte solution used to manufacture the non-aqueous electrolyte secondary battery may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more. The content of the sulfur-containing compound in the electrolyte solution used to manufacture the non-aqueous electrolyte secondary battery may be, for example, 5% by mass or less, 3% by mass or less, or 2% by mass or less. These lower and upper limits can be combined arbitrarily.

[0079] The electrolyte may contain other additives, such as vinyl ethylene carbonate and cyclohexyl benzene.

[0080] (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. The separator may be, for example, a microporous thin film, a woven fabric, a nonwoven fabric, or a laminate of at least two selected from these. The separator is preferably made of polyolefin (e.g., polypropylene, polyethylene).

[0081] (others) 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 is housed in an exterior body together with an electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0082] FIG. 1 is a schematic perspective view, partially cut away, of a prismatic nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure. The battery includes a bottomed prismatic battery case 4, an electrode group 1, and an electrolyte 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 therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an electrolyte injection hole, which is closed with a seal 8 after injection.

[0083] [Example] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0084] Examples 1 to 7 and Comparative Examples 1 to 7 A non-aqueous electrolyte secondary battery was fabricated and evaluated according to the following procedure.

[0085] (1) Preparation of the 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 binder, and a conductive agent.

[0086] The negative electrode active material was a mixture of Si-containing material and graphite (average particle size (D50) 25 μm). The Si-containing material was Li-ion battery whose surface was coated with a conductive layer containing conductive carbon. 2y SiO 2+y Particles (y = 0.5, average particle size (D50) 10 μm) and SiO2 whose surface is coated with a conductive layer containing conductive carbon x Particles (x = 1, average particle size (D50) 5 μm) were used. 2y SiO 2+y SiO excluding particles and conductive layer x The mass ratio of the particles to the Si-containing material was 1: 1. In the negative electrode active material, the mass ratio of the Si-containing material excluding the conductive layer to the graphite was 6:94.

[0087] The binders used were sodium polyacrylate (PAA-Na), sodium salt of CMC (CMC-Na), and SBR.The conductive agent used was CNTs (average diameter approximately 1.6 nm, average length approximately 5 μm) containing more than 90% SWCNTs.

[0088] The CNT content in the negative electrode mixture was set to the value shown in Table 1. The PAA-Na, CMC-Na, and SBR contents in the negative electrode mixture were each set to 1 mass %.

[0089] 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.

[0090] (2) Preparation of the positive electrode Lithium-containing composite oxide (LiNi 0.8 Co0.18 Al 0.02 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP were added to 95 parts by mass of ethanol (O2) and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, the coating was dried, and then 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.

[0091] (3) Preparation of electrolyte An electrolyte solution was prepared by dissolving LiPF6 and, if necessary, the additives shown in Table 1 in a mixed solvent of EC, DMC, and MA (EC:DMC:MA = 20:60:20 (volume ratio)). The concentration of LiPF6 in the electrolyte solution was 1.35 mol / L. The concentrations (initial concentrations) of the additives in the electrolyte solution were the values ​​(mass %) shown in Table 1.

[0092] (4) 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 having an Al layer, and a predetermined amount of the above-mentioned electrolyte solution was injected, and the exterior body was sealed to produce a nonaqueous electrolyte secondary battery. Note that when the electrode assembly was housed in the exterior body, a portion of the positive electrode lead and the negative electrode lead were each exposed to the outside from the exterior body.

[0093] (evaluation) The nonaqueous electrolyte secondary battery was subjected to charge / discharge cycles according to the following procedure, and the remaining amount of electrolyte and the capacity retention rate after the cycles were determined.

[0094] In a 45°C environment, the nonaqueous electrolyte secondary battery was charged at a constant current of 0.5 C (180 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C (18 mA). After a 10-minute rest, the battery was discharged at a constant current of 0.7 C (252 mA) until the voltage reached 2.5 V. The discharge capacity (Ci) at this time was determined. This cycle of charge, rest, and discharge was repeated 400 times, and the discharge capacity (Cc) at the 400th cycle was determined. The capacity retention rate was calculated as the ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci, which was taken as 100%.

[0095] In addition, the nonaqueous electrolyte secondary battery was disassembled after 400 cycles, and the remaining electrolyte solution was recovered and its volume was determined. The volume ratio (%) of the remaining electrolyte solution was calculated, assuming that the initial volume of the electrolyte solution was 100%. Table 1 shows this ratio as the remaining amount of electrolyte solution (%).

[0096] The results of the examples and comparative examples are shown in Table 1. Table 1 also shows the CNT content (mass%) in the negative electrode mixture, and the types and amounts (mass%) of additives added to the electrolyte. In Table 1, E1 to E7 correspond to examples 1 to 7, and C1 to C7 correspond to comparative examples 1 to 7.

[0097] The additives in the table are indicated as follows:

[0098] MAL: Maleic anhydride SUC: succinic anhydride ES: Ethylene sulfite LiFSI: Lithium bisfluorosulfonyl imide FEC: Fluoroethylene carbonate In the batteries E5 and C5, the concentration of LiPF6 was reduced from 1.35 mol / L to 1.25 mol / L, and LiFSI was added instead to a concentration of 0.10 mol / L.

[0099] In the batteries E6 and C6, 50% by volume of the EC was replaced with FEC.

[0100] [Table 1]

[0101] As shown in Table 1, when the negative electrode mixture contains CNTs, the remaining amount of electrolyte is reduced by 3.3% compared to when it does not contain CNTs (comparison of C1 and C2). Furthermore, when the negative electrode mixture does not contain CNTs, the remaining amount of electrolyte is almost unchanged even when acid anhydride is added to the electrolyte (+0.5% for C3 and +0.1% for C4 compared to C1). In other words, when the negative electrode mixture does not contain CNTs, acid anhydride contributes little to suppressing the decrease in electrolyte.

[0102] On the other hand, when a negative electrode mixture containing CNTs is combined with an electrolyte containing an acid anhydride, the reduction in electrolyte loss is suppressed. Specifically, compared to C2, which uses an electrolyte without an acid anhydride, E1 to E7 can maintain the remaining electrolyte amount comparable to when the negative electrode mixture does not contain CNTs (E1 +4.2%, E2 +2.6%, E5 +3.8%, E6 +3.4%, and E7 +3.6% compared to C2).

[0103] Comparing C1 and C5, neither of which uses CNT, shows that the decrease in electrolyte is more pronounced when LiFSI is used (C5 is -0.1% compared to C1). Comparing E5 and C5, on the other hand, shows that even when CNT is used, the use of acid anhydride ensures a remaining amount of electrolyte that far exceeds that of C5, which does not use CNT (E5 is +0.6% compared to C5). Comparing E7 and C7, a similar trend is seen when ES is used.

[0104] Furthermore, comparing C2 and C6, both of which use CNT, it can be seen that when FEC is used, the decrease in electrolyte is more pronounced (C6 is -0.4% compared to C2). In contrast, comparing E6 and C6, it can be seen that when acid anhydride is used, the remaining electrolyte exceeds that of C1, which does not use CNT (+0.1% compared to C1), and is significantly improved over C6 (+3.8% compared to C6).

[0105] Furthermore, E1 to E7 can ensure a higher capacity retention rate than those without CNT or acid anhydride, and even after 400 charge-discharge cycles at high temperature (45°C) are achieved (comparison of C1 to C7 and E1 to E7). This is thought to be because the inclusion of CNT in the anode mixture prevents the conductive path from being broken during repeated charge-discharge cycles, and the use of acid anhydride prevents side reactions that occur when the anode mixture contains CNT, allowing electrons to be preferentially consumed in the charge-discharge reaction.

[0106] When the CNT content in the negative electrode mixture was 0.005 mass % or more and 0.1 mass % or less, the same tendency as above was observed. [Industrial Applicability]

[0107] The nonaqueous electrolyte secondary battery of the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, etc. However, the uses of the nonaqueous electrolyte secondary battery are not limited to these. [Explanation of symbols]

[0108] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal

Claims

1. The battery comprises a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and an electrolyte solution; the electrolyte solution contains an acid anhydride, The content of the acid anhydride in the electrolytic solution is 5% by mass or less, the negative electrode comprises a negative electrode mixture containing a negative electrode active material and carbon nanotubes, the content of the carbon nanotubes in the negative electrode mixture is 0.005% by mass or more and 0.1% by mass or less, The negative electrode active material of the nonaqueous electrolyte secondary battery includes a silicon-containing material and a carbonaceous material.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the acid anhydride contains a carbon-carbon unsaturated bond.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the acid anhydride includes maleic anhydride.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon nanotubes include single-walled carbon nanotubes.

5. 5. The nonaqueous electrolyte secondary battery according to claim 4, wherein the proportion of the single-walled carbon nanotubes in the carbon nanotubes is 90% or more.

6. 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolytic solution contains a chain carboxylic acid ester.

7. The nonaqueous electrolyte secondary battery according to claim 6 , wherein the chain carboxylic acid ester contains at least methyl acetate.

8. 8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolytic solution contains a sulfur-containing compound.

9. 9. The nonaqueous electrolyte secondary battery according to claim 8, wherein the sulfur-containing compound includes at least one selected from the group consisting of sulfate esters, sulfite esters, and sulfonate esters.

10. 10. The nonaqueous electrolyte secondary battery according to claim 8, wherein the content of the sulfur-containing compound in the electrolyte solution is 5 mass % or less.

11. 11. The nonaqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing material accounts for 4 mass % or more of the negative electrode active material.

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