Non-aqueous electrolyte secondary batteries

Abstract

To provide a nonaqueous electrolytic solution secondary battery that enhances electron conductivity of electrodes thereby improving input / output characteristics.SOLUTION: The nonaqueous electrolytic solution secondary battery comprises: a positive electrode having, as an active material, a transition metal composite oxide containing lithium; a negative electrode; and a nonaqueous electrolytic solution. The negative electrode contains: titanium oxide particles which are particles of a titanium oxide represented by general formula H2Ti12O25; a binder; and a single carbon nanotube in an amount from 0.3 wt.% to 5.0 wt.% inclusive based on the amount of the titanium oxide. The secondary particle size D50 of the titanium oxide particles is from 1 μm to 15 μm inclusive, and the secondary particle size D90 thereof is 50 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. [Background technology]

[0002] Lithium-ion secondary batteries are widely used in portable devices such as personal computers and mobile phones. In recent years, large-capacity batteries have been developed for electric vehicles (EVs), industrial robots, mega-solar systems, and stationary power sources for homes, and this market is expected to continue expanding. These new markets, particularly EVs, demand high-capacity lithium-ion secondary batteries. However, the current battery capacity limit has already been reached with the conventional combination of a Li-transition metal composite oxide cathode and a graphite anode. Even with efforts to develop higher-voltage cathodes and high-capacity Si compound anodes, cycle times equivalent to or exceeding those of current batteries have not been achieved, preventing practical application. Furthermore, while progress has been made in the development of new battery technologies, such as solid-state electrolyte batteries and organic sulfur / Li metal batteries, many challenges, including productivity, remain, and large-scale practical applications remain in the early stages. As part of the infrastructure for lithium-ion secondary batteries, the development of charging environments, including contactless charging, is progressing. Therefore, if lithium-ion secondary batteries with excellent input / output characteristics can be developed and charged in a short time, the above-mentioned challenge of achieving high capacity can be resolved. For example, Toshiba SCiB (registered trademark) shown in Non-Patent Document 1 (Toshiba Review Vol. 71 No. 2 pp. 44) is compatible with charging at 20 C and has been developed as a battery that can be charged to nearly 100% in a few minutes. This battery also uses the general formula Li4Ti5O 12 The active material shown in the figure is used for the negative electrode, but the charge / discharge potential of the material is 1.5V (vs. Li / Li +) or more, the problem of Li deposition, which was an issue with conventional graphite, is completely eliminated during normal charging and discharging. This problem is particularly important at high inputs, where overvoltage becomes large. On the other hand, Li4Ti5O 12 Since Li4Ti5O has low electronic conductivity, it is necessary to add conductive additives such as graphite or carbon black to the electrode. 12 The original theoretical capacity of the graphite is 175 mAh / g, which is lower than the theoretical capacity of graphite, 372 mAh / g, and there is a further problem that the capacity is significantly reduced by adding the auxiliary agent. [Prior art documents] [Patent documents]

[0003] [Non-Patent Document 1] Toshiba Review Vol.71 No.2 pp44 [Patent Document 1] Japanese Patent Application Publication No. 6-275263 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-255000 [Patent Document 3] Japanese Patent Publication No. 2020-117416 [Patent Document 4] Patent No. 6030708 Summary of the Invention [Problem to be solved by the invention]

[0004] As a material that solves the problems of the above-mentioned negative electrode material, for example, Patent Document 2 (JP 2008-255000 A) discloses a material having the general formula H2Ti 12 O 25 A new titanium oxide, H2Ti, has been reported. 12 O 25 The charge / discharge potential is about 1.5V (vs. Li / Li + ) and conventional spinel-based lithium titanate Li4Ti5O 12 Since the solubility of Li in the alloy is also high, the problem of Li precipitation can be avoided. 12The theoretical capacity of H2Ti is approximately 175mAh / g. 12 O 25 It has been shown that the capacity is about 200 to 230 mAh / g. Furthermore, Patent Document 3 (JP 2020-117416 A) describes H2Ti 12 O 25 It has been shown that this new manufacturing method improves capacity to 270 to 320 mAh / g. Furthermore, Patent Document 4 (Japanese Patent No. 6030708) shows a new titanium oxide represented by the general formula TiNb2O7. In Patent Document 4, TiNb2O7 has been shown to provide a capacity comparable to that of graphite. However, the problem of low electronic conductivity of the active material remains unresolved for any of the negative electrode materials.

[0005] An object of the present invention is to improve the electronic conductivity of electrodes and improve input / output characteristics in non-aqueous electrolyte secondary batteries. [Means for solving the problem]

[0006] Below, several aspects will be described as means for solving the problems. These aspects can be combined as needed. A non-aqueous electrolyte secondary battery according to one aspect of the present invention includes a positive electrode having a lithium-containing transition metal composite oxide as an active material, a negative electrode, and a non-aqueous electrolyte. 12 O 25 The titanium oxide particles have a secondary particle diameter D50 of 1 μm or more and 15 μm or less and a secondary particle diameter D90 of 50 μm or less. The non-aqueous electrolyte secondary battery uses titanium oxide particles with a specific surface area of ​​15m 2 / g or more 150m 2 / g or less. [Effects of the Invention]

[0007] The non-aqueous electrolyte secondary battery according to the present invention has improved electronic conductivity of the electrodes and improved input / output characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0008] (1) Overall structure A non-aqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode having a lithium-containing transition metal composite oxide as an active material, a negative electrode, and a non-aqueous electrolyte. 12 O 25 The titanium oxide particles are titanium oxide particles represented by the formula (1), a binder, and single carbon nanotubes in an amount of 0.3 wt% to 5.0 wt% relative to the titanium oxide. The secondary particles preferably have a D50 of 1 μm to 15 μm, and a D90 of 50 μm or less.

[0009] H2Ti used in this embodiment 12 O 25 In the case of H2Ti, primary particles are granulated to form secondary particles. If the secondary particle diameter D50 is less than 1 μm, dispersibility deteriorates during electrode paste preparation, making it necessary to add excessive amounts of binder and / or solvent, resulting in a relative decrease in the active material concentration. On the other hand, if the secondary particle diameter D50 exceeds 15 μm or if D90 exceeds 50 μm, the smoothness of the electrode sheet is impaired and the H2Ti, which does not receive the contribution of carbon coating to electronic conductivity, is degraded. 12 O 25 If the proportion of is increased, the characteristics may be deteriorated.

[0010] H2Ti 12 O 25Although ZnO has a large specific surface area as a negative electrode active material and contributes to improved utilization, its poor electronic conductivity suggests the addition of a conductive additive such as graphite or carbon black in larger amounts. Therefore, in order to improve input / output performance, we investigated electrodes containing existing conductive additives such as graphite, carbon black, Ketjen black, vapor-grown carbon fiber (VGCF®), and multi-walled carbon nanotubes. We found that significantly increasing the additive amount to 10 wt% or more improved performance. However, due to the large specific surface area of ​​these additives, the mechanical strength of the electrode could not be achieved without also increasing the binder amount. Generally, binders such as polyvinylidene fluoride (PVDF), polyimide, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyacrylic acid all have low electronic conductivity, so increasing their amount reduces input / output performance. These studies revealed that using single carbon nanotubes as a conductive additive dramatically improved input / output performance at lower additive amounts compared to the existing conductive additives, achieving capacity not possible with conventional additives. This is because single carbon nanotubes have a large specific surface area. 12 O 25 It is believed that by covering the particle surface, electronic conductivity can be improved efficiently. Ordinary conductive additives such as carbon black are present not only on the surface of the active material but also in large amounts in the spaces between the active materials, so a large amount must be added to sufficiently increase the surface conductivity mentioned above. Multi-walled carbon nanotubes are mostly present on the surface of the active material, but because their fiber length is short, a large amount must be added to increase the surface conductivity, and they do not contribute much to the conductivity between the active materials. In contrast, single carbon nanotubes have long fiber length, so even a small amount contributes to the conductivity not only on the surface but also between the active materials as mentioned above, and it is believed that a small amount can satisfy the desired characteristics. Furthermore, the specific surface area is small, at 15m 2 / g, there is no problem even if the probability of the conductive additive existing on the surface is small. 12 O 25In the case of particles, it is thought that the conductivity between the primary particles that make up the active material is reflected in the input / output characteristics of charging and discharging at high current densities. When single carbon nanotubes are used, the mechanical strength of the electrode does not decrease even if the amount of binder is reduced by reducing the amount of addition. Therefore, this single carbon nanotube was used as a binder. 12 O 25 By incorporating or supporting H2Ti during synthesis, the input / output characteristics are improved without adding it during electrode production. 12 O 25 The input / output characteristics were further improved compared to when added to

[0011] Single carbon nanotube with H2Ti 12 O 25 As a method for containing or supporting H2Ti 12 O 25 A method of mixing and coating single carbon nanotubes onto the powder using a disperser or mechanochemicals, or H2Ti 12 O 25 In the manufacturing process of H2Ti 12 O 25 Alternatively, single carbon nanotubes may be wet mixed with the proton-exchanged lithium titanate obtained in the previous step in a slurry state, followed by simultaneous drying. 12 O 25 In order to disperse and adhere the single carbon nanotubes more uniformly to the particle surfaces, it is more preferable to wet mix them in a slurried state and then simultaneously dry them. The non-aqueous electrolyte secondary battery having the above-described configuration has improved electronic conductivity of the electrodes and improved input / output characteristics.

[0012] (2) Overall negative electrode configuration The specific surface area of ​​the active material used in the negative electrode according to this embodiment is not particularly limited. However, as will be described later, the above-mentioned negative electrode is made of an active material having a specific surface area of ​​15 m 2 / g or more, 150m 2 It is preferable to form the binder containing an active material having a SiO2 / g or less. 12 O25 is Li4Ti5O 12 It has the same low electronic conductivity and a specific surface area as Li4Ti5O 12 It is several times larger than H2Ti. 12 O 25 The surface area of ​​the graphite is more than 10 times larger than that of commonly used graphite. H2Ti 12 O 25 In a preferred manufacturing process, which will be described later, a titanium raw material and a lithium raw material are mixed and the precursor lithium titanate is synthesized at a relatively low temperature by hydrothermal synthesis. In order to complete the reaction sufficiently, it is preferable to use a titanium compound raw material having a particle size of 5 nm or more and 200 nm or less. 12 O 25 The titanium compound, which is the raw material, retains its shape as primary particles, and the specific surface area is 15m 2 / g or more, 150m 2 / g or less.

[0013] (3) Detailed configuration (3-1)H2Ti 12 O 25 Method for synthesizing and mixing single carbon nanotubes H2Ti 12 O 25 The synthesis method and the method for mixing single carbon nanotubes are described below. (3-1-1)H2Ti 12 O 25 Synthesis of H2Ti used in this embodiment 12 O 25 The synthesis method includes a lithium titanate synthesis step, a lithium titanate heat treatment step, a lithium / proton exchange step, and a proton exchanger heat treatment step. In the lithium titanate synthesis process, a titanium raw material containing a titanium compound and a lithium raw material containing a lithium compound are mixed, and the mixture is subjected to crystal growth by heat treatment or the like to obtain lithium titanate. More specifically, the mixture containing the titanium raw material and the lithium raw material is subjected to crystal growth by hydrothermal synthesis or the like. The titanium raw material is not particularly limited as long as it contains a titanium compound, and examples thereof include oxides such as TiO, Ti2O3, and TiO2, titanium oxide hydrates represented by TiO(OH)2 and TiO2·xH2O (x is optional), inorganic titanium compounds such as titanium chloride and titanium sulfate, and organic titanium compounds such as titanium isopropoxide and titanium butoxide. Of these, titanium oxide or titanium oxide hydrate is particularly preferred. When the titanium compound is particulate, the primary particle size is preferably 5 nm or more and 200 nm or less. By appropriately selecting the reaction conditions in the hydrothermal synthesis method, it is possible to obtain H2Ti while maintaining the primary particle shape of the titanium raw material. 12 O 25 Furthermore, if the primary particle size of the titanium compound is smaller than 5 nm, the particles will aggregate strongly, and if the aggregation cannot be broken down, there is a risk that unreacted portions will remain. If the primary particle size is larger than 200 nm, there is also a risk that the reaction will not progress to the interior of the particles. The lithium raw material is not particularly limited as long as it contains a lithium compound, and examples thereof include oxides such as LiO and LiO, salts such as LiCO and LiNO, and hydroxides such as LiOH. Among these, hydroxides such as LiOH are particularly preferred. The mixture containing the titanium raw material and the lithium raw material may be obtained by dry mixing the titanium raw material and the lithium raw material, or by dissolving or suspending the titanium raw material and the lithium raw material in a liquid such as water or ethanol.

[0014] The lithium titanate synthesis process includes a step of growing crystals by heat-treating a mixture containing the titanium and lithium raw materials. As a crystal growth method, a solid-phase reaction method, which is a general method for synthesizing ceramic fine particles, or a liquid-phase method such as a precipitation method, a sol-gel method, or a hydrothermal synthesis method can be used, but among them, the hydrothermal synthesis method is particularly preferred. When crystallizing using hydrothermal synthesis, TiO2 is the preferred titanium raw material, and LiOH·H2O is the preferred lithium raw material. Furthermore, it is preferable that the weight ratio of the lithium raw material to the titanium raw material be 1 or more (ratio of the amount of lithium raw material to the amount of titanium raw material is approximately 2.3 times). There are no particular restrictions on the reaction temperature and reaction time in hydrothermal synthesis, but a reaction temperature of 150°C or higher and a reaction time of 3 hours or more are preferred. Lithium titanate is obtained by crystal growth using hydrothermal synthesis. Lithium titanate includes Li2TiO3, Li2Ti2O4, LiTi2O4, and Li4Ti5O 12 Among them, Li2TiO3 is preferable. The lithium titanate obtained by hydrothermal synthesis can be recovered by known methods such as filtration, natural sedimentation, and centrifugation. Because the recovered lithium titanate contains unreacted LiOH, it is preferable to wash it. The solvent used for washing may be water or an inorganic acid such as low-concentration hydrochloric acid or nitric acid. After washing, the lithium titanate is dried by known methods such as a box dryer or spray dryer.

[0015] In the lithium titanate heat treatment process, the lithium titanate obtained in the lithium titanate synthesis process is heat-treated. This heat treatment removes solvent molecules that have infiltrated into the lithium titanate's crystal structure, and simultaneously converts the lithium titanate, whose main phase is Li2TiO3 with a rock-salt crystal structure, to Li2TiO3 with a monoclinic crystal structure. The composite crystal structure results in a more irregular arrangement of titanium atom lattice sites than Li2TiO3 with a single structure, either rock-salt or monoclinic. Therefore, this composite structure Li2TiO3 is less likely to convert to titanium dioxide, such as anatase or rutile, during the dehydration process in the subsequent heat treatment of the lithium titanate proton exchanger, compared to single-structure Li2TiO3. The heat treatment process for lithium titanate is carried out in air or an inert gas atmosphere such as nitrogen or argon. The heat treatment temperature is preferably 100°C or higher and 600°C or lower. At firing temperatures below 100°C, the phase change from the rock salt crystal structure to the monoclinic crystal structure is difficult to proceed, while at temperatures above 600°C, most of the rock salt crystal structure is transformed into the monoclinic crystal structure. The heat treatment temperature is more preferably 200°C to 500°C. The heat treatment time is preferably between 0.5 and 100 hours, more preferably between 1 and 30 hours.

[0016] In the lithium / proton exchange process, the lithium in the heat-treated lithium titanate is exchanged for protons. That is, the heat-treated lithium titanate is immersed in an acidic aqueous solution and subjected to a proton exchange reaction, thereby obtaining a proton-exchanged lithium titanate in which almost all of the lithium in the heat-treated lithium titanate has been exchanged for hydrogen. In this process, it is preferable to disperse the lithium titanate in the acidic aqueous solution, hold it for a certain period of time, and then separate it by filter filtration, centrifugation, or the like, and dry it. The acid used in the lithium / proton exchange step is preferably an aqueous solution of any concentration containing one or more of hydrochloric acid, sulfuric acid, and nitric acid, and more preferably dilute hydrochloric acid with a concentration of 0.1N to 1.0N. The treatment time when exchanging lithium with protons is 10 hours to 10 days, preferably 1 to 7 days. The treatment temperature when exchanging lithium with protons is preferably room temperature (20°C) or higher and lower than 100°C.

[0017] The proton-exchanged lithium titanate can be dried using a known method such as a box dryer or a spray dryer. A conductive additive such as carbon may be added to the proton-exchanged lithium titanate before drying. The conductive additive may be added by stirring the proton-exchanged lithium titanate in a slurry state. A dispersant may be added or a disperser may be used, if necessary. In the proton exchanger heat treatment step, the lithium titanate proton exchanger obtained in the lithium / proton exchange step is heat treated. The heat treatment causes the dehydration reaction of the proton exchanger to progress, resulting in titanium oxide H2Ti 12 O 25The heat treatment atmosphere can be air, an inert gas atmosphere such as nitrogen or argon, a hydrogen-containing atmosphere, or a reduced pressure, but an inert gas atmosphere or a reduced pressure atmosphere is preferred. The heat treatment temperature is preferably 200°C or higher and 600°C or lower, more preferably 260°C or higher and 500°C or lower. The heat treatment time is usually 0.5 to 100 hours, more preferably 1 to 30 hours. Since firing in an oxygen-containing atmosphere or at a high temperature of 600°C or higher promotes the formation of side reactions such as anatase and rutile, it is preferable to perform the heat treatment in the above-mentioned atmosphere, temperature, and time.

[0018] H2Ti obtained by proton exchanger heat treatment process 12 O 25 In powder XRD measurement using Cu-Kα as a radiation source, the peaks may be at the same positions as those in Patent Document 3 (JP 2008-255000 A). The peak intensity ratios may also be different. The difference in peak intensity ratios is due to the reduced crystal growth of certain crystal planes caused by the miniaturization of primary particles. In particular, the peaks originating from the (110) plane, which appears near 25°, and the peaks originating from the (020) plane, which appears near 48°, may be significantly weaker in intensity or may overlap with neighboring peaks, making them difficult to distinguish. In addition, titanium dioxide such as anatase and rutile contains H2Ti as an impurity. 12 O 25 It may be contained in small amounts, but if it is a small amount, it is H2Ti 12 O 25 It has almost no effect on the battery characteristics of H2Ti. 12 O 25 The titanium dioxide content of H2Ti is determined by powder XRD measurement. 12 O 25 It is calculated as the ratio I1 / I0 of the peak height I0 that appears at around 28° on the (003) plane of H2Ti to the peak height I1 of the main peak of titanium dioxide (the (101) plane that appears at around 25° on anatase, and the (110) plane that appears at around 27° on rutile). The peak height is measured from the base of a straight line connecting the heights of the minimum points before and after the peak to the peak apex.12 O 25 The I1 / I0 ratio is preferably 5 times or less, and more preferably 3 times or less.

[0019] H2Ti 12 O 25 The particle shape is not particularly limited, but isotropic shapes such as spheres and polyhedrons are preferred in order to increase the packing density of the negative electrode layer. Also, H2Ti 12 O 25 The particle shape of the H2Ti is preferably a secondary particle formed by aggregation of primary particles. The secondary particle shape makes it possible to easily separate the H2Ti, which is an active material in the production of the negative electrode layer of a lithium ion battery, from the H2Ti. 12 O 25 This improves handling and powder properties such as fluidity, adhesion, and packing, leading to further improvements in battery properties. The preferred average secondary particle diameter D50 is in the range of 1 μm to 15 μm, with D90 being 50 μm or less. If the secondary particle diameter D50 is less than 1 μm, dispersibility during electrode paste preparation deteriorates. On the other hand, if the secondary particle diameter D90 exceeds 50 μm, the smoothness of the electrode sheet is impaired, and the H2Ti, which does not receive the contribution of electronic conductivity from carbon coating, is not obtained. 12 O 25 The specific surface area, which depends on the particle size, is 15m 2 / g or more, 150m 2 / g or less is desirable. 2 If the saturation is less than 150m / g, the primary particle size will be large, and the current density will increase when the battery is made, resulting in a decrease in input / output characteristics. 2 When the HTO bulk density exceeds 1 / g, the bulk density of the HTO becomes too high, so that a large amount of binder is required to maintain mechanical strength, and the capacity decreases significantly due to a decrease in electrode density. A preferred method for granulating and forming secondary particles is to spray-dry a slurry containing a proton exchanger of lithium titanate using a spray dryer or the like.

[0020] (3-1-2) Mixing of single carbon nanotubes Single carbon nanotube with H2Ti 12 O25 As a method of mixing and depositing H2Ti 12 O 25 In the manufacturing process of H2Ti 12 O 25 Alternatively, single carbon nanotubes may be wet mixed with the proton-exchanged lithium titanate obtained in the previous step in a slurry state, followed by simultaneous drying. 12 O 25 In order to more uniformly disperse and adhere the single carbon nanotubes to the particles, it is more preferable to wet mix the single carbon nanotubes into the slurry and dry them simultaneously. In particular, it is preferable that, immediately before the drying of the lithium titanate proton exchanger, the proton exchanger is made into a slurry, and the single carbon nanotubes are mixed and stirred, and then spray-dried using a spray dryer or the like, so that the single carbon nanotubes are mixed and adhered to the particle surfaces of the lithium titanate proton exchanger. When single carbon nanotubes are added to a slurry of a proton-exchanged material of lithium titanate, a dispersant or a surfactant may be added, and dispersion using a wet bead mill or a media-less disperser may also be used. There is no particular limitation on the content or loading amount of single carbon nanotubes, but H2Ti 12 O 25 It is preferable that the content of H2Ti is 0.3 wt% or more and 5 wt% or less. 12 O 25 However, if the amount is less than 0.3 wt%, the input and output will be equal to or less than when conventional additives are added to the electrode, and if the amount exceeds 5 wt%, the concentration of the negative electrode active material will be relatively low, leading to a decrease in capacity, as well as a decrease in adhesive strength and a decrease in conductivity.On the other hand, if the amount of binder is increased in order to improve strength, this will cause a further decrease in capacity.

[0021] (3-2) Battery manufacturing method A method for producing a battery having the configuration of this embodiment will be described below. H2Ti containing or supported on single carbon nanotubes 12 O 25Examples of binders for the negative electrode include polyvinyl alcohol, polyacrylic acid, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), hydroxypropyl cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyamideimide, and polyamide. These may be used alone or in combination. Conductive additives are generally unnecessary, but since the electronic conductivity of single carbon nanotubes is limited to adjacent particles, carbon blacks such as acetylene black and Ketjen black, or VGCF (registered trademark), may be mixed in to enhance long-range electronic conductivity across multiple particles.

[0022] The active material for the positive electrode is a lithium-containing transition metal composite oxide (lithium-containing transition metal composite oxide) capable of absorbing and releasing lithium ions. Examples of lithium-containing transition metal composite oxides include layered lithium-containing transition metal composite oxides such as LiCoO2, LiNiO2, LiNiCoMnO2, and LiMn2O4. These may be used alone or in combination of two or more. Examples of binders include polyvinyl alcohol, polyacrylic acid, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), PVDF, polyamideimide, and polyimide. These may be used alone or in combination of two or more. Examples of conductive additives include carbon blacks such as acetylene black and ketjen black, and single carbon nanotubes. These may be used alone or in combination of two or more. The positive electrode is manufactured by dispersing the positive electrode active material, binder, conductive additive, etc. in a solvent such as N-methyl-2-pyrrolidone (NMP), applying the dispersion to one or both sides of a current collector, drying, and then, if necessary, performing a pressing process such as a calendaring process. However, the manufacturing method of the positive electrode is not limited to the above-mentioned method, and other manufacturing methods may also be used. Conventionally known materials such as aluminum, aluminum alloys, and stainless steel can be used as the current collector. The thickness of the current collector is not particularly limited, but is usually 1 to 50 μm.

[0023] (3-2-1) Example of battery configuration The nonaqueous electrolyte secondary battery of this embodiment may have a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte. The non-aqueous electrolyte may be prepared by dissolving a lithium salt in an organic solvent. Examples of suitable organic solvents include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethylformamide, dioxolane, and acetonitrile. These may be used alone or in combination. Examples of suitable lithium salts include LiClO4, LiBF4, LiPF6, LiCF3SO3, LiCF3CO2, and LiAsF6. One or more of these may be used. In addition, 3-propane sultone, diphenyl disulfide, cyclohexylbenzene, and vinylene carbonate can be added to improve cycle performance. Microporous membranes made of polyolefins such as polyethylene (PE) and polypropylene (PP), and nonwoven fabrics such as cellulose can be used as separators. Laminated separators composed of a porous layer primarily containing inorganic fillers can also be used.

[0024] (3-2-2) Battery type The shape of the nonaqueous electrolyte secondary battery according to this embodiment is not particularly limited, and may be, for example, any of a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, a prismatic type, a large type used in an electric vehicle, etc. [Example]

[0025] Although the method for producing a battery according to the embodiment will be described in detail below, the present invention is not limited thereto. Example 1 (Preparation of positive electrode) The positive electrode was fabricated by applying LiNiCoMn (5:2:3)O2 as the positive electrode active material, carbon black as the conductive additive, and PVDF as the binder in a weight ratio of 96:2:2 to aluminum foil, drying, and pressing. 12 O 25 (hereinafter sometimes referred to as HTO / SCNT) was synthesized as follows. 12 O 25 The content of single carbon nanotubes was set to 1 wt %. (Negative electrode active material H2Ti 12 O 25 Synthesis of (lithium titanate synthesis process) 1000 g of lithium hydroxide monohydrate (Wako Pure Chemical Industries, Ltd.) and 5000 mL of ion-exchanged water were placed in a 10 L titanium hydrothermal reaction vessel and stirred with a stirrer until all of the lithium hydroxide monohydrate was dissolved. 1000 g of titanium dioxide (Sakai Chemical Industry Co., Ltd. SSP-25, crystalline form: anatase, primary particle size: approximately 5 nm) was added and mixed with stirring to obtain a mixed slurry of titanium oxide and lithium hydroxide monohydrate. The mixed slurry was stirred while undergoing a hydrothermal reaction in an autoclave at 180°C for 24 hours. The slurry after the hydrothermal reaction was subjected to suction filtration using a Buchner funnel and 5C filter paper, and the solids on the filter paper were collected. The collected solids were resuspended in a 0.05 mol / L hydrochloric acid solution to a concentration of approximately 10 g / L and stirred for 1 hour. The entire volume was then subjected to suction filtration using a Buchner funnel and 5C filter paper, and the solids on the filter paper were collected. The filtered solids were placed in a porcelain dish and dried overnight in a box dryer set at 130 °C. The dried solids were crushed in a dry coffee mill to obtain lithium titanate. (lithium titanate heat treatment process) The lithium titanate was placed in an alumina firing container and heated to 300°C in the atmosphere at a heating rate of 200°C / h using a box-type firing furnace, held at 300°C for 5 hours, and then naturally cooled to room temperature in the furnace to obtain a heat-treated lithium titanate product. The crystalline phase of the heat-treated lithium titanate was identified using a powder X-ray diffractometer (Rigaku RINT TTR-III, X-ray source: CuKα), and it was confirmed that the structure was a mixture of rock salt crystal structure and monoclinic crystal structure of Li2TiO3.

[0026] (Lithium / proton exchange process for heat-treated lithium titanate) The heat-treated lithium titanate product was suspended in a 0.5 mol / L aqueous hydrochloric acid solution to a concentration of approximately 25 g / L, and the suspension was stirred for 12 hours using a stirrer. The stirring was then stopped, and the suspension was allowed to stand for another 12 hours. The slurry was then filtered under suction using a Buchner funnel and 5C filter paper to obtain a solid. The resulting solid was resuspended in ion-exchanged water and filtered under suction again for washing. The washing process was repeated until the conductivity of the filtrate finally fell below 100 μS / cm. The proton-exchanged product is Li2TiO3 in which the lithium has been replaced by protons, and therefore corresponds to the composition of H2TiO3. Heat treatment of the proton-exchanged product causes dehydration, resulting in the formation of H2Ti 12 O 25 The solid content (cake) after the water washing is formed. 12 O 25 To calculate the H2Ti content, a small amount of solid matter was weighed into a porcelain crucible, dried overnight at 130°C in a box dryer, and then fired in an atmospheric firing furnace at 350°C for 5 hours in a nitrogen stream. After cooling to room temperature, the solid matter was removed from the furnace and weighed. After washing with water, the H2Ti content in the solid matter was 12 O 25 The content (concentration) is given by the following formula 1. Formula 1: H2Ti 12 O 25 Content (concentration) = Powder weight after firing (g) / Solid weight after washing (g) × 100 (%) After firing, the powder was lightly crushed in a mortar and pestle, and then the H2Ti was measured using the BET single-point method (Mountec: Macsorb HM-1220). 12 The specific surface area of ​​O25 was measured. 12 O 25 The specific surface area of 2 / g. The crystal phase was identified by powder X-ray diffraction (Rigaku RINT TTR-III, X-ray source: CuKα). 12 O 25 and anatase TiO2. 12 O 25 The ratio I1 / I0 of the peak height I0 appearing at about 28° on the (003) plane of TiO2 to the peak height I1 on the (101) plane appearing at about 25° on anatase TiO2 was 0.7.

[0027] (Mixing of single carbon nanotubes) H2Ti 12 O 25 The solids (cake) after washing were suspended in ion-exchanged water and stirred to a concentration of approximately 200 g / L, to prepare a proton exchanger slurry. Furthermore, an ammonium polyacrylate dispersant (KF Chemicals, Dispersant A40) was added as a dispersant at 5 wt% relative to the solids, and the mixture was stirred. The slurry was passed through a standard SUS sieve with a mesh size of 45 μm, and it was confirmed that no particles remained on the sieve. A single carbon nanotube dispersion liquid (Kusumoto Chemicals) was used, converted to a solid content of H2Ti. 12 O 25 The mixture was weighed out so as to be 1 wt % based on the total weight of the powder, added to the slurry, and the mixture was stirred for 1 hour. The mixed slurry of proton exchange material and single carbon nanotubes was dried and granulated using a spray dryer (Fujisaki Electric: Four-fluid nozzle type micromist spray dryer MDL-050M) under the operating conditions of a hot air inlet temperature of 250°C, an outlet temperature of 110°C, and an air flow rate of 30 L / min. (Heat treatment process of proton exchanger) The resulting granulated mixture of proton exchange material and carbon nanotubes was fired in an atmospheric firing furnace in a nitrogen stream at 350°C for 5 hours.

[0028] (Measurement of secondary particle size) H2Ti obtained by the above process 12 O 25 The secondary particle size of the HTO / SCNT mixture was measured using a laser diffraction particle size analyzer (HORIBA, Ltd.: LA-950). 0.025% sodium hexametaphosphate was used as the dispersion medium, ultrasonic dispersion was performed on the LA-950 for 1 minute, and the refractive index was set to 2.52. The particle size distribution measured using the particle size analyzer showed a single peak ranging from approximately 0.5 μm to 20 μm, with a D50 of 2.6 μm and a D90 of 8.1 μm. Regarding the definitions of D50 and D90, the particle size distribution was expressed as the 50th and 90th smallest particle sizes in the volume-based particle size distribution, respectively. Here, the particle size distribution measurements reflect the state of aggregation, not the dispersion of primary particles, and are therefore expressed as secondary particle sizes. Furthermore, particle shape was observed using a scanning electron microscope (JEOL Ltd.: JSM-7000F), and it was confirmed that granulated secondary particles of about 1 μm to 10 μm were formed.

[0029] (Preparation of negative electrode) The HTO / SCNT thus prepared was applied to Cu foil using SBR and CMC as binders in a weight ratio of 97:1:2, followed by drying and pressing to prepare a negative electrode. (Fabrication of card-type cells) Each electrode was cut to the specified size, stacked with a polyethylene separator between them, and packaged in resin-laminated aluminum foil. The electrolyte was then poured into the packaging container using 1M LiPF6 / EC:DEC (3:7), and the container was sealed to create a card-type cell. In this cell, the ratio of the negative electrode capacity to the opposing positive electrode capacity was set to 0.9, and the cell capacity was regulated by the positive electrode capacity.

[0030] Example 2 The positive electrode active material was LiNiCoMn (5:2:3)O2, the conductive additive carbon black, and the binder PVDF. The positive electrode was fabricated by applying the mixture to an Al foil in a weight ratio of 96:2:2, drying, and pressing. The negative electrode active material was H2Ti (as in Example 1). 12 O 25 In the single carbon nanotube mixing process in the synthesis of H2Ti 12 O 25 HTO / SCNT was synthesized in the same manner as in Example 1, except that the content of HTO was 0.5 wt %. The resulting HTO / SCNT was applied to an Al foil using PVDF as a binder to prepare a negative electrode. The weight ratio of (HTO / SCNT) / PVDF was 96 / 4. A card-type cell was then prepared in the same manner as in Example 1.

[0031] Example 3 The positive electrode active material was LiNiCoMn (5:2:3)O2, the conductive additive carbon black, and the binder PVDF. The positive electrode was fabricated by applying the mixture to an Al foil in a weight ratio of 96:2:2, drying, and pressing. The negative electrode active material was H2Ti (as in Example 1). 12 O 25 In the single carbon nanotube mixing process in the synthesis of H2Ti 12 O 25 HTO / SCNT was synthesized in the same manner as in Example 1, except that the content of HTO / SCNT was 3 wt %. The resulting HTO / SCNT was applied to an Al foil using PVDF as a binder to prepare a negative electrode. The weight ratio of (HTO / SCNT) / PVDF was 96 / 4. A card-type cell was then prepared in the same manner as in Example 1.

[0032] (Comparative Example 1) H2Ti in Example 1 12 O 25 In the synthesis of H2Ti, the same procedure as in Example 1 was used except that single carbon nanotubes were not added in the mixing step of single carbon nanotubes. 12 O25 was synthesized. In Example 1, H2Ti 12 O 25 Electrodes and batteries were fabricated in the same manner except that a negative electrode containing a single carbon nanotube, SBR, and CMC in a weight ratio of 96:1:1:2 was used. (Comparative Example 2) In Comparative Example 1, H2Ti 12 O 25 Electrodes and batteries were fabricated in the same manner except that a negative electrode containing acetylene black and PVDF in a weight ratio of 80:10:10 was used. (Comparative Example 3) In Comparative Example 1, H2Ti 12 O 25 Electrodes and batteries were fabricated in the same manner except that a negative electrode containing acetylene black, SBR, and CMC in a weight ratio of 90:5:2:3 was used.

[0033] Comparative Example 4 An electrode and a battery were produced in the same manner as in Example 1, except that a multi-wall carbon nanotube dispersion liquid was used instead of a single carbon nanotube dispersion liquid. (Comparative Example 5) An electrode and a battery were produced in the same manner as in Example 1, except that the content of the single carbon nanotube was set to 0.2 wt %. (Comparative Example 6) An electrode and a battery were produced in the same manner as in Example 1, except that the content of the single carbon nanotube was set to 6 wt %.

[0034] (Comparative Example 7) The positive electrode was fabricated by applying LiNiCoMn (5:2:3)O2 as the positive electrode active material, carbon black as the conductive additive, and PVDF as the binder in a weight ratio of 96:2:2 to an Al foil, drying, and pressing. 12 O 25 (HTO / SCNT) was synthesized as follows: H2Ti 12 O 25The content of single carbon nanotubes was set to 1 wt %. As the negative electrode active material, H2Ti of Example 1 was used. 12 O 25 In the lithium titanate synthesis step in the synthesis of H2Ti, the same procedure as in Example 1 was repeated except that titanium dioxide (R-310 manufactured by Sakai Chemical Industry Co., Ltd., crystal form: rutile, primary particle size: approximately 150 nm) was used as the titanium raw material. 12 O 25 H2Ti was synthesized. 12 O 25 The specific surface area of 2 / g. The diffraction pattern obtained by powder X-ray diffraction was 12 O 25 , and were assigned to anatase TiO2 and rutile TiO2. 12 O 25 The ratio I1 / I0 of the peak height I0 of the (003) plane of anatase TiO2, which appears at around 28°, to the peak height I1 of the (101) plane of anatase TiO2, which appears at around 25°, was 4.8. The ratio I1 / I0 of the peak height I1 of the (110) plane of rutile TiO2, which appears at around 27°, was 2.9. H2Ti 12 O 25 and single carbon nanotubes were mixed in the same manner as in Example 1. The particle size distribution of the obtained HTO / SCNT was a bimodal distribution ranging from approximately 0.1 μm to approximately 30 μm, with D50 being 0.7 μm and D90 being 9.3 μm. The HTO / SCNT thus produced was applied to Cu foil using SBR and CMC as binders in a weight ratio of 97:1:2, followed by drying and pressing to produce a negative electrode. However, due to significant peeling of the coating from the Cu foil, the amount of CMC was increased to 93:1:6 to produce an electrode. Thereafter, electrodes and batteries were produced in the same manner as in Example 1.

[0035] (Comparative Example 8) The positive electrode was fabricated by applying LiNiCoMn (5:2:3)O2 as the positive electrode active material, carbon black as the conductive additive, and PVDF as the binder in a weight ratio of 96:2:2 to an Al foil, drying, and pressing. 12 O 25 (HTO / SCNT) was synthesized as follows: H2Ti 12 O 25 The content of single carbon nanotubes was set to 1 wt %. As the negative electrode active material, H2Ti of Example 1 was used. 12 O 25 In the single carbon nanotube mixing step in the synthesis of (1), the slurry containing the proton exchanger and single carbon nanotubes was filtered under suction using a Buchner funnel and 5C filter paper, and the solid matter on the filter paper was collected. The filtered solid matter was dried overnight in a box dryer set at 130°C, and the dried solid matter was crushed in a dry coffee mill to obtain HTO / SCNT. The particle size distribution of the obtained HTO / SCNT was bimodal, ranging from approximately 0.5 μm to 300 μm, with a D50 of 3.8 μm and a D90 of 56.8 μm. The HTO / SCNT thus prepared was used to coat Cu foil with SBR and CMC as binders in a weight ratio of 97:1:2, followed by drying and pressing to prepare a negative electrode. However, the two-dimensional continuity of the coating film decreased, resulting in large variations in the amount of coating and reduced in-plane uniformity. Subsequently, electrodes and batteries were prepared in the same manner as in Example 1.

[0036] (Comparison between Examples and Comparative Examples) For each battery in these examples and comparative examples, three cycles of 3V 0.2C CCCV charging and 1V termination 0.2C discharge were performed, followed by 3V 1C CCCV charging, 1V termination 20C discharge, and 1V 0.2C CCCV discharge, followed by 3V 20C CCCV charging. Table 1 shows the initial discharge capacity, the ratio of 20C discharge capacity to 0.2C discharge capacity as an output evaluation, and the ratio of CC charge capacity to total charge capacity at 20C charge as an input evaluation. In the examples, input and output at 20C were both 60% or more and capacities of 70 mAh or more were obtained, whereas in Comparative Example 1, where only single carbon nanotubes were mixed, input and output decreased. In Comparative Examples 2 and 3, other conductive additives significantly decreased the input and output, and the initial capacity was also low. In Comparative Example 4, multi-walled carbon nanotubes were used during synthesis, but no improvement in input and output was observed. On the other hand, when the content during synthesis was low, as in Comparative Example 5, the effect was difficult to obtain. Conversely, in Comparative Example 6, when the content was too high, the capacity and input / output decreased due to a decrease in conductivity, which is thought to be due to a decrease in the adhesive strength between the active material layer and the Al current collector. In Comparative Example 7, the primary particle size of HTO increased, which reduced the specific surface area and increased the current density, slowing the insertion and desorption of lithium ions into the active material particles during charge and discharge, resulting in a significant decrease in both the initial capacity and the 20°C input / output. In Comparative Example 8, the 20°C input / output decreased due to variations in the coating amount on the negative electrode. [Table 1]

Claims

1. a positive electrode having a lithium-containing transition metal composite oxide as an active material; a negative electrode; Non-aqueous electrolyte Equipped with The negative electrode is The general formula is H ２ Ti １２ O ２５ a mixture of titanium oxide particles and single carbon nanotubes, which are titanium oxide particles represented by the formula: Binder and Including, the mixture of titanium oxide particles and single carbon nanotubes contains or supports single carbon nanotubes in an amount of 0.3 wt % to 5.0 wt % relative to the titanium oxide; The secondary particle diameter D50 of the titanium oxide particles is 1 μm or more and 15 μm or less, and the secondary particle diameter D90 is 50 μm or less. Nonaqueous electrolyte secondary battery.

2. The titanium oxide particles have a specific surface area of ​​15 m ２ / g or more 150m ２ / g or less, 2. The nonaqueous electrolyte secondary battery according to claim 1.

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