Lithium iron phosphate composite material, preparation method thereof, positive electrode sheet and energy storage device

Abstract

The application provides a lithium iron phosphate composite material, a preparation method thereof, a positive electrode sheet and an energy storage device. The lithium iron phosphate composite material comprises a core, an amorphous carbon layer and a mesoporous carbon layer. The core is a lithium iron phosphate particle. The amorphous carbon layer is coated on the surface of the core. The mesoporous carbon layer is coated on the surface of the amorphous carbon layer away from the core. The lithium iron phosphate composite material provided by the application can be used as a positive electrode material, ensures high capacity and high compaction density of the positive electrode material, and solves the technical bottleneck that capacity and other key performances of the existing positive electrode material are difficult to balance.

Description

Technical Field

[0001] This application relates to the field of energy storage device technology, and in particular to a lithium iron phosphate composite material and its preparation method, a positive electrode sheet, and an energy storage device. Background Technology

[0002] In recent years, with the rapid growth of global renewable energy installations, the energy storage market has ushered in a period of broad development. As a core component of energy storage devices, batteries' continuous technological iteration has a profound impact on the development of the energy storage industry. In line with the rapid development trend of the global energy storage industry, the requirements for energy storage batteries in terms of efficiency and cost competitiveness are constantly increasing, and breakthroughs in the energy density of single-cell batteries are the core path to achieving these goals.

[0003] Among them, the cathode material, as the core component of energy storage batteries, directly determines the energy density, production cost, and long-term reliability of the battery, and is a key breakthrough for the technological upgrade of energy storage batteries. Under the technical constraint of fixed battery casing volume, increasing the specific capacity of the cathode active material is the most direct and efficient technical means to improve the energy density of a single cell. However, in existing technologies, simply pursuing capacity performance improvement often leads to prominent problems such as low compaction density, cycle performance degradation, and poor rate characteristics, which seriously restrict the large-scale application and market promotion of high-energy-density energy storage batteries. Summary of the Invention

[0004] The purpose of this application is to provide a lithium iron phosphate composite material and its preparation method, positive electrode sheet and energy storage device. The lithium iron phosphate composite material can be used as a positive electrode material, ensuring high specific capacity, high efficiency and high compaction density of the positive electrode material, and solving the technical bottleneck of existing positive electrode materials where capacity and other key performance are difficult to balance.

[0005] This application provides a lithium iron phosphate composite material, which includes a core, an amorphous carbon layer and a mesoporous carbon layer. The core is a lithium iron phosphate particle, the amorphous carbon layer covers the surface of the core, and the mesoporous carbon layer covers the surface of the amorphous carbon layer opposite to the core.

[0006] This application provides a lithium iron phosphate composite material with a double-layer carbon-coated lithium iron phosphate core. The inner amorphous carbon layer constructs continuous electron channels and is covalently bonded to the lithium iron phosphate core to ensure high specific capacity and efficient operation of the composite material. The outer mesoporous carbon layer provides ion transport channels without generating ineffective voids, achieving high compaction density in the lithium iron phosphate composite material. The amorphous and mesoporous carbon layers synergistically isolate the electrolyte, buffer the volume effect, and suppress ion dissolution and structural degradation, meeting the long-cycle requirements of energy storage devices. This application achieves synergistic optimization of high specific capacity, high compaction density, and long cycle life by precisely controlling the composition and morphology of the double-layer carbon coating structure. The high specific capacity significantly improves the energy density of a single cell, the high compaction density further optimizes the volumetric energy density of the battery, and the long cycle life meets the long-term stable operation requirements of energy storage batteries. This effectively solves the technical bottleneck of existing cathode materials where capacity and other key performance aspects are difficult to balance, providing new core material support for the development of high-energy-density and high-reliability energy storage batteries.

[0007] In one possible implementation, the thickness of the amorphous carbon layer is 1.5 nm to 3 nm.

[0008] In one possible implementation, the thickness of the mesoporous carbon layer is 4 nm to 6 nm; and / or, the mesoporous carbon layer has mesopores with a pore size of 3 nm to 5 nm.

[0009] This application also provides a method for preparing a lithium iron phosphate composite material, wherein the lithium iron phosphate composite material is the aforementioned lithium iron phosphate composite material, and the method for preparing the lithium iron phosphate composite material includes: A mixed raw material is prepared, comprising lithium iron phosphate precursor, sucrose, amphiphilic block copolymer and polar solvent, wherein the mass ratio of sucrose to amphiphilic block copolymer in the mixed raw material is 1.5:1 to 5:2, and the polar solvent comprises a first strong polar solvent and a second strong polar solvent, wherein the first strong polar solvent is water, the dielectric constant of the second strong polar solvent is ε, the dipole moment of the second strong polar solvent is μ, ε > 15 F / m and μ > 2.5 D; The mixed raw materials were sintered to obtain a lithium iron phosphate composite material.

[0010] In one possible implementation, the mixed raw materials also include hydroxypentanoic acid.

[0011] In one possible implementation, the amphiphilic block copolymer is a PEOn-PPOm-PEOn triblock copolymer, wherein n≥80 and m≥60; and / or, the second strongly polar solvent is anhydrous ethanol.

[0012] In one possible implementation, the volume ratio of the first strongly polar solvent to the second strongly polar solvent is 4:1 to 8:1.

[0013] In one possible implementation, the step of sintering the mixed raw materials includes: placing the mixed raw materials in an inert atmosphere for a first sintering step and a second sintering step, wherein the temperature of the first sintering step is 280°C to 320°C and the temperature of the second sintering step is 650°C to 700°C.

[0014] This application embodiment also provides a positive electrode sheet, including a positive current collector and a positive electrode material layer. The positive electrode material layer is disposed on at least one side of the positive current collector along the thickness direction. The positive electrode material layer includes the lithium iron phosphate composite material as described above, or includes the lithium iron phosphate composite material prepared according to the preparation method of the lithium iron phosphate composite material described above.

[0015] This application also provides an energy storage device, including a negative electrode, a separator, and a positive electrode as described above, wherein the separator is disposed between the negative electrode and the positive electrode. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the structure of an energy storage system according to an embodiment of this application; Figure 2 Here is an HRTEM image of the lithium iron phosphate composite material from Example 1; Figure 3 Here is an HRTEM image of the lithium iron phosphate composite material in Comparative Example 2; Figure 4 HRTEM image of the lithium iron phosphate composite material in Comparative Example 3; Figure 5 Here is an HRTEM image of the lithium iron phosphate composite material in Comparative Example 4. Figure 6 HRTEM image of the lithium iron phosphate composite material of Comparative Example 7; Figure 7 The particle size distribution diagrams are for the lithium iron phosphate composite materials of Example 1 and Comparative Example 1. Figure 8 The nitrogen adsorption-desorption isotherm and pore size distribution curve of the lithium iron phosphate composite material in Example 1 are shown. Figure 9 Nitrogen adsorption-desorption isotherms and pore size distribution curves of the lithium iron phosphate composite material in Comparative Example 1 are shown. Figure 10Nitrogen adsorption-desorption isotherms and pore size distribution curves of the lithium iron phosphate composite material in Comparative Example 2. Figure 11 Nitrogen adsorption-desorption isotherms and pore size distribution curves of the lithium iron phosphate composite material of Comparative Example 3 are shown. Figure 12 Electrochemical impedance spectroscopy of lithium iron phosphate composite materials from Example 1 and Comparative Examples 1-3; Figure 13 The charge / discharge curves of the lithium-ion batteries of Example 1 and Comparative Examples 1-3 at 25°C @ 0.1P are shown. Figure 14 The charge / discharge curves of the lithium-ion batteries of Example 2, Comparative Example 4, and Comparative Example 7 at 25°C @ 0.1P are shown. Figure 15 The charge / discharge curves of the lithium-ion batteries of Example 4 and Comparative Examples 10-12 at 25°C @ 0.1P are shown.

[0018] Reference numerals: 5000, energy storage system; 2000, high-voltage cable; 3000, first power conversion device; 4000, second power conversion device; 1000, energy storage device. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] Because the energy people need is highly time- and space-dependent, in order to make rational use of energy and improve energy efficiency, it is necessary to use a medium or device to store one form of energy in the same way or by converting it into another form of energy, and then release it in a specific form of energy based on future application needs.

[0021] Currently, the generation of green electricity generally relies on solar, wind, and hydropower. However, wind and solar power are generally characterized by strong intermittency and large fluctuations, which can cause grid instability, insufficient power during peak demand periods, and excessive power during off-peak periods. Unstable voltage can also damage the power grid. Therefore, insufficient electricity demand or insufficient grid capacity may lead to the problem of "wind and solar curtailment." Solving these problems requires energy storage. This involves converting electrical energy into other forms of energy through physical or chemical means and storing it. When needed, this energy can be converted back into electrical energy and released. Simply put, energy storage is like a large "power bank," storing electrical energy when solar and wind power are abundant and releasing the stored electricity when needed.

[0022] Taking electrochemical energy storage as an example, this solution provides an energy storage device 1000, which is applied to an energy storage system. The energy storage device 1000 is equipped with a set of chemical batteries, which mainly use the chemical elements in the batteries as energy storage medium. The charging and discharging process is accompanied by the chemical reaction or change of the energy storage medium. Simply put, the electrical energy generated by wind and solar energy is stored in the chemical batteries. When the use of external electrical energy reaches its peak, the stored electrical energy is released for use, or transferred to places with a shortage of electricity for use.

[0023] Current energy storage applications are quite widespread, including generation-side energy storage, grid-side energy storage, and consumption-side energy storage. The corresponding energy storage devices include: (1) Large-scale energy storage power stations applied to wind power and photovoltaic power stations can help renewable energy power generation meet grid connection requirements and improve the utilization rate of renewable energy. As a high-quality active / reactive power regulation power source on the power supply side, energy storage power stations can achieve load matching of power in time and space, enhance the absorption capacity of renewable energy, reduce instantaneous power changes, reduce the impact on the power grid, improve the absorption of new energy power generation, and are of great significance in power grid system backup, alleviating peak load power supply pressure and peak regulation and frequency regulation. (2) Energy storage containers applied on the grid side mainly function as peak shaving, frequency regulation and relief of grid congestion. In terms of peak shaving, they can realize peak shaving and valley filling of electricity load, that is, charging the energy storage battery when the electricity load is low and releasing the stored electricity during the peak electricity load period, thereby achieving a balance between power production and consumption. (3) Small energy storage cabinets applied to the electricity consumption side mainly function as self-consumption of electricity, peak-valley price arbitrage, capacity cost management, and improvement of power supply reliability. Depending on the application scenario, electricity consumption side energy storage can be divided into industrial and commercial energy storage cabinets, household energy storage devices, energy storage charging piles, etc., which are generally used in conjunction with distributed photovoltaics. Industrial and commercial users can use energy storage for peak-valley price arbitrage and capacity cost management. In the electricity market implementing peak-valley pricing, by charging the energy storage system when the electricity price is low and discharging the energy storage system when the electricity price is high, peak-valley price arbitrage can be achieved, reducing electricity costs. In addition, industrial enterprises subject to two-part tariffs can use energy storage systems to store energy during off-peak hours and discharge during peak loads, thereby reducing peak power and the maximum demand declared, achieving the goal of reducing capacity costs. Household photovoltaics with energy storage can improve the level of self-consumption of electricity. Due to high electricity prices and poor power supply stability, the demand for household photovoltaic installations is driven. Given that photovoltaic power generation occurs during the day, while user load is generally higher at night, configuring energy storage can better utilize photovoltaic power, improve self-consumption levels, and reduce electricity costs. Furthermore, energy storage is needed in areas such as communication base stations and data centers for backup power.

[0024] In some embodiments, see Figure 1 , Figure 1 This is a schematic diagram of the structure of an energy storage system 5000 according to an embodiment of this application, and this application Figure 1 The embodiments are illustrated using a shared energy storage scenario on the generation / distribution side as an example. The energy storage device 1000 of this application is not limited to its generation / distribution side energy storage scenario.

[0025] This application provides an energy storage system 5000, which includes: a high-voltage cable 2000, a first power conversion device 3000, a second power conversion device 4000, and an energy storage device 1000 provided in this application. In some embodiments of the power generation scenario, the second power conversion device 4000 can be a wind power conversion device. Since the electricity generated by wind power conversion is volatile, random, and intermittent, the unstable electricity output by the wind power conversion device can be stored in the energy storage device 1000 through grid connection. The energy storage device 1000 is connected to the high-voltage cable 2000 and outputs smooth electricity to supply the power consumption side of the distribution network, realizing peak shaving and frequency regulation, and stable grid operation; or, wind... The wind power conversion device is always connected to the high-voltage cable 2000. Under normal power generation conditions, the power output of the wind power conversion device is supplied to the power consumption side of the distribution network through the high-voltage cable. When the current power load is low and the wind power conversion device generates excess power, the excess power is first stored in the energy storage device 1000 to reduce wind and solar curtailment rates and improve the problem of new energy power generation consumption. When the power load is high, the power grid issues an instruction to transmit the power stored in the energy storage device 1000 together with the high-voltage cable 2000 in grid-connected mode to supply power to the power consumption side. This provides the power grid with various services such as peak shaving, frequency regulation, and backup, giving full play to the peak shaving role of the power grid, promoting peak shaving and valley filling, and alleviating the power supply pressure of the power grid.

[0026] In some embodiments on the distribution network side, the first power conversion device 3000 can be a photovoltaic power conversion device. The energy storage device 1000 is connected to the high-voltage cable 2000 and installed downstream of the high-voltage cable 2000 between the user load and the user load. The electrical energy output by the photovoltaic power conversion device is stored in the energy storage device 1000, which can respond in a timely manner to act as a backup power source when the power grid / distribution network fails. Alternatively, it can provide power supply support to alleviate line congestion when the high-voltage cable 2000 transmission line is blocked, and to delay the economic pressure caused by the expansion of the power grid / distribution capacity when the power grid is planned to be expanded.

[0027] Optionally, the first power conversion device 3000 may include, but is not limited to, a wind power conversion device, and the second power conversion device 4000 may include, but is not limited to, a photovoltaic power conversion device. The first power conversion device 3000 and the second power conversion device 4000 can convert at least one of solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy and mechanical energy into electrical energy.

[0028] Optionally, the energy storage device 1000 may include, but is not limited to, energy storage applications such as energy storage power stations, hydropower / thermal / wind power generation systems, solar power generation systems, mobile power systems, smart home systems, or temporary power supply systems, and may also be applied in multiple fields such as data centers, military equipment, aerospace, charging piles, and electric vehicles.

[0029] Optionally, the energy storage device 1000 may include, but is not limited to, single-cell batteries, or battery modules, battery packs, battery clusters, power banks, energy storage cabinets / containers, and other battery integrated systems composed of single-cell batteries. The actual application form of the energy storage device 1000 provided in this application embodiment may be, but is not limited to, the listed products, and may also be other application forms. This application embodiment does not strictly limit the application form of the energy storage device 1000. This application embodiment only uses a multi-cell battery as an example for illustration.

[0030] Optionally, when the energy storage device 1000 is a single battery cell, the energy storage device 1000 can be, but is not limited to, at least one of cylindrical, square, prismatic, or other shaped batteries.

[0031] Optionally, the battery cell can be a rechargeable battery, which refers to a battery cell that can be recharged after discharge to activate the active materials and continue to be used. The battery cell can be a lithium-ion battery, sodium-ion battery, sodium-lithium-ion battery, lithium metal battery, sodium metal battery, lithium-sulfur battery, magnesium-ion battery, nickel-metal hydride battery, nickel-cadmium battery, lead-acid battery, etc., and this application does not specifically limit it.

[0032] Specifically, this application provides an energy storage device, which includes a casing, a battery cell, and an electrolyte. The battery cell is installed inside the casing, and the electrolyte is injected into the inside of the casing and wets the battery cell. In this application embodiment, the energy storage device is a secondary battery. For example, the secondary battery can be a lithium-ion battery. For example, the electrolyte includes at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0033] A battery cell includes a negative electrode, a separator, a positive electrode, a negative tab, and a positive tab. The separator is disposed between the negative electrode and the positive electrode. The negative tab is electrically connected to the negative electrode, and the positive tab is electrically connected to the positive electrode. For example, the negative electrode, separator, and positive electrode can be stacked sequentially, wound to form a bare battery cell, and then the negative and positive tabs can be soldered to obtain the battery cell.

[0034] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer, with the negative electrode material layer disposed on at least one side of the negative electrode current collector along its thickness direction. In this embodiment, the negative electrode material layer is disposed on both surfaces of the negative electrode current collector along its thickness direction. The negative electrode material layer includes materials such as additives and negative electrode active materials. Additives to the negative electrode material layer include conductive agents, binders, and dispersants. For example, the conductive agent includes conductive carbon black SP (Super-P). For example, the binder includes at least one of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). For example, the negative electrode active material includes artificial graphite.

[0035] The separator can be an existing battery separator; for example, a polyethylene (PE) membrane. The positive electrode includes a positive current collector and a positive electrode material layer, the positive electrode material layer being disposed on at least one side of the positive current collector along its thickness direction. In this embodiment, the positive electrode material layer is disposed on both surfaces of the positive current collector along its thickness direction. The positive electrode material layer includes materials such as additives and positive electrode active materials. For example, the additives in the positive electrode material layer include conductive agents, binders, and dispersants. For example, the conductive agent includes conductive carbon black SP (Super-P). For example, the binder includes polyvinylidene fluoride (PVDF).

[0036] In this embodiment, the positive electrode active material in the positive electrode sheet includes a lithium iron phosphate composite material (hereinafter referred to as LiFePO4@C@3D-OMC). The lithium iron phosphate composite material includes a core and a carbon coating layer, with the carbon coating layer covering the surface of the core. The core is a lithium iron phosphate particle, and the carbon coating layer includes an amorphous carbon layer and a mesoporous carbon layer. The amorphous carbon layer covers the surface of the core, and the mesoporous carbon layer covers the surface of the amorphous carbon layer facing away from the core. In this embodiment, the amorphous carbon layer is covalently connected to the core, and the mesoporous carbon layer is covalently connected to the amorphous carbon layer. The amorphous carbon layer is composed of disordered amorphous carbon (also referred to as disordered carbon). For example, the thickness of the amorphous carbon layer is 1.5 nm to 3 nm. Specifically, the thickness of the amorphous carbon layer can be 1.5 nm, 2 nm, 3 nm, or any value between these values. The amorphous carbon layer of this thickness has the following functions: 1. Flexible stress buffering and interface stabilization: Employing an ultra-thin, dense coating design, the amorphous carbon layer can precisely absorb and disperse the mechanical stress generated during the lithium intercalation / deintercalation cycle of LiFePO4 (3%~5% volume expansion and contraction) through its own flexible deformation and stress relaxation effect. This overcomes the shortcomings of uneven buffering in traditional carbon layers, effectively suppressing particle cracking and pulverization. Simultaneously, through chemical bonding, it achieves integrated bonding with LiFePO4 and the outer mesoporous carbon layer, preventing interface debonding and innovatively constructing a dual protection system of "flexible buffering - strong bonding," ensuring long-term stability of the electrode structure; 2. Synergistic regulation of electron / ion transport and interface stabilization: The 1.5nm~3nm thickness design constructs a low-impedance electron transport network to accelerate electron conduction while avoiding the Li-induced degradation caused by excessively thick carbon layers in traditional methods. + This addresses the issues of diffusion obstruction and rate performance degradation. Simultaneously, a dense structure is used to construct an integrated barrier of "physical isolation and side reaction suppression," actively intercepting the contact between the electrolyte and the LiFePO4 active surface, thus inhibiting Fe³⁺... + 3. Design for synergistic optimization of structure protection and capacity: breaking through the traditional dilemma of "thick carbon layers leading to incompatibility and thin layers leading to incompatibility", achieving a balance between structural protection and electrochemical capacity in the thickness range of 1.5nm~3nm - avoiding the problem of carbon layer coating defects and ineffective protection when the thickness is <1.5nm, and solving the drawbacks of excessive non-active carbon content, decreased energy density, and increased polarization when the thickness is >3nm. While giving full play to the core protective role of the carbon layer, it maximizes the retention of the specific capacity of the active material, and achieves an innovative improvement in structural stability and electrochemical performance.

[0037] For example, the thickness of the mesoporous carbon layer is 4nm to 6nm. Specifically, the thickness of the mesoporous carbon layer can be 4nm, 5nm, 6nm, or any value between these values. This thickness allows the mesoporous carbon layer to fully exert its synergistic core functions of structural support, pore stability, and ion transport, providing structural assurance for the excellent electrochemical performance of the composite material. Its specific functions are as follows: 1. Rigid framework support and spatial confinement control: The 4nm to 6nm thick mesoporous carbon layer can construct a continuous and mechanically adapted three-dimensional rigid framework, providing stable mechanical constraint and support for the internal LiFePO4@C composite particles. During lithium-ion intercalation / deintercalation cycling, this rigid framework effectively suppresses the disordered expansion of active particles, alleviates internal stress concentration caused by volume effects, and avoids structural damage caused by stress accumulation. Simultaneously, the mesoporous carbon network, through spatial confinement, firmly fixes LiFePO4@C particles, significantly inhibiting particle pulverization, detachment, and structural breakage during long-term cycling, ensuring the continuity and long-term stability of the overall electrode structure. 2. Stable ordered mesoporous structure and improved ion transport kinetics: This thickness range precisely matches the structural requirements of 3-5 nm ordered mesoporous pores, providing stable support for the mesoporous channels and effectively preventing collapse, distortion, or blockage of the mesoporous pores during precursor carbonization, electrode fabrication, and charge / discharge cycling, ensuring the integrity of the ordered mesoporous structure. Based on this stable mesoporous structure, a continuous and interconnected lithium-ion fast transport channel can be constructed, significantly shortening the lithium-ion intercalation / deintercalation cycle. + Diffusion pathways are improved, ion transport impedance is reduced, and ion transport kinetics are optimized, thereby significantly improving the rate performance of composite materials. Specifically, when the thickness is less than 4 nm, the mesoporous carbon skeleton lacks strength, easily leading to mesoporous structure collapse; when the thickness exceeds 6 nm, it significantly increases the ion diffusion barrier, reduces the effective utilization rate of active materials, and degrades electrochemical performance. 3. High specific surface area and synergistic optimization of structure-electrochemical performance: A thickness design of 4 nm to 6 nm achieves a synergistic balance between the high specific surface area of ​​the mesoporous carbon layer and structural stability. The high specific surface area promotes full electrolyte wetting, enhances interfacial ion adsorption capacity, and provides sufficient active sites for electrochemical reactions. Simultaneously, this thickness effectively avoids excessive accumulation of inactive carbon phases, reducing dilution of active materials and ensuring the energy density of the composite material.

[0038] Mesoporous carbon layers possess mesopores. For example, the pore size is 3 nm to 5 nm. Specifically, the pore size can be 3 nm, 4 nm, 5 nm, or any value between these values. This pore size is a core prerequisite for the mesoporous carbon layer to perform its structural support, ion transport, and performance optimization functions, directly determining the pore characteristics and electrochemical performance of the composite material. Its specific functions are as follows: 1. Constructing ordered three-dimensional interconnected channels, significantly optimizing ion transport kinetics: A pore size of 3-5 nm falls within the typical ordered mesoporous range, highly consistent with the H1-type hysteresis loop characteristic observed in nitrogen adsorption-desorption tests, ensuring the formation of a highly ordered, uniformly sized, three-dimensional interconnected pore network structure in the mesoporous carbon layer. This pore system enables rapid and uniform wetting of the electrolyte, facilitating lithium-ion (Li...)... + It provides an unobstructed and efficient transmission channel, effectively shortening the Li + 1. Diffusion pathways reduce interfacial transport impedance and concentration polarization, significantly improving the rate performance and electrochemical reaction kinetics efficiency of the composite material, providing a guarantee for high-rate energy storage applications; 2. Enhanced spatial confinement effect, synergistically suppressing structural degradation and active ion dissolution: The 3-5 nm mesoporous pore size allows for precise spatial confinement control of the internal LiFePO4@C composite particles, effectively suppressing the aggregation and pulverization of active particles during charge-discharge cycles, and providing sufficient buffer space for the 3%-5% volume change generated during LiFePO4 lithium intercalation / deintercalation, alleviating internal stress concentration, and maintaining the integrity of the interfacial bonding between the particles and the carbon layer. Simultaneously, the mesoporous carbon layer, relying on its high specific surface area, can capture a small amount of dissolved Fe³⁺ through physical adsorption. + Active ions further block ion dissolution pathways, inhibit interfacial side reactions, delay long-term degradation of electrode structures, and improve material cycle stability; 3. Synergistic optimization of specific surface area and pore volume to achieve a precise balance between structural stability and electrochemical performance: that is, a mesoporous pore size of 3~5nm can enable composite materials to achieve the optimal synergistic effect of high specific surface area and suitable pore volume: high specific surface area can ensure that electrochemical reaction active sites are fully exposed, strengthen the interfacial contact between electrolyte and active material, and improve interfacial reaction efficiency; the design of suitable pore volume avoids the problem of decreased mechanical strength of mesoporous carbon skeleton and easy structural collapse caused by excessively large pore size, and also prevents the problem of Li caused by excessively small pore size. + This pore size range represents the optimal choice that balances material adsorption capacity, mass transfer efficiency, and structural stability, providing solid structural support for the long-term stable operation of composite materials. It also addresses drawbacks such as impaired transport and increased polarization.

[0039] During lithium-ion insertion / extraction, LiFePO4 undergoes an isotropic volume change of 3% to 5%. Long-term cycling leads to accumulated volume stress, which can cause microcracks in the particles and interfacial debonding, thus compromising the integrity of the electrode structure. Amorphous carbon layers and mesoporous carbon layers can synergistically buffer the volume effect of the LiFePO4 core. Specifically: 1) Amorphous carbon, with its low modulus, high ductility, and lack of a fixed crystal form, can tightly coat the surface of LiFePO4 particles to form an ultrathin flexible layer. When LiFePO4 expands in volume, the amorphous carbon layer absorbs most of the mechanical stress generated by the volume change through its own stretching and deformation, preventing stress from directly acting on the interior of the LiFePO4 particles and suppressing the generation of microcracks from the source. When the volume shrinks, the amorphous carbon layer can elastically rebound to tightly adhere to the LiFePO4 surface, maintaining the interfacial bonding between the LiFePO4 particles and the amorphous carbon layer, eliminating stress concentration caused by interfacial gaps. 2) The mesoporous carbon layer (3D-OMC) serves as a rigid three-dimensional framework with high mechanical strength and a continuous, ordered pore structure. It covers the outside of the amorphous carbon layer. On the one hand, its rigid framework provides reverse support for the composite material, restricts the unconstrained expansion of LiFePO4 particles, disperses local stress, and prevents excessive stress concentration. On the other hand, the ordered mesoporous structure of 3D-OMC provides space to accommodate volume changes, avoids the overall structure from being squeezed and collapsed due to volume expansion, and maintains the integrity of the porous structure of the electrode.

[0040] In addition, Fe³⁺ in LiFePO₄ + Li + Plasma easily detaches from the electrode structure and enters the electrolyte. Amorphous carbon layers and mesoporous carbon layers can synergistically inhibit the ion dissolution of LiFePO4. Specifically: 1) The amorphous carbon layer promotes the formation of a stable, thin, and dense interfacial passivation layer, preventing the continuous growth and rupture of the SEI film, reducing ion dissolution caused by interfacial damage, and lowering the probability of ion dissolution from the source. 2) The three-dimensional ordered mesoporous structure of the mesoporous carbon layer (3D-OMC) creates a nano-confinement effect on the LiFePO4 particles coated with amorphous carbon. Even if the LiFePO4 particles develop tiny cracks due to volume changes, their fragments are confined within the mesoporous channels and cannot detach into the electrolyte. Furthermore, 3D-OMC has a high specific surface area and abundant pore structure, which can fix a small amount of dissolved Fe³⁺ through physical adsorption. + Li + Plasma prevents free migration and consumption of active ions in the electrolyte, further suppressing the loss of active ions. The dense isolation of amorphous carbon reduces ion dissolution at the source, while the confined adsorption of 3D-OMC intercepts potential dissolved ions. Together, they form a dual protection system of "source blocking + end interception," significantly reducing the loss of active substances by ions and ensuring the stability of the electrochemical activity of LiFePO4.

[0041] Furthermore, the ultimate failure modes of cathode materials during long-term cycling include LiFePO4 particle pulverization, carbon layer peeling, overall electrode collapse, and blockage of ion transport channels. The amorphous carbon layer and mesoporous carbon layer in the cathode active material provided in this application can synergistically suppress electrode structural degradation. Specifically: 1) The amorphous carbon layer tightly binds LiFePO4 particles to the mesoporous carbon layer (3D-OMC) framework, forming a high-strength, high-stability porous conductive network. This conductive network can effectively resist mechanical stress during cycling, preventing LiFePO4 particle detachment and carbon layer peeling, maintaining the integrity of the overall electrode structure, and fundamentally suppressing structural collapse. 2) The synergistic coating of amorphous carbon and mesoporous carbon layer (3D-OMC) forms a complete protective layer, which significantly reduces the penetration rate of electrolyte into the electrode interior, inhibits the corrosion and damage of LiFePO4 particles and carbon skeleton by electrolyte, and at the same time, the ordered mesoporous structure ensures uniform distribution of electrolyte and rapid ion transport, reduces local polarization and side reactions, avoids structural damage caused by local overheating and stress concentration, and improves the long-term stability of interface bonding.

[0042] This application provides a lithium iron phosphate composite material with a double-layer carbon-coated lithium iron phosphate core. The inner amorphous carbon layer constructs continuous electron channels and is covalently bonded to the lithium iron phosphate core to ensure high specific capacity and efficient operation of the composite material. The outer mesoporous carbon layer provides ion transport channels without generating ineffective voids, achieving high compaction density in the lithium iron phosphate composite material. The amorphous and mesoporous carbon layers synergistically isolate the electrolyte and buffer the volume effect of LiFePO4 particles, suppressing LiFePO4 ion dissolution and electrode structure degradation, thus meeting the long-cycle requirements of energy storage devices. This application achieves synergistic optimization of high specific capacity, high compaction density, and long cycle life by precisely controlling the composition and morphology of the double-layer carbon coating structure. The high specific capacity significantly improves the energy density of a single cell, the high compaction density further optimizes the volumetric energy density of the battery, and the long cycle life meets the requirements for long-term stable operation of energy storage batteries. This effectively solves the technical bottleneck of existing cathode materials where capacity and other key performance aspects are difficult to balance, providing new core material support for the development of high-energy-density and high-reliability energy storage batteries.

[0043] This application also provides a method for preparing the above-mentioned lithium iron phosphate composite material, comprising: S10. Prepare a mixed raw material, wherein the mixed raw material includes a lithium iron phosphate precursor, sucrose, an amphiphilic block copolymer, and a polar solvent, wherein the mass ratio of sucrose to amphiphilic block copolymer in the mixed raw material is 1.5:1 to 5:2, and the polar solvent includes a first strong polar solvent and a second strong polar solvent, wherein the first strong polar solvent is water, the dielectric constant of the second strong polar solvent is ε, the dipole moment of the second strong polar solvent is μ, ε > 15 F / m, and μ > 2.5D. Here, "F / m" is the unit of dielectric constant ε, representing farads per meter. "D" is the unit of dipole moment μ, representing Debye.

[0044] In this embodiment, iron source, phosphorus source, and lithium source are weighed according to the stoichiometric ratio of lithium iron phosphate (LiFePO4) and mixed to obtain a lithium iron phosphate precursor (hereinafter referred to as LFP precursor). For example, the iron source and phosphorus source are iron phosphate, and the lithium source is lithium carbonate.

[0045] For example, the mass ratio of sucrose to the amphiphilic block copolymer is 1.5:1, 2:1, 5:2, or any value between these ratios. The amphiphilic block copolymer includes hydrophilic and hydrophobic segments. In this embodiment, the amphiphilic block copolymer is an amphiphilic nonionic block copolymer. For example, the amphiphilic nonionic block copolymer is a PEOn-PPOm-PEOn (polyoxyethylene-polyoxypropylene-polyoxyethylene) triblock copolymer, where n ≥ 80 and m ≥ 60. Specifically, in this embodiment, the amphiphilic block copolymer is PEO. 100 -PPO 65 -PEO 100 (Pluronic F127, F127). The reasons for choosing the PEOn-PPOm-PEOn triblock copolymer are: 1) The PEO segments provide sufficient hydration and steric hindrance, resulting in stable micelles that do not aggregate and guide uniform sucrose deposition; 2) The PPO hydrophobic core is sufficiently large to form stable spherical micelles with high backbone strength and no collapse. When n < 80, the PEO segments are too short, resulting in insufficient hydration capacity and weak steric hindrance on the micelle surface, making micelle aggregation and fusion likely, preventing the formation of stable spherical micelles. Furthermore, the excessively short PEO segments cannot provide a sufficient solvation layer, leading to phase separation and complete disruption of the self-assembly process. When m < 60, the hydrophobic core is too small, resulting in insufficient micelle core strength, making it prone to disintegration in the solvent or excessive swelling by the PEO segments, failing to form stable spherical micelles. Ultimately, after carbonization, it cannot form mesopores of sufficient size, failing to meet the requirements for rapid lithium-ion transport. Only when the above conditions are met simultaneously can a regular three-dimensional cubic micelle array be formed by self-assembly in a water-ethanol mixed solvent, and finally carbonized to obtain a three-dimensional ordered mesoporous carbon structure with stable structure, interconnected channels, and uniform pore size distribution.

[0046] In this embodiment, sucrose and PEOn-PPOm-PEOn triblock copolymer are used together as carbon sources, which has the following advantages: 1. As a single hydrophilic carbon source, sucrose can be completely dissolved in polar solvents into molecular form, enabling gradient directional adsorption of the carbon source. Sucrose preferentially adsorbs onto the surface of the LFP precursor, while simultaneously filling the micelle gaps of the PEOn-PPOm-PEOn triblock copolymer, providing a basis for precise carbon source distribution for the bilayer carbon structure. 2. The sucrose adsorbed on the surface of the LFP precursor is carbonized at high temperature during the subsequent sintering process to form an inner disordered amorphous carbon layer, forming COM covalent bonds with the LFP precursor to construct continuous electron transport channels, which is beneficial for improving the bulk conductivity of the lithium iron phosphate composite material and strengthening interfacial bonding. 3. Sucrose filling the intercellular spaces of the PEOn-PPOm-PEOn triblock copolymer micelles can undergo ordered carbonization simultaneously with the directional thermal desorption of the PEOn-PPOm-PEOn triblock copolymer micelles, replicating the ordered micelle structure to form an outer mesoporous carbon layer (3D-OMC). This provides a rapid transport channel for lithium ions and achieves integrated connection with the inner disordered amorphous carbon layer through CC covalent bonds. 4. Sucrose and the PEOn-PPOm-PEOn triblock copolymer synergistically adapt to a one-step process, eliminating the need for an additional carbon source. This allows for the simultaneous formation of both the amorphous and mesoporous carbon layers, simplifying the process and ensuring its operability and repeatability. 5. The bilayer carbon structure formed by sucrose carbonization, consisting of the amorphous and mesoporous carbon layers, can buffer the volume expansion of the cathode material and inhibit ion dissolution, synergistically improving the rate performance, cycle stability, and energy density of the cathode material, thus meeting the needs of industrial applications. 6. The high hydroxyl content of sucrose allows it to form a multi-hydrogen bond network with hydroxypentanoic acid, PEOn-PPOm-PEOn triblock copolymer and LFP precursor, thereby enhancing the interface modification effect and micellar structure stability and avoiding carbon layer defects.

[0047] The reasons why amphiphilic block copolymers can form mesoporous carbon layers in subsequent sintering processes are mainly as follows: 1. The hydrophilic segments in the amphiphilic block copolymers are highly soluble in polar solvents, forming a strong solvation reaction with the polar solvent molecules, thus reducing part of the system's energy. 2. The hydrophobic segments in the amphiphilic block copolymers have poor compatibility with polar solvents; if dispersed in polar solvents, they would increase the interfacial energy of the system, becoming a thermodynamically unstable factor. 3. To stabilize, the system spontaneously drives the aggregation of hydrophobic segments to reduce the contact area with the polar solvent, while the hydrophilic segments spread out and wrap around the hydrophobic aggregates, forming "hydrophilic shell-hydrophobic core" micelles. 4. The micellar structure minimizes the total energy of the system, achieving stable existence and providing a template for the preparation of ordered mesoporous carbon layers.

[0048] The main reasons why amphiphilic block copolymers cannot form mesoporous carbon layers in nonpolar solvents during subsequent sintering are as follows: 1. In nonpolar solvents, the hydrophobic segments of the amphiphilic block copolymer are highly compatible with the nonpolar solvent through weak dispersion forces. The molecular chains are fully extended and uniformly dispersed, and there is no aggregation driving force to "reduce the contact area with the solvent." The hydrophobic segments have no aggregation motive and lose the basis for the formation of micelle "cores." 2. The hydrophilic segments of the amphiphilic block copolymer have a large difference in polarity with the nonpolar solvent and are completely incompatible. They can only reduce the interfacial energy through intramolecular curl-up and self-entanglement, and cannot extend to form a continuous micelle shell as in polar solvents, thus failing to construct a stable micelle "shell" layer. 3. The combination of hydrophobic dissolution and hydrophilic curl-up has already reduced the interfacial energy of the system to a low level, and the thermodynamically stable state does not require a core-shell structure.

[0049] The selection logic for polar solvents in the mixed raw materials is shown in Table 1.

[0050] Table 1. Selection Logic of Polar Solvents

[0051] In this embodiment, the first strongly polar solvent is water, and the second strongly polar solvent is anhydrous ethanol, which has the following advantages: 1. Water has strong polarity, which can ensure the full solubilization of the PEO segment in the PEOn-PPOm-PEOn triblock copolymer, thereby driving the self-assembly of the PEOn-PPOm-PEOn triblock copolymer micelles to form a long-range ordered structure, realizing the gradient dispersion at the sucrose molecular level, while ensuring the nanoscale dispersion of LFP precursor particles and avoiding hard agglomeration. 2. Ethanol has a slightly weaker isopolarity than water, which can moderately control the size of the PEOn-PPOm-PEOn triblock copolymer micelles, avoiding insufficient mesoporous pore size (<10nm) in the subsequently formed mesoporous carbon layer due to excessively small micelle size, while reducing the viscosity of the mixed raw materials, improving the diffusion uniformity of sucrose molecules, and avoiding local aggregation.

[0052] The volume ratio of the first strongly polar solvent to the second strongly polar solvent is 4:1 to 8:1. Specifically, the volume ratio of the first strongly polar solvent to the second strongly polar solvent is 4:1, 5:1, 6:1, 7:1, 8:1, or any value between these ratios. For example, the volume ratio of deionized water to anhydrous ethanol is 4:1 to 8:1. The main functions of this volume ratio range are as follows: 1. At this ratio, deionized water fully dissolves hydrophilic sucrose, providing a homogeneous carbon precursor for carbonization; anhydrous ethanol efficiently dissolves F127 and forms a stable mixed system with water, avoiding component aggregation and phase separation. This ratio balances the solubility of both components, ensuring complete dissolution of sucrose and stable dispersion of F127, laying the foundation for uniform mixing of the precursors. Secondly, solvent polarity is a core influencing factor for F127 self-assembly. The polarity of the mixed solvent in this ratio matches the amphiphilic characteristics of F127, promoting hydrogen bonding between its PEO segment and water, and self-polymerization of its PPO segment, resulting in a uniform and ordered micelle template. This template guides sucrose to fill the micelle gaps, forming a uniform, three-dimensionally interconnected 3D-OMC mesoporous structure after carbonization, meeting the design requirements of composite materials. In this embodiment, deionized water and anhydrous ethanol were measured and mixed according to a volume ratio of 4:1 to 8:1 to obtain the polar solvent. The preferred volume ratio of deionized water to anhydrous ethanol is 6:1. At this ratio, the volume fraction of ethanol in the aqueous ethanol solution is 16%, which can achieve the optimal balance between the micelle order of the PEOn-PPOm-PEOn triblock copolymer and the mesoporous size of the mesoporous carbon layer. The mesoporous pore size of the mesoporous carbon layer can be precisely controlled within 3nm~5nm, which is suitable for the rapid lithium-ion transport requirements.

[0053] In this embodiment, the mixed raw materials also include hydroxypentanoic acid. Hydroxypentanoic acid, as a co-solvent, has the following advantages: 1. As a specific and highly efficient co-solvent for PEOn-PPOm-PEOn triblock copolymers, hydroxypentanoic acid can improve the solubility and dispersion stability of PEOn-PPOm-PEOn triblock copolymers in polar solvents, preventing the formation of random aggregates due to localized concentration enrichment of PEOn-PPOm-PEOn triblock copolymers. 2. Hydroxypentanoic acid can selectively chelate Fe in lithium iron phosphate precursors. 3+ This inhibits the aggregation of lithium iron phosphate precursors, improving the dispersibility and confinement effect of lithium iron phosphate. Hydroxypentanoic acid only reacts with Fe... 3+ Forms stable complexes, does not react with Li + PO4 3-Coordination occurs, preventing lithium loss at its source and ensuring the accuracy of the Li:Fe:P molar ratio in the lithium iron phosphate precursor. 3. Hydroxypentanoic acid assists in the self-assembly of ordered micelles in the PEOn-PPOm-PEOn triblock copolymer, enhancing the dual confinement effect of sucrose and the lithium iron phosphate precursor. This facilitates the formation of a continuous and complete coating layer on the lithium iron phosphate particles by mesoporous carbon during subsequent high-temperature sintering. 4. It optimizes the interfacial compatibility of the system, reduces the interfacial tension between components in the mixed raw materials, and promotes the self-assembly process of the PEOn-PPOm-PEOn triblock copolymer. The polar functional groups (hydroxyl + carboxyl) of hydroxypentanoic acid can adsorb at the interface between the PEOn-PPOm-PEOn triblock copolymer micelles / lithium iron phosphate precursor / sucrose molecules and the polar solvent, effectively reducing the interfacial tension between each solid / polymer component and the polar solvent, ensuring that the final PEOn-PPOm-PEOn triblock copolymer micelle structure has high order and uniformity.

[0054] S20. Take the mixed raw materials and sinter them to prepare lithium iron phosphate composite material.

[0055] In this embodiment, the step of sintering the mixed raw materials includes: placing the mixed raw materials in an inert atmosphere for a first sintering step and a second sintering step. The temperature of the first sintering step is 280℃~320℃, and the temperature of the second sintering step is 650℃~700℃. The first sintering step decomposes the amphiphilic block copolymer and initially carbonizes the carbon source, while the second sintering step enables the crystallization and carbothermic reduction of LiFePO4, resulting in a lithium iron phosphate composite material (LiFePO4@C@3D-OMC).

[0056] For example, the temperature of the first sintering step is 280℃, 300℃, 320℃, or any value between these values. For example, the temperature of the second sintering step is 650℃, 670℃, 690℃, 700℃, or any value between these values. In this embodiment, step S20 specifically involves: first heating the temperature to 280℃~320℃ at a first heating rate, holding at this temperature for the first sintering step, then heating the temperature to 650℃~700℃ at a second heating rate, holding at this temperature for the second sintering step, wherein the second heating rate is greater than the first heating rate. For example, the first heating rate is 1℃ / min~3℃ / min, and the second heating rate is 4℃ / min~6℃ / min.

[0057] In the preparation method of the lithium iron phosphate composite material provided in this application embodiment, sucrose and an amphiphilic block copolymer are used as carbon sources. The amphiphilic block copolymer includes hydrophilic and hydrophobic segments. The hydrophilic segments are highly soluble in polar solvents, while the hydrophobic segments have poor compatibility with polar solvents, spontaneously driving the aggregation of hydrophobic segments to reduce the contact area with polar solvents. At the same time, the hydrophilic segments spread out and wrap around the outside of the hydrophobic segment aggregates, thereby forming micelles with a "hydrophilic shell and hydrophobic core". Sucrose, as a single hydrophilic carbon source, can be completely dissolved in polar solvents into a molecular state and preferentially adsorbed on the surface of the lithium iron phosphate precursor, while filling the gaps in the micelles formed by the amphiphilic block copolymer. The lithium iron phosphate precursor is sintered to form a lithium iron phosphate core, the sucrose adsorbed on the surface of the lithium iron phosphate precursor is sintered to form an amorphous carbon layer, and the sucrose filling the micelle gaps of the amphiphilic block copolymer is sintered to form a mesoporous carbon layer, thereby preparing a lithium iron phosphate composite material with a double carbon coating layer.

[0058] Furthermore, in this embodiment, water is selected as the first strongly polar solvent. Water is an excellent solvent for sucrose (a polyhydroxy hydrophilic carbon source), ensuring complete dissolution and uniform dispersion of the carbon source. Simultaneously, water can form hydrogen bonds with the hydrophilic segments of the amphiphilic block copolymer, driving the self-assembly behavior of the amphiphilic block copolymer template and ensuring the subsequent formation of an ordered mesoporous structure. The second strongly polar solvent used, such as ethanol, can effectively dissolve the amphiphilic block copolymer and is infinitely miscible with water, thus adjusting the solvent polarity. The use of the second strongly polar solvent, such as ethanol, can also reduce the surface tension of the polar solvent, improving the wetting ability of lithium iron phosphate particles and carbon precursors, and preventing the agglomeration of active materials.

[0059] The following describes in detail the performance of the lithium iron phosphate composite material and the energy storage device made using the lithium iron phosphate composite material provided in the embodiments of this application, with reference to the examples.

[0060] Example 1: Example 1 provides a lithium iron phosphate composite material, prepared according to the following steps: (1) Weigh iron phosphate and lithium carbonate according to the chemical molar ratio of lithium iron phosphate (LiFePO4), premix them in a ball mill, add deionized water as a solvent, transfer to a high-speed ball mill for fine grinding for 2h~4h, and then dry at 80℃ for 10h to obtain lithium iron phosphate (LFP) precursor powder.

[0061] (2) Deionized water and anhydrous ethanol are measured at a volume ratio of 6:1. The solvents are mixed and stirred for 10 minutes to obtain the desired polar solvent.

[0062] (3) Weigh out lithium iron phosphate precursor powder, hydrophilic carbon source sucrose, block copolymer F127, and hydroxypentanoic acid in a mass ratio of 100:2:1:0.1. Stir F127 in a polar solvent for 5 min, then add hydroxypentanoic acid and stir for 5 min, then add sucrose and stir continuously at 40℃ for 30 min to form a transparent mixed solution; finally add lithium iron phosphate precursor powder and stir at high speed at 60℃ for 6 h to obtain a mixed raw material. Transfer the mixed raw material to an ultrasonic dispersion instrument and ultrasonically disperse it for 30 min at 400W power under a water bath protection temperature of 45℃. The ultrasonic mode is intermittent ultrasonic (5s working / 3s pause).

[0063] (4) Place the ultrasonically mixed raw material in a glass petri dish, and then dry it in a vacuum oven at a temperature of 60°C and a vacuum degree of ≤-0.09MPa for 16 hours. Stir it slightly every 4 hours during the period to avoid surface crusting, and obtain the required dry mixed raw material powder.

[0064] (5) The mixed raw material powder obtained in step (4) is heated to 300°C at 2°C / min and held for 2 hours in a tube furnace under N2 atmosphere protection for the first sintering. Then, the temperature is increased to 680°C at 5°C / min and held for 8 hours for the second sintering to obtain composite lithium iron phosphate (LiFePO4@C@3D-OMC) cathode material.

[0065] Example 2: Example 2 provides a lithium iron phosphate composite material, prepared according to steps (1) to (5) in Example 1, except that only the mass ratio of hydrophilic carbon source sucrose in the mixed raw materials in step (3) is adjusted. After adjustment, lithium iron phosphate precursor powder, hydrophilic carbon source sucrose, block copolymer F127, and hydroxypentanoic acid are weighed in sequence according to a mass ratio of 100:1.5:1:0.1.

[0066] Example 3: Example 3 provides a lithium iron phosphate composite material, which is prepared according to steps (1) to (5) in Example 1. The difference is that only the mass ratio of hydrophilic carbon source sucrose in the mixed raw materials in step (3) is adjusted. After adjustment, lithium iron phosphate precursor powder, hydrophilic carbon source sucrose, block copolymer F127 and hydroxypentanoic acid are weighed in sequence according to a mass ratio of 100:5:2:0.1.

[0067] Example 4: Example 4 provides a lithium iron phosphate composite material, prepared according to steps (1) to (5) in Example 1, except that the volume ratio of deionized water to anhydrous ethanol in step (2) is adjusted to 4:1.

[0068] Example 5: Example 5 provides a lithium iron phosphate composite material, prepared according to steps (1) to (5) in Example 1, except that the volume ratio of deionized water to anhydrous ethanol in step (2) is adjusted to 8:1.

[0069] Comparative Example 1: Comparative Example 1 provides a lithium iron phosphate composite material, prepared according to the following steps: (1) Weigh iron phosphate and lithium carbonate according to the chemical molar ratio of lithium iron phosphate (LiFePO4), premix them in a ball mill, add deionized water as a solvent, transfer to a high-speed ball mill for fine grinding for 2h~4h, and then dry at 80℃ for 10h to obtain lithium iron phosphate (LFP) precursor powder.

[0070] (2) Deionized water and anhydrous ethanol were weighed at a volume ratio of 6:1, the solvents were mixed and stirred for 10 min to obtain the desired polar solvent. Lithium iron phosphate precursor powder and hydrophilic carbon source sucrose were weighed at a mass ratio of 100:2, and stirred in the deionized water-anhydrous ethanol mixed solvent for 2 h to obtain a transparent solution. The transparent solution was transferred to a glass dish and dried in an 80℃ drying oven for 12 h to obtain powder.

[0071] (3) The powder obtained in step (2) is heated to 680°C at 5°C / min in a tube furnace for 8 hours under N2 atmosphere to obtain LiFePO4 / C cathode material.

[0072] (4) Weigh a certain amount of template agent block copolymer F127, dissolve it in 2 mol / L hydrochloric acid solution at room temperature, then slowly add tetraethyl orthosilicate (TEOS) and deionized water, continue stirring for 12 h, transfer this solution to a reactor, hydrothermally age it at 120℃ for 24 h, wash it with water, filter it, dry it, and then carbonize it under N2 protection. The temperature is raised from room temperature to 550℃ and maintained for 5 h. After calcination, the temperature is naturally cooled to room temperature to obtain mesoporous SiO2 hard template.

[0073] (5) Dissolve sucrose in water, weigh a certain amount of the SiO2 hard template obtained in step (4), stir at 80℃ for 12h to obtain a suspension, dry the suspension in a drying oven at 120℃ for 12h, and then carbonize it at 800℃ under N2 protection. After carbonization is completed, grind it, remove the template with 5% HF solution, wash it with distilled water, dry it at 120℃, and finally obtain three-dimensional ordered mesoporous carbon (3D-OMC).

[0074] (6) Weigh the LiFePO4 / C cathode material from step (3) and the three-dimensional ordered mesoporous carbon (3D-OMC) from step (5) at a mass ratio of 100:1. Add the two materials to deionized water and stir thoroughly. Then transfer the mixed solution to an ultrasonic dispersion instrument and ultrasonically disperse it for 30 minutes at a power of 400W under a water bath protection temperature of 45℃. The ultrasonic mode is intermittent ultrasonic (working for 5 seconds and pausing for 3 seconds). After drying in water, the lithium iron phosphate composite material with three-dimensional ordered mesoporous carbon coated with LiFePO4 / C is obtained.

[0075] Comparative Example 2: Comparative Example 2 provides a lithium iron phosphate composite material, which is prepared according to steps (1) to (5) in Example 1. The difference is that only the mass ratio of hydrophilic carbon source sucrose in the mixed raw materials in step (3) is adjusted. After adjustment, lithium iron phosphate precursor powder, hydrophilic carbon source sucrose, block copolymer F127 and hydroxypentanoic acid are weighed in sequence according to a mass ratio of 100:5:1:0.1.

[0076] Comparative Example 3: Comparative Example 3 provides a lithium iron phosphate composite material, prepared according to steps (1) to (5) in Example 1, except that only the mass ratio of hydrophilic carbon source sucrose in the mixed raw materials in step (3) was adjusted. After adjustment, lithium iron phosphate precursor powder, hydrophilic carbon source sucrose, block copolymer F127, and hydroxypentanoic acid were weighed in sequence according to a mass ratio of 100:1:1:0.1.

[0077] Comparative Example 4: Comparative Example 4 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) in Example 1, except that the mixed polar solvent made of deionized water and anhydrous ethanol in step (2) is replaced with pure deionized water.

[0078] Comparative Example 5: Comparative Example 5 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) in Example 1, except that the mixed polar solvent made of deionized water and anhydrous ethanol in step (2) is replaced with pure anhydrous ethanol.

[0079] Comparative Example 6: Comparative Example 6 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) of Example 1, except that the mixed polar solvent made of deionized water and anhydrous ethanol in step (2) is replaced with methanol.

[0080] Comparative Example 7: Comparative Example 7 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) of Example 1, except that the mixed polar solvent made of deionized water and anhydrous ethanol in step (2) is replaced with pure anhydrous acetone.

[0081] Comparative Example 8: Comparative Example 8 provides a lithium iron phosphate composite material, prepared according to steps (1) to (5) in Example 1, except that in step (2) the deionized water and anhydrous ethanol are in a volume ratio of 1:1.

[0082] Comparative Example 9: Comparative Example 9 provides a lithium iron phosphate composite material, prepared according to steps (1) to (5) in Example 1, except that in step (2) the deionized water-anhydrous ethanol volume ratio is 10:1.

[0083] Comparative Example 10: Comparative Example 10 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) of Example 1, except that hydroxypentanoic acid is not added in step (5).

[0084] Comparative Example 11: Comparative Example 11 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) of Example 1, except that the block copolymer F127 in step (3) is replaced with P123 (PEO). 20 -PPO 70 -PEO 20 ).

[0085] Comparative Example 12: Comparative Example 12 provides a lithium iron phosphate composite material prepared according to steps (1) to (5) of Example 1, except that the block copolymer F127 in step (3) is replaced with F68 (PEO). 80 -PPO 30 -PEO 80 ).

[0086] See Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 6 , Figure 2 The image shown is an HRTEM image of the lithium iron phosphate composite material from Example 1. Figure 3 The image shows the HRTEM image of the lithium iron phosphate composite material in Comparative Example 2. Figure 4 Here is an HRTEM image of the lithium iron phosphate composite material of Comparative Example 3. Figure 5 Here is an HRTEM image of the lithium iron phosphate composite material in Comparative Example 4. Figure 6The image shown is an HRTEM image of the lithium iron phosphate composite material in Comparative Example 7. The carbon layer thickness was measured using a multi-point statistical method: a continuous carbon layer region was selected along the surface of the LiFePO4 grains, and five measurement points were randomly selected. The carbon layer thickness values ​​at each point were recorded, and the minimum and maximum values ​​were used as the distribution range of the carbon layer thickness for that sample. Furthermore, in the HRTEM image, if the carbon structure exhibits a regularly arranged array of channels and long-range ordered stripes, it indicates high mesoporous order. If the channels are disordered, non-periodic, or locally collapsed, it indicates low mesoporous order. If no periodic mesoporous channel array, no ordered channel stripes, and no honeycomb structure are observed, it indicates a non-mesoporous structure. Further, HRTEM was used to observe the continuity of the carbon layer. Multiple areas were randomly selected on the particle surface for observation. If the carbon layer was uniformly coated, without breaks or exposed matrix, it was considered to have high carbon layer continuity. If the carbon layer was discontinuous, locally missing, or exposed matrix, it was considered to have low or discontinuous carbon layer continuity.

[0087] The carbon layer thickness and mesopore size in the lithium iron phosphate composite materials of Examples 1-5 and Comparative Examples 1-12 are summarized in Table 2.

[0088] Table 2. Carbon layer thickness and mesopore size in lithium iron phosphate composite materials of Examples 1-5 and Comparative Examples 1-12.

[0089] from Figure 2 As can be seen from the HRTEM (High Resolution Transmission Electron Microscopy) image of the lithium iron phosphate composite material in Example 1, the surface of the LiFePO4 core particles is coated with an amorphous carbon layer with a thickness of 2 nm to 2.5 nm. This amorphous carbon layer is continuous and uniform, closely adheres to the lithium iron phosphate core, and has a clear interface. Further, a three-dimensionally ordered mesoporous carbon (3D-OMC) layer with a thickness of 4.5 nm to 5.2 nm is grown in situ on the amorphous carbon layer. The mesoporous structure in the mesoporous carbon layer is complete, with interconnected pores, and seamlessly connected with the inner amorphous carbon layer, together forming an integrated "core-shell-network" composite structure.

[0090] from Figure 3As can be seen from the HRTEM image of the lithium iron phosphate composite material in Comparative Example 2, the carbon coating layer on the LiFePO4 core is excessively thick and dense. The mesoporous channels of the carbon coating layer are blocked by amorphous carbon, resulting in no obvious ordered channel structure. This makes the carbon coating layer appear as amorphous carbon as a whole. The main reason is that the sucrose carbon source is excessive, exceeding the self-assembly capacity of the template agent block copolymer F127. The excess sucrose carbon source cannot enter the ordered structure of the F127 micelle template and can only be adsorbed and accumulated on the particle surface and inside the channels in a free state. During the high-temperature carbonization process, the free sucrose that does not participate in the template self-assembly is directly converted into dense and disordered amorphous carbon. These amorphous carbons fill and cover the entrance and interior of the mesoporous channels in large quantities, causing the originally ordered channels to be blocked and collapsed.

[0091] from Figure 4 As can be seen from the HRTEM image of the lithium iron phosphate composite material in Comparative Example 3, the surface of the LiFePO4 particles shows almost no uniform carbon coating. The amorphous carbon layer thickens and densifies due to the excessively high polarity of the deionized water system, resulting in a decrease in carbon layer uniformity. Although the mesoporous carbon layer retains some framework structure, the mesoporous order is significantly reduced, and the regularity and connectivity of the pores are inferior to those in Example 1. The main reasons are: the polarity of the pure deionized water system increases significantly, leading to abnormal aggregation and disordered arrangement of F127 micelles, making it impossible to construct a regular mesoporous template; at the same time, the high polarity environment causes local aggregation and excessive deposition of the sucrose carbon source, resulting in the densification and thickening of the amorphous carbon layer, partial blockage of the mesoporous channels, and shrinkage in size, ultimately destroying the synergistic structure of the inner amorphous carbon and the outer three-dimensional ordered mesoporous carbon, making it impossible to obtain the uniform, regular, and highly interconnected double-layer carbon coating structure of the example.

[0092] from Figure 5 As can be seen from the HRTEM image of the lithium iron phosphate composite material in Comparative Example 4, the carbon layer on the surface of the LiFePO4 particles is discontinuous and unevenly coated. The thickness of the inner disordered carbon layer is about 3.5~4.5 nm, and the outer mesoporous carbon structure is incomplete and irregular, with an overall thickness of about 5.8~6.8 nm. The mesoporous order is significantly reduced, and the regularity and connectivity of the pores are inferior to those in Example 1. The main reason is that the polarity of the pure deionized water system is greatly increased, which leads to the over-hydration and stretching of the PEO segments of the block copolymer F127, resulting in abnormal micelle aggregation and disordered arrangement, making it impossible to construct a regular three-dimensional mesoporous template. At the same time, the high polarity environment causes local aggregation and excessive deposition of the sucrose carbon source, resulting in the densification of the inner disordered carbon layer, partial blockage of the outer mesoporous channels, and size shrinkage. Ultimately, this destroys the synergistic structure of the inner amorphous carbon and the outer three-dimensional ordered mesoporous carbon, making it impossible to obtain the uniform, regular, and highly interconnected double-layer carbon coating structure of the example.

[0093] from Figure 6As can be seen from the HRTEM image of the lithium iron phosphate composite material in Comparative Example 7, only a thick and disordered amorphous carbon layer is formed on the surface of the LiFePO4 particles, without a uniform and continuous double-layer carbon coating structure; the carbon layer thickness reaches 6-8 nm, and the whole is dense, agglomerated, and irregular blocky in shape, completely lacking the three-dimensional ordered mesoporous carbon skeleton structure and pore characteristics, and the mesoporous order is completely lost. The main reason is that the polarity of the pure anhydrous acetone system is too low, which prevents the PEO segments of the block copolymer F127 from being fully hydrated and extended. The hydrophobic aggregation effect of the PPO segments is completely ineffective, and the micelle self-assembly process collapses completely, making it impossible to form any regular mesoporous template. At the same time, the sucrose carbon source has extremely poor solubility in acetone, resulting in severe local aggregation and disordered deposition. It can only form a thick and discontinuous amorphous carbon layer on the surface of LiFePO4, which completely destroys the synergistic construction of the inner amorphous carbon and the outer three-dimensional ordered mesoporous carbon. Ultimately, it is impossible to obtain the uniform, regular, and highly interconnected double-layer carbon coating structure in the examples.

[0094] See Figure 7 , Figure 7 The image shows the particle size distribution of the lithium iron phosphate composite materials of Example 1 and Comparative Example 1. Figure 7 The horizontal axis represents particle size in μm, and the vertical axis represents volume percentage in %. The particle size distribution is determined by using a laser diffraction particle size distribution measuring instrument (Malvern Mastersizer 3000) according to the particle size distribution laser diffraction method (GB / T19077 2016) to measure the particle size distribution of the material, thereby measuring the material's Dv10, Dv50, and Dv90.

[0095] from Figure 7 As can be seen from the particle size distribution diagram of Example 1 (dot curve), the particle size distribution is narrow and concentrated, with the main peak located in the 0.5-1μm range. There are almost no large particles larger than 5μm, indicating that the lithium iron phosphate composite material of Example 1 has small and uniform particle size and no obvious agglomeration.

[0096] In the particle size distribution diagram of Comparative Example 1 (triangular curve), the particle size distribution is wide and multi-peaked, with two main peaks at 0.5-1 μm and 3-5 μm. The proportion of large particles is significantly increased, indicating that the lithium iron phosphate composite material of Comparative Example 1 has severe particle agglomeration and poor size uniformity. The experimental results show that the two-step synthesis process of Comparative Example 1, which first prepares 3D-OMC as a support, makes it easy for the subsequently loaded LiFePO4 / C particles to agglomerate and grow on the surface of 3D-OMC, forming larger secondary particles, thus resulting in a wider particle size distribution of the prepared lithium iron phosphate composite material. In contrast, the embodiment of this application uses a one-step synthesis process, which allows LiFePO4 nucleation, carbon source coating and 3D-OMC network formation to occur simultaneously, effectively suppressing the agglomeration of the synthesized lithium iron phosphate composite material particles, resulting in more uniform and finer particle sizes.

[0097] See Figure 8 and Figure 9 , Figure 8 The nitrogen adsorption-desorption isotherm and pore size distribution curve of the lithium iron phosphate composite material in Example 1 are shown below. Figure 9 The nitrogen adsorption-desorption isotherms and pore size distribution curves of the lithium iron phosphate composite material in Comparative Example 1 are shown. The nitrogen adsorption-desorption isotherms were tested using a fully automated specific surface area and porosity analyzer (TriStarII 3020) and the Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption method.

[0098] from Figure 8 It can be seen that the isotherm of Example 1 shows a significant H1-type hysteresis loop in the medium-high pressure region (P / P0>0.4), with the largest adsorption volume and narrow pore size distribution, concentrated in 3.5-4.2 nm, indicating that the lithium iron phosphate composite material of Example 1 has a high specific surface area and an ordered interconnected 3D mesoporous structure.

[0099] from Figure 9It can be seen that the hysteresis loop in the isotherm of Comparative Example 1 is significantly weaker, the adsorption volume is significantly reduced, the pore size distribution is wider, and the proportion of mesopores is decreased. This indicates that some of the mesopore channels in the lithium iron phosphate composite material of Comparative Example 1 are blocked by LiFePO4 particles, resulting in a decrease in pore structure utilization. A possible reason is that Comparative Example 1 uses a method of first synthesizing 3D-OMC and then composited with LiFePO4 / C. The subsequently synthesized LiFePO4 / C particles enter and block the mesopore channels of 3D-OMC, thus damaging the pore structure in 3D-OMC and reducing the utilization rate of the mesopore channels. Furthermore, during the composite process of 3D-OMC and LiFePO4 / C, mechanical stirring and heat treatment also disrupt some of the ordered mesopore structure, leading to a wider pore size distribution and a lower proportion of mesopores in the prepared lithium iron phosphate composite material. This results in a significant reduction in the adsorption volume, a weakening of the hysteresis loop, and a comprehensive deterioration of the pore structure performance.

[0100] Comprehensive reference Figure 7-9 By comparing Example 1 and Comparative Example 1, compared to the method in Comparative Example 1 where LiFePO4 / C cathode material and mesoporous carbon were synthesized separately and then combined to form a lithium iron phosphate composite material with mesoporous carbon coating LiFePO4 / C, the present application embodiment, through a one-step sintering method of mixing LiFePO4 precursor with carbon source sucrose and block copolymer F127, produces composite lithium iron phosphate (LiFePO4@C@3D-OMC) cathode material with uniform and fine particle size. The amorphous carbon layer in the lithium iron phosphate composite material is continuous and uniform, and the mesoporous structure of the mesoporous carbon layer is complete. (See also...) Figure 10 and Figure 11 , Figure 10 The nitrogen adsorption-desorption isotherm and pore size distribution curve of the lithium iron phosphate composite material in Comparative Example 2 are shown below. Figure 11 The nitrogen adsorption-desorption isotherm and pore size distribution curve of the lithium iron phosphate composite material in Comparative Example 3 are shown.

[0101] from Figure 10 As can be seen, in the isotherm of Comparative Example 2, the adsorption volume of nitrogen by the lithium iron phosphate composite material decreased significantly, with almost no obvious hysteresis loop, the mesoporous peak almost disappeared, and there were no obvious ordered channels. This indicates that the mesoporous channels were completely blocked by amorphous carbon, and the material as a whole exhibited a dense and non-porous state. Excess sucrose could not participate in the construction of the ordered mesoporous structure and formed a large amount of dense amorphous carbon after high-temperature carbonization. Furthermore, this amorphous carbon filled and blocked the mesoporous channels, destroying the regular structure of the three-dimensional ordered mesoporous carbon.

[0102] from Figure 11It can be seen that in the isotherm of Comparative Example 3, the lithium iron phosphate composite material has the smallest adsorption volume for nitrogen, the hysteresis loop completely disappears, and there is no obvious mesoporous peak, indicating that the 3D-OMC framework collapses due to insufficient carbon source, and the pore structure is completely lost. The excessive amount of block copolymer F127 leads to a severe lack of sucrose carbon source. The sucrose carbon source is insufficient to support a stable 3D-OMC framework structure, causing the mesoporous channels of block copolymer F127 to collapse and break on a large scale due to the lack of carbon wall support during high-temperature carbonization. After sintering, the ordered mesoporous structure is completely lost, and the material as a whole presents a loose and non-porous state.

[0103] See also Figure 3-4 and Figure 10-11 By comparing Example 1 and Comparative Examples 2-3, excessive sucrose addition during the preparation process leads to blockage of the formed mesopores, while insufficient sucrose addition makes it difficult to support the pore walls of the formed mesopores. In this application, the embodiments achieved controllable synthesis of an ordered mesoporous structure by precisely controlling the mass ratio of sucrose to block copolymer F127. The sucrose carbon source and the F127 micelle template self-assemble through hydrogen bonding to form a stable ordered mesoporous precursor. Furthermore, after high-temperature carbonization, the F127 template is removed, precisely replicating a 3-5 nm ordered mesoporous network with interconnected channels and a complete structure. Extensive experimental research in this application has shown that by controlling the mass ratio of sucrose to amphiphilic block copolymer to 1.5:1~5:2, the mesoporous channels of the mesoporous carbon layer in the formed lithium iron phosphate composite material can be ensured to be orderly interconnected and not blocked, thereby guaranteeing the quality of the prepared lithium iron phosphate composite material and, consequently, the performance of the assembled lithium-ion battery.

[0104] See Figure 12 , Figure 12 The images show the electrochemical impedance spectroscopy of the lithium iron phosphate composite materials of Example 1 and Comparative Examples 1-3.

[0105] from Figure 12 It can be seen that in the electrochemical impedance spectroscopy (EIS Nyquist) spectrum of the lithium iron phosphate composite material in Example 1, the semicircle diameter is smallest in the high-frequency region, indicating the lowest charge transfer resistance (Rct), while the slope of the straight line is largest in the low-frequency region. This indicates that Li + The diffusion resistance is minimal and the diffusion efficiency is highest, thanks to the integrated "core-shell-network" structure of the lithium iron phosphate composite material in Example 1, which achieves optimal synergy between electron conduction and ion diffusion.

[0106] In the electrochemical impedance spectroscopy of the lithium iron phosphate composite material in Comparative Example 1, the increased semicircle diameter in the high-frequency region indicates higher charge transfer impedance, while the decreased slope of the straight line in the low-frequency region indicates lower Li +Increased diffusion resistance. This stems from the discontinuity of the carbon coating, poor interfacial bonding, and pore blockage resulting from the two-step method used in Comparative Example 1. In the electrochemical impedance spectroscopy of the lithium iron phosphate composite material in Comparative Example 2, the semicircle diameter in the high-frequency region significantly increases, the charge transfer impedance rises sharply, and the slope of the straight line in the low-frequency region further decreases. + The diffusion resistance increased significantly. This is due to the excessively thick and dense carbon coating and mesoporous channel blockage caused by the excess sucrose in Comparative Example 2. In the electrochemical impedance spectroscopy of the lithium iron phosphate composite material in Comparative Example 3, the semicircle diameter was the largest and the charge transfer impedance was the highest in the high-frequency region; the slope of the straight line was the smallest in the low-frequency region, indicating that Li... + The diffusion resistance was the greatest. This is due to the discontinuity of the carbon coating, the breakage of the conductive network, and the collapse of the 3D-OMC skeleton caused by the excess of block copolymer F127 in Comparative Example 3.

[0107] The lithium iron phosphate composite materials from Examples 1-5 and Comparative Examples 1-12 were used to prepare lithium-ion batteries according to the following steps: Preparation of the positive electrode sheet: Step 1: The prepared cathode material, conductive carbon black (Super-P), binder (PVDF), and dispersant are thoroughly mixed in a homogenizing tank at a ratio of 97%:1.2%:1.6%:0.2%. The cathode material is the lithium iron phosphate composite material of Examples 1-5 or Comparative Examples 1-12.

[0108] Step 2: N-Methylpyrrolidone (NMP) is added to the powder from Step 1 as a solvent and stirred for 4-6 hours to obtain the positive electrode slurry.

[0109] Step 3: Coat the positive electrode slurry evenly on the positive electrode current collector with a thickness of 13um, and dry it in an oven at 105℃ to obtain the first positive electrode film.

[0110] Step 4: Repeat steps 1-3 to obtain a positive electrode film coated with positive electrode material on both sides; Step 5: The positive electrode film is rolled to obtain a positive electrode sheet with a thickness of 185um.

[0111] Preparation of negative electrode sheet: Step 1: Mix the negative electrode material artificial graphite, conductive carbon black (Super-P), binder 1 (CMC), binder 2 (SBR), and dispersant in a ratio of 97.1:0.8:1.2:0.7:0.1, add deionized water, and stir thoroughly in a homogenizing tank for 5 hours to obtain the negative electrode slurry.

[0112] Step 2: Coat the negative electrode slurry evenly on the 6µm negative electrode current collector and dry it in an oven at 105℃ to obtain the first negative electrode film.

[0113] Step 3: Repeat steps 1 and 2 to obtain a negative electrode film coated with negative electrode paste on both sides.

[0114] Step 4: The negative electrode film from Step 3 is rolled to obtain a negative electrode sheet with a thickness of 145μm.

[0115] Electrolyte preparation: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 1:1:1, dissolved, and thoroughly stirred. Then, lithium salt LiPF6 (Lithium Hexafluorophosphate) was added, and the mixture was stirred until homogeneous to obtain the electrolyte. The concentration of LiPF6 in the electrolyte was 1.0 mol / L.

[0116] Preparation of the diaphragm: A polyethylene film (PE) with a thickness of 3 μm and a ceramic coating of 9 PE + 3 CCS was used as the diaphragm. "9 PE + 3 CCS" refers to a 9 μm polyethylene film coated with a 3 μm thick ceramic coating.

[0117] The manufacturing process of lithium-ion batteries: Step 1: The prepared positive electrode sheet and negative electrode sheet are die-cut to obtain a positive electrode sheet with a width of 84 mm and a length of 686 mm, and a negative electrode sheet with a width of 90 mm and a length of 670 mm, respectively. Step 2: Using a 5cm radius winding needle as the central axis, wind the negative electrode, separator, and positive electrode in sequence to obtain a wound bare cell.

[0118] Step 3: Place the bare battery cell in an aluminum-plastic film, and obtain the welded bare battery cell through stamping and tab welding processes.

[0119] Step 4: Inject the prepared electrolyte into the welded bare cell, and then use a packaging machine to obtain the desired soft-pack battery.

[0120] The performance of lithium-ion batteries assembled from lithium iron phosphate composite materials in Examples 1-5 and Comparative Examples 1-12 is shown in Table 3. The test method for the 25℃ 0.1P specific capacity in Table 3 is as follows: using a nebula meter, according to the GB / T36276-2023 test standard, the finished lithium-ion batteries were subjected to an initial charge-discharge performance test at 25℃ 0.1P. The charge / discharge capacity of the lithium-ion battery was calculated using the charging capacity and discharging capacity of the second cycle (charge / discharge specific capacity = charging capacity or discharging capacity / weight of positive electrode material). The test method for the 25℃ 0.5P energy efficiency and rate capability is as follows: using a nebula meter, according to the GB / T36276-2023 test standard, the finished lithium-ion batteries were subjected to an initial charge-discharge performance test at 25℃. The energy efficiency was calculated using the charging energy and discharging energy data of the second cycle (energy efficiency = discharging energy / charging energy).

[0121] The test method for energy retention rate at 45℃ 100% SOC (State of Charge) storage is as follows: Using a Byte tester, according to the GB / T36276-2023 test standard, the finished lithium-ion battery is tested for energy retention and recovery capabilities at 45℃. After 30 days of storage at 45℃, the lithium-ion battery undergoes a complete charge-discharge cycle, then is fully charged to 3.65V and stored again. This test cycle is repeated twice to obtain the energy retention rate after 60 days of storage. The test method for 25℃ 0.5P cycle is as follows: Using a Nebula tester, according to the GB / T36276-2023 test standard, the finished lithium-ion battery is tested for 25℃ 0.5P cycle performance.

[0122] Test method for 2P energy retention rate: Using a Byte tester, according to the GB / T36276-2023 test standard, the 2P energy retention rate of the finished lithium-ion battery is tested. First, the initial charge and discharge performance test at 25℃ and 0.5P is performed. The discharge energy of the second cycle is taken as the benchmark. Then, a charge and discharge test is performed at 25℃ and 2P power for one cycle. 2P energy retention rate = 2P discharge energy / 0.5P discharge energy of the second cycle.

[0123] Table 3 Performance of lithium-ion batteries assembled in Examples 1-5 and Comparative Examples 1-12

[0124] As shown in Table 2, the lithium iron phosphate composite material (LiFePO4@C@3D-OMC material) prepared in this application demonstrates excellent comprehensive performance at both the powder and full-cell levels when used as a positive electrode active material. Specifically, at the powder level, the compaction density of the lithium iron phosphate composite material reaches 2.62 g / cm³, and the 0.1P charging capacity is as high as 162 mAh / g, laying a core foundation for high-energy-density cell design. At the full-cell level, the lithium-ion battery assembled using the lithium iron phosphate composite material achieves an energy efficiency of 96.2% at 25℃ and 0.5P, significantly improving the economic benefits of the energy storage device throughout its entire life cycle. After 60 days of storage at 45℃ and 100% SOC, the energy retention rate of the lithium-ion battery assembled using the lithium iron phosphate composite material remains as high as 98.7%, ensuring that the discharge energy of the lithium-ion battery stably meets design requirements during application. The lithium-ion battery assembled using lithium iron phosphate composite material exhibits a capacity retention rate (discharge capacity on the 500th cycle / discharge capacity on the 1st cycle) of 99.4% after 500 cycles at 25℃ and 0.5P, demonstrating excellent cycle stability.

[0125] In this embodiment, the compaction density of the lithium iron phosphate composite material is significantly superior to that of most comparative lithium iron phosphate composite materials. The core mechanism is that this embodiment achieves LiFePO4 particle nucleation, carbon coating deposition, and in-situ self-assembly of the 3D-OMC network simultaneously in a one-step process, forming a dense and stable integrated "core-shell-network" structure. This structure can control the uniformity and dispersion of LiFePO4 particles, achieving close packing and resulting in a compaction density of 2.62 g / cm³. Simultaneously, the continuous amorphous carbon layer constructs an efficient electron conduction pathway, and the ordered mesoporous carbon layer's mesoporous network reduces the LiFePO4 particle density. + Diffusion resistance, together with the other two factors, enables the lithium iron phosphate composite material to perform optimally under 0.1P discharge conditions, specifically with a specific capacity of 162 mAh / g, a 2P rate capacity retention of 93%, and a 500-cycle capacity retention of 99.4%.

[0126] In Comparative Example 1, the lithium iron phosphate composite material was prepared using a two-step process, resulting in poor continuity of the carbon coating layer, weak interfacial bonding, and LiFePO4 particles clogging the mesopores. Its compaction density of 2.5 g / cm³ resulted in lower electrochemical performance compared to Example 1. In Comparative Example 2, excessive sucrose, while increasing the apparent compaction density to 2.61 g / cm³ through carbonization, caused irreversible blockage of the mesopores, leading to Li... + Diffusion deteriorated, with a 0.1P specific capacity of 147 mAh / g, a 2P retention rate of 76%, and a cycle retention rate of 81.5%, showing a significant deterioration in performance compared to Example 1. In Comparative Example 3, the excessive amount of block copolymer F127 led to insufficient sucrose carbon source, preventing the formation of a complete carbon layer and a stable 3D-OMC backbone. The compaction density decreased to 2.35 g / cm³, electron and ion channels were broken, and performance deteriorated across the board.

[0127] In Comparative Example 4, the only polar solvent used was pure deionized water, resulting in a high viscosity of the mixed raw materials. This led to the agglomeration and uneven particle distribution of the block copolymer F127 in the mixed raw materials. The compaction density of the resulting lithium iron phosphate composite material as a cathode material was 2.4 g / cm³, which was lower than the compaction density of the cathode material in Example 1. Furthermore, the insufficient continuity of the carbon layer in the lithium iron phosphate composite material increased the material's impedance, slightly reducing the performance of the assembled lithium-ion battery. In Comparative Example 5, the only polar solvent used was pure anhydrous ethanol. Insufficient solvation of the block copolymer F127 led to particle agglomeration. Although the apparent compaction density was 2.5 g / cm³, mesopore blockage caused Li... +Diffusion obstruction significantly reduced the performance of assembled lithium-ion batteries. In Comparative Example 6, methanol was used as the polar solvent. The excessively high polarity of methanol caused the dissociation and agglomeration of the block copolymer F127, resulting in a compaction density of 2.5 g / cm³. Simultaneously, the carbon layer in the lithium iron phosphate composite material became unstable, and side reactions intensified, leading to a substantial decrease in the performance of the assembled lithium-ion battery. Furthermore, methanol is highly toxic and has poor industrial applicability. In Comparative Example 7, acetone was used as the polar solvent. Its low polarity caused the self-assembly failure of the block copolymer F127 and severe agglomeration of sucrose particles, forming a thick, dense, discontinuous, and heavily accumulated carbon layer. This resulted in an increased compaction density of the prepared lithium iron phosphate composite material as a cathode material, reaching 2.61 g / cm³. The carbon layer in the lithium iron phosphate composite material was discontinuous, and the 3D-OMC framework collapsed, resulting in poor performance of the assembled lithium-ion battery. Comparative Example 8 used a mixed solvent of deionized water and anhydrous ethanol, with an excessive volume of anhydrous ethanol. This reduced the solvent polarity, leading to severe micellar aggregation and disordered agglomeration of the block copolymer F127. This resulted in the loss of its ability to form a template for three-dimensional ordered mesoporous carbon, resulting in a compaction density of 2.4 g / cm³ for the lithium iron phosphate composite material. Furthermore, the lack of mesopores caused Li... + The diffusion resistance increased dramatically, resulting in a significant deterioration in the rate performance and energy efficiency of the assembled lithium-ion battery, failing to achieve efficient ion transport characteristics. Comparative Example 9 used a mixed solvent of deionized water and anhydrous ethanol, with an excess of deionized water. This significantly increased the polarity of the system, leading to excessive hydration and stretching of the PEO segments in the block copolymer F127. This resulted in abnormally large and disordered micelle sizes, preventing F127 from forming a uniform micelle array and ultimately disrupting the construction of the three-dimensional ordered mesoporous carbon. Simultaneously, the high polarity environment caused localized aggregation and densification of the sucrose carbon source, resulting in insufficient carbon layer continuity. When used as a cathode material, this composite material had a compaction density of 2.35 g / cm³. Due to the disordered micelle arrangement, the carbon layer structure was unstable, leading to Li… + When diffusion channels are blocked, the high-temperature storage performance and long-cycle retention of assembled lithium-ion batteries are significantly reduced.

[0128] In Comparative Example 10, hydroxypentanoic acid was not added. Due to the lack of hydroxypentanoic acid in stabilizing and dispersing F127 micelles, the number of F127 micelles was unstable, resulting in a coating structure with insufficient carbon layer continuity and severe densification, with a compaction density of only 2.45 g / cm³. Simultaneously, carbon layer defects led to increased interfacial impedance, resulting in electrochemical performance with a 0.1P specific capacity of 158 mAh / g, a 2P retention rate of 89%, and a cycle retention rate of 93.5%, significantly deteriorating compared to the examples. In Comparative Example 11, P123 was used instead of F127. The compaction density of this lithium iron phosphate cathode composite material was only 2.45 g / cm³, far lower than the examples, and the electrochemical performance showed a significant decrease compared to the examples. This was mainly because the PEO segment of P123 was too short and its hydrophobicity too strong, making it difficult to form stable micelles in this polar system and unable to effectively match with the sucrose carbon source, leading to uneven carbon layer coating and the inability to form a mesoporous framework. In Comparative Example 12, F127 was replaced with F68. The PPO segment of F68 was too short, and the hydrophobic interaction force was insufficient, which prevented the construction of a stable spherical micelle template. As a result, the carbon source could not be deposited in an orderly manner to form a three-dimensional mesoporous carbon structure. Consequently, the lithium iron phosphate composite material exhibited a low compaction density (compaction density of 2.35 g / cm³) and poor electrochemical performance.

[0129] See Figures 13-15 , Figure 13 The above are charge / discharge curves of the lithium-ion batteries of Example 1 and Comparative Examples 1-3 at 25°C @ 0.1P. Figure 14 The graphs show the charge / discharge curves of the lithium-ion batteries of Example 2, Comparative Example 4, and Comparative Example 7 at 25°C @ 0.1P. Figure 15 The charge / discharge curves of the lithium-ion batteries of Example 4 and Comparative Examples 10-12 at 25°C @ 0.1P are shown.

[0130] from Figure 13 It can be seen that the lithium-ion battery of Example 1 has the highest discharge specific capacity, reaching 162 mAh / g, which is close to the theoretical specific capacity of LFP (lithium iron phosphate). Furthermore, the voltage plateau of the lithium-ion battery is the longest and most stable in the 3.3V~3.4V range, indicating that the lithium-ion battery of Example 1 has the least electrochemical polarization and the highest utilization rate of the lithium iron phosphate composite material as the active material. The lithium-ion battery of Comparative Example 1 has a discharge specific capacity of approximately 153 mAh / g, lower than that of the lithium-ion battery of Example 1. At the same time, the voltage plateau of the lithium-ion battery of Comparative Example 1 is shorter and slightly tilted, and the polarization is slightly increased. This is due to the increased interfacial impedance of the lithium iron phosphate composite material caused by the two-step method used in Comparative Example 1, and the Li... +Diffusion was hindered. The discharge specific capacity of the lithium-ion battery in Comparative Example 2 was approximately 147 mAh / g, significantly lower than that of the lithium-ion battery in Example 1. Furthermore, the voltage plateau of the lithium-ion battery in Comparative Example 2 was noticeably shorter and more tilted, with significantly increased polarization. This was due to the excessive sucrose in Comparative Example 2, resulting in an overly thick carbon coating layer, pore blockage, and a substantial decrease in the utilization rate of the active material. The discharge specific capacity of the lithium-ion battery in Comparative Example 3 was only 145 mAh / g, with the shortest and most severely tilted voltage plateau and the greatest polarization. This was due to the excessive block copolymer F127 in Comparative Example 3, leading to the breakage of the conductive network in the carbon coating layer and the collapse of the 3D-OMC framework, resulting in low utilization of the active material.

[0131] from Figure 14 It can be seen that the discharge specific capacity of the lithium-ion battery in Comparative Example 4 is 152 mAh / g, the voltage plateau is significantly shorter than that in Example 2, and a significant "collapse" phenomenon occurs at the end of the plateau, indicating increased polarization. Simultaneously, the uneven distribution of LiFePO4 particles in the lithium iron phosphate composite material and insufficient continuity of the carbon coating layer increase the interfacial impedance of the material, reducing electronic conduction efficiency and active material utilization. The discharge specific capacity of the lithium-ion battery in Comparative Example 7 is 150 mAh / g. The excessively weak polarity of acetone solvent causes complete failure of the self-assembly process of the block copolymer F127. Sucrose is almost insoluble, leading to rapid sedimentation and severe agglomeration of LiFePO4 particles. The carbon coating layer is extremely thin and discontinuous, and the 3D-OMC framework completely collapses, resulting in a disruption of the electronic conduction path and the Li... + The diffusion channels are completely broken, reducing the utilization rate of active substances.

[0132] from Figure 15 It can be seen that the discharge specific capacity of the lithium-ion battery in Comparative Example 10 is 158 mAh / g, slightly lower than that in Example 4. The voltage plateau length is shortened, and there is a significant collapse at the end of the plateau, with a slight increase in polarization. Its core defect stems from the absence of hydroxypentanoic acid as an interface modifier, which weakens the hydrogen bonding coordination between the carbon source and the LiFePO4 particle surface. This leads to a decrease in the uniformity of the carbon layer coating, localized densification and insufficient continuity, deterioration of the regularity and connectivity of the mesopore channels, and an increase in interfacial impedance and Li... + Diffusion resistance increases simultaneously, ultimately leading to a decrease in the utilization rate of active materials. The lithium-ion battery in Comparative Example 11 has a discharge specific capacity of 153 mAh / g, a significantly shortened voltage plateau with increased slope, and a greatly enhanced polarization effect. The root cause of the performance degradation in this comparative example is: PEO... 20 -PPO 70 -PEO 20(P123) has an excessively high proportion of PPO segments and a too short hydrophilic segment of PEO, resulting in micelles that are predominantly columnar / rod-shaped. This only allows for the formation of two-dimensional hexagonal ordered mesoporous carbon, failing to construct the three-dimensional cubic interconnected 3D-OMC framework found in the embodiments. Simultaneously, the bonding force between the carbon source and the particle interface is weakened, leading to uneven carbon layer coating, localized exposure, and impaired continuity of the electronic conductivity network. + The transmission exhibits significant anisotropy, resulting in a comprehensive deterioration in cell dynamics and capacity. The discharge specific capacity of Comparative Example 12 lithium-ion battery is only 143 mAh / g, the lowest among all comparative examples. The fundamental reason for this is: PEO... 80 -PPO 30 -PEO 80 The excessively long PEO chains and weak hydrophobic PPO cores result in extremely poor micelle stability and a completely disordered self-assembly process, making it impossible to form an effective mesoporous template. The highly hydrophilic micelles lead to uncontrolled carbon source distribution, ultimately forming a thin and discontinuous amorphous carbon layer on the particle surface. After high-temperature carbonization, the framework collapses over a large area, disrupting electron conduction and Li... + The diffusion pathway is severely blocked, the interfacial impedance increases sharply, the utilization rate of active materials decreases significantly, and the electrochemical performance deteriorates most significantly.

[0133] The method for preparing lithium iron phosphate composite materials provided in this application involves setting mixed raw materials including lithium iron phosphate precursor, sucrose, amphiphilic block copolymer, and polar solvent. The amphiphilic block copolymer is arranged in an orderly manner in the polar solvent to form micelles. Sucrose preferentially adsorbs onto the surface of the lithium iron phosphate precursor and simultaneously fills the intercellular spaces formed by the amphiphilic block copolymer. Through a one-step sintering process, the lithium iron phosphate precursor is sintered to form a lithium iron phosphate core. The sucrose adsorbed on the surface of the lithium iron phosphate precursor is carbonized at high temperature to form an amorphous carbon layer. The sucrose filling the intercellular spaces of the amphiphilic block copolymer is also carbonized at high temperature to form a mesoporous carbon layer. This achieves a continuous and uniform double-layer carbon coating structure on the surface of the lithium iron phosphate core, constructing a highly efficient electron conduction network and reducing the interfacial impedance of the lithium iron phosphate composite material. Simultaneously, by precisely controlling the mass ratio of sucrose to the amphiphilic block copolymer, the mesoporous channels of the mesoporous carbon layer are prevented from being blocked or collapsing, thus constructing an ordered and interconnected three-dimensional mesoporous carbon network, which provides a high-efficiency electron conduction network for LiFePO4. +This method provides a short-range, rapid diffusion channel while ensuring the small and uniform particle size of the prepared lithium iron phosphate composite material. This improves the utilization rate of the lithium iron phosphate composite material as a positive electrode active material and optimizes the compaction characteristics of the positive electrode sheet. The preparation method of the lithium iron phosphate composite material provided in this application overcomes the technical bottlenecks of discontinuous carbon coating and easy collapse of mesoporous structure in traditional processes. It achieves synergistic optimization of microstructure and electrochemical performance. The prepared lithium iron phosphate composite material combines the high compaction density and high specific capacity of powder with the excellent energy efficiency, storage stability and cycle life of the entire cell. As a positive electrode material, it can effectively meet the core requirements of energy storage devices for high energy density and long life.

[0134] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims

1. A method for preparing a lithium iron phosphate composite material, characterized in that, The lithium iron phosphate composite material comprises a core, an amorphous carbon layer, and a mesoporous carbon layer. The core is a lithium iron phosphate particle. The amorphous carbon layer coats the surface of the core, and the mesoporous carbon layer coats the surface of the amorphous carbon layer facing away from the core. The preparation method of the lithium iron phosphate composite material includes: A mixed raw material is prepared, comprising lithium iron phosphate precursor, sucrose, amphiphilic block copolymer and polar solvent, wherein the mass ratio of sucrose to amphiphilic block copolymer in the mixed raw material is 1.5:1 to 5:2, and the polar solvent comprises a first strong polar solvent and a second strong polar solvent, wherein the first strong polar solvent is water and the second strong polar solvent is anhydrous ethanol; The mixed raw materials are sintered to obtain the lithium iron phosphate composite material.

2. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The mixed raw materials also include hydroxypentanoic acid.

3. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The amphiphilic block copolymer is a PEOn-PPOm-PEOn triblock copolymer, wherein n≥80 and m≥60.

4. The method for preparing the lithium iron phosphate composite material according to claim 1, characterized in that, The volume ratio of the first strongly polar solvent to the second strongly polar solvent is 4:1 to 8:

1.

5. The method for preparing the lithium iron phosphate composite material according to any one of claims 1 to 4, characterized in that, The step of sintering the mixed raw materials includes: placing the mixed raw materials in an inert atmosphere for a first sintering step and a second sintering step, wherein the temperature of the first sintering step is 280℃~320℃ and the temperature of the second sintering step is 650℃~700℃.

6. A positive electrode sheet, characterized in that, It includes a positive current collector and a positive electrode material layer, wherein the positive electrode material layer is disposed on at least one side of the positive current collector along the thickness direction, and the positive electrode material layer includes a lithium iron phosphate composite material prepared by the method for preparing lithium iron phosphate composite material according to any one of claims 1 to 5.

7. An energy storage device, characterized in that, It includes a negative electrode, a separator, and a positive electrode as described in claim 6, wherein the separator is disposed between the negative electrode and the positive electrode.

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