A binary-doped iron-based sodium fluorophosphate cathode material and its preparation method

Abstract

The present invention relates to a dual-doped iron-based sodium-ion cathode material of fluorophosphate, expressed as Na<subgt;2< / subgt;Fe<subgt;1-x-y< / subgt;M<subgt;x< / subgt;N<subgt;y< / subgt>PO<subgt;4< / subgt>F@C, which is obtained by surface carbon coating of Na<subgt;2< / subgt>Fe<subgt;1-x-y< / subgt>M<subgt;x< / subgt;N<subgt;y< / subgt>PO<subgt;4< / subgt>F, where M is Mn, 0.02 ≤ x ≤ 0.10; N is selected from at least one of Ni, Co, Cu, Zn, Ti, 0.005 ≤ y ≤ 0.05, and @C represents carbon coating. By dual doping, the reversible specific capacity of the prepared cathode material is significantly improved, especially the reversible specific capacity at high rates, which is beneficial to the industrialization of sodium-ion batteries; in addition, the cathode material of the present invention is carbon coated, and polysaccharide and imidazoline surfactant are used as a compound carbon source, and a uniform and dense carbon coating layer can be obtained, significantly improving the cycle stability.

Description

Technical Field

[0001] The present invention relates to a cathode material for sodium-ion batteries, and particularly to a binary-doped iron-based fluorophosphate sodium-ion cathode material and a preparation method thereof. Background Art

[0002] Sodium-ion batteries have advantages such as rich resources, low cost, and high cost performance, and have good application prospects. The working principle of sodium-ion batteries is similar to that of lithium-ion batteries, and the cathode material determines the energy density of sodium-ion batteries. Compared with transition metal layered oxide cathode materials, polyanion-type compounds have a higher working voltage, a stable three-dimensional structural framework, relatively low cost, and high safety. Polyanion-type sodium-ion cathode materials have a high theoretical specific capacity and good safety, and are currently a research hotspot for sodium-ion battery cathode materials.

[0003] Polyanion-type compounds are composed of polyanion groups and transition metal elements, and are framework structure materials composed of anion polyhedra [XOx] n- and transition metal-oxygen polyhedra. Applying it to the cathode material of sodium-ion batteries has the following advantages: 1), the framework structure of the X-O covalent bond has higher stability, so the safety of the prepared sodium-ion battery is further improved; 2) polyanion-type compounds have rich lattice vacancies, which can alleviate the decrease in battery capacity caused by the repeated insertion and extraction of sodium ions during charge and discharge; 3) polyanions have an inductive effect, which increases the redox potential of transition metal ions. However, in polyanion-type sodium-ion cathode materials, the polyhedra composed of transition metals and oxygen atoms are blocked by anion tetrahedrons, and the electron clouds of transition metals are in a discontinuous state, which hinders electron exchange and lacks an electron transport pathway, resulting in low electronic conductivity and slow sodium-ion diffusion kinetics, limiting the specific capacity and rate performance of sodium-ion batteries. It can be seen that for polyanion sodium-ion cathode materials, the intrinsic properties of the materials determine the lower limit of the rate performance of the cathode materials. Therefore, improving the electronic conductivity is the key to improving polyanion cathode materials, which is of great significance for further improving their rate performance and increasing the capacity.

[0004] The fluorophosphate-type cathode material Na2MPO4F has a high theoretical capacity, a high working voltage, and inexpensive raw materials, and is a promising sodium-ion cathode material at present. Among them, the iron-based fluorophosphate Na2FePO4F has a high theoretical specific capacity of 124 mAh·g -1 However, Na2FePO4F has low electronic conductivity and poor rate performance, which limits the exertion of its electrochemical performance.

[0005] Generally, in the prior art, the electrochemical performance of fluorophosphate cathode materials is improved by carbon coating, element doping, and reducing the particle size of the materials. Carbon coating can improve the surface conductivity of particles and inhibit grain growth, and it is a relatively mature modification technology currently in application. There are also reports of improving the electron and ion conductivities of fluorophosphate cathode materials by doping with Ni or Mn. However, although the discharge specific capacity increases after doping, the cycle stability decreases. However, it is difficult to obtain a uniformly coated layer on the surface by carbon coating, and the amorphous carbon coated has insufficient improvement on the electron conductivity of the material and cannot achieve the theoretical high capacity of the cathode material. Doping often leads to a decrease in structural stability or affects the energy density of the material. And the nanosizing of the material makes the primary particles prone to agglomeration. Therefore, the above modification methods have their own characteristics. To obtain a fluorophosphate sodium ion cathode material that can meet practical applications, multiple factors need to be comprehensively considered. Summary of the Invention

[0006] In order to overcome the defect that the electrochemical performance of the polyanionic sodium ion cathode material of iron-based fluorophosphate in the prior art still needs to be further improved, especially the low electron conductivity and sodium ion diffusion coefficient, the present invention provides an improved iron-based fluorophosphate sodium ion cathode material and its preparation method. Through binary doping with iron-based phosphate salts, the present invention obtains a sodium ion cathode material with excellent electrochemical performance. Through carbon coating, especially using polysaccharide and imidazoline surfactant as carbon sources, the cycle stability of the sodium ion battery cathode material is significantly improved. The binary-doped iron-based fluorophosphate sodium ion cathode material prepared by the present invention has excellent comprehensive performance, and the raw materials are cheap, easy to obtain, and rich in sources. It is a new type of sodium ion battery cathode material with great application potential.

[0007] The object of the present invention is achieved through the following technical solutions:

[0008] The first object of the present invention is to provide a binary-doped iron-based fluorophosphate sodium ion cathode material, expressed as Na2Fe 1-x-y M x N y PO4F@C, which is obtained by surface carbon coating of Na2Fe 1-x-y M x N y PO4F, where M is Mn, 0.02 ≤ x ≤ 0.10; N is selected from at least one of Ni, Co, Cu, Zn, Ti, 0.005 ≤ y ≤ 0.05, and @C represents carbon coating.

[0009] Further, 0.03 ≤ x ≤ 0.07; N is Co, 0.01 ≤ y ≤ 0.03.

[0010] Further, the particle size of the sodium ion cathode material is 200 - 1000 nm, preferably 500 - 800 nm, the thickness of the carbon coating layer is 1 - 10 nm, preferably 4 - 7 nm, and the carbon coating layer accounts for 5 - 10 wt% of the sodium ion cathode material.

[0011] In the present invention, binary doping of Mn and transition metal N (especially Co) is carried out on Na2PO4F. By regulating the doping ratio, the electronic conductivity and sodium ion diffusion performance of the sodium ion cathode material can be significantly improved, which is beneficial to the high-rate performance of sodium ion batteries. At the same time, the reversible specific capacity of the sodium ion cathode material obtained in the present invention is as high as 100 mAh·g at 0.1C -1 or more, and the reversible specific capacity is also 80 mAh·g -1 or more at a high rate of 5C, and the cycle stability is good. It is a sodium ion battery cathode material with great potential for practical industrial applications.

[0012] The second object of the present invention is to provide a preparation method for the binary-doped iron-based fluorophosphate sodium ion cathode material, including the following steps:

[0013] (S1) Add a sodium source, a fluorine source, an iron source, a phosphorus source, a manganese source, a transition metal N source, and a reducing agent to a dispersant, adjust the pH to 9 - 11, react the mixed solution in a hydrothermal reaction kettle, cool, wash, and dry to obtain Na2Fe 1-x-y M x N y PO4F material;

[0014] (S2) Perform carbon coating on the material obtained in step (S1).

[0015] In step (S1), when adjusting the pH to 9 - 11, if the alkalinity is too strong, such as pH > 11, Fe 2+ is easily oxidized to F e3+ , and if the alkalinity is too weak, it is not conducive to the hydrolysis of Fe 2+ .

[0016] Further, in step (S1), the sodium source is NaOH, Na2CO3, NaF, the fluorine source is NaF, the iron source is at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, and their hydrates, the manganese source is at least one of manganese chloride and manganese nitrate, the transition metal N source is a salt of divalent transition metal, such as a halide salt, nitrate; the phosphorus source is at least one of phosphoric acid, ammonium dihydrogen phosphate, and ammonium hydrogen phosphate.

[0017] Further, in step (S1), the feeding ratio of the sodium source, the fluorine source, the iron source, the phosphorus source, the manganese source, the transition metal N source, and the reducing agent satisfies that Na:F:Fe:P:Mn:element N satisfies Na2Fe 1-x-y Mx N y The equivalent ratio of PO4F@C. The ratio of each element of the polyanionic phosphate obtained in the present invention is calculated according to the equivalent of the feeding ratio. Although there is a certain difference from the actually measured value, the difference is not significant.

[0018] Further, the hydrothermal reaction conditions in step (S1) are programmed heating. First, preheat at 80 - 100 °C for 1 - 2 h, then heat up to 120 - 130 °C at a rate of 0.5 - 2 °C / min, keep the temperature for reaction for 2 - 4 h, and then heat up to 150 - 180 °C at a rate of 3 - 5 °C / min, keep the temperature for reaction for 6 - 10 h. According to the programmed heating process of the present invention, the obtained cathode material structure framework has multi-dimensional sodium ion channels. The structure framework has little hindrance to the diffusion of sodium ions, which is beneficial to the migration of sodium ions in multiple directions. The ion channel size is large and the path is straight, which is more suitable for the smooth diffusion of sodium ions in the channel, reduces the sodium ion diffusion barrier, and improves the sodium ion diffusion coefficient.

[0019] In the initial stage of the solvothermal reaction, first preheat at 80 - 100 °C, and then slowly heat up to 120 - 130 °C, which is beneficial to adjusting the crystal structure of the obtained Na2Fe 1-x-y M x N y PO4F material, making the material framework structure stable. Then heat up to 150 - 180 °C at a faster rate and keep the temperature for reaction, finally obtaining a framework structure that is stable and beneficial to the diffusion of sodium ions. The finally obtained material particles have good dispersibility, and the primary particle size D90 is 200 - 1000 nm, preferably 500 - 800 nm.

[0020] Further, the dispersant in step (S1) is a mixed solvent of water and ethylene glycol in a mass ratio of 1:2 - 5.

[0021] Further, the antioxidant in step (S1) is not particularly limited. It is to prevent Fe 2+ from being oxidized. Common water-soluble antioxidants in the art can be used, such as ascorbic acid, vitamin C, etc. The addition amount of the antioxidant is 0.05 - 0.1 equivalent of the Fe source (calculated as Fe).

[0022] Further, the carbon coating in step (S2) is gas-phase deposition carbon coating or impregnation calcination carbon coating. Preferably, it is the impregnation calcination carbon coating process. Specifically, the material obtained in step (S1) and the carbon source are mixed evenly in the dispersant, dried, and then calcined at 600 - 800 °C for 6 - 10 h under the protection of an inert atmosphere to obtain the carbon-coated material.

[0023] The carbon source is a compound of polysaccharide and imidazoline surfactant in a mass ratio of 100:5-10. The polysaccharide is selected from at least one of glucose and sucrose. The imidazoline surfactant is a long-chain alkyl imidazoline, such as at least one of undecyl carboxymethyl hydroxyethyl imidazoline, undecyl carboxyethyl hydroxyethyl imidazoline, dodecyl carboxymethyl hydroxyethyl imidazoline, tetradecyl carboxymethyl hydroxyethyl imidazoline, and hexadecyl carboxymethyl hydroxyethyl imidazoline.

[0024] In the prior art, surfactants are used in the preparation of cathode materials for lithium-ion or sodium-ion batteries, but they are all added together with other raw materials in hydrothermal and sol-gel methods. The micelles formed by the surfactants in the dispersion phase are used as template agents to obtain porous or hollow microspheres. The method of the present invention is different from these patents. The surfactant is added together with the carbon source during carbon coating after the cathode material particles are formed.

[0025] In the present invention, the polysaccharide is used as the carbon source, which is rich in sources and low in price. On the one hand, the imidazoline surfactant makes the polysaccharide more evenly attached to the surface of the cathode material particles, forming a more uniform and dense carbon coating layer during the subsequent calcination process; on the other hand, it also serves as a supplement to the carbon source. Although the reason is not clear, when adding other surfactants during carbon coating, the cycle stability of the sodium-ion battery cannot be significantly improved, such as adding SDS, dodecyl trimethyl ammonium bromide, oleic acid, etc. Moreover, replacing the carbon source with other carbon sources, such as citric acid, phenolic resin, etc., cannot achieve the effect of polysaccharide and imidazoline surfactant, indicating that the polysaccharide as the carbon source plays a certain synergistic compounding role with the imidazoline surfactant.

[0026] During carbon coating, the dispersant is at least one of ethanol, ethylene glycol, and propanol, and the dosage of the dispersant is 2-3 times the total mass of the material obtained in step (S1) and the carbon source.

[0027] The usage amount of the carbon source is 30-40 wt% of the Na2Fe 1-x-y M x N y PO4F material. Although adding more carbon source can more effectively inhibit volume expansion and also improve the electronic conductivity of the material to a certain extent, carbon is not an active substance in the electrode material. If the carbon coating layer is too thick, or the mass content of the carbon coating layer is too large, the mass energy density of the battery will become lower. Therefore, it is appropriate to control the usage amount of the carbon source in the present invention to be 30-40 wt% of the material obtained in step (S1). Within this range, a sodium-ion battery cathode material with better comprehensive performance can be obtained.

[0028] The present invention has achieved the following beneficial effects:

[0029] 1. The present invention prepares a binary-doped polyanionic sodium-ion battery cathode material, where the doping elements are metallic manganese and another transition metal. The binary doping significantly improves the reversible specific capacity of the prepared cathode material, especially the reversible specific capacity at high rates, which is beneficial for the industrialization of sodium-ion batteries.

[0030] 2. The present invention performs carbon coating on the cathode material. Using polysaccharide and imidazoline-based surfactants as a compound carbon source, a uniform and dense carbon coating layer can be obtained, significantly improving the cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is the electron microscope photograph of the cathode material of Preparation Example 1 of the present invention;

[0032] Figure 2 It is the electron microscope photograph of the cathode material of Preparation Example 9 of the present invention;

[0033] Figure 3 It is the electron microscope photograph of the cathode material of Preparation Example 11 of the present invention.

[0034] 2. The present invention performs carbon coating on the cathode material. Using polysaccharide and imidazoline-based surfactants as a compound carbon source, a uniform and dense carbon coating layer can be obtained, significantly improving the cycle stability.

[0035] Figure 1 It is the electron microscope photograph of the cathode material of Preparation Example 1 of the present invention;

[0036] Figure 2 It is the electron microscope photograph of the cathode material of Preparation Example 9 of the present invention;

[0037] Figure 3 It is the electron microscope photograph of the cathode material of Preparation Example 11 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The following further elaborates the present application through examples.

[0039] The mass of the carbon coating layer in the sodium-ion battery cathode material is measured by thermogravimetric (TGA) method.

[0040] Preparation Example 1

[0041] (S1) Add NaOH, NaF, FeSO4·7H2O, H3PO4, Mn(NO3)2, Co(NO3)2 and ascorbic acid in a molar ratio of 1:0.93:1:1:0.05:0.02:0.1. Add a dispersant which is 12 times the mass of the above raw materials. The dispersant is a mixed solvent of deionized water and ethylene glycol in a mass ratio of 1:3. Adjust the pH to 10, stir evenly, transfer to a hydrothermal reaction kettle with a polytetrafluoroethylene inner lining, preheat at 80 °C for 1 h, then heat up to 120 °C at a rate of 1 °C / min, hold the reaction for 3 h, then heat up to 170 °C at a rate of 4 °C / min, hold the reaction for 8 h. After cooling, wash 3 times with deionized water and ethanol respectively, filter, and vacuum dry at 0.01 Mpa and 90 °C for 15 h to obtain Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F.

[0042] (S2) Add the Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F and a carbon source to ethanol as the dispersant. The carbon source is a compound of glucose and undecyl carboxymethyl hydroxyethyl imidazoline in a mass ratio of 100:8. The dosage of the carbon source is 34 wt% of Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F. The dosage of the dispersant ethanol is 2.2 times that of the Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F material and the carbon source. Ultrasonic vibrate for 1 h, mix evenly, dry at 0.01 MPa and 110 °C, grind and then load into a muffle furnace. Calcinate at 700 °C for 6 h under an argon atmosphere. Finally, obtain carbon-coated Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F, expressed as Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F@C, @C indicates carbon coating. After testing, the particle size D90 of the obtained Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F@C is about 570 nm, the thickness of the carbon coating layer is 6 nm, and the mass of the carbon coating layer is 7.3 wt%.

[0043] Preparation Example 2

[0044] (S1) Add NaOH, FeSO4·7H2O, NaF, H3PO4, Mn(NO3)2, Co(NO3)2 and ascorbic acid in a molar ratio of 1:0.94:1:1:0.03:0.03:0.1. Add a dispersant which is 12 times the mass of the above raw materials. The dispersant is a mixed solvent of deionized water and ethylene glycol in a mass ratio of 1:5. Adjust the pH to 10, stir evenly, transfer to a hydrothermal reaction kettle with a polytetrafluoroethylene inner lining, preheat at 90°C for 1 h, then heat up to 120°C at a rate of 0.5°C / min, hold the reaction for 3 h, then heat up to 160°C at a rate of 3°C / min, hold the reaction for 10 h. After cooling, wash with deionized water and ethanol 3 times respectively, filter, and vacuum dry at 0.01 Mpa and 90°C for 15 h to obtain Na2Fe 0.94 Mn 0.03 Co 0.03 PO4F.

[0045] (S2) Add the Na2FeMn 0.05 Co 0.02 PO4F obtained in step (S1) and a carbon source to ethylene glycol as the dispersant. The carbon source is a compound of sucrose and 1-undecyl carboxyethyl hydroxyethyl imidazoline in a mass ratio of 100:5. The dosage of the carbon source is 30 wt% of Na2Fe 0.94 Mn 0.03 Co 0.03 PO4F. The dosage of the dispersant is 2.2 times the sum of the Na2Fe 0.94 Mn 0.03 Co 0.03 PO4F material and the carbon source. Ultrasonic oscillation for 1 h, mix evenly, dry at 0.01 MPa and 110°C, grind and then load into a muffle furnace, calcine at 700°C for 6 h under an argon atmosphere, and finally obtain carbon-coated Na2Fe 0.94 Mn 0.03 Co 0.03 PO4F, expressed as Na2Fe 0.94 Mn 0.03 Co 0.03 PO4F@C, where @C represents carbon coating. After testing, the particle size D90 of the obtained Na2Fe 0.94 Mn 0.03 Co 0.03 PO4F@C is about 680 nm, the thickness of the carbon coating layer is 5 nm, and the mass of the carbon coating layer is 6.8 wt%.

[0046] Preparation Example 3

[0047] (S1) Add NaOH, FeSO4·7H2O, NaF, H3PO4, Mn(NO3)2, Co(NO3)2 and ascorbic acid in a molar ratio of 1:0.92:1:1:0.07:0.01:0.1. Add a dispersant which is 12 times the mass of the above raw materials. The dispersant is a mixed solvent of deionized water and ethylene glycol in a mass ratio of 1:3. Adjust the pH to 10, stir evenly, transfer to a hydrothermal reaction kettle with a polytetrafluoroethylene inner lining, preheat at 90 °C for 1 h, then heat up to 120 °C at a rate of 2 °C / min, hold the reaction for 3 h, then heat up to 180 °C at a rate of 3 °C / min, hold the reaction for 6 h. After cooling, wash 3 times with deionized water and ethanol respectively, filter, and vacuum dry at 0.01 Mpa and 90 °C for 15 h to obtain Na2Fe 0.92 Mn 0.07 Co 0.01 PO4F.

[0048] (S2) Add the Na2Fe 0.92 Mn 0.07 Co 0.01 PO4F and the carbon source to the dispersant ethanol. The carbon source is a compound of glucose and undecyl carboxymethyl hydroxyethyl imidazoline in a mass ratio of 100:10. The dosage of the carbon source is 40 wt% of Na2Fe 0.92 Mn 0.07 Co 0.01 PO4F. The dosage of the dispersant is 2.2 times that of Na2Fe 0.92 Mn 0.07 Co 0.01 material and the carbon source. Ultrasonic oscillation for 1 h, mix thoroughly, dry at 0.01 MPa and 110 °C, grind and then load into a muffle furnace, calcine at 700 °C for 6 h in an argon atmosphere, and finally obtain carbon-coated Na2Fe 0.92 Mn 0.07 Co 0.01 PO4F, expressed as Na2Fe 0.92 Mn 0.07 Co 0.01 PO4F@C, @C represents carbon coating. After testing, the obtained Na2Fe 0.92 Mn 0.07 Co 0.01 PO4F@C has a particle size D90 of about 700 nm, a carbon coating layer thickness of 8 nm, and a carbon coating layer mass of 8.1 wt%.

[0049] Preparation Example 4

[0050] Other conditions and operations are the same as those in Preparation Example 1, except that NaOH, FeSO4·7H2O, NaF, H3PO4, Mn(NO3)2, Co(NO3)2 and ascorbic acid are added in a molar ratio of 1:0.9:1:1:0.05:0.05:0.1, and finally Na2Fe 0.9 Mn 0.05 Co 0.05 PO4F@C is obtained.

[0051] Preparation Example 5

[0052] Other conditions and operations are the same as those in Preparation Example 1, except that NaOH, FeSO4·7H2O, NaF, H3PO4, Mn(NO3)2, Co(NO3)2 and ascorbic acid are added in a molar ratio of 1:0.9:1:1:0.08:0.02:0.1, and finally Na2Fe 0.9 Mn 0.08 Co 0.02 PO4F@C is obtained.

[0053] Preparation Example 6

[0054] Other conditions and operations are the same as those in Preparation Example 1, except that Co(NO3)2 is replaced with an equimolar amount of Cu(NO3)2, and finally Na2Fe 0.93 Mn 0.05 Cu 0.02 PO4F@C is obtained.

[0055] Preparation Example 7

[0056] Other conditions and operations are the same as those in Preparation Example 1, except that Co(NO3)2 is replaced with an equimolar amount of Ni(NO3)2, and finally Na2Fe 0.93 Mn 0.05 Ni 0.02 PO4F@C is obtained.

[0057] Preparation Example 8

[0058] Other conditions and operations are the same as those in Preparation Example 1, except that Co(NO3)2 is replaced with an equimolar amount of Zn(NO3)2, and finally Na2Fe 0.93 Mn 0.05 Zn 0.02 PO4F@C is obtained.

[0059] Preparation Example 9

[0060] Other conditions and operations are the same as those in Preparation Example 1, except that in step (S2), the carbon source is a compound of glucose and undecyl carboxymethyl hydroxyethyl imidazoline in a mass ratio of 100:2.

[0061] Preparation Example 10

[0062] Other conditions and operations are the same as those in Preparation Example 1, except that in step (S2), the carbon source is a compound of glucose and undecyl carboxymethyl hydroxyethyl imidazoline in a mass ratio of 100:15.

[0063] Preparation Example 11

[0064] Other conditions and operations are the same as those in Preparation Example 1, except that in step (S2), the carbon source is glucose, that is, undecyl carboxymethyl hydroxyethyl imidazoline is not added.

[0065] Figure 1 It is the electron microscope photograph of the positive electrode material obtained in the preparation example. Figure 2 It is the electron microscope photograph of the positive electrode material obtained in Preparation Example 9. Figure 3 It is the electron microscope photograph of the positive electrode material obtained in Preparation Example 11. It can be seen that in Preparation Example 1, glucose and undecyl carboxymethyl hydroxyethyl imidazoline are used together as the carbon source, and the thickness of the obtained carbon coating layer is uniform; the carbon coating thickness of the positive electrode material obtained in Preparation Example 9 is uneven, while in Preparation Example 11, without adding the undecyl carboxymethyl hydroxyethyl imidazoline surfactant, the carbon coating thickness is uneven and obvious exposure appears. It shows that the present invention uses a compound of a certain proportion of polysaccharide and imidazoline surfactant as the carbon source, which can achieve good carbon coating for sodium ion batteries.

[0066] Preparation Example 12

[0067] Other conditions and operations are the same as those in Preparation Example 1, except that in step (S1), after preheating at 80 °C for 1 h, the temperature is raised to 170 °C at a heating rate of 4 °C / min and kept for reaction for 12 h.

[0068] Comparative Example 1

[0069] Other conditions and operations are the same as those in Preparation Example 1, except that in step (S1), the materials are fed in a molar ratio of 1:0.95:1:1:0.05:0.1 for NaOH, FeSO4·7H2O, NaF, H3PO4, Mn(NO3)2 and ascorbic acid, that is, Co doping is not carried out. Finally, the sodium ion battery positive electrode material Na2Fe 0.95 Mn 0.05 PO4F@C.

[0070] Comparative Example 2

[0071] Other conditions and operations are the same as those in Preparation Example 1, except that in step (S1), the material feeding is carried out by adding NaOH, FeSO4·7H2O, NaF, H3PO4, Co(NO3)2 and ascorbic acid in a molar ratio of 1:0.98:1:1:0.02:0.1, that is, Mn doping is not carried out. Finally, the sodium-ion battery cathode material Na2Fe 0.98 Co 0.02 PO4F@C is obtained.

[0072] Comparative Example 3

[0073] The cathode material is Na2Fe 0.93 Mn 0.05 Co 0.02 PO4F obtained in step (S1) of Preparation Example 1, that is, the cathode material is not carbon-coated.

[0074] Application Example

[0075] The cathode material powder obtained in the preparation example, conductive agent super P and PVDF are mixed at a mass ratio of 8:1:1, and NMP is used as a dispersant to prepare a slurry, which is coated on an aluminum foil and vacuum dried. After compaction, a cathode sheet is obtained. Using metallic sodium as the anode, a commercial polypropylene separator, and 1mol / L NaPF6 (the molar ratio of the solvents EC and DEC is 1:1) as the electrolyte, and the above-prepared cathode sheet as the cathode, a 2032 button cell is assembled in an inert atmosphere in a glove box. The battery performance is tested: the charge-discharge voltage is set to 2.5 - 4.0V, and a constant current charge-discharge test is carried out with a battery tester. The results are shown in Table 1 below:

[0076] Table 1

[0077]

[0078]

[0079] From the data in Table 1, it can be seen that the sodium-ion battery cathode material prepared by the present invention has excellent electrochemical performance. In the preferred embodiment, at a current density of 0.1C, the reversible specific capacity is above 110mAh g -1 or more; at a high-rate current density of 5C, it also has a reversible specific capacity of above 90mAh g -1 or more; and it has good cycle stability. After operating for 100 cycles, it can maintain a capacity retention rate of more than 90%.

Claims

1. A method for preparing a binary doped iron-based sodium fluorophosphate positive electrode material, characterized in that: The following steps are involved: (S1) adding a sodium source, a fluorine source, an iron source, a phosphorus source, a manganese source, a transition metal nitrogen source, and an antioxidant to a dispersant, adjusting the pH to alkaline, reacting the mixture in a hydrothermal reactor, cooling, washing, and drying to obtain Na2Fe 1-x-y M x N y PO4F material; (S2) carbon-coating the material obtained in step (S1); the carbon-coating process comprises the following steps: uniformly mixing the material obtained in step (S1) and a carbon source in a dispersant, drying, and calcining at 600-800° C. for 6-10 hours under inert atmosphere to obtain a carbon-coated material; the carbon source is a compound of a polysaccharide and an imidazoline-type surfactant in a mass ratio of 100:5-10; the polysaccharide is selected from at least one of glucose and sucrose, and the imidazoline-type surfactant is selected from at least one of undecylcarboxymethylhydroxyethylimidazoline, undecylcarboxyethylhydroxyethylimidazoline, dodecylcarboxymethylhydroxyethylimidazoline, tetradecylcarboxymethylhydroxyethylimidazoline, and hexadecylcarboxymethylhydroxyethylimidazoline; The binary doped iron-based sodium fluorophosphate cathode material is expressed as Na2Fe 1-x-y M x N y PO4F@C, which is Na2Fe 1-x-y M x N y PO4F is obtained by surface carbon coating, wherein M is Mn, 0.02≤x≤0.10; N is selected from at least one of Ni, Co, Cu, and Zn, 0.005≤y≤0.05, and @C represents carbon coating.

2. The preparation method according to claim 1, characterized in that 0.03≤x≤0.07; N is Co, 0.01≤y≤0.

03.

3. The preparation method according to claim 1, characterized in that The particle size of the sodium ion positive electrode material is 200-1000 nm, the thickness of the carbon coating layer is 1-10 nm, and the carbon coating layer accounts for 5-10 wt % of the sodium ion positive electrode material.

4. The preparation method according to claim 3, characterized in that The particle size of the sodium ion positive electrode material is 500-800 nm, and the thickness of the carbon coating layer is 4-7 nm.

5. The preparation method according to claim 1, characterized in that In step (S1), the sodium source is NaOH, Na2CO3, or NaF, the fluorine source is NaF, the iron source is at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, and hydrates thereof, the manganese source is at least one of manganese chloride and manganese nitrate, and the transition metal N source is a salt of a divalent transition metal; and the phosphorus source is at least one of phosphoric acid, ammonium dihydrogen phosphate, and ammonium monohydrogen phosphate.

6. The preparation method according to claim 1, characterized in that The hydrothermal reaction conditions in step (S1) are programmed temperature rise, first preheating at 80-100°C for 1-2 hours, then heating to 120-130°C at 0.5-2°C / min, keeping the temperature for 2-4 hours, then heating to 150-180°C at 3-5°C / min, and keeping the temperature for 6-10 hours.

7. The preparation method according to claim 1, characterized in that The dispersant in step (S1) is a mixed solvent of water and ethylene glycol in a mass ratio of 1:2-5; the antioxidant is a water-soluble antioxidant, and the added amount of the antioxidant is 0.05-0.1 equivalent of the Fe source; the Fe source is calculated as Fe.

8. The preparation method according to claim 1, characterized in that The amount of carbon source used is the Na2Fe 1-x-y M x N y PO4F material 30-40wt%.

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