A lithium iron manganese phosphate positive electrode material, a preparation method thereof and a lithium ion battery

Abstract

This invention provides a lithium manganese iron phosphate (LMP) cathode material, its preparation method, and a lithium-ion battery. The LMP cathode material includes a core, and a first shell and a second shell sequentially coated on the surface of the core. The core comprises boron-doped LMP, the first shell is a boron-doped alumina layer, and the second shell is a lithium niobate tantalate layer. The boron doping level in the LMP cathode material gradually decreases from the core to the second shell. This invention constructs a multi-layered structure for synergistic protection and performs gradient boron doping on the core and the first shell. This not only improves the intrinsic electronic / ionic conductivity of LMP but also avoids interfacial stress caused by abrupt changes in elemental distribution between different material layers, achieving a balance between "high bulk conductivity and low interfacial impedance," thus improving overall electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a lithium manganese iron phosphate cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] As a promising candidate cathode material for lithium-ion batteries, lithium manganese iron phosphate (LMFP) has shown broad application prospects in power batteries and energy storage batteries due to its high theoretical specific capacity, advantageous voltage platform, controllable cost, and excellent thermal stability. However, the intrinsic electronic conductivity of LMFP is much lower than that of lithium iron phosphate. Furthermore, the diffusion of lithium ions in the crystal lattice is limited, resulting in a low lithium-ion diffusion coefficient, which restricts the high-rate charge-discharge performance of LMFP. Secondly, LMFP suffers from insufficient surface chemical stability, primarily due to the easy dissolution of transition metal ions such as manganese ions in the electrolyte. This disrupts the lattice integrity of LMFP, leading to particle cracking. Additionally, LMFP has poor interfacial compatibility with the electrolyte, easily forming an unstable SEI film on the surface during charge-discharge, resulting in severe interfacial side reactions that can easily cause battery swelling and rapid capacity decay.

[0003] To address the aforementioned issues, existing technologies primarily improve performance through carbon coating (to enhance conductivity) and metal ion doping (such as magnesium ions and aluminum ions to suppress Mn dissolution). However, these technologies still have limitations: the carbon coating layer is prone to detachment during cycling, single ion doping cannot simultaneously solve the problems of electronic conduction and surface stability, and excessive carbon coating layers can reduce the specific capacity of the cathode material, leading to a decrease in rate performance and cycle performance.

[0004] Therefore, it is of great significance to develop a modification strategy that can synergistically improve the electronic / ionic conductivity and surface stability of LMFP. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a lithium manganese iron phosphate cathode material, its preparation method, and a lithium-ion battery. The present invention constructs a multi-layered shell for synergistic protection and performs gradient boron doping on the core and the first outer shell. This not only improves the intrinsic electronic / ionic conductivity of lithium manganese iron phosphate but also avoids interfacial stress caused by abrupt changes in elemental distribution between different material layers, achieving a balance between "high bulk conductivity and low interfacial impedance," thus improving overall electrochemical performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a lithium manganese iron phosphate cathode material, the lithium manganese iron phosphate cathode material comprising a core, and a first outer shell and a second outer shell sequentially covering the surface of the core.

[0008] The core comprises boron-doped lithium manganese iron phosphate material, the first shell is a boron-doped alumina layer, and the second shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell.

[0009] This invention constructs a multi-layered shell for synergistic protection. The boron-doped alumina layer exhibits excellent chemical compatibility with the core, tightly coating the core surface and protecting its structural integrity. Simultaneously, its dense structure further blocks the diffusion of transition metal ions into the electrolyte and forms a dual barrier with the lithium niobate tantalate layer, significantly reducing interfacial side reactions. The lithium niobate tantalate layer possesses excellent chemical stability and resistance to electrolyte corrosion, and as the outermost shell, it directly blocks the contact between the lithium manganese iron phosphate cathode material and the electrolyte.

[0010] This invention involves boron doping of the core and the first shell, allowing boron to enter the lithium manganese iron phosphate lattice and form electron hopping channels. This reduces lithium-ion migration resistance and improves bulk conductivity. Boron doping in the alumina layer allows the first shell to form an Al-OB bond network, increasing its electronic conductivity and reducing the interfacial impedance between the core and the second shell. Furthermore, the boron doping level gradually decreases from the core to the second shell, avoiding interfacial stress caused by abrupt changes in elemental distribution between different material layers. This ensures sufficient boron doping in the core to improve bulk conductivity while leveraging the structural integrity of the boron-free second shell, achieving a balance between high bulk conductivity and low interfacial impedance.

[0011] The lithium manganese iron phosphate cathode material designed in this invention not only solves the problem of low intrinsic electronic / ionic conductivity of lithium manganese iron phosphate, but also improves its insufficient surface stability, achieving a comprehensive improvement in electrochemical performance with high rate, long cycle and high safety, providing an effective solution for the industrial application of lithium-ion batteries.

[0012] Preferably, the particle size D50 of the core is 100-500nm, for example, it can be 100nm, 200nm, 300nm, 400nm or 500nm, etc.

[0013] Preferably, in the boron-doped lithium manganese iron phosphate material, the boron doping amount is 0.3-1.0 wt%, for example, it can be 0.3 wt%, 0.5 wt%, 0.8 wt%, or 1.0 wt%.

[0014] Preferably, in the core, the amount of boron doping gradually decreases in the radial direction from the core center.

[0015] The radial gradient of "high boron in the core and low boron in the surface" designed in this invention avoids internal particle cracks caused by the overall expansion of the core during uniform doping. At the same time, the high boron core region can still retain sufficient lattice defects and electron jumping channels, ensuring that the bulk electronic conductivity is not affected. Furthermore, the low boron surface region has a more complete lattice, further reducing the difference in boron doping amount with the first shell, resulting in better interface matching and forming a tighter gradient bond, avoiding interface delamination. Meanwhile, the boron content transitions smoothly from the core surface to the first shell, allowing for smooth electron transport through the BO bond network and improving the interface electronic conductivity.

[0016] Preferably, the thickness of the first outer shell is 10-30nm, for example, it can be 10nm, 20nm or 30nm.

[0017] Preferably, the boron doping amount in the boron-doped alumina layer is 0.1-0.2 wt%, for example, it can be 0.1 wt%, 0.12 wt%, 0.14 wt%, 0.16 wt%, 0.18 wt%, or 0.2 wt%.

[0018] Preferably, the thickness ratio of the first outer shell to the second outer shell is 1:(1.5-3), for example, it can be 1:1.5, 1:2, 1:2.5 or 1:3, etc.

[0019] In this invention, a suitable thickness ratio is beneficial for the first shell to fully play its role as a conductive bridge, ensuring efficient ion / electron transport between the core and shell. At the same time, it allows the second shell to form a sufficiently dense protective barrier, suppressing the dissolution of transition metal (manganese ions) and side reactions of the electrolyte. The two work together to reduce the interface impedance and improve the cycle capacity retention rate of the cathode material, avoiding the performance shortcomings caused by an excessively thick single shell.

[0020] Preferably, the lithium niobate tantalate layer is doped with rare earth elements.

[0021] Preferably, based on the mass of the lithium niobate tantalate layer, the doping amount of the rare earth element is 0.2-1 wt%, for example, it can be 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, or 1 wt%.

[0022] In this invention, the doping of a suitable amount of rare earth elements can optimize the lithium-ion transport channel through the lattice regulation effect of rare earth ions. This content range can avoid lattice distortion or second phase formation caused by excessive rare earth ions, ensuring the structural integrity of the lithium niobate tantalate layer, and further improving its interface compatibility with the first shell while maintaining excellent barrier performance.

[0023] Preferably, the rare earth element includes lanthanum and / or cerium.

[0024] Secondly, the present invention provides a method for preparing lithium manganese iron phosphate cathode material as described in the first aspect, the method comprising the following steps:

[0025] A core is provided, the core comprising boron-doped lithium manganese iron phosphate material.

[0026] The boron-doped lithium manganese iron phosphate material is coated with a first shell and a second shell in sequence to obtain the lithium manganese iron phosphate cathode material.

[0027] The first outer shell is a boron-doped alumina layer; the second outer shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell.

[0028] Preferably, the preparation method of the boron-doped lithium manganese iron phosphate material includes a solid-phase dispersion method.

[0029] Preferably, the specific steps of the solid-phase dispersion method include:

[0030] (a1) The lithium manganese iron phosphate material precursor and the first boron source are mixed and sintered to obtain the boron-doped precursor material.

[0031] (b1) The boron-doped precursor material and the lithium source are mixed and calcined in an inert atmosphere to obtain the boron-doped lithium manganese iron phosphate material.

[0032] It should be noted that the present invention does not limit the specific type of inert atmosphere; for example, it may be nitrogen or argon.

[0033] Preferably, the general chemical formula of the lithium manganese iron phosphate material precursor is Mn. x Fe 1-x PO4, where 0 < x < 1. For example, it could be 0.2, 0.4, 0.6, or 0.8, etc.

[0034] Preferably, the first boron source includes any one or a combination of at least two of boric acid, boron oxide, or lithium borate.

[0035] It should be noted that the present invention does not specifically limit the type of lithium source. For example, it can be lithium carbonate, lithium hydroxide, lithium nitrate, or lithium acetate.

[0036] Preferably, the sintering process is constant-temperature sintering or variable-temperature sintering.

[0037] Preferably, the isothermal sintering temperature is 700-750℃, for example, 700℃, 710℃, 720℃, 730℃, 740℃ or 750℃, and the time is 4-8h, for example, 4h, 5h, 6h, 7h or 8h.

[0038] Preferably, the variable-temperature sintering is cooling sintering.

[0039] This invention employs a cooling sintering method to construct a gradient distribution of boron doping in the formed core. This method does not require additional complex equipment; the boron gradient distribution can be fixed solely through temperature control. This ensures that the core center has sufficient boron doping to improve bulk conductivity, while reducing lattice distortion through low boron content in the surface layer. Furthermore, it forms a coherent overall gradient system with the boron doping of the subsequent first shell layer, further enhancing the performance synergy of the core-shell structure.

[0040] Preferably, during the cooling sintering process, the initial temperature is 750-800℃, for example, 750℃, 760℃, 770℃, 780℃, 790℃ or 900℃, and the final temperature is 600-650℃, for example, 600℃, 610℃, 620℃, 630℃, 640℃ or 650℃, the cooling rate is 1-10℃ / min, for example, 1℃ / min, 3℃ / min, 5℃ / min, 7℃ / min or 9℃ / min, and the cooling time is 0.5-2.5h, for example, 0.5h, 1h, 1.5h, 2h or 2.5h.

[0041] Preferably, the calcination treatment temperature is 650-800℃, for example, 650℃, 700℃, 750℃ or 800℃, and the time is 5-15h, for example, 5h, 8h, 10h, 12h or 15h.

[0042] Preferably, the coating method for the boron-doped alumina layer includes the sol-gel method.

[0043] In this invention, the boron-doped alumina layer is prepared by the sol-gel method, which not only forms a coating layer with controllable thickness and continuity, but also ensures that boron elements in the first shell do not agglomerate, further stabilizing the conductive network and ion diffusion channels of the Al-OB bond. In addition, the deposition can be completed at low temperature, which can avoid the boron gradient already formed in the core from being disrupted by diffusion at high temperature. Therefore, it can ensure the dual function of "conductivity and barrier" of the first shell, while also taking into account the stability of the original structure and gradient distribution of the core.

[0044] Preferably, the specific steps of the sol-gel method include:

[0045] (a2) Mix the aluminum source, the second boron source, the chelating agent, the dispersant and the solvent to obtain boron-doped aluminum sol.

[0046] (b2) The boron-doped lithium manganese iron phosphate material and the boron-doped aluminum sol are mixed, dried, and then the dried material is heat-treated to obtain the boron-doped aluminum oxide layer.

[0047] It should be noted that the present invention does not limit the type of solvent. For example, it can be deionized water, ethanol, or isopropanol.

[0048] Preferably, the second boron source includes any one or a combination of at least two of boric acid, boron oxide, or lithium borate.

[0049] Preferably, the aluminum source includes Al(NO3)3·9H2O and / or aluminum isopropoxide.

[0050] Preferably, the chelating agent includes any one or a combination of at least two of citric acid, EDTA (ethylenediaminetetraacetic acid), acetylacetone, or tartaric acid.

[0051] Preferably, the dispersant comprises any one or a combination of at least two of polyvinylpyrrolidone, polyethylene glycol, or sodium dodecylbenzenesulfonate.

[0052] Preferably, the pH value of the boron-doped aluminum sol is 5-7, for example, it can be 5, 5.5, 6, 6.5 or 7.

[0053] Preferably, the heat treatment temperature is 450-750℃, for example, 450℃, 550℃, 650℃ or 750℃, and the time is 5-10h, for example, 5h, 6h, 7h, 8h, 9h or 10h.

[0054] Preferably, the coating method for the lithium niobate tantalate layer includes a wet coating method.

[0055] Preferably, the specific steps of the wet coating method include:

[0056] (a3) Mix the niobium source, tantalum source, lithium source and solvent to obtain a precursor solution.

[0057] (b3) The precursor solution is sprayed onto the outer surface of the boron-doped lithium manganese iron phosphate material with a boron-doped alumina layer deposited, and then calcined to obtain the lithium niobate tantalate layer.

[0058] Preferably, the solvent includes any one of ethanol, isopropanol, or an ethanol-citric acid mixture.

[0059] Preferably, the niobium source includes any one or a combination of at least two of niobium ethanol, niobium oxide, or niobium isopropoxide.

[0060] Preferably, the tantalum source includes any one or a combination of at least two of tantalum ethoxide, tantalum oxide, or tantalum isopropoxide.

[0061] It should be noted that the present invention does not limit the type of lithium source. For example, it can be lithium nitrate, lithium acetate, lithium ethanol, or lithium isopropoxide.

[0062] Preferably, the molar ratio of the niobium source and the tantalum source is (0.15-0.85):(0.85-0.15), wherein the niobium source selection range "0.15-0.85" can be, for example, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, or 0.85, and the tantalum source selection range "0.85-0.15" can be, for example, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, or 0.85.

[0063] Preferably, the calcination temperature is 300-600℃, for example, 300℃, 400℃, 500℃ or 600℃, and the time is 2-6h, for example, 2h, 3h, 4h, 5h or 6h.

[0064] Preferably, the atmosphere for the calcination treatment is an inert atmosphere. For example, it could be nitrogen or argon.

[0065] Preferably, a rare earth element source is also added during the mixing process of the niobium source, tantalum source, lithium source and solvent.

[0066] Preferably, the preparation method includes the following steps:

[0067] (1) Kernel preparation:

[0068] (a1) A lithium manganese iron phosphate precursor and a first boron source are mixed and sintered in an inert atmosphere, then cooled to room temperature to obtain a boron-doped precursor material; wherein the general chemical formula of the lithium manganese iron phosphate precursor is Mn x Fe 1- x PO4, where 0 < x < 1; the first boron source includes any one or a combination of at least two of boric acid, boron oxide or lithium borate; the sintering treatment is cooling sintering, with an initial temperature of 750-800℃, a final temperature of 600-650℃, a cooling rate of 1-10℃ / min, and a cooling time of 0.5-2.5h.

[0069] (b1) The boron-doped precursor material and the lithium source are mixed and calcined in an inert atmosphere to obtain the boron-doped lithium manganese iron phosphate material; wherein the calcination temperature is 650-800℃ and the time is 5-15h.

[0070] (2) Preparation of the first shell:

[0071] (a2) An aluminum source, a second boron source, a chelating agent, a dispersant, and a solvent are mixed to obtain a boron-doped aluminum sol with a pH of 5-7; wherein the aluminum source includes Al(NO3)3·9H2O and / or aluminum isopropoxide, the second boron source includes any one or a combination of at least two of boric acid, boron oxide, or lithium borate, the chelating agent includes any one or a combination of at least two of citric acid, EDTA, acetylacetone, or tartaric acid, and the dispersant includes any one or a combination of at least two of polyvinylpyrrolidone, polyethylene glycol, or sodium dodecylbenzenesulfonate.

[0072] (b2) Under ultrasonic-assisted dispersion, the boron-doped lithium manganese iron phosphate material is immersed in the boron-doped aluminum sol, stirred evenly, dried, and then heat-treated to obtain the boron-doped alumina layer; wherein the heat treatment temperature is 400-750℃ and the time is 5-10h.

[0073] (3) Preparation of the second shell:

[0074] (a3) Under stirring conditions, a niobium source, a tantalum source, a lithium source and a solvent are mixed to obtain a precursor solution; wherein the niobium source includes any one or a combination of at least two of niobium ethoxide, niobium oxide or niobium isopropoxide, and the tantalum source includes any one or a combination of at least two of tantalum ethoxide, tantalum oxide or tantalum isopropoxide; the molar ratio of the niobium source and the tantalum source is (0.15-0.85):(0.85-0.15).

[0075] (b3) The precursor solution is sprayed onto the outer surface of the boron-doped alumina layer and then calcined in an inert atmosphere to obtain the lithium niobate tantalate layer; wherein the calcination temperature is 300-600℃ and the time is 2-6h.

[0076] Thirdly, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery includes the lithium manganese iron phosphate positive electrode material as described in the first aspect.

[0077] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0078] Compared with the prior art, the present invention has the following beneficial effects:

[0079] (1) The present invention constructs a multi-layer shell for synergistic protection. The boron-doped alumina layer has excellent chemical compatibility with the core and can tightly cover the core surface to protect the structural integrity of the core. At the same time, its dense structure can further block the diffusion of transition metal ions into the electrolyte and form a double barrier with the lithium niobate tantalate layer, which greatly reduces the interfacial side reactions. The lithium niobate tantalate layer has excellent chemical stability and resistance to electrolyte corrosion. As the outermost shell layer, it can directly block the contact between the lithium manganese iron phosphate cathode material and the electrolyte.

[0080] (2) In this invention, boron doping is applied to the core and the first shell, allowing boron to enter the lithium manganese iron phosphate lattice, forming an electron hopping channel, reducing lithium-ion migration resistance, and improving the bulk conductivity. Furthermore, boron doping also enters the alumina layer, enabling the first shell to form an Al-OB bond network through boron doping, thus improving the electronic conductivity of the first shell and reducing the interfacial impedance between the core and the second shell. Moreover, the amount of boron doping gradually decreases from the core to the second shell, avoiding interfacial stress caused by abrupt changes in elemental distribution between different material layers. This ensures that the core has sufficient boron doping to improve bulk conductivity, while leveraging the structural integrity of the boron-free second shell, achieving a balance between "high bulk conductivity and low interfacial impedance."

[0081] (3) The lithium manganese iron phosphate cathode material designed in this invention not only solves the problem of low intrinsic electronic / ionic conductivity of lithium manganese iron phosphate, but also improves its insufficient surface stability, thus achieving a comprehensive improvement in electrochemical performance with high rate, long cycle and high safety, providing an effective solution for the industrial application of lithium-ion batteries. Detailed Implementation

[0082] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0083] Example 1

[0084] This embodiment provides a lithium manganese iron phosphate cathode material, which includes a core and a first outer shell and a second outer shell sequentially covering the surface of the core.

[0085] The core comprises boron-doped lithium manganese iron phosphate material, the first shell is a boron-doped alumina layer, and the second shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell; in the core, the boron doping amount gradually decreases radially from the center of the core.

[0086] The core has a particle size D50 of 300 nm; the boron doping content in the boron-doped lithium manganese iron phosphate material is 0.65 wt%; the thickness of the first shell is 20 nm; the boron doping content in the boron-doped alumina layer is 0.15 wt%; and the thickness ratio of the first shell to the second shell is 1:2.

[0087] This embodiment also provides a method for preparing the above-mentioned lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0088] (1) Kernel preparation:

[0089] (a1) A lithium manganese iron phosphate precursor and boron oxide are mixed and sintered under a nitrogen atmosphere at a decreasing temperature, and then cooled to room temperature to obtain a boron-doped precursor material; wherein, the general chemical formula of the lithium manganese iron phosphate precursor is Mn x Fe 1-x PO4, where x = 0.7; the sintering treatment method is cooling sintering, with an initial temperature of 775℃, a final temperature of 625℃, a cooling rate of 5℃ / min, and a cooling time of 0.5h.

[0090] (b1) The boron-doped precursor material and lithium carbonate are mixed and calcined in a nitrogen atmosphere to obtain the boron-doped lithium manganese iron phosphate material; wherein the calcination temperature is 750°C and the time is 10h.

[0091] (2) Preparation of the first shell:

[0092] (a2) Al(NO3)3·9H2O, boric acid, citric acid, polyvinylpyrrolidone and ethanol solvent were mixed to obtain boron-doped aluminum sol with a pH of 6.

[0093] (b2) Under ultrasonic-assisted dispersion, the boron-doped lithium manganese iron phosphate material is immersed in the boron-doped aluminum sol, stirred evenly, dried, and then heat-treated to obtain the boron-doped alumina layer; wherein the heat treatment temperature is 550°C and the time is 8h.

[0094] (3) Preparation of the second shell:

[0095] (a3) Under stirring conditions, niobium ethoxide, tantalum ethoxide, lithium ethoxide and ethanol solvent are mixed to obtain a precursor solution; wherein the molar ratio of niobium ethoxide and tantalum ethoxide is 0.5:0.5.

[0096] (b3) The precursor solution is sprayed onto the outer surface of the boron-doped alumina layer and then calcined in a nitrogen atmosphere to obtain the lithium niobate tantalate layer; wherein the calcination temperature is 450°C and the time is 4h.

[0097] Example 2

[0098] This embodiment provides a lithium manganese iron phosphate cathode material, which includes a core and a first outer shell and a second outer shell sequentially covering the surface of the core.

[0099] The core comprises boron-doped lithium manganese iron phosphate material, the first shell is a boron-doped alumina layer, and the second shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell; in the core, the boron doping amount gradually decreases radially from the center of the core.

[0100] The core has a particle size D50 of 100 nm; the boron doping amount in the boron-doped lithium manganese iron phosphate material is 0.3 wt%; the thickness of the first shell is 10 nm; the boron doping amount in the boron-doped alumina layer is 0.1 wt%; and the thickness ratio of the first shell to the second shell is 1:3.

[0101] This embodiment also provides a method for preparing the above-mentioned lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0102] (1) Kernel preparation:

[0103] (a1) A lithium manganese iron phosphate precursor and boron oxide are mixed and sintered under a nitrogen atmosphere at a decreasing temperature, and then cooled to room temperature to obtain a boron-doped precursor material; wherein, the general chemical formula of the lithium manganese iron phosphate precursor is Mn x Fe 1-x PO4, where x = 0.7; the sintering treatment method is cooling sintering, with an initial temperature of 750℃, a final temperature of 650℃, a cooling rate of 1℃ / min, and a cooling time of 1.7h.

[0104] (b1) The boron-doped precursor material and lithium carbonate are mixed and calcined in a nitrogen atmosphere to obtain the boron-doped lithium manganese iron phosphate material; wherein the calcination temperature is 650°C and the time is 15h.

[0105] (2) Preparation of the first shell:

[0106] (a2) Al(NO3)3·9H2O, boric acid, citric acid, polyvinylpyrrolidone and ethanol solvent were mixed to obtain boron-doped aluminum sol with a pH of 5.

[0107] (b2) Under ultrasonic-assisted dispersion, the boron-doped lithium manganese iron phosphate material is immersed in the boron-doped aluminum sol, stirred evenly, dried, and then heat-treated to obtain the boron-doped alumina layer; wherein the heat treatment temperature is 400℃ and the time is 10h.

[0108] (3) Preparation of the second shell:

[0109] (a3) Under stirring conditions, niobium ethoxide, tantalum ethoxide, lithium ethoxide and ethanol solvent are mixed to obtain a precursor solution; wherein the molar ratio of niobium ethoxide and tantalum ethoxide is 0.15:0.85.

[0110] (b3) The precursor solution is sprayed onto the outer surface of the boron-doped alumina layer and then calcined in a nitrogen atmosphere to obtain the lithium niobate tantalate layer; wherein the calcination temperature is 300°C and the time is 6h.

[0111] Example 3

[0112] This embodiment provides a lithium manganese iron phosphate cathode material, which includes a core and a first outer shell and a second outer shell sequentially covering the surface of the core.

[0113] The core comprises boron-doped lithium manganese iron phosphate material, the first shell is a boron-doped alumina layer, and the second shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell; in the core, the boron doping amount gradually decreases radially from the center of the core.

[0114] The core has a particle size D50 of 500 nm; the boron doping content in the boron-doped lithium manganese iron phosphate material is 1.0 wt%; the thickness of the first shell is 30 nm; the boron doping content in the boron-doped alumina layer is 0.2 wt%; and the thickness ratio of the first shell to the second shell is 1:1.5.

[0115] This embodiment also provides a method for preparing the above-mentioned lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0116] (1) Kernel preparation:

[0117] (a1) A lithium manganese iron phosphate precursor and boron oxide are mixed and sintered under a nitrogen atmosphere at a decreasing temperature, and then cooled to room temperature to obtain a boron-doped precursor material; wherein, the general chemical formula of the lithium manganese iron phosphate precursor is Mn x Fe 1-x PO4, where x = 0.7; the sintering treatment method is cooling sintering, with an initial temperature of 800℃, a final temperature of 600℃, a cooling rate of 4℃ / min, and a cooling time of 0.8h.

[0118] (b1) The boron-doped precursor material and lithium carbonate are mixed and calcined in a nitrogen atmosphere to obtain the boron-doped lithium manganese iron phosphate material; wherein the calcination temperature is 800°C and the time is 5h.

[0119] (2) Preparation of the first shell:

[0120] (a2) Al(NO3)3·9H2O, boric acid, citric acid, polyvinylpyrrolidone and ethanol solvent were mixed to obtain boron-doped aluminum sol with a pH of 7.

[0121] (b2) Under ultrasonic-assisted dispersion, the boron-doped lithium manganese iron phosphate material is immersed in the boron-doped aluminum sol, stirred evenly, dried, and then heat-treated to obtain the boron-doped aluminum oxide layer; wherein the heat treatment temperature is 750°C and the time is 5h.

[0122] (3) Preparation of the second shell:

[0123] (a3) Under stirring conditions, niobium ethanol, tantalum ethanol, lithium ethanol and ethanol solvent are mixed to obtain a precursor solution; wherein the molar ratio of niobium ethanol and tantalum ethanol is 0.85:0.15.

[0124] (b3) The precursor solution is sprayed onto the outer surface of the boron-doped alumina layer and then calcined in a nitrogen atmosphere to obtain the lithium niobate tantalate layer; wherein the calcination temperature is 600°C and the time is 2h.

[0125] Example 4

[0126] The difference between this embodiment and embodiment 1 is that cooling sintering in step (1) is replaced by constant temperature sintering, and the constant temperature sintering temperature is 725°C and the time is 6 hours.

[0127] The remaining preparation methods and parameters are consistent with those in Example 1.

[0128] Example 5

[0129] The difference between this embodiment and Embodiment 1 is that the lithium niobate tantalate layer is doped with lanthanum, and the doping amount of lanthanum is 0.6 wt% (based on the mass of the lithium niobate tantalate layer), that is, lanthanum isopropoxide is added during the mixing process in step (3).

[0130] The remaining preparation methods and parameters are consistent with those in Example 1.

[0131] Example 6

[0132] The difference between this embodiment and Embodiment 1 is that the thickness ratio of the first outer shell to the second outer shell is 1:1.

[0133] The remaining preparation methods and parameters are consistent with those in Example 1.

[0134] Example 7

[0135] The difference between this embodiment and Embodiment 1 is that the thickness ratio of the first outer shell to the second outer shell is 1:4.

[0136] The remaining preparation methods and parameters are consistent with those in Example 1.

[0137] Example 8

[0138] The difference between this embodiment and Embodiment 5 is that the doping amount of lanthanum is 2 wt%.

[0139] The remaining preparation methods and parameters are consistent with those in Example 5.

[0140] Example 9

[0141] The difference between this embodiment and embodiment 1 is that, in the cooling sintering process described in step (1), the cooling rate is 15℃ / min.

[0142] The remaining preparation methods and parameters are consistent with those in Example 1.

[0143] Example 10

[0144] The difference between this embodiment and embodiment 1 is that the cooling sintering in step (1) is replaced by heating sintering, that is, the initial temperature is 625℃, the final temperature is 775℃, the heating rate is 5℃ / min, and the heating time is 0.5h.

[0145] The remaining preparation methods and parameters are consistent with those in Example 1.

[0146] Comparative Example 1

[0147] The difference between this comparative example and Example 1 is that step (a1) is not performed in step (1), that is, boron doping is not performed on the core.

[0148] The remaining preparation methods and parameters are consistent with those in Example 1.

[0149] Comparative Example 2

[0150] The difference between this comparative example and Example 1 is that the boron-doped alumina layer is replaced with a boron-doped manganese-based oxide layer, and Al(NO3)3·9H2O in step (2) is replaced with manganese nitrate.

[0151] The remaining preparation methods and parameters are consistent with those in Example 1.

[0152] Comparative Example 3

[0153] The difference between this comparative example and Example 1 is that step (2) is omitted.

[0154] The remaining preparation methods and parameters are consistent with those in Example 1.

[0155] Comparative Example 4

[0156] The difference between this comparative example and Example 1 is that step (3) is omitted.

[0157] The remaining preparation methods and parameters are consistent with those in Example 1.

[0158] Comparative Example 5

[0159] The difference between this comparative example and Example 1 is that the boron doping amount in the lithium manganese iron phosphate cathode material is uniformly distributed from the core to the first shell layer.

[0160] The remaining preparation methods and parameters are consistent with those in Example 1.

[0161] Performance testing

[0162] Lithium-ion batteries were prepared based on the lithium manganese iron phosphate cathode material provided in the above embodiments and comparative examples: The lithium manganese iron phosphate cathode material, conductive carbon black and polyvinylidene fluoride prepared above were weighed in a mass ratio of 8:1:1 and added to N-methylpyrrolidone to make a slurry. The slurry was then coated on aluminum foil, dried, and sliced ​​to obtain a cathode sheet. A lithium metal sheet was used as the counter electrode, a polypropylene microporous membrane was used as the separator, and an electrolyte with a solute concentration of 1 mol / L was prepared (wherein, the solvent is a mixed solution of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1, and the solute is LiPF6). The CR2025 button battery was assembled in an argon glove box.

[0163] The CR2025 button cell was subjected to charge-discharge performance tests, cycle performance tests, and rate performance tests.

[0164] Test conditions for charge and discharge performance: After the assembled battery is left to stand for 2 hours, the first cycle specific capacity and first efficiency are tested at 25℃, voltage range of 2.5-4.5V, and 0.1C rate.

[0165] Cyclic performance test conditions: 300 cycles were performed at 25℃, 2.5-4.5V, and 2C rate to obtain the cycle capacity retention rate.

[0166] Rate performance test conditions: at 25℃, 2.5-4.5V, 5 cycles each at 0.2C, 0.5C, 1C, 2C, and 5C.

[0167] The test results are shown in Table 1.

[0168] Table 1

[0169]

[0170]

[0171] analyze:

[0172] As shown in Table 1, the lithium manganese iron phosphate cathode material designed in this invention not only solves the problem of low intrinsic electronic / ionic conductivity of lithium manganese iron phosphate, but also improves its insufficient surface stability, achieving a comprehensive improvement in electrochemical performance with high rate, long cycle and high safety, providing an effective solution for the industrial application of lithium-ion batteries.

[0173] A comparison of Examples 1 and 4 shows that using a cooling sintering method to construct a boron doping gradient distribution in the formed core eliminates the need for additional complex equipment. The boron gradient distribution can be fixed solely through temperature control, ensuring sufficient boron doping at the core center to improve bulk conductivity while reducing lattice distortion through low boron levels on the surface. Furthermore, it forms a coherent overall gradient system with the boron doping of the subsequent first shell layer, further enhancing the performance synergy of the core-shell structure. The resulting lithium manganese iron phosphate cathode material achieves a comprehensive improvement in electrochemical performance, including high rate capability, long cycle life, and high safety.

[0174] A comparison of Examples 1 and 5 shows that doping with an appropriate amount of rare earth elements can optimize the lithium-ion transport channel through the lattice regulation effect of rare earth ions. At the same time, this content range can avoid lattice distortion or second phase formation caused by excessive rare earth ions, ensuring the structural integrity of the lithium niobate tantalate layer, and further improving its interface compatibility with the first shell while maintaining excellent barrier performance. The prepared lithium manganese iron phosphate cathode material achieves improved electrochemical performance.

[0175] As can be seen from the comparison between Example 1 and Examples 6-7, if the thickness ratio of the first shell and the second shell is too small, the first shell will be too thin. As a result, the first shell will not be able to fully play its role as a conductive bridge, which will lead to the obstruction of electron / ion transport between the core and the second shell. At the same time, the excessively thick second shell will increase the length of the lithium ion diffusion path, which will ultimately reduce the performance of the lithium manganese iron phosphate cathode material.

[0176] A comparison of Examples 5 and 8 shows that excessive rare earth element doping leads to severe lattice distortion, disrupts lithium-ion diffusion channels, reduces the ion diffusion coefficient, and significantly degrades rate performance. Furthermore, excess rare earth elements tend to precipitate at grain boundaries, forming impurity phases that not only reduce the density of the lithium niobate tantalate layer but also weaken its barrier effect on the electrolyte, resulting in decreased cycle performance. In addition, the high valence of excess rare earth elements creates localized charge imbalances in the lattice, exacerbating interactions with anions in the electrolyte, increasing interfacial impedance, and reducing the battery's charge / discharge efficiency.

[0177] A comparison of Examples 1 and 9 shows that if the cooling rate is too fast during the cooling sintering process, the boron gradient distribution in the core will be disordered, disrupting the continuity of the gradient and causing a "local open circuit" in the bulk conductive network. Furthermore, excessively rapid cooling will cause thermal stress concentration, leading to cracks in the core particles. These cracks will hinder the diffusion of lithium ions in subsequent cycles, while increasing the contact area with the electrolyte and accelerating the dissolution of transition metals, ultimately resulting in a decrease in the performance of the lithium manganese iron phosphate cathode material.

[0178] A comparison of Example 1 and Example 10 shows that if cooling sintering is replaced by heating sintering, the amount of boron doping in the core gradually increases from the center of the core in the radial direction. This leads to a decrease in the structural stability of the core particles, obstruction of lithium ion transport, and an increase in the difference in boron doping between the core surface layer and the first shell layer, resulting in a worse interface bonding effect, increased interface impedance, and ultimately a decrease in the performance of the lithium manganese iron phosphate cathode material.

[0179] As can be seen from the comparison between Example 1 and Comparative Example 1, if the core is not doped with boron, the bulk electronic conductivity and lithium-ion diffusion coefficient of the core will decrease significantly, resulting in the obstruction of lithium-ion transport. At the same time, due to the lack of boron's lattice stabilizing effect, the core is prone to cracking due to volume changes during cycling, making the transition metal easier to dissolve, triggering electrolyte side reactions, and causing a sharp drop in battery cycle performance. Furthermore, since there is no boron element connecting the core and the first shell, the interface impedance increases, further aggravating the overall electrochemical performance degradation.

[0180] As can be seen from the comparison between Example 1 and Comparative Example 2, if the boron-doped alumina layer is replaced with a boron-doped manganese-based oxide layer, the chemical stability of the boron-doped manganese-based oxide layer is poor, which will lead to an increase in the dissolution probability of manganese ions and a decrease in cycle performance. Furthermore, the interface matching degree between the boron-doped manganese-based oxide layer and the core and the second shell layer is poor, which is not conducive to improving the overall performance of the lithium manganese iron phosphate cathode material.

[0181] As can be seen from the comparison between Example 1 and Comparative Example 3, if the first shell is not provided, there is a lack of boron gradient connection and conductive buffer between the core and the second shell, the interface impedance increases sharply, and the core is directly exposed to the high lattice stress of the second shell, the particle cracking rate increases, and the cycle stability deteriorates significantly.

[0182] As can be seen from the comparison between Example 1 and Comparative Example 4, if the second shell is not provided, the first shell loses its outer protection and is easily corroded by the electrolyte. Furthermore, it cannot suppress the long-distance migration of transition metal ions, which leads to the aggravation of side reactions and a significant decrease in cycle performance.

[0183] As can be seen from the comparison between Example 1 and Comparative Example 5, if the boron doping amount in the lithium manganese iron phosphate cathode material is uniformly distributed from the core to the first shell, it is difficult to exert the synergistic effect of boron doping and multilayer shells. As a result, the electronic / ionic conductivity and surface stability of the lithium manganese iron phosphate cathode material cannot be effectively improved. The conductivity enhancement effect of boron doping and the interface protection effect of multilayer shells cannot be fully exerted, resulting in a significant deterioration in the high-rate capacity retention and cycle stability of the material, and a significant decrease in electrochemical performance.

[0184] It should be noted that the technical solution of the present invention is illustrated through the above embodiments, but the present invention is not limited to the above embodiments, that is, it does not mean that the present invention must rely on the above embodiments to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A lithium manganese iron phosphate cathode material, characterized in that, The lithium manganese iron phosphate cathode material includes a core, and a first outer shell and a second outer shell sequentially covering the surface of the core; The core comprises boron-doped lithium manganese iron phosphate material, the first shell is a boron-doped alumina layer, and the second shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The particle size D50 of the core is 100-500nm.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, In the boron-doped lithium manganese iron phosphate material, the boron doping amount is 0.3-1.0 wt%.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that, In the core, the amount of boron doping gradually decreases in the radial direction from the core center.

5. The lithium iron phosphate cathode material according to claim 1, characterized in that, The thickness of the first outer shell is 10-30 nm.

6. The lithium iron phosphate cathode material according to claim 1, characterized in that, In the boron-doped alumina layer, the boron doping amount is 0.1-0.2 wt%.

7. The lithium iron phosphate cathode material according to claim 1, characterized in that, The thickness ratio of the first outer shell to the second outer shell is 1:(1.5-3).

8. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium niobate tantalate layer is doped with rare earth elements.

9. The lithium iron phosphate cathode material according to claim 8, characterized in that, Based on the mass of the lithium niobate tantalate layer, the doping amount of the rare earth element is 0.2-1 wt%.

10. The lithium iron phosphate cathode material according to claim 8, characterized in that, The rare earth elements include lanthanum and / or cerium.

11. A method for preparing lithium manganese iron phosphate cathode material as described in any one of claims 1-10, characterized in that, The preparation method includes the following steps: A core is provided, the core comprising boron-doped lithium manganese iron phosphate material; A first outer shell and a second outer shell are sequentially coated onto the surface of the boron-doped lithium manganese iron phosphate material to obtain the lithium manganese iron phosphate cathode material. The first outer shell is a boron-doped alumina layer; the second outer shell is a lithium niobate tantalate layer; the boron doping amount in the lithium manganese iron phosphate cathode material gradually decreases from the core to the second shell.

12. The preparation method according to claim 11, characterized in that, The preparation method of the boron-doped lithium manganese iron phosphate material includes the solid-phase dispersion method.

13. The preparation method according to claim 12, characterized in that, The specific steps of the solid-phase dispersion method include: (a1) The lithium manganese iron phosphate material precursor and the first boron source are mixed and sintered to obtain the boron-doped precursor material; (b1) The boron-doped precursor material and the lithium source are mixed and calcined in an inert atmosphere to obtain the boron-doped lithium manganese iron phosphate material.

14. The preparation method according to claim 13, characterized in that, The general chemical formula of the lithium manganese iron phosphate material precursor is Mn. x Fe 1-x PO4, where 0 < x < 1.

15. The preparation method according to claim 13, characterized in that, The first boron source includes any one or a combination of at least two of boric acid, boron oxide, or lithium borate.

16. The preparation method according to claim 13, characterized in that, The sintering process is either isothermal sintering or variable-temperature sintering.

17. The preparation method according to claim 16, characterized in that, The isothermal sintering temperature is 700-750℃, and the time is 4-8 hours.

18. The preparation method according to claim 16, characterized in that, The variable-temperature sintering is cooling sintering.

19. The preparation method according to claim 18, characterized in that, During the cooling sintering process, the initial temperature is 750-800℃, the final temperature is 600-650℃, the cooling rate is 1-10℃ / min, and the cooling time is 0.5-2.5h.

20. The preparation method according to claim 13, characterized in that, The calcination treatment is carried out at a temperature of 650-800℃ for 5-15 hours.

21. The preparation method according to claim 11, characterized in that, The coating method for the boron-doped alumina layer includes the sol-gel method.

22. The preparation method according to claim 21, characterized in that, The specific steps of the sol-gel method include: (a2) The aluminum source, the second boron source, the chelating agent, the dispersant and the solvent are mixed to obtain boron-doped aluminum sol; (b2) The boron-doped lithium manganese iron phosphate material and the boron-doped aluminum sol are mixed, dried, and then the dried material is heat-treated to obtain the boron-doped aluminum oxide layer.

23. The preparation method according to claim 22, characterized in that, The aluminum source includes Al(NO3)3·9H2O and / or aluminum isopropoxide.

24. The preparation method according to claim 22, characterized in that, The chelating agent includes any one or a combination of at least two of citric acid, EDTA, acetylacetone, or tartaric acid.

25. The preparation method according to claim 22, characterized in that, The dispersant includes any one or a combination of at least two of polyvinylpyrrolidone, polyethylene glycol, or sodium dodecylbenzenesulfonate.

26. The preparation method according to claim 22, characterized in that, The pH value of the boron-doped aluminum sol is 5-7.

27. The preparation method according to claim 22, characterized in that, The heat treatment is performed at a temperature of 400-750℃ for 5-10 hours.

28. The preparation method according to claim 11, characterized in that, The coating method for the lithium niobate tantalate layer includes a wet coating method.

29. The preparation method according to claim 28, characterized in that, The specific steps of the wet coating method include: (a3) Mix the niobium source, tantalum source, lithium source and solvent to obtain a precursor solution; (b3) The precursor solution is sprayed onto the outer surface of the boron-doped lithium manganese iron phosphate material coated with a boron-doped alumina layer, and then calcined to obtain the lithium niobate tantalate layer.

30. The preparation method according to claim 29, characterized in that, The niobium source includes any one or a combination of at least two of niobium ethanol, niobium oxide, or niobium isopropoxide.

31. The preparation method according to claim 29, characterized in that, The tantalum source includes any one or a combination of at least two of tantalum ethoxide, tantalum oxide, or tantalum isopropoxide.

32. The preparation method according to claim 29, characterized in that, The molar ratio of the niobium source to the tantalum source is (0.15-0.85):(0.85-0.15).

33. The preparation method according to claim 29, characterized in that, The roasting process is carried out at a temperature of 300-600℃ for 2-6 hours.

34. The preparation method according to claim 29, characterized in that, The atmosphere for the roasting treatment is an inert atmosphere.

35. The preparation method according to claim 29, characterized in that, During the mixing process of the niobium source, tantalum source, lithium source and solvent, a rare earth element source is also added.

36. The preparation method according to claim 11, characterized in that, The preparation method includes the following steps: (1) Kernel preparation: (a1) A lithium manganese iron phosphate precursor and a first boron source are mixed and sintered in an inert atmosphere, then cooled to room temperature to obtain a boron-doped precursor material; wherein the general chemical formula of the lithium manganese iron phosphate precursor is Mn x Fe 1-x PO4, where 0 < x < 1; the first boron source includes any one or a combination of at least two of boric acid, boron oxide or lithium borate; the sintering treatment is cooling sintering, with an initial temperature of 750-800℃, a final temperature of 600-650℃, a cooling rate of 1-10℃ / min, and a cooling time of 0.5-2.5h. (b1) The boron-doped precursor material and the lithium source are mixed and calcined in an inert atmosphere to obtain the boron-doped lithium manganese iron phosphate material; wherein the calcination temperature is 650-800℃ and the time is 5-15h. (2) Preparation of the first shell: (a2) Mix an aluminum source, a second boron source, a chelating agent, a dispersant, and a solvent to obtain a boron-doped aluminum sol with a pH of 5-7; wherein the aluminum source includes Al(NO3)3·9H2O and / or aluminum isopropoxide, the second boron source includes any one or a combination of at least two of boric acid, boron oxide, or lithium borate, the chelating agent includes any one or a combination of at least two of citric acid, EDTA, acetylacetone, or tartaric acid, and the dispersant includes any one or a combination of at least two of polyvinylpyrrolidone, polyethylene glycol, or sodium dodecylbenzenesulfonate; (b2) Under ultrasonic-assisted dispersion, the boron-doped lithium manganese iron phosphate material is immersed in the boron-doped aluminum sol, stirred evenly, dried, and then heat-treated to obtain the boron-doped alumina layer; wherein the heat treatment temperature is 400-750℃ and the time is 5-10h. (3) Preparation of the second shell: (a3) Under stirring conditions, a niobium source, a tantalum source, a lithium source, and a solvent are mixed to obtain a precursor solution; wherein the niobium source includes any one or a combination of at least two of niobium ethoxide, niobium oxide, or niobium isopropoxide, and the tantalum source includes any one or a combination of at least two of tantalum ethoxide, tantalum oxide, or tantalum isopropoxide; the molar ratio of the niobium source to the tantalum source is (0.15-0.85):(0.85-0.15); (b3) The precursor solution is sprayed onto the outer surface of the boron-doped alumina layer and then calcined in an inert atmosphere to obtain the lithium niobate tantalate layer; wherein the calcination temperature is 300-600℃ and the time is 2-6h.

37. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery includes the lithium manganese iron phosphate positive electrode material as described in any one of claims 1-10.

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