Multi-cation doped ammonium manganese iron phosphate precursor, carbon-coated modified lithium manganese iron phosphate, and preparation method and application of multi-cation doped ammonium manganese iron phosphate precursor and carbon-coated modified lithium manganese iron phosphate

Abstract

The invention belongs to the technical field of new energy materials, and particularly relates to a multi-cation doped ammonium manganese ferric phosphate precursor, carbon-coated modified lithium manganese ferric phosphate and a preparation method and application thereof. According to the multi-cation doped ammonium manganese ferric phosphate precursor prepared by the invention, a step-by-step synthesis method is adopted, an iron phosphate compound is firstly prepared, then manganese ion exchange and metal cation doping are synchronously performed to obtain a precursor, and the obtained precursor, a lithium source and a carbon source are subjected to solid-phase sintering to obtain the multi-cation doped ammonium manganese ferric phosphate precursor. And the prepared carbon-coated modified lithium manganese iron phosphate is used for constructing a lithium ion battery. An electrochemical performance test result shows that the obtained lithium ion battery has excellent capacity retention ratio, cycling stability, rate capability and charge-discharge capacity; moreover, the preparation method of the carbon-coated modified lithium manganese iron phosphate is simple and convenient to operate, few in by-products, nanocrystallized in particle size and high in raw material conversion rate, ensures that all doping elements are uniformly distributed in the material and the sulfur content reaches 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, and more specifically, relates to a polynary cation-doped ammonium manganese iron phosphate precursor, a carbon-coated modified lithium manganese iron phosphate, and a preparation method and application thereof. 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. To this end, 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, hoping 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] In order to overcome these challenges, researchers have proposed a variety of modification strategies, including cation doping, carbon coating, and particle size nano-sizing. However, in the industrialization process of lithium manganese iron phosphate, the existing technical routes still face many challenges. Although the preparation method based on the solid phase method is mature, the lithium manganese iron phosphate prepared by physical mixing is prone to uneven element distribution, incomplete conversion of doped elements, more by-products, excessive particle size, and excessive sulfur content. For example, Chinese patent application CN117440928A discloses the preparation of lithium manganese iron phosphate by the technical route of carbon coating and co-precipitation, which effectively solves the problem of uneven manganese iron phosphate precursor. The specific capacity and cycle performance of lithium manganese iron phosphate further prepared by the precursor are effectively improved. However, on the one hand, the above method requires the addition of a co-solvent containing potassium chloride and sodium chloride, which will additionally introduce alkali metal impurities such as K and Na; on the other hand, the one-step co-precipitation method of the patent is easy to form an irregular block precursor with a large particle size, which is not conducive to the subsequent grinding process to prepare nano-scale lithium manganese iron phosphate, thereby affecting the electrochemical properties of the further prepared positive electrode material. Chinese patent application CN116675204A discloses a method for preparing a compact ammonium manganese iron phosphate precursor, a positive electrode material and a battery, by mixing a phosphorus source with ammonia water and adding alkali solution to adjust the pH to alkalinity, and then reacting with ferromanganese metal to obtain a spherical or urchin-like ammonium manganese iron phosphate monohydrate, the ammonium manganese iron phosphate precursor obtained by the above method has low Na / S impurity content, high tapping, and good product crystallinity. However, it is necessary to modulate the reaction bottom liquid first, and simultaneously pump the mixed solution of the phosphorus source and ammonia water and the configured ferromanganese metal aqueous solution into the reaction bottom liquid for reaction, that is, the precursor is still prepared by one-step coprecipitation, and there is no relevant data to show the effect of the precursor after metal doping, and the sulfur impurity content of the precursor obtained is still high (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, so as to improve the comprehensive performance of lithium manganese iron phosphate and promote 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 obtained by the preparation method.

[0008] Another object of the present invention is to provide a lithium-ion battery positive electrode, the positive electrode comprising 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 positive electrode of the lithium ion battery described above.

[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 iron phosphate precursor, and the preparation method 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 and the doped metal salt solution M are prepared into a mixed solution under a protective gas atmosphere;

[0014] S3. The mixed solution obtained in step S2 is added to the reaction system I obtained in step S1 at least twice, the pH of the system is maintained at 7 to 11, mixed, fully reacted at 50 to 60 ° C, and post-treated 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 the next addition is performed after sufficient reaction at 50-60°C; the amount of the mixed solution added for the first time is 10%-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, respectively, and are not repeated with each other;

[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 source, carbon source, manganese salt and iron salt, significantly improving the structural stability of the material; the sulfur impurity content is strictly controlled at ≤100ppm, reaching the battery-grade standard, thus eliminating the cumbersome secondary desulfurization process; in the preparation process, the present application innovatively realizes the uniform substitution and coordination of a variety of different metal cations in ammonium manganese iron phosphate, and there is 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 the 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 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 0.1<x<0.9, 0<a≤0.05, 0<b≤0.05, 0<c≤0.05, a+b+c=0.1.

[0020] Preferably, the polyvalent cation-doped ammonium manganese iron phosphate precursor is expressed as 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 polyvalent cation-doped ammonium manganese iron phosphate precursor is expressed as 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] Further, 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] Further, 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 7-11 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] Further, 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] Further, 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 vanadium oxalate, vanadium sulfate, vanadium 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] Further, 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] Further, 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] Further, in step S2, 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 doped 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 doped 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 doped 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 doped 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 to separate the solid and liquid, 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 above-obtained polyvalent cation-doped ammonium manganese iron phosphate precursor, the carbon source and water are thoroughly mixed to obtain a slurry, and the obtained slurry is 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 modified lithium iron manganese phosphate positive electrode materials, this application cleverly uses precursors of atomic-level distribution of various elements as iron salts, manganese salts and multi-metal cation doping sources for direct fine mixing. This strategy fundamentally blocks the intervention path of sulfur impurities, effectively avoiding their direct contact with the electrolyte, thereby reducing the generation of sulfur-containing byproducts and showing good capacity retention during the cycle; in addition, this application combines a highly conductive carbon layer with lithium iron manganese phosphate, which not only significantly improves the electronic conduction efficiency of the material, but also further optimizes its rate performance and cycle 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 Si, 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 outlet 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-type 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 screen 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 above-mentioned 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 positive electrode of the lithium-ion battery 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, then coating it on aluminum foil, and vacuum drying it at 80 to 140° C. for 12 to 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 lithium-ion battery mentioned above, 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 rate, 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 multi-cation doped ammonium manganese iron phosphate precursor prepared in this application uses a step-by-step synthesis method to first prepare an iron phosphate complex, then simultaneously perform manganese ion exchange and metal cation doping to obtain a precursor, and solid-phase sinter the obtained precursor with a lithium source and a carbon source to prepare a 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, cycle stability, rate performance, and charge and discharge capacity; and the preparation method of the carbon-coated modified lithium manganese iron phosphate is not only simple to operate, has few by-products, nano-sized particles, and a high raw material conversion rate, but also ensures that each doping element is evenly distributed in the material and the sulfur content reaches the battery level, which is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0085] Figure 3 This is the 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 is further described below in conjunction with the accompanying drawings and specific examples, but the examples do not limit the present invention in any form. 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 are 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0094] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0101] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 9.5, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 60°C for 2 hours to form a reaction system I.

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

[0107] Use a peristaltic pump to pump 11.7 L of diammonium phosphate solution A into the 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 hours, slowly pump in 23.4 L of mixed salt solution C, and finally react at 60°C for 2 hours to obtain a 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) with a mass of PO4 precursor was used as a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 38%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 5°C / min and kept at this temperature for 9h.

[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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0114] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 titanium 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 9, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 55°C for 2 hours to form a 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 diammonium phosphate solution A into the 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 hours, slowly pump in 40 L of mixed salt solution C, and finally react at 55°C for 2 hours to obtain a 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%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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°C at a heating rate of 3°C / min and kept 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0127] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 polyvalent cation-doped ammonium manganese iron phosphate precursor, modified lithium manganese iron phosphate positive electrode 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 30 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to obtain a mixed slurry I.

[0133] After the reactor was cooled to room temperature, the mixed slurry I 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.

[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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0139] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 positive electrode 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] 15L of diammonium phosphate solution A was pumped into a high-efficiency sealed synthesis reactor using a peristaltic pump, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Subsequently, the pH of the solution system in the reactor was adjusted to 9.5 with ammonia water, and 30L of iron salt solution B was slowly pumped into the reactor using a peristaltic pump. After sufficient stirring, the system was reacted at 60°C for 2h to obtain a mixed slurry I.

[0145] After the reactor was cooled to room temperature, the mixed slurry I was filtered to separate the solid and liquid, and washed with ultrapure water for 3 times 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 PO4 precursor mass of PEG was used as a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 38%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 5°C / min and kept at this temperature for 9h.

[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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0151] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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] Comparative Example 3 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 polyvalent cation-doped ammonium manganese iron phosphate precursor is synthesized using a one-step method.

[0154] 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

[0155] 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, ammonium oxytitanium oxalate, magnesium sulfate, and cobalt sulfate heptahydrate were dissolved in ultrapure water at a molar ratio of 20:70:3:3:4 to prepare a 1 mol / L mixed salt solution B.

[0156] A peristaltic pump was used to pump 15 L of ammonium dihydrogen phosphate solution A into a high-efficiency sealed synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 9, and a peristaltic pump was used to slowly pump 30 L of iron salt solution B into the reactor, and finally the reaction was carried out at 55°C for 2 hours to obtain a mixed slurry I.

[0157] After the reactor was cooled to room temperature, the mixed slurry I 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.

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

[0159] 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 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 mass of PO4 precursor was used as a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 44%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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°C at a heating rate of 3°C / min and kept 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0163] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

[0169] Use a peristaltic pump to pump 11.7 L of diammonium phosphate solution A into the reactor system I, adjust the pH of the solution system in the reactor to 10 with ammonia water, and after sufficient stirring for 2 hours, slowly pump 33.4 L of mixed salt solution C. After sufficient stirring, the system reacts at 50°C for 2 hours to form a 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 less than or equal to 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. The feed amount is adjusted according to the outlet temperature and the particle size of the discharge (22 μm ≤ D50 ≤ 25 μm) to obtain a spray powder. The above-mentioned spray powder is loaded into a graphite sagger and placed in a nitrogen atmosphere box furnace (oxygen content < 10 ppm), and the temperature is raised to 700°C at a heating rate of 2°C / min and kept warm 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0176] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0183] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 40 L of mixed salt solution C. 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ D 50The spray powder was placed in a graphite sagger and placed in a nitrogen atmosphere box furnace (oxygen content <10ppm), and heated to 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0190] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 iron 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0197] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0204] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 iron 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0211] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0218] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 iron 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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0225] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0232] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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. Monocation-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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0239] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0246] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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. Monocation-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 synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

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

[0253] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / min and kept at this temperature for 10 hours.

[0257] 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.

[0258] 3. Preparation of Lithium-ion Batteries

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

[0260] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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.

[0261] Comparative Example 11 Preparation of monovalent cation-doped ammonium manganese iron phosphate precursor, carbon-coated modified lithium manganese iron phosphate and lithium ion battery

[0262] 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.

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

[0264] 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 magnesium sulfate were dissolved in ultrapure water at a molar ratio of 6:1 to prepare a 0.7 mol / L mixed salt solution C.

[0265] A peristaltic pump was used to pump 5 L of ammonium dihydrogen phosphate solution A into a high-efficiency sealed synthesis reactor, and nitrogen was passed into the reactor to exhaust the air in the sealed reactor. Ammonia water was then used to adjust the pH of the solution system in the reactor to 10, and a peristaltic pump was used to slowly pump 10 L of iron salt solution B into the reactor. After sufficient stirring, the system was reacted at 50°C for 2 hours to form a reaction system I.

[0266] A peristaltic pump was used to slowly pump 10 L of mixed salt solution C into the reaction system I in the reactor. After sufficient stirring, the system was reacted at 50° C. for 2 h to form a reaction system II.

[0267] Use a peristaltic pump to pump 11.7 L of diammonium 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 in 23.4 L of mixed salt solution C, and 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 a carbon source and dispersed in ultrapure water to prepare a slurry with a solid content of 40%, and 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 0.3 μm, the slurry is transferred to the spray drying equipment. The spray drying system is set with an inlet temperature of 220°C and an outlet temperature of 110°C. According to the outlet temperature and the particle size of the discharge (22 μm ≤ 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 700°C at a heating rate of 2°C / 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, the conductive agent carbon black, and the binder polyvinylidene fluoride are dissolved in N-methylpyrrolidone dispersant in a mass ratio of 94:4:2 and mixed evenly to obtain a positive electrode slurry, which is then coated on aluminum foil and dried in vacuum at 100°C for 24 hours to obtain a lithium-ion battery positive electrode.

[0274] The button lithium-ion battery was assembled in a glove box filled with argon gas. 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 scanning electron microscopy. 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 PO4 precursor and carbon-coated modified lithium iron manganese 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, it can be seen that the NH4Mn 0.6 Fe 0.3 Co 0.05 V 0.03 Mg 0.02 After solid-liquid separation by filter pressing, 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 measurement

[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 corresponds to the diffraction peaks of LiMnPO4 and LiFePO4, and no other impurity peaks exist. This indicates that Mn, Fe and multi-doped cations are uniformly distributed in the lattice.

[0280] (3) Sulfur content and doping cation content detection

[0281] Take 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, respectively, add appropriate amounts of hydrochloric acid to heat and digest for 2 hours, then cool and dilute, and measure 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] From the data in Table 1, it can be seen that the sulfur content of the multi-cation-doped ammonium manganese iron phosphate precursor synthesized by the step-by-step method in Examples 1 to 3 is much lower than that of Comparative Examples 1 to 4 which do not adopt the step-by-step method, thereby significantly reducing the sulfur content in the prepared carbon-coated modified lithium manganese iron phosphate to the battery-grade standard of sulfur content ≤100ppm, eliminating the cumbersome secondary desulfurization step. Further comparison shows that whether it is the step-by-step method to synthesize excessive ternary cation doping (Comparative Example 5), binary cation doping (Comparative Examples 6 to 8) or monocyclic cation doping (Comparative Examples 9 to 11), the sulfur impurity content of the precursor and the modified lithium manganese iron phosphate has no obvious increase, 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 elements.

[0289] The ICP test results show that the main element content of the precursors prepared in Examples 1 to 3 and Comparative Examples 1 to 11 is basically consistent with the target doping molar ratio, and the deviation remains within an acceptable range. This result verifies the accuracy of the corresponding element ratio in the precursor expression, further confirms that the multi-cation doped ammonium manganese iron phosphate precursor is successfully prepared according to the preset doping metal element molar ratio, and confirms the uniform distribution of the doping elements therein.

[0290] In the process of preparing carbon-coated modified lithium manganese iron phosphate, the present invention cleverly uses atomically uniformly distributed iron source, manganese source and multi-metal cation doping source as precursors to directly perform fine mixing. This strategy fundamentally blocks the intervention path of sulfur impurities, effectively avoiding their direct contact with the electrolyte, thereby reducing the generation of harmful sulfur-containing byproducts and showing good capacity retention during the cycle.

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

[0292] (1) Electrochemical test

[0293] The carbon-coated modified lithium manganese iron phosphate material obtained in Examples 1 to 3 and Comparative Examples 1 to 11 was used as the positive electrode active material, and was weighed and dispersed in N-methylpyrrolidone dispersant according to the mass ratio of active material: conductive agent carbon black: binder polyvinylidene fluoride = 94:4:2 to form a uniformly dispersed positive electrode slurry, which was then coated on aluminum foil, vacuum dried at 120°C for 12 hours, and then punched and weighed to obtain a circular pole piece. A metal lithium sheet was used as the negative electrode, the obtained pole piece was used as the positive electrode, a polypropylene film was used as the diaphragm, and a solvent volume ratio of 1:1:1 dimethyl carbonate (DMC) / diethyl carbonate (DEC) / ethylene carbonate (EC) and an electrolyte of 1 mol / LLiPF6 were used to assemble a 2032-type lithium-ion button cell in a mikaelona glove box (O2≤0.01ppm, H2O≤0.01ppm), and the electrochemical performance test was performed at room temperature after standing for 12 hours. 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 an oven at 60° C. 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, and the manganese content was tested using an inductively coupled plasma 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 properties test of lithium-ion batteries based on modified lithium iron manganese sulfate cathode materials

[0302]

[0303] From the data in Table 2, it can be seen that in terms of compaction density, since the ammonium manganese iron phosphate precursor of Comparative Examples 1 to 4 was not synthesized by a step-by-step method, the primary nanoparticles failed to present a better particle size ratio state during the solid phase sintering process, that is, the physical phase composition was impure due to uneven distribution or multi-phase composition during the one-step synthesis, which was not conducive to the regular growth and molding of the primary particles; since the types and dosages of metal cations doped in the lithium manganese iron phosphate of Comparative Examples 5 to 11 were different from those in Example 1, the synergistic optimization effect between the doping elements could not be exerted, and the primary nanoparticles failed to present a better particle size ratio state during the solid phase sintering process, resulting in a lower compaction density of the positive electrode material after crushing.

[0304] In terms of powder resistance, in Comparative Examples 1 to 4, due to the excessively high sulfur content, sulfur impurities react with the material to form a sulfurization reaction, and the sulfide layer generated on the contact surface significantly increases the contact resistance, thereby increasing the powder resistance; in Comparative Example 5, due to the excessive doping of cations, the capacity of the active components in the material is weakened, and an excessive amount of inactive material phase is generated, which seriously hinders the conductivity of the material; similarly, in Comparative Examples 6 to 11, due to the changes in the content and type of doped cations, the carbon-coated modified lithium manganese iron phosphate cannot play the synergistic effect of the components, and this change also hinders the conductivity of the material, thereby enhancing the powder resistance; in contrast, in Examples 1 to 3, the range of sulfur impurity content in the carbon-coated modified lithium manganese iron phosphate material is successfully controlled (≤100ppm), and the ternary cations are ensured to be uniformly distributed in the precursor lattice, which significantly promotes the transfer of charges, thereby effectively reducing the powder resistance (<100Ω·cm).

[0305] The electrochemical performance test further shows that in Comparative Examples 1 to 4, due to the sulfur impurity content in the one-step synthesis precursor exceeding the battery-grade standard and the failure of manganese, iron and various doped metal elements to be distributed at the atomic level, the formation of a multiphase composition in the comparative examples leads to a 0.1C capacity lower than that of the corresponding Examples 1 to 3, and the capacity decay and manganese dissolution are aggravated during the large current 1C and 100 cycles. On the one hand, this is attributed to the ultra-high sulfur impurity content in the precursor being introduced as a sulfur source into the carbon-coated lithium manganese iron phosphate material, which will form sulfur-containing byproducts that are not conducive to maintaining cycle stability during the charge and discharge process, thereby affecting the rate performance and cycle performance. On the other hand, due to the uneven distribution of the doping elements, even a local coating layer is formed on the surface, which makes it impossible to regulate the crystal structure stability of lithium manganese iron phosphate from a microscopic level, which is not conducive to suppressing Mn 3+dissolution and Jahn-Teller effect, and thus the capacity decay is aggravated. 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 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, and the stable presence of manganese in the crystal structure of lithium iron manganese 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 equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A polyvalent cation-doped ammonium manganese iron phosphate precursor, characterized in that: The preparation method comprises the following steps: 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; S2. The manganese salt solution and the doped metal salt M solution are prepared into a mixed solution under a protective gas atmosphere; S3. The mixed solution obtained in step S2 is added to the reaction system I obtained in step S1 at least twice, the pH of the system is maintained at 7 to 11, mixed, fully reacted at 50 to 60 ° C, and post-treated 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 the next addition is performed after sufficient reaction at 50-60°C; 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, and A, B, and C are respectively selected from non-repeating ones of Mg, Ti, V, Co, Ni, Cr, Zn, Cu, Zr, Nb, and Al. 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.

2. The polyvalent cation-doped ammonium manganese iron phosphate precursor according to claim 1, characterized in that: 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, and lithium phosphate.

3. The polyvalent cation-doped ammonium manganese iron 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.

4. 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 iron phosphate precursor according to any one of claims 1 to 3, the carbon source and water are fully 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.

5. The preparation method according to claim 4, characterized in that: 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.

6. The preparation method according to claim 4, 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 to 3 is 1: (0.9 to 1.1): (0.8 to 1.2).

7. The preparation method according to claim 4, 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).

8. Carbon-coated modified lithium manganese iron phosphate prepared by the preparation method according to any one of claims 4 to 7.

9. A positive electrode for a lithium ion battery, characterized in that: It 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 as claimed in claim 8.

10. 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 9.

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