Positive electrode composite material, electrochemical device and electronic device

Abstract

A positive electrode composite material, an electrochemical device and an electronic device. The positive electrode composite material comprises lithium cobalt oxide having a crystal structure that belongs to the P63mc space group and a first substance having a crystal structure that belongs to the Pnma space group, wherein the lithium cobalt oxide contains an element M, the element M comprising at least one of Al, Mg, La, Ca, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr, Y or Zr; and the first substance is located on the surface of the lithium cobalt oxide. The positive electrode composite material can prevent direct contact between Co and an electrolyte, which is beneficial for improving the cycle life and capacity retention rate of a battery.

Description

Cathode composite material, electrochemical device, and electronic device Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a positive electrode composite material, an electrochemical device using the positive electrode composite material, and an electronic device using the electrochemical device. Background Art

[0002] With the popularization of electronic devices, lithium-ion batteries have been widely used in commercial products due to their advantages such as high energy density, good safety and no memory effect.

[0003] However, in lithium-ion batteries, the LiCoO2 cathode material with P63mc crystal structure is + When the LiCoO2 cathode material has a surface area of ​​100 nm and a charge / discharge ratio of 0.1 / Li, the spinel phase transition does not occur on the surface, and the material has high structural reversibility and very high reversible capacity at high voltage. However, during charging, because the inert layer cannot spontaneously form on the surface of the LiCoO2 cathode material, direct contact between Co and the electrolyte triggers side reactions, leading to high-temperature storage failure and rapid cycle decay, which limits the commercial application of the material.

[0004] Summary of the Invention

[0005] The present application provides a positive electrode composite material capable of inhibiting direct contact between Co and electrolyte.

[0006] In addition, the present application also provides an electrochemical device using the positive electrode composite material and an electronic device using the electrochemical device.

[0007] In a first aspect, the present application provides a positive electrode composite material, comprising a lithium cobalt oxide having a P63mc crystal structure and a first substance having a Pnma crystal structure, wherein the lithium cobalt oxide contains an M element, and the M element includes at least one of Al, Mg, La, Ca, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr, Y or Zr, and the first substance is located on the surface of the lithium cobalt oxide.

[0008] In this application, the first substance is located on the surface of the lithium cobalt oxide, which can block direct contact between the lithium cobalt oxide and the electrolyte, reduce the occurrence of side reactions, and reduce Co dissolution, thereby improving the cycle life and capacity retention of the electrochemical device. At the same time, the first substance with a Pnma crystal structure itself has high thermal stability, which helps to improve the interfacial stability between the positive electrode composite material and the electrolyte, reduce Co dissolution, and thus help improve the high-temperature storage performance and capacity retention of the positive electrode composite material.

[0009] Based on the first aspect, in some possible implementations, the chemical formula of the first substance is LiFe (1-z) Nz PO4, the N element is selected from at least one of Sr, Na, Si, Co, Mn, Cu, W, Nb, Zn, B, Mg, Y, Al, Zr, K, Ca, V, Ti, Ni, or Sn, and 0≤z<1. The iron and phosphorus in the first substance form FeO6 octahedra and PO4 hexahedrons with oxygen, respectively. The octahedra and hexahedrons are alternately linked to form a stable spatial skeleton. Lithium atoms fill the gaps in this skeleton to form LiO octahedra, thus forming a stable structure that helps improve the stability of the positive electrode composite material. Furthermore, the first substance can also contribute to gram capacity, reducing the impact of the first substance on the gram capacity of the positive electrode composite material, thereby helping to increase the energy density of the electrochemical device. At the same time, the first substance can also form phosphates when charged to a certain voltage. The phosphates react with the electrolyte to form a CEI film, thereby further enhancing the interfacial stability between the positive electrode composite material and the electrolyte, and improving the stability of the positive electrode material. Furthermore, the doped N element can further improve the high-temperature storage stability and high-temperature cycling stability of the electrochemical device.

[0010] Based on the first aspect, in some possible implementations, the mass content of the first substance in the positive electrode composite material is ω, 0<ω≤3%. This helps maintain a good gram capacity of the positive electrode composite material and also enables the electrochemical device to have excellent capacity retention and rate performance.

[0011] Based on the first aspect, in some possible embodiments, the median particle size D of the lithium cobalt oxide is 50 The thickness is 3 μm to 15 μm, which is beneficial to improving the uniformity and firmness of the first substance on the surface of the lithium cobalt oxide, and also beneficial to improving the uniform dispersion of the positive electrode material slurry.

[0012] Based on the first aspect, in some possible embodiments, the lithium cobalt oxide is a single-crystal particle having cracks. The presence of the cracks can reduce and release stress generated by the expansion and contraction of the lithium cobalt oxide lattice during charge and discharge, thereby inhibiting the rapid expansion of lattice defects and improving the stability of the lithium cobalt oxide.

[0013] Based on the first aspect, in some possible embodiments, the amount of the M element in the lithium cobalt oxide is m, the sum of the amounts of the Co element and the M element in the lithium cobalt oxide is n, and 0.05≤m / n≤0.15. This is beneficial to further improve the structural stability of the lithium cobalt oxide.

[0014] Based on the first aspect, in some possible embodiments, the median particle size D of the first substance 50Less than or equal to 300 nm. This helps shorten the diffusion path of lithium ions in the positive electrode composite material, thereby improving the rate performance of the electrochemical device, and also helps increase the contact area between the first substance and the lithium cobalt oxide, thereby improving the firmness of the first substance on the surface of the lithium cobalt oxide.

[0015] Based on the first aspect, in some possible embodiments, the median particle size D of the first substance 50 Less than or equal to 200 nm, which is beneficial to further improve the rate performance of electrochemical devices.

[0016] A second aspect of the present application provides an electrochemical device comprising a positive electrode sheet and a negative electrode sheet. The positive electrode sheet comprises a current collector and a positive electrode active layer disposed on the current collector. The positive electrode active layer comprises a positive electrode composite material. The positive electrode composite material comprises a lithium cobalt oxide having a P63mc crystal structure and a first substance having a Pnma crystal structure on its surface, thereby reducing Co dissolution and facilitating improved cycle life, charge-discharge efficiency, and high-temperature performance of the electrochemical device.

[0017] A third aspect of the present application provides an electronic device comprising an electrochemical device. The electrochemical device supplies power to the electronic device, and the electrochemical device comprises the aforementioned positive electrode composite material. The positive electrode composite material comprises a lithium cobalt oxide having a P63mc crystal structure and a first substance having a Pnma crystal structure on its surface. This improves the high-temperature storage performance, cycle life, and capacity retention of the positive electrode composite material, reduces Co dissolution, and improves the cycle life and high-temperature performance of the electrochemical device, thereby facilitating an increase in the service life and high-temperature applicability of the electronic device. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0019] FIG1 is an X-ray diffraction pattern of lithium cobalt oxide prepared in Example 1.

[0020] FIG2 is a scanning electron microscope image of the lithium cobalt oxide material with a P63mc structure prepared in Example 1.

[0021] FIG3 is a scanning electron microscope image of LiFePO4 used in the positive electrode composite material of Example 1.

[0022] FIG4 is an X-ray diffraction pattern of the positive electrode composite material prepared in Example 1.

[0023] FIG5 is a scanning electron microscope image of the positive electrode composite material prepared in Example 1.

[0024] FIG6 is an EDS test image of the surface of the positive electrode composite material prepared in Example 1.

[0025] FIG7 is a scanning electron microscope image of a cross section of the lithium cobalt oxide prepared in Example 1. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application are described clearly and in detail below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0027] An embodiment of the present application provides an electrochemical device, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0028] The outer shell can be a packaging bag encapsulated with an encapsulation film (such as an aluminum-plastic film), such as when the electrochemical device is a soft-pack battery. In other embodiments, the electrochemical device can also be a steel-shell battery, an aluminum-shell battery, etc.

[0029] The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator, with the separator being disposed between the positive and negative electrode sheets. The electrode assembly can be a laminated structure, formed by stacking the positive electrode sheet, separator, and negative electrode sheet. In other embodiments, the electrode assembly can also be a wound structure, formed by stacking the positive electrode sheet, separator, and negative electrode sheet and then winding them.

[0030] Positive electrode

[0031] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer includes a positive electrode active material, a binder and a conductive agent.

[0032] According to some embodiments of the present application, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or nickel foil may be used. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0033] The positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions (ie, a lithiated intercalation compound). The positive electrode active material includes a positive electrode composite material.

[0034] The present application provides a positive electrode composite material, which includes a lithium cobalt oxide having a P63mc crystal structure and a first substance having a Pnma crystal structure. The lithium cobalt oxide contains an M element, and the M element includes at least one of Al, Mg, La, Ca, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr, Y or Zr. The first substance is located on the surface of the lithium cobalt oxide.

[0035] In this application, the first substance is located on the surface of the lithium cobalt oxide, which can block direct contact between the lithium cobalt oxide and the electrolyte, reduce the occurrence of side reactions, and reduce Co dissolution, thereby improving the cycle life and capacity retention of the electrochemical device. At the same time, the first substance with a Pnma crystal structure itself has high thermal stability, which helps to improve the interfacial stability between the positive electrode composite material and the electrolyte, reduce Co dissolution, and thus help improve the high-temperature storage performance and capacity retention of the positive electrode composite material.

[0036] For example, if the first substance is lithium iron phosphate with a Pnma crystal structure, the oxygen ions of the lithium iron phosphate form a hexagonal close-packed (hcp) arrangement, and the iron ions form chain octahedra on alternating basal planes, bridged by tetrahedral phosphate (PO4). The strong covalent bond of PO in the structure gives it better stability, which is beneficial to improving the stability of the positive electrode composite material.

[0037] At the same time, lithium iron phosphate has a high operating voltage (about 3.4V) and a high theoretical specific capacity (about 170mAh g -1 ), the lithium iron phosphate formed on the lithium cobalt oxide also contributes a certain gram capacity to the positive electrode composite material, reducing the degree of reduction in the gram capacity of the positive electrode composite material. The delithiation voltage of the first substance provided in the present application is lower than the voltage of lithium cobalt oxide. When charged to 4.55V, phosphate is formed in the first substance. The phosphate can serve as a raw material for forming a solid electrolyte interface (CEI film) on the surface of the lithium cobalt oxide and participate in the formation of a more stable CEI film, thereby improving the interfacial stability between the positive electrode material and the electrolyte and reducing the dissolution of Co.

[0038] Doping the lithium cobalt oxide with a metal M element is beneficial to synthesizing a lithium cobalt oxide with a stable P63mc crystal structure and improving the purity of the P63mc crystal structure.

[0039] In some embodiments, the chemical formula of the first substance is LiFe (1-z) N z PO4, N element is selected from at least one of Sr, Na, Si, Co, Mn, Cu, W, Nb, Zn, B, Mg, Y, Al, Zr, K, Ca, V, Ti, Ni or Sn, 0≤z<1. LiFe (1-z) N zPO4 has a Pnma crystal structure. The first substance contains phosphate and iron ions. The iron and phosphorus in the first substance form FeO6 octahedrons and PO4 hexahedrons with oxygen respectively. The octahedrons and hexahedrons are alternately linked to form a stable spatial skeleton. Lithium atoms fill the gaps in this skeleton to form LiO octahedrons, thereby forming a stable structure, which is beneficial to improving the stability of the positive electrode composite material. In addition, the above-mentioned first substance can also contribute gram capacity, reducing the impact of the first substance on the gram capacity of the positive electrode composite material, thereby helping to improve the energy density of the electrochemical device; at the same time, the above-mentioned first substance can also form phosphate when charged to a certain voltage. The phosphate reacts with the electrolyte to form a CEI film, thereby further improving the interface stability of the positive electrode composite material and the electrolyte, and improving the stability of the positive electrode material. In addition, the doped N element can further improve the high-temperature storage stability and high-temperature cycle stability of the electrochemical device.

[0040] In some embodiments, the mass content of the first substance in the positive electrode composite material is ω, where 0 < ω ≤ 3%. This helps maintain a good gram capacity of the positive electrode composite material while also enabling the electrochemical device to have excellent capacity retention and rate performance. In some embodiments, ω can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or any value within a range formed by any two of the foregoing values.

[0041] In some embodiments, the median particle size D of the lithium cobalt oxide is 50 The median particle size D of the lithium cobalt oxide is 3 μm to 15 μm. This is beneficial to improving the uniformity and firmness of the first substance on the surface of the lithium cobalt oxide, and also to improving the uniform dispersion of the positive electrode material slurry. In some embodiments, the median particle size D of the lithium cobalt oxide is 50 It can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm or any value within the range formed by any two of the above values.

[0042] In some embodiments, the lithium cobalt oxide is a single crystal particle, and the lithium cobalt oxide has cracks, as shown in Figure 7. The lithium cobalt oxide single crystal particles have cracks, some of which are located inside the grains, and some of which extend from the interior of the grains to the surface. The direction of the cracks is parallel to the transition metal layer (Co layer). The presence of these cracks can reduce and release the stress generated by the lattice expansion and contraction of the lithium cobalt oxide during charge and discharge, thereby inhibiting the rapid expansion of lattice defects, improving the stability of the lithium cobalt oxide, and facilitating the realization of high electrochemical performance, such as cycle performance and rate performance.

[0043] In some embodiments, the amount of the M element in the lithium cobalt oxide is m, the sum of the amounts of the Co element and the M element in the lithium cobalt oxide is n, and 0.05 ≤ m / n ≤ 0.15. This further improves the structural stability of the lithium cobalt oxide. In some embodiments, the ratio of m / n can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, or any value within a range formed by any two of the foregoing values.

[0044] In some embodiments, the median particle size D of the first material is 50 Less than or equal to 300nm. The median particle size of the first substance is set in the nanometer range, which is beneficial to shortening the diffusion path of lithium ions in the positive electrode composite material, improving the rate performance of the electrochemical device, and also helping to increase the contact area between the first substance and the lithium cobalt oxide, and improving the firmness of the first substance on the surface of the lithium cobalt oxide. In some embodiments, the median particle size D of the first substance is less than or equal to 300nm. 50 It can be 20nm, 30nm, 50nm, 70nm, 90nm, 100nm, 120nm, 140nm, 160nm, 180nm, 200nm, 220nm, 240nm, 260nm, 280nm, 300nm or any value within the range formed by any two of the above values.

[0045] In some embodiments, the median particle size D of the first material is preferably 50 Less than or equal to 200 nm, which is beneficial to further improve the rate performance of electrochemical devices.

[0046] The present application also provides a method for preparing a positive electrode composite material, comprising the following steps:

[0047] (1) Lithium cobalt oxide and a first substance having a Pnma crystal structure are dispersed in a solvent to obtain a mixed suspension.

[0048] (2) Pre-drying the mixed suspension in step (1) to remove the solvent, and then performing a secondary drying process to obtain a positive electrode composite material.

[0049] Pre-drying can be done by spray drying, rotary evaporation, or fluidized bed drying, with a drying temperature of 90-180°C to remove most of the moisture and obtain a pre-dried material. A secondary high-temperature drying process, at a temperature not less than 200°C, further removes trace amounts of solvent. Deionized water is an ideal solvent.

[0050] The binder is used to bond the positive electrode active material particles to facilitate the formation of a film layer, while also being able to improve the bonding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder may include, but is not limited to, a binder polymer, such as polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane. In some embodiments, the polyolefin binder includes at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid.

[0051] In some embodiments, the conductive agent includes a carbon-based material, a metal-based material, a conductive polymer, or a mixture thereof. Carbon-based materials include natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, or carbon fiber; metal-based materials include metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; and conductive polymers include polyphenylene derivatives.

[0052] In some embodiments, the mass content of the positive electrode composite material in the positive electrode active material layer is greater than 95%, and the compaction density of the positive electrode composite material is greater than 4.0 g / cm 3 .

[0053] Negative electrode

[0054] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0055] The negative electrode current collector includes copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, a polymer substrate coated with a conductive metal, or any combination thereof.

[0056] The negative electrode current collector includes copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, a polymer substrate coated with a conductive metal, or any combination thereof.

[0057] The negative electrode active material includes a material that reversibly intercalates / deintercalates lithium ions. In some embodiments, the material that reversibly intercalates / deintercalates lithium ions includes a carbon material. In some embodiments, the carbon material can be any carbon-based negative electrode active material commonly used in lithium-ion rechargeable batteries. In some embodiments, the carbon material includes, but is not limited to: crystalline carbon, amorphous carbon, or a mixture thereof. Crystalline carbon can be amorphous, flaky, platelet-shaped, spherical, or fibrous natural graphite or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbide, calcined coke, etc.

[0058] In some embodiments, the negative electrode active material layer includes a negative electrode active material. The specific types of negative electrode active materials are not subject to specific restrictions and can be selected according to needs. In some embodiments, the negative electrode active material includes, but is not limited to: lithium metal, structured lithium metal, natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structured lithiated TiO2-Li4Ti5O 12 , Li-Al alloy or any combination thereof. The silicon-carbon composite refers to a composite containing at least about 5 wt% silicon based on the weight of the silicon-carbon negative electrode active material.

[0059] In some embodiments, the negative electrode active material comprises at least one of artificial graphite, natural graphite, hard carbon, soft carbon, silicon alloy, or silicon oxide.

[0060] When the negative electrode includes a silicon-carbon compound, the ratio of silicon to carbon is about 1:10 to 10:1 based on the total weight of the negative electrode active material, and the median particle size Dv of the silicon-carbon compound is about 1:10 to 10:1. 50 The negative electrode active material layer may be formed using a method such as evaporation, sputtering, or plating. When the negative electrode comprises lithium metal, the negative electrode active material layer may be formed using, for example, a spherical, twisted conductive skeleton and metal particles dispersed within the conductive skeleton. In some embodiments, the spherical, twisted conductive skeleton may have a porosity of approximately 5% to approximately 85%. In some embodiments, a protective layer may also be provided on the lithium metal negative electrode active material layer.

[0061] In some embodiments, the negative electrode active material layer may include a binder and, optionally, a conductive material. The binder improves the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

[0062] In some embodiments, the conductive material includes, but is not limited to, a carbon-based material, a metal-based material, a conductive polymer, or mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, and silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0063] In some embodiments, the current collector includes, but is not limited to, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and any combination thereof.

[0064] The negative electrode can be prepared by methods known in the art. For example, the negative electrode can be obtained by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition, and then coating the active material composition on a current collector. In some embodiments, the solvent may include water, etc., but is not limited thereto.

[0065] Isolation film

[0066] The material and shape of the separator used in the electrochemical device 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 substance formed of a material that is stable to the electrolyte of the present application.

[0067] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a porous nonwoven fabric, film, or composite film, 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 nonwoven fabric, polyethylene nonwoven fabric, or polypropylene-polyethylene-polypropylene porous composite film may be used.

[0068] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by a mixture of a 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.

[0069] The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer comprises a polymer, and the polymer material is selected from at least one of polyamide, polyacrylonitrile, an acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0070] electrolyte

[0071] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and optional additives.

[0072] 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. The electrolyte used in the electrolyte according to the present application is not limited, and it can 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) or ethyl propionate (EP).

[0073] In some embodiments, the organic solvent includes an ether solvent, for example, 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.

[0074] According to some embodiments of the present application, the electrochemical device of the present application includes, but is not limited to: a lithium-ion battery. In some embodiments, the electrochemical device includes a lithium-ion battery.

[0075] This application also applies the electrochemical device to an electronic device to power a load in the electronic device. The positive electrode composite material in the electrochemical device of the electronic device comprises a first substance having a Pnma crystal structure, which can block contact between lithium cobalt oxide and the electrolyte, reduce the occurrence of side reactions, and reduce Co dissolution. The first substance itself has high thermal stability, which helps improve the interfacial stability between the positive electrode composite material and the electrolyte, thereby improving the cycle life, high-temperature performance, and charge-discharge efficiency of the electrochemical device, further improving the service life, charge-discharge efficiency, and high-temperature performance of the electronic device.

[0076] The electronic 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.

[0077] The present application is described below by way of specific examples and comparative examples. It should be understood by those skilled in the art that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0078] Example 1

[0079] (1) Preparation of P63mc structured lithium cobalt oxide

[0080] Sodium carbonate, cobalt acetate, and aluminum oxide were weighed and added to a ball mill at a molar ratio of Na:Co:Al = 0.75:0.94:0.06. Milling balls and an appropriate amount of deionized water were then added to the mixture at a mass ratio of 10:1. The mixture was ball milled at 300 rpm for 10 hours. After ball milling, the mixture was removed and spray-dried to remove the solvent, resulting in a uniform raw material mixture.

[0081] The raw material mixture is pre-calcined at 650°C in a muffle furnace to decompose it into oxides. The pre-calcined oxides are then dispersed again and calcined in a muffle furnace at 900°C for 72 hours. After calcination, the material is cooled to room temperature.

[0082] The calcined sintered product was jaw crushed and jet milled to obtain the target particle size of the precursor material. Ion exchange was then performed using lithium bromide dissolved in hexanol. The lithium bromide concentration in the solution was 5 mol / L, and the Li / Na molar ratio was controlled at approximately 10. The ion exchange reaction was carried out at a temperature of approximately 170°C and a reaction time of 46 hours.

[0083] After the ion exchange reaction, the solid powder was filtered out and washed with excess methanol. Finally, it was dried under vacuum at 180 ° C to obtain Na 0.014 Li 0.9 Co 0.94 Al 0.06 O2 lithium cobalt oxide material.

[0084] To Na 0.014 Li 0.9 Co 0.94 Al 0.06 The element content of O2 material was tested by ICP, and normalized by the sum of the molar masses of Co and Al. The molar number of oxygen was defaulted to 2 for processing. The obtained material composition was Na 0.014 Li 0.9 Co 0.94 Al 0.06 O2.

[0085] The obtained Na 0.014 Li 0.9 Co 0.94 Al 0.06 The median particle size D50 of the O2 material is 11.2 μm. XRD test of the obtained material is performed, as shown in Figure 1, indicating that the Na 0.014 Li 0.9 Co 0.94 Al 0.06 The O2 material has a P63mc crystal structure [reference Solid State Ionics (2001) 144, 263-276].

[0086] (2) Preparation of positive electrode composite materials

[0087] The cathode composite material is Na 0.014 Li 0.9 Co 0.94 Al 0.06 The O2 material is obtained by combining lithium cobalt oxide and LiFePO4 material.

[0088] The LiFePO4 used in this example was commercially available, and its SEM morphology is shown in Figure 3. The preparation methods of the first substance in other examples (such as Examples 11 to 17) can be referred to as follows: [Reference Electrochimica Acta 50 (2005) 2955–2958].

[0089] The preparation method of the positive electrode composite material includes: sand milling or ball milling the LiFePO4 material to reduce the median particle size to the target size, which can improve the uniformity and conductivity. The median particle size D of the LiFePO4 after the refinement treatment is measured by a laser particle size tester. 50 The refined nano-LiFePO4 material and Na 0.014 Li 0.9 Co 0.94 Al 0.06O2 material is dispersed in deionized water, and the LiFePO4 material added with deionized water is ultrasonically treated to depolymerize the LiFePO4 material and react with Na 0.014 Li 0.9 Co 0.94 Al 0.06 The O2 materials were uniformly mixed to obtain a mixed suspension. The suspension was pre-dried by spray drying at a drying temperature of 120°C. Subsequently, the pre-dried material was subjected to a secondary high-temperature treatment at 180°C to remove trace amounts of residual moisture on the surface, obtaining Na2O4 with LiFePO4 on the surface. 0.014 Li 0.9 Co 0.94 Al 0.06 The O2 positive electrode composite material comprises LiFePO4 having a mass content of 1% based on the mass of the positive electrode composite material.

[0090] (3) Preparation of lithium-ion soft-pack batteries

[0091] Preparation of positive electrode sheet: The positive electrode composite material prepared in Example 1, conductive carbon (SP) and adhesive polyvinylidene fluoride (PVDF) were mixed in N-methylpyrrolidone solvent (NMP) at a mass ratio of 97:2:1 and stirred thoroughly to obtain positive electrode slurry. After being processed through a 200-mesh screen, the above positive electrode slurry was coated on an aluminum foil with a thickness of 9 μm. After drying, cold pressing and cutting, a sheet with a thickness of about 90 μm and a compaction density of 4.15 g / cm was obtained. 3 The positive electrode.

[0092] Preparation of the negative electrode sheet: Artificial graphite, conductive carbon (SP), sodium carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were stirred and dispersed uniformly in deionized water at a mass ratio of 96:1:1.5:1.5 to produce a negative electrode slurry. After passing through a 200-mesh screen, the slurry was coated onto 6µm thick copper foil. After drying, cold pressing, and cutting, the negative electrode sheet was obtained. The negative electrode / positive electrode capacity ratio (CB) was approximately 1.055.

[0093] Isolation membrane: A polyethylene porous polymer membrane coated with double-sided ceramic powder, with a thickness of 9um.

[0094] Electrolyte Preparation: The electrolyte mixture consists of 12.5 wt.% LiPF6 dissolved in a solution of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) in a mass ratio of 1:0.8:1.1:2. The electrolyte also contains 5 wt.% fluoroethylene carbonate (FEC) and 2 wt.% 1,3,6-hexanitrile (HTCN).

[0095] Assembling lithium-ion batteries: The above-mentioned positive electrode sheets, negative electrode sheets and isolation films are made into wound cells according to the winding cell process, and placed in an aluminum-plastic film. Vacuum drying is carried out at 100°C until the water content is less than 100ppm. Then, the above-mentioned electrolyte is injected and the sealing is completed. The electrolyte is allowed to stand at high temperature to completely infiltrate the electrode sheets, and then the process of formation, molding, liquid extraction and trimming is carried out to obtain a lithium-ion battery. Examples 2 to 4 Examples 2 to 4 have roughly the same steps as Example 1. The difference is the mass content of LiFePO4. The remaining steps are exactly the same as the material preparation process in Example 1. The specific parameters of the first substance are shown in Table 1.

[0096] Examples 5 to 10

[0097] The steps of Examples 5 to 10 are substantially the same as those of Example 1. The difference is that the median particle size D of LiFePO4 is 50 The remaining steps are exactly the same as the material preparation process in Example 1. The parameters of the specific median particle size material are shown in Table 1.

[0098] Examples 11 to 17

[0099] Examples 5 to 10 follow substantially the same procedures as Example 1. The differences lie in the doping elements and particle size parameters of the P63mc structured lithium cobalt oxide material, and the doping elements, particle size parameters, and content of the Pnma structured first material. The Ti, La, Zr, Mg, Mn, Ni, and Ca doped in the P63mc structured lithium cobalt oxide are obtained by replacing the aluminum oxide in Example 1 with their corresponding oxides. Specific material parameters are shown in Table 1.

[0100] Comparative Example 1

[0101] The difference between the material preparation process of Comparative Example 1 and Example 1 is that Na 0.014 Li 0.9 Co 0.94 Al 0.06 The surface of the O2 lithium cobalt oxide does not contain the first substance. The remaining steps are exactly the same as the material preparation process in Example 1.

[0102] FIG1 is an SEM image of the material of Comparative Example 1, which shows the lamellar single crystal particle characteristics of the lithium cobalt oxide material, and its surface is relatively smooth.

[0103] Comparative Example 2

[0104] The difference between Comparative Example 2 and Example 1 is that P63mc structured lithium cobalt oxide and LiFePO4 are directly added as positive electrode active materials (physical mixing) during the preparation of the positive electrode slurry, and the remaining steps are the same as those of Example 1.

[0105] Performance Testing

[0106] The performance of lithium-ion batteries prepared with positive electrode active materials according to Examples 1 to 17 and Comparative Examples 1 to 2 was evaluated using the following method. The specific test results are listed in Table 2.

[0107] Discharge capacity in grams and rate performance: Place the lithium-ion battery in a 25°C environment and let it rest for at least 2 hours. Then, charge it at a constant current rate of 0.2C until the voltage reaches 4.55V, and continue charging at a constant voltage rate until the cutoff current reaches 0.025C. Discharge it at a constant current rate of 0.2C until the voltage drops to 3V. Repeat these steps twice, and use the discharge capacity of the second discharge to calculate the discharge capacity in grams of the lithium-ion battery cathode material at a rate of 0.2C. Under the same conditions, increase the charge and discharge rate to 1.5C, and calculate the discharge capacity in grams at a rate of 1.5C. Since the battery design is identical except for the difference in the cathode active material, the 1.5C / 0.2C capacity in grams ratio of the full battery can be used as an indicator to judge the rate performance or kinetic properties of the cathode active material. The larger the ratio, the better the kinetic properties of the cathode active material.

[0108] High temperature storage performance: Place the lithium-ion battery in a 25°C environment and let it stand for at least 2 hours. Then, charge it at a constant current rate of 0.2C until the voltage reaches 4.55V, and continue to charge it at a constant voltage until the cut-off current is 0.025C to obtain a fully charged lithium-ion battery, and test the thickness of the battery T0. Then, place the fully charged battery in an 80°C environment and let it stand for 24 hours, then cool it to 25°C, and test the thickness of the battery after high temperature storage T1. The percentage increase in the thickness of the battery can be calculated by (T1-T0) / T0 and used to judge the high temperature storage performance of the battery. The larger the value, the more gas is generated during the high temperature storage process, and the worse the high temperature storage performance of the battery.

[0109] High temperature cycle performance and anode Co dissolution: The lithium-ion battery was placed in a 45°C constant temperature box and charged at a constant current rate of 1C until the voltage reached 4.55V, and then continued to charge at a constant voltage to 0.025C. Then, it was discharged at a constant current rate of 1C until the voltage dropped to 3V. Taking the first cycle discharge capacity as the benchmark, the capacity retention rate of the battery after 200 cycles was recorded. After the cycle was completed, the battery was disassembled, and the negative electrode was removed for inductively coupled plasma mass spectrometry (ICP) testing to obtain the Co deposition concentration at the anode. Among them, C represents the rate, and 1C = 200mA / g.

[0110] Test method:

[0111] The inductively coupled plasma mass spectrometry (ICP) testing method involves using an inductively coupled plasma mass spectrometer (PE Optima 7000DV) to measure the content of elements such as lithium and transition metals in the cathode material. First, an appropriate amount of powder sample is weighed, and approximately 10 mL of aqua regia is added. The sample is heated on a flat-plate heating device at approximately 185°C for 30 to 50 minutes to fully digest the sample before testing.

[0112] XRD testing methods include testing cathode materials using an X-ray powder diffractometer (Bruker D8 ADVANCE) with a Cu Kα target, a test voltage of 40 kV, a test current of 35 mA, a scan angle range of 10° to 90°, and a scan rate of 0.02° / s. The intensity of the strongest diffraction peak must be greater than 10,000 counts. The collected XRD spectra are refined using Fullprof software to determine the phase structure and dopant element occupancy.

[0113] The SEM and EDS testing methods include: taking material powder, observing the surface morphology of the powder using a scanning electron microscope (ZEISS SEM, Sigma-02-33), and testing the elemental composition of the first substance on the surface by EDS.

[0114] Take the positive electrode sheet and process it with a slicer to obtain a cross section perpendicular to the surface of the positive electrode current collector. Use a scanning electron microscope to photograph the cross section to obtain a cross-sectional backscattered image. Cracks inside the grains can be observed on the cross section. In the cross-sectional image, closed areas with a different color from the surrounding area are cracks.

[0115] Refer to Figure 2, the present application of the Na prepared in Example 1 0.014 Li 0.9 Co 0.94 Al 0.06 O2 lithium cobalt oxide was subjected to SEM testing. As shown in FIG2 , the obtained material was a lamellar single crystal particle with a smooth particle surface.

[0116] Referring to Figure 3, the present application conducted an SEM test on the LiFePO4 used in Example 1, showing that the LiFePO4 material particles were uniform in size. Referring to Figure 4, the present application conducted an XRD test on the positive electrode composite material prepared in Example 1. The XRD diffraction peaks showed characteristic peaks of lithium cobalt oxide with a P63mc crystal structure, as well as characteristic peaks of LiFePO4 with a Pnma crystal structure. This indicates that the positive electrode composite material prepared in Example 1 contains lithium cobalt oxide with a P63mc crystal structure and LiFePO4 with a Pnma crystal structure.

[0117] Referring to Figures 5 and 6, the present application also conducted SEM and EDS tests on the positive electrode composite material prepared in Example 1. In Figure 5, the surface of the single-crystalline lithium cobalt oxide is evenly dispersed and covered with LiFePO4 nanoparticles, and the positive electrode composite material is formed with single-crystalline lithium cobalt oxide as the core and LiFePO4 nanoparticles as the shell. As can be seen from Figure 6, there are iron and phosphorus elements on the surface of the single-crystalline lithium cobalt oxide, which further indicates that LiFePO4 exists on the surface of the single-crystalline lithium cobalt oxide. The lithium cobalt oxide material and the LiFePO4 first substance are combined with each other to form a core-shell structured positive electrode composite material, which maintains the characteristics of their respective crystal structures.

[0118] Referring to FIG7 , a cross-section of the positive electrode sheet was subjected to an SEM test. The test results showed that cracks existed in the lithium cobalt oxide single crystal particles (such as the cracks within the dotted box in FIG7 ). Some cracks were entirely inside the grains, while others extended to the surface, and the direction of the cracks was parallel to the transition metal layer (Co layer).

[0119] Table 1

[0120] Table 2

[0121] As shown in Tables 1 and 2, compared to Comparative Examples 1 and 2, the batteries assembled from lithium cobalt oxide with LiFePO4 on the surface in Examples 1 to 4 exhibit reduced Co dissolution from the anode, improving high-temperature storage performance and capacity retention. Lithium iron phosphate, as the first substance present on the surface of the lithium cobalt oxide, is inherently electrochemically active, so the P63mc structured lithium cobalt oxide material still exhibits good discharge capacity per gram.

[0122] In Comparative Example 1, there is no first substance on the surface of the P63mc structure lithium cobalt oxide. After the assembled battery cell is stored at 80°C for 24 hours at full charge, the thickness of the battery cell increases by 67.5%, indicating that the positive electrode interface side reaction is serious and the gas production is relatively large. In contrast, in Examples 1 to 4, the battery cell with lithium cobalt oxide having LiFePO4 on the surface as the positive electrode active material has a battery cell thickness increase of only about 20%, indicating that the first substance significantly improves the cathode interface stability and high-temperature storage performance at full charge. Moreover, the battery cell assembled with the positive electrode active material in Comparative Example 1 has a capacity retention rate of only 30.6% after 200 cycles at 45°C, which is much lower than the capacity retention rate of the assembled battery cells in Examples 1 to 4 (more than 80%), indicating that the presence of LiFePO4 on the surface of lithium cobalt oxide can significantly improve the cycle stability of the positive electrode material. Compared with Comparative Example 1, the concentration of Co deposited on the anode of the assembled battery cells in Examples 1 to 4 is reduced after 200 cycles at 45°C, indicating that the cathode Co dissolution is reduced and the cathode interface is more stable, thereby exhibiting better high-temperature storage performance and cycle stability. Therefore, the first substance with a Pnma crystal structure on the surface of the P63mc structure lithium cobalt oxide is beneficial to reducing the dissolution of Co and can also improve the high-temperature storage performance and capacity retention rate of the P63mc structure lithium cobalt oxide.

[0123] Compared with the direct mixing of LiFePO4 and lithium cobalt oxide in Comparative Example 2, in Examples 1 to 4, LiFePO4 is located on the surface of lithium cobalt oxide. The high-temperature storage performance, capacity retention rate and Co dissolution amount of the battery cells assembled with this positive electrode composite material are all better than those in Comparative Example 2.

[0124] In Examples 1 to 4, as the LiFePO4 concentration increases, in addition to a slight loss in specific capacity, the rate performance of the positive electrode composite material also gradually decreases. In particular, the cell assembled in Example 4, with a first substance content of 4% by mass, saw its capacity retention and 1.5C / 0.2C capacity ratio decrease by 5.5 mAh / g and 7.2%, respectively, compared to the cell assembled in Example 1. This is because excessive first substance on the surface of the lithium cobalt oxide increases lithium ion diffusion resistance, increases interfacial impedance, and leads to increased battery polarization, thus affecting discharge specific capacity and kinetics.

[0125] From Examples 5 to 10, it can be seen that the LiFePO4 grain size has little effect on the discharge capacity and rate performance of the prepared battery, but has a greater impact on the high-temperature storage performance, high-temperature cycle capacity retention rate and Co dissolution. 50As the LiFePO4 crystal size increases, the gas production in high-temperature storage increases, the high-temperature cycle capacity retention rate gradually decreases, and the Co dissolution content in the anode gradually increases. The smaller the LiFePO4 grain size in the positive electrode composite material, the tighter it is combined with the surface of the lithium cobalt oxide material, the more uniform it is on the surface of the lithium cobalt oxide material, and the better the stability. When the LiFePO4 grain size is larger, it is more uneven, the surface of the positive electrode composite material is more exposed, and the interfacial side reaction with the electrolyte is more serious, thereby affecting the high-temperature storage performance, high-temperature cycle capacity retention rate and Co dissolution amount of the positive electrode composite material. When the grain size of LiFePO4 is D 50 When the thickness is less than or equal to 300 nm, it is more conducive to improving the high temperature storage performance, high temperature cycle capacity retention rate and Co dissolution amount of the positive electrode composite material, and further preferably the D of LiFePO4 50 Below 200nm.

[0126] In Examples 11 to 17, the lithium cobalt oxide matrix with a P63mc crystal structure and the first material with a Pnma crystal structure were modified by doping with different elements. Compared with Comparative Examples 1 and 2, the positive electrode composite materials in Examples 11 to 17 significantly improved in terms of high-temperature storage stability, high-temperature cycle capacity retention, and Co dissolution.

[0127] The above disclosure is only a preferred embodiment of the present application and certainly cannot be used to limit the present application. Therefore, equivalent changes made based on the present application are still within the scope covered by the present application.

Claims

1. A positive electrode composite material, wherein: It includes a lithium cobalt oxide having a P63 mc crystal structure and a first substance having a Pnma crystal structure, wherein the lithium cobalt oxide contains an M element, and the M element includes at least one of Al, Mg, La, Ca, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr, Y or Zr, and the first substance is located on the surface of the lithium cobalt oxide.

2. The positive electrode composite material according to claim 1, wherein The chemical formula of the first substance is LiFe (1-z) N z PO4, N element is selected from at least one of Sr, Na, Si, Co, Mn, Cu, W, Nb, Zn, B, Mg, Y, Al, Zr, K, Ca, V, Ti, Ni or Sn, 0≤z<1.

3. The positive electrode composite material according to claim 1 or 2, wherein The mass content of the first substance in the positive electrode composite material is ω, 0<ω≤3%.

4. The cathode composite material according to any one of claims 1 to 3, wherein The median particle size D of the lithium cobalt oxide 50 3μm to 15μm.

5. The cathode composite material according to any one of claims 1 to 4, wherein The lithium cobalt oxide is a single crystal particle and has cracks.

6. The cathode composite material according to any one of claims 1 to 5, wherein The amount of substance of the M element in the lithium cobalt oxide is m, the sum of the amounts of the Co element and the M element in the lithium cobalt oxide is n, and 0.05≤m / n≤0.

15.

7. The cathode composite material according to any one of claims 1 to 6, wherein The median particle size D of the first substance 50 Less than or equal to 300nm.

8. The positive electrode composite material according to claim 7, wherein The median particle size D of the first substance 50 Less than or equal to 200nm.

9. An electrochemical device comprising a positive electrode sheet and a negative electrode sheet, wherein: The positive electrode sheet includes a current collector and a positive electrode active layer disposed on the current collector, and the positive electrode active layer includes the positive electrode composite material according to any one of claims 1 to 8.

10. An electronic device, wherein: Comprising the electrochemical device according to claim 9.

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