Method for preparing phosphoric pyrophosphoric composite salt precursor by using iron phosphate slag and application thereof

Abstract

The application provides a method for preparing a phosphoric pyrophosphoric composite salt precursor from iron phosphate slag and application thereof. The method comprises the following steps: mixing iron phosphate slag, a complexing agent and water, and adjusting the pH value of the obtained mixture to 3-7 by using a pH regulator, so that iron ions and phosphate ions in the iron phosphate slag are transferred to the liquid phase to obtain a suspension mixture, wherein the complexing agent comprises a combination of one or more of sodium polyphosphate, sodium hexametaphosphate, sodium metaphosphate, sodium pyrophosphate or sodium hydrogen pyrophosphate, and the pH regulator comprises an organic weak acid; performing solid-liquid separation on the suspension mixture, removing solid impurities therefrom and collecting the solution; and performing drying treatment on the solution to form the phosphoric pyrophosphoric composite salt precursor. The method provided by the application can convert phosphorus and iron elements in the iron phosphate slag into a liquid phase under the action of the complexing agent and the pH regulator, remove impurities through solid-liquid separation, and obtain a liquid phase which can be directly used to prepare the phosphoric pyrophosphoric composite salt precursor.

Description

Technical Field

[0001] This invention belongs to the field of materials production technology, specifically relating to a method and application for preparing a phosphate pyrophosphate composite salt precursor using ferric phosphate slag. Background Technology

[0002] Iron phosphate slag is a solid waste generated during the lithium extraction and production of lithium carbonate from retired lithium iron phosphate batteries. With the large-scale application of lithium iron phosphate batteries, the amount of iron phosphate slag generated from the recycling of retired batteries is increasing, which not only pollutes the environment but also wastes phosphorus and iron resources.

[0003] The composition of iron phosphate slag varies depending on the recycling process and source. Generally, it mainly includes iron phosphate, carbon materials (such as residual carbon from lithium iron phosphate coating, conductive carbon black, carbon nanotubes, conductive graphite and other conductive additives), and binders (such as polyvinylidene fluoride, PTFE, etc.). Aluminum materials (For example, from residual current collector aluminum foil), some iron phosphate slag may also contain graphite anode, conductive agent CMC / SBR, separator (such as PP, PE, etc.) and other components.

[0004] Currently, the utilization rate of iron phosphate slag is low, with most of it being treated as solid waste or used as building filler. For example, iron phosphate slag is used as a raw material for sintered ceramics and refractory materials, or as one of the raw materials for cement. This utilization method yields products with low value. Only a small portion of iron phosphate slag is used to re-prepare iron phosphate. For instance, existing technology CN116177512B discloses the preparation of iron phosphate material using waste lithium battery iron-phosphate slag, and existing technology CN118359179A discloses the secondary calcination and refining of anhydrous iron phosphate from iron phosphate slag. However, the iron phosphate prepared by these methods has a high impurity content, resulting in lower product quality compared to iron phosphate prepared using conventional raw materials. Furthermore, due to the diverse types of impurities in iron phosphate slag, the existing technology for removing various impurities from the slag is lengthy and complex, with overall costs not significantly different from traditional iron phosphate preparation methods. Therefore, this utilization method suffers from insufficient economic viability. If iron phosphate slag is directly used to prepare lithium iron phosphate, it will contain a large amount of unusable products from the waste slag (such as graphite, carbon, and aluminum), and the resulting lithium iron phosphate will fall far short of the commercial demand for lithium iron phosphate. It is evident that the current phosphorus-iron slag has not been effectively and efficiently utilized. From an economic perspective, this is because the value generated by its utilization is not high, while the cost of its treatment is not low, resulting in a small gap between value and cost. From a technical perspective, no efficient and high-value-added treatment method has emerged. Summary of the Invention

[0005] To solve all or part of the above-mentioned technical problems, the present invention provides the following technical solutions: One objective of this invention is to provide a method for preparing a phosphate pyrophosphate complex precursor using ferric phosphate slag, comprising: Ferric phosphate slag, a complexing agent, and a solvent are mixed, and the pH of the resulting mixture is adjusted to 4-6 using a pH adjuster, so that the phosphorus and iron elements in the ferric phosphate slag are complexed with the complexing agent and separated into the liquid phase, resulting in a suspension mixture. The complexing agent includes one or more combinations of sodium polyphosphate, sodium hexametaphosphate, sodium metaphosphate, sodium pyrophosphate, or sodium hydrogen pyrophosphate, and the pH adjuster includes a weak organic acid. The suspension mixture is subjected to solid-liquid separation to remove solid impurities and collect the solution; The solution is dried to form a phosphate pyrophosphate complex precursor.

[0006] This invention employs a method of dissolving iron phosphate slag using a weak organic acid-assisted complexing agent. The complexing agent can complex the iron ions in the slag, reducing the concentration of free iron ions in the solution. The weak organic acid-assisted complexing agent promotes the entry of iron and phosphorus elements into the solution. The resulting solution can be directly used to prepare a phosphate pyrophosphate composite salt precursor. After supplementing with a sodium source, calcination yields sodium iron phosphate pyrophosphate cathode material, eliminating the need to synthesize battery-grade iron phosphate.

[0007] Furthermore, the use of the complexing agent facilitates obtaining a solution with suitable viscosity, avoiding excessively high viscosity, thus enabling the direct preparation of the phosphate pyrophosphate composite salt precursor. The complexing agent reacts with orthophosphate to generate pyrophosphate, which is convenient for subsequent cathode material preparation processes. Additionally, the use of a weak organic acid can inhibit aluminum dissolution, improve solution purity, and reduce the introduction of unwanted aluminum ions; it is also more environmentally friendly and produces less pollution compared to inorganic acids. Based on this, the present invention can effectively remove impurities such as carbon, graphite, aluminum, and binders from iron phosphate slag through simple solid-liquid separation, achieving a utilization rate of over 90% for iron and phosphorus in the iron phosphate slag, with some preferred embodiments reaching close to 100%.

[0008] Compared to existing technologies that directly use ferric phosphate waste as raw material for sintered ceramics and refractory materials, this invention reprocesses it into a salt precursor for use in the synthesis of cathode materials, which is more economically valuable. Compared to existing technologies that use inorganic acids to dissolve ferric phosphate and then resynthesize battery-grade ferric phosphate, the process route of this patent is simpler.

[0009] In some embodiments, the pH of the resulting mixture is adjusted to 4-6 using the organic acid. The weak organic acid enables the gentle leaching of iron and phosphorus. If the acidity is too strong or a strong acid is used, aluminum in the iron phosphate slag will dissolve, increasing the impurity content in the solution.

[0010] In some embodiments, the organic weak acid includes one or a combination of oxalic acid, tartaric acid, citric acid, or acetic acid. These organic weak acids can form a carbon coating layer on the cathode material during the subsequent sintering process (i.e., the process of preparing the cathode material by calcining the phosphate pyrophosphate composite salt precursor). This improves the conductivity of the cathode material; and the use of these organic weak acids does not introduce additional impurity elements.

[0011] In some preferred embodiments, the pH adjuster includes oxalic acid and / or citric acid. Using oxalic acid results in less residual carbon during subsequent sintering to prepare the cathode material, which helps reduce the carbon content of the cathode material and increase its capacity. Using citric acid yields a lower solution viscosity, facilitating the direct solution synthesis of the cathode material.

[0012] In some embodiments, the method specifically includes: preparing an aqueous solution containing the complexing agent, adjusting the pH of the solution to 4-6 using the organic weak acid, adding ferric phosphate residue and stirring for 0.5-24 hours to dissolve the iron and phosphorus elements, thereby obtaining the suspension mixture.

[0013] In some preferred embodiments, the stirring time is 2-6 hours. If the time is too short, the leaching of phosphorus and iron will be incomplete. After a certain period of time, the leaching of phosphorus and iron will no longer change significantly. Considering both the element recovery rate and the time cost, 2-6 hours is preferred.

[0014] The equipment used for mixing can be a mixer, homogenizer, ball mill, emulsifier, disperser, etc., and this invention does not impose any particular limitation on it.

[0015] In some embodiments, the ratio of the amount of complexing agent to the total amount of phosphorus and iron in the ferric phosphate slag is 0.1-1:1.

[0016] In some embodiments, the total mass ratio of the iron phosphate slag and complexing agent to the mass ratio of water is 1:1-4.

[0017] In some embodiments, the ratio of the amount of pH adjuster to the total amount of phosphorus and iron in the ferric phosphate slag is 0.1-3:1, preferably 0.5-1:1.

[0018] In some embodiments, the method further includes: performing elemental analysis on the solution and, according to the chemical formula Na... h Fe i P j O k Adjust the ratio of Na, Fe, P, and O in the solution, where 0.2 < h ≤ 4, 1 < i < 3, 2 < j ≤ 4, and 7 < k ≤ 20.

[0019] In some embodiments, the solvent includes water.

[0020] In some embodiments, the drying process includes one or a combination of rake drying, flash drying, spray drying, vacuum drying, and freeze drying.

[0021] In some preferred embodiments, the drying process includes spray drying. The spray drying process conditions include: a gas flow rate of 20-30 L / min, an inlet temperature of 180-240°C, and a fan speed of 60-80%.

[0022] In some embodiments, the freeze-drying uses liquid nitrogen as the freezing source, and the freezing time is 2-12 hours.

[0023] In some embodiments, the vacuum drying temperature is 80-100℃ and the drying time is 4-24h.

[0024] The second objective of this invention is to provide a phosphate pyrophosphate complex salt precursor, which is prepared by any of the methods described herein.

[0025] A third objective of this invention is to provide the application of the aforementioned phosphate pyrophosphate composite salt precursor in the preparation of sodium iron phosphate pyrophosphate cathode material for sodium-ion batteries.

[0026] The fourth objective of this invention is to provide a method for preparing sodium iron phosphate pyrophosphate cathode material for sodium-ion batteries, comprising: The phosphate pyrophosphate complex salt precursor was prepared by any one of the methods described above; Add a sodium source, or add both a sodium source and a carbon source, to the phosphate pyrophosphate complex salt precursor and mix them uniformly to obtain a mixture. The mixture is calcined at 500℃-600℃ for 6h-12h to form sodium iron phosphate pyrophosphate cathode material.

[0027] The sodium source and carbon source can be any sodium source (e.g., sodium carbonate) and carbon source (e.g., glucose) commonly used in the preparation of sodium iron phosphate pyrophosphate cathode materials in this field, and the present invention does not make any particular limitation thereto.

[0028] The preparation method may include: determining the proportion of each element in the phosphate pyrophosphate composite salt precursor, and supplementing it with a sodium source, or a sodium source and a carbon source, according to the chemical formula of the desired cathode material, and then sintering.

[0029] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The method provided by the present invention can gently leach and transfer phosphorus and iron elements in iron phosphate slag to the liquid phase, while avoiding the dissolution of aluminum impurities. Thus, a large number of impurities such as aluminum metal, carbon, graphite, binder, and separator in iron phosphate slag can be removed by simple solid-liquid separation. The solution obtained by solid-liquid separation is dried to directly prepare a phosphate pyrophosphate composite salt precursor. The phosphate pyrophosphate composite salt precursor is calcined to directly obtain battery cathode material, which has the characteristics of low raw material cost, simple process and excellent product performance. (2) The method of the present invention can achieve a utilization rate of iron and phosphorus elements in iron phosphate slag of more than 90%, or even close to 100%. This method uses iron phosphate slag to prepare sodium-ion battery cathode materials, and the estimated raw material cost is reduced by more than 50%, realizing the efficient and high-value utilization of iron phosphate slag, reducing solid waste emissions, and is conducive to the sustainable use of environment and resources. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a charge-discharge curve of the cathode material prepared using the phosphate pyrophosphate composite salt of Example 1 of the present invention as a precursor under 0.1C conditions. Detailed Implementation

[0032] The technical solutions of the present invention will be described in detail below with reference to specific embodiments, so that those skilled in the art can better understand and implement the technical solutions of the present invention. The specific functional details disclosed herein should not be construed as limiting, but are merely intended to form the basis of the claims and to teach those skilled in the art to employ the representative basis of the invention in different ways in any suitable detailed embodiment.

[0033] In addition, unless otherwise specified, all raw materials used in the following embodiments can be purchased from the market or other sources, and all production and testing equipment used are known in the art.

[0034] Example 1 This embodiment provides a method for preparing a phosphate pyrophosphate complex salt precursor using iron phosphate slag, including the following steps: The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 51g of sodium metaphosphate were dispersed in 0.5L of deionized water and mixed evenly. 9g of oxalic acid was added to the mixture to adjust the pH to 4. After stirring continuously for 2 hours, a suspension mixture was obtained. The suspension mixture was filtered to obtain filter residue and solution. The obtained solution was subjected to elemental analysis by ICP method, and the ratio of sodium, iron and phosphorus was found to be approximately 1:3:4. This solution was directly used as a precursor solution to prepare phosphate pyrophosphate complex salt. The precursor solution was spray-dried at a gas flow rate of 25 ml / min, a fan speed of 60%, and an inlet temperature of 220°C to obtain a powder, which is a phosphoric acid pyrophosphate complex salt with the chemical formula NaFe3P4O. 15 .

[0035] Example 2 This embodiment provides a method for preparing a phosphate pyrophosphate complex salt precursor using iron phosphate slag, including the following steps: The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 66.5g of sodium pyrophosphate were dispersed in 0.5L of deionized water and mixed evenly. 9g of oxalic acid was added to the mixture to adjust the pH to 4. The mixture was stirred continuously for 2 hours to obtain a suspension. The suspension was filtered to obtain filter residue and solution. The obtained solution was subjected to elemental analysis by ICP method, and the ratio of sodium, iron and phosphorus elements was found to be approximately 2:3:4. This solution was directly used as a precursor solution to prepare phosphate pyrophosphate complex salt. The precursor solution was spray-dried at a gas flow rate of 25 ml / min, a fan speed of 60%, and an inlet temperature of 220℃ to obtain a powder, which is a phosphoric acid pyrophosphate complex salt with the chemical formula Na₂Fe₃P₄O₂. 15.5 .

[0036] Example 3 This embodiment provides a method for preparing a phosphate pyrophosphate complex salt precursor using iron phosphate slag, including the following steps: The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 51g of sodium metaphosphate were dispersed in 0.5L of deionized water and mixed evenly. 19.2g of citric acid was added to the mixture to adjust the pH to 5. The mixture was stirred continuously for 2 hours to obtain a suspension. The suspension was filtered to obtain filter residue and solution. Elemental analysis of the solution was performed using the ICP method, and the ratio of sodium, iron, and phosphorus was found to be approximately 1:3:4. This solution was then used directly as a precursor solution to prepare phosphate pyrophosphate complex salt. The precursor solution was spray-dried at a gas flow rate of 30 ml / min, a fan speed of 80%, and an inlet temperature of 200℃ to obtain a powder, which is a phosphoric acid pyrophosphate complex salt with the chemical formula NaFe3P4O. 15 .

[0037] Example 4 This embodiment provides a method for preparing a phosphate pyrophosphate complex salt precursor using iron phosphate slag, including the following steps: The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 51g of sodium metaphosphate were dispersed in 0.5L of deionized water and mixed evenly. 4.5g of oxalic acid and 9.6g of citric acid were added to the mixture to adjust the pH to 4. The mixture was stirred continuously for 2 hours to obtain a suspension. The suspension was filtered to obtain filter residue and solution. Elemental analysis of the solution was performed using the ICP method, and the ratio of sodium, iron, and phosphorus was found to be approximately 1:3:4, thus obtaining the precursor solution. The precursor solution was spray-dried at a gas flow rate of 30 ml / min, a fan speed of 80%, and an inlet temperature of 200℃ to obtain a powder, which is a phosphoric acid pyrophosphate complex salt with the chemical formula NaFe3P4O. 15 .

[0038] Example 5 This embodiment provides a method for preparing a phosphate pyrophosphate complex salt precursor using iron phosphate slag, including the following steps: The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 51g of sodium metaphosphate were dispersed in 0.5L of deionized water and mixed evenly. 15g of tartaric acid was added to the mixture to adjust the pH value to 6. The mixture was stirred continuously for 0.5h to obtain a suspension. The suspension was filtered to obtain filter residue and solution. Elemental analysis of the solution was performed using the ICP method, and the ratio of sodium, iron, and phosphorus was found to be approximately 1:3:4. This solution was then used directly as a precursor solution to prepare phosphate pyrophosphate complex salt. The precursor solution was freeze-dried using liquid nitrogen for 6 hours to obtain a powder, which is the phosphate pyrophosphate complex salt with the chemical formula NaFe3P4O. 15 .

[0039] Example 6 This embodiment provides a method for preparing a phosphate pyrophosphate complex salt precursor using iron phosphate slag, including the following steps: The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 106g of sodium hexametaphosphate were dispersed in 0.5L of deionized water and mixed evenly. 9g of oxalic acid was added to the mixture to adjust the pH value to 4. The mixture was stirred continuously for 10h to obtain a suspension. The suspension was filtered to obtain filter residue and solution. Elemental analysis of the solution was performed using the ICP method, and the ratio of sodium, iron, and phosphorus was found to be approximately 1:2:3. This solution was then used directly as a precursor solution to prepare a phosphoric acid pyrophosphate complex. The precursor solution was vacuum dried at 90°C for 12 hours to obtain a powder, which is the phosphate pyrophosphate complex salt with the chemical formula NaFe₂P₃O. 10 .

[0040] Example 7 The ferric phosphate slag was taken and elemental analysis was performed. It was found that the ferric phosphate slag used in this embodiment contained 33.4 wt% iron and 18.5 wt% phosphorus. 151g of ferric phosphate slag and 55.5g of sodium pyrophosphate were dispersed in 0.5L of deionized water and mixed evenly. 9g of oxalic acid was added to the mixture to adjust the pH to 4. After stirring continuously for 24h, a suspension was obtained. The suspension was filtered to obtain filter residue and solution. Elemental analysis of the solution was performed using the ICP method, and the ratio of sodium, iron, and phosphorus was found to be approximately 1:2:3. This solution was then used directly as a precursor solution to prepare a phosphoric acid pyrophosphate complex. The precursor solution was vacuum dried at 90°C for 12 hours to obtain a powder, which is the phosphate pyrophosphate complex salt with the chemical formula NaFe₂P₃O. 10 .

[0041] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not add the complexing agent sodium metaphosphate. After mixing iron phosphate slag with water, only oxalic acid is used to adjust the pH value to 4. The rest of the process is the same as in Example 1.

[0042] Compared with Example 1 and Comparative Example 1, it was found that without the addition of a complexing agent, ferric phosphate could not be completely dissolved, and the yield decreased significantly.

[0043] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not add oxalic acid, but only sodium metaphosphate; otherwise, the procedures are the same as in Example 1.

[0044] Compared with Example 1 and Comparative Example 1, it was found that the absence of oxalic acid resulted in limited dissolution of iron phosphate, and the conductivity of the cathode material prepared directly from the solution decreased. This may be because without oxalic acid, a uniform carbon coating layer cannot be formed in the preparation of the cathode material.

[0045] The positive electrode material prepared using the phosphate pyrophosphate composite salt obtained in the above examples and comparative examples is as follows: Based on the chemical formula Na4Fe3P4O 15 A sodium source of the corresponding proportion and a portion of carbon source were added to the obtained phosphate pyrophosphate complex precursor. After uniform mixing using a mixer, the mixture was placed in a tube furnace and sintered at 550℃ for 10 hours to obtain the positive electrode material. Subsequently, a slurry was prepared by mixing the positive electrode material, conductive agent, and binder in a ratio of 8:1:1. After drying in an oven, the positive electrode sheet was obtained using a cutting machine.

[0046] Sodium-ion coin cells are assembled using the above-mentioned positive electrode: the negative electrode is sodium metal, the electrolyte is a sodium perchlorate solution of 95% PC (5% FEC), and the separator is a glass fiber separator.

[0047] Figure 1 Table 2 shows the charge-discharge curves of the cathode material prepared using the phosphate pyrophosphate composite salt as a precursor in Example 1 at 0.1C. The performance of sodium-ion batteries from other examples was also tested, and the results are shown in Table 2.

[0048] This invention also prepares a phosphoric acid pyrophosphate composite salt with the same chemical formula as in Example 1 using conventional sodium, iron, and phosphorus sources. The preparation method is based on prior art CN118993017A. Following the same method described above, the phosphoric acid pyrophosphate composite salt is used to prepare a cathode material and assemble it into a sodium-ion battery. The relevant performance tests of the sodium-ion battery are shown in Table 2.

[0049] Table 2 Relevant Performance of Sodium-ion Batteries

[0050] In summary, the method provided by this invention can effectively remove impurities such as carbon, graphite, aluminum, and binders from iron phosphate slag through simple solid-liquid separation. This solves the problems of complex, lengthy, and costly removal processes caused by the variety of impurities in iron phosphate slag. Usable elements such as phosphorus and iron are retained in the separated liquid phase and, after processing, can be used as phosphate pyrophosphate complex salts as raw materials for battery materials, achieving high-value utilization. Furthermore, this invention prepares phosphate pyrophosphate complex salt precursors under liquid-phase conditions, achieving atomic-level uniform mixing of various elements such as iron and phosphorus, with uniform proportions and distributions. When used as a precursor in battery materials, this not only improves the reaction rate but also results in battery materials with low impurities and excellent electrochemical performance. The method provided by this invention has a short process flow and requires mild process conditions, eliminating the need for heating or other methods. It features low hardware investment and low process cost, enabling large-scale production.

[0051] All aspects, embodiments, features, and examples of this invention are to be regarded as illustrative in all respects and are not intended to limit the invention, the scope of which is defined only by the claims. Other embodiments, modifications, and uses will become apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0052] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0053] Although the invention has been described with reference to illustrative embodiments, those skilled in the art will understand that various other changes, omissions, and / or additions can be made without departing from the spirit and scope of the invention, and that elements of the embodiments can be substituted with substantially equivalents. Furthermore, many modifications can be made without departing from the scope of the invention to adapt particular situations or materials to the teachings of the invention. Therefore, this invention is not intended to be limited to the specific embodiments disclosed for carrying out the invention, but rather is intended to encompass all embodiments falling within the scope of the appended claims. Moreover, unless specifically stated otherwise, any use of the terms first, second, etc., does not indicate any order or importance, but is used to distinguish one element from another.

Claims

1. A method for preparing a phosphate pyrophosphate complex precursor using ferric phosphate slag, characterized in that, include: Ferric phosphate slag, complexing agent and solvent are mixed, and the pH value of the resulting mixture is adjusted to 4-6 using a pH adjuster to transfer iron ions and phosphate ions in the ferric phosphate slag into the liquid phase to obtain a suspension mixture. The complexing agent includes one or more combinations of sodium polyphosphate, sodium hexametaphosphate, sodium metaphosphate, sodium pyrophosphate or sodium hydrogen pyrophosphate, and the pH adjuster includes a weak organic acid. The suspension mixture is subjected to solid-liquid separation to remove solid impurities and collect the solution; The solution is dried to form a phosphate pyrophosphate complex precursor.

2. The method according to claim 1, characterized in that: The pH of the resulting mixture was adjusted to 4-6 using the aforementioned weak organic acid.

3. The method according to claim 2, characterized in that: The organic weak acid includes one or a combination of oxalic acid, tartaric acid, citric acid or acetic acid.

4. The method according to claim 3, characterized in that: The organic weak acids include oxalic acid and / or citric acid.

5. The method according to claim 3, characterized in that, Specifically, it includes: Prepare an aqueous solution containing the complexing agent, adjust the pH of the solution to 4-6 using the organic weak acid, add ferric phosphate residue and stir for 0.5-24 h to dissolve the iron and phosphorus elements, and obtain the suspension mixture.

6. The method according to claim 1, characterized in that: The ratio of the amount of complexing agent to the total amount of phosphorus and iron in the ferric phosphate slag is 0.1-1:

1. And / or, the total mass ratio of the iron phosphate slag and complexing agent to the mass ratio of water is 1:1-4; And / or, the ratio of the amount of the organic weak acid to the total amount of phosphorus and iron in the ferric phosphate slag is 0.1-3:

1.

7. The method according to claim 1, characterized in that: It also includes elemental analysis of the solution, and according to the chemical formula Na h Fe i P j O k Adjust the ratio of Na, Fe, P, and O in the solution, wherein 0.2 < h ≤ 4, 1 < i < 3, 2 < j ≤ 4, and 7 < k ≤ 20; And / or, the solvent includes water; And / or, the drying process includes one or a combination of rake drying, flash drying, spray drying, vacuum drying, and freeze drying.

8. A phosphate pyrophosphate complex precursor, characterized in that: It is prepared by the method described in any one of claims 1-7.

9. The application of the phosphoric acid pyrophosphate composite salt precursor according to claim 8 in the preparation of sodium iron phosphate pyrophosphate cathode material for sodium-ion batteries.

10. A method for preparing sodium iron phosphate (SO4) pyrophosphate cathode material for sodium-ion batteries, characterized in that, include: The phosphate pyrophosphate complex precursor was prepared by the method according to any one of claims 1-7; Add a sodium source, or add both a sodium source and a carbon source, to the phosphate pyrophosphate complex salt precursor and mix them uniformly to obtain a mixture. The mixture is calcined at 500℃-600℃ for 6h-12h to form sodium iron phosphate pyrophosphate cathode material.

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