Composite carbon coated polyanion positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion batteries, and particularly relates to a composite carbon coated polyanion positive electrode material as well as a preparation method and application thereof. The composite carbon-coated polyanion positive electrode material provided by the invention comprises a pyrophosphate ferric phosphate sodium inner core, an ordered carbon coating layer on the surface of the inner core and a disordered carbon coating layer on the surface of the ordered carbon coating layer. According to the ordered-disordered composite carbon-coated polyanion material provided by the invention, the rate capability is considered while the capacity development, the first effect and the cycle performance of a battery are improved.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion batteries, specifically relating to a composite carbon-coated polyanion cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion battery technology has seen slow development since its inception in the 1980s. In recent years, however, it has regained attention due to increasing demand for renewable energy storage and concerns about lithium supply and cost. Sodium-ion batteries, with their low cost, high safety, and excellent high and low temperature performance, have become a powerful complement to lithium-ion batteries, leading to continuous growth in market demand.

[0003] The main cathode material routes for sodium-ion batteries are layered oxides, polyanionic materials, and Prussian blue. Among them, sodium iron pyrophosphate (NFPP), a polyanionic material, has an olivine structure similar to lithium iron phosphate, a stable framework structure, and theoretically a long cycle life. It also has the advantages of low manufacturing cost, environmental friendliness, and high theoretical specific capacity, which has attracted a lot of attention and made it the most watched cathode material route in the sodium-ion battery market this year.

[0004] However, the challenge is that, due to Na + With Li + Compared to other sodium-ion batteries, which have larger radii, higher masses, and lower redox potentials, sodium-ion batteries are more difficult to structurally tune in traditional electrode materials, resulting in lower overall energy density. The cathode material is a crucial component of sodium-ion batteries, serving as the primary site for sodium ion storage and decisively influencing battery safety, reliability, electrochemical performance, and future prospects. Iron-based materials, due to their readily available and widely sourced raw materials, have become commercially valuable cathode material systems for sodium-ion batteries. Sodium iron pyrophosphate (NFPP) is easy to prepare and exhibits good cycle performance, but its electronic conductivity and ion mobility are lower than other sodium-ion cathode materials. NFPP possesses a typical sodium ultrafast conductor structure, which presents challenges for electron conduction and ion transport, severely limiting its performance and representing its most significant weakness.

[0005] Therefore, how to improve sodium iron phosphate pyrophosphate to meet the requirements of sodium-ion batteries has received increasing attention. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the technical problems in related technologies. To this end, embodiments of the present invention propose a composite carbon-coated polyanionic cathode material.

[0007] The composite carbon-coated polyanionic cathode material of this invention includes a sodium iron phosphate pyrophosphate core, an ordered carbon coating layer on the surface of the core, and a disordered carbon coating layer on the surface of the ordered carbon coating layer.

[0008] The advantages and technical effects of the composite carbon-coated polyanion cathode material in this invention are as follows: 1. In this invention, by coating sodium iron phosphate pyrophosphate with an ordered carbon coating layer, the conductivity of the material can be improved, thereby obtaining a sodium-ion battery with high capacity, high initial efficiency, and excellent cycle performance; 2. In this invention, based on improving the conductivity of the polyanion cathode material by coating sodium iron phosphate pyrophosphate with an ordered carbon coating layer, a secondary carbon coating is performed by a disordered carbon coating layer to obtain an ordered-disorder composite carbon-coated polyanion material, which improves the battery's capacity, initial efficiency, and cycle performance while also taking into account rate performance.

[0009] In some embodiments, the D50 of the sodium iron phosphate pyrophosphate core particle size is 8~10 μm.

[0010] In some embodiments, the thickness of the ordered carbon coating layer is 3~4 μm; And / or, the thickness of the disordered carbon coating layer is 4~6 μm.

[0011] This invention also provides a method for preparing a composite carbon-coated polyanion cathode material, comprising the following steps: (1) After weighing sodium source, phosphorus source and iron source in proportion, they are mixed by sand milling to obtain mixed powder; (2) The mixed powder obtained in step (1) is mixed with a supplementary carbon source and sintered in a reducing atmosphere to obtain sodium iron phosphate pyrophosphate cathode material; (3) After the sodium iron phosphate pyrophosphate cathode material obtained in step (2) is mixed evenly with the first type of carbon source solid phase, it is sintered at high temperature in an inert atmosphere to obtain the first carbon-coated composite material. (4) The first carbon-coated composite material obtained in step (3) is mixed with the second type of carbon source, spray granulation is performed to obtain a mixed material, and the mixed material is sintered at low temperature in an inert atmosphere to obtain a composite carbon-coated polyanion cathode material.

[0012] The advantages and technical effects of the preparation method of composite carbon-coated polyanionic cathode material in this invention are as follows: 1. The method of this invention prepares sodium iron phosphate pyrophosphate cathode material, mixes it with a first type of carbon source, and then sintersects it at high temperature to obtain a highly conductive ordered carbon coating layer; 2. The method of this invention mixes the first carbon-coated composite material with a second type of carbon source and performs low-temperature sintering to obtain a carbon coating layer with a larger interlayer spacing (interlayer spacing 0.395nm) that is porous and rich in defects, which is beneficial to electron transfer and ion diffusion kinetics, and greatly improves the cycle life and rate performance of the sodium iron phosphate pyrophosphate system cell.

[0013] In some embodiments, in step (1), the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate, or sodium acetate. And / or, in step (1), the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, or sodium trihydrogen pyrophosphate. And / or, in step (1), the iron source includes at least one of iron oxide, ferric nitrate, ferric phosphate, ferric oxide, ferrous carbonate, ferrous oxalate, ferrous acetate, or ferrous citrate.

[0014] In some embodiments, in step (1), the molar ratio of sodium, iron, and phosphorus in the sodium source, iron source, and phosphorus source is (4~5):3:4; And / or, in step (1), the rotation speed of the sand mill is 500 to 3000 rpm, the sand milling time is 0.5 h to 2 h, and the D50 of the mixed powder after sand milling is 200 to 700 nm.

[0015] In some embodiments, in step (2), the supplementary carbon source includes at least one of soluble starch, ascorbic acid, citric acid, sucrose, or glucose; the mass ratio of the mixed powder to the supplementary carbon source is (9~10):1; And / or, in step (2), the reducing atmosphere is a nitrogen-hydrogen mixture, wherein the hydrogen content is 5-10% (V%). And / or, in step (2), the sintering adopts a two-stage sintering: first, the temperature is raised from room temperature to 250℃~400℃ at a heating rate of 1~2℃ / min, and held for 10~12h, and then the temperature is raised to 500~800℃ at a heating rate of 2~3℃ / min, and held for 15~24h. And / or, in step (2), the D50 of the sodium iron phosphate pyrophosphate cathode material is 8~10μm.

[0016] In some embodiments, in step (3), the first type of carbon source includes at least one of carbon black, acetylene black, conductive graphite, graphene or carbon nanotubes, and the mass ratio of the sodium iron phosphate pyrophosphate cathode material to the first type of carbon source is (5~15):1. And / or, in step (3), the inert atmosphere includes at least one of nitrogen or argon, the high-temperature sintering temperature is 600℃~800℃, the high-temperature sintering time is 8h~15h, and the heating rate is 0.5~5℃ / min; And / or, in step (3), the D50 of the first carbon-coated composite material is 10 to 12 μm.

[0017] In some embodiments, in step (4), the second type of carbon source includes at least one of phenolic resin, biomass sintering powder, or polyacrylonitrile. And / or, in step (4), the mass ratio of the first carbon-coated composite material and the second type of carbon source is (10~20):1, the D50 of the first carbon-coated composite material is 6~8μm, and the D50 of the mixed material obtained after spray granulation is 11~14μm; And / or, in step (4), the low-temperature sintering adopts segmented sintering, the temperature of the first stage sintering is 200~300℃, the time is 1~3h, the heating rate is 2~3℃ / min, the temperature of the second stage sintering is 300~500℃, the time is 2~5h, and the heating rate is 1~2℃ / min; the inert atmosphere includes at least one of argon or nitrogen.

[0018] The present invention also provides the application of the above-mentioned composite carbon-coated polyanion cathode material or the composite carbon-coated polyanion cathode material prepared by the above-mentioned preparation method in sodium-ion batteries. Detailed Implementation

[0019] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0020] The composite carbon-coated polyanionic cathode material of this invention includes a sodium iron phosphate pyrophosphate core, an ordered carbon coating layer on the surface of the core, and a disordered carbon coating layer on the surface of the ordered carbon coating layer.

[0021] The composite carbon-coated polyanion cathode material of this invention improves the conductivity of sodium iron phosphate pyrophosphate by coating it with an ordered carbon coating layer, thereby obtaining a sodium-ion battery with high capacity, high initial efficiency, and excellent cycle performance. In this invention embodiment, based on improving the conductivity of the polyanion cathode material by coating sodium iron phosphate pyrophosphate with an ordered carbon coating layer, a secondary carbon coating is performed by a disordered carbon coating layer to obtain an ordered-disorder composite carbon-coated polyanion material, which improves the battery's capacity, initial efficiency, and cycle performance while also taking into account rate performance.

[0022] In some embodiments, preferably, the core particle size of the sodium iron phosphate pyrophosphate has a D50 of 8~10 μm.

[0023] In some embodiments, preferably, the thickness of the ordered carbon coating layer is 3~4 μm, for example 3 μm, 3.5 μm or 4 μm, etc. And / or, the thickness of the disordered carbon coating layer is 4~6 μm, for example 4μm, 5μm or 6μm.

[0024] In this embodiment of the invention, the thickness of the ordered carbon coating layer is preferred, which is beneficial to maximizing the conductivity of the polyanionic material without affecting the electron migration rate. If the ordered carbon coating layer is too thick, it will actually affect the electron migration rate; if the ordered carbon coating layer is too thin, the effect of improving electron conductivity is not significant. Similarly, the thickness of the disordered carbon coating layer is preferred, which can maximize the migration rate of sodium ions. If the disordered carbon coating layer is too thick, it will actually affect the migration rate of sodium ions; if the disordered carbon coating layer is too thin, the effect of improving sodium ion conductivity is not significant.

[0025] This invention also provides a method for preparing a composite carbon-coated polyanion cathode material, comprising the following steps: (1) After weighing sodium source, phosphorus source and iron source in proportion, they are mixed by sand milling to obtain mixed powder; (2) The mixed powder obtained in step (1) is mixed with a supplementary carbon source and sintered in a reducing atmosphere to obtain sodium iron phosphate pyrophosphate cathode material; (3) After the sodium iron phosphate pyrophosphate cathode material obtained in step (2) is mixed evenly with the first type of carbon source solid phase, it is sintered at high temperature in an inert atmosphere to obtain the first carbon-coated composite material. (4) The first carbon-coated composite material obtained in step (3) is mixed with the second type of carbon source, spray granulation is performed to obtain a mixed material, and the mixed material is sintered at low temperature in an inert atmosphere to obtain a composite carbon-coated polyanion cathode material.

[0026] The advantages and technical effects of the preparation method of composite carbon-coated polyanionic cathode material in this invention are as follows: 1. The method of this invention prepares sodium iron phosphate pyrophosphate cathode material, mixes it with a first type of carbon source, and then sintersects it at high temperature to obtain a highly conductive ordered carbon coating layer; 2. The method of this invention mixes the first carbon-coated composite material with a second type of carbon source and performs low-temperature sintering to obtain a carbon coating layer with a larger interlayer spacing (interlayer spacing 0.395nm) that is porous and rich in defects, which is beneficial to electron transfer and ion diffusion kinetics, and greatly improves the cycle life and rate performance of the sodium iron phosphate pyrophosphate system cell.

[0027] In some embodiments, preferably, in step (1), the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate, or sodium acetate. And / or, in step (1), the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, or sodium trihydrogen pyrophosphate. And / or, in step (1), the iron source includes at least one of iron oxide, ferric nitrate, ferric phosphate, ferric oxide, ferrous carbonate, ferrous oxalate, ferrous acetate, or ferrous citrate.

[0028] In some embodiments, preferably, in step (1), the molar ratio of sodium, iron, and phosphorus in the sodium source, iron source, and phosphorus source is (4~5):3:4; And / or, in step (1), the rotation speed of the sand mill is 500 to 3000 rpm, the sand milling time is 0.5 h to 2 h, and the D50 of the mixed powder after sand milling is 200 to 700 nm.

[0029] In some embodiments, preferably, in step (2), the supplementary carbon source includes at least one of soluble starch, ascorbic acid, citric acid, sucrose, or glucose; the mass ratio of the mixed powder to the supplementary carbon source is (9~10):1; And / or, in step (2), the reducing atmosphere is a nitrogen-hydrogen mixture, wherein the hydrogen content is 5-10V.

[0030] In some embodiments, preferably, in step (2), the sintering adopts a two-stage sintering: first, the temperature is raised from room temperature to 250℃~400℃ at a heating rate of 1~2℃ / min, and held for 10~12h, and then the temperature is raised to 500~800℃ at a heating rate of 2~3℃ / min, and held for 15~24h. And / or, in step (2), the D50 of the sodium iron phosphate pyrophosphate cathode material is 8~10μm.

[0031] In this embodiment of the invention, a segmented sintering method is used to prepare sodium iron phosphate pyrophosphate cathode material, which can improve the specific capacity and rate performance of the material. This is because the segmented heating method can avoid the sudden decomposition of some organic substances and low-boiling-point impurities at high temperatures in the early stage of sintering, which would cause particle breakage or contamination of the target phase, thereby obtaining a purer target phase.

[0032] In some embodiments, preferably, in step (3), the first type of carbon source includes at least one of carbon black, acetylene black, conductive graphite, graphene or carbon nanotubes, and the mass ratio of the sodium iron phosphate pyrophosphate cathode material to the first type of carbon source is (5~15):1. And / or, in step (3), the inert atmosphere includes at least one of nitrogen or argon, the high-temperature sintering temperature is 600℃~800℃, the high-temperature sintering time is 8h~15h, and the heating rate is 0.5~5℃ / min; And / or, in step (3), the D50 of the first carbon-coated composite material is 10 to 12 μm.

[0033] In this embodiment of the invention, a higher sintering temperature is used when preparing the ordered carbon coating layer. The higher sintering temperature can ensure the uniformity and density of the carbon coating as much as possible.

[0034] In some embodiments, preferably, in step (4), the second type of carbon source includes at least one of phenolic resin, biomass sintering powder, or polyacrylonitrile; the biomass sintering powder includes at least one of coconut shell powder, walnut shell powder, apricot shell powder, or bamboo powder. And / or, in step (4), the mass ratio of the first carbon-coated composite material and the second type of carbon source is (10~20):1, the D50 of the first carbon-coated composite material is 6~8μm, and the D50 of the mixed material obtained after spray granulation is 11~14μm; And / or, in step (4), the low-temperature sintering adopts segmented sintering, the temperature of the first stage sintering is 200~300℃, the time is 1~3h, the heating rate is 2~3℃ / min, the temperature of the second stage sintering is 300~500℃, the time is 2~5h, and the heating rate is 1~2℃ / min; the inert atmosphere includes at least one of argon or nitrogen.

[0035] In this embodiment of the invention, segmented sintering is used to prepare disordered carbon coating, which facilitates the formation of a more uniform, continuous, and conductive coating layer, significantly improving the electronic conductivity and rate performance of the material. Furthermore, sintering is performed at a low temperature to prevent the disordered carbon from graphitizing at high temperatures, which would partially transform it into ordered carbon and negatively impact the performance.

[0036] The present invention also provides the application of the above-mentioned composite carbon-coated polyanion cathode material or the composite carbon-coated polyanion cathode material prepared by the above-mentioned preparation method in sodium-ion batteries.

[0037] The technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0038] Example 1 (1) Sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate were mixed and milled for 2 hours at a mill speed of 1200 rpm. The molar ratio of sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate was 4:3:4, and the particle size D50 of the material after milling was 500 nm. (2) The mixed powder after sand milling was mixed with ground sucrose and sintered in a tube furnace to obtain a polyanion cathode material with a particle size D50 of 8.2 μm. The mass ratio of the mixed powder to sucrose was 9:1. The sintering was carried out in two stages: the temperature was increased from room temperature to 350℃ at a heating rate of 1.5℃ / min and held for 12 h, and then increased to 700℃ at a heating rate of 2.5℃ / min and held for 20 h. The reducing atmosphere was a nitrogen-hydrogen mixture with a hydrogen content of 8% V%. (3) After crushing the polyanionic cathode material obtained in step (2), it is mixed evenly with conductive graphite in a high-speed mixer at a mass ratio of 10:1. The mixture is then placed in a tube furnace under an argon atmosphere for sintering at a temperature of 750℃, a heating rate of 2.5℃ / min, and a sintering time of 10h to obtain the first carbon-coated composite material with a particle size D50 of 10μm. (4) The first carbon-coated composite material and phenolic resin were added to the solvent ethanol and mixed evenly. The mass ratio of the precursor to the phenolic resin was 12:1. The D50 of the mixed material after spray granulation was 12.5 μm. The mixed material was added to a tube furnace under an argon atmosphere for segmented sintering. The temperature of the first stage sintering was 250℃, the time was 2h, and the heating rate was 2℃ / min. The temperature of the second stage sintering was 450℃, the time was 3.5h, and the heating rate was 1.5℃ / min.

[0039] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 3.6 μm, and the thickness of the disordered carbon coating layer is 5.2 μm.

[0040] Example 2 The preparation method of this embodiment is the same as that of Example 1, except that the sintering method in step (2) is changed to heating to 700°C at a heating rate of 1.5°C / min and holding for 32 hours.

[0041] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 2 μm, and the thickness of the disordered carbon coating layer is 5 μm.

[0042] Example 3 The preparation method of this embodiment is the same as that of Example 1, except that the sintering method in step (4) is changed to heating to 450°C at a heating rate of 2°C / min and holding for 5.5h.

[0043] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 3.6 μm, and the thickness of the disordered carbon coating layer is 4.4 μm.

[0044] Example 4 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of polyanionic cathode material to graphite in step (3) is 8:1.

[0045] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 3.9 μm, and the thickness of the disordered carbon coating layer is 5.2 μm.

[0046] Example 5 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of polyanionic cathode material to graphite in step (3) is 12:1.

[0047] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 3.3 μm, and the thickness of the disordered carbon coating layer is 5.2 μm.

[0048] Example 6 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of the first carbon-coated composite material and the phenolic resin in step (4) is 10:1.

[0049] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 3.6 μm, and the thickness of the disordered carbon coating layer is 5.6 μm.

[0050] Example 7 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of the first carbon-coated composite material and the phenolic resin in step (4) is 14:1.

[0051] In the composite carbon-coated polyanionic cathode material prepared in this embodiment, the particle size of the sodium iron phosphate pyrophosphate core is 8.5 μm, the thickness of the ordered carbon coating layer is 3.6 μm, and the thickness of the disordered carbon coating layer is 4.8 μm.

[0052] Comparative Example 1 (1) Mix sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate, and mill them in a sand mill for 2 hours at a speed of 1200 rpm. The molar ratio of sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate is 4:3:4. (2) The mixed powder after sand milling was mixed with ground sucrose and sintered in a tube furnace to obtain a polyanion cathode material with a particle size D50 of 8.2 μm. The mass ratio of the mixed powder to sucrose was 9:1. The sintering was carried out in two stages: the temperature was increased from room temperature to 350℃ at a heating rate of 1.5℃ / min and held for 12 h, and then increased to 700℃ at a heating rate of 2.5℃ / min and held for 20 h. The reducing atmosphere was a nitrogen-hydrogen mixture with a hydrogen content of 8 V.

[0053] Comparative Example 2 (1) Mix sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate, and mill them in a sand mill for 2 hours at a speed of 1200 rpm. The molar ratio of sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate is 4:3:4. (2) The powder after sand milling was mixed with the ground sucrose and sintered in a tube furnace to obtain a polyanion cathode material with a particle size D50 of 8.2 μm. The mass ratio of the mixed powder to sucrose was 9:1. The sintering was carried out in two stages: the temperature was increased from room temperature to 350℃ at a heating rate of 1.5℃ / min and held for 12 h, and then increased to 700℃ at a heating rate of 2.5℃ / min and held for 20 h. The reducing atmosphere was a nitrogen-hydrogen mixture with a hydrogen content of 8%. (3) After crushing the polyanionic cathode material obtained in step (2), it is mixed evenly with graphite in a high-speed mixer at a mass ratio of 10:1. The mixture is then placed in a tube furnace under an argon atmosphere for sintering at a temperature of 750℃, a heating rate of 2.5℃ / min, and a sintering time of 10h to obtain a polyanionic cathode material and a first carbon-coated composite precursor with a particle size D50 of 10μm.

[0054] Comparative Example 3 (1) Mix sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate, and mill them in a sand mill for 2 hours at a speed of 1200 rpm. The molar ratio of sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate is 4:3:4. (2) The powder after sand milling was mixed with the ground sucrose and sintered in a tube furnace to obtain a polyanion cathode material with a particle size D50 of 8.2 μm. The mass ratio of the mixed powder to sucrose was 9:1. The sintering was carried out in two stages: the temperature was increased from room temperature to 350℃ at a heating rate of 1.5℃ / min and held for 12 h, and then increased to 700℃ at a heating rate of 2.5℃ / min and held for 20 h. The reducing atmosphere was a nitrogen-hydrogen mixture with a hydrogen content of 8%. (3) After crushing the polyanionic cathode material obtained in step (2), it is mixed evenly with phenolic resin in solvent ethanol. The mass ratio of precursor to phenolic resin is 12:1. The powder after spray granulation is added to a tube furnace under argon atmosphere for segmented sintering. The temperature of the first stage sintering is 250℃, the time is 2h, and the heating rate is 2℃ / min. The temperature of the second stage sintering is 450℃, the time is 3.5h, and the heating rate is 1.5℃ / min.

[0055] The materials prepared in Examples 1-7 and Comparative Examples 1-3 were used as positive electrode materials. The positive electrode materials, carbon black conductive agent and PVDF binder were added to NMP and mixed evenly in a mass ratio of 90:5:5 to prepare a battery positive electrode slurry with a solid content of 45%. The slurry was coated on an aluminum foil with a thickness of 10 μm, and then vacuum dried and rolled to form a positive electrode sheet. A sodium metal sheet was used as the negative electrode, and a solution of 1.15M NaPF6 and EC:DMC (1:1 vol%) was used as the electrolyte to assemble a coin cell.

[0056] The button cells prepared above were tested using a Blue Battery testing system. The test temperature was 25℃, and the test voltage was 1.5V-3.4V. The charge-discharge capacity and first-cycle efficiency of the cells were tested at 0.1C / 0.33C / 0.5C. The cells were then cycled for 200 cycles at 0.5C / 1C. The cycle retention rate after 200 cycles was obtained by dividing the discharge capacity at the 200th cycle by the discharge capacity at the first cycle. The test results are shown in Table 1. Table 1

[0057] Based on the charge-discharge tests and 0.5C / 1C cycling at different rates in Table 1, and comparing Examples 1-7 with Comparative Examples 1-3, it can be seen that the introduction of ordered conductive carbon coating layers and disordered defect-rich carbon coating layers in this invention can effectively improve the battery's capacity and rate performance, and also improve the cycle life of sodium batteries at room temperature. Comparing Examples 1 and 2-3, it can be seen that the segmented heating method can improve the specific capacity and rate performance of the material. This is because the segmented heating method can avoid the sudden decomposition of some organic substances and low-boiling-point impurities at high temperatures during the early stage of sintering, which could lead to particle breakage or contamination of the target phase, thus obtaining a purer target phase. Furthermore, a more uniform, continuous, and conductive coating layer is formed during the carbon coating process, significantly improving the material's electronic conductivity and rate performance. Examples 1 and Examples 4-5 and 6-7 show that further increasing the amount of ordered and disordered carbon sources cannot significantly improve the specific capacity and cycle life of the material. Excessive ordered carbon coating forms an overly thick coating layer, which actually affects the sodium battery's performance. + The transport and transfer of electrons deteriorate the properties of the material.

[0058] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0059] Although the above embodiments have been shown and described, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made to the above embodiments by those skilled in the art are within the protection scope of the present invention.

Claims

1. A composite carbon-coated polyanionic cathode material, characterized in that, The sodium pyrophosphate ferric phosphate core, the ordered carbon coating layer on the surface of the core, and the disordered carbon coating layer on the surface of the ordered carbon coating layer.

2. The composite carbon-coated polyanion cathode material of claim 1, wherein, The D50 of the sodium pyrophosphate ferric phosphate core is 8-10 μm.

3. The composite carbon-coated polyanion cathode material of claim 1, wherein, The thickness of the ordered carbon coating layer is 3-4 μm. And / or, the thickness of the disordered carbon coating layer is 4-6 μm.

4. The method for preparing the composite carbon-coated polyanionic cathode material according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: (1) After the sodium source, the phosphorus source and the iron source are weighed in proportion, the mixed powder is obtained by sand milling; (2) The mixed powder obtained in step (1) is mixed with a supplementary carbon source, and sintered in a reducing atmosphere to obtain a sodium pyrophosphate ferric phosphate positive electrode material; (3) The sodium pyrophosphate ferric phosphate positive electrode material obtained in step (2) and the first type of carbon source are mixed uniformly in solid phase, and then sintered at high temperature in an inert atmosphere to obtain a first carbon-coated composite material; (4) The first carbon-coated composite material obtained in step (3) is mixed with a second type of carbon source, spray granulated to obtain a mixed material, and then sintered at low temperature in an inert atmosphere to obtain a composite carbon-coated polyanion positive electrode material.

5. The method for preparing the composite carbon-coated polyanionic cathode material according to claim 4, characterized in that, In step (1), the sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate or sodium acetate; And / or, in step (1), the phosphorus source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate or monosodium pyrophosphate; And / or, in step (1), the iron source includes at least one of iron oxide, iron nitrate, iron phosphate, magnetite, ferrous carbonate, ferrous oxalate, ferrous acetate or ferrous citrate.

6. The method for preparing the composite carbon-coated polyanionic cathode material according to claim 4 or 5, characterized in that, In step (1), the molar ratio of sodium, iron and phosphorus in the sodium source, the iron source and the phosphorus source is (4-5):3:4; And / or, in step (1), the rotation speed of the sand mill is 500-3000 rpm, the sand milling time is 0.5-2 h, and the D50 of the mixed powder after sand milling is 200-700 nm.

7. The method for preparing the composite carbon-coated polyanionic cathode material according to claim 4, characterized in that, In step (2), the supplementary carbon source includes at least one of soluble starch, ascorbic acid, citric acid, sucrose or glucose; and the mass ratio of the mixed powder and the supplementary carbon source is (9-10):1; And / or, in step (2), the reducing atmosphere is a nitrogen-hydrogen mixed gas, wherein the hydrogen content is 5-10 V%; And / or, in step (2), the sintering adopts two-stage sintering: first, the temperature is raised from room temperature to 250-400℃ at a rate of 1-2 ℃ / min, and then the temperature is raised to 500-800℃ at a rate of 2-3 ℃ / min after holding for 10-12 h, and holding for 15-24 h; And / or, in step (2), the D50 of the sodium pyrophosphate ferric phosphate positive electrode material is 8-10 μm.

8. The method for preparing the composite carbon-coated polyanionic cathode material according to claim 4, characterized in that, In step (3), the first type of carbon source includes at least one of carbon black, acetylene black, conductive graphite, graphene or carbon nanotubes, and the mass ratio of the sodium pyrophosphate ferric phosphate positive electrode material and the first type of carbon source is (5-15):1; And / or, in the step (3), the inert atmosphere comprises at least one of nitrogen or argon, the temperature of the high-temperature sintering is 600-800℃, the high-temperature sintering time is 8-15h, and the heating rate is 0.5-5℃ / min; And / or, in the step (3), the D50 of the first carbon-coated composite material is 10-12μm.

9. The method for preparing the composite carbon-coated polyanionic cathode material according to claim 4, characterized in that, In the step (4), the second type of carbon source comprises at least one of phenolic resin, biomass sintering powder or polypropylene. And / or, in the step (4), the mass ratio of the first carbon-coated composite material to the second type of carbon source is (10-20):1, and the D50 of the mixed material obtained after the spray granulation is 11-14μm. And / or, in the step (4), the low-temperature sintering adopts segmented sintering, the temperature of the first-stage sintering is 200-300℃, the time is 1-3h, and the heating rate is 2-3℃ / min, the temperature of the second-stage sintering is 300-500℃, the time is 2-5h, and the heating rate is 1-2℃ / min; the inert atmosphere comprises at least one of argon or nitrogen.

10. Use of the composite carbon-coated polyanion positive electrode material according to any one of claims 1-3 or prepared by the preparation method of any one of claims 4-9 in a sodium ion battery.

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