A multi-element cation-doped ammonium manganese iron phosphate precursor, carbon-coated modified lithium manganese iron phosphate, and a preparation method and application thereof

Abstract

The application belongs to the technical field of new energy materials, and particularly relates to a multi-cation doped ammonium manganese iron phosphate precursor, carbon-coated modified lithium manganese iron phosphate and a preparation method and application thereof. The multi-cation doped ammonium manganese iron phosphate precursor prepared by the application is prepared by using a step-by-step synthesis method, i.e., first preparing a ferric phosphate compound, then synchronously performing manganese ion exchange and metal cation doping on the precursor, and then performing solid-phase sintering on the obtained precursor, a lithium source and a carbon source to prepare the carbon-coated modified lithium manganese iron phosphate for constructing a lithium ion battery. The electrochemical performance test results show that the obtained lithium ion battery has excellent capacity retention rate, cycle stability, rate performance and charge-discharge capacity. Moreover, the preparation method of the carbon-coated modified lithium manganese iron phosphate not only has the advantages of simple operation, less by-products, nanometer particle size, high raw material conversion rate, but also ensures the uniform distribution of each doping element in the material and the sulfur content reaching the battery level, and is suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy materials. More specifically, it relates to a polynary cation-doped ammonium manganese iron phosphate precursor, carbon-coated modified lithium manganese iron phosphate, and their preparation methods and applications. Background Art

[0002] With the widespread application of lithium-ion batteries in electric vehicles, portable electronic devices and other fields, the market demand for battery performance is becoming increasingly diversified, especially the demand for improving energy density, rate performance and low-temperature performance is becoming more and more urgent. As a key factor in determining the energy density of lithium-ion batteries, the performance optimization of positive electrode materials has become a research hotspot. Although traditional lithium iron phosphate positive electrode materials have good safety and cycle stability, their energy density is close to the theoretical limit and it is difficult to meet the growing performance requirements. For this reason, researchers have turned their attention to lithium manganese phosphate with an olivine structure and developed lithium manganese iron phosphate (LiFe α Mn 1-α PO4, 0<α<1) is a solid phosphate system cathode material, in order to optimize the electrochemical performance by partially replacing iron atoms with manganese atoms. Compared with the current mainstream iron phosphate and ternary cathode materials, lithium manganese iron phosphate is regarded as a new cathode material to solve the limited energy density of lithium iron phosphate due to its advantages such as high working voltage platform, high thermal stability, low cost and green environmental protection. However, its inherent poor conductivity and low Li + Diffusion coefficient, Mn 3+ The induced Jahn-Teller effect and manganese dissolution during the cycling process severely restrict its electrochemical performance.

[0003] To overcome these challenges, researchers have proposed a variety of modification strategies, including cation doping, carbon coating, and particle nanosizing. However, existing technical routes still face numerous challenges in the industrialization of lithium manganese iron phosphate. While the solid-phase method is a mature process, the lithium manganese iron phosphate prepared by physical mixing is prone to problems such as uneven element distribution, incomplete conversion of doping elements, excessive byproduct formation, excessive particle size, and excessive sulfur content. For example, Chinese patent application CN117440928A discloses a technical route for preparing lithium manganese iron phosphate using carbon coating and co-precipitation. This effectively solves the problem of unevenness in the precursor. The specific capacity and cycling performance of the lithium manganese iron phosphate further prepared using this precursor are significantly improved. However, on the one hand, this method requires the addition of a co-solvent containing potassium chloride and sodium chloride, which will introduce additional alkali metal impurities such as K and Na. On the other hand, the one-step co-precipitation method in this patent easily forms a precursor with large, irregular particles, which is not conducive to the subsequent grinding process to prepare nano-sized lithium manganese iron phosphate, thereby affecting the electrochemical performance of the subsequent cathode material. Chinese patent application CN116675204A discloses a preparation method, cathode material and battery of a dense ammonium manganese ferrous phosphate precursor, by mixing a phosphorus source with ammonia and adding alkali solution to adjust pH to alkaline, then reacting with ferromanganese metal mixture to obtain a particle morphology of spherical or sea urchin-shaped ammonium manganese ferrous phosphate monohydrate, the ammonium manganese ferrous phosphate precursor obtained by the above method has low Na / S impurity content, high tap density and good product crystallinity. However, it needs to first modulate the reaction bottom liquid, and simultaneously the mixed solution of the phosphorus source and ammonia and the ferromanganese metal aqueous solution of the configuration are respectively pumped into the reaction bottom liquid and reacted, that is, it is still prepared by one-step coprecipitation. In addition, there is no relevant data showing the effect of the precursor after doping metal, and the sulfur impurity content of its resulting precursor is still higher (greater than 450ppm).

[0004] Therefore, although lithium manganese iron phosphate has made certain progress in modification strategies, further modification is still needed to achieve more efficient and uniform doping and coating, thereby improving the overall performance of lithium manganese iron phosphate and promoting its widespread application in the field of lithium-ion batteries. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects and shortcomings of the modification strategy of lithium manganese iron phosphate, such as uneven distribution of doping elements, incomplete reaction, large amount of by-products, excessive particle size and excessive sulfur content, and to provide a multi-cation doped ammonium manganese iron phosphate precursor.

[0006] Another object of the present invention is to provide a method for preparing carbon-coated modified lithium manganese iron phosphate.

[0007] Another object of the present invention is to provide carbon-coated modified lithium manganese iron phosphate prepared by the preparation method.

[0008] Another object of the present invention is to provide a lithium-ion battery positive electrode, wherein the positive electrode comprises the aforementioned carbon-coated modified lithium manganese iron phosphate.

[0009] Another object of the present invention is to provide a lithium-ion battery, wherein the positive electrode of the lithium-ion battery comprises the above-mentioned positive electrode of the lithium-ion battery.

[0010] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0011] The present invention provides a polynary cation-doped ammonium manganese ferric phosphate precursor, the preparation method of which comprises the following steps:

[0012] S1. The phosphorus source solution and the iron salt solution are mixed at pH 7 to 11 and reacted at 50 to 60 ° C to obtain a reaction system I;

[0013] S2. The manganese salt solution, the doped metal salt solution M is prepared into a mixed solution under a protective gas atmosphere;

[0014] S3. The mixed solution obtained in step S2 was added to the reaction system I obtained in step S1 at least twice, maintaining the pH of the system at 7 to 11, mixing, and reacting at 50 to 60 ° C., and post-processing to obtain a polyvalent cation-doped ammonium manganese iron phosphate precursor;

[0015] Each time the mixed solution is added, it needs to be mixed with the reaction system I and fully reacted at 50-60°C before the next addition; the amount of the mixed solution added for the first time is 10% to 45% of the total volume of the mixed solution;

[0016] The metal salt M solution contains three metal ions A, B, and C, wherein A, B, and C are selected from Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr, Nb, and Al, which are not repeated.

[0017] The expression of the polyvalent cation-doped ammonium manganese iron phosphate precursor is NH4Mn x Fe 1-x-a-b-c A a B b C c PO4, and 0<x<0.9, 0<a≤0.05, 0<b≤0.05, 0<c≤0.05, a+b+c=0.1.

[0018] This application synthesized a multi-metal cation-doped NH4Mn x Fe 1-x-a-b-c A a B b C cPO4 precursor. This design effectively avoids the problems of uneven atomic distribution and irregular surface deposition after physical mixing of conventional oxide dopants with phosphorus sources, carbon sources, manganese salts and iron salts, significantly improving the structural stability of the material. The sulfur impurity content is strictly controlled at ≤100ppm, meeting battery-grade standards, thus eliminating the tedious secondary desulfurization process. During the preparation process, this application innovatively achieves uniform substitution and coordination of multiple different metal cations in ammonium manganese iron phosphate, and a synergistic effect between different metal atoms, ensuring that the doped metal, manganese and iron atoms are regularly distributed at the atomic level in the precursor lattice. Through a step-by-step process, the raw materials can be maximized. In the key step of manganese ion exchange in step S3, the phosphorus source and the mixed solution are cleverly used to fully exchange manganese ions with the excess iron ion raw materials in the solution, achieving a metal ion utilization rate close to 100%, thereby significantly improving the overall yield.

[0019] Furthermore, the expression of the polynary cation doped ammonium manganese iron phosphate precursor is NH4Mn x Fe 1-x-a-b- c A a B b C c PO4, and 0.1<x<0.9, 0<a≤0.05, 0<b≤0.05, 0<c≤0.05, a+b+c=0.1.

[0020] Preferably, the expression of the polyvalent cation-doped ammonium manganese iron phosphate precursor is NH4Mn x Fe 1-x-a-b- c A a B b C c PO4, and 0.4≤x<0.9, 0<a≤0.05, 0<b≤0.05, 0<c≤0.05, a+b+c=0.1.

[0021] More preferably, the expression of the polynary cation-doped ammonium manganese iron phosphate precursor is NH4Mn x Fe 1-x-a-b- c A a B b C c PO4, and 0.5≤x≤0.8, 0<a≤0.05, 0<b≤0.05, 0<c≤0.05, a+b+c=0.1.

[0022] Furthermore, in step S1, the phosphorus source includes one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, lithium dihydrogen phosphate, dilithium hydrogen phosphate, lithium acetate, and lithium phosphate, preferably ammonium dihydrogen phosphate.

[0023] Furthermore, in step S1, the iron salt includes ferrous sulfate, ferric chloride, ferrous chloride, ferric nitrate, ferric acetate, ammonium ferric sulfate, ferric citrate, ferrous oxalate or a hydrate of any of the above iron salts, preferably ferrous sulfate heptahydrate.

[0024] Furthermore, in step S1, the molar ratio of the phosphorus source to the iron salt is 1:(0.5-3).

[0025] Furthermore, the molar ratio of the phosphorus source to the iron salt is 1:1.

[0026] Furthermore, in step S1, the pH value of 7 to 11 is adjusted by adding an alkaline reagent.

[0027] Furthermore, the alkaline reagent includes one or more of ammonia water, sodium hydroxide, lithium hydroxide, potassium hydroxide, trimethylamine, and pyridine, preferably ammonia water.

[0028] Furthermore, in step S1, the reaction time is 0.5 to 10 hours.

[0029] Furthermore, in step S2, the protective gas includes one or more of nitrogen, argon, and helium.

[0030] Furthermore, in step S2, the manganese salt includes manganese sulfate, manganese chloride, manganese nitrate, manganese carbonate, manganese acetate, manganese oxalate or a hydrate of any of the above manganese salts, preferably manganese sulfate monohydrate.

[0031] Furthermore, in step S2, the metal salt M solution includes one or more of sulfate, nitrate, acetate, oxalate, and citrate.

[0032] Furthermore, in step S2, the Mg in the metal salt M solution includes magnesium sulfate, magnesium nitrate, magnesium carbonate, magnesium chloride, magnesium acetate, magnesium oxalate, magnesium citrate or a hydrate of any of the above magnesium salts, preferably magnesium sulfate.

[0033] Furthermore, in step S2, the Ti in the metal salt M solution includes ammonium titanyl oxalate, titanium sulfate, titanyl sulfate, titanium chloride or a hydrate of any of the above titanium salts, preferably ammonium titanyl oxalate.

[0034] Furthermore, in step S2, V in the metal salt M solution includes vanadyl oxalate, vanadyl sulfate, vanadyl dichloride or a hydrate of any of the above vanadium salts, preferably vanadium oxalate.

[0035] Furthermore, in step S2, the Co in the metal salt M solution includes cobalt sulfate, cobalt acetate, cobalt chloride, cobalt nitrate or a hydrate of any of the above cobalt salts, preferably cobalt sulfate heptahydrate.

[0036] Furthermore, in step S2, the Nb in the metal salt M solution includes ammonium niobium oxalate, niobium oxalate or a hydrate of any of the above niobium salts, preferably ammonium niobium oxalate.

[0037] Furthermore, in step S2, the Zn in the metal salt M solution includes zinc sulfate, zinc acetate, zinc chloride, zinc nitrate or a hydrate of any of the above zinc salts, preferably zinc sulfate.

[0038] Furthermore, in step S2, the Cu in the metal salt M solution includes copper sulfate, copper acetate, copper chloride, copper nitrate or a hydrate of any of the above copper salts, preferably copper sulfate.

[0039] Furthermore, in step S2, the Zr in the metal salt M solution includes zirconium sulfate, zirconium tetrachloride, zirconium oxychloride, zirconium nitrate or a hydrate of any of the above zirconium salts, preferably zirconium sulfate.

[0040] Furthermore, in step S2, the Al in the metal salt M solution includes aluminum sulfate, aluminum chloride, aluminum nitrate or a hydrate of any of the above aluminum salts, preferably aluminum sulfate.

[0041] Furthermore, in step S2, the molar ratio of manganese in the manganese salt and metals A, B, and C in the doping metal salt M solution is d:e:f:g, wherein d is 0<d<0.9, e is 0<e≤0.05, 0<f≤0.05, and 0<g≤0.05.

[0042] Furthermore, in step S2, the molar ratio of manganese in the manganese salt and metals A, B, and C in the doping metal salt M solution is d:e:f:g, wherein d is 0.1≤d<0.9, e is 0<e≤0.05, 0<f≤0.05, and 0<g≤0.05.

[0043] Preferably, in step S2, the molar ratio of manganese in the manganese salt and metals A, B, and C in the doping metal salt M solution is d:e:f:g, wherein d is 0.4≤d<0.9, e is 0<e≤0.05, 0<f≤0.05, and 0<g≤0.05.

[0044] More preferably, in step S2, the molar ratio of manganese in the manganese salt and metals A, B, and C in the doping metal salt M solution is d:e:f:g, wherein d is 0.5≤d≤0.8, e is 0<e≤0.05, 0<f≤0.05, and 0<g≤0.05.

[0045] Furthermore, in step S2, the concentration of the mixed solution is the molar concentration of total metal cations.

[0046] Furthermore, in step S2, the concentration of the mixed solution is 0.5-2 mol / L, preferably 0.7 mol / L.

[0047] Furthermore, in step S3, the reaction time is 0.5 to 10 hours.

[0048] Furthermore, in step S3, the post-treatment includes filter pressing and washing.

[0049] Furthermore, the filter pressing is to filter the product and then separate the solid and liquid.

[0050] Furthermore, the washing is performed using water for 3 to 5 times.

[0051] Specifically, in step S3, the post-treatment includes filtering the product, performing solid-liquid separation, and washing with water for 3 to 5 times.

[0052] The present invention provides a method for preparing carbon-coated modified lithium manganese iron phosphate, comprising the following steps:

[0053] Si. The lithium salt, the phosphorus source, the polyvalent cation-doped ammonium manganese ferric phosphate precursor obtained above, the carbon source and water were thoroughly mixed to obtain a slurry, and the resulting slurry was ground and dried to obtain a powder;

[0054] Sii. Under a protective gas atmosphere, the powder obtained in step Si is calcined at 600-900° C. and post-treated to obtain a modified lithium manganese iron phosphate positive electrode material.

[0055] In the process of preparing the modified lithium iron manganese phosphate cathode material, this application cleverly uses precursors with atomic-level distribution of various elements as doping sources for iron salts, manganese salts, and multi-metal cations, directly performing fine mixing. This strategy fundamentally blocks the entry of sulfur impurities, effectively preventing their direct contact with the electrolyte, thereby reducing the formation of sulfur-containing byproducts and demonstrating good capacity retention during cycling. Furthermore, this application combines a highly conductive carbon layer with the lithium iron manganese phosphate, significantly improving the material's electronic conduction efficiency and further optimizing its rate capability and cycling stability.

[0056] Furthermore, in step Si, the lithium salt includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium nitrate, lithium acetate, and lithium phosphate, preferably lithium carbonate.

[0057] Furthermore, in step Si, the phosphorus source includes one or more of lithium dihydrogen phosphate, dilithium hydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid, preferably lithium dihydrogen phosphate.

[0058] Furthermore, in step Si, the carbon source includes one or more of glucose, polyethylene glycol, polyvinylidene fluoride, polyacrylamide, polyvinyl alcohol, sucrose, starch, acetylene black, graphite powder, graphene, and carbon nanotubes, preferably glucose.

[0059] Furthermore, in step Si, the molar ratio of the lithium salt, the phosphorus source, and the multi-metal cation-doped ammonium manganese iron phosphate precursor obtained above is 1:(0.9-1.1):(0.8-1.2).

[0060] Furthermore, in step Si, the mass ratio of the multi-metal cation-doped ammonium manganese iron phosphate precursor to the carbon source is 1:(0.05-0.11).

[0061] Furthermore, in step Si, the solid content of the slurry is 15% to 48%.

[0062] Furthermore, in step Si, the particle size of the ground slurry is 0.28 μm ≤ D 50 ≤0.3μm.

[0063] Furthermore, in step S1, the drying equipment is a spray drying equipment.

[0064] Furthermore, the conditions of the spray drying equipment are as follows: the inlet air temperature is 200-260°C, the outlet air temperature is 100-150°C, and the discharge particle size is 22 μm ≤ D 50 ≤25μm.

[0065] Furthermore, in step Sii, the protective gas includes one or more of nitrogen, helium, and argon.

[0066] Furthermore, the oxygen content of the protective gas is less than 10 ppm.

[0067] Furthermore, in step Sii, the calcining equipment is a box furnace, a roller kiln or a tube furnace, preferably a box furnace.

[0068] Furthermore, the heating rate of the box furnace is 2-5°C.

[0069] Furthermore, in step Sii, the calcination time is 2 to 20 hours.

[0070] Furthermore, in step Sii, the post-processing includes crushing and screening.

[0071] Furthermore, the particle size of the crushed particles is 0.5 μm ≤ D 50 ≤1μm.

[0072] Furthermore, the mesh size of the sieving sieve is 200 to 400 meshes.

[0073] The present invention protects the carbon-coated modified lithium manganese iron phosphate prepared by the above preparation method.

[0074] The present invention protects a lithium-ion battery positive electrode, comprising a current collector and a positive electrode active material loaded on the current collector, wherein the positive electrode active material comprises the aforementioned carbon-coated modified lithium manganese iron phosphate.

[0075] Preferably, the current collector is unmodified aluminum foil or carbon-coated aluminum foil.

[0076] Furthermore, as a preferred manner, the preparation method of the lithium-ion battery positive electrode comprises the following steps: dispersing carbon-coated modified lithium manganese iron phosphate, conductive agent carbon black, and binder polyvinylidene fluoride in N-methylpyrrolidone dispersant to obtain a positive electrode slurry, which is then coated on aluminum foil, and vacuum dried at 80-140°C for 12-24 hours to obtain a positive electrode.

[0077] Furthermore, the mass ratio of the carbon-coated modified lithium manganese iron phosphate, the conductive carbon black, and the binder polyvinylidene fluoride is 1: (0.005-0.285): (0.01-0.14).

[0078] The present invention protects a lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode is the positive electrode of the above-mentioned lithium ion battery.

[0079] Furthermore, as a preferred embodiment, the positive electrode of the lithium-ion battery is the positive electrode of the aforementioned lithium-ion battery, the negative electrode is a metal lithium sheet, the separator is a polyethylene film, and the electrolyte is LiPF6.

[0080] The lithium ion battery prepared by the present invention exhibits excellent capacity retention, cycle stability, rate performance and charge and discharge capacity.

[0081] Compared with the prior art, the present invention has the following beneficial effects:

[0082] The polynary cation-doped ammonium manganese iron phosphate precursor prepared in this application uses a step-by-step synthesis method. First, an iron phosphate complex is prepared. Then, manganese ion exchange and metal cation doping are simultaneously performed to obtain a precursor. The obtained precursor is solid-phase sintered with a lithium source and a carbon source to prepare a carbon-coated modified lithium manganese iron phosphate for use in constructing lithium-ion batteries. Electrochemical performance test results show that the resulting lithium-ion battery has excellent capacity retention, cycle stability, rate performance, and charge-discharge capacity. Moreover, the preparation method of the carbon-coated modified lithium manganese iron phosphate is not only simple to operate, produces few by-products, has nano-sized particles, and has a high raw material conversion rate, but also ensures the uniform distribution of each doping element in the material and that the sulfur content reaches battery-grade levels, making it suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Figure 1This is an SEM image of the multi-cation doped ammonium manganese iron phosphate precursor prepared in Example 1.

[0084] Figure 2 This is an SEM image of the carbon-coated modified lithium manganese iron phosphate prepared in Example 1.

[0085] Figure 3 This is an SEM image of the carbon-coated modified lithium manganese iron phosphate prepared in Example 1.

[0086] Figure 4 This is the XRD pattern of the carbon-coated modified lithium manganese iron phosphate prepared in Example 1. DETAILED DESCRIPTION

[0087] The present invention will be further described below with reference to the accompanying drawings and specific examples, but the examples do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0088] Unless otherwise specified, the reagents and materials used in the following examples were commercially available.

[0089] Example 1 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate and Lithium Ion Battery

[0090] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 Preparation of PO4

[0091] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, cobalt sulfate heptahydrate, vanadium oxalate, and magnesium sulfate were dissolved in ultrapure water at a molar ratio of 60:5:3:2 to prepare a 0.7 mol / L (referring to the molar concentration of total metal cations) mixed salt solution C.

[0092] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0093] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0094] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0095] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the polyvalent cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4.

[0096] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0097] According to the molar ratio of Li:P:(Fe+Mn+Co+V+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0098] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0099] 3. Preparation of lithium-ion batteries

[0100] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0101] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0102] Example 2 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate and Lithium Ion Battery

[0103] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.02 Zn 0.03 V 0.05 Preparation of PO4

[0104] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, cobalt sulfate heptahydrate, zinc sulfate, and vanadium oxalate were dissolved in ultrapure water at a molar ratio of 60:2:3:5 to prepare a 0.7 mol / L mixed salt solution C.

[0105] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the solution to 9.5. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 60°C for 2 h to form Reaction System I.

[0106] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 60° C. for 2 h to form reaction system II.

[0107] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 9.5 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 60°C for 2 h to obtain mixed slurry III.

[0108] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain NH4Mn 0.6 Fe0.3 Co 0.02 Zn 0.03 V 0.05 PO4 precursor.

[0109] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0110] According to the molar ratio of Li:P:(Fe+Mn+Co+Zn+V)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.02 Zn 0.03 V 0.05 PO4 precursor, 11% NH4Mn 0.6 Fe 0.3 Co 0.02 Zn 0.03 V 0.05 Polyethylene glycol (PEG) of PO4 precursor quality was used as a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 38%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and the temperature was increased to 700℃ at a heating rate of 5℃ / min and kept at this temperature for 9 hours.

[0111] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0112] 3. Preparation of lithium-ion batteries

[0113] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0114] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0115] Example 3 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate and Lithium Ion Battery

[0116] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04 Preparation of PO4

[0117] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, ammonium titanyl oxalate, magnesium sulfate, and cobalt sulfate heptahydrate were dissolved in ultrapure water at a molar ratio of 70:3:3:4 to prepare a 0.8 mol / L mixed salt solution C.

[0118] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the solution to 9. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 55°C for 2 h to form Reaction System I.

[0119] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 55° C. for 2 h to form reaction system II.

[0120] Use a peristaltic pump to pump 20 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 40 L of mixed salt solution C into it. Finally, react at 55 ° C for 2 h to obtain mixed slurry III.

[0121] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04 PO4 precursor.

[0122] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0123] According to the molar ratio of Li:P:(Fe+Mn+Ti+Mg+Co)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.7 Fe0.2 Ti 0.03 Mg 0.03 Co 0.04 PO4 precursor, 7% NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04 Polyvinylidene fluoride (PVDF) of PO4 precursor quality was used as a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 44%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite sagger and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 730℃ at a heating rate of 3℃ / min and kept at this temperature for 9 hours.

[0124] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0125] 3. Preparation of lithium-ion batteries

[0126] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0127] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0128] Comparative Example 1 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Modified Lithium Manganese Iron Phosphate Cathode Material and Lithium Ion Battery

[0129] The difference from Example 1 is that the polyvalent cation-doped ammonium manganese iron phosphate precursor is synthesized using a one-step method.

[0130] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02Preparation of PO4

[0131] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate, manganese sulfate monohydrate, cobalt sulfate heptahydrate, vanadium oxalate, and magnesium sulfate were dissolved in ultrapure water at a molar ratio of 30:60:5:3:2 to prepare a 1 mol / L mixed salt solution B.

[0132] A peristaltic pump was used to pump 15 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the solution to 10. A peristaltic pump was then used to slowly pump 30 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to produce mixed slurry I.

[0133] After the reactor was cooled to room temperature, the mixed slurry I was subjected to filter press for solid-liquid separation and washed three times with ultrapure water to obtain NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor.

[0134] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0135] According to the molar ratio of Li:P:(Fe+Mn+Co+V+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0136] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0137] 3. Preparation of lithium-ion batteries

[0138] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0139] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0140] Comparative Example 2 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Modified Lithium Manganese Iron Phosphate Cathode Material and Lithium Ion Battery

[0141] The difference from Example 2 is that the polyvalent cation-doped ammonium manganese iron phosphate precursor is synthesized using a one-step method.

[0142] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.02 Zn 0.03 V 0.05 Preparation of PO4

[0143] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate, manganese sulfate monohydrate, cobalt sulfate heptahydrate, zinc sulfate, and vanadium oxalate were dissolved in ultrapure water at a molar ratio of 30:60:2:3:5 to prepare a 1 mol / L mixed salt solution B.

[0144] A peristaltic pump was used to pump 15 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the solution to 9.5. A peristaltic pump was then used to slowly pump 30 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 60°C for 2 h to produce mixed slurry I.

[0145] After the reactor was cooled to room temperature, the mixed slurry I was subjected to filter press for solid-liquid separation and washed three times with ultrapure water to obtain NH4Mn 0.6 Fe 0.3 Co0.02 Zn 0.03 V 0.05 PO4 precursor.

[0146] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0147] According to the molar ratio of Li:P:(Fe+Mn+Co+Zn+V)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.02 Zn 0.03 V 0.05 PO4 precursor, 11% NH4Mn 0.6 Fe 0.3 Co 0.02 Zn 0.03 V 0.05 PEG of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 38%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and the temperature was increased to 700℃ at a heating rate of 5℃ / min and kept at this temperature for 9 hours.

[0148] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0149] 3. Preparation of lithium-ion batteries

[0150] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0151] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0152] Preparation of multi-cation doped ammonium manganese iron phosphate precursor, carbon-coated modified lithium manganese iron phosphate and lithium ion battery

[0153] The difference from Example 3 is that the multi-cation doped ammonium manganese iron phosphate precursor is synthesized by a one-step method.

[0154] 1. Preparation of multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04 PO4

[0155] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate is dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate, manganese sulfate monohydrate, oxotitanium ammonium oxalate, magnesium sulfate, and cobalt sulfate heptahydrate are dissolved in ultrapure water according to a molar ratio of 20:70:3:3:4 to prepare a 1 mol / L mixed salt solution B.

[0156] Using a peristaltic pump, 15 L of the ammonium dihydrogen phosphate solution A is pumped into a high-efficiency closed synthesis reactor, and nitrogen gas is passed into the reactor to evacuate the air in the closed reactor. Subsequently, the pH of the solution system in the reactor is adjusted to 9 using ammonia water, and then 30 L of the iron salt solution B is slowly pumped into the reactor using a peristaltic pump. Finally, the reaction is carried out at 55°C for 2 h to prepare a mixed slurry I.

[0157] After the reactor is cooled to room temperature, the mixed slurry I is subjected to pressure filtration solid-liquid separation, and washed 3 times with ultrapure water to obtain the NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04 PO4 precursor.

[0158] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0159] According to a molar ratio of Li:P:(Fe+Mn+Ti+Mg+Co)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (wherein Li is provided by lithium dihydrogen phosphate and lithium carbonate, and P is provided by lithium dihydrogen phosphate), NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04 PO4 precursor, 7% NH4Mn 0.7 Fe 0.2 Ti 0.03 Mg 0.03 Co 0.04PVDF with a PO4 precursor mass is used as a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 44%. The mixed slurry is fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite sagger and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 730℃ at a heating rate of 3℃ / min and kept at this temperature for 9 hours.

[0160] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0161] 3. Preparation of lithium-ion batteries

[0162] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0163] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0164] Comparative Example 4 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate and Lithium Ion Battery

[0165] The difference from Example 1 is that the mixed salt solution C is not added in batches to the multi-cation doped ammonium manganese iron phosphate precursor prepared in the reaction system I.

[0166] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 Preparation of PO4

[0167] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, cobalt sulfate heptahydrate, vanadium oxalate, and magnesium sulfate were dissolved in ultrapure water at a molar ratio of 60:5:3:2 to prepare a 0.7 mol / L mixed salt solution C.

[0168] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0169] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into the reaction kettle system I. Use ammonia water to adjust the pH of the solution system in the reaction kettle to 10. After sufficient stirring for 2 hours, 33.4 L of mixed salt solution C is slowly pumped into it. After sufficient stirring, the system is reacted at 50°C for 2 hours to form reaction system II.

[0170] After the reactor was cooled to room temperature, the mixed slurry II was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor.

[0171] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0172] According to the molar ratio of Li:P:(Fe+Mn+Co+V+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50When the particle size is ≤0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set at an inlet temperature of 220°C and an outlet temperature of 110°C. The feed rate is adjusted according to the outlet temperature and the particle size (22μm≤D50≤25μm) to obtain a spray powder. The spray powder is placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm). The temperature is increased to 700°C at a heating rate of 2°C / min and maintained at this temperature for 10 hours.

[0173] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0174] 3. Preparation of lithium-ion batteries

[0175] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0176] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0177] Comparative Example 5 Preparation of Multi-Cation Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate and Lithium Ion Battery

[0178] The difference from Example 1 is that an excess of ternary cations is used to dope with ammonium manganese ferric phosphate to obtain a multi-cation doped ammonium manganese ferric phosphate precursor.

[0179] 1. Multi-cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.2 Co 0.1 V 0.06 Mg 0.04 Preparation of PO4

[0180] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, cobalt sulfate heptahydrate, vanadium oxalate, and magnesium sulfate were dissolved in ultrapure water at a molar ratio of 30:5:3:2 to prepare a 0.8 mol / L mixed salt solution C.

[0181] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0182] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0183] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 40 L of mixed salt solution C into it. Finally, react at 50 ° C for 2 h to obtain mixed slurry III.

[0184] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the polyvalent cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.2 Co 0.1 V 0.06 Mg 0.04 PO4.

[0185] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0186] According to the molar ratio of Li:P:(Fe+Mn+Co+V+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.2 Co 0.1 V 0.06 Mg 0.04 PO4 precursor, 8% NH4Mn 0.6 Fe 0.2 Co 0.1 V 0.06 Mg 0.04 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0187] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0188] 3. Preparation of lithium-ion batteries

[0189] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0190] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0191] Comparative Example 6 Preparation of Binary Cation-Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate, and Lithium Ion Battery

[0192] The difference from Example 1 is that a binary cation-doped ammonium manganese ferric phosphate precursor is prepared by doping binary cations with ammonium manganese ferric phosphate.

[0193] 1. Binary cation doped ammonium manganese ferrous phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.05 Preparation of PO4

[0194] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, cobalt sulfate heptahydrate, and vanadium oxalate were dissolved in ultrapure water at a molar ratio of 12:1:1 to prepare a 0.7 mol / L mixed salt solution C.

[0195] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0196] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0197] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0198] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the binary cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.05 PO4.

[0199] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0200] According to the molar ratio of Li:P:(Fe+Mn+Co+V)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.05 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.05 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0201] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0202] 3. Preparation of lithium-ion batteries

[0203] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0204] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0205] Comparative Example 7 Preparation of Binary Cation-Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate, and Lithium Ion Battery

[0206] The difference from Example 1 is that a binary cation-doped ammonium manganese ferric phosphate precursor is prepared by doping binary cations with ammonium manganese ferric phosphate.

[0207] 1. Binary cation doped ammonium manganese ferrous phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 Mg 0.05 Preparation of PO4

[0208] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, cobalt sulfate heptahydrate, and magnesium sulfate were dissolved in ultrapure water at a molar ratio of 12:1:1 to prepare a 0.7 mol / L mixed salt solution C.

[0209] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0210] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0211] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0212] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the binary cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.05 Mg 0.05 PO4.

[0213] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0214] According to the molar ratio of Li:P:(Fe+Mn+Co+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.05 Mg 0.05 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 Co 0.05 Mg 0.05 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0215] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0216] 3. Preparation of lithium-ion batteries

[0217] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0218] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0219] Comparative Example 8 Preparation of Binary Cation-Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate, and Lithium Ion Battery

[0220] The difference from Example 1 is that a binary cation-doped ammonium manganese ferric phosphate precursor is prepared by doping binary cations with ammonium manganese ferric phosphate.

[0221] 1. Binary cation doped ammonium manganese ferrous phosphate precursor NH4Mn 0.6 Fe 0.3 V 0.05 Mg 0.05 Preparation of PO4

[0222] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate, vanadium oxalate, and magnesium sulfate were dissolved in ultrapure water at a molar ratio of 12:1:1 to prepare a 0.7 mol / L mixed salt solution C.

[0223] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0224] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0225] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0226] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the binary cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 V 0.05 Mg 0.05 PO4.

[0227] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0228] According to the molar ratio of Li:P:(Fe+Mn+V+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 V 0.05 Mg 0.05 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 V 0.05 Mg 0.05 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0229] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0230] 3. Preparation of lithium-ion batteries

[0231] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0232] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0233] Comparative Example 9 Preparation of Monovalent Cation-Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate, and Lithium Ion Battery

[0234] The difference from Example 1 is that a monovalent cation-doped ammonium manganese ferric phosphate precursor is prepared by doping ammonium manganese ferric phosphate with monovalent cations.

[0235] 1. Monobasic cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.1 Preparation of PO4

[0236] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate and cobalt sulfate heptahydrate were dissolved in ultrapure water at a molar ratio of 6:1 to prepare a 0.7 mol / L mixed salt solution C.

[0237] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0238] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0239] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0240] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the monovalent cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Co 0.1 PO4.

[0241] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0242] According to the molar ratio of Li:P:(Fe+Mn+Co)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Co 0.1 Precursor, 8% NH4Mn 0.6 Fe 0.3 Co 0.1Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0243] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0244] 3. Preparation of lithium-ion batteries

[0245] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0246] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0247] Comparative Example 10 Preparation of Monovalent Cation-Doped Ammonium Manganese Iron Phosphate Precursor, Carbon-Coated Modified Lithium Manganese Iron Phosphate, and Lithium Ion Battery

[0248] The difference from Example 1 is that a monovalent cation-doped ammonium manganese ferric phosphate precursor is prepared by doping ammonium manganese ferric phosphate with monovalent cations.

[0249] 1. Monobasic cation doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 V 0.1 Preparation of PO4

[0250] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate was dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate was dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; manganese sulfate monohydrate and vanadium oxalate were dissolved in ultrapure water at a molar ratio of 6:1 to prepare a 0.7 mol / L mixed salt solution C.

[0251] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency, sealed, synthetic reactor. Nitrogen was then introduced into the reactor to evacuate the air. Ammonia was then used to adjust the pH of the reactor to 10. A peristaltic pump was then used to slowly pump 10 L of iron salt solution B into the reactor. After thorough stirring, the system was reacted at 50°C for 2 h to form Reaction System I.

[0252] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form reaction system II.

[0253] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0254] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the monovalent cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 V 0.1 PO4.

[0255] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0256] According to the molar ratio of Li:P:(Fe+Mn+V)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 V 0.1 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 V 0.1 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0257] After the nitrogen atmosphere box furnace is naturally reduced to room temperature, the sintered powder is airflow pulverized to a particle size of 0.5 μm≤D 50 ≤1 μm, and then sieved using a 200-mesh screen to obtain carbon-coated modified lithium manganese iron phosphate.

[0258] 3. Preparation of a lithium ion battery

[0259] The carbon-coated modified lithium manganese iron phosphate obtained above, carbon black as a conductive agent, and polyvinylidene fluoride as a binder are dissolved in N-methylpyrrolidone dispersant and mixed uniformly in a mass ratio of 94:4:2 to obtain a positive electrode slurry, which is then coated on an aluminum foil. After vacuum drying at 100°C for 24 h, a lithium ion battery positive electrode is obtained.

[0260] A button lithium ion battery is assembled in an argon-filled glove box, the positive electrode is the lithium ion battery positive electrode described above, the negative electrode is a metal lithium sheet, the separator is a polyethylene film, and the electrolyte is LiPF6.

[0261] Preparation of a lithium ion battery using a monocation-doped ammonium manganese iron phosphate precursor and carbon-coated modified lithium manganese iron phosphate

[0262] The difference from Example 1 is that a monocation-doped ammonium manganese iron phosphate precursor is prepared by doping a monocation with ammonium manganese iron phosphate.

[0263] 1. Preparation of a monocation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Mg 0.1 PO4

[0264] First, under a nitrogen protective atmosphere, ammonium dihydrogen phosphate is dissolved in ultrapure water to prepare a 2 mol / L ammonium dihydrogen phosphate solution A; ferrous sulfate heptahydrate is dissolved in ultrapure water to prepare a 1 mol / L iron salt solution B; and manganese sulfate monohydrate and magnesium sulfate are dissolved in ultrapure water in a molar ratio of 6:1 to prepare a 0.7 mol / L mixed salt solution C.

[0265] Using a peristaltic pump, 5 L of the ammonium dihydrogen phosphate solution A is pumped into a high-efficiency closed synthesis reactor, and nitrogen gas is passed into the reactor to evacuate the air in the closed reactor. Then, the pH of the solution system in the reactor is adjusted to 10 using ammonia water, and 10 L of the iron salt solution B is slowly pumped into the reactor using a peristaltic pump. After sufficient stirring, the system is reacted at 50°C for 2 h to form a reaction system I.

[0266] Using a peristaltic pump, 10 L of the mixed salt solution C is slowly pumped into the reaction system I in the reactor, and after sufficient stirring, the system is reacted at 50°C for 2 h to form a reaction system II.

[0267] Use a peristaltic pump to pump 11.7 L of ammonium dihydrogen phosphate solution A into reaction system II in the reactor, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 h, slowly pump 23.4 L of mixed salt solution C into it. Finally, react at 50°C for 2 h to obtain mixed slurry III.

[0268] After the reactor was cooled to room temperature, the mixed slurry III was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times to obtain the monovalent cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.6 Fe 0.3 Mg 0.1 PO4.

[0269] 2. Preparation of carbon-coated modified lithium manganese iron phosphate

[0270] According to the molar ratio of Li:P:(Fe+Mn+Mg)=1.03:1:1, lithium dihydrogen phosphate, lithium carbonate (Li is provided by lithium dihydrogen phosphate and lithium carbonate, P is provided by lithium dihydrogen phosphate), NH4Mn 0.6 Fe 0.3 Mg 0.1 PO4 precursor, 8% NH4Mn 0.6 Fe 0.3 Mg 0.1 Glucose of PO4 precursor quality was used as carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%. The mixed slurry was fully ground in a sand mill to a slurry particle size of 0.28 μm ≤ D 50 When the particle size is less than 0.3μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet air temperature of 220℃ and an outlet air temperature of 110℃. According to the outlet air temperature and the particle size of the material (22μm≤D 50 The spray powder was placed in a graphite crucible and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700℃ at a heating rate of 2℃ / min and kept at this temperature for 10 hours.

[0271] After the nitrogen atmosphere box furnace naturally cools to room temperature, the sintered powder is air flow crushed to a particle size of 0.5μm≤D 50 ≤1μm, and then sieved with a 200-mesh sieve to obtain carbon-coated modified lithium manganese iron phosphate.

[0272] 3. Preparation of lithium-ion batteries

[0273] The carbon-coated modified lithium manganese iron phosphate obtained above was dissolved in N-methylpyrrolidone dispersant with conductive agent carbon black and binder polyvinylidene fluoride in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which was then coated on aluminum foil and vacuum dried at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0274] The button lithium-ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the lithium-ion battery mentioned above, the negative electrode was a metal lithium sheet, the separator was a polyethylene film, and the electrolyte was LiPF6.

[0275] Experimental Example 1 Material composition and microstructure characterization

[0276] (1) SEM measurement

[0277] The NH4Mn prepared in Experimental Example 1 was observed by emission scanning electron microscopy. 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor and carbon-coated modified lithium manganese iron phosphate LiMn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 was characterized, and the results were as follows Figures 1 to 3 As shown in the figure, the NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 After solid-liquid separation by filter press, the PO4 precursor is evenly distributed in micron size, while the carbon-coated modified lithium manganese iron phosphate LiMn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 After air flow crushing and sieving, PO4 is evenly distributed in nanometer size.

[0278] (2) XRD determination

[0279] The carbon-coated modified LiMn prepared in Example 1 was measured by X-ray diffractometer. 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 was characterized, and the results were as follows Figure 4 As shown, the carbon-coated modified lithium manganese iron phosphate prepared in Example 1 has diffraction peaks corresponding to LiMnPO4 and LiFePO4, and no other impurity peaks are present, which indicates that Mn, Fe and multi-element doping cations are uniformly distributed in the lattice.

[0280] (3) Detection of sulfur content and doping cation content

[0281] Appropriate amounts of the polyvalent cation-doped ammonium manganese iron phosphate precursors and modified lithium manganese iron phosphate obtained in Examples 1 to 3 and Comparative Examples 1 to 11 were respectively taken into a centrifuge tube, added with appropriate amount of hydrochloric acid, heated for digestion for 2 hours, and then cooled and diluted, and measured using an inductively coupled plasma emission spectrometer.

[0282] (4) Determination of main element content in precursor (ICP)

[0283] The main element contents of the polyvalent cation-doped ammonium manganese iron phosphate precursors obtained in Examples 1 to 3 and Comparative Examples 1 to 11 were determined using an inductively coupled plasma emission spectrometer.

[0284] The test data are shown in Table 1.

[0285] Table 1 Sulfur content in precursor / lithium manganese iron phosphate and main / doping element content in precursor

[0286]

[0287] Note: ND means the corresponding element content was not detected.

[0288] As can be seen from the data in Table 1, the sulfur content of the multi-cation-doped ammonium manganese iron phosphate precursors synthesized by the step-by-step method in Examples 1 to 3 is much lower than that of Comparative Examples 1 to 4 in which the step-by-step method is not used, thereby significantly reducing the sulfur content in the prepared carbon-coated modified lithium manganese iron phosphate to a battery-grade standard of sulfur content ≤100ppm, eliminating the tedious secondary desulfurization step. Further comparison shows that whether it is the step-by-step synthesis of excessive ternary cation doping (Comparative Example 5), binary cation doping (Comparative Examples 6 to 8) or monovalent cation doping (Comparative Examples 9 to 11), the sulfur impurity content of the precursor and the modified lithium manganese iron phosphate does not increase significantly, indicating that the step-by-step method is the fundamental reason for reducing the sulfur impurity content, rather than the type and dosage of the doping element.

[0289] ICP test results show that the main element contents of the precursors prepared in Examples 1-3 and Comparative Examples 1-11 are essentially consistent with the target doping molar ratio, with deviations remaining within an acceptable range. This result verifies the accuracy of the corresponding element ratios in the precursor formula and further confirms the successful preparation of the multi-cation-doped ammonium manganese ferrous phosphate precursor according to the preset doping metal element molar ratio, confirming the uniform distribution of the doping elements therein.

[0290] In the preparation of carbon-coated modified lithium manganese iron phosphate, this invention cleverly uses atomically uniformly distributed iron and manganese sources, as well as multiple metal cation doping sources, as precursors for direct, fine mixing. This strategy fundamentally blocks the entry pathway for sulfur impurities, effectively preventing direct contact with the electrolyte, thereby reducing the formation of harmful sulfur-containing byproducts and demonstrating excellent capacity retention during cycling.

[0291] Experimental Example 2 Physical and Chemical Performance Test of Lithium-ion Battery Based on Modified Lithium Manganese Iron Phosphate Cathode Material

[0292] (1) Electrochemical testing

[0293] The carbon-coated modified lithium manganese iron phosphate materials obtained in Examples 1-3 and Comparative Examples 1-11 were used as the positive electrode active material. The materials were weighed and dispersed in N-methylpyrrolidone dispersant at a mass ratio of 94:4:2 (active material: conductive agent carbon black: binder polyvinylidene fluoride) to form a uniformly dispersed positive electrode slurry. The slurry was then coated on aluminum foil and vacuum-dried at 120°C for 12 hours. The resulting circular electrode pieces were then punched and weighed to produce circular electrode pieces. 2032-inch lithium-ion button cells were assembled in a Mikrona glove box (O₂ ≤ 0.01 ppm, H₂O ≤ 0.01 ppm) using a lithium metal sheet as the negative electrode, the prepared electrode piece as the positive electrode, a polypropylene film as the separator, and an electrolyte solution containing a solvent ratio of 1:1:1 (dimethyl carbonate (DMC) / diethyl carbonate (DEC) / ethylene carbonate (EC) by volume and 1 mol / L LiPF₆ as the electrolyte). After standing for 12 hours, the electrochemical performance of the cells was tested at room temperature. The test voltage range is set to 2~4.5V, and the charge and discharge test is performed at a current rate of 0.1C / 1C. The calculation formula involved is as follows:

[0294] First cycle discharge efficiency = first cycle discharge specific capacity / first cycle charge specific capacity × 100%;

[0295] Capacity retention rate = 100th cycle discharge capacity / first cycle discharge capacity × 100%.

[0296] (2) Negative electrode manganese content test

[0297] The batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 11 were fully charged for the 100th cycle at a current rate of 1 C and stored in a 60° C. oven for 30 days. After naturally cooling to room temperature, the metallic lithium negative electrode was treated with a 0.1 mol / L HCl solution for 2 h. The manganese content was measured using an inductively coupled plasma optical emission spectrometer to obtain manganese dissolution data.

[0298] (3) Powder compaction and powder resistance test methods

[0299] The carbon-coated modified lithium manganese iron phosphate prepared in Examples 1 to 3 and Comparative Examples 1 to 11 were used as test samples, and the powder compaction was tested using a Yuanneng powder compaction equipment, and the powder conductivity was tested using a powder resistivity tester.

[0300] The above measurement results are shown in Table 2.

[0301] Table 2 Physical and chemical performance test of lithium-ion battery based on modified lithium manganese iron sulfate cathode material

[0302]

[0303] From the data in Table 2, it can be seen that, in terms of the compaction density, the comparative examples 1-4 failed to present a better particle size distribution of the primary nanoparticles in the solid phase sintering process due to the absence of a step-by-step synthesis of the ammonium manganese iron phosphate precursor, i.e., the uneven distribution or multiphase composition of the one-step synthesis resulted in impure phase composition, which was not conducive to the regular growth of the primary particles. The comparative examples 5-11 failed to exhibit the synergistic optimization effect between the doped elements due to the difference in the type and dosage of the doped metal cations from the lithium manganese iron phosphate, which failed to present a better particle size distribution of the primary nanoparticles in the solid phase sintering process, thereby resulting in a smaller compaction density of the positive electrode material after crushing.

[0304] In terms of the powder resistance, the comparative examples 1-4 resulted in a significant increase in the sulfide layer generated at the contact surface due to the high sulfur content, which increased the contact resistance and in turn increased the powder resistance. The comparative example 5 weakened the capacity of the active components in the material due to the excessive doping of cations, and at the same time, generated excessive non-active material phases, which seriously hindered the conductivity of the material. Similarly, the comparative examples 6-11 failed to exhibit the component synergistic effect of the carbon-coated modified lithium manganese iron phosphate due to the change in the content and type of the doped cations, which hindered the conductivity of the material and in turn increased the powder resistance. In comparison, the comparative examples 1-3 successfully controlled the range of the sulfur impurity content (≤100 ppm) in the carbon-coated modified lithium manganese iron phosphate material, and ensured the uniform distribution of the ternary cations in the precursor lattice, which significantly promoted the charge transfer and in turn effectively reduced the powder resistance (<100 Ω·cm).

[0305] The electrochemical performance test further showed that the comparative examples 1-4 had a lower 0.1C capacity than the corresponding examples 1-3 due to the formation of multiphase components in the comparative examples, and the capacity decay was intensified and the manganese dissolution was intensified during the 1C and 100-cycle processes. On the one hand, this was due to the introduction of the sulfur impurity content in the precursor as a sulfur source into the carbon-coated lithium manganese iron phosphate material, which formed sulfur-containing byproducts that were not conducive to maintaining the cycle stability during the charging and discharging process, thereby affecting the rate capability and cycle performance. On the other hand, due to the uneven distribution of the doped elements, even the formation of a local coating layer on the surface, the crystal structure stability of the lithium manganese iron phosphate could not be regulated from the microscopic level, which was not conducive to the suppression of Mn 3+dissolution and Jahn-Teller effect, thereby exacerbating capacity decay. The excessive ternary cation doping in Comparative Example 5, the binary cation doping and the monocation doping in Comparative Examples 6 to 11 cannot efficiently adjust the electronic structure inside the lithium manganese iron phosphate lattice, resulting in poor intrinsic conductivity, and then showing poor capacity, cycle stability, rate performance and increased manganese dissolution. In contrast, the appropriate amount of three different metal cation doping in Examples 1 to 3 can effectively exert the synergistic effect between the elements to improve the compaction density, cycle performance and rate performance of the material and reduce the amount of manganese dissolution.

[0306] In summary, Examples 1 to 3 have excellent overall performance. These materials not only have a high compaction density, but also have excellent electronic conductivity, which ensures the efficient flow of current inside the battery, thereby reducing energy loss; in terms of charge and discharge specific capacity, they far exceed similar materials, can store and release more electrical energy under the same conditions, and significantly extend the battery life; in terms of cycle stability, even after multiple charge and discharge cycles, their performance degradation is extremely small; in addition, these materials have the characteristic of low manganese dissolution. The stable presence of manganese in the crystal structure of lithium manganese iron phosphate not only maintains the structural stability of the material, but also effectively reduces the side reactions that may be caused by manganese dissolution, further improving the safety and reliability of the battery.

[0307] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A polyvalent cation-doped ammonium manganese ferric phosphate precursor, characterized in that: Preparation method thereof The following steps are included: S1. Mix the phosphorus source solution and the iron salt solution at pH 7-11 and allow to react at 50-60°C to obtain reaction system I. S2. The manganese salt solution, the doped metal salt solution M is prepared into a mixed solution under a protective gas atmosphere; S3. The mixed solution obtained in step S2 was added to the reaction system I obtained in step S1 at least twice, maintaining the pH of the system at 7~11, mixing, and reacting at 50~60 ℃ for a long time, and post-processing to obtain a polyvalent cation-doped ammonium manganese iron phosphate precursor; Each time the mixed solution is added, it needs to be mixed with the reaction system I and fully reacted at 50-60 °C before the next addition. The amount of the mixed solution added for the first time is 10%-45% of the total volume of the mixed solution. The doped metal salt M solution contains three metal ions A, B, and C, wherein A, B, and C are selected from Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr, Nb, and Al, which are not repeated. The expression of the polyvalent cation-doped ammonium manganese iron phosphate precursor is NH4Mn x Fe 1-x-a-b-c A a B b C c PO4, and 0<x<0.9, 0<a≤0.05, 0<b≤0.05, 0<c≤0.05, a+b+c=0.1; The phosphorus source includes one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

2. The polyvalent cation-doped ammonium manganese ferric phosphate precursor according to claim 1, characterized in that: In step S1, the iron salt includes ferrous sulfate, ferric chloride, ferrous chloride, ferric nitrate, ferric acetate, ammonium ferric sulfate, ferric citrate, ferrous oxalate or a hydrate of any of the above iron salts.

3. A method for preparing carbon-coated modified lithium manganese iron phosphate, characterized in that: The following steps are involved: Si. The lithium salt, the phosphorus source, the polyvalent cation-doped ammonium manganese ferric phosphate precursor according to any one of claims 1 or 2, the carbon source and water are thoroughly mixed to obtain a slurry, and the obtained slurry is ground and dried to obtain a powder; Sii. Under a protective gas atmosphere, the powder obtained in step Si is calcined at 600-900° C. and post-treated to obtain carbon-coated modified lithium manganese iron phosphate.

4. The preparation method according to claim 3, characterized in that In step S1, the lithium salt includes one or more of lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium nitrate, lithium acetate, and lithium phosphate.

5. The preparation method according to claim 3, characterized in that: In step Si, the molar ratio of the lithium salt, the phosphorus source, and the polyvalent cation-doped ammonium manganese iron phosphate precursor according to any one of claims 1 or 2 is 1: (0.9-1.1): (0.8-1.2).

6. The preparation method according to claim 3, characterized in that: In step Si, the mass ratio of the polynary cation-doped ammonium manganese iron phosphate precursor to the carbon source is 1: (0.05-0.11).

7. Carbon-coated modified lithium manganese iron phosphate prepared by the preparation method according to any one of claims 3 to 6.

8. A lithium ion battery positive electrode, characterized in that The invention comprises a current collector and a positive electrode active material loaded on the current collector, wherein the positive electrode active material comprises the carbon-coated modified lithium manganese iron phosphate according to claim 7.

9. A lithium-ion battery, characterized in that: The invention comprises an electrolyte, a separator, a positive electrode and a negative electrode, wherein the positive electrode comprises the positive electrode of the lithium-ion battery according to claim 8.

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