Polyanion-based sodium battery positive electrode material, preparation method therefor, and application thereof

Abstract

A polyanion-based sodium battery positive electrode material, a preparation method therefor, and an application thereof, relating to the technical field of sodium-ion batteries. The polyanion-based sodium battery positive electrode material comprises a polyanionic compound and a carbon coating layer located on the surface of the polyanionic compound, the chemical formula of the polyanionic compound being Na 4Fe aM b(PO 4) 2P 2O 7, wherein 1.4≤a<3, 0.005<b≤1.7, 2.7<a+b<3, and M is a transition metal. Performing doping with a transition metal and engineering iron defects greatly reduces the impurity-phase content in the polyanionic material. Furthermore, the sodium-ion migration capability, the reversible capacity, and the energy density indicator of the positive electrode material are significantly improved.

Description

A polyanion-based sodium battery positive electrode material and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 29, 2023, with application number 202311868205.7 and invention name “A polyanion-based sodium battery positive electrode material, its preparation method and application”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of sodium ion batteries, and in particular to a polyanion-based sodium battery positive electrode material, a preparation method thereof, and an application thereof. Background Art

[0003] Lithium-ion batteries are currently widely used in various smart devices, electric vehicles, and energy storage applications due to their high energy density and long lifespan. However, the scarcity of lithium has led to high prices for lithium-ion batteries. To better promote energy conservation and environmental protection while meeting market demand, reducing battery costs is imperative. Sodium-ion batteries, which share the same operating principle as lithium-ion batteries and benefit from the inexhaustible availability of sodium resources, theoretically offer a low-cost advantage and are worthy of further development.

[0004] The mainstream sodium-ion batteries in the current market use layered oxides, Prussian white and polyanion materials as the main positive electrode materials. Among them, polyanion materials have attracted much attention due to their cost advantages and ultra-long cycle life. Among polyanion materials, Na4Fe3(PO4)P2O7 has a theoretical capacity of 129mAh / g and a moderate voltage (3.1V) and is considered to be one of the most promising materials. However, the large-scale synthesis process of this material is often accompanied by the formation of impurities NaFePO4 and Na2FeP2O7, and the electrochemical activity of NaFePO4 and Na2FeP2O7 is low. The presence of these two by-products greatly affects the yield of Na4Fe3(PO4)P2O7, which has an adverse effect on the overall capacity of the material. Summary of the Invention

[0005] The present application discloses a polyanion-based sodium battery positive electrode material, a preparation method and an application thereof, which can overcome the problems existing in the prior art of high impurity content and low charge and discharge capacity in the iron-based polyanion sodium battery positive electrode material during large-scale production.

[0006] In a first aspect, the present application provides a polyanion-based sodium battery positive electrode material, the polyanion-based sodium battery positive electrode material comprising a polyanion compound and a carbon coating layer located on the surface of the polyanion compound;

[0007] The chemical formula of the polyanion compound is Na4Fe a M b(PO4)2P2O7, where 1.4 ≤ a < 3, 0.005 < b ≤ 1.7, 2.7 < a + b < 3.0, and M is a transition metal.

[0008] Optionally, based on the total weight of the polyanion-based sodium battery cathode material, the carbon content is 1 wt% - 5 wt%.

[0009] Optionally, based on the total weight of the polyanion-based sodium battery cathode material, the carbon content is 1 wt% - 3 wt%.

[0010] Optionally, M is selected from one or more of Mn, Co, Ni, Cr, Ti, and V.

[0011] Optionally, 1.5 ≤ a < 2.99 and 0.01 ≤ b < 1.

[0012] In a second aspect, the present application discloses a method for preparing the polyanion-based sodium battery cathode material as described in any one of the above, the method comprising the following steps:

[0013] (1) Grinding a sodium source, a phosphorus source, a carbon source, an iron source, and a transition metal source to obtain a mixed material;

[0014] (2) Spray-drying the mixed material to obtain a precursor powder;

[0015] (3) Calcining the precursor powder at 450°C - 650°C in the presence of a protective gas.

[0016] Optionally, in step (1), the conditions for the grinding include: a rotation speed of 1000 r / min - 3000 r / min and a time of 2 h - 10 h.

[0017] Optionally, in step (2), the conditions for the spray-drying include: a temperature of 150°C - 300°C.

[0018] Optionally, 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.

[0019] Optionally, the iron source is selected from one or more of iron phosphate, iron nitrate, iron oxide, magnetite, iron sulfate, and ferrous sulfate.

[0020] Optionally, the transition metal source is selected from a phosphate or an oxide of a transition metal.

[0021] Optionally, 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 monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, and monosodium trihydrogen pyrophosphate.

[0022] Optionally, the carbon source is selected from one or more of graphene, carbon black, carbon nanotubes, SuperP, carbon fiber, ascorbic acid, citric acid, glucose, polyethylene glycol, formaldehyde, acetaldehyde, citric acid, malic acid, oxalic acid, adipic acid, starch and sucrose.

[0023] In a third aspect, the present application discloses the application of the polyanion-based sodium battery positive electrode material described in any of the above items in a sodium ion battery.

[0024] In a fourth aspect, the present application discloses a positive electrode sheet, which contains any of the polyanion-based sodium battery positive electrode materials described above.

[0025] In a fifth aspect, the present application discloses a sodium ion battery, which includes the positive electrode sheet described in any one of the above items.

[0026] Compared with the prior art, this application has the following beneficial effects:

[0027] 1. This application discloses a polyanion-based sodium-ion battery cathode material. By doping with transition metals and creating iron defects, the impurity content in the polyanion material is greatly reduced. Simultaneously, the sodium ion migration ability, reversible capacity, and energy density of the cathode material are significantly improved. Using this cathode material is conducive to obtaining a sodium-ion battery with high specific capacity and high capacity retention rate.

[0028] 2. This application also discloses a method for preparing the aforementioned polyanion-based sodium-ion battery cathode material. This method comprises grinding and spray-drying the raw materials to obtain a precursor powder, which is then calcined under a protective atmosphere to obtain the polyanion-based sodium-ion battery cathode material. This preparation method is simple to operate, utilizes readily available raw materials, and requires no additional equipment, enabling large-scale mass production using conventional lithium iron phosphate production lines.

[0029] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the following is a brief introduction to the drawings required for use in the embodiments or related technical descriptions. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0031] FIG1 is an XRD test result diagram of the product prepared according to Comparative Example 1 of the present application;

[0032] FIG2 is a SEM characterization result diagram of the product prepared according to Example 1 of the present application;

[0033] FIG3 is an XRD test result diagram of the product prepared according to Example 1 of the present application;

[0034] FIG4 is an XRD test result diagram of the product prepared according to Example 2 of the present application;

[0035] FIG5 is an XRD test result diagram of the product prepared according to Example 3 of the present application;

[0036] FIG6 is an XRD test result diagram of the product prepared according to Example 4 of the present application;

[0037] FIG7 is an XRD test result diagram of the product prepared according to Example 5 of the present application;

[0038] FIG8 is a charge and discharge curve diagram of a button battery made from the product prepared in Comparative Example 1 of Test Example 1 of the present application;

[0039] FIG9 is a charge and discharge curve diagram of a button battery made from the product prepared in Example 1 of Test Example 1 of the present application;

[0040] FIG10 is a charge and discharge curve diagram of a button battery made from the product prepared in Example 2 of Test Example 1 of the present application;

[0041] FIG11 is a charge and discharge curve diagram of a button battery made from the product prepared in Example 3 of Test Example 1 of the present application;

[0042] FIG12 is a charge and discharge curve diagram of a button battery made from the product prepared in Example 4 of Test Example 1 of the present application;

[0043] FIG13 is a charge and discharge curve diagram of a button battery made from the product prepared in Example 5 of Test Example 1 of the present application. Specific embodiments

[0044] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0045] The following describes in detail a polyanion-based sodium battery positive electrode material provided by the present application, its preparation method and application in conjunction with the accompanying drawings.

[0046] In some embodiments of the present application, the present application discloses a polyanion-based sodium battery positive electrode material, the polyanion-based sodium battery positive electrode material comprising a polyanion compound and a carbon coating layer located on the surface of the polyanion compound;

[0047] 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.

[0048] This application is based on the Na4Fe a M b Transition metal elements and iron defects are introduced into the (PO4)2P2O7 structure. Through appropriate doping of transition metal elements and coordination with iron defects, the sodium ion migration ability of the positive electrode material, the reversible capacity of the material and the energy density index of the material are significantly improved.

[0049] In some embodiments of the present application, the carbon content is 1 wt%-5 wt% based on the total weight of the polyanion-based sodium battery positive electrode material.

[0050] In some embodiments of the present application, the carbon content is 1 wt% to 3 wt%. Too low a carbon coating content can lead to decreased electrical conductivity, while too high a carbon coating content can also affect the energy density of the material. The present application preferably controls the carbon coating content to ensure that both electrical conductivity and energy density are within a favorable range.

[0051] In some embodiments of the present application, in order to further improve the coordination between the iron element and the transition metal element, and thereby improve the sodium ion migration ability of the positive electrode material, the transition metal M is selected from one or more of Mn, Co, Ni, Cr, Ti and V.

[0052] In some embodiments of the present application, in order to further improve the sodium ion migration ability and reversible capacity of the positive electrode material, the doping amount of the transition metal element and the amount of iron defects can be further controlled. Specifically, 1.5≤a<2.99, 0.01≤b<1, 2.9<a+b<3.

[0053] In some embodiments of the present application, the present application further discloses a method for preparing the polyanion-based sodium battery positive electrode material as described in any of the above embodiments, the method comprising the following steps:

[0054] (1) grinding a sodium source, a phosphorus source, a carbon source, an iron source, and a transition metal source to obtain a mixture;

[0055] (2) spray drying the mixture to obtain a precursor powder;

[0056] (3) In the presence of a protective gas, the precursor powder is calcined at 450°C-650°C.

[0057] The method for preparing a polyanion-based sodium battery positive electrode material described in the present application comprises adding sodium, phosphorus, iron and transition metal elements in a stoichiometric ratio, then performing a grinding and spray drying process to obtain a precursor powder, and then calcining the precursor powder in the presence of a protective gas to obtain the polyanion-based sodium battery positive electrode material described in the present application.

[0058] In some embodiments of the present application, the grinding is mechanical grinding, and the equipment used for grinding is a sand mill. During the grinding process, the solid content is controlled to be 40wt%-80wt%.

[0059] In some embodiments of the present application, the grinding speed is 1000 r / min-3000 r / min, and the grinding time is 2 h-10 h. Specifically, the grinding speed can be set to any value such as 1000 r / min, 1500 r / min, 2000 r / min, 2500 r / min, 3000 r / min, or a range between any two values; the grinding time can be set to any value such as 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, or a range between any two values.

[0060] In some embodiments of the present application, in step (2), the spray drying conditions include: a temperature of 150°C-300°C.

[0061] Specifically, the spray drying temperature can be set to any value such as 150°C, 180°C, 210°C, 240°C, 230°C, 250°C, 260°C, 300°C, or a range between any two values.

[0062] In some embodiments of the present application, 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.

[0063] In some embodiments of the present application, the iron source is selected from one or more of ferric phosphate, ferric nitrate, ferric oxide, ferroferric oxide, ferric sulfate and ferrous sulfate.

[0064] In some embodiments of the present application, the transition metal source is selected from phosphates or oxides of transition metals.

[0065] In some embodiments of the present application, 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 monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate and monosodium trihydrogen pyrophosphate.

[0066] In specific applications, there are no specific requirements for the amount of sodium source, phosphorus source, iron source, and transition metal source, as long as the elements in the raw materials meet the requirements of this application. For example, sodium pyrophosphate can serve as both a phosphorus source and a sodium source, and iron phosphate can provide both phosphorus and iron.

[0067] In some embodiments of the present application, the carbon source is selected from one or more of graphene, carbon black, carbon nanotubes, SuperP, carbon fiber, ascorbic acid, citric acid, glucose, polyethylene glycol, formaldehyde, acetaldehyde, citric acid, malic acid, oxalic acid, adipic acid, starch and sucrose.

[0068] In some embodiments of the present application, in the raw material, the ratio of the amount of carbon source to the sum of the amounts of iron and transition metal elements is 0.3-5:1, wherein the carbon source is calculated as carbon element.

[0069] In some embodiments of the present application, in step (3), the protective gas may be one of N2, CO2 and Ar, preferably N2.

[0070] In specific applications, in step (3), the calcination temperature can be set to any value such as 450°C, 470°C, 500°C, 520°C, 550°C, 570°C, 600°C, 620°C, 650°C, or a range between any two values.

[0071] In some embodiments of the present application, the present application also discloses the application of the polyanion-based sodium battery positive electrode material described in any of the above embodiments in a sodium ion battery.

[0072] Applying the polyanion-based sodium battery positive electrode material proposed in this application to sodium ion batteries can further improve the battery's gram capacity and capacity retention rate.

[0073] In some embodiments of the present application, the present application further discloses a positive electrode sheet, which contains the polyanion-based sodium battery positive electrode material described in any of the above embodiments.

[0074] In some embodiments of the present application, the positive electrode sheet further comprises a conductive agent and a binder. Based on the total weight of the polyanion-based sodium battery positive electrode material, the conductive agent, and the binder, the amount of the polyanion-based sodium battery positive electrode material is 80-98wt%, the amount of the conductive agent is 1-15wt%, and the amount of the binder is 1-15wt%. The conductive agent and the binder can be conventionally selected in the art.

[0075] In some embodiments of the present application, the positive electrode sheet further includes a positive electrode current collector. There are no special requirements for the selection of the positive electrode current collector, which is a conventional choice in the art, for example, aluminum foil.

[0076] In some embodiments of the present application, the present application further discloses a sodium ion battery, which includes the positive electrode sheet described in any one of the above embodiments.

[0077] The present application will be described in detail below through comparative examples, embodiments and test examples, but the protection scope of the present application is not limited thereto.

[0078] Comparative Example 1:

[0079] Preparation of polyanion-based sodium battery cathode material D1:

[0080] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide and glucose were used as sodium source, phosphorus source, iron source and carbon source respectively, and the molar ratio of sodium, iron and phosphorus in the raw materials was controlled to be 4:3:4, and the molar ratio of carbon source (calculated as carbon element) to iron 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 during grinding was 50 wt%;

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

[0083] The precursor powder was placed in a box furnace and calcined at 550 °C for 12 h under N2 atmosphere and then cooled naturally.

[0084] The carbon content of the prepared product was detected to be 1.8 wt %. The Fe / P molar ratio of the prepared product was 0.749 by ICP testing and calculation, as shown in FIG1 . According to XRD testing, the product contained impurity phases NaFePO4 and Na2FeP2O7. Further analysis and calculation showed that the contents of the two impurity phases were 7% and 8%, respectively.

[0085] Example 1:

[0086] Preparation of polyanion-based sodium battery cathode material S1:

[0087] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, manganese dioxide and glucose were used as the sodium source, phosphorus source, iron source, manganese source and carbon source, respectively. The molar ratio of sodium, iron, manganese and phosphorus 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 as carbon element) to the sum of the molar amounts of iron and manganese 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 during grinding was 50 wt%;

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

[0090] The precursor powder was placed in a box furnace and calcined at 550 ° C for 12 h under N2 atmosphere and cooled naturally to obtain the product Na4Fe 2.98 Mn 0.01 (PO4)2P2O7 and coated on Na4Fe 2.98 Mn 0.01 Carbon coating on the surface of (PO4)2P2O7.

[0091] The carbon content of the prepared product was determined to be 1.9 wt%. ICP analysis revealed an Fe / P molar ratio of 0.743 and a Mn / P molar ratio of 0.007. SEM characterization results, shown in Figure 2, reveal secondary spherical particles formed by the agglomeration of primary particles, with a size ranging from 1 to 10 μm. XRD analysis, as shown in Figure 3, revealed virtually no detectable impurities in the product.

[0092] Example 2:

[0093] Preparation of polyanion-based sodium battery cathode material S2:

[0094] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, manganese dioxide and glucose were respectively used as the sodium source, phosphorus source, iron source, transition metal source and carbon source, and the molar ratio of sodium, iron, manganese and phosphorus in the raw materials was controlled to be 4:1.996:1:4; the ratio of the molar amount of the carbon source (calculated as carbon element) to the sum of the molar amounts of iron and manganese was 0.5:1;

[0095] 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 during grinding was 50 wt%;

[0096] The mixture was prepared into a dry precursor powder at 250 °C using a spray drying method;

[0097] The precursor powder was placed in a box furnace and calcined at 550 ° C for 12 h under N2 atmosphere and cooled naturally to obtain the product Na4Fe 1.996Mn(PO4)2P2O7 and coated on Na4Fe 1.996 Carbon coating on the surface of Mn(PO4)2P2O7.

[0098] The carbon content of the prepared product was 2.1 wt %. ICP analysis of the prepared product revealed an Fe / P molar ratio of 0.497 and a Mn / P molar ratio of 0.252. As shown in Figure 4, XRD analysis revealed the presence of a small amount of NaFePO4 impurity phase in the product, which, according to analysis and calculation, is approximately 5%.

[0099] Example 3:

[0100] Preparation of polyanion-based sodium battery cathode material S3:

[0101] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, manganese dioxide and glucose were respectively used as the sodium source, phosphorus source, iron source, transition metal source and carbon source, and the molar ratio of sodium, iron, manganese and phosphorus in the raw materials was controlled to be 4:1.97:1:4; the ratio of the molar amount of the carbon source (calculated as carbon element) to the sum of the molar amounts of iron and manganese was 0.5:1;

[0102] 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 during grinding was 50 wt%;

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

[0104] The precursor powder was placed in a box furnace and calcined at 550 ° C for 12 h under N2 atmosphere and cooled naturally to obtain the product Na4Fe 1.97 Mn(PO4)2P2O7 and coated on Na4Fe 1.97 Carbon coating on the surface of Mn(PO4)2P2O7.

[0105] The carbon content of the prepared product was determined to be 1.8 wt %. Inductively coupled plasma spectroscopy (ICP) analysis revealed an Fe / P molar ratio of 0.491 and a Mn / P molar ratio of 0.252. As shown in Figure 5 , XRD analysis revealed that the product contained essentially no detectable impurities.

[0106] Example 4:

[0107] Preparation of polyanion-based sodium battery cathode material S4:

[0108] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, manganese dioxide, cobalt oxide and glucose were respectively used as the sodium source, phosphorus source, iron source, manganese source, cobalt source and carbon source, and the molar ratio of sodium, iron, manganese, cobalt and phosphorus in the raw materials was controlled to be 4:1.48:0.75:0.75:4, and the ratio of the molar amount of the carbon source (calculated as carbon element) to the sum of the molar amounts of iron, manganese and cobalt was 0.6:1;

[0109] 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 during grinding was 50 wt%;

[0110] The mixture was prepared into a dry precursor powder at 250 °C using a spray drying method;

[0111] The precursor powder was placed in a box furnace and calcined at 550 ° C for 12 h under N2 atmosphere, and then cooled naturally.

[0112] The product obtained is Na4Fe 1.48 Mn 0.75 Co 0.75 (PO4)2P2O7 and coated on Na4Fe 1.48 Mn 0.75 Co 0.75 Carbon coating on the surface of (PO4)2P2O7.

[0113] The carbon content of the prepared product was determined to be 2.2 wt%. ICP analysis of the prepared product revealed an Fe / P molar ratio of 0.369, a Mn / P ratio of 0.182, and a Co / P ratio of 0.189. As shown in Figure 6, XRD analysis revealed that the product contained essentially no detectable impurities.

[0114] Example 5:

[0115] Preparation of polyanion-based sodium battery cathode material S5:

[0116] Sodium carbonate, ammonium dihydrogen phosphate, ferric oxide, titanium dioxide and glucose were used as sodium source, phosphorus source, iron source, titanium source and carbon source, respectively. The molar ratio of sodium, iron, titanium and phosphorus in the raw materials was controlled to be 4:2.97:0.02:4, and the ratio of the molar amount of the carbon source (calculated as carbon element) to the sum of the molar amounts of iron and titanium was 0.5:1.

[0117] 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 during grinding was 50 wt%;

[0118] The mixture was prepared into a dry precursor powder at 250 °C using a spray drying method;

[0119] The precursor powder was placed in a box furnace and calcined at 550 ° C for 12 h under N2 atmosphere and cooled naturally to obtain the product Na4Fe 2.97 Ti 0.02 (PO4)2P2O7 and coated Na4Fe 2.97 Ti 0.02 Carbon coating of (PO4)2P2O7 on the surface.

[0120] The carbon content of the prepared product was determined to be 1.9 wt %. ICP analysis of the prepared product revealed an Fe / P ratio of 0.740 and a Ti / P ratio of 0.007. As shown in Figure 7 , XRD analysis revealed that virtually no impurities were detected in the product.

[0121] Comparative Example 2:

[0122] Preparation of polyanion-based sodium battery cathode material D2:

[0123] The method described in Comparative Example 1 was followed, except that the molar ratio of sodium, iron and phosphorus in the raw materials was 4:2.7:4.

[0124] Comparative Example 3:

[0125] Preparation of polyanion-based sodium battery cathode material D3:

[0126] The method described in Comparative Example 1 was followed, except that the molar ratio of sodium, iron and phosphorus in the raw materials was 4:2.9:4.

[0127] Comparative Example 4:

[0128] Preparation of polyanion-based sodium battery cathode material D4:

[0129] The method described in Example 1 was followed, except that the molar ratio of sodium, iron, manganese and phosphorus in the raw materials was 4:1.6:1:4.

[0130] Comparative Example 5:

[0131] Preparation of polyanion-based sodium battery cathode material D5:

[0132] The method described in Example 1 was followed, except that the molar ratio of sodium, iron, manganese and phosphorus in the raw materials was 4:1.24:1.75:4.

[0133] Test Example 1:

[0134] The positive electrode materials of the above embodiments and the above comparative examples were respectively made into positive electrode sheets, which were then assembled into button batteries to test the charge and discharge performance.

[0135] The testing process is as follows:

[0136] The sample to be tested, acetylene black and PVDF binder are mixed uniformly, then coated on aluminum foil, and vacuum-dried at 80°C to obtain an electrode sheet; 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 is 80wt%, the amount of acetylene black is 10wt%, and the amount of PVDF binder is 10wt%;

[0137] The obtained electrode sheet, metallic sodium, and 0.8M NaPF6 / EC-DMC electrolyte were used to assemble a button cell to test the charge and discharge characteristics. The charge and discharge current was 0.1C, where 1C = 129mA / g.

[0138] The charge and discharge curves of Comparative Example 1 and Examples 1-5 are shown in Figures 8 to 13, respectively. The discharge capacity and average discharge voltage of Comparative Examples 1-5 and Examples 1-5 are shown in Table 1.

[0139] Table 1

[0140] From Figures 8-13 and Table 1, it can be seen that the button batteries prepared using the positive electrode materials prepared in Examples 1-5 have higher discharge capacity in grams, discharge average voltage and capacity retention rate, while the button batteries prepared using the positive electrode materials prepared in the comparative example have significantly lower discharge capacity in grams, discharge average voltage and capacity retention rate than those in the examples.

[0141] The preferred embodiments of the present application are described in detail above, 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 solution of the present application, including combining the various technical features in any other appropriate manner. These simple modifications and combinations should also be regarded as the contents disclosed in the present application and fall within the scope of protection of the present application.

[0142] References herein to "one embodiment," "an embodiment," or "one or more embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present application. Furthermore, please note that examples of the phrase "in one embodiment" herein do not necessarily all refer to the same embodiment. In the description provided herein, numerous specific details are described. However, it is understood that embodiments of the present application may be practiced without these specific details. In some instances, well-known methods, structures, and techniques are not shown in detail so as not to obscure the understanding of this description.

[0143] In the claims, any reference signs placed between brackets shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps not listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The present application may be implemented by means of hardware comprising several different elements and by means of a suitably programmed computer. In a unit claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The use of the words first, second, and third etc. does not indicate any order. These words may be interpreted as names.

[0144] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A cathode material for a polyanion-based sodium battery, wherein, The cathode material for a sodium battery based on polyanion includes 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, where 1.4 ≤ a < 3, 0.005 < b ≤ 1.7, 2.7 < a + b < 3, and M is a transition metal.

2. The polyanion-based sodium battery cathode material according to claim 1, wherein, Based on the total weight of the cathode material for a sodium battery based on polyanion, the carbon content is 1 wt% - 5 wt%.

3. The polyanion-based sodium battery cathode material according to claim 2, wherein, Based on the total weight of the cathode material for a sodium battery based on polyanion, the carbon content is 1 wt% - 3 wt%.

4. The polyanion-based sodium battery cathode material according to claim 1 or 2, wherein, M is selected from one or more of Mn, Co, Ni, Cr, Ti, and V.

5. The polyanion-based sodium battery cathode material according to claim 1, wherein, 1.5 ≤ a < 2.99, 0.01 ≤ b < 1.

6. A method for preparing a polyanion-based sodium battery cathode material as described in any one of claims 1-5, wherein, The method includes 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°C - 650°C in the presence of a protective gas.

7. The method according to claim 6, wherein, In step (1), the grinding conditions include: a rotation speed of 1000 r / min - 3000 r / min and a time of 2 h - 10 h.

8. The method according to claim 6, wherein In step (2), the spray-drying conditions include: a temperature of 150°C - 300°C.

9. The method according to claim 6 or 7, wherein 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.

10. The method according to claim 9, wherein, The iron source is selected from one or more of iron phosphate, iron nitrate, iron oxide, magnetite, iron sulfate, and ferrous sulfate.

11. The method according to claim 9, wherein The transition metal source is selected from phosphates or oxides of transition metals.

12. The method according to claim 9, 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 monohydrogen pyrophosphate, disodium dihydrogen pyrophosphate, and sodium trihydrogen monopyrophosphate.

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

14. Application of the cathode material for a sodium battery based on polyanion according to any one of claims 1 - 5 in a sodium ion battery.

15. A positive electrode sheet, wherein, The positive electrode sheet contains the cathode material for a sodium battery based on polyanion according to any one of claims 1 - 5.

16. A sodium-ion battery, wherein, The sodium ion battery includes the positive electrode sheet according to claim 15.

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