Secondary battery and electronic device

Abstract

A secondary battery and an electronic device. Specifically, the secondary battery comprises a positive electrode, a negative electrode and an electrolyte; the positive electrode comprises a lithium cobalt oxide, a binder and an inorganic additive; the electrolyte comprises lithium difluorophosphate, a dinitrile compound and a trinitrile compound. The positive electrode resistance can be reduced, warping can be suppressed, and the safety and high-temperature storage characteristics of the secondary battery can also be improved.

Description

Secondary batteries and electronic devices Technical Field

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

[0002] The rapid development of electronic products such as smartphones, tablets, and smart wearables has placed higher demands on the service life and safety of secondary batteries, taking into account the varying usage times and operating temperatures of these products. The batteries for the new generation of high-end electronic products have been upgraded to high-voltage cells with a charge cutoff voltage of 4.4V. As the charge cutoff voltage increases, the battery's energy density significantly improves. However, in existing technologies, increasing the secondary battery voltage leads to a series of safety issues, such as poor high-temperature storage performance and gassing during high-temperature storage.

[0003] Summary of the Invention

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

[0005] The inventors discovered that the positive electrode includes lithium cobalt oxide, a binder and an inorganic additive, and the electrolyte includes lithium difluorophosphate, a dinitrile compound and a trinitrile compound, which can not only improve the positive electrode resistance and warping suppression, but also improve the safety and high-temperature storage characteristics of the secondary battery, thereby completing the present application.

[0006] In a secondary battery whose positive electrode includes lithium cobalt oxide, a binder and an inorganic additive, the electrolyte contains (I) lithium difluorophosphate, (II) a dinitrile compound and (III) a trinitrile compound. It is important that the total content of (I), (II) and (III) is not less than 1.5% by mass and not more than 10% by mass based on the mass of the electrolyte. In particular, it is critical to set the total content of (I) to (III) within a specific range for the stability of the positive electrode structure and significantly suppress the increase in interface resistance under high voltage. This design can not only improve the positive electrode resistance and warpage suppression, but also improve the safety and high-temperature storage characteristics of the secondary battery.

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

[0008] The present application uses a specific combination of positive electrode structure and electrolyte, which can not only improve the positive electrode resistance and warpage suppression, but also enhance the safety and high-temperature storage characteristics of the secondary battery.

[0009] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the following description through 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 interpreted as limiting the present application.

[0011] Unless expressly stated otherwise, the following terms used in this application have the meanings indicated below.

[0012] The present application uses a specific combination of positive electrode structure and electrolyte, which can not only improve the positive electrode resistance and warpage suppression, but also enhance the safety and high-temperature storage characteristics of the secondary battery.

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

[0014] I. Electrolyte

[0015] The electrolyte used in the secondary battery of the present application includes an electrolyte and a solvent for dissolving the electrolyte. In some embodiments, the electrolyte of the present application includes (I) lithium difluorophosphate, (II) a dinitrile compound, and (III) a trinitrile compound.

[0016] In the prior art, dinitrile compounds are commonly used to stabilize the surface structure of positive electrode materials. However, the film formed by dinitrile compounds on the surface of the positive electrode material is easily decomposed at high voltages (4.6V and above), especially in the presence of inorganic additives, which can decompose at 4.4V. Although the cause of decomposition is unclear, it is speculated that the inorganic additives may catalyze the decomposition of the film. The inventors have discovered that when the electrolyte also contains lithium difluorophosphate and trinitrile compounds, the decomposition of the film can be significantly suppressed, thereby improving not only the positive electrode resistance and warpage suppression, but also the safety and high-temperature storage characteristics of the secondary battery.

[0017] Specifically, from the viewpoint of improving the positive electrode resistance characteristics of secondary battery, based on electrolyte quality, the content of lithium difluorophosphate is more than 0.01 mass %, preferably the content of lithium difluorophosphate is more than 0.04 mass %, preferably more than 0.06 mass %, more preferably more than 0.08 mass %. In addition, as the upper limit of the content of lithium difluorophosphate, from the viewpoint of generating heat when suppressing internal short circuit, the content of lithium difluorophosphate is 1 mass % or less, preferably less than 0.9 mass %, more preferably less than 0.8 mass %, further preferably less than 0.7 mass %, particularly preferably less than 0.6 mass %. When within the above range, it contributes to generating heat when further suppressing internal short circuit.

[0018] In the present application, dinitrile compound refers to a nitrile compound containing two nitrile groups, specifically, dinitrile compound is selected from at least one of succinonitrile, adiponitrile, ethylene glycol di(propionitrile) ether or 1,4-dicyano-2-butene. As dinitrile compound, it can be only one kind, or it can be more than two kinds. Specifically, from the viewpoint of improving positive electrode resistance characteristics, based on the quality of the electrolyte, the content of dinitrile compound is more than 0.3% by mass, preferably the content of dinitrile compound is more than 0.9% by mass, preferably more than 1.2% by mass, more preferably more than 2.5% by mass. In addition, as the upper limit of the content of dinitrile compound, from the viewpoint of suppressing heat generation during internal short circuit, the content of dinitrile compound is less than 8.5% by mass, preferably less than 6.1% by mass, more preferably less than 5.4% by mass, further preferably less than 4.4% by mass, particularly preferably less than 4% by mass. When within the above range, it helps to further suppress heat generation during internal short circuit.

[0019] In the present application, a trinitrile compound refers to a nitrile compound containing three nitrile groups. Specifically, the trinitrile compound is selected from at least one of 1,2,3-propanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,2,4-tris(2-cyanoethoxy)butane, 1,2,3-tris(2-cyanoethoxy)propane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, or 1,2,5-tris(cyanoethoxy)pentane. The trinitrile compound may be only one or more. From the viewpoint of improving the high temperature storage characteristics of the secondary battery, based on the mass of the electrolyte, the content of the trinitrile compound is 0.5% by mass or more, preferably 0.8% by mass or more, preferably 1.4% by mass or more, preferably 1.7% by mass or more, more preferably 2.1% by mass or more. In addition, as the upper limit of the content of the trinitrile compound, from the viewpoint of suppressing the increase in positive electrode resistance, the content of the trinitrile compound is 5% by mass or less, preferably 4.1% by mass or less, preferably 3.5% by mass or less, more preferably 3.1% by mass or less, further preferably 2.6% by mass or less, particularly preferably 1.8% by mass or less. When within the above range, it helps to further suppress the increase in positive electrode resistance.

[0020] Furthermore, the total content of (I), (II) and (III) is 1.5% by mass or more, preferably 1.8% by mass or more, based on the mass of the electrolyte, from the viewpoint of suppressing heat generation during internal short circuit. In addition, the upper limit of the total content of (I), (II) and (III) is 10% by mass or less, preferably 9.5% by mass or less, from the viewpoint of improving electrochemical characteristics under high temperature environments.

[0021] In some embodiments, the total content of (I), (II), and (III) is a mass %, where a is 1.5, 1.8, 2.66, 3.11, 3.63, 4, 5.1, 5.45, 5.6, 5.8, 6.5, 7.1, 7.36, 7.4, 8.1, 9.4, 9.5, 9.7, or 10, or within a range consisting of any two of these values. Within this range, heat generation during an internal short circuit can be further suppressed.

[0022] In addition, the electrolyte may further include a first compound, such as lithium monofluorophosphate, 1,2-bis(difluorophosphine)ethane, vinylene carbonate, lithium fluorosulfonate, 1,3-propane sultone, 1,3-propylene sultone, 1,3-propylene glycol cyclic sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilane)phosphate or tris(trimethylsilane)borate. The inventors also unexpectedly discovered that the first compound can improve the stability of the aforementioned film at high temperature and high voltage and increase the ion conduction rate of lithium ions, thereby further improving the high temperature storage characteristics. The above-mentioned first compound may be only one or more than two.

[0023] Specifically, from the perspective of improving lithium ion conduction in the aforementioned coating, the content of the first compound is 0.01% by mass or more, preferably 0.1% by mass or more, preferably 0.3% by mass or more, and more preferably 0.5% by mass or more, based on the mass of the electrolyte. Furthermore, from the perspective of improving initial resistance, the upper limit of the first compound content is 10% by mass or less, preferably 9% by mass or less, more preferably 8% by mass or less, further preferably 7% by mass or less, and particularly preferably 5% by mass or less. Within this range, high-temperature storage characteristics are further improved.

[0024] In some embodiments, the electrolyte is not particularly limited, and any substance known as an electrolyte can be used arbitrarily. The quality of the electrolyte is not particularly limited, as long as it does not damage the effect 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% by mass based on the mass of the electrolyte, preferably 9 to 13% by mass, and more preferably 9 to 12% by mass. By setting the above content range, it is possible to more balancedly improve the high temperature storage characteristics and improve the low temperature discharge characteristics.

[0025] In some embodiments, the electrolyte further comprises a non-aqueous solvent known in the prior art that can serve as a solvent for the electrolyte. In some embodiments, the non-aqueous solvent includes, but is not limited to, one or more of the following: cyclic carbonates, linear carbonates, cyclic carboxylates, linear carboxylates, cyclic ethers, linear ethers, phosphorus-containing organic solvents, and sulfur-containing organic solvents.

[0026] II. Positive Electrode

[0027] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material, and the positive electrode mixture layer can be one layer or multiple layers. The positive electrode mixture layer includes a positive electrode active material layer, and optionally includes a bottom coating layer or an edge layer. The positive electrode active material is any material that can reversibly intercalate and deintercalate lithium ions.

[0028] For example, as the positive electrode active material for a secondary battery, a composite metal oxide containing one or more selected from the group consisting of cobalt, manganese, and nickel and lithium, or a lithium-containing olivine-type phosphate containing one 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.

[0029] As such a lithium composite metal oxide, for example, those selected from 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 a transition metal such as Co, Ni, Mn, Fe, etc.), LiNi 1 / 2 Mn 3 / 2 O4, LiFePO4, LiMnPO4, and LiMn 1-x FexPO4 (0.01 < x < 1), etc., are preferably one or more, more preferably two or more. A part of these composite metal oxides with lithium or lithium-containing olivine-type phosphates can be replaced by other elements. For example, a part of cobalt, nickel, manganese, and iron can be replaced by one or two or more substituting elements selected from Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W, and Zr, or coated with a compound or carbon material containing these substituting elements.

[0030] For example, the positive electrode includes lithium cobalt oxide containing at least one element A selected from lanthanum, yttrium, cerium, tungsten, or strontium. From the perspective of improving the high-temperature storage characteristics of the lithium-ion battery, the content of any one of the elements A 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 the lithium cobalt oxide. In addition, as an upper limit for the content of element A, the content of any one of the elements A 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.

[0031] From the viewpoint of increasing the voltage during charging, the positive electrode potential is preferably 4.4 V (vs. Li / Li + ) or higher, more preferably 4.5 V (vs. Li / Li + ) or higher, and particularly preferably 4.6 V (vs. Li / Li + ) or higher.

[0032] 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 graphites such as natural graphite (e.g., flaky graphite) or artificial graphite, and carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black. In addition, graphite and carbon black may be appropriately mixed for use. The amount of the conductive agent added to the positive electrode mixture is preferably 1 to 10% by mass, particularly preferably 1.5 to 5% by mass.

[0033] The positive electrode binder includes at least one of polyvinylidene fluoride, polyacrylonitrile, and polyacrylic acid. Polyacrylonitrile is preferred from the viewpoint of improving the positive electrode resistance characteristics and the high-temperature storage stability of the secondary battery.

[0034] From the perspective of improving the positive electrode resistance characteristics and suppressing positive electrode warpage, the positive electrode binder may further include a dispersant, and the dispersant includes at least one of glycerol trimethacrylate, trimethylolpropane trimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, di(trimethylolpropane) trimethacrylate, di(trimethylolpropane) tetramethacrylate, di(pentaerythritol) tetramethacrylate, di(pentaerythritol) pentamethacrylate, di(pentaerythritol) hexamethacrylate, di(glycerol) trimethacrylate, di(glycerol) tetramethacrylate, or tri(hydroxymethylpropane) alkylene oxide-modified trimethacrylate. The dispersant, when combined with the components in the positive electrode and the components in the electrolyte of the present application, is also beneficial for improving the positive electrode resistance characteristics and suppressing positive electrode warpage.

[0035] From the perspective of improving the positive electrode resistance characteristics and suppressing the warping of the positive electrode, the inorganic additives contained in the positive electrode include at least one metal element selected from the group consisting of aluminum, magnesium, titanium, zirconium, niobium, indium, tungsten, tin, zinc, and antimony.

[0036] Specifically, the inorganic additive can be at least one of an inorganic metal oxide or an inorganic metal hydroxide. Examples include aluminum oxide, aluminum hydroxide, boehmite, magnesium oxide, magnesium hydroxide, titanium dioxide, zirconium dioxide, niobium monoxide, niobium dioxide, niobium trioxide, niobium pentoxide, indium trioxide, tungsten trioxide, tin dioxide, zinc oxide, and antimony trioxide. 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 perspective of improving internal short-circuit characteristics, it is preferably included in the edge layer; from the perspective of suppressing positive electrode warpage, it is preferably included in the undercoat layer.

[0037] There are three ways to make the positive electrode:

[0038] Method 1: Inorganic additives are included in the active material layer

[0039] The above-mentioned positive electrode active material, inorganic additives, conductive agents such as acetylene black and carbon black, and binders such as polyvinylidene fluoride or polyacrylonitrile are mixed, and a high-boiling point solvent such as 1-methyl-2-pyrrolidone is added and kneaded to form a slurry, which is then coated on an aluminum foil of a current collector, dried, and pressed to form a positive electrode.

[0040] Method 2: Inorganic additives are included in the base coat

[0041] Inorganic additives are mixed with conductive agents such as acetylene black and carbon black, and binders such as polyvinylidene fluoride or polyacrylonitrile, deionized water is added and mixed evenly to form a slurry, which is then coated on the aluminum foil of the current collector and dried to form a primer layer.

[0042] The above-mentioned positive electrode active material is mixed with a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile, and a high-boiling point solvent such as 1-methyl-2-pyrrolidone is added and kneaded to form a slurry, which is then applied to a primer layer, dried, and pressed to form a positive electrode.

[0043] Method 3: Inorganic additives are included in the edge layer

[0044] An inorganic additive is mixed with a conductive agent such as acetylene black, carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile, and deionized water is added thereto and mixed evenly to prepare a slurry 1.

[0045] The above-mentioned positive electrode active material is mixed with a conductive agent such as acetylene black or carbon black, and a binder such as polyvinylidene fluoride or polyacrylonitrile, and a high boiling point solvent such as 1-methyl-2-pyrrolidone is added thereto and kneaded to prepare slurry 2 .

[0046] Slurry 1 and slurry 2 are coated in parallel on the aluminum foil of the current collector, with slurry 1 located at one or two outer edges of slurry 2 (along the direction of coating), and then dried and pressed to form a positive electrode, in which slurry 1 forms the edge layer and slurry 2 forms the active material layer.

[0047] The density of the positive electrode excluding the current collector is usually 3.5 g / cm 3 In order to further increase the capacity of the battery, it is preferably 3.8 g / cm 3 More than 4 g / cm 3 More preferably, 4.1 g / cm 3 In addition, as its upper limit, it is preferably 4.6 g / cm 3 the following.

[0048] The type of positive electrode current collector is not particularly limited and can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and 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.

[0049] III. Negative electrode

[0050] The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on a surface of the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material. In some embodiments, the charge capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent unintentional deposition of lithium metal on the negative electrode during charging.

[0051] The negative electrode active material is not particularly limited, and examples thereof include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials obtained by combining these.

[0052] Carbon-based negative electrode active materials

[0053] Here, the carbon-based negative electrode active material refers to an active material having a carbon-based skeleton that can insert lithium. Examples of the carbon-based negative electrode active material include carbonaceous materials and graphite materials.

[0054] Examples of carbonaceous materials include easily graphitized carbon and non-graphitizable carbon having an amorphous structure, such as glassy carbon. Examples of easily graphitized carbon include carbon materials obtained from petroleum or coal using tar pitch as a raw material. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, and pyrolysis vapor-grown carbon fibers. Examples of non-graphitizable carbon include phenolic resin sintered bodies, polyacrylonitrile-based carbon fibers, quasi-isotropic carbon, furfuryl alcohol resin sintered bodies (PFA), and hard carbon.

[0055] Furthermore, examples of graphite materials include natural graphite and artificial graphite. Examples of artificial graphite include artificial graphite obtained by heat-treating carbon containing graphitized carbon at 2800°C or higher, graphite MCMB obtained by heat-treating MCMB at 2000°C or higher, and graphite mesophase pitch-based carbon fibers obtained by heat-treating mesophase pitch-based carbon fibers at 2000°C or higher. Furthermore, in the present application, natural graphite (amorphous-coated natural graphite) at least a portion of its surface is coated with amorphous carbon may be used as the carbon-based negative electrode active material.

[0056] In addition, the metal-based negative electrode active material is an active material containing a metal, generally referring to an active material having an element capable of inserting lithium or alloying with lithium in the structure, an element inserted into lithium or alloyed with lithium, and a theoretical current capacity of 500 mAh / g or more per unit mass. As a metal-based negative electrode active material, for example, lithium metal, a single metal that can form a lithium alloy (such as Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.) and its alloys, and their oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. can be used. Among them, as a metal-based negative electrode active material, an active material containing silicon (silicon-based negative electrode active material) is preferably used. This is because the use of a silicon-based negative electrode active material can increase the capacity of the secondary battery.

[0057] Examples of silicon-based negative electrode active materials include silicon (Si), alloys containing silicon, SiO, and SiO. x , a composite of silicon-containing material and conductive carbon in which a silicon-containing material is coated or composited with conductive carbon.

[0058] From the viewpoint of improving battery capacity, silicon-carbon materials, for example, porous carbon-supported silicon composite materials are preferred.

[0059] The negative electrode active material may be used alone or in combination of two or more at any ratio.

[0060] The volume average particle size of the negative electrode active material is preferably 1 μm or greater, more preferably 5 μm or greater, and preferably 30 μm or less, more preferably 20 μm or less. If the volume average particle size of the negative electrode active material is above the lower limit, heat generation during internal short circuits can be effectively suppressed. If the volume average particle size of the negative electrode active material is below the upper limit, an increase in the initial resistance of the resulting battery can be effectively suppressed.

[0061] The negative electrode mixture layer may further include a negative electrode binder. The negative electrode binder can improve the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. The type of negative electrode binder is not particularly limited, as long as it is a material that is stable to the electrolyte or the solvent used in electrode manufacturing. In some embodiments, the negative electrode binder includes a resin binder. Examples of resin binders include, but are not limited to, fluororesins, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, and the like. When an aqueous solvent is used to prepare the negative electrode mixture slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, polyvinyl alcohol, and the like.

[0062] As a current collector for retaining the negative electrode active material, any known current collector can be used. Examples of negative electrode current collectors include, but are not limited to, metal materials such as copper, nickel, stainless steel, and nickel-plated steel. In some embodiments, the negative electrode current collector is copper.

[0063] The negative electrode can be prepared by applying a negative electrode mixture slurry containing a negative electrode active material, a resin binder, etc. on a negative electrode current collector, drying it, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0064] IV. Isolation membrane

[0065] In order to prevent short circuit, a separator is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte of the present application is usually used by permeating the separator.

[0066] There is no particular restriction on the material and shape of the isolation membrane, as long as the effect of the present application is not significantly impaired. The isolation membrane may be a resin, glass fiber, inorganic substance, etc. formed of a material that is stable to the electrolyte of the present application. In some embodiments, the isolation membrane includes a porous sheet or a non-woven fabric-like material with excellent liquid retention. Examples of materials for resin or glass fiber isolation membranes may include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the above-mentioned isolation membranes can be used alone or in any combination.

[0067] The isolation film may also be a material formed by laminating the above materials, and examples thereof include, but are not limited to, a three-layer isolation film formed by laminating polypropylene, polyethylene, and polypropylene in this order.

[0068] Examples of inorganic materials include, but are not limited to, oxides such as aluminum oxide and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates (e.g., barium sulfate, calcium sulfate, etc.) The inorganic material may be in the form of, but is not limited to, particles or fibers.

[0069] The separator may be in the form of a thin film, examples of which include, but are not limited to, nonwoven fabrics, woven fabrics, and microporous films. In the thin film form, the separator has a pore size of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm. In addition to the above-mentioned independent thin film separators, separators may also be used: separators formed by forming a composite porous layer containing the above-mentioned inorganic particles on the surface of the positive electrode and / or negative electrode using a resin-based binder. For example, a separator formed by forming a porous layer on both sides of the positive electrode using fluororesin as a binder with aluminum oxide particles having a particle size of 90% less than 1 μm.

[0070] The thickness of the separator is arbitrary. In some embodiments, the separator has a thickness greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the separator has a thickness less than 50 μm, less than 40 μm, or less than 30 μm. When the separator has a thickness within the above range, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0071] The present application further provides an electronic device, which includes the secondary battery according to the present application.

[0072] The use of the secondary battery of the present application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the secondary battery of the present application can be used for, but not limited to, laptop computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and lithium-ion capacitors, etc.

[0073] The preparation of a secondary battery is described below with reference to specific embodiments. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0074] Example

[0075] Examples of the secondary battery of the present application are shown below, but the present application is not limited to these examples.

[0076] Production of secondary batteries

[0077] Production of positive electrode:

[0078] Method 1: Inorganic additives are included in the active material layer

[0079] The positive electrode active material (97% by mass) listed in Table 1-1, the inorganic additive (0.5% by mass) listed in Table 1-1, and acetylene black (1.0% by mass) were mixed and added to a solution prepared by dissolving the binder (1.5% by mass) listed in Table 1-1 in 1-methyl-2-pyrrolidone. This mixture was mixed to prepare a positive electrode slurry. This positive electrode slurry was applied to both sides of aluminum foil, dried, and pressurized to form an active material layer. The foil was then cut into a desired size to produce a positive electrode.

[0080] Method 2: Inorganic additives are included in the base coat

[0081] Mix the inorganic additive (65 mass%) of Table 1-1, acetylene black (20 mass%), and the binder (15 mass%) of Table 1-1 (if a dispersant is used, adjust the binder to 12 mass% and the dispersant in Table 1-1 to 3 mass%), add deionized water and mix evenly, make a slurry, apply it on both sides of the aluminum foil, and dry it to form a primer layer.

[0082] The positive electrode active material (97% by mass) and acetylene black (1.5% by mass) listed in Table 1-1 were mixed and added to a solution of the binder (1.5% by mass) listed in Table 1-1 dissolved in 1-methyl-2-pyrrolidone. This mixture was mixed to prepare a positive electrode slurry. This positive electrode slurry was applied to the surface of the undercoat layer, dried, pressurized, and then cut into a specified size to produce a positive electrode.

[0083] Method 3: Inorganic additives are included in the edge layer

[0084] Mix the inorganic additive (65 mass%) in Table 1-1, acetylene black (20 mass%), and the binder (15 mass%) in Table 1-1 (if there is a dispersant, adjust it to 12 mass% of the binder and 3 mass% of the dispersant in Table 1-1), add deionized water and mix evenly to make a positive electrode slurry 1, apply it on the edge of the aluminum foil with the pole ear side, dry it, and form an edge layer.

[0085] The positive electrode active material (97% by mass) and acetylene black (1.5% by mass) listed in Table 1-1 were mixed and added to a solution prepared by dissolving the binder (1.5% by mass) listed in Table 1-1 in 1-methyl-2-pyrrolidone. The mixture was mixed to prepare positive electrode slurry 2. Slurry 1 and slurry 2 were applied parallel to each other on aluminum foil, with slurry 1 located at the outer edge of slurry 2 (along the direction of coating). After drying and pressurizing, the slurry was cut into the specified size to produce the positive electrode.

[0086] Negative electrode production:

[0087] A negative electrode paste was prepared by mixing 96% artificial graphite and 96% silicon carbon (90:10 mass ratio) with 2% styrene-butadiene rubber as the negative electrode active materials. This paste was then added to a solution of 2% lithium carboxymethyl cellulose dissolved in deionized water. This paste was then applied to one side of a copper foil (current collector), dried, pressurized, and cut to the desired size to produce the negative electrode.

[0088] Battery production:

[0089] The positive electrode and negative electrode prepared as above were each connected to a wire. The layers were stacked via a polypropylene porous membrane having a thickness of 10 μm to obtain a laminate. In addition, LiPF6 as a supporting electrolyte was dissolved in a solution comprising (I) lithium difluorophosphate, (II) a dinitrile compound, and (III) a trinitrile compound. Based on 100 parts by mass of the total mass of the non-aqueous electrolyte, the contents and components of (I) to (III) and the first compound are shown in Table 1-2, and the content of LiPF6 was 14%.

[0090] The stacked body was then placed in an aluminum laminate case along with 3.2g of electrolyte. The opening of the case was heat-sealed, and the battery was completed through formation and capacity testing. This secondary battery was in the form of a bag, 35mm wide, 48mm high, and 5mm thick.

[0091] Tables 1-1 and 1-2 show the positive electrode active material, inorganic additives, binder composition, and electrolyte component codes of the prepared secondary batteries. The positive electrode active material is lithium cobalt oxide containing element A. The mass content of element A is calculated based on the mass of the positive electrode active material.

[0092] The abbreviation code of polyvinylidene fluoride is PVDF, the abbreviation of polyacrylonitrile is PAN, and the abbreviation of polyacrylic acid is PAA;

[0093] Dispersants: glycerol trimethacrylate code S1, trimethylolpropane trimethacrylate code S2, pentaerythritol trimethacrylate code S3, pentaerythritol tetramethacrylate code S4, di(trimethylolpropane) trimethacrylate code S5, di(trimethylolpropane) tetramethacrylate code S6, di(pentaerythritol) tetramethacrylate code S7, di(pentaerythritol) pentamethacrylate code S8, di(pentaerythritol) hexamethacrylate code S9, di(glycerol) trimethacrylate code S10.

[0094] Table 1-1 Positive electrode

[0095] The values ​​in the above table are all in mass %

[0096] Table 1-2 Electrolyte

[0097] The values ​​in () in the above table are all mass %

[0098] The abbreviations in the above table represent the following substances:

[0099] Test Method

[0100] Positive electrode resistor

[0101] After disassembling the secondary batteries obtained in the Examples and Comparative Examples, the positive electrodes were punched into circular shapes with a diameter of 12 mm. The volume resistivity of the positive electrodes was measured at 25°C using a tensile-compression tester (manufactured by IMADA SEISAKUSHO CO., LTD., model "SV-301NA") and an electrochemical measuring apparatus (manufactured by HOKUTO DENKO CORPORATION, model "HSV-110"). A pressure of 2 kN and a current of 10 mA were applied. The voltage was read after 10 minutes and evaluated according to the following criteria.

[0102] A: 200Ω·cm or less;

[0103] B: more than 200Ω·cm and less than 300Ω·cm;

[0104] C: more than 300Ω·cm and less than 400Ω·cm;

[0105] D: more than 400 Ω·cm.

[0106] Positive electrode warpage suppression

[0107] A narrow strip with a size of 2 cm in width (coating width direction) and 5 cm in length (coating direction) was cut from the positive electrode as a test piece. The test piece was placed on a horizontal plane with the positive electrode mixture layer side facing downward. Then, a laser displacement meter ("LJV-7080" manufactured by Keyence Corporation) was used to measure the height (warping amount) of the edges of the width ends of the test piece from the horizontal plane when the center of the width direction of the two ends of the length direction of the test piece was pressed from above to the horizontal plane, and the evaluation was performed according to the following criteria. The smaller the warping amount, the more the warping of the positive electrode can be suppressed.

[0108] A: The warpage is less than 1.2mm;

[0109] B: Warpage is greater than 1.2mm and less than 1.6mm;

[0110] C: Warpage is greater than 1.6mm and less than 2mm;

[0111] D: The warping is greater than 2 mm, or the positive electrode mixture layer has cracks.

[0112] Suppressing heat generation during internal short circuits in secondary batteries

[0113] The secondary batteries obtained in the examples and comparative examples were discharged at a temperature of 25°C by a constant current method at 0.2C until the cell voltage reached 3.0V. Then, CC-CV charging was performed by a constant current method at 0.2C (the upper limit of the cell voltage was 4.5V), and CC discharge was performed by a constant current method at 0.2C to 3.0V. This 0.2C charge and discharge was repeated three times. Then, at a temperature of 25°C, the battery was charged to 4.5V by a constant voltage and constant current (CC-CV) method at a charge rate of 0.2C (cut-off condition: 0.02C). Then, an iron nail with a diameter of 3mm and a length of 10cm was penetrated near the center of the secondary battery at a speed of 5m / min, thereby forcing it to short-circuit. The same operation was performed on 5 secondary batteries produced, and the forced short-circuiting was performed according to the number of test bodies that did not rupture or catch fire, and the evaluation was performed according to the following criteria. The greater the number of test bodies that did not rupture or catch fire, the better the heat suppression during the internal short circuit of the secondary battery, that is, the better the safety performance.

[0114] A: The number of test pieces without rupture or fire is 4 or 5;

[0115] B: The number of test pieces without rupture or fire is 3;

[0116] C: The number of test pieces without rupture or fire is 2;

[0117] D: The number of test pieces in which no cracking or ignition occurred was 1 or 0.

[0118] High-temperature storage characteristics of secondary batteries

[0119] The secondary batteries obtained in the examples and comparative examples were discharged at a constant current of 0.2 C at a temperature of 25°C until the cell voltage reached 3.0 V. They were then charged in a CC-CV manner at a constant current of 0.2 C (upper cell voltage 4.5 V, cutoff condition: 0.02 C), and then discharged in a CC manner at a constant current of 0.2 C until the cell voltage reached 3.0 V.

[0120] Next, the initial IV resistance R1 was measured. Specifically, in a 25°C atmosphere, the battery was charged at 1.0C (C is a value expressed as rated capacity (mA) / 1h) to 50% SOC (State of Charge). Then, with 50% SOC as the center, charging and discharging were performed at 0.5C, 1.0C, 1.5C, and 2.0C for 20 seconds and 20 seconds, respectively. The battery voltage after 20 seconds for each case (charge side and discharge side) was plotted against the current value, and the slope was calculated as the initial IV resistance R1 (Ω) (IV resistance during charge and IV resistance during discharge).

[0121] Then, CC-CV charging was performed at a constant current of 0.2 C (upper limit cell voltage 4.5 V). Next, the secondary battery was stored in a non-oxidizing oven with a nitrogen atmosphere at 60° C. in the processing chamber for 4 weeks.

[0122] After storage, the IV resistance R2 after high-temperature storage was measured using the same method as the initial IV resistance R1.

[0123] The IV resistance increase rate was calculated using the obtained initial IV resistance R1 and the IV resistance R2 after high-temperature storage using the following formula.

[0124] IV resistance increase rate (%) = (R2-R1) / R1×100

[0125] The IV resistance increase rate (%) and IV resistance R2 after high-temperature storage were used to evaluate the battery according to the following criteria. Smaller IV resistance increase rate (%) and IV resistance R2 (Ω) after high-temperature storage indicate a long-term decrease in internal resistance and better battery characteristics.

[0126] A: The IV resistance increase rate is less than 40%, and the IV resistance R2 is less than 3.1Ω;

[0127] B: The IV resistance increase rate is greater than 40% and less than 50%, and the IV resistance R2 is less than 3.5Ω;

[0128] C: The IV resistance increase rate is greater than 50% and less than 65%, and the IV resistance R2 is less than 3.7Ω;

[0129] D: The IV resistance increase rate is 65% or more.

[0130] Test results

[0131] Table 2-1

[0132] Table 2-2

[0133] Table 2-3

[0134] In a secondary battery whose positive electrode includes lithium cobalt oxide, a binder and an inorganic additive, the electrolyte contains (I) lithium difluorophosphate, (II) a dinitrile compound and (III) a trinitrile compound. It is important that the total content of (I), (II) and (III) is greater than 1.5% by mass and less than 10% by mass. In particular, it is critical that the total content of (I) to (III) is set within a specific range, which is very critical for the stability of the positive electrode structure and significantly suppresses the increase in interfacial resistance under high voltage. This design can not only improve the positive electrode resistance and warpage suppression, but also enhance the safety and high-temperature storage characteristics of the secondary battery.

[0135] When the positive electrode binder further includes a dispersant such as glycerol trimethacrylate, trimethylolpropane trimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, etc., the positive electrode resistance characteristics can be further improved and the positive electrode warping can be suppressed.

[0136] When the electrolyte further contains a first compound such as lithium monofluorophosphate, 1,2-bis(difluorophosphine)ethane, vinylene carbonate, lithium fluorosulfonate, 1,3-propane sultone, 1,3-propylene sultone, 1,3-propylene glycol cyclic sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl)phosphate, or tris(trimethylsilyl)borate, further improved high-temperature storage characteristics are obtained.

[0137] References throughout the specification to “an embodiment,” “some embodiments,” “one embodiment,” “another example,” “an example,” “a specific example,” or “a portion of an example” mean that at least one embodiment or example in the present application includes the specific features, structures, materials, or characteristics described in that embodiment or example. Therefore, descriptions appearing throughout the specification, such as, for example, “in some embodiments,” “in an embodiment,” “in one embodiment,” “in another example,” “in an example,” “in a specific example,” or “an example,” are not necessarily references to the same embodiment or example in the present application. In addition, the specific features, structures, materials, or characteristics in the present application may be combined in any suitable manner in one or more embodiments or examples.

[0138] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present application.

Claims

1. A secondary battery comprising: A positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises lithium cobalt oxide, a binder, and an inorganic additive, and the metal element in the inorganic additive comprises at least one of aluminum, magnesium, titanium, zirconium, niobium, indium, tungsten, tin, zinc, or antimony; The electrolyte solution contains (I) lithium difluorophosphate, (II) a dinitrile compound, and (III) a trinitrile compound, wherein the total content of (I), (II), and (III) is 1.5% by mass or more and 10% by mass or less based on the mass of the electrolyte solution. 2 . The secondary battery according to claim 1 , wherein the total content of (I), (II) and (III) is 2.66% by mass or more and 6.5% by mass or less. 3 . The secondary battery according to claim 1 , wherein the total content of (I), (II) and (III) is 5.1% by mass or more and 9.5% by mass or less. 4 . The secondary battery according to claim 1 , wherein the binder comprises at least one of polyvinylidene fluoride, polyacrylonitrile, or polyacrylic acid.

5. The secondary battery according to claim 1 , wherein the binder further comprises a dispersant, the dispersant comprising at least one of glycerol trimethacrylate, trimethylolpropane trimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, di(trimethylolpropane) trimethacrylate, di(trimethylolpropane) tetramethacrylate, di(pentaerythritol) tetramethacrylate, di(pentaerythritol) pentamethacrylate, di(pentaerythritol) hexamethacrylate, di(glycerol) trimethacrylate, di(glycerol) tetramethacrylate, or tri(hydroxymethylpropane) alkylene oxide-modified trimethacrylate.

6. The secondary battery according to any one of claims 1 to 5, wherein the dinitrile compound is at least one selected from succinonitrile, adiponitrile, ethylene glycol di(propionitrile) ether, or 1,4-dicyano-2-butene; and / or The trinitrile compound is selected from 1,2,3-propanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,3,6- At least one of hexanetricarboxylic acid nitrile, 1,2,4-tris(2-cyanoethoxy)butane, 1,2,3-tris(2-cyanoethoxy)propane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane or 1,2,5-tris(cyanoethoxy)pentane.

7. The secondary battery according to any one of claims 1 to 5, wherein the mass of the lithium difluorophosphate is 0.01 mass % or more and 1 mass % or less based on the mass of the electrolyte; The mass of the dinitrile compound is greater than or equal to 0.3 mass % and less than or equal to 8.5 mass % based on the mass of the electrolyte; The mass of the trinitrile compound is 0.5 mass % or more and 5 mass % or less based on the mass of the electrolyte.

8. The secondary battery according to any one of claims 1 to 5, wherein The electrolyte also includes a first compound, which includes at least one of lithium monofluorophosphate, 1,2-bis(difluorophosphine)ethane, vinylene carbonate, lithium fluorosulfonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, 1,3-propylene glycol cyclic sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl)phosphate or tris(trimethylsilyl)borate.

9. The secondary battery according to any one of claims 1 to 5, wherein the lithium cobalt oxide includes element A, wherein element A includes at least one of lanthanum, yttrium, cerium, tungsten, or strontium; and the content of any one of element A is greater than or equal to 0.01% by mass and less than or equal to 1% by mass based on the mass of the lithium cobalt oxide. 10 . The secondary battery according to claim 1 , wherein the positive electrode comprises an undercoat layer and an active material layer provided on a current collector, and the inorganic additive is contained in the active material layer or the undercoat layer. 11 . The secondary battery according to claim 1 , wherein the positive electrode comprises an edge layer and an active material layer provided on a current collector, and the inorganic additive is contained in the edge layer. 12 . An electronic device comprising the secondary battery according to claim 1 .

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