Phosphate positive electrode material and preparation method thereof, positive electrode plate and battery

Abstract

The invention discloses a phosphate positive electrode material and a preparation method thereof, a positive electrode plate and a battery. The phosphate positive electrode material is sheet-shaped, the length of the phosphate positive electrode material is d1, the thickness of the phosphate positive electrode material is d2, the length-thickness ratio of the phosphate positive electrode material is (10-100): 1, and the length of the phosphate positive electrode material is the size of a large surface of the phosphate positive electrode material. The phosphate positive electrode material can effectively shorten the diffusion path of lithium ions, so that the time required for the lithium ions to migrate from the interior of the material to the surface is shortened, and the improvement of the electrochemical performance of the phosphate positive electrode material under a high-rate condition is promoted.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a phosphate-based cathode material and its preparation method, cathode sheet, and battery. Background Technology

[0002] In fast charging or high-power discharging scenarios, lithium ions need to migrate from the positive electrode to the negative electrode (charging) or in the reverse direction (discharging) within a short time. If the diffusion path is too long, the lithium ion migration rate is limited, leading to increased battery internal resistance and intensified polarization, manifested as a sudden voltage drop or excessively rapid temperature rise. With conventional phosphate-based cathode materials (such as LiMnPO4 and LiFePO4), it is difficult to precisely control the grain morphology and particle size, and the grains are prone to agglomeration, forming micron-sized secondary particles, which significantly prolongs the lithium ion diffusion path. Summary of the Invention

[0003] The purpose of this invention is to provide a phosphate-based cathode material, its preparation method, cathode sheet, and battery, thereby solving the problem of long lithium-ion diffusion paths in phosphate-based cathode materials.

[0004] To achieve the objectives of this invention, the following technical solution is provided: In a first aspect, the present invention provides a phosphate-based cathode material, wherein the phosphate-based cathode material is in sheet form, the length of the phosphate-based cathode material is d1, the thickness of the phosphate-based cathode material is d2, the length-to-thickness ratio of the phosphate-based cathode material is (10-100):1, and the length of the phosphate-based cathode material is the size of its larger surface area.

[0005] In one embodiment, the length d1 of the phosphate-based cathode material is the maximum length of its large surface area, and the length d1 of the phosphate-based cathode material satisfies: 1μm≤d1≤10μm.

[0006] In one embodiment, the thickness of the phosphate-based cathode material is d2, which satisfies: 50nm≤d2≤500nm.

[0007] In one embodiment, the phosphate-based cathode material includes a lithium manganese iron phosphate core and a coating layer, wherein the coating layer covers the outer surface of the lithium manganese iron phosphate core.

[0008] In one embodiment, the thickness of the coating layer is d3, which satisfies: 5nm≤d3≤10nm.

[0009] In a second aspect, the present invention provides a method for preparing a phosphate-based cathode material, used to prepare the phosphate-based cathode material as described in any one of the various embodiments of the first aspect, comprising: Add a surfactant to the mixture to obtain a mixed solution; The mixed solution was subjected to a microwave hydrothermal reaction to obtain a precursor; The precursor is subjected to secondary processing to obtain the phosphate-based cathode material.

[0010] In one embodiment, the surfactant includes one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and hexadecyltrimethylammonium bromide.

[0011] In one embodiment, the concentration of the surfactant is 0.01 mol / L to 0.3 mol / L.

[0012] In one embodiment, the precursor undergoes a secondary processing, including: The carbon source and the precursor are mixed and calcined.

[0013] Thirdly, the present invention provides a positive electrode sheet comprising a phosphate-based positive electrode material as described in any one of the various embodiments of the first aspect or a phosphate-based positive electrode material obtained by any one of the preparation methods described in any one of the various embodiments of the second aspect.

[0014] Fourthly, the present invention provides a battery comprising a positive electrode sheet as described in any of the various embodiments of the third aspect.

[0015] In conventional phosphate-based cathode materials, lithium ions mainly diffuse one-dimensionally along the thickness direction. The phosphate-based cathode material of this invention, by limiting the length-to-thickness ratio of the phosphate-based cathode material, directionally suppresses crystal growth in the thickness direction, which can effectively shorten the diffusion path of lithium ions, thereby reducing the time required for them to migrate from the interior of the material to the surface. Since the lithium ion diffusion rate is closely related to rate performance, the reduction of diffusion time directly promotes the improvement of the electrochemical performance of the phosphate-based cathode material of this invention under high rate conditions. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of a phosphate-based cathode material according to one embodiment; Figure 2 This is a schematic diagram of a phosphate-based cathode material according to another embodiment; Figure 3 A scanning electron microscope image of a phosphate-based cathode material according to one embodiment; Figure 4 This is a flowchart of a method for preparing a phosphate-based cathode material according to one embodiment. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0020] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used in this invention includes any and all combinations of one or more of the associated listed items.

[0021] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0022] Please refer to Figure 1 , Figure 2 and Figure 3 The present invention provides a phosphate-based cathode material, which is in sheet form, with a length of d1, a thickness of d2, and a length-to-thickness ratio of (10-100):1. The length of the phosphate-based cathode material is the size of its larger surface area.

[0023] Optionally, phosphate-based cathode materials include lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium manganese phosphate, etc., without restriction.

[0024] Optionally, the length-to-thickness ratio of the phosphate-based cathode material can be 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1, etc., without limitation.

[0025] If the aspect ratio of a phosphate-based cathode material is too small, it tends to be nearly spherical or blocky, easily resulting in dense packing. This makes it difficult for the electrolyte to penetrate into the interparticle spaces, forming "dead zones" and reducing the effective utilization rate of the active material. Lithium ions inside the agglomerates must diffuse to the surface through a tortuous path, increasing transport resistance, especially under low temperature or high rate conditions. If the aspect ratio is too large, the phosphate-based cathode material tends to be elongated or needle-shaped particles, requiring lithium ions to travel a longer path along their length to reach the surface and participate in the reaction. When the aspect ratio of a phosphate-based cathode material is moderate, it is less prone to agglomeration and the path required for lithium ions to participate in the reaction is reduced.

[0026] The phosphate-based cathode material is sheet-like, comprising a first large surface and a second large surface facing away from each other, preferably with the first and second large surfaces parallel. The direction perpendicular to the first and second large surfaces is the thickness direction of the phosphate-based cathode material, and the directions parallel to the first large surface are the length and width directions of the phosphate-based cathode material, where the length direction is the direction containing the longest point of the phosphate-based cathode material. The length direction is denoted as the a-axis, the width direction as the b-axis, and the thickness direction as the c-axis. Figure 3 As shown in Figure 3-4, the direction of the longest part of the phosphate-based cathode material is the a-axis.

[0027] Please refer to Figure 1 Optionally, multiple phosphate-based cathode materials can be arranged with their long sides (large surfaces) parallel to the electrode surface, which may improve the conductivity and ion transport efficiency of the electrode.

[0028] In conventional phosphate-based cathode materials, lithium ions mainly diffuse one-dimensionally along the thickness direction. The phosphate-based cathode material of this invention, by limiting the length-to-thickness ratio of the phosphate-based cathode material, directionally suppresses crystal growth in the thickness direction, which can effectively shorten the diffusion path of lithium ions, thereby reducing the time required for them to migrate from the interior of the material to the surface. Since the lithium ion diffusion rate is closely related to rate performance, the reduction of diffusion time directly promotes the improvement of the electrochemical performance of the phosphate-based cathode material of this invention under high rate conditions.

[0029] In one embodiment, the length d1 of the phosphate-based cathode material is the maximum length of its large surface area, and the length d1 of the phosphate-based cathode material satisfies: 1μm≤d1≤10μm.

[0030] Optionally,

[010] the crystal plane is perpendicular to the b-axis and parallel to the ac plane formed by the a-axis and c-axis. The phosphate-based cathode material of the present invention achieves preferred alignment on this crystal plane, which substantially promotes the growth of the crystal along the a-axis and c-axis directions, while inhibiting the growth along the b-axis direction. This orientation regulation can effectively shorten the diffusion path of lithium ions, thereby improving the rate performance of the phosphate-based cathode material.

[0031] Optionally, the maximum length of the large surface area of ​​the phosphate-based cathode material is the size of the longest side of the material particle. Specifically, d1 is the size of the phosphate-based cathode material in the a-axis direction.

[0032] Optionally, d1 can be observed and measured using scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Specifically, based on the SEM image, software such as Nano Measurer or ImageJ can be used to import the SEM image, set the actual length and unit, manually or automatically mark the particle boundaries, and the software records the size of each particle and exports the average length d1 of the particles. Alternatively, d1 can be measured through threshold segmentation and morphological operations. The main steps are: adjusting image contrast and brightness to remove background noise; separating particles from the background through grayscale differences to generate a binary image; using operations such as dilation, erosion, opening, and closing to eliminate noise and connect particle edges; and identifying and measuring the equivalent length d1 of each particle.

[0033] Optionally, d1 can be 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc., without restriction.

[0034] Optionally, the width d4 of the phosphate-based cathode material satisfies: 1μm≤d4≤10μm. d4 is the dimension of the phosphate-based cathode material in the c-axis direction. d4 can be 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc., without limitation.

[0035] The length d1 directly affects the diffusion path of lithium ions within the material. If d1 is too large, it may increase the diffusion resistance of lithium ions, thus affecting the battery's charge / discharge rate and rate performance. If d1 is too small, the size of the phosphate-based cathode material decreases, which may shorten the diffusion path of lithium ions within the phosphate-based cathode material, but it also reduces the contact area between phosphate-based cathode materials, leading to obstructed lithium ion transport between them. A moderate d1 helps form more continuous lithium ion transport channels, improving ion transport efficiency and ensuring transport between phosphate-based cathode materials.

[0036] In one embodiment, the thickness of the phosphate-based cathode material is d2, which satisfies: 50nm≤d2≤500nm.

[0037] Optionally, d2 can be 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, etc., without restriction.

[0038] When d2 is too large, lithium-ion transport in phosphate-based cathode materials mainly occurs along one-dimensional channels on specific crystal planes (such as the

[010] crystal plane). Excessive thickness requires lithium ions to traverse longer paths to complete insertion / extraction, leading to a significant increase in diffusion resistance and potentially affecting the battery's charge / discharge rate and rate performance. When d2 is too small, the mechanical strength of the phosphate-based cathode material decreases due to its thinness, making it prone to breakage or deformation during charge / discharge cycles, and placing high demands on the manufacturing process. When d2 is moderate, the appropriate thickness facilitates lithium-ion insertion / extraction within continuous lithium-ion transport channels and ensures uniform stress distribution within the phosphate-based cathode material during volume expansion / contraction during charge / discharge.

[0039] For reference Figure 2 , Figure 2 In one embodiment, multiple phosphate-based cathode materials are stacked in the b-axis direction. The phosphate-based cathode material includes a lithium manganese iron phosphate core and a coating layer, with the coating layer covering the outer surface of the lithium manganese iron phosphate core.

[0040] Specifically, when the phosphate-based cathode material is lithium manganese iron phosphate, the chemical formula of lithium manganese iron phosphate is LiMnxFe1-xPO4, and the molar ratio of each element is Li:Fe:Mn:P=1:(1-x):x:1, where x=0.1-0.9, without limitation. x can specifically be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc., without limitation. Lithium manganese iron phosphate not only retains the stability of the olivine structure but also exhibits multiple advantages: its Mn²... + / Mn³ + The redox couple boosts the voltage platform to 4.1V, enabling a theoretical energy density of 230Wh / kg, which is 15%-20% higher than that of lithium iron phosphate. In terms of low-temperature performance, the capacity retention rate can reach 75% at -20℃ (compared to only 60% for lithium iron phosphate), thanks to the superior low-temperature activity of the manganese platform. In addition, since manganese resources are 50 times more abundant than cobalt, the raw material cost of lithium manganese iron phosphate is 40% lower than that of ternary materials, giving it a significant cost advantage.

[0041] Optionally, the coating layer is carbon coating.

[0042] Optionally, a coating layer is constructed along the layered stacking direction of the phosphate-based cathode material, that is, a continuous coating layer is constructed along the layered stacking direction of the sheet-like phosphate-based cathode material to achieve a continuous conductive network. The coating layer uniformly covers all surfaces of each lithium manganese iron phosphate core. The sheet-like lithium manganese iron phosphate cores form an integral structure through physical stacking, and an efficient conductive network is constructed in three-dimensional space with the help of the coating layer, thereby significantly enhancing the electronic conductivity of the material.

[0043] Please refer to Figure 3 , specifically Figure 3 Images 3-1 to 3-4 in the image are scanning electron microscope (SEM) images of phosphate-based cathode materials with a coating layer covering a lithium manganese iron phosphate core. Figure 3 The data shows that all phosphate-based cathode materials have a sheet-like structure, and the stacking direction of multiple phosphate-based cathode materials is the direction of the large surface area of ​​the phosphate-based cathode material.

[0044] Forming a coating layer on the surface of the lithium manganese iron phosphate core helps create a continuous conductive network, significantly improving the electron transport efficiency within the phosphate-based cathode material. This network structure helps reduce polarization during battery charging and discharging, thereby enhancing the overall battery performance. The coating layer further reduces the transport resistance of lithium ions within the phosphate-based cathode material, improving transport efficiency and contributing to higher rate performance, allowing the battery to release or absorb more energy in a shorter time.

[0045] In one embodiment, the thickness of the coating layer is d3, which satisfies: 5nm≤d3≤10nm.

[0046] Optionally, d3 can be 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, etc., without restriction.

[0047] Optionally, the ratio of the thickness of the coating layer to the thickness of the lithium manganese iron phosphate core is 1:(10-100), specifically 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:70, 1:80, 1:90, 1:100, etc., without limitation.

[0048] When d3 is too large, an excessively thick coating layer prolongs the diffusion path of lithium ions within the phosphate-based cathode material, increasing transport resistance and leading to a decrease in battery charge / discharge rate. This performance degradation is particularly pronounced under low-temperature or high-rate charge / discharge conditions. In fast-charging scenarios, an excessively thick coating layer may restrict the rapid insertion and extraction of lithium ions, prolonging charging time. Phosphate-based cathode materials undergo volume expansion and contraction during charge / discharge, and an excessively thick coating layer may lack the elasticity to effectively buffer these stress changes. Conversely, when d3 is too small, an excessively thin coating layer cannot form a continuous conductive network, resulting in increased internal resistance and reduced charge / discharge efficiency. Furthermore, an excessively thin coating layer offers limited protection for the lithium manganese iron phosphate core, failing to reduce electrolyte erosion of the phosphate-based cathode material and the shedding of active materials. A moderate d3 ensures that the coating layer can construct a continuous conductive network and protect the lithium manganese iron phosphate core without hindering the rapid insertion and extraction of lithium ions.

[0049] Please refer to Figure 4 This invention provides a method for preparing a phosphate-based cathode material, used to prepare the phosphate-based cathode material as described in any of the foregoing embodiments, comprising: Step S10: Add the surfactant to the mixture to obtain a mixed solution.

[0050] Step S20: Perform a microwave hydrothermal reaction on the mixed solution to obtain the precursor.

[0051] Step S30: The precursor is subjected to secondary processing to obtain a phosphate-based cathode material.

[0052] Optionally, in step S10, the mixture includes an iron source, a manganese source, a phosphorus source, and a lithium source. The iron source, manganese source, phosphorus source, and lithium source are weighed according to a molar ratio of Li:Fe:Mn:P = 1:(1-x):x:1. The iron source includes one or more of ferric sulfate, ferric phosphate, ferric chloride, and ferric nitrate. The manganese source includes one or more of manganese sulfate, manganese oxide, manganese carbonate, and manganese phosphate. The phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and lithium dihydrogen phosphate. The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium sulfate, lithium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0053] Optionally, in step S10, the process of obtaining the mixed solution can also use mechanical stirring to ensure that the mixture is evenly distributed in the mixed solution, and also to ensure that the sheet-like phosphate-based cathode material is evenly distributed in the liquid phase during the formation process.

[0054] Optionally, in step S20, the temperature of microwave hydrothermal heating can be 130℃-180℃, and the duration of microwave hydrothermal heating can be 5min-15min.

[0055] Optionally, step S20 may further include filtering and washing the microwave reaction product to obtain the precursor. Specifically, distilled water is used for washing.

[0056] Optionally, the secondary processing method in step S30 can be chemical vapor deposition (CVD, which decomposes carbon source gases such as methane and acetylene at high temperature to deposit and form a uniform carbon layer), solution method (using organic substances such as glucose and dopamine as carbon sources, and forming a carbon coating layer by high temperature carbonization after hydrothermal reaction or ball milling), high temperature solid phase method (mixing carbon source with material and then carbonizing at high temperature), etc., without limitation.

[0057] Traditional phosphate-based cathode materials are generally synthesized using a hydrothermal method. The reaction rate is limited by liquid-phase ion diffusion kinetics and requires maintenance under high pressure for 12-48 hours, resulting in a lengthy reaction time. This invention employs a microwave hydrothermal method to prepare phosphate-based cathode materials, utilizing a microwave field to interact with polar molecules (H₂O / Mn²⁺). + / Fe² + The dipole interaction of the precursor solution enables targeted energy transfer at the molecular scale, resulting in a local hot spot effect at high frequencies. Compared to the crystallization cycle of 12-48 hours in the traditional hydrothermal method, the present invention uses microwave hydrothermal method to compress the nucleation-growth process to 5-15 minutes, which greatly improves the preparation efficiency of phosphate-based cathode materials.

[0058] In one embodiment, the surfactant includes one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and hexadecyltrimethylammonium bromide.

[0059] Optionally, the surfactants include anionic surfactants and cationic surfactants, wherein sodium dodecylbenzenesulfonate and sodium dodecyl sulfate are anionic surfactants, and hexadecyltrimethylammonium bromide is a cationic surfactant.

[0060] In step S10, the surfactant can make the crystals more inclined to extend along the a-axis or c-axis (i.e., along the length and width directions), control the crystal growth trend, directionally suppress crystal growth in the b-axis direction (i.e., the thickness direction), and promote the preferred arrangement of crystals on the

[010] crystal plane, so that the phosphate-based cathode material of the present invention is generally plate-like, and the diffusion path of lithium ions inside the material will be significantly shortened.

[010] The preferred arrangement of crystal planes can also enable the phosphate-based cathode material to form a more compact crystal structure, reducing structural damage caused by volume expansion and contraction during charging and discharging.

[0061] In one embodiment, the concentration of the surfactant is 0.01 mol / L to 0.3 mol / L.

[0062] Optionally, the concentration of the surfactant can be 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, etc., without limitation.

[0063] Surfactants prevent direct contact between particles by adsorbing onto the particle surface of a mixture, creating steric hindrance or electrostatic repulsion. If the surfactant concentration is too low, the particle surface cannot be completely covered, causing solute molecules to aggregate on the particle surface, promoting particle growth and agglomeration. If the surfactant concentration is too high, although it can inhibit initial particle growth, the small particles have a high specific surface area and are prone to re-agglomeration after washing via van der Waals forces or surface hydrogen bonds, forming secondary large particles. When the surfactant concentration is moderate, the particle dispersibility in the mixed solution is optimal, and the degree of agglomeration is minimal.

[0064] In one embodiment, step S30 includes: mixing the carbon source and the precursor and calcining them.

[0065] Optionally, the carbon source includes one or more of glucose, sucrose, polyethylene glycol, carbon nanotubes, and graphite.

[0066] Optionally, the mass ratio of the precursor to the carbon source is 1:(0.01-0.15). Specifically, the mass ratio of the precursor to the carbon source is 1:0.01, 1:0.03, 1:0.06, 1:0.09, 1:0.1, 1:0.12, 1:0.15, etc., without limitation.

[0067] Optionally, the mixing of the carbon source and the precursor can be done by grinding, specifically by ball milling.

[0068] Optionally, the calcination temperature can be 700℃-800℃, and the calcination time can be 5h-15h. Specifically, the calcination temperature can be 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, etc., without limitation. The calcination time can be 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, etc., without limitation. If the calcination time is too long, the phosphate-based cathode material will continue to grow, resulting in obvious agglomeration and sintering, leading to uneven particle size distribution and the maximum particle size exceeding a certain range, which is very detrimental to the high-current performance of the phosphate-based cathode material. If the calcination time is too short, the crystal structure of the material cannot grow completely, affecting the crystallinity and electrochemical performance. When the calcination temperature is too low, the crystal structure of the material cannot grow completely, resulting in insufficient crystallinity and greater resistance to lithium-ion diffusion in the channels, thus affecting the electrochemical performance of the phosphate-based cathode material. Conversely, excessively high calcination temperatures exacerbate particle agglomeration, leading to particle growth and even sintering, reducing the specific surface area of ​​the phosphate-based cathode material and hindering lithium-ion migration and diffusion. Therefore, a suitable calcination temperature and time are conducive to the formation of the crystal structure of phosphate-based cathode materials.

[0069] This invention employs a "dispersion-then-coating" strategy for coating layer formation. After microwave hydrothermal reaction, well-dispersed sheet-like precursor particles are formed. Carbon source coating is then applied to these particles, and a coating layer is generated in situ on the surface of each individual particle through a subsequent calcination process. The coating layer forms a porous structure at high temperatures, preventing the layered lithium manganese iron phosphate core from growing during heating, refining the grains, and increasing the conductive area. The porous coating layer can accommodate the volume changes of the lithium manganese iron phosphate core during charge and discharge, preventing structural collapse. The coating layer also acts as a reducing agent to prevent the layered lithium manganese iron phosphate core from being oxidized, stabilizing the interface structure.

[0070] The present invention provides a positive electrode sheet, comprising a phosphate-based positive electrode material as described in any one of the foregoing embodiments or a phosphate-based positive electrode material obtained by any one of the foregoing embodiments.

[0071] The positive electrode sheet includes a positive current collector and a positive active material layer, with the positive active material layer disposed on the positive current collector. The positive current collector includes, but is not limited to, any one of aluminum foil, composite aluminum foil, or carbon-coated aluminum foil. The positive active material layer includes, as described in any of the foregoing embodiments, a phosphate-based positive electrode material, a conductive agent, a binder, and other components. This invention does not specifically limit the materials such as the conductive agent and binder; suitable materials can be selected according to actual application requirements. The conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes, and the content of the conductive agent in the positive active layer is 3wt%-5wt%. The binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0072] This invention provides a battery comprising a positive electrode sheet as described in any of the foregoing embodiments. The battery also includes a negative electrode sheet and a separator, wherein the positive electrode sheet, the separator, and the negative electrode sheet are stacked.

[0073] Optionally, the separator can be any of the following: woven membrane, nonwoven membrane (non-woven fabric), microporous membrane, composite membrane, rolled membrane, etc., without limitation. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector includes, but is not limited to, any of the following: copper foil, composite copper foil, or carbon-coated copper foil. The negative electrode active material layer includes components such as negative electrode material, conductive agent, and binder. This invention does not specifically limit these materials, and appropriate materials can be selected according to actual application requirements. The negative electrode material can be carbon-based compound, silicon-based compound, titanium-based compound, etc., without limitation.

[0074] The technical solution of this application will be described in detail below through specific embodiments.

[0075] Example 1 This embodiment provides a phosphate-based cathode material. The phosphate-based cathode material has a length of d1, a thickness of d2, and a length-to-thickness ratio of 50:1. The length of the phosphate-based cathode material is its larger surface dimension. The phosphate-based cathode material includes a lithium manganese iron phosphate core and a coating layer, with the coating layer covering the outer surface of the lithium manganese iron phosphate core. The length d1 of the phosphate-based cathode material is 5 μm, the thickness d2 is 100 nm, and the thickness d3 of the coating layer is 5 nm.

[0076] The preparation method of this embodiment is as follows: iron phosphate, iron phosphate, phosphoric acid, and lithium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:Mn:P=1:(1-x):x:1, where x=0.5; sodium dodecylbenzenesulfonate is added as a surfactant to obtain a mixed solution with a surfactant concentration of 0.2mol / L; the mixed solution is subjected to a microwave hydrothermal reaction at a reaction temperature of 150℃ for 10min to obtain a precursor; the precursor and glucose are calcined at a ratio of 1:0.1 at a calcination temperature of 750℃ for 15h to obtain a phosphate-based cathode material.

[0077] Example 2 This embodiment provides a phosphate-based cathode material. The length of the phosphate-based cathode material is d1, the thickness is d2, the length-to-thickness ratio is 10:1, the length d1 is 5 μm, the thickness d2 is 500 nm, and the thickness d3 of the coating layer is 10 nm. All other parameters are the same as in Example 1.

[0078] The preparation method of this embodiment differs from that of Example 1 in that: iron phosphate, iron phosphate, phosphoric acid, and lithium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:Mn:P=1:(1-x):x:1, where x=0.5; sodium dodecylbenzenesulfonate is added as a surfactant to obtain a mixed solution with a surfactant concentration of 0.1mol / L; the mixed solution is subjected to a microwave hydrothermal reaction at a reaction temperature of 150℃ for 5min to obtain a precursor; the precursor and glucose are calcined at a ratio of 1:0.2 to obtain a phosphate-based cathode material.

[0079] Example 3 This embodiment provides a phosphate-based cathode material. The length of the phosphate-based cathode material is d1, the thickness is d2, and the length-to-thickness ratio is 50:1. The length d1 of the phosphate-based cathode material is 10 μm, the thickness d2 is 200 nm, and the thickness d3 of the coating layer is 10 nm. All other parameters are the same as in Example 1.

[0080] The preparation method of this embodiment differs from that of Example 1 in that: iron phosphate, iron phosphate, phosphoric acid, and lithium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:Mn:P=1:(1-x):x:1, where x=0.7; sodium dodecylbenzenesulfonate is added as a surfactant to obtain a mixed solution with a surfactant concentration of 0.2 mol / L; the mixed solution is subjected to a microwave hydrothermal reaction at a reaction temperature of 160℃ for 15 min to obtain a precursor; the precursor and glucose are calcined at a ratio of 1:0.2 to obtain a phosphate-based cathode material.

[0081] Example 4 This embodiment provides a phosphate-based cathode material. The length of the phosphate-based cathode material is d1, the thickness is d2, and the length-to-thickness ratio is 100:1. The length d1 of the phosphate-based cathode material is 5 μm, the thickness d2 is 50 nm, and the thickness d3 of the coating layer is 5 nm. All other parameters are the same as in Example 1.

[0082] The preparation method of this embodiment differs from that of Example 1 in that: iron phosphate, iron phosphate, phosphoric acid, and lithium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:Mn:P=1:(1-x):x:1, where x=0.1; sodium dodecylbenzenesulfonate is added as a surfactant to obtain a mixed solution with a surfactant concentration of 0.3mol / L; the mixed solution is subjected to a microwave hydrothermal reaction at a reaction temperature of 150℃ for 15min to obtain a precursor; the precursor and glucose are calcined at a ratio of 1:0.1 to obtain a phosphate-based cathode material.

[0083] Example 5 This embodiment provides a phosphate-based cathode material. The length of the phosphate-based cathode material is d1, the thickness is d2, and the length-to-thickness ratio is 100:1. The length d1 of the phosphate-based cathode material is 20 μm, the thickness d2 is 200 nm, and the thickness d3 of the coating layer is 10 nm. All other parameters are the same as in Example 1.

[0084] The preparation method of this embodiment differs from that of Example 1 in that: iron phosphate, iron phosphate, phosphoric acid, and lithium dihydrogen phosphate are weighed according to the molar ratio of Li:Fe:Mn:P=1:(1-x):x:1, where x=0.9; sodium dodecylbenzenesulfonate is added as a surfactant to obtain a mixed solution with a surfactant concentration of 0.3mol / L; the mixed solution is subjected to a microwave hydrothermal reaction at a reaction temperature of 140℃ for 15min to obtain a precursor; the precursor and glucose are calcined at a ratio of 1:0.2 to obtain a phosphate-based cathode material.

[0085] Example 6 This embodiment provides a phosphate-based cathode material with a coating thickness d3 of 10 nm. All other parameters are the same as in Example 1.

[0086] The difference between the preparation method of this embodiment and that of Example 1 is that the precursor and glucose are calcined in a ratio of 1:0.2 to obtain a phosphate-based cathode material.

[0087] Example 7 This embodiment provides a phosphate-based cathode material with a coating thickness d3 of 20 nm. All other parameters are the same as in Example 1.

[0088] The difference between the preparation method of this embodiment and that of Example 1 is that the precursor and glucose are calcined in a ratio of 1:0.3 to obtain a phosphate-based cathode material.

[0089] Example 8 This embodiment provides a phosphate-based cathode material with a coating thickness d3 of 1 nm. All other parameters are the same as in Example 1.

[0090] The difference between the preparation method of this embodiment and that of Example 1 is that the precursor and glucose are calcined in a ratio of 1:0.02 to obtain a phosphate-based cathode material.

[0091] Example 9 This embodiment provides a phosphate-based cathode material with the same parameters as in Example 1.

[0092] The preparation method of this embodiment differs from that of Example 1 in that the mixed solution is synthesized by hydrothermal method at a reaction temperature of 150°C, a reaction pressure of 100 MPa, and a reaction time of 48 h to obtain the precursor.

[0093] Comparative Example 1 This comparative example provides a phosphate-based cathode material with a length of d1, a thickness of d2, and a length-to-thickness ratio of 5:1. All other parameters are the same as in Example 1.

[0094] The difference between the preparation method of this comparative example and that of Example 1 is that the concentration of the added surfactant is 0.005 mol / L.

[0095] Comparative Example 2 This comparative example provides a phosphate-based cathode material with a length of d1, a thickness of d2, and a length-to-thickness ratio of 200:1. All other parameters are the same as in Example 1.

[0096] The preparation method of this comparative example differs from that of Example 1 in that sodium dodecylbenzenesulfonate is added as a surfactant to obtain a mixed solution with a surfactant concentration of 0.4 mol / L; the mixed solution is subjected to a microwave hydrothermal reaction at a temperature of 150°C for 60 min to obtain the precursor.

[0097] The phosphate-based cathode materials provided in Examples 1-9 and Comparative Examples 1-2 were assembled to form cathode sheets and batteries. The remaining components of the batteries are the same, as shown below: Negative electrode: Lithium sheet.

[0098] Separator: PE / PP membrane.

[0099] Electrolyte: Ethylene carbonate and ethyl methyl carbonate are mixed in a volume ratio of 3:7, and LiPF6 (lithium hexafluorophosphate) is added to form an electrolyte with a concentration of 1 mol / L.

[0100] The electrochemical performance of each battery assembled in the above examples and comparative examples was tested for cycle stability and EIS (electrochemical impedance spectroscopy) under the following conditions: Cyclic stability test: The battery is placed in the Blue Battery test cabinet for charging and discharging. The charging and discharging current is 1C=150mA / g, and the voltage range is 2.00V-4.25V.

[0101] EIS testing: The battery was discharged to 50% SOC (state of charge) on a Gammary electrochemical workstation at a frequency of 100 kHz - 0.05 Hz.

[0102] The test results are shown in the table below.

[0103]

[0104] The test results and experimental procedures of Comparative Examples 1 and 9 show that the phosphate-based cathode material prepared by the microwave hydrothermal method requires a significantly shorter reaction time compared to the traditional hydrothermal method. Furthermore, the battery corresponding to the phosphate-based cathode material prepared in Example 1 exhibits better charge-discharge specific capacity, coulombic efficiency, capacity retention, and impedance than that of Example 9. This indicates that the microwave hydrothermal preparation method not only accelerates the production process but also significantly improves the battery's capacity and impedance performance. The microwave hydrothermal preparation method can further improve the carbon coating in the phosphate-based cathode material. However, the batteries corresponding to the phosphate-based cathode materials prepared in Examples 1 and 9 show better initial charge-discharge specific capacity and initial coulombic efficiency than the phosphate-based cathode materials and batteries of Comparative Examples 1 and 2.

[0105] The test results of Comparative Example 1, Comparative Example 1 and Comparative Example 2 show that when other parameters are the same, an excessively large or small length-to-thickness ratio of the phosphate-based cathode material will affect the capacity and impedance performance of the battery. An excessively large length-to-thickness ratio of the phosphate-based cathode material will prolong the transport path of lithium ions in the length direction, while an excessively small length-to-thickness ratio of the phosphate-based cathode material will increase the transport resistance of lithium ions. Therefore, it is necessary to control the length-to-thickness ratio of the phosphate-based cathode material within a suitable range.

[0106] The test results from Comparative Examples 1 and 3, and Examples 4 and 5 show that, with other parameters remaining the same, increasing the overall length and thickness of the phosphate-based cathode material increases the battery's initial charge-discharge specific capacity, initial coulombic rate, and impedance, but decreases the battery's capacity retention after 200 cycles. This is because an excessively large overall size of the phosphate-based cathode material increases the lithium-ion transport path, leading to excessively high local current density, causing electrode material cracking, increased side reactions, and increased impedance. Conversely, an excessively small overall size of the phosphate-based cathode material may generate stress due to volume changes during charge-discharge, leading to particle cracking and structural damage. Therefore, it is crucial to control the overall length and thickness of the phosphate-based cathode material within a suitable range. However, all results are superior to the phosphate-based cathode materials and batteries of Comparative Examples 1 and 2.

[0107] The test results from Comparative Examples 1, 6, 7, and 8 show that, with other parameters remaining constant, as the thickness of the coating layer increases, the battery's charge-discharge specific capacity and 200-cycle capacity retention rate both exhibit a trend of first increasing and then decreasing, while the battery's impedance value shows a trend of first decreasing and then increasing. This is because an excessively thick coating layer affects the rapid insertion and extraction of lithium ions, while an excessively thin coating layer cannot construct a continuous conductive network and protect the lithium manganese iron phosphate core. Therefore, the thickness of the coating layer must be controlled within a suitable range. However, the battery's charge-discharge specific capacity and capacity retention rate are superior to those of the phosphate-based cathode materials and batteries in Comparative Examples 1 and 2.

[0108] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0109] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A phosphate-based cathode material, characterized in that, The phosphate-based cathode material is in sheet form, with a length of d1 and a thickness of d2. The length-to-thickness ratio of the phosphate-based cathode material is (10-100):1, and the length of the phosphate-based cathode material is the size of its larger surface area.

2. The phosphate-based cathode material according to claim 1, characterized in that, The length d1 of the phosphate-based cathode material is the maximum length of its large surface area, and the length d1 of the phosphate-based cathode material satisfies: 1μm≤d1≤10μm.

3. The phosphate-based cathode material according to claim 1, characterized in that, The thickness of the phosphate-based cathode material is d2, which satisfies the following condition: 50nm≤d2≤500nm.

4. The phosphate-based cathode material according to any one of claims 1-3, characterized in that, The phosphate-based cathode material includes a lithium manganese iron phosphate core and a coating layer, wherein the coating layer covers the outer surface of the lithium manganese iron phosphate core.

5. The phosphate-based cathode material according to claim 4, characterized in that, The thickness of the coating layer is d3, which satisfies: 5nm≤d3≤10nm.

6. A method for preparing a phosphate-based cathode material, used to prepare the phosphate-based cathode material as described in any one of claims 1 to 5, characterized in that, include: Add a surfactant to the mixture to obtain a mixed solution; The mixed solution was subjected to a microwave hydrothermal reaction to obtain a precursor; The precursor is subjected to secondary processing to obtain the phosphate-based cathode material.

7. The method for preparing the phosphate-based cathode material according to claim 6, characterized in that, The surfactant includes one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and hexadecyltrimethylammonium bromide.

8. The method for preparing the phosphate-based cathode material according to claim 7, characterized in that, The concentration of the surfactant is 0.01 mol / L to 0.3 mol / L.

9. The method for preparing the phosphate-based cathode material according to claim 8, characterized in that, The precursor is subjected to secondary processing, including: The carbon source and the precursor are mixed and calcined.

10. A positive electrode plate, characterized in that, Includes phosphate-based cathode materials as described in any one of claims 1-5 or phosphate-based cathode materials obtained by the preparation method described in any one of claims 6 to 9.

11. A battery, characterized in that, Includes the positive electrode sheet as described in claim 10.

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