Carbon-coated mixed phosphate cathode material, preparation method and application thereof

Abstract

The application provides a carbon-coated mixed phosphate positive electrode material, a preparation method and application thereof. The preparation method comprises the following steps: preparing an acid solution, dissolving a sodium source, a phosphorus source, a carbon source and a metal source in water, and then adding the metal source into the acid solution to obtain a mixed solution; wherein the metal elements in the metal source include Fe, Mn and M elements, and the M elements are one or more of Ni, Al, Ti, V, Mg or Zn; drying the mixed solution to obtain a precursor; and performing sectional heat treatment on the precursor under an inert atmosphere to obtain the carbon-coated mixed phosphate positive electrode material; and the molecular formula of the mixed phosphate is Na4Fe x Mn y M z (PO4)2P2O7, wherein x+y+z=3, and x, y and z are not all 0. The obtained carbon-coated mixed phosphate positive electrode material has high crystallinity, no impurity phases such as NaMPO4 and Na2MP2O7, and the phase purity and electrochemical performance are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, and more specifically, to a carbon-coated mixed phosphate cathode material, its preparation method, and its application. Background Technology

[0002] In recent years, renewable clean energy sources such as solar, wind, tidal, and geothermal energy have developed rapidly. However, these clean energy sources are intermittent and cannot provide continuous power. Therefore, to fully utilize these renewable energy sources, it is necessary to develop matching energy storage systems. Lithium-ion batteries dominate the energy storage market due to their high energy density and mature technology. However, the shortage and uneven distribution of lithium resources have led to a significant increase in the price of lithium carbonate, and the resulting high cost limits its application in large-scale energy storage systems. Sodium-ion batteries, on the other hand, have a similar working mechanism to lithium-ion batteries, and sodium metal is widely distributed globally and inexpensive. Therefore, sodium-ion batteries are considered the most promising alternative to lithium-ion batteries in the energy storage field.

[0003] Mixed phosphate Na4M3(PO4)2P2O7 (M = Fe, Mn, Ni, Co) is a sodium-ion battery cathode material with a polyanionic structure, combining the advantages of phosphate and pyrophosphate. In applications, it exhibits high redox potential and excellent cycle stability. The theoretical reversible specific capacity of the mixed phosphate Na4Fe3(PO4)2P2O7 cathode material is 129 mAh g. -1 The average operating voltage is 3.1V (vs. Na). + Sodium ion battery cathode material (Na) has excellent electrochemical sodium storage performance and low raw material cost, making it a promising material for sodium-ion batteries.

[0004] However, the mixed phosphate Na4Fe3(PO4)2P2O7 cathode material has three main problems: First, the material has low electronic conductivity. Currently, researchers often improve its electronic conductivity by combining Na4Fe3(PO4)2P2O7 with carbon materials, which has achieved significant results. Second, the sodium ion diffusion coefficient is low. To shorten the ion diffusion path, the material needs to be nano-sized. Currently, nanomaterials are mainly obtained by controlling the particle size of raw materials and heat treatment processes. Third, impurity phases such as NaFePO4 and Na2FeP2O7 are easily generated during the preparation process, which seriously affects the reversible specific capacity and cycle stability of the material. To address this problem, Professor Cao Yuliang's research group designed an iron-phosphorus ratio and introduced an iron defect regulation strategy to prepare a cathode material Na4Fe3(PO4)2P2O7 with good electrochemical performance. 2.91(PO4)2P2O7. However, most reported methods for preparing Na4Fe3(PO4)2P2O7 use Fe(NO3)3·9H2O and FePO4 as iron sources. The Na4Fe3(PO4)2P2O7 products prepared from these raw materials inevitably contain impurity phases such as NaFePO4 and Na2FeP2O7. The main reasons for this are: Fe(NO3)3·9H2O easily decomposes into FeO at low temperatures, and FeO readily reacts with NaH2PO4 to form the NaFePO4 impurity phase; FePO4 readily reacts with NaH2PO4 to form NaFePO4 and Na2FeP2O7 impurity phases; furthermore, the introduction of crystallization water from the raw materials during sintering and the insufficient control and optimization of high and low temperature heat treatment processes also contribute to this problem.

[0005] Therefore, there is an urgent need in the field to provide a simple and efficient preparation method to solve the problem that the phase purity of the mixed phosphate cathode material obtained by the existing preparation method is low and cannot meet the requirements of sodium-ion battery applications. Summary of the Invention

[0006] The main objective of this invention is to provide a carbon-coated mixed phosphate cathode material, its preparation method, and its application, in order to solve the problem that the phase purity of the mixed phosphate cathode material obtained by the existing preparation method is low and cannot meet the requirements of sodium-ion battery applications.

[0007] To achieve the above objectives, the present invention provides a method for preparing a carbon-coated mixed phosphate cathode material, comprising: step S1, preparing an acid solution, and dissolving a sodium source, a phosphorus source, a carbon source, and a metal source in water and adding the solution to the acid solution to obtain a mixed solution; wherein the metal source includes Fe, Mn, and M elements, and the M element is one or more of Ni, Al, Ti, V, Mg, or Zn; step S2, drying the mixed solution to obtain a precursor; and step S3, subjecting the precursor to segmented heat treatment under an inert atmosphere to obtain a carbon-coated mixed phosphate, i.e., a carbon-coated mixed phosphate cathode material; wherein the molecular formula of the mixed phosphate in the carbon-coated mixed phosphate cathode material is Na4Fe. x Mn y M z (PO4)2P2O7, where x+y+z=3, and x, y and z are all not 0.

[0008] Further, the element M is Mg, Al or Zn; preferably, 0 < x < 3, 0 < y < 3, 0 < z ≤ 0.2.

[0009] Further, in step S1, the acid solution is a mixed solution of organic acid and inorganic acid, wherein the organic acid is selected from one or more of citric acid monohydrate, oxalic acid, formic acid, acetic acid, ascorbic acid, tartaric acid and malic acid, and the inorganic acid is selected from one or more of concentrated nitric acid, hydrochloric acid and sulfuric acid; preferably, the acid solution is a mixed solution of citric acid monohydrate and concentrated nitric acid; more preferably, the molar ratio of citric acid monohydrate to HNO3 in concentrated nitric acid in the acid solution is 1:(2.19~2.40).

[0010] Further, in step S1, the carbon source is selected from one or more of glucose, sucrose, chitosan, citric acid monohydrate, carbon nanotubes, graphene, carbon black, mesoporous carbon, soluble starch, corn dextrin, methylcellulose, phenolic resin, polypropylene, polyacrylonitrile, polyethylene, and polyvinyl alcohol; preferably, the carbon source is a mixture of glucose and citric acid monohydrate; more preferably, the molar ratio of glucose to citric acid monohydrate in the carbon source is 1:(1.8-2.2).

[0011] Further, in step S1, the total acid mass concentration of the acid solution is 13-17%; the volume ratio of the acid solution to water is (0.2-0.35):1; the total mass ratio of the sodium source, phosphorus source, carbon source and metal source to water is (0.2-0.3):1; preferably, the molar ratio of the carbon source and the metal source is (1.1-1.2):1.

[0012] Further, in step S3, the segmented heat treatment includes: a first stage heat treatment at a temperature of 250–350°C, a heating rate of 1–2°C / min, and a holding time of 5–10 h; and a second stage heat treatment at a temperature of 500–700°C, a heating rate of 5–10°C / min, and a holding time of 10–20 h.

[0013] Furthermore, in step S1, the dissolution standard for the metal source is a solid particle size D50 of 0.2–0.5 μm.

[0014] Furthermore, the metal source is added in the form of one or more of nitrates, phosphates, sulfates, acetates, chlorides, oxides, and hydroxides; and / or the sodium source is selected from one or more of sodium carbonate, sodium hydroxide, sodium nitrate, sodium chloride, sodium citrate, sodium oxalate, sodium acetate, sodium pyrophosphate, trisodium phosphate, sodium dihydrogen phosphate, and sodium monohydrogen phosphate; and / or the phosphorus source is selected from one or more of sodium pyrophosphate, trisodium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid.

[0015] Furthermore, the drying method in step S2 is selected from blower drying, flash drying or spray drying; preferably, the drying method is spray drying; more preferably, the inlet air temperature of spray drying is 140-250°C and the outlet air temperature is 80-120°C.

[0016] Another aspect of the present invention provides a carbon-coated mixed phosphate cathode material, which is prepared by the above-described preparation method.

[0017] Another aspect of the present invention provides a sodium-ion battery including a positive electrode sheet, the positive electrode sheet being a carbon-coated mixed phosphate positive electrode material.

[0018] Compared with other preparation methods, the carbon-coated mixed phosphate cathode material prepared by applying the technical solution of this invention has higher crystallinity and is free of impurity phases such as NaMPO4 and Na2MP2O7, thus significantly improving its phase purity. When the carbon-coated mixed phosphate cathode material prepared by this invention is applied to sodium-ion batteries, the battery cycle performance can be significantly improved. Attached Figure Description

[0019] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0020] Figure 1 Carbon-coated Na4Mn 1.5 Fe 1.4 Al 0.1 X-ray diffraction pattern of (PO4)2P2O7 cathode material;

[0021] Figure 2 Carbon-coated Na4Mn 1.5 Fe 1.4 Al 0.1 Scanning electron microscope image of (PO4)2P2O7 cathode material;

[0022] Figure 3 Carbon-coated Na4Mn 1.5 Fe 1.4 Mg 0.1 X-ray diffraction pattern of (PO4)2P2O7 cathode material;

[0023] Figure 4 Carbon-coated Na4Mn 1.5 Fe 1.4 Mg 0.1 Scanning electron microscope image of (PO4)2P2O7 cathode material;

[0024] Figure 5 Carbon-coated Na4Mn 1.5 Fe 1.4 Zn 0.1 X-ray diffraction pattern of (PO4)2P2O7 cathode material;

[0025] Figure 6Carbon-coated Na4Mn 1.5 Fe 1.4 Zn 0.1 Scanning electron microscope image of (PO4)2P2O7 cathode material;

[0026] Figure 7 Carbon-coated Na4Mn 1.5 Fe 1.4 Mg 0.05 Zn 0.05 X-ray diffraction pattern of (PO4)2P2O7 cathode material;

[0027] Figure 8 Carbon-coated Na4Mn 1.5 Fe 1.4 Mg 0.05 Zn 0.05 Scanning electron microscope image of (PO4)2P2O7 cathode material. Detailed Implementation

[0028] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0029] As described in the background section, existing methods for preparing mixed phosphate cathode materials suffer from low phase purity, failing to meet the requirements for sodium-ion battery applications. To address these issues, this application provides a method for preparing a carbon-coated mixed phosphate cathode material, comprising: step S1, preparing an acid solution, and dissolving a sodium source, phosphorus source, carbon source, and metal source in water before adding the solution to obtain a mixed solution; wherein the metal source includes Fe, Mn, and M elements, and M element is one or more of Ni, Al, Ti, V, Mg, or Zn; step S2, drying the mixed solution to obtain a precursor; and step S3, subjecting the precursor to segmented heat treatment under an inert atmosphere to obtain a carbon-coated mixed phosphate, i.e., a carbon-coated mixed phosphate cathode material; wherein the molecular formula of the mixed phosphate in the carbon-coated mixed phosphate cathode material is Na4Fe. x Mn y M z (PO4)2P2O7, where x+y+z=3, and x, y and z are all not 0.

[0030] The carbon-coated mixed phosphate cathode material prepared using the method provided by this invention exhibits high crystallinity and is free of impurity phases such as NaMPO4 and Na2MP2O7, resulting in significantly improved phase purity and enhanced electrochemical performance. Specifically, preparing the precursor in an acidic solution prevents the formation of metal phosphate precipitates, improves the mixing uniformity of elements such as Na, P, Fe, Mn, and M, thereby increasing the phase purity of the mixed phosphate in the carbon-coated mixed phosphate cathode material and avoiding the formation of impurity phases such as NaMPO4 and Na2MP2O7. Furthermore, the carbon-coated Na4Fe prepared under these acidic conditions... x Mn y M z (PO4)2P2O7 mixed phosphate materials have wider applicability. Meanwhile, the metal elements introduced into the mixed solution used in this application to prepare the precursor include Fe, Mn, and M, and these elements, along with the sodium and phosphorus sources, satisfy the Na4Fe... x Mn y M z The formula (PO4)2P2O7, where x+y+z=3 and x, y, and z are all non-zero, requires a specific quantitative requirement. Firstly, by simultaneously doping Fe, Mn, and one or more of Ni, Al, Ti, V, Mg, or Zn into the carbon-coated mixed phosphate cathode material according to the above quantitative requirements, the electronic conductivity of the cathode material can be improved more effectively compared to doping only Fe and Mn. In particular, controlling the M element doping amount within the above range is beneficial for further improving its electrochemical performance. Furthermore, this invention utilizes acid solution dissolution, drying, and segmented heat treatment to ensure that the above doping elements are more uniformly incorporated into the mixed phosphate cathode material, while the carbon coating is more uniform. This has a better promoting effect on ensuring the electrochemical performance of the material, thus providing higher cycle stability when applied to the cathode of sodium-ion batteries.

[0031] In actual production, the inert atmosphere used in this invention can be of a type commonly used in the art, such as nitrogen. The molecular formula of the mixed phosphate is Na₄Fe₂O₃. x Mn y M z (PO4)2P2O7. Technicians can determine the relationship between the amounts of sodium, phosphorus, carbon and metal sources added during the preparation process, which will not be elaborated here.

[0032] More specifically, in the preparation method provided by the present invention, the preferred experimental steps for obtaining the mixed solution in step S1 are as follows: (1) adding the metal source to deionized water to form solution A; (2) dissolving the acid in deionized water to form solution B (acid solution); (3) dissolving the sodium source, phosphorus source, and carbon source in deionized water to form solution C; (4) adding solution C to solution B to form solution D; (5) adding solution A to solution D to obtain the desired solution E, i.e., the mixed solution. Adding the above materials in this order is intended to reduce the probability of insoluble metal phosphate formation, thereby improving the phase purity of the obtained mixed phosphate. Furthermore, the inventors have discovered through extensive experiments that mixing the solution containing the sodium source, phosphorus source, and carbon source with the acid solution before the solution containing the metal source further enhances the mixing uniformity of the elements in the system, thereby minimizing the formation of impurity phases and improving the cycle stability of the obtained material.

[0033] In a preferred embodiment, element M is Mg, Al, or Zn. Due to the special electronic structure of these three metal elements, they can have a better synergistic effect with Fe and Mn metals, thereby significantly optimizing the electrochemical performance of the resulting mixed phosphate. On this basis, it is preferred that 0 < x < 3, 0 < y < 3, and 0 < z ≤ 0.2, limiting the metal components in the mixed phosphate except for Na to this ratio, can give full play to the advantages of each metal, thereby obtaining a mixed phosphate component with superior microstructure and electrochemical performance.

[0034] It is worth mentioning that, through extensive experimentation, the inventors creatively obtained a mixed phosphate, Na4Fe, in which M is simultaneously a two-metal compound. x Mn y M1 0.05 M2 .0.05 (PO4)2P2O7, where M1 is Al and M2 is Mg, the resulting mixed phosphate has the molecular formula Na4Fe. x Mn y Al 0.05 Mg .0.05 (PO4)2P2O7. This compound, due to the presence of Al... 3+ and Mg 2+ The presence of Mn can inhibit 3+ The Ginger-Tyler effect occurs, hindering the disproportionation reaction of manganese and slowing down the conversion of manganese to Mn. 2+ It dissolves in the electrolyte in a form that improves the crystal structure stability of the material during charge and discharge, thus exhibiting more outstanding electrochemical cycle stability. When applied as a cathode material for sodium-ion batteries, it can significantly improve the capacity and cycle stability of sodium-ion batteries.

[0035] In a typical embodiment, the acid solution in step S1 is a mixed solution of organic and inorganic acids. Mixing the organic and inorganic acids allows the stronger inorganic acid to compensate for the insufficient acidity of the organic acid, resulting in a lower pH value in the system. This improves experimental results while simultaneously dissolving and etching the solid components, reducing their particle size, and facilitating the nanostructuring of the material to achieve superior structural properties. The organic acid can be selected from commonly used types in the art, including but not limited to one or more selected from citric acid monohydrate, oxalic acid, formic acid, acetic acid, ascorbic acid, tartaric acid, and malic acid. Similarly, the inorganic acid can be selected from commonly used types in the art, including but not limited to one or more selected from concentrated nitric acid, hydrochloric acid, and sulfuric acid. Since concentrated nitric acid is an oxidizing acid and can provide a large amount of H₂... + Therefore, in a preferred embodiment, the acid solution is a mixed solution of citric acid monohydrate and concentrated nitric acid, where concentrated nitric acid refers to a nitric acid solution with a mass concentration of 65%. Furthermore, in order to set the pH within a range more conducive to the dissolution of solid components, achieve better acid dissolution, and reduce the risk of changes in the valence state of each metal element due to oxidation, thereby obtaining a mixed phosphate with a more uniform structure, more preferably, the molar ratio of citric acid monohydrate to HNO3 in the concentrated nitric acid solution is 1:(2.19–2.40). The mixed acid solution prepared with this molar ratio can maintain a clear state for a longer period, thereby improving the stability and uniformity of the system, which is beneficial for obtaining a mixed phosphate with a superior structure and enhancing the long-cycle stability of the subsequently obtained material during battery applications.

[0036] Furthermore, the carbon source in step S1 can be one or more commonly used in the art, including but not limited to glucose, sucrose, chitosan, citric acid monohydrate, carbon nanotubes, graphene, carbon black, mesoporous carbon, soluble starch, corn dextrin, methylcellulose, phenolic resin, polypropylene, polyacrylonitrile, polyethylene, and polyvinyl alcohol. Through extensive experimentation, the inventors discovered that in a preferred embodiment, selecting a mixture of glucose and citric acid monohydrate as the carbon source results in a carbon-coated mixed phosphate cathode material with a more uniform carbon coating morphology and a more suitable thickness. This allows the sodium-ion battery to exhibit a larger specific capacity and superior long-cycle stability when used as a cathode material. More preferably, the molar ratio of glucose to citric acid monohydrate in the carbon source is 1:(1.8–2.2). Selecting this molar ratio within this range allows for a more complete utilization of the synergistic effect of glucose and citric acid monohydrate, as well as the carbonization temperature during subsequent heat treatment, resulting in a cathode material with a more uniform coating layer.

[0037] In a preferred embodiment, in step S1, the total acid mass concentration of the acid solution is 13-17%. This concentration provides a pH value more favorable for system homogeneity and reaction, while also better protecting Mn ions from precipitating as MnPO4. To improve dispersion, the volume ratio of acid solution to water is chosen to be (0.2-0.35):1. To ensure better dissolution of the sodium, phosphorus, carbon, and metal sources, the weight ratio of the total mass of these solid components before dissolution to water is chosen to be (0.2-0.3):1. To improve carbon coating and obtain a more complete and uniform cathode material, the molar ratio of carbon source to metal source is preferably (1.1-1.2):1. Choosing this molar ratio also helps the citric acid monohydrate in the carbon source to protect the metal in its internal mixed phosphate from oxidation during subsequent heat treatment, resulting in a more stable structure, higher phase purity, and higher microscopic uniformity in the carbon-coated Na4Fe. x Mn y M z (PO4)2P2O7 material (where x+y+z=3, and x, y and z are all not 0).

[0038] Furthermore, in order to improve the overall effect of heat treatment, in step S3, the segmented heat treatment includes: a first stage heat treatment at a temperature of 250-350℃, a heating rate of 1-2℃ / min, and a holding time of 5-10h; and a second stage heat treatment at a temperature of 500-700℃, a heating rate of 5-10℃ / min, and a holding time of 10-20h. By adopting the segmented heat treatment process with the above parameters, carbon-coated mixed phosphate materials with better crystallinity and phase purity can be obtained. In the first stage of heat treatment, the carbonization temperature of the mixed carbon source selected in this invention is chosen to be 250–350°C, with a relatively slow heating rate of 1–2°C / min. Compared to heating rates higher than 2°C / min, this allows for more complete carbonization and more effectively prevents the oxidation of metal ions in the precursor, resulting in a better structure. A longer holding time of 5–10 hours also promotes complete carbonization, thus obtaining a cathode material with a complete coating layer. In the second stage of heat treatment, a faster heating rate of 5–10°C / min is chosen. Compared to heating rates lower than 5°C / min, this rate reduces thermal diffusion and allows the set heat treatment temperature to be reached more quickly, thereby improving phase purity and structural uniformity. Furthermore, the higher heat treatment temperature of 500–700°C ensures that the various metal ions selected in this invention, especially Al, are properly heated. 3+ It can react better with PO4 during solid-state reactions. 3-The combined effect of a longer holding time of 10–20 h and a higher heat treatment time of 500–700 °C can improve the crystallinity of the resulting carbon-coated phosphate cathode material, thereby enhancing the long-cycle stability of the final sodium-ion battery.

[0039] In order to enable the metal source to dissolve better and provide a richer supply of metal ions to the system, in a preferred embodiment, the dissolution standard of the metal source in step S1 is a solid particle size D50 of 0.2 to 0.5 μm. Compared with larger particle sizes of 0.5 μm and above, this particle size range allows metals that are not completely dissolved in deionized water to dissolve more quickly after entering the acid solution and enter the system in the form of metal ions. This improves the dispersion uniformity of each element in the entire system and is beneficial to improving the structural uniformity of the mixed phosphate components in the final carbon-coated mixed phosphate cathode material.

[0040] In several typical embodiments, the metal source is added in the form of one or more of nitrates, phosphates, sulfates, acetates, chlorides, oxides, and hydroxides; and / or the sodium source is selected from those commonly used in the art, including but not limited to one or more of sodium carbonate, sodium hydroxide, sodium nitrate, sodium chloride, sodium citrate, sodium oxalate, sodium acetate, sodium pyrophosphate, trisodium phosphate, sodium dihydrogen phosphate, and sodium monohydrogen phosphate; and / or the phosphorus source is also selected from those commonly used in the art, including but not limited to one or more of sodium pyrophosphate, trisodium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid.

[0041] Furthermore, the drying method in step S2 is selected from commonly used methods in the art, such as forced-air drying, flash drying, or spray drying. In a typical embodiment, the drying method is spray drying, and more preferably, the inlet air temperature is 140-250°C and the outlet air temperature is 80-120°C, so as to achieve the drying of the precursor in a faster time, thereby saving costs and obtaining a precursor with better drying properties, which is beneficial to obtaining a carbon-coated mixed phosphate material with better uniformity in subsequent segmented heat treatment.

[0042] Another aspect of the present invention provides a carbon-coated mixed phosphate cathode material, which is prepared by the above-described preparation method. The obtained cathode material has high crystallinity and is free of impurity phases such as NaMPO4 and Na2MP2O7, i.e., high phase purity; at the same time, it has uniform particle size and stable structure, and can provide a number of excellent electrochemical properties, including specific capacity and long-cycle stability, when used as a cathode in sodium-ion batteries.

[0043] Another aspect of the present invention provides a sodium-ion battery, including a positive electrode sheet, and the positive electrode sheet is the above carbon-coated mixed phosphate positive electrode material. The obtained sodium-ion battery has both good sodium storage performance and excellent long-cycle stability, and can be well applied to various electrochemical scenarios.

[0044] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0045] Unless otherwise defined, all the technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0046] Example 1

[0047] Preparation of a carbon-coated mixed phosphate positive electrode material:

[0048] Add 36.7635 g of Mn(CH3COO)2·4H2O, 56.56 g of Fe(NO3)3·9H2O and 0.78 g of Al(OH)3 into 200 mL of deionized water to form solution A (before adding each metal salt into deionized water, first ball mill it with a ball mill for 8 h until 0.2 μm < D50 < 0.5 μm, and wash the ball mill tank with deionized water 3 - 5 times, and transfer all the washing solutions to the beaker); add 63.042 g of citric acid monohydrate and 50 mL of concentrated nitric acid (mass concentration is 65%) into 200 mL of deionized water to form solution B; add 62.404 g of NaH2PO4·2H2O, 21.6 g of glucose and 50.4 g of citric acid monohydrate into 200 mL of deionized water to form solution C; add solution C into solution B to form solution D; add solution A into solution D, and adjust the solution to 900 mL with deionized water to form a uniform light yellow transparent clear solution E, in which the solid particle size D50 is 0.3 - 0.5 μm. Spray dry the above transparent solution, with an inlet air temperature of 220 °C and an outlet air temperature of 110 °C, to obtain a powdery and dry light yellow precursor. Heat the precursor in a nitrogen atmosphere at a heating rate of 1 °C / min to 300 °C, hold for 5 h, then heat at a heating rate of 5 °C / min to 650 °C, hold for 12 h, and after cooling to room temperature, obtain carbon-coated Na4Mn 1.5 Fe 1.4 Al 0.1 (PO4)2P2O7 positive electrode material.

[0049] Figure 1 For the obtained carbon-coated Na4Mn 1.5 Fe 1.4 Al0.1 The X-ray diffraction pattern of the (PO4)2P2O7 cathode material shows that the product has high phase purity and crystallinity, and no impurity phases such as NaFePO4, Na2FeP2O7, NaMnPO4, and NaMnP2O7.

[0050] Figure 2 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Al 0.1 Scanning electron microscope image of (PO4)2P2O7 cathode material, showing that it consists of near-spherical particles of varying sizes from 1 to 5 μm, with smooth surfaces and no tiny pores.

[0051] Example 2

[0052] Preparation of a carbon-coated mixed phosphate cathode material:

[0053] The only difference between this example and Example 1 is that 0.78g of Al(OH)3 was not added to solution A, but 1.4832g of Mg(NO3)2 was added instead.

[0054] Figure 3 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Mg 0.1 The X-ray diffraction pattern of the (PO4)2P2O7 cathode material shows that the product has high phase purity and crystallinity, and no impurity phases such as NaFePO4, Na2FeP2O7, NaMnPO4, and NaMnP2O7.

[0055] Figure 4 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Mg 0.1 Scanning electron microscope image of (PO4)2P2O7 cathode material, showing that it is an irregularly shaped thin sheet with pores on the surface and varying sizes.

[0056] Example 3

[0057] Preparation of a carbon-coated mixed phosphate cathode material:

[0058] The only difference between this example and Example 1 is that 0.78g of Al(OH)3 was not added to solution A, but 1.894g of Zn(NO3)2 was added instead.

[0059] Figure 5 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Zn 0.1The X-ray diffraction pattern of the (PO4)2P2O7 cathode material shows that the product has high phase purity and crystallinity, and no impurity phases such as NaFePO4, Na2FeP2O7, NaMnPO4, and NaMnP2O7.

[0060] Figure 6 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Zn 0.1 Scanning electron microscope image of (PO4)2P2O7 cathode material, showing that it is an irregularly shaped thin sheet with tiny pores on the surface, ranging in size from 1 to 5 μm.

[0061] Example 4

[0062] Preparation of a carbon-coated mixed phosphate cathode material:

[0063] The only difference between this example and Example 1 is that 0.78g of Al(OH)3 was not added to solution A, but 0.947g of Zn(NO3)2 and 0.7416g of Mg(NO3)2 were added instead.

[0064] Figure 7 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Mg 0.05 Zn 0.05 The X-ray diffraction pattern of the (PO4)2P2O7 cathode material shows that the product has high phase purity and crystallinity, and no impurity phases such as NaFePO4, Na2FeP2O7, NaMnPO4, and NaMnP2O7.

[0065] Figure 8 The resulting carbon is coated with Na4Mn 1.5 Fe 1.4 Mg 0.05 Zn 0.05 Scanning electron microscope image of (PO4)2P2O7 cathode material, showing that it is an irregularly shaped thin sheet with pores of varying sizes on the surface, and the particle size is 1-10 μm.

[0066] Example 5

[0067] Preparation of a carbon-coated mixed phosphate cathode material:

[0068] The only difference between this example and Example 1 is that no 50 mL of concentrated nitric acid was added to solution B.

[0069] Example 6

[0070] Preparation of a carbon-coated mixed phosphate cathode material:

[0071] The only difference between this embodiment and Example 1 is that 63.042g of citric acid monohydrate and 22.83mL of concentrated nitric acid (mass concentration of 65%) are added to 200mL of deionized water to form solution B, that is, the molar ratio of citric acid to HNO3 in concentrated nitric acid is 1:1.

[0072] Example 7

[0073] Preparation of a carbon-coated mixed phosphate cathode material:

[0074] The only difference between this embodiment and Example 1 is that 63.042g of citric acid monohydrate and 68.49mL of concentrated nitric acid (mass concentration of 65%) are added to 200mL of deionized water to form solution B, that is, the molar ratio of citric acid to HNO3 in concentrated nitric acid is 1:3.

[0075] Example 8

[0076] Preparation of a carbon-coated mixed phosphate cathode material:

[0077] The only difference between this embodiment and Example 1 is that glucose was not added to solution C, and was replaced with an equimolar amount (25.2 g) of citric acid monohydrate.

[0078] Example 9

[0079] Preparation of a carbon-coated mixed phosphate cathode material:

[0080] The only difference between this embodiment and Example 1 is that 62.404g NaH2PO4·2H2O, 31.5g glucose and 36.75g citric acid monohydrate are added to 200mL of deionized water to form solution C, that is, the molar ratio of glucose to citric acid monohydrate is 1:1.

[0081] Example 10

[0082] Preparation of a carbon-coated mixed phosphate cathode material:

[0083] The only difference between this embodiment and Example 1 is that 62.404g NaH2PO4·2H2O, 15.75g glucose and 55.125g citric acid monohydrate are added to 200mL of deionized water to form solution C, that is, the molar ratio of glucose to citric acid monohydrate is 1:3.

[0084] Example 11

[0085] Preparation of a carbon-coated mixed phosphate cathode material:

[0086] The only difference between this embodiment and Example 1 is that 62.404g NaH2PO4·2H2O, 18.00g glucose and 42.00g citric acid monohydrate are added to 200mL of deionized water to form solution C, that is, the molar ratio of carbon source to metal source is 1:1.

[0087] Example 12

[0088] Preparation of a carbon-coated mixed phosphate cathode material:

[0089] The only difference between this embodiment and Example 1 is that 62.404g NaH2PO4·2H2O, 36.00g glucose and 84.00g citric acid monohydrate are added to 200mL of deionized water to form solution C, that is, the molar ratio of carbon source to metal source is 2:1.

[0090] Example 13

[0091] Preparation of a carbon-coated mixed phosphate cathode material:

[0092] The only difference between this embodiment and Embodiment 1 is that the precursor is heated to 300°C in a nitrogen atmosphere at a heating rate of 5°C / min and held at that temperature for 3 hours, and then heated to 650°C at a heating rate of 5°C / min and held at that temperature for 12 hours. That is, in the segmented heat treatment, the heating rate of the first heating process is increased and the holding time is shortened.

[0093] Example 14

[0094] Preparation of a carbon-coated mixed phosphate cathode material:

[0095] The only difference between this embodiment and Embodiment 1 is that the precursor is heated to 300°C in a nitrogen atmosphere at a heating rate of 1°C / min and held at that temperature for 5 hours, and then heated to 650°C at a heating rate of 2°C / min and held at that temperature for 10 hours. That is, in the segmented heat treatment, the heating rate of the first heating process is increased and the holding time is shortened.

[0096] Example 15

[0097] Preparation of a carbon-coated mixed phosphate cathode material:

[0098] The only difference between this embodiment and Example 1 is that the solid particle size D50 contained in solution E is 0.1 to 0.2 μm.

[0099] Example 16

[0100] Preparation of a carbon-coated mixed phosphate cathode material:

[0101] The only difference between this embodiment and Embodiment 1 is that the weight of each metal source is adjusted so that the molecular formula of the mixed phosphate in the final carbon-coated mixed phosphate cathode material is Na4Mn. 0.9 Fe 2.0 Al 0.1 (PO4)2P2O7.

[0102] Example 17

[0103] Preparation of a carbon-coated mixed phosphate cathode material:

[0104] The only difference between this embodiment and Embodiment 1 is that the weight of each metal source is adjusted so that the molecular formula of the mixed phosphate in the final carbon-coated mixed phosphate cathode material is Na4Mn. 2.0 Fe 0.9 Al 0.1 (PO4)2P2O7.

[0105] Comparative Example 1

[0106] Preparation of a carbon-coated mixed phosphate cathode material:

[0107] 45.2448 g of FePO4 was added to 100 mL of deionized water to form solution A; 42.028 g of citric acid monohydrate was added to 200 mL of deionized water to form solution B; 15.8985 g of Na2CO3, 15.601 g of NaH2PO4·2H2O, and 19.817 g of glucose monohydrate were sequentially added to 200 mL of deionized water to form solution C; solution C was added to solution B to form solution D; solution A was added to solution D, and the solution was adjusted to 900 mL with deionized water to obtain the desired solution E. The solution was dispersed using a high-speed emulsifying shear mill for 4 hours to obtain a uniformly dispersed, grayish-white, opaque suspension. The water in the above opaque suspension was instantly evaporated using a spray dryer with an inlet air temperature of 200℃ and an outlet air temperature of 110℃ to obtain a powdered, dried, light-colored precursor. The precursor was heated to 280°C in a nitrogen atmosphere furnace at a heating rate of 1°C / min and held for 5 hours. Then, it was heated to 580°C at a heating rate of 5°C / min and held for 15 hours. After cooling to room temperature, carbon-coated Na4Fe3(PO4)2P2O7 cathode material was obtained.

[0108] Comparative Example 2

[0109] Preparation of a carbon-coated mixed phosphate cathode material:

[0110] 73.527 g of Mn(CH3COO)2·4H2O was added to 200 mL of deionized water to form solution A; 50 mL of 65% concentrated nitric acid was dissolved in 200 mL of deionized water to form solution B; 26.59 g of Na4P2O7, 23.0056 g of NH4H2PO4, and 63.042 g of citric acid monohydrate were sequentially added to 200 mL of deionized water to form solution C; solution C was added to solution B to form solution D; solution A was added to solution D, and the solution was adjusted to 900 mL with deionized water to obtain the desired solution E. The above transparent solution was spray-dried at an inlet air temperature of 230℃ and an outlet air temperature of 120℃ to obtain a powdered, dried, light yellow precursor. The precursor was heated to 300℃ in a nitrogen atmosphere at a heating rate of 1℃ / min and held for 5h. Then it was heated to 700℃ at a heating rate of 5℃ / min and held for 20h. After cooling to room temperature, carbon-coated Na4Mn3(PO4)2P2O7 cathode material was obtained.

[0111] Comparative Example 3

[0112] Preparation of a carbon-coated mixed phosphate cathode material:

[0113] 22.0581 g of Mn(CH3COO)2·4H2O and 84.84 g of Fe(NO3)3·9H2O were added to 200 mL of deionized water to form solution A; 73.549 g of citric acid monohydrate was added to 200 mL of deionized water to form solution B; 62.404 g of NaH2PO4·2H2O was added to 200 mL of deionized water to form solution C; solution C was added to solution B to form solution D; solution A was added to solution D, and the solution was adjusted to 800 mL with deionized water to form a uniform, pale yellow, transparent, and clear solution E. The above transparent solution was spray-dried at an inlet air temperature of 220℃ and an outlet air temperature of 110℃ to obtain a powdered, dried, pale yellow precursor. The precursor was heated to 300℃ in a nitrogen atmosphere at a heating rate of 1℃ / min and held at that temperature for 5 h. Then, it was heated to 600℃ at a heating rate of 5℃ / min and held at that temperature for 10 h. After cooling to room temperature, carbon-coated Na4Mn was obtained. 0.9 Fe 2.1 (PO4)2P2O7 cathode material.

[0114] Comparative Example 4

[0115] Preparation of a carbon-coated mixed phosphate cathode material:

[0116] 36.7635g of Mn(CH3COO)2·4H2O and 60.6g of Fe(NO3)3·9H2O were added to 200mL of deionized water to form solution A; 73.549g of citric acid monohydrate was added to 200mL of deionized water to form solution B; 62.404g of NaH2PO4·2H2O was added to 200mL of deionized water to form solution C; solution C was added to solution B to form solution D; solution A was added to solution D, and the solution was adjusted to 800mL with deionized water to form a uniform, pale yellow, transparent, and clear solution E. The above transparent solution was spray-dried at an inlet air temperature of 220℃ and an outlet air temperature of 110℃ to obtain a powdered, dried, pale yellow precursor. The precursor was heated to 300℃ in a nitrogen atmosphere at a heating rate of 1℃ / min and held for 5 h, then heated to 650℃ at a heating rate of 5℃ / min and held for 10 h. After cooling to room temperature, carbon-coated Na4Mn was obtained. 1.5 Fe 1.5 (PO4)2P2O7 cathode material.

[0117] Performance testing:

[0118] The obtained carbon-coated mixed phosphate material, acetylene black, and PVDF were weighed in a mass ratio of 85:10:5, and a small amount of NMP was added to form a uniform slurry. The slurry was coated onto aluminum foil and then dried in a vacuum at 120°C for 12 hours. Electrode sheets were punched into φ12mm round electrodes. Using a sodium metal sheet as the counter electrode, Cellgard 2035 as the separator, and 1mol / L NaPF6EC+DMC (1:1 vol%)+5% FEC as the electrolyte, a CR2025 type button cell was assembled in a glove box. The above button cell was subjected to constant current charge-discharge cycle tests at current densities of 0.1C and 0.5C (1C = 129 mAh·g). -1 The voltage test range is 1.5–4.0V (vs. Na / Na). + ).

[0119] The electrochemical performance test results of each example and comparative example are shown in Table 1. The performance tests include the first charge-discharge specific capacity at 0.1C rate, the first discharge specific capacity at 0.5C rate, and the capacity retention rate after 100 charge-discharge cycles.

[0120] Table 1

[0121]

[0122]

[0123] As can be seen from the above description, the embodiments of the present invention exhibit higher crystallinity and are free of impurity phases such as NaMPO4 and Na2MP2O7, resulting in significantly improved phase purity and thus enhanced electrochemical performance. Example 4, in particular, involves carbon-coated Na4Fe. x Mn y Al 0.05 Mg .0.05 (PO4)2P2O7, because Al is present in it 3+ and Mg 2+ The presence of Mn can inhibit 3+ The Ginger-Tyler effect occurs, hindering the disproportionation reaction of manganese and slowing down the conversion of some manganese to Mn. 2+ The form dissolves in the electrolyte, improving the crystal structure stability of the material during charge and discharge processes, thus exhibiting more outstanding electrochemical cycle stability.

[0124] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0125] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon-coated mixed phosphate cathode material, characterized in that, The method comprises the following steps: S1, adding a metal source into deionized water to form a solution A; adding an acid into deionized water to form an acid solution B, dissolving a sodium source, a phosphorus source and a carbon source into deionized water to form a solution C; adding the solution C into the solution B to form a solution D; adding the solution A into the solution D to obtain a mixed solution; wherein the metal elements in the metal source include Fe, Mn and M elements, and the M elements are one or more of Ni, Al, Ti, V, Mg or Zn; the acid solution is a mixed solution of citric acid monohydrate and concentrated nitric acid, and the molar ratio of citric acid monohydrate to HNO3 in the acid solution is 1:(2.19-2.40); S2, drying the mixed solution to obtain a precursor; Step S3, performing a segmented heat treatment on the precursor under an inert atmosphere to obtain a carbon-coated mixed phosphate, i.e., the carbon-coated mixed phosphate positive electrode material; and the molecular formula of the mixed phosphate in the carbon-coated mixed phosphate positive electrode material is Na4Fe x Mn y M z (PO4)2P2O7, wherein x+y+z=3, and x, y, and z are not all 0.

2. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1, characterized in that, the M elements are Mg, Al or Zn.

3. The method of claim 2, wherein the carbon-coated mixed phosphate cathode material is prepared by the steps of: mixing a lithium source, a transition metal source, and a phosphate source to form a mixture; and heating the mixture at a temperature of 600-800°C for 6-24 hours in a non-oxidizing atmosphere. 0 4. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1 or 2, characterized in that, In S1, the carbon source is selected from one or more of glucose, sucrose, chitosan, citric acid monohydrate, carbon nanotubes, graphene, carbon black, mesoporous carbon, soluble starch, corn dextrin, methyl cellulose, phenolic resin, polypropylene, polyacrylonitrile, polyethylene and polyvinyl alcohol.

5. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1, characterized in that, The carbon source is a mixture of glucose and citric acid monohydrate.

6. The method of claim 5, wherein the carbon-coated mixed phosphate cathode material is prepared by the steps of: mixing a lithium source, a transition metal source, and a phosphate source to form a mixture; and heating the mixture at a temperature of 600-800°C for 6-24 hours in a non-oxidizing atmosphere. The molar ratio of the glucose to the citric acid monohydrate in the carbon source is 1:(1.8-2.2).

7. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1 or 2, characterized in that, In S1, the total acid mass concentration of the acid solution is 13-17%, the volume ratio of the acid solution to water is (0.2-0.35):1, and the weight ratio of the total mass of the sodium source, the phosphorus source, the carbon source and the metal source to water is (0.2-0.3):

1.

8. The method of claim 7, wherein the carbon-coated mixed phosphate cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphate source; and heating the mixture to a temperature of 600-800°C for 6-24 hours in a non-oxidizing atmosphere. The molar ratio of the carbon source to the metal source is (1.1-1.2):

1.

9. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1 or 2, characterized in that, In S3, the segmented heat treatment comprises: first segment heat treatment, temperature 250-350℃, heating rate 1-2℃ / min, holding time 5-10h; second segment heat treatment, temperature 500-700℃, heating rate 5-10℃ / min, holding time 10-20h.

10. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1 or 2, characterized in that, In S1, the dissolution standard of the metal source is solid particle size D50=0.2-0.5μm.

11. The preparation method of the carbon-coated mixed phosphate positive electrode material according to claim 1 or 2, characterized in that, the metal source is added in the form of one or more of nitrate, phosphate, sulfate, acetate, chloride, oxide and hydroxide; and / or, the sodium source is selected from one or more of sodium carbonate, sodium hydroxide, sodium nitrate, sodium chloride, sodium citrate, sodium oxalate, sodium acetate, sodium pyrophosphate, trisodium phosphate, sodium dihydrogen phosphate and sodium monohydrogen phosphate; and / or, the phosphorus source is selected from one or more of sodium pyrophosphate, trisodium phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium phosphate and phosphoric acid.

12. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 1 or 2, characterized in that, The drying is achieved by means of air blowing drying, flash drying or spray drying.

13. The method of claim 12, wherein the carbon-coated mixed phosphate cathode material is prepared by a process comprising: mixing a lithium source, a transition metal source, and a phosphate source to form a mixture; and heating the mixture to form the carbon-coated mixed phosphate cathode material. The drying is achieved by means of spray drying.

14. The method for preparing the carbon-coated mixed phosphate cathode material according to claim 13, characterized in that, The inlet air temperature of the spray drying is 140-250 DEG C, and the outlet air temperature is 80-120 DEG C.

15. A carbon-coated mixed phosphate cathode material, characterized in that, The carbon-coated mixed phosphate positive electrode material is prepared by the preparation method in any one of claims 1 to 14.

16. A sodium-ion battery comprising a positive electrode sheet, characterized by, The positive electrode sheet comprises the carbon-coated mixed phosphate positive electrode material in claim 15.

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