A polyanion-based sodium battery cathode material, a preparation method and application thereof

Abstract

The application relates to the technical field of sodium ion batteries, in particular to a polyanion-based sodium battery positive electrode material and a preparation method and application thereof. The polyanion-based sodium battery positive electrode material comprises a polyanion compound and a carbon coating layer on the surface of the polyanion compound; the chemical formula of the polyanion compound is Na4Fe a M b (PO4)2P2O7, wherein 1.4<=a<3, 0.005<b<=1.7, 2.7<=a+b<3, and M is a transition metal. By doping the transition metal and manufacturing iron defects, the content of impurities in the polyanion material is greatly reduced. Meanwhile, the sodium ion migration capacity, reversible capacity and energy density indexes of the positive electrode material are significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion batteries, in particular to a polyanion-based sodium battery cathode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries are widely used in various intelligent devices, electric vehicles and energy storage due to their high energy density and long life characteristics. However, the price of lithium ion batteries is high due to the scarcity of lithium element. In order to better promote the concept of energy saving and environmental protection and meet market demand, it is imperative to reduce the cost of batteries. Sodium ion batteries have the same working principle as lithium ion batteries, and sodium resources are inexhaustible, which theoretically has the advantage of low cost, and is worth further development.

[0003] The current market mainstream sodium ion battery adopts layered oxide, prussian white and polyanion material as the main material of the positive electrode. Among them, polyanion material is concerned due to its cost advantage and super long cycle life. Among the polyanion materials, Na4Fe3(PO4)P2O7 has a theoretical capacity of 129 mAh / g and a moderate voltage (3.1 V), and is considered to be one of the most promising materials. However, the generation of impurities NaFePO4 and Na2FeP2O7 often accompanies the large-scale synthesis of the material, and the electrochemical activity of NaFePO4 and Na2FeP2O7 is low. The existence of the two by-products greatly affects the yield of Na4Fe3(PO4)P2O7 and adversely affects the overall capacity of the material. SUMMARY

[0004] The purpose of the present application is to overcome the problems of high content of impurities in iron-based polyanion sodium battery cathode materials in the large-scale production process, low charge and discharge capacity and other problems in the prior art, and to provide a polyanion-based sodium battery cathode material and a preparation method and application thereof

[0005] In order to achieve the above-mentioned purpose, the first aspect of the present application provides a polyanion-based sodium battery cathode material, which comprises a polyanion compound and a carbon coating layer on the surface of the polyanion compound.

[0006] The chemical formula of the polyanion compound is Na4Fe a M b (PO4)2P2O7, wherein 1.4≤a<3, 0.005<b≤1.7, 2.7<a+b<3.0, and M is a transition metal.

[0007] Preferably, the carbon content is 1-5%, more preferably 1-3%, based on the total weight of the polyanion-based sodium battery cathode material.

[0008] Preferably, M is selected from one or more of Mn, Co, Ni, Cr, Ti and V.

[0009] Preferably, 1.5≤a<2.99, 0.01≤b<1.

[0010] The second aspect of the present application provides a method for preparing the polyanion-based sodium battery cathode material described above, which comprises the following steps:

[0011] (1) grinding a sodium source, a phosphorus source, a carbon source, an iron source and a transition metal source to obtain a mixture;

[0012] (2) spray drying the mixture to obtain a precursor powder;

[0013] (3) calcining the precursor powder at 450-650℃ in the presence of a protective gas.

[0014] Preferably, in step (1), the grinding conditions include a rotation speed of 1000-3000r / min and a time of 2-10h.

[0015] Preferably, in step (2), the spray drying conditions include a temperature of 150-300℃.

[0016] Preferably, the sodium source is selected from one or more of sodium acetate, sodium oxalate, sodium citrate, trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium carbonate and sodium bicarbonate;

[0017] Preferably, the iron source is selected from one or more of iron phosphate, iron nitrate, iron oxide, magnetite, iron sulfate and ferrous sulfate.

[0018] Preferably, the transition metal source is selected from a phosphate or an oxide of a transition metal.

[0019] Preferably, the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium pyrophosphate monohydrate, disodium pyrophosphate dihydrate and monosodium pyrophosphate trihydrate.

[0020] Preferably, the carbon source is selected from one or more of graphene, carbon black, carbon nanotubes, SuperP, carbon fibers, ascorbic acid, citric acid, glucose, polyethylene glycol, formaldehyde, acetaldehyde, citric acid, malic acid, oxalic acid, adipic acid, starch and sucrose.

[0021] The third aspect of the present application provides the use of the polyanion-based sodium battery cathode material described above in a sodium ion battery.

[0022] The fourth aspect of the present application provides a positive electrode sheet, which contains the polyanion-based sodium battery positive electrode material described above.

[0023] The fourth aspect of the present application provides a sodium ion battery, which includes the positive electrode sheet described above.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The present application provides a polyanion-based sodium ion battery positive electrode material, which greatly reduces the content of impurities in the polyanion material by doping transition metals and creating iron defects, and significantly improves the sodium ion migration ability, reversible capacity, and energy density indicators of the positive electrode material. Using the positive electrode material is conducive to obtaining a sodium ion battery with high specific capacity and high capacity retention rate.

[0026] 2. The present application also provides a method for preparing the polyanion-based sodium ion battery positive electrode material described above, which obtains a precursor powder by grinding and spray drying the raw materials, and then calcines under a protective gas to obtain the polyanion-based sodium ion battery positive electrode material. The preparation method is simple to operate, the raw materials are easy to obtain, and no additional equipment is needed. Large-scale production can be realized by borrowing the traditional lithium iron phosphate production line. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is the XRD detection result graph of the product prepared in Comparative Example 1;

[0028] Figure 2 is the SEM characterization result graph of the product prepared in Example 1;

[0029] Figure 3 is the XRD detection result graph of the product prepared in Example 1;

[0030] Figure 4 is the XRD detection result graph of the product prepared in Example 2;

[0031] Figure 5 is the XRD detection result graph of the product prepared in Example 3;

[0032] Figure 6 is the XRD detection result graph of the product prepared in Example 4;

[0033] Figure 7 is the XRD detection result graph of the product prepared in Example 5;

[0034] Figure 8 is the charge-discharge curve graph of the button cell made of the product prepared in Comparative Example 1 in Test Example 1;

[0035] Figure 9is a charge-discharge curve of a button cell made of the product prepared in Example 1 of Test Example 1;

[0036] Figure 10 is a charge-discharge curve of a button cell made of the product prepared in Example 2 of Test Example 1;

[0037] Figure 11 is a charge-discharge curve of a button cell made of the product prepared in Example 3 of Test Example 1;

[0038] Figure 12 is a charge-discharge curve of a button cell made of the product prepared in Example 4 of Test Example 1;

[0039] Figure 13 is a charge-discharge curve of a button cell made of the product prepared in Example 5 of Test Example 1. DETAILED DESCRIPTION

[0040] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the present application.

[0041] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The endpoints of the ranges and any values are understood to be approximate values. The exact values are understood to be within the range of values. Any numerical value, however, can be expressed as approximately or approximately.

[0042] The first aspect of the present application provides a polyanion-based sodium battery cathode material, which comprises a polyanion compound and a carbon coating layer on the surface of the polyanion compound.

[0043] The chemical formula of the polyanion compound is Na4Fe a M b (PO4)2P2O7, wherein 1.4≤a<3, 0.005<b≤1.7, 2.7<a+b<3, and M is a transition metal.

[0044] The present application introduces transition metal elements and iron defects into the Na4Fe3(PO4)2P2O7 structure of the sodium battery cathode material. Through appropriate transition metal element doping and coordination with iron defects, the sodium ion migration ability, reversible capacity and energy density of the cathode material are significantly improved.

[0045] In a preferred embodiment, the carbon content is 1-5%, more preferably 1-3%, based on the total weight of the sodium-ion battery cathode material. Too low a content of the carbon coating layer will result in a decrease in the electrical conductivity, while too high a content of the carbon coating layer will also affect the energy density of the material. The content of the carbon coating layer is preferably controlled in the present application to ensure that both the electrical conductivity and the energy density are within a good range.

[0046] In a preferred embodiment, in order to further improve the coordination between the iron element and the transition metal element, and thus improve the sodium ion migration ability of the cathode material, the transition metal M is selected from one or more of Mn, Co, Ni, Cr, Ti and V.

[0047] In a preferred embodiment, in order to further improve the sodium ion migration ability and reversible capacity of the cathode material, the doping amount of the transition metal element and the iron defect amount can be further controlled, specifically, 1.5≤a<2.99, 0.01≤b<1, 2.9

[0048] The second aspect of the present application provides a method for preparing the sodium-ion battery cathode material described above, which comprises the following steps:

[0049] (1) grinding a sodium source, a phosphorus source, a carbon source, an iron source and a transition metal source to obtain a mixture;

[0050] (2) spray drying the mixture to obtain a precursor powder;

[0051] (3) calcining the precursor powder at 450-650℃ in the presence of a protective gas.

[0052] The method described in the present application involves feeding sodium elements, phosphorus elements, iron elements and transition metal elements according to stoichiometric ratios, then grinding and spray drying to obtain a precursor powder, and then calcining the precursor powder in the presence of a protective gas to obtain the sodium-ion battery cathode material described in the present application.

[0053] In a preferred embodiment, the grinding is mechanical grinding, and the equipment is a sand mill. The solid content is controlled to be 40-80% by weight during the grinding process. Further preferably, the rotation speed of the grinding is 1000-3000 r / min, and the grinding time is 2-10 h. Specifically, the rotation speed of the grinding can be 1000 r / min, 1500 r / min, 2000 r / min, 2500 r / min or 3000 r / min; the grinding time can be 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h.

[0054] In a preferred embodiment, in step (2), the spray-drying conditions include a temperature of 150-300℃.

[0055] Preferably, the sodium source is selected from one or more of sodium acetate, sodium oxalate, sodium citrate, trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium carbonate and sodium bicarbonate.

[0056] Preferably, the iron source is selected from one or more of ferric phosphate, ferric nitrate, iron oxide, magnetite, ferrous sulfate and ferrous sulfate.

[0057] Preferably, the transition metal source is selected from a phosphate or an oxide of a transition metal.

[0058] Preferably, the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium pyrophosphate, disodium dihydrogen pyrophosphate and monosodium trihydrogen pyrophosphate.

[0059] In a specific embodiment, there is no special requirement for the amount of the sodium source, the phosphorus source, the iron source and the transition metal source, as long as the elements in the raw materials meet the requirements of the present application. For example, sodium pyrophosphate can be used as both a phosphorus source and a sodium source, and ferric phosphate can provide both phosphorus and iron.

[0060] In a preferred embodiment, the carbon source is selected from one or more of graphene, carbon black, carbon nanotubes, SuperP, carbon fibers, ascorbic acid, citric acid, glucose, polyethylene glycol, formaldehyde, acetaldehyde, citric acid, malic acid, oxalic acid, adipic acid, starch and sucrose.

[0061] In a preferred embodiment, in the raw materials, the ratio of the amount of substance of the carbon source to the sum of the amount of substance of iron and transition metal elements is 0.3-5:1, wherein the carbon source is calculated as carbon element.

[0062] In the method described in the present application, in step (3), the protective gas can be one of N2, CO2 and Ar, preferably N2.

[0063] In a specific embodiment, in step (3), the calcination temperature can be 450℃, 500℃, 550℃, 600℃ or 650℃.

[0064] The third aspect of the present application provides the use of the polyanion-based sodium battery cathode material described above in a sodium ion battery.

[0065] The use of the polyanion-based sodium battery cathode material described in the present application in a sodium ion battery can further improve the specific capacity and capacity retention rate of the battery.

[0066] The fourth aspect of the present application provides a positive electrode sheet containing the polyanion-based sodium battery positive electrode material described above.

[0067] Preferably, the positive electrode sheet further contains a conductive agent and a binder, and the polyanion-based sodium battery positive electrode material accounts for 80-98 wt%, the conductive agent accounts for 1-15 wt%, and the binder accounts for 1-15 wt% based on the total weight of the polyanion-based sodium battery positive electrode material, the conductive agent, and the binder. The conductive agent and the binder can be conventional choices in the art.

[0068] In the present application, the positive electrode sheet further includes a positive electrode current collector, and there is no special requirement for the selection of the positive electrode current collector, which can be a conventional choice in the art, for example, it can be an aluminum foil.

[0069] The fourth aspect of the present application provides a sodium ion battery including the positive electrode sheet described above.

[0070] The present application will be described in detail below through examples, but the protection scope of the present application is not limited thereto.

[0071] Comparative Example 1

[0072] Preparation of polyanion-based sodium battery positive electrode material D1:

[0073] Sodium carbonate, ammonium dihydrogen phosphate, diiron trioxide, and glucose were taken as sodium source, phosphorus source, iron source, and carbon source respectively, and the molar ratio of sodium element, iron element, and phosphorus element in the raw materials was controlled to be 4:3:4, and the molar ratio of carbon source (calculated by carbon element) to iron element was 0.5:1.

[0074] The raw materials were put into a sand mill and ground at a speed of 2000 r / min for 6 h, and the solid content was 50 wt% during grinding.

[0075] The mixture was prepared into a dry precursor powder at 250°C by using a spray drying method.

[0076] The precursor powder was placed in a box furnace and calcined at 550°C for 12 h under N2 atmosphere, and then naturally cooled.

[0077] The carbon content of the prepared product was 1.8 wt%, the Fe / P molar ratio of the prepared product was calculated to be 0.749 by ICP test, and it was known from XRD test (as shown in Figure 1 ) that the product contained impurities NaFePO4 and Na2FeP2O7, and further analysis and calculation showed that the contents of the two impurities were 7% and 8% respectively.

[0078] Example 1

[0079] Preparation of polyanion-based sodium battery positive electrode material S1:

[0080] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, manganese dioxide and glucose were taken as sodium source, phosphorus source, iron source, manganese source and carbon source, respectively. The molar ratio of sodium element, iron element, manganese element and phosphorus element in the raw materials was controlled to be 4:2.98:0.01:4, and the ratio of the molar amount of the carbon source (calculated by carbon element) to the sum of the molar amounts of iron element and manganese element was 0.5:1.

[0081] The raw materials were put into a sand mill and ground at a speed of 2000 r / min for 6 h, and the solid content was 50 wt% during the grinding;

[0082] The mixture was prepared into a dry precursor powder at 250°C by using a spray drying method;

[0083] The precursor powder was placed in a box furnace and calcined at 550°C for 12 h under N2 atmosphere, and naturally cooled, and the obtained product was Na4Fe 2.98 Mn 0.01 (PO4)2P2O7and a carbon coating layer coated on the surface of Na4Fe 2.98 Mn 0.01 (PO4)2P2O7.

[0084] The carbon content in the prepared product was 1.9 wt%, the Fe / P molar ratio of the prepared product was 0.743, and the Mn / P molar ratio was 0.007 by ICP test. The SEM characterization result is shown in Figure 2 , and it can be seen that the morphology is secondary spherical particles formed by agglomeration of primary particles, and the size of the secondary particles is 1-10 μm. According to the XRD test (as shown in Figure 3 ), it can be seen that there is basically no impurity phase detectable in the product.

[0085] Example 2

[0086] Preparation of a polyanion-based sodium battery cathode material S2:

[0087] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, manganese dioxide and glucose were taken as sodium source, phosphorus source, iron source, transition metal source and carbon source, respectively. The molar ratio of sodium element, iron element, manganese element and phosphorus element in the raw materials was controlled to be 4:1.996:1:4, and the ratio of the molar amount of the carbon source (calculated by carbon element) to the sum of the molar amounts of iron element and manganese element was 0.5:1.

[0088] The raw materials were put into a sand mill and ground at a speed of 2000 r / min for 6 h, and the solid content was 50 wt% during the grinding;

[0089] The mixture was prepared into a dry precursor powder at 250°C by using a spray drying method;

[0090] The precursor powder was placed in a box furnace and calcined at 550°C for 12h under N2atmosphere, and naturally cooled to obtain the product Na4Fe 1.996 Mn(PO4)2P2O7and coated on the surface of Na4Fe 1.996 Mn(PO4)2P2O7.

[0091] The carbon content of the prepared product was 2.1wt%, and the prepared product was detected by ICP, and the molar ratio of Fe / P was 0.497 and the molar ratio of Mn / P was 0.252. According to the XRD test (such as Figure 4 ), it can be known that a small amount of NaFePO4impurity phase exists in the product, and according to the analysis and calculation, the content is about 5%.

[0092] Example 3

[0093] Preparation of polyanion-based sodium battery cathode material S3:

[0094] Take sodium carbonate, ammonium dihydrogen phosphate, ferric sesquioxide, manganese dioxide and glucose as sodium source, phosphorus source, iron source, transition metal source and carbon source respectively, control the molar ratio of sodium element, iron element, manganese element and phosphorus element in the raw materials to be 4:1.97:1:4, and the molar ratio of carbon source (calculated by carbon element) to the sum of the molar amount of iron element and manganese element is 0.5:1;

[0095] Put the raw materials into the sand mill and grind at a speed of 2000r / min for 6h, and the solid content is 50wt% during grinding;

[0096] Use spray drying method to prepare dry precursor powder from the mixture at 250°C;

[0097] Put the precursor powder into a box furnace and calcine at 550°C for 12h under N2atmosphere, and naturally cool to obtain the product Na4Fe 1.97 Mn(PO4)2P2O7and coated on the surface of Na4Fe 1.97 Mn(PO4)2P2O7.

[0098] The carbon content of the prepared product was 1.8%, and the prepared product was detected by ICP, and according to the ICP detection, the molar ratio of Fe / P was 0.491 and the molar ratio of Mn / P was 0.252. According to the XRD test (such as Figure 5 ), it can be known that there is basically no impurity phase in the product.

[0099] Example 4

[0100] Preparation of polyanion-based sodium battery cathode material S4:

[0101] Na2CO3, (NH4)2HPO4, Fe2O3, MnO2, Co3O4 and glucose were taken as sodium source, phosphorus source, iron source, manganese source, cobalt source and carbon source respectively, the molar ratio of sodium element, iron element, manganese element, cobalt element and phosphorus element in the raw materials was controlled to be 4:1.48:0.75:0.75:4, and the molar ratio of the molar amount of the carbon source (calculated by carbon element) to the sum of the molar amounts of iron element, manganese element and cobalt element was 0.6:1;

[0102] The raw materials were put into a sand mill and ground at a speed of 2000 r / min for 5 h, and the solid content was 50 wt% during grinding;

[0103] The mixture was prepared into a dry precursor powder at 250 ℃ by using a spray drying method;

[0104] The precursor powder was placed in a box furnace and calcined at 550 ℃ for 12 h under N2 atmosphere, and naturally cooled,

[0105] The obtained product was Na4Fe 1.48 Mn 0.75 Co 0.75 (PO4)2P2O7and a carbon coating layer coated on the surface of Na4Fe 1.48 Mn 0.75 Co 0.75 (PO4)2P2O7.

[0106] The carbon content in the prepared product was 2.2 wt%, the Fe / P molar ratio, the Mn / P ratio and the Co / P ratio of the prepared product were 0.369, 0.182 and 0.189 respectively by ICP test. According to the XRD test (as shown in FIG. 1), it can be seen that there is basically no impurity phase detectable in the product. Figure 6

[0107] Example 5

[0108] Preparation of a polyanion-based sodium battery cathode material S5:

[0109] Na2CO3, (NH4)2HPO4, Fe2O3, TiO2 and glucose were taken as sodium source, phosphorus source, iron source, titanium source and carbon source respectively, the molar ratio of sodium element, iron element, titanium element and phosphorus element in the raw materials was controlled to be 4:2.97:0.02:4, and the molar ratio of the molar amount of the carbon source (calculated by carbon element) to the sum of the molar amounts of iron element and titanium element was 0.5:1;

[0110] The raw materials were put into a sand mill and ground at a speed of 2000 r / min for 6 h, and the solid content was 50 wt% during grinding;

[0111] The mixture was prepared into a dry precursor powder at 250 ℃ by using a spray drying method;

[0112] ​The precursor powder was calcined at 550°C for 12h under N2atmosphere in a box furnace, and the obtained product was Na4Fe 2.97 Ti 0.02 (PO4)2P2O7and coated Na4Fe 2.97 Ti 0.02 (PO4)2P2O7in the surface carbon coating layer.

[0113] The carbon content in the prepared product was 1.9wt%, and the Fe / P value and Ti / P value of the prepared product were 0.740 and 0.007 respectively tested by ICP. According to the XRD test (as shown in Figure 7 ), it can be seen that there is basically no impurity phase detectable in the product.

[0114] Comparative Example 2

[0115] Preparation of polyanion-based sodium battery cathode material D2:

[0116] According to the method described in Comparative Example 1, the difference is that the molar ratio of sodium element, iron element and phosphorus element in the raw material is 4:2.7:4.

[0117] Comparative Example 3

[0118] Preparation of polyanion-based sodium battery cathode material D3:

[0119] According to the method described in Comparative Example 1, the difference is that the molar ratio of sodium element, iron element and phosphorus element in the raw material is 4:2.9:4.

[0120] Comparative Example 4

[0121] Preparation of polyanion-based sodium battery cathode material D4:

[0122] According to the method described in Example 1, the difference is that the molar ratio of sodium element, iron element, manganese element and phosphorus element in the raw material is 4:1.6:1:4.

[0123] Comparative Example 5

[0124] Preparation of polyanion-based sodium battery cathode material D5:

[0125] According to the method described in Example 1, the difference is that the molar ratio of sodium element, iron element, manganese element and phosphorus element in the raw material is 4:1.24:1.75:4.

[0126] Test Example 1

[0127] The cathode materials of the examples and comparative examples were made into positive electrode sheets, and then assembled into button cells to test the charge and discharge performance.

[0128] The test process is as follows:

[0129] The sample to be tested, acetylene black and PVDF binder were mixed uniformly, and then coated on an aluminum foil. After vacuum drying at 80°C, an electrode sheet was obtained; wherein, based on the total weight of the sample to be tested, acetylene black and PVDF binder, the amount of the sample to be tested was 80wt%, the amount of acetylene black was 10wt%, and the amount of PVDF binder was 10wt%.

[0130] The obtained electrode sheet, metallic sodium and 0.8M NaPF6 / EC-DMC electrolyte were used to assemble a button cell to test the charge-discharge characteristics, and the charge-discharge current was 0.1C, wherein 1C=129mA / g.

[0131] The charge-discharge curves of Comparative Example 1 and Examples 1-5 are shown in Figures 8-13 The discharge gram capacity and discharge average voltage of Comparative Examples 1-5 and Examples 1-5 are shown in Table 1.

[0132] Table 1

[0133]

[0134]

[0135] It can be seen from Figures 8-13 and Table 1 that the button cell prepared by using the positive electrode material prepared by Examples 1-5 has higher discharge gram capacity, discharge average voltage and capacity retention rate, while the discharge gram capacity, discharge average voltage and capacity retention rate of the button cell prepared by using the positive electrode material prepared by Comparative Examples are significantly lower than those of Examples.

[0136] The above describes the preferred embodiments of the present application in detail, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as disclosed by the present application, and all fall within the protection scope of the present application.

Claims

1. A polyanion-based sodium battery cathode material, characterized in that, The polyanion-based sodium battery cathode material comprises a polyanion compound and a carbon coating layer on the surface of the polyanion compound. The poly-anionic compound has the chemical formula Na4Fe a M b (PO4)2P2O7, wherein 1.5≤a<2.99, 0.01≤b<1, 2.9<a+b<3, M is Mn and / or Ti; The carbon content is 1-3% based on the total weight of the polyanion-based sodium battery cathode material.

2. A method of preparing the polyanion-based sodium battery cathode material of claim 1, characterized in that, The method comprises the following steps: (1) grinding a sodium source, a phosphorus source, a carbon source, an iron source and a transition metal source to obtain a mixture; (2) spray drying the mixture to obtain a precursor powder; (3) calcining the precursor powder at 450-650°C in the presence of a protective gas.

3. The method of claim 2, wherein, In step (1), the grinding conditions include a rotation speed of 1000-3000 r / min and a time of 2-10 h.

4. The method of claim 2, wherein, In step (2), the spray drying conditions include a temperature of 150-300°C.

5. The method according to claim 2 or 3, characterized in that, The sodium source is selected from one or more of sodium acetate, sodium oxalate, sodium citrate, trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium carbonate and sodium bicarbonate.

6. The method of claim 5, wherein, The iron source is selected from one or more of iron phosphate, iron nitrate, iron oxide, magnetite, iron sulfate and ferrous sulfate.

7. The method of claim 5, wherein, The transition metal source is selected from a phosphate or an oxide of a transition metal.

8. The method of claim 5, wherein, The phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, trisodium pyrophosphate monohydrate, disodium pyrophosphate dihydrate and monosodium pyrophosphate trihydrogen.

9. The method of claim 5, wherein, The carbon source is selected from one or more of graphene, carbon black, carbon nanotubes, SuperP, carbon fibers, ascorbic acid, citric acid, glucose, polyethylene glycol, formaldehyde, acetaldehyde, citric acid, malic acid, oxalic acid, adipic acid, starch and sucrose.

10. Use of the polyanion-based sodium battery cathode material of claim 1 in a sodium ion battery.

11. A positive electrode sheet characterized by comprising: The positive electrode sheet contains the polyanion-based sodium battery cathode material of claim 1.

12. A sodium-ion battery, characterized in that, The sodium ion battery comprises the positive electrode sheet of claim 11.

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