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

Positive electrode composite materials, electrochemical devices and electronic devices Technical Field

[0001] This 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 Technology

[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 a P63mc crystal structure exhibits limitations when charged to 4.55V (vs. Li). + When the concentration of Li is above a certain level, no spinel phase transition occurs on the surface, exhibiting high structural reversibility and very high reversible capacity at high voltages. However, during charging, because the surface of the LiCoO2 cathode material cannot spontaneously form an inert layer, Co directly contacts the electrolyte, triggering side reactions. This leads to problems such as high-temperature storage failure and rapid cycle decay, limiting the commercial application of the material.

[0004] Summary of the Invention

[0005] This application provides a positive electrode composite material that can suppress direct contact between Co and the electrolyte.

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

[0007] The first aspect of this 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 element M, and the element M includes at least one selected from 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 material is located on the surface of lithium cobalt oxide, which can prevent direct contact between lithium cobalt oxide and 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 material with the Pnma crystal structure itself has high thermal stability, which is beneficial to improving the interfacial stability between the cathode composite material and the electrolyte, reducing Co dissolution, and thus improving the high-temperature storage performance and capacity retention of the cathode 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 and N are 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, where 0 ≤ z < 1. In the first substance, iron and phosphorus react with oxygen to form FeO6 octahedra and PO4 hexahedrons, respectively. The alternating links of octahedra and hexahedrons form a stable spatial framework. Lithium atoms fill the gaps in this framework, forming LiO octahedra, thus creating a stable structure that improves the stability of the cathode composite material. Furthermore, the first substance contributes to the specific capacity, reducing its impact on the specific capacity of the cathode composite material, thereby improving the energy density of the electrochemical device. Simultaneously, the first substance, when charged to a certain voltage, can form phosphate ions. These phosphate ions react with the electrolyte to form a CEI film, further enhancing the interfacial stability between the cathode composite material and the electrolyte, thus improving the stability of the cathode material. Additionally, 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 cathode composite material is ω, where 0 < ω ≤ 3%. This is beneficial for maintaining a good specific capacity of the cathode 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 implementations, the median particle size D of lithium cobalt oxide 50 The thickness ranges from 3 μm to 15 μm. This is beneficial for improving the uniformity and stability of the first material on the lithium cobalt oxide surface, and also for improving the uniform dispersion of the cathode material slurry.

[0012] Based on the first aspect, in some possible implementations, the lithium cobalt oxide is a single crystal particle, and the lithium cobalt oxide has cracks. The presence of these cracks can reduce and release the stress generated by the lattice expansion and contraction process of the lithium cobalt oxide during charging and discharging, thereby suppressing the rapid propagation of lattice defects and improving the stability of the lithium cobalt oxide.

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

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

[0015] Based on the first aspect, in some possible implementations, the median particle size D of the first substance 50 200 nm or less. This is beneficial for further improving the rate performance of electrochemical devices.

[0016] A second aspect of this application provides an electrochemical device including a positive electrode and a negative electrode. The positive electrode includes a current collector and a positive electrode active layer disposed on the current collector. The positive electrode active layer includes a positive electrode composite material. The surface of the lithium cobalt oxide with a P63mc crystal structure in the positive electrode composite material has a first substance with a Pnma crystal structure, which reduces Co dissolution and improves the cycle life, charge / discharge efficiency, and high-temperature performance of the electrochemical device.

[0017] A third aspect of this application provides an electronic device, including an electrochemical device. The electrochemical device supplies power to the electronic device and comprises the aforementioned positive electrode composite material. The surface of the lithium cobalt oxide with a P63mc crystal structure in the positive electrode composite material has a first substance with a Pnma crystal structure, which can improve the high-temperature storage performance, cycle life, and capacity retention of the positive electrode composite material, reduce Co leaching, and improve the cycle life and high-temperature performance of the electrochemical device, thereby improving the service life and high-temperature applicability of the electronic device. Attached Figure Description

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

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

[0020] Figure 2 is a scanning electron microscope image of the lithium cobalt oxide material with P63mc structure prepared in Example 1.

[0021] Figure 3 is a scanning electron microscope image of LiFePO4 used in the cathode composite material of Example 1.

[0022] Figure 4 shows the X-ray diffraction pattern of the positive electrode composite material prepared in Example 1.

[0023] Figure 5 is a scanning electron microscope image of the positive electrode composite material prepared in Example 1.

[0024] Figure 6 shows the EDS test image of the surface of the positive electrode composite material prepared in Example 1.

[0025] Figure 7 is a scanning electron microscope image of the cross section of the lithium cobalt oxide prepared in Example 1. Detailed Implementation

[0026] The technical solutions in the embodiments of this application are described clearly and in detail below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0027] One embodiment of this application provides an electrochemical device, which includes a housing, an electrode assembly, and an electrolyte. Both the electrode assembly and the electrolyte are located within the housing.

[0028] The outer casing can be a packaging bag sealed with an encapsulating film (such as aluminum-plastic film), for example, the electrochemical device is a pouch battery. In other embodiments, the electrochemical device can also be a steel-cased battery, an aluminum-cased battery, etc.

[0029] The electrode assembly includes a positive electrode, a negative electrode, and a separator, with the separator disposed between the positive and negative electrode. The electrode assembly can be a stacked structure, formed by layering the positive electrode, separator, and negative electrode. In other embodiments, the electrode assembly can also be a wound structure, formed by winding the stacked positive electrode, separator, and negative electrode.

[0030] Positive electrode sheet

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

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

[0033] Positive electrode active materials include compounds that reversibly insert and deintercalate lithium ions (i.e., lithiation intercalation compounds). Positive electrode active materials include positive electrode composite materials.

[0034] This 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 element M, which 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 material is located on the surface of lithium cobalt oxide, which can prevent direct contact between lithium cobalt oxide and 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 material with the Pnma crystal structure itself has high thermal stability, which is beneficial to improving the interfacial stability between the cathode composite material and the electrolyte, reducing Co dissolution, and thus improving the high-temperature storage performance and capacity retention of the cathode composite material.

[0036] For example, lithium iron phosphate, with its first material being a Pnma crystal structure, has oxygen ions arranged in a hexagonal close-packed (hcp) configuration, and iron ions forming chain-like octahedra on alternating basal planes, which are bridged by tetrahedral phosphate groups (PO4). The strong covalent bonds of PO in the structure give it better stability, which in turn helps to improve the stability of the cathode composite material.

[0037] Meanwhile, lithium iron phosphate has a high operating voltage (approximately 3.4V) and a high theoretical specific capacity (approximately 170mAh g). -1 The lithium iron phosphate formed on lithium cobalt oxide also contributes a certain specific capacity to the cathode composite material, reducing the degree of specific capacity reduction of the cathode composite material. The delithiation voltage of the first material provided in this application is lower than that of lithium cobalt oxide. When charged to 4.55V, phosphate ions are formed in the first material. Phosphate ions can serve as raw materials for forming a solid electrolyte interface (CEI film) on the surface of lithium cobalt oxide, and participate in the formation of a more stable CEI film, improving the interfacial stability between the cathode material and the electrolyte, and reducing the dissolution of Co.

[0038] Doping lithium cobalt oxide with metallic element M facilitates the synthesis of lithium cobalt oxide with a stable P63mc crystal structure and improves 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, where N 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 Phosphate-N2 crystal structure. The first component contains phosphate and iron ions. Iron and phosphorus in the first component react with oxygen to form FeO6 octahedra and PO4 hexahedrons, respectively. The alternating links of octahedra and hexahedrons form a stable spatial framework. Lithium atoms fill the gaps in this framework, forming LiO octahedra, thus creating a stable structure and improving the stability of the cathode composite material. Furthermore, the first component contributes to the specific capacity, reducing its impact on the specific capacity of the cathode composite material, thereby improving the energy density of the electrochemical device. Simultaneously, when charged to a certain voltage, the first component can form phosphate ions. These phosphate ions react with the electrolyte to form a CEI film, further enhancing the interfacial stability between the cathode composite material and the electrolyte, thus improving the stability of the cathode material. Additionally, the doping with nitrogen can further improve the high-temperature storage stability and high-temperature cycling stability of the electrochemical device.

[0040] In some embodiments, the mass content of the first substance in the cathode composite material is ω, where 0 < ω ≤ 3%. This is beneficial for maintaining a good specific capacity of the cathode composite material and also allows 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 the range of any two of the above values.

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

[0042] In some embodiments, the lithium cobalt oxide is a single crystal particle with cracks, as shown in Figure 7. The presence of cracks in the single crystal lithium cobalt oxide particles, some located inside the grain and others extending from the grain to the surface, with the crack direction parallel to the transition metal layer (Co layer), can reduce and release the stress generated by the lattice expansion and contraction of the lithium cobalt oxide during charging and discharging. This, in turn, inhibits the rapid propagation of lattice defects, improves the stability of the lithium cobalt oxide, and is beneficial for achieving high electrochemical performance, such as cycle performance and rate performance.

[0043] In some embodiments, the amount of element M in lithium cobalt oxide is m, and the sum of the amounts of elements Co and M in lithium cobalt oxide is n, where 0.05 ≤ m / n ≤ 0.15. This is beneficial for further improving the structural stability of 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 the range of any two of the above values.

[0044] In some embodiments, the median particle size D of the first material 50 The median particle size of the first material is less than or equal to 300 nm. Setting the median particle size of the first material within the nanometer range is beneficial for shortening the diffusion path of lithium ions in the cathode composite material, improving the rate performance of the electrochemical device, and also for increasing the contact area between the first material and lithium cobalt oxide, thus enhancing the adhesion of the first material to the lithium cobalt oxide surface. In some embodiments, the median particle size D of the first material... 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 of any two of the above values.

[0045] In some embodiments, the median particle size D of the first material is preferred. 50 200 nm or less. This is beneficial for further improving the rate performance of electrochemical devices.

[0046] This 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) The mixed suspension in step (1) is pre-dried to remove the solvent, and then a second drying process is performed to obtain the positive electrode composite material.

[0049] Pre-drying can be achieved through processes such as spray drying, rotary evaporation drying, and fluidized bed drying, at temperatures ranging from 90 to 180°C, to remove most of the moisture and obtain a pre-dried material. A secondary high-temperature drying process, at a temperature not lower than 200°C, further removes trace amounts of solvent, such as deionized water.

[0050] Adhesives are used to bond positive electrode active material particles, thereby facilitating the formation of a film layer and improving the adhesion between the positive electrode active layer and the positive electrode current collector. In some embodiments, the adhesive may include, but is not limited to, adhesive polymers, 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, polyolefin adhesives include at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid.

[0051] In some embodiments, the conductive agent includes carbon-based materials, metal-based materials, conductive polymers, or mixtures thereof. Examples of carbon-based materials include natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, or carbon fibers; examples of metal-based materials include metal powders or fibers of copper, nickel, aluminum, silver, etc.; and examples of 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 / cm³. 3 .

[0053] Negative electrode sheet

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

[0055] Negative current collectors include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrates coated with conductive metal, or any combination thereof.

[0056] Negative current collectors include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrates coated with conductive metal, or any combination thereof.

[0057] Negative electrode active materials include materials that reversibly insert / deintercalate lithium ions. In some embodiments, the materials that reversibly insert / deintercalate lithium ions include carbon materials. 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 mixtures thereof. Crystalline carbon can be amorphous, flake-shaped, flake-shaped, spherical, or fibrous natural or artificial graphite. Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbides, calcined coke, etc.

[0058] In some embodiments, the negative electrode active material layer includes a negative electrode active material. The specific type of negative electrode active material is not limited and can be selected according to requirements. 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 spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Li-Al alloys or any combination thereof. The silicon-carbon composite refers to a silicon-carbon anode active material containing at least about 5 wt% silicon by weight.

[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 silicon-carbon compounds, based on the total weight of the negative electrode active material, the silicon:carbon ratio is approximately 1:10-10:1, and the median particle size Dv of the silicon-carbon compound is... 50 The micrometer size ranges from approximately 0.1 micrometers to 20 micrometers. When the negative electrode comprises an alloy material, the negative electrode active material layer can be formed using methods such as vapor deposition, sputtering, or plating. When the negative electrode comprises lithium metal, the negative electrode active material layer is formed, for example, using a conductive framework with a spherical twisted structure and metal particles dispersed within the conductive framework. In some embodiments, the spherical twisted conductive framework 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 an adhesive and optionally a conductive material. The adhesive enhances 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 adhesive includes, but is not limited to: polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, 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, carbon-based materials, metal-based materials, conductive polymers, 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, or 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, 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 onto a current collector. In some embodiments, the solvent may include, but is not limited to, water.

[0065] Separating membrane

[0066] The material and shape of the separator used in the electrochemical device of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application.

[0067] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected.

[0068] A surface treatment layer is disposed 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 mixture of polymer and inorganic materials. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, 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 polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, the polymer material of which is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0070] electrolyte

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

[0072] The organic solvent in the electrolyte of this application may be any organic solvent known in the prior art that can be used as an electrolyte solvent. There are no limitations on the electrolyte used in the electrolyte according to this application; it may be any electrolyte known in the prior art. The additives in the electrolyte according to this application may be any additives known in the prior art that can be used as electrolyte additives. 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 ether solvents, such as at least one selected from 1,3-dioxapentane (DOL) and dimethyl glycol ether (DME). In some embodiments, the lithium salt includes at least one selected from organic lithium salts or inorganic lithium salts. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(fluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate)borate LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one selected from fluoroethylene carbonate and adiponitrile.

[0074] According to some embodiments of this application, the electrochemical device of this 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 electronic devices to power loads within those devices. The positive electrode composite material in the electrochemical device of the aforementioned electronic device possesses a first substance with a Pnma crystal structure, which can block contact between lithium cobalt oxide and the electrolyte, reducing side reactions and Co dissolution. The first substance itself has high thermal stability, which is beneficial for improving 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, and further enhancing the lifespan, charge / discharge efficiency, and high-temperature performance of the electronic device.

[0076] The electronic devices described in this application are not particularly limited. In some embodiments, the electronic devices described in this application include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0077] The present application will be described below through specific embodiments and comparative examples. Those skilled in the art should understand 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 lithium cobalt oxide with P63mc structure

[0080] Sodium carbonate, cobalt acetate, and alumina were weighed according to a molar ratio of Na:Co:Al = 0.75:0.94:0.06 and added separately to a ball mill jar. With the addition of grinding balls and an appropriate amount of deionized water, the materials were mixed at a grinding ball to material mass ratio of 10:1. The mixture was ball-milled at 300 rpm for 10 hours. After ball milling, the material was removed and spray-dried to remove the solvent, obtaining a homogeneous raw material mixture.

[0081] The above-mentioned raw material mixture was placed in a muffle furnace for pre-calcination at 650°C to decompose it into oxides. The pre-calcined oxides underwent a secondary dispersion treatment and were then placed in a muffle furnace for high-temperature calcination at 900°C for 72 hours. After calcination, the material was allowed to cool to room temperature.

[0082] The calcined sintered product was subjected to jaw crusher and air jet milling to obtain precursor material with the target particle size. Then, ion exchange was performed using lithium bromide dissolved in hexanol. In the solution, the concentration of lithium bromide was 5 mol / L, the Li / Na molar ratio was controlled at approximately 10, the ion exchange reaction temperature was approximately 170℃, and the reaction time was 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 materials.

[0084] To Na 0.014 Li 0.9 Co 0.94 Al 0.06 The elemental composition of the O2 material was determined by ICP testing, normalized to the sum of the molar masses of Co and Al, with the number of moles of oxygen assumed to be 2. The resulting material composition was Na. 0.014 Li 0.9 Co 0.94 Al 0.06 O2.

[0085] The obtained Na was measured using a laser particle size analyzer. 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 analysis of the obtained material, as shown in Figure 1, indicates that the Na... 0.014 Li 0.9 Co 0.94 Al 0.06 O2 materials have a P63mc crystal structure [Reference: Solid State Ionics (2001) 144, 263-276].

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

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

[0088] The LiFePO4 used in this embodiment was commercially available, and the SEM morphology of the material is shown in Figure 3. The preparation method of the first substance in other embodiments (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 cathode composite material includes: sand milling or ball milling LiFePO4 material to reduce its median particle size to the target size, which can improve uniformity and conductivity. The median particle size D of the refined LiFePO4 is measured by a laser particle size analyzer. 50 The nanometer size is 67 nm. The refined nano-LiFePO4 material was then mixed with Na... 0.014 Li 0.9 Co 0.94 Al 0.06O2 material is dispersed in deionized water. The LiFePO4 material in the deionized water is then ultrasonically treated to depolymerize the LiFePO4 material, and then reacted with Na... 0.014 Li 0.9 Co 0.94 Al 0.06 O2 materials were uniformly mixed to obtain a mixed suspension. The suspension was pre-dried by spray drying at 120℃. Subsequently, the pre-dried material underwent a second high-temperature treatment at 180℃ to remove trace amounts of residual surface moisture, yielding Na+ with LiFePO4 on its surface. 0.014 Li 0.9 Co 0.94 Al 0.06 The O2 cathode composite material contains LiFePO4 at a mass content of 1% based on the mass of the cathode composite material.

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

[0091] Preparation of the positive electrode sheet: The positive electrode composite material prepared in Example 1, conductive carbon (SP), and binder polyvinylidene fluoride (PVDF) were mixed thoroughly in an N-methylpyrrolidone solvent (NMP) at a mass ratio of 97:2:1 to obtain a positive electrode slurry. After being screened through a 200-mesh sieve, the positive electrode slurry was coated onto an aluminum foil with a thickness of 9 μm. After drying, cold pressing, and cutting, a sheet with a thickness of approximately 90 μm and a compaction density of 4.15 g / cm³ was obtained. 3 The positive electrode sheet.

[0092] Preparation of the negative electrode sheet: Artificial graphite, conductive carbon (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were dispersed evenly in deionized water at a mass ratio of 96:1:1.5:1.5 to obtain a negative electrode slurry. After being processed through a 200-mesh sieve, the negative electrode slurry was coated onto a 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] Separating membrane: A porous polyethylene polymer membrane coated with ceramic powder on both sides, with a thickness of 9µm.

[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-hexamethylenetrionitrile (HTCN).

[0095] Assembling Lithium-ion Batteries: The above-mentioned positive electrode, negative electrode, and separator are fabricated into a wound cell using a wound cell process and placed in an aluminum-plastic film. Vacuum drying is performed at 100°C until the water content is below 100 ppm. Then, the above-mentioned electrolyte is injected and the cell is sealed. High-temperature standing allows the electrolyte to completely wet the electrode, followed by formation, molding, liquid extraction, and edge trimming processes to obtain the lithium-ion battery. Examples 2 to 4: The steps in Examples 2 to 4 are largely the same as those in Example 1. The difference lies in the mass content of LiFePO4. The remaining steps are exactly the same as the material preparation process in Example 1. Specific parameters of the first substance are shown in Table 1.

[0096] Examples 5 to 10

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

[0098] Examples 11 to 17

[0099] Examples 5 to 10 follow largely the same steps as Example 1. The difference lies in the doping elements, particle size parameters, and content of the P63mc structure lithium cobalt oxide material and the first substance with the Pnma structure. Specifically, the Ti, La, Zr, Mg, Mn, Ni, and Ca doped into the P63mc structure lithium cobalt oxide are obtained by replacing the alumina in Example 1 with their corresponding oxides. Specific material parameters are shown in Table 1.

[0100] Comparative Example 1

[0101] The difference between Comparative Example 1 and Example 1 is that the material preparation process involves 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] Figure 1 shows the SEM image of the material in Comparative Example 1, which illustrates the lamellar single-crystal particle characteristics of the lithium cobalt oxide material with a relatively smooth surface.

[0103] Comparative Example 2

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

[0105] Performance testing

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

[0107] Discharge specific capacity and rate performance: The lithium-ion battery was placed in a 25°C environment and left to stand for at least 2 hours. Then, it was charged at a constant current rate of 0.2C until the voltage reached 4.55V, and then charged at a constant voltage rate until the cutoff current was 0.025C. It was then discharged at a constant current rate of 0.2C until the voltage dropped to 3V. This process was repeated twice, and the discharge specific capacity of the lithium-ion battery cathode material at 0.2C was calculated based on the discharge capacity from the second discharge. Under the same conditions, the charge / discharge rate was increased to 1.5C, and the discharge specific capacity at 1.5C was calculated. Since the battery design is identical except for the cathode active material, the 1.5C / 0.2C specific capacity ratio of the entire battery can be used as an indicator of the rate performance or kinetic characteristics of the cathode active material. The larger this ratio, the better the kinetic characteristics of the cathode active material.

[0108] High-temperature storage performance: The lithium-ion battery was placed in a 25°C environment and left to stand for at least 2 hours. Then, it was charged at a constant current rate of 0.2C until the voltage reached 4.55V, and then charged at a constant voltage rate until the cutoff current was 0.025C, obtaining a fully charged lithium-ion battery. The battery thickness T0 was then measured. The fully charged battery was then placed in an 80°C environment and left to stand for 24 hours, then cooled to 25°C, and the battery thickness T1 after high-temperature storage was measured. The percentage increase in battery thickness can be calculated by (T1-T0) / T0 and used to evaluate the battery's high-temperature storage performance. The larger this value, the more gas is generated during high-temperature storage, and the worse the battery's high-temperature storage performance.

[0109] High-temperature cycling performance and anode Co dissolution: The lithium-ion battery was placed in a 45°C constant-temperature chamber and charged at a 1C rate with constant current until the voltage reached 4.55V, and then charged at a constant voltage until 0.025C. It was then discharged at a 1C rate with constant current until the voltage dropped to 3V. Using the initial discharge capacity as a baseline, the capacity retention rate of the battery was recorded after 200 cycles. After cycling, the battery was disassembled, and the negative electrode was removed for inductively coupled plasma mass spectrometry (ICP) to obtain the Co deposition concentration at the anode. Here, C represents the rate, 1C = 200 mA / g.

[0110] Test method:

[0111] The inductively coupled plasma mass spectrometry (ICP) testing method includes: using an inductively coupled plasma spectrometer (ICP, instrument model: PE Optima 7000DV) to test the content of elements such as Li and transition metals in the cathode material. First, weigh an appropriate amount of powder sample, add about 10 mL of aqua regia, and heat in a plate heater at about 185℃ for 30 to 50 minutes to ensure complete digestion of the sample, and then perform the test on the instrument.

[0112] The XRD testing method included: using an X-ray powder diffractometer (XRD instrument model: Bruker D8 ADVANCE) to test the cathode material, with Cu Kα as the target, a test voltage of 40 kV, a test current of 35 mA, a scanning angle range of 10° to 90°, a scanning rate of 0.02° / s, and requiring the strongest diffraction peak intensity to be greater than 10,000 (in counts). The acquired XRD patterns were then refined using Fullprof software to determine the phase structure and dopant site 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 using 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 current collector. Use a scanning electron microscope to photograph the cross section to obtain a backscattered image of the cross section. Cracks inside the grains can be observed on the cross section. In the cross section image, the closed area with a different color from the surrounding area is the crack.

[0115] Referring to Figure 2, this application describes the Na prepared in Example 1. 0.014 Li 0.9 Co 0.94 Al 0.06 The O2 lithium cobalt oxide was subjected to SEM testing, as shown in Figure 2. The obtained material is a layered single crystal particle with a smooth particle surface.

[0116] Referring to Figure 3, SEM testing was performed on the LiFePO4 used in Example 1, showing that the LiFePO4 material particles are of uniform size. Referring to Figure 4, XRD testing was performed on the cathode 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 cathode composite material prepared in Example 1 contains both lithium cobalt oxide with a P63mc crystal structure and LiFePO4 with a Pnma crystal structure.

[0117] Referring to Figures 5 and 6, this application also performs SEM and EDS tests on the cathode composite material prepared in Example 1. In Figure 5, LiFePO4 nanoparticles are uniformly dispersed and coated on the surface of the single-crystal lithium cobalt oxide, forming a cathode composite material with the single-crystal lithium cobalt oxide as the core and the LiFePO4 nanoparticles as the shell. Figure 6 shows the presence of iron and phosphorus elements on the surface of the single-crystal lithium cobalt oxide, further indicating the presence of LiFePO4 on the surface. The lithium cobalt oxide material and the LiFePO4 first substance combine to form a core-shell structured cathode composite material, which retains the characteristics of their respective crystal structures.

[0118] Referring to Figure 7, SEM testing was performed on the cross-section of the positive electrode. The test results showed that there were cracks in the lithium cobalt oxide single crystal particles (cracks within the dashed box in Figure 7). Some cracks were entirely inside the grains, while others extended to the surface. 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, in Examples 1 to 4, the amount of Co dissolution at the anode of the batteries assembled from lithium cobalt oxide with LiFePO4 on the surface is reduced, thus improving high-temperature storage performance and capacity retention. Since lithium iron phosphate exists as the primary substance on the surface of the lithium cobalt oxide and is inherently electrochemically active, the P63mc structure lithium cobalt oxide material still exhibits good discharge specific capacity.

[0122] In Comparative Example 1, the P63mc structure lithium cobalt oxide surface lacked the first substance. After 24 hours of storage at 80°C in a fully charged state, the cell thickness increased by 67.5%, indicating severe side reactions and significant gas production at the cathode interface. In contrast, the cells in Examples 1 to 4, using lithium cobalt oxide with LiFePO4 on its surface as the cathode active material, only showed an increase in cell thickness of about 20%, demonstrating that the first substance significantly improved the cathode interface stability and high-temperature storage performance in a fully charged state. Furthermore, the cell assembled with the cathode active material in Comparative Example 1 exhibited a capacity retention rate of only 30.6% after 200 cycles at 45°C, far lower than the capacity retention rate (over 80%) of the cells assembled in Examples 1 to 4, indicating that the presence of LiFePO4 on the lithium cobalt oxide surface significantly improves the cycle stability of the cathode material. Compared to Comparative Example 1, the cells assembled in Examples 1 to 4 showed a lower concentration of Co deposited at the anode after 200 cycles at 45°C, indicating reduced Co dissolution at the cathode and a more stable cathode interface, thus exhibiting better high-temperature storage performance and cycle stability. Therefore, the first material with the Pnma crystal structure on the surface of 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 of P63mc structure lithium cobalt oxide.

[0123] Compared to 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, and Co dissolution of the battery cells assembled from this cathode composite material are all superior to those of Comparative Example 2.

[0124] In Examples 1 to 4, with the increase of LiFePO4 concentration, in addition to a slight loss in specific capacity, the rate performance of the cathode composite material also gradually decreased. In particular, the cell assembled in Example 4, with a first material content of 4%, showed a decrease in capacity retention and 1.5C / 0.2C capacity ratio of 5.5 mAh / g and 7.2%, respectively, compared to the cell assembled in Example 1. This is because excessive first material on the surface of lithium cobalt oxide increases lithium-ion diffusion resistance and interfacial impedance, leading to increased battery polarization, thereby affecting discharge specific capacity and kinetics.

[0125] As shown in Examples 5 to 10, the LiFePO4 grain size has a relatively small impact on the discharge specific capacity and rate performance of the prepared batteries, but a significant impact on high-temperature storage performance, high-temperature cycle capacity retention, and Co dissolution. With the increase of LiFePO4 grain size D... 50With increasing density, high-temperature storage gas production increases, high-temperature cycling capacity retention gradually decreases, and the anode Co dissolution content gradually increases. Smaller LiFePO4 grain sizes in the cathode composite material result in tighter bonding with the lithium cobalt oxide surface, greater uniformity, and better stability. Conversely, larger LiFePO4 grain sizes lead to greater inhomogeneity, more exposed cathode composite surface, and more severe interfacial side reactions with the electrolyte, thus affecting the high-temperature storage performance, high-temperature cycling capacity retention, and Co dissolution. When the LiFePO4 grain size D... 50 When the wavelength is less than or equal to 300 nm, it is more conducive to improving the high-temperature storage performance, high-temperature cycling capacity retention, and Co dissolution of the cathode composite material, further optimizing the D content 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 different elemental doping. Compared with Comparative Examples 1 and 2, the cathode composite materials in Examples 11 to 17 showed significantly better performance in terms of high-temperature storage stability, high-temperature cycling capacity retention, and Co dissolution.

[0127] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Therefore, any equivalent variations made in accordance with this application are still within the scope of this application.

Claims

1. A positive electrode composite material, wherein, The invention comprises 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 element M, and the element M 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 as described in claim 1, wherein, The chemical formula of the first substance is LiFe. (1-z) N z PO4, where N 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.

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

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

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

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

15.

7. The positive electrode 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 as described in 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 and a negative electrode, wherein, The positive electrode sheet includes a current collector and a positive active layer disposed on the current collector, wherein the positive active layer includes a positive composite material as described in any one of claims 1 to 8.

10. An electronic device, wherein, Includes the electrochemical device as described in claim 9.

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