A heteroelement-doped carbon-coated polyanion cathode material and preparation method thereof

Abstract

The present invention provides a heteroelement-doped carbon-coated polyanion cathode material, the general structural formula of which is Na 3+x M 2+x (PO4) 1+ x P2O7@P‑N / C, wherein 0≤x≤2, M is one or more of Ni, Fe, Co, and Mn, and the polyanion cathode material is a core-shell structure, Na 3+x M 2+x (PO4) 1+x P2O7 is the core of the core-shell structure of the polyanion cathode material, @P-N / C represents the coating layer formed on the surface of the core material, and P-N / C is a carbon coating layer co-doped with phosphorus and nitrogen. The polyanion cathode material doped with heteroelement carbon in the present invention is a core-shell structure. 3+x M 2+x (PO4) 1+ x The carbon coating layer on the surface of P2O7 material co-doped with phosphorus and nitrogen is a sheet-like structure with high sp 2 Hybrid carbon structure, excellent conductivity, and phosphorus and nitrogen co-doped carbon coating cooperate with each other to reasonably activate the heterogeneous phase, improve capacity and cycle performance, high conductivity P-N / C, phosphorus and nitrogen co-doped carbon coating, and core Na 3+x M 2+x (PO4) 1+x The synergistic effect of P2O7 improves the overall performance of the material, including capacity and cycle performance.

Description

Technical Field

[0001] The present invention relates to the field of battery positive electrode materials, and in particular to a heteroelement-doped carbon-coated polyanion positive electrode material and a preparation method thereof. Background Art

[0002] In recent years, sodium-ion batteries (SIBs), constrained by limited lithium resources and high costs, have attracted global attention in large-scale energy storage and small-scale power applications due to their relatively high natural abundance, potentially low raw material costs, and excellent electrochemical performance. They are gradually becoming an important technological option in the "post-lithium battery era." Because hard carbon anodes in SIBs systems can provide a reversible capacity of approximately 300 mAh / g, the primary challenge in the practical application of SIBs lies in the lack of low-cost, high-performance cathode materials.

[0003] In the early stages of research, inspired by the successful experience of LiFePO4 in lithium-ion batteries, NaFePO4 became the focus of research due to its high specific capacity of 155 mAh / g. Unfortunately, unlike LiFePO4, the thermodynamically stable maricite-NaFePO4 is electrochemically inert, while the electrochemically active olivine-NaFePO4 is not a thermodynamically stable phase structure, and the preparation process is very complicated (Vacuum, 2023, 210:111853). In addition, Na2FeP2O7, derived from NaFePO4, can be directly synthesized via a solid-state route. Its lattice distortion is small during the charge and discharge process, resulting in good cycle performance and strong structural stability. However, this cathode material is limited by its single-electron reaction, resulting in a low theoretical capacity (approximately 97.1 mAh / g), which greatly hinders its commercial application. Iron-based composite phosphate (NFPP), which combines the advantages of both NaFePO4 and Na2FeP2O7, can produce a theoretical specific capacity of about 128.9 mAh / g. Coupled with its low volumetric strain, high thermal stability and higher working potential, it is considered to be a promising SIBs positive electrode material with NASICON structure.

[0004] In recent years, a large number of studies on NFPP have been published. For example, Professor Xia Yongyao's team at Fudan University reported a Na3Fe2PO4P2O7 cathode material composed of NaFePO4 and Na2FeP2O7 in a 1:1 molar ratio. The addition of r-GO enhanced the material's conductivity, but its actual specific capacity was low, reaching 110.2 mAh / g at 0.1C (ACS Energy Lett. 2020, 5, 3788-3796). Professor Cao Yuliang's team at Wuhan University further increased the molar ratio of NaFePO4 and Na2FeP2O7 to 2:1, resulting in a slightly improved specific capacity of the prepared material, reaching 110.9 mAh / g at 0.2C (Nano Energy 91(2022)106680). Increasing the proportion of NaFePO4 in the composite phosphate increases the material's theoretical specific capacity, but in most current NFPP research, simply changing the NaFePO4 ratio in the composite phosphate is not sufficient to significantly improve the battery performance of iron-based composite phosphate (NFPP) cathode materials. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a heteroelement-doped carbon-coated polyanion cathode material and a preparation method thereof.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a heteroelement-doped carbon-coated polyanion cathode material, the structural formula of the polyanion cathode material is

[0007] Na 3+x M 2+x (PO4) 1+x P2O7@PN / C, wherein 0≤x≤2, M is one or more of Ni, Fe, Co, and Mn, and the polyanion cathode material is a core-shell structure, Na 3+x M 2+x (PO4) 1+x P2O7 is the core of the core-shell structure of the polyanion positive electrode material, @PN / C represents a coating layer formed on the surface of the core material, and PN / C is a carbon coating layer co-doped with phosphorus and nitrogen elements.

[0008] The above-mentioned heteroelement-doped carbon-coated polyanion cathode material is a core-shell structure. 3+x M 2+x (PO4) 1+ x The surface of the P2O7 material forms a PN / C coating layer, a carbon coating layer co-doped with phosphorus and nitrogen, which is a sheet structure and has a high sp 2Hybrid carbon structure, excellent conductivity, and phosphorus and nitrogen co-doped carbon coating cooperate with each other, reasonably activate the heterogeneous phase, improve capacity and cycle performance, high conductivity PN / C, phosphorus and nitrogen co-doped carbon coating, and core Na 3+x M 2+x (PO4) 1+x The synergistic effect of P2O7 improves the overall performance of the material, including capacity and cycle performance. Moreover, the doping of phosphorus is different from that of other elements in the coating layer. The core Na 3+x M 2+x (PO4) 1+x The P2O7 material is mainly composed of phosphate pyrophosphate, and phosphorus is doped into the coating layer. 3+x M 2+x (PO4) 1+x The phase composition of P2O7 does not cause adverse interference, and the high conductivity PN / C is achieved. The carbon coating layer co-doped with phosphorus and nitrogen elements is consistent with the core Na 3+ x M 2+x (PO4) 1+x P2O7 produces synergy and overcomes the adverse effects of phosphorus doping on the phase composition of the core material.

[0009] In heteroelement-doped carbon-coated polyanion cathode materials, Na 3+x M 2+x (PO4) 1+x P2O7 materials may be composed of a multiphase structure, specifically including one or more phases such as NaMPO4, Na2MP2O7, and Na4M3(PO4)2P2O7. When 0≤x<1, the cathode material is a mixed phase structure composed of Na2MP2O7 and Na4M3(PO4)2P2O7. When x=1, the cathode material is a pure phase structure of Na4M3(PO4)2P2O7. When 1<x≤2, the cathode material is a mixed phase structure composed of NaMPO4 and Na4M3(PO4)2P2O7. The phase of the material can be controlled by controlling the ratio of the source materials during the preparation process.

[0010] Preferably, the preparation method of PN / C comprises uniformly dispersing an organic carbon source, an organic phosphorus source, and an organic nitrogen source, heating the mixture to 600-1000° C. in an inert gas atmosphere, and then sintering the mixture at a constant temperature for 5-20 hours to obtain a PN / C carbon material.

[0011] The above-mentioned heteroelement-doped carbon-coated polyanion cathode material is prepared by sintering an organic carbon source, an organic phosphorus source, and an organic nitrogen source to prepare a phosphorus and nitrogen co-doped carbon material (PN / C), and then 3+x M2+x (PO4) 1+x The mixed sintering of the source materials of P2O7 material overcomes the adverse effect of phosphorus doping on the phase composition of the core material.

[0012] Preferably, M is one or more of Ni, Fe, and Co.

[0013] Preferably, 1≤x≤2.

[0014] Preferably, the mass of the PN / C coating layer accounts for 0.1% to 5.0% of the mass of the polyanion positive electrode material.

[0015] Preferably, the mass of the PN / C coating layer accounts for 0.5% to 3.0% of the mass of the polyanion positive electrode material.

[0016] The above-mentioned heteroelement-doped carbon-coated polyanion cathode material has been found to have better capacity and cycle performance when the mass of the PN / C coating layer accounts for 0.5% to 3.0% of the mass of the polyanion cathode material.

[0017] Preferably, in the process of preparing the PN / C carbon material, the weight ratio of phosphorus, nitrogen and carbon in the organic phosphorus source, organic nitrogen source and organic carbon source is a:b:100, wherein 1.0≤a≤5.0; 20≤b≤50.

[0018] Preferably, in the process of preparing the PN / C carbon material, the weight ratio of phosphorus, nitrogen and carbon in the organic phosphorus source, organic nitrogen source and organic carbon source is a:b:100, wherein 3≤a≤8; 30≤b≤45.

[0019] The study found that when the weight ratio of phosphorus, nitrogen and carbon elements in the organic phosphorus source, organic nitrogen source and organic carbon source meets the above requirements, the polyanion positive electrode material doped with heteroelement carbon has better capacity and cycle performance.

[0020] Preferably, the organic carbon source is one or more of glucose, chitosan, sucrose, starch, citric acid, sodium citrate, lauric acid, and sodium laurate.

[0021] Preferably, the organic phosphorus source is one of triphenylphosphine and phytic acid.

[0022] Preferably, the organic nitrogen source is one or more of melamine, urea, and chitosan.

[0023] Preferably, the preparation method of the polyanion positive electrode material comprises the following steps: 3+x M 2+x (PO4) 1+xAfter the precursor materials of P2O7 are dispersed and mixed, the temperature is raised to 420-600℃ under an inert gas atmosphere and then sintered at a constant temperature for 5-20 hours to obtain Na 3+x M 2+x (PO4) 1+x P2O7@PN / C.

[0024] Preferably, Na 3+x M 2+x (PO4) 1+x The source materials of P2O7 include sodium source, M source and phosphorus source.

[0025] Preferably, the sodium source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium nitrate, and sodium citrate.

[0026] More preferably, the sodium source is one or more of sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate and sodium citrate.

[0027] Preferably, the M source is one or more of phosphates, nitrates, sulfates, oxalates, acetates and oxides corresponding to one or more elements of Ni, Fe, Co and Mn.

[0028] Preferably, the phosphorus source is one or more of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, ferric phosphate, and ferric pyrophosphate.

[0029] More preferably, the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, and iron phosphate.

[0030] The present invention also provides a method for preparing any of the above-mentioned heteroelement-doped carbon-coated polyanion positive electrode materials, the method comprising the following steps:

[0031] (1) grinding and mixing an organic carbon source, an organic nitrogen source, and an organic phosphorus source according to a weight ratio, and then sintering in one step or two steps under an inert gas atmosphere to obtain a PN / C carbon material;

[0032] (2) According to the preset x value, the PN / C carbon material, the sodium source, the M source and the dispersant are dispersed or dissolved in a solvent in a stoichiometric ratio and ground to obtain a mixed slurry H2. After removing the solvent and drying H2, a precursor material to be sintered H3 in which all elements are evenly dispersed is obtained;

[0033] (3) The obtained H3 is subjected to a one-step or two-step sintering treatment in an inert atmosphere to obtain a polyanion positive electrode material doped with hetero-element or carbon-coated with hetero-element.

[0034] Preferably, in step (1), the grinding process is a ball milling process, wherein the rotation speed of the ball milling process is 400-800 rpm, the ball-to-material ratio is 2-20:1, the diameter of the zirconia ball is 0.1-0.5 mm, and the ball milling time is 2-5 h.

[0035] Preferably, in step (2), the grinding process includes a ball milling process and a sand milling process in sequence, wherein the ball milling speed is 400-800 rpm; the ball-to-material ratio is 2-20:1, and the diameter of the zirconia ball is 0.1-0.5 mm; the solid content of the sand milling is 15%-40%, and the sand milling speed is 1000-2400 rpm; the particle size of the solid in the slurry after sand milling is D50≤6μm and Dmax≤30μm.

[0036] Preferably, in step (2), the dispersant is one or more of PVP, PVA, NPEOs, PEG, APG, and PAA.

[0037] Preferably, in step (2), the solvent is one of water, methanol, ethanol, acetone, ethylene glycol and n-butanol.

[0038] Preferably, in step (2), the drying method is spray drying, freeze drying, high temperature drying or vacuum drying, and the drying temperature is not higher than 280°C.

[0039] More preferably, the drying method is spray drying; the inlet air temperature of the spray drying is 180-280°C, and the outlet air temperature is 80-120°C.

[0040] Preferably, in step (1), in the one-step sintering or two-step sintering, the procedure for the one-step sintering is: in an inert protective atmosphere, heating to 600-1000°C at a heating rate of 1-5°C / min, and sintering at a constant temperature for 5-20 hours; then cooling to room temperature at a cooling rate of 2-10°C / min; the procedure for the two-step sintering is: in an inert protective atmosphere, heating to a pre-sintering temperature of 400-550°C at a heating rate of 1-5°C / min for 2-4 hours; then heating to 600-1000°C at a heating rate of 3-6°C / min for 4-10 hours; then cooling to room temperature at a cooling rate of 2-10°C / min.

[0041] Preferably, in step (3), in the one-step sintering or two-step sintering, the procedure for the one-step sintering is: in an inert protective atmosphere, heating to 420-600°C at a heating rate of 1-5°C / min, and sintering at a constant temperature for 5-20 hours; then cooling to room temperature at a cooling rate of 2-10°C / min; the procedure for the two-step sintering is: in an inert protective atmosphere, heating to a pre-sintering temperature of 250-420°C at a heating rate of 1-5°C / min for 2-4 hours; then heating to 520-600°C at a heating rate of 3-6°C / min for 4-10 hours; then cooling to room temperature at a cooling rate of 2-10°C / min.

[0042] The beneficial effects of the present invention are as follows: the present invention provides a heteroelement-doped carbon-coated polyanion cathode material and a preparation method thereof, wherein the heteroelement-doped carbon-coated polyanion cathode material of the present invention is a core-shell structure. 3+x M 2+x (PO4) 1+x The surface of P2O7 material, PN / C, phosphorus and nitrogen co-doped carbon coating, is a sheet structure with high sp 2 Hybrid carbon structure, excellent conductivity, and phosphorus and nitrogen co-doped carbon coating cooperate with each other, reasonably activate the heterogeneous phase, improve capacity and cycle performance, high conductivity PN / C, phosphorus and nitrogen co-doped carbon coating, and core Na 3+x M 2+x (PO4) 1+x The synergistic effect of P2O7 improves the overall performance of the material, including capacity and cycle performance. Moreover, the doping of phosphorus is different from that of other elements in the coating layer. The core Na 3+x M 2+x (PO4) 1+x P2O7 material is based on phosphate pyrophosphate, and phosphorus is doped as a coating layer to reduce the Na content in the polyanion positive electrode material. 3+x M 2+x (PO4) 1+x The phase composition of P2O7 does not cause adverse interference, and the high conductivity PN / C is achieved. The carbon coating layer co-doped with phosphorus and nitrogen elements is consistent with the core Na 3+x M 2+x (PO4) 1+x P2O7 produces synergy and overcomes the adverse effects of phosphorus doping on the phase composition of the core material. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the preparation process of the hetero-element doped carbon-coated polyanion positive electrode material of the present invention.

[0044] Figure 2This is a scanning electron microscope image of the PN / C prepared in Example 1 of the hetero-element doped carbon-coated polyanion positive electrode material of the present invention.

[0045] Figure 3 The Na prepared in Example 1 of the heteroelement-doped carbon-coated polyanion cathode material of the present invention 4.3 M 3.3 (PO4) 2.3 Scanning electron microscope image of P2O7@PN / C.

[0046] Figure 4 These are X-ray diffraction patterns of the heteroelement-doped carbon-coated polyanion positive electrode materials of Example 1, Example 14, and Example 15; Intensity in the figure represents intensity.

[0047] Figure 5 The charge-discharge curves of sodium ion button cells assembled from the samples of Example 1 and Example 10 of the heteroelement-doped carbon-coated polyanion cathode material of the present invention at a rate of 0.1C, with an operating voltage of 1.5-4.0 V vs. Na + / Na; in the figure, Voltage represents voltage, and Specific capacity represents specific capacity.

[0048] Figure 6 Cycling curves of sodium ion button batteries assembled from the heteroelement-doped carbon-coated polyanion cathode material Example 1 and Comparative Examples 1, 2, and 3 of the present invention; Specific capacity represents the specific capacity, and Cycle number represents the number of cycles. DETAILED DESCRIPTION

[0049] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0050] Example 1

[0051] As a heteroelement-doped carbon-coated polyanion cathode material according to an embodiment of the present invention, the general structural formula of the polyanion cathode material is Na 3+x M 2+x (PO4) 1+x P2O7@PN / C, wherein x=1.3, M is Fe, the polyanion cathode material is a core-shell structure, Na 3+x M 2+x (PO4) 1+xP2O7 forms the core of the core-shell structure of the polyanion cathode material. @PN / C represents the coating formed on the surface of the core material. PN / C is a carbon coating co-doped with phosphorus and nitrogen. Thermogravimetric analysis determined that the mass of the PN / C coating accounted for 1.80% of the mass of the polyanion cathode material.

[0052] The different contents of phosphorus, nitrogen, and carbon in the PN / C carbon material are controlled by varying the amounts of organic carbon source, organic nitrogen source, and organic phosphorus source added during the preparation process. Specifically, the weight ratio of phosphorus, nitrogen, and carbon in the organic carbon source, organic nitrogen source, and organic phosphorus source is 2:30:100.

[0053] The preparation method of the heteroelement-doped carbon-coated polyanion cathode material comprises the following steps:

[0054] (1) grinding and mixing an organic carbon source, an organic nitrogen source, and an organic phosphorus source according to a weight ratio, and then sintering them in an inert gas atmosphere to obtain a PN / C carbon material; the organic carbon source is chitosan, the organic phosphorus source is triphenylphosphine, and the organic nitrogen source is urea; the grinding process used is a ball milling process, wherein the ball milling process speed is set to 600 rpm, the ball-to-material ratio is 10:1, the diameter of the zirconia ball is 0.2 mm, and the ball milling time is 2 h;

[0055] The one-step sintering procedure is as follows: in a nitrogen inert atmosphere, the temperature is raised to 900°C at a heating rate of 3°C / min, sintered at a constant temperature for 8 h, and then cooled to room temperature at a cooling rate of 5°C / min;

[0056] (2) PN / C carbon material, sodium source, M source and PVA dispersant are dispersed or dissolved in methanol solvent according to the stoichiometric ratio and ground to obtain a mixed slurry H2. After H2 is subjected to solvent removal and drying treatment, a precursor material to be sintered H3 in which all elements are uniformly dispersed is obtained; the drying method is spray drying, the inlet air temperature of the spray drying is 250°C, and the outlet air temperature is 110°C; the grinding process includes a ball milling process and a sand milling process in sequence, wherein the solid content of the ball milling and ball milling is 25%; the ball milling speed is 600 rpm; the ball-to-material ratio is 6:1, and the diameter of the zirconia ball is 0.3 mm; the sand milling speed is 1500 rpm; the particle size of the solid in the slurry after sand milling is D50≤6μm, Dmax≤30μm; the sodium source is sodium dihydrogen phosphate, the M source is Ni oxalate; and the phosphorus source is ammonium dihydrogen phosphate;

[0057] (3) The obtained H3 is subjected to a one-step sintering treatment in a nitrogen inert atmosphere to obtain a polyanion positive electrode material coated with heteroelement-doped element-doped carbon; the one-step sintering procedure is as follows: in a nitrogen inert protective atmosphere, the temperature is increased to 520°C at a heating rate of 3°C / min, and the temperature is kept constant for 9 hours; and then the temperature is cooled to room temperature at a cooling rate of 5°C / min.

[0058] Example 2

[0059] As a heteroelement-doped carbon-coated polyanion positive electrode material of the present invention, the only difference between this embodiment and embodiment 1 is that: by controlling the preparation method step (2), the PN / C carbon material and Na 3+x M 2+x (PO4) 1+x The weight proportion of the P2O7 source material was determined by thermogravimetric analysis to be 0.21% of the weight of the PN / C coating layer of the polyanion positive electrode material.

[0060] Example 3

[0061] As a heteroelement-doped carbon-coated polyanion positive electrode material of the present invention, the only difference between this embodiment and embodiment 1 is that: by controlling the preparation method step (2), the PN / C carbon material and Na 3+x M 2+x (PO4) 1+x The weight proportion of the P2O7 source material was determined by thermogravimetric analysis to be 0.53% of the weight of the PN / C coating layer of the polyanion positive electrode material.

[0062] Example 4

[0063] As a heteroelement-doped carbon-coated polyanion positive electrode material of the present invention, the only difference between this embodiment and embodiment 1 is that: by controlling the preparation method step (2), the PN / C carbon material and Na 3+x M 2+x (PO4) 1+x The weight proportion of the P2O7 source material was determined by thermogravimetric analysis to be 1.18% of the weight of the PN / C coating layer of the polyanion positive electrode material.

[0064] Example 5

[0065] As a heteroelement-doped carbon-coated polyanion positive electrode material of the present invention, the only difference between this embodiment and embodiment 1 is that: by controlling the preparation method step (2), the PN / C carbon material and Na 3+x M 2+x (PO4) 1+xThe weight proportion of the P2O7 source material was determined by thermogravimetric analysis to be 3.27% of the weight of the PN / C coating layer of the polyanion positive electrode material.

[0066] Example 6

[0067] As a heteroelement-doped carbon-coated polyanion positive electrode material of the present invention, the only difference between this embodiment and embodiment 1 is that: by controlling the preparation method step (2), the PN / C carbon material and Na 3+x M 2+x (PO4) 1+x The weight proportion of the P2O7 source material was determined by thermogravimetric analysis to be 4.18% of the weight of the PN / C coating layer of the polyanion positive electrode material.

[0068] Example 7

[0069] As a heteroelement-doped carbon-coated polyanion positive electrode material of an embodiment of the present invention, the only difference between this embodiment and Example 1 is that when preparing the PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic carbon source, organic nitrogen source and organic phosphorus source is 4.5:30:100.

[0070] Example 8

[0071] As a heteroelement-doped carbon-coated polyanion positive electrode material of an embodiment of the present invention, the only difference between this embodiment and Example 1 is that when preparing the PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic carbon source, organic nitrogen source and organic phosphorus source is 7.5:40:100.

[0072] Example 9

[0073] As a heteroelement-doped carbon-coated polyanion positive electrode material of an embodiment of the present invention, the only difference between this embodiment and Example 1 is that when preparing the PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic carbon source, organic nitrogen source and organic phosphorus source is 10:35:100.

[0074] Example 10

[0075] As a heteroelement-doped carbon-coated polyanion positive electrode material of an embodiment of the present invention, the only difference between this embodiment and Example 1 is that when preparing the PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic carbon source, organic nitrogen source and organic phosphorus source is 5:20:100.

[0076] Example 11

[0077] As a heteroelement-doped carbon-coated polyanion positive electrode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is that M is Ni.

[0078] Example 12

[0079] As a heteroelement-doped carbon-coated polyanion positive electrode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is that M is Mn.

[0080] Example 13

[0081] As a heteroelement-doped carbon-coated polyanion positive electrode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is that M is Co.

[0082] Example 14

[0083] As a heteroelement-doped carbon-coated polyanion positive electrode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is that x=0.6.

[0084] Example 15

[0085] As a heteroelement-doped carbon-coated polyanion positive electrode material according to an embodiment of the present invention, the only difference between this embodiment and Example 1 is that x=1.

[0086] Comparative Example 1

[0087] As a comparative example of the present invention, a polyanion cathode material coated with hetero-element doped carbon is prepared. The only difference between this comparative example and Example 1 is that the coating layer is made of an organic carbon source and does not contain phosphorus or nitrogen doping. Specifically, in step (1) of the preparation method, except that the organic nitrogen source and organic phosphorus source are not added, the remaining grinding and sintering processes are exactly the same.

[0088] Comparative Example 2

[0089] As a comparative example of the present invention, a heteroelement-doped carbon-coated polyanion positive electrode material, the only difference between this comparative example and Example 1 is that the coating layer material is different, does not contain phosphorus, and the phosphorus content is supplemented by nitrogen.

[0090] Comparative Example 3

[0091] As a comparative example of the present invention, a heteroelement-doped carbon-coated polyanion positive electrode material, the only difference between this comparative example and Example 1 is that the coating layer material is different, does not contain nitrogen, and the nitrogen content is supplemented by phosphorus.

[0092] Experimental methods

[0093] 1. Material Characterization

[0094] The heteroelement-doped carbon-coated polyanion positive electrode materials of Example 1, Example 2, Example 3, Example 10, Example 14, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were characterized.

[0095] The PN / C and positive electrode samples prepared in Example 1 of the present invention were subjected to scanning electron microscopy, and the results were as follows: Figure 2 and Figure 3 shown.

[0096] In order to explore the different values ​​of x, the prepared Na 3+x M 2+x (PO4) 1+x The specific structure and composition of the P2O7 positive electrode material are shown in the following figure: Figure 4 As shown; Intensity in the figure represents intensity.

[0097] In order to explore the effects of carbon materials doped with different weight ratios of impurity elements on the performance of positive electrode materials, electrochemical performance tests were carried out on the samples of Example 1 and Example 10. Figure 5 The charge and discharge curves of the sodium ion button battery assembled for the above sample at a rate of 0.1C, with an operating voltage of 1.5~4.0V vs.Na + / Na; in the figure, Voltage represents voltage, and Specificcapacity represents specific capacity.

[0098] In order to explore the effect of impurity doping on the performance of the positive electrode material, the samples of Example 1 and Comparative Examples 1-3 were subjected to charge and discharge cycle performance tests at 1C, and the voltage range of the constant current charge and discharge test was 1.5~4.0V; Figure 6 1 and 2 are cycle curves of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3; in the figure, Specific capacity represents specific capacity, and Cycle number represents the number of cycles.

[0099] 2. Battery performance test

[0100] Battery Performance Testing Method: Using the mixed phosphate materials prepared in Examples 1-15 and Comparative Examples 1-4 as the positive electrode material for sodium-ion batteries, the positive electrode active material, conductive agent (super-P), and binder (PVDF) were added to NMP in a mass ratio of 80:5:15 in a dry room (humidity below 15%) for slurrying, with the solid content controlled at 50%. After slurrying, the slurry was evenly coated on an aluminum foil current collector using an automatic coating machine and transferred to a vacuum drying oven at 120°C for 12 hours. The material was removed and naturally cooled to room temperature. It was then cut into pieces using a manual battery slicer. The cut positive electrode sheets were collected, weighed, and placed in a ziplock bag. The bag was then placed in an inert gas-filled glove box for later use. A sodium metal sheet was used as the negative electrode, a GF / A glass fiber separator was used as the separator, and an electrolyte was added to assemble a CR2032 button cell. After standing, the cells were subjected to electrochemical performance testing. The electrochemical performance tests were conducted using a CT-2001A battery testing system (1C = 129 mA / g). The voltage range of the constant current charge and discharge test was 1.5 to 4.0 V.

[0101] The results are shown in Table 1.

[0102] Table 1 Battery performance of heteroelement-doped carbon-coated polyanion cathode materials

[0103]

[0104]

[0105] As shown in Table 1, the polyanion cathode material doped with heteroelement carbon of the present invention has a high 3+x M 2+x (PO4) 1+ x The surface of P2O7 material, PN / C, phosphorus and nitrogen co-doped carbon coating, is a sheet structure with high sp 2 Hybrid carbon structure, excellent conductivity, and phosphorus and nitrogen co-doped carbon coating cooperate with each other, reasonably activate the heterogeneous phase, improve capacity and cycle performance, high conductivity PN / C, phosphorus and nitrogen co-doped carbon coating, and core Na 3+x M 2+x (PO4) 1+x The synergistic effect of P2O7 improves the overall performance of the material, including capacity and cycle performance. Moreover, the doping of phosphorus is different from that of other elements in the coating layer. The core Na 3+x M 2+x (PO4) 1+x P2O7 material is based on phosphate pyrophosphate, and phosphorus is doped as a coating layer to reduce the Na content in the polyanion positive electrode material. 3+x M 2+x(PO4) 1+x The phase composition of P2O7 does not cause adverse interference, and the high conductivity PN / C is achieved. The carbon coating layer co-doped with phosphorus and nitrogen elements is consistent with the core Na 3+x M 2+x (PO4) 1+x P2O7 produces synergy and overcomes the adverse effects of phosphorus doping on the phase composition of the core material.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A heteroelement-doped carbon-coated polyanion cathode material, characterized in that: The general structural formula of the polyanion positive electrode material is Na 3+x Fe 2+x (PO4) 1+x P2O7@PN / C, wherein 1<x≤2, the polyanion cathode material is a core-shell structure, Na 3+x Fe 2+x (PO4) 1+x P2O7 is the core of the core-shell structure of the polyanion cathode material, @PN / C represents a coating layer formed on the surface of the core material, and PN / C is a carbon coating layer co-doped with phosphorus and nitrogen elements; The preparation method of the heteroelement-doped carbon-coated polyanion positive electrode material comprises the following steps: (1) After the organic carbon source, organic phosphorus source, and organic nitrogen source are uniformly dispersed, the temperature is raised to 600-1000°C under an inert gas atmosphere and then constant-temperature sintered for 5-20 hours to obtain PN / C carbon material; (2) According to the preset x value, the PN / C carbon material, sodium source, iron source and dispersant are dispersed or dissolved in a solvent according to the stoichiometric ratio and ground to obtain a mixed slurry H2. After removing the solvent and drying H2, a precursor material to be sintered H3 in which all elements are evenly dispersed is obtained; (3) sintering the obtained H3 in an inert atmosphere in one or two steps to obtain a polyanion cathode material doped with heteroelement carbon; In the process of preparing PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic phosphorus source, organic nitrogen source and organic carbon source used is a:b:100, wherein 1.0≤a≤8; 20≤b≤50; the organic carbon source is one or more of glucose, chitosan, sucrose, starch, citric acid, sodium citrate, lauric acid and sodium laurate; the organic phosphorus source is one of triphenylphosphine and phytic acid; and the organic nitrogen source is one or more of melamine, urea and chitosan.

2. The heteroelement-doped carbon-coated polyanion cathode material according to claim 1, characterized in that: The mass of the PN / C coating layer accounts for 0.1% to 5.0% of the mass of the polyanion positive electrode material.

3. The heteroelement-doped carbon-coated polyanion cathode material according to claim 2, characterized in that: The mass of the PN / C coating layer accounts for 0.5% to 3.0% of the mass of the polyanion positive electrode material.

4. The heteroelement-doped carbon-coated polyanion cathode material according to claim 2, characterized in that: In the process of preparing PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic phosphorus source, organic nitrogen source and organic carbon source is a:b:100, wherein 3≤a≤8; 20≤b≤50.

5. The heteroelement-doped carbon-coated polyanion cathode material according to claim 4, characterized in that: In the process of preparing PN / C carbon material, the weight ratio of phosphorus element, nitrogen element and carbon element in the organic phosphorus source, organic nitrogen source and organic carbon source is a:b:100, wherein 3≤a≤8; 30≤b≤45.

6. The heteroelement-doped carbon-coated polyanion cathode material according to claim 1, characterized in that: The preparation method of the polyanion positive electrode material comprises the steps of: sintering the PN / C carbon material and Na 3+x Fe 2+x (PO4) 1+x The precursor materials of P2O7 are dispersed and mixed, and then heated to 420~600℃ under an inert gas atmosphere and sintered at a constant temperature for 5~20 hours to obtain Na 3+x Fe 2+x (PO4) 1+x P2O7.

7. The heteroelement-doped carbon-coated polyanion cathode material according to claim 6, characterized in that: Na 3+x Fe 2+x (PO4) 1+x The precursor of P2O7 includes a sodium source, an iron source, and a phosphorus source. The sodium source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium nitrate, and sodium citrate; the iron source is one or more of phosphates, nitrates, sulfates, oxalates, acetates, and oxides corresponding to the Fe element; and the phosphorus source is one or more of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, iron phosphate, and iron pyrophosphate.

8. The method for preparing the heteroelement-doped carbon-coated polyanion cathode material according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: (1) grinding and mixing an organic carbon source, an organic nitrogen source, and an organic phosphorus source according to a weight ratio, and then sintering them in one step or two steps under an inert gas atmosphere to obtain a PN / C carbon material; (2) According to the preset x value, the PN / C carbon material, sodium source, iron source and dispersant are dispersed or dissolved in a solvent according to the stoichiometric ratio and ground to obtain a mixed slurry H2. After removing the solvent and drying H2, a precursor material to be sintered H3 in which all elements are evenly dispersed is obtained; (3) The obtained H3 is subjected to a one-step or two-step sintering treatment in an inert atmosphere to obtain a polyanion positive electrode material doped with heteroelement carbon.

9. The method for preparing the heteroelement-doped carbon-coated polyanion cathode material according to claim 8, characterized in that: In step (1), the grinding process is a ball milling process, wherein the rotation speed of the ball milling process is 400-800 rpm, the ball-to-material ratio is 2-20:1, the diameter of the zirconia ball is 0.1-0.5 mm, and the ball milling time is 2-5 h; In step (2), the grinding process includes a ball milling process and a sand milling process in sequence, wherein the ball milling speed is 400-800 rpm; the ball-to-material ratio is 2-20:1, and the diameter of the zirconia ball is 0.1-0.5 mm; the solid content of the sand milling is 15%-40%, and the sand milling speed is 1000-2400 rpm; the particle size of the solid in the slurry after sand milling is D50≤6μm, Dmax≤30μm; In step (2), the dispersant is one or more of PVP, PVA, NPEOs, PEG, APG, and PAA; In step (2), the solvent is one of water, methanol, ethanol, acetone, ethylene glycol and n-butanol; In step (2), the drying method is spray drying, freeze drying or vacuum drying, and the drying temperature is not higher than 280°C; In step (3), in the one-step sintering or two-step sintering, the procedure for the one-step sintering is to heat the material to 420-600°C at a heating rate of 1-5°C / min in an inert protective atmosphere, and sinter at a constant temperature for 5-20 h; then cool it to room temperature at a cooling rate of 2-10°C / min; the procedure for the two-step sintering is to heat the material to a pre-sintering temperature of 250-420°C at a heating rate of 1-5°C / min in an inert protective atmosphere for 2-4 h; then heat it to 520-600°C at a heating rate of 3-6°C / min for 4-10 h; then cool it to room temperature at a cooling rate of 2-10°C / min.

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