A low-sulfur type metal cation-doped ammonium manganese iron phosphate precursor, a carbon-coated modified ammonium manganese iron sodium pyrophosphate, and a preparation method and application thereof

Abstract

The application belongs to the technical field of new energy materials, and particularly relates to a low-sulfur type metal cation doped ammonium manganese iron phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate phosphate and a preparation method and application thereof. A low-sulfur type metal cation doped ammonium manganese iron phosphate precursor is successfully prepared. By adding an appropriate amount of a phosphoric acid source to the prepared metal cation doped ammonium manganese iron phosphate precursor, not only the sulfur content of the precursor is significantly reduced, but also the molar ratio of manganese, iron, a doped element and phosphorus is controlled to be slightly less than 1. This not only realizes the atomic level distribution of the elements in the precursor, but also improves the tap density, which is beneficial to the subsequent preparation of a carbon-coated modified sodium manganese iron pyrophosphate phosphate material with high performance. The sodium ion battery constructed based on the material has excellent charge-discharge capacity, capacity retention rate and cycle stability. In addition, the preparation method of the precursor is simple and easy to control, and has wide application advantages.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of new energy materials. More specifically, it relates to a low-sulfur type metal cation doped ammonium manganese iron phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate phosphate and a preparation method and application thereof. BACKGROUND

[0002] Lithium-ion batteries, as the current mainstream energy storage technology, realize energy storage through the reversible intercalation and deintercalation process of lithium ions between the positive and negative electrodes. However, the energy storage capacity of lithium-ion batteries in the earth's crust is limited, coupled with the growing global demand for clean energy and portable electronic devices, leading to a shortage of lithium resources and rising prices. Although lithium ions have excellent performance in energy density, cycle life and comprehensive performance, their high cost and strong resource dependence have prompted researchers to explore alternative solutions. In this context, sodium-ion batteries have become a strong competitor for lithium-ion batteries due to their similar energy storage principles, abundant crustal reserves and lower cost potential. In particular, polyanion-type sodium-ion battery cathode materials are considered an ideal choice for high-energy-density sodium-ion batteries due to their abundant raw materials, low cost, stable structure, simple synthesis, high theoretical specific capacity and environmental friendliness. Among them, sodium iron pyrophosphate phosphate (Na4Fe3(PO4)2P2O7) as one of the mixed iron-based phosphates has a higher theoretical specific capacity due to the higher alkali metal ion content in the material structure than sodium iron phosphate (NaFePO4). However, its poor intrinsic conductivity and low discharge platform limit its full energy density.

[0003] To improve the voltage platform of sodium iron pyrophosphate phosphate, researchers have attempted to introduce manganese elements into sodium iron pyrophosphate phosphate to replace part of the iron elements, thereby forming a solid solution of sodium manganese iron pyrophosphate phosphate. This strategy combines the high voltage platform of Mn 2+ with the high theoretical specific capacity of Fe 2+The advantage of similar ionic radii effectively improves the voltage plateau and energy density of the mixed iron-based phosphate. However, the conventional manganese doping process is often achieved by adding low-cost sulfate, which is economical but inevitably leads to an increase in sulfur content in the material, thereby adversely affecting its electrochemical performance. In addition, sodium manganese iron pyrophosphate phosphate also faces problems such as intensified manganese leaching, low ion and electron conductivity, and difficulty in sodium ion intercalation and deintercalation, which all adversely affect the electrochemical performance of the material. In view of the above problems, the prior art has attempted strategies such as ion doping and carbon coating. On the one hand, Chinese Patent Application CN118295543A discloses a high-compaction-density sodium manganese iron pyrophosphate phosphate / carbon composite positive electrode material, which is synthesized by a three-step sintering method to prepare nanospherical particles. This material improves the electrochemical performance such as electron and ion conductivity and charge and discharge capacity to some extent. However, the process is relatively complicated and has high energy consumption, and the carbon coating as an independent step may not be conducive to the in-situ formation of a dense carbon coating layer, thereby affecting the full play of the compaction density and ensuring the uniformity of the carbon coating. On the other hand, Chinese Patent Application CN116404154A discloses a high-entropy sodium pyrophosphate sodium ion battery positive electrode material, which is formed by introducing five elements of Mg, Ca, Al, Cr and Mn in equal molar ratio at the Fe site to form a combined crystal. Although the material has a single phase and good crystallinity, it still faces the problem of difficulty in sodium ion intercalation and deintercalation, which not only affects the ion conductivity and structural stability of the material, but also may exacerbate the Jahn-Teller distortion effect, thereby adversely affecting the electrochemical performance of the material. In addition, as can be observed from the SEM image, the positive electrode material has obvious stacking phenomenon, which may significantly reduce the specific surface area of the electrolyte contact and prolong the sodium ion migration path, which is not conducive to the full utilization of active substances and the acceleration of sodium ion migration kinetics, thereby affecting the full play of its electrochemical performance. SUMMARY

[0004] The technical problem to be solved by the present application is to overcome the defects and deficiencies of the prior art sodium manganese iron pyrophosphate phosphate material, such as intensified manganese leaching, high sulfur impurity content, small specific surface area, and Jahn-Teller structural distortion caused by difficulty in sodium ion intercalation and deintercalation, and to provide a preparation method of a low-sulfur type metal cation doped sodium manganese iron ammonium phosphate precursor.

[0005] Another object of the present application is to provide a low-sulfur type metal cation doped sodium manganese iron ammonium phosphate precursor prepared by the above preparation method.

[0006] Another object of the present application is to provide a preparation method of a carbon-coated modified sodium manganese iron pyrophosphate phosphate.

[0007] Another object of the present application is to provide the carbon-coated modified sodium manganese iron pyrophosphate phosphate prepared by the above preparation method.

[0008] Another object of the present application is to provide a sodium ion battery anode comprising the aforementioned carbon-coated modified sodium manganese iron pyrophosphate phosphate.

[0009] Another object of the present application is to provide a sodium ion battery, wherein the anode of the sodium ion battery comprises the aforementioned sodium ion battery anode.

[0010] The above objects of the present application are achieved by the following technical solutions.

[0011] The present application protects a preparation method of a low-sulfur type metal cation-doped manganese iron ammonium phosphate precursor, comprising the following steps:

[0012] S1. Using manganese sulfate to provide manganese elements, and preparing a metal cation-doped manganese iron ammonium phosphate precursor;

[0013] S2. Mixing the metal cation-doped manganese iron ammonium phosphate precursor obtained in step S1 with a phosphoric acid source, keeping the solution pH at 7-8, and fully reacting at 30-80℃, filtering, washing the precipitate, and drying to obtain a low-sulfur type metal cation-doped manganese iron ammonium phosphate precursor;

[0014] The metal cation is selected from at least one of Ca, Mg, Al, Ti, Ni, Cr, Zn, Mo, Cu, Zr, Nb, Y, Sn, Co, Ce, V, and W, without repeating each other;

[0015] The low-sulfur type metal cation-doped manganese iron ammonium phosphate precursor has the expression NH4Mn x Fe 1-x-y M y PO4, 0

[0016] The addition amount of the phosphoric acid source (the amount of substance is calculated based on phosphate) is at least 0.1 times the amount of substance of the metal cation-doped manganese iron ammonium phosphate precursor;

[0017] The steps S1-S2 are both carried out in a protective gas atmosphere.

[0018] In the process of manganese doping, sodium manganese pyrophosphate (NH4Mn) materials are often produced using low-cost manganese sulfate. While economical, this approach inevitably leads to excessively high sulfur content in the material. This is because sulfur impurities (sulfate ions) are either encapsulated within the prepared NH4Mn precursor particles or adsorbed on their surface, resulting in a high sulfur impurity content in the precursor. The inventors discovered that adding an appropriate amount of phosphoric acid source to the synthesized metal cation-doped NH4Mn precursor can induce recrystallization, exposing and removing sulfur impurities from the precursor particles. Subsequent filtration and washing processes can then transfer these sulfur impurities, thus achieving the goal of sulfur removal. Furthermore, the addition of the phosphoric acid source can regulate the temperature of the NH4Mn precursor. x Fe 1-x-y M y The molar ratio of manganese, iron, and doped metal elements in PO4 to phosphorus is kept slightly less than 1, achieving uniform substitution and coordination of different elements in the precursor. This regulation ensures that the doped metal, manganese, and iron elements are regularly distributed at the atomic level in the precursor lattice, forming a single solid solution phase, thereby further improving the performance of NH4Mn. x Fe 1-x-y M y The structure of PO4 is stable, and its sulfur content is greatly reduced.

[0019] Furthermore, as a common expression, the NH4Mn x Fe 1-x-y M y In PO4, M represents the doping of multiple (e.g., 4) metal cations, and A, B, C, and D represent the 4 different metal cations respectively.

[0020] Furthermore, as a common expression, when there are four types of doped metal cations, the formula for the low-sulfur metal cation-doped manganese iron ammonium phosphate precursor is NH4Mn. x Fe 1-x-a-b-c-d A a B b C c D d PO4, and 0<x<0.9, 0<a<0.1, 0<b<0.1, 0<c<0.1, 0<d<0.1, a+b+c+d<0.1.

[0021] Furthermore, as a common expression, the NH4Mn x Fe 1-x-y M y In PO4, M indicates that when multiple (e.g., 5) metal cations are doped, A, B, C, D, and E represent the 5 different metal cations respectively.

[0022] Further, as a common expression, when the doped metal cation is 5 kinds, the expression of the low-sulfur metal cation-doped manganese iron ammonium phosphate precursor is NH4Mn x Fe 1-x-a-b-c-d-e A a B b C c D d E e PO4, and 0

[0023] Further, in step S1, the preparation method of the metal cation-doped manganese iron ammonium phosphate precursor includes the following steps:

[0024] SI. Manganese sulfate and a doped metal cation salt M solution are mixed to form a mixed solution I under a protective gas atmosphere;

[0025] SII. Iron, a phosphoric acid source solution are fully reacted under a protective gas atmosphere to obtain a ferrous phosphate solution system II;

[0026] SIII. The mixed solution I obtained in step SI and an oxidizing agent are added to the ferrous phosphate solution system II obtained in step SII under a protective gas atmosphere, mixed, and fully reacted at 50-80°C, so that the divalent manganese ions and the divalent iron ions are oxidized to trivalent manganese ions and trivalent iron ions, respectively, to obtain an intermediate system III. After adjusting the pH of the intermediate system III to 7-8, the reaction is carried out at 50-80°C, and the post-treatment is carried out to obtain the metal cation-doped manganese iron ammonium phosphate precursor.

[0027] Further, in step SI, the metal cation salt M solution includes one or more of a sulfate, a citrate, a nitrate, a carbonate, a bicarbonate, an ammonium salt, an acetate, a phosphate, and an oxalate.

[0028] Further, in step SI, the Ca is selected from calcium chloride, calcium nitrate, calcium acetate, calcium bicarbonate, calcium dihydrogen phosphate, or a hydrate of any of the above calcium salts.

[0029] Further, in step SI, the Mg is selected from magnesium sulfate, magnesium chloride, magnesium nitrate, magnesium carbonate, magnesium oxalate, magnesium acetate, magnesium citrate, or a hydrate of any of the above magnesium salts.

[0030] Further, in step SI, the Al is selected from aluminum sulfate, aluminum chloride, aluminum nitrate, or a hydrate of any of the above aluminum salts.

[0031] Further, in step SI, the Ti is selected from the group consisting of ammonium oxalato titanium, titanium sulfate, titanium oxysulfate, titanium chloride, or a hydrate of any of the aforementioned titanium salts.

[0032] Further, in step SI, the Ni is selected from the group consisting of nickel sulfate, nickel nitrate, nickel acetate, nickel chloride, or a hydrate of any of the aforementioned nickel salts.

[0033] Further, in step SI, the Cr is selected from the group consisting of chromium sulfate, chromium chloride, chromium oxalate, chromium acetate, or a hydrate of any of the aforementioned chromium salts.

[0034] Further, in step SI, the Zn is selected from the group consisting of zinc sulfate, zinc acetate, zinc chloride, zinc nitrate, or a hydrate of any of the aforementioned zinc salts.

[0035] Further, in step SI, the Mo is selected from the group consisting of ammonium molybdate, molybdenum sulfate, molybdenum chloride, molybdenum nitrate, molybdenum acetate, or a hydrate of any of the aforementioned molybdenum salts.

[0036] Further, in step SI, the Cu is selected from the group consisting of copper sulfate, copper acetate, copper chloride, copper nitrate, or a hydrate of any of the aforementioned copper salts.

[0037] Further, in step SI, the Zr is selected from the group consisting of zirconium sulfate, zirconium tetrachloride, zirconium oxychloride, zirconium nitrate, zirconium acetate, or a hydrate of any of the aforementioned zirconium salts.

[0038] Further, in step SI, the Nb is selected from the group consisting of ammonium oxalato niobate, niobium oxalate, or a hydrate of any of the aforementioned niobium salts.

[0039] Further, in step SI, the Y is selected from the group consisting of yttrium sulfate, yttrium nitrate, yttrium chloride, yttrium acetate, or a hydrate of any of the aforementioned yttrium salts.

[0040] Further, in step SI, the Sn is selected from the group consisting of stannous sulfate, stannous chloride, stannous acetate, tin nitrate, or a hydrate of any of the aforementioned tin salts.

[0041] Further, in step SI, the Co is selected from the group consisting of cobalt sulfate, cobalt acetate, cobalt chloride, cobalt nitrate, or a hydrate of any of the aforementioned cobalt salts.

[0042] Further, in step SI, the Ce is selected from the group consisting of cerium sulfate, cerium nitrate, cerium acetate, cerium oxalate, cerium chloride, or a hydrate of any of the aforementioned cerium salts.

[0043] Further, in step SI, the V is selected from the group consisting of vanadyl oxalate, vanadyl sulfate, vanadyl dichloride, or a hydrate of any of the aforementioned vanadium salts.

[0044] Further, in step SI, the W is selected from the group consisting of tungsten nitrate, tungsten chloride, tungsten phosphate, or a hydrate of any of the aforementioned tungsten salts.

[0045] Further, in step SI, the molar ratio of manganese in the manganese sulfate and the doping cation in the doping metal cation salt M solution is f: g, wherein f is 0 < f < 0.9, and g is 0 < g < 0.1.

[0046] Further, in step SI, the molar concentration of the mixed solution I is the molar concentration of the total metal cations.

[0047] Further, in step SI, the concentration of the mixed solution I is 0.1-5 mol / L, preferably 3 mol / L.

[0048] Further, in step SI, the protective gas includes one or more of nitrogen, helium, and argon.

[0049] Further, in step SII, the iron source is one or more of iron blocks, iron powder, iron slag, iron foil, and iron foil.

[0050] Further, in step SII, the phosphoric acid source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, lithium phosphate, sodium phosphate, ammonium hypophosphite, ammonium polyphosphate, sodium hypophosphite, sodium metaphosphate, and sodium tripolyphosphate.

[0051] Further, in step SII, the molar ratio of the iron source to the phosphoric acid source is 1: (1-3).

[0052] Further, in step SII, the reaction time is 0.5-10 h.

[0053] Further, in step SII, due to the weak oxidizing property of the phosphoric acid source and the protective gas atmosphere, the iron source is only oxidized to the divalent state, and the resulting product is ferrous dihydrogen phosphate.

[0054] Further, in step SIII, the oxidizing agent includes one or more of hydrogen peroxide, oxygen, and ozone. One of the purposes of adding the oxidizing agent is to oxidize the Mn 2+ in the divalent manganese salt in the mixed solution I obtained in step SI to Mn 3+ ; the other purpose is to oxidize the Fe 2+ in the ferrous dihydrogen phosphate solution system II obtained in step SII to Fe 3+ .

[0055] Further, in step SIII, the molar ratio of the oxidizing agent is at least 0.5 times the molar amount of manganese sulfate in the mixed solution I obtained in step SI.

[0056] Further, in step SIII, the reaction time is 0.5-10 h.

[0057] Further, in step SIII, the pH 7-8 is adjusted by adding a basic reagent.

[0058] Still further, the basic reagent includes one or more of urea, ammonia, urea phosphate. By using these basic reagents to adjust the pH of the solution, Mn 3+ and Fe 3+ and the doping metal cation are co-precipitated in the phosphate system to form a uniform metal cation-doped manganese iron ammonium phosphate precursor, while also avoiding the use of sodium hydroxide, potassium hydroxide, etc. to introduce Na, K, and other alkali metal impurities.

[0059] Further, in step SIII, the reaction time is 0.5-10 h.

[0060] Further, in step SIII, the post-treatment includes filtration, washing, and drying.

[0061] Still further, the filtration is pressure filtration, i.e., the solution containing the metal cation-doped manganese iron ammonium phosphate precursor is subjected to pressure filtration for solid-liquid separation.

[0062] Still further, the washing is washing the filter cake after filtration with water 3-5 times.

[0063] Still further, the drying is flash evaporation, i.e., the filter cake after washing is transferred into a flash evaporator for heat exchange to rapidly evaporate the surface moisture.

[0064] Specifically, in step SIII, the post-treatment includes subjecting the solution containing the metal cation-doped manganese iron ammonium phosphate precursor to pressure filtration for solid-liquid separation, washing the filter cake after pressure filtration with water 3-5 times, and transferring the filter cake after washing into a flash evaporator for heat exchange to rapidly evaporate the surface moisture.

[0065] Further, in step S2, the phosphate source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, ammonium phosphate, lithium phosphate, sodium phosphate, ammonium hypophosphite, ammonium polyphosphate, sodium phosphite, sodium metaphosphate, and sodium tripolyphosphate.

[0066] Still further, in step S2, the molar ratio of the metal cation-doped manganese iron ammonium phosphate precursor to the phosphate source is 1:(0.1-20), which is intended to ensure that the phosphate source is excessive relative to the precursor. If the amount of the phosphate source added is too much, it will cause unnecessary waste of resources.

[0067] Further, in step S2, the mixing time is 10-180 min.

[0068] Further, in step S2, the pH is 7-8 is adjusted by adding a basic reagent.

[0069] Further, the basic reagent is one or more of urea, ammonia, urea phosphate.

[0070] Further, in step S2, the reaction time is 0.5-10h.

[0071] Further, in step S2, the filtration is pressure filtration, i.e. the solution containing the low-sulfur metal cation-doped ammonium manganese iron phosphate precursor is subjected to pressure filtration for solid-liquid separation.

[0072] Further, in step S2, the washing of the precipitate is washing the precipitate after filtration with water for 3-5 times.

[0073] Further, in step S2, the drying is flash evaporation, i.e. the precipitate after washing is transferred into a flash evaporator for heat exchange to rapidly evaporate the surface moisture.

[0074] The application protects the low-sulfur metal cation-doped ammonium manganese iron phosphate precursor prepared by the above preparation method.

[0075] The application synthesizes a low-sulfur metal cation-doped ammonium manganese iron phosphate precursor NH4Mn x Fe 1-x-y M y PO4. The precursor significantly reduces the sulfur impurity content by treating the prepared metal cation-doped ammonium manganese iron phosphate precursor with an appropriate amount of a phosphoric acid source, reaching the battery grade standard. At the same time, the addition of the phosphoric acid source makes the actual ratio of the molar ratio of manganese, iron and doped metal elements to phosphorus element in the precursor slightly less than 1, which effectively improves the stability of the precursor structure, makes each doped element and the main elements (manganese and iron) in the precursor in atomic level distribution, and greatly improves the tap density, which is beneficial to further prepare a positive electrode material with excellent battery capacity, long cycle life and high safety.

[0076] The application protects a preparation method of a carbon-coated modified sodium manganese iron pyrophosphate phosphate, comprising the following steps:

[0077] Si. The sodium salt, the phosphoric acid source, the aforementioned low-sulfur metal cation-doped ammonium manganese iron phosphate precursor, the carbon source and the alcohol solution are fully mixed to obtain a slurry, and the obtained slurry is ground and dried to obtain a powder;

[0078] Sii. The powder obtained in step Si is precalcined at 300-400℃ for 0.5-8h under a protective gas atmosphere, and then fully calcined by increasing the temperature to 450-750℃, and then treated to obtain the carbon-coated modified sodium manganese iron pyrophosphate phosphate.

[0079] The application can prepare carbon-coated modified sodium manganese iron phosphate positive electrode material by co-precipitation-one-step sintering method, which is innovative and efficient. In the preparation process, low-sulfur metal cation doped manganese iron ammonium phosphate precursor is used as metal source and phosphoric acid source, which can accurately control the periodic distribution between each doped element and main element in the precursor. After combining carbon layer, sodium salt and grinding and spraying treatment, calcination is carried out, so that manganese, iron and doped metal cations can be uniformly distributed on the crystal layer. This preparation method not only significantly enhances the stability of the crystal structure, but also improves the conductivity of the material, ensuring the single solid solution phase characteristics of the positive electrode material.

[0080] Further, in step Si, the sodium salt includes one or more of sodium carbonate, sodium dihydrogen phosphate, sodium bicarbonate, sodium citrate, sodium hydroxide, sodium citrate, sodium pyrophosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, sodium nitrate, sodium chloride, sodium oxalate, sodium ethoxide.

[0081] Further, in step Si, the phosphoric acid source includes one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, ammonium phosphate, lithium dihydrogen phosphate, lithium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, sodium hexafluorophosphate.

[0082] Further, in step Si, the carbon source includes one or more of polyethylene glycol, glucose, fructose, sucrose, starch, cyclodextrin, citric acid, tartaric acid, ascorbic acid, malic acid, lactic acid, polyvinylpyrrolidone, polyvinylidene fluoride, epoxy resin, phenolic resin.

[0083] Further, in step Si, the molar ratio of the phosphoric acid source, the sodium salt and the obtained low-sulfur metal cation doped manganese iron ammonium phosphate precursor is 1:(1.3-1.4):(1.3-1.4). The sodium salt and the phosphoric acid source are appropriately over-added to ensure that the reaction proceeds fully and completely.

[0084] Further, in step Si, the mass ratio of the low-sulfur metal cation doped manganese iron ammonium phosphate precursor to the carbon source is 1:(0.01-1).

[0085] Further, in step S1, the alcohol solution is a mixed solution of alcohol reagent and water. The alcohol reagent not only acts as a dispersion medium, but also helps the solution to dry quickly due to its fast evaporation rate.

[0086] Further, the alcohol reagent includes one or more of methanol, ethanol, ethylene glycol.

[0087] Preferably, the alcohol reagent is ethanol.

[0088] More preferably, the volume ratio of the alcohol reagent and water is 1:(1-5).

[0089] Further, in step Si, the solid content of the slurry is 10%-50%.

[0090] Further, in step Si, the particle size of the ground slurry is 0.2 μm≤D 50 ≤0.3 μm.

[0091] Further, in step Si, the drying is spray drying.

[0092] Still further, the conditions of the spray drying are an inlet temperature of 190-250 ℃, an outlet temperature of 90-140 ℃, and an outlet particle size of 18 μm≤D 50 ≤25 μm.

[0093] Further, in step Sii, the protective gas comprises one or more of nitrogen, helium, and argon.

[0094] Further, in step Sii, the calcination device is a box furnace, a roller kiln, or a tube furnace, preferably a box furnace.

[0095] Still further, the heating rate of the calcination is 1-20 ℃ / min.

[0096] Further, in step Sii, the time of the sufficient calcination is 2-20 h. The stepwise calcination method is conducive to the formation of a single phase of the carbon-coated modified sodium manganese iron pyrophosphate phosphate.

[0097] Further, in step Sii, the post-treatment comprises crushing and sieving.

[0098] Still further, the crushing is crushing the calcined powder to a particle size of 0.6 μm≤D 50 ≤0.85 μm.

[0099] Still further, the sieving is sieving the crushed powder with a mesh size of 200-400 mesh.

[0100] Specifically, in step Sii, the post-treatment comprises crushing the calcined powder to a particle size of 0.6 μm≤D 50 ≤0.85 μm, and then sieving with a mesh size of 200-400 mesh.

[0101] The present application protects the carbon-coated modified sodium manganese iron pyrophosphate phosphate prepared by the above preparation method.

[0102] The present application protects a sodium-ion battery positive electrode, comprising a current collector and a positive electrode active material loaded on the current collector, wherein the positive electrode active material comprises the aforementioned carbon-coated modified sodium manganese iron pyrophosphate phosphate.

[0103] Preferably, the current collector is aluminum foil or carbon-coated aluminum foil.

[0104] Further, as a preferred manner, the preparation method of the sodium ion battery positive electrode comprises the following steps: dispersing the carbon-coated modified sodium manganese iron pyrophosphate, conductive agent carbon black and binder polyvinylidene fluoride in N-methyl pyrrolidone dispersant to obtain a positive electrode slurry, then coating the positive electrode slurry on an aluminum foil, and after vacuum drying at 80-140℃ for 12-24h, the positive electrode is obtained.

[0105] Further, the mass ratio of the carbon-coated sodium manganese iron pyrophosphate, conductive agent carbon black and binder polyvinylidene fluoride is 1:(0.01-0.29):(0.01-0.14).

[0106] The application protects a sodium ion battery, which comprises a positive electrode, a negative electrode, a separator and an electrolyte, and the positive electrode is the aforementioned sodium ion battery positive electrode.

[0107] The sodium ion battery prepared by the application has excellent charge and discharge capacity, capacity retention rate and cycle stability.

[0108] Further, as a preferred manner, the positive electrode of the sodium ion battery is the aforementioned sodium ion battery positive electrode, the negative electrode is a metal sodium sheet, the separator is glass fiber, and the electrolyte is 1mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0109] Compared with the prior art, the application has the following beneficial effects:

[0110] The application provides a low-sulfur type metal cation doped manganese iron ammonium phosphate precursor. By adding an appropriate amount of a phosphoric acid source to the prepared metal cation doped manganese iron ammonium phosphate precursor, not only the sulfur content of the precursor is significantly reduced, but also the molar ratio of manganese, iron, doped elements and phosphorus elements is slightly less than 1, which not only realizes the atomic level distribution of elements in the precursor, but also improves the tap density, which is beneficial to the subsequent preparation of high-performance carbon-coated modified sodium manganese iron pyrophosphate material. The sodium ion battery based on the material has excellent charge and discharge capacity, capacity retention rate and cycle stability. In addition, the preparation method of the precursor is simple and easy to control, and has wide application advantages. BRIEF DESCRIPTION OF DRAWINGS

[0111] Figure 1 The low-sulfur type metal cation doped manganese iron ammonium phosphate precursor NH4Mn 0.7 Fe 0.25 Mg 0.01 Ti 0.01 Co0.01 Ni 0.01 Cu 0.01 SEM image of PO4. DETAILED DESCRIPTION

[0112] The present application is further illustrated by the following description with reference to the accompanying drawings and specific examples, but the examples do not limit the present application in any form. Unless otherwise specified, the reagents, methods and equipment used in the present application are conventional reagents, methods and equipment in the technical field.

[0113] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0114] Example 1 Preparation of low-sulfur metal cation-doped ammonium manganese iron phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate phosphate and sodium ion battery

[0115] 1. Preparation of low-sulfur metal cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.7 Fe 0.25 Mg 0.01 Ti 0.01 Co 0.01 Ni 0.01 Cu 0.01 PO4.

[0116] Into a nitrogen-purged dissolving tank, 7 L of ultrapure water was added, and 3.39 kg of manganese sulfate monohydrate, 34.51 g of anhydrous magnesium sulfate, 79.12 g of oxotitanium ammonium oxalate, 80.59 g of cobalt sulfate heptahydrate, 75.36 g of nickel sulfate hexahydrate, and 45.76 g of anhydrous copper sulfate were dissolved in the dissolving tank to prepare a mixed metal source solution I;

[0117] Into a closed synthesis reactor containing 8.49 L of ultrapure water, 401.38 g of an iron block and iron powder mixture was added, and nitrogen gas was introduced into the reactor while stirring. Then, 3.31 kg of 85wt% concentrated phosphoric acid solution was slowly pumped in, and stirred uniformly for 1 h to prepare a ferrous phosphate dihydrogen solution system II. The ferrous phosphate dihydrogen solution system II was slowly heated to 70°C, and the metal: hydrogen peroxide (2.1 kg of 27.5wt%) in the aforementioned mixed metal source solution I was simultaneously pumped into the ferrous phosphate dihydrogen solution system II at a molar ratio of 2:1, and stirred and reacted at 70°C for 2 h under a nitrogen protective atmosphere to prepare an intermediate system III. The intermediate system III was adjusted to pH 7 using a urea solution, and aged at 70°C for 2 h under a nitrogen protective atmosphere to obtain a manganese iron phosphate doping modification slurry. The metal cation-doped ammonium manganese iron phosphate precursor (NH4Mn 0.7 Fe 0.25 Mg 0.01 Ti 0.01 Co0.01 Ni 0.01 Cu 0.01 PO4)。

[0118] The above metal cation doped manganese iron ammonium phosphate precursor was dispersed in 20 L of ultrapure water under a nitrogen protective atmosphere. A 85 wt% concentrated phosphoric acid solution was slowly pumped into the reaction kettle at a molar ratio of precursor: phosphoric acid = 3: 1. The slurry was stirred for 30 min to obtain intermediate system V. Urea was used to adjust the pH of intermediate system V to 7. The reaction was aged at 70°C for 4 h under a nitrogen protective atmosphere. The low-sulfur metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.7 Fe 0.25 Mg 0.01 Ti 0.01 Co 0.01 Ni 0.01 Cu 0.01 PO4)。

[0119] 2. Carbon-coated modified manganese iron sodium pyrophosphate phosphate Na4Mn 2.1 Fe 0.75 Mg 0.03 Ti 0.03 Co 0.03 Ni 0.03 Cu 0.03 Preparation of (PO4)2P2O7

[0120] The corresponding sodium carbonate, low-sulfur metal cation doped manganese iron ammonium phosphate precursor, and sodium dihydrogen phosphate were weighed in an ultrapure water and ethanol (volume ratio of ultrapure water and ethanol = 1:1) mixed solution in a quantity ratio of Na:(Mn+Fe+Mg+Ti+Co+Ni+Cu):P = 4.08:3:4.05. Organic carbon source polyethylene glycol (5.1 wt% of the mass of the low-sulfur metal cation doped manganese iron ammonium phosphate precursor) was added, and the slurry was stirred for 15 min until it was uniformly dispersed. The slurry with a solid content of 40% was transferred to a sand mill and ground to a particle size D 50 = 0.25 μm. The slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, the spray powder was subjected to stepwise temperature sintering using an atmosphere box furnace under a nitrogen protective atmosphere. The temperature was raised to 380°C at a rate of 5°C / min and held for 3 h. Then the temperature was raised to 500°C at a rate of 5°C / min and held for 8 h.

[0121] After the atmosphere box furnace naturally cooled to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified manganese iron sodium pyrophosphate phosphate (Na4Mn 2.1 Fe 0.75 Mg0.03 Ti 0.03 Co 0.03 Ni 0.03 Cu 0.03 (PO4)2P2O7)。

[0122] 3. Preparation of sodium-ion battery

[0123] The carbon-coated modified manganese iron sodium pyrophosphate phosphate obtained above, carbon black, and polyvinylidene fluoride binder were dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5, mixed uniformly to obtain a positive electrode slurry, and then coated on an aluminum foil. After vacuum drying at 120°C for 12h, a sodium-ion battery positive electrode was obtained.

[0124] The sodium-ion battery was assembled in an argon-filled glove box. The positive electrode was the above sodium-ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1mol / L NaClO4 dissolved in a volume ratio of 1:1 dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0125] Preparation of low-sulfur metal cation-doped ammonium manganese iron phosphate precursor, carbon-coated modified manganese iron sodium pyrophosphate phosphate, and sodium-ion battery

[0126] 1. Preparation of low-sulfur metal cation-doped ammonium manganese iron phosphate precursor NH4Mn 0.75 Fe 0.2 W 0.01 Nb 0.01 Co 0.01 Zn 0.01 Cu 0.01 Preparation of PO4

[0127] Into a nitrogen-purged dissolving tank, 9L of ultrapure water was added, and 4.54kg of manganese sulfate monohydrate, 105.3g of sodium tungstate, 192.8g of niobium oxalate, 100.7g of cobalt sulfate hexahydrate, 64.38g of zinc sulfate monohydrate, and 57.2g of anhydrous copper sulfate were dissolved in the dissolving tank to prepare a mixed metal source solution I;

[0128] A mixture of 401.38 g of iron chunks and iron powder was added into a closed synthesis reactor containing 7.67 L of ultrapure water, and nitrogen was bubbled into the reactor while stirring. Then, 4.13 kg of 85 wt% concentrated phosphoric acid solution was slowly pumped into the reactor, and stirred for 1 h to obtain a ferrous phosphate solution system II. The ferrous phosphate solution system II was slowly heated to 76 °C, and the metal source solution I was simultaneously pumped into the ferrous phosphate solution system II at a molar ratio of metal to hydrogen peroxide (2.1 kg of 27.5 wt%) of 2:1. After stirring and reacting at 76 °C for 2 h under a nitrogen atmosphere, an intermediate system III was obtained. The intermediate system III was adjusted to pH 7.5 using a urea solution and aged at 76 °C for 2 h under a nitrogen atmosphere to obtain a doped modified manganese iron phosphate slurry. The metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.75 Fe 0.2 W 0.01 Nb 0.01 Co 0.01 Zn 0.01 Cu 0.01 PO4) was obtained by pressure filtration, washing, and flash evaporation.

[0129] Under a nitrogen atmosphere, the metal cation doped manganese iron ammonium phosphate precursor was dispersed in 20 L of ultrapure water in a high-efficiency closed synthesis reactor. An 85 wt% phosphoric acid solution was slowly pumped into the precursor slurry at a molar ratio of precursor to phosphoric acid of 4:1, and stirred for 30 min to obtain an intermediate system V. The system V was adjusted to pH 7.5 using urea and aged at 76 °C for 4 h under a nitrogen atmosphere. A low-sulfur metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.75 Fe 0.2 W 0.01 Nb 0.01 Co 0.01 Zn 0.01 Cu 0.01 PO4) was obtained by pressure filtration, washing, and flash evaporation.

[0130] 2. Preparation of carbon-coated modified manganese iron sodium pyrophosphate Na4Mn 2.25 Fe 0.6 W 0.03 Nb 0.03 Co 0.03 Zn 0.03 Cu 0.03 PO4)2P2O7

[0131] The corresponding sodium carbonate, low-sulfur metal cation-doped manganese iron ammonium phosphate precursor and sodium dihydrogen phosphate were weighed in a quantity relationship of the amount-of-substance ratio of Na:(Mn+Fe+W+Nb+Co+Zn+Cu):P being 4.06:3:4.06 in a mixed solution of ultrapure water and ethanol (volume ratio of ultrapure water and ethanol being 1:1), and an organic carbon source polyethylene glycol (4.8wt% of the low-sulfur metal cation-doped manganese iron ammonium phosphate precursor) was added, and after stirring for 15 min until uniformly dispersed, the slurry with a solid content of 38% was transferred into a sand mill for grinding to a particle size D 50 0.28pm, the slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen protective atmosphere, the spray powder was subjected to staged temperature sintering using an atmosphere box furnace. The temperature was raised to 360°C at a rate of 5°C / min and held for 2h, and then the temperature was raised to 520°C at a rate of 5°C / min and held for 9h. After naturally decreasing to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 2.25 Fe 0.6 W 0.03 Nb 0.03 Co 0.03 Zn 0.03 Cu 0.03 (PO4)2P2O7).

[0132] 3. Preparation of sodium ion battery

[0133] The carbon-coated modified sodium manganese iron pyrophosphate obtained above, a conductive agent carbon black, and a binder polyvinylidene fluoride were dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5 to mix uniformly to obtain a positive electrode slurry, which was then coated on an aluminum foil. After vacuum drying at 100°C for 24h, a sodium ion battery positive electrode was obtained.

[0134] The assembly of a button sodium ion battery was carried out in an argon-filled glove box, the positive electrode was the above-mentioned sodium ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0135] Preparation of low-sulfur metal cation-doped manganese iron ammonium phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate, and sodium ion battery

[0136] 1. Preparation of low-sulfur metal cation-doped manganese iron ammonium phosphate precursor NH4Mn 0.25 Fe 0.7 Mg 0.01 Ti 0.02 Co 0.01 Zn 0.01 PO4

[0137] Into a nitrogen purged dissolving tank, 1 L of ultrapure water was added, 432.64 g of manganese sulfate monohydrate, 12.32 g of anhydrous magnesium sulfate, 56.51 g of oxotitanium ammonium oxalate, 28.79 g of cobalt sulfate hexahydrate, and 18.4 g of zinc sulfate monohydrate were dissolved in the dissolving tank to prepare a mixed metal source solution I;

[0138] Into a closed synthesis reactor containing 10.62 L of ultrapure water, a mixture of 401.38 g of iron block and iron powder was added, nitrogen was purged into the reactor and continuously stirred, then 1.18 kg of 85wt% concentrated phosphoric acid solution was slowly pumped in, and stirred uniformly for 1 h to prepare a ferrous biphosphate solution system II. The ferrous biphosphate system II was slowly heated to 74°C, and the metal: hydrogen peroxide (2.1 kg of 27.5wt%) in the form of a molar ratio of 2:1 in the aforementioned mixed metal source solution I was simultaneously slowly pumped into the ferrous biphosphate solution system II, and after being fully stirred and reacted at a reaction temperature of 74°C for 2 h under a nitrogen protective atmosphere, an intermediate system III was prepared. After adjusting the intermediate system III to a pH of 7.2 with a urea solution and aging at a reaction temperature of 74°C for 2 h under a nitrogen protective atmosphere, a doped modified manganese iron phosphate slurry was obtained. Through pressure filtration, washing, and flash evaporation treatment, a metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.25 Fe 0.7 Mg 0.01 Ti 0.02 Co 0.01 Zn 0.01 PO4) was obtained.

[0139] Under a nitrogen protective atmosphere, the aforementioned metal cation doped manganese iron ammonium phosphate precursor was dispersed in 20 L of ultrapure water in a high-efficiency closed synthesis reactor, and 85wt% concentrated phosphoric acid solution was slowly pumped into the reactor at a molar ratio of precursor: phosphoric acid = 3.5:1 to prepare an intermediate system V, which was stirred for 30 min in the precursor slurry, and then urea was used to adjust the system V to a pH of 7.2 and age at a reaction temperature of 74°C for 4 h under a nitrogen protective atmosphere. Through pressure filtration, washing, and flash evaporation treatment, a low-sulfur metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.25 Fe 0.7 Mg 0.01 Ti 0.02 Co 0.01 Zn 0.01 PO4) was obtained.

[0140] 2. Carbon-coated modified manganese iron sodium pyrophosphate phosphate Na4Mn 0.75 Fe 2.1 Mg 0.03 Ti 0.06 Co 0.03 Zn 0.03Preparation of (PO4)2P2O7

[0141] The corresponding sodium carbonate, low-sulfur metal cation-doped manganese iron ammonium phosphate precursor, and sodium dihydrogen phosphate were weighed in a quantity ratio of 4.09:3:4.09 of the amount-of-substance ratio of Na:(Mn+Fe+Mg+Ti+Co+Zn):P in an ultrapure water and ethanol (volume ratio of ultrapure water and ethanol is 1:1) mixed solution, and an organic carbon source, polyethylene glycol (4.8wt% of the mass of the low-sulfur metal cation-doped manganese iron ammonium phosphate precursor), was added. After stirring for 15 min until uniformly dispersed, the slurry with a solid content of 38% was transferred to a sand mill for grinding to a particle size D 50 0.26μm, the slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, the spray powder was subjected to staged temperature rising sintering using an atmosphere box furnace under a nitrogen protective atmosphere. The temperature was raised to 360℃ at a rate of 5℃ / min and held for 2h, and then raised to 550℃ at a rate of 5℃ / min and held for 9h. After naturally decreasing to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 0.75 Fe 2.1 Mg 0.03 Ti 0.06 Co 0.03 Zn 0.03 (PO4)2P2O7).

[0142] 3. Preparation of a sodium ion battery

[0143] The carbon-coated modified sodium manganese iron pyrophosphate obtained above, a conductive agent carbon black, and a binder polyvinylidene fluoride were dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5, mixed uniformly to obtain a positive electrode slurry, and then coated on an aluminum foil. After vacuum drying at 100℃ for 24h, a positive electrode of a sodium ion battery was obtained.

[0144] A button sodium ion battery was assembled in an argon-filled glove box. The positive electrode was the positive electrode of the sodium ion battery described above, the negative electrode was a metal sodium sheet, the separator was a glass fiber, and the electrolyte was 1mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0145] Preparation of a low-sulfur metal cation-doped manganese iron ammonium phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate, and a sodium ion battery

[0146] 1. Preparation of a low-sulfur metal cation-doped manganese iron ammonium phosphate precursor NH4Mn 0.7 Fe 0.25 V 0.05 PO4

[0147] Into a nitrogen purged dissolving tank, 7L of ultrapure water was added, 3.39kg of manganese sulfate monohydrate and 443.18g of vanadyl oxalate were dissolved in the above dissolving tank to prepare a mixed metal source solution I;

[0148] Into a closed synthesis reactor containing 8.49L of ultrapure water, 401.38g of iron block and iron powder mixture was added, nitrogen was purged into the reactor and continuously stirred, then 3.31kg of 85wt% concentrated phosphoric acid solution was slowly pumped in, stirred uniformly for 1h, to prepare a ferrous phosphate dihydro solution system II. The ferrous phosphate dihydro system II was slowly heated to 74℃, and the metal: hydrogen peroxide (2.1kg of 27.5wt%) in the form of a molar ratio of 2:1 in the aforementioned mixed metal source solution I was simultaneously slowly pumped into the ferrous phosphate dihydro solution system II, after being fully stirred and reacted at a reaction temperature of 74℃ for 2h under a nitrogen protective atmosphere, an intermediate system III was prepared. After adjusting the intermediate system III to a pH of 7.2 with a urea solution and aging at a reaction temperature of 74℃ for 2h under a nitrogen protective atmosphere, a doped modified manganese iron phosphate slurry was obtained. Through pressure filtration, washing, and flash evaporation treatment, a metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.7 Fe 0.25 V 0.05 PO4).

[0149] Under a nitrogen protective atmosphere, the above metal cation doped manganese iron ammonium phosphate precursor was dispersed in 20L of ultrapure water in a high-efficiency closed synthesis reactor, 85wt% concentrated phosphoric acid solution was slowly pumped into the reactor at a molar ratio of precursor: phosphoric acid = 3:1, stirred for 30min to obtain an intermediate system V, and then urea was used to adjust the pH of the system V to 7.2 and aged at a reaction temperature of 74℃ for 4h under a nitrogen protective atmosphere. Through pressure filtration, washing, and flash evaporation treatment, a low-sulfur metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.7 Fe 0.25 V 0.05 PO4).

[0150] 2. Preparation of carbon-coated modified manganese iron sodium pyrophosphate phosphate Na4Mn 2.1 Fe 0.75 V 0.15 (PO4)2P2O7

[0151] The corresponding sodium carbonate, low-sulfur metal cation-doped manganese iron ammonium phosphate precursor and sodium dihydrogen phosphate were weighed in a quantity ratio of 4.09:3:4.09 of the amount-of-substance ratio of Na:(Mn+Fe+V):P in a mixed solution of ultrapure water and ethanol (volume ratio of ultrapure water to ethanol is 1:1), and an organic carbon source, polyethylene glycol (4.8wt% of the mass of the low-sulfur metal cation-doped manganese iron ammonium phosphate precursor), was added, and the slurry with a solid content of 38% was transferred into a sand mill for grinding to a particle size D 50 0.26pm, the slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen protective atmosphere, the spray powder was subjected to staged temperature sintering using an atmosphere box furnace. The temperature was raised to 360°C at a rate of 5°C / min and held for 2h, and then the temperature was raised to 550°C at a rate of 5°C / min and held for 9h. After naturally decreasing to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 2.1 Fe 0.75 V 0.15 (PO4)2P2O7).

[0152] 3. Preparation of sodium-ion battery

[0153] The carbon-coated modified sodium manganese iron pyrophosphate obtained above, a conductive agent carbon black, and a binder polyvinylidene fluoride were dissolved in N-methylpyrrolidone dispersant in a mass ratio of 90:5:5 to obtain a positive electrode slurry, which was then coated on an aluminum foil. After vacuum drying at 100°C for 24h, a sodium-ion battery positive electrode was obtained.

[0154] The assembly of the button sodium-ion battery was carried out in an argon-filled glove box. The positive electrode was the above-mentioned sodium-ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0155] Comparative Example 1: Preparation of metal cation-doped manganese iron phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate, and sodium-ion battery

[0156] The difference from Example 1 is that in Step 1, the metal cation-doped manganese iron ammonium phosphate precursor was not treated with a phosphoric acid source solution, i.e., a low-sulfur metal cation-doped manganese iron ammonium phosphate precursor NH4Mn 0.7 Fe 0.25 Mg 0.0 1Ti 0.01 Co 0.01 Ni 0.01 Cu 0.01 PO4. The specific steps are as follows:

[0157] 1. Metal cation doped ammonium manganese iron phosphate precursor NH4Mn 0.7 Fe 0.25 Mg 0.01 Ti 0.01 Co 0.01 Ni 0.01 Cu 0.01 PO4) preparation

[0158] Into a nitrogen purged dissolving tank, 7L of ultrapure water was added, 3.39kg of manganese sulfate monohydrate, 34.51g of anhydrous magnesium sulfate, 79.12g of titanium oxalate, 80.59g of cobalt sulfate heptahydrate, 75.36g of nickel sulfate hexahydrate, 45.76g of copper sulfate anhydrous were dissolved in the dissolving tank to prepare a mixed metal source solution I;

[0159] Into a closed synthesis reactor containing 8.49L of ultrapure water, 401.38g of iron block and iron powder mixture was added, nitrogen was purged into the reactor and continuously stirred, then 3.31kg of 85wt% concentrated phosphoric acid solution was slowly pumped in, stirred uniformly for 1h to prepare a ferrous phosphate dihydrogen solution system II. The ferrous phosphate dihydrogen system II was slowly heated to 70°C, and the metal: hydrogen peroxide (2.1kg of 27.5wt%) in the form of a molar ratio of 2:1 in the aforementioned mixed metal source solution I was simultaneously slowly pumped into the ferrous phosphate dihydrogen solution system II, and after fully stirring and reacting at 70°C for 2h under a nitrogen protective atmosphere, an intermediate system III was prepared. The intermediate system III was adjusted to pH 7 with urea solution and aged at 70°C for 2h under a nitrogen protective atmosphere to obtain a doped modified manganese iron phosphate slurry. Through pressure filtration, washing, and flash evaporation treatment, a metal cation doped ammonium manganese iron phosphate precursor (NH4Mn 0.7 Fe 0.25 Mg 0.01 Ti 0.01 Co 0.01 Ni 0.01 Cu 0.01 PO4) was obtained.

[0160] 2. Carbon-coated modified manganese iron sodium pyrophosphate Na4Mn 2.1 Fe 0.75 Mg 0.03 Ti 0.03 Co 0.03 Ni 0.03 Cu 0.03 (PO4)2P2O7) preparation

[0161] The corresponding sodium carbonate, metal cation doped manganese iron ammonium phosphate precursor and sodium dihydrogen phosphate were weighed in a quantity ratio of 4.08:3:4.05 of the amount-of-substance ratio of Na:(Mn+Fe+Mg+Ti+Co+Ni+Cu):P, in a mixed solution of ultrapure water and ethanol (volume ratio of ultrapure water and ethanol is 1:1), and an organic carbon source polyethylene glycol (5.1wt% of the mass of the metal cation doped manganese iron ammonium phosphate precursor) was added, and after stirring for 15 min until uniformly dispersed, the slurry with a solid content of 40% was transferred to a sand mill for grinding to a particle size D 50 2.1 Fe 0.75 Mg 0.03 Ti 0.03 Co 0.03 Ni 0.03 Cu 0.03 (PO4)2P2O7).

[0162] 3. Preparation of sodium ion battery

[0163] The carbon-coated modified sodium manganese iron pyrophosphate phosphate obtained above was dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5 with the conductive agent carbon black and the binder polyvinylidene fluoride, and mixed uniformly to obtain a positive electrode slurry, which was then coated on an aluminum foil. After vacuum drying at 100°C for 24 h, a sodium ion battery positive electrode was obtained.

[0164] The assembly of the button sodium ion battery was carried out in an argon-filled glove box. The positive electrode was the above-mentioned sodium ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1 mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0165] Preparation of metal cation doped manganese iron ammonium phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate phosphate and sodium ion battery of Comparative Example 2

[0166] The difference from Example 2 is that in step 1, the metal cation doped manganese iron ammonium phosphate precursor was not treated with a phosphoric acid source solution, i.e., a low-sulfur type metal cation doped manganese iron ammonium phosphate precursor NH4Mn 0.75 Fe 0.2 W 0.01 Nb 0.01 ​Co 0.01 Zn 0.01 Cu 0.01 PO4. The specific steps are as follows:

[0167] 1. Preparation of metal cation doped manganese iron ammonium phosphate precursor NH4Mn 0.75 Fe 0.2 W 0.01 Nb 0.01 Co 0.01 Zn 0.01 Cu 0.01 PO4

[0168] Into a nitrogen gas purged dissolving tank, 9L of ultrapure water was added, and 4.54kg of manganese sulfate monohydrate, 105.3g of sodium tungstate, 192.8g of niobium oxalate, 100.7g of cobalt sulfate hexahydrate, 64.38g of zinc sulfate monohydrate, and 57.2g of anhydrous copper sulfate were dissolved in the dissolving tank to prepare a mixed metal source solution I;

[0169] Into a sealed synthesis reactor containing 7.67L of ultrapure water, 401.38g of an iron block and iron powder mixture was added, and nitrogen gas was purged into the reactor while continuously stirring. Then, 4.13kg of 85wt% concentrated phosphoric acid solution was slowly pumped in, and stirred uniformly for 1h to prepare a ferrous phosphate dihydrogen solution system II. The ferrous phosphate dihydrogen system II was slowly heated to 76℃, and the aforementioned mixed metal source solution I was simultaneously slowly pumped into the ferrous phosphate dihydrogen solution system II in the form of a metal: hydrogen peroxide (2.1kg of 27.5wt%) molar ratio of 2:1. After fully stirring and reacting at a reaction temperature of 76℃ for 2h under a nitrogen gas protective atmosphere, an intermediate system III was prepared. The intermediate system III was then adjusted to a pH of 7.5 using a urea solution and aged for 2h at a reaction temperature of 76℃ under a nitrogen gas protective atmosphere to obtain a doped modified manganese iron phosphate slurry. Through pressure filtration, washing, and flash evaporation treatment, a metal cation doped manganese iron ammonium phosphate precursor (NH4Mn 0.75 Fe 0.2 W 0.01 Nb 0.01 Co 0.01 Zn 0.01 Cu 0.01 PO4) was obtained.

[0170] 2. Preparation of carbon-coated modified manganese iron sodium phosphate Na4Mn 2.25 Fe 0.6 W 0.03 Nb 0.03 Co 0.03 Zn 0.03 Cu 0.03 (PO4)2P2O7

[0171] The corresponding sodium carbonate, metal cation doped manganese iron ammonium phosphate precursor and sodium dihydrogen phosphate were weighed in a quantity relationship of 4.06:3:4.06 of the amount-of-substance ratio of Na:(Mn+Fe+W+Nb+Co+Zn+Cu):P in an ultrapure water and ethanol (volume ratio of ultrapure water and ethanol is 1:1) mixed solution, and an organic carbon source polyethylene glycol (4.8wt% of the mass of the metal cation doped manganese iron ammonium phosphate precursor) was added, and after stirring for 15 min until uniformly dispersed, the slurry with a solid content of 38% was transferred to a sand mill for grinding to a particle size D 50 0.28pm, the slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen protective atmosphere, the spray powder was subjected to staged temperature sintering using an atmosphere box furnace. The temperature was raised to 360°C at a rate of 5°C / min and held for 2h, and then the temperature was raised to 520°C at a rate of 5°C / min and held for 9h. After naturally decreasing to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 2.25 Fe 0.6 W 0.03 Nb 0.03 Co 0.03 Zn 0.03 Cu 0.03 (PO4)2P2O7).

[0172] 3. Preparation of sodium ion battery

[0173] The carbon-coated modified sodium manganese iron pyrophosphate obtained above, a conductive agent carbon black, and a binder polyvinylidene fluoride were dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5 to mix uniformly to obtain a positive electrode slurry, which was then coated on an aluminum foil. After vacuum drying at 100°C for 24h, a sodium ion battery positive electrode was obtained.

[0174] The assembly of the button sodium ion battery was carried out in an argon-filled glove box. The positive electrode was the above-mentioned sodium ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0175] Preparation of metal cation doped manganese iron ammonium phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate, and sodium ion battery of Comparative Example 3

[0176] The difference from Example 3 is that in step 1, the metal cation doped manganese iron ammonium phosphate precursor was not treated with a phosphoric acid source solution, i.e., a low-sulfur type metal cation doped manganese iron ammonium phosphate precursor NH4Mn 0.25 Fe 0.7 Mg 0.0 1Ti 0.02Co 0.01 Zn 0.01 PO4. The specific steps are as follows:

[0177] 1. Metal cation-doped manganese iron ammonium phosphate precursor NH4Mn 0.25 Fe 0.7 Mg 0.01 Ti 0.02 Co 0.01 Zn 0.01 Preparation of PO4

[0178] Add 1L of ultrapure water to a nitrogen-purified dissolving tank, and dissolve 432.64g of manganese sulfate monohydrate, 12.32g of anhydrous manganese sulfate, 56.51g of titanium oxalate ammonium, 28.79g of cobalt sulfate hexahydrate, and 18.4g of zinc sulfate monohydrate in the above dissolving tank to prepare mixed metal source solution I.

[0179] A mixture of 401.38 g of iron blocks and iron powder was added to a sealed reactor containing 10.62 L of ultrapure water. Nitrogen gas was introduced into the reactor while stirring continuously. Then, 1.18 kg of 85 wt% concentrated phosphoric acid solution was slowly pumped in and stirred uniformly for 1 h to obtain ferrous dihydrogen phosphate solution system II. Ferrous dihydrogen phosphate system II was slowly heated to 74 °C, and the metal:hydrogen peroxide (2.1 kg 27.5 wt%) from the aforementioned mixed metal source solution I was simultaneously and slowly pumped into ferrous dihydrogen phosphate solution system II in a molar ratio of 2:1. After stirring thoroughly at 74 °C under a nitrogen protective atmosphere for 2 h, intermediate system III was obtained. Intermediate system III was then adjusted to pH 7.2 with urea solution and aged at 74 °C for 2 h under a nitrogen protective atmosphere to obtain doped modified manganese iron phosphate slurry. The metal cation doped manganese iron ammonium phosphate precursor (NH4Mn) was obtained by pressure filtration, washing, and flash evaporation. 0.25 Fe 0.7 Mg 0.01 Ti 0.02 Co 0.01 Zn 0.01 PO4).

[0180] 2. Carbon-coated modified sodium manganese pyrophosphate (Na4Mn) 0.75 Fe 2.1 Mg 0.03 Ti 0.06 Co 0.03 Zn 0.03 Preparation of (PO4)2P2O7

[0181] The corresponding sodium carbonate, metal cation doped manganese iron ammonium phosphate precursor and sodium dihydrogen phosphate were weighed in a quantity ratio of 4.09:3:4.09 of the amount-of-substance ratio of Na:(Mn+Fe+Mg+Ti+Co+Zn):P in a mixed solution of ultrapure water and ethanol (volume ratio of ultrapure water and ethanol is 1:1), and an organic carbon source polyethylene glycol (4.8wt% of the mass of the metal cation doped manganese iron ammonium phosphate precursor) was added, and the slurry with a solid content of 38% was transferred into a sand mill for grinding to a particle size D 50 0.26pm, the slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, under a nitrogen protective atmosphere, the spray powder was subjected to staged temperature sintering using an atmosphere box furnace. The temperature was raised to 360°C at a rate of 5°C / min and held for 2h, and then the temperature was raised to 550°C at a rate of 5°C / min and held for 9h. After naturally cooling to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 0.75 Fe 2.1 Mg 0.03 Ti 0.06 Co 0.03 Zn 0.03 (PO4)2P2O7).

[0182] 3. Preparation of sodium ion battery

[0183] The carbon-coated modified sodium manganese iron pyrophosphate obtained above, a conductive agent carbon black, and a binder polyvinylidene fluoride were dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5 to obtain a positive electrode slurry, which was then coated on an aluminum foil. After vacuum drying at 100°C for 24h, a sodium ion battery positive electrode was obtained.

[0184] The assembly of the button sodium ion battery was carried out in an argon-filled glove box. The positive electrode was the above-mentioned sodium ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0185] Preparation of metal cation doped manganese iron ammonium phosphate precursor, carbon-coated modified sodium manganese iron pyrophosphate, and sodium ion battery

[0186] The difference from Example 1 is that in step 1, an excess of metal cation doped manganese iron ammonium phosphate precursor was prepared by doping with metal cation doped manganese iron ammonium. The specific steps are as follows:

[0187] 1. Metal cation doped manganese iron ammonium phosphate precursor NH4Mn 0.5 Fe 0.25 Mg0.05 Ti 0.05 Co 0.05 Ni 0.05 Cu 0.05 PO4

[0188] Dissolve 2.42 kg of manganese sulfate monohydrate, 172.55 g of anhydrous magnesium sulfate, 420.42 g of ammonium titanyl oxalate, 402.95 g of cobalt sulfate heptahydrate, 376.8 g of nickel sulfate hexahydrate, and 228.8 g of anhydrous copper sulfate in 7 L of ultrapure water in a nitrogen-purged dissolving tank to prepare a mixed metal source solution I.

[0189] Add 401.38 g of an iron block and iron powder mixture to a closed synthesis reactor containing 8.49 L of ultrapure water, and then slowly pump 3.31 kg of 85 wt% concentrated phosphoric acid solution into the reactor while continuously stirring. After 1 h of uniform stirring, a ferrous phosphate dihydrogen solution system II is prepared. Slowly heat the ferrous phosphate dihydrogen system II to 70°C, and then slowly pump the metal: hydrogen peroxide (2.1 kg of 27.5 wt%) in the form of a 2:1 molar ratio from the aforementioned mixed metal source solution I into the ferrous phosphate dihydrogen solution system II. After 2 h of sufficient stirring and reaction under a nitrogen protective atmosphere at a reaction temperature of 70°C, an intermediate system III is prepared. Adjust the intermediate system III to a pH of 7 using a urea solution, and then age it under a nitrogen protective atmosphere at a reaction temperature of 70°C for 2 h to obtain a doped modified manganese iron phosphate slurry. After pressure filtration, washing, and flash evaporation, a metal cation-doped manganese iron ammonium phosphate precursor (NH4Mn 0.5 Fe 0.25 Mg 0.05 Ti 0.05 Co 0.05 Ni 0.05 Cu 0.05 PO4

[0190] Under a nitrogen protective atmosphere, disperse the aforementioned metal cation-doped manganese iron ammonium phosphate precursor in 20 L of ultrapure water in a high-efficiency closed synthesis reactor, and then slowly pump 85 wt% phosphoric acid solution into the precursor slurry in the reactor at a precursor: phosphoric acid molar ratio of 3:1. After 30 min of stirring, an intermediate system V is obtained, and then adjust the system V to a pH of 7 using urea and age it under a nitrogen protective atmosphere at a reaction temperature of 70°C for 4 h. After pressure filtration, washing, and flash evaporation, a low-sulfur metal cation-doped manganese iron ammonium phosphate precursor (NH4Mn 0.5 Fe 0.25 Mg 0.05 Ti 0.05 Co 0.05 Ni 0.05 Cu 0.05 PO4

[0191] 2. Carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 1.5 Fe 0.75 Mg 0.15 Ti 0.15 Co 0.15 Ni 0.15 Cu 0.15 Preparation of (PO4)2P2O7

[0192] The corresponding sodium carbonate, metal cation-doped ammonium manganese iron phosphate precursor and sodium dihydrogen phosphate were weighed in an amount ratio of 4.08:3:4.05 of the amount-of-substance ratio of Na:(Mn+Fe+Mg+Ti+Co+Ni+Cu):P in an ultrapure water and ethanol (volume ratio of ultrapure water and ethanol is 1:1) mixed solution, and an organic carbon source polyethylene glycol (5.1wt% of the mass of the metal cation-doped ammonium manganese iron phosphate precursor) was added. After stirring for 15 min until uniformly dispersed, the slurry with a solid content of 40% was transferred to a sand mill for grinding to a particle size D 50 0.25 μm, the slurry was transferred to a spray granulation dryer for spray drying to obtain a spray powder. Subsequently, the spray powder was subjected to staged temperature rising sintering using an atmosphere box furnace under a nitrogen protective atmosphere. The temperature was raised to 380°C at a rate of 5°C / min and held for 3 h, and then the temperature was raised to 500°C at a rate of 5°C / min and held for 8 h. After naturally decreasing to room temperature, the obtained sintered powder was subjected to jet milling and sieving to obtain carbon-coated modified sodium manganese iron pyrophosphate (Na4Mn 1.5 Fe 0.75 Mg 0.15 Ti 0.15 Co 0.15 Ni 0.15 Cu 0.15 (PO4)2P2O7).

[0193] 3. Preparation of a sodium ion battery

[0194] The above obtained carbon-coated modified sodium manganese iron pyrophosphate, conductive agent carbon black and binder polyvinylidene fluoride were dissolved in N-methyl pyrrolidone dispersant in a mass ratio of 90:5:5 and mixed uniformly to obtain a positive electrode slurry, which was then coated on an aluminum foil. After vacuum drying at 100°C for 24 h, a sodium ion battery positive electrode was obtained.

[0195] The assembly of a button sodium ion battery was carried out in an argon-filled glove box. The positive electrode was the above sodium ion battery positive electrode, the negative electrode was a metal sodium sheet, the separator was glass fiber, and the electrolyte was 1 mol / L NaClO4 dissolved in a volume ratio of 1:1 of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution.

[0196] Characterization of material composition and microstructure of experimental example 1

[0197] (1) SEM measurement

[0198] The low-sulfur metal cation-doped manganese iron ammonium phosphate precursor prepared in Example 1 was characterized by using a transmission scanning electron microscope, and the results are shown in FIG. 1. The precursor prepared in Example 1 had a sheet structure after flash evaporation, and the specific surface area of the sheet structure was larger than that of a stacked structure, which was more conducive to subsequent grinding and refinement, and the target particle size was reached faster, thereby improving the synthesis efficiency of the material. Figure 1

[0199] (2) Sulfur content and doped cation content detection

[0200] The low-sulfur metal cation-doped manganese iron ammonium phosphate precursor and the carbon-coated modified manganese iron sodium pyrophosphate phosphate prepared in Examples 1-4 and Comparative Examples 1-4 were taken into a centrifuge tube, and an appropriate amount of hydrochloric acid was added for heating digestion for 2 h, then cooled and diluted. The inductively coupled plasma emission spectrometer was used to measure the sulfur content and doped cation content.

[0201] (3) Iron element content and phosphorus element content measurement

[0202] The potassium dichromate titration method was used to measure the iron content. After the metal cation-doped manganese iron ammonium phosphate precursor / lithium manganese iron phosphate prepared in Examples 1-4 and Comparative Examples 1-4 was decomposed by perchloric acid and the carbon in the sample was removed, in an acidic medium, the orthophosphate reacted with the quinoline molybdate precipitant to form a yellow quinoline phosphomolybdate precipitate. After filtration, washing, drying, and weighing, the weight of the quinoline phosphomolybdate precipitate was obtained, and the phosphorus element content in the sample was calculated.

[0203] (4) Tap density

[0204] The metal cation-doped manganese iron ammonium phosphate precursor samples prepared in Examples 1-4 and Comparative Examples 1-4 were placed in a three-sided graduated cylinder and placed in a tap density tester. The test time of the density instrument was adjusted. After the tap density tester stopped vibrating, the graduated cylinder was removed and the reading was taken to calculate the tap density of the powder.

[0205] The detection data are shown in Table 1 and FIG. 2. Figure 1

[0206] Table 1: Tap density, sulfur content, main doped element content ratio, and doped element content in the precursor

[0207]

[0208]

[0209] Note: 1438 / Mg indicates that the content of metal Mg in the precursor is 1438 ppm, and the others are the same.

[0210] ​​From the data in Table 1, it can be seen that the low-sulfur metal cation-doped manganese iron ammonium phosphate precursors in Examples 1-4 were prepared by using a phosphoric acid source to treat the synthesized metal cation-doped manganese iron ammonium phosphate precursors, and the sulfur content was much lower than that of the metal cation-doped manganese iron ammonium phosphate precursors in Comparative Examples 1-3 which were not treated with a phosphoric acid source. This can be attributed to the fact that the addition of an appropriate amount of a phosphoric acid source to the synthesized metal cation-doped manganese iron ammonium phosphate precursors in Examples 1-4 can induce recrystallization to expose sulfur impurities and separate them from the precursor particles, and then through subsequent post-processing of filtration and washing, the transfer of sulfur impurities is achieved, thereby achieving the purpose of removing sulfur impurities and improving the crystallinity of the precursor, which is beneficial to avoiding the introduction of sulfur impurities in the subsequent preparation of carbon-coated modified sodium manganese iron pyrophosphate. In Comparative Example 4, excessive metal doping can affect the electrochemical performance of the positive electrode material. In addition, the addition of a phosphoric acid source can also regulate the content of Mn, Fe, doped metal elements and P elements in the precursor, so that the actual n(Mn+Fe+M):n(P) is slightly less than 1. This regulation optimizes the manganese iron defects in the lattice of sodium manganese iron pyrophosphate at the molecular level, provides an effective channel for the insertion and extraction of sodium ions, and at the same time, appropriately expands the channel, alleviates the volume change or structure collapse during sodium storage, and significantly improves the cycle stability of the material. The ICP test results show that the actual content of the main elements in the precursors synthesized in Examples 1-4 and Comparative Examples 1-4 is highly consistent with the target doping molar ratio, with a deviation within a reasonable range. This confirms the accuracy of the element ratio in the precursor chemical formula and proves that the metal cation-doped manganese iron ammonium phosphate precursor has been successfully synthesized according to the preset molar ratio of the doped metal elements. At the same time, the ICP test also confirms the uniform distribution of the doped elements in the precursor, which lays a solid foundation for the subsequent preparation of positive electrode materials with excellent performance.

[0211] Experimental Example 2 Physical properties of carbon-coated sodium manganese iron pyrophosphate positive electrode material and physicochemical performance test of sodium ion battery based on the material

[0212] (1) Electrochemical test

[0213] The carbon-coated modified sodium manganese iron pyrophosphate phosphate material obtained in Examples 1-4 and Comparative Examples 1-4 was used as a positive electrode active material. The active material, carbon black as a conductive agent, and polyvinylidene fluoride as a binder were weighed in a mass ratio of 90:5:5, dispersed in an N-methylpyrrolidone dispersant to form a uniformly dispersed positive electrode slurry, and then coated on an aluminum foil. After vacuum drying at 120°C for 12h, the sheet was punched and weighed to obtain a round electrode sheet. A metal sodium sheet was used as the negative electrode, the prepared electrode sheet was used as the positive electrode, glass fiber was used as the separator, and a 1 mol / L NaClO4 solution dissolved in a 1:1 volume ratio of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution was used as the electrolyte. A 2032 type sodium ion button cell was assembled in a Milan glove box (O2≤0.01 ppm, H2O≤0.01 ppm), and after standing for 12h, the electrochemical performance test was carried out at room temperature. The test voltage range was set to 1.7-4.3V, and the charge-discharge test was carried out at a 0.2C / 1C rate current. The calculation formula is as follows:

[0214] First cycle discharge efficiency = first cycle discharge specific capacity / first cycle charge specific capacity x 100%;

[0215] Capacity retention rate = 100th cycle electric specific capacity / first cycle discharge specific capacity x 100%.

[0216] (2) Negative electrode manganese content test

[0217] The amount of Mn dissolved in the electrolyte after the battery assembled with the carbon-coated modified sodium manganese iron pyrophosphate phosphate positive electrode material obtained in Examples 1-4 and Comparative Examples 1-4 was tested using an inductively coupled plasma test method at a 1C current density after 100 cycles.

[0218] (3) Powder compaction density test method

[0219] The carbon-coated modified sodium manganese iron lithium phosphate positive electrode material prepared in Examples 1-4 and Comparative Examples 1-4 was used as the test sample. A powder compaction density instrument was used to apply a pressure of 5T to the powder sample, and the volume and mass were measured to obtain the compaction density data.

[0220] The above determination results are shown in Table 2.

[0221] Table 2 Physical properties of carbon-coated sodium manganese iron pyrophosphate phosphate positive electrode material and physicochemical performance test of sodium ion battery based on the material

[0222]

[0223]

[0224] From the data in Table 2, in terms of compaction density, since Comparative Examples 1-3 did not use a phosphoric acid source to treat the metal cation-doped ammonium manganese iron phosphate precursor, the actual n(Mn+Fe+M):n(P) was slightly greater than 1, and the tap density was <1, which greatly affected the performance of the subsequent processed material, and in turn affected the compaction density of the positive electrode material. Comparative Example 4 is due to the excessive doping of metal cations in the metal cation-doped ammonium manganese iron phosphate precursor, the excessive ion doping caused the particles to grow excessively to form large-size micron particles, thereby making the compaction density of the positive electrode material after crushing smaller.

[0225] The electrochemical performance test further showed that since Comparative Examples 1-3 did not use a phosphoric acid source to treat the metal cation-doped ammonium manganese iron phosphate precursor, the sulfur impurity content was far beyond the battery grade standard, and the actual n(Mn+Fe+M):n(P) in the precursor was slightly greater than 1, resulting in a lower 0.2C capacity than Comparative Examples 1-3, and the capacity decay was more severe during the 1C large current and 100 cycle processes, and the manganese dissolution was more severe. On the one hand, this is due to the excessively high ratio of manganese, iron and doped metal elements to phosphorus, which is not conducive to the construction of manganese iron site defects in the phosphate pyrophosphate manganese iron sodium lattice at the molecular level, and cannot provide a favorable channel for sodium ion migration. On the other hand, since the sodium ion radius is large, the smaller sodium ion channel cannot better alleviate the stress changes caused by the embedding and de-embedding process, thereby the rate performance and cycle stability are poor, and the capacity decay is more severe. The excessive metal cation doping in Comparative Example 4 greatly reduces the content of active metal sites in the material, thereby reducing the sodium storage capacity of the material; at the same time, the stress changes caused by the embedding and de-embedding process of sodium ions make the material rate performance and cycle life performance worse. In contrast, the appropriate amount of metal cation doping in Examples 1-4 can effectively exert the synergistic effect between the elements to improve the compaction density (>2.4 g / cm 3 ), the initial charge specific capacity (>113 mAh / g), the initial coulombic efficiency (>94%), and the cycle performance (>96%), while effectively alleviating the manganese dissolution (<170 ppm) problem caused by the Jahn-Teller effect.

[0226] In summary, the present application successfully prepared a low-sulfur metal cation-doped ammonium manganese iron phosphate precursor. By adding an appropriate amount of phosphoric acid source to the prepared metal cation-doped ammonium manganese iron phosphate precursor, not only the sulfur content of the precursor was significantly reduced, but also the molar ratio of manganese, iron, doped elements to phosphorus was slightly less than 1, which not only realized the atomic-level distribution of elements in the precursor, but also improved the tap density, which was conducive to the subsequent preparation of high-performance carbon-coated modified phosphate pyrophosphate manganese iron sodium material. The sodium ion battery based on this material has excellent charge and discharge capacity, cycle stability and rate performance.

[0227] The preferred embodiments of the present application are described above with reference to the drawings, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications, and the like, which are not deviated from the spirit and principles of the present application, are equivalent replacement modes, and are included in the protection scope of the present application.

Claims

1. A method for preparing a low-sulfur type metal cation-doped ammonium manganese iron phosphate precursor, characterized in that, The method comprises the following steps: S1. Preparing a metal cation doped manganese iron ammonium phosphate precursor by using manganese sulfate to provide manganese elements; S2. Mixing the metal cation doped manganese iron ammonium phosphate precursor obtained in step S1 with a phosphoric acid source solution, keeping the pH of the solution at 7-8, and fully reacting at 30-80 ℃, filtering, washing and drying the precipitate to obtain a low-sulfur metal cation doped manganese iron ammonium phosphate precursor; The metal cation is at least one selected from the group consisting of Ca, Mg, Al, Ti, Ni, Cr, Zn, Mo, Cu, Zr, Nb, Y, Sn, Co, Ce, V and W, but not mutually repeated; The low-sulfur type metal cation-doped ammonium manganese iron phosphate precursor has an expression of NH4Mn x Fe 1-x-y M y PO4, 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 In step S2, the phosphoric acid source comprises one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, ammonium phosphate, lithium phosphate, sodium phosphate, ammonium hypophosphite, ammonium polyphosphate, sodium phosphite, sodium metaphosphate and sodium tripolyphosphate; In step S2, the addition amount of the phosphoric acid source is at least 0.1 times the amount of substance of the metal cation doped manganese iron ammonium phosphate precursor; The steps S1-S2 are all carried out in a protective gas atmosphere; In step S1, the preparation method of the metal cation doped manganese iron ammonium phosphate precursor specifically comprises the following steps: SI. Preparing a mixed solution I by mixing manganese sulfate and a metal cation doping salt M solution in a protective gas atmosphere; SII. Fully reacting iron single element and a phosphoric acid source solution in a protective gas atmosphere to obtain a ferrous dihydrogen phosphate solution system II; SIII. Adding the mixed solution I obtained in step SI and an oxidizing agent to the ferrous dihydrogen phosphate solution system II obtained in step SII in a protective gas atmosphere, mixing, fully reacting at 50-80 ℃, oxidizing the divalent manganese ions and the divalent iron ions into trivalent manganese ions and trivalent iron ions respectively, adjusting the pH of the intermediate system III to 7-8 with ammonia water, and then reacting at 50-80 ℃ to obtain the metal cation doped manganese iron ammonium phosphate precursor after post-treatment; In step SII, the phosphoric acid source comprises one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, lithium phosphate, sodium phosphate, ammonium hypophosphite, ammonium polyphosphate, sodium phosphite, sodium metaphosphate and sodium tripolyphosphate; In step SII, the molar ratio of the iron single element to the phosphoric acid source is 1: (1-3).

2. The low-sulfur metal cation doped manganese iron ammonium phosphate precursor prepared by the preparation method of claim 1.

3. A method for preparing carbon-coated modified sodium manganese iron pyrophosphates, characterized by, The method comprises the following steps: Si. Fully mixing sodium salt, a phosphoric acid source, the low-sulfur metal cation doped manganese iron ammonium phosphate precursor of claim 2, a carbon source and an alcohol solution to obtain a slurry, grinding the slurry and drying to obtain a powder; Sii. Pre-calcining the powder obtained in step Si at 300-400 ℃ for 0.5-8 h in a protective gas atmosphere, fully calcining by increasing the temperature to 450-750 ℃, and post-treating to obtain a carbon-coated modified manganese iron sodium pyrophosphate phosphate.

4. The preparation method according to claim 3, characterized in that, In step S1, the molar ratio of the phosphoric acid source, the sodium phosphate salt, and the low-sulfur metal cation-doped ammonium manganese iron phosphate precursor of claim 3 is 1:(1.3-1.4):(1.3-1.4).

5. The preparation method according to claim 3, characterized in that, In step S1, the carbon source includes one or more of polyethylene glycol, glucose, fructose, sucrose, starch, cyclodextrin, citric acid, tartaric acid, ascorbic acid, malic acid, lactic acid, polyvinylpyrrolidone, polyvinylidene fluoride, epoxy resin, and phenolic resin.

6. The preparation method according to claim 3, characterized in that, In step S1, the mass ratio of the low-sulfur metal cation-doped ammonium manganese iron phosphate precursor and the carbon source is 1:(0.01-1).

7. The carbon-coated modified sodium manganese iron pyrophosphate phosphate prepared by the preparation method of any one of claims 3-6.

8. A sodium-ion battery cathode, characterized in that, The sodium ion battery of claim 8 includes an electrolyte, a separator, a positive electrode, and a negative electrode, wherein the positive electrode includes the carbon-coated modified sodium manganese iron pyrophosphate phosphate of claim 7.

9. A sodium-ion battery, characterized in that, The sodium ion battery of claim 8 includes an electrolyte, a separator, a positive electrode, and a negative electrode, wherein the positive electrode includes the carbon-coated modified sodium manganese iron pyrophosphate phosphate of claim 7.

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