Electrochemical devices and electronic devices

Abstract

An electrochemical device and an electronic device, the positive electrode of the electrochemical device comprising a positive electrode current collector and a positive electrode active material layer located on one side or both sides of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode material, the positive electrode material comprising lithium manganate and lithium iron phosphate; the mass percentage content a of lithium iron phosphate based on the total mass of the positive electrode material satisfies: 5wt%≤a≤20wt%, the Dv50 of lithium iron phosphate is 0.8μm to 2.0μm, the Dn10 of lithium iron phosphate is 0.2μm to 0.5μm, and the specific surface area of lithium iron phosphate is not less than 5m 2 / g. By reasonably controlling the mass content, particle size and specific surface area of lithium iron phosphate, the problems of lithium iron phosphate floating and electrode drying cracking are solved while the safety of the electrochemical device is improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and more particularly to an electrochemical device and an electronic device. Background Technology

[0002] Electrochemical devices, such as lithium-ion batteries, have advantages such as high energy density, high power, and long cycle life, and are widely used in various fields. To reduce the cost of electrochemical devices, lithium manganese oxide is usually used as the positive electrode material. However, lithium manganese oxide may react with hydrofluoric acid in the electrolyte, causing manganese to dissolve and deposit on the negative electrode. This ultimately leads to the destruction of the solid electrolyte interphase (SEI) film on the positive electrode, resulting in lithium plating during charge and discharge, and creating safety hazards. Summary of the Invention

[0003] Some embodiments of this application propose an electrochemical device comprising: a positive electrode, the positive electrode including a positive electrode current collector and a positive electrode active material layer located on one or both sides of the positive electrode current collector, the positive electrode active material layer including a positive electrode material, the positive electrode material including lithium manganese oxide and lithium iron phosphate; based on the total mass of the positive electrode material, the mass percentage 'a' of lithium iron phosphate satisfies: 5wt% ≤ a ≤ 20wt%, the Dv50 of lithium iron phosphate is 0.8μm to 2.0μm, the Dn10 of lithium iron phosphate is 0.2μm to 0.5μm, and the specific surface area of ​​lithium iron phosphate is not less than 5m². 2 / g. This application improves safety by inhibiting manganese leaching through lithium iron phosphate, and solves the problems of lithium iron phosphate flotation and electrode drying cracking by rationally controlling the mass content, particle size and specific surface area of ​​lithium iron phosphate.

[0004] In some embodiments of this application, the mass percentage content of lithium iron phosphate satisfies 10wt% ≤ a ≤ 20wt%; in some embodiments, the Dv50 of lithium iron phosphate is 1.1μm to 1.8μm; in some embodiments, the Dn10 of lithium iron phosphate is 0.2μm to 0.4μm; and in some embodiments, the specific surface area of ​​lithium iron phosphate is greater than 8m². 2 / g.

[0005] In some embodiments of this application, the specific surface area of ​​lithium manganese oxide is 0.2 m². 2 / g to 0.7m 2 / g. In some embodiments, lithium iron phosphate adheres to the surface of lithium manganate. The specific surface area of lithium manganate affects the buoyancy and sinking force of lithium iron phosphate. When the specific surface area of lithium manganate is too small, lithium manganate and lithium iron phosphate are likely to sink in the slurry. When the specific surface area of lithium manganate is too large, lithium manganate and lithium iron phosphate are likely to float in the slurry. When the specific surface area of lithium manganate is within the above range, the buoyancy and sinking force reach equilibrium, avoiding the problems of uneven distribution caused by slurry deposition and slurry floating.

[0006] In some embodiments of the present application, the positive electrode material further includes: lithium nickel cobalt manganate. Based on the total mass of the positive electrode material, the mass percentage content b of lithium nickel cobalt manganate satisfies: 0 wt% < b ≤ 10 wt%. In some embodiments, lithium nickel cobalt manganate can form a coating on lithium manganate, reducing the reaction activity between the electrolyte and lithium manganate. At the same time, controlling the lithium nickel cobalt manganate within the above mass range can reduce costs while ensuring the effect.

[0007] In some embodiments of the present application, at least part of the positive electrode material has a core-shell structure, with lithium manganate located in the core of the core-shell structure, and lithium iron phosphate and lithium nickel cobalt manganate located in the shell layer of the core-shell structure, thereby preventing manganese dissolution. In some embodiments of the present application, at least part of the positive electrode material has a core-shell structure, with lithium manganate located in the core of the core-shell structure and lithium iron phosphate located in the shell layer of the core-shell structure. In some embodiments, lithium iron phosphate forms a coating on lithium manganate, reducing the reaction active sites between the electrolyte and lithium manganate, thereby preventing manganese dissolution.

[0008] In some embodiments of the present application, the positive electrode material contains at least one of B, Mg, Al, Si, S, Ti, Cr, Cu, Zn, Ga, Ge, Y, Zr, Mo, Ag, Ba, W, In, Sn, Pb or Sb, thereby improving conductivity.

[0009] In some embodiments of the present application, the positive electrode active material layer includes a first layer and a second layer. The first layer is located between the positive electrode current collector and the second layer. The difference in the mass percentage content of iron element between the first layer and the second layer is not greater than 0.5%, which indicates that lithium iron phosphate is evenly distributed in the positive electrode active material layer in the present application, without the problems of sinking and floating. In some embodiments of the present application, the difference in thickness between the first layer and the second layer is not greater than 20%. In some embodiments of the present application, the first layer includes lithium manganate and lithium iron phosphate, and the second layer includes lithium manganate and lithium iron phosphate; or the first layer includes lithium manganate, lithium iron phosphate or lithium nickel cobalt manganate, and the second layer includes lithium manganate, lithium iron phosphate and lithium nickel cobalt manganate.

[0010] In some embodiments of this application, the mass percentage of lithium manganese oxide in the second layer is d based on the total mass of the cathode material; the mass percentage of lithium manganese oxide in the first layer is e based on the total mass of the cathode material; wherein, d < e.

[0011] In some embodiments of this application, the positive electrode active material layer further includes a binder and a conductive agent. In some embodiments of this application, an electronic device is also proposed, comprising any of the above-mentioned electrochemical devices.

[0012] This application also proposes an electronic device comprising the electrochemical device of any of the claims in this application. The positive electrode of this application comprises a positive electrode current collector and a positive electrode active material layer located on one or both sides of the positive electrode current collector. The positive electrode active material layer comprises a positive electrode material, which comprises lithium manganese oxide and lithium iron phosphate. Based on the total mass of the positive electrode material, the mass percentage 'a' of lithium iron phosphate satisfies: 5wt% ≤ a ≤ 20wt%, the Dv50 of lithium iron phosphate is 0.8μm to 2.0μm, the Dn10 of lithium iron phosphate is 0.2μm to 0.5μm, and the specific surface area of ​​lithium iron phosphate is not less than 5m². 2 / g. This application improves the safety of electrochemical devices while solving the problems of lithium iron phosphate flotation and electrode drying cracking by rationally controlling the mass content, particle size and specific surface area of ​​lithium iron phosphate. Detailed Implementation

[0013] The following embodiments are intended to enable those skilled in the art to fully understand this application, but do not limit this application in any way.

[0014] In this application, "equal" or "identical" means that the difference between the two objects is within 30%, 20%, 10%, or 5%. The difference is calculated by subtracting the smaller object from the larger object, dividing the difference by the value of the smaller object, and then multiplying by 100%.

[0015] Electrochemical devices, such as lithium-ion batteries, are widely used in various fields. In order to reduce the cost of electrochemical devices, lithium manganese oxide is used as the positive electrode material. The spinel structure of lithium manganese oxide has the characteristics of low temperature resistance, good rate performance and easy preparation. However, lithium manganese oxide material itself is unstable. Lithium manganese oxide is prone to react with hydrofluoric acid in the electrolyte, which leads to the dissolution of manganese, which in turn leads to the destruction of the SEI film of the negative electrode, resulting in lithium plating and causing safety hazards. The high temperature performance and cycle performance of lithium manganese oxide are poor. These characteristics are all caused by the properties of manganese.

[0016] Some embodiments of this application propose an electrochemical device, which can be a lithium-ion battery. The electrochemical device includes a positive electrode, and may also include a negative electrode and a separator. The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on one or both sides of the positive electrode current collector. The positive electrode current collector can be copper foil or aluminum foil. The positive electrode active material layer includes a positive electrode material, which includes lithium manganese oxide and lithium iron phosphate. Based on the total mass of the positive electrode material, the mass percentage 'a' of lithium iron phosphate satisfies: 5wt% ≤ a ≤ 20wt%. The Dv50 of lithium iron phosphate is 0.8μm to 2.0μm, and the Dn10 is 0.2μm to 0.5μm. Dn10 is the particle size corresponding to a cumulative number distribution percentage of 10%, i.e., the particle size of 10% of the particles is not greater than this particle size. The specific surface area of ​​lithium iron phosphate is not less than 5m². 2 / g.

[0017] In this application, lithium iron phosphate and lithium manganese oxide are used simultaneously in the cathode material. Lithium iron phosphate can effectively absorb hydrofluoric acid and coat lithium manganese oxide, reducing the reactive activity of the electrolyte and lithium manganese oxide, thereby effectively inhibiting manganese dissolution. However, because lithium iron phosphate has a nano-sized particle size, it is prone to floating during slurry preparation and cracking of the cathode active material layer during cathode drying. To solve the problems of lithium iron phosphate floating and cracking, this application controls the mass percentage of lithium iron phosphate, Dv50, Dn10, and specific surface area. When the above characteristics of lithium iron phosphate meet the limits of this application, lithium iron phosphate floating can be suppressed and cracking can be prevented. When these limits cannot be fully met, lithium iron phosphate floating or cracking of the cathode active material layer will occur, leading to poor performance of the electrochemical device. The electrochemical device proposed in the embodiments of this application, using the above-mentioned cathode material, effectively solves the problems of electrode cracking and lithium iron phosphate floating while reducing the cost of the cathode material and preventing manganese dissolution.

[0018] In the embodiments of this application, the mass percentage of lithium iron phosphate in the cathode material is not less than 5 wt%, thereby ensuring sufficient lithium iron phosphate to suppress manganese dissolution. When the mass percentage of lithium iron phosphate in the cathode material does not exceed 20 wt%, the electrochemical device exhibits superior low-temperature performance. When the mass percentage of lithium iron phosphate exceeds 20%, the low-temperature performance of the electrochemical device will significantly deteriorate. In some embodiments of this application, the electrochemical device is charged from 0% SOC (state of charge) to 3.65V at a current of 2A at room temperature of 20°C to 25°C to obtain a charging capacity C1. After standing for 2 hours in an environment of -20°C, it is discharged at a rate of 0.5C to 2.5V to obtain a discharge capacity C2. The ratio of the discharge capacity C2 to C1 of the electrochemical device is greater than 75%, indicating that the electrochemical device in this application has good low-temperature performance.

[0019] In some embodiments of this application, the mass percentage content of lithium iron phosphate satisfies 10wt%≤a≤20wt%. In some embodiments, if the mass percentage content of lithium iron phosphate is too low, it may not be able to absorb hydrofluoric acid well. Therefore, the mass percentage content of lithium iron phosphate is controlled to be no less than 10% to ensure the protection of lithium manganese oxide.

[0020] The mass content of compounds (e.g., lithium iron phosphate, lithium manganese oxide, or lithium nickel cobalt manganese oxide) in the active material layer can be measured using X-ray photoelectron spectroscopy. It can also be tested using X-ray diffraction. The content of characteristic elements is tested to deduce the content of the active material. Specific testing principles and details are prior art and will not be elaborated upon here.

[0021] In some embodiments, the Dv50 of lithium iron phosphate is 1.1 μm to 1.8 μm, and in some embodiments, the Dn10 of lithium iron phosphate is 0.2 μm to 0.4 μm. In some embodiments, Dv50 and Dn10 characterize the particle size of lithium iron phosphate. If the particle size of lithium iron phosphate is too small, it will consume the electrolyte more quickly; if the particle size is too large, it will be detrimental to rate performance and will not protect lithium manganese oxide well. In some embodiments, the specific surface area of ​​lithium iron phosphate is greater than 8 m². 2 / g, In some embodiments, the specific surface area of ​​lithium iron phosphate affects both its contact area with the electrolyte and its contact area with lithium manganese oxide. Therefore, the specific surface area of ​​lithium iron phosphate cannot be too small; otherwise, it cannot effectively cover lithium manganese oxide and absorb hydrofluoric acid. In some embodiments, the specific surface area of ​​lithium iron phosphate is greater than 8m². 2 / g less than 17m 2 / g. In some embodiments, the specific surface area of ​​lithium iron phosphate is greater than 8m². 2 / g less than 15m 2 / g.

[0022] In some embodiments of this application, the specific surface area of ​​lithium manganese oxide is 0.2 m². 2 / g to 0.7m 2 / g. In some embodiments, lithium iron phosphate is attached to the surface of lithium manganese oxide. The specific surface area of ​​lithium manganese oxide affects the buoyancy and sinking force of lithium iron phosphate. When the specific surface area of ​​lithium manganese oxide is too small, lithium manganese oxide and lithium iron phosphate tend to sink in the slurry. When the specific surface area of ​​lithium manganese oxide is too large, lithium manganese oxide and lithium iron phosphate tend to float in the slurry. When the specific surface area of ​​lithium manganese oxide is within the above range, the buoyancy and sinking force are basically balanced, avoiding uneven distribution caused by slurry deposition and the problem of slurry floating.

[0023] In some embodiments of this application, the specific surface area of ​​the particles can be measured using a specific surface area meter. Specific testing details will not be elaborated further.

[0024] In some embodiments of the present application, the positive electrode material further includes: lithium nickel cobalt manganese oxide. Based on the total mass of the positive electrode material, the mass percentage content b of lithium nickel cobalt manganese oxide satisfies: 0 wt% < b ≤ 10 wt%. In some embodiments, lithium nickel cobalt manganese oxide can form a coating on lithium manganate, reducing the reaction activity between the electrolyte and lithium manganate. At the same time, controlling lithium nickel cobalt manganese oxide within the above mass range can reduce costs while ensuring the effect. In some embodiments of the present application, 1 wt% < b ≤ 10 wt%.

[0025] In some embodiments of the present application, the positive electrode material contains at least one of B, Mg, Al, Si, S, Ti, Cr, Cu, Zn, Ga, Ge, Y, Zr, Mo, Ag, Ba, W, In, Sn, Pb or Sb. In some embodiments, the impurity atoms in the positive electrode material form carriers, thereby improving the conductivity of the positive electrode material, which is beneficial to improving the rate performance and cycling performance of the electrochemical device.

[0026] In some embodiments, the positive electrode active material layer includes a first layer and a second layer. The first layer is located between the positive electrode current collector and the second layer, and the thicknesses of the first layer and the second layer can be the same or different. The types of active materials included in the first layer and the second layer can be the same or different.

[0027] In some embodiments, the types of active materials included in the first layer and the second layer are the same. For example, the first layer includes lithium manganate and lithium iron phosphate, and the second layer includes lithium manganate and lithium iron phosphate. Or the first layer includes lithium manganate, lithium iron phosphate or lithium nickel cobalt manganese oxide, and the second layer includes lithium manganate, lithium iron phosphate and lithium nickel cobalt manganese oxide.

[0028] In some embodiments of the present application, the first layer is located between the positive electrode current collector and the second layer, and the difference between the mass percentage content of iron element in the first layer and the mass percentage content of iron element in the second layer is not greater than 0.5%. In some embodiments, the thicknesses of the first layer and the second layer can be the same. For a target area on the positive electrode current collector, the difference between the mass percentage content of iron element in the first layer and the mass percentage content of iron element in the second layer in the target area is not greater than 0.5%. In some embodiments, for the positive electrode active material layer on either side of the positive electrode current collector, when it is divided into the first layer and the second layer along the thickness direction, for any target area, it is detected that the mass percentage content of iron element in the first layer in the target area and the mass percentage content of iron element in the second layer in the target area are almost the same, which indicates that lithium iron phosphate is uniformly distributed in the positive electrode active material layer and there is no problem of lithium iron phosphate floating up. In some embodiments, the difference degree of the thicknesses of the first layer and the second layer is not greater than 20%.

[0029] In some embodiments, the first and second layers contain different types of active materials. The mass percentage of lithium manganese oxide in the second layer is d, based on the total mass of the cathode material. The mass percentage of lithium manganese oxide in the first layer is e, based on the total mass of the cathode material. Wherein, d < e. This configuration results in the first layer, closer to the current collector side, containing more lithium manganese oxide than the second layer, and exhibiting a lower hydrofluoric acid concentration, which helps reduce manganese dissolution from the cathode active material layer.

[0030] In some embodiments of this application, at least a portion of the cathode material has a core-shell structure, with lithium manganese oxide located in the core and lithium iron phosphate located in the shell. In some embodiments, lithium iron phosphate coats lithium manganese oxide, reducing the reactive sites of the electrolyte and lithium manganese oxide, thereby preventing manganese leaching and reducing electrolyte erosion of lithium manganese oxide. In some embodiments of this application, at least a portion of the cathode material has a core-shell structure, with lithium manganese oxide located in the core and lithium iron phosphate and lithium nickel cobalt manganese oxide located in the shell, thereby further preventing manganese leaching.

[0031] In some embodiments of this application, the positive electrode active material layer further includes a binder and a conductive agent. The binder may be polyvinylidene fluoride, and the conductive agent may be one or more of conductive carbon black, carbon nanotubes, and graphite.

[0032] In some embodiments, the electrochemical device comprises a separator between the positive and negative electrodes. In some embodiments, the separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene comprises at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Polyethylene and polypropylene, in particular, are effective in preventing short circuits and can improve battery stability through a turn-off effect. In some embodiments, the thickness of the separator is in the range of about 5 μm to 50 μm.

[0033] In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the separator. The porous layer comprises inorganic particles and a binder. The inorganic particles are selected from at least one of alumina (Al₂O₃), silicon oxide (SiO₂), magnesium oxide (MgO), titanium oxide (TiO₂), hafnium dioxide (HfO₂), tin oxide (SnO₂), cerium dioxide (CeO₂), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the separator have a diameter in the range of about 0.01 μm to 1 μm. The binder for the porous layer is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer on the surface of the separator can improve the separator's heat resistance, oxidation resistance, and electrolyte wetting properties, and enhance the adhesion between the separator and the electrode.

[0034] In some embodiments of this application, the electrochemical device may be of a wound or stacked type. In some embodiments, the positive and / or negative electrodes of the electrochemical device may be multilayer structures formed by winding or stacking, or they may be single-layer structures consisting of a single-layer positive electrode, a separator, and a single-layer negative electrode.

[0035] In some embodiments, the electrochemical device includes a lithium-ion battery, but this application is not limited thereto. In some embodiments, the electrochemical device may also include an electrolyte. The electrolyte may be one or more of a gel electrolyte, a solid electrolyte, and an electrolyte solution, wherein the electrolyte solution includes a lithium salt and a non-aqueous solvent. The lithium salt is selected from one or more of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB, or lithium difluoroborate. For example, LiPF6 is selected as the lithium salt because it has high ionic conductivity and can improve cycle characteristics.

[0036] The non-aqueous solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof. The carbonate compound may be a chain carbonate compound, a cyclic carbonate compound, a fluorocarbonate compound, or a combination thereof.

[0037] Examples of chain carbonate compounds are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof. Examples of cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), or combinations thereof. Examples of fluorinated carbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0038] Examples of carboxylic acid ester compounds are methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerate lactone, mevalonic acid lactone, caprolactone, methyl formate, or combinations thereof.

[0039] Examples of ether compounds are dibutyl ether, tetraethylene dimethyl ether, diethylene dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or combinations thereof.

[0040] Examples of other organic solvents include dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters or combinations thereof.

[0041] In some embodiments of this application, taking a lithium-ion battery as an example, the positive electrode, separator, and negative electrode are sequentially wound or stacked into electrode components, then encapsulated in, for example, an aluminum-plastic film, and then injected with electrolyte. Formation and encapsulation are then performed to manufacture a lithium-ion battery. The prepared lithium-ion battery is then subjected to performance testing.

[0042] Those skilled in the art will understand that the methods for preparing the electrochemical devices (e.g., lithium-ion batteries) described above are merely examples. Other methods commonly used in the art can be employed without departing from the disclosure of this application.

[0043] This application discloses an electronic device, including an electrochemical device; the electrochemical device is any of the electrochemical devices described in this application. The electronic device in the embodiments of this application is not particularly limited, and can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is 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, drones, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, or large household batteries, etc.

[0044] The following are some specific embodiments and comparative examples to better illustrate this application, wherein a lithium-ion battery is used as an example.

[0045] Preparation of the positive electrode: The positive electrode material, conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were dissolved in an N-methylpyrrolidone solution at a weight ratio of 96.2:0.8:0.6:2.4 to form a positive electrode slurry. Aluminum foil (16 μm) was used as the positive electrode current collector, and the positive electrode slurry was coated onto the current collector (coating weight 0.23 kg / m²). 2 After drying, cold pressing (electrode thickness 0.17mm), and cutting, the positive electrode is obtained.

[0046] Preparation of negative electrode: Graphite, styrene acrylate and lithium carboxymethyl cellulose are mixed in a mass ratio of 98:1:1 with deionized water as solvent to form a slurry of negative electrode active material layer. Copper foil is used as negative electrode current collector. The slurry of negative electrode active material layer is coated on the negative electrode current collector and dried at 90°C to obtain negative electrode sheet.

[0047] Preparation of the separator: The separator is made of 8μm thick polyethylene (PE).

[0048] Preparation of electrolyte: Under an environment with a water content of less than 10 ppm, lithium hexafluorophosphate and a non-aqueous organic solvent (ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC): propyl propionate (PP): ethylene carbonate (VC) = 20:30:20:28:2, by weight) were mixed at a weight ratio of 8:92 to form an electrolyte.

[0049] Lithium-ion battery fabrication: The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to form an electrode assembly. This assembly is placed in an outer aluminum-plastic film package, and after dehydration at 80°C, the electrolyte is injected and the battery is sealed. Following formation, degassing, and edge trimming processes, the lithium-ion battery is obtained.

[0050] The testing methods for this application are described below.

[0051] 1. Dn10 test

[0052] Scanning electron microscopy (SEM) is used to scan the cross-section of the electrode along its thickness direction to obtain SEM images. Specifically, sampling involves disassembling the electrochemical device under test, removing the electrode, and immersing it in dimethyl carbonate (DMC) solution for 6 hours to remove residual electrolyte. The electrode is then dried in a drying oven. Sample preparation involves cutting the tested cross-section (the section along the thickness direction of the active material layer) from the dried electrode with a blade. The test sample is then adhered to paraffin wax using a heating plate and polished with an ion polisher until the surface is smooth, yielding the SEM test sample. Testing involves obtaining SEM images of the active material layer of the electrochemical device under test using SEM. From the obtained SEM images, the average particle size of the active material is determined. Specifically, for a single particle, the length of its longest diagonal and shortest diagonal are determined in the SEM image containing the particle. The particle size is then calculated as the arithmetic mean of the lengths of the longest and shortest diagonals. In the SEM image, randomly select n (e.g., 100) active material particles, and determine the particle size of each active material particle to obtain n particle sizes. Sort the n particles by their particle sizes from smallest to largest, and define the particle size of the 10th percentile particle in ascending order as Dn10.

[0053] 2. Dv50 Test

[0054] The cumulative particle size distribution curve of the negative electrode active material was tested using a MasterSizer 2000. Dv50 represents the cumulative diameter of the 50% of the volumetric baseline distribution obtained by laser scattering particle size analyzer.

[0055] 3. Electrode crack detection: Take any positive electrode from the electrochemical device. If the crack in the positive electrode active material layer is no larger than 0.5 mm, it is considered that there is no crack; otherwise, it is considered that there is a crack.

[0056] 4. Detection of uniform distribution of lithium iron phosphate (uniform distribution of Fe ions):

[0057] The positive electrode active material layer on one side of the positive electrode current collector is uniformly divided into an upper layer (second layer) and a lower layer (first layer) in the thickness direction. The difference in Fe mass content between the two layers is obtained by scanning with an energy dispersive spectrometer and randomly selecting a region on the positive electrode current collector.

[0058] 5. Low-temperature discharge performance test at -20℃:

[0059] An electrochemical device is charged from 0% SOC (state of charge) to 3.65V with a current of 2A at room temperature of 20℃ to 25℃ to obtain a charging capacity C1. After standing for 2 hours in an environment of -20℃, it is discharged to 2.5V at a rate of 0.5C to obtain a discharge capacity C2. Calculate the ratio of the discharge capacity C2 to C1 of the electrochemical device.

[0060] The differences between the various embodiments and comparative examples lie in the composition and material properties of the cathode material. Specific differences and performance tests are shown in the table below.

[0061] Table 1

[0062]

[0063] The cathode materials used in the examples and comparative examples in Table 1 are mixtures of lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese oxide. Table 1 shows the mass percentage of lithium iron phosphate and lithium nickel cobalt manganese oxide in the cathode materials, with the remainder being lithium manganese oxide.

[0064] Please refer to Example 1 and Comparative Examples 1 to 3. In Example 1, no electrode cracking occurred, and there was almost no difference in Fe content between the upper and lower layers, indicating that lithium iron phosphate is uniformly distributed in the positive electrode active material layer and will not cause cracking of the positive electrode active material layer, and it has good low-temperature discharge performance.

[0065] In Comparative Example 1, the mass percentage of lithium iron phosphate was too low, which would lead to manganese leaching (not recorded in the table, but manganese leaching occurred in Comparative Example 1 after testing), posing a safety hazard. In Example 1, however, no manganese leaching occurred, indicating high safety performance.

[0066] In Comparative Examples 2 and 3, the mass percentage of lithium iron phosphate was too high. As can be seen, this led to a decline in low-temperature cycling performance. The higher the content, the lower the low-temperature cycling performance. Furthermore, the positive electrode active material layer began to show problems such as uneven Fe distribution and cracking. This may be because the mass percentage of lithium iron phosphate was too high. At this time, there was too much lithium iron phosphate, which could not be fully adsorbed on the surface of lithium manganese oxide. The excess lithium iron phosphate floated to the surface, resulting in a large difference in Fe content between the upper and lower layers, causing the electrode to crack. In addition, the low-temperature performance of the electrochemical device was affected when there was too much lithium iron phosphate.

[0067] Therefore, the mass percentage 'a' of lithium iron phosphate needs to be controlled within 5wt% ≤ a ≤ 20wt%.

[0068] Please refer to Examples 2 to 3 and Comparative Examples 4 to 7. In Examples 2 to 3, the Dv50 is 0.8 μm and 2.0 μm, respectively. No cracks larger than 0.3 mm appear on the electrode, and the difference in Fe content between the upper and lower layers is small. At the same time, it has good low-temperature discharge performance.

[0069] In Comparative Examples 4, 6, and 7, the Dv50 of the lithium iron phosphate was too small, leading to electrode cracking and uneven Fe element distribution. In Comparative Example 5, the Dv50 of the lithium iron phosphate was too large, resulting in a significant decrease in low-temperature discharge performance.

[0070] Therefore, the Dv50 of lithium iron phosphate needs to be controlled between 0.8μm and 2.0μm.

[0071] Please refer to Examples 4 and 5 and Comparative Example 8. In Examples 4 and 5, the Dn10 of lithium iron phosphate is 0.2 μm and 0.5 μm, respectively. No cracks appear on its electrode, and the difference in Fe content between the upper and lower layers is small. It also has good low-temperature discharge performance.

[0072] In Comparative Example 8, the Dn10 of lithium iron phosphate was too small, which caused the electrode to crack and the Fe element to be unevenly distributed. This may be because the lithium iron phosphate particles are small and easily float in the positive electrode slurry, resulting in uneven distribution of lithium iron phosphate in the positive electrode active material layer, which in turn causes cracks to appear on the positive electrode active material layer.

[0073] Therefore, the Dn10 of lithium iron phosphate needs to be controlled between 0.2μm and 0.5μm.

[0074] Please refer to Example 6 and Comparative Examples 9 and 10. In Example 6, the specific surface area of ​​lithium iron phosphate is greater than 5 m². 2 / g, while the specific surface area of ​​lithium iron phosphate in comparative examples 9 and 10 is less than 5m². 2 / g, the low-temperature discharge performance of lithium iron phosphate in Comparative Examples 9 and 10 was significantly reduced. This may be because lithium iron phosphate may not have coated lithium manganese oxide well, resulting in manganese dissolution and reduced low-temperature performance.

[0075] Therefore, the specific surface area of ​​lithium iron phosphate needs to be greater than 5m². 2 / g.

[0076] In Examples 4, 7 to 12, all samples met the requirements of 5wt% ≤ a ≤ 20wt% as defined in this application, with a Dv50 of 0.8 μm to 2.0 μm, a Dn10 of 0.2 μm to 0.5 μm, and a specific surface area of ​​not less than 5 m². 2 / g, while in Examples 4, 7 to 12 there were no cracks, and the difference in Fe content between the upper and lower layers in each example was no more than 0.3%, and the low-temperature discharge performance at -20°C was good. This indicates that within the scope defined in this application, lithium iron phosphate did not float up, resulting in differences in Fe element distribution, and the electrode did not crack, while ensuring low-temperature performance.

[0077] In Examples 4, 7 to 12, it can also be seen that the difference in Fe content between the upper and lower layers is the greatest in Example 4. This may be because the lithium iron phosphate particles in Example 4 have a smaller particle size and a higher mass percentage content, as well as a larger specific surface area. In this case, small-sized lithium iron phosphate particles are more likely to float. It can be seen that the mass percentage content of lithium iron phosphate, along with the particle size and specific surface area, jointly affect the performance of the cathode. Only when the characteristic range defined in this application is met simultaneously can the overall performance be better.

[0078] In Example 4, the mass percentage of lithium nickel cobalt manganese oxide was 0. It can be seen that when the cathode material contains only lithium iron phosphate and lithium manganese oxide, as long as the requirements of 5wt% ≤ a ≤ 20wt% as defined in this application are met, the Dv50 of lithium iron phosphate is 0.8 μm to 2.0 μm, the Dn10 of lithium iron phosphate is 0.2 μm to 0.5 μm, and the specific surface area of ​​lithium iron phosphate is not less than 5 m². 2 / g can ensure electrode quality, uniformity of Fe element distribution, and low-temperature performance.

[0079] In Example 12, the lithium nickel cobalt manganese oxide (LiCO) content was 5% by mass, and the lithium-ion battery achieved a capacity retention of 90% during low-temperature discharge, demonstrating excellent low-temperature discharge performance. This is because its 10wt%≤a≤20wt% content, the Dv50 of lithium iron phosphate (LFP) being 1.1μm to 1.8μm, and the Dn10 of LFP being 0.2μm to 0.4μm, all contribute to this excellent performance. Furthermore, LFP has a specific surface area greater than 8m². 2 At this point, the parameters of lithium iron phosphate are optimal, resulting in the best low-temperature performance and a more uniform distribution of Fe elements.

[0080] The above description is merely a selection of preferred embodiments of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in the embodiments of this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in the embodiments of this disclosure.

Claims

1. An electrochemical device, characterized in that, include: The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on one or both sides of the positive electrode current collector. The positive electrode active material layer includes a positive electrode material, which includes lithium manganese oxide and lithium iron phosphate. Based on the total mass of the cathode material, the mass percentage 'a' of lithium iron phosphate satisfies: 5wt% ≤ a ≤ 20wt%, the Dv50 of the lithium iron phosphate is 0.8μm to 2.0μm, the Dn10 of the lithium iron phosphate is 0.2μm to 0.5μm, and the specific surface area of ​​the lithium iron phosphate is not less than 5m². 2 / g.

2. The electrochemical device according to claim 1, characterized in that, It satisfies at least one of the following: (a) 10wt% ≤ a ≤ 20wt%; (b) The Dv50 of the lithium iron phosphate is from 1.1 μm to 1.8 μm; (c) The Dn10 of the lithium iron phosphate is 0.2 μm to 0.4 μm; (d) The specific surface area of ​​the lithium iron phosphate is greater than 8 m². 2 / g.

3. The electrochemical device according to claim 1, characterized in that, The specific surface area of ​​the lithium manganese oxide is 0.2 m². 2 / g to 0.7m 2 / g.

4. The electrochemical device according to claim 1, characterized in that, The cathode material also includes: lithium nickel cobalt manganese oxide; Based on the total mass of the cathode material, the mass percentage b of the lithium nickel cobalt manganese oxide satisfies: 0 wt%. <b≤10wt%。 5. The electrochemical device according to claim 4, characterized in that, At least a portion of the cathode material has a core-shell structure, with lithium manganese oxide located in the core of the core-shell structure and lithium iron phosphate and lithium nickel cobalt manganese oxide located in the shell layer of the core-shell structure.

6. The electrochemical device according to claim 1, characterized in that, At least a portion of the cathode material has a core-shell structure, with lithium manganese oxide located in the core of the core-shell structure and lithium iron phosphate located in the shell layer of the core-shell structure.

7. The electrochemical device according to claim 1, characterized in that, The cathode material contains at least one of B, Mg, Al, Si, S, Ti, Cr, Cu, Zn, Ga, Ge, Y, Zr, Mo, Ag, Ba, W, In, Sn, Pb, or Sb.

8. The electrochemical device according to claim 1, characterized in that, The positive electrode active material layer includes a first layer and a second layer. The first layer is located between the positive electrode current collector and the second layer. The difference between the mass percentage of iron in the first layer and the mass percentage of iron in the second layer is not greater than 0.5%.

9. The electrochemical device according to claim 8, characterized in that, satisfy: The first layer comprises lithium manganese oxide and lithium iron phosphate, and the second layer comprises lithium manganese oxide and lithium iron phosphate; or The first layer includes lithium manganese oxide, lithium iron phosphate, or lithium nickel cobalt manganese oxide, and the second layer includes lithium manganese oxide, lithium iron phosphate, and lithium nickel cobalt manganese oxide.

10. The electrochemical device according to claim 8, characterized in that, Based on the total mass of the cathode material, the mass percentage of lithium manganese oxide in the second layer is d; Based on the total mass of the cathode material, the mass percentage of lithium manganese oxide in the first layer is e; where d < e.

11. An electronic device comprising an electrochemical device according to any one of claims 1 to 10.

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