Non-aqueous electrolyte secondary battery

Abstract

To provide a nonaqueous electrolyte secondary battery having excellent low-temperature characteristics and cycle characteristics by suppressing reduction reaction of an electrolyte and reducing generation of gas.SOLUTION: The present invention relates to a non-aqueous electrolytic solution secondary cell comprising a positive electrode, a negative electrode containing H2Ti12O25, and a non-aqueous electrolytic solution. The nonaqueous electrolyte solution contains a solvent and a fluorinated cyclic phosphate ester in an amount of 1 to 8% by weight relative to the solvent. The solvent includes any of a cyclic carbonate, an acyclic carbonate, and a lactone.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 applications such as electric vehicles (EVs), industrial robots, mega-solar systems, and stationary home power sources, 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 even 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 remain, including productivity, and large-capacity 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 is used for the negative electrode, but the charge / discharge potential of this 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 increases. On the other hand, Li4Ti5O 12 However, there was a problem in that the theoretical capacity of the original battery was 175 mAh / g, which was lower than the theoretical capacity of graphite, which was 372 mAh / g. [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 Summary of the Invention [Problem to be solved by the invention]

[0004] Titanium oxide H2Ti is a material that can solve the problems of the above-mentioned negative electrode materials. 12 O 25 There is H2Ti. 12 O 25 The theoretical capacity is 300mAh / g, which is comparable to that of graphite. However, the negative electrode material is H2Ti 12 O 25 When a common cyclic carbonate, acyclic carbonate, lactone, or the like is used as the 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 has an object to obtain a nonaqueous electrolyte secondary battery having good low-temperature characteristics and good cycle characteristics by suppressing the reduction reaction of the electrolyte and reducing gas generation. [Means for solving the problem]

[0006] In order to achieve the above object, the first invention provides a positive electrode having a lithium-containing transition metal composite oxide as an active material, and a titanium oxide, H2Ti 12 O 25 and a non-aqueous electrolyte solution containing 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.

[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 of 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 non-aqueous 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. [Effects of the Invention]

[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. DETAILED DESCRIPTION OF THE INVENTION

[0017] (1) Overall structure The non-aqueous 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 H2Ti 12 O 25 The battery is provided with a negative electrode containing the above-mentioned compound and a non-aqueous electrolyte solution. The non-aqueous electrolyte solution contains a solvent and 1 to 8% by weight 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 negative electrode configuration The specific surface area of ​​the active material used in the negative electrode according to this embodiment is not particularly limited. 2 / g or more, 150m 2 It is preferable to form the binder containing an active material having a content of H2Ti / g or less. 12 O 25 The surface area of ​​H2Ti is more than 10 times larger than that of commonly used graphite. 12 O 25In 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.

[0019] (3) Detailed configuration (3-1)H2Ti 12 O 25 Synthesis and Mixing of Single Carbon Nanotubes (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 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, crystal growth is performed on the mixture containing the titanium raw material and the lithium raw material 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 titanium 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. 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, H2Ti can be obtained 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 does not dissolve, 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 Li2O and Li2O2, salts such as Li2CO3 and LiNO3, 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 involves heat-treating a mixture containing the titanium and lithium raw materials to grow crystals. Crystal growth can be achieved by solid-state reaction, a common method for synthesizing ceramic microparticles, or liquid-phase methods such as precipitation, sol-gel, and hydrothermal synthesis. Hydrothermal synthesis is particularly preferred. When crystallizing using hydrothermal synthesis, TiO2 is preferred as the titanium raw material, and LiOH·H2O is preferred as the lithium raw material. Furthermore, the weight ratio of the lithium raw material to the titanium raw material is preferably 1:1 (approximately 2.3:1) or greater. While there are no particular restrictions on the reaction temperature or time during hydrothermal synthesis, a reaction temperature of 150°C or higher and a reaction time of 3 hours or longer are preferred. Crystal growth by hydrothermal synthesis yields lithium titanate. Examples of lithium titanate include Li2TiO3, Li2Ti2O4, LiTi2O4, and Li4Ti5O. 12Among these, Li2TiO3 is preferred. 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 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 of 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.

[0022] In the lithium / proton exchange process, the lithium in the heat-treated lithium titanate is exchanged for protons. Specifically, 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 for hydrogen. In this process, the lithium titanate is preferably dispersed in the acidic aqueous solution, held for a certain period of time, and then separated by filtration or centrifugation, followed by drying. 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 at a concentration of 0.1N to 1.0N being more preferred. The treatment time for exchanging lithium for protons is 10 hours to 10 days, preferably 1 to 7 days. The treatment temperature for exchanging lithium for protons is preferably room temperature (20°C) or higher and less than 100°C.

[0023] The lithium titanate proton-exchanged material can be dried using known methods such as a box dryer or spray dryer. A conductive additive such as carbon may be added to the proton-exchanged material before drying. The conductive additive may be added by stirring the proton-exchanged material into a slurry. If necessary, a dispersant may be added or a disperser may be used. In the proton-exchanged material heat-treatment step, the lithium titanate proton-exchanged material obtained in the lithium / proton-exchange step is heat-treated. The heat treatment promotes a dehydration reaction of the proton-exchanged material, resulting in titanium oxide H2Ti 12 O 25 The 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.

[0024] H2Ti obtained by proton exchanger heat treatment process 12 O 25 In powder XRD measurement using Cu-Kα as a radiation source, it is sufficient for the peaks to be 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 certain crystal planes is poor due to the miniaturization of primary particles. In particular, the peak originating from the (110) plane, which appears around 25°, and the peak originating from the (020) plane, which appears 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 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.

[0025] H2Ti 12 O 25 The particle shape of H2Ti is not particularly limited, but isotropic shapes such as spheres and polyhedrons are preferred to increase the packing density of the negative electrode layer. 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 25This 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.

[0026] Also, H2Ti 12 O 25 Alternatively, single carbon nanotubes may be mixed and attached to the surface.

[0027] (3-1-2) Mixing of single carbon nanotubes Single carbon nanotube H2Ti 12 O 25 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 25In 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 to form a slurry of the proton exchanger immediately before drying the lithium titanate proton exchanger, mix the single carbon nanotubes with the slurry, and spray-dry the mixture with a spray dryer or the like to adhere the single carbon nanotubes to the particle surfaces of the lithium titanate proton exchanger. When adding the single carbon nanotubes to the slurry of the lithium titanate proton exchanger, a dispersant or a surfactant may be added, and dispersion using a wet bead mill, a media-less disperser, or the like may also be used in combination. 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% by weight or more and 5% by weight or less. 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 negative electrode active material concentration 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, thereby suppressing a decrease in conductivity.

[0028] (3-2) Example of battery configuration The nonaqueous electrolyte secondary battery of this embodiment may include 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 CoO2 (where 0≦y≦1.1), Li z NiO2 (where 0≦z≦1.1), Li e MnO2 (where 0≦e≦1.1), Li a Cob M 1 1-b O2 (where 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 O2 (where 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 O2 (where 0 ≦ f ≦ 1.1, 0 < g < 1.0, 0 < h < ......

[0030] Examples of the binder for the positive electrode composite material layer include polysaccharides such as starch, polyvinyl alcohol, polyacrylic acid, CMC, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, and their modified products, thermoplastic resins such as polyvinyl chloride, polyvinyl pyrrolidone, polytetrafluoroethylene (PTFE), PVDF, polyethylene, polypropylene, polyamideimide, polyamide, and their modified products, polymers having rubber-like elasticity such as ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, SBR, butadiene rubber, polybutadiene, fluorine rubber, polyethylene oxide, and their modified products, and polyimide. One of these can be used, or two or more can be used in combination.

[0031] It should be noted that there seems to be an incomplete expression in the original text at the end of line 23 where "0 < h <......" is shown. Please check and correct it if necessary.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 electron conductor in the resulting lithium secondary battery. For example, aluminum, 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, aluminum or aluminum alloy is particularly preferred due to its light weight and high electron conductivity. For example, foils, films, sheets, nets, punched sheets, laths, porous bodies, foams, and molded bodies of fiber groups made of the above materials can be used as the positive electrode current collector. 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 60 to 98 wt % of positive electrode active material, 1 to 15 wt % of binder, and 0.1 to 10 wt % of 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 producing the 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 composition. The positive electrode mixture composition is then applied to one or both sides of a current collector and dried. The positive electrode is then manufactured through a press process such as calendering, if necessary. However, the manufacturing method for the positive electrode is not limited to the above 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, H2Ti, which can absorb and release lithium ions. 12 O 25 can be used.

[0036] Examples of binders that can be used 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 additive 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 electron 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 electron conductivity. For the positive electrode current collector, for example, foil, film, sheet, net, punched sheet, lath, porous body, foam, or fiber molded 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, 60 to 98 wt % of active material, 1 to 15 wt % of binder, and 0.1 to 10 wt % of conductive additive, and the thickness of the negative electrode mixture layer is preferably 10 to 100 μm per side of the current collector.

[0040] Next, a method for producing the negative electrode will be described. H2Ti, the negative electrode active material 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 The separator can be made of a material that is strong enough to hold a large amount of nonaqueous electrolyte. For example, a microporous membrane made of polyolefin, such as polyethylene (PE) or polypropylene (PP), having 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 made of, for example, only PE or only PP, or may contain an ethylene-propylene copolymer. It may also be a laminated separator composed of a porous layer mainly made of a resin with a melting point of 140°C or lower and a porous layer mainly 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. 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 membrane made of polyolefin or a laminated separator) is more preferably 10 to 30 μm.

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

[0043] Examples of the organic solvent that can be used include cyclic carbonates, acyclic carbonates, and lactones. Examples of the 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 the 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 the 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, and the low viscosity of the acyclic carbonate (or EMC) facilitates the mobility of lithium ions, resulting in a non-aqueous electrolyte secondary battery with good electrical conductivity. When a mixed solvent of PC and EMC is used, it is more preferable to contain MEC in an amount of 15 to 80% by volume of the total volume of the mixed solvent. This configuration improves 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 nonaqueous electrolyte as a fluorinated cyclic phosphate ester. TFEP is added at 1 to 8 wt % relative to the organic solvent. When the amount added is within this range, gas generation during charge and discharge is suppressed, and good low-temperature characteristics and cycle characteristics are obtained. The effects of adding TFEP will be described later.

[0045] The lithium salt may be an inorganic lithium salt or an organic lithium salt. Examples of the inorganic lithium salt include LiClO4, LiBF4, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, and LiB 10 Cl 10 Examples of the organic lithium salts that can be used include LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiN(CF3SO2)2, LiC(CF3SO2)3, LiC n F 2n+1 SO3 (2≦n≦7) and LiN(RfOSO2)2 (Rf represents a fluoroalkyl group) can be used. The materials described as inorganic lithium salts and organic lithium salts can 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 high. 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 high and a decrease in the viscosity of the non-aqueous electrolyte can be suppressed.

[0046] Among the materials used for the nonaqueous electrolyte, it is preferable to use an electrolyte in which LiPF6 is dissolved in a solvent containing propylene carbonate, a cyclic carbonate, and at least one acyclic carbonate selected from dimethyl carbonate, diethyl carbonate, and ethyl methyl 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, propylene carbonate 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 due to their boiling points 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 will be described. The TFEP used in this invention is commonly used as a solvent. Because TFEP is flame-retardant, numerous studies have shown its ability to prevent battery fires. However, its high viscosity makes it unsuitable for producing batteries with excellent input / output performance. Therefore, in this invention, we used TFEP as a solvent additive, fabricating nonaqueous electrolyte secondary batteries with varying amounts of TFEP added, with no additive as the standard. High-temperature storage tests were conducted on the fabricated nonaqueous electrolyte secondary batteries under input / output, charge / discharge cycling, and charge / discharge conditions. The results showed that increasing the additive amount reduced the amount of gas generated after each test, particularly after charge / discharge cycling and high-temperature storage tests in the charged state. This reduced gas generation is expected to prevent 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 (Self-Intercalated Insulator). It is inferred that the SEI suppresses the reduction of the electrolyte. Measurements of evolved gases after each test showed that, in addition to 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, CH4 and CO, which result from the reductive decomposition of the electrolyte, decreased with increasing TFEP content. On the other hand, when TFEP content exceeded 8%, the amount of gas increased compared to when TFEP content was 8% or less. Furthermore, CO decreased while CH4 increased, suggesting the occurrence of other reactions. Regarding charge-discharge cycling, the rate of discharge capacity decline was found to decrease with TFEP content of 1% or more. Furthermore, low-temperature characteristics improved with TFEP content of 1% or more, but decreased with TFEP content of 8% or more. This is thought to be due to increased film resistance caused by the increased SEI film content. From the above, it was found that it is preferable to add 1 to 8 wt % of TFEP to the non-aqueous electrolyte solution, which is made of an organic solvent and a lithium salt.

[0048] (3-4) 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 rectangular type, a large type used in an electric vehicle, etc. [Example]

[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 The positive electrode active material was LiNiCoMn(5:2:3)O2, carbon black as a conductive additive, and PVDF as a binder. The positive electrode was fabricated by applying the LiNiCoMn(5:2:3)O2:carbon black:PVDF weight ratio to an Al foil, drying, and pressing. The negative electrode active material was LiNiCoMn(5:2:3)O2:carbon black:PVDF in a weight ratio of 96:2:2. 2 / g H2Ti 12 O 25, SCNT as a conductive additive, SBR and CMC mixed in a 1:1 ratio as a binder, and H2Ti 12 O 25 A negative electrode was fabricated by applying SCNT:SBR and CMC (1:1) to a Cu foil in a weight ratio of 97:1:2, drying, and pressing. Each electrode was cut to the required size, stacked with a polyethylene separator between them, and packaged in resin-laminated aluminum foil. Next, the electrolyte was 1M LiPF6 / PC:DEC (3:7) with 3% TFEP added by weight, poured into the packaging container, and sealed to produce 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 An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte used was 1M LiPF6 / PC:DEC (3:7) to which 1.5 wt % of TFEP was added.

[0052] Example 3 An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte used was 1M LiPF6 / PC:DEC (3:7) to which 6% by weight of TFEP had been added.

[0053] Example 4 In Example 1, a negative electrode active material having a specific surface area of ​​30 m 2 / g H2Ti 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 H2Ti 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 m2 / g H2Ti 12 O 25 An electrode and a battery were fabricated in the same manner except that the above was used.

[0056] Example 7 An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte used was 1M LiPF6 / PC:EMC (3:7) to which 3 wt % of TFEP was added.

[0057] Example 8 An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte used was 1M LiPF6 / GBL:EMC (3:7) to which 3 wt % of TFEP was added.

[0058] (Comparative Example 1) An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte solution was changed to 1M LiPF6 / PC:DEC (3:7).

[0059] (Comparative Example 2) An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte used was 1M LiPF6 / PC:DEC (3:7) to which 0.5 wt % of TFEP was added.

[0060] (Comparative Example 3) An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte used was 1M LiPF6 / PC:DEC (3:7) to which 10 wt % TFEP was added.

[0061] Comparative Example 4 An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte solution was changed to 1M LiPF6 / PC:MEC (3:7).

[0062] (Comparative Example 5) An electrode and a battery were fabricated in the same manner as in Example 1, except that the electrolyte solution was changed to 1M LiPF6 / 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] (Low temperature characteristic evaluation by low temperature discharge) For each battery in each of the above examples and comparative examples, three cycles of CCCV charging at 2.8 V, 0.2 C, and 1 V termination 0.2 C discharge were performed, followed by CCCV charging at 2.8 V, 1 C, and then a 1 V termination 20 C discharge at -20 °C, and the discharge capacity was measured. On the other hand, after three cycles under the same conditions, three cycles of CCCV charging at 2.8 V, 1 C, and then a 1 V termination 0.2 C discharge at -20 °C were performed, and the discharge capacity was measured. The discharge capacity ratio of the 20 C discharge at -20 °C to the discharge capacity at 0.2 C discharge 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] (Low temperature characteristic evaluation by low temperature charging) For each battery in each of the above examples and comparative examples, three cycles of CCCV charging at 2.8 V, 0.2 C, and 1 V termination 0.2 C discharge were performed at 25 ° C., followed by CCCV discharging at 1 V, 0.2 C at 25 ° C. for the third cycle, followed by CCCV charging at 2.8 V, 10 C at -20 ° C., and the charge capacity was measured. On the other hand, three cycles were performed under the same conditions, followed by CCCV discharging at 1 V, 0.2 C at 25 ° C. for the third cycle, followed by CCCV charging at 2.8 V, 0.2 C at -20 ° C., and the charge capacity was measured. The charge capacity ratio when charging at 10 C at -20 ° C to the charge capacity when charging at 0.2 C at -20 ° C (charge capacity at 10 C charge / charge capacity at 0.2 C charge) is shown in Table 1 as "-20 ° C 10 Ccc Capacity Ratio (%)".

[0067] (Cycle characteristics) Each battery in each of the above examples and comparative examples was subjected to three cycles of CCCV charging at 2.8 V and 0.2 C at 25° C. and termination 0.2 C discharge at 1 V, followed by 500 cycles of charging at 2.8 V and 1 C at 25° C. and discharging at 1 V and 1 C, 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 3000-kJ / cm2 battery with a specific surface area of ​​30 m was used as the negative electrode active material. 2 / g H2Ti 12 O 25 and 1.5 wt% TFEP, which has a specific surface area of ​​70 m 2 / g H2Ti 12 O 25 Although the cycle characteristics were slightly inferior to those of Comparative Example 3, which used 10 wt% TFEP, they showed almost the same characteristics. 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 still sufficient. From the above, it can be seen that all Examples exhibit good low-temperature characteristics and cycle characteristics.

[0069] [Table 1]

[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, followed by a 1 V termination 0.2 C discharge. After CCCV charging at 2.8 V and 0.2 C, the battery was then stored at high temperature for 20 days at 60°C, followed by a 1 V 0.2 C discharge. After high-temperature storage, the battery was opened and placed in liquid paraffin. The released gas was collected in a measuring cylinder and the total amount was measured. The battery 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 measured and the type of gas was identified for the battery used to evaluate the cycle characteristics in the same manner as for the battery after high-temperature storage. The results are shown in Table 3. The "Gas volume after storage (%)" in Table 2 and the "Gas volume after cycling (%)" in Table 3 indicate the ratio of the gas volume to that of Comparative Example 1. The "CH4 ratio (%)" and "CO ratio (%)" in Tables 2 and 3 indicate the ratio of CH4 and CO to the total gas, respectively. In all examples, the gas volume was suppressed, and it is believed that the reduction of gas thought to be associated with the electrolyte reduction reaction suppressed the reaction. The detailed mechanism is as described above in "(3-3) Effect of TFEP."

[0072] [Table 2]

[0073] [Table 3]

Claims

1. a positive electrode having a lithium-containing transition metal composite oxide as an active material; Titanium oxide H ２ Ti １２ O ２５ a negative electrode comprising a non-aqueous electrolyte; 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; The solvent contains any one of a cyclic carbonate, an acyclic carbonate, and a lactone.

2. 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. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the cyclic carbonate is ethylene carbonate or propylene carbonate.

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

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

Images