Secondary battery and electronic device

Abstract

The secondary battery comprises a positive pole piece, the positive pole piece comprises a positive current collector, a first material layer and a second material layer which are stacked, the positive current collector comprises a metal layer, and the first material layer is arranged between the positive current collector and the second material layer; the first material layer comprises first material particles, the first material particles comprise a matrix and a carbon coating layer located on the surface of the matrix, Dv10 of the first material particles is D1 [mu] m, 0.3 < = D1 < = 2.0, the matrix comprises at least one of LiFekM (1-k) PO4, 0 < = k < = 1, and the M element is selected from at least one of manganese, cobalt, magnesium, calcium, zinc, chromium or lead. The secondary battery of the present application has low internal resistance and internal resistance growth rate. An electronic device including the secondary battery is also provided.

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] When a lithium-ion battery is punctured by foreign objects, it short-circuits and generates abnormal heat. If the rate of heat generation within the battery exceeds the rate of heat dissipation, the accumulated heat raises the overall temperature and accelerates side reactions, potentially causing thermal runaway and, in extreme cases, fire or explosion.

[0003] Currently, existing technologies often use a high-adhesion safety coating on the positive electrode current collector, such as aluminum foil, to reduce short-circuit points and, in turn, reduce heat generation during a short circuit. However, lithium-ion batteries with high-adhesion safety coatings generally have a higher internal resistance, and after tests such as high-temperature storage, high-temperature and high-humidity, and thermal shock, the internal resistance growth rate is much higher than that of lithium-ion batteries without high-adhesion safety coatings. Therefore, it is necessary to develop a new electrode design to address the problem of excessive internal resistance growth in lithium-ion batteries coated with high-adhesion safety coatings under high-temperature conditions.

[0004] Summary of the Invention

[0005] In view of the above-mentioned problems existing in the prior art, the present application provides a secondary battery and an electronic device to reduce the internal resistance and internal resistance growth rate of the secondary battery, thereby improving the electrical performance and life of the secondary battery.

[0006] In a first aspect, the present application provides a secondary battery comprising a positive electrode plate, the positive electrode plate comprising a positive electrode current collector, a first material layer, and a second material layer stacked, the positive electrode current collector comprising a metal layer, the first material layer being disposed between the positive electrode current collector and the second material layer; the first material layer comprising first material particles, the first material particles comprising a matrix and a carbon coating layer located on the surface of the matrix, and the second material layer comprising a positive electrode active material. Wherein, the Dv10 of the first material particles is D1μm, 0.3≤D1≤2.0, the matrix comprises LiFe k M( 1-k )PO4, wherein 0≤k≤1, and the M element is selected from at least one of manganese, cobalt, magnesium, calcium, zinc, chromium or lead.

[0007] In the prior art, the internal resistance of secondary batteries with a first material layer (safety coating) is relatively large, and after high-temperature storage, high-temperature and high-humidity, thermal shock and other tests, the internal resistance growth rate is much higher than that of secondary batteries without a safety coating. Excessive internal resistance growth rate will have a serious impact on the performance and life of the secondary battery. The inventors of this application have found through research that when a secondary battery with a safety coating is stored at high temperature, the positive electrode current collector is affected by the first material particles in the first material layer, and the gas will accumulate at the interface between the first material layer and the metal layer, corroding the metal layer, thereby increasing the interface resistance between the first material layer and the metal layer; in addition, the carbon coating layer on the surface of the substrate increases side reactions at high temperatures, causing the side reaction products to accumulate on the surface of the substrate and in the first material layer, obstructing the electron channel, thereby causing the internal resistance of the secondary battery to continue to increase. To address this problem, the present application reasonably controls the particle size of the first material particles in the first material layer, so that the first material layer and the metal layer have a higher peel strength, thereby reducing the risk of exposure of the metal layer when the secondary battery is subjected to external force. The above-mentioned matrix has a larger short-circuit resistance, which can reduce the short-circuit current and improve safety performance. At the same time, it can also effectively reduce the gas enriched in the first material layer and reduce the occurrence of side reactions of the carbon coating layer, thereby reducing the internal resistance and internal resistance growth rate of the secondary battery during charging and discharging.

[0008] In some embodiments, 0.7 ≤ D1 ≤ 1.5. If the Dv10 value of the first material particles is too small, more gas will accumulate at the interface between the first material layer and the metal layer, causing more side reactions in the carbon coating layer at high temperatures, thereby increasing the internal resistance of the secondary battery. If the Dv10 value of the first material particles is too large, although large particles can reduce the occurrence of interfacial side reactions to a certain extent, the number of large particles in the particle distribution is difficult to control, which will increase the number of bad points in the electrode and reduce the yield rate.

[0009] In some embodiments, the Dv90 of the first material particles is D2μm, 5.0≤D2≤10.0. If the Dv90 value of the first material particles is too small, the number of small particles in the particle distribution is excessive, and the number of small particles is difficult to control. This can lead to gas enrichment at the cross-section of the metal layer and the first material layer at high temperatures and intensify side reactions in the carbon coating layer, causing an increase in the internal resistance growth rate of the secondary battery. If the Dv90 value of the first material particles is too large, the number of large particles in the particle distribution is difficult to control, which can increase the number of bad spots in the electrode.

[0010] In some embodiments, 6.0≤D2≤8.0, which can further reduce the internal resistance growth rate and improve the yield rate.

[0011] In some embodiments, the mass content of the carbon coating is C%, based on the mass of the first material particles, with 0.1 ≤ C ≤ 3.0. The carbon coating can increase the conductivity of the substrate and hinder the aggregation and growth of the substrate. However, if the carbon coating content is too high, after high-temperature storage, due to the different thermal expansion coefficients of the carbon coating and the substrate, and the high-temperature electrolyte, the carbon coating will partially decompose, resulting in an uneven layer of sediment on the substrate surface. This will reduce the contact between the first material particles and increase the resistance of the secondary battery. The synergistic effect of the appropriate carbon coating content and particle size can significantly reduce the internal resistance and internal resistance growth rate of the secondary battery while improving its safety performance.

[0012] In some embodiments, 0.5≤C≤2.0, which can further reduce the internal resistance growth rate.

[0013] In some embodiments, the Dv90 of the first material particles is D2 μm, and the mass content of the carbon coating layer is C%, based on the mass of the first material particles, 3.0 ≤ D1 × D2 / C ≤ 12.0, 0.7 ≤ D1 ≤ 1.5, 6.0 ≤ D2 ≤ 8.0, and 0.5 ≤ C ≤ 2.0. When the particle size of the first material particles and the amount of carbon coating satisfy the above relationship, the internal resistance growth rate of the secondary battery can be further reduced.

[0014] In some embodiments, 4.7≤D1×D2 / C≤12.0, which can further reduce the internal resistance growth rate, and achieve lower internal resistance and higher yield.

[0015] In some embodiments, the matrix includes at least one of lithium iron phosphate or lithium iron manganese phosphate. Lithium iron phosphate and lithium iron manganese phosphate have low electrical conductivity. When the secondary battery short-circuits due to external force, they can increase short-circuit resistance, reduce short-circuit current, and improve safety.

[0016] In some embodiments, the first material layer further comprises at least one of inorganic particles, a conductive agent, and a binder. In some embodiments, the inorganic particles comprise at least one of aluminum oxide, magnesium oxide, calcium oxide, magnesium hydroxide, boehmite, silicon oxide, or calcium oxide. The inorganic particles can further reduce the short-circuit current of the secondary battery during a short circuit, thereby improving safety.

[0017] In some embodiments, the mass content of the first material particles is 70% to 85% based on the mass of the first material layer.

[0018] In some embodiments, in the first material layer, the mass ratio of the conductive agent to the binder is 10:1 to 3:1.

[0019] In some embodiments, the thickness of the first material layer is 1 μm to 6 μm. When the thickness of the first material layer is within the above range, the secondary battery has both high energy density and safety performance.

[0020] In some embodiments, the second material layer includes a positive electrode active material, and the positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide.

[0021] In some embodiments, the Dv50 of the positive electrode active material is greater than the Dv50 of the first material particles. In some embodiments, the Dv50 of the positive electrode active material is D3 μm, with 4 ≤ D3 ≤ 15. A Dv50 value greater than the Dv50 value of the first material particles helps increase the lithium ion conduction rate in the positive electrode active material and enhances the rate performance of the secondary battery. Furthermore, a D3 value between 4 and 15 facilitates electrode coating and can reduce the number of bad spots on the electrode.

[0022] In some embodiments, the metal layer is an aluminum layer.

[0023] In some embodiments, the secondary battery further includes a negative electrode plate, the negative electrode plate includes a negative electrode active material, and the negative electrode active material includes graphite.

[0024] In some embodiments, the first material layer is in direct contact with the metal layer, which is beneficial for simplifying the process, reducing manufacturing costs, and facilitating electron conduction between the first material layer and the metal layer.

[0025] In a second aspect, the present application provides an electronic device comprising the secondary battery of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 is a schematic structural diagram of a positive electrode sheet in a secondary battery according to some embodiments of the present application, wherein 1 is a positive electrode current collector, 2 is a first material layer, and 3 is a second material layer.

[0027] Figure 2 is a schematic diagram of the high-temperature storage mechanism of the positive electrode plate in the secondary battery in the prior art, wherein A is a schematic diagram before storage, B is a schematic diagram after storage, 1 is the positive electrode current collector, 2 is the first material layer, 3 is the second material layer, 4 is the isolation membrane, 5a is the first material particle before storage, 5b is the first material particle after storage, 6a is the by-product before storage, and 6b is the by-product after storage. DETAILED DESCRIPTION

[0028] 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.

[0029] In addition, amounts, ratios, and other numerical values ​​are sometimes presented in this application in a range format. It should be understood that such a range format is used for convenience and brevity and should be interpreted flexibly to include not only the values ​​explicitly specified as limits of the range, but also all individual values ​​or sub-ranges encompassed within the range, as if each value and sub-range were explicitly specified.

[0030] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0031] Primary and secondary batteries

[0032] The secondary battery provided by the present application includes a positive electrode plate, the positive electrode plate includes a positive electrode current collector, a first material layer and a second material layer stacked, the positive electrode current collector includes a metal layer, the first material layer is arranged between the positive electrode current collector and the second material layer; the first material layer includes first material particles, the first material particles include a matrix and a carbon coating layer located on the surface of the matrix, wherein Dv10 of the first material particles is D1μm, 0.3≤D1≤2.0, and the matrix includes LiFe k M( 1-k )PO4, wherein 0≤k≤1, and the M element is selected from at least one of manganese, cobalt, magnesium, calcium, zinc, chromium or lead.

[0033] As shown in FIG1 , some embodiments of the present application provide a secondary battery comprising a positive electrode sheet, which includes a positive electrode current collector 1 (surface metal layer not shown), a first material layer 2, and a second material layer 3. It should be understood that although the first material layer is shown as being located on both sides of the positive electrode current collector in FIG1 , this is merely exemplary, and the first material layer 2 and the second material layer 3 may also be located on one side of the positive electrode current collector.

[0034] As shown in Figure 2, before the secondary battery in the prior art is stored at high temperature, the first material layer contains a small amount of by-products 6a, and the battery performance is not greatly affected. However, after high-temperature storage, the positive electrode current collector 1 is affected by the first material particles 5 in the first material layer 2, and the gas will gather at the interface between the first material layer 2 and the positive electrode current collector 1, corroding the metal layer, thereby increasing the interface resistance between the first material layer and the metal layer; in addition, the carbon coating layer on the surface of the substrate increases the side reactions at high temperatures, causing the side reaction products 6b to accumulate on the surface of the substrate and in the first material layer, obstructing the electron channel, thereby causing the internal resistance of the secondary battery to continue to increase. To address this problem, the present application reasonably controls the particle size of the first material particles in the first material layer, thereby enabling the first material layer and the metal layer to have a higher peel strength, reducing the risk of the metal layer being exposed when the secondary battery is subjected to external force, and having a higher short-circuit current, thereby improving safety performance; at the same time, it can also effectively reduce the gas enriched in the first material layer and reduce the occurrence of side reactions in the carbon coating layer, thereby reducing the internal resistance and internal resistance growth rate of the secondary battery.

[0035] In some embodiments, the first material layer is in direct contact with the metal layer, which is beneficial for simplifying the process, reducing manufacturing costs, and facilitating electron conduction between the first material layer and the metal layer.

[0036] In some embodiments, D1 is 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 μm, 1.7, 1.8, 1.9 or a range consisting of any two of these values. In some embodiments, 0.7≤D1≤1.5. When the Dv10 value of the first material particles is too small, the gas gathered at the interface between the first material layer and the metal layer will increase, causing the carbon coating layer to have more side reactions at high temperatures, thereby causing the internal resistance of the secondary battery to continue to increase. When the Dv10 value of the first material particles is too large, although the large particles can reduce the occurrence of interface side reactions to a certain extent, the number of large particles in the particle distribution is not easy to control, which will increase the number of bad points in the electrode.

[0037] In some embodiments, the Dv90 of the first material particles is D2μm, 5.0≤D2≤10.0. In some embodiments, D2 is 5.3, 5.5, 5.7, 6.0, 6.3, 6.5, 6.7, 7.0, 7.3, 7.5, 7.7, 8.0, 8.3, 8.5, 8.7, 9.0, 9.3, 9.5, 9.7 or a range consisting of any two of these values. If the Dv90 value of the first material particles is too small, the number of small particles in the particle distribution is too large, and the number of small particles is difficult to control, which will cause the internal resistance growth rate of the secondary battery to increase. When the Dv90 value of the first material particles is too large, the number of large particles in the particle distribution is not easy to control, which will increase the number of bad points in the electrode. In some embodiments, 6.0≤D2≤8.0.

[0038] In some embodiments, 0.7≤D1≤1.5, 6.0≤D2≤8.0.

[0039] As used herein, Dv50 represents the value below which 50% of the particles in a volume-based particle size distribution are smaller than this value. Dv10 represents the value below which 10% of the particles in a volume-based particle size distribution are smaller than this value. Dv90 represents the value below which 90% of the particles in a volume-based particle size distribution are smaller than this value.

[0040] In some embodiments, the carbon coating layer has a mass content of C% based on the mass of the first material particles, 0.1≤C≤3.0. In some embodiments, C is 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or a range consisting of any two of these values. In some embodiments, 0.5≤C≤2.0. The carbon coating increases the conductivity of the substrate and inhibits its aggregation and growth. However, if the carbon coating content is too high, after high-temperature storage, the different thermal expansion coefficients of the carbon coating and the substrate, combined with the high-temperature electrolyte, can partially decompose the carbon coating, resulting in an uneven layer of sediment deposited on the surface of the first material particles. This impairs contact between the first material particles and increases the resistance of the secondary battery. The synergistic effect of the appropriate carbon coating content and particle size can significantly reduce the internal resistance and internal resistance growth rate, while improving the safety performance of the secondary battery.

[0041] In some embodiments, the Dv90 of the first material particles is D2μm, and based on the mass of the first material particles, the mass content of the carbon coating layer is C%, 3.0≤D1×D2 / C≤12.0, 0.7≤D1≤1.5, 6.0≤D2≤8.0, and 0.5≤C≤2.0. When the particle size of the first material particles and the amount of carbon coating satisfy the above relationship, the internal resistance growth rate of the secondary battery can be further reduced. In some embodiments, D1×D2 / C is 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, or a range consisting of any two of these values. In some embodiments, 4.7≤D1×D2 / C≤12.0.

[0042] In some embodiments, the matrix includes at least one of lithium iron phosphate or lithium iron manganese phosphate. Lithium iron phosphate and lithium iron manganese phosphate have low electrical conductivity. When the secondary battery short-circuits due to external force, they can increase short-circuit resistance, reduce short-circuit current, and improve safety.

[0043] In some embodiments, the first material layer further comprises at least one of inorganic particles, a conductive agent, and a binder. In some embodiments, the inorganic particles comprise at least one of aluminum oxide, magnesium oxide, calcium oxide, magnesium hydroxide, boehmite, silicon oxide, or calcium oxide. The addition of inorganic particles can increase short-circuit resistance and improve safety performance.

[0044] In some embodiments, based on the mass of the first material layer, the mass content of the first material particles is 70% to 85%, for example, 72%, 75%, 77%, 80% or 83%.

[0045] In some embodiments, in the first material layer, the mass ratio of the conductive agent to the binder is 10:1 to 3:1, for example, 4:1, 5:1, 6:1, 7:1, 8:1 or 9:1.

[0046] In some embodiments, the thickness of the first material layer is 1 μm to 6 μm, for example 2 μm, 3 μm, 4 μm or 5 μm. When the thickness of the first material layer is within the above range, the secondary battery has both high energy density and safety performance.

[0047] In some embodiments, the second material layer includes a positive electrode active material, and the positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide.

[0048] In some embodiments, the Dv50 of the positive electrode active material is greater than the Dv50 of the first material particles. In some embodiments, the Dv50 of the positive electrode active material is D3 μm, 4 ≤ D3 ≤ 15, and D3 is, for example, 6, 8, 10, 12, or 14. A Dv50 value greater than the Dv50 value of the first material particles is beneficial for increasing the conduction rate of lithium ions in the positive electrode active material and improving the rate performance of the secondary battery. Furthermore, a D3 value between 4 and 15 facilitates electrode coating and can reduce the number of bad spots on the electrode.

[0049] In some embodiments, the metal layer is an aluminum layer. In some embodiments, the secondary battery further comprises a negative electrode plate, the negative electrode plate comprises a negative electrode active material, and the negative electrode active material comprises graphite.

[0050] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used. The composite current collector may be formed by combining a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0051] In some embodiments, the second material layer further comprises a binder and a conductive agent. In some embodiments, the binder comprises an adhesive polymer, such as at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane. In some embodiments, the polyolefin binder comprises at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid. In some embodiments, the conductive agent comprises a carbon-based material, such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, or carbon fiber; a metal-based material, such as metal powder or metal fiber of copper, nickel, aluminum, silver, etc.; a conductive polymer, such as a polyphenylene derivative; or a mixture thereof.

[0052] In some embodiments, the secondary battery further comprises a negative electrode, the negative electrode comprising a negative electrode active material layer, the negative electrode active material layer comprising a negative electrode active material, the negative electrode active material comprising graphite. In some embodiments, the negative electrode active material layer further comprises a binder and an optional conductive agent.

[0053] In some embodiments, the binder comprises at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamide-imide, polyvinylidene fluoride, polyvinyl difluoride, polytetrafluoroethylene, water-based acrylic resin, polyvinyl formal or styrene-acrylic acid copolymer resin. In some embodiments, any conductive material can be used as the conductive material as long as it does not cause chemical changes. In some embodiments, the conductive material comprises at least one of conductive carbon black, acetylene black, carbon nanotubes, Ketjen black or graphene.

[0054] In some embodiments, the negative electrode further includes a negative electrode current collector, which includes copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0055] The secondary battery of the present application also includes a separator. The material and shape of the separator used in the secondary battery of the present application are not particularly limited and can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material formed from a material that is stable to the electrolyte of the present application.

[0056] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film, or composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric, or polypropylene-polyethylene-polypropylene porous composite film may be used.

[0057] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic material. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer includes a polymer, and the polymer material is selected from at least one of polyamide, polyacrylonitrile, an acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0058] The secondary of the present application also includes an electrolyte. The electrolyte that can be used in the present application can be an electrolyte known in the prior art.

[0059] In some embodiments, the electrolyte includes an organic solvent, a lithium salt and an optional additive. The organic solvent in the electrolyte of the present application may be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no restriction on the electrolyte used in the electrolyte according to the present application, and it may be any electrolyte known in the prior art. The additive of the electrolyte according to the present application may be any additive known in the prior art that can be used as an electrolyte additive. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, for example, including at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate) LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0060] In some embodiments, the secondary battery of the present application includes, but is not limited to, a lithium-ion battery or a sodium-ion battery. In some embodiments, the secondary battery includes a lithium-ion battery.

[0061] 2. Electronic Devices

[0062] The present application further provides an electronic device, which includes the secondary battery according to the first aspect of the present application.

[0063] The electronic devices or devices of the present application are not particularly limited. In some embodiments, the electronic devices of the present application include, but are not limited to, laptop computers, pen-type 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.

[0064] In the following examples and comparative examples, all reagents, materials and instruments used are commercially available unless otherwise specified.

[0065] Examples and Comparative Examples

[0066] Lithium iron phosphate material preparation

[0067] Lithium iron phosphate is prepared by solid phase method, and the process can be divided into three parts:

[0068] 1. Preparation of precursor lithium iron phosphate

[0069] The raw materials are iron blocks, sulfuric acid, phosphoric acid, hydrogen peroxide and sodium carbonate. After two reactions, washing and filtration, flash drying, and dehydration of dihydrated ferric phosphate at 500 to 900 degrees Celsius, anhydrous ferric phosphate is obtained.

[0070] 2. Secondary processing

[0071] ① Mixing: Precursor iron phosphate is mixed with lithium carbonate or lithium hydroxide, deionized water is added, the mixture is thoroughly mixed and stirred, and then a protective gas such as nitrogen or argon is introduced, and the mixture is treated at a relatively low temperature for 1 to 5 hours.

[0072] ②Spray drying: The stirred slurry is sprayed out by pressure and turned into particles after passing through the spray dryer. Spherical particles of a certain size are obtained according to the process requirements.

[0073] ③ Sintering: Sintering at 550°C to 750°C for 5h to 20h to obtain lithium iron phosphate.

[0074] ④ Crushing: Use air flow milling equipment to crush the sintered lithium iron phosphate.

[0075] ⑤ Mixing and classification: After the crushed lithium iron phosphate particles are mixed, they are classified according to particle size.

[0076] ⑥ Baking: Bake the qualified lithium iron phosphate to remove moisture.

[0077] 3. Carbon coating

[0078] Using carbon black, glucose, urea, citric acid, and other carbon sources, the reducibility of carbon sources at high temperatures is exploited to reduce ferric iron to ferrous iron, while simultaneously coating the surface of lithium iron phosphate with the resulting pyrolyzed carbon. This carbon coating not only forms a porous carbon film, enhancing conductivity, but also prevents particle aggregation and growth.

[0079] By adjusting the pressure, flow rate, and nozzle size of the spray dryer's liquid pump, lithium iron phosphate particles of varying sizes can be produced. Adjusting the mass fraction of the carbon source can adjust the carbon coating content. Adjusting the particle size of the lithium iron phosphate material can adjust the specific surface area of ​​the lithium iron phosphate material. Adjusting the carbon coating amount can also adjust the powder resistivity of the lithium iron phosphate material.

[0080] Example 1

[0081] Preparation of positive electrode:

[0082] Lithium iron phosphate, ceramic (boehmite), conductive agent (carbon nanotube), binder (PAA) and dispersant (CMC-Li) are fully stirred and mixed in an appropriate amount of N-methylpyrrolidone (NMP) solvent in a weight ratio of 85:10:3:2, and evenly stirred to obtain a first material layer slurry. The slurry is coated on aluminum foil, vacuum-dried at 80°C, and then cold-pressed to a thickness of 6 μm as the first material layer (safety coating).

[0083] Secondly, the positive electrode active material (LiCoO2), conductive agent (conductive carbon black), and binder (PVDF) are fully stirred and mixed in an appropriate amount of N-methylpyrrolidone (NMP) solvent in a weight ratio of 97.5:1:1.5 to obtain a second material layer slurry, and the second material layer slurry is coated on the upper surface of the first material layer to form a second material layer (active material layer).

[0084] After vacuum drying at 80℃, the positive electrode sheets are produced by cold pressing, die-cutting and slitting.

[0085] Preparation of negative electrode sheet

[0086] The negative electrode active material (graphite), conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC-Na) are mixed in a weight ratio of 95.7:1.5:1.8:1, and then fully stirred and mixed in an appropriate amount of deionized water solvent to form a uniform negative electrode slurry; this slurry is coated on the current collector Cu foil, dried, and cold pressed to obtain the negative electrode sheet.

[0087] Preparation of electrolyte

[0088] In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 1:3:3:3. Fluorinated ethylene carbonate and 1,3-propane sultone were then added, dissolved, and thoroughly stirred. Lithium hexafluorophosphate (LiPF6) was then added and mixed uniformly to obtain an electrolyte. The mass percentage of LiPF6 was 12.5%, the mass percentage of fluoroethylene carbonate was 2%, and the mass percentage of 1,3-propane sultone was 2%. The mass percentages of each substance were calculated based on the mass of the electrolyte.

[0089] Preparation of lithium-ion batteries

[0090] A polyethylene (PE) porous polymer film is selected as the separator, and the above-mentioned negative electrode sheet and positive electrode sheet are wound together with the separator and placed in an aluminum-plastic film. After that, liquid is injected, allowed to stand, and formed to make a lithium-ion secondary battery.

[0091] Examples 2 to 8, Examples 10 to 24, Comparative Examples 1 to 2

[0092] Examples 2 to 23 and Comparative Examples 1 to 2 are achieved on the basis of Example 1 by adjusting the pressure, flow rate, nozzle size, mass fraction of the carbon source, particle size of the lithium iron phosphate, etc. of the spray dryer liquid pump.

[0093] Example 9

[0094] Example 9 is achieved by adjusting the base material based on Example 1.

[0095] Test Method

[0096] Matrix (lithium iron phosphate, lithium iron manganese phosphate) related parameter test

[0097] Take a fully discharged lithium-ion battery (voltage <3V), disassemble it, remove the positive electrode, soak it in DMC (ethylene carbonate) for 20 minutes, then rinse it with DMC and acetone in turn to remove the electrolyte. Then place it in an oven and bake it at 80℃ for 12 hours to obtain the treated positive electrode. Then use tape to peel off the first material layer (safety coating) and scrape off the material. The scraped material is processed as follows:

[0098] ① Soak in NMP solvent at 85℃ for 4h;

[0099] ②Ultrasonic cleaning for 30 minutes;

[0100] ③ Soak in DI deionized water at 25℃ and ultrasonicate for 30min;

[0101] ④ Repeat steps 1 to 3 three times;

[0102] ⑤ Dry in an oven at 80°C to obtain lithium iron phosphate powder material.

[0103] The obtained lithium iron phosphate material was tested as follows

[0104] 1. Particle size test

[0105] The particle size test method is based on GB / T 19077-2016. The specific process involves weighing 1g of sample, mixing it with 20mL of deionized water and a trace amount of dispersant, ultrasonicating it for 5 minutes, and then pouring the solution into the sample injection system Hydro2000SM for testing. The testing equipment used is a Mastersizer 3000 produced by Malvern. During the test, when the laser beam passes through the dispersed particle sample, the particle size is measured by measuring the intensity of the scattered light. The data is then used to analyze and calculate the particle size distribution that forms the scattering spectrum. The particles used in the test have a refractive index of 1.8. Each sample is tested three times, and the final particle size is the average of the three tests.

[0106] 2. Carbon coating content test

[0107] The carbon coating content is tested by boiling and drying a dilute hydrochloric acid solution. The principle is that carbon is insoluble in hydrochloric acid, which can separate lithium iron phosphate and carbon. Take M1g of lithium iron phosphate and add dilute hydrochloric acid solution. Boil the mixed solution on a heating furnace for 30 minutes, use a circulating pump to filter and dry the solid, and weigh the mass of the residual carbon M2g. Carbon coating content = M2 / M1×100%. Lithium-ion battery related parameter test

[0108] 3. Lithium-ion battery internal resistance test

[0109] Use a resistance meter to test the AC internal resistance of the lithium-ion battery using a sinusoidal, 1000Hz frequency wave.

[0110] 4. Lithium-ion battery internal resistance growth rate test

[0111] Storage conditions (85°C for 6 hours): In an environment of 25±3°C, charge the lithium-ion battery at a constant current of 0.2C to 4.45V, then charge it at a constant voltage of 4.45V to 0.025C. The initial internal resistance of the lithium-ion battery is recorded as IMP0. Place the lithium-ion battery in an oven at 85±3°C for 6 hours, then take it out. After the temperature of the lithium-ion battery drops to 25±3°C, test its internal resistance and record it as "IMP6h".

[0112] The IMP growth rate of a lithium-ion battery placed at 85°C for 6 hours is (IMP6h-IMP0) / IMP0×100%.

[0113] 5. Test the number of bad points on the electrode

[0114] After the first material layer is dried and before cold pressing, CCD photography is performed along the electrode strip direction, 100 samples are taken, and the number of samples with abnormal convex points on the electrode surface is counted.

[0115] Test results

[0116] Table 1 shows the effect of the particle size of the lithium iron phosphate material on the battery performance, wherein the Dv10 of the lithium iron phosphate material is D1 μm, the Dv90 of the lithium iron phosphate material is D2 μm, and the mass content of the carbon coating layer is C% based on the mass of the lithium iron phosphate material.

[0117] Table 1

[0118] From the data in Table 1, it can be seen that the Dv10 value of lithium iron phosphate will affect the internal resistance growth rate of lithium-ion batteries and the number of bad points in the pole piece. When Dv10 is in the range of 0.3μm to 2.0μm, the secondary battery has a smaller internal resistance growth rate while ensuring a low number of bad points. As shown in the data of Comparative Example 1, when the Dv10 value is too low, although it is beneficial to the coating process and can reduce the number of bad points in the pole piece, it will enrich more gas at the interface and induce excessive carbon coating layer side reactions, thereby causing the internal resistance to grow too fast. As shown in the data of Comparative Example 2, when the Dv10 value is too high, although large particles can reduce the internal resistance growth rate to a certain extent, the number of large particles in the lithium iron phosphate particles is not easy to control, which will increase the number of bad points in the pole piece.

[0119] Table 2 further studies the effect of Dv90 of lithium iron phosphate material on battery performance based on Example 4.

[0120] Table 2

[0121] As can be seen from the data in Table 2, the Dv10 value of lithium iron phosphate is within the range of 0.3μm to 2.0μm. Further controlling the Dv90 value within the range of 5μm to 10μm can enable lithium-ion batteries to have both a low number of bad points in the electrode and a low internal resistance growth rate. Furthermore, controlling the Dv90 value within the range of 6μm to 8μm can comprehensively balance the number of bad points in the electrode and the internal resistance growth rate within a relatively good range.

[0122] Table 3 further studies the effect of the carbon coating amount of the lithium iron phosphate material on the battery performance based on Example 12.

[0123] Table 3

[0124] It can be seen from the data in Table 3 that by controlling the mass content of the carbon coating layer within the range of 0.1% to 3.0%, the lithium-ion battery has a lower internal resistance and internal resistance growth rate. As shown in the data of Example 17, when the carbon coating amount is low, the conductivity of the lithium iron phosphate material is not significantly improved, and the internal resistance of the lithium-ion battery is large. As shown in the data of Example 24, when the carbon coating amount is high, there are too many side reactions at the interface, and the internal resistance growth rate of the lithium-ion battery is high. Furthermore, by controlling the mass content of the carbon coating layer within the range of 0.5% to 2.0%, the internal resistance and internal resistance growth rate of the lithium-ion battery can be comprehensively balanced within a better range.

[0125] Based on the data from the examples in Tables 1 to 3, it can be concluded that when 3.0 ≤ D1 × D2 / C ≤ 12.0, 0.7 ≤ D1 ≤ 1.5, 6.0 ≤ D2 ≤ 8.0, and 0.5 ≤ C ≤ 2.0 are met, the number of electrode bad points is low, the internal resistance of the lithium-ion battery is small, and the internal resistance growth rate is below 30%. Furthermore, when 4.7 ≤ D1 × D2 / C ≤ 12.0 is met, the internal resistance growth rate can be significantly reduced to below 25%.

[0126] 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 principles and scope of the present application.

Claims

1. A secondary battery, comprising a positive electrode plate, the positive electrode plate comprising a positive electrode current collector, a first material layer and a second material layer stacked, the positive electrode current collector comprising a metal layer, the first material layer being arranged between the positive electrode current collector and the second material layer; the first material layer comprising first material particles, the first material particles comprising a matrix and a carbon coating layer located on a surface of the matrix, and the second material layer comprising a positive electrode active material; The Dv10 of the first material particles is D1 μm, 0.3≤D1≤2.0; The matrix includes LiFe k M( 1-k )PO4, wherein 0≤k≤1, the M element is selected from at least one of manganese, cobalt, magnesium, calcium, zinc, chromium or lead.

2. The secondary battery according to claim 1, wherein 0.7≤D1≤1.5。 3. The secondary battery according to claim 1, wherein The Dv90 of the first material particles is D2 μm, 5.0≤D2≤10.

0.

4. The secondary battery according to claim 3, wherein 6.0≤D2≤8.0。 5. The secondary battery according to any one of claims 1 to 4, wherein: Based on the mass of the first material particles, the mass content of the carbon coating layer is C%, and 0.1≤C≤3.

0.

6. The secondary battery according to claim 5, wherein 0.5≤C≤2.0。 7. The secondary battery according to claim 1, wherein The Dv90 of the first material particles is D2 μm. Based on the mass of the first material particles, the mass content of the carbon coating layer is C%, 3.0≤D1×D2 / C≤12.0, 0.7≤D1≤1.5, 6.0≤D2≤8.0, and 0.5≤C≤2.

0.

8. The secondary battery according to claim 7, wherein 4.7≤D1×D2 / C≤12.

0.

9. The secondary battery according to claim 1, wherein The matrix includes at least one of lithium iron phosphate or lithium iron manganese phosphate.

10. The secondary battery according to claim 1, wherein The first material layer further comprises at least one of inorganic particles, a conductive agent and a binder; the inorganic particles comprise at least one of aluminum oxide, magnesium oxide, calcium oxide, magnesium hydroxide, boehmite, silicon oxide or calcium oxide; and / or The positive electrode active material comprises lithium cobalt oxide and / or lithium nickel cobalt manganese oxide; and / or The metal layer is an aluminum layer.

11. The secondary battery according to claim 1, wherein The thickness of the first material layer is 1 μm to 6 μm; and / or, The Dv50 of the positive electrode active material is greater than the Dv50 of the first material particles, and the Dv50 of the positive electrode active material is D3 μm, 4≤D3≤15. 12 . The secondary battery according to claim 1 , further comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode active material, and the negative electrode active material comprises graphite.

13. The secondary battery according to claim 1, wherein The first material layer is in direct contact with the metal layer. 14 . An electronic device comprising the secondary battery according to claim 1 .

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