Pyrophosphate and ferric phosphate sodium composite positive electrode material, preparation method thereof, positive plate and sodium ion battery

Abstract

The invention provides a sodium ferric pyrophosphate composite positive electrode material and a preparation method thereof, a positive plate and a sodium ion battery, and relates to the technical field of batteries. The pyrophosphate and sodium ferric phosphate composite positive electrode material has a core-shell structure, the pyrophosphate and sodium ferric phosphate composite positive electrode material is equivalent to an inner core, and a carbon coating layer and a PEDOT coating layer are equivalent to an outer shell and coat the surface of the inner core; the core-shell structure can uniformly coat the sodium ferric pyrophosphate material, so that the conductivity of the material can be improved, the rate capability and the cycle performance can be improved, and side reactions can be hindered or slowed down. The preparation method of the sodium ferric phosphate pyrophosphate composite positive electrode material comprises the following steps: grinding the raw materials, carrying out spray drying to prepare a sodium ferric phosphate pyrophosphate precursor, forming a carbon coating layer on the surface of the sodium ferric phosphate pyrophosphate through a sintering process, and forming a uniform, complete and stable PEDOT coating layer on the surface of the carbon coating layer through an in-situ oxidation polymerization method. The preparation method is simple and convenient to operate and easy to implement.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and relates to a sodium iron pyrophosphate composite cathode material, a preparation method thereof, a cathode sheet, and a sodium ion battery. Background Art

[0002] In recent years, in order to meet the growing demand for periodic sustainable energy, high-performance and cost-effective large-scale electrical energy storage technologies (EESs) have made remarkable progress. Sodium ion batteries (SIBs) have shown great potential for large-scale energy storage due to their high safety, abundant sodium resources, and low cost. Most of the research on polyanion cathode materials has focused on vanadium-based and iron-based compounds. Iron-based polyanion cathode materials have become a research hotspot in the academic and industrial fields due to their environmental friendliness, low cost, and stable structure. Among them, sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 (hereinafter referred to as NFPP) has a long cycle life, excellent high and low temperature performance, strong thermal stability, and low cost, and is an ideal cathode material for large-scale energy storage applications. In the future, with the breakthrough of technology and preparation process, and the drive of energy storage demonstration projects, its industrialization is expected to accelerate. In the iron-based polyanion family, NFPP has an open framework with large tunnels, small volume change during charge and discharge, and a theoretical specific capacity as high as 129 mAh·g -1 , combining the advantages of phosphates and pyrophosphates, and is considered to be the most attractive member. However, the practical application of NFPP is still in its infancy, mainly hindered by its poor intrinsic electronic conductivity.

[0003] The native conductivity of NFPP materials is relatively low, which is a major limitation of their electrochemical performance. Due to the strong covalent bond between phosphate anions and oxygen in the crystal structure of NFPP, a stable framework structure is formed, which although helps to improve the chemical and thermal stability of the material, also limits the free migration path of electrons within the material, increases the internal resistance, and thus affects the overall electronic conductivity. The low conductivity results in poor charge transfer efficiency inside the material under high-rate charge and discharge conditions, leading to a decrease in the power density of the battery. In addition, the larger internal resistance not only exacerbates the heating of the battery, but also causes an increase in energy loss during the battery cycle, thereby shortening the service life of the battery.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] Aiming at the deficiencies and defects existing in the prior art, the present invention aims to provide a sodium iron pyrophosphate composite cathode material, a preparation method thereof, a cathode sheet, and a sodium ion battery.

[0006] To achieve the above object, the following technical solutions are adopted:

[0007] The first object of the present invention is to provide a sodium iron pyrophosphate composite cathode material, which comprises a sodium iron pyrophosphate material, and a carbon coating layer and a PEDOT coating layer which are sequentially coated on the surface of the sodium iron pyrophosphate material from the inside to the outside.

[0008] Further, on the basis of the above technical solution of the present invention, the particle size of the sodium iron pyrophosphate material is 5-15 μm;

[0009] and / or, the coating thickness of the carbon coating layer is 1-4 μm;

[0010] and / or, the coating thickness of the PEDOT coating layer is 1-4 μm;

[0011] and / or, the mass fraction ratio of the sodium iron pyrophosphate material, the carbon coating layer and the PEDOT coating layer is (95-97%):(1-2.5%):(1-3%).

[0012] The second object of the present invention is to provide a preparation method of a sodium iron pyrophosphate composite cathode material, which comprises the following steps:

[0013] (a) A feed liquid formed by mixing a sodium source, an iron source, a phosphorus source, a carbon source and a solvent is first ground and then spray-dried to obtain a sodium iron pyrophosphate precursor;

[0014] (b) The sodium iron pyrophosphate precursor is sintered to obtain a sodium iron pyrophosphate material with a carbon coating layer on the surface;

[0015] (c) The sodium iron pyrophosphate material with a carbon coating layer on the surface, EDOT monomer, organic acid and solvent are mixed, and then an initiator is added to carry out an oxidative polymerization reaction to coat a PEDOT coating layer on the surface of the carbon coating layer to obtain a sodium iron pyrophosphate composite cathode material.

[0016] Further, on the basis of the above technical solution of the present invention, in step (a), the sodium source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium acetate, sodium sulfate, sodium hydroxide, sodium citrate, sodium pyrophosphate and sodium dihydrogen pyrophosphate;

[0017] and / or, the iron source includes one or more of iron powder, iron citrate, ferrous citrate, iron nitrate, ferrous nitrate, iron sulfate, ferrous sulfate, iron chloride, ferrous chloride, magnetite, iron oxide, ferrous oxide, iron oxalate, ferrous oxalate, iron acetate, iron phosphate, iron pyrophosphate and ammonium ferrous sulfate;

[0018] and / or, the phosphorus source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium hydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, pyrophosphoric acid, sodium pyrophosphate and sodium dihydrogen pyrophosphate;

[0019] And / or, the carbon source includes one or more of aqueous carbon nanotubes, oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, adipic acid, soluble starch, sucrose, and glucose;

[0020] And / or, the mass of the carbon source accounts for 0.5-5% of the total mass of the iron source, phosphorus source, and sodium source.

[0021] Further, on the basis of the above technical solution of the present invention, in step (a), the grinding is ball milling, the rotation speed is 100-500 rpm, and the time is 1-10 h;

[0022] And / or, in step (a), the process parameters of spray drying include: the inlet temperature of the slurry is 210-230 °C, the outlet temperature is 90-100 °C; and / or, the feeding rate of the slurry is 50-70 mL / min; and / or, the frequency of the atomizing disk is 5500-6500 Hz.

[0023] Further, on the basis of the above technical solution of the present invention, in step (b), the sintering includes pre-sintering and high-temperature sintering. The sodium iron pyrophosphate precursor is first pre-sintered and then high-temperature sintered;

[0024] And / or, the sintering atmosphere includes at least one of nitrogen and argon.

[0025] Further, on the basis of the above technical solution of the present invention, in step (b), the pre-sintering temperature is 500-600 °C, the sintering time at the pre-sintering temperature is 2-10 h, and the temperature rise rate to the pre-sintering temperature is 2-10 °C / min;

[0026] And / or, the high-temperature sintering temperature is 650-800 °C, the sintering time at the high-temperature sintering temperature is 2-10 h, and the temperature rise rate from the pre-sintering temperature to the high-temperature sintering temperature is 2-10 °C / min.

[0027] Further, on the basis of the above technical solution of the present invention, in step (c), the mass of the EDOT monomer accounts for 0.5-5% of the total mass of the iron source, phosphorus source, and sodium source;

[0028] And / or, the organic acid includes dodecylbenzenesulfonic acid;

[0029] And / or, the molar ratio of the organic acid to the EDOT monomer is (0.5-1.0):1;

[0030] And / or, the initiator includes at least one of azo initiators, organic peroxide initiators, or redox initiator systems;

[0031] And / or, the mass of the initiator is 0.01-2.0% of the mass of the EDOT monomer;

[0032] And / or, the temperature of the oxidative polymerization reaction is 20-70°C, and the time is 5-36 h.

[0033] The third object of the present invention is to provide a positive electrode sheet, which comprises the sodium iron pyrophosphate composite positive electrode material provided by the first object of the present invention or the sodium iron pyrophosphate composite positive electrode material prepared by using the preparation method provided by the second object of the present invention.

[0034] The fourth object of the present invention is to provide a sodium ion battery, which comprises the sodium iron pyrophosphate composite positive electrode material provided by the first object of the present invention, the sodium iron pyrophosphate composite positive electrode material prepared by using the preparation method provided by the second object of the present invention, or the positive electrode sheet provided by the third object of the present invention.

[0035] Compared with the prior art, the technical solution of the present invention has at least the following technical effects:

[0036] (1) The present invention provides a sodium iron pyrophosphate composite positive electrode material, which has a core-shell structure, wherein the sodium iron pyrophosphate material is equivalent to the inner core, and the carbon coating layer and the PEDOT coating layer are equivalent to the outer shell and are coated on the surface of the inner core; this core-shell structure can uniformly coat the sodium iron pyrophosphate material, improve the conductivity of the material, enhance the rate performance and the cycle performance, and hinder or slow down the occurrence of side reactions.

[0037] (2) The present invention provides a preparation method of the above sodium iron pyrophosphate composite positive electrode material. First, each raw material is ground and then made into a sodium iron pyrophosphate precursor by spray drying. Then, a carbon coating layer is formed on the surface of the sodium iron pyrophosphate through a sintering process, and then a uniform, complete and stable PEDOT coating layer is formed on the surface of the carbon coating layer by an in-situ oxidative polymerization method; this preparation method is simple to operate, can adjust the thickness of the coating layer and the morphology of the material to obtain a core-shell structure, thereby improving the comprehensive performance of the material. Moreover, the prepared sodium iron pyrophosphate composite positive electrode material has excellent conductivity and the first-cycle discharge specific capacity, high material utilization efficiency, and significantly improved electrochemical performance.

[0038] (3) The present invention provides a sodium ion battery, which comprises the above sodium iron pyrophosphate composite positive electrode material or a positive electrode sheet prepared from the above sodium iron pyrophosphate composite positive electrode material. Due to the advantages of the above sodium iron pyrophosphate composite positive electrode material, the sodium ion battery containing it has good cycle performance and rate performance, and at the same time, the heat generation of the battery is low and the cycle life is long. Description of the Drawings

[0039] Figure 1XRD pattern of the sodium iron pyrophosphate composite cathode material provided in Embodiment 2 of the present invention;

[0040] Figure 2 SEM images of the sodium iron pyrophosphate composite cathode material provided in Embodiment 2 of the present invention;

[0041] Figure 3 Rate performance test charts of the sodium iron pyrophosphate composite cathode material (PEDOT@NFPP@C) provided in Embodiment 1 of the present invention and the sodium iron pyrophosphate composite cathode material (NFPP@C) provided in Comparative Example 1. Detailed implementation manners

[0042] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0043] In the present invention, the endpoints and any values within the disclosed ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0044] According to the first aspect of the present invention, a sodium iron pyrophosphate composite cathode material is provided, which includes a sodium iron pyrophosphate material, and a carbon coating layer and a PEDOT coating layer sequentially coated on the surface of the sodium iron pyrophosphate material from the inside out, that is, the surface of the sodium iron pyrophosphate material is coated with a carbon coating layer, and the surface of the carbon coating layer is coated with a PEDOT coating layer.

[0045] In the present invention, a carbon coating layer is formed on the surface of the sodium iron pyrophosphate material. The carbon coating layer can improve the conductivity of the composite cathode material, enhance the rate performance and cycling performance, and hinder or slow down the occurrence of side reactions. If no coating is carried out, the conductivity of the material will be low, and the rate performance and cycling performance will also decline.

[0046] A PEDOT coating layer is coated on the surface of the carbon coating layer. The PEDOT coating layer is mainly polymerized from 3,4-ethylenedioxythiophene (EDOT) monomers. The PEDOT coating layer mainly has the following functions:

[0047] (1) It has improved the electronic conductivity. The conduction mechanism of PEDOT mainly involves the following key steps:

[0048] Formation of conjugated structure: Through the alternating single and double bonds and C=C double bonds in its main chain, as well as the formation of π bonds, PEDOT forms a delocalized electron cloud. This structure allows electrons to move freely in the polymer chain.

[0049] Doping process: In PEDOT, doping occurs through the transfer of electrons from the highest occupied molecular orbital (homo) of the molecule to the polymer main chain, forming a hole. This P-type doping enables the polymer to lose electrons in the oxidation reaction, thus achieving charge balance.

[0050] Change in electronic structure: During the doping and dedoping processes, the electronic structure of PEDOT changes, generating new polaron energy levels, thereby achieving electronic conduction.

[0051] (2) The PEDOT coating layer buffers the structural changes of NFPP during the sodiation and desodiation processes, thereby improving its rate performance and cycling stability.

[0052] (3) The PEDOT conductive polymer has both electron / ion dual conductivity and the chemical functional groups required to hinder interfacial side reactions, and can effectively prevent adverse battery performance phase changes, transition metal dissolution, and intergranular cracks and a large amount of oxygen release at grain boundaries in NFPP during high charge states and high-temperature aging processes. The in-situ polymerization to form a conductive polymer protective layer significantly improves the electrochemical performance and thermal stability of the layered oxide cathode material. This surface coating method plays an important role in stabilizing the crystal and interfacial structures under long-term cycling and high-temperature heating.

[0053] When the battery is charged and discharged, this polymer provides a protective layer for the positive electrode, protecting it from damage by the battery electrolyte. The protection of the traditional coating on the surface of the positive electrode particles is only in the micron size, and it is easy to crack inside. Different from this, the PEDOT coating can penetrate into the interior of the positive electrode particles, increasing an additional shielding layer. In addition, although PEDOT prevents chemical reactions between the battery and the electrolyte, it can ensure the necessary sodium ion and electron transport, which is required for the operation of the battery.

[0054] (4) PEDOT exists in an amorphous state in the composite material, and the addition of PEDOT does not change the original structure of the bulk material.

[0055] The sodium iron pyrophosphate composite cathode material of the present invention has a core-shell structure, where the sodium iron pyrophosphate material is equivalent to the inner core, and the carbon coating layer and the PEDOT coating layer are equivalent to the outer shell and coat the surface of the inner core. This core-shell structure can uniformly coat the sodium iron pyrophosphate material, improve the conductivity of the material, enhance the rate performance and cycling performance, and hinder or slow down the occurrence of side reactions.

[0056] As an alternative embodiment of the technical solution of the present invention, the particle size of the sodium iron pyrophosphate material is 5-15 μm, and typical but non-limiting particle sizes are 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm or 15 μm, etc.

[0057] As an alternative embodiment of the technical solution of the present invention, the coating thickness of the carbon coating layer is 1-4 μm. If the thickness of the carbon coating layer is too large, it will reduce the comprehensive specific capacity of the composite material, increase the liquid absorption amount, and increase the resistance to sodium ion deintercalation. If the thickness is too small, the coating effect will not be achieved, and the problem of low conductivity of the material cannot be solved. Therefore, the typical but non-limiting thickness of the carbon coating layer is 1 μm, 2 μm, 3 μm or 4 μm, etc.

[0058] As an alternative embodiment of the technical solution of the present invention, the coating thickness of the PEDOT coating layer is 1-4 μm. If the thickness of the PEDOT coating layer is too large, it will cause an increase in the resistance to sodium ion transmission and a decrease in the effective capacity, thus affecting the improvement of the rate performance and cycling performance. If the thickness is too small, the coating effect will not be achieved, the problem of low conductivity of the material cannot be solved, and there is also the problem that the improvement of the rate performance and cycling performance is not obvious. Therefore, the typical but non-limiting thickness of the PEDOT coating layer is 1 μm, 2 μm, 3 μm or 4 μm, etc.

[0059] As an alternative embodiment of the technical solution of the present invention, the mass fraction ratio of the sodium iron pyrophosphate material, the carbon coating layer and the PEDOT coating layer is (95-97%):(1-2.5%):(1-3%), so the typical but non-limiting mass ratios are 95%:2.5%:2.5%, 95%:2%:3%, 96%:1%:3%, 96%:2%:2%, 96%:2.5%:1.5%, 97%:1%:2%, 97%:1.5%:1.5% or 97%:2%:1%, etc.

[0060] According to the second aspect of the present invention, a preparation method of a sodium iron pyrophosphate composite cathode material is provided, including the following steps:

[0061] (a) First, grind the feed liquid formed by mixing an iron source, a phosphorus source, a sodium source, a carbon source and a solvent, and then perform spray drying to obtain a sodium iron pyrophosphate precursor;

[0062] (b) Sinter the sodium iron pyrophosphate precursor to obtain a sodium iron pyrophosphate material with a carbon coating layer on its surface;

[0063] (c) Mix the sodium iron pyrophosphate material with a carbon coating layer on its surface, EDOT monomer, organic acid and solvent, and then add an initiator to carry out an oxidative polymerization reaction to coat a PEDOT coating layer on the surface of the carbon coating layer, obtaining a sodium iron pyrophosphate composite cathode material.

[0064] The present invention provides a preparation method of a sodium iron pyrophosphate composite cathode material. First, each raw material is ground and then made into a sodium iron pyrophosphate precursor by spray drying. Then, a carbon coating layer is formed on the surface of the sodium iron pyrophosphate through a sintering process. Next, a uniform, complete and stable PEDOT coating layer is formed on the surface of the carbon coating layer by an in-situ oxidative polymerization method. The preparation method is simple to operate, can adjust the thickness of the coating layer and the morphology of the material to obtain a core-shell structure, thereby improving the comprehensive performance of the material. And the prepared sodium iron pyrophosphate composite cathode material has excellent conductivity and first-cycle discharge specific capacity, high material utilization efficiency, and significantly improved electrochemical performance.

[0065] Specifically, the raw materials used for the sodium iron pyrophosphate composite cathode material are further described.

[0066] As an optional implementation manner of the technical solution of the present invention, in step (a), the sodium source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium acetate, sodium sulfate, sodium hydroxide, sodium citrate, sodium pyrophosphate and sodium dihydrogen pyrophosphate.

[0067] As an optional implementation manner of the technical solution of the present invention, in step (a), the iron source includes one or more of iron powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, magnetite, iron oxide, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate and ammonium ferrous sulfate.

[0068] As an optional implementation manner of the technical solution of the present invention, in step (a), the phosphorus source includes one or more of sodium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, pyrophosphoric acid, sodium pyrophosphate and sodium dihydrogen pyrophosphate.

[0069] As an optional implementation manner of the technical solution of the present invention, in step (a), the dosage ratios of the iron source, phosphorus source and sodium source can be calculated according to the stoichiometric ratio of sodium iron pyrophosphate.

[0070] As an alternative embodiment of the technical solution of the present invention, in step (a), the carbon source includes one or more of aqueous carbon nanotubes, oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butanal, lactic acid, citric acid, malic acid, adipic acid, soluble starch, sucrose, and glucose.

[0071] As an alternative embodiment of the technical solution of the present invention, in step (a), the mass of the carbon source accounts for 0.5-5% of the total mass of the iron source, phosphorus source, and sodium source. For example, the typical but non-limiting mass ratios of the carbon source are 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%, etc.

[0072] As an alternative embodiment of the technical solution of the present invention, in step (a), the grinding is ball milling, the rotation speed is 100-500 rpm (such as 100 rpm, 200 rpm, 300 rpm, 400 rpm, or 500 rpm, etc.), and the time is 1-10 h (such as 1 h, 2 h, 4 h, 5 h, 6 h, 8 h, or 10 h, etc.).

[0073] The above-ground slurry after grinding is spray-dried, which can effectively control the sphericity of sodium iron pyrophosphate precursor (sodium iron pyrophosphate) particles, thereby adjusting the sphericity of the sodium iron pyrophosphate composite cathode material, and further improving the tap density of the material. As an alternative embodiment of the technical solution of the present invention, in step (a), during spray drying, the slurry is spray-dried under the conditions that the inlet temperature is 210-230 °C (such as 210 °C, 220 °C, or 230 °C, etc.), the outlet temperature is 90-100 °C (such as 90 °C, 95 °C, or 100 °C, etc.), the feeding rate of the slurry is 50-70 mL / min (such as 50 mL / min, 60 mL / min, or 70 mL / min, etc.), and the frequency of the atomization disk is 5500-6500 Hz (such as 5500 Hz, 5800 Hz, 6000 Hz, 6200 Hz, or 6500 Hz, etc.).

[0074] As an alternative embodiment of the technical solution of the present invention, in step (b), the sintering includes pre-sintering and high-temperature sintering. The sodium iron pyrophosphate precursor is first pre-sintered, and after pre-sintering, high-temperature sintering is carried out. The sintering is carried out in stages. If pre-sintering is not carried out and high-temperature sintering is directly carried out, the temperature may rise too fast, which may cause uneven surface carbonization of the precursor powder, form an uneven carbon coating layer, affect the structure of the conductance network, and reduce the conductivity of the material. As an alternative embodiment of the technical solution of the present invention, in step (b), the sintering atmosphere includes at least one of nitrogen and argon.

[0075] The sintering temperature, temperature rise rate, sintering time, etc. are crucial for the formation of the material and the uniformity of the carbon coating layer.

[0076] As an alternative embodiment of the technical solution of the present invention, in step (b), the temperature rise rate to the pre-sintering temperature is 2-10 °C / min (for example, 2 °C / min, 4 °C / min, 5 °C / min, 6 °C / min, 8 °C / min, 10 °C / min, etc.), the pre-sintering temperature is 500-600 °C (for example, 500 °C, 520 °C, 550 °C, 580 °C or 600 °C, etc.), and the sintering time at the pre-sintering temperature is 2-10 h (for example, 2 h, 4 h, 5 h, 6 h, 8 h or 10 h, etc.).

[0077] Too fast a temperature rise rate or inappropriate temperature will cause undesirable phase changes or rapid grain growth of the material. A lower initial temperature and a higher later temperature change (for example, below 500 °C and then rising to above 800 °C) will cause non-uniform phase changes and accumulation of internal stress in different stages, making the internal structure and morphology of the material unstable and leading to deterioration of the electrochemical performance.

[0078] As an alternative embodiment of the technical solution of the present invention, in step (b), the temperature rise rate from the pre-sintering temperature to the high-temperature sintering temperature is 2-10 °C / min (for example, 2 °C / min, 4 °C / min, 5 °C / min, 6 °C / min, 8 °C / min, 10 °C / min, etc.), the high-temperature sintering temperature is 650-800 °C (for example, 650 °C, 680 °C, 700 °C, 720 °C, 750 °C, 780 °C or 800 °C, etc.), and the sintering time at the high-temperature sintering temperature is 2-10 h (for example, 2 h, 4 h, 5 h, 6 h, 8 h or 10 h, etc.).

[0079] The coking process of the carbon source at different temperatures is the key to controlling the carbon layer thickness and uniformity. The change in temperature requires sufficient time and control. Simply heating to 750 °C (i.e., direct heating without pre-sintering) or extreme temperature changes will cause the carbon source to not decompose or coke sufficiently, and an ideal uniform carbon coating layer cannot be formed, thus affecting the conductivity and the + transport efficiency of Na and reducing the discharge capacity.

[0080] As an alternative embodiment of the technical solution of the present invention, in step (c), the mass of the EDOT monomer accounts for 0.5-5% of the total mass of the iron source, phosphorus source and sodium source. For example, the typical but non-limiting mass ratio of the EDOT monomer is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, etc.

[0081] As an alternative embodiment of the technical solution of the present invention, in step (c), the organic acid includes dodecylbenzenesulfonic acid (DBSA). DBSA is an organic protonic acid. Compared with inorganic protonic acids (such as hydrochloric acid, sulfuric acid, and phosphoric acid, etc.), it has good environmental stability, is not prone to migration, and will not affect the polarity in the formed molecular chain after doping. Moreover, as an organic sulfonic acid, DBSA contains both polar and non-polar groups in its molecular structure, has a relatively large molecular weight, and greatly improves conductivity, solubility, etc. Its use in the in-situ oxidative polymerization of EDOT monomers on the surface of sodium iron pyrophosphate material coated with a carbon coating layer is conducive to the formation of a uniform PEDOT coating layer, making the obtained polyanion composite material have significantly improved electrochemical performance and structural stability.

[0082] Preferably, in step (c), the molar ratio of the organic acid to the EDOT monomer is (0.5 - 1.0):1, and for example, it can be 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, or 0.95:1, etc.

[0083] As an alternative embodiment of the technical solution of the present invention, in step (c), the initiator includes at least one of azo initiators, organic peroxide initiators, or redox initiation systems;

[0084] Among them, common azo initiators usually include the following several:

[0085] Azodiisobutyronitrile: The use temperature range is 50 - 65 °C, the decomposition is uniform, only one kind of free radical is formed, and there are no other side reactions;

[0086] Azodiisooctanenitrile: It has greater activity and high initiation efficiency, and can replace azodiisobutyronitrile;

[0087] Dimethyl azodiisobutyrate (AIBN): The initiation activity is moderate, the polymerization reaction is easy to control, there is no residue during the polymerization process, the product conversion rate is high, and the decomposition products are harmless.

[0088] Common organic peroxide initiators usually include the following several: persulfates (such as ammonium persulfate), tert-butyl hydroperoxide, sodium metabisulfite, etc. These initiators can also initiate polymerization reactions at lower temperatures.

[0089] The redox initiation system mainly uses the free radicals generated by the electron transfer between the oxidant and the reductant to initiate the polymerization reaction, and is suitable for low-temperature polymerization reactions.

[0090] And / or, the mass of the initiator is 0.01 - 2.0% of the mass of the EDOT monomer, such as 0.01%, 0.05%, 0.1%, 0.2%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.5%, 1.6%, 1.8% or 2.0%, etc.

[0091] As an alternative embodiment of the technical solution of the present invention, the temperature of the oxidative polymerization reaction is 20 - 70 °C and the time is 5 - 36 h. Typical but non-limiting temperatures are 20 °C, 30 °C, 40 °C, 50 °C, 60 °C or 70 °C, and typical but non-limiting times are 5 h, 10 h, 12 h, 18 h, 20 h, 24 h, 28 h, 30 h, 32 h, 35 h or 36 h, etc.

[0092] As an alternative embodiment of the technical solution of the present invention, in step (c), after the oxidative polymerization reaction is completed, the obtained reaction mixture product is separated, and the separated solid product is washed, dried and pulverized to obtain the sodium iron pyrophosphate composite cathode material.

[0093] According to the third aspect of the present invention, a cathode sheet is provided, which comprises the sodium iron pyrophosphate composite cathode material provided in the first aspect of the present invention, or the sodium iron pyrophosphate composite cathode material prepared by the preparation method provided in the second aspect of the present invention.

[0094] In view of the advantages of the above-mentioned sodium iron pyrophosphate composite cathode material of the present invention, the cathode sheet containing it has high conductivity and can improve the rate performance of the battery.

[0095] According to the fourth aspect of the present invention, a sodium ion battery is provided, which comprises the sodium iron pyrophosphate composite cathode material provided in the first aspect of the present invention, the sodium iron pyrophosphate composite cathode material prepared by the preparation method provided in the second aspect of the present invention, or the cathode sheet provided in the third aspect of the present invention.

[0096] In view of the advantages of the above-mentioned sodium iron pyrophosphate composite cathode material or the cathode sheet prepared therefrom of the present invention, the sodium ion battery containing it has good cycle performance and rate performance, and at the same time, the heat generation of the battery is low and the cycle life is long.

[0097] The present invention will be further described in detail below with specific examples and comparative examples. Among them, the aqueous carbon nanotubes are multi-walled, with a diameter of 20 - 40 nm, a length of 100 - 200 μm, 10 - 15 layers, a specific surface area of 200 - 250 m 2 / g, a density of 1.5 g / cm 3 , and a concentration of 10 mg / mL.

[0098] Example 1

[0099] This embodiment provides a sodium iron pyrophosphate composite cathode material, which comprises a sodium iron pyrophosphate material, and a carbon coating layer and a PEDOT coating layer that are sequentially coated on the surface of the sodium iron pyrophosphate material from the inside to the outside;

[0100] Among them, the particle size of the sodium iron pyrophosphate material is 5-15 μm; the average coating thickness of the carbon coating layer is 1.5 μm; the average coating thickness of the PEDOT coating layer is 2.5 μm. The coating amount of the carbon coating layer (i.e., the percentage of the mass of the carbon coating layer in the mass of the sodium iron pyrophosphate composite cathode material) is about 2%, and the PEDOT coating amount (i.e., the percentage of the mass of PEDOT in the mass of the sodium iron pyrophosphate composite cathode material) is about 1.5%.

[0101] The preparation method of the sodium iron pyrophosphate composite cathode material (abbreviated as PEDOT@NFPP / C) in this embodiment includes the following steps:

[0102] (a) Weigh 1 mol (380 g) of sodium phosphate dodecahydrate, 2 mol (301 g) of iron phosphate, and a composite carbon source composed of 0.03 mol (5.76 g) of citric acid and 13 g of aqueous carbon nanotubes and add them to 2.5 L of deionized water. After stirring evenly to form a solution, then add 1 mol (151 g) of iron phosphate and 0.03 mol (8.37 g) of iron nitrate, and stir evenly to form a feed liquid;

[0103] Pour the feed liquid into a storage tank, grind it in a sand mill at a rotation speed of 350 r / min and a flow rate of 40 L / h for 240 min, and control the particle size after ball milling to be 200-400 nm; the molar ratio of sodium, iron, and phosphorus elements in the mixture is 3:2:3;

[0104] Perform spray drying treatment on the ball-milled slurry at an inlet temperature of 220 °C and an outlet temperature of 95 °C. The feeding rate of spray drying is 60 mL / min, and the frequency of the atomization disk is 6000 Hz to obtain precursor powder;

[0105] (b) Heat the precursor powder obtained after spray drying treatment to 550 °C at a temperature rise rate of 10 °C / min in a nitrogen atmosphere, hold for 2 h, and then heat it to 750 °C for calcination at a temperature rise rate of 10 °C / min and hold for 5 h to obtain carbon-coated sodium iron pyrophosphate (Na3Fe2(PO4)(P2O7) / C composite cathode material);

[0106] (c) Then, in-situ polymerization is carried out on the Na3Fe2(PO4)(P2O7) / C composite cathode material. Specifically, 10 g of dodecylbenzenesulfonic acid is dissolved in 2 L of deionized water, and 8 g of EDOT monomer and 500 g of carbon-coated sodium iron pyrophosphate are added successively for secondary coating, and stirring is continued for 30 min; 0.08 g of ammonium persulfate is dissolved in deionized water and added dropwise to the above mixed solution, and polymerization is carried out at room temperature for 24 h to coat a PEDOT coating layer on the surface of the carbon coating layer. Then, the mixture is centrifuged and washed with deionized water until it is colorless; after the obtained material is dried, it is crushed with a crusher to obtain a sodium iron pyrophosphate composite cathode material, and the average coating thickness of the PEDOT coating layer is 2.5 μm.

[0107] Example 2

[0108] This example provides a sodium iron pyrophosphate composite cathode material. Except that the PEDOT coating amount in the sodium iron pyrophosphate composite cathode material is 1.25%, and the average coating thickness of the PEDOT coating layer is 1.7 μm, the rest of the structural composition is the same as that in Example 1.

[0109] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 except that the mass of the EDOT monomer added in step (c) is 6.5 g and the mass of the organic acid dodecylbenzenesulfonic acid added is 8.1 g, and the rest of the steps and process parameters are the same as those in Example 1.

[0110] Example 3

[0111] This example provides a sodium iron pyrophosphate composite cathode material. Except that the PEDOT coating amount in the sodium iron pyrophosphate composite cathode material is 1.35%, and the average coating thickness of the PEDOT coating layer is 2.3 μm, the rest of the structural composition is the same as that in Example 1.

[0112] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 except that the mass of the EDOT monomer added in step (c) is 10 g and the mass of the organic acid dodecylbenzenesulfonic acid added is 12.5 g, and the rest of the steps and process parameters are the same as those in Example 1.

[0113] Example 4

[0114] This example provides a sodium iron pyrophosphate composite cathode material. Except that the PEDOT coating amount in the sodium iron pyrophosphate composite cathode material is 1.75%, and the average coating thickness of the PEDOT coating layer is 3.2 μm, the rest of the structural composition is the same as that in Example 1.

[0115] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 in terms of the remaining steps and process parameters, except that the mass of the EDOT monomer added in step (c) is 12 g and the mass of the organic acid dodecylbenzenesulfonic acid added is 15 g.

[0116] Example 5

[0117] This example provides a sodium iron pyrophosphate composite cathode material, which is the same as that in Example 1 in terms of the remaining structural components, except that the PEDOT coating amount in the sodium iron pyrophosphate composite cathode material is 1.95% and the average coating thickness of the PEDOT coating layer is 3.8 μm.

[0118] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 in terms of the remaining steps and process parameters, except that the mass of the EDOT monomer added in step (c) is 15 g and the mass of the organic acid dodecylbenzenesulfonic acid added is 18.75 g.

[0119] Example 6

[0120] This example provides a sodium iron pyrophosphate composite cathode material, which is the same as that in Example 1 in terms of the remaining structural components, except that the PEDOT coating amount in the sodium iron pyrophosphate composite cathode material is 0.5% and the coating thickness of the PEDOT coating layer is 0.6 μm.

[0121] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 in terms of the remaining steps and process parameters, except that the mass of the EDOT monomer added in step (c) is 5 g and the mass of the organic acid dodecylbenzenesulfonic acid added is 6.25 g.

[0122] Example 7

[0123] This example provides a sodium iron pyrophosphate composite cathode material, which is the same as that in Example 1 in terms of the remaining structural components, except that the PEDOT coating amount in the sodium iron pyrophosphate composite cathode material is 4% and the coating thickness of the PEDOT coating layer is 7.5 μm.

[0124] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 in terms of the remaining steps and process parameters, except that the mass of the EDOT monomer added in step (c) is 50 g and the mass of the organic acid dodecylbenzenesulfonic acid added is 62.5 g.

[0125] Example 8

[0126] This example provides a sodium iron pyrophosphate composite cathode material, which is the same as that in Example 1 in terms of the remaining structural components, except that the coating amount of the carbon coating layer in the sodium iron pyrophosphate composite cathode material is about 1.5% and the average coating thickness of the carbon coating layer is 0.7 μm.

[0127] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 in the remaining steps and process parameters, except that the mass of citric acid added in step (a) is 4.32 g and the mass of aqueous carbon nanotubes is 10 g.

[0128] Example 9

[0129] This example provides a sodium iron pyrophosphate composite cathode material. Except that the coating amount of the carbon coating layer in the sodium iron pyrophosphate composite cathode material is about 2.5% and the average coating thickness of the carbon coating layer is 2.0 μm, the remaining structural compositions are the same as those in Example 1.

[0130] The preparation method of the sodium iron pyrophosphate composite cathode material in this example is the same as that in Example 1 in the remaining steps and process parameters, except that the mass of citric acid added in step (a) is 7.20 g and the mass of aqueous carbon nanotubes is 16.25 g.

[0131] Example 10

[0132] This example provides a sodium iron pyrophosphate composite cathode material, which includes a sodium iron pyrophosphate material, and a carbon coating layer and a PEDOT coating layer that are sequentially coated on the surface of the sodium iron pyrophosphate material from the inside to the outside;

[0133] Among them, the particle size of the sodium iron pyrophosphate material is 5 - 15 μm; the average coating thickness of the carbon coating layer is 1.5 μm; the average coating thickness of the PEDOT coating layer is 1.5 μm. The coating amount of the carbon coating layer (i.e., the percentage of the mass of the carbon coating layer in the mass of the sodium iron pyrophosphate composite cathode material) is about 2%, and the PEDOT coating amount (i.e., the percentage of the mass of PEDOT in the mass of the sodium iron pyrophosphate composite cathode material) is about 1.5%.

[0134] The preparation method of the sodium iron pyrophosphate composite cathode material (abbreviated as PEDOT@NFPP / C) in this example includes the following steps:

[0135] (a) Weigh 1 mol 380 g of sodium phosphate dodecahydrate, 2 mol 301 g of iron phosphate, and a composite carbon source composed of 0.03 mol 10.26 g of sucrose and 13 g of Ketjenblack and add them to 2.5 L of deionized water. After stirring evenly to form a solution, then add 1 mol 151 g of iron phosphate and 0.03 mol 8.37 g of iron nitrate, and stir evenly to form a feed liquid;

[0136] Pour the said feed liquid into the storage tank, grind it in a sand mill at a rotation speed of 350 r / min and a flow rate of 40 L / h for 240 min, and control the particle size after ball milling to be 200 - 400 nm; the molar ratio of sodium, iron, and phosphorus elements in the mixture is 3∶2∶3;

[0137] Perform spray drying on the ball-milled slurry at an inlet temperature of 220 °C and an outlet temperature of 95 °C. The feeding rate of spray drying is 60 mL / min, and the frequency of the atomization disk is 6000 Hz to obtain the precursor powder;

[0138] (b) Heat the precursor powder obtained after spray drying treatment to 550 °C at a temperature rise rate of 10 °C / min under a nitrogen atmosphere, hold for 2 h, and then heat to 750 °C for calcination at a temperature rise rate of 10 °C / min and hold for 5 h to obtain sodium iron pyrophosphate phosphate coated with carbon (Na3Fe2(PO4)(P2O7) / C composite cathode material);

[0139] (c) Then perform in-situ polymerization on the Na3Fe2(PO4)(P2O7) / C composite cathode material. Specifically, dissolve 10 g of dodecylbenzenesulfonic acid in 2 L of deionized water, add 8 g of EDOT monomer and 500 g of carbon-coated sodium iron pyrophosphate phosphate in sequence for secondary coating, and continue stirring for 30 min; dissolve 0.08 g of ammonium persulfate in deionized water, and add it dropwise to the above mixed solution. After room temperature polymerization for 30 h, centrifuge the mixture and wash it with deionized water until it is colorless; after drying the obtained material, crush it with a crusher to obtain the PEDOT-coated sodium iron pyrophosphate phosphate composite cathode material, and the average coating thickness of the PEDOT coating layer is 1.5 μm.

[0140] Comparative Example 1

[0141] This comparative example provides a sodium iron pyrophosphate phosphate composite cathode material (abbreviated as NFPP / C), which includes a sodium iron pyrophosphate phosphate material and a carbon coating layer coated on the surface of the sodium iron pyrophosphate phosphate material;

[0142] Among them, the particle size of the sodium iron pyrophosphate phosphate material is 5 - 15 μm; the average coating thickness of the carbon coating layer is 1.5 μm.

[0143] The preparation method of the sodium iron pyrophosphate phosphate composite cathode material in this comparative example is the same as that of Example 1 in all other steps and process parameters except that step (c) is not carried out.

[0144] Comparative Example 2

[0145] This comparative example provides a sodium iron pyrophosphate composite cathode material (abbreviated as PEDOT@NFPP), which includes a sodium iron pyrophosphate material and a PEDOT coating layer coated on the surface of the sodium iron pyrophosphate material;

[0146] Among them, the particle size of the sodium iron pyrophosphate material is 5 - 15 μm; the average coating thickness of the PEDOT coating layer is 2.5 μm.

[0147] The preparation method of the sodium iron pyrophosphate composite cathode material in this comparative example includes the following steps:

[0148] (a) Weigh 1 mol (380 g) of sodium phosphate dodecahydrate and 2 mol (301 g) of iron phosphate according to the stoichiometric ratio, add them to 2.5 L of deionized water, stir evenly to form a solution, then add 1 mol (151 g) of iron phosphate and 0.03 mol (8.37 g) of iron nitrate, and stir evenly to form a feed liquid;

[0149] Pour the feed liquid into a storage tank, grind it in a sand mill at a rotation speed of 350 r / min and a flow rate of 40 L / h for 240 min, and control the particle size after ball milling between 200 - 400 nm; the molar ratio of sodium, iron, and phosphorus elements in the mixture is 3∶2∶3;

[0150] Perform spray drying on the ball - milled slurry at an inlet temperature of 220 °C and an outlet temperature of 95 °C, with a feed rate of spray drying of 60 mL / min and a frequency of the atomization disk of 6000 Hz to obtain a precursor powder;

[0151] (b) Heat the precursor powder obtained after spray drying to 550 °C at a temperature rise rate of 10 °C / min in a nitrogen atmosphere, hold for 2 h, and then heat to 750 °C for calcination at a temperature rise rate of 10 °C / min and hold for 5 h to obtain a sodium iron pyrophosphate (Na3Fe2(PO4)(P2O7) cathode material);

[0152] (c) Then perform in - situ polymerization on the Na3Fe2(PO4)(P2O7) cathode material. Specifically, dissolve 10 g of dodecylbenzenesulfonic acid in 2 L of deionized water, add 8 g of EDOT monomer and 500 g of sodium iron pyrophosphate for coating in sequence, and continue stirring for 30 min; dissolve 0.08 g of ammonium persulfate in deionized water, and add it dropwise to the above - mentioned mixed solution. After room - temperature polymerization for 24 h, centrifuge the mixture and wash it with deionized water until it is colorless; then perform drying treatment on the obtained material and crush it with a crusher to obtain a PEDOT - coated sodium iron pyrophosphate composite cathode material, and the average coating thickness of the PEDOT coating layer is 2.5 μm.

[0153] In order to compare the technical effects of each example and comparative example, the following experimental examples are specially set up.

[0154] Experimental Example 1

[0155] Taking Example 2 as an example, the sodium iron pyrophosphate composite cathode material was subjected to XRD detection, and the specific results are as Figure 1 shown. It can be seen from Figure 1 that the crystal phase is sodium iron pyrophosphate, and the crystal phase remains unchanged after coating, that is, the coating will not damage the crystal structure.

[0156] In addition, the sodium iron pyrophosphate composite cathode material was subjected to SEM detection, and the specific results are as Figure 2 shown. It can be seen from Figure 2 that the morphology of the sodium iron pyrophosphate composite cathode material is spherical-like.

[0157] Experimental Example 2

[0158] The sodium iron pyrophosphate composite cathode materials provided in Examples 1 to 10 and Comparative Examples 1 to 2 were made into sodium ion batteries, and the specific production method is as follows:

[0159] Positive electrode sheet: The sodium iron pyrophosphate composite cathode material was mixed with acetylene black and PVDF in a mass ratio of 70:20:10, and then the solvent NMP was added to make a slurry (solid content: 58 - 60%), which was coated on an aluminum foil with a diameter of 19 mm, and then the aluminum foil was vacuum dried at 120°C for 12 hours to obtain the positive electrode sheet;

[0160] Using metallic sodium as the counter electrode, 1 mol / L NaClO4 ethylene carbonate / diethyl carbonate (volume ratio 1:1) as the electrolyte, and cellgard2035 as the separator, a button battery was assembled in a glove box, and the battery model was CR2016. The charge-discharge rate was 1C, and the voltage range was 1.7V - 4.3V. The discharge capacity of the battery was detected, and the specific results are shown in Table 1.

[0161] Table 1

[0162]

[0163]

[0164] From the data of Examples 1-7 in Table 1, it can be seen that the sodium iron pyrophosphate composite cathode material provided by the present invention comprises a sodium iron pyrophosphate material, and a carbon coating layer and a PEDOT coating layer sequentially coated on the surface of the sodium iron pyrophosphate material from the inside to the outside. When the thickness of the carbon coating layer is determined, as the thickness of the PEDOT coating layer increases, the discharge capacity of the battery increases and the initial efficiency increases. When the thickness of the PEDOT coating layer continues to increase to 2.5 μm, both the discharge capacity and the initial efficiency of the battery decrease instead, indicating that if the thickness of the PEDOT coating layer is too small, the coating effect cannot be achieved and the problem of low conductivity of the material cannot be solved. However, if the thickness of the PEDOT coating layer is too large, it will lead to an increase in the sodium ion transport resistance and a decrease in the effective capacity.

[0165] By comparing the data of Example 1 and Examples 8-9, it can be seen that when the thickness of the carbon coating layer is less than 1.5 μm (such as 0.7 μm) or greater than 1.5 μm (such as 2.0 μm), the battery shows the phenomena of small discharge capacity and reduced initial efficiency, indicating that if the thickness of the carbon coating layer is too small, the coating effect cannot be achieved and the problem of low conductivity of the material cannot be solved. While if the thickness of the carbon coating layer is too large, it will reduce the comprehensive specific capacity of the composite material, increase the sodium ion insertion / extraction resistance.

[0166] From Comparative Example 1 and Example 1, for the sodium iron pyrophosphate composite cathode material, the PEDOT coating plays an obvious role, which can improve the initial efficiency, specific capacity performance, conductivity of the material, and enhance the rate performance, as specifically Figure 3 shown.

[0167] From Comparative Example 1 and Comparative Example 2, it can be seen that for the sodium iron pyrophosphate composite cathode material, both the carbon coating layer and the PEDOT coating layer play obvious roles, and both can improve the conductivity of the material, enhance the rate performance and cycle performance, and improve the performance of the NFPP cathode material.

[0168] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are within the scope of protection of the pending claims of the present invention.

Claims

1. A sodium iron pyrophosphate composite cathode material, characterized in that, It contains sodium iron pyrophosphate material, and a carbon coating layer and a PEDOT coating layer that are sequentially coated on the surface of the sodium iron pyrophosphate material from the inside to the outside.

2. The sodium iron pyrophosphate composite cathode material according to claim 1, characterized in that, The particle size of the sodium iron pyrophosphate material is 5 - 15 μm; and / or, the coating thickness of the carbon coating layer is 1 - 4 μm; and / or, the coating thickness of the PEDOT coating layer is 1 - 4 μm; and / or, the mass fraction ratio of the sodium iron pyrophosphate material, the carbon coating layer and the PEDOT coating layer is (95 - 97%):(1 - 2.5%):(1 - 3%); 3. The preparation method of the sodium iron pyrophosphate composite cathode material according to claim 1 or 2, characterized in that, It includes the following steps: (a) Grind the feed liquid formed by mixing a sodium source, an iron source, a phosphorus source, a carbon source and a solvent first, and then perform spray drying to obtain a sodium iron pyrophosphate precursor; (b) Sinter the sodium iron pyrophosphate precursor to obtain a sodium iron pyrophosphate material with a carbon coating layer on the surface; (c) Mix the sodium iron pyrophosphate material with a carbon coating layer on the surface, EDOT monomer, organic acid and solvent, and then add an initiator to carry out an oxidative polymerization reaction to coat a PEDOT coating layer on the surface of the carbon coating layer to obtain a sodium iron pyrophosphate composite cathode material.

4. The preparation method of the sodium iron pyrophosphate composite cathode material according to claim 3, characterized in that, In step (a), the sodium source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium acetate, sodium sulfate, sodium hydroxide, sodium citrate, sodium pyrophosphate and sodium dihydrogen pyrophosphate; and / or, the iron source includes one or more of iron powder, iron citrate, ferrous citrate, iron nitrate, ferrous nitrate, iron sulfate, ferrous sulfate, iron chloride, ferrous chloride, magnetite, iron oxide, ferrous oxide, iron oxalate, ferrous oxalate, iron acetate, iron phosphate, iron pyrophosphate and ammonium ferrous sulfate; and / or, the phosphorus source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium hydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, pyrophosphoric acid, sodium pyrophosphate and sodium dihydrogen pyrophosphate; and / or, the carbon source includes one or more of aqueous carbon nanotubes, oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, adipic acid, soluble starch, sucrose and glucose; and / or, the mass of the carbon source accounts for 0.5 - 5% of the total mass of the iron source, the phosphorus source and the sodium source.

5. The preparation method of the sodium iron pyrophosphate composite cathode material according to claim 3, characterized in that, In step (a), the grinding is ball milling, the rotation speed is 100 - 500 rpm, and the time is 1 - 10 h; and / or, in step (a), the process parameters of spray drying include: the inlet temperature of the slurry is 210 - 230 °C, and the outlet temperature is 90 - 100 °C; and / or, the feeding rate of the slurry is 50 - 70 mL / min; and / or, the frequency of the atomizing disk is 5500 - 6500 Hz.

6. The preparation method of the sodium iron pyrophosphate composite cathode material according to claim 3, wherein In step (b), the sintering includes pre-sintering and high-temperature sintering. First pre-sinter the sodium iron pyrophosphate precursor, and then perform high-temperature sintering; and / or, the sintering atmosphere includes at least one of nitrogen and argon.

7. The preparation method of the sodium iron pyrophosphate composite cathode material according to claim 6, characterized in that, In step (b), the pre-sintering temperature is 500 - 600 °C, the sintering time at the pre-sintering temperature is 2 - 10 h, and the temperature rise rate to the pre-sintering temperature is 2 - 10 °C / min; And / or, the high-temperature sintering temperature is 650 - 800 °C, the sintering time at the high-temperature sintering temperature is 2 - 10 h, and the temperature rise rate from the pre-sintering temperature to the high-temperature sintering temperature is 2 - 10 °C / min.

8. The preparation method of the sodium iron pyrophosphate composite cathode material according to claim 3, wherein In step (c), the mass of the EDOT monomer accounts for 0.5 - 5% of the total mass of the iron source, phosphorus source, and sodium source; And / or, the organic acid includes dodecylbenzenesulfonic acid; And / or, the molar ratio of the organic acid to the EDOT monomer is (0.5 - 1.0):1; And / or, the initiator includes at least one of azo initiators, organic peroxide initiators, or redox initiator systems; And / or, the mass of the initiator is 0.01 - 2.0% of the mass of the EDOT monomer; And / or, the temperature of the oxidative polymerization reaction is 20 - 70 °C, and the time is 5 - 36 h.

9. A positive electrode sheet, characterized in that, Comprising the sodium iron pyrophosphate composite cathode material according to claim 1 or 2, or the sodium iron pyrophosphate composite cathode material prepared by the preparation method according to any one of claims 3 - 8.

10. A sodium-ion battery, characterized in that, Comprising the sodium iron pyrophosphate composite cathode material according to claim 1 or 2, the sodium iron pyrophosphate composite cathode material prepared by the preparation method according to any one of claims 3 - 8, or the cathode sheet according to claim 9.

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