Non-aqueous electrolyte secondary battery

Abstract

The purpose of the present invention is to obtain a non-aqueous electrolyte secondary battery having favorable low-temperature characteristics and cycle characteristics by suppressing reduction reaction of an electrolyte and reducing generation of gas.　A non-aqueous electrolyte secondary battery according to the present invention comprises: a positive electrode containing, as an active material, a transition metal composite oxide containing lithium; a negative electrode containing H2Ti12O25; and a non-aqueous electrolyte. The non-aqueous electrolyte contains a solvent and 1-8 wt%, with respect to the solvent, of a fluorinated cyclic phosphoric acid ester, the solvent having any one of a cyclic carbonate, a non-cyclic carbonate, and a lactone.

Description

Nonaqueous electrolyte secondary battery

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

[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. High capacity is a key requirement for lithium-ion secondary batteries in these new markets, especially EVs. 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. Development of higher-voltage cathodes and higher-capacity Si-compound anodes has not yet achieved cycle times equivalent to or superior to current batteries, preventing their practical application. Furthermore, while new battery technologies such as solid electrolyte batteries and organosulfur / Li-metal batteries have been developed recently, many challenges remain, including productivity, and large-scale practical applications remain in the early stages. Development of charging infrastructure, including wireless charging, is also underway for lithium-ion secondary batteries as part of their infrastructure. Therefore, if a lithium ion secondary battery with excellent input / output characteristics can be developed and charged in a short time, the above-mentioned problem of increasing capacity can be solved. For example, Toshiba SCiB (registered trademark) shown in Non-Patent Document 1 (Toshiba Review Vol. 71 No. 2 pp. 44) is compatible with charging equivalent to 20 C, and a battery capable of charging to nearly 100% in a few minutes has been developed. In addition, this battery uses the general formula Li ４ Ti ５ O １２ The active material shown in is used for the negative electrode, but the charge / discharge potential of this material is 1.5 V (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, as overvoltage increases. On the other hand, ４ Ti ５ O １２However, there was a problem in that the original theoretical capacity was 175 mAh / g, which was lower than the theoretical capacity of graphite, 372 mAh / g.

[0003] Toshiba Review Vol. 71 No. 2 pp. 44 JP-A-6-275263

[0004] As a material that solves the problems of the above-mentioned negative electrode materials, titanium oxide H ２ Ti １２ O ２５ There is. ２ Ti １２ O ２５ The theoretical capacity of H is 300 mAh / g, which is comparable to that of graphite. ２ Ti １２ O ２５ When a battery is used with a common cyclic carbonate, acyclic carbonate, lactone, or the like as an electrolyte, gas is generated due to reductive decomposition of the electrolyte during charging and discharging, which can lead to performance degradation and reduced safety.

[0005] The present invention has been made to solve the above-mentioned problems, and aims to obtain a nonaqueous electrolyte secondary battery with good low-temperature characteristics and cycle characteristics by suppressing the reduction reaction of the electrolyte and reducing gas generation.

[0006] In order to achieve the above object, the first invention is a positive electrode using a transition metal composite oxide containing lithium as an active material, and a titanium oxide H ２ Ti １２ O ２５ and a nonaqueous electrolyte solution, the nonaqueous electrolyte solution containing a solvent, an electrolyte, and 1 to 8% by weight of a fluorinated cyclic phosphate ester relative to the solvent, the solvent containing any one of a cyclic carbonate, an acyclic carbonate, and a lactone.

[0007] When configured in this manner, the fluorinated cyclic phosphate ester forms a solid electrolyte interface (SEI) on the negative electrode during charge and discharge, suppressing the reduction reaction of the solvent. This suppresses gas generation and results in a nonaqueous electrolyte secondary battery with good low-temperature characteristics and cycle characteristics.

[0008] A second invention is the nonaqueous electrolyte secondary battery according to the first invention, wherein the solvent is a mixed solvent of a cyclic carbonate and an acyclic carbonate or a mixed solvent of a lactone and an acyclic carbonate.

[0009] With this configuration, the high dielectric constant cyclic carbonate or lactone facilitates ionization of lithium ions, and the low viscosity acyclic carbonate facilitates the movement of lithium ions, resulting in a non-aqueous electrolyte secondary battery with good electrical conductivity.

[0010] A third invention is the nonaqueous electrolyte secondary battery according to the first invention, wherein the cyclic carbonate is ethylene carbonate or propylene carbonate.

[0011] Such a configuration provides a high dielectric constant that is more suitable as a solvent for a non-aqueous electrolyte secondary battery, resulting in a non-aqueous electrolyte secondary battery with good electrical conductivity.

[0012] A fourth invention is the nonaqueous electrolyte secondary battery according to the first invention, wherein the acyclic carbonate is diethyl carbonate, dimethyl carbonate, or ethyl methyl carbonate.

[0013] This configuration provides a low viscosity that is suitable as a solvent for non-aqueous electrolyte secondary batteries, resulting in a non-aqueous electrolyte secondary battery with good electrical conductivity.

[0014] A fifth invention is the nonaqueous electrolytic secondary battery of the first invention, wherein the lactone is γ-butyrolactone.

[0015] Such a configuration provides a high dielectric constant that is more suitable as a solvent for a non-aqueous electrolyte secondary battery, resulting in a non-aqueous electrolyte secondary battery with good electrical conductivity.

[0016] According to this invention, a non-aqueous electrolyte secondary battery can be obtained that suppresses gas generation and has good low-temperature characteristics and cycle characteristics.

[0017] (1) Overall Configuration The nonaqueous electrolyte secondary battery according to one embodiment of the present invention comprises a positive electrode having a lithium-containing transition metal composite oxide as an active material, and a H ２ Ti １２ O ２５and a non-aqueous electrolyte solution. The non-aqueous electrolyte solution contains a solvent and 1 to 8 wt % of a fluorinated cyclic phosphate ester relative to the solvent. The solvent contains any one of a cyclic carbonate, an acyclic carbonate, and a lactone.

[0018] (2) Overall Structure of the Negative Electrode The specific surface area of ​​the active material used in the negative electrode according to this embodiment is not particularly limited. However, the above-described negative electrode is ２ / g or more, 150m ２ It is preferable to form the electrode using a binder containing an active material having a densitometric value of 1 / g or less. ２ Ti １２ O ２５ The surface area of ​​H is more than 10 times larger than that of commonly used graphite. ２ Ti １２ O ２５ In a preferred manufacturing process of the present invention, which will be described later, a titanium raw material and a lithium raw material are mixed and a precursor lithium titanate is synthesized at a relatively low temperature by a hydrothermal synthesis method, and in order to complete the reaction sufficiently, it is preferable to use a titanium compound as a raw material having a particle size of 5 nm or more and 200 nm or less. ２ Ti １２ O ２５ The shape of the raw material titanium compound remains as primary particles, and the specific surface area is 15m ２ / g or more, 150m ２ / g or less.

[0019] (3) Detailed configuration (3-1)H ２ Ti １２ O ２５ Synthesis of and mixing of single carbon nanotubes (3-1-1)H ２ Ti １２ O ２５ Synthesis of H used in this embodiment ２ Ti １２ O ２５The 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 step, 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 TiO, Ti ２ O ３ , TiO ２ Titanium oxide such as TiO(OH) ２ , TiO ２ ・xH ２ Examples of titanium compounds include titanium oxide hydrate represented by the formula (x is arbitrary), inorganic titanium compounds such as titanium chloride and titanium sulfate, and organic titanium compounds such as titanium isopropoxide and titanium butoxide. Among these, titanium oxide or titanium oxide hydrate is particularly preferred. When the titanium compound is in particulate form, the primary particle diameter is preferably 5 nm or more and 200 nm or less. By appropriately selecting reaction conditions in the hydrothermal synthesis method, it is possible to produce titanium compounds by hydrothermal synthesis while maintaining the primary particle shape of the titanium raw material. ２ Ti １２ O ２５ Furthermore, when the primary particle size of the titanium compound is smaller than 5 nm, the particles tend to aggregate strongly, and if the aggregation does not dissolve, there is a risk that unreacted portions will remain. When the primary particle size is larger than 200 nm, there is also a risk that the reaction will not progress to the inside of the particles. The lithium raw material is not particularly limited as long as it contains a lithium compound, and for example, Li ２ O, Li ２ O ２ oxides such as Li ２ CO ３ , LiNO ３ 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.

[0020] The lithium titanate synthesis process includes a step of growing crystals by heat-treating a mixture containing the titanium raw material and the lithium raw material. As a method for growing crystals, 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 by the hydrothermal synthesis method, TiO is used as the titanium raw material. ２ is preferred, and the lithium raw material is LiOH.H ２ O is preferred. Furthermore, it is preferred that the weight ratio of the lithium raw material to the titanium raw material is 1 time or more (ratio of the amount of material of the lithium raw material to the amount of material of the titanium raw material is about 2.3 times) or more. There are no particular restrictions on the reaction temperature and reaction time in hydrothermal synthesis, but a reaction temperature of 150°C or more and a reaction time of 3 hours or more are preferred. Lithium titanate can be obtained by crystal growth through hydrothermal synthesis. As lithium titanate, Li ２ TiO ３ , Li ２ Ti ２ O ４ , LiTi ２ O ４ , Li ４ Ti ５ O １２ Among them, Li ２ TiO ３ Preferably, the lithium titanate obtained by hydrothermal synthesis can be recovered by known methods such as filtration, natural sedimentation, and centrifugation. Since 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.

[0021] In the lithium titanate heat treatment step, the lithium titanate obtained in the lithium titanate synthesis step is heat treated. This heat treatment removes solvent molecules that have penetrated into the crystalline structure of the lithium titanate, and at the same time, the lithium titanate is converted into Li (Li ) ... ２ TiO ３is the main phase, but some of it has a monoclinic crystal structure ２ TiO ３ By providing a composite crystal structure, the arrangement of the lattice sites of titanium atoms changes to that of Li, which has a single structure of a rock salt crystal structure or a monoclinic crystal structure. ２ TiO ３ Therefore, the Li ２ TiO ３ is a single-structure Li ２ TiO ３ Compared to the above, the lithium titanate proton-exchanged material is less likely to change to titanium dioxide such as anatase or rutile during the dehydration process in the subsequent heat treatment step of the lithium titanate proton-exchanged material. The lithium titanate heat treatment step 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 a firing temperature 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 converted to 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.

[0022] In the lithium / proton exchange process, the lithium in the heat-treated lithium titanate is exchanged with protons. That is, by immersing the heat-treated lithium titanate in an acidic aqueous solution and applying a proton exchange reaction, a proton-exchanged lithium titanate is obtained in which almost all of the lithium in the heat-treated lithium titanate has been exchanged with 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 filtration or centrifugation, and dry it. The acid used in the lithium / proton exchange process is preferably an aqueous solution of any concentration containing one or more of hydrochloric acid, sulfuric acid, and nitric acid, with dilute hydrochloric acid having a concentration of 0.1 N to 1.0 N being more preferable. The treatment time for exchanging lithium with protons is 10 hours to 10 days, preferably 1 to 7 days. The treatment temperature for exchanging lithium with protons is preferably room temperature (20°C) or higher and less than 100°C.

[0023] The proton-exchanged lithium titanate can be dried using known methods 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 into a slurry. A dispersant may be added or a disperser may be used as needed. In the proton-exchanged lithium titanate heat-treatment step, the proton-exchanged lithium titanate obtained in the lithium / proton-exchange step is heat-treated. The heat treatment promotes a dehydration reaction of the proton-exchanged lithium titanate to form titanium oxide H. ２ Ti １２ O ２５ The heat treatment atmosphere can be air, an inert gas atmosphere such as nitrogen or argon, a hydrogen gas-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 and firing at a high temperature of 600°C or higher promotes the formation of anatase, rutile, and other side reactions, it is preferable to perform the heat treatment in the above-mentioned atmosphere, temperature, and time.

[0024] H obtained by the proton exchanger heat treatment process ２ Ti １２ O ２５ In powder XRD measurement using Cu-Kα as a radiation source, it is sufficient that the peaks are at the same positions as those in JP 2008-255000 A. The peak intensity ratio may also be different. The difference in peak intensity ratio is due to the fact that the crystal growth of a certain crystal plane is poor due to the miniaturization of the primary particles, and in particular, the peak derived from the (110) plane appearing around 25° and the peak derived from the (020) plane appearing around 48° become significantly weaker in intensity or overlap with neighboring peaks, making them difficult to distinguish. In addition, titanium dioxide such as anatase and rutile contains H as an impurity. ２ Ti １２ O ２５ It may be contained in small amounts, but if it is a small amount, ２ Ti １２O ２５ It has almost no effect on the battery characteristics. ２ Ti １２ O ２５ The titanium dioxide content is determined by powder XRD measurement. ２ Ti １２ O ２５ The peak height I appears at around 28° on the (003) plane of ０ and the peak height I of the main peak of titanium dioxide (the (101) plane that appears around 25° in anatase and the (110) plane that appears around 27° in rutile). １ Ratio I １ / I ０ The peak height is calculated as follows: The peak height is the height from the base of a straight line connecting the heights of the minimum points before and after the peak to the peak apex. ２ Ti １２ O ２５ I １ / I ０ is preferably 5 times or less, and more preferably 3 times or less.

[0025] H ２ Ti １２ O ２５ The particle shape of H is not particularly limited, but isotropic shapes such as spherical or polyhedral shapes are preferred in order to increase the packing density of the negative electrode layer. ２ Ti １２ O ２５ The particle shape of the secondary particles is preferably a secondary particle formed by aggregation of primary particles. By having the secondary particle shape, it is possible to easily obtain the active material H ２ Ti １２ O ２５ The handling and powder properties, such as fluidity, adhesion, and packing properties, are improved, leading to further improvements in battery properties. The preferred average secondary particle diameter D50 is in the range of 1 μm to 15 μm, and D90 is 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 H, which does not receive the contribution of electronic conductivity from carbon coating, is not obtained. ２ Ti １２ O ２５The 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 particle size is less than 150 m / g, the primary particle size becomes large, and the current density when used in a battery increases, resulting in a decrease in input / output characteristics. ２ When the bulk density of the HTO exceeds 1 / g, the bulk density of the HTO becomes too high, so that the amount of binder must be increased to maintain the mechanical strength, and the capacity decreases significantly due to a decrease in the electrode density. A preferred method for granulating and forming the secondary particles is to spray-dry a slurry containing a proton exchanger of lithium titanate using a spray dryer or the like.

[0026] Also, H ２ Ti １２ O ２５ Alternatively, single carbon nanotubes may be mixed and attached to the surface.

[0027] (3-1-2) Mixing of single carbon nanotubes ２ Ti １２ O ２５ As a method of mixing and depositing ２ Ti １２ O ２５ In the manufacturing process of ２ Ti １２ O ２５ Alternatively, single carbon nanotubes may be wet mixed with a proton-exchanged lithium titanate obtained in a previous step in a slurry state, followed by simultaneous drying. ２ Ti １２ O ２５In order to more uniformly disperse and coat the single carbon nanotubes on the particles, a method of wet mixing with a slurry and co-drying is more preferred. In particular, just before drying the lithium titanate proton-exchanged material, it is preferred to make the proton-exchanged material into a slurry form, stir and mix the single carbon nanotubes, and spray-dry using a spray dryer or the like, so that the single carbon nanotubes are mixed and coated on the particle surfaces of the lithium titanate proton-exchanged material. When adding single carbon nanotubes to the slurry of lithium titanate proton-exchanged material, a dispersant or surfactant may be added, or dispersion using a wet bead mill, medialess disperser, or the like may be used in combination. There are no particular restrictions on the content or loading amount of single carbon nanotubes, but H ２ Ti １２ O ２５ It is preferable that the content of H is 0.3% by weight or more and 5% by weight or less. ２ Ti １２ O ２５ If the amount is 0.3% by weight or more, input and output equal to or greater than that obtained when a conventional auxiliary is added to an electrode can be obtained, and if the amount is 5% by weight or less, the concentration of the negative electrode active material does not become too low relatively, making it possible to suppress a decrease in capacity and at the same time suppress a decrease in adhesive strength, and conversely, to suppress a decrease in conductivity.

[0028] (3-2) 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.

[0029] (3-2-1) Positive Electrode The active material of the positive electrode can be a lithium-containing transition metal composite oxide (lithium-containing transition metal composite oxide) that can absorb and release lithium ions. The lithium-containing transition metal composite oxide can be any of those used in conventional lithium secondary batteries, such as Li ｙ CoO ２ (where 0≦y≦1.1), Li ｚ NiO ２ (where 0≦z≦1.1), Li ｅ MnO ２ (where 0≦e≦1.1), Li ａ Co ｂ M１ １－ｂ O ２ (However, M １ is at least one metal element selected from the group consisting of Mg, Mn, Fe, Ni, Cu, Zn, Al, Ti, Ge, and Cr, and 0≦a≦1.1, 0<b<1.0.), Li ｃ Ni １－ｄ M ２ ｄ O ２ (However, M ２ is at least one metal element selected from the group consisting of Mg, Mn, Fe, Ni, Cu, Zn, Al, Ti, Ge, and Cr, and 0≦c≦1.1, 0<d<1.0.), Li ｆ Mn ｇ Ni ｈ Co １－ｇ－ｈ O ２ (wherein 0≦f≦1.1, 0<g<1.0, and 0<h<1.0) and the like. Only one of these may be used, or two or more may be used in combination.

[0030] Examples of binders that can be used for the positive electrode mixture layer include polysaccharides such as starch, polyvinyl alcohol, polyacrylic acid, CMC, hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose, and modified products thereof; thermoplastic resins such as polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), PVDF, polyethylene, polypropylene, polyamideimide, and polyamide, and modified products thereof; polymers having rubber-like elasticity such as ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, SBR, butadiene rubber, polybutadiene, fluororubber, and polyethylene oxide, and modified products thereof; and polyimide. One or more of these may be used.

[0031] Examples of the conductive additive for the positive electrode mixture layer include carbon blacks such as carbon nanotubes, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as aluminum powder, nickel powder, copper powder, and silver powder; conductive whiskers made of carbon fluoride, zinc oxide, potassium titanate, and the like; conductive metal oxides such as titanium oxide; and organic conductive materials such as polyphenylene derivatives. One or more of these may be used.

[0032] The positive electrode current collector can be similar to those used in the positive electrodes of conventional lithium secondary batteries. The material of the positive electrode current collector is not particularly limited, as long as it is a chemically stable electronic conductor in the resulting lithium secondary battery. Examples include aluminum, aluminum alloys, stainless steel, nickel, titanium, carbon, conductive resins, and composites of aluminum, aluminum alloys, or stainless steel with a carbon or titanium layer formed on the surface. Among these, aluminum or aluminum alloys are particularly preferred due to their light weight and high electronic conductivity. The positive electrode current collector can be, for example, a foil, film, sheet, net, punched sheet, lath, porous body, foam, or molded fiber body made from the above materials. The surface of the positive electrode current collector may also be subjected to a surface treatment to create irregularities. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 1 to 500 μm.

[0033] The positive electrode mixture layer preferably contains, for example, 60 to 98 wt % of the positive electrode active material, 1 to 15 wt % of the binder, and 0.1 to 10 wt % of the conductive additive, and the thickness of the positive electrode mixture layer is preferably 10 to 100 μm per side of the positive electrode current collector.

[0034] Next, a method for manufacturing a positive electrode will be described. A positive electrode active material, a binder, and a conductive additive are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) to prepare a paste or slurry-like positive electrode mixture-containing composition. Next, the positive electrode mixture-containing composition is applied to one or both sides of a current collector and then dried. Thereafter, the positive electrode is manufactured through a process of performing a pressing process such as a calendaring process as necessary. However, the manufacturing method for the positive electrode is not limited to the above-described method, and other manufacturing methods may also be used.

[0035] (3-2-2) Negative Electrode The active material of the negative electrode is titanium oxide, which can absorb and release lithium ions. ２ Ti １２ O ２５ can be used.

[0036] Examples of the binder for the negative electrode mixture layer include polysaccharides such as starch, polyvinyl alcohol, polyacrylic acid, CMC, hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose, and modified products thereof; thermoplastic resins such as polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), PVDF, polyethylene, polypropylene, polyamideimide, and polyamide, and modified products thereof; polymers having rubber-like elasticity such as ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, SBR, butadiene rubber, polybutadiene, fluororubber, and polyethylene oxide, and modified products thereof; and polyimide. One or more of these may be used.

[0037] Examples of the conductive auxiliary agent for the negative electrode mixture layer include carbon black such as carbon nanotubes, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as aluminum powder, nickel powder, copper powder, and silver powder; conductive whiskers made of carbon fluoride, zinc oxide, potassium titanate, and the like; conductive metal oxides such as titanium oxide; and organic conductive materials such as polyphenylene derivatives. One or more of these may be used.

[0038] The negative electrode current collector can be similar to those used in negative electrodes of conventional lithium secondary batteries. The material is not particularly limited as long as it is a chemically stable electronic conductor in the resulting lithium secondary battery. For example, copper, aluminum or aluminum alloy, stainless steel, nickel, titanium, carbon, conductive resin, or a composite material in which a carbon or titanium layer is formed on the surface of aluminum, aluminum alloy, or stainless steel can be used. Among these, copper, aluminum, or aluminum alloy is particularly preferred due to its light weight and high electronic conductivity. For the positive electrode current collector, for example, foil, film, sheet, net, punched sheet, lath, porous body, foam, or molded fiber body made of the above materials can be used. The surface of the current collector can also be roughened by surface treatment. The thickness of the current collector is not particularly limited, but is typically 1 to 500 μm.

[0039] The negative electrode mixture layer preferably contains, for example, an active material in an amount of 60 to 98 wt %, a binder in an amount of 1 to 15 wt %, and a conductive additive in an amount of 0.1 to 10 wt %. The thickness of the negative electrode mixture layer is preferably, for example, 10 to 100 μm per side of the current collector.

[0040] Next, a method for manufacturing the negative electrode will be described. ２ Ti １２ O ２５ A paste or slurry-like negative electrode mixture-containing composition is prepared by dispersing a binder and a conductive additive in a solvent such as water. The negative electrode mixture-containing composition is then applied to one or both sides of a current collector and dried. The negative electrode is then manufactured through a press process such as calendering, if necessary. However, the manufacturing method for the negative electrode is not limited to the above-described method, and other manufacturing methods may also be used.

[0041] (3-2-3) Separator A separator material that is strong enough and can hold a large amount of nonaqueous electrolyte can be used. For example, a microporous membrane made of polyolefins such as polyethylene (PE) or polypropylene (PP) with a thickness of 5 to 50 μm and an aperture ratio of 30 to 70% can be used. The microporous membrane constituting the separator may be, for example, one made solely of PE or PP, or may contain an ethylene-propylene copolymer. It may also be a laminate of a microporous membrane made of PE and a microporous membrane made of PP. Furthermore, a laminated separator composed of a porous layer primarily made of a resin with a melting point of 140°C or less and a porous layer primarily made of a resin with a melting point of 150°C or higher or an inorganic filler with a heat-resistant temperature of 150°C or higher can also be used. The heat-resistant temperature means that deformation such as softening does not occur at least at that temperature. The thickness of the separator (a separator made of a microporous polyolefin film or a laminated separator) is more preferably 10 to 30 μm.

[0042] (3-2-4) Nonaqueous Electrolyte The nonaqueous electrolyte may contain an organic solvent, a fluorinated cyclic phosphate ester, a lithium salt, and an additive.

[0043] Examples of organic solvents that can be used include cyclic carbonates, acyclic carbonates, and lactones. Examples of cyclic carbonates that can be used include ethylene carbonate (EC) and propylene carbonate (PC). The use of these cyclic carbonate materials results in a high dielectric constant, thereby achieving good electrical conductivity. Examples of acyclic carbonates that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The use of these acyclic carbonate materials results in good electrical conductivity due to the low viscosity of the acyclic carbonates. Examples of lactones that can be used include γ-butyrolactone (GBL). The use of the above lactone materials results in a high dielectric constant, thereby achieving good electrical conductivity. These materials may be used alone or in combination. In particular, it is preferable to use a mixed solvent of a cyclic carbonate and an acyclic carbonate or a mixed solvent of a lactone and an acyclic carbonate, and it is even more preferable to use a mixed solvent of PC and EMC. The high dielectric constant of the cyclic carbonate (or PC) or lactone facilitates ionization of lithium ions, while the low viscosity of the acyclic carbonate (or EMC) facilitates lithium ion mobility, resulting in a nonaqueous electrolyte secondary battery with good electrical conductivity. When a mixed solvent of PC and EMC is used, it is more preferable to include MEC in an amount of 15% by volume or more and 80% by volume or less relative to the total volume of the mixed solvent. This configuration can improve the stability of the solvent during high-voltage charging while maintaining high low-temperature characteristics and charge / discharge cycle characteristics of the battery.

[0044] 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEP) is added to the non-aqueous electrolyte as a fluorinated cyclic phosphate ester. TFEP is added at 1 to 8% by weight relative to the organic solvent. If the amount added is within this range, gas generation during charging and discharging is suppressed, and good low-temperature characteristics and cycle characteristics are obtained. The effects and benefits of adding TFEP will be described later.

[0045] As the lithium salt, inorganic lithium salts and organic lithium salts can be used. Examples of inorganic lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiSbF ６ , LiB １０ Cl １０ , lower aliphatic carboxylic acid Li, LiAlCl ４ , LiCl, LiBr, LiI, chloroborane Li, tetraphenylborate Li, etc. Examples of organic lithium salts that can be used include LiCF ３ SO ３ , LiCF ３ CO ２ , Li ２ C ２ F ４ (SO ３ ) ２ , LiN(CF ３ SO ２ ) ２ , LiC(CF ３ SO ２ ) ３ , LiC ｎ F ２ｎ＋１ SO ３ (2≦n≦7), LiN(RfOSO ２ ) 2 (Rf represents a fluoroalkyl group) can be used. Furthermore, the materials described as inorganic lithium salts and organic lithium salts may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, preferably 0.2 to 3.0 mol / L, more preferably 0.8 to 2.0 mol / L, and even more preferably 0.9 to 1.6 mol / L. When the lithium ion concentration is within this range, the ionic conductivity is increased. In particular, when the lithium ion concentration is 0.2 mol / L or more, 0.8 mol / L or more, or 0.9 mol / L or more, a significant decrease in ionic conductivity can be suppressed. When the lithium ion concentration is 3.0 mol / L or less, 2.0 mol / L or less, or 1.6 mol / L or less, the ionic conductivity is increased and a decrease in the viscosity of the non-aqueous electrolyte can be suppressed.

[0046] Among the materials used for the non-aqueous electrolyte, a solvent containing propylene carbonate, which is a cyclic carbonate, and at least one acyclic carbonate selected from dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate is used. ６ It is preferable to use an electrolyte solution containing a solution of propylene carbonate. Propylene carbonate has a dielectric constant equivalent to that of ethylene carbonate (EC), which is used in batteries with typical graphite negative electrodes. However, unlike EC, which is solid at room temperature, it does not become sherbet-like at low temperatures and suffer from reduced low-temperature characteristics. Furthermore, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate have low viscosity and are less likely to gasify because their boiling points are above 90°C, which is within the practical range.

[0047] (3-3) Effect of TFEP Next, the effect of adding TFEP to a non-aqueous electrolyte solution will be described. The TFEP used in the present invention is generally used as a solvent. Because TFEP is flame-retardant, numerous reports have shown that it can prevent battery ignition. On the other hand, its high viscosity makes it unsuitable for producing batteries with excellent input / output performance. Therefore, in the present invention, TFEP was used as an additive to the solvent, and non-aqueous electrolyte secondary batteries were fabricated with varying amounts of TFEP added, with no additive as the standard. High-temperature storage tests were conducted on the fabricated non-aqueous electrolyte secondary batteries in input / output, charge / discharge cycles, and charge / discharge states. It was found that the amount of gas generated after each test, particularly after charge / discharge cycles and high-temperature storage tests in the charged state, decreased with increasing additive amount. This reduction in the amount of gas generated is expected to suppress performance degradation and safety degradation. Initial cyclic voltammetry measurements revealed a reduction peak during charging that was not observed without additives. This suggests the formation of a film equivalent to the SEI. It was inferred that the SEI suppressed the reduction of the electrolyte solution. After each test, the gas species generated were measured. The results showed that CO2, which is generated by the reaction between water trapped in the battery and alkaline components present on the electrolyte and the surface of the positive electrode active material, ２ In addition, CH ４It was found that the amount of CH and CO decreased as the amount of TFEP added increased. On the other hand, when the amount of TFEP added exceeded 8%, the amount of gas increased compared to when the amount of TFEP added was 8% or less, and while CO decreased, CH ４ The increase in TFEP content indicates that a different reaction is occurring. Regarding charge-discharge cycles, it was found that the rate of discharge capacity decline decreases when the amount of TFEP added is 1% or more. Low-temperature characteristics also improve when the amount of TFEP added is 1% or more, but decrease when the amount of TFEP added is 8% or more. This is thought to be due to the increase in film resistance caused by the increase in the amount of SEI film. From the above, it was found that adding 1 to 8% by weight of TFEP to an electrolyte consisting of an organic solvent and a lithium salt is effective for non-aqueous electrolytes.

[0048] (3-4) Battery Shape The shape of the nonaqueous electrolyte secondary battery according to this embodiment is not particularly limited. The shape of the nonaqueous electrolyte secondary battery according to this embodiment may be any of a coin shape, a button shape, a sheet shape, a laminated shape, a cylindrical shape, a flat shape, a prismatic shape, a large shape for use in an electric vehicle, etc.

[0049] Although the method for producing a battery according to the embodiment will be described in detail below, the present invention is not limited thereto.

[0050] Example 1: LiNiCoMn(5:2:3)O as a positive electrode active material ２ , carbon black as a conductive additive, PVDF as a binder, and LiNiCoMn(5:2:3)O ２ The carbon black and PVDF were applied to an Al foil in a weight ratio of 96:2:2, dried, and pressed to prepare a positive electrode. ２ / g of H ２ Ti １２ O ２５ , SCNT is used as a conductive additive, and SBR and CMC are mixed in a 1:1 ratio as a binder. ２ Ti １２ O ２５The negative electrode was fabricated by applying a 1:1 mixture of SCNT, SBR, and CMC to a Cu foil in a weight ratio of 97:1:2, drying, and pressing. Each electrode was cut to a predetermined size, stacked with a polyethylene separator between them, and packaged in resin-laminated aluminum foil. Next, 1M LiPF was used as the electrolyte. ６ A PC:DEC (3:7) mixture containing 3% by weight of TFEP was poured into a packaging container and sealed to prepare 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.

[0051] (Example 2) In Example 1, the electrolyte was changed to 1M LiPF ６ Electrodes and batteries were fabricated in the same manner except that 1.5% by weight of TFEP was added to PC:DEC (3:7).

[0052] (Example 3) In Example 1, the electrolyte was changed to 1M LiPF ６ Electrodes and batteries were fabricated in the same manner except that 6% by weight of TFEP was added to PC:DEC (3:7).

[0053] Example 4: In Example 1, a negative electrode active material having a specific surface area of ​​30 m ２ / g of H ２ Ti １２ O ２５ An electrode and a battery were fabricated in the same manner except that the above was used.

[0054] Example 5: In Example 2, a negative electrode active material having a specific surface area of ​​30 m ２ / g of H ２ Ti １２ O ２５ An electrode and a battery were fabricated in the same manner except that the above was used.

[0055] Example 6: In Example 3, a negative electrode active material having a specific surface area of ​​30 m ２ / g of H ２ Ti １２ O ２５ An electrode and a battery were fabricated in the same manner except that the above was used.

[0056] Example 7: In Example 1, 1M LiPF was used as the electrolyte. ６ Electrodes and batteries were fabricated in the same manner except that 3% by weight of TFEP was added to PC:EMC (3:7).

[0057] (Example 8) In Example 1, 1M LiPF was used as the electrolyte. ６ Electrodes and batteries were fabricated in the same manner except that 3% by weight of TFEP was added to GBL:EMC (3:7).

[0058] Comparative Example 1: In Example 1, the electrolyte was changed to 1M LiPF ６ Electrodes and batteries were fabricated in the same manner except that the ratio was changed to PC:DEC (3:7).

[0059] Comparative Example 2: In Example 1, 1M LiPF was used as the electrolyte. ６ Electrodes and batteries were fabricated in the same manner except that 0.5 wt % of TFEP was added to PC:DEC (3:7).

[0060] Comparative Example 3: In Example 1, 1M LiPF was used as the electrolyte. ６ Electrodes and batteries were fabricated in the same manner except that 10% by weight of TFEP was added to PC:DEC (3:7).

[0061] (Comparative Example 4) In Example 1, the electrolyte was changed to 1M LiPF ６ Electrodes and batteries were fabricated in the same manner except that the ratio was changed to PC:MEC (3:7).

[0062] Comparative Example 5: In Example 1, the electrolyte was changed to 1M LiPF ６ Electrodes and batteries were fabricated in the same manner except that the ratio was changed to GBL:MEC (3:7).

[0063] The low-temperature characteristics and cycle characteristics of each battery in each of the above examples and comparative examples were evaluated. The low-temperature characteristics were evaluated by two methods: charging at room temperature (25°C) and then discharging at a low temperature (-20°C) (evaluation of low-temperature characteristics by low-temperature discharge), and discharging at room temperature (25°C) and then charging at a low temperature (-20°C) (evaluation of low-temperature characteristics by low-temperature charge).

[0064] Each evaluation method will be explained below.

[0065] (Evaluation of Low-Temperature Characteristics by Low-Temperature Discharge) For each battery of each of the above examples and comparative examples, three cycles of CCCV charging at 2.8 V, 0.2 C at 25 ° C and 1 V termination 0.2 C discharge were performed, followed by CCCV charging at 2.8 V, 1 C at 25 ° C, followed by 1 V termination 20 C discharge at −20 ° C, and the discharge capacity was measured. On the other hand, three cycles were performed under the same conditions, followed by CCCV charging at 2.8 V, 1 C at 25 ° C, followed by 1 V termination 0.2 C discharge at −20 ° C, and the discharge capacity was measured. The discharge capacity ratio when 20 C discharge at −20 ° C was performed relative to the discharge capacity when 0.2 C discharge was performed at −20 ° C (discharge capacity at 20 C discharge / discharge capacity at 0.2 C discharge) is shown in Table 1 as "−20 ° C 20 C discharge capacity ratio (%)".

[0066] (Evaluation of low-temperature characteristics by low-temperature charging) For each battery of each of the above examples and comparative examples, three cycles of CCCV charging at 2.8 V, 0.2 C at 25° C. and terminal 0.2 C discharge to 1 V were performed, and after the third cycle, CCCV discharging at 1 V, 0.2 C at 25° C., CCCV charging at 2.8 V, 10 C was performed at −20° C., and the charge capacity was measured. On the other hand, three cycles were performed under the same conditions, and after the third cycle, CCCV discharging at 1 V, 0.2 C at 25° C., CCCV charging at 2.8 V, 0.2 C was performed at −20° C., and the charge capacity was measured. The ratio of the charge capacity when charging at 10 C at −20° C. to the charge capacity when charging at 0.2 C at −20° C. (charge capacity when charging at 10 C / charge capacity when charging at 0.2 C) is shown in Table 1 as “−20° C. 10 Ccc capacity ratio (%).”

[0067] (Cycle Characteristics) Each battery of the above Examples and Comparative Examples was subjected to 3 cycles of CCCV charging at 2.8 V, 0.2 C at 25° C. and 1 V termination 0.2 C discharge, and then 500 cycles of charging at 2.8 V, 1 C at 25° C. and discharging at 1 V, 1 C were repeated, and the discharge capacity after 500 cycles was measured. The evaluation results of the discharge capacity after 500 cycles are shown in Table 1 as "Capacity retention rate (%) after 500 cycles."

[0068] Referring to Table 1, it can be seen that in all Examples, the "-20°C 20C discharge capacity ratio (%)" and "-20°C 20C discharge capacity ratio (%)" are improved compared to the Comparative Examples. It can be seen that the "capacity retention rate (%) after 500 cycles" is improved in Examples 1 to 4 and Example 6 compared to the Comparative Examples. In Example 5, a negative electrode active material having a specific surface area of ​​30 m was used. ２ / g of H ２ Ti １２ O ２５ and 1.5 wt% TFEP, but the negative electrode active material has a specific surface area of ​​70 m ２ / g of H ２ Ti １２ O ２５ Although the cycle characteristics were slightly inferior to those of Comparative Example 3 using 10 wt % TFEP, they were almost the same. ２ Ti １２ O ２５ This is thought to be due to the small specific surface area of ​​the TFEP and the fact that TFEP is consumed in the formation of SEI during cycling, resulting in a decrease in the retention rate over long periods of cycling. However, the battery characteristics are satisfactory. From the above, it can be seen that all Examples exhibit good low-temperature characteristics and cycle characteristics.

[0069]

[0070] Next, the amount of gas generated was evaluated.

[0071] Each battery in the Examples and Comparative Examples was subjected to three cycles of CCCV charging at 2.8 V and 0.2 C at 25°C and discharging at 1 V and 0.2 C to a terminal voltage of 1 V. After CCCV charging at 2.8 V and 0.2 C, the battery was stored at 60°C for 20 days as a high-temperature storage, followed by a discharge at 1 V and 0.2 C. After high-temperature storage, the battery was opened in liquid paraffin, and the released gas was collected in a measuring cylinder to measure the total amount. The gas was then diluted with He gas and analyzed by GCMS to identify the gas species. The results are shown in Table 2. The total amount of gas was also measured and the gas species identified for the battery used to evaluate cycle performance in the same manner as for the battery after high-temperature storage. The results are shown in Table 3. The "Gas Amount After Storage (%)" in Table 2 and the "Gas Amount After Cycling (%)" in Table 3 represent the ratio of the gas amount to that of Comparative Example 1. The "CH4 Ratio (%)" and "CO Ratio (%)" in Tables 2 and 3 represent the CH4 Ratio (%) and CO Ratio (%), respectively. ４ The ratio of CO to the total gas is shown. In all examples, the amount of gas is suppressed, and it is considered that the gas that is thought to be associated with the electrolyte reduction reaction is reduced, thereby suppressing the reaction. The detailed mechanism is as described in the above "(3-3) Effect of TFEP".

[0072]

[0073]

Claims

1. The positive electrode uses a transition metal composite oxide containing lithium as the active material, and the positive electrode uses titanium oxide as the active material. ２ Ti １２ O ２５ and a nonaqueous electrolyte solution, wherein the nonaqueous electrolyte solution contains a solvent, an electrolyte, and 1 to 8% by weight of a fluorinated cyclic phosphate ester relative to the solvent, and the solvent contains any one of a cyclic carbonate, an acyclic carbonate, and a lactone.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the solvent is a mixed solvent of a cyclic carbonate and an acyclic carbonate or a mixed solvent of a lactone and an acyclic carbonate.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the cyclic carbonate is ethylene carbonate or propylene carbonate.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the acyclic carbonate is diethyl carbonate, dimethyl carbonate or ethyl methyl carbonate.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lactone is γ-butyrolactone.

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