A composite modified lithium iron manganese phosphate positive electrode material, a preparation method thereof, a positive electrode sheet and a battery

Abstract

This application relates to the field of battery technology, and discloses a composite modified lithium manganese iron phosphate cathode material, its preparation method, cathode sheet, and battery. The lithium manganese iron phosphate cathode material includes a core and a coating layer covering at least a portion of the surface of the core. The coating layer includes a fast ion conductor layer and a carbon coating layer. The carbon coating layer covers at least a portion of the surface of the core, and the fast ion conductor layer covers at least a portion of the surface of the carbon coating layer. The fast ion conductor layer includes a fast ion conductor material, and the cation in the fast ion conductor material includes at least element M. In the lithium manganese iron phosphate cathode material, the total molar amount of Mn, Fe, and M elements is e; the molar amount of M element in the core is c1; the molar amount of M element in the coating layer is d1; c = c1 / e, d = d1 / e; d > c. This scheme improves cycle performance while maintaining the capacity of the lithium manganese iron phosphate cathode material.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a composite modified lithium manganese iron phosphate cathode material, its preparation method, cathode sheet, and battery. Background Technology

[0002] Lithium manganese iron phosphate (LiMnPO4, or LMFP) cathode material is considered a core material for next-generation high-power lithium-ion batteries due to its high voltage platform and high theoretical energy density, and is widely used in electric vehicles, electric aviation, energy storage systems, and high-energy-density portable electronic devices. In the electric vehicle field, the high voltage characteristics of LMFP can be directly matched with high-nickel cathode materials (such as NCM811) to construct multi-level energy density battery systems. Meanwhile, its lower raw material cost and excellent thermal stability give it a significant advantage in scenarios with stringent safety requirements (such as power battery packs). Furthermore, LMFP can serve as a high-power battery material with long cycle life in energy storage systems, meeting the needs of grid peak shaving and frequency regulation, distributed energy storage, and other applications.

[0003] However, the practical application of LMFPs is currently limited by their inherent material defects: low electronic / ionic conductivity leads to poor rate performance, manganese dissolution causes insufficient cycle stability, the Jahn-Teller effect causes structural distortion, and sulfur impurities (such as sulfate raw materials) introduced in traditional processes pose potential safety risks to batteries. These problems severely restrict the commercialization of LMFPs in high-power, long-life batteries.

[0004] To address the aforementioned issues, existing modification strategies primarily involve doping LMFPs, with the dopant elements evenly distributed within the grains to improve LMFP structural stability and enhance cycling performance. However, this approach fails to form a high-concentration protective layer on the surface to suppress manganese dissolution, while the high internal doping concentration occupies active sites, reducing specific capacity.

[0005] Therefore, providing a lithium manganese iron phosphate cathode material that balances high specific capacity and cycle performance is an urgent technical problem to be solved. Summary of the Invention

[0006] This application provides a composite modified lithium manganese iron phosphate cathode material, its preparation method, cathode sheet, and battery to solve the aforementioned technical problems. This lithium manganese iron phosphate cathode material, while ensuring the high energy density advantage of the main material, also considers ion / electron transport capability and structural stability, thereby improving the problem of cathode materials struggling to achieve both high specific capacity and cycle stability.

[0007] In a first aspect, embodiments of this application provide a lithium manganese iron phosphate cathode material, the lithium manganese iron phosphate cathode material comprising a core and a coating layer covering at least a portion of the surface of the core; the coating layer comprises a fast ion conductor layer and a carbon coating layer;

[0008] The carbon coating layer covers at least a portion of the surface of the core, and the fast ion conductor layer covers at least a portion of the surface of the carbon coating layer;

[0009] The chemical formula of the core includes LiMn x Fe 1-x M y PO4, where 0.3 ≤ x ≤ 0.7, 0 < y ≤ 0.02;

[0010] The M element includes at least one of Ti, Zr, Al, La, and Ge;

[0011] The fast ion conductor layer includes a fast ion conductor material, wherein the cations in the fast ion conductor material include at least the element M.

[0012] In the lithium manganese iron phosphate cathode material, the total molar amount of Mn, Fe and M elements is e; the molar amount of M element in the core is c1; the molar amount of M element in the coating layer is d1; c=c1 / e, d=d1 / e; d>c.

[0013] In one possible embodiment, the fast ion conductor material includes at least one of lithium titanium phosphate, lithium zirconium phosphate, NASICON-type lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, garnet-type lithium lanthanum zirconate, and perovskite-type lithium lanthanum titanate.

[0014] In one possible embodiment, the (010) crystal plane strength of the lithium manganese iron phosphate cathode material is a, the (100) crystal plane strength of the lithium manganese iron phosphate cathode material is b, and 3.5≤a / b≤3.9.

[0015] In one possible embodiment, 2.5 ≤ d / c ≤ 3.7;

[0016] And / or; the thickness of the fast ion conductor layer is 2-10 nm;

[0017] And / or; in lithium manganese iron phosphate cathode materials, the carbon coating layer accounts for 1.0-2.5% of the total mass;

[0018] And / or; in lithium manganese iron phosphate cathode materials, the mass percentage of sulfur is less than 50 ppm.

[0019] Secondly, embodiments of this application provide a method for preparing a lithium manganese iron phosphate cathode material. The method is used to prepare the lithium manganese iron phosphate cathode material provided above, and the method includes:

[0020] S1: Manganese, iron and carbon sources are dispersed in a buffer solution, and phosphorus source is added to carry out a co-precipitation reaction to obtain a precursor suspension;

[0021] S2: Dissolve the metal alkoxide ionic conductor precursor containing element M in an organic solvent and add it dropwise to the precursor suspension to obtain the first slurry;

[0022] S3: Add a lithium source to the first slurry and perform a hydrothermal reaction to obtain a second slurry, wherein the temperature of the hydrothermal reaction is 90-135℃;

[0023] S4: The second slurry is spray-dried, sintered in the first, second, and third sintering processes to obtain lithium manganese iron phosphate cathode material.

[0024] The first sintering temperature is 300-400℃ and the first sintering time is 1-4h; the second sintering temperature is 500-600℃ and the second sintering time is 1-3h; the third sintering temperature is 700-750℃ and the third sintering time is 0.1-1.5h.

[0025] In one possible embodiment, in step S1, a manganese source, an iron source, an organic directing agent, and a carbon source are dispersed in a buffer solution. The organic directing agent includes at least one of tartaric acid, citric acid, polyethylene glycol, polyvinyl alcohol, starch, and amino acids.

[0026] In one possible embodiment, the mass of the organic directing agent is 0.5%-2% of the total mass of the manganese and iron sources.

[0027] In one possible embodiment, the metal alkoxide ionic conductor precursor containing element M includes at least one of tetrabutyl titanate, zirconium isopropoxide, aluminum isopropoxide, germanium isopropoxide, and lanthanum isopropoxide.

[0028] And / or; the temperature of the hydrothermal reaction is 100-120℃, and the time of the hydrothermal reaction is 1-4h;

[0029] And / or; the temperature of the third sintering is 710-740℃, and the time of the third sintering is 0.5-1.0h;

[0030] And / or; in the first slurry, the mass of element M is 0.5%-2.5% of the sum of the masses of elements Mn and Fe;

[0031] And / or; the manganese source does not contain sulfur, and the iron source does not contain sulfur.

[0032] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the lithium manganese iron phosphate positive electrode material provided above.

[0033] Fourthly, embodiments of this application provide a battery, which includes the positive electrode sheet provided above.

[0034] This application provides a composite modified lithium manganese iron phosphate cathode material, its preparation method, cathode sheet, and battery. It employs a multi-layer composite structure including a core, a carbon coating layer covering the core surface, and a fast-ion conductor layer coating the carbon coating layer. The core is doped with element M, and the cations in the fast-ion conductor material include at least element M; and d>c. The core, as the lithium storage host, undertakes the main reversible lithium insertion / extraction function. The fast-ion conductor layer is on the outer layer. Its chemically stable properties directly isolate the electrolyte from contact with the inner carbon coating layer and core, preventing oxidative decomposition of the electrolyte under high pressure and also preventing electrolyte corrosion of the core. Furthermore, the fast-ion conductor layer, in direct contact with the electrolyte, can capture lithium ions in the electrolyte at the fastest speed and "pump" the lithium ions into the core through its three-dimensional channels. The carbon coating layer further constructs a continuous electronic conductive network and isolates the core from direct erosion by the external electrolyte. If the carbon coating layer were on the outermost layer, the carbon would easily be oxidized to CO2 during high-voltage charging, thus being consumed and rendered ineffective. By sandwiching a carbon coating layer in the middle, its excellent electronic conductivity (connecting the core and the fast ion conductor layer) is preserved while protecting the carbon coating layer from electrolyte corrosion. This constructs a dual electron / ion conduction network. The inner carbon coating layer is responsible for electron transport, while the outer fast ion conductor layer is responsible for ion transport. Their distinct roles reduce the charge transfer impedance at the interface. Furthermore, d>c, which is beneficial for achieving a synergistic effect of "surface stability and high internal capacity." The high-concentration M-doped layer on the surface (fast ion conductor layer) greatly enhances structural stability and suppresses manganese dissolution and electrolyte side reactions; the low M-doped layer inside retains complete active sites and ideal lattice parameters, maximizing specific capacity and voltage plateau, improving rate performance and cycle life. Finally, without the intermediate carbon coating layer, allowing the core and the outermost fast ion conductor layer to directly contact each other would present the following problems: due to the significant difference in crystal structure and lattice constant between the core and the fast ion conductor layer, a distinct heterogeneous interface would form, easily leading to stress concentration during charging and discharging, causing shell peeling. This application incorporates a carbon coating layer between the core and the outermost fast ion conductor, which greatly enhances the bonding force between the coating layer and the substrate, preventing the outer shell from detaching during cycling. Attached Figure Description

[0035] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0036] Figure 1 XRD patterns of lithium manganese iron phosphate cathode materials provided in Examples 1 and 11 of this application;

[0037] Figure 2HRTEM image of the lithium manganese iron phosphate cathode material of Example 1 provided in this application.

[0038] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0039] To enable those skilled in the art to better understand the present invention, this application will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the invention and are not intended to limit its scope. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. When the following description relates to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements.

[0040] In a first aspect, embodiments of this application provide a composite modified lithium manganese iron phosphate cathode material, the lithium manganese iron phosphate cathode material comprising a core and a coating layer covering at least a portion of the surface of the core; the coating layer comprising a fast ion conductor layer and a carbon coating layer; the carbon coating layer covering at least a portion of the surface of the core, and the fast ion conductor layer covering at least a portion of the surface of the carbon coating layer;

[0041] The core's chemical formula includes LiMn x Fe 1-x M y PO4, where 0.3 ≤ x ≤ 0.7, 0 < y ≤ 0.02;

[0042] The M element includes at least one of Ti, Zr, Al, La, and Ge;

[0043] The fast ion conductor layer includes a fast ion conductor material, wherein the cations in the fast ion conductor material include at least the M element;

[0044] In lithium manganese iron phosphate cathode material, the total molar amount of Mn, Fe and M elements is e; the molar amount of M element in the core is c1; the molar amount of M element in the coating layer is d1; c=c1 / e, d=d1 / e; d>c.

[0045] The lithium manganese iron phosphate cathode material of this application can be constructed into core-shell multilayer composite particles, as follows:

[0046] The core acts as the main lithium storage component, undertaking the primary function of reversible lithium insertion / extraction.

[0047] The fast ion conductor layer is located on the outer layer. Its chemically stable properties allow it to directly isolate the electrolyte from the internal carbon coating and core, preventing oxidation and decomposition of the electrolyte under high pressure and inhibiting corrosion of the core. Furthermore, the fast ion conductor layer's direct contact with the electrolyte enables it to capture lithium ions from the electrolyte at the fastest speed and "pump" them into the core through its three-dimensional channels.

[0048] The carbon coating further constructs a continuous electronic conductivity network and isolates the core from direct erosion by the external electrolyte. If the carbon coating were on the outermost layer, it would be easily oxidized to CO2 during high-voltage charging, thus being consumed and rendered ineffective. By sandwiching the carbon coating in the middle, its excellent electronic conductivity (connecting the core and the fast ion conductor layer) is preserved, while the carbon coating is protected from electrolyte erosion. This creates a dual electronic / ionic conductivity network. The inner carbon coating is responsible for electron transport, while the outer fast ion conductor is responsible for ion transport; their functions are clearly defined, and together they reduce the charge transfer impedance at the interface.

[0049] Furthermore, d>c, which is beneficial for achieving the synergistic effect of "surface stability and high internal capacity". The high-concentration M-doped layer (fast ion conductor layer) on the surface greatly enhances the structural stability and suppresses manganese dissolution and electrolyte side reactions; the low-M-doped region inside retains complete active sites and ideal lattice parameters, maximizing specific capacity and voltage plateau, and improving rate performance and cycle life.

[0050] Finally, without an intermediate carbon coating layer, allowing the core and the outermost fast ion conductor to directly contact each other presents the following problems: due to the significant differences in their crystal structures and lattice constants, a distinct heterogeneous interface forms, which can easily lead to stress concentration and shell peeling during charging and discharging. This application, by incorporating a carbon coating layer between the core and the outermost fast ion conductor, greatly enhances the bonding force between the coating layer and the substrate, preventing the shell from detaching during cycling.

[0051] It is understandable that 0.3 ≤ x ≤ 0.7, for example, 0.3, 0.4, 0.5, 0.6, 0.7 or any two of these values.

[0052] It is understandable that 0 < y ≤ 0.02, such as 0.005, 0.01, 0.015, 0.02, or any two of these values.

[0053] In one possible embodiment, the fast ion conductor material includes at least one of lithium titanium phosphate, lithium zirconium phosphate, NASICON-type lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, garnet-type lithium lanthanum zirconate, and perovskite-type lithium lanthanum titanate.

[0054] In one possible embodiment, the (010) crystal plane strength of the lithium manganese iron phosphate cathode material is a, the (100) crystal plane strength of the lithium manganese iron phosphate cathode material is b, and 3.5≤a / b≤3.9.

[0055] It is understandable that 3.5 ≤ a / b ≤ 3.9, such as 3.5, 3.6, 3.7, 3.8, 3.9, or any two of these values.

[0056] In this embodiment, the (010) crystal plane intensity *a* and (100) crystal plane intensity *b* of the lithium manganese iron phosphate cathode material can be obtained by X-ray diffraction testing. The ratio *a* / *b* is used to characterize the preferred orientation of the crystal planes of the lithium manganese iron phosphate cathode material. By limiting this ratio, the lithium manganese iron phosphate cathode material can have a more obvious (010) crystal plane exposure feature. Appropriately increasing the intensity ratio of the (010) crystal plane relative to the (100) crystal plane can reduce lithium-ion diffusion resistance and improve polarization behavior. When *a* / *b* is within a preset ratio range, the ion transport channels inside the material are smoother, and the co-migration efficiency of electrons and lithium ions is improved, thereby reducing interface impedance and concentration polarization during charging and discharging. Furthermore, the synergistic effect of 3.5≤a / b≤3.9 with the outer fast ion conductor layer and carbon coating layer can enhance the interfacial stability after electrolyte wetting, suppress side reactions and structural collapse, so that the material can still maintain a high capacity utilization rate under high-rate discharge and fast charging conditions, and reduce the decay rate during long cycles, thereby improving the rate performance, cycle life and batch consistency of the electrode material.

[0057] In one possible embodiment, 2.5 ≤ d / c ≤ 3.7.

[0058] In this application, the M element is highly enriched within the coating layer, while the core region remains relatively low-doped, thus balancing the stability of the bulk lattice and the activity of the surface interface. Furthermore, this allows the core to retain the high capacity characteristics of lithium manganese iron phosphate, while the doping distribution is controlled within the thinner coating layer region, reducing interfacial impedance.

[0059] In addition, 2.5≤d / c≤3.7 can more effectively improve the electronic structure and ion migration channels on the particle surface, and suppress the erosion of the surface lattice by the electrolyte and the dissolution of manganese; at the same time, the low doping of the core avoids occupying too many active sites, thereby maintaining the plateau capacity and structural order.

[0060] It is understandable that 2.5 ≤ d / c ≤ 3.7, such as 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7 or any two of these values.

[0061] In one possible embodiment, the thickness of the fast ion conductor layer is 2-10 nm. The fast ion conductor layer within the above range can form a stable ion conduction interface while maintaining a short lithium ion migration path, thereby reducing charge transfer resistance. If the thickness is too small, the coverage will be discontinuous and it will be difficult to suppress side reactions. If it is too thick, it will increase diffusion resistance and occupy the active volume. Therefore, this range takes into account both ion conduction capability and interface protection.

[0062] Understandably, the thickness of the fast ion conductor layer is 2-10 nm, for example, 2 nm, 4 nm, 6 nm, 8 nm, 10 nm or any two of these values.

[0063] In one possible embodiment, the carbon coating layer accounts for 1.0-2.5% of the mass of the lithium manganese iron phosphate cathode material; the carbon coating layer within the above range can form a uniform electronic conductive network on the outer surface of the core, thereby improving conductivity.

[0064] It is understandable that the mass percentage of the carbon coating is 1.0-2.5%, for example, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.5%, or any two of these values.

[0065] In one possible embodiment, the sulfur content in the lithium manganese iron phosphate cathode material is less than 50 ppm by mass. By limiting the sulfur content to extremely low levels, the risks of impurity-induced interfacial side reactions, gas evolution, and metal dissolution can be reduced, thereby ensuring material purity, batch consistency, and safety during long-term storage and cycling. The synergistic effect of the fast ion conductor layer, carbon coating layer, and low sulfur control enables the material to possess efficient ion transport, good electronic conductivity, and a stable interface, thereby improving rate performance, cycle life, and suitability for industrial applications.

[0066] Secondly, embodiments of this application provide a method for preparing a lithium manganese iron phosphate cathode material. The method is used to prepare the lithium manganese iron phosphate cathode material provided above, and the method includes:

[0067] S1: Manganese, iron and carbon sources are dispersed in a buffer solution, and phosphorus source is added to carry out a co-precipitation reaction to obtain a precursor suspension;

[0068] S2: Dissolve the metal alkoxide ionic conductor precursor containing element M in an organic solvent and add it dropwise to the precursor suspension to obtain the first slurry;

[0069] S3: Add a lithium source to the first slurry and perform a hydrothermal reaction to obtain the second slurry, wherein the temperature of the hydrothermal reaction is 90-135℃;

[0070] S4: The second slurry is spray-dried, sintered in the first, second, and third sintering processes to obtain lithium manganese iron phosphate cathode material.

[0071] The first sintering temperature is 300-400℃ and the first sintering time is 1-4h; the second sintering temperature is 500-600℃ and the second sintering time is 1-3h; the third sintering temperature is 700-750℃ and the third sintering time is 0.1-1.5h.

[0072] In this application, manganese, iron, and carbon sources are first dispersed in a buffer solution, which facilitates the uniform adsorption of the carbon source on the surface and in the interstices of the precursor particles. Subsequently, a phosphorus source is added for co-precipitation, which allows the Mn, Fe, and P components to precipitate simultaneously at the microscale, forming a precursor suspension with a relatively uniform composition, thus providing a basis for the subsequent formation of the olivine main phase. The buffer solution is used to maintain pH stability during the precipitation process to control the co-precipitation rate of manganese, iron, and phosphorus and to suppress particle agglomeration caused by local supersaturation.

[0073] Then, the metal alkoxide ionic conductor precursor containing element M is added dropwise to the precursor suspension. Utilizing the self-hydrolysis reaction of the metal alkoxide, an amorphous colloidal layer rich in element M is generated in situ on the surface of the precursor particles. Furthermore, the dropwise addition method avoids excessively rapid local hydrolysis that could lead to the formation of large-sized byproducts, allowing the ionic conductor component to adhere to the particle surface and near-surface region in a finer and more uniform manner. This facilitates the formation of a continuous, fast-acting ionic conductor layer during subsequent heat treatment.

[0074] Secondly, after adding a lithium source and carrying out a hydrolysis reaction, the lithium source can further and uniformly contact the phosphate and metal components in the precursor, so that the metal alkoxide ion conductor precursor containing the M element can be adsorbed, penetrated and initially complexed on the particle surface to form a second slurry with high wettability and uniformity.

[0075] Finally, the second slurry undergoes spray drying to rapidly remove the solvent after instantaneous atomization, forming micron-sized secondary spherical particles. This process improves powder flowability and bulk density, and fixes the precursor components dispersed in the liquid phase into intermediates with better sphericity. The first sintering temperature is controlled at 300-400℃ for 1-4 hours to remove some solvent, slowly decompose organic matter, and perform preliminary carbonization, transforming organic ligands and some free groups on the particle surface into a uniform carbonaceous layer, while preventing premature sintering and agglomeration of particles at high temperatures. The second sintering temperature is controlled at 500-600℃ for 1-3 hours to promote the nucleation and grain growth of the Li, Mn, Fe, and P main phases, gradually forming the olivine structure and improving crystal integrity, while simultaneously driving the diffusion of some M-containing metal alkoxide ionic conductor precursors from the particle surface to the interior. The third sintering temperature is controlled at 700-750℃ and held for 0.1-1.5 hours to complete the main phase shaping, surface ion conductor layer crystallization, and locking of the diffusion degree of M element, so that the fast ion conductor layer forms a continuous and dense thin-layer structure and coexists synergistically with the outer carbon coating layer. By using a staged heating method, the sufficient crystallization of the main phase and the preservation of the surface structure can be balanced, avoiding lithium loss, particle coarsening, and homogenization of doping distribution caused by single-stage high temperature.

[0076] Furthermore, the hydrothermal reaction temperature in this application is 90-135°C, which is relatively low. The hydrothermal reaction in this application is only used to control the pre-formation of the material, so the hydrothermal temperature cannot be too high. Specifically, if the temperature is too low (<90°C), it will lead to incomplete lithium intercalation and poor crystallinity; if the temperature is too high (>135°C), it will lead to excessively rapid grain growth, which will damage the morphology of the nanosheets and easily induce the formation of impurity phases. Secondly, if the hydrothermal reaction temperature is too high, the M element will diffuse inward in this step, which is not conducive to maintaining d>c.

[0077] It is understandable that the temperature of the hydrothermal reaction is 90-135℃, such as 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, or any two of these values.

[0078] It is understandable that the first sintering temperature is 300-400℃, such as 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃ or any two of these values.

[0079] It is understandable that the first sintering time is 1-4 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours or any two of these values.

[0080] It is understandable that the second sintering temperature is 500-600℃, such as 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃ or any two of these values.

[0081] It is understandable that the second sintering time is 1-3 hours, for example, 1 hour, 2 hours, 3 hours or any two of these values.

[0082] Understandably, the temperature for the third sintering is 700-750℃, such as 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, or any two of these values.

[0083] It is understandable that the third sintering time is 0.1-1.5h, for example, 0.1h, 0.5h, 1h, 1.5h or any two of these values.

[0084] In one possible embodiment, in step S1, a manganese source, an iron source, an organic directing agent, and a carbon source are dispersed in a buffer solution. The organic directing agent includes at least one of tartaric acid, citric acid, polyethylene glycol, polyvinyl alcohol, starch, and amino acids.

[0085] This application mixes an organic directing agent with manganese, iron, and carbon sources, allowing organic molecules to form complexes or adsorb onto metal ions through carboxyl, hydroxyl, amino, or ether bonds, thereby regulating the nucleation rate and crystal growth orientation. When the organic directing agent is tartaric acid, citric acid, or an amino acid, its carboxyl and hydroxyl groups preferentially chelate Mn. 2+ and Fe 2+ It reduces the activity of free ions and promotes uniform nucleation of particles; when the organic directing agent is polyethylene glycol, polyvinyl alcohol or starch, its long chain structure can generate steric hindrance on the particle surface, inhibit disordered aggregation and induce primary particle thinning.

[0086] In this application, the synergistic dispersion of manganese source, iron source, organic directing agent, and carbon source in buffer solution ensures that metal ions are in a controlled complexing state before co-precipitation, thereby reducing the risk of large particle agglomeration caused by local supersaturation. Furthermore, the precursor morphology is controlled by the crystal facet selective adsorption of the organic directing agent, making it easier for the subsequently converted lithium manganese iron phosphate cathode material to form a higher (010) crystal facet exposure ratio. Since the organic directing agent can form an adsorption layer or weak complexing layer on the particle surface, it can also improve the dispersion stability of the system, reduce hard agglomeration between particles, and result in precursor particles with a more concentrated particle size distribution and more stable sphericity after spray drying.

[0087] Specifically, organic directing molecules typically possess specific functional groups (such as hydroxyl and carboxyl groups). In the early stages of crystal nucleation and growth, these molecules preferentially and specifically adsorb onto high surface energy crystal faces (typically the (100) and (001) faces for olivine-structured lithium manganese iron phosphate) using spatial matching or electrostatic interactions. This adsorption significantly reduces the surface energy of the (100) and (001) faces, making them relatively thermodynamically stable. The (100) and (001) faces covered by organic molecule adsorption are subject to strong steric hindrance. This means that external lithium ions, iron / manganese ions, and phosphate ions have difficulty crossing this organic molecular barrier to accumulate on the (100) and (001) faces. Therefore, the growth rate of these "blocked" faces is greatly slowed down or even halted. Since the growth of the (100) and (001) faces is inhibited, crystal growth can only proceed rapidly on low surface energy crystal faces with no (or very few) organic molecule adsorption, namely the (010) faces. This results in preferential growth of crystals along the b-axis (

[010] direction), ultimately forming nanosheets, nanorods, or layered morphologies extending along the

[010] direction. Under this morphology, the outer surface with the largest area is naturally the (010) crystal plane, thus achieving a high proportion of exposure of the (010) crystal plane.

[0088] In one possible embodiment, the mass of the organic directing agent is 0.5%-2% of the sum of the masses of the manganese and iron sources, for example, 0.5%, 1%, 1.5%, 2%, or any two of these values.

[0089] In one possible embodiment, the manganese source comprises a manganese salt solution.

[0090] In one possible embodiment, the manganese source includes at least one of manganese carbonate, basic manganese carbonate, manganese hydroxide, manganese acetate, manganese oxalate, and manganese dihydrogen phosphate.

[0091] In one possible embodiment, the iron source includes at least one of ferrous oxalate, ferrous hydroxide, ferrous acetate, ferrous citrate, and ferric dihydrogen phosphate.

[0092] In one possible embodiment, the phosphorus source includes at least one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium dihydrogen phosphate.

[0093] In one possible embodiment, the carbon source includes at least one of glucose, sucrose, polyvinyl alcohol, polyethylene glycol, polydopamine, ascorbic acid, ethylenediaminetetraacetic acid, and starch.

[0094] In one possible embodiment, the lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium nitrate, lithium acetate, and lithium dihydrogen phosphate.

[0095] In one possible embodiment, the hydrothermal reaction temperature is 100-120°C, and the reaction time is 1-4 hours.

[0096] In one possible embodiment, the heating rate of the first sintering is 1-5°C / min; the heating rate of the second sintering is 3-8°C / min; and the heating rate of the third sintering is 5-10°C / min.

[0097] Understandably, the heating rate for the first sintering is 1-5℃ / min, for example, 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or any two of these values.

[0098] Understandably, the heating rate for the second sintering is 3-8℃ / min, for example, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min or any two of these values.

[0099] Understandably, the heating rate for the third sintering is 5-10℃ / min, for example, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min or any two of these values.

[0100] In one possible embodiment, the third sintering temperature is 710-740℃, and the third sintering time is 0.5-1.0h. If the third sintering temperature is too low, the surface conductor layer will not be sufficiently crystallized, and the interface resistance will be too high; if the temperature is too high or the holding time is too long, the M element will easily migrate into the deep core, affecting the d / c value, and may lead to particle growth and loss of active sites. Therefore, the above-mentioned narrow window can balance the integrity of crystallization and the preservation of good performance.

[0101] In one possible embodiment, the metal alkoxide ionic conductor precursor containing element M includes at least one of tetrabutyl titanate, zirconium isopropoxide, aluminum isopropoxide, germanium isopropoxide, and lanthanum isopropoxide.

[0102] In one possible embodiment, the mass of element M in the first slurry is 0.5%-2.5% of the sum of the masses of elements Mn and Fe, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, or any two of these values.

[0103] In one possible embodiment, the manganese source does not contain sulfur, and the iron source does not contain sulfur.

[0104] In one possible embodiment, the buffer solution comprises at least one of the following: acetate-sodium acetate / ammonium / lithium, citric acid-sodium citrate / ammonium, tartaric acid-sodium tartrate / potassium, and dihydrogen phosphate-hydrogen phosphate buffer systems.

[0105] In one possible embodiment, when the phosphorus source is phosphoric acid, the pH value is controlled at 1.5-3.0 when the phosphorus source is phosphoric acid; when the phosphorus source is phosphate, the pH value is controlled at 4.5-6.0 when the phosphorus source is added.

[0106] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the lithium manganese iron phosphate positive electrode material provided above.

[0107] Fourthly, embodiments of this application provide a battery, which includes the positive electrode sheet provided above.

[0108] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will now be described with reference to the accompanying drawings.

[0109] Example 1

[0110] The preparation method of the lithium manganese iron phosphate cathode material in this embodiment is as follows:

[0111] S1 Room Temperature Coprecipitation: At room temperature, manganese carbonate and ferrous oxalate were dispersed in an acetate-sodium acetate buffer solution (pH 5.0), and L-tartaric acid (organic directing agent) and glucose (carbon source) were added. While stirring and ultrasonically dispersing, ammonium dihydrogen phosphate solution was slowly added dropwise, maintaining the pH at 5.0, and reacted at room temperature for 2 hours to obtain a precursor suspension. The molar ratio of Mn / Fe was controlled at 1:1; the molar ratio of (Mn+Fe) / P was controlled at 1:1.035; the amount of L-tartaric acid added was 1.5% of the total mass of manganese carbonate and ferrous oxalate; and the amount of carbon source added was 7% of the total mass of manganese carbonate and ferrous oxalate.

[0112] Construction of the S2 surface doped layer: While stirring, tetrabutyl titanate and aluminum isopropoxide were dissolved in anhydrous ethanol and rapidly added dropwise to the above precursor suspension slurry to obtain the first slurry. Tetrabutyl titanate and aluminum isopropoxide undergo instantaneous self-hydrolysis, forming a Ti / Al-rich amorphous colloidal shell on the particle surface. At this point, the precursor interior contains almost no Ti / Al. The mass of Ti in the first slurry was controlled to be 1.078% of the sum of the masses of Mn and Fe, and the mass of Al in the first slurry was controlled to be 0.107% of the sum of the masses of Mn and Fe.

[0113] S3 Low-Temperature Hydrothermal Lithium Intercalation: A measured amount of lithium hydroxide monohydrate (Li / (Mn+Fe) molar ratio of 1.015:1) is added to the first slurry, which is then transferred to an autoclave and reacted at 110°C for 2 hours to obtain the second slurry. At this point, lithium-ion intercalation induces crystal transformation, and the surface Ti / Al partially diffuses into the shallow layer of the crystal lattice.

[0114] S4 Spray Drying: The second slurry is spray dried, and then heated to 350°C at 2°C / min under N2 atmosphere (first sintering), and held for 2 hours. The temperature is then increased to 580°C at 5°C / min (second sintering), and held for 2 hours. Finally, the temperature is rapidly increased to 730°C at 8°C / min (third sintering), and held for 30 minutes to obtain the lithium manganese iron phosphate cathode material of this embodiment.

[0115] The spray dryer has an inlet temperature of 190℃ and an outlet temperature of 95℃.

[0116] Figure 2 The image shows an HRTEM image of the lithium manganese iron phosphate cathode material of Example 1 provided in this application. It can be seen from the image that the lithium manganese iron phosphate cathode material of Example 1 includes an LMFP core, a carbon coating layer, and a fast ion conductor layer.

[0117] Example 2

[0118] The preparation method of the lithium manganese iron phosphate cathode material in this embodiment is as follows:

[0119] S1 Room Temperature Coprecipitation: At room temperature, manganese carbonate and ferrous oxalate were dispersed in an acetate-sodium acetate buffer solution (pH 5.0), and citric acid (organic directing agent) and glucose (carbon source) were added. While stirring and ultrasonically dispersing, ammonium dihydrogen phosphate solution was slowly added dropwise, maintaining the pH at 5.0, and reacted at room temperature for 2 hours to obtain a precursor suspension. The molar ratio of Mn / Fe was controlled at 0.7:0.3; the molar ratio of (Mn+Fe) / P was 1:1.035; the amount of citric acid added was 0.5% of the total mass of manganese carbonate and ferrous oxalate; and the amount of carbon source added was 7% of the total mass of manganese carbonate and ferrous oxalate.

[0120] Construction of the S2 surface doped layer: While stirring, zirconium isopropoxide was dissolved in anhydrous ethanol and rapidly added dropwise to the above precursor suspension slurry to obtain the first slurry. The mass of zirconium in the first slurry was controlled to be 0.5% of the sum of the masses of Mn and Fe.

[0121] S3 Low-Temperature Hydrothermal Lithium Intercalation: A measured amount of lithium hydroxide monohydrate (Li / (Mn+Fe) molar ratio of 1.015:1) is added to the first slurry, which is then transferred to an autoclave and reacted at 90°C for 4 hours to obtain the second slurry. Lithium-ion intercalation induces crystal transformation, and the surface Ti / Al partially diffuses into the shallow layer of the crystal lattice.

[0122] S4 Spray Drying: The second slurry is spray dried, and then heated to 300℃ at 2℃ / min under N2 atmosphere (first sintering), and held for 4 hours. The temperature is then increased to 500℃ at 5℃ / min (second sintering), and held for 3 hours. Finally, the temperature is rapidly increased to 700℃ at 8℃ / min (third sintering), and held for 1.5 hours to obtain the lithium manganese iron phosphate cathode material of this embodiment.

[0123] The spray dryer has an inlet temperature of 190℃ and an outlet temperature of 95℃.

[0124] Example 3

[0125] The preparation method of the lithium manganese iron phosphate cathode material in this embodiment is as follows:

[0126] S1 Room Temperature Coprecipitation: At room temperature, manganese carbonate and ferrous oxalate were dispersed in an acetate-sodium acetate buffer solution (pH 5.0), and polyethylene glycol (PEG-2000) (organic directing agent) and glucose (carbon source) were added. While stirring and ultrasonically dispersing, ammonium dihydrogen phosphate solution was slowly added dropwise, maintaining the pH at 5.0, and reacted at room temperature for 2 hours to obtain a precursor suspension. The molar ratio of Mn / Fe was controlled at 0.3:0.7; the molar ratio of (Mn+Fe) / P was controlled at 1:1.035; the amount of polyethylene glycol added was 2% of the total mass of manganese carbonate and ferrous oxalate; and the amount of carbon source added was 7% of the total mass of manganese carbonate and ferrous oxalate.

[0127] Construction of the S2 surface doped layer: While stirring, dissolve tetrabutyl titanate in anhydrous ethanol and rapidly add it dropwise to the above precursor suspension slurry to obtain the first slurry. Control the mass of Ti in the first slurry to be 2.5% of the sum of the masses of Mn and Fe.

[0128] S3 Low-Temperature Hydrothermal Lithium Intercalation: A measured amount of lithium hydroxide monohydrate (Li / (Mn+Fe) molar ratio of 1.015:1) is added to the first slurry, which is then transferred to an autoclave and reacted at 135°C for 1 hour to obtain the second slurry. Lithium-ion intercalation induces crystal transformation, and the surface Ti / Al partially diffuses into the shallow layer of the crystal lattice.

[0129] S4 Spray Drying: The second slurry is spray dried, and then heated to 400℃ at 2℃ / min under N2 atmosphere (first sintering), and held for 1 hour. The temperature is then increased to 600℃ at 5℃ / min (second sintering), and held for 1 hour. Finally, the temperature is rapidly increased to 750℃ at 8℃ / min (third sintering), and held for 0.1 h to obtain the lithium manganese iron phosphate cathode material of this embodiment.

[0130] The spray dryer has an inlet temperature of 190℃ and an outlet temperature of 95℃.

[0131] Example 4

[0132] The only difference between this embodiment and Embodiment 1 is that the hydrothermal reaction temperature is 100°C.

[0133] Example 5

[0134] The only difference between this embodiment and Embodiment 1 is that the hydrothermal reaction temperature is 120°C.

[0135] Example 6

[0136] The only difference between this embodiment and Embodiment 1 is that the third sintering temperature is 710℃ and the time is 1.0h.

[0137] Example 7

[0138] The only difference between this embodiment and Embodiment 1 is that the third sintering temperature is 740℃ and the time is 0.5h.

[0139] Example 8

[0140] The only difference between this embodiment and Example 1 is that: manganese acetate is used as the manganese source, ferrous carbonate is used as the iron source; citric acid-sodium citrate (pH 5.2) is used as the buffer solution; polyethylene glycol (PEG-2000) is used as the directing agent; ascorbic acid is used as the carbon source; and ammonium dihydrogen phosphate is used as the phosphorus source.

[0141] Tetrabutyl titanate was replaced with zirconium isopropoxide, aluminum isopropoxide was replaced with lanthanum isopropoxide, and the molar amount of zirconium was controlled to be the same as that of titanium in Example 1, and the molar amount of lanthanum was controlled to be the same as that of aluminum in Example 1.

[0142] Example 9

[0143] The only difference between this embodiment and Example 1 is that the amount of L-tartaric acid added is 0.1% of the total mass of manganese carbonate and ferrous oxalate.

[0144] Example 10

[0145] The only difference between this embodiment and Embodiment 1 is that the amount of L-tartaric acid added is 3% of the total mass of manganese carbonate and ferrous oxalate.

[0146] Example 11

[0147] The only difference between this embodiment and Embodiment 1 is that L-tartaric acid is not added during the process.

[0148] Figure 1 XRD patterns of lithium manganese iron phosphate cathode materials from Examples 1 and 11 provided in this application. Figure 1 It can be seen that the a / b value of Example 1 is greater than that of Example 11, indicating that the addition of the organic directing agent is beneficial to the exposure of the (010) crystal plane.

[0149] Comparative Example 1

[0150] The preparation method of this comparative example is as follows: S1 Room temperature coprecipitation: At room temperature, manganese carbonate, ferrous oxalate, titanium nitrate, and aluminum nitrate were dispersed in an acetate-sodium acetate buffer solution (pH 5.0), and L-tartaric acid (organic directing agent) and glucose (carbon source) were added. While stirring and ultrasonically dispersing, ammonium dihydrogen phosphate solution was slowly added dropwise, maintaining the pH at 5.0, and reacted at room temperature for 2 hours to obtain the first slurry. Specifically, the molar ratio of Mn / Fe was controlled to be 1:1; the molar ratio of (Mn+Fe) / P was 1:1.035; the amount of L-tartaric acid added was 1.5% of the total mass of manganese carbonate and ferrous oxalate; the amount of carbon source added was 7% of the total mass of manganese carbonate and ferrous oxalate; the mass of Ti was controlled to be 1.078% of the sum of the masses of Mn and Fe; and the mass of Al was controlled to be 0.107% of the sum of the masses of Mn and Fe.

[0151] S2 Low-temperature hydrothermal lithium intercalation: Add metered amounts of lithium hydroxide monohydrate (Li / (Mn+Fe) molar ratio of 1.015:1) to the first slurry, transfer to a high-pressure reactor, and react at 110°C for 2 hours to obtain the second slurry.

[0152] S3 Spray Drying: The second slurry was spray dried, and then heated to 350℃ at 2℃ / min under N2 atmosphere (first sintering), and held for 2 hours. The temperature was then increased to 580℃ at 5℃ / min (second sintering), and held for 2 hours. Finally, the temperature was rapidly increased to 730℃ at 8℃ / min (third sintering), and held for 30 minutes to obtain the lithium manganese iron phosphate cathode material of this comparative example.

[0153] The spray dryer has an inlet temperature of 190℃ and an outlet temperature of 95℃.

[0154] Comparative Example 2

[0155] The only difference between this embodiment and Embodiment 1 is that the third sintering time is 2 hours.

[0156] Comparative Example 3

[0157] The only difference between this embodiment and Embodiment 1 is that the temperature of the third sintering is 800℃.

[0158] Comparative Example 4

[0159] The only difference between this embodiment and Embodiment 1 is that the hydrothermal reaction temperature is 145°C.

[0160] Comparative Example 5

[0161] The only difference between this comparative example and Example 1 is that the mass of Ti in the first slurry is controlled to be 2.5% of the sum of the masses of Mn and Fe, and the mass of Al in the first slurry is controlled to be 0.46% of the sum of the masses of Mn and Fe.

[0162] Test case

[0163] 1) Testing of lithium iron phosphate cathode materials

[0164] Selective leaching of the outer fast-ion conductor layer: A citric acid-acetic acid buffer leaching solution was prepared, with glacial acetic acid at a mass fraction of 5% and the pH adjusted to 3.4. The lithium iron phosphate cathode materials of each example and comparative example were added to the above leaching solution at a solid-liquid ratio of 1:30, and leached at a constant temperature of 45°C with stirring for 5 hours. After leaching, the solid and liquid were separated to obtain filtrate and filter cake. The filtrate is the fast-ion conductor layer solution, and the filter cake is the lithium manganese iron phosphate cathode material coated with a carbon coating layer.

[0165] Low-temperature air calcination decarburization: The above filter cake is placed in an air atmosphere furnace and heated to 500°C at a heating rate of 5°C / min, and held at that temperature for 2.5 hours for low-temperature oxidation decarburization; after cooling to room temperature with the furnace, the core is obtained. This method achieves complete separation of the carbon coating layer and the fast ion conductor layer, thus obtaining the core.

[0166] Core chemical formula test (ICP-OES): Weigh approximately 0.1g of the core obtained after the above treatment. Add aqua regia to the core sample and place it in a microwave digester for heating and digestion until the acid on the temperature-controlled hot plate evaporates to dryness. Wash repeatedly with ultrapure water and transfer to a volumetric flask for dilution. Use ICP-OES to determine the emission intensity of elements such as Li, Mn, Fe, P, and M in the solution. Calculate the concentration of each element using a standard curve to obtain the chemical formula of the core.

[0167] Fast ion conductor material testing (XRD): Lithium manganese iron phosphate cathode material powder was thoroughly ground until free of particles. 0.2g of powder was filled into the sample cell and flattened with a glass slide, ensuring the sample surface was flat and flush with the cell surface. A Cu Kα radiation source was used, and the scanning range was set to 10. ° ~90 ° Scanning speed 5 ° / min; After obtaining the diffraction pattern, search the PDF card library to confirm the composition of the fast ion conductor layer.

[0168] Tests of the (010) crystal plane strength a and (100) crystal plane strength b of lithium manganese iron phosphate cathode material: The lithium manganese iron phosphate cathode material powder was thoroughly ground until there were no particles. 0.2g of powder was filled into the sample cell and flattened with a glass slide to ensure that the sample surface was flat and flush with the cell surface. A Cu Kα radiation source was used, and the scanning range was set to 10. ° ~90 ° Scanning speed 5 ° / min; After obtaining the diffraction pattern, find the characteristic diffraction peaks corresponding to the (010) and (100) crystal planes. The absolute intensity values ​​of these two diffraction peaks can be directly read or integrated to obtain a and b. The peak intensity ratio can be further calculated to evaluate the degree of preferred orientation.

[0169] Tests for c and d:

[0170] Core sample (c1 value determination): Accurately weigh 0.1g of the core after low-temperature air calcination and decarburization, place it in a clean microwave digestion vessel, add aqua regia and heat it in a microwave digestion apparatus. After digestion, remove the acid to near dryness on a temperature-controlled hot plate, wash it several times with ultrapure water and transfer it to a volumetric flask for dilution. Use ICP-OES to determine the emission intensity of element M in the solution, and calculate the c1 value of element M content through a standard curve.

[0171] Coated layer sample (d1 value determination): The clear filtrate obtained after leaching with citric acid-acetic acid buffer and solid-liquid separation was directly transferred, and the emission intensity of element M in the solution was determined by ICP-OES. The content of element M, d1 value, was calculated by converting it through the standard curve.

[0172] Measurement of e in lithium manganese iron phosphate cathode material: The total molar amount e of Mn, Fe and M elements in lithium manganese iron phosphate cathode material was measured by ICP.

[0173] Calculate c using c1 / e, and calculate d using d1 / e.

[0174] Fast ion conductor layer thickness test: Lithium manganese iron phosphate cathode material was dispersed in ethanol, ultrasonically dispersed, and then dropped onto a copper grid. After the solvent evaporated, the thickness was measured directly on the photograph based on the contrast difference between the carbon coating layer and the fast ion conductor layer under HRTEM high magnification.

[0175] Mass fraction test of carbon coating: Using a 0.01% balance, accurately weigh 0.1500g of lithium manganese iron phosphate cathode material, add an appropriate amount of pure tungsten granules or tin granules as flux, place the crucible in a high-frequency infrared carbon-sulfur analyzer, introduce pure oxygen, and allow the sample to burn completely at a high temperature (>1500℃). Carbon elements are converted into CO2 gas and released. The instrument measures the absorbance of CO2 gas using an infrared detector, automatically calculates and outputs the mass fraction of the carbon coating.

[0176] Test of the mass percentage of sulfur in lithium manganese iron phosphate cathode material: Weigh 0.1g of lithium manganese iron phosphate cathode material sample into a microwave digestion vessel, add aqua regia and heat in a microwave digester. After digestion, remove acid to near dryness on a temperature-controlled hot plate, wash repeatedly with ultrapure water and transfer to a volumetric flask for volume adjustment; determine the emission intensity of S in the solution using ICP-OES, calculate the concentration through a standard curve, and calculate its mass.

[0177] 2) Battery testing:

[0178] The lithium manganese iron phosphate cathode materials of each embodiment and comparative example were assembled into batteries according to the following method and tested as follows.

[0179] Battery assembly method: After fabricating the lithium manganese iron phosphate cathode material from all the above embodiments and comparative examples into a cathode sheet, it is assembled with a counter electrode, separator, and electrolyte according to the following method to obtain a coin cell. The method includes:

[0180] The lithium manganese iron phosphate cathode materials from the examples and comparative examples were mixed with conductive carbon black and PVDF at a mass ratio of 95:5:5. N-methylpyrrolidone (NMP) was added to adjust the viscosity and disperse the mixture to form a uniform cathode slurry. The slurry was coated on one side of the cathode aluminum foil current collector, pre-dried at 80°C, and vacuum-dried at 120°C for 12 hours. The resulting material was then punched into circular electrode sheets with a diameter of 12 mm. Using a lithium metal sheet as the counter electrode, Celgard 2400 as the separator, and an EC / DEC / EMC (volume ratio 1:1:1) solution as the electrolyte, 13% lithium hexafluorophosphate was added to the electrolyte. The mixture was then assembled into a CR2032 coin cell in an argon-filled glove box (water and oxygen content <0.1 ppm).

[0181] 0.2C specific capacity and first-time efficiency: The coin cells assembled in the above examples and comparative examples were left to stand at 25°C for 12 hours, and then charged and discharged using a battery charge and discharge tester. The test regime was as follows: constant current charging at 0.2C to 4.35V, then constant voltage charging until the current drops to 0.05C cutoff, left to stand for 5 minutes, and then constant current discharging at 0.2C to 2.5V. The first discharge specific capacity (0.2C discharge specific capacity) and the first charge specific capacity (0.2C charge specific capacity) were recorded. The first-time efficiency was the ratio of the first discharge specific capacity to the first charge specific capacity.

[0182] Cycle retention rate at 0.5C at 45℃: Fully charged coin cells were placed in a high and low temperature test chamber at 45℃ for 10 hours. Then, they were charged at 0.5C constant current to 4.35V at 45℃, followed by constant voltage charging until the current dropped to 0.05C (cutoff). After standing for 5 minutes, they were discharged at 0.5C constant current to 2.5V. The initial discharge specific capacity at 45℃ was recorded. This process was repeated, and the discharge specific capacity after the 200th cycle was recorded. The cycle retention rate at 0.5C at 45℃ was calculated as the discharge specific capacity after the 200th cycle divided by the initial discharge specific capacity.

[0183] The test results are shown in Tables 1 and 2.

[0184] Table 1

[0185]

[0186] Table 2

[0187]

[0188] As shown in Tables 1-2, compared with Comparative Example 1, although fast ion conductor layers can be generated due to the simultaneous addition of titanium nitrate, aluminum nitrate, manganese carbonate, and ferrous oxalate, the uniform distribution of M element doping results in d not being greater than c, leading to a decrease in capacity and cycle performance.

[0189] Compared with Example 1 and Comparative Example 2, due to the long third sintering time, the diffusion of M element is intensified, resulting in d / c being less than 1, which leads to a decrease in the capacity and cycle performance of the cathode material.

[0190] Compared with Example 1 and Comparative Example 3, the diffusion of M element was intensified due to the excessively high temperature of the third sintering, resulting in d / c being less than 1, which led to a decrease in the capacity and cycle performance of the cathode material.

[0191] Compared with Example 1 and Comparative Example 4, the excessively high temperature of the hydrothermal reaction caused some M elements to diffuse more rapidly during the hydrothermal process, resulting in d / c being less than 1, which reduced the capacity and cycle performance of the cathode material.

[0192] Compared with Example 1 and Comparative Example 5, the excessive addition of M element resulted in a slightly higher M doping level in the core, leading to a decrease in electrical performance.

[0193] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A composite modified lithium iron phosphate cathode material, characterized in that, The lithium manganese iron phosphate cathode material includes a core and a coating layer covering at least a portion of the surface of the core; The coating layer includes a fast ion conductor layer and a carbon coating layer; the carbon coating layer covers at least a portion of the surface of the core, and the fast ion conductor layer covers at least a portion of the surface of the carbon coating layer; The chemical formula of the core includes LiMn x Fe 1-x M y PO4, where 0.3 ≤ x ≤ 0.7, 0 < y ≤ 0.02; The M element includes at least one of Ti, Zr, Al, La, and Ge; The fast ion conductor layer includes a fast ion conductor material, wherein the cations in the fast ion conductor material include at least the element M. In the lithium manganese iron phosphate cathode material, the total molar amount of Mn, Fe and M elements is e; the molar amount of M element in the core is c1; the molar amount of M element in the coating layer is d1; c=c1 / e, d=d1 / e; d>c.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The fast ion conductor material includes at least one of lithium titanium phosphate, lithium zirconium phosphate, NASICON-type lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, garnet-type lithium lanthanum zirconate, and perovskite-type lithium lanthanum titanate.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The (010) crystal plane strength of the lithium manganese iron phosphate cathode material is a, and the (100) crystal plane strength of the lithium manganese iron phosphate cathode material is b, with 3.5 ≤ a / b ≤ 3.

9.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that, 2.5≤d / c≤3.7; And / or; the thickness of the fast ion conductor layer is 2-10 nm; And / or; in the lithium manganese iron phosphate cathode material, the carbon coating layer accounts for 1.0-2.5% by mass; And / or; in the lithium manganese iron phosphate cathode material, the mass percentage of sulfur is less than 50 ppm.

5. A method for preparing lithium manganese iron phosphate cathode material according to any one of claims 1-4, characterized in that, The method includes: S1: Manganese, iron and carbon sources are dispersed in a buffer solution, and phosphorus source is added to carry out a co-precipitation reaction to obtain a precursor suspension; S2: Dissolve the metal alkoxide ionic conductor precursor containing element M in an organic solvent and add it dropwise to the precursor suspension to obtain the first slurry; S3: Add a lithium source to the first slurry and perform a hydrothermal reaction to obtain a second slurry, wherein the temperature of the hydrothermal reaction is 90-135℃; S4: The second slurry is spray-dried, sintered in the first, second, and third sintering processes to obtain the lithium manganese iron phosphate cathode material. The first sintering temperature is 300-400℃ and the first sintering time is 1-4h; the second sintering temperature is 500-600℃ and the second sintering time is 1-3h; the third sintering temperature is 700-750℃ and the third sintering time is 0.1-1.5h.

6. The preparation method according to claim 5, characterized in that, In step S1, a manganese source, an iron source, an organic directing agent, and a carbon source are dispersed in a buffer solution. The organic directing agent includes at least one of tartaric acid, citric acid, polyethylene glycol, polyvinyl alcohol, starch, and amino acids.

7. The preparation method according to claim 6, characterized in that, The mass of the organic directing agent is 0.5%-2% of the sum of the masses of the manganese source and the iron source.

8. The preparation method according to claim 5, characterized in that, The metal alkoxide ionic conductor precursor containing element M includes at least one of tetrabutyl titanate, zirconium isopropoxide, aluminum isopropoxide, germanium isopropoxide, and lanthanum isopropoxide. And / or; the temperature of the hydrothermal reaction is 100-120℃, and the time of the hydrothermal reaction is 1-4h; And / or; the temperature of the third sintering is 710-740℃, and the time of the third sintering is 0.5-1.0h; And / or; in the first slurry, the mass of element M is 0.5%-2.5% of the sum of the masses of elements Mn and Fe; And / or; the manganese source does not contain sulfur, and the iron source does not contain sulfur.

9. A positive electrode plate, characterized in that, The positive electrode includes lithium manganese iron phosphate positive electrode material according to any one of claims 1-4.

10. A battery, characterized in that, The battery includes the positive electrode according to claim 9.

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