A positive electrode active material, a secondary battery, and an electric device
By coating the surface of the active core of lithium manganese iron phosphate with carbide and oxide layers, the problems of thermal runaway and electrolyte wettability in lithium manganese iron phosphate secondary batteries are solved, resulting in batteries with high heat resistance and long cycle life.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SUNWODA MOBILITY ENERGY TECHNOLOGY CO LTD
- Filing Date
- 2026-03-31
- Publication Date
- 2026-06-02
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Abstract
Description
Technical Field
[0001] This application relates to the field of battery technology, and in particular to a positive electrode material and its preparation method, a secondary battery, and an electrical device. Background Technology
[0002] In lithium-ion secondary batteries, lithium manganese iron phosphate (LFP) has attracted widespread attention in the industry due to its advantages of high voltage platform and high energy density. However, secondary batteries prepared with LFP as the positive electrode active material have several drawbacks: (1) Firstly, high heat generation makes the secondary battery prone to thermal runaway, leading to violent combustion reactions and a low safety factor; (2) Secondly, the LFP positive electrode has poor electrolyte wettability, severely affecting its electrochemical performance, especially causing a significant capacity drop after long-term cycling. Therefore, a secondary battery with both high heat resistance and long cycle performance is needed. Summary of the Invention
[0003] The purpose of this application is to improve the lithium manganese iron phosphate positive electrode active material to solve the problems of thermal runaway and cycle failure in secondary batteries using lithium manganese iron phosphate as the positive electrode active material, and to provide a positive electrode active material that allows secondary batteries prepared using this positive electrode active material to have both high heat resistance and long cycle performance.
[0004] A first aspect of this application provides a positive electrode active material, the positive electrode active material comprising: An active core, wherein the active core comprises lithium iron manganese phosphate; A first coating layer is disposed on the surface of the active core, and the first coating layer includes carbides; A second coating layer is disposed on the surface of the first coating layer, and the second coating layer includes an oxide.
[0005] In some embodiments of this application, the carbide includes at least one carbide having the following chemical structural formula: M y C x , 1≤x≤2, 1≤y≤5, M represents Fe, Ca, Cr, Ta, V, Zr, W, P or Si.
[0006] In some embodiments of this application, the carbide includes at least one of iron carbide, calcium carbide, zirconium carbide, and tungsten carbide.
[0007] In some embodiments of this application, the oxide comprises at least one oxide having the chemical structural formula shown below: DO z z≤9, D represents Zr, Ti, Al, or Si.
[0008] In some embodiments of this application, the oxide includes at least one of titanium oxide, aluminum oxide, and zirconium oxide.
[0009] In some embodiments of this application, the positive electrode active material satisfies: r:d1:d2=1:(0.1~1.0):(0.1~3.0); where r represents the radius of the active core in nm; d1 represents the thickness of the first coating layer in nm; and d2 represents the thickness of the second coating layer in nm.
[0010] In some embodiments of this application, the particle size Dv50 nm of the positive electrode active material satisfies: 20nm≤Dv50nm≤500nm, where Dv50 represents the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 50%.
[0011] In some embodiments of this application, the average particle size of the carbide 1 D nm satisfies: 8nm≤ 1 D nm≤15nm.
[0012] In some embodiments of this application, the average particle size of the oxide is... 2 D nm satisfies: 2nm≤ 2 D nm≤10nm.
[0013] In some embodiments of this application, the active core further includes a first lithium salt, which includes at least one of lithium iron phosphate, lithium vanadate, and lithium manganese phosphate.
[0014] A second aspect of this application provides a secondary battery, which includes a positive electrode, a separator, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes the positive active material described in the first aspect of this application.
[0015] A third aspect of this application provides an electrical device that includes the secondary battery described in the second aspect of this application.
[0016] Compared with the prior art, the beneficial effects of this application are: The positive electrode active material of this application comprises a first coating layer formed by coating an easily oxidizable carbide onto the surface of an active core containing lithium manganese iron phosphate, and a second coating layer formed by coating the surface of the first coating layer with an oxide containing thermal stability and capable of capturing oxygen. After the above-mentioned coating improvement, the oxygen generated by the thermal decomposition of lithium manganese iron phosphate can be absorbed by the two coating layers, which can effectively reduce the violent reaction between oxygen and the fully charged negative electrode active material, thereby avoiding the occurrence of high-temperature thermal runaway in the secondary battery. In addition, due to the polarity of the oxide particles in the second coating layer, it can wet the electrolyte more efficiently, greatly reducing the high-temperature wetting time or eliminating the high-temperature wetting process. The improved wettability can effectively reduce electrolyte loss and reduce the probability of capacity drop in the later stages of cycling of the secondary battery.
[0017] The secondary batteries prepared using the positive electrode active material of this application have the following characteristics: the number of cycles at 25℃@80%SOH is over 1900, the thermal diffusion time is over 190 min, and the rate performance is over 80%. Detailed Implementation
[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0019] In this application, the technical features described in an open-ended manner include both closed-ended technical solutions comprised of the listed features and open-ended technical solutions that include the listed features. In this application, numerical ranges are involved; unless otherwise specified, these ranges are considered continuous and include the minimum and maximum values of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to an integer, it includes every integer between the minimum and maximum values of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, these ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.
[0020] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.
[0021] In a first aspect, embodiments of this application provide a positive electrode active material, the positive electrode active material comprising: An active core, wherein the active core comprises lithium iron manganese phosphate; A first coating layer is disposed on the surface of the active core, and the first coating layer includes carbides; A second coating layer is disposed on the surface of the first coating layer, and the second coating layer includes an oxide.
[0022] The positive electrode active material of this application comprises a first coating layer formed by coating an easily oxidizable carbide onto the surface of an active core containing lithium manganese iron phosphate, and a second coating layer formed by coating the surface of the first coating layer with an oxide containing a material with good thermal stability and capable of capturing oxygen. When lithium manganese iron phosphate in the positive electrode active material decomposes upon heating, it produces oxygen. After the aforementioned coating improvement, the carbide in the first coating layer can react with the oxygen, consuming most of it. A small amount of oxygen that passes through the first coating layer to the second coating layer can be effectively adsorbed and captured by the oxide particles in the second coating layer. Therefore, the oxygen produced by the thermal decomposition of lithium manganese iron phosphate is absorbed by the two coating layers, effectively reducing the violent reaction between oxygen and the fully charged negative electrode active material, thereby fundamentally preventing the occurrence of high-temperature thermal runaway in the secondary battery. In addition, due to the polarity of the oxide particles in the second coating layer, they can wet the electrolyte more efficiently, greatly reducing the high-temperature wetting time or eliminating the high-temperature wetting process. The improved wettability can effectively reduce electrolyte loss and reduce the probability of capacity drop in the secondary battery in the later stages of cycling.
[0023] In some embodiments of this application, the carbide includes at least one carbide having the following chemical structural formula: M y C x , 1≤x≤2, 1≤y≤5, M represents Fe, Ca, Cr, Ta, V, Zr, W, P or Si.
[0024] In some embodiments of this application, the carbide includes at least one of iron carbide, calcium carbide, zirconium carbide, and tungsten carbide.
[0025] In some embodiments of this application, the oxide includes at least one oxide having the chemical structural formula shown below: DO z z≤9, D represents Zr, Al, Ti or Si.
[0026] In some embodiments of this application, the oxide includes at least one of titanium oxide, aluminum oxide, and zirconium oxide.
[0027] In some embodiments of this application, the positive electrode active material satisfies: r:d1:d2 = 1:(0.1~1.0):(0.1~3.0); where r represents the radius of the active core in nm; d1 represents the thickness of the first coating layer in nm; and d2 represents the thickness of the second coating layer in nm. A reasonable r:d1:d2 ratio can maximize the oxygen consumption and capture efficiency of the two coating layers and suppress high-temperature thermal runaway of the secondary battery. Specifically: Maintaining a suitable ratio between d1 and r ensures the integrity and uniformity of the carbide coating layer, with sufficient thickness to support its full reaction with oxygen without increasing the overall impedance of the positive electrode active material. When d2 is properly matched with r and d1, the second coating layer can tightly cover the surface of the first coating layer, forming a continuous and dense protective layer. This effectively traps the small amount of oxygen that penetrates the first coating layer and prevents the coating layer from becoming too thick, leading to increased brittleness, easy detachment, and loss of protective effect. A suitable d2 thickness also ensures the exposed area of oxide particles, further improving the electrolyte wettability of the positive electrode active layer and enhancing the cycle life of the secondary battery. Therefore, a reasonable ratio between the core radius and the thickness of the two coating layers can synergistically optimize heat resistance, electrolyte wettability, and structural stability, significantly improving the heat resistance and cycle performance of the secondary battery, extending battery life, and simultaneously achieving high energy density.
[0028] In some embodiments of this application, the thickness d1 of the first coating layer satisfies: 12nm≤d1nm≤50nm, specifically it can be any one of 12nm, 18nm, 20nm, 25nm, and 50nm or any range formed by any two values.
[0029] In some embodiments of this application, the thickness d2 of the second coating layer satisfies: 6nm≤d1nm≤150nm, specifically it can be any one of 6nm, 30nm, and 150nm or any range formed by two values.
[0030] In some embodiments of this application, the particle size Dv50 nm of the positive electrode active material satisfies: 20 nm ≤ Dv50 nm ≤ 500 nm, where Dv50 represents the particle size corresponding to a cumulative volume percentage of 50% for the positive electrode active material. When the Dv50 particle size of the positive electrode active material is within the aforementioned suitable range, a uniform porous structure can be formed between the particles, facilitating rapid electrolyte penetration and filling of the pores, shortening the ion diffusion path, and increasing the ion conduction rate. This, in turn, improves the rate performance of the battery, enabling it to maintain stable capacity output during high-rate charging and discharging. Simultaneously, the suitable particle size, combined with the flexibility of the carbides in the first coating layer and the stability of the oxides in the second coating layer, effectively alleviates stress damage to the coating layer, preventing coating layer detachment and cracking, and preventing electrolyte erosion of the active core leading to dissolution and loss of active materials, thereby delaying battery capacity decay and extending cycle life. A suitable particle size can also balance the packing density and specific surface area of the positive electrode active material, prevent heat accumulation caused by uneven dispersion or agglomeration, and reduce the internal resistance of the electrode while ensuring high energy density, thereby reducing polarization during charge and discharge and further improving the cycle stability and safety of the battery. Specifically, the Dv50 particle size of the positive electrode active material can be any one or any two values within a range of 20nm, 152nm, 160nm, 184nm, 200nm, 232nm, 260nm, 440nm, and 500nm.
[0031] In some embodiments of this application, the average particle size of the carbides in the first coating layer is 1 D nm, satisfying: 8nm≤ 1 D nm≤15nm. 1 The value of D can be any one of 8nm, 10nm, and 15nm, or any range formed by two values.
[0032] In some embodiments of this application, the average particle size of the oxides in the second coating layer is 2 D nm, satisfying: 2nm≤ 2 D nm≤10nm. 2 The value of D can be any one of 2nm, 5nm, 8nm, 250nm or any range formed by two values.
[0033] In some embodiments of this application, the positive electrode active material satisfies: 1 < 1 D / 2 For D≤5, the ratio can be any one of 1.1, 1.6, 3, or 5, or any interval formed by any two values. 1 < 1 D / 2D indicates that the particle size of the carbide in the first coating layer is larger than that of the oxide in the second active layer. In the double-layer coating structure of the positive electrode active material, the first coating layer located in the inner layer uses carbide with a larger particle size, and the second coating layer located in the outer layer uses oxide with a smaller particle size: (1) There are appropriate gaps between the larger carbide particles in the first coating layer, which provides sufficient reaction space and diffusion channels for the carbide and oxygen generated by the thermal decomposition of lithium manganese iron phosphate. (2) The smaller oxide particles in the second coating layer can form a dense "barrier film", which can not only physically block the small amount of oxygen that has not been completely consumed, but also efficiently capture the trace oxygen that penetrates the first layer through the strong adsorption characteristics of the oxide; the smaller oxide particles in the second coating layer also have a large specific surface area, which increases the contact sites with the electrolyte and improves the electrolyte wettability of the positive electrode active material.
[0034] In some embodiments of this application, the active core may further include a first lithium salt, which includes at least one of lithium iron phosphate (LFP), lithium vanadate phosphate (Li3V2(PO4)3), and lithium manganese phosphate (LiMnPO4). The mass ratio of lithium manganese iron phosphate to the first lithium salt within this suitable range can further improve thermal runaway resistance while ensuring good cycle performance.
[0035] In some embodiments of this application, the chemical formula of the lithium manganese iron phosphate is LiMn. γ Fe 1-γ PO4, 0 < γ < 1.
[0036] In some embodiments of this application, depending on performance requirements, the first coating layer and the second coating layer may also independently contain doping elements, the doping elements including non-metallic elements, the non-metallic elements including at least one of N, O, F, and H.
[0037] In some embodiments of this application, the mass percentage of the dopant element is 1 to 20% based on the total weight of the first or second coating layer.
[0038] In this application, the method of coating the positive electrode active material is not particularly limited. Any method that can form a first coating layer and a second coating layer on the surface of the active core can be used to prepare the positive electrode active material in this application. For example, the coating method can be calcination, in-situ coating, etc.
[0039] For example, the positive electrode active material is prepared by a method comprising the following steps: S1: The raw materials and carbides used for the active core are dispersed in water to form the first slurry. After ball milling and stirring for 4 to 10 hours, the slurry is dried and calcined in an air atmosphere at 950 to 1200°C for 3 to 6 hours to form the first coating layer on the surface of the active core. S2: The product of step S1, the ester compound containing element D, and the alcohol solvent (such as ethanol) are mixed to obtain a second slurry. Then, water is added dropwise to the second slurry under stirring to obtain a precursor dispersion. The ester compound containing element D is hydrolyzed in situ on the surface of the product of step S1 to form a second coating layer precursor. The product is centrifuged to remove unreacted compounds. The precipitate after centrifugation is calcined in air at 950~1200℃ for 3~6 hours to obtain the positive electrode active material.
[0040] In some embodiments of this application, the positive electrode active material has a wetting angle of <90° with respect to the electrolyte at 25±1℃. The electrolyte is a LiPF6 solution with a mass percentage of 12wt%, and the solvent of the electrolyte is a mixed solvent formed by ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC) in a mass ratio of 1:1:1. The smaller the wetting angle, the better the wettability.
[0041] In a second aspect of this application, in some embodiments of this application, a secondary battery is provided, the secondary battery including a positive electrode, a separator, a negative electrode, and an electrolyte, the positive electrode including a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector, the positive active layer including the positive active material described in the first aspect of this application.
[0042] In some embodiments of this application, the positive electrode active layer further includes a conductive agent and a binder. This application does not limit the types of conductive agents and binders, but the conductive agents include, but are not limited to, at least one of superconducting carbon (Super P), acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers. The binders include, but are not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).
[0043] In some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector, wherein the negative active layer includes a negative active material.
[0044] This application does not specifically limit the type of negative electrode active material. Any negative electrode active material commonly used in the art for preparing secondary batteries can be used in this application. For example, the negative electrode active material includes, but is not limited to, at least one of natural graphite, artificial graphite, soft carbon, hard carbon, nano-carbon, and silicon-carbon composites.
[0045] In some embodiments of this application, the negative electrode active layer further includes a conductive agent, a thickener, and a binder. This application does not limit the types of conductive agents, thickeners, and binders in the negative electrode active layer. The conductive agents include, but are not limited to, at least one of superconducting carbon (Super P), acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers. The thickeners include, but are not limited to, sodium carboxymethyl cellulose (CMC). The binders include, but are not limited to, polyacrylic acid (PAA).
[0046] In some embodiments of this application, there are no restrictions on the type of solvent used to form the positive electrode slurry and / or negative electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode material, the negative electrode active material, the conductive agent, the binder, or the dispersant.
[0047] Commonly used positive and negative current collectors in this field can be used to prepare the secondary battery described in this application. The negative current collector can be copper foil or carbon-coated copper foil. The positive current collector can be made of metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; carbon materials such as carbon cloth and carbon paper; or composite materials formed by polymers and metal layers. In some embodiments, aluminum foil is used as the positive current collector.
[0048] In some embodiments of this application, the type of diaphragm is not particularly limited and can be selected according to actual needs. For example, the diaphragm can be a polypropylene membrane, a polyethylene membrane, a polyvinylidene fluoride membrane, a spandex membrane, an aramid membrane, or a multilayer composite membrane modified with a coating.
[0049] In some embodiments of this application, the electrolyte includes lithium salts and organic solvents, and may also contain additives. The types and compositions of the lithium salts and organic solvents are not particularly limited and can be selected according to actual needs. The lithium salts may include lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, etc.; the solvents may include ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC), dimethyl carbonate, propyl propionate, etc.; and the additives may include lithium difluorophosphate, lithium bis(oxalato)borate, succinate, 1,3-propanesulfonyl lactone, and vinyl sulfate, etc.
[0050] In some embodiments of this application, the preparation of the secondary battery includes: stacking the positive electrode, separator, and negative electrode in sequence, so that the separator is positioned between the positive and negative electrode to act as a separator, then winding it into a square bare cell, installing it into a battery casing, then baking it at 65~95°C to remove water, injecting electrolyte, sealing it, and then undergoing processes such as standing, hot and cold pressing, formation, clamping, and capacity testing to obtain the secondary battery.
[0051] In some embodiments of this application, the secondary battery may include an outer packaging, which can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic, such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, or an aluminum-plastic film, such as an aluminum-plastic film formed by a composite of a PA layer, an aluminum layer, and a PP layer. The shape of the secondary battery is not particularly limited; it can be cylindrical, square, or any other arbitrary shape.
[0052] In a third aspect, some embodiments of this application also provide an electrical device, which includes the secondary battery described in the second aspect of this application. Non-limiting examples of the electrical device may include vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools. Vehicles may be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. This application does not impose special limitations on the above-mentioned devices.
[0053] The following are specific embodiments of this application, and the technical solutions of this application are further described in conjunction with the embodiments. However, this application is not limited to these embodiments. Unless otherwise specified, the reagents, methods, and equipment used in this application are all conventional reagents, methods, and equipment in this technical field.
[0054] Example 1 This embodiment provides a positive electrode active material, the preparation method of which includes the following steps: S1: The raw materials for the active core (see Table 1 for details), carbides (M) y C x(The types and particle sizes are shown in Table 1) are dispersed in water to form a first slurry (solid content is 6wt%). After stirring at 500 r / min for 6 h in a planetary ball mill, it is dried in an oven at 60℃ for 24 h, and then transferred to a tube furnace and calcined in an air atmosphere at 1000℃ for 3 h to form a first coating layer on the surface of the active core. S2: The product of step S1, an ester compound containing element D (in Example 1, element D is Si, and the ester compound containing element D is tetraethyl orthosilicate), and ethanol are mixed to obtain a second slurry. Then, water is added dropwise to the second slurry under stirring to obtain a precursor dispersion. The ester compound containing element D undergoes in-situ hydrolysis on the surface of the product of step S1 to form a second coating layer precursor. After centrifugation, unreacted ester compounds and solvents (ethanol and water) are removed. The precipitate after centrifugation is transferred to a tube furnace and calcined in air at 1000°C for 3 hours to synthesize an oxide (DO) on the surface of the first coating layer in situ. y The second coating layer (the types and particle sizes are shown in Table 1) is used to obtain the positive electrode active material. The particle size, coating layer thickness and other parameters of the positive electrode active material are detailed in Table 1. It should be noted that in Table 1: (1) The Dv50 particle size of the positive electrode active material of this application can be obtained by particle size analyzer; (2) The core, first coating layer and second coating layer of the positive electrode active material can be distinguished by transmission electron microscopy (TEM); Using TEM to perform a cross-diameter (elemental) line scan on a single positive electrode active particle, observe the signal of the coating element (M or D) from "0 → rise → plateau → fall". Through this method, the thickness d1 of the first coating layer, the thickness d2 of the second coating layer, and the radius r of the active core of the positive electrode active material can be obtained. Carbide M in the first coating layer y C x average particle size 1 D. Oxides (DO) in the second coating layer z average particle size 2 D can also be obtained through TEM testing. Specifically, first find the corresponding coating layer, then select the coating layer, use image analysis software to analyze the particle size of the particles in the selected coating layer area, and then calculate the average particle size of the particles in that area.
[0055] (3) Wetting angle (unit: °): Ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC) are mixed in a mass ratio of 1:1:1 to form a solvent. LiPF6 is then added to prepare an electrolyte with a lithium salt concentration of 12wt%. The positive electrode active material is pressed into a sheet to obtain a test sample. Then, the electrolyte is dropped onto the surface of the sheet to be tested, and the contact angle between the electrolyte and the test sample is measured using a contact angle meter.
[0056] Examples 2-22, Comparative Examples 1-5 A series of positive electrode active materials are provided and prepared according to the method of Example 1. By adjusting parameters such as the type and amount of raw materials and the calcination temperature, a series of positive electrode active materials as shown in Table 1 are obtained; specifically: Examples 2-7 involve doping the first or second coating layer with a dopant element. The dopant element can be added via calcination or in-situ doping, and the specific element can be conventionally selected based on its type. For example: Nitrogen doping is performed using a calcination method. After the desired coating layer is prepared (in Example 2, doping occurs in the first coating layer, immediately after step S1), the product to be doped is placed in a tube furnace, and a mixed gas of NH3 and N2 is introduced. The mixture is then reacted at 500°C for 2 hours. The amount of N doping can be controlled by adjusting the proportion of N in the mixed gas. The higher the proportion of NH3 in the mixed gas, the more N is doped. For example, in Example 2, the volume ratio of NH3 to N2 is 10:90. O doping (as in Example 6) also uses the calcination method: after the coating layer to be doped is prepared (in Example 6, the doping is done in the first coating layer, after step S1), the product to be doped is placed in a tube furnace, and a mixed gas formed by mixing O2 and N2 in a volume ratio of 30:70 is introduced, and the reaction is carried out at 300°C for 2 hours. The doping of element F is carried out by in-situ doping. Taking Example 7 as an example, in step S2, NaF, tetraethyl orthosilicate, and the product of step S1 are simultaneously added to ethanol to prepare the second coating layer. The doping amount of the dopant elements in Examples 2-7 can be obtained by XPS elemental analysis. Examples 8-9 replaced the type of carbide in the first coating layer; Examples 10-11 replaced the type of oxide in the second coating layer. In Example 10, the oxide in the second coating layer was TiO2, and the reaction raw material "ester compound containing element D" in step S2 was tetrabutyl titanate (TBOT). In Example 11, the oxide in the second coating layer was ZrO2, and the reaction raw material "ester compound containing element D" in step S2 was tetra-n-propylzirconate. Examples 12-13 involve changing the ball milling speed in step S1. In Example 12, the ball milling speed is 800 r / min, and in Example 13, it is 300 r / min. It should be noted that the ball milling speed affects the core size and the average particle size of the first coating layer. 1 D), thereby changing the particle size (Dv50) of the prepared positive electrode active material and the ratio r:d1:d2 of each layer in the positive electrode and active material; wherein, the faster the ball milling speed, the smaller the core size, which in turn leads to a smaller particle size Dv50 of the prepared positive electrode active material particles, and at the same time, the average particle size in the first coating layer ( 1 The smaller the value of D), the denser the first coating layer is, which leads to a corresponding decrease in the thickness d1 of the first coating layer under the same amount of carbide. Therefore, by changing the rotation speed of the ball mill in step S1, positive electrode active materials with different particle sizes and different r:d1:d2 ratios can be obtained. In Examples 14-15, in step S2, the average particle size of the oxides in the prepared second coating layer can be controlled by adjusting the stirring speed when water is added. 2 D. The faster the stirring speed, the smaller the average particle size of the oxides in the second coating layer; In Examples 16-17, by changing the calcination time in step S1, a first coating layer of different thicknesses can be obtained. The longer the calcination time, the greater the thickness of the first coating layer. In Examples 18-19, by changing the hydrolysis reaction time in step S2, a second coating layer of different thicknesses can be obtained; Examples 20-22 use different types of kernel materials; In Comparative Example 1, no coating layer was applied; In Comparative Example 2, Fe3C in step S1 was replaced with the organic carbon source glucose; In Comparative Example 3, the precursor dispersion in step S2 was replaced with the organic carbon source glucose. In Comparative Example 4, step S2 was not performed; In Comparative Example 5, step S1 was not performed; instead, the "product of step S1" mentioned in step S2 was replaced with LiMn. 0.5 Fe 0.5 PO4.
[0057] Table 1 Note: In Table 1, (1) the doping element A of the first coating layer is doped in C, and the mass percentage of the doping is based on the total mass of C and doping element A; the doping element B of the second coating layer is doped in O, and the mass percentage of the doping is based on the total mass of O and doping element B; (2) the radius of the active core is calculated according to the formula r=Dv50 / 2-d1-d2.
[0058] Application Examples 1-22, Comparative Examples 1-5 The cathode materials prepared in the above embodiments and comparative examples were used to prepare a series of secondary batteries, and the cycle performance of the secondary batteries was tested. The preparation method of the secondary batteries is as follows: Preparation of positive electrode sheet The positive electrode active material, conductive carbon black, and polyvinylidene fluoride (PVDF) binder prepared in the above examples and comparative examples were thoroughly mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 96:3:1 to obtain a positive electrode slurry. The prepared positive electrode slurry was coated onto a positive electrode current collector Al foil with a double-sided conductive carbon layer, wherein the Al foil thickness was 12 μm and the single-sided conductive carbon layer thickness was 1 μm. The conductive carbon layer can increase the electronic conductivity between the positive electrode active material and the current collector. After drying (drying under vacuum at 85°C), cold pressing, slitting, and welding of electrode tabs, a positive electrode sheet loaded with a positive electrode active layer was obtained, with a compaction density of 2.6 g / cm³. 3 ; Negative electrode preparation Graphite (negative electrode active material), Super P (conductive agent), and PVDF (binder) were mixed at a mass ratio of 95.7:1:3.3. Deionized water was added as solvent, and the mixture was stirred until homogeneous to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated onto a copper foil (6 μm thick) for the negative electrode current collector and dried in an oven (85℃ under vacuum). After rolling, slitting, and welding of tabs, a negative electrode sheet with a negative electrode active layer was obtained. The compacted density of the negative electrode sheet was 1.6 g / cm³. 3 ; diaphragm The diaphragm uses a 7μm thick PE base film; electrolyte At room temperature (25°C), in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), ethylene carbonate (EC), dimethyl carbonate (DMC), and propylene carbonate (PC) were mixed uniformly in a mass ratio of 1:1:1 to obtain a mixed solvent. Water was removed using a 4 Å molecular sieve to obtain the mixed solvent. LiPF6 was added to the obtained mixed solvent, and the mixture was continuously stirred and cooled with dry ice to ensure that the electrolyte temperature rise did not exceed 2°C. Finally, a colorless and transparent liquid was obtained, and an electrolyte with a lithium salt content of 12 wt% was obtained. Assemble secondary batteries The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. After winding, hot pressing and shaping, and electrode tab welding, a bare cell (a small 1P5S module) is obtained. The bare cell is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24 hours. The electrolyte prepared above is injected into the dried battery. After top-side sealing, electrolyte injection (injection coefficient 1.5g / Ah), formation, and sorting, a second sealing is performed (liquid retention coefficient 1.1g / Ah) to obtain a secondary battery.
[0059] Application Comparative Example 6 A secondary battery was prepared according to the preparation method of the secondary battery in the above application example, but it differs from the secondary battery in application example 1 in that: The preparation of the positive electrode differs; in the secondary battery used in this application's comparative example, LiMn is used as the positive electrode active material. 0.5 Fe 0.5 A positive electrode active layer was prepared by PO4, and then Fe3C slurry was coated on the surface of the positive electrode active layer to prepare a carbide layer with a thickness of 20 nm. Then, a slurry containing SiO2 was coated on the surface of the carbide layer to prepare an oxide layer with a thickness of 30 nm. Other steps were the same as in Application Example 1.
[0060] 1. Thermal diffusion performance test of secondary batteries: At a constant temperature of 25℃, the cells are first charged to 100% SOC at 1C. The cells are then assembled into a small module (cell) of 1P5S. The heating element is attached to the edge of the secondary battery, and the heating causes the secondary battery to thermal runaway. Five cells are grouped together, and the thermal diffusion time (min) of the adjacent cells from the edge cell is recorded. The thermal diffusion time is defined as the interval between the thermal runaway time of the edge cell and the thermal runaway time of the adjacent cell. 2. Cycle performance test of secondary batteries: Place the secondary battery in a charge / discharge test chamber at a constant temperature of 25°C, with a voltage range of 2.5~4.25V, a charge rate of 1C, a discharge rate of 1C, and perform cyclic testing. When the capacity drops to 80% of the initial capacity (80% SOH), stop the test and record the number of cycles.
[0061] 3. Ratio performance (%): At a constant temperature of 25℃, the capacitor is charged at a constant current of 0.33C to 100% SOC, then charged at a constant voltage of 0.05C (voltage range 2.5~4.25V), and discharged at a constant current of 0.33C to 2.5V. The discharge capacity is defined as Q1. Similarly, the capacitor is charged at a constant current of 0.33C to 100% SOC, then charged at a constant voltage of 0.05C (voltage range 2.5~4.25V), and discharged at a constant current of 3C to 2.5V. The discharge capacity is defined as Q2. Rate performance = Q2 / Q1*100%.
[0062] The test results are detailed in Table 2.
[0063] Table 2 The results above show that: The comparative results of Examples 1 and 2-7 show that doping a small amount of doping elements into the coating layer can reduce the contact angle between the positive electrode active material and the electrolyte, thereby improving the cycle performance of the secondary battery.
[0064] The comparison results between the examples and comparative examples show that the positive electrode active material of this application, by coating an easily oxidizable carbide onto the surface of the active core containing lithium manganese iron phosphate to form a first coating layer, and then coating the surface of the first coating layer with an oxide containing thermal stability and capable of capturing oxygen to form a second coating layer, improves the coating process. After the above-mentioned coating improvement, the oxygen generated by the thermal decomposition of lithium manganese iron phosphate can be absorbed by the two coating layers, effectively reducing the violent reaction between oxygen and the fully charged negative electrode active material, thereby fundamentally preventing the occurrence of high-temperature thermal runaway in the secondary battery. In addition, due to the polarity of the oxide particles in the second coating layer, it can wet the electrolyte more efficiently, greatly reducing the high-temperature wetting time or eliminating the high-temperature wetting process. The improved wettability can effectively reduce electrolyte loss and reduce the probability of capacity drop in the later stages of cycling.
[0065] This application utilizes a specific coating treatment of lithium manganese iron phosphate core as the positive electrode active material to prepare secondary batteries with the following characteristics: the number of cycles at 25℃@80%SOH is all above 1900 cycles, the thermal diffusion time is all above 190 min, and the rate performance is all above 80%.
[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.
Claims
1. A positive electrode active material, characterized in that, The positive electrode active material includes: An active core, wherein the active core comprises lithium iron manganese phosphate; A first coating layer is disposed on the surface of the active core, and the first coating layer includes carbides; A second coating layer is disposed on the surface of the first coating layer, and the second coating layer includes an oxide.
2. The positive electrode active material according to claim 1, characterized in that, The carbide includes at least one carbide having the following chemical structural formula: M y C x , 1≤x≤2, 1≤y≤5, M represents Fe, Ca, Cr, Ta, V, Zr, W, P or Si.
3. The positive electrode active material according to claim 1 or 2, characterized in that, The carbide includes at least one of iron carbide, calcium carbide, zirconium carbide, and tungsten carbide.
4. The positive electrode active material according to claim 1, characterized in that, The oxide includes at least one oxide having the following chemical structural formula: DO z z≤9, D represents Zr, Al, Ti or Si.
5. The positive electrode active material according to claim 1 or 4, characterized in that, The oxide includes at least one of titanium oxide, aluminum oxide, and zirconium oxide.
6. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material satisfies: r:d1:d2=1:(0.1~1.0):(0.1~3.0); Where r represents the radius of the active core in nm; d1 represents the thickness of the first coating layer in nm; and d2 represents the thickness of the second coating layer in nm.
7. The positive electrode active material according to claim 6, characterized in that, The positive electrode active material satisfies at least one of the following characteristics: (1) The particle size Dv50 nm of the positive electrode active material satisfies: 20nm≤Dv50 nm≤500nm, where Dv50 represents the particle size corresponding to the cumulative volume percentage of the positive electrode active material reaching 50%. (2) The average particle size of the carbide 1 D nm satisfies: 8nm≤ 1 D nm≤15nm; (3) The average particle size of the oxide 2 D nm satisfies: 2nm≤ 2 D nm≤10nm.
8. The positive electrode active material according to claim 1, characterized in that, The active core also includes a first lithium salt, which includes at least one of lithium iron phosphate, lithium vanadate, and lithium manganese phosphate.
9. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a separator, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes the positive active material according to any one of claims 1 to 8.
10. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in claim 9.