Non-aqueous electrolyte secondary battery

Abstract

The non-aqueous electrolyte secondary battery includes: a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and an electrolytic solution. The electrolytic solution contains an acid anhydride. The negative electrode includes a negative electrode binder, and the negative electrode binder contains a negative electrode active material and a carbon nanotube. The negative electrode active material contains a silicon-containing material and a carbonaceous material.

Description

Technical Field

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

[0002] For non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries, they include: a positive electrode, a negative electrode, and a non-aqueous electrolyte. As the non-aqueous electrolyte, a non-aqueous electrolyte solution is mainly used. The negative electrode includes a negative electrode mixture, and the negative electrode mixture contains a negative electrode active material capable of electrochemically storing and releasing lithium ions. As the negative electrode active material, a material capable of electrochemically storing and releasing lithium ions is used. As such a material, for example, a carbonaceous material, a silicon-containing material, etc. are used. In addition, carbonaceous materials that do not store and release lithium ions such as carbon fibers and carbon nanotubes are sometimes added to the negative electrode mixture.

[0003] Patent Document 1 proposed a scheme of using a composite electrode agent in a lithium ion secondary battery, and the composite electrode agent includes: particles containing an element capable of storing / releasing lithium ions, carbon particles capable of storing / releasing lithium ions, multi-walled carbon nanotubes, and carbon nanofibers.

[0004] Patent Document 2 proposed a scheme of using an electrode in a lithium ion battery, and the electrode is manufactured as follows: a mixture is obtained by dry mixing an active material, carbon fibers with a fiber diameter of 50 nm or more and 300 nm or less, carbon fibers with a fiber diameter of 5 nm or more and 400 nm or less, and carbon black with a binder, a liquid medium is added to the mixture and kneaded, and the kneaded product is formed into a sheet shape.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-146519

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2014-160590 Summary of the Invention

[0009] The volume change of the silicon-containing material is large with the storage and release of lithium ions. Therefore, if a silicon-containing material is used as the negative electrode active material, the conductive path between the negative electrode active material particles is cut off, the negative electrode active material particles are isolated, and the cycle characteristics are likely to deteriorate.

[0010] If a negative electrode active material containing a silicon-containing material and carbon nanotubes are combined, the conductivity between the negative electrode active material particles can be easily ensured at the initial stage of the charge-discharge cycle. On the other hand, by using carbon nanotubes, the utilization rate of the negative electrode is increased, the expansion and contraction of the negative electrode active material are increased, and the negative electrode active material is likely to break. Therefore, if long-term charge-discharge cycles are performed at high temperatures, side reactions gradually increase, and the consumption of the electrolyte becomes significant.

[0011] One aspect of the present invention relates to a non-aqueous electrolyte secondary battery, which includes: a positive electrode, a separator, a negative electrode disposed opposite to the positive electrode with the separator therebetween, and an electrolytic solution. The electrolytic solution contains an acid anhydride. The negative electrode includes a negative electrode binder, and the negative electrode binder contains a negative electrode active material and carbon nanotubes. The negative electrode active material contains a silicon-containing material and a carbonaceous material.

[0012] According to the present invention, it is possible to suppress the reduction of the electrolytic solution when the charge-discharge cycles of the non-aqueous electrolyte secondary battery are repeatedly performed at a high temperature for a long time, and improve the capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A perspective view showing a part of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention cut away. DETAILED DESCRIPTION

[0014] For the non-aqueous electrolyte secondary battery of the present invention, it includes: a positive electrode, a separator, a negative electrode disposed opposite to the positive electrode with the separator therebetween, and an electrolytic solution. The negative electrode includes a negative electrode binder, and the negative electrode binder contains a negative electrode active material and carbon nanotubes. The negative electrode active material contains a silicon-containing material and a carbonaceous material. Hereinafter, the silicon-containing material may sometimes be referred to as a Si-containing material, and the carbon nanotubes may be referred to as CNTs.

[0015] The electrolytic solution contains an acid anhydride. The acid anhydride reacts rapidly with the negative electrode active material. When the negative electrode active material is broken during the charge-discharge cycle, the acid anhydride reacts rapidly on the newly formed surface generated by the breakage to form a low-resistance protective film. By quickly protecting the newly formed surface, the progress of further side reactions accompanied by the decomposition of the electrolytic solution is suppressed. Therefore, by including the Si-containing material and CNTs, even when the negative electrode active material is likely to be broken due to expansion and contraction, the consumption of the electrolytic solution can be suppressed. As a result, even when the charge-discharge cycle is performed at a high temperature for a long time, the capacity retention rate can be significantly improved.

[0016] The acid anhydride contributes little to suppressing the reduction of the electrolytic solution when using a negative electrode binder that does not contain CNTs, and also does not contribute much to improving the capacity retention rate when performing a long-term charge-discharge cycle at a high temperature. However, if the electrolytic solution containing an acid anhydride is combined with a negative electrode binder containing CNTs, the reduction of the electrolytic solution is significantly suppressed and the capacity retention rate is significantly improved when performing a long-term charge-discharge cycle at a high temperature.

[0017] In the non-aqueous electrolyte secondary battery of the present invention, by using CNTs, the capacity retention rate at the initial stage of the charge-discharge cycle is improved. In addition, by using an acid anhydride, the capacity retention rate is also improved even when performing a long-term charge-discharge cycle at a high temperature.

[0018] The content of the acid anhydride in the electrolyte is, for example, 5% by mass or less, and may also be 3% by mass or less. When the content of the acid anhydride is in such a range, the functions required for the electrolyte are not impaired, and the function of forming a protective film on the newly formed surface generated by the rupture of the negative electrode active material is continuously maintained over a long period of time. Therefore, when performing charge and discharge cycles at high temperature for a long time, a further reduction in the electrolyte can be suppressed. When further suppressing the reduction of the electrolyte, the content of the acid anhydride in the electrolyte is preferably 2% by mass or less.

[0019] It should be noted that for a non-aqueous electrolyte secondary battery, the content of the acid anhydride in the electrolyte changes during the storage period or the charge and discharge cycle. Therefore, in the electrolyte taken from the non-aqueous electrolyte secondary battery, it is sufficient that the acid anhydride remains at a concentration equal to or higher than the detection limit. The content of the acid anhydride in the electrolyte can be 0.01% by mass or more, can be 0.1% by mass or more, and can also be 0.5% by mass or more.

[0020] On the other hand, the content of the acid anhydride in the electrolyte used for manufacturing a non-aqueous electrolyte secondary battery can be 0.1% by mass or more, and can also be 0.3% by mass or more or 0.5% by mass or more. The content of the acid anhydride in the electrolyte used for manufacturing a non-aqueous electrolyte secondary battery is, for example, 5% by mass or less, and may also be 3% by mass or less or 2% by mass or less. These lower limit values and upper limit values can be arbitrarily combined.

[0021] The type of the acid anhydride is not particularly limited, but from the viewpoint of being able to react more rapidly on the newly formed surface generated by the rupture of the negative electrode active material to form a protective film, an acid anhydride containing a carbon-carbon unsaturated bond is preferred.

[0022] From the viewpoint of making as many constituent elements of the acid anhydride molecule as possible effectively utilized for the formation of the protective film, the molecule of the acid anhydride preferably has as simple a structure as possible. Examples of such acid anhydrides include maleic anhydride, succinic anhydride, acetic anhydride, phthalic anhydride, benzoic anhydride, etc. Among them, from the viewpoint of forming a balance of excellent stability and reactivity and a film with lower resistance, maleic anhydride, succinic anhydride, etc. are preferred. However, the acid anhydride can be used alone or in combination of two or more.

[0023] For the content of each component in the electrolyte, for example, it is determined by using gas chromatography under the following conditions.

[0024] Measuring device: GC-2010Plus manufactured by Shimadzu Corporation

[0025] Column: HP-1 (1μm × 60m) manufactured by J&W Company

[0026] Linear velocity: 30.0 cm / second

[0027] Inlet temperature: 270 °C

[0028] Detector: FID 290 °C (sens.10 1 )

[0029] Hereinafter, the non-aqueous electrolyte secondary battery of the present invention will be specifically described according to each component.

[0030] (Negative electrode)

[0031] The negative electrode includes a negative electrode mixture. The negative electrode may also include: a negative electrode mixture and a negative electrode current collector that holds the negative electrode mixture. The negative electrode usually has a layered negative electrode mixture (hereinafter referred to as a negative electrode mixture layer). The negative electrode mixture contains a negative electrode active material and CNT. The negative electrode mixture may further contain a binder, a thickener, a conductive agent other than CNT, etc.

[0032] (Negative electrode active material)

[0033] The negative electrode active material contains a Si-containing material and a carbonaceous material. The degree of expansion and contraction of the carbonaceous material during charge and discharge is smaller than that of the Si-containing material. By using the Si-containing material and the carbonaceous material together, the contact state between the negative electrode active material particles and between the negative electrode mixture and the negative electrode current collector can be better maintained during repeated charge and discharge. Therefore, by combining the carbonaceous material with the Si-containing material, both the high capacity of the Si-containing material can be ensured and high cycle characteristics can be easily ensured. If necessary, the negative electrode active material may contain other negative electrode active materials in addition to the Si-containing material and the carbonaceous material. As other negative electrode active materials, for example, at least one selected from the group consisting of Sn metal, Sn alloys, and Sn compounds such as Sn oxides can be cited.

[0034] (Si-containing material)

[0035] Examples of the Si-containing material include Si metal, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase (fine Si phase) is dispersed in a lithium ion conducting phase (matrix). As the silicon oxide, SiO x .x can be, for example, 0.5 ≤ x < 2, or can be 0.8 ≤ x ≤ 1.6.

[0036] From the viewpoint of ensuring higher cycle characteristics, the Si-containing material preferably contains the above composite material. The lithium ion conducting phase preferably contains at least one selected from the group consisting of SiO 2 phase and silicate phase. The lithium ion conducting phase may further contain a carbon phase. The lithium ion conducting phase can form an amorphous phase. The Si-containing material may contain a composite material in which a silicon phase is dispersed in a SiO 2 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, etc.

[0037] SiO2 is an amorphous phase containing 95% by mass or more of silicon dioxide. Silicon particles are dispersed in SiO 2 phase of the composite material is represented by SiO x , and x can be within the above range, for example. SiO x is obtained, for example, by heat-treating silicon monoxide and separating it into SiO 2 phase and fine Si phase through disproportionation reaction. Using a transmission electron microscope (TEM: Transmission Electron Microscope) to observe the particle cross-section of SiO x , the silicon phase dispersed in SiO 2 phase can be confirmed.

[0038] For the silicate phase, it preferably contains at least one of an alkali metal element (Group 1 element other than hydrogen in the long-period type periodic table) and a Group 2 element in the long-period type 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 can be 1 / 2 or 1. For the composite material in which silicon particles are dispersed in the silicate phase, for example, a mixture of silicate and raw silicon can be stirred and pulverized using a ball mill or the like, and after being micronized, the mixture is heat-treated in an inert atmosphere to obtain it.

[0039] The content of the silicon phase dispersed in the silicate phase can be 30% by mass or more and 95% by mass or less, or 35% by mass or more and 75% by mass or less, relative to the whole of the composite material.

[0040] The carbon phase contains, for example, amorphous carbon with low crystallinity. The amorphous carbon can be, for example, graphitizable carbon (hard carbon) or non-graphitizable carbon (soft carbon). For the composite material in which silicon particles are dispersed in the carbon phase, for example, a mixture of a carbon source and raw silicon can be stirred and pulverized using a ball mill or the like, and after being micronized, the mixture is heat-treated in an inert atmosphere to obtain it. The carbon source can be, for example, sugars such as carboxymethyl cellulose (CMC) and water-soluble resins such as polyvinylpyrrolidone.

[0041] Regarding the composition of the Si-containing material, for example, a reflected electron image of the cross-section of the negative electrode mixture layer is obtained by a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), the particles of the Si-containing material are observed, and it is obtained by elemental analysis of the observed particles of the Si-containing material. Elemental analysis can be, for example, analysis using an electron probe microanalyzer (EPMA: Electron Probe Micro Analyzer). Through the above analysis, the composition of the lithium ion conduction phase can also be obtained.

[0042] The Si-containing material can be used alone as one kind, or two or more kinds can be used in combination.

[0043] The Si-containing material is, for example, a particulate material. The average particle diameter (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 easily obtained.

[0044] It should be noted that in this specification, the average particle diameter (D50) refers to the particle diameter (volume average particle diameter) at which the volume cumulative value becomes 50% in the particle size distribution measured by the laser diffraction scattering method. As the measuring device, for example, "LA-750" manufactured by HORIBA, Ltd. can be used.

[0045] From the viewpoint of improving conductivity, at least a part of the surface of the particles of the Si-containing material can also be covered with a conductive layer. The conductive layer contains a conductive material such as conductive carbon. The covering amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less with respect to the total of 100 parts by mass of the Si-containing material particles and the conductive layer. For the Si-containing material particles having a conductive layer on the surface, for example, they are obtained by mixing coal tar pitch or the like with the Si-containing material particles and performing heat treatment in an inert atmosphere.

[0046] The Si-containing material has a large volume change during expansion and contraction during charge and discharge. Therefore, if the ratio of the Si-containing material in the negative electrode active material increases, the cycle characteristics are likely to deteriorate. On the other hand, according to the present invention, since the negative electrode mixture contains a specific content of CNT, even when the ratio of the Si-containing material in the negative electrode active material is relatively large, the disconnection of the conductive path can be suppressed, and high cycle characteristics can be easily ensured. The ratio of the Si-containing material in the negative electrode active material is preferably 4% by mass or more, and can also be 5% by mass or more. The ratio of the Si-containing material is preferably 15% by mass or less, and can also be 10% by mass or less. These lower limit values and upper limit values can be arbitrarily combined.

[0047] (Carbonaceous material)

[0048] As carbonaceous materials, for example, graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon) can be mentioned. The carbonaceous materials can be used alone as one kind, or two or more kinds can be used in combination.

[0049] From the viewpoints of excellent charge-discharge stability and less irreversible capacity, among them, graphite is preferably used as the carbonaceous material. As graphite, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles can be mentioned. The graphite particles may partially contain amorphous carbon, graphitizable carbon, and non-graphitizable carbon.

[0050] Graphite is a carbonaceous material with a well-developed graphite-type crystal structure. The interplanar spacing d002 of the (002) plane of graphite measured by X-ray diffraction method can be, for example, 0.340 nm or less, or can be 0.3354 nm or more and 0.340 nm or less. In addition, the crystallite size Lc(002) of graphite can be, for example, 5 nm or more, or can be 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.

[0051] The ratio of the carbonaceous material in the negative electrode active material can be, for example, 97% by mass or less, can be 96% by mass or less, or can be 95% by mass or less. The ratio of the carbonaceous material in the negative electrode active material can be, for example, 76% by mass or more, can be 80% by mass or more, 85% by mass or more, or can be 90% by mass or more. These lower limit values and upper limit values can be arbitrarily combined.

[0052] In the negative electrode active material, the ratio of the total amount of the Si-containing material and the carbonaceous material is preferably 90% by mass or more, can be 95% by mass or more, or can be 98% by mass or more. In the negative electrode active material, the ratio of the total amount of the Si-containing material and the carbonaceous material is 100% by mass or less. The negative electrode active material can be composed only of the Si-containing material and the carbonaceous material.

[0053] (CNT)

[0054] CNT is a carbonaceous material with a diameter in the nanometer range and having a structure in which a sheet (graphene) of a six-membered ring network formed by carbon atoms is wound into a tube shape. CNT has excellent electrical conductivity. When the number of layers of graphene constituting the tube structure is 1, it is called single-walled carbon nanotube (SWCNT). In the case where the number of the above layers is plural, it is called multi-walled carbon nanotube (MWCNT).

[0055] The CNT preferably contains SWCNT. In this case, in addition to easily ensuring higher cycle characteristics, it is also easy to further exert the effect of suppressing the reduction of the electrolyte brought about by the acid anhydride.

[0056] The proportion of SWCNT in the CNT is, for example, 50% or more, may be 75% or more, or may be 90% or more. The proportion of SWCNT in the CNT is 100% or less. It should be noted that the proportion of SWCNT in the CNT is the ratio of the number of SWCNT to the whole CNT.

[0057] For example, it can be confirmed that the negative electrode mixture contains CNT by a scanning electron microscope (SEM: Scanning Electron Microscope) image of the cross section of the negative electrode mixture layer.

[0058] The proportion of SWCNT in the CNT contained in the negative electrode mixture is obtained by the following method.

[0059] Use SEM to obtain an image of the cross section of the negative electrode mixture layer or CNT. In the SEM image, arbitrarily select multiple (for example, 50 to 200) CNTs for observation, obtain the number of SWCNT, and calculate the proportion of the number of SWCNT to the total number of selected CNTs.

[0060] The quantitative analysis of CNT is performed in combination with Raman spectroscopy and thermogravimetric analysis, for example.

[0061] From the viewpoint of reducing the disconnection of the conductivity path during charge and discharge, the average diameter of the CNT is, for example, 1 nm or more and 10 nm or less, and may also be 1 nm or more and 5 nm or less.

[0062] From the viewpoint of reducing the disconnection of the conductivity path during charge and discharge, the average length of the CNT is, for example, 1 μm or more and 100 μm or less, and may also be 5 μm or more and 20 μm or less.

[0063] The average length and average diameter of the CNT can be determined by using at least one of SEM and TEM from an image of the cross section of the negative electrode mixture layer or CNT. More specifically, in the captured image, arbitrarily select multiple (for example, 50 to 200) CNTs, measure the length and diameter, and average them respectively, thereby obtaining the average length and average diameter. It should be noted that the length of the CNT refers to the length when the CNT is extended into a straight line.

[0064] Regarding the content of CNT in the negative electrode mixture, for example, it is 0.005% by mass or more and 0.1% by mass or less, it can be 0.01% by mass or more and 0.05% by mass or less, or it can be 0.02% by mass or more and 0.05% by mass or less. By making the content of CNT in the negative electrode mixture 0.005% by mass or more, the improvement effect of the conductivity of the negative electrode and the capacity retention rate at the initial stage of charge and discharge cycle is enhanced. In addition, in the negative electrode mixture containing CNT, when combined with an electrolyte containing an acid anhydride, the effect of suppressing electrolyte reduction is also significantly enhanced. On the other hand, by making the content of CNT in the negative electrode mixture 0.1% by mass or less (further 0.05% by mass or less), the effect of suppressing electrolyte reduction brought by the acid anhydride is significantly enhanced.

[0065] (Other)

[0066] As the binder, for example, a resin material is used. As the binder, for example, fluororesins (such as polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (such as polyethylene, polypropylene), polyamide resins (such as aromatic polyamide resins), polyimide resins (such as polyimide, polyamideimide), acrylic resins (such as polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer or their salts), vinyl resins (such as polyvinyl acetate), rubber-like materials (such as styrene-butadiene rubber (SBR)) can be cited. The binder can be used alone as one kind, or two or more kinds can be used in combination.

[0067] As the thickener, for example, cellulose derivatives such as cellulose ether can be cited. As the cellulose derivative, CMC and its modified products, methyl cellulose, etc. can be cited. The modified product of CMC also includes the salt of CMC. As the salt, alkali metal salts (such as sodium salt), ammonium salts, etc. can be cited. The thickener can be used alone as one kind, or two or more kinds can be combined and used.

[0068] As the conductive agent other than CNT, for example, conductive fibers and conductive particles other than CNT can be cited. As the conductive fiber, carbon fiber, metal fiber, etc. can be cited. As the conductive particle, conductive carbon (such as carbon black), metal powder, etc. can be cited. The conductive agent can be used alone as one kind, or two or more kinds can be used in combination.

[0069] The negative electrode current collector is selected according to the type of non-aqueous electrolyte secondary battery. As the negative electrode current collector, for example, a sheet-like current collector can be cited. As the current collector, a metal foil or the like can also be used. In addition, a porous one can also be used as the current collector. As the porous current collector, for example, a net-like body, a perforated thin plate, a porous metal net can be cited.

[0070] As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy can be exemplified.

[0071] There is no particular limitation on the thickness of the negative electrode current collector, but for example, it is 1 to 50 μm, and it may also be 5 to 30 μm.

[0072] The negative electrode can be formed, for example, by coating a negative electrode slurry obtained by dispersing the constituent components of a negative electrode mixture in a dispersion medium on the surface of a negative electrode current collector and drying it. The dried coating film can be calendered as needed.

[0073] 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), or a mixed solvent thereof.

[0074] (Positive electrode)

[0075] 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 coating a positive electrode slurry obtained by dispersing a positive electrode mixture in a dispersion medium on the surface of the positive electrode current collector and drying it. The dried coating film can be calendered as needed. The positive electrode mixture contains a positive electrode active material as an essential component, and as optional components, it may contain a binder, a conductive agent, etc. As the dispersion medium, for example, it can be selected from the dispersion media exemplified for the negative electrode.

[0076] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal can be used. As the transition metal, for example, Ni, Co, Mn, etc. can be cited. As the composite oxide containing lithium and a transition metal, for example, Li a CoO 2 、Li a NiO 2 、Li a MnO 2 、Li a Co b1 Ni 1-b1 O 2 、Li a Co b1 M 1-b1 O c1 、Li a Ni 1-b1 M b1 O c1 、Li a Mn 2 O 4 、Li a Mn 2- b1 M b1 O 4Here, 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. It should be noted that the value of a representing the molar ratio of lithium increases or decreases through charge and discharge.

[0077] Among them, Li is preferred a Ni b2 M 1-b2 O 2 (0 < a ≤ 1.2, 0.3 ≤ b2 ≤ 1, and M is at least one selected from the group consisting of Mn, Co, and Al.) The lithium nickel composite oxide shown. From the viewpoint of high capacity, it is more preferably satisfied that 0.85 ≤ b2 ≤ 1. From the viewpoint of the stability of the crystal structure, Li is further preferred a Ni b2 Co c2 Al d O 2 (0 < a ≤ 1.2, 0.85 ≤ b2 < 1, 0 < c2 ≤ 0.15, 0 < d ≤ 0.1, b2 + c2 + d = 1).

[0078] As the binder, resin materials exemplified for the negative electrode can be used. As the conductive agent, for example, it can be selected from those exemplified for the negative electrode. As the conductive agent, graphite can also be used.

[0079] For the shape and thickness of the positive electrode current collector, they can be selected respectively from the shapes and ranges described for the negative electrode current collector. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, and titanium can be cited.

[0080] (Electrolyte)

[0081] As the electrolyte, it is usually directly used in a liquid state, but it can also be in a state where the fluidity is restricted by a gelling agent or the like. The electrolyte usually contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. In addition, it also 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.

[0082] (Non-aqueous solvent)

[0083] As non-aqueous solvents, cyclic carbonates, chain carbonates, cyclic carboxylates, and chain carboxylates can be mentioned, for example. As cyclic carbonates, propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), etc. can be mentioned. As chain carbonates, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. can be mentioned. As cyclic carboxylates, γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. can be mentioned. As chain carboxylates, methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, etc. can be mentioned. The electrolytic solution may contain one kind of non-aqueous solvent, or may contain two or more kinds in combination.

[0084] When the electrolytic solution contains FEC and a chain carboxylate, side reactions are likely to occur when combined with a negative electrode binder containing CNT. However, even in such a case, the effect produced by using an acid anhydride can be significantly obtained. Therefore, side reactions can be suppressed and high cycle characteristics can be ensured. When the electrolytic solution contains at least MA as FEC or a chain carboxylate, such an effect is further significant.

[0085] (Lithium salt)

[0086] As lithium salts, for example, 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 , lithium salts of lower aliphatic carboxylic acids, LiCl, LiBr, LiI, phosphates, borates, imide salts. As phosphates, lithium difluorophosphate (LiPO 2 F 2 ), lithium difluoro bis(oxalate) phosphate (LiDFBOP), lithium tetrafluoro(oxalate) phosphate, etc. can be mentioned. As borates, lithium bis(oxalate) borate (LiBOB), lithium difluoro(oxalate) borate (LiDFOB), etc. can be mentioned. As imide salts, lithium bis(fluorosulfonyl)imide (LiN(FSO 2 )) 2 , lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 )) 2) Lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 )(C 4 F 9 SO 2 ))、Lithium bis(pentafluoroethanesulfonyl)imide (LiN(C 2 F 5 SO 2 ) 2 ) etc. The electrolyte can contain one lithium salt or a combination of two or more.

[0087] When the electrolyte contains lithium bis(fluorosulfonyl)imide (LiFSI), side reactions are likely to occur when combined with a negative electrode binder containing CNT. However, even in such a case, the effects produced by using acid anhydrides can be significantly obtained. Therefore, side reactions can be suppressed and high cycle characteristics can be ensured.

[0088] The concentration of the lithium salt in the electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0089] (Sulfur-containing compound)

[0090] As the sulfur-containing compound, at least one selected from the group consisting of sulfate esters, sulfite esters, and sulfonate esters can be used. Sulfate esters have an -O-S(=O) 2 -O- structure. Sulfate esters can be cyclic, linear, or form salts. Sulfite esters also have an -O-S(=O)-O- structure. Sulfite esters can be cyclic, linear, or form salts. Sulfonate esters have a -S(=O) 2 -O- structure. Sulfonate esters can be cyclic, linear, or form salts. The electrolyte can contain one sulfur-containing compound or a combination of two or more.

[0091] As the sulfate ester, C 2-4 alkyl sulfate esters are preferred. Specifically, ethylene sulfite, propylene sulfite, trimethylene sulfite, butylene sulfite, vinyl sulfite, ethyl sulfate, and methyl sulfate can be cited.

[0092] As the sulfite ester, C 2-4 alkylene sulfite esters are preferred. Specifically, ethylene sulfite (ES), propylene sulfite, trimethylene sulfite, butylene sulfite, vinyl sulfite, etc. can be cited.

[0093] As the sulfonate ester, those selected from the group consisting of C 3-5 alkanesulfonic acid lactones and C 3-5At least one selected from the group consisting of olefin sulfonic acid lactones. Specifically, 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, etc. can be mentioned.

[0094] In the sulfur-containing compound, one or more hydrogen atoms in the above-exemplified compounds may be substituted with substituents. Examples of the substituents include alkyl groups, hydroxyalkyl groups, hydroxyl groups, alkoxy groups, halogen atoms, etc. The carbon number of the substituents may be 1 to 4 or 1 to 3. Examples of the halogen atoms include chlorine atoms, fluorine atoms, etc.

[0095] The content of the sulfur-containing compound in the electrolyte is, for example, 5% by mass or less, may be 3% by mass or less, or may be 2% by mass or less. When the content of the sulfur-containing compound is within such a range, the effect of suppressing the reduction of the electrolyte is further improved. In this case, the viscosity of the electrolyte can be suppressed to be low, and the charge-discharge reaction can proceed more uniformly. Therefore, it can be considered that the consumption of the electrolyte as a whole is suppressed.

[0096] It should be noted that the content of the sulfur-containing compound in the electrolyte of the non-aqueous electrolyte secondary battery changes during the storage period or the charge-discharge cycle. Therefore, in the electrolyte taken from the non-aqueous electrolyte secondary battery, it is sufficient that the sulfur-containing compound remains at a concentration above the detection limit. The content of the sulfur-containing compound in the electrolyte may be 0.01% by mass or more, may be 0.1% by mass or more, or may be 0.5% by mass or more.

[0097] Regarding the content of the sulfur-containing compound in the electrolyte for manufacturing a non-aqueous electrolyte secondary battery, it may be 0.1% by mass or more, may be 0.3% by mass or more, or may be 0.5% by mass or more. Regarding the content of the sulfur-containing compound in the electrolyte for manufacturing a non-aqueous electrolyte secondary battery, it is, for example, 5% by mass or less, may be 3% by mass or less, or may be 2% by mass or less. These lower limit values and upper limit values can be arbitrarily combined.

[0098] The electrolyte may contain other additives. Examples of such additives include vinylene carbonate, cyclohexylbenzene, etc.

[0099] (Separator)

[0100] Generally, it is preferably to interpose a separator between the positive electrode and the negative electrode. The separator has a high ion permeability and has appropriate mechanical strength and insulation properties. As the separator, for example, a microporous film, a woven fabric or a non-woven fabric, or a laminate of at least two selected from them can be used. As the material of the separator, polyolefin (e.g., polypropylene, polyethylene) is preferred.

[0101] (Others)

[0102] As an example of the structure of a non-aqueous electrolyte secondary battery, a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator therebetween and an electrolyte are housed in a housing can be cited. However, it is not limited thereto, and other types of electrode groups can also be applied. For example, it can also be a laminated electrode group in which a positive electrode and a negative electrode are laminated with a separator therebetween. The form of the non-aqueous electrolyte secondary battery is not limited either. For example, it can be a cylindrical type, a square type, a coin type, a button type, a laminated type, etc.

[0103] Figure 1 FIG. 4 is a schematic perspective view of a part of a square non-aqueous electrolyte secondary battery according to an embodiment of the present invention cut away. The battery includes: a bottomed square battery case 4, an electrode group 1 housed in the battery case 4, and an electrolyte. The electrode group 1 includes: a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator sandwiched therebetween. The negative electrode current collector of the negative electrode is electrically connected to a negative terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4 that also serves as a positive terminal. The periphery of the sealing plate 5 is fitted into the opening end of the battery case 4, and the fitting portion is laser welded. There is an injection hole for the electrolyte on the sealing plate 5, which is blocked by a plug 8 after the injection of the electrolyte.

[0104] [Examples]

[0105] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0106] 《Examples 1 to 7 and Comparative Examples 1 to 7》

[0107] Non-aqueous electrolyte secondary batteries were produced and evaluated in the following order.

[0108] (1) Production of negative electrode

[0109] An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture is a mixture of a negative electrode active material, a binder, and a conductive agent.

[0110] In the negative electrode active material, a mixture of a Si-containing material and graphite (average particle diameter (D50) 25 μm) was used. In the Si-containing material, Li 2y SiO 2+y particles (y = 0.5, average particle diameter (D50) 10 μm) and SiO x particles (x = 1, average particle diameter (D50) 5 μm) whose surfaces are covered with a conductive layer containing conductive carbon were used. Li 2y SiO 2+yThe mass ratio of the particles to SiO with the conductive layer removed x is set to 1:1. In the negative electrode active material, the mass ratio of the Si-containing material excluding the conductive layer to graphite is set to 6:94.

[0111] As the binder, sodium polyacrylate (PAA-Na), the sodium salt of CMC (CMC-Na), and SBR are used. As the conductive agent, CNT containing 90% or more of SWCNT (average diameter of about 1.6 nm and average length of about 5 μm) is used.

[0112] The content of CNT in the negative electrode mixture is the value shown in Table 1. The contents of PAA-Na, CMC-Na, and SBR in the negative electrode mixture are each set to 1% by mass.

[0113] Next, the negative electrode slurry is coated on the surface of the copper foil. After drying the coating film, rolling is performed to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm 3 ) on both sides of the copper foil, obtaining a negative electrode.

[0114] (2) Fabrication of the positive electrode

[0115] In 95 parts by mass of a lithium-containing composite oxide (LiNi 0.8 Co 0.18 Al 0.02 O 2 ), 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP are added and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry is coated on the surface of the aluminum foil. After drying the coating film, rolling is performed to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm 3 ) on both sides of the aluminum foil, obtaining a positive electrode.

[0116] (3) Preparation of the electrolyte

[0117] The electrolyte is prepared by dissolving LiPF 6 and additives as required as shown in Table 1 in a mixed solvent of EC, DMC, and MA (EC:DMC:MA = 20:60:20 (volume ratio)). The concentration of LiPF 6 in the electrolyte is set to 1.35 mol / L. The concentration (initial concentration) of the additives in the electrolyte is set to the value (mass%) shown in Table 1.

[0118] (4) Fabrication of the non-aqueous electrolyte secondary battery

[0119] An aluminum positive electrode lead is attached to the obtained positive electrode, and a nickel negative electrode lead is attached to the obtained negative electrode. In an inert gas atmosphere, the positive electrode and the negative electrode are wound into a spiral shape with a polyethylene film (separator) in between to fabricate a wound-type electrode assembly. The electrode assembly is housed in a bag-shaped outer casing formed of a laminated sheet having an Al layer. After injecting a prescribed amount of the above electrolyte, the outer casing is sealed to fabricate a non-aqueous electrolyte secondary battery. It should be noted that when the electrode assembly is housed in the outer casing, a part of the positive electrode lead and the negative electrode lead protrude from the outer casing to the outside.

[0120] (Evaluation)

[0121] The charge-discharge cycle of the non-aqueous electrolyte secondary battery is carried out in the following order, and the residual amount and capacity retention rate of the electrolyte after the cycle are determined.

[0122] In an environment at 45 °C, constant current charging is carried out at a current of 0.5C (180 mA) until the voltage of the non-aqueous electrolyte secondary battery reaches 4.2 V, and then constant voltage charging is carried out at a voltage of 4.2 V until the current reaches 0.05C (18 mA). After stopping for 10 minutes, constant current discharging is carried out at a current of 0.7C (252 mA) until the voltage of the non-aqueous electrolyte secondary battery reaches 2.5 V. The discharge capacity (Ci) at this time is determined. One cycle of such charging, stopping, and discharging is defined as one cycle, and 400 cycles are repeated. The discharge capacity (Cc) of the 400th cycle is determined. The ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci when the initial discharge capacity Ci is set to 100% is determined as the capacity retention rate.

[0123] In addition, the non-aqueous electrolyte secondary battery after 400 cycles is disassembled, the remaining electrolyte is recovered, and the volume is determined. The ratio (%) of the volume of the remaining electrolyte to the initial volume of the electrolyte when the initial volume of the electrolyte is set to 100% is calculated. In Table 1, this ratio is shown as the electrolyte residual amount (%).

[0124] The results of the examples and comparative examples are shown in Table 1. The content (mass%) of CNT in the negative electrode mixture, the type and addition amount (mass%) of the additive added to the electrolyte are also shown in Table 1. In Table 1, E1 to E7 are Examples 1 to 7, and C1 to C7 are Comparative Examples 1 to 7.

[0125] The representation of the additives in the table is as follows.

[0126] MAL: Maleic anhydride

[0127] SUC: Succinic anhydride

[0128] ES: Ethylene sulfite

[0129] LiFSI: Lithium bis(fluorosulfonyl)imide

[0130] FEC: Fluoroethylene Carbonate

[0131] In batteries E5 and C5, the concentration of LiPF 6 was decreased from 1.35 mol / L to 1.25 mol / L, and instead, LiFSI was added at a concentration of 0.10 mol / L.

[0132] In batteries E6 and C6, 50 vol% of EC was replaced with FEC.

[0133] [Table 1]

[0134]

[0135] As shown in Table 1, when the negative electrode mixture contains CNT, the remaining amount of the electrolyte is reduced by 3.3% compared with the case where CNT is not contained (comparison between C1 and C2). In addition, when CNT is not contained in the negative electrode mixture, even if an acid anhydride is added to the electrolyte, the remaining amount of the electrolyte hardly changes (for C3, it is +0.5% and for C4, it is +0.1% relative to C1). In other words, when CNT is not contained in the negative electrode mixture, the acid anhydride hardly contributes to suppressing the reduction of the electrolyte.

[0136] On the other hand, when an electrolyte containing an acid anhydride is combined with a negative electrode mixture containing CNT, the effect of suppressing the reduction of the electrolyte is exerted. Specifically, compared with C2 using an electrolyte without an acid anhydride, in E1 to E7, the remaining amount of the electrolyte comparable to that when the negative electrode mixture does not contain CNT can be ensured (for E1, it is +4.2%, for E2, it is +2.6%, for E5, it is +3.8%, for E6, it is +3.4%, and for E7, it is +3.6% relative to C2).

[0137] It should be noted that when C1 and C5 that do not use CNT are compared, it can be seen that when LiFSI is used, the reduction of the electrolyte becomes more significant (for C5, it is -0.1% relative to C1). In contrast, when E5 and C5 are compared, it can be seen that even when CNT is used, if an acid anhydride is used, the remaining amount of the electrolyte significantly exceeding that of C5 without using CNT can be ensured (for E5, it is +0.6% relative to C5). It should be noted that when E7 and C7 are compared, the same tendency can be seen when ES is used.

[0138] Note that, when comparing C2 and C6 both using CNT, it can be seen that the reduction of the electrolyte becomes more significant when using FEC (C6 is -0.4% relative to C2). In contrast, when comparing E6 and C6, it can be seen that when using an acid anhydride, it is possible to ensure an electrolyte residue that exceeds that of C1 without using CNT (+0.1% relative to C1) and is significantly improved compared to C6 (+3.8% for E6 relative to C6).

[0139] Furthermore, among E1 to E7, compared with the case of not using CNT or not using an acid anhydride, a high capacity retention rate can be ensured. Even when 400 charge-discharge cycles are repeatedly performed at a high temperature (45 °C), an excellent capacity retention rate can be obtained (comparison between C1 to C7 and E1 to E7). This can be considered that since the negative electrode mixture contains CNT, the disconnection of the conduction path in the negative electrode mixture during repeated charge and discharge is suppressed. In addition, by using an acid anhydride, the side reaction when the negative electrode mixture contains CNT is suppressed, and electrons are preferentially consumed in the charge-discharge reaction.

[0140] Note that when the content of CNT in the negative electrode mixture is 0.005 mass% or more and 0.1 mass% or less, the same tendency as above is obtained.

[0141] Industrial Applicability

[0142] The non-aqueous electrolyte secondary battery of the present invention can be used as the main power source for mobile communication devices, portable electronic devices, etc. However, the use of the non-aqueous electrolyte secondary battery is not limited thereto.

[0143] Explanation of Reference Numerals

[0144] 1 Electrode group

[0145] 2 Positive electrode lead

[0146] 3 Negative electrode lead

[0147] 4 Battery case

[0148] 5 Sealing plate

[0149] 6 Negative terminal

[0150] 7 Gasket

[0151] 8 Plug

Claims

1. A non-aqueous electrolyte secondary battery, comprising: a positive electrode, a separator, a negative electrode disposed opposite to the positive electrode with the separator therebetween, and an electrolyte solution. The electrolyte solution contains an acid anhydride, and the content of the acid anhydride in the electrolyte solution is 0.01% by mass or more and 5% by mass or less. The negative electrode includes a negative electrode binder, the negative electrode binder contains a negative electrode active material and carbon nanotubes, and the content of the carbon nanotubes in the negative electrode binder is 0.005% by mass or more and 0.1% by mass or less. The negative electrode active material includes a silicon-containing material and a carbonaceous material.

2. The non-aqueous electrolyte secondary battery according to claim 1. Wherein, The acid anhydride contains a carbon-carbon unsaturated bond.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2. Wherein, The acid anhydride contains maleic anhydride.

4. The non-aqueous electrolyte secondary battery according to claim 1 or 2. Wherein, The carbon nanotubes include single-walled carbon nanotubes.

5. The non-aqueous 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. The non-aqueous electrolyte secondary battery according to claim 1 or 2. Wherein, The electrolyte solution contains a chain carboxylic acid ester.

7. The non-aqueous electrolyte secondary battery according to claim 6. Wherein, The chain carboxylic acid ester contains at least methyl acetate.

8. The non-aqueous electrolyte secondary battery according to claim 1 or 2. Wherein, The electrolyte solution contains a sulfur-containing compound.

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

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

11. The non-aqueous electrolyte secondary battery according to claim 1 or 2. Wherein, The proportion of the silicon-containing material in the negative electrode active material is 4% by mass or more.

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