A lithium manganese iron phosphate material, its preparation method and application

Abstract

This invention belongs to the field of lithium-ion batteries, specifically relating to a lithium manganese iron phosphate material, its preparation method, and its applications. The lithium manganese iron phosphate material of this invention comprises a core and a coating layer covering the surface of the core. The core comprises lithium manganese iron phosphate doped with nickel and cobalt, and the coating layer comprises graphene and Al2O3. This invention simultaneously improves the electronic conductivity and high-temperature cycling stability of the material through the synergistic effect of nickel-cobalt dual-element bulk doping and the coating layer containing graphene and Al2O3.

Description

Technical Field

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

[0002] Lithium manganese iron phosphate (LMFP), as a cathode material for lithium-ion batteries, has broad application prospects in power batteries and energy storage due to its relatively high voltage platform (approximately 4.1V), good safety performance, and abundant raw material reserves. However, traditional LMFP materials face two major problems that restrict their development: one is its poor electronic conductivity (typically below 10). -9 The first problem is that the manganese ion is easily dissolved in high-temperature environments (such as 60°C and above), which reduces the stability of the material structure and significantly shortens the battery cycle life, making it impossible to meet the requirements of long-term stable operation of large-scale energy storage systems.

[0003] In existing technologies, some studies improve the conductivity of lithium manganese iron phosphate through single-element doping. For example, doping with nickel can improve the electronic conductivity of the material, but the effect of single doping on improving high-temperature cycling stability is limited. Other technologies use a single oxide (such as Al₂O₃) to coat lithium manganese iron oxide. While this can suppress manganese dissolution to some extent, the coating layer increases ion transport resistance, affecting the battery's rate performance. Furthermore, some preparation processes, such as the high-temperature solid-state method, suffer from problems such as uneven product particle size and uneven distribution of doping elements, further affecting the electrochemical performance of the material.

[0004] Therefore, it is of great significance to develop a lithium manganese iron phosphate material that has high electronic conductivity, excellent high-temperature cycling stability and simple preparation process. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention proposes a lithium manganese iron phosphate material, its preparation method, and its applications.

[0006] Specifically, one aspect of the present invention provides a lithium manganese iron phosphate material, the lithium manganese iron phosphate material comprising a core and a coating layer covering the surface of the core, the core comprising lithium manganese iron phosphate doped with nickel and cobalt, and the coating layer comprising graphene and Al2O3, wherein the lithium manganese iron phosphate doped with nickel and cobalt has the chemical formula LiMn. 1-x-y- z Fe x Ni y Co z PO4, where 0.4≤x≤0.7, 0.01≤y≤0.03, and 0.005≤z≤0.02.

[0007] In one or more embodiments, x satisfies 0.5≤x≤0.7, and z satisfies 0.01≤z≤0.02.

[0008] In one or more embodiments, the mass ratio of graphene to Al2O3 in the coating layer is 1:(1-5).

[0009] In some preferred embodiments, the mass ratio of graphene to Al2O3 in the coating layer is 1:(4-5).

[0010] In one or more embodiments, the thickness of the coating layer is 8-20 nm.

[0011] In some preferred embodiments, the thickness of the coating layer is 11-20 nm.

[0012] In some preferred embodiments, the thickness of the coating layer is 15-20 nm.

[0013] In one or more embodiments, the particle size of the lithium manganese iron phosphate material is 1.4-2 μm.

[0014] Another aspect of the present invention provides a method for preparing the lithium manganese iron phosphate material according to any embodiment of the present invention, comprising: (1) Add lithium source, manganese source, iron source, nickel source, cobalt source, phosphorus source and polycarboxylic acid to water, mix evenly to obtain sol; (2) The sol is dried, pre-calcined, and pulverized to obtain a nickel-cobalt-doped lithium manganese iron phosphate precursor; (3) Graphene is dispersed in water to obtain a graphene dispersion; (4) Add the aluminum source to the graphene dispersion and mix evenly to obtain a graphene / Al2O3 coated solution; (5) The nickel-cobalt doped lithium manganese iron phosphate precursor is added to the graphene / Al2O3 coating solution, and after stirring, filtering, drying and calcining, the lithium manganese iron phosphate material is obtained.

[0015] In one or more embodiments, the polycarboxylic organic acid is selected from one or more of oxalic acid, tartaric acid, citric acid, succinic acid, and phthalic acid.

[0016] In some preferred embodiments, the polycarboxylic organic acid is selected from one or more of oxalic acid, tartaric acid, and citric acid.

[0017] In one or more embodiments, the molar ratios of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, and cobalt in the cobalt source satisfy: n(Li):(n(Mn)+n(Fe)+n(Ni)+n(Co))=1:1, and 0.4≤n(Fe):n(Li)≤0.7, 0.01≤n(Ni):n(Li)≤0.03, 0.005≤n(Co):n(Li)≤0.02.

[0018] In some preferred embodiments, 0.5 ≤ n(Fe):n(Li) ≤ 0.7.

[0019] In some preferred embodiments, 0.01 ≤ n(Co):n(Li) ≤ 0.02.

[0020] In one or more embodiments, the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, cobalt in the cobalt source, and phosphorus in the phosphorus source satisfies: n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2.

[0021] In one or more embodiments, the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, cobalt in the cobalt source, and the polycarboxylic organic acid satisfies: n(polycarboxylic organic acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=(1.2-1.5):1.

[0022] In one or more embodiments, the lithium source is selected from one or more of lithium carbonate, lithium acetate, and lithium nitrate.

[0023] In one or more embodiments, the manganese source is selected from one or more of manganese sulfate, manganese carbonate, manganese nitrate, manganese oxalate, manganese acetate, and manganese chloride.

[0024] In one or more embodiments, the iron source is selected from one or more of ferrous sulfate, ferrous chloride, ferrous bromide, ferrous oxalate, ferrous acetate, and ferrous nitrate.

[0025] In one or more embodiments, the nickel source is selected from one or more of nickel acetate, nickel nitrate, nickel sulfate, and nickel chloride.

[0026] In one or more embodiments, the cobalt source is selected from one or more of cobalt acetate, cobalt nitrate, cobalt sulfate, and cobalt chloride.

[0027] In one or more embodiments, the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and potassium dihydrogen phosphate.

[0028] In one or more embodiments, in step (1), the pH of the solution is 5.5-6.5.

[0029] In one or more embodiments, in step (1), mixing is carried out by stirring for 4-6 hours and at a speed of 200-250 rpm.

[0030] In one or more embodiments, in step (1), the length of the sol filaments is 5-9 cm.

[0031] In one or more embodiments, in step (2), the drying time is 8-10 hours and the drying temperature is 120-150°C.

[0032] In one or more embodiments, in step (2), the fire is pre-fired in an air atmosphere for 3-4 hours and at a temperature of 400-450°C.

[0033] In one or more embodiments, in step (2), the material is pulverized by ball milling for 0.5-2 hours and at a speed of 1500-200 rpm.

[0034] In one or more embodiments, in step (2), the particle size of the nickel-cobalt doped lithium manganese iron phosphate precursor is 1.4-2.0 μm.

[0035] In one or more embodiments, in step (3), ultrasonic dispersion is used, with an ultrasonic power of 300-500W and an ultrasonic time of 1-2h.

[0036] In one or more embodiments, in step (3), the concentration of the graphene dispersion is 0.5-1.0 g / L, and the graphene particle size in the graphene dispersion is 0.5-1 μm.

[0037] In one or more embodiments, in step (4), the aluminum source is aluminum isopropoxide or aluminum nitrate.

[0038] In one or more embodiments, in step (4), mixing is carried out by stirring for 3-4 hours and at a speed of 200-250 rpm.

[0039] In one or more embodiments, in step (4), the pH of the graphene / Al2O3 coating solution is 8.0-9.0.

[0040] In one or more embodiments, in step (5), the stirring time is 3-4 hours and the stirring speed is 200-250 rpm.

[0041] In one or more embodiments, in step (5), the drying temperature is 100-120°C and the drying time is 6-8 hours.

[0042] In one or more embodiments, in step (5), calcination is carried out in an inert atmosphere, wherein the inert atmosphere is nitrogen or argon, the calcination temperature is 750-800℃, and the calcination time is 10-12h.

[0043] Another aspect of the present invention also provides lithium manganese iron phosphate materials prepared using the methods described in any of the embodiments herein.

[0044] Another aspect of the invention also provides the application of the lithium iron phosphate material described in any embodiment herein in lithium-ion batteries.

[0045] The present invention also provides a cathode material comprising lithium manganese iron phosphate as described in any embodiment herein.

[0046] The present invention also provides a lithium-ion battery comprising the positive electrode of the present invention.

[0047] The present invention also provides an electrical device comprising the lithium-ion battery of the present invention.

[0048] This invention provides a lithium manganese iron phosphate material and its preparation method. This invention improves the electronic conductivity and high-temperature cycling stability of the material simultaneously through the synergistic effect of nickel-cobalt dual-element bulk doping and graphene and Al2O3 nanocomposite coating. Attached Figure Description

[0049] Figure 1 The image shows the XRD pattern of the lithium manganese iron phosphate material prepared in Example 1.

[0050] Figure 2 The image shows the SEM image of the micron-sized secondary particles of the lithium manganese iron phosphate material prepared in Example 1. Detailed Implementation

[0051] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used herein are explained and defined in general terms below. Unless otherwise specified, all technical and scientific terms used herein have the common meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0052] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0053] In this document, the terms “contains,” “includes,” “containing,” and similar terms encompass the meanings of “basically composed of” and “composed of.” For example, when this document discloses “A contains B and C,” “A is basically composed of B and C” and “A is composed of B and C” should be considered as having been disclosed in this document.

[0054] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0055] Unless otherwise specified, percentages refer to mass percentages and proportions refer to mass ratios in this article.

[0056] In this article, the sum of the percentages of all components in the composition is 100%.

[0057] In this document, when describing embodiments or examples, it should be understood that it is not intended to limit the invention to those embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein are covered within the scope of this invention.

[0058] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0059] This invention provides a lithium manganese iron phosphate material, which includes a core and a coating layer covering the surface of the core. The core comprises lithium manganese iron phosphate doped with nickel and cobalt, and the coating layer comprises graphene and Al2O3. The chemical formula of the lithium manganese iron phosphate doped with nickel and cobalt is LiMn. 1-x-y-z Fe x Ni y Co z PO4, where 0.4≤x≤0.7, 0.01≤y≤0.03, and 0.005≤z≤0.02.

[0060] In this invention, the chemical formula of the lithium manganese iron phosphate material can be LiMn. 1-x-y-z Fe x Ni y Co zPO4@(Graphene / Al2O3), where 0.4≤x≤0.7, 0.01≤y≤0.03, 0.005≤z≤0.02, and @(Graphene / Al2O3) indicates that graphene and Al2O3 are coated on LiMn. 1-x-y-z Fe x Ni y Co z The surface of PO4.

[0061] The reason for choosing a nickel-cobalt dual-element bulk-phase doped lithium manganese iron phosphate matrix in this invention is that the introduction of nickel can increase the concentration of electron carriers in the material lattice and significantly improve electronic conductivity; cobalt can optimize the crystal structure of the material, enhance lattice stability, and inhibit the migration and dissolution of manganese ions, thus ensuring the high-temperature cycling performance of the material.

[0062] In some implementations, the chemical formula of lithium manganese iron phosphate material is LiMn. 1-x-y-z Fe x Ni y Co z PO4@(Graphene / Al2O3), preferably x satisfies 0.4≤x≤0.7, more preferably 0.5≤x≤0.7, for example 0.5, 0.6, 0.7. Controlling the iron doping ratio within the above range can improve the 0.1C first-cycle discharge capacity of lithium manganese iron phosphate material.

[0063] Preferably, y satisfies 0.01 ≤ y ≤ 0.03, for example, 0.01, 0.012, 0.014, 0.016, 0.018, 0.02, 0.024, 0.026, 0.028, 0.03. Controlling the nickel doping ratio within the above range can increase the electron carrier concentration in the material lattice and significantly improve electronic conductivity.

[0064] The optimal value of z is 0.005 ≤ z ≤ 0.02, more preferably 0.01 ≤ z ≤ 0.02, such as 0.01, 0.015, and 0.02. Controlling the cobalt doping ratio within the above range can optimize the crystal structure of the material, enhance lattice stability, suppress the migration and dissolution of manganese ions, and ensure the high-temperature cycling performance of the material.

[0065] The reason for choosing a graphene / Al2O3 nanocomposite coating layer in this invention is that graphene has extremely high electronic conductivity and can form a continuous electron transport network on the material surface, further reducing electron transport resistance; Al2O3 has good chemical stability and can effectively block direct contact between the electrolyte and the material matrix, reducing the occurrence of manganese ion dissolution and electrolyte decomposition reaction; the composite coating layer formed by the two in synergy can improve electronic conductivity and ensure the interfacial stability of the material.

[0066] In this invention, the mass ratio of graphene to Al2O3 can be 1:(1-5), preferably 1:(4-5), for example 1:4 or 1:5. Controlling the mass ratio of graphene to Al2O3 within the above range can improve electronic conductivity while ensuring the interfacial stability of the material. In this invention, the thickness of the coating layer can be 8-20 nm, preferably 11-20 nm, more preferably 15-20 nm, for example 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, or 20 nm. Controlling the thickness of the coating layer within the above range can reduce the dissolution of manganese ions. In this invention, the particle size of the lithium manganese iron phosphate material can be 1.4-2 μm, for example 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, or 2 μm.

[0067] The present invention also provides a method for preparing the lithium manganese iron phosphate material of the present invention, the method comprising: (1) Add lithium source, manganese source, iron source, nickel source, cobalt source, phosphorus source and polycarboxylic acid to water, mix evenly to obtain sol; (2) The sol was dried, pre-calcined and pulverized to obtain a nickel-cobalt-doped lithium manganese iron phosphate precursor; (3) Graphene is dispersed in water to obtain a graphene dispersion; (4) Add the aluminum source to the graphene dispersion and mix evenly to obtain a graphene / Al2O3 coated solution; (5) Add the nickel-cobalt doped lithium manganese iron phosphate precursor to the graphene / Al2O3 coating solution, and after stirring, filtering, drying and calcining, obtain the lithium manganese iron phosphate material.

[0068] In this invention, the polycarboxylic organic acid may be selected from one or more of oxalic acid, tartaric acid, citric acid, succinic acid, and phthalic acid.

[0069] In some preferred embodiments, the polycarboxylic organic acid may be selected from one or more of oxalic acid, tartaric acid, and citric acid.

[0070] In some preferred embodiments, the polycarboxylic acid can be citric acid. In the process of preparing lithium manganese iron phosphate materials, the polycarboxylic acid can act as a chelating agent, carbon source and antioxidant, which is the key to achieving uniform doping, high conductivity and high purity of the material.

[0071] In this invention, the molar ratios of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, and cobalt in the cobalt source can satisfy: n(Li):(n(Mn)+n(Fe)+n(Ni)+n(Co))=1:1, and 0.4≤n(Fe)≤0.7, 0.01≤n(Ni)≤0.03, 0.005≤n(Co)≤0.02.

[0072] In some preferred embodiments, the molar ratios of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, and cobalt in the cobalt source can satisfy: n(Li):(n(Mn)+n(Fe)+n(Ni)+n(Co))=1:1, and 0.5≤n(Fe)≤0.7, 0.01≤n(Ni)≤0.03, 0.01≤n(Co)≤0.02.

[0073] In this invention, the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, cobalt in the cobalt source, and phosphorus in the phosphorus source can satisfy: n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2.

[0074] In this invention, the molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, cobalt in the cobalt source, and polycarboxylic organic acid can satisfy: n(polycarboxylic organic acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=(1.2-1.5):1, for example 1.2:1, 1.3:1, 1.4:1, 1.5:1.

[0075] In this invention, the lithium source can be selected from one or more of lithium carbonate, lithium acetate, and lithium nitrate.

[0076] In this invention, the manganese source can be selected from one or more of manganese sulfate, manganese carbonate, manganese nitrate, manganese oxalate, manganese acetate, and manganese chloride.

[0077] In this invention, the iron source can be selected from one or more of ferrous sulfate, ferrous chloride, ferrous bromide, ferrous oxalate, ferrous acetate, and ferrous nitrate.

[0078] In this invention, the nickel source can be selected from one or more of nickel acetate, nickel nitrate, nickel sulfate, and nickel chloride.

[0079] In this invention, the cobalt source can be selected from one or more of cobalt acetate, cobalt nitrate, cobalt sulfate, and cobalt chloride.

[0080] In this invention, the phosphorus source may be selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and potassium dihydrogen phosphate.

[0081] In this invention, in step (1), the pH of the solution can be 5.5-6.5, for example 5.5, 6, 6.5.

[0082] In some implementations, diammonium hydrogen phosphate and citric acid can be added to adjust the pH.

[0083] In step (1), mixing can be done by stirring. The mixing time can be 4-6 hours, for example, 4 hours, 5 hours, or 6 hours. The mixing speed can be 200-250 rpm, for example, 200 rpm, 210 rpm, 220 rpm, 230 rpm, 240 rpm, or 250 rpm.

[0084] In step (1), the length of the sol filaments can be 5-9cm, for example 5cm, 6cm, 7cm, 8cm, or 9cm.

[0085] In step (2), the drying time can be 8-10 hours and the drying temperature can be 120-150℃.

[0086] In step (2), pre-firing can be carried out in an air atmosphere for 3-4 hours and at a temperature of 400-450℃.

[0087] In step (2), ball milling can be used for pulverization. The ball milling time can be 0.5-2 hours and the ball milling speed can be 1500-2000 rpm.

[0088] In step (2), the particle size of the nickel-cobalt doped lithium manganese iron phosphate precursor can be 1.4-2.0 μm, for example 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm.

[0089] In step (3), ultrasonic dispersion can be used, with an ultrasonic power of 300-500W and an ultrasonic time of 1-2h.

[0090] In step (3), the concentration of the graphene dispersion can be 0.5-1.0 g / L, and the graphene particle size in the graphene dispersion can be 0.5-1 μm, for example 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm.

[0091] In step (4), the aluminum source can be aluminum isopropoxide or aluminum nitrate.

[0092] In step (4), mixing can be done by stirring, the mixing time can be 3-4 hours, and the mixing speed can be 200-250 rpm.

[0093] In step (4), the pH of the graphene / Al2O3 coating solution can be 8.0-9.0, for example 8.0, 8.5, or 9.

[0094] In some implementations, the pH of the solution can be adjusted by adding ammonia.

[0095] In step (5), the stirring time can be 3-4 hours and the stirring speed can be 200-250 rpm.

[0096] In step (5), the drying temperature can be 100-120℃ and the drying time can be 6-8h; In step (5), calcination can be carried out in an inert atmosphere, which can be nitrogen or argon. The calcination temperature can be 750-800℃ and the calcination time can be 10-12h.

[0097] In the preparation process of this invention, a multi-element doped precursor can be prepared by sol-gel method. This method can achieve uniform distribution of doped elements, avoiding the problem of uneven element distribution in traditional solid-state methods. Furthermore, the preparation of graphene and Al2O3 composite coating liquid by hydrothermal synthesis can ensure uniform deposition of the coating layer on the material surface. The subsequent high-temperature calcination process can promote the improvement of the material crystallinity and enhance the bonding strength between the coating layer and the substrate, preventing the coating layer from falling off.

[0098] The present invention also provides lithium manganese iron phosphate material prepared by the above method.

[0099] This invention also provides the application of the lithium iron phosphate material of this invention in lithium-ion batteries.

[0100] The present invention also provides a positive electrode, which includes the lithium manganese iron phosphate material of the present invention.

[0101] The present invention also provides a lithium-ion battery comprising the above-described positive electrode.

[0102] The present invention also provides an electrical device comprising the aforementioned lithium-ion battery.

[0103] The beneficial effects of this invention include: (1) Significantly improved electronic powder conductivity: The synergistic effect of nickel-cobalt dual element doping and coating layers including graphene and Al2O3 improves the electronic conductivity and 10C rate discharge capacity of the material compared with traditional undoped lithium manganese iron phosphate materials, meeting the requirements of high power applications.

[0104] (2) Excellent high-temperature cycling stability: The stabilizing effect of cobalt on the crystal structure and the protective effect of the coating layer including graphene and Al2O3 on the interface enable the material to maintain a capacity retention rate of ≥80.5% and a manganese leaching amount of ≤32ppm after 300 cycles at 1C in a high-temperature environment of 60℃.

[0105] (3) Balanced electrochemical performance: Nickel-cobalt dual-element doping makes the material have a first-cycle discharge capacity of ≥149mAh / g at 0.1C, close to the theoretical capacity, and the discharge voltage plateau is stable at 4.05-4.15V. The energy density (discharge capacity) is 11%-21% higher than that of traditional undoped lithium iron phosphate materials.

[0106] The present invention will be described below by way of specific embodiments. It should be understood that these embodiments are merely illustrative and are not intended to limit the scope of the invention. The methods, reagents, and materials used in the embodiments and comparative examples are conventional methods, reagents, and materials in the art, unless otherwise stated. The starting material compounds in the embodiments and comparative examples are all commercially available.

[0107] Example 1

[0108] The preparation steps of the lithium manganese iron phosphate material in Example 1 are as follows: (1) Preparation of nickel-cobalt doped lithium manganese iron phosphate precursor: According to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Co)=1:0.45:0.5:0.03:0.02, 12.24g of lithium acetate, 13.23g of manganese acetate, 14.76g of ferrous acetate, 0.896g of nickel acetate, and 0.598g of cobalt acetate were weighed. 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1.3:1) were weighed as raw materials. The above raw materials were dissolved in deionized water, and the pH of the solution was adjusted to 6.0 using diammonium hydrogen phosphate and citric acid. The solution was stirred at 75°C for 5 hours at a stirring speed of 200 rpm to form a sol with a liquid filament length of 6 cm. The sol was dried at 130℃ for 9 hours to obtain a dry gel. The dry gel was pre-calcined in air at 420℃ for 3.5 hours to obtain a nickel-cobalt-doped lithium manganese iron phosphate precursor. The precursor was then ball-milled. The D50 of the ball-milled precursor was 1.6 μm. The ball-milling time was 0.5 hours and the ball-milling speed was 1900 rpm.

[0109] (2) Preparation of graphene / Al2O3 composite coating solution: 0.16 g of graphene (purity ≥99%) was dispersed in deionized water to prepare a suspension with a concentration of 0.8 g / L. The suspension was ultrasonically treated at 400 W for 1.5 h to obtain a graphene dispersion with a D50 of 0.6 μm. 2.56 g of aluminum isopropoxide (to satisfy the mass ratio of graphene to Al2O3 of 1:4) was added to the graphene dispersion, and the pH of the solution was adjusted to 8.5 with ammonia. Then the solution was stirred at 55 °C for 2.5 h at a stirring speed of 200 rpm to form a graphene / Al2O3 composite coating solution.

[0110] (3) Preparation of lithium manganese iron phosphate materials: 100g of nickel-cobalt-doped lithium manganese iron phosphate precursor was added to 200mL of graphene / Al2O3 composite coating solution, stirred for 3.5h and then filtered. The stirring speed was 200rpm. After washing with deionized water 3 times, the filter residue was dried at 110℃ for 7h to obtain the dried material. The dried material was placed in an argon atmosphere furnace and calcined at 780℃ for 11 hours at a heating rate of 2℃ / min. After cooling, LiMn was obtained. 0.45 Fe 0.5 Ni 0.03 Co 0.02 PO4@(Graphene / Al2O3) material.

[0111] The battery fabrication process in Example 1 is as follows: (1) Preparation of positive electrode: The prepared lithium manganese iron phosphate material, conductive agent (conductive carbon black) and binder (polyvinylidene fluoride PVDF) were mixed in a mass ratio of 90:5:5. An appropriate amount of 1-methyl-2-pyrrolidone (NMP) solvent was added, and the mixture was stirred evenly to obtain a positive electrode slurry. The viscosity and solid content of the positive electrode slurry were adjusted to a suitable coating. The positive electrode slurry was uniformly coated on an aluminum foil current collector, and the aluminum foil was dried at 80°C for 60 minutes, followed by vacuum drying at 100°C for 12 hours. The dried aluminum foil was cut into circular pieces with a diameter of 12 mm to obtain the positive electrode. The LMFP loading on the positive electrode was approximately 5 mg / cm³. 2 .

[0112] (2) Preparation of negative electrode: Lithium metal is used as the negative electrode of the standard half cell.

[0113] (3) Preparation of electrolyte: Prepare lithium-ion battery electrolyte in a glove box (in an argon protective atmosphere). Dissolve LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl acetate (EMC) (the volume ratio of EC, DMC and EMC is 1:1:1) to prepare a 1 mol / L LiPF6 solution as electrolyte.

[0114] (4) Battery assembly was carried out using a standard CR2016 button mold in an argon-filled glove box.

[0115] Testing revealed that the lithium manganese iron phosphate material prepared in Example 1 had a particle size of 1.6 μm, a graphene / Al₂O₃ coating thickness of 15 nm, and an electronic conductivity of 3.1 × 10⁻⁶. -2 S / cm.

[0116] The electrochemical performance test results of the battery prepared in Example 1 are as follows: the first discharge capacity at 0.1C is 157.43 mAh / g; the capacity retention rate after 800 cycles at 1C is 92%; and the capacity retention rate after 300 cycles at 60℃ is 87%.

[0117] Example 2

[0118] The preparation steps of the lithium manganese iron phosphate material in Example 2 are as follows: (1) Preparation of nickel-cobalt doped lithium manganese iron phosphate precursor: According to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Co)=1:0.27:0.7:0.01:0.02, 12.24g of lithium acetate, 7.94g of manganese acetate, 20.66g of ferrous acetate, 0.299g of nickel acetate, and 0.598g of cobalt acetate were weighed, along with 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1.3:1), as raw materials; The above raw materials were dissolved in deionized water, and the pH of the solution was adjusted to 5.8 using diammonium hydrogen phosphate and citric acid. The solution was stirred at 72°C for 4.5 hours at a stirring speed of 240 rpm to form a sol with a filament length of 6.5 cm. The sol was dried at 130℃ for 9 hours to obtain a dry gel. The dry gel was pre-calcined in air at 430℃ for 3 hours to obtain a nickel-cobalt-doped lithium manganese iron phosphate precursor. The precursor was then ball-milled. The D50 of the precursor after ball milling was 1.7 μm. The ball milling time was 0.5 hours and the ball milling speed was 1900 rpm.

[0119] (2) Preparation of graphene / Al2O3 composite coating solution: 0.16 g of graphene (purity ≥ 99%) was dispersed in deionized water to prepare a suspension with a concentration of 0.8 g / L. The suspension was ultrasonically treated at 400 W for 1.5 h to obtain a graphene dispersion with a D50 of 0.7 μm. 1.61 g of aluminum isopropoxide (to satisfy the mass ratio of graphene to Al2O3 of 1:3) was added to the graphene dispersion, and the pH of the solution was adjusted to 8.5 with ammonia. The solution was then stirred at 55°C for 2.5 h at a stirring speed of 200 rpm to form a graphene / Al2O3 composite coating solution.

[0120] (3) Preparation of lithium manganese iron phosphate materials: 100g of nickel-cobalt-doped lithium manganese iron phosphate precursor was added to 200mL of graphene / Al2O3 composite coating solution, stirred for 3h and then filtered. The stirring speed was 200rpm. After washing with deionized water 3 times, the filter residue was dried at 115℃ for 6.5h to obtain the dried material. The dried material was placed in an argon atmosphere furnace and calcined at 750℃ for 11 hours at a heating rate of 2℃ / min. After cooling, LiMn was obtained. 0.27 Fe 0.7 Ni 0.01 Co 0.02 PO4@(Graphene / Al2O3) material.

[0121] The battery preparation process in Example 2 is basically the same as that in Example 1, except that the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Example 2 in the preparation of the positive electrode.

[0122] Testing revealed that the lithium manganese iron phosphate material prepared in Example 2 had a particle size of 1.7 μm, a graphene / Al₂O₃ coating thickness of 12 nm, and an electronic conductivity of 2.9 × 10⁻⁶. -2 S / cm.

[0123] The electrochemical performance test results of the battery prepared in Example 2 are as follows: the first discharge capacity at 0.1C is 150.79 mAh / g; the capacity retention rate after 800 cycles at 1C is 90%; and the capacity retention rate after 300 cycles at 1C at 60℃ is 85%.

[0124] The performance test results of the lithium manganese iron phosphate materials and batteries prepared in Examples 1 and 2 are shown in Table 1.

[0125] Table 1: Performance test results of lithium manganese iron phosphate materials and batteries prepared in Examples 1 and 2

[0126] As shown in Table 1, compared with Example 1, the reduction in nickel doping ratio in Example 2 lowered the electron carrier concentration, resulting in a slight decrease in conductivity. The low-temperature sintering at 750℃ in Example 2 resulted in slightly lower crystallinity, leading to a slight decrease in capacity and cycle performance compared to Example 1. However, it still meets the following requirements: ≥85% retention rate after 300 cycles at 60℃ and ≥150mAh / g discharge capacity in the first cycle at 0.1C. This demonstrates that the process exhibits stability over a wide parameter range, and the doping ratio and sintering temperature can be adjusted according to actual needs.

[0127] Example 3

[0128] The preparation steps of the lithium manganese iron phosphate material in Example 3 are as follows: (1) Preparation of nickel-cobalt doped lithium manganese iron phosphate precursor: According to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Co)=1:0.55:0.4:0.03:0.02, 12.24g of lithium acetate, 16.18g of manganese acetate, 11.81g of ferrous acetate, 0.896g of nickel acetate, and 0.598g of cobalt acetate were weighed, along with 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1.3:1), as raw materials; The above raw materials were dissolved in deionized water, and the pH of the solution was adjusted to 6.2 using diammonium hydrogen phosphate and citric acid. The solution was stirred at 78°C for 5.5 hours at a stirring speed of 250 rpm to form a sol with a liquid filament length of 7 cm. The sol was dried at 130℃ for 9 hours to obtain a dry gel. The dry gel was pre-calcined in air at 430℃ for 3 hours to obtain a nickel-cobalt-doped lithium manganese iron phosphate precursor. The precursor was then ball-milled. The D50 of the ball-milled precursor was 1.7 μm. The ball-milling time was 0.5 hours and the ball-milling speed was 1900 rpm.

[0129] (2) Preparation of graphene / Al2O3 composite coating solution: 0.16 g of graphene (purity ≥ 99%) was dispersed in deionized water to prepare a suspension with a concentration of 0.8 g / L. The suspension was ultrasonically treated at 450 W for 1.8 h to obtain a graphene dispersion with a D50 of 0.8 μm. 2.69 g of aluminum isopropoxide (to satisfy the mass ratio of graphene to Al2O3 of 1:5) was added to the graphene dispersion, and the pH of the solution was adjusted to 8.5 with ammonia. The solution was then stirred at 55 °C for 2.5 h at a stirring speed of 200 rpm to form a graphene / Al2O3 composite coating solution.

[0130] (3) Preparation of lithium manganese iron phosphate materials: 100g of nickel-cobalt-doped lithium manganese iron phosphate precursor was added to 200mL of graphene / Al2O3 composite coating solution, stirred for 3h and then filtered. The stirring speed was 200rpm. After washing with deionized water 4 times, the filter residue was dried at 105℃ for 7.5h to obtain the dried material. The dried material was placed in an argon atmosphere furnace and calcined at 780℃ for 12 hours at a heating rate of 2℃ / min. After cooling, LiMn was obtained. 0.55 Fe 0.4 Ni 0.03 Co 0.02 PO4@(Graphene / Al2O3) material.

[0131] The battery preparation process in Example 3 is basically the same as that in Example 1, except that the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Example 3 in the preparation of the positive electrode.

[0132] Testing revealed that the lithium manganese iron phosphate material prepared in Example 3 had a particle size of 1.7 μm, a graphene / Al₂O₃ coating thickness of 20 nm, and an electronic conductivity of 3.3 × 10⁻⁶. -2 S / cm.

[0133] The electrochemical performance test results of the battery prepared in Example 3 are as follows: the first discharge capacity at 0.1C is 149.02 mAh / g; the capacity retention rate after 800 cycles at 1C is 93%; and the capacity retention rate after 300 cycles at 60℃ is 86%.

[0134] The performance test results of the lithium manganese iron phosphate materials and batteries prepared in Examples 1 and 3 are shown in Table 2.

[0135] Table 2: Performance test results of lithium manganese iron phosphate materials and batteries prepared in Examples 1 and 3

[0136] As shown in Table 2, with the nickel-cobalt doping amount remaining constant, increasing the manganese content can improve the material's voltage plateau (measured at 4.12V, 0.02V higher than Example 1), while decreasing the iron content leads to a slight decrease in the 0.1C first-cycle discharge capacity. The thickened coating layer enhances the interface protection effect, resulting in better cycle stability and resistance to manganese leaching of the lithium manganese iron phosphate material compared to Example 1. This demonstrates that increasing the proportion of Al2O3 can further optimize high-temperature performance, providing a process adaptation solution for high-voltage applications.

[0137] Example 4

[0138] The preparation steps of the lithium manganese iron phosphate material in Example 4 are basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Co)=1:0.465:0.5:0.03:0.005, 12.24g of lithium acetate, 13.52g of manganese acetate, 14.76g of ferrous acetate, 0.896g of nickel acetate, and 0.149g of cobalt acetate are weighed, and 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1.3:1) are weighed as raw materials to finally obtain LiMn 0.465 Fe 0.5 Ni 0.03 Co 0.005 PO4@(Graphene / Al2O3) material.

[0139] The battery preparation process in Example 4 is basically the same as that in Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Example 4.

[0140] Testing revealed that the lithium manganese iron phosphate material prepared in Example 4 had a particle size of 1.48 μm, a graphene / Al₂O₃ coating thickness of 16 nm, and an electronic conductivity of 3.0 × 10⁻⁶. -2 S / cm.

[0141] Example 5

[0142] The preparation steps of the lithium manganese iron phosphate material in Example 5 are basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Co)=1:0.46:0.5:0.03:0.01, 12.24g of lithium acetate, 13.37g of manganese acetate, 14.76g of ferrous acetate, 0.896g of nickel acetate, and 0.299g of cobalt acetate are weighed, and 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1.3:1) are weighed as raw materials to finally obtain LiMn 0.46 Fe 0.5 Ni 0.03 Co 0.01 PO4@(Graphene / Al2O3) material.

[0143] The battery preparation process in Example 5 is basically the same as that in Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Example 5.

[0144] Testing revealed that the lithium manganese iron phosphate material prepared in Example 5 had a particle size of 1.72 μm, a graphene / Al₂O₃ coating thickness of 17 nm, and an electronic conductivity of 2.8 × 10⁻⁶. -2 S / cm.

[0145] Example 6

[0146] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 6 were basically the same as those in Example 1, except that in step (2), 0.64 g of aluminum isopropoxide was added to the graphene dispersion (to satisfy the mass ratio of graphene to Al2O3 as 1:1), and LiMn was finally obtained. 0.45 Fe 0.5 Ni 0.03 Co 0.02 PO4@(Graphene / Al2O3) material.

[0147] The battery preparation process in Example 6 is basically the same as that in Example 1, except that the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Example 6 in the preparation of the positive electrode.

[0148] Testing revealed that the lithium manganese iron phosphate material prepared in Example 6 had a particle size of 1.6 μm, a graphene / Al₂O₃ coating thickness of 11 nm, and an electronic conductivity of 3.0 × 10⁻⁶. -2 S / cm.

[0149] Comparative Example 1

[0150] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 1 were basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe) = 1:0.3:0.7, 12.24 g of lithium acetate, 8.82 g of manganese acetate, and 20.66 g of ferrous acetate were weighed, along with 31.69 g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe))=1:2) and 59.94 g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe))=1.3:1), as raw materials, and finally LiMn was obtained. 0.3 Fe 0.7 PO4@(Graphene / Al2O3) material.

[0151] The battery preparation process of Comparative Example 1 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Example 3.

[0152] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 1 had a particle size of 1.82 μm, a graphene / Al₂O₃ coating thickness of 12.5 nm, and an electronic conductivity of 2.5 × 10⁻⁶. -2 S / cm.

[0153] The performance test results of the lithium manganese iron phosphate material and battery prepared in Comparative Example 1 and Example 1 are shown in Table 3.

[0154] Table 3: Performance test results of lithium manganese iron phosphate materials and batteries prepared in Comparative Example 1 and Example 1

[0155] As can be seen from the test results in Table 3, the electrochemical performance of the material in Comparative Example 1 is lower than that in Example 1. This shows that nickel-cobalt dual-element doping is the core of improving the conductivity and lattice stability of the material. Without it, the electrochemical performance of the material deteriorates significantly.

[0156] Comparative Example 2

[0157] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 2 are basically the same as those in Example 1, except that step (2) is skipped and the nickel-cobalt doped lithium manganese iron phosphate precursor prepared in step (1) is directly sintered to finally obtain LiMn 0.45 Fe 0.5 Ni 0.03 Co 0.02 PO4 material.

[0158] The battery preparation process of Comparative Example 2 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 2.

[0159] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 2 had a particle size of 1.78 μm and an electronic conductivity of 1.7 × 10⁻⁶. - ²S / cm.

[0160] The performance test results of the lithium manganese iron phosphate materials and batteries prepared in Comparative Example 2 and Example 1 are shown in Table 4.

[0161] Table 4: Performance test results of lithium manganese iron phosphate materials and batteries prepared in Comparative Example 2 and Example 1

[0162] As can be seen from the test results in Table 4, the electrochemical performance of the material prepared in Comparative Example 2 is lower than that in Example 1. This indicates that the graphene / Al2O3 composite coating layer is the key to protecting the material interface and reducing impedance. Without it, the high-temperature cycling and storage stability are severely reduced.

[0163] Comparative Example 3

[0164] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 3 were basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Co)=1:0.445:0.5:0.03:0.025, 12.24g of lithium acetate, 13.09g of manganese acetate, 14.76g of ferrous acetate, 0.896g of nickel acetate, and 0.747g of cobalt acetate were weighed, along with 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1.3:1), as raw materials, and finally LiMn was obtained. 0.445 Fe 0.5 Ni 0.03 Co 0.025 PO4@(Graphene / Al2O3) material.

[0165] The battery preparation process of Comparative Example 3 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 3.

[0166] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 3 had a particle size of 1.55 μm, a graphene / Al₂O₃ coating thickness of 13.5 nm, and an electronic conductivity of 2.0 × 10⁻⁶. -2 S / cm.

[0167] Comparative Example 4

[0168] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 4 were basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni)=1:0.45:0.5:0.05, 12.24g of lithium acetate, 13.09g of manganese acetate, 14.76g of ferrous acetate, and 1.49g of nickel acetate were weighed, along with 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni))=1.3:1) as raw materials, finally obtaining LiMn 0.45 Fe 0.5 Ni 0.05 PO4@(Graphene / Al2O3) material.

[0169] The battery preparation process of Comparative Example 4 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 4.

[0170] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 4 had a particle size of 1.68 μm, a graphene / Al₂O₃ coating thickness of 13.2 nm, and an electronic conductivity of 1.8 × 10⁻⁶. -2 S / cm.

[0171] Comparative Example 5

[0172] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 5 were basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Co)=1:0.45:0.5:0.05, 12.24g of lithium acetate, 13.09g of manganese acetate, 14.76g of ferrous acetate, and 1.49g of cobalt acetate were weighed, and 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Co))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Co))=1.3:1) were weighed as raw materials to finally obtain LiMn 0.45 Fe 0.5 Co 0.05PO4@(Graphene / Al2O3) material.

[0173] The battery preparation process of Comparative Example 5 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 5.

[0174] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 5 had a particle size of 1.70 μm, a graphene / Al₂O₃ coating thickness of 13.8 nm, and an electronic conductivity of 1.9 × 10⁻⁶. - ²S / cm.

[0175] Comparative Example 6

[0176] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 6 are basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Zr)=1:0.45:0.5:0.03:0.02, 12.24g of lithium acetate, 13.09g of manganese acetate, 14.76g of ferrous acetate, 0.896g of nickel acetate, and 1.28g of zirconium acetate are weighed, and 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Zr))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Zr))=1.3:1) are weighed as raw materials to finally obtain LiMn 0.45 Fe 0.5 Ni 0.03 Zr 0.02 PO4@(Graphene / Al2O3) material.

[0177] The battery preparation process of Comparative Example 6 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 6.

[0178] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 6 had a particle size of 1.95 μm, a graphene / Al₂O₃ coating thickness of 18 nm, and an electronic conductivity of 2.0 × 10⁻⁶. - ²S / cm.

[0179] Comparative Example 7

[0180] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 7 were basically the same as those in Example 1, except that in step (1), according to the molar ratio of n(Li):n(Mn):n(Fe):n(Ni):n(Mg)=1:0.45:0.5:0.03:0.02, 12.24g of lithium acetate, 13.09g of manganese acetate, 14.76g of ferrous acetate, 0.896g of nickel acetate, and 0.545g of magnesium acetate were weighed, and 31.69g of diammonium hydrogen phosphate (satisfying n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Mg))=1:2) and 59.94g of citric acid (satisfying n(citric acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Mg))=1.3:1) were weighed as raw materials to finally obtain LiMn 0.45 Fe 0.5 Ni 0.03 Mg 0.02 PO4@(Graphene / Al2O3) material.

[0181] The battery preparation process of Comparative Example 7 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 7.

[0182] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 7 had a particle size of 1.82 μm, a graphene / Al₂O₃ coating thickness of 19 nm, and an electronic conductivity of 1.9 × 10⁻⁶. - ²S / cm.

[0183] Comparative Example 8

[0184] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 8 were basically the same as those in Example 1, except that in step (2), 3.84 g of aluminum isopropoxide was added to the graphene dispersion (to satisfy the mass ratio of graphene to Al2O3 of 1:6), and LiMn was finally obtained. 0.45 Fe 0.5 Ni 0.03 Co 0.02 PO4@(Graphene / Al2O3) material.

[0185] The battery preparation process of Comparative Example 8 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 8.

[0186] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 8 had a particle size of 1.76 μm, a graphene / Al₂O₃ coating thickness of 22 nm, and an electronic conductivity of 1.2 × 10⁻⁶. - ²S / cm.

[0187] Comparative Example 9

[0188] The preparation steps of the lithium manganese iron phosphate material in Comparative Example 9 were basically the same as those in Example 1, except that in step (2), 0.64 g of aluminum isopropoxide was added to the graphene dispersion (to satisfy the mass ratio of graphene to Al2O3 of 3:1), and LiMn was finally obtained. 0.45 Fe 0.5 Ni 0.03 Co 0.02 PO4@(Graphene / Al2O3) material.

[0189] The battery preparation process of Comparative Example 9 is basically the same as that of Example 1, except that in the preparation of the positive electrode, the lithium manganese iron phosphate material prepared in Example 1 is replaced with the lithium manganese iron phosphate material prepared in Comparative Example 9.

[0190] Testing revealed that the lithium manganese iron phosphate material prepared in Comparative Example 9 had a particle size of 1.85 μm, a graphene / Al₂O₃ coating thickness of 7 nm, and an electronic conductivity of 1.1 × 10⁻⁶. - ²S / cm.

[0191] The composition of the lithium manganese iron phosphate materials prepared in Examples 1-6 and Comparative Examples 1-9 is shown in Table 5.

[0192] Table 5: Composition of lithium manganese iron phosphate materials prepared in Examples 1-6 and Comparative Examples 1-9

[0193] Test case

[0194] I. XRD Characterization

[0195] The testing equipment was a Bruker D8 Advance X-ray diffractometer from Germany.

[0196] Detection conditions: CuK α Radiation (λ=0.15418nm), tube voltage 40kV, tube current 200mA, scanning range 10°-90°, step size 0.04°, dwell time 0.4s, a graphite monochromator was used to remove background interference, and the sample was a powder tablet (pressure 5MPa, diameter 10mm, thickness 1mm).

[0197] Figure 1The image shows the XRD pattern of the lithium manganese iron phosphate material prepared in Example 1. The diffraction pattern is basically consistent with the spectrum of LiFePO4, which also has an olivine structure, indicating that the prepared modified lithium manganese iron phosphate cathode material belongs to an orthorhombic crystal system compound with an ordered olivine structure. At the same time, no obvious impurity phase peaks and carbon peaks were observed, indicating that the prepared lithium manganese iron phosphate material has high-purity lithium manganese iron phosphate phase and low carbon content, and the composite coating layer is nanoscale and its content is appropriately controlled, without affecting the matrix crystal structure.

[0198] II. Scanning Electron Microscopy (SEM) Characterization

[0199] Testing equipment model: JSM-7800F field emission scanning electron microscope (JEOL).

[0200] Detection conditions: accelerating voltage 15kV, working distance 8mm, secondary electron imaging mode, sample is treated with gold sputtering (gold layer thickness 5-8nm) to eliminate charge accumulation.

[0201] Figure 2 The image shows the SEM image of the micron-sized secondary particles of the lithium manganese iron phosphate material prepared in Example 1. The image shows that the precursor particles are regular spherical with a particle size distribution concentrated in the range of 1.6-10 μm (D50=1.6μm±0.2μm). There is no obvious agglomeration between particles, only a small number of irregular pores (pore size 100-200nm) formed due to drying. The pores are evenly distributed and do not penetrate into the interior of the particles. There is no coating layer attached, the lattice edges are sharp, and there are no obvious structural defects, which confirms that the precursor has good crystallinity.

[0202] III. Specific Surface Area Characterization

[0203] The Brunauer–Emmett–Teller (BET) method was used, and a specific surface area and pore size analyzer (model: ASAP 2460, Micromeritics Instrument Corporation, USA) was employed to determine the nitrogen adsorption-desorption isotherm of the samples at liquid nitrogen temperature (77 K). The specific surface area was calculated using the BET equation, and the pore size distribution and pore volume were analyzed using the Barrett-Joyner-Halenda (BJH) model. Before testing, the samples were degassed under vacuum at 150 °C for 6 hours to remove surface adsorbed impurities. The specific surface area of ​​the lithium manganese iron phosphate materials prepared in the examples and comparative examples was then measured.

[0204] IV. Material Electronic Conductivity Testing

[0205] Testing equipment model: Keithley 2400 source meter with four-probe test station (USA).

[0206] Testing conditions: DC four-probe method was used. The sample was a powder tablet (pressure 10MPa, size 10mm×10mm×2mm). The test temperature was 25℃ (temperature controlled by constant temperature chamber, accuracy ±0.5℃). The voltage scan range was 0-5V, the scan rate was 0.1V / s, and each voltage point was held for 10s to ensure data stability.

[0207] V. Electrochemical Performance Testing

[0208] (1) Charge and discharge test: At 25℃, after resting for 10 min, charge to 4.5V at constant current of 0.1C, 1C and 10C respectively, rest for 10 min, and then discharge to 2.0V at constant current of 0.1C, 1C and 10C respectively. Record the charge and discharge capacity, and test the charge and discharge specific capacity of the battery under test at the corresponding rate: Specific charging capacity (mAh / g) = Charging capacity / Mass of active material; Discharge specific capacity (mAh / g) = discharge capacity / mass of active material.

[0209] (2) First Coulombic Efficiency Test at 0.1C: At 25℃, after resting for 10 min, the battery was charged to 4.5V at a constant current of 0.1C, rested for 10 min, and then discharged to 2.0V at a constant current of 0.1C. The first-cycle Coulombic efficiency of the battery under test at 0.1C was then tested. Initial coulombic efficiency = (0.1C initial discharge specific capacity / 0.1C initial charge specific capacity) 100%.

[0210] (2) Room temperature cycling test: At 25℃, after resting for 10 min, charge at 1C constant current to 4.5V, rest for 10 min, discharge at 1C constant current to 2.0V, cycle for 800 times, and calculate the capacity retention rate of the battery after 800 cycles: Capacity retention rate of the battery after 800 cycles = (Discharge capacity after the 800th cycle / Discharge capacity of the first cycle) 100%.

[0211] (3) High temperature cycling test: At 60℃, after resting for 10 min, charge at 1C constant current to 4.5V, rest for 10 min, discharge at 1C constant current to 2.0V, cycle for 300 times, and calculate the capacity retention rate of the battery after 300 cycles: Capacity retention rate of the battery after 300 cycles = (Discharge capacity after the 300th cycle / Discharge capacity of the first cycle) 100%.

[0212] (4) Stability test at room temperature: The room temperature storage stability test was conducted according to the following method: Material samples were sealed in aluminum-plastic bags and placed in a constant temperature and humidity chamber. They were stored under both dry inert conditions (25±2℃, relative humidity ≤10%, argon protection) and simulated production environment conditions (25±2℃, relative humidity 50±5%, air atmosphere). Sampling points were 0, 7, 15, 30, 60, 90, and 180 days. At each sampling point, samples were taken for moisture content determination (Karl Fischer method), pH value testing, specific surface area analysis (BET method, using a Micron ASAP 2460 specific surface area and pore size analyzer), crystal structure analysis (X-ray diffraction, Bruker D8 Advance, Germany), surface elemental valence state analysis (X-ray photoelectron spectroscopy), and metal leaching detection (inductively coupled plasma atomic emission spectrometry, Agilent 5110, USA). Meanwhile, the stored materials were assembled into CR2016 coin cells according to the battery preparation steps in Example 1 (the electrode formulation was the stored material: conductive carbon black: PVDF = 90:5:5, the loading was 5 mg / cm², the electrolyte was a 1 mol / L LiPF6 solution (the volume ratio of EC:DMC:EMC in the solvent was 1:1:1), and the counter electrode was a lithium metal sheet).

[0213] The assembled battery was tested for its first discharge capacity within a voltage range of 2.0-4.5V (charged to 4.5V at a constant current of 0.1C, left to stand for 30 min, and then discharged to 2.0V at a constant current of 0.1C, and the first discharge capacity was recorded). The first discharge capacity was compared with the initial performance (the first discharge capacity of the battery prepared from the unstored material was used as the initial performance), and the capacity retention rate was calculated. The room temperature storage stability of the sample was evaluated by the capacity retention rate after 30 days of storage.

[0214] (5) Discharge voltage plateau test

[0215] According to the "Methods for Testing Electrochemical Performance of Lithium-ion Battery Electrode Materials" (SJ / T 11793-2022), the discharge voltage plateau of the materials prepared in Examples 1-6 was tested. The discharge voltage plateau of the materials was in the range of 4.05-4.15V.

[0216] VI. Interface Impedance Test (EIS Test)

[0217] Interfacial impedance testing was performed using electrochemical impedance spectroscopy (EIS) on an electrochemical workstation (Solartron 1470E, Ametek, UK). Tests were conducted at room temperature (25±2℃), with a frequency range of 0.01 Hz to 100 kHz and an AC excitation signal amplitude of 5 mV. Tests were performed after the battery's initial charge-discharge activation and after different cycle counts (e.g., 100, 300, 500, and 1000 cycles) at a fully charged state (4.3V). The battery was allowed to stand for 2 hours before testing to reach equilibrium.

[0218] The test data were fitted to the equivalent circuit using ZView software, and the equivalent circuit model Rs+(Rsf) was adopted. CPEsf)+(Rct CPEdl)+WRs+(Rsf CPEsf)+(Rct CPEdl)+W, where Rs is the ohmic resistance (resistance of electrolyte, diaphragm, and electrode contact), Rsf is the SEI film resistance, Rct is the charge transfer resistance, CPE is the constant phase angle element, and WW is the Warburg impedance. The values ​​of each impedance component are obtained through fitting, and the rate of impedance change during the cycling process is calculated.

[0219] VII. Manganese leaching test

[0220] First, prepare the manganese standard solution: Take 10.00 mL of a 1000 μg / mL manganese standard solution and place it in a 100 mL volumetric flask. Add 2.00 mL of concentrated hydrochloric acid, dilute to the mark with water, and shake well to obtain a stock standard solution with a concentration of 100.00 μg / mL. Take 0 mL, 0.50 mL, 1.00 mL, and 2.00 mL of this stock standard solution and place them in four 100 mL volumetric flasks. Add 2.00 mL of concentrated hydrochloric acid to each flask, dilute to the mark with water, and shake well to obtain a series of standard solutions with concentrations of 0 μg / mL, 0.50 μg / mL, 1.00 μg / mL, and 2.00 μg / mL, respectively.

[0221] The emission intensity of manganese in a series of standard solutions was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, model: Agilent 5110, Agilent Technologies, USA). Standard curves were plotted with manganese concentration on the x-axis and emission intensity on the y-axis.

[0222] The test sample solution was prepared as follows: 5.00 g ± 0.05 g of lithium manganese iron phosphate cathode material was placed in a beaker, 100 mL of deionized water was added, and the mixture was stirred until homogeneous. The mixture was kept at 25℃ for 2 hours, and then filtered through a microporous membrane with a pore size of 0.45 μm. The resulting filtrate was the test sample solution. The blank sample solution was prepared using the same procedure, but without the addition of lithium manganese iron phosphate cathode material.

[0223] The emission intensity of manganese in the test sample solution and blank sample solution was measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). The concentration of manganese in the test sample solution was automatically calculated by computer from a standard curve, which is the amount of manganese dissolved in the lithium iron manganese phosphate cathode material. The results are expressed in ppm (mg / kg). Each sample was tested in triplicate, and the average value was taken as the final result. The relative standard deviation was controlled within 5% to ensure the reliability of the data.

[0224] The performance test results of the samples prepared in Examples 1-6 and Comparative Examples 1-9 are shown in Tables 6-1 and 6-2.

[0225] Table 6-1: Performance test results of samples prepared in Examples 1-5 and Comparative Examples 1-9

[0226] Table 6-2: Performance test results of samples prepared in Example 6 and Comparative Examples 1-9

[0227] As can be seen from the experimental data of Examples 1-6 and Comparative Examples 3-7 in Tables 6-1 and 6-2, the Ni-Co dual-phase doping synergistic effect is significant: the performance of Ni-Co dual-phase doped lithium manganese iron phosphate materials is better than that of lithium manganese iron phosphate materials doped with Ni alone, Co alone, and other metal combinations (Ni+Zr, Ni+Mg), and they all show better performance in terms of capacity, cycle stability, interfacial impedance, and manganese dissolution suppression.

[0228] Compared to Comparative Examples 1 and 2, the composite lithium manganese iron phosphate cathode materials prepared in Examples 1 to 6 of this invention have higher electronic conductivity (≥2.8 × 10⁻⁶). - ²S / cm), and Example 1 can achieve a first-cycle discharge specific capacity of >155 mAh / g at 0.1C, and a first-cycle coulombic efficiency of >97%, while also having better high-rate performance.

[0229] Furthermore, the dual modification process of "nickel-cobalt dual-element doping + graphene / Al2O3 composite coating" of the present invention has a synergistic effect: doping solves the problems of bulk conductivity and lattice stability, while coating solves the problems of interface protection and impedance. The combination of the two makes the overall performance of the material far exceed that of a single modification scheme, and the process parameters (such as doping ratio and coating layer ratio) can be adjusted to suit different application scenarios, verifying the innovation and practicality of the technology.

[0230] As can be seen from the experimental data of Example 1 and Comparative Example 1, under the same formulation, the lithium manganese iron phosphate material of the present invention achieves a significant improvement in electronic conductivity. The reasons are: (1) The continuous electron transport network constructed by graphene / Al2O3 coating on the surface of the material can effectively reduce the electron transport resistance; (2) Nickel-cobalt doping makes the material have a higher specific surface area within a certain range, increasing the contact area between the cathode material and the electrolyte, thereby improving the electrochemical performance of the material. In addition, nickel-cobalt doping can not only reduce the cell size of the material, making the material more stable, but also broaden the Li ion transport channel, making the lithium insertion and extraction reaction kinetics better, thereby improving the electrochemical performance of the material, so that the material can still maintain high capacity when discharged at high rate, which can meet the application requirements of new energy vehicles such as "rapid acceleration, hill climbing and other high power output", and solve the technical pain point of "poor rate performance" of traditional lithium manganese iron phosphate materials.

Claims

1. A lithium manganese iron phosphate material, characterized in that, The lithium manganese iron phosphate material comprises a core and a coating layer covering the surface of the core. The core comprises lithium manganese iron phosphate doped with nickel and cobalt, and the coating layer comprises graphene and Al2O3. The chemical formula of the lithium manganese iron phosphate doped with nickel and cobalt is LiMn. 1-x-y-z Fe x Ni y Co z PO4, where 0.4≤x≤0.7, 0.01≤y≤0.03, and 0.005≤z≤0.

02.

2. The lithium manganese iron phosphate as described in claim 1, characterized in that, The x satisfies 0.5≤x≤0.7, and the z satisfies 0.01≤z≤0.

02.

3. The lithium manganese iron phosphate material as described in claim 1, characterized in that, The lithium manganese iron phosphate material has one or more of the following characteristics: In the coating layer, the mass ratio of graphene to Al2O3 is 1:(1-5), preferably 1:(4-5); The thickness of the coating layer is 8-20 nm, preferably 11-20 nm, and more preferably 15-20 nm; The particle size of the lithium manganese iron phosphate material is 1.4-2 μm.

4. A method for preparing lithium manganese iron phosphate material according to any one of claims 1-3, characterized in that, The method includes: (1) Add lithium source, manganese source, iron source, nickel source, cobalt source, phosphorus source and polycarboxylic acid to water, mix evenly to obtain sol; (2) The sol is dried, pre-calcined, and pulverized to obtain a nickel-cobalt-doped lithium manganese iron phosphate precursor; (3) Graphene is dispersed in water to obtain a graphene dispersion; (4) Add the aluminum source to the graphene dispersion and mix evenly to obtain a graphene / Al2O3 coated solution; (5) The nickel-cobalt doped lithium manganese iron phosphate precursor is added to the graphene / Al2O3 coating solution, and after stirring, filtering, drying and calcining, the lithium manganese iron phosphate material is obtained.

5. The method as described in claim 4, characterized in that, The method has one or more of the following characteristics: The polycarboxylic acid is selected from one or more of oxalic acid, tartaric acid, citric acid, succinic acid and phthalic acid, preferably selected from one or more of oxalic acid, tartaric acid and citric acid; The molar ratios of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, and cobalt in the cobalt source satisfy: n(Li):(n(Mn)+n(Fe)+n(Ni)+n(Co))=1:1, and 0.4≤n(Fe):n(Li)≤0.7, 0.01≤n(Ni):n(Li)≤0.03, 0.005≤n(Co):n(Li)≤0.02; preferably, 0.5≤n(Fe):n(Li)≤0.7; preferably, 0.01≤n(Co):n(Li)≤0.02; The molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, cobalt in the cobalt source, and phosphorus in the phosphorus source satisfies: n(P):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=1:2; The molar ratio of lithium in the lithium source, manganese in the manganese source, iron in the iron source, nickel in the nickel source, cobalt in the cobalt source, and the polycarboxylic organic acid satisfies: n(polycarboxylic organic acid):(n(Li)+n(Mn)+n(Fe)+n(Ni)+n(Co))=(1.2-1.5):1; The lithium source is selected from one or more of lithium carbonate, lithium acetate, and lithium nitrate; The manganese source is selected from one or more of manganese sulfate, manganese carbonate, manganese nitrate, manganese oxalate, manganese acetate, and manganese chloride; The iron source is selected from one or more of ferrous sulfate, ferrous chloride, ferrous bromide, ferrous oxalate, ferrous acetate, and ferrous nitrate. The nickel source is selected from one or more of nickel acetate, nickel nitrate, nickel sulfate, and nickel chloride; The cobalt source is selected from one or more of cobalt acetate, cobalt nitrate, cobalt sulfate, and cobalt chloride; The phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and potassium dihydrogen phosphate.

6. The method as described in claim 4, characterized in that, The method has one or more of the following characteristics: In step (1), the pH of the solution is 5.5-6.5; In step (1), the mixture is stirred for 4-6 hours and at a speed of 200-250 rpm. In step (1), the length of the sol filaments is 5-9 cm; In step (2), the drying time is 8-10 hours and the drying temperature is 120-150℃; In step (2), the fire is pre-fired in an air atmosphere for 3-4 hours at a temperature of 400-450°C. In step (2), the material is pulverized by ball milling for 0.5-2 hours and at a speed of 1500-200 rpm. In step (2), the particle size of the nickel-cobalt doped lithium manganese iron phosphate precursor is 1.4-2.0 μm; In step (3), ultrasonic dispersion is used, with an ultrasonic power of 300-500W and an ultrasonic time of 1-2 hours; In step (3), the concentration of the graphene dispersion is 0.5-1.0 g / L, and the particle size of the graphene in the graphene dispersion is 0.5-1 μm; In step (4), the aluminum source is aluminum isopropoxide or aluminum nitrate; In step (4), the mixture is stirred for 3-4 hours and the mixing speed is 200-250 rpm. In step (4), the pH of the graphene / Al2O3 coating solution is 8.0-9.

0. In step (5), the stirring time is 3-4 hours and the stirring speed is 200-250 rpm; In step (5), the drying temperature is 100-120℃ and the drying time is 6-8h; In step (5), calcination is carried out in an inert atmosphere, wherein the inert atmosphere is nitrogen or argon, the calcination temperature is 750-800℃, and the calcination time is 10-12h.

7. Lithium manganese iron phosphate material prepared by any one of claims 4-6.

8. A positive electrode plate, characterized in that, The positive electrode comprises lithium manganese iron phosphate material as described in any one of claims 1-3 and 7.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode sheet as described in claim 8.

10. An electrical device, characterized in that, The electrical device includes the lithium-ion battery as described in claim 9.

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