Positive electrode active material, secondary battery, and electric device

Abstract

Embodiments of the present application provide a positive electrode active material, a secondary battery, and an electric device. The positive electrode active material includes composite particles and a carbon-containing high molecular polymer material coated on the outside of the composite particles. The composite particles include lithium-iron-manganese phosphate particles and metal oxide particles at least partially coated on the outside of the lithium-iron-manganese phosphate particles. The carbon-containing high molecular polymer material includes a conjugated high molecule derived from a substituted or unsubstituted aliphatic alkyne. A heteroatom is doped in the conjugated high molecule, and the heteroatom includes a Group IA element or a Group VIIA element. Thus, the capacity of the secondary battery can be improved, and the thermal stability can be improved.

Description

Technical Field

[0001] The exemplary embodiments of this application generally relate to the field of secondary batteries, and particularly to positive electrode active materials, secondary batteries, and electrical devices. Background Technology

[0002] Lithium manganese iron phosphate (LFP) lithium-ion batteries are a derivative of lithium iron phosphate (LFP) lithium-ion batteries. Due to their relatively high voltage platform, the energy density of LFP batteries falls between that of LFP and ternary lithium-ion batteries, limiting their market application. Therefore, further improving the energy density of LFP batteries can enhance their technological advantages and expand their application scenarios.

[0003] Currently, one way to improve energy density is by blending lithium manganese iron phosphate with ternary lithium batteries. However, this results in an aging-related shift in the State of Charge (SOC) - Open Circuit Voltage (OCV) curve at the battery pack level, significantly limiting its application. Furthermore, lithium-enriched lithium manganese iron phosphate systems suffer from reduced thermal stability and earlier thermal runaway temperatures because lithium fails to occupy a stable crystal lattice. Summary of the Invention

[0004] In a first aspect of this application, a positive electrode active material is provided. The positive electrode active material comprises composite particles and a carbon-containing polymer material coating the outer surface of the composite particles. The composite particles include lithium manganese iron phosphate particles and metal oxide particles at least partially coating the outer surface of the lithium manganese iron phosphate particles. The carbon-containing polymer material comprises a conjugated polymer derived from substituted or unsubstituted aliphatic alkynes, and the conjugated polymer is doped with heteroatoms, including Group IA elements or Group VIIA elements.

[0005] The positive electrode active material of this application is obtained by coating high-temperature resistant metal oxide particles onto the surface of lithium manganese iron phosphate particles to obtain composite particles, and then coating the outside of the composite particles with a carbon-containing polymer material, thereby obtaining a positive electrode active material with a double-layer coating structure. This double-layer coating structure can protect the core lithium manganese iron phosphate particles, so that the positive electrode active material can have both high energy density and good thermal stability.

[0006] Specifically, metal oxides can at least partially coat the surface of lithium manganese iron phosphate (LFP) particles, thereby establishing a thermal barrier. This not only blocks heat transfer to the LFP particles but also physically prevents direct contact between the active material (i.e., LFP) and the electrolyte at high temperatures, significantly suppressing catalytic side reactions on the cathode surface (such as electrolyte oxidative decomposition) and thus improving the material's thermal stability. Furthermore, the metal oxide particles can reduce side reactions between the LFP surface and the electrolyte, inhibit the dissolution of transition metals (Mn / Fe), and extend the cycle life of the full battery.

[0007] Furthermore, metal oxide particles can form a rigid shell on the surface of lithium manganese iron phosphate (LFP) particles. This rigid shell provides appropriate constraint on the LFP. During charging and discharging, if the LFP core undergoes lattice expansion / contraction due to lithium-ion intercalation / deintercalation, this rigid shell can mechanically constrain the volume change of the core, limiting excessive anisotropic expansion and making the volume change more uniform and controllable. In this way, the integrity of LFP particles can be improved, reducing electrical contact failures and exposure of new side reaction interfaces caused by breakage and pulverization of active materials, thereby extending the battery's cycle life.

[0008] Carbon-containing polymer materials coat the outer surface of the composite particles, forming a continuous conductive network. This accelerates electron conduction between composite particles and from the surface to the core, thereby improving the electrical performance of the positive electrode active material. Furthermore, the polymeric protective layer constructed from carbon-containing polymer materials provides the positive electrode active material with appropriate elasticity, thus buffering volume changes during cycling and maintaining the integrity of the electrode structure. Simultaneously, the polymeric protective layer partially blocks the electrolyte from corroding the internal materials, synergistically forming a more stable protective interface with the inner metal oxide layer to ensure the stable operation of the lithium manganese iron phosphate particles.

[0009] It should be noted that simple aliphatic alkyne polymer particles, such as polyacetylene and polypropyne, have poor electrical conductivity. Therefore, heteroatom doping is necessary to improve the conductivity of these polymer particles. This is achieved by injecting charges into the conjugated backbone of the polymer particles, for example, through p-type doping with halogen elements (i.e., Group VIIA elements) or n-type doping with alkali metal elements (i.e., Group IA elements), thereby generating charge carriers on the molecular chain and altering the conductivity of the polymer particles. The heteroatom-modified carbon-containing polymer material is then coated onto the outside of the composite particles, constructing a three-dimensional conductive network for the positive electrode active material. This improves the ion exchange efficiency between the positive electrode active material and the electrolyte, contributing to increased energy density.

[0010] In some embodiments, the heteroatom includes at least one selected from lithium, sodium, potassium, fluorine, chlorine, bromine, and iodine.

[0011] Group IA elements such as lithium, sodium, and potassium act as electron donors, doping them into conjugated polymer chains to inject electrons into them. This significantly increases the carrier concentration in the polymer and constructs a highly efficient electron transport layer on the particle surface, thereby improving the lithium-ion insertion / extraction and exchange kinetics between the lithium manganese iron phosphate material at the core and the electrolyte.

[0012] In some embodiments, heteroatoms can be introduced into aliphatic alkyne polymers by exposing the polymer to alkali metal vapor under the protection of an inert gas (such as helium), allowing alkali metal atoms to diffuse and dope into the polymer backbone. For example, elemental sodium is heated to 200°C, and the resulting sodium vapor is mixed with helium, the partial pressure of sodium vapor in the mixed atmosphere being 1%–2%. Polyacetylene is exposed to a continuous, directional flow of the mixed atmosphere at a flow rate of 0.1 L / min. The doping amount of sodium atoms can be controlled by controlling the exposure time of the polyacetylene. The doping method for lithium or potassium atoms is similar, only requiring adjustment of the heating temperature of the alkali metal element according to the properties of the elemental lithium or potassium; for example, the elemental lithium can be heated to 400°C, and the elemental potassium to 180°C, etc.

[0013] Group VIIA elements such as fluorine, chlorine, bromine, and iodine are highly electronegative and act as electron-withdrawing agents, capable of stealing electrons from conjugated structures, thereby enabling the formation of quasi-particle charge carriers such as hole-type polarons and bipolarons in polymer chains. This increases the charge carrier concentration of the polymer, thereby improving the intrinsic conductivity of the conjugated polymer. Furthermore, halogen modification of polymer chains can introduce strongly polar bonds such as CF and C-Cl, enhancing the electrochemical stability of the polymer layer and effectively suppressing the oxidative decomposition of the electrolyte under high voltage.

[0014] In some embodiments, halogen atoms can be doped into aliphatic alkyne polymers using either gas-phase doping or liquid-phase doping methods, depending on the specific halogen atom. Taking polyacetylene as an example, for highly electronegative halogen atoms (e.g., F or Cl), polyacetylene can be placed in a hydrogen halide atmosphere (e.g., HCl or HF) to allow the hydrogen halide gas to react with the aliphatic alkyne polymer. The reaction temperature is maintained at approximately 260°C, allowing the hydrogen halide to interact with the polymer chains, thereby doping chlorine or fluorine atoms into the polymer backbone. For less electronegative halogen atoms (e.g., Br or I), polyacetylene can be immersed in a carbon tetrachloride solution containing a halogen element (such as iodine or bromine). The concentration of the halogen in the carbon tetrachloride solution is typically controlled at (0.2~0.8) g / 100 mL. The doping amount of halogen atoms can be adjusted by precisely controlling the immersion time of the polymer. Alternatively, the doping amount of halogen atoms can also be controlled by adjusting the solution concentration.

[0015] In some embodiments, the heteroatom doping amount is 0.05wt% to 2wt%, based on the total mass of the conjugated polymer.

[0016] By controlling the doping amount of heteroatoms in conjugated polymers within the range of 0.05wt% to 2wt%, the concentration of charge carriers in the polymer can be increased while ensuring the stability of the conjugated structure of the polymer molecular chain, thereby improving the conductivity of the conjugated polymer.

[0017] In some embodiments, the doping amount of heteroatoms may be one of 0.05wt%, 0.1wt%, 0.2wt%, 0.4wt%, 0.8wt%, 1wt%, 1.5wt%, or 2wt%, or a range of any two.

[0018] In some embodiments, the conjugated polymer includes at least one of polyacetylene, polypropyne, and polybutyne.

[0019] These chain-like aliphatic alkyne polymers form a linear conjugated structure with continuous sp² hybridized carbon chains, which lays the foundation for conductive structures with high carrier concentrations obtained after heteroatom doping. Taking polyacetylene as an example, the electron-donating or electron-withdrawing properties of heteroatoms can inject or extract electrons into the conjugated system, thereby generating electronic or hole carriers. This significantly alters the type, concentration, and transport characteristics of carriers in the system, achieving effective control over conductivity.

[0020] In some embodiments, the conjugated polymer can be cis-structured, trans-structured, or a hybrid of cis- and trans-structured. For example, the conjugated polymer can include cis-polyacetylene and / or trans-polyacetylene.

[0021] In some embodiments, appropriate side chains can be introduced into the polyaliphatic alkyne polymer to alter the physical and chemical properties of the conjugated polymer. For example, the introduction of side chains can improve the elasticity of the conjugated polymer to a certain extent, enabling it to better buffer the volume change stress of the active particles during charging and discharging, and protect the integrity of the inner rigid metal oxide coating.

[0022] In some embodiments, the number-average molecular weight of the conjugated polymer is 100~200000 g / mol.

[0023] By controlling the number-average molecular weight of conjugated polymers, the delocalization and transport of electrons on the conjugated backbone are optimized, thereby constructing a more efficient and stable three-dimensional conductive network. At the same time, it can also improve the mechanical strength of carbon-containing polymer materials coated on the outside of composite particles, enabling them to more effectively buffer volume change stress and maintain structural integrity during long-term battery cycling.

[0024] In some embodiments, the number-average molecular weight of the conjugated polymer can be one of 100 g / mol, 500 g / mol, 1000 g / mol, 5000 g / mol, 10000 g / mol, 15000 g / mol, 50000 g / mol, 100000 g / mol, 150000 g / mol, or 200000 g / mol, or a range of any two.

[0025] In some embodiments, the lithium manganese iron phosphate particles are lithium-rich lithium manganese iron phosphate, and the chemical formula of lithium-rich lithium manganese iron phosphate includes Li. 1+X Mn Y Fe 1-Y PO4, where 0 < X ​​≤ 0.7 and 0 < Y ≤ 0.5.

[0026] Generally, lithium-rich manganese iron phosphate (LFP) refers to LFP materials where the molar amount of lithium is at least greater than the sum of the molar amounts of manganese and iron. Reflected in the above chemical formula, this means the stoichiometric coefficient of lithium is greater than the sum of the stoichiometric coefficients of manganese and iron. By introducing excess lithium into the material's crystal lattice, the lithium ions consumed during charging due to the formation of the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) films can be replenished, thus improving the battery's charge and discharge capacity and consequently increasing its energy density.

[0027] However, excessive lithium enrichment (e.g., exceeding 0.7%) may prevent lithium ions from occupying a stable crystal lattice, thus reducing the thermal stability of lithium manganese iron phosphate materials. By controlling the lithium enrichment level of lithium manganese iron phosphate at an appropriate level, a balance can be established between energy density and thermal stability, ensuring battery safety while achieving high-capacity batteries.

[0028] In some embodiments, in the chemical formula of lithium manganese iron phosphate, X (hereinafter also referred to as lithium enrichment) can be a value of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 or 0.7 or a range of any two.

[0029] In some embodiments, in the chemical formula of lithium manganese iron phosphate, the value of Y can be one of 0.1, 0.2, 0.3, 0.4 and 0.5 or a range of any two.

[0030] In some embodiments, lithium-rich manganese iron phosphate further includes modifying elements, including at least one of scandium, titanium, and vanadium.

[0031] Introducing modifiers such as scandium, titanium, and vanadium into lithium-rich manganese iron phosphate (LFP) can effectively regulate its crystal structure, ion transport kinetics, and electrochemical stability. Specifically, scandium, titanium, and vanadium ions can dope into the material's crystal lattice, increasing the internal carrier concentration and improving the material's electronic conductivity. Furthermore, they can suppress lattice volume distortion and structural collapse during charge and discharge, enhancing the material's structural stability and cycle reversibility. In addition, these modifiers can reduce transition metal ion dissolution, suppress side reactions of the electrolyte on the material surface, and improve the cycle stability and cycle life of the cathode active material.

[0032] In some embodiments, the doping of modified elements into lithium manganese iron phosphate can be achieved by mixing the chloride corresponding to the modified element with lithium manganese iron phosphate particles in a certain proportion, and reacting the mixture in a tube furnace under a nitrogen or argon atmosphere, thereby incorporating the modified element into the lithium manganese iron phosphate. The amount of modified element doped can be controlled by controlling the reaction time.

[0033] In some embodiments, the content of the modifying element is 200 ppm to 2000 ppm based on the total mass of the lithium manganese iron phosphate particles.

[0034] In some embodiments, based on the total mass of lithium manganese iron phosphate particles, the content of the modifying element is one of 200ppm, 400ppm, 600ppm, 800ppm, 1000ppm, 1200ppm, 1400ppm, 1600ppm, 1800ppm or 2000ppm or a range of any two.

[0035] In some embodiments, the metal oxide particles include at least one of alumina particles, zinc oxide particles, chromium oxide particles, and titanium oxide particles.

[0036] These metal oxide particles, through a dual mechanism of physical isolation and chemical stabilization, not only significantly improve the overall thermal stability and structural integrity of the material, but also reduce transition metal dissolution and side reactions at the source. The combination of these metal oxide particles with lithium-rich manganese iron phosphate ensures high energy density while providing a stable substrate for the outer carbon-containing polymer coating, thus guaranteeing the conductivity of the positive electrode active material.

[0037] In some embodiments, the mass ratio of lithium manganese iron phosphate particles to metal oxide particles is (80~180):1.

[0038] In some embodiments, the mass ratio of lithium manganese iron phosphate particles to metal oxide particles can be one of 80, 100, 120, 140, 160 or 180 or a range of any two.

[0039] In some embodiments, the mass ratio of lithium manganese iron phosphate particles to conjugated polymer is (10~35):1.

[0040] The mass ratio of lithium manganese iron phosphate particles to conjugated polymer can be one of 10, 15, 20, 25, 30 or 35 or any range of both.

[0041] In a second aspect of this application, a secondary battery is provided. The secondary battery includes: a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including the positive active material provided in the first aspect of this application.

[0042] In some embodiments, the secondary battery further includes a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector.

[0043] In some embodiments, the secondary battery further includes a separator. The type of separator can be selected according to actual needs. Exemplarily, the separator can be a polypropylene membrane, a polyethylene membrane, a polyvinylidene fluoride membrane, a spandex membrane, an aramid membrane, or a coated and modified multilayer composite membrane.

[0044] In some embodiments, the secondary battery further includes an electrolyte, which may be an organic or inorganic solution with lithium salt as the solute.

[0045] In some embodiments, the secondary battery also includes an outer packaging, which can be rigid packaging, such as a hard plastic shell, aluminum shell, steel shell, etc. The outer packaging of the secondary battery can also be flexible packaging, such as a pouch-type soft pack, and the material of the soft pack can be polypropylene, polybutylene terephthalate, and / or polybutylene succinate.

[0046] In some embodiments, the secondary battery may be constrained by its outer packaging into any suitable shape. For example, the secondary battery as a whole may be cylindrical, prismatic, or other regular or irregular shapes to match the battery space of the electrical device.

[0047] In a third aspect of this application, an electrical device is provided. This device includes the secondary battery provided in the second aspect of this application, and serves as a power source for the electrical device.

[0048] In some embodiments, the electrical equipment can be application devices such as vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools. Vehicles can 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.

[0049] It should be understood that the content described in this content section is not intended to limit the key or essential features of the embodiments of this application, nor is it intended to restrict the scope of this application. Other features of this application will become readily apparent from the following description. Attached Figure Description

[0050] The above and other features, advantages, and aspects of the embodiments of this application will become more apparent from the accompanying drawings and the following detailed description. In the drawings, the same or similar reference numerals denote the same or similar elements, wherein: Figure 1 A schematic diagram of the coating structure of the positive electrode active material according to an embodiment of this application is shown. Reference numerals: 1-Lithium manganese iron phosphate particles, 2-Metal oxide particles, 3-Carbon-containing polymer material. Detailed Implementation

[0051] Embodiments of this application will now be described in more detail with reference to the accompanying drawings. While some embodiments of this application are shown in the drawings, it should be understood that this application can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this application. It should be understood that the drawings and embodiments of this application are for illustrative purposes only and are not intended to limit the scope of protection of this application.

[0052] It should be noted that the headings of any section / subsection provided herein are not limiting. Various embodiments are described throughout this document, and embodiments of any type may be included under any section / subsection. Furthermore, embodiments described in any section / subsection may be combined in any way with any other embodiments described in the same section / subsection and / or different sections / subsections.

[0053] In the description of embodiments of this application, the term "comprising" and similar terms should be understood as open-ended inclusion, i.e., "including but not limited to". The term "based on" should be understood as "at least partially based on". The term "one embodiment" or "the embodiment" should be understood as "at least one embodiment". The term "some embodiments" should be understood as "at least some embodiments". Other explicit and implicit definitions may also be included below. The terms "first", "second", etc., may refer to different or the same objects. Other explicit and implicit definitions may also be included below.

[0054] As mentioned earlier, for lithium batteries based on lithium manganese iron phosphate, as the lithium content increases, some lithium elements in the lithium manganese iron phosphate material cannot occupy a stable crystal lattice, which leads to a decrease in the thermal stability of the lithium manganese iron phosphate material.

[0055] This application provides a positive electrode active material, a secondary battery, and an electrical device to solve or at least partially solve the above-mentioned technical problems or other potential technical problems.

[0056] Figure 1 A schematic diagram of the coating structure of the positive electrode active material according to an embodiment of the present disclosure is shown. Figure 1 As shown, metal oxide particles 2 coat the outer side of lithium manganese iron phosphate particles 1, and carbon-containing polymer material 3 coats the outer side of metal oxide particles 2. Thus, the positive electrode active material forms a multi-layered coating structure with lithium manganese iron phosphate particles 1 as the core, metal oxide particles 2 as the inner shell, and carbon-containing polymer material 3 as the outer shell. The shells formed by metal oxide particles 2 and carbon-containing polymer material 3 respectively provide modification and protection to the lithium manganese iron phosphate particles 1.

[0057] According to the positive electrode active material provided in the embodiments of this application, metal oxide particles are at least partially coated on the outer side of lithium manganese iron phosphate particles, thereby providing physical isolation for the lithium manganese iron phosphate. The metal oxide particles can establish a thermal isolation barrier for the lithium manganese iron phosphate, reducing heat transfer to the lithium manganese iron phosphate particles, and also reducing direct contact between the active material and the electrolyte at high temperatures, thereby improving the thermal stability of the lithium manganese iron phosphate particles. This improves the defects of the lithium manganese iron phosphate system, such as reduced thermal stability and earlier thermal runaway temperature, caused by increased lithium content, providing a basis for lithium enrichment of lithium manganese iron phosphate particles.

[0058] Furthermore, embodiments of this application also coat the outer surface of the composite particles with a carbon-containing polymer material, thereby improving the conductivity of the positive electrode active material. The carbon-containing polymer material of this application is obtained by doping a conjugated polymer with heteroatoms that have electron-donating and / or electron-withdrawing capabilities. The carbon-containing polymer material coats the outer surface of the composite particles, forming a continuous conductive network. This accelerates electron conduction between composite particles and from the surface to the core of the composite particles, thereby improving the electrical performance of the positive electrode active material.

[0059] The positive electrode active material and secondary battery of this application will be further described below with reference to specific embodiments.

[0060] Example 1 Preparation of the positive active material Lithium manganese iron phosphate (Li 1.2 Mn 0.5 Fe 0.5 PO4 powder (with a lithium enrichment X of 0.2) was added to 180 mL of aluminum chloride aqueous solution and stirred to form a mixed system. The concentration of aluminum chloride in the aluminum chloride aqueous solution was 0.5 mol / L.

[0061] Under continuous stirring, a 0.5 mol / L sodium hydroxide solution was slowly added to the mixture, and the reaction was allowed to proceed for 45 minutes. The amount of sodium hydroxide added was three times the amount of aluminum chloride to ensure that the aluminum hydroxide produced by the reaction of aluminum chloride and sodium hydroxide fully precipitated on the surface of the lithium manganese iron phosphate particles. After the reaction was complete, the precipitate was washed with ethanol by centrifugation.

[0062] The washed product was placed in a tube furnace under a protective gas atmosphere and calcined at 500°C for 4.5 hours. The protective gas was argon, with a flow rate of 0.1 L / min. This process converts aluminum hydroxide into an alumina (Al₂O₃) coating, resulting in composite particles of lithium manganese iron phosphate coated with metal oxide. In the aforementioned steps, the mass ratio of lithium manganese iron phosphate particles to alumina particles was controlled to be 130:1 by controlling the amount of lithium manganese iron phosphate added to the aluminum chloride solution.

[0063] The composite particles and conjugated polymer were mixed at a mass ratio of 20:1, and 0.35 L of ethyl methyl carbonate (EMC) solvent was added. The mixture was stirred at 40 °C for 6.5 h at a stirring speed of 350 rpm. After washing with ethyl methyl carbonate and centrifuging three times, the product was finally vacuum dried at 60 °C for 24 h to obtain the positive electrode active material.

[0064] The conjugated polymer was fluorine-doped polyacetylene. The conjugated polymer was prepared as follows: polyacetylene (number-average molecular weight 50,000 g / mol) was placed in a hydrogen fluoride (HF) atmosphere and heated to 260°C to react with the hydrogen fluoride. The doping amount of fluorine atoms was controlled to be 1 wt% based on the reaction time.

[0065] Preparation of the positive electrode sheet The positive electrode current collector is a carbon-coated aluminum foil with a thickness of 12 μm. A carbon coating is applied to both sides of the aluminum foil, with a thickness of 1 μm on each side.

[0066] The prepared positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent carbon black were mixed at a mass ratio of 97:2:1. N-methylpyrrolidone (NMP) was added to adjust the solid content to 65%, yielding the positive electrode active slurry. The positive electrode active slurry was coated on both sides of the positive electrode current collector, with a coating amount of 0.298 g / 1540.25 mm. 2 The coated positive current collector is placed in an oven to dry, and then rolled and slit to obtain the positive electrode sheet.

[0067] Preparation of the negative electrode sheet The negative electrode current collector is a copper foil with a thickness of 4.5 μm. Artificial graphite (negative electrode material), carbon black (conductive agent), and sodium carboxymethyl cellulose (binder) were mixed at a mass ratio of 97:2:1, and deionized water was added to adjust the solid content to 50%. The mixture was stirred until homogeneous to obtain a negative electrode slurry. This slurry was then uniformly coated onto the copper foil used as the negative electrode current collector. The coating weight of the negative electrode slurry was 0.140 g / 1540.25 mm. 2 The coated negative electrode current collector is placed in an oven to dry, then rolled and slit to obtain the negative electrode sheet.

[0068] Preparation of the electrolyte An organic solvent was prepared by mixing ethylene carbonate, diethyl carbonate, and dimethyl carbonate in a volume ratio of 7:1.5:1.5. In a glove box under an argon atmosphere with a water content below 10 ppm, thoroughly dried lithium hexafluorophosphate was dissolved in the organic solvent. After the lithium hexafluorophosphate was completely dissolved, an additive prepared from vinylene carbonate and fluoroethylene carbonate in a volume ratio of 1:1 was added, wherein the weight ratio of organic solvent, lithium hexafluorophosphate, and additive was 80:15:5. The mixture was then thoroughly mixed to obtain the electrolyte.

[0069] Assembly of the secondary battery The positive electrode, PE separator, and negative electrode are stacked in sequence and wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, vacuum dried, and then injected with electrolyte. After standing, formation, and capacity testing, a secondary battery is obtained.

[0070] Examples 2-4 The difference from Example 1 is that the heteroatoms doped in the conjugated polymer polyacetylene are different.

[0071] Depending on the heteroatoms, different doping methods can be used to treat polyacetylene. Specifically, in Example 2, the conjugated polymer is polyacetylene doped with chlorine atoms. This conjugated polymer was prepared by the following method: polyacetylene (number average molecular weight 50,000 g / mol) was placed in an atmosphere of hydrogen chloride (HCl) and heated to 260°C to allow the hydrogen chloride to react with the polyacetylene. The amount of fluorine atoms doped was controlled to be 1 wt% based on the reaction time.

[0072] Example 3: The conjugated polymer was lithium-doped polyacetylene. This conjugated polymer was prepared by heating elemental lithium to 400°C and mixing the resulting lithium vapor with helium gas, wherein the partial pressure of the lithium vapor in the mixed atmosphere was 1%. The polyacetylene was exposed to a continuous, directional gas flow of the mixed atmosphere at a flow rate of 0.1 L / min. The lithium doping amount was controlled to 1 wt% by controlling the exposure time of the polyacetylene.

[0073] Example 4: The conjugated polymer was sodium-doped polyacetylene. This conjugated polymer was prepared by heating elemental sodium to 200°C and mixing the resulting sodium vapor with helium gas, wherein the partial pressure of the sodium vapor in the mixed atmosphere was 1%. The polyacetylene was exposed to a continuous, directional gas flow in the mixed atmosphere at a flow rate of 0.1 L / min. The sodium doping amount was controlled to 1 wt% by controlling the exposure time of the polyacetylene.

[0074] Examples 5-8 The difference from Example 1 is that the amount of heteroatom doping in the conjugated polymer polyacetylene is different.

[0075] Examples 9-11 The difference from Example 1 is that the number-average molecular weight of the conjugated polymer polyacetylene is different.

[0076] Examples 12-14 The difference from Example 1 is that the lithium enrichment X of the lithium manganese iron phosphate particles is different.

[0077] Examples 15-18 The difference from Example 1 is that the lithium manganese iron phosphate particles are also doped with modifying elements. The types and amounts of modifying elements in the lithium manganese iron phosphate in Examples 15-18 are different. In Examples 15-18, the lithium manganese iron phosphate can be pretreated to allow the modifying elements to be incorporated into the lithium manganese iron phosphate particles.

[0078] Specifically, in Examples 15, 17 and 18, the chloride (TiCl4) corresponding to the modified element titanium (Ti) can be mixed with lithium manganese iron phosphate particles in a certain proportion and placed in a tube furnace to react under an argon atmosphere (the temperature of the tube furnace is controlled at 100°C). By controlling the reaction time, the content of titanium element in the lithium manganese iron phosphate particles is 200ppm, 1000ppm and 2000ppm respectively.

[0079] In Example 16, the chloride (VCl3) corresponding to the modified element vanadium (V) was mixed with lithium manganese iron phosphate particles in a certain proportion and placed in a tube furnace for reaction under an argon atmosphere (the temperature of the tube furnace was controlled at 425°C). The vanadium content in the lithium manganese iron phosphate particles was controlled to be 200 ppm by controlling the reaction time.

[0080] Examples 19 and 20 The difference from Example 1 is that the mass ratio k1 of lithium manganese iron phosphate particles to metal oxide particles is different.

[0081] Examples 21 and 22 The difference from Example 1 is that the mass ratio k2 of lithium manganese iron phosphate particles to carbon-containing polymer materials is different.

[0082] Comparative Example 1 The difference from Example 1 is that the composite particles are not coated with carbon-containing polymer materials on the outside.

[0083] Comparative Example 2 The difference from Example 1 is that the conjugated polymer is not doped with heteroatoms.

[0084] Comparative Example 3 The difference from Example 1 is that the heteroatom doped in the conjugated polymer is an oxygen atom. Based on the total mass of the conjugated polymer, the oxygen atom doping amount is 1 wt%.

[0085] Examples 1-22 and Comparative Examples 1-3 are shown in Table 1: Table 1

[0086] Note: The lithium enrichment level X in Table 1 can be determined based on lithium manganese iron phosphate (Li... 1+X Mn Y Fe 1-Y The molecular weight of lithium in PO4 is used to determine its composition. k1 represents the mass ratio of lithium manganese iron phosphate particles to metal oxide particles. k2 represents the mass ratio of lithium manganese iron phosphate particles to carbon-containing polymer materials; " / " indicates that no corresponding processing was performed in this embodiment.

[0087] Performance test Test content 1) Capacity C0 test of secondary batteries At an ambient temperature of 25℃, the secondary battery was charged at a constant current of 0.33C to 4.25V, then charged at a constant voltage until the current dropped below 0.05C, and subsequently discharged at 0.33C to 2.5V. After this cycle was repeated three times, the capacity of the third discharge was taken as the C0 capacity.

[0088] 2) Cycle performance test of secondary batteries The secondary battery was placed in a charge / discharge test chamber and subjected to cyclic testing under constant temperature of 45℃, voltage range of 2.5~4.25V, charge rate of 1C, and discharge rate of 1C. The test was stopped when the capacity dropped to 80% of the initial capacity, and the number of cycles was recorded.

[0089] 3) Gas generation performance test of secondary batteries At an ambient temperature of 25℃, within a voltage range of 4.25~2.5V, and a charge / discharge rate of 1C, the secondary battery was subjected to 200 charge / discharge cycles. During the cycles, the gas production volume (i.e., the battery expansion volume) of the secondary battery was tested using the water displacement method.

[0090] The detection results of Examples 1-22 and Comparative Examples 1-3 are shown in Table 2: Table 2

[0091] Referring to Table 2 and combining it with Table 1, it can be seen that coating the lithium manganese iron phosphate (LFP) particles with metal oxide particles and carbon-containing polymer materials can improve the charge transfer efficiency between LFP and the electrolyte while providing protection. This gives LFP higher chemical and thermal stability, reduces gas production during battery cycling, and thus improves battery cycle performance and extends battery life. Furthermore, the establishment of the protective barrier and conductive network provides a foundation for lithium enrichment of LFP materials. LFP can utilize lithium-rich materials with higher lithium content, thereby increasing the capacity of the secondary battery and giving LFP-based secondary batteries better application prospects.

[0092] Referring to Examples 1-22 and Comparative Examples 1-3, the coating of lithium manganese iron phosphate particles with metal oxide particles and carbon-containing polymer materials can significantly improve the cycle performance of the secondary battery and reduce the gas production. The secondary batteries prepared in Examples 1-22 can achieve more than 2000 cycles at 80% capacity retention, with the secondary battery prepared in Example 13 achieving an even higher 3059 cycles at 80% capacity retention. Regarding gas production performance, the secondary batteries prepared in Examples 1-22 can control the gas production volume to within 7.9 mL after 200 cycles, while the secondary battery prepared in Example 13 has a gas production volume of only 0.5 mL after 200 cycles. Furthermore, the coating of lithium manganese iron phosphate particles with metal oxide particles and carbon-containing polymer materials can also improve the capacity of the secondary battery to a certain extent.

[0093] Referring to Examples 1-3 and Comparative Example 3, doping conjugated polymers with atoms of Group IA or Group VIIA elements can significantly enhance the conductivity of carbon-containing polymer materials. This improves the cycle performance of secondary batteries. The positive electrode active material in Example 1 used a fluorine-doped conjugated polymer, achieving a cycle count of 2600 and a gas production volume controlled at 2 mL.

[0094] Referring to Examples 1 and 5-8, variations in the heteroatom doping amount can affect the structure of carbon-containing polymers, enabling them to construct stable, interconnected conductive networks. This improves the charge exchange efficiency between the positive electrode active material and the electrolyte, allowing for rapid insertion and extraction of lithium ions in the lithium manganese iron phosphate particles. Consequently, this increases the capacity and cycle life of the secondary battery while reducing its gas production performance. The secondary battery in Example 7 achieves a capacity of 162.3 Ah, a cycle life of 2876, and a gas production volume controlled at 0.7 mL.

[0095] Referring to Examples 1 and 9-11, the number-average molecular weight of the conjugated polymer can affect the cycle performance and gas production performance of the secondary battery. Selecting a conjugated polymer with a suitable number-average molecular weight can optimize the delocalization and transport of electrons on the conjugated backbone, thereby constructing a more efficient and stable three-dimensional conductive network. The secondary battery in Example 10 has a capacity of 161.7 Ah, a cycle count of 2779, and a gas production volume that can be controlled at 1.1 mL.

[0096] Referring to Examples 1 and 12-14, it can be seen that by coating lithium manganese iron phosphate with two layers, the lithium manganese iron phosphate particles can be lithium-rich particles, and the lithium enrichment of lithium manganese iron phosphate can be combined with the two-layer coating structure. When the lithium enrichment X of lithium manganese iron phosphate is in the range of (0, 0.7), the secondary battery prepared thereby can achieve superior performance in terms of capacity, cycle count, and gas production volume. The secondary battery of Example 13 has a capacity of 163.3 Ah, a cycle count of 3059, and a gas production volume that can be controlled at 0.5 mL.

[0097] In addition, referring to Examples 1 and 15-22, the capacity, cycle performance, gas production performance and other parameters of the secondary battery can be further optimized by doping modified elements into lithium manganese iron phosphate, changing the mass ratio of lithium manganese iron phosphate particles to metal oxide particles and carbon-containing polymer materials.

[0098] Various implementations of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed implementations. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described implementations. The terminology used herein is chosen to best explain the principles, practical applications, or improvements to technology in the market, or to enable others skilled in the art to understand the various implementations disclosed herein.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes composite particles and a carbon-containing polymer material coating the outside of the composite particles. The composite particles include lithium manganese iron phosphate particles and metal oxide particles that at least partially coat the outer surface of the lithium manganese iron phosphate particles. The carbon-containing polymer material includes conjugated polymers derived from substituted or unsubstituted aliphatic alkynes, wherein the conjugated polymers are doped with heteroatoms, and the heteroatoms include Group IA elements or Group VIIA elements.

2. The positive electrode active material according to claim 1, characterized in that, The heteroatom includes at least one of lithium, sodium, potassium, fluorine, chlorine, bromine, and iodine.

3. The positive electrode active material according to claim 2, characterized in that, Based on the total mass of the conjugated polymer, the doping amount of the heteroatom is 0.05wt%~2wt%.

4. The positive electrode active material according to claim 1, characterized in that, The conjugated polymer includes at least one of polyacetylene, polypropyne, and polybutyne.

5. The positive electrode active material according to claim 1, characterized in that, The number-average molecular weight of the conjugated polymer is 100~200000 g / mol.

6. The positive electrode active material according to claim 1, characterized in that, The lithium manganese iron phosphate particles are lithium-rich lithium manganese iron phosphate, and the chemical formula of the lithium-rich lithium manganese iron phosphate includes Li. 1+X Mn Y Fe 1-Y PO4, where 0 < X ​​≤ 0.7 and 0 < Y ≤ 0.

5.

7. The positive electrode active material according to claim 6, characterized in that, The lithium-rich manganese iron phosphate also includes modifying elements, which include at least one of scandium, titanium and vanadium.

8. The positive electrode active material according to claim 7, characterized in that, Based on the total mass of the lithium manganese iron phosphate particles, the content of the modifying element is 200ppm to 2000ppm.

9. The positive electrode active material according to claim 1, characterized in that, The metal oxide particles include at least one of aluminum oxide particles, zinc oxide particles, chromium oxide particles, and titanium oxide particles.

10. The positive electrode active material according to claim 1, characterized in that, The mass ratio of the lithium manganese iron phosphate particles to the metal oxide particles is (80~180):

1.

11. The positive electrode active material according to claim 1, characterized in that, The mass ratio of the lithium manganese iron phosphate particles to the conjugated polymer is (10~35):

1.

12. A secondary battery, characterized in that, The invention includes a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising the positive active material according to any one of claims 1-11.

13. An electrical appliance, characterized in that, include: The secondary battery of claim 12, wherein the secondary battery serves as the power supply for the electrical device.

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