Sodium ferric sulfate positive electrode material, preparation method thereof, positive electrode sheet and sodium ion battery

Abstract

The application relates to the technical field of sodium ion batteries, in particular to a sodium ferrosulfate positive electrode material, a preparation method and a positive electrode sheet and a sodium ion battery, wherein the sodium ferrosulfate positive electrode material comprises a sodium ferrosulfate base body, and the surface of the sodium ferrosulfate base body is coated with a carbon coating layer; wherein the sodium ferrosulfate base body is doped with P elements, and the carbon coating layer is doped with N elements. The P elements doped in the sodium ferrosulfate base body and the N elements doped on the carbon coating layer improve the rate performance and the cycle stability of the sodium ferrosulfate positive electrode material.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a sodium iron sulfate cathode material, its preparation method, cathode sheet, and sodium-ion battery. Background Technology

[0002] Sodium-ion batteries, as a promising technology for electric vehicles and stationary energy storage, offer advantages over lithium-ion batteries, including better low-temperature performance, lower cost, and abundant materials. Furthermore, large-scale production of sodium-ion batteries is feasible using existing lithium-ion battery manufacturing infrastructure.

[0003] Currently, promising sodium-ion cathode materials for commercial application include layered oxides, polyanionic compounds, and Prussian blue compounds. Among them, the structural phase transition of layered oxides introduces many unstable factors for sodium ion insertion and extraction, and the introduction of expensive Ni and Cu elements to improve performance results in high costs, which is not conducive to large-scale market application in the future. Prussian blue materials contain a large number of Fe(CN)6 defects and water of crystallization in their structure, which leads to structural failure and gas generation during sodium ion insertion and extraction, making commercial application difficult. In contrast, polyanionic compounds have a rigid open framework that can stably store sodium ions. In terms of resource costs, the abundance of Fe element in the Earth's crust is 5.00%, which is much higher than that of Mn, V, and Co. Iron-based polyanionic compounds have cost-effectiveness advantages and environmental friendliness.

[0004] Among various iron-based polyanionic compounds, sodium ferric sulfate cathode has the highest redox potential, approximately 3.8V, significantly higher than sodium ferric pyrophosphate (3.10V) and sodium ferric phosphate (2.90V). Due to its similar reversible capacity, wide availability of raw materials, and simple manufacturing process, sodium ferric sulfate material offers far superior energy density, raw material cost, and production cost compared to other polyanionic compounds. However, Fe... 2+ Sodium ferric sulfate is easily oxidized, which affects its reactivity, leading to decreased rate performance and cycle stability. Simultaneously, the material itself has poor electronic conductivity, which may cause difficulties in sodium ion insertion / extraction, thus limiting its rate capability. Currently, carbon coating is widely used to improve the electrochemical performance of sodium ferric sulfate materials; however, the rate performance and cycle performance of conventional carbon-coated sodium ferric sulfate cathode materials remain poor.

[0005] Therefore, improving the rate performance and cycle stability of sodium ferric sulfate cathode materials is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the aforementioned shortcomings in the existing technology, the present invention aims to provide a sodium iron sulfate cathode material, its preparation method, cathode sheet, and sodium-ion battery. The present invention improves the rate performance and cycle stability of the sodium iron sulfate cathode material by doping P element in the sodium iron sulfate matrix and N element on the carbon coating layer.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a sodium ferric sulfate cathode material, the sodium ferric sulfate cathode material comprising a sodium ferric sulfate matrix, the surface of the sodium ferric sulfate matrix being coated with a carbon coating layer, wherein the sodium ferric sulfate matrix is ​​doped with P element, and the carbon coating layer is doped with N element.

[0009] This invention improves the rate performance and cycle stability of sodium ferric sulfate cathode materials by doping phosphorus (P) into the sodium ferric sulfate matrix and nitrogen (N) into the carbon coating layer.

[0010] Furthermore, the chemical formula of the sodium ferric sulfate matrix is ​​Na. 6-2x Fe x (SO4)3, where x = 1.5 to 2.0;

[0011] And / or, the median particle size Dv50 of the sodium ferric sulfate cathode material is 2–6 μm;

[0012] And / or, the thickness of the carbon coating layer is 1 to 10 nm.

[0013] In a second aspect, the present invention provides a method for preparing the sodium ferric sulfate cathode material as described in the first aspect, the method comprising the following steps:

[0014] S1. Mix the iron source, sodium source and sulfur source to obtain sodium ferric sulfate precursor A;

[0015] S2. The sodium ferric sulfate precursor A, carbon source, and amphiphilic molecule are uniformly dispersed in an aqueous organic solvent for reaction. The organic solvent is then removed by evaporation to obtain a solid mixture B. The polar hydrophilic end of the amphiphilic molecule includes a P-containing hydrophilic end and an N-containing hydrophilic end, and the nonpolar hydrophobic end of the amphiphilic molecule includes a long hydrocarbon chain.

[0016] S3. The solid mixture B is sintered at low temperature under a protective gas atmosphere, and the sintered material is post-processed to obtain the sodium ferric sulfate cathode material.

[0017] Furthermore, the iron source includes at least one of FeSO4, ferrous nitrate, ferrous oxalate, and ferrous chloride; the sodium source includes at least one of sodium sulfate, sodium carbonate, sodium bicarbonate, sodium fluoride, sodium dihydrogen phosphate, and organic acid salts of sodium; and the sulfur source includes at least one of FeSO4, sodium sulfate, ammonium sulfate, and sodium bisulfate.

[0018] And / or, in step S1, the molar ratio of iron in the iron source, sodium in the sodium source and sulfur in the sulfur source is consistent with the molar ratio of iron, sodium and sulfur in the sodium ferric sulfate matrix;

[0019] And / or, in step S1, the mixing includes ball milling, wherein the ball-to-material ratio of the ball milling is (15-20):1, the rotation speed of the ball milling is 200-500 r / min, and the ball milling time is 4-6 h.

[0020] Further, in step S2, the carbon source includes at least one of acetylene black, biomass nanocarbon, glucose, sucrose, caramel, starch, CNTs, carbon quantum dots, and dextrin; the amphiphilic molecule includes a nitrogen-containing phosphate ester amphiphilic molecule, or the amphiphilic molecule includes P-containing amphiphilic organic compounds and N-containing amphiphilic organic compounds, wherein the nitrogen-containing phosphate ester amphiphilic molecules include at least one of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, aminopolyethylene glycol phospholipids, and sphingomyelin; the P-containing amphiphilic organic compounds include at least one of N-free phospholipid compounds, phosphatidic acid, and cardiolipin; and the N-containing amphiphilic organic compounds include at least one of polyoxyethylene castor oil and polyacrylamide.

[0021] Furthermore, in step S2, the mass ratio of the sodium ferric sulfate precursor A, the carbon source, and the amphiphilic molecule is (95-98):(1-2):(1-3).

[0022] Furthermore, in step S2, the dispersion rotation speed is 3000-5000 r / min, and the dispersion time is 2-4 h;

[0023] And / or, in step S2, the reaction temperature is 60–80°C, and the reaction time is 5–6 hours;

[0024] And / or, in step S2, the aqueous organic solvent includes at least one of ethanol, propanol, tetrahydrofuran, dipropylene glycol butyl ether, and propylene glycol methyl ether.

[0025] Furthermore, in step S3, the protective gas includes at least one of nitrogen, argon, hydrogen, and ammonia; the sintering temperature of the low-temperature sintering is 300–400°C, and the sintering time is 8–15 h; the post-treatment includes at least one of crushing, grinding, pulverizing, sieving, and demagnetizing.

[0026] Thirdly, the present invention provides a positive electrode sheet, the positive electrode sheet comprising the sodium ferric sulfate positive electrode material described in the first aspect or the sodium ferric sulfate positive electrode material prepared by the preparation method described in the second aspect.

[0027] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the positive electrode sheet described in the third aspect.

[0028] Compared with the prior art, the beneficial effects of the present invention include at least one of the following:

[0029] (1) The present invention improves the rate performance and cycle stability of sodium iron sulfate cathode material by doping P element in sodium iron sulfate matrix and N element on carbon coating layer.

[0030] (2) This invention provides a low-cost, high-rate-performance, and high-cycle-performance polyanionic cathode material for sodium-ion batteries. The surface of the sodium ferric sulfate cathode material precursor is modified with amphiphilic molecules containing both phosphorus (P) and nitrogen (N) hydrophilic ends to form an N / P-doped carbon-coated sodium ferric sulfate cathode material. Anionic doping of the sodium ferric sulfate cathode material with phosphorus (P) and doping of the carbon coating with nitrogen (N) allows the amphiphilic molecules to form a stable carbon film through low-temperature carbonization. This not only improves the air stability of sodium ferric sulfate but also constructs better ion / electron transport channels, making it easier to stably utilize the cathode material's capacity and enhancing its structural stability, thus significantly improving the rate-performance and cycle-performance. This preparation method is simple, environmentally friendly, and highly safe, possessing broad research prospects and application value. Attached Figure Description

[0031] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the structure of solid mixture B prepared in Example 1 of the present invention;

[0033] Figure 2 The XRD diffraction pattern of the sodium ferric sulfate cathode material prepared in Example 1 of this invention;

[0034] Figure 3 This is a SEM image of the sodium ferric sulfate cathode material prepared in Example 1 of the present invention;

[0035] Figure 4 This is a comparison chart of the discharge specific capacity of sodium ferric sulfate cathode materials prepared in Example 1 and Comparative Example 1 of the present invention at different rates. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0037] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0038] In a first aspect, the present invention provides a sodium ferric sulfate cathode material, the sodium ferric sulfate cathode material comprising a sodium ferric sulfate matrix, the surface of the sodium ferric sulfate matrix being coated with a carbon coating layer, wherein the sodium ferric sulfate matrix is ​​doped with P element, and the carbon coating layer is doped with N element.

[0039] This invention improves the rate performance and cycle stability of sodium ferric sulfate cathode materials by doping phosphorus (P) into the sodium ferric sulfate matrix and nitrogen (N) into the carbon coating layer.

[0040] In the above-mentioned sodium ferric sulfate cathode material, as an optional embodiment, the chemical formula of the sodium ferric sulfate matrix is ​​Na. 6-2x Fe x (SO4)3, where x = 1.5 to 2.0.

[0041] In the above-mentioned sodium ferric sulfate cathode material, as an optional embodiment, the median particle size Dv50 of the sodium ferric sulfate cathode material is 2 to 6 μm, for example, it can be 2 μm, 4 μm or 6 μm.

[0042] In the above-mentioned sodium ferric sulfate cathode material, as an optional embodiment, the thickness of the carbon coating layer is 1 to 10 nm, for example, it can be 1 nm, 3 nm, 5 nm, 7 nm or 10 nm.

[0043] In a second aspect, the present invention provides a method for preparing the sodium ferric sulfate cathode material as described in the first aspect, the method comprising the following steps:

[0044] S1. Mix the iron source, sodium source and sulfur source to obtain sodium ferric sulfate precursor A;

[0045] S2. The sodium ferric sulfate precursor A, carbon source, and amphiphilic molecule are uniformly dispersed in an aqueous organic solvent for reaction. The organic solvent is then removed by evaporation to obtain a solid mixture B. The polar hydrophilic end of the amphiphilic molecule includes a P-containing hydrophilic end and an N-containing hydrophilic end, and the nonpolar hydrophobic end of the amphiphilic molecule includes a long hydrocarbon chain.

[0046] S3. The solid mixture B is sintered at low temperature under a protective gas atmosphere, and the sintered material is post-processed to obtain the sodium iron sulfate cathode material (doped carbon-coated sodium iron sulfate cathode material).

[0047] In step S2, the present invention uses a solvothermal method to prepare the product, which can avoid the serious side reactions of the hydrothermal method.

[0048] The preparation method provided by this invention uses aqueous organic solvents for material synthesis, which is green and environmentally friendly. All aqueous organic solvents used can be evaporated and recovered. Furthermore, the preparation method has a simple process flow and low cost.

[0049] The N / P-doped carbon-coated sodium ferric sulfate cathode material provided by this invention introduces amphiphilic molecules containing N and P during the preparation process, resulting in a uniform coating of amphiphilic molecules on the surface of the prepared sodium ferric sulfate precursor A. The polar hydrophilic ends of these amphiphilic molecules consist of P-containing and N-containing hydrophilic ends, while the non-polar hydrophobic ends consist of long hydrocarbon chains. The polar hydrophilic ends can bind to inorganic materials, while the non-polar hydrophobic ends maintain stable steric hindrance between particles. The P-containing hydrophilic ends can act as anion dopersants during the synthesis of sodium ferric sulfate. P in the P-containing hydrophilic ends can partially replace S, and since the ionic radius of P is larger than that of S, P doping into the sodium ferric sulfate matrix can expand the crystal lattice, facilitating the migration of sodium ions in the diffusion channels and improving the rate performance of the material. Simultaneously, P doping improves the stability of the sodium ferric sulfate material. The N-containing hydrophilic ends can form defects in the carbon coating layer on the surface, which is beneficial for the adsorption and storage of sodium ions, further improving the conductivity and increasing the Na+ content of the material. + The migration rate is improved, thus enhancing the rate performance of the material. The phosphorus-containing hydrophilic end of the same phospholipid molecule is located inside the material, while the hydrophobic end is in the carbon layer. The structural continuity of the carbon layer strengthens the tight bond between sodium ferric sulfate and the coated carbon layer. In other words, the phosphorus-containing and nitrogen-containing hydrophilic ends promote the tight bond between sodium ferric sulfate and the carbon layer, ensuring the structural stability of the material. The amphiphilic molecules, after low-temperature carbonization, form a stable carbon film on the material surface, isolating it from air and moisture, which is beneficial to the material's conductivity and air stability.

[0050] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, the iron source includes at least one of divalent iron compounds, such as at least one of FeSO4, ferrous nitrate, ferrous oxalate, and ferrous chloride; the sodium source includes at least one of sodium sulfate, sodium carbonate, sodium bicarbonate, sodium fluoride, sodium dihydrogen phosphate, and organic acid salts of sodium; and the sulfur source includes at least one of FeSO4, sodium sulfate, ammonium sulfate, and sodium bisulfate.

[0051] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S1, the molar ratio of iron in the iron source, sodium in the sodium source, and sulfur in the sulfur source is consistent with the molar ratio of iron, sodium, and sulfur in the sodium ferric sulfate matrix.

[0052] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S1, the mixing includes ball milling, the ball-to-material ratio of the ball milling is (15-20):1, the rotation speed of the ball milling is 200-500 r / min (for example, it can be 200 r / min, 300 r / min, 400 r / min or 500 r / min), and the ball milling time is 4-6 h.

[0053] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S2, the carbon source includes at least one of acetylene black, biomass nanocarbon, glucose, sucrose, caramel, starch, CNTs, carbon quantum dots, and dextrin. Preferably, the carbon source is low-cost acetylene black. The amphiphilic molecule includes phosphate ester amphiphilic molecules containing nitrogen, or the amphiphilic molecule includes P-containing amphiphilic organic compounds and N-containing amphiphilic organic compounds. The nitrogen-containing phosphate ester amphiphilic molecules include at least one of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, aminopolyethylene glycol phospholipids, and sphingomyelin. The P-containing amphiphilic organic compounds include at least one of N-free phospholipid compounds, phosphatidic acid, and cardiolipin. The N-containing amphiphilic organic compounds include at least one of polyoxyethylene castor oil and polyacrylamide.

[0054] Specifically, the structural formula of the phosphatidylcholine (DLPC) is shown below:

[0055]

[0056] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S2, the mass ratio of the sodium ferric sulfate precursor A, the carbon source, and the amphiphilic molecule is (95-98):(1-2):(1-3). If the amphiphilic molecule has too high a molecular weight, the coating layer will be too thick, but the coating layer will lack capacity. Although the coating layer can enhance electron / ion transport and improve the material's capacity, an excessively thick coating layer will lead to a decrease in its own capacity. If the amphiphilic molecule has too low a molecular weight, it will not play a role in coating and doping.

[0057] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S2, the dispersion speed is 3000-5000 r / min (for example, it can be 3000 r / min, 3500 r / min, 4000 r / min, 4500 r / min or 5000 r / min), and the dispersion time is 2-4 h.

[0058] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S2, the aqueous organic solvent can be recycled and includes at least one of ethanol, propanol, tetrahydrofuran, dipropylene glycol butyl ether, and propylene glycol methyl ether.

[0059] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S2, the reaction temperature is 60-80℃ (for example, it can be 60℃, 65℃, 70℃, 75℃ or 80℃), and the reaction time is 5-6h.

[0060] In the above-mentioned method for preparing sodium ferric sulfate cathode material, as an optional embodiment, in step S3, the protective gas includes at least one of nitrogen, argon, hydrogen, and ammonia, preferably argon and hydrogen; the sintering temperature of the low-temperature sintering is 300-400℃ (for example, 300℃, 320℃, 340℃, 360℃, 380℃, or 400℃), and the sintering time is 8-15h (for example, 8h, 10h, 12h, or 15h); the post-treatment includes at least one of crushing, grinding, pulverizing, sieving, and demagnetizing.

[0061] Thirdly, the present invention provides a positive electrode sheet, which comprises the sodium iron sulfate positive electrode material described in the first aspect or the sodium iron sulfate positive electrode material prepared by the preparation method described in the second aspect. This improves the rate performance and cycle stability of the battery.

[0062] Fourthly, the present invention provides a sodium-ion battery comprising the positive electrode sheet described in the third aspect, thereby exhibiting high rate performance and cycle stability.

[0063] In the above-mentioned sodium-ion battery, as an optional embodiment, the sodium-ion battery further includes a negative electrode, a separator, and an electrolyte.

[0064] In the aforementioned sodium-ion battery, as an optional embodiment, the negative electrode active material on the negative electrode sheet includes at least one of graphene, graphene oxide, soft carbon, hard carbon, expanded graphite, silicon, and mesophase carbon microspheres, preferably hard carbon.

[0065] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0066] Example 1

[0067] This embodiment provides a method for preparing sodium ferric sulfate cathode material, including the following steps:

[0068] S1. Ferrous sulfate (FeSO4, iron and sulfur sources) and sodium sulfate (Na2SO4, sodium and sulfur sources) are ball-milled and mixed to obtain sodium ferric sulfate precursor Na. 2.26 Fe 1.87 (SO4)3 (sodium ferric sulfate precursor A), wherein the molar ratio of iron in ferrous sulfate to sodium in sodium sulfate is 1.87:2.26, the ball-to-material ratio in the ball milling mixture is 15:1, the rotation speed of the ball milling mixture is 300 r / min, and the ball milling time is 5 h;

[0069] S2. Sodium ferric sulfate precursor A, acetylene black, and phosphatidylcholine were added to a sealed reactor at a mass ratio of 96:2:2, and uniformly dispersed in anhydrous n-propanol at a solid-liquid ratio of 1:1.5. The mixture was heated to 70°C and reacted for 5 hours. After the reaction was completed, the anhydrous n-propanol was evaporated to obtain solid mixture B. The structural diagram of solid mixture B is shown below. Figure 1 As shown, the dispersion rotation speed is 4000 r / min and the dispersion time is 3 h;

[0070] S3. The solid mixture B is sintered at 380°C for 12 hours in a 97% Ar + 3% H2 atmosphere (Ar:H2 volume ratio is 97:3). The sintered material is then pulverized to obtain an N / P doped carbon-coated sodium iron sulfate cathode material (sodium iron sulfate cathode material). The median particle size Dv50 of this cathode material is 3 μm, and the thickness of the carbon coating layer is 3-5 nm.

[0071] Figure 2 The image shows the XRD diffraction pattern of the sodium ferric sulfate cathode material prepared in this embodiment. Figure 2 It can be seen that the characteristic peaks of the synthesized sodium ferric sulfate cathode material correspond well.

[0072] Figure 3This is a SEM image of the sodium ferric sulfate cathode material prepared in this embodiment. Figure 3 It can be seen that the synthesized sodium ferric sulfate cathode material has a spherical structure with a particle size of about 3 μm.

[0073] Example 2

[0074] The preparation method of the sodium ferric sulfate cathode material provided in this embodiment is basically the same as that in Example 1. The difference is that in step S2, the mass ratio of sodium ferric sulfate precursor A to acetylene black and phosphatidylcholine is 97:2:1.

[0075] Example 3

[0076] The preparation method of the sodium ferric sulfate cathode material provided in this embodiment is basically the same as that in Example 1. The difference is that in step S2, the mass ratio of sodium ferric sulfate precursor A to acetylene black and phosphatidylcholine is 95:2:3.

[0077] Example 4

[0078] The preparation method of the sodium ferric sulfate cathode material provided in this embodiment is basically the same as that in Example 1, except that in step S2, phosphatidylcholine is replaced with phosphatidylserine.

[0079] Example 5

[0080] The preparation method of the sodium ferric sulfate cathode material provided in this embodiment is basically the same as that in Example 1, except that in step S3, the 97% Ar + 3% H2 atmosphere is replaced with a 100% Ar atmosphere.

[0081] Example 6

[0082] The preparation method of the sodium ferric sulfate cathode material provided in this embodiment is basically the same as that in Embodiment 1. The difference is that in step S1, the ball-to-material ratio of the ball milling is 18:1, the rotation speed of the ball milling is 500 r / min, and the ball milling time is 4 h; in step S2, the dispersion speed is 5000 r / min, and the dispersion time is 2 h.

[0083] Comparative Example 1

[0084] This comparative example provides a method for preparing sodium ferric sulfate cathode material, including the following steps:

[0085] S1. Ferrous sulfate (FeSO4, iron and sulfur sources) and sodium sulfate (Na2SO4, sodium and sulfur sources) are ball-milled and mixed to obtain sodium ferric sulfate precursor Na. 2.26 Fe 1.87(SO4)3 (sodium ferric sulfate precursor A), wherein the molar ratio of iron in ferrous sulfate to sodium in sodium sulfate is 1.87:2.26, the ball-to-material ratio in the ball milling mixture is 15:1, the rotation speed of the ball milling mixture is 300 r / min, and the ball milling time is 5 h;

[0086] S2. Add sodium ferric sulfate precursor A and acetylene black to a sealed reactor at a mass ratio of 96:4, and disperse them uniformly in anhydrous n-propanol at a solid-liquid ratio of 1:1.5. Heat to 70°C and react for 5 hours. After the reaction is complete, evaporate the anhydrous n-propanol to obtain solid mixture B. The dispersion speed is 4000 r / min and the dispersion time is 3 hours.

[0087] S3. The solid mixture B is sintered at 380°C for 12 hours in a 97% Ar + 3% H2 atmosphere (the volume ratio of Ar to H2 is 97:3). The sintered material is then pulverized to obtain carbon-coated sodium iron sulfate cathode material.

[0088] Comparative Example 2

[0089] The preparation method of the sodium ferric sulfate cathode material provided in this comparative example is basically the same as that in Example 1, except that in step S2, phosphatidylcholine is replaced with polyoxyethylene castor oil.

[0090] The structure of polyoxyethylene castor oil in this comparative example is shown below:

[0091]

[0092] Comparative Example 3

[0093] The preparation method of the sodium ferric sulfate cathode material provided in this comparative example is basically the same as that in Example 1, except that in step S2, anhydrous n-propanol is replaced with deionized water.

[0094] Performance testing

[0095] Preparation of button batteries: In a glove box, the positive electrode sheet, glass fiber separator and electrolyte prepared from the positive electrode materials of the examples and comparative examples were assembled in sequence to obtain CR2032 button batteries. The electrolyte used was EC+DEC+EMC of 1.15mol / L NaPF6 (volume ratio 1:1:1), and the additive DFP was 2%.

[0096] Preparation of soft-pack batteries: The positive electrode sheet, hard carbon negative electrode sheet and polyethylene separator (PE+OBS separator) prepared by the positive electrode materials of the examples and comparative examples are stacked and assembled, and then baked until the moisture content is qualified. Electrolyte is injected, and after hot pressing formation and high temperature standing, they are packaged. After capacity testing, they are left to stand at room temperature to obtain soft-pack 505060 finished batteries. The electrolyte used is EC+DEC+EMC of 1.15mol / L NaPF6 (volume ratio 1:1:1), and the additive DFP is 2%.

[0097] Air stability evaluation: The cathode materials of the examples and comparative examples were placed in air for 0 months and 1 month, respectively, and then assembled into button cells. The 0.1C discharge capacity of the materials was tested, and the charge and discharge voltage window was 2.0V to 4.4V. The test results are shown in Table 1.

[0098] Material rate stability: The rate stability of the coin cell material was tested at 25°C using an electrochemical workstation battery testing system. The tested current densities were 0.1C, 0.5C, 1C, 5C, 10C, 20C, and 0.1C, with a charge / discharge voltage window of 2.0V–4.4V. The average discharge specific capacity over 10 charge / discharge cycles at the corresponding current density was taken as the discharge specific capacity at that current density. The test results are shown in Table 1 and [Table data would be inserted here]. Figure 4 .

[0099] Material cycle stability: The material cycle stability of the pouch cell was tested at 25°C on an electrochemical workstation battery testing system. The test charge / discharge current density was 1C / 1C, and the charge / discharge voltage window was 2.0V to 4.3V. The test results are shown in Table 1.

[0100] Table 1

[0101]

[0102]

[0103] From Table 1, it can be seen at least that:

[0104] (1) Comparing Example 1 with Comparative Example 1, it can be seen that the present invention improves the rate performance and cycle stability of sodium ferric sulfate cathode material by doping P element in sodium ferric sulfate matrix and N element on carbon coating layer. N / P doped carbon-coated sodium ferric sulfate cathode material, on the one hand, its phospholipid amphiphilic molecules form a stable carbon film on the material surface after low-temperature carbonization, which improves the air stability of sodium ferric sulfate, so that the cathode material has no capacity decay after being placed in air for 1 month. On the other hand, the sodium ferric sulfate cathode material can be modified by anion doping with P, and N can form defects in carbon coating layer to build better ion / electron transport channels, so that the material still has a high capacity at a high rate of 20C. At the same time, the P-containing hydrophilic end and the N-containing hydrophilic end can promote the tight bonding between sodium ferric sulfate and carbon layer, ensure the structural stability of the material, and optimize the cycle performance of the material.

[0105] (2) Comparing Example 1 with Comparative Example 2, it can be seen that only N-doped carbon coating layer is introduced, and no anion doping modification is performed on sodium iron sulfate cathode material. The stability and rate performance of the material are reduced, which is manifested as low specific capacity at high rate of 20C and poor 1C / 1C cycle performance.

[0106] (3) Comparing Example 1 with Comparative Example 3, it can be seen that when solid mixture B is prepared by hydrothermal method in step S2, the stability and rate performance of the material both decrease. This is manifested in low specific capacity at a high rate of 20C and poor 1C / 1C cycle performance. The reason may be that the hydrothermal method has serious side reactions and high impurity content, which reduces the electrochemical performance of the prepared cathode material.

[0107] (4) Comparing Example 1 and Example 5, it can be seen that when the 97% Ar + 3% H2 atmosphere is replaced with a 100% Ar atmosphere, the air stability, rate performance, and cycle performance of the material all decrease. The reason is that: Fe 2+ It is easily oxidized, which affects the material's reactivity. Adding reducing gas H2 can reduce trivalent Fe, ensuring the Fe is reduced during sintering. 2+ Its stable existence.

[0108] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sodium ferric sulfate cathode material, characterized in that, The sodium ferric sulfate cathode material includes a sodium ferric sulfate matrix, the surface of which is coated with a carbon coating layer, wherein the sodium ferric sulfate matrix is ​​doped with P element, and the carbon coating layer is doped with N element; The method for preparing the sodium ferric sulfate cathode material includes the following steps: S1. Mix the iron source, sodium source and sulfur source to obtain sodium ferric sulfate precursor A; S2. The sodium ferric sulfate precursor A, carbon source, and amphiphilic molecules are uniformly dispersed in an aqueous organic solvent for reaction. The organic solvent is then removed by evaporation to obtain a solid mixture B. The amphiphilic molecules include phosphate ester amphiphilic molecules containing nitrogen. The polar hydrophilic ends of the amphiphilic molecules include P-containing hydrophilic ends and N-containing hydrophilic ends. The nonpolar hydrophobic ends of the amphiphilic molecules include long hydrocarbon chains. S3. The solid mixture B is sintered at low temperature under a protective gas atmosphere, and the sintered material is post-processed to obtain the sodium ferric sulfate cathode material.

2. The sodium ferric sulfate cathode material according to claim 1, characterized in that, The chemical formula of the sodium ferric sulfate matrix is ​​Na. 6-2x Fe x (SO4)3, where x = 1.5~2.0; And / or, the median particle size Dv50 of the sodium ferric sulfate cathode material is 2~6 μm; And / or, the thickness of the carbon coating layer is 1~10nm.

3. The sodium ferric sulfate cathode material according to claim 1, characterized in that, The iron source includes at least one of FeSO4, ferrous nitrate, ferrous oxalate, and ferrous chloride; the sodium source includes at least one of sodium sulfate, sodium carbonate, sodium bicarbonate, sodium fluoride, sodium dihydrogen phosphate, and an organic acid salt of sodium; the sulfur source includes at least one of FeSO4, sodium sulfate, ammonium sulfate, and sodium bisulfate. And / or, in step S1, the molar ratio of iron in the iron source, sodium in the sodium source and sulfur in the sulfur source is consistent with the molar ratio of iron, sodium and sulfur in the sodium ferric sulfate matrix; And / or, in step S1, the mixing includes ball milling, wherein the ball-to-material ratio of the ball milling is (15-20):1, the rotation speed of the ball milling is 200-500 r / min, and the ball milling time is 4-6 h.

4. The sodium ferric sulfate cathode material according to claim 1, characterized in that, In step S2, the carbon source includes at least one of acetylene black, biomass nanocarbon, glucose, sucrose, caramel, starch, CNTs, carbon quantum dots, and dextrin; the nitrogen-containing phosphate amphiphilic molecules include at least one of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, aminopolyethylene glycol phospholipids, and sphingomyelin.

5. The sodium ferric sulfate cathode material according to claim 1, characterized in that, In step S2, the mass ratio of the sodium ferric sulfate precursor A, the carbon source and the amphiphilic molecule is (95~98):(1~2):(1~3).

6. The sodium ferric sulfate cathode material according to claim 1, characterized in that, In step S2, the dispersion rotation speed is 3000-5000 r / min, and the dispersion time is 2-4 h; And / or, in step S2, the reaction temperature is 60~80℃, and the reaction time is 5~6h; And / or, in step S2, the aqueous organic solvent includes at least one of ethanol, propanol, tetrahydrofuran, dipropylene glycol butyl ether, and propylene glycol methyl ether.

7. The sodium ferric sulfate cathode material according to claim 1, characterized in that, In step S3, the protective gas includes at least one of nitrogen, argon, hydrogen, and ammonia; the sintering temperature of the low-temperature sintering is 300~400℃, and the sintering time is 8~15h; the post-treatment includes at least one of crushing, screening, and demagnetization.

8. A positive electrode sheet, characterized in that, The positive electrode sheet includes the sodium ferric sulfate positive electrode material as described in any one of claims 1-7.

9. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode sheet as described in claim 8.

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