A modified sodium iron sulfate cathode material and its preparation method and application

Abstract

The present invention discloses a modified sodium ferric sulfate cathode material and its preparation method and application, belonging to the technical field of sodium ion batteries. The preparation method comprises the following steps: step S1, mixing graphene oxide, a sodium source, an antioxidant, a ferrous source, a sulfur source and water to obtain a mixed material; step S2, vacuum freeze-drying the mixed material and crushing it once to obtain a precursor; step S3, microwave sintering the precursor and crushing it twice to obtain a modified sodium ferric sulfate cathode material; microwave sintering is carried out at temperatures T1 and T2 in sequence; wherein T1 is 200-250°C, the holding time at T1 is 10-15 minutes, and T2 is 320-380°C, the holding time at T2 is 12-24 minutes. The above method can produce a modified sodium ferric sulfate cathode material with less impurities and uniform carbon coating, thereby helping to improve its electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and in particular relates to a modified sodium ferric sulfate positive electrode material, a preparation method thereof, and an application thereof. Background Art

[0002] Sodium-ion batteries have become a hot topic in recent battery technology research and development due to their high safety, abundant raw materials, and low cost. They are widely used in low-speed electric vehicles and energy storage devices. The cathode material is a key component of the battery and plays a crucial role in determining its electrochemical performance.

[0003] Currently, sodium-ion cathode materials are primarily classified into three categories based on their structure: polyanions, Prussian blue, and oxides. Polyanion cathode materials, with their low cost, excellent cycle performance, and environmental friendliness, have attracted widespread attention due to their promising applications in energy storage and low-speed vehicles. Among polyanion cathode materials, sodium iron sulfate (SFS) boasts a voltage plateau of up to 3.8V, significantly increasing the battery's power density. However, its structural characteristics result in poor electronic conductivity and sodium ion diffusion, limiting its application.

[0004] At present, most of the methods are to carbon-coat sodium ferric sulfate to improve its electronic conductivity and sodium ion diffusion rate. However, the carbon-coated sodium ferric sulfate materials prepared by the current modification methods have defects such as many impurity phases and uneven carbon coating, which have a great impact on the capacity and rate performance of the material. Summary of the Invention

[0005] The main purpose of the present invention is to provide a modified sodium ferric sulfate positive electrode material and its preparation method and application, so as to solve the problems of multiple impurities and uneven carbon coating in the sodium ferric sulfate material prepared in the prior art.

[0006] In order to achieve the above object, the first aspect of the present invention provides a method for preparing a modified sodium ferric sulfate positive electrode material, comprising the following steps:

[0007] Step S1, mixing graphene oxide, a sodium source, an antioxidant, a ferrous source, a sulfur source and water to obtain a mixed material;

[0008] Step S2, vacuum freeze-drying the mixed material and pulverizing it once to obtain a precursor;

[0009] Step S3, subjecting the precursor to microwave sintering and secondary crushing to obtain a modified sodium ferric sulfate cathode material;

[0010] Microwave sintering is carried out at temperatures T1 and T2 in sequence; wherein, T1 is 200-250° C., the holding time at T1 is 10-15 minutes, T2 is 320-380° C., the holding time at T2 is 12-24 minutes.

[0011] Through the synergistic effect between each step, first, graphene oxide is used as a carbon source. Since graphene oxide contains more hydrophilic oxygen-containing functional groups, it is promoted to be evenly dispersed with other raw materials in aqueous solvents. Secondly, in the subsequent vacuum freeze-drying, graphene oxide can maintain its original uniform dispersion state in the mixed material, avoiding stratification while achieving drying. Finally, through two-stage microwave-assisted sintering, on the one hand, microwave-assisted heating overcomes the shortcomings of uneven conventional heating, and has the advantages of shortening reaction time, improving work efficiency, fast heating speed, uniform heating, no temperature gradient, and no hysteresis effect. At the same time, graphene oxide has excellent microwave absorption properties, can quickly react with microwaves, and directly reduce to high-quality, micron-level graphene sheets without any reducing agent or atmosphere. This process is short in time and highly efficient. On the other hand, through two-stage sintering, free water and crystallization water in the raw materials are first removed under relatively low temperature conditions to avoid affecting the phase formation and crystallinity of the material and reduce the formation of impurity phases. Then, nano-level sintering is promoted under relatively high temperature conditions. The formation of sodium ferric sulfate phase and graphene coating layer, the introduction of graphene oxide will not affect the structure of sodium ferric sulfate, nano-scale sodium ferric sulfate helps to improve the diffusion coefficient of sodium ions, and at the same time, nano-scale sodium ferric sulfate is coated by micron-scale graphene sheets, which helps to form a uniform carbon coating, greatly improving the utilization rate of graphene oxide, improving the structural stability of the modified sodium ferric sulfate positive electrode material, and avoiding the occurrence of moisture absorption and decomposition. In addition, the formed graphene coating has a high degree of graphitization, which can provide a high-speed channel for the rapid transfer of electrons and improve the electronic conductivity of the material, thereby helping to improve its electrochemical properties such as the first coulomb efficiency (first effect), specific capacity, and rate performance.

[0012] Further, the water is divided into a first part and a second part, and step S1 includes:

[0013] Step S11, mixing graphene oxide with a first portion of water, and after ultrasonic dispersion, adding a sodium source and a first portion of a sulfur source thereto, and mixing them uniformly to obtain material A;

[0014] Step S12, dissolving the antioxidant in the second portion of water, adding the ferrous source and the second portion of the sulfur source thereto, and mixing them uniformly to obtain material B;

[0015] Step S13, adding the material B of step S12 to the material A of step S11 to obtain a mixed material.

[0016] By controlling the order of adding the raw materials into the mixture, the uniform dispersion of the raw materials can be further promoted.

[0017] Furthermore, the frequency of ultrasound is 20 to 50 Hz, and the time is 0.5 to 1.5 h.

[0018] By controlling the physical parameters of ultrasound, it is beneficial to promote the uniform dispersion of various raw materials.

[0019] Furthermore, the primary pulverization controls the D50 particle size to be 5 to 15 μm; and / or the secondary pulverization controls the D50 particle size to be 0.5 to 1.5 μm.

[0020] By controlling the particle size of the primary and secondary crushing, adhesion between particles can be prevented.

[0021] Furthermore, the vacuum freeze-drying temperature is -20 to 0°C and the time is 12 to 24 hours; and / or, the microwave sintering is carried out under an inert atmosphere, the inert atmosphere includes at least one of nitrogen and argon, and the power of the microwave sintering is 1200 to 1800W.

[0022] By controlling the conditions of vacuum freeze drying and microwave sintering, it is beneficial to achieve effective dehydration of the material to achieve drying and the formation of the final product.

[0023] Furthermore, in the mixed material, the molar ratio of sodium element, iron element and sulfur element is 2:2:3, the molar ratio of iron element to antioxidant is 10:(0.8~1.5), and the mass ratio of the sum of the masses of graphene oxide, sodium source, antioxidant, ferrous source and sulfur source to graphene oxide is 100:(2.0~3.0).

[0024] By limiting the molar ratio of sodium, iron and sulfur in the mixture, modified sodium ferric sulfate positive electrode materials with different stoichiometric ratios can be prepared. By limiting the mass ratio of graphene oxide, modified sodium ferric sulfate positive electrode materials with different carbon coating amounts can be obtained.

[0025] Furthermore, the sodium source is selected from at least one of sodium sulfate, sodium bisulfate, sodium nitrate, sodium carbonate, and sodium bicarbonate; and / or the antioxidant is selected from at least one of ascorbic acid and citric acid; and / or the ferrous source is selected from at least one of ferrous sulfate, ammonium ferrous sulfate, ferrous nitrate, and hydrates thereof; and / or the sulfur source is selected from at least one of sodium sulfate, sodium bisulfate, ferrous sulfate, ammonium ferrous sulfate, ammonium bisulfate, and ammonium sulfate.

[0026] The second aspect of the present invention provides a modified sodium ferric sulfate positive electrode material, which is prepared by the preparation method provided by the first aspect.

[0027] The modified sodium ferric sulfate positive electrode material prepared by the above method has the advantages of less impurity phase and uniform carbon coating, which makes the modified sodium ferric sulfate positive electrode material have high structural stability and avoids phenomena such as moisture absorption and decomposition. At the same time, it has excellent electronic conductivity and sodium ion diffusion rate, which helps to improve its electrochemical properties such as first efficiency, specific capacity, and rate performance.

[0028] Furthermore, the modified sodium ferric sulfate positive electrode material includes a sodium ferric sulfate core and a carbon coating layer coated on the surface of the sodium ferric sulfate core, wherein the chemical formula of the sodium ferric sulfate core material is Na2Fe2(SO4)3, and the carbon content in the modified sodium ferric sulfate positive electrode material is 1.0wt% to 3.0wt%.

[0029] Furthermore, the D50 particle size of the modified sodium ferric sulfate positive electrode material is 0.5 to 1.5 μm.

[0030] Furthermore, the modified sodium ferric sulfate positive electrode material is subjected to charge and discharge tests under conditions of 25°C and 2.0-4.5V, and its 0.2C first coulombic efficiency is ≥97%; and / or, its 0.2C discharge specific capacity is ≥87mAh / g; and / or, its 0.5C discharge specific capacity is ≥86mAh / g; and / or, its 1C discharge specific capacity is ≥86mAh / g; and / or, its 1C discharge rate capacity retention rate is ≥98%.

[0031] The third aspect of the present invention provides a sodium ion battery, comprising the modified sodium ferric sulfate positive electrode material provided by the second aspect.

[0032] Due to the inclusion of the modified sodium ferric sulfate positive electrode material, the sodium ion battery of the present invention has excellent electrochemical properties, such as first efficiency, specific capacity and rate performance.

[0033] The implementation of the present invention has at least the following beneficial effects:

[0034] The preparation method of the modified sodium ferric sulfate positive electrode material provided by the present invention can synthesize a high-purity sodium ferric sulfate phase by introducing graphene oxide as a carbon source and through steps such as vacuum freeze drying and microwave-assisted two-stage sintering, thereby reducing the formation of impurity phases. At the same time, the graphene oxide is reduced to graphene with better and more stable conductivity, and the graphene is uniformly coated on the surface of the sodium ferric sulfate to form a modified sodium ferric sulfate positive electrode material with less impurity phases and uniform carbon coating. In this way, the structural stability of the modified sodium ferric sulfate positive electrode material can be significantly improved, and phenomena such as moisture absorption and decomposition can be avoided. At the same time, the electronic conductivity and sodium ion diffusion rate can be effectively improved, thereby helping to improve its electrochemical properties such as first efficiency, specific capacity, and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is an SEM image of the modified sodium iron sulfate positive electrode material prepared in Example 1 of the present invention;

[0036] Figure 2 This is an SEM image of the modified sodium iron sulfate positive electrode material prepared in Comparative Example 1 of the present invention;

[0037] Figure 3 The XRD patterns of the modified sodium ferric sulfate positive electrode materials prepared in Example 1 and Comparative Example 1 of the present invention are shown;

[0038] Figure 4 This is a graph showing the first charge and discharge curves of a battery assembled with the modified sodium ferric sulfate positive electrode material prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0039] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0040] As described in the background of the present invention, the sodium ferric sulfate material prepared in the prior art has the problems of multiple impurities and uneven carbon coating. In order to solve the above problems, the first aspect of the present invention provides a method for preparing a modified sodium ferric sulfate positive electrode material, comprising the following steps:

[0041] Step S1, mixing graphene oxide, a sodium source, an antioxidant, a ferrous source, a sulfur source and water to obtain a mixed material;

[0042] Step S2, vacuum freeze-drying the mixed material and pulverizing it once to obtain a precursor;

[0043] Step S3, subjecting the precursor to microwave sintering and secondary crushing to obtain a modified sodium ferric sulfate cathode material;

[0044] Microwave sintering is carried out at temperatures T1 and T2 in sequence; wherein, T1 is 200-250° C., the holding time at T1 is 10-15 minutes, T2 is 320-380° C., the holding time at T2 is 12-24 minutes.

[0045] In the preparation method of the modified sodium ferric sulfate positive electrode material of the present invention, first, graphene oxide is used as a carbon source. Since graphene oxide contains more hydrophilic oxygen-containing functional groups, it is promoted to be uniformly dispersed with other raw materials in an aqueous solvent.

[0046] Secondly, in the subsequent vacuum freeze-drying, graphene oxide can maintain its original uniform dispersion state in the mixed material, avoiding stratification while achieving drying.

[0047] Finally, through two-stage microwave-assisted sintering, on the one hand, microwave-assisted heating overcomes the shortcomings of conventional uneven heating, and has the advantages of shortening reaction time, improving work efficiency, fast heating speed, uniform heating, no temperature gradient, and no hysteresis effect. At the same time, graphene oxide has excellent microwave absorption properties, can quickly react with microwaves, and directly reduce to high-quality, micron-level graphene sheets without any reducing agent or atmosphere. This process is short in time and highly efficient. On the other hand, through two-stage sintering, free water and crystallization water in the raw materials are first removed under relatively low temperature conditions to avoid affecting the phase formation and crystallinity of the material and reduce the formation of impurity phases. Then, the graphene oxide is promoted under relatively high temperature conditions. The formation of nano-scale sodium ferric sulfate phase and graphene coating layer, the introduction of graphene oxide will not affect the structure of sodium ferric sulfate, nano-scale sodium ferric sulfate helps to improve the diffusion coefficient of sodium ions, and at the same time, nano-scale sodium ferric sulfate is coated with micron-scale graphene sheets, which helps to form a uniform carbon coating, greatly improving the utilization rate of graphene oxide, improving the structural stability of the modified sodium ferric sulfate positive electrode material, and avoiding the occurrence of moisture absorption and decomposition. In addition, the formed graphene coating has a high degree of graphitization, which can provide a high-speed channel for the rapid transfer of electrons, improve the electronic conductivity of the material, and thus help to improve its electrochemical properties such as first effect, specific capacity, and rate performance.

[0048] The present invention does not limit the specific steps of mixing the raw materials with water, and can be achieved by conventional methods in the art. For example, in some embodiments, the water is divided into a first part and a second part, and step S1 includes:

[0049] Step S11, mixing graphene oxide with a first portion of water, and after ultrasonic dispersion, adding a sodium source and a first portion of a sulfur source thereto, and mixing them uniformly to obtain material A;

[0050] Step S12, dissolving the antioxidant in the second portion of water, adding the ferrous source and the second portion of the sulfur source thereto, and mixing them uniformly to obtain material B;

[0051] Step S13, adding the material B of step S12 to the material A of step S11 to obtain a mixed material.

[0052] The amount of the first part of water and the amount of the second part of water added is equal to the total amount of water added, and the volume ratio of the first part of water to the second part of water is (4-6):(6-4). The sum of the amount of the first part of sulfur source and the amount of the second part of sulfur source added is equal to the total amount of sulfur source added, and the molar ratio of the first part of sulfur source to the second part of sulfur source is (4-6):(6-4).

[0053] Ultrasonic dispersion can further promote uniform dispersion of the raw materials. In some embodiments, the frequency of the ultrasound is 20 to 50 Hz, such as 20 Hz, 30 Hz, 40 Hz, 50 Hz, or any two thereof, and the time is 0.5 to 1.5 h, such as 0.5 h, 1 h, 1.5 h, or any two thereof.

[0054] In order to avoid the phenomenon of particle adhesion, primary crushing and secondary crushing can further ensure that the particles do not stick together. In some embodiments, the primary crushing controls the D50 particle size to be 5-15 μm; and / or, the secondary crushing controls the D50 particle size to be 0.5-1.5 μm.

[0055] Vacuum freeze drying can be performed in a vacuum freeze dryer. Vacuum freeze drying is performed by freezing the mixed material into a solid and then dehydrating the material under low temperature and low pressure conditions to achieve drying. In some embodiments, the vacuum freeze drying temperature is -20 to 0°C, for example, -20°C, -15°C, -10°C, -5°C, 0°C, or any two thereof, and the time is 12 to 24 hours, for example, 12 hours, 15 hours, 20 hours, 24 hours, or any two thereof.

[0056] Microwave sintering utilizes microwave heating to sinter materials. During microwave sintering, microwaves directly interact with material particles, absorbing the microwave energy and heating them. In some embodiments, microwave sintering is performed under an inert atmosphere (e.g., nitrogen or argon) at a power of 1200-1800W.

[0057] The present invention does not limit the amount of each raw material added and can be adjusted according to actual needs. For example, in some embodiments, the molar ratio of sodium, iron, and sulfur in the mixture is 2:2:3, the molar ratio of iron to antioxidant is 10:(0.8-1.5), and the mass ratio of the sum of the mass of graphene oxide, sodium source, antioxidant, ferrous source, and sulfur source to the mass of graphene oxide is 100:(2.0-3.0).

[0058] By limiting the molar ratio of sodium, iron and sulfur in the mixture, modified sodium ferric sulfate positive electrode materials with different stoichiometric ratios can be prepared. By limiting the mass ratio of graphene oxide, modified sodium ferric sulfate positive electrode materials with different carbon coating amounts can be obtained.

[0059] In some embodiments, the sodium source is selected from at least one of sodium sulfate, sodium bisulfate, sodium nitrate, sodium carbonate, and sodium bicarbonate; and / or the antioxidant is selected from at least one of ascorbic acid and citric acid; and / or the ferrous source is selected from at least one of ferrous sulfate, ammonium ferrous sulfate, ferrous nitrate, and hydrates thereof; and / or the sulfur source is selected from at least one of sodium sulfate, sodium bisulfate, ferrous sulfate, ammonium ferrous sulfate, ammonium bisulfate, and ammonium sulfate.

[0060] It should be noted that sodium sulfate and sodium bisulfate can be used as both sodium sources and sulfur sources; ferrous sulfate and ammonium ferrous sulfate can be used as both ferrous sources and sulfur sources, as long as the molar ratio of sodium, iron and sulfur in the mixture meets the above requirements.

[0061] The second aspect of the present invention provides a modified sodium ferric sulfate positive electrode material, which is prepared by the preparation method provided by the first aspect.

[0062] The modified sodium ferric sulfate positive electrode material prepared by the above method has the advantages of less impurity phase and uniform carbon coating, which makes the modified sodium ferric sulfate positive electrode material have high structural stability and avoids phenomena such as moisture absorption and decomposition. At the same time, it has excellent electronic conductivity and sodium ion diffusion rate, which helps to improve its electrochemical properties such as first efficiency, specific capacity, and rate performance.

[0063] The modified sodium ferric sulfate positive electrode material of the present invention comprises a sodium ferric sulfate core and a carbon coating layer coated on the surface of the sodium ferric sulfate core, wherein the sodium ferric sulfate core is formed by the reaction of a sodium source, a ferrous source, and a sulfur source, and the carbon coating layer is formed by stacking graphene sheets formed by reducing graphene oxide.

[0064] In some embodiments, the chemical formula of the sodium ferric sulfate core material is Na2Fe2(SO4)3, and the carbon content in the modified sodium ferric sulfate positive electrode material is 1.0 wt% to 3.0 wt%.

[0065] Wherein, based on the total mass of the modified sodium ferric sulfate positive electrode material being 100%, the carbon content is 1.0 to 3.0%, for example, 1.0%, 2.0%, 3.0%, or a range consisting of any two thereof. Preferably, the chemical formula of the sodium ferric sulfate core material in the modified sodium ferric sulfate positive electrode material is Na2Fe2(SO4)3, and the carbon content in the sodium ferric sulfate positive electrode material is 2 wt%.

[0066] In some embodiments, the D50 particle size of the modified sodium ferric sulfate positive electrode material is 0.5-1.5 μm.

[0067] In some embodiments, the modified sodium ferric sulfate positive electrode material is subjected to charge and discharge tests under conditions of 25°C and 2.0-4.5V, and its 0.2C first efficiency is ≥97%; and / or, its 0.2C discharge specific capacity is ≥87mAh / g; and / or, its 0.5C discharge specific capacity is ≥86mAh / g; and / or, its 1C discharge specific capacity is ≥86mAh / g; and / or, its 1C discharge rate capacity retention rate is ≥98%.

[0068] The third aspect of the present invention provides a sodium ion battery, comprising the modified sodium ferric sulfate positive electrode material provided by the second aspect.

[0069] Due to the inclusion of the modified sodium ferric sulfate positive electrode material, the sodium ion battery of the present invention has excellent electrochemical properties, such as first efficiency, specific capacity and rate performance.

[0070] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0071] Example 1

[0072] The preparation method of the modified sodium iron sulfate positive electrode material in this embodiment includes the following steps:

[0073] 1) Add 0.5 g of graphene oxide to 50 mL of deionized water, ultrasonically disperse at 30 Hz for 1 h, then add 0.025 mol of sodium sulfate, and mix well to obtain material A;

[0074] 2) Add 0.0025 mol of ascorbic acid to 50 mL of deionized water and dissolve completely, then add 0.05 mol of ferrous sulfate heptahydrate and stir evenly to obtain material B;

[0075] 3) adding material B from step 2) to material A from step 1), mixing well, drying in a vacuum freeze dryer at -15°C for 24 hours, and then pulverizing once to a D50 particle size of 9.5 μm to obtain precursor C;

[0076] 4) Precursor C was placed in a crucible and heated using microwaves in a nitrogen atmosphere. In the first heating stage, the temperature was 240° C. for 12 min, and in the second heating stage, the temperature was 350° C. for 12 min to obtain material D.

[0077] 5) Material D was secondary crushed to a D50 particle size of 0.8 μm to obtain a modified sodium ferric sulfate positive electrode material having a sodium ferric sulfate core and a carbon coating layer of this embodiment, wherein the carbon content was 1.5 wt %. The chemical formula of the sodium ferric sulfate core material was Na2Fe2(SO4)3.

[0078] Example 2

[0079] The difference from Example 1 is that "0.025 mol sodium sulfate" in step 1) is replaced by "0.025 mol sodium bisulfate, 0.025 mol sodium nitrate".

[0080] Example 3

[0081] The difference from Example 1 is that "0.05 mol of ferrous sulfate heptahydrate" in step 1) is replaced by "0.05 mol of ammonium bisulfate and 0.05 mol of ferrous nitrate".

[0082] Example 4

[0083] The difference from Example 1 is that in step 1), "heating in the first stage, the temperature is 240°C, the heating time is 12 min, heating in the second stage, the temperature is 350°C, and the heating time is 12 min" is replaced by "heating in the first stage, the temperature is 220°C, the heating time is 12 min, heating in the second stage, the temperature is 330°C, and the heating time is 12 min".

[0084] Example 5

[0085] The difference from Example 1 is that "0.0025 mol ascorbic acid" in step 1) is replaced by "0.0025 mol citric acid".

[0086] Example 6

[0087] The difference from Example 1 is that "0.5 g of graphene oxide" in step 1) is replaced by "0.68 g of graphene oxide", and other conditions remain unchanged, and the modified sodium ferric sulfate positive electrode material having a sodium ferric sulfate core and a carbon coating layer of this embodiment is obtained, the carbon content of which is 3 wt%, and the chemical formula of the sodium ferric sulfate core material is Na2Fe2(SO4)3.

[0088] Comparative Example 1

[0089] The difference from Example 1 is that "0.5 g of graphene oxide" in step 1) is replaced by "1 g of glucose".

[0090] Comparative Example 2

[0091] The difference from Example 1 is that in step 1), "placing in a vacuum freeze dryer and drying at -15°C for 24 hours" is replaced by "placing in a vacuum oven and drying at 80°C for 24 hours".

[0092] Comparative Example 3

[0093] The difference from Example 1 is that in step 1), "heating in a nitrogen atmosphere by using microwaves" is replaced by "heating in a nitrogen atmosphere in a tubular furnace".

[0094] Comparative Example 4

[0095] The difference from Example 1 is that in step 1), "heating in the first stage, the temperature is 240°C, the heating time is 12 min, heating in the second stage, the temperature is 350°C, and the heating time is 12 min" is replaced by "heating temperature is 350°C, and heating time is 24h".

[0096] Test example

[0097] 1. Button production

[0098] The modified sodium ferric sulfate positive electrode material prepared in the embodiment and the comparative example was used as the positive electrode active material. The modified sodium ferric sulfate positive electrode material, the conductive agent SP, and the binder PVDF were mixed in a mass ratio of 8:1:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersant for slurry preparation. The mixture was coated on aluminum foil to make a positive electrode sheet, and a CR2032 button half-cell was made in an argon protective atmosphere, wherein the negative electrode was a metal sodium sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L NaClO4 solution.

[0099] 2. Power-off test

[0100] 1. After the battery has been allowed to stand at 25°C for 4 hours, the first charge and discharge capacity test was performed. The test conditions were as follows: charge to 4.5V at 0.2C, stand for 1 minute, and then discharge to 2.0V at 0.2C. The charge and discharge curves were obtained, and the first charge capacity (C0) and the first discharge capacity (D0) from 2.0 to 4.5V were recorded respectively. The first coulombic efficiency (first efficiency) was calculated according to D0 / C0×100%.

[0101] 2. Use a battery charge and discharge tester to test the battery at 25°C. Charge the battery to 4.5V at 0.2C constant current. After standing for 1 minute, discharge the battery to 2.0V at 0.2C constant current. Record the discharge capacity Q. 0.2c After standing for 1 minute, charge the battery to 4.5V at 0.5C constant current. After standing for 1 minute, discharge the battery to 2.0V at 0.5C constant current. Record the discharge capacity Q. 0.5c After standing for 1 minute, charge at 1C constant current to 4.5V. After standing for 1 minute, discharge at 1C constant current to 2.0V. Record the discharge capacity Q. 1c , the 1C discharge rate capacity retention rate can be calculated according to the following formula:

[0102] 1C discharge rate capacity retention rate = Q 1c / Q 0.2c ×100%

[0103] The test results are shown in Table 1 and Figure 4 .

[0104] Table 1

[0105]

[0106]

[0107] According to Table 1, compared with Example 1, the graphene oxide in Comparative Example 1 was replaced with an organic carbon source. Figure 1 and Figure 2 The SEM image shows that the particles of Example 1 are small and uniform, with a faintly rounded outer layer, which is the result of graphene coating. On the one hand, it reduces the side reactions caused by excessive contact between the active material and the electrolyte, and on the other hand, it helps to reduce the water absorption of the positive electrode material. The particles of Comparative Example 1 are relatively large and uneven in size, with agglomeration and obvious edges and corners, indicating that the carbon coating of the organic carbon source may be uneven, and the active material is partially exposed. Figure 3 From the XRD patterns of Example 1 and Comparative Example 1, it is found that there is an obvious impure phase peak in the overall phase of Comparative Example 1. From Table 1, it can be concluded that the performance of Comparative Example 1 is significantly reduced, indicating that it is difficult to achieve similar performance under the same conditions by replacing the organic carbon source. On the one hand, the phase is impure, and on the other hand, the low-temperature decomposition of the organic carbon source has a low degree of graphitization and poor carbon coating, resulting in limited performance improvement of the final product. Comparative Example 2 changes the slurry drying method and adopts oven drying, which has poor performance, mainly because the conventional drying method will cause stratification, and the carbon and the raw materials are unevenly distributed after drying, the sintered product is relatively poor, and the carbon coating is uneven; Comparative Example 3 adopts the conventional tubular furnace heating method, and graphene oxide fails to be converted into graphene. On the one hand, the conductivity of graphene oxide is not as good as graphene. On the other hand, without this conversion process, the distribution of graphene oxide is not uniform, and ultimately the carbon coating is still poor; Comparative Example 4 adopts one-step microwave heating, and the performance of the obtained finished product is poor, mainly because the characteristic of microwave heating is that the temperature rises rapidly, and there is still free water and crystal water in the material. The separation of sodium iron sulfate and water may occur simultaneously, resulting in the final product being impure, with poor crystallinity, and the final electrical properties will also be poor.

[0108] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a modified sodium ferric sulfate positive electrode material, characterized in that: The following steps are involved: Step S1, mixing graphene oxide, a sodium source, an antioxidant, a ferrous source, a sulfur source and water to obtain a mixed material; Step S2, vacuum freeze-drying the mixed material and pulverizing it once to obtain a precursor; Step S3, subjecting the precursor to microwave sintering and secondary crushing to obtain a modified sodium ferric sulfate positive electrode material; The microwave sintering is sequentially kept warm at temperatures T1 and T2; wherein, the T1 is 200-250°C, the holding time at the T1 temperature is 10-15 minutes, the T2 is 320-380°C, and the holding time at the T2 temperature is 12-24 minutes.

2. The preparation method according to claim 1, characterized in that The water is divided into a first part and a second part, and the step S1 comprises: Step S11, mixing graphene oxide with a first portion of water, and after ultrasonic dispersion, adding a sodium source and a first portion of a sulfur source thereto, and mixing them uniformly to obtain material A; Step S12, dissolving the antioxidant in the second portion of water, adding the ferrous source and the second portion of the sulfur source thereto, and mixing them uniformly to obtain material B; Step S13, adding the material B of step S12 to the material A of step S11 to obtain the mixed material.

3. The preparation method according to claim 2, characterized in that The frequency of the ultrasound is 20 to 50 Hz, and the time is 0.5 to 1.5 h; and / or, the primary crushing controls the D50 particle size to be 5 to 15 μm; and / or, the secondary crushing controls the D50 particle size to be 0.5 to 1.5 μm; and / or, the temperature of the vacuum freeze-drying is -20 to 0°C, and the time is 12 to 24 h; and / or, the microwave sintering is carried out under an inert atmosphere, the inert atmosphere includes at least one of nitrogen and argon, and the power of the microwave sintering is 1200 to 1800 W.

4. The preparation method according to any one of claims 1 to 3, characterized in that In the mixed material, the molar ratio of sodium element, iron element and sulfur element is 2:2:3, the molar ratio of iron element to antioxidant is 10:(0.8~1.5), and the mass ratio of the sum of the masses of the graphene oxide, sodium source, antioxidant, ferrous source and sulfur source to the graphene oxide is 100:(2.0~3.0).

5. The preparation method according to any one of claims 1 to 3, characterized in that The sodium source is selected from at least one of sodium sulfate, sodium bisulfate, sodium nitrate, sodium carbonate, and sodium bicarbonate; and / or the antioxidant is selected from at least one of ascorbic acid and citric acid; and / or the ferrous source is selected from at least one of ferrous sulfate, ammonium ferrous sulfate, ferrous nitrate, and hydrates thereof; and / or the sulfur source is selected from at least one of sodium sulfate, sodium bisulfate, ferrous sulfate, ammonium ferrous sulfate, ammonium bisulfate, and ammonium sulfate.

6. A modified sodium iron sulfate positive electrode material, characterized in that: Prepared by the preparation method according to any one of claims 1 to 5.

7. The modified sodium ferric sulfate cathode material according to claim 6, characterized in that The modified sodium ferric sulfate positive electrode material includes a sodium ferric sulfate core and a carbon coating layer coated on the surface of the sodium ferric sulfate core, wherein the chemical formula of the sodium ferric sulfate core material is Na2Fe2(SO4)3, and the carbon content in the modified sodium ferric sulfate positive electrode material is 1.0wt% to 3.0wt%.

8. The modified sodium ferric sulfate cathode material according to claim 6 or 7, characterized in that: The D50 particle size of the modified sodium ferric sulfate positive electrode material is 0.5 to 1.5 μm.

9. The modified sodium ferric sulfate cathode material according to claim 8, characterized in that The modified sodium ferric sulfate positive electrode material is subjected to charge and discharge tests under conditions of 25° C. and 2.0-4.5V, and its 0.2C first coulombic efficiency is ≥97%; and / or its 0.2C discharge specific capacity is ≥87mAh / g; and / or its 0.5C discharge specific capacity is ≥86mAh / g; and / or its 1C discharge specific capacity is ≥86mAh / g; and / or its 1C discharge rate capacity retention rate is ≥98%.

10. A sodium ion battery, characterized in that: The modified sodium iron sulfate positive electrode material comprises the modified sodium iron sulfate positive electrode material according to any one of claims 6 to 9.

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