Low-alkalinity sodium ferric pyrophosphate positive electrode material and preparation method thereof

Abstract

The invention belongs to the technical field of energy material sodium ion batteries, and particularly relates to a low-alkalinity ferric sodium pyrophosphate positive electrode material and a preparation method thereof. The preparation method of the low-alkalinity ferric sodium pyrophosphate positive electrode material comprises the following steps: uniformly mixing an iron source compound, a phosphorus source compound, a sodium source compound, a carbon source and the like in deionized water according to a stoichiometric ratio, and carrying out ball milling and / or sanding to obtain uniform micro-nano slurry; successively drying and sintering the slurry to obtain the polyanion type ferric sodium pyrophosphate positive electrode material. And adding ferrous sulfate, sodium sulfate and carbon powder into a certain amount of sodium ferric pyrophosphate, uniformly mixing, and sintering to obtain the low-alkalinity sodium ferrous sulfate coated sodium ferric pyrophosphate positive electrode material. The preparation route provided by the invention is simple in process flow, the product performance is obviously improved, and the preparation method has a great application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of battery manufacturing, and relates to a preparation method of a sodium ion battery, in particular to a low-alkaline sodium iron pyrophosphate phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] In recent years, with the rapid development of the electric vehicle and energy storage system industries, the demand for high-performance and low-cost batteries is rising. Sodium ion batteries are considered as one of the ideal substitutes for lithium ion batteries due to their advantages of abundant resources, low cost and safety, and show great market potential. The market demand for polyanion-type positive electrode materials, which are the core components of sodium ion batteries, is also rapidly expanding.

[0003] Among the three routes of sodium ion battery positive electrodes, polyanion compounds have stable structure, high cycle life, high thermal stability and high working voltage, and become the most suitable route for long-term energy storage. The cycle life of the polyanion system is basically above 3000 times, and the theoretical cycle number can reach 10000 times. Polyanion compounds can be divided into phosphate, sulfate, pyrophosphate, fluorophosphate, mixed phosphate and silicate according to the types of anions. Phosphate has fast diffusion rate, but small capacity and high toxicity of vanadium compounds; sulfate has high working voltage, but is prone to thermal decomposition.

[0004] Iron-based polyanion compounds have wide raw material sources and low cost. The theoretical specific capacity of sodium iron pyrophosphate phosphate (Na4Fe3(PO4)2P2O7, NFPP) is 129 mAh / g, and the voltage platform is 3.1 V. NFPP has wide raw material sources, and the material cost is 11-13 thousand yuan / ton. Its three-dimensional crystal structure provides fast and stable three-dimensional channels for sodium ions, and provides excellent cycle performance and high safety performance, so the best use scenario of NFPP material is energy storage, such as small storage, household storage, outdoor base station energy storage, photovoltaic energy storage, etc. Of course, some low-speed vehicles, bicycles, and scenarios with high safety requirements, such as airport sweeper vehicles, are also suitable.

[0005] None of the existing patents on sodium iron pyrophosphate phosphate mention the improvement of pH value, and the pH value of most commercial NFPP is higher than 10.0. The high pH value can easily cause PVDF to undergo HF reaction and form conjugated double bonds during slurry preparation, and the molecules are glued together due to the conjugated double bonds, thereby forming a gel. In addition, high residual alkali can cause hydrogen gas and alkane in the battery, so it is necessary to develop modified NFPP materials with low pH value. Sodium iron sulfate, as another potential polyanion-type positive electrode material, has a general formula of Na x Fe y (SO4) x / 2+yDue to the higher electronegativity of the sulfate group, NFS exhibits a higher voltage plateau than NFPP, often reaching ~3.6V. However, during phase formation, it tends to form an impurity phase, Na6Fe(SO4)4, which is detrimental to the material's electrochemical performance. Notably, the material is acidic, so combining NFPP with NFS is expected to lower the high pH of NFPP. NFS has simple raw materials and a simple process, requiring only a stoichiometric mixture of sodium ferrous sulfate, sodium sulfate, and CNTs, followed by drying and low-temperature sintering. Therefore, NFS coating NFPP is cost-effective and feasible.

[0006] Compared with the prior art CN119495724A which discloses a carbon-coated sodium iron phosphate pyrophosphate positive electrode material, its preparation method and application, the use of NFS to coat and modify NFPP can overcome the problem of its high pH value. In addition, compared with the carbon coating layer, NFS itself can provide capacity, and the increase in the amount of carbon coating will inevitably reduce the capacity of NFPP itself.

[0007] In addition, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the inventor studied a large number of documents and patents when making the present invention, but due to space limitations, not all details and contents are listed in detail. However, this does not mean that the present invention does not have the characteristics of these prior arts. On the contrary, the present invention already has all the characteristics of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0008] Based on the above technical problems, one of the objectives of the present invention is to provide a low-alkalinity sodium iron pyrophosphate positive electrode material and a preparation method thereof to overcome the shortcomings of the prior art.

[0009] To achieve the above object, the present invention adopts the following technical solutions: A method for preparing a low-alkalinity sodium iron pyrophosphate positive electrode material, characterized in that the preparation method comprises the following steps: 0.5-3 parts of ferrous sulfate, 0.5-2.5 parts of sodium sulfate, 0.05-0.2 parts of carbon nanotubes and 50 parts of sodium ferric pyrophosphate are uniformly mixed. The mixed material is heat-treated at 350-370°C in a protective atmosphere and then naturally cooled to room temperature to obtain a composite material of sodium ferrous sulfate [Na6Fe4(SO4)7] coated with sodium ferric pyrophosphate.

[0010] Preferably, 50 parts of sodium ferric pyrophosphate, 0.89 parts of ferrous sulfate, 0.62 parts of sodium sulfate and 0.06 The CNTs are mixed, and after uniform mixing, the mixture is heated to 360 DEG C at a heating rate of 3 DEG C / min under a nitrogen atmosphere, and the product is collected after natural cooling to room temperature in the furnace, to obtain a Na6Fe4(SO4)7 modified sodium pyrophosphate iron phosphate composite positive electrode material.

[0011] Preferably, 50 parts of sodium pyrophosphate iron phosphate, 1.18 parts of ferrous sulfate, 0.83 parts of sodium sulfate, and 0.08 The CNTs are mixed, and after uniform mixing, the mixture is heated to 360 DEG C at a heating rate of 3 DEG C / min under a nitrogen atmosphere, and the product is collected after natural cooling to room temperature in the furnace, to obtain a Na6Fe4(SO4)7 modified sodium pyrophosphate iron phosphate composite positive electrode material.

[0012] Preferably, 50 parts of sodium pyrophosphate iron phosphate, 1.18 parts of ferrous sulfate, 0.83 parts of sodium sulfate, and 0.1 The CNTs are mixed, and after uniform mixing, the mixture is heated to 360 DEG C at a heating rate of 3 DEG C / min under a nitrogen atmosphere, and the product is collected after natural cooling to room temperature in the furnace, to obtain a Na6Fe4(SO4)7 modified sodium pyrophosphate iron phosphate composite positive electrode material.

[0013] According to a preferred embodiment, the preparation method of sodium pyrophosphate iron phosphate comprises the following steps: Iron phosphate dihydrate, disodium hydrogen phosphate, sodium carbonate, glucose, polyvinylpyrrolidone, and deionized water are mixed; The mixed liquid is ball milled, dried, crushed, and sieved; The temperature is increased to 350 DEG C at a heating rate of 3 DEG C / min, and the temperature is maintained for 4 h. After the temperature maintenance, the temperature is increased to 500 DEG C at a rate of 3 DEG C / min, and the temperature is maintained for 10 h. The temperature is then cooled to room temperature.

[0014] According to a preferred embodiment, the protective atmosphere is selected from one or more of nitrogen, argon, and hydrogen.

[0015] According to a preferred embodiment, the preparation method of the low-alkali sodium pyrophosphate iron phosphate positive electrode material comprises the following steps: S1. Iron source, phosphorus source, sodium source, carbon source, and additive are weighed and sequentially added to deionized water, and stirred to obtain a mixed slurry; S2. The mixed slurry obtained in step S1 is ground to obtain a refined and uniform slurry; S3. The slurry in step S2 is dried, crushed, and sieved, and the undersize material is collected to obtain a precursor; S4. The precursor powder obtained in step S3 is heat treated in a protective atmosphere and naturally cooled to room temperature to obtain the sodium pyrophosphate iron phosphate positive electrode material; S5, a certain amount of the pyrophosphate sodium iron phosphate obtained in S4 is weighed, and then a proper amount of ferrous sulfate, sodium sulfate, an additive and a carbon source are added and mixed uniformly; S6, the mixture in S5 is naturally cooled to room temperature after heat treatment in a protective atmosphere to obtain a composite positive electrode material of sodium ferrous sulfate coated pyrophosphate sodium iron phosphate.

[0016] According to a preferred embodiment, the preparation method of the sodium pyrophosphate iron phosphate comprises the following steps: The iron source, the phosphorus source, the sodium source, the carbon source, polyvinylpyrrolidone and deionized water are mixed; The mixed solution is ball milled, dried and sieved; The temperature is increased to 350℃ at a rate of 3℃ / min, and the temperature is kept for 4 h. After the temperature keeping, the temperature is continuously increased to 500℃ at a rate of 3℃ / min, and the temperature is kept for 10 h. Then the temperature is cooled to room temperature.

[0017] According to a preferred embodiment, in step S1, the iron source is one or more of iron phosphate, ferrous oxalate, iron oxide and iron nitrate.

[0018] According to a preferred embodiment, in step S1, the phosphorus source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid and disodium dihydrogen pyrophosphate.

[0019] According to a preferred embodiment, in step S1, the sodium source is one or more of sodium carbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium citrate and sodium hydroxide.

[0020] According to a preferred embodiment, in steps S1 and S5, the carbon source is one or more of citric acid, sucrose, glucose, starch, carbon black and graphitized carbon.

[0021] According to a preferred embodiment, in steps S1 and S5, the additive is one or more of polyethylene glycol, polyvinylpyrrolidone, ascorbic acid, malt dextrin and starch polysaccharide.

[0022] According to a preferred embodiment, in step S2, the grinding method is one or more of ball milling and / or sand milling, the ball milling speed is 200-400 rpm, the ball milling time is 6-12 h, the sand milling speed is 1000-3000 rpm, the sand milling time is 1-3 h, and the slurry particle size D50 is 0.1-0.5 μm.

[0023] According to a preferred embodiment, in step S3, the drying method is air drying, spray drying and freeze drying. The air drying temperature is 70-120℃. The spray drying inlet temperature is 100-250℃. The outlet temperature is 60-120℃. The feeding rate is 0.5-40 mL / min. The freeze drying temperature is -60--20℃. The vacuum pressure is 1-5 Pa. The drying time is 12-24 h.

[0024] According to a preferred embodiment, the protective atmosphere is one or more of nitrogen, argon, helium, nitrogen-hydrogen mixed gas and argon-hydrogen mixed gas. The heat treatment temperature is 300-700℃, the heating rate is 1-10℃ / min, and the holding time is 2-14 h.

[0025] According to a preferred embodiment, in step S5, the ferrous sulfate is anhydrous ferrous sulfate, monohydrate ferrous sulfate, tetrahydrate ferrous sulfate, pentahydrate ferrous sulfate and heptahydrate ferrous sulfate, and the particle size of the ferrous sulfate, sodium sulfate, additives and carbon source is 0.1-10 μm.

[0026] One of the purposes of the present application is also to provide a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material prepared based on the above-mentioned low-alkaline sodium pyrophosphate ferric phosphate positive electrode material preparation method.

[0027] One of the purposes of the present application is also to provide a sodium ion battery positive electrode using a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material prepared based on the above-mentioned low-alkaline sodium pyrophosphate ferric phosphate positive electrode material preparation method.

[0028] One of the purposes of the present application is also to provide a sodium ion battery using a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material prepared based on the above-mentioned low-alkaline sodium pyrophosphate ferric phosphate positive electrode material preparation method. The sodium ion battery comprises a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode uses a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material prepared based on the above-mentioned low-alkaline sodium pyrophosphate ferric phosphate positive electrode material preparation method.

[0029] The present patent technical solution solves the problem of high alkalinity of sodium pyrophosphate ferric phosphate by secondary low-temperature coating modification treatment, minimizes the negative impact on electrochemical performance, and improves the cycle stability of the material, as described below: (1) The coating layer involved in the present application can neutralize the alkaline residues or components on the surface of sodium pyrophosphate ferric phosphate, so that the final obtained composite material exhibits a significantly reduced pH value.

[0030] (2) The selected coating material, sodium ferrous sulfate, is not an inert substance itself, but a polyanionic positive electrode material with electrochemical activity. Therefore, when it exists as a coating layer, it can also undergo reversible sodium ion deintercalation during the charging and discharging process of the battery, contributing positively or having little effect on the overall capacity of the composite material. This is a significant distinction from the problem of capacity reduction that may occur when using an inert coating layer, making this method maximize the electrochemical capacity and rate performance of the host material while solving the alkaline problem. In addition, sodium ferrous sulfate itself has a high sodium ion diffusion coefficient, and when introduced into the composite material system, it can effectively improve the ion transport rate on the particle surface and interface, optimizing the electrode reaction kinetics. This characteristic directly translates into more excellent cycle stability, which is manifested in the composite material having a higher capacity retention rate after multiple charging and discharging cycles, prolonging the service life of the battery. Figure 1 and Table 2). BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 pH value and electrical performance data graphs of the materials prepared for Examples 2, 6 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0032] In the description of the present application, the terms are used only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified.

[0033] The raw materials used in the following examples are commercially available or self-made; unless otherwise specified, the mass percentage is indicated.

[0034] The preparation method of the sodium iron pyrophosphate phosphate precursor involved in the following examples is as follows: S1 55.07 g of iron phosphate dihydrate, 14.16 g of disodium hydrogen phosphate, 10.65 g of sodium carbonate, 9.28 g of glucose and 0.45 g of polyvinylpyrrolidone were weighed and added to 200 mL of deionized water in sequence and stirred to obtain a uniformly dispersed mixture liquid; S2 The mixture liquid was transferred to a ball mill tank, and after adding an appropriate amount of zirconium balls, it was ball milled at a speed of 300 rpm for 10 h; after the end, the slurry was dried at 80℃, crushed by a high-speed crusher and sieved through a 300 mesh sieve, and the undersize material was collected to obtain the precursor; S3 The precursor was placed in a nitrogen atmosphere for heat treatment, with a heating rate of 3℃ / min to 350℃, and kept for 4h, then continued to heat at a rate of 3℃ / min to 500℃, and kept for 10 h, and then the product was collected after natural cooling to room temperature in the furnace, to obtain sodium iron pyrophosphate phosphate.

[0035] The preparation of the precursor comprises 55.07 parts by weight of iron phosphate dihydrate, 14.16 parts by weight of disodium hydrogen phosphate, 10.65 parts by weight of sodium carbonate, 9.28 parts by weight of glucose, and 0.45 parts by weight of polyvinylpyrrolidone.

[0036] Example 1 The embodiment provides a preparation method of a low-alkaline sodium iron pyrophosphate positive electrode material, which comprises the following steps: 50 g of sodium iron pyrophosphate, 0.89 g of ferrous sulfate, 0.62 g of sodium sulfate and 0.06 g of CNTs are mixed, and then uniformly mixed and heated to 360 DEG C at a heating rate of 3 DEG C / min under a nitrogen atmosphere for 24 h, and then the product is collected after the furnace is naturally cooled to room temperature to obtain a composite positive electrode material of modified sodium iron pyrophosphate sodium pyrophosphate.

[0037] Example 2 The embodiment provides a preparation method of a low-alkaline sodium iron pyrophosphate positive electrode material, which has the same preparation method and steps as those of the embodiment 1, and the difference lies in that the amounts of ferrous sulfate, sodium sulfate and CNTs are adjusted to 1.18 g of ferrous sulfate, 0.83 g of sodium sulfate and 0.08 g of CNTs.

[0038] Example 3 The embodiment provides a preparation method of a low-alkaline sodium iron pyrophosphate positive electrode material, which has the same preparation method and steps as those of the embodiment 1, and the difference lies in that the amounts of ferrous sulfate, sodium sulfate and CNTs are adjusted to 1.47 g of ferrous sulfate, 1.03 g of sodium sulfate and 0.1 g of CNTs.

[0039] Example 4 The embodiment provides a preparation method of a low-alkaline sodium iron pyrophosphate positive electrode material, which has the same preparation method and steps as those of the embodiment 1, and the difference lies in that the amounts of ferrous sulfate, sodium sulfate and CNTs are adjusted to 2.94 g of ferrous sulfate, 2.06 g of sodium sulfate and 0.2 g of CNTs.

[0040] Example 5 The embodiment provides a preparation method of a low-alkaline sodium iron pyrophosphate positive electrode material, which has the same preparation method and steps as those of the embodiment 2, and the difference lies in that the amount of CNTs is adjusted to 0.06 g of CNTs.

[0041] Example 6 The embodiment provides a preparation method of a low-alkaline sodium iron pyrophosphate positive electrode material, which has the same preparation method and steps as those of the embodiment 2, and the difference lies in that the amount of CNTs is adjusted to 0.1 g of CNTs.

[0042] Example 7 The embodiment provides a preparation method of a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material, and the preparation method and steps are similar to those of the embodiment 2, except that the sintering temperature is adjusted to 350 DEG C.

[0043] Embodiment 8 The embodiment provides a preparation method of a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material, and the preparation method and steps are similar to those of the embodiment 2, except that the sintering temperature is adjusted to 370 DEG C.

[0044] Embodiment 9 The embodiment provides a preparation method of a low-alkaline sodium pyrophosphate ferric phosphate positive electrode material, and the preparation method and steps are similar to those of the embodiment 1, except that the sodium ferrous sulfate is synthesized separately first, and then the sodium ferrous sulfate is mixed with the sodium pyrophosphate ferric phosphate.

[0045] Comparative Example 1 The sodium pyrophosphate ferric phosphate positive electrode material provided by the comparative example is not subjected to any modification treatment.

[0046] Comparative Example 2 The comparative example replaces the sodium source on the basis of the comparative example 1 to synthesize a low-pH sodium pyrophosphate ferric phosphate positive electrode material, and comprises the following steps. S1 55.07 g of iron phosphate dihydrate, 14.16 g of sodium phosphate dibasic, 17.34 g of trisodium citrate, 9.28 g of glucose and 0.45 g of polyvinylpyrrolidone are weighed and sequentially added into 200 mL of deionized water and stirred to obtain a uniformly dispersed mixed solution; S2 the mixed solution is transferred into a ball mill tank, and after adding appropriate zirconium balls, the ball milling is performed at a rotating speed of 300 rpm for 10 h; after the end, the slurry is dried at 80 DEG C, and after being crushed by a high-speed crusher, the sodium pyrophosphate ferric phosphate positive electrode material is obtained by sieving through a 300 mesh screen.

[0047] The detailed ingredient and sintering procedure information is shown in Table 1.

[0048] Table 1. Sodium ion battery performance data

[0049] To verify the use effect of the positive electrode material described in the application, the materials of each example and comparative example were prepared into positive electrode sheets. The preparation method was as follows: the positive electrode material, acetylene black and polyvinylidene fluoride were mixed into a uniform slurry at a mass ratio of 94:3:3, then the slurry was uniformly coated on an aluminum foil using a 100 μm four-side coater, then the film was placed in a 100 ℃ air drying oven for drying for 8 hours. The electrode film was punched into a circular sheet with a diameter of 14 mm using a sheet punching machine. The cut electrode circular sheet was placed in a 100 ℃ vacuum drying oven for drying for 8 hours and then transferred to a glove box. A CR2016 type button cell was assembled in the glove box using metallic Na as the counter electrode, NaPF6 as the electrolyte and Whatman GF / D glass fiber separator, and constant current charge-discharge test was carried out at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C.

[0050] Table 2. pH value and electrical performance data table of examples 1-9 and comparative example 1

[0051] Figure 1 The first charge-discharge curves of examples 2, 6 and comparative example 1 at 0.2 C rate are shown in the figure. It can be seen that a small amount of sodium ferrous sulfate coating has no effect on the charge-discharge performance of sodium iron pyrophosphate.

[0052] It can be seen from the data in table 2 that examples 2 (pH = 9.39, capacity 101.51 mAh / g) and 6 (pH = 10.26, capacity 102.2 mAh / g) represent the lower limit and upper limit of the interval respectively, and the capacities of both are higher than 101 mAh / g, which are obviously better than the original materials of comparative example 1 (pH = 10.99, capacity 99.52 mAh / g) and comparative example 2 (pH = 9.82, capacity 96.24 mAh / g). At the same time, the pH of the central samples such as examples 5 (pH = 9.58, capacity 98.7), 7 (pH = 9.45, capacity 98.86) and 8 (pH = 9.76, capacity 97.33) generally remain above 98 mAh / g. It can be seen that the effect of different amounts of sodium ferrous sulfate on the pH of the NFPP sample is obviously reduced compared with comparative examples 1 and 2. In example 1, when the amount reaches 3%, the pH value decreases from 10.99 to 10.22. With further increase of the amount, the decrease of the pH value is more significant, and when the amount reaches 10%, the pH value reaches 4.72. However, with the increase of the amount, the specific discharge capacity gradually decreases, which is due to the increase of the content of sodium ferrous sulfate, thereby reducing the proportion of NFPP and causing the decrease of the capacity. The results prove that the coating technology involved in the application can reduce the alkalinity while maintaining good electrochemical performance.

[0053] Both insufficient and excessive pH will lead to performance degradation. The high alkalinity (pH = 10.99) sample of Comparative Example 1 has a capacity of only 99.52 mAh / g, which is lower than 80% of the coated examples, highlighting the performance bottleneck of the original material. Example 4 (pH = 4.72) has a capacity of 93.25 mAh / g, which is at least 5% lower than other examples. This indicates that an excessively acidic environment severely damages the material structure or ion diffusion path, and also demonstrates the infeasibility of simply pursuing low pH. Meanwhile, Comparative Example 2 uses sodium citrate as the sodium source instead of sodium carbonate, resulting in a pH of 9.82 for the sodium pyrophosphate iron phosphate cathode material. However, the specific discharge capacity is only 96.24 mAh / g.

[0054] In addition, there is still a significant difference in capacity among Examples 1-9 at similar pH levels: for example, in the pH ≈ 9.4-9.6 range, Example 2 (capacity 101.51) is 2.65 mAh / g higher than Example 7 (capacity 98.86). This result demonstrates that raw material mixing uniformity, heat treatment conditions, and carbon nanotube dispersion are crucial to the material.

[0055] Sodium ferrous sulfate itself has poor conductivity, so CNTs need to be added to improve conductivity. Based on Example 2, the CNTs addition ratio is adjusted, and the specific discharge capacity of Example 5 is lower than that of Example 2 and Comparative Example 1. This is because the amount of CNTs added is small, and the conductivity is poor. In Example 6, the amount of CNTs added is increased to 5%, and the specific discharge capacity is increased to 102.2 mAh / g, but the pH is only 10.26. Examples 7 and 8 are composite materials obtained at different temperatures, and the specific discharge capacity is significantly reduced. This is because sodium ferrous sulfate is prone to produce Na6Fe(SO4)4 impurities during phase formation. This impurity has low activity, with a theoretical capacity of only 46.4 mAh / g, resulting in a decrease in the specific discharge capacity of the composite material. Example 9 directly physically mixes sodium pyrophosphate iron phosphate with sodium ferrous sulfate. This method significantly reduces the pH, but the specific discharge capacity is low. Obviously, the preparation route of the sodium ferrous sulfate coated sodium pyrophosphate iron phosphate cathode material provided by the present application can significantly reduce the pH of the composite material while ensuring the charge and discharge performance and improving the capacity retention rate, and has great application prospect.

[0056] The foregoing basic examples and each further selected example of the present application can be freely combined to form a plurality of embodiments, all of which are embodiments that can be employed and claimed by the present application. In the present application, each selected example can be combined with any basic example and selected example. A person skilled in the art can know numerous combinations.

[0057] It should be noted that the above-mentioned embodiments illustrate rather than limit the application, and that those skilled in the art will be able to devise modifications which, though perhaps not explicitly described or shown herein, nonetheless fall within the scope of the application. Accordingly, the patentable scope of the application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a low-alkalinity sodium iron pyrophosphate positive electrode material, characterized in that: The preparation method comprises the following steps: 0.5-3 parts of ferrous sulfate, 0.5-2.5 parts of sodium sulfate, 0.05-0.2 parts of carbon nanotubes and 50 parts of sodium ferric pyrophosphate are uniformly mixed, the mixed material is heat-treated at 350-370°C in a protective atmosphere, and then naturally cooled to room temperature to obtain a composite material of sodium ferrous sulfate coated sodium ferric pyrophosphate.

2. The preparation method according to claim 1, characterized in that The preparation method of the sodium ferric pyrophosphate comprises the following steps: Mixing an iron source, a phosphorus source, a sodium source, a carbon source, polyvinyl pyrrolidone and deionized water; After ball milling, the mixture is dried and sieved; The temperature was raised to 350°C at a rate of 3°C / min and kept at that temperature for 4 h. After the end of the heat preservation, the temperature was further raised to 500°C at a rate of 3°C / min and kept at that temperature for 10 h, and then cooled to room temperature.

3. The preparation method according to claim 1 or 2, characterized in that The iron source is one or more of ferric phosphate, ferrous phosphate dihydrate, ferrous oxalate, ferric oxide, and ferric nitrate; the phosphorus source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, pyrophosphoric acid, and disodium dihydrogen pyrophosphate; the sodium source is one or more of sodium carbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium citrate, and sodium hydroxide; and the carbon source is one or more of citric acid, sucrose, glucose, starch, carbon black, and graphitized carbon.

4. The preparation method according to claim 1, characterized in that The protective atmosphere is one or more of nitrogen, argon, helium, nitrogen-hydrogen mixed gas, and argon-hydrogen mixed gas.

5. The preparation method according to claim 1, characterized in that The ferrous sulfate is the remaining substance after free water and bound water are removed from anhydrous ferrous sulfate, ferrous sulfate monohydrate, ferrous sulfate tetrahydrate, ferrous sulfate pentahydrate and ferrous sulfate heptahydrate.

6. The preparation method according to claim 1, characterized in that The particle size of the substance participating in the reaction is 0.1-10 μm.

7. The preparation method according to claim 1, characterized in that 0.89 parts of ferrous sulfate, 0.62 parts of sodium sulfate, 0.06 parts of carbon nanotubes and 50 parts of sodium ferric pyrophosphate were uniformly mixed.

8. A low-alkalinity sodium iron pyrophosphate positive electrode material, characterized in that The low-alkalinity sodium ferric pyrophosphate positive electrode material is prepared based on the low-alkalinity sodium ferric pyrophosphate positive electrode material preparation method according to any one of claims 1 to 7.

9. A sodium ion battery positive electrode, characterized in that A low-alkalinity sodium ferric pyrophosphate positive electrode material prepared by the method for preparing a low-alkalinity sodium ferric pyrophosphate positive electrode material according to any one of claims 1 to 7.

10. A sodium ion battery, characterized in that: A low-alkalinity sodium ferric pyrophosphate positive electrode material prepared by the method for preparing a low-alkalinity sodium ferric pyrophosphate positive electrode material according to any one of claims 1 to 7.

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