Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. The secondary battery comprises a positive electrode, a negative electrode and an electrolyte. The positive electrode comprises lithium cobalt oxide, a binder and an inorganic additive. The electrolyte comprises: (I) lithium difluorophosphate; (II) a boron-containing lithium salt; and (III) ethyl propionate, wherein on the basis of the mass of the electrolyte, the total content of (I) and (II) is 0.03% by mass or more, but 1.5% by mass or less, and the content of (III) is 12% by mass or more, but 37% by mass or less. By means of a combination of the specific positive electrode composition and the electrolyte, not only can the through-plane resistance and peel strength of the positive electrode be improved, but the low-temperature output characteristic and the low-temperature rate capability of the secondary battery can also be improved.

Description

Secondary battery and electronic device TECHNICAL FIELD

[0001] The present application relates to the field of energy storage, in particular to a secondary battery and an electronic device. BACKGROUND

[0002] With the rapid development of electronic products such as smart phones, tablets and smart wear, considering the use time of electronic digital products and the difference of working environment temperature, higher requirements are put forward for the service life and safety performance of secondary batteries. With the increase of charging cut-off voltage, the energy density of the battery has been significantly improved. However, in the prior art, with the increase of the voltage of the secondary battery, the cycle performance of the high-voltage secondary battery becomes poor. After using high-voltage positive electrode film-forming additives to improve the cycle characteristics, the low-temperature characteristics are greatly affected.

[0003] SUMMARY

[0004] The embodiments of the present application adjust the composition of the positive electrode applied in the secondary battery and the components in the electrolyte to solve the problems existing in the prior art to some extent.

[0005] The present inventors have found that the positive electrode comprising lithium cobalt oxide, binder and inorganic additive, and the electrolyte comprising lithium difluorophosphate, lithium salt containing boron and ethyl propionate can not only improve the positive electrode penetration resistance and peel strength, but also improve the low-temperature output characteristics and low-temperature rate characteristics of the secondary battery, thereby completing the present application.

[0006] In the secondary battery comprising lithium cobalt oxide, binder and inorganic additive, the electrolyte simultaneously contains (I) lithium difluorophosphate, (II) lithium salt containing boron and (III) ethyl propionate, the total content of (I) and (II) is 0.03 mass% or more and 1.5 mass% or less, and the content of (III) is 12 mass% or more and 37 mass% or less based on the mass of the electrolyte, which is important for the stability of the positive electrode structure, significantly inhibits the increase of interface resistance at low temperature, and this design not only improves the positive electrode penetration resistance and peel strength, but also improves the low-temperature output characteristics and low-temperature rate characteristics of the secondary battery.

[0007] In another aspect of the present application, the present application provides an electronic device comprising the secondary battery described in the present application.

[0008] The present application uses a specific combination of positive electrode structure and electrolyte, which not only improves the positive electrode penetration resistance and peel strength, but also improves the low-temperature output characteristics and low-temperature rate characteristics of the secondary battery.

[0009] Additional aspects and advantages of the embodiments of the present application will be described, apparent, or elucidated in the following description, shown in part, or by implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0010] The embodiments of the present application will be described in detail below. The embodiments of the present application should not be construed as limiting the present application.

[0011] The following terms used in the present application have the meanings indicated below, unless explicitly indicated otherwise.

[0012] The present application can improve the positive electrode penetration resistance and the peeling strength, and can improve the low-temperature output characteristics and the low-temperature rate characteristics of the secondary battery by using a specific combination of the positive electrode and the electrolyte.

[0013] In one embodiment, the present application provides a secondary battery including a positive electrode, a negative electrode, and an electrolyte as described below.

[0014] I. Electrolyte

[0015] The electrolyte in the secondary battery of the present application includes an electrolyte and a solvent in which the electrolyte is dissolved.

[0016] In some embodiments, the electrolyte of the present application includes (I) lithium difluorophosphate, (II) a lithium salt containing boron, and (III) ethyl propionate.

[0017] When ethyl propionate is used in the electrolyte to improve the output power at low temperatures, the coating film formed on the surface of the positive electrode material is thick, and particularly in the presence of inorganic additives, the ethyl propionate is catalyzed to form a film, which increases the impedance of the coating film and affects the low-temperature characteristics of the secondary battery. The inventors have found that when the electrolyte further contains lithium difluorophosphate and a lithium salt containing boron, the increase in the impedance of the coating film can be significantly suppressed, thereby not only improving the positive electrode penetration resistance and the peeling strength, but also improving the low-temperature output characteristics and the low-temperature rate characteristics of the secondary battery.

[0018] Specifically, from the viewpoint of improving the positive electrode penetration resistance characteristics of the secondary battery, the content of lithium difluorophosphate is 0.02% by mass or more, preferably 0.03% by mass or more, preferably 0.06% by mass or more, and more preferably 0.08% by mass or more, based on the mass of the electrolyte. In addition, as an upper limit of the content of lithium difluorophosphate, from the viewpoint of improving the low-temperature rate, the content of lithium difluorophosphate is 1.15% by mass or less, preferably 0.89% by mass or less, more preferably 0.67% by mass or less, further preferably 0.45% by mass or less, and particularly preferably 0.39% by mass or less. When it is within the above range, it is helpful to further improve the low-temperature rate.

[0019] In some embodiments, the boron-containing lithium salt is at least one of lithium tetrafluoroborate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium tetracyanoborate, lithium tetrakis(trifluoromethyl)borate, lithium (trifluoromethyl)trifluoroborate, lithium bis(trifluoromethyl)difluoroborate, lithium pentafluoroethyltrifluoroborate, lithium dicyano(oxalato)borate, lithium bis(malonato)borate, lithium (2-fluoromalonato)difluoroborate, lithium malonato(oxalato)borate, lithium bis(waterosalicylato)borate, lithium methoxytricyanoborate, lithium ethoxytricyanoborate, lithium tetramethoxyborate, lithium tetraethoxyborate, lithium tetrakis(trifluoromethoxy)borate, lithium tetrakis(2,2,2-trifluoroethoxy)borate, lithium poly(tetra-p-phenolato)borate, lithium bis(trifluoroborate) sulfate, lithium difluoroborate, lithium methanedisulfonato difluoroborate, lithium difluorophosphoryloxytrifluoroborate, lithium bis(difluorophosphoryloxy)difluoroborate, lithium tetrakis(difluorophosphoryloxy)borate.

[0020] Specifically, from the viewpoint of improving the positive electrode through resistance characteristics, the content of the boron-containing lithium salt is 0.01 mass% or more, preferably 0.05 mass% or more, preferably 0.12 mass% or more, more preferably 0.23 mass% or more, based on the mass of the electrolyte. Further, as the upper limit of the content of the boron-containing lithium salt, from the viewpoint of improving the low temperature rate, the content of the boron-containing lithium salt is 0.95 mass% or less, preferably 0.88 mass% or less, more preferably 0.72 mass% or less, further preferably 0.56 mass% or less, particularly preferably 0.48 mass% or less. When it is within the above range, it is further helpful to improve the low temperature rate.

[0021] Specifically, from the viewpoint of suppressing the increase in the positive electrode through resistance, the content of ethyl propionate is 12 mass% or more, preferably 15 mass% or more, preferably 16 mass% or more, more preferably 22 mass% or more, based on the mass of the electrolyte. Further, as the upper limit of the content of ethyl propionate, from the viewpoint of suppressing the increase in the positive electrode through resistance, the content of ethyl propionate is 37 mass% or less, preferably 35 mass% or less, more preferably 32 mass% or less, further preferably 31 mass% or less, particularly preferably 28 mass% or less. When it is within the above range, it is further helpful to suppress the increase in the positive electrode through resistance.

[0022] Further, the total content of (I) and (II) is 0.03 mass% or more, preferably 0.08 mass% or more, based on the mass of the electrolyte, from the viewpoint of improving the low temperature rate. Further, as the upper limit of the total content of (I) and (II), from the viewpoint of improving the electrochemical characteristics in a low temperature environment, it is 1.5 mass% or less, preferably 1.24 mass% or less.

[0023] In some embodiments, the total content of (I) and (II) is a mass % based on the mass of the electrolyte, a being 0.03, 0.04, 0.08, 0.17, 0.29, 0.43, 0.6, 0.71, 0.94, 1.19, 1.24, 1.34, 1.5, or within a range defined by any two of the aforementioned values. When within the aforementioned range, it is helpful to further improve the low-temperature rate.

[0024] Further, the electrolyte can also include other additives, such as at least one of 1,2-bis(difluorophosphoryl)ethane, fluoroethylene carbonate, vinylene carbonate, lithium fluorosulfonate, butanedinitrile, hexanedinitrile, ethylene sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate. The inventors have also unexpectedly found that the other additives can improve the stability of the aforementioned coating at high voltage and increase the ionic conduction rate of lithium ions, thereby further improving the low-temperature rate characteristics. The aforementioned other additives can be only one, or two or more.

[0025] Specifically, from the viewpoint of improving the conduction of lithium ions in the aforementioned coating, the content of the other additives is 0.01 mass % or more based on the mass of the electrolyte, preferably the content of the other additives is 0.1 mass % or more, preferably 0.3 mass % or more, more preferably 0.5 mass % or more. Further, as the upper limit of the content of the other additives, from the viewpoint of improving the low-temperature rate, the content of the other additives is 10 mass % or less, preferably 9 mass % or less, more preferably 8 mass % or less, further preferably 7 mass % or less, particularly preferably 5 mass % or less. When within the aforementioned range, it is helpful to further improve the low-temperature rate characteristics.

[0026] In some embodiments, the electrolyte is not particularly limited and can be arbitrarily used as a substance known as an electrolyte. The mass of the electrolyte is not particularly limited as long as it does not impair the effects of the present application. For example, the lithium salt used in the electrolyte of the present application includes lithium hexafluorophosphate, and the content of lithium hexafluorophosphate is 9 to 15 mass %, preferably 9 to 13 mass %, more preferably 9 to 12 mass % based on the mass of the electrolyte. By being in the aforementioned content range, it is possible to more balancedly exert the improvement of the low-temperature output characteristics and the low-temperature rate characteristics.

[0027] In some embodiments, the electrolyte further comprises any non-aqueous solvent known in the art as a solvent for an electrolyte. In some embodiments, the non-aqueous solvent includes, but is not limited to, one or more of: cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, cyclic ethers, chain ethers, phosphorus-containing organic solvents, and sulfur-containing organic solvents.

[0028] II. Cathode

[0029] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer provided on a surface of the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material, and can be one or more layers. The positive electrode mixture layer includes a positive electrode active material layer, and optionally includes a base coat layer or an edge layer. The base coat layer is located between the current collector and the positive electrode active material layer, and serves to improve mechanical adhesion between the current collector and the active material layer and to reduce electrical resistance therebetween. The edge layer is provided at the edge of the current collector and is juxtaposed with the positive electrode active layer, and serves to improve insulation of the positive electrode edge and the negative electrode edge. The positive electrode active material is any material capable of reversibly intercalating and deintercalating lithium ions.

[0030] For example, as the positive electrode active material for a secondary battery, a lithium-containing olivine-type phosphate containing one or two or more selected from the group consisting of cobalt, manganese, and nickel, or a lithium-containing olivine-type phosphate containing one or two or more selected from iron, cobalt, nickel, and manganese is used. These positive electrode active materials can be used alone or in combination of two or more.

[0031] As such lithium complex metal oxides, one or more selected from the group consisting of LiCoO2, LiMn2O4, LiNiO2, LiCo 1-x Ni x O2(0.01 < x < 1), LiNi x Mn y Co z O2(x + y + z = 1), a solid solution of Li2MnO3 and LiMO2(M is Co, Ni, Mn, Fe, or the like transition metal), LiNi 1 / 2 Mn 3 / 2 O4, LiFePO4, LiMnPO4, and LiMn 1-x Fe x PO4(0.01 < x < 1) are more preferable. A part of these lithium complex metal oxides or lithium-containing olivine-type phosphates can be substituted with other elements, or a part of cobalt, nickel, manganese, and iron can be substituted with one or two or more selected from Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W, and Zr, or can be coated with a compound containing these other elements or a carbon material. Lithium cobalt oxide is preferable.

[0032] For example, the positive electrode contains lithium cobalt oxide having at least one of lanthanum, iridium, cerium, tungsten, and strontium as a foreign element. From the viewpoint of improving the low-temperature output characteristics of the lithium-ion battery, the content of any one of the doping elements is preferably 0.01% by mass or more, preferably 0.03% by mass or more, and more preferably 0.05% by mass or more, based on the mass of lithium cobalt oxide. In addition, as the upper limit of the content of the doping elements, the content of any one of the doping elements is 1% by mass or less, preferably 0.5% by mass or less, more preferably 0.3% by mass or less, further preferably 0.15% by mass or less, and particularly preferably 0.1% by mass or less.

[0033] As the voltage at the time of charging, from the viewpoint of high-voltage, the positive electrode potential is preferably 4.4 V (vs. Li / Li+) or more, more preferably 4.5 V (vs. Li / Li+) or more, and particularly preferably 4.6 V (vs. Li / Li+) or more.

[0034] The conductive agent of the positive electrode is not particularly limited as long as it is an electron-conducting material that does not cause chemical changes. Examples include natural graphite (flaky graphite and the like), artificial graphite, acetylene black, ketjen black, slot method carbon black, furnace black, lamp black, or thermal cracking method carbon black, and the like. In addition, graphite and carbon black can be appropriately mixed and used. The amount of the conductive agent added to the positive electrode mixture is preferably 1 to 10% by mass, and particularly preferably 1.5 to 5% by mass.

[0035] The binder of the positive electrode includes at least one of polyvinylidene fluoride, polyacrylonitrile, and polyacrylic acid. From the viewpoint of improving the through resistance characteristics of the positive electrode and the low-temperature output stability of the secondary battery, polyacrylonitrile is preferred.

[0036] From the viewpoint of improving the resistance through properties and the peeling strength of the positive electrode, the positive electrode binder can further include a dispersant including at least one of bis(2,2,6,6-tetramethylpiperidin-4-yl) sebacate, bis(2,2,6,6-tetramethylpiperidin-4-yl) succinate, bis(l,2,2,6,6-pentamethylpiperidin-4-yl) sebacate, bis(l-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, bis(l,2,2,6,6-pentamethylpiperidin-4-yl) n-butyl-3,5-di-t-butyl 4-hydroxybenzylmalonate, a condensate of 1-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4-hydroxypiperidine and succinic acid, 2,2,6,6-tetramethylpiperidin-4-yl stearate, 2,2,6,6-tetramethylpiperidin-4-yl dodecanoate, 1,2,2,6,6-pentamethylpiperidin-4-yl stearate, 1,2,2,6,6-pentamethylpiperidin-4-yl dodecanoate, N,N'-bis(2,2,6,6-tetramethylpiperidin-4-yl) hexamethylenediamine, a condensate of 4-t-octylamino-2,6-dichloro-l,3,5-triazine, tris(2,2,6,6-tetramethylpiperidin-4-yl) nitrilotriacetate.

[0037] From the viewpoint of improving the resistance through properties and the peeling strength of the positive electrode, the positive electrode includes an inorganic additive. The metal element in the inorganic additive includes at least one of aluminum, magnesium, titanium, zirconium, niobium, indium, tungsten, tin, zinc, antimony.

[0038] Specifically, the inorganic additive can be at least one of an inorganic metal oxide, an inorganic metal hydroxide. For example, aluminum oxide, aluminum hydroxide, boehmite, magnesium oxide, magnesium hydroxide, titanium dioxide, zirconium dioxide, niobium monoxide, niobium dioxide, di-niobium trioxide, di-niobium pentoxide, di-indium trioxide, tungsten trioxide, tin dioxide, zinc oxide, di-antimony trioxide.

[0039] The inorganic additive can be included in any one or more of the active material layer, the undercoat layer, or the edge layer. From the viewpoint of improving the resistance through properties of the positive electrode, it is preferable to be included in the undercoat layer or the edge layer; from the viewpoint of improving the peeling strength, it is preferable to be included in the active material layer.

[0040] There are three ways to manufacture the positive electrode as follows:

[0041] Method 1: The inorganic additive is included in the active material layer

[0042] The positive electrode is manufactured by mixing the above-mentioned positive electrode active material, the inorganic additive, and a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile, adding a high-boiling solvent such as 1-methyl-2-pyrrolidone thereto, and performing kneading to manufacture a slurry, coating the slurry on an aluminum foil serving as a current collector, drying, and applying pressure.

[0043] Method 2: Inorganic additive is contained in the undercoat layer

[0044] The inorganic additive, a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile are mixed, deionized water is added thereto and mixed uniformly to prepare a slurry, which is applied to an aluminum foil of the current collector, dried, and formed into an undercoat layer.

[0045] The positive electrode active material, a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile are mixed, a high-boiling solvent such as 1-methyl-2-pyrrolidinone is added thereto and mixed to prepare a slurry, which is applied to the undercoat layer, dried, and formed into a positive electrode by pressing.

[0046] Method 3: Inorganic additive is contained in the edge layer

[0047] The inorganic additive, a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile are mixed, deionized water is added thereto and mixed uniformly to prepare a slurry 1.

[0048] The positive electrode active material, a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile are mixed, a high-boiling solvent such as 1-methyl-2-pyrrolidinone is added thereto and mixed to prepare a slurry 2.

[0049] The slurry 1 and the slurry 2 are applied in parallel to an aluminum foil of the current collector or the like, with the slurry 1 being located at one or both outer edges of the slurry 2 (in the direction of the applied tape), and then dried and formed into a positive electrode by pressing.

[0050] The density of the positive electrode active material layer is generally 3.5 g / cm 3 Preferably, the density of the positive electrode active material layer is 3.8 g / cm 3 More preferably, the density of the positive electrode active material layer is 4 g / cm 3 Further preferably, the density of the positive electrode active material layer is 4.1 g / cm 3 Further, as an upper limit, the density of the positive electrode active material layer is preferably 4.6 g / cm 3 Further.

[0051] The positive electrode current collector is not particularly limited and can be any material known to be suitable for use as a positive electrode current collector. Examples of the positive electrode current collector can include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and carbon materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0052] III. Negative electrode

[0053] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on a surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material. In some embodiments, the chargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent lithium metal from inadvertently depositing on the negative electrode during charging.

[0054] Further, as the negative electrode active material, there is no particular limitation, and carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials combining them, and the like can be given.

[0055] Carbon-based negative electrode active materials

[0056] Here, the carbon-based negative electrode active material refers to an active material having a carbon-based skeleton into which lithium can be inserted, and as the carbon-based negative electrode active material, for example, carbonaceous materials and graphitic materials can be given.

[0057] As the carbonaceous material, for example, easy graphitizable carbon, difficult graphitizable carbon having a structure with an amorphous-like structure represented by glassy carbon, and the like can be given. Here, as the easy graphitizable carbon, for example, carbon materials obtained from tar pitch as a raw material from petroleum or coal can be given. When a specific example is given, for example, coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, and pyrolytic vapor grown carbon fibers can be given. Further, as the difficult graphitizable carbon, for example, phenol resin sintered bodies, polyacrylonitrile-based carbon fibers, quasi-isotropic carbon, furfuryl alcohol resin sintered bodies (PFA), hard carbon, and the like can be given.

[0058] Further, as the graphitic material, for example, natural graphite, artificial graphite, and the like can be given. Among them, as the artificial graphite, for example, artificial graphite obtained by heat-treating carbon containing easy graphitizable carbon at 2800°C or higher, graphitic MCMB obtained by heat-treating MCMB at 2000°C or higher, graphitic mesophase pitch-based carbon fibers obtained by heat-treating mesophase pitch-based carbon fibers at 2000°C or higher, and the like can be given. In addition, in the present application, as the carbon-based negative electrode active material, natural graphite in which at least a part of the surface thereof is covered with amorphous carbon (amorphous-coated natural graphite) can be used.

[0059] Further, the metal-based negative electrode active material is an active material containing a metal, and generally refers to an active material having a theoretical current capacity of 500 mAh / g or more per unit mass in a case where an element capable of intercalating lithium or alloying with lithium is contained in the structure and intercalates lithium or alloy with lithium. As the metal-based negative electrode active material, for example, lithium metal, elemental metals capable of forming lithium alloys (for example, Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, and the like), alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, and the like thereof can be used. Among them, as the metal-based negative electrode active material, an active material containing silicon (silicon-based negative electrode active material) is preferable. This is because the use of the silicon-based negative electrode active material can allow the secondary battery to be high in capacity.

[0060] As the silicon-based negative electrode active material, for example, silicon (Si), an alloy containing silicon, silicon oxide, a composite of a silicon-containing material and a conductive carbon in which the silicon-containing material is coated or compounded with the conductive carbon can be given.

[0061] From the viewpoint of improving the capacity of the battery, a silicon-carbon material, for example, a composite of porous carbon supporting silicon, is preferable.

[0062] In addition, the negative electrode active material can be used alone or two or more kinds thereof can be used in combination at an arbitrary ratio.

[0063] The negative electrode mixture layer can further include a negative electrode binder. The negative electrode binder can improve the binding between the negative electrode active material particles and the binding between the negative electrode active material and the current collector. The kind of the negative electrode binder is not particularly limited, and can be a material stable to an electrolyte or a solvent used at the time of manufacturing the electrode. In some embodiments, the negative electrode binder includes a resin binder. Examples of the resin binder include, but are not limited to, a fluorine resin, a polyacrylonitrile (PAN), a polyimide resin, an acrylic resin, a polyolefin resin, and the like. When a water-based solvent is used to prepare a negative electrode mixture slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or a salt thereof, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, and the like.

[0064] As the current collector that holds the negative electrode active material, a publicly known current collector can be arbitrarily used. Examples of the negative electrode current collector include, but are not limited to, metal materials such as copper, nickel, stainless steel, nickel-plated steel, and the like. In some embodiments, the negative electrode current collector is copper.

[0065] The negative electrode can be prepared by coating a negative electrode mixture slurry containing a negative electrode active material, a resin binder, and the like on a negative electrode current collector, drying, and then calendering to form a negative electrode mixture layer on both surfaces of the negative electrode current collector, whereby the negative electrode can be obtained.

[0066] IV. Separation Film

[0067] In order to prevent short-circuiting, a separation film is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte solution of the present application is usually impregnated into the separation film and used.

[0068] The material and shape of the separation film are not particularly limited as long as the effects of the present application are not significantly impaired. The separation film can be a resin, a glass fiber, an inorganic substance, or the like formed of a material stable to the electrolyte solution of the present application. In some embodiments, the separation film includes a porous sheet or a nonwoven fabric-like substance having excellent liquid retention, or the like. Examples of the material of the resin or glass fiber separation film can include, but are not limited to, polyolefin, aramid, polytetrafluoroethylene, polyethersulfone, or the like. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The above-mentioned materials of the separation film can be used alone or in any combination.

[0069] The separation film can also be a material in which the above-mentioned materials are layered, examples of which include, but are not limited to, a three-layer separation film in which polypropylene, polyethylene, and polypropylene are layered in this order, or the like.

[0070] Examples of the material of the inorganic substance can include, but are not limited to, oxides such as alumina and silica, nitrides such as aluminum nitride and silicon nitride, sulfates (e.g., barium sulfate, calcium sulfate, and the like). The form of the inorganic substance can include, but is not limited to, a granular or fibrous form.

[0071] The shape of the separation film can be a film shape, examples of which include, but are not limited to, a nonwoven fabric, a woven fabric, a microporous film, or the like. In the film shape, the separation film has a pore diameter of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm. In addition to the above-mentioned independent film-shaped separation film, a separation film formed by forming a composite porous layer containing the above-mentioned inorganic substance particles on the surface of the positive electrode and / or the negative electrode using a resin-based adhesive can also be used, for example, a separation film in which 90% of alumina particles having a particle size of less than 1 μm are formed into a porous layer on both sides of the positive electrode using a fluororesin as an adhesive.

[0072] The thickness of the separation film is arbitrary. In some embodiments, the thickness of the separation film is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separation film is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separation film is within the above-mentioned range, the insulating property and the mechanical strength can be ensured, and the direct current resistance property and the energy density of the secondary battery can be ensured.

[0073] The present application also provides an electronic device including the secondary battery according to the present application.

[0074] The use of the secondary battery of the present application is not particularly limited, and it can be used for any electronic device known in the art. In some embodiments, the secondary battery of the present application can be used for, but not limited to, notebook computers, pen-input computers, mobile computers, electronic book players, portable telephones, portable facsimile machines, portable copying machines, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, hand-held cleaners, portable CD players, mini-disc players, transceivers, electronic organizers, calculators, memory cards, portable audio players, radios, backup power supplies, motors, automobiles, motorcycles, power-assisted bicycles, bicycles, lighting appliances, toys, game machines, timepieces, power tools, flashlights, cameras, home-use large-size storage batteries, and lithium-ion capacitors, and the like.

[0075] The preparation of the secondary battery will be described below in connection with specific examples, and those skilled in the art will understand that the preparation methods described in the present application are only examples, and any other suitable preparation method is within the scope of the present application.

[0076] Examples

[0077] Hereinafter, examples of the non-aqueous electrolyte solution of the present application will be shown, but the present application is not limited to these examples.

[0078] 1. Preparation of the secondary battery

[0079] (1) Preparation of the positive electrode:

[0080] Method 1: Inorganic additive is contained in the active material layer

[0081] The positive electrode active material of Table 1-1 : 97 mass%, the inorganic additive of Table 1-1 : 0.5 mass%, and acetylene black: 1.0 mass% were mixed, added to a solution in which the binder of Table 1-1 : 1.5 mass% was previously dissolved in 1-methyl-2-pyrrolidone, and mixed to prepare a positive electrode slurry. The positive electrode slurry was coated on both sides of an aluminum foil, dried, and pressure-treated, and then cut into a prescribed size to prepare a positive electrode.

[0082] Method 2: Inorganic additive is contained in the primer layer

[0083] The inorganic additive of Table 1-1 : 65 mass%, acetylene black: 20 mass%, and the binder of Table 1-1 : 15 mass% were mixed, deionized water was added thereto, and mixed uniformly to prepare a slurry, which was coated on both sides of an aluminum foil, dried, and formed into a primer layer.

[0084] The positive electrode active material of Table 1-1 : 97 mass%, acetylene black: 1.5 mass% were mixed, added to a solution in which the binder of Table 1-1 : 1.5 mass% was previously dissolved in 1-methyl-2-pyrrolidinone and mixed to prepare a positive electrode slurry. The positive electrode slurry was coated on the surface of the undercoat layer, dried, pressure treated and cut into a prescribed size to make a positive electrode.

[0085] Method three: inorganic additive is contained in the edge layer

[0086] The inorganic additive of Table 1-1 : 65 mass%, acetylene black: 20 mass% and the binder of Table 1-1 : 15 mass% were mixed, deionized water was added thereto and mixed uniformly to prepare a positive electrode slurry 1, which was coated on the edge of the side of the aluminum foil having a tab, dried and formed into an edge layer.

[0087] The positive electrode active material of Table 1-1 : 97 mass%, acetylene black: 1.5 mass% were mixed, added to a solution in which the binder of Table 1-1 : 1.5 mass% was previously dissolved in 1-methyl-2-pyrrolidinone and mixed to prepare a positive electrode slurry 2. The slurry 1 and the slurry 2 were coated on the aluminum foil in parallel at the same time, the slurry 1 was located at the outer edge of the slurry 2 (in the direction of the coating run), dried, pressure treated and cut into a prescribed size to make a positive electrode.

[0088] (2) Negative electrode production:

[0089] The negative electrode active material artificial graphite and silicon carbon (mass ratio 90: 10): 96 mass%, butadiene rubber: 2 mass% were mixed, added to a solution in which the lithium carboxymethyl cellulose: 2 mass% was previously dissolved in deionized water and mixed to prepare a negative electrode mixture paste. The negative electrode mixture paste was coated on one side of the copper foil (current collector), dried, pressure treated and cut into a prescribed size to make a negative electrode.

[0090] (3) Battery production:

[0091] The positive electrode and the negative electrode each produced as above were connected with a lead wire. Lamination was performed via a polypropylene porous film having a thickness of 10 μm. Furthermore, LiPF6 as a supporting electrolyte was dissolved in a solution containing (I) lithium difluorophosphate, (II) a lithium salt containing boron and (III) ethyl propionate, and propyl propionate, ethylene carbonate and propylene carbonate (mass ratio 2.3: 1.2: 0.9). The contents and components of (I) to (III), other additives based on the total mass of the nonaqueous electrolyte solution were as shown in Table 2, and the content of LiPF6 was 14%.

[0092] Then, the laminate was housed in an aluminum laminate case together with 3.2 g of electrolyte. The opening of the case was heat-sealed, and a secondary battery was produced through steps of formation, capacity, etc. The secondary battery was a pouch-shaped battery having a width of 35 mm, a height of 48 mm, and a thickness of 5 mm.

[0093] The codes of the positive active material, the inorganic additive, the binder, and the electrolyte components of the secondary battery produced are shown in Tables 1-1 and 1-2. The positive active material was lithium cobaltate containing a hetero element, and the mass content of the hetero element was calculated based on the mass of the positive active material.

[0094] Polyvinylidene fluoride is abbreviated as PVDF, polyacrylonitrile is abbreviated as PAN, and polyacrylic acid is PAA;

[0095] The dispersants included bis(2,2,6,6-tetramethylpiperidin-4-yl) sebacate (code Al), bis(2,2,6,6-tetramethylpiperidin-4-yl) succinate (code A2), bis(l,2,2,6,6-pentamethylpiperidin-4-yl) sebacate (code A3), bis(l-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate (code A4), and bis(l,2,2,6,6-pentamethylpiperidin-4-yl) n-butyl-3,5-di-t-butyl-4-hydroxybenzylmalonate (code A5). The mass of the dispersant was 10% of the total weight of the binder.

[0096] Table 1-1 Positive Electrode

[0097] Note: The values in Table 1-1 above are mass %.

[0098] Table 1-2 Electrolyte

[0099] Note: The values in parentheses in Table 1-2 above are mass %.

[0100] The abbreviations in Table 1-2 above represent the following substances

[0101] 2. Test Methods

[0102] (1) Positive Electrode Penetration Resistance

[0103] The fabricated secondary battery was disassembled, and the positive electrode was taken out. The positive electrode was clamped with a gold-plated clamp, and was pressurized at a pressure of 10 MPa. Then, using a terminal attached to the gold-plated clamp and a multichannel potentiostat, the voltage at the time when a current of 10 mA was flowing was measured. The voltage after 10 minutes was read, and the volume resistivity p (Ω-cm) was calculated from the calculated resistance value, the thickness, and the area of the positive electrode. The following criteria were used for evaluation. The smaller the volume resistivity p, the lower the penetration resistance.

[0104] A: Volume resistivity p is less than 90 Ω-cm

[0105] B: Volume resistivity p is 90 Ω-cm or more and less than 180 Ω-cm

[0106] C: Volume resistivity p is 180 Ω-cm or more and less than 270 Ω-cm

[0107] D: Volume resistivity p is 270 Ω-cm or more

[0108] (2) Positive electrode peeling strength

[0109] The fabricated secondary battery was disassembled, and a laminated body in which a positive electrode on which a mixture layer was coated on one surface and a separator were adhered to each other under pressurization conditions of a temperature of 25°C and a pressure of 1 MPa for 10 seconds was selected as a test piece.

[0110] The surface of the positive electrode on the side of the current collector of the test piece was pasted with a glass paper tape with the surface of the positive electrode on the side of the current collector facing downward. At this time, as the glass paper tape, a glass paper tape specified in JIS Z1522 was used. In addition, the glass paper tape was preliminarily fixed to a horizontal test stand. Then, one end of the separator was stretched in the vertical direction at a stretching speed of 50 mm / minute, and the stress at the time of peeling was measured.

[0111] This measurement was performed a total of 6 times, and the average value of the stress was taken as the peeling strength, and the adhesion of the positive electrode and the separator was evaluated according to the following criteria. The greater the peeling strength, the higher the adhesion between the electrode (positive electrode) and the separator.

[0112] A: Peeling strength is 5.0 N / m or more

[0113] B: Peeling strength is 4.0 N / m or more and less than 5.0 N / m

[0114] C: Peeling strength is 3.0 N / m or more and less than 4.0 N / m

[0115] D: Peeling strength is 2.0 N / m or more and less than 3.0 N / m

[0116] (3) Low-temperature rate characteristics

[0117] The prepared secondary battery was charged to 4.5 V at a constant current-constant voltage (CCCV) at a temperature of 25°C to prepare a battery cell.

[0118] A constant current charge-discharge was carried out at 0.2 C between 4.5 and 3.0 V of the battery cell voltage at a temperature of 25°C, and the discharge capacity at this time was defined as CO. Thereafter, the battery cell was charged at a constant current of 0.2 C, and a discharge was carried out at a constant current of 0.5 C to 3.0 V at a temperature of -20°C, and the discharge capacity at this time was defined as CI. Then, as a rate characteristic, a capacity retention rate represented by ΔC = (CI / CO) x 100 (%) was calculated, and was evaluated according to the following criteria. The larger the value of the capacity retention rate ΔC, the higher the discharge capacity at a high current in a low temperature environment, and the lower the internal resistance.

[0119] A: The capacity retention rate ΔC is 70% or more

[0120] B: The capacity retention rate ΔC is more than 60% and less than 70%

[0121] C: The capacity retention rate ΔC is more than 50% and less than 60%

[0122] D: The capacity retention rate ΔC is less than 50%

[0123] (4) Low temperature output characteristic

[0124] The prepared secondary battery was charged to 4.5 V at a constant current-constant voltage (CCCV) at a temperature of 25°C to prepare a battery cell. The prepared battery cell was discharged to 3.0 V at a constant current of 0.2 C and 1 C at a temperature of -20°C, and the capacity was calculated. Then, a discharge capacity retention rate represented by a ratio (= (capacity at 1 C / capacity at 0.2 C) x 100 (%)) of the capacities was calculated. These measurements were carried out for five battery cells, and the average value of the calculated discharge capacity retention rates was used as an output characteristic, and was evaluated according to the following criteria. The larger the value, the more excellent the output characteristic.

[0125] A: The average value of the discharge capacity retention rates is 85% or more

[0126] B: The average value of the discharge capacity retention rates is more than 80% and less than 85%

[0127] C: The average value of the discharge capacity retention rates is more than 75% and less than 80%

[0128] D: The average value of the discharge capacity retention rates is less than 75%

[0129] Test results

[0130] Table 2-1

[0131] Table 2-2

[0132] Table 2-3

[0133] In a secondary battery in which a positive electrode includes a lithium cobalt oxide, a binder, and an inorganic additive, an electrolyte contains (I) lithium difluorophosphate, (II) a lithium salt containing boron, and (III) ethyl propionate, the total content of the (I) and the (II) is 0.03% by mass or more and 1.5% by mass or less based on the mass of the electrolyte; and the content of the (III) is 12% by mass or more and 37% by mass or less, which is very critical for the stability of the positive electrode structure, significantly inhibits an increase in interface resistance at a high voltage, and this design not only improves the positive electrode penetration resistance and the peeling strength, but also improves the low-temperature output characteristics and the low-temperature rate characteristics of the secondary battery.

[0134] When the positive electrode binder further includes a condensate of bis(2,2,6,6-tetramethylpiperidin-4-yl) sebacate, bis(2,2,6,6-tetramethylpiperidin-4-yl) succinate, bis(1,2,2,6,6-pentamethylpiperidin-4-yl) sebacate, bis(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, bis(1,2,2,6,6-pentamethylpiperidin-4-yl) n-butyl-3,5-di-t-butyl-4-hydroxybenzyl malonate, a condensate of 1-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4-hydroxypiperidine and succinic acid, 2,2,6,6-tetramethylpiperidin-4-yl stearate, 2,2,6,6-tetramethylpiperidin-4-yl dodecanoate, 1,2,2,6,6-pentamethylpiperidin-4-yl stearate, 1,2,2,6,6-pentamethylpiperidin-4-yl dodecanoate, N,N'-bis(2,2,6,6-tetramethylpiperidin-4-yl) hexamethylenediamine, a condensate of 4-t-octylamino-2,6-dichloro-1,3,5-triazine, tris(2,2,6,6-tetramethylpiperidin-4-yl) nitrilotriacetate, the positive electrode penetration resistance characteristics and the peeling strength can be further improved.

[0135] When the electrolyte further contains other additives such as 1,2-bis(difluorophosphoroxo)ethane, fluoroethylene carbonate, vinylene carbonate, lithium fluorosulfate, butanedinitrile, hexanedinitrile, ethylene sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl) phosphate, tris(trimethylsilyl) borate, the low-temperature output characteristics and the low-temperature rate characteristics are further improved.

[0136] References throughout this specification to "an embodiment", "particular embodiments", "one embodiment", "another embodiment", "certain embodiments", "some embodiments", "one example" or "an example" mean that a particular feature, structure, material or characteristic is included in at least one embodiment or example of the present application. Thus, the appearances of the phrases such as "in some embodiments", "in an embodiment", "in one embodiment", "in another embodiment", "in one example", "in particular embodiments" or "in certain embodiments" in various places throughout this specification are not necessarily referring to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments or examples.

[0137] While the illustrative embodiments have been described and illustrated, it will be understood by those skilled in the art that the above-described embodiments are not the only ways in which the present application can be practiced. Changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the application.

Claims

1. A secondary battery comprising: a positive electrode, a negative electrode, and an electrolyte solution, the positive electrode includes lithium cobalt oxide, a binder, and an inorganic additive, and a metal element included in the inorganic additive includes at least one of aluminum, magnesium, titanium, zirconium, niobium, indium, tungsten, tin, zinc, and antimony; the electrolyte solution contains (I) lithium difluorophosphate, (II) a lithium salt containing boron, and (III) ethyl propionate, the total content of the (I) and the (II) is 0.03 mass% or more and 1.5 mass% or less based on the mass of the electrolyte solution, and the content of the (III) is 12 mass% or more and 37 mass% or less.

2. The secondary battery according to claim 1, wherein the total content of the (I) and the (II) is 0.04 mass% or more and 1.24 mass% or less.

3. The secondary battery according to claim 1, wherein the content of the (III) is 15 mass% or more and 35 mass% or less.

4. The secondary battery according to claim 1, wherein the binder includes at least one of polyvinylidene fluoride, polyacrylonitrile, and polyacrylic acid.

5. The secondary battery according to claim 1, wherein the binder further includes a dispersant including at least one of bis(2,2,6,6-tetramethylpiperidin-4-yl) sebacate, bis(2,2,6,6-tetramethylpiperidin-4-yl) succinate, bis(l,2,2,6,6-pentamethylpiperidin-4-yl) sebacate, bis(l-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate, bis(l,2,2,6,6-pentamethylpiperidin-4-yl) n-butyl-3,5-di-t-butyl 4-hydroxybenzyl malonate, a condensate of 1-(2-hydroxyethyl)-2,2,6,6-tetramethyl-4-hydroxypiperidine and succinic acid, 2,2,6,6-tetramethylpiperidin-4-yl stearate, 2,2,6,6-tetramethylpiperidin-4-yl dodecanoate, 1,2,2,6,6-pentamethylpiperidin-4-yl stearate, 1,2,2,6,6-pentamethylpiperidin-4-yl dodecanoate, N,N'-bis(2,2,6,6-tetramethylpiperidin-4-yl) hexamethylenediamine, a condensate of 4-t-octylamino-2,6-dichloro-l,3,5-triazine, tris(2,2,6,6-tetramethylpiperidin-4-yl) nitrilotriacetate.

6. The secondary battery according to claims 1 to 4, wherein the boron-containing lithium salt is at least one selected from the group consisting of lithium tetrafluoroborate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium tetracyanoborate, lithium tetrakis(trifluoromethyl)borate, lithium (trifluoromethyl)trifluoroborate, lithium bis(trifluoromethyl)difluoroborate, lithium pentafluoroethyltrifluoroborate, lithium dicyanooxalato borate, lithium bis(malonato)borate, lithium (2-fluoromalonato)difluoroborate, lithium malonato(oxalato)borate, lithium bis(p-salicylato)borate, lithium methoxytricyanoborate, lithium ethoxytricyanoborate, lithium tetramethoxyborate, lithium tetraethoxyborate, lithium tetrakis(trifluoromethoxy)borate, lithium tetrakis(2,2,2-trifluoroethoxy)borate, lithium poly(t-p-salicyloxy)borate, lithium bis(trifluoroborate) sulfate, lithium difluoroborate, lithium methanedisulfonato difluoroborate, lithium difluorophosphoryloxytrifluoroborate, lithium bis(difluorophosphoryloxy)difluoroborate, lithium tetra(difluorophosphoryloxy)borate.

7. The secondary battery according to claims 1 to 4, wherein the mass of the lithium difluorophosphate is 0.02 mass% or more and 1.15 mass% or less based on the mass of the electrolyte solution; the mass of the boron-containing lithium salt is 0.01 mass% or more and 0.95 mass% or less based on the mass of the electrolyte solution; the mass of the ethyl propionate is 16 mass% or more and 32 mass% or less based on the mass of the electrolyte solution.

8. The secondary battery according to claims 1 to 4, wherein the electrolyte solution further includes other additives including at least one of 1,2-bis(difluorophosphoryl)ethane, fluoroethylene carbonate, vinylene carbonate, lithium fluorosulfonate, butanedinitrile, hexanedinitrile, ethylene sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate.

9. The secondary battery according to claims 1 to 4, wherein the lithium cobalt oxide has at least one of lanthanum, iridium, cerium, tungsten, strontium as a foreign element; the content of any of the foreign elements is 0.01 mass% or more and 1 mass% or less based on the mass of the lithium cobalt oxide.

10. The secondary battery according to claims 1 to 4, wherein the positive electrode includes a base coat layer and an active material layer disposed on a current collector, and the inorganic additive is contained in the active material layer or the base coat layer.

11. The secondary battery according to claims 1 to 4, wherein the positive electrode includes an edge layer and an active material layer disposed on a current collector, and the inorganic additive is contained in the edge layer.

12. An electronic device including the secondary battery according to any one of claims 1 to 11.

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