Positive electrode material, preparation method therefor, positive electrode plate and sodium ion battery

Abstract

Provided in the present application are a positive electrode material, a preparation method therefor, a positive electrode plate, and a sodium ion battery. The positive electrode material comprises sodium iron fluoropyrophosphate, the molecular formula of the sodium iron fluoropyrophosphate being NamFe3(PO4)p(P2O7)qFw, where 5.02≤m≤5.05, 4.02≤p+2q≤4.05, 0.5p≤q≤0.55p, 1.01≤w≤1.05, and 1.87≤p≤2.05. The positive electrode material has a low impurity content and microscopic particles of high sphericity and excellent morphology, and exhibits excellent electrochemical performance.

Description

Positive electrode materials, their preparation methods, positive electrode sheets and sodium-ion batteries Technical Field

[0001] This application relates to the field of sodium-ion battery technology, and more specifically, to a positive electrode material, its preparation method, a positive electrode sheet, and a sodium-ion battery. Background Technology

[0002] Due to the constraints imposed by the scarcity of lithium resources on the development of lithium-ion batteries, sodium-ion batteries are currently considered one of the most viable alternatives, especially in the field of power batteries for new energy vehicles. However, the performance of the most commonly used iron-based sodium-ion battery cathode material, especially its electrochemical activity and stability, still has considerable room for improvement. More precise composition matching, purity optimization, and morphology design are necessary.

[0003] Meanwhile, the resource utilization and recycling of waste lithium iron phosphate (LFP) batteries have received increasing attention due to the rapid increase in the amount of scrapped LFP batteries. The conventional chemical treatment method for waste LFP involves chemical dissolution followed by impurity removal to remove iron and phosphorus before extracting lithium. This process suffers from drawbacks such as high cost, low added value of the product (especially the very low added value of phosphorus and iron), large wastewater generation, and low element utilization. Although existing technologies exist for preparing LFP cathode materials from waste LFP batteries, the overall impurity removal effect is unsatisfactory, resulting in cathode materials with generally high impurity content and poor performance.

[0004] For the reasons mentioned above, how to provide a sodium-ion battery cathode material with high phase purity and excellent morphology based on the resource utilization of waste lithium iron phosphate, so that it exhibits high electrochemical activity and stability, is one of the important technical problems that need to be solved in this field. Summary of the Invention

[0005] The main objective of this application is to provide a cathode material, its preparation method, cathode sheet, and sodium-ion battery, in order to solve the problem that existing technologies are unable to prepare sodium-ion battery cathode materials with excellent morphology and good electrochemical activity and stability based on the resource utilization of waste lithium iron phosphate.

[0006] To achieve the above objectives, the first aspect of this application provides a positive electrode material comprising sodium ferric fluoropyrophosphate, wherein the molecular formula of sodium ferric fluoropyrophosphate is Na. m Fe3(PO4) p (P2O7) q F w , where 5.02≤m≤5.05, 4.02≤p+2q≤4.05, 0.5p≤q≤0.55p, 1.01≤w≤1.05, and 1.87≤p≤2.05.

[0007] The cathode material provided in this application has a unique chemical composition, enabling it to significantly improve the energy density of corresponding sodium-ion batteries while maintaining good cycle stability and excellent rate performance. In particular, the introduction of fluorine, with its stronger electronegativity, can increase the voltage plateau of the cathode material and simultaneously act as an anion dopant, thereby further enhancing the ionic conductivity of the resulting cathode material.

[0008] Furthermore, the cathode material also includes sodium iron phosphate, and the mass fraction of sodium iron phosphate in the cathode material is ≤1.1%. By controlling the content of sodium iron phosphate in the cathode material, the electrochemical performance of the cathode material can be further optimized, because sodium iron phosphate is an electrochemically inert substance, and reducing its content can improve capacity, cycle life, and safety. More preferably, in the XRD pattern of the cathode material, the main peak height of sodium iron phosphate (fluoropyrophosphate) H1 and the main peak height of sodium iron phosphate impurities H2 satisfy: H2 / H1≤1.5%. The cathode material provided in this application exhibits superior electrochemical performance due to its high purity, and when applied to sodium-ion batteries, it exhibits higher capacity and stronger long-cycle stability.

[0009] In some preferred embodiments, the sphericity of the cathode material is 0.94–0.99. Through extensive experimentation, the inventors optimized the sphericity of the cathode material, resulting in a near-spherical microstructure. This leads to better electrode contact performance. Furthermore, cathode materials with high sphericity exhibit shorter and more uniform sodium ion migration distances, better electrolyte wetting, and better particle gradation (due to improved fluidity, facilitating interlocking between particles of different sizes), effectively improving the charging and discharging efficiency of the sodium-ion battery in which it is located.

[0010] The second aspect of this application provides a method for preparing a cathode material, comprising: providing waste lithium iron phosphate cathode material; dissolving the waste lithium iron phosphate cathode material to obtain pretreated filter residue; mixing the pretreated filter residue with a pyrophosphate solution to obtain a first filtrate; mixing the first filtrate with a precipitant to obtain a second filtrate; mixing the second filtrate with a pH adjuster, an ferrous source, and an oxidant to obtain a third filtrate and a third filter residue; subjecting the third filtrate to adsorption treatment to obtain an adsorption residue; mixing the adsorption residue, the third filter residue, a sodium source, a phosphorus source, and a fluorine source to obtain a mixed slurry; and sequentially grinding, drying, and calcining the mixed slurry to obtain the cathode material.

[0011] The preparation method described in this application utilizes recycled waste lithium iron phosphate cathode material to prepare the cathode material as described above. This not only effectively reduces the production cost of cathode material but also achieves the recycling of waste battery materials, resulting in significant environmental benefits. The prepared cathode material exhibits excellent electrochemical performance, significantly improving the energy density and cycle stability of sodium-ion batteries, providing strong support for the commercial application of sodium-ion batteries. Furthermore, the obtained cathode material demonstrates stable performance, meeting the application requirements of sodium-ion batteries in large-scale energy storage systems.

[0012] Further, in the step of dissolving waste lithium iron phosphate cathode material to obtain pretreated filter residue, an alkaline agent is used to dissolve the waste lithium iron phosphate cathode material; and / or, the step of mixing the pretreated filter residue with a pyrophosphate solution to obtain a first filtrate includes: mixing the pretreated filter residue with a pyrophosphate solution, subjecting it to a first reaction time at a first reaction temperature, and then subjecting it to a first solid-liquid separation treatment to obtain the first filtrate; and / or, the step of mixing the first filtrate with a precipitant to obtain a second filtrate includes: mixing the first filtrate with a precipitant, subjecting it to a second reaction time at a second reaction temperature, and then subjecting it to a second solid-liquid separation treatment to obtain the second filtrate; and / or, the step of mixing the second filtrate with a pH adjuster, a ferrous source, and an oxidant to obtain a third filtrate and a third filter residue includes: mixing the second filtrate with a pH adjuster, a ferrous source, and an oxidant, subjecting it to a third reaction time after reaching the third reaction endpoint pH value, and then subjecting it to a third solid-liquid separation treatment to obtain the third filtrate and the third filter residue; and / or, the step of adsorption treatment includes: using an ion exchange resin to adsorb and treat the third filtrate.

[0013] Based on the above process flow, further optimization of specific procedures is carried out to more accurately control the reaction conditions in the recycling and reprocessing process, such as reaction temperature, time and final pH value, so as to more effectively remove impurities in waste battery materials and significantly improve the purity and electrochemical performance of the final cathode material, making it more suitable for high energy density sodium-ion batteries.

[0014] Further, the mass ratio of the alkali agent to the waste lithium iron phosphate cathode material is (3-5):1; the molar ratio of phosphate to pyrophosphate in the first filtrate is 1:(0.45-0.49); the molar ratio of the precipitant to the sum of the molar amounts of nickel, cobalt, copper, and zinc ions in the first filtrate is (1.2-1.5):1; the molar ratio of iron in the ferrous source to the molar ratio of pyrophosphate in the second filtrate is (0.95-0.98):1; and the molar ratio of Na, Fe, P, and F in the mixed slurry is (5.02-5.05):3:(4.02-4.05):(1.01-1.05).

[0015] By optimizing the mass relationships and molar ratios in each step, the recovery rate of waste lithium iron phosphate cathode materials can be further improved. At the same time, the purity of the obtained cathode materials can be optimized, their electrochemical performance can be enhanced, and their application in sodium-ion batteries can be made more widespread.

[0016] Furthermore, the concentration of the alkali agent is 3 mol / L to 5 mol / L; the concentration of the pyrophosphate solution is 3 mol / L to 5 mol / L; the mass fraction of Al in the pretreated filter residue is <0.1%; and the Li in the adsorption residue is <0.1%. + Concentration < 100 ppm.

[0017] Optimizing the concentration of raw materials and controlling the component content of the resulting products during the preparation process is beneficial to producing cathode materials with high purity and stability, reducing the performance degradation of cathode materials during charge-discharge processes, and extending the service life of the corresponding sodium-ion batteries.

[0018] Furthermore, the first reaction temperature is 70℃~90℃, and the first reaction time is 1h~2h; the second reaction temperature is 40℃~60℃, and the second reaction time is 30min~60min; the third reaction endpoint pH value is 2.8~3.2, and the third reaction time is 30min~60min; the calcination temperature is 550℃~650℃, and the calcination holding time is 10h~13h.

[0019] By controlling the reaction conditions in each of the above steps, it is beneficial to more significantly optimize the crystal structure and morphology of the obtained cathode material, thereby improving its electrochemical performance and ultimately enhancing the cycle performance and energy density of the corresponding sodium-ion battery.

[0020] Further, the alkaline agent is a sodium hydroxide solution; the precipitant is selected from one or more of sodium sulfide, sodium hydrogen sulfide, and sodium N,N-dimethyldithiocarbamate; the pH adjuster is selected from one or more of sodium hydroxide, ammonia, ammonium bicarbonate, and ammonium carbonate; the ferrous source is selected from one or more of ferrous acetate, ferrous citrate, and ferrous gluconate; and the oxidant is selected from one or more of hydrogen peroxide, oxygen, and ozone.

[0021] Based on the optimization of the above reaction steps, reaction conditions, and dosage relationships, suitable chemical reagents are further selected for different steps as described above, so as to further optimize the preparation process, comprehensively improve the various properties of the cathode material, and enhance the electrical performance of the corresponding sodium-ion battery.

[0022] A third aspect of this application provides a positive electrode sheet comprising the aforementioned positive electrode material, or a positive electrode material prepared by the aforementioned method for preparing the positive electrode material. Because the positive electrode material obtained in this application has a superior morphology, higher phase purity, and fewer impurities, its corresponding positive electrode sheet also possesses superior electrochemical performance.

[0023] The fourth aspect of this application provides a sodium-ion battery comprising the aforementioned positive electrode. Because the sodium iron phosphate fluoropyrophosphate positive electrode material obtained in this application possesses excellent microstructure and extremely high phase purity, when it is used as a positive electrode component in a sodium-ion battery, the resulting sodium-ion battery exhibits comprehensively improved electrochemical performance, particularly in initial discharge capacity and cycle stability.

[0024] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0026] Figure 1 shows the SEM results of the cathode material obtained in Example 1;

[0027] Figure 2 shows the XRD patterns of Examples 1 to 3. Detailed Implementation

[0028] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0030] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0031] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0033] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0034] In the description of the embodiments of this application, unless otherwise specified, the solvent used for "solution", "filtrate", etc. is water, preferably at least one of distilled water, deionized water, pure water, and ultrapure water.

[0035] As described in the background section, existing technologies struggle to utilize waste lithium iron phosphate resources to prepare sodium-ion battery cathode materials with excellent morphology, electrochemical activity, and stability. To address this technical problem, the first aspect of this application provides a cathode material comprising sodium iron fluoride pyrophosphate, wherein the molecular formula of sodium iron fluoride pyrophosphate is Na. m Fe3(PO4) p (P2O7) q F w , where 5.02≤m≤5.05, 4.02≤p+2q≤4.05, 0.5p≤q≤0.55p, 1.01≤w≤1.05, and 1.87≤p≤2.05.

[0036] The cathode material provided in this application has a unique chemical composition, enabling it to significantly improve the energy density of corresponding sodium-ion batteries while maintaining good cycle stability and excellent rate performance. In particular, the introduction of fluorine, with its stronger electronegativity, can increase the voltage plateau of the cathode material and simultaneously act as an anion dopant, thereby further enhancing the ionic conductivity of the resulting cathode material.

[0037] Furthermore, 5.02≤m≤5.05, 2.0≤p≤2.01, 1.01≤q≤1.02, 1.01≤w≤1.05.

[0038] Furthermore, the molecular formula of sodium ferric fluoropyrophosphate in the cathode material is Na. 5.03 Fe3(PO4) 2.0 (P2O7) 1.02 F 1.03 Na 5.02 Fe3(PO4) 2.0 (P2O7) 1.01 F 1.01 Or Na 5.05 Fe3(PO4) 2.01 (P2O7) 1.01 F 1.05 .

[0039] Furthermore, the cathode material also includes sodium iron phosphate, and the mass fraction of sodium iron phosphate in the cathode material is ≤1.1%, specifically 0.14%, 0.21%, 0.23%, 0.24%, 0.25%, 0.46%, or 0.98%; preferably 0.1% to 1.01%; more preferably 0.10% to 0.5%; and even more preferably 0.10% to 0.23%. By controlling the content of sodium iron phosphate in the cathode material, the electrochemical performance of the cathode material can be further optimized, and the capacity, cycle life, and safety can be improved.

[0040] Furthermore, in the XRD pattern of the cathode material, the peak height H1 of sodium fluoropyrophosphate and the peak height H2 of sodium iron phosphate impurities satisfy the following: H2 / H1 ≤ 1.5%, specifically 0.2%, 0.3%, 0.4%, 0.42%, 0.45%, 0.46%, 0.57%, 1.22%, preferably 0.2%–1.22%, more preferably 0.2%–0.36%, and even more preferably 0.2%–0.4%. The cathode material provided in this application exhibits superior electrochemical performance due to its high purity, resulting in higher capacity and stronger long-cycle stability when applied to sodium-ion batteries.

[0041] In some preferred embodiments, the sphericity of the cathode material is 0.94–0.99, specifically 0.945, 0.946, 0.961, 0.968, 0.969, 0.972, 0.974, and 0.985, preferably 0.945–0.99, more preferably 0.96–0.99, and even more preferably 0.97–0.99. The inventors have optimized the sphericity of the cathode material through extensive experiments, resulting in a near-circular microstructure, which in turn exhibits better particle contact performance and electrolyte wettability, effectively improving the charge-discharge efficiency of the sodium-ion battery in which it is located.

[0042] Furthermore, the resistivity of the cathode material powder is 30 Ω·cm to 76 Ω·cm, such as 32 Ω·cm, 33 Ω·cm, 56 Ω·cm, 59 Ω·cm, 63 Ω·cm, 65 Ω·cm, and 69 Ω·cm, preferably 30 Ω·cm to 65 Ω·cm, and more preferably 30 Ω·cm to 60 Ω·cm. The specific surface area is 12.0 m². 2 / g~17.5m 2 / g, for example, 12.1m 2 / g, 12.3m 2 / g, 12.7m 2 / g, 12.9m 2 / g, 16.8m 2 / g, 17.3m 2 / g. The compacted density is 2.15g / mL to 2.25g / mL, such as 2.15g / mL, 2.16g / mL, 2.17g / mL, 2.20g / mL, 2.21g / mL, 2.22g / mL, 2.23g / mL, and 2.25g / mL.

[0043] Furthermore, the cathode material also includes carbon elements, with a mass fraction of 1.54% to 2.2%, for example, 1.56%, 1.59%, 1.61%, 1.62%, 1.63%, 2.14%, 2.19%, etc.

[0044] Furthermore, the cathode material includes a core and a carbon coating layer, with the carbon coating layer coating the surface of the core. The core includes sodium iron phosphate fluoropyrophosphate, or the core includes sodium iron phosphate fluoropyrophosphate and sodium iron phosphate.

[0045] Furthermore, the mass fraction of the carbon coating layer in the cathode material is 1.54% to 2.2%, for example, it can be 1.56%, 1.59%, 1.61%, 1.62%, 1.63%, 2.14%, 2.19%, etc.

[0046] Furthermore, at 25°C, the first charge specific capacity of the cathode material at a 0.2C rate is greater than or equal to 112 mAh / g, preferably 112 mAh / g to 122 mAh / g, more preferably 115 mAh / g to 122 mAh / g, for example, it can be 112.4 mAh / g, 113.5 mAh / g, 115.4 mAh / g, 118.2 mAh / g, 118.5 mAh / g, 118.9 mAh / g, 120.1 mAh / g, 120.4 mAh / g, 121.7 mAh / g, etc.

[0047] Furthermore, at 25°C, the initial discharge specific capacity of the cathode material at a 0.2C rate is greater than or equal to 103 mAh / g, preferably 103 mAh / g to 114 mAh / g, more preferably 106 mAh / g to 114 mAh / g, for example, it can be 103.2 mAh / g, 104.1 mAh / g, 106.3 mAh / g, 107.2 mAh / g, 109.6 mAh / g, 110.3 mAh / g, 110.6 mAh / g, 111.8 mAh / g, 113.4 mAh / g, etc.

[0048] Furthermore, at 25°C, the median voltage of the first discharge of the positive electrode material at a 0.2C rate is greater than or equal to 3.18V, preferably 3.15V to 3.35V, more preferably 3.2V to 3.32V, for example, it can be 3.18V, 3.21V, 3.23V, 3.25V, 3.26V, 3.27V, 3.32V, etc.

[0049] Furthermore, at 25°C, the initial discharge specific capacity of the cathode material at a 1C rate is greater than or equal to 101 mAh / g, preferably 101 mAh / g to 110 mAh / g, more preferably 105 mAh / g to 110 mAh / g, for example, it can be 101.8 mAh / g, 103.6 mAh / g, 105.2 mAh / g, 106.8 mAh / g, 107.5 mAh / g, 108.1 mAh / g, 108.6 mAh / g, 109.1 mAh / g, 109.5 mAh / g, etc.

[0050] Furthermore, at 25°C, the capacity retention rate of the cathode material after 500 charge-discharge cycles at a 1C rate is greater than or equal to 97.5%, preferably 97.5% to 99%, more preferably 98% to 99%, for example, it can be 97.5%, 97.6%, 98%, 98.3%, 98.6%, 98.7%, 98.9%, etc.

[0051] In some embodiments, the first charge specific capacity, first discharge specific capacity, and first discharge median voltage of the positive electrode material are obtained by preparing a positive electrode sheet having the positive electrode material, which participates in the assembly to form a coin cell and is then tested.

[0052] The positive electrode sheet comprises a positive electrode active material, a binder, and a conductive agent. The positive electrode active material is the lithium supplement agent as described above. The binder is preferably PVDF (polyvinylidene fluoride), but other adhesive materials suitable for the positive electrode sheet can also be used. The conductive agent is preferably a carbon conductive material, such as conductive carbon black (SP). The above-mentioned positive electrode active material, binder, and conductive agent are mixed in a suitable mass ratio, using an aluminum sheet as the current collector. The mass ratio of the positive electrode active material, binder, and conductive agent to obtain the positive electrode sheet can be 85:7:8, and the compaction density of the mixture of the positive electrode active material, binder, and conductive agent can be 2.2 g / mL.

[0053] The second aspect of this application provides a method for preparing a cathode material, comprising: providing waste lithium iron phosphate cathode material; dissolving the waste lithium iron phosphate cathode material to obtain pretreated filter residue; mixing the pretreated filter residue with a pyrophosphate solution to obtain a first filtrate; mixing the first filtrate with a precipitant to obtain a second filtrate; mixing the second filtrate with a pH adjuster, an ferrous source, and an oxidant to obtain a third filtrate and a third filter residue; subjecting the third filtrate to adsorption treatment to obtain an adsorption residue; mixing the adsorption residue, the third filter residue, a sodium source, a phosphorus source, and a fluorine source to obtain a mixed slurry; and sequentially grinding, drying, and calcining the mixed slurry to obtain the cathode material.

[0054] The preparation method described in this application utilizes recycled waste lithium iron phosphate cathode material to prepare the cathode material as described above. This not only effectively reduces the production cost of cathode material but also achieves the recycling of waste battery materials, resulting in significant environmental benefits. The prepared cathode material exhibits excellent electrochemical performance, significantly improving the energy density and cycle stability of sodium-ion batteries, providing strong support for the commercial application of sodium-ion batteries. Furthermore, the obtained cathode material demonstrates stable performance, meeting the application requirements of sodium-ion batteries in large-scale energy storage systems.

[0055] Furthermore, in the step of dissolving the waste lithium iron phosphate cathode material to obtain pretreated filter residue, an alkaline agent is used to dissolve the waste lithium iron phosphate cathode material.

[0056] Further, the step of mixing the pretreated filter residue with the pyrophosphate solution to obtain the first filtrate includes: mixing the pretreated filter residue with the pyrophosphate solution, subjecting the mixture to a first reaction temperature and a first reaction time, and then subjecting it to a first solid-liquid separation treatment to obtain the first filtrate.

[0057] Further, the step of mixing the first filtrate with a precipitant to obtain the second filtrate includes: mixing the first filtrate with a precipitant, subjecting the mixture to a second reaction temperature and a second reaction time, and then subjecting it to a second solid-liquid separation treatment to obtain the second filtrate.

[0058] Further, the step of mixing the second filtrate with a pH adjuster, a ferrous source, and an oxidant to obtain a third filtrate and a third filter residue includes: mixing the second filtrate with a pH adjuster, a ferrous source, and an oxidant, and after reaching the third reaction endpoint pH value, undergoing a third reaction time, and then undergoing a third solid-liquid separation treatment to obtain the third filtrate and the third filter residue.

[0059] Furthermore, the adsorption treatment step includes: using an ion exchange resin to adsorb and treat the third filtrate.

[0060] The method provided in this application effectively utilizes elements such as iron and phosphorus in waste lithium iron phosphate cathode materials, while simultaneously utilizing sodium elements, to obtain cathode materials with relatively high added value and excellent performance for sodium-ion batteries. It also generates minimal wastewater and is environmentally friendly. Specifically, in the alkaline dissolution process, aluminum is primarily removed using an alkaline agent. Taking NaOH as an example, the following reaction occurs: NaOH + Al + H₂O → NaAlO₂ + H₂. The remaining pretreated filtrate obtained after filtration is mainly a sodium-containing solution. Pyrophosphate is added to this solution. Pyrophosphate ions have strong coordination properties and are highly acidic, making them more readily soluble in lithium iron phosphate. Other impurities, such as carbon, are removed by filtration because they are insoluble. Furthermore, since no oxidant is added in the alkaline dissolution step, some inert metals, such as elemental copper, are not easily dissolved and are thus easily removed by filtration. The following reaction mainly occurs during this process: LiFePO₄ + 0.5H₄P₂O₇ → Li + +Fe 2+ +H2PO4 2- +0.5P₂O₇ 4-The first filtrate obtained by filtering the first reaction solution mainly contains elements such as lithium, iron, and phosphorus. A precipitant (such as sodium sulfide, sodium hydrogen sulfide, or sodium N,N-dimethyldithiocarbamate) is added to remove heavy metals from the first filtrate. The aforementioned sulfides are chosen as precipitants because heavy metal ions, such as nickel, cobalt, copper, and zinc ions, react with sulfide ions to form water-insoluble sulfide precipitates. Furthermore, the pH value at which precipitation occurs is relatively low. Simultaneously, the reducing properties of sulfides can reduce the small amount of ferric ions present, thus preventing the oxidation of ferrous ions to form ferric phosphate precipitate, which helps reduce the loss of iron and phosphorus. After the first filtrate reacts with the precipitant, a second reaction solution is obtained. This second reaction solution is then filtered to obtain a second filtrate from which heavy metals have been removed. A pH adjuster (such as sodium hydroxide, ammonia, ammonium bicarbonate, and ammonium carbonate), a ferrous source (such as ferrous acetate, ferrous citrate, and ferrous gluconate), and an oxidant (such as hydrogen peroxide, oxygen, and ozone) are added to the second filtrate to react and produce ferric phosphate / ferrous phosphate and ferric pyrophosphate / ferrous pyrophosphate. The iron, phosphate, and pyrophosphate anions in the second filtrate are precipitated to obtain the third filter residue, which is then separated from the lithium ions in the third filtrate (a lithium-containing solution, where the main cation is lithium ions), providing a basis for subsequent lithium adsorption and recovery. The adsorption residue obtained by further adsorbing and recovering lithium ions from the third filtrate is mixed with the third filter residue. Using the third filter residue as an iron source, and further supplemented with sodium, phosphorus, and fluorine sources, sodium ferric phosphate fluoropyrophosphate is prepared as the positive electrode material.

[0061] Compared with traditional preparation processes, the recovery preparation method provided in this application has a shorter process and a higher recovery rate of each element. In the aqueous solution formed by the third filtrate, the mixing of sodium, iron and phosphorus elements is more uniform than that of the solid phase method, and the final cathode material has better morphology, higher purity and better performance.

[0062] Specifically, in the method provided in this application, pyrophosphate is used for the dissolution process of lithium iron phosphate. Pyrophosphate has stronger acidity and can be used at a higher concentration than phosphoric acid (the mass fraction of pyrophosphate in the pyrophosphate solution can reach 95%, while the mass fraction of phosphoric acid in the phosphoric acid solution is generally only 80%). Simultaneously, pyrophosphate can provide pyrophosphate ions to the cathode material obtained in this application, which is beneficial for the cathode material to have fewer impurities, higher purity, and higher phase purity. Compared with some solid-phase processes, i.e., processes that use phosphate to convert and prepare sodium iron phosphate pyrophosphate, the cathode material prepared in this application has higher purity. This is because the process of converting phosphate to pyrophosphate needs to occur completely at a certain temperature; otherwise, a certain amount of electrochemically inactive impurity sodium iron phosphate phase will be generated, resulting in a high impurity content and low electrical performance in the final cathode material product. As mentioned above, this application does not require the conversion of phosphate and pyrophosphate, thus avoiding the generation of sodium iron phosphate impurity phase, resulting in a cathode material with lower impurities, higher phase purity, and therefore superior electrochemical performance.

[0063] Meanwhile, using pyrophosphate to dissolve lithium iron phosphate ensures full utilization of anions without generating other impurities, thus avoiding increased costs and reduced element recovery rates caused by separating other impurities. Furthermore, it effectively avoids generating other waste, resulting in good environmental performance. The high recovery rate of each element also leads to higher purity of the cathode material obtained from re-calcination, resulting in excellent electrochemical performance.

[0064] Compared to the traditional process, which involves first preparing lithium iron phosphate waste into iron phosphate and lithium carbonate, and then sintering the iron phosphate with sodium source, the process provided in this application is shorter, lower in cost, generates less wastewater, and produces a cathode material with superior performance.

[0065] Furthermore, when the pH adjuster is sodium hydroxide, the concentration of the sodium hydroxide solution is preferably 5 mol / L to 10 mol / L.

[0066] Furthermore, the ferrous source can be mixed with the second filtrate in the form of a solution, and the concentration of ferrous element in the ferrous source is preferably 1.5 mol / L to 2.5 mol / L.

[0067] Furthermore, the oxidant is selected from at least one of hydrogen peroxide, oxygen, and ozone.

[0068] Furthermore, the concentration of the alkaline solution used for alkaline dissolution is 3 mol / L to 5 mol / L; the mass ratio of the alkaline solution to the waste lithium iron phosphate cathode material is (3 to 5): 1, which is beneficial to effectively improve the dissolution efficiency of aluminum in the waste lithium iron phosphate cathode material, reduce the generation of by-products, and thus improve the purity of the obtained cathode material.

[0069] Furthermore, the mass fraction of Al in the pretreated filter residue is <0.1%.

[0070] Furthermore, the preparation method of the cathode material also includes introducing CO2 into the pretreatment filtrate to obtain aluminum hydroxide. Specifically, in this application, aluminum is leached with an alkaline agent, and then carbon dioxide is introduced into the pretreatment filtrate. This further reduces impurities in the pretreatment filtrate while converting a small amount of aluminum into aluminum hydroxide precipitate. The aluminum hydroxide precipitate can be used as a catalyst support, realizing its resource utilization. The reaction occurring in this process is: 2NaAlO2 + CO2 + 3H2O → 2Al(OH)3 + Na2CO3.

[0071] Furthermore, the concentration of the pyrophosphate solution is 3 mol / L to 5 mol / L. At this concentration, the pyrophosphate solution has more suitable coordination and acidity, which is beneficial for the efficient dissolution of lithium iron phosphate and the effective extraction of elements.

[0072] Furthermore, the reaction temperature of the first reaction is 70℃~90℃, and the time is 1h~2h, so as to facilitate more complete dissolution of lithium iron phosphate and improve the recovery rate of each element, especially Fe.

[0073] Furthermore, in the first filtrate, the molar ratio of phosphate to pyrophosphate is 1:(0.45-0.49), which facilitates the formation of sodium iron fluoride pyrophosphate with higher phase purity during subsequent calcination to obtain the cathode material, thereby improving its electrochemical performance.

[0074] Furthermore, in order to improve the removal effect of heavy metal ions and thus improve the purity and electrochemical performance of the obtained cathode material, the preferred ratio of the amount of precipitant to the total amount of nickel ions, cobalt ions, copper ions and zinc ions in the first filtrate is (1.2~1.5):1.

[0075] Furthermore, the reaction temperature of the second reaction is 40℃~60℃, and the reaction time is 30min~60min. Preferably, the pH value of the second reaction is 1.8~2.5. The selection of the above reaction conditions is beneficial for more effectively removing impurity metal ions, improving the purity of the obtained cathode material and the corresponding cycle stability of the sodium-ion battery.

[0076] Furthermore, in order to more efficiently precipitate anions such as iron, phosphate, and pyrophosphate in the second filtrate, improve the purity of the resulting third filtrate, and more effectively reduce impurities, increase phase purity, and enhance electrochemical performance in the obtained cathode material, the preferred endpoint pH value for the third reaction is 2.8–3.2. Additionally, in the third reaction, after reaching the endpoint pH value, the reaction continues for 30–60 minutes.

[0077] Furthermore, the ratio of the amount of iron in the ferrous source (e.g., ferrous acetate) to the amount of pyrophosphate in the second filtrate is (0.95–0.98):1, thereby more effectively reducing the formation of by-products and improving the phase purity of the cathode material.

[0078] Further, the adsorption treatment includes: adding the third filtrate to an ion exchange resin for adsorption treatment until the Li in the adsorption residue is reduced. + Concentration <100ppm. This application utilizes ion exchange resin to separate and adsorb lithium from waste lithium iron phosphate materials, resulting in a lower residual amount of lithium in the adsorption residue and a higher lithium recovery rate. Furthermore, the extremely low lithium content adsorption residue obtained from the separation and adsorption treatment can be recycled as a solution and mixed with the third filter residue, sodium source, phosphorus source, and fluorine source to obtain a mixed slurry, thereby preparing a cathode material. The resulting cathode material has fewer impurities, higher purity and phase purity, and superior electrochemical performance.

[0079] Furthermore, in the adsorption process, before adding the third filtrate to the ion exchange resin, the pH value of the third filtrate is adjusted to 5-7, and the temperature of the third filtrate is maintained at 55℃-65℃, so that the adsorption effect of the ion exchange resin on lithium ions in the third filtrate can reach the best.

[0080] Furthermore, when adding ion exchange resin to the third filtrate for adsorption treatment, the adsorption treatment temperature is 55℃~65℃ to maintain the adsorption effect of ion exchange resin on lithium ions in the third filtrate.

[0081] Furthermore, the ion exchange resin is a cation exchange resin, preferably HPL900 or GC-18. Also, when the lithium content adsorbed by the ion exchange resin is 4 g / L to 6 g / L, lithium is recovered by eluting with 2 mol / L to 4 mol / L sulfuric acid, followed by precipitation of the eluent. The regenerated ion exchange resin can then be reused.

[0082] Furthermore, the molar ratio of Na, Fe, P, and F in the mixed slurry is (5.02–5.05):3:(4.02–4.05):(1.01–1.05). This molar ratio, obtained through extensive experimentation, combined with grinding and drying processes, results in a cathode material with higher phase purity, better morphology, and superior electrochemical performance after calcination.

[0083] To further improve the crystal phase purity, crystallinity, and stability of the obtained cathode material, the calcination holding temperature is 550℃~650℃, and the calcination holding time is 10h~13h. Preferably, the calcination heating rate is 60℃ / h~120℃ / h.

[0084] In some embodiments, the sodium source may be sodium citrate and / or sodium gluconate, the phosphorus source may be phosphoric acid and / or ammonium dihydrogen phosphate, and the fluorine source may be one or more of ammonium fluoride, hydrogen fluoride, and sodium fluoride.

[0085] During the grinding process of the mixed slurry, it is preferable to grind it to a particle size of 100 nm to 200 nm. Sodium hexadecylbenzenesulfonate can be added as a dispersant during the grinding process. The amount of dispersant added can be 1% to 3% of the total weight of the mixed slurry.

[0086] During the spray drying process, the preferred particle size of the spray-dried material is 3μm to 8μm, and the moisture content is less than 0.5%.

[0087] During the calcination process, a roller furnace can be used. In some embodiments, nitrogen is introduced during calcination for atmosphere protection. Simultaneously, the oxygen content in the holding section is maintained below 2 ppm, the CO content below 100 ppm, the CO2 content below 500 ppm, and the humidity ≤2% by adjusting the nitrogen introduction rate. The material is discharged after cooling to a temperature ≤100°C after sintering.

[0088] In some embodiments, the process further includes sieving, iron removal, and packaging of the calcined material to obtain the cathode material. Preferably, a 100-200 mesh ultrasonic vibrating screen is used for sieving, and the packaging process is carried out in a constant temperature and humidity room to control the moisture content of the product to ≤600ppm.

[0089] A third aspect of this application provides a positive electrode sheet comprising the aforementioned positive electrode material, or a positive electrode material prepared by the aforementioned method for preparing the positive electrode material. Because the positive electrode material obtained in this application has a superior morphology, higher phase purity, and fewer internal impurities, its corresponding positive electrode sheet also possesses superior electrochemical performance.

[0090] The fourth aspect of this application provides a sodium-ion battery comprising the aforementioned positive electrode. Because the sodium iron phosphate fluoropyrophosphate positive electrode material obtained in this application possesses excellent microstructure and extremely high phase purity, when it is used as a positive electrode component in a sodium-ion battery, the resulting sodium-ion battery exhibits comprehensively improved electrochemical performance, particularly in initial discharge capacity and cycle stability.

[0091] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0092] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of this application.

[0093] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0094] I. Preparation Method

[0095] Example 1

[0096] Preparation method of cathode material based on waste lithium iron phosphate cathode material:

[0097] 1. Waste lithium iron phosphate material was dissolved in an alkaline solution (alkali agent). The mass ratio of waste lithium iron phosphate material to alkaline solution was 1:4. The alkaline solution was sodium hydroxide solution with a concentration of 4 mol / L. The reaction was stopped when the aluminum content in the filter residue was less than 0.1%. After filtration, pretreated filtrate and pretreated filter residue were obtained. Carbon dioxide was passed through the pretreated filtrate to obtain aluminum hydroxide precipitate and filtrate. The aluminum hydroxide precipitate and the filtrate after filtering out the aluminum hydroxide precipitate were tested. The results are shown in Table 1.

[0098] Table 1

[0099] 2. Add 4 mol / L pyrophosphoric acid to the pretreated filter residue to carry out the first reaction. The first reaction temperature is 80℃ and the first reaction time is 1.5h. After filtration, the first filtrate and the first filter residue are obtained. The molar ratio of phosphate to pyrophosphoric acid in the first filtrate is 1:0.48.

[0100] 3. Add sodium sulfide (precipitant) to the first filtrate to carry out the second reaction. The amount of sodium sulfide added to the first filtrate is 1.4 times the total amount of nickel, cobalt, copper, and zinc ions in the first filtrate. The second reaction temperature is 50℃, the second reaction time is 50 min, and the pH value is maintained at 2.1 during the reaction. After the heavy metals are precipitated by the reaction, filter to obtain the second filtrate and the second filter residue.

[0101] 4. Add sodium hydroxide (pH adjuster), ferrous acetate (ferrous source), and oxidant to the obtained second filtrate to carry out the third reaction. The molar ratio of ferrous acetate to pyrophosphate in the second filtrate is 0.98:1. Add sodium hydroxide to adjust the pH value of the solution. The concentration of sodium hydroxide solution is 8 mol / L, the concentration of ferrous acetate solution is 2 mol / L, and the oxidant is hydrogen peroxide with a concentration of 9 mol / L. Stir the reaction until the endpoint pH value is 3.2. Continue stirring for the third reaction time for 45 minutes, and then filter to obtain the third filtrate and the third filter residue.

[0102] 5. The third filtrate was added to an HPL900 weakly acidic cation exchange resin for adsorption, with a saturated adsorption capacity of 5.5 g / L (the amount of lithium ions adsorbed by 1 L of ion exchange resin). Before treatment with the ion exchange resin, sodium hydroxide was added to adjust the pH of the third filtrate to 6.2. Then, the temperature of the pH-adjusted third filtrate was raised to 60°C. The lithium content before entering the ion exchange resin was 12.24 g / L, and the adsorption temperature was 60°C. After passing through five consecutive ion exchange resin columns, the lithium ion concentration was reduced to 67 ppm, yielding the adsorption residue. The adsorption residue and the third filter residue were mixed and slurried to obtain a slurry. The concentrations of Na, Fe, P and F in the slurry were sampled and tested. The molar ratio of the aforementioned elements was found to be 0.45:3:4:0.0006. A sodium source (a mixture of sodium acetate and sodium citrate in a mass ratio of 5:5), a phosphorus source (ammonium dihydrogen phosphate), and a fluorine source (sodium fluoride) were added to the slurry to obtain a mixed slurry, and the molar ratio of Na, Fe, P and F in the mixed slurry was maintained at 5.03:3:4.04:1.03.

[0103] 6. The above-mentioned mixed slurry is ground until the particle size of the solid components is 130 nm. A dispersant, sodium hexadecylbenzenesulfonate, is added during the grinding process at 2.1% of the mass of the mixed slurry. The ground mixed slurry is then spray-dried under nitrogen atmosphere protection. The resulting spray-dried material has a particle size of 6.2 μm and a moisture content of less than 0.5%. The spray-dried material is then calcined in a roller furnace at a heating rate of 100 °C / h, reaching 600 °C and holding for 12 hours. The oxygen content is maintained below 2 ppm, CO content below 100 ppm, CO2 content below 500 ppm, and humidity ≤2% during the holding period by adjusting the nitrogen supply. After calcination, the material is cooled to a temperature ≤100 °C before being discharged. The discharged material is screened using a 100-mesh ultrasonic vibrating screen. Iron removal and packaging are carried out in a constant temperature and humidity chamber, controlling the moisture content of the final product to ≤600 ppm, thus obtaining the cathode material.

[0104] 7. After lithium is adsorbed by the ion exchange resin, the lithium content in the ion exchange resin is 5.3 g / L. Then, 3 mol / L sulfuric acid is added for elution. The lithium is recovered by precipitation of the eluent to obtain lithium carbonate with a purity higher than 98.5% (mass fraction). The regenerated ion exchange resin after elution can be recycled.

[0105] The obtained cathode material was subjected to XRD testing, as shown in Figure 1. Analysis of the obtained spectrum reveals that the cathode material contains Na... 5.03 Fe3(PO4) 2.0 (P2O7) 1.02 F 1.03The cathode material contains sodium iron phosphate with the molecular formula NaFePO4 and sodium iron phosphate as impurities. The mass fraction of sodium iron phosphate impurities in the cathode material is 0.14%.

[0106] The SEM image of the obtained cathode material is shown in Figure 1, and the XRD pattern is shown in Figure 2.

[0107] Meanwhile, in the XRD pattern of the cathode material, the main peak height H1 of sodium fluoropyrophosphate and the main peak height H2 of sodium iron phosphate impurity satisfy H2 / H1 = 0.2% (where the peak position of the main peak height H2 of sodium iron phosphate impurity is shown by the dashed line, and the main peak height H1 of sodium fluoropyrophosphate is marked).

[0108] Example 2

[0109] Preparation method based on waste lithium iron phosphate cathode material:

[0110] 1. Waste lithium iron phosphate material is dissolved in an alkaline solution (alkali agent), then filtered to obtain pretreated filtrate and pretreated filter residue. The mass ratio of waste lithium iron phosphate material to alkaline solution is 1:3. The alkaline solution is sodium hydroxide solution with a concentration of 5 mol / L. The reaction is stopped when the aluminum content in the filter residue is less than 0.1%. Carbon dioxide is passed through the pretreated filtrate to obtain aluminum hydroxide precipitate and filtrate.

[0111] 2. Add 5 mol / L pyrophosphate to the pretreated filter residue to carry out the first reaction. The first reaction temperature is 70℃ and the first reaction time is 2h. Filter to obtain the first filtrate and the first filter residue. The molar ratio of phosphate to pyrophosphate in the first filtrate is 1:0.45.

[0112] 3. Add sodium sulfide (precipitant) to the first filtrate to carry out the second reaction. The amount of sodium sulfide added to the first filtrate is 1.2 times the total amount of nickel, cobalt, copper, and zinc ions in the first filtrate. The second reaction temperature is 60℃, the second reaction time is 60 min, and the pH value is maintained at 2.5 during the reaction. After the heavy metals are precipitated by the reaction, filter to obtain the second filtrate and the second filter residue.

[0113] 4. Add sodium hydroxide (pH adjuster), ferrous acetate (ferrous source), and ozone to the obtained second filtrate to carry out the third reaction. The molar ratio of ferrous acetate to pyrophosphate in the second filtrate is 0.95:1, and the concentration of the ferrous acetate solution is 1.5 mol / L. First, add ozone to oxidize 50% of the ferrous ions in the solution to ferric ions. Then, add a 10 mol / L sodium hydroxide solution. Stir the reaction until the final pH value is 2.8. Continue stirring for the third reaction time for 60 minutes, and then filter to obtain the third filtrate and the third filter residue.

[0114] 5. The third filtrate is added to a GC-18 type weakly acidic cation exchange resin for adsorption until the lithium ion concentration is reduced to 100 pm, resulting in an adsorption residue. The adsorption residue and the third filter residue are mixed and slurried to obtain a slurry. The concentrations of Na, Fe, P, and F in the slurry are sampled and tested. Sodium gluconate (sodium source), phosphoric acid (phosphorus source), and ammonium fluoride (fluorine source) are added to the slurry to obtain a mixed slurry, maintaining the molar ratio of Na, Fe, P, and F in the mixed slurry at 5.02:3:4.02:1.01.

[0115] 6. The above-mentioned mixed slurry is ground until the particle size of the solid components is 100 nm. A dispersant, sodium hexadecylbenzenesulfonate, is added during the grinding process at 1% of the mass of the mixed slurry. The ground mixed slurry is then spray-dried under nitrogen atmosphere protection. The resulting spray-dried material has a particle size of 3 μm and a moisture content of less than 0.5%. The spray-dried material is then calcined in a roller furnace at a heating rate of 60 °C / h, reaching 550 °C and holding for 13 hours. The oxygen content is maintained below 2 ppm, CO content below 100 ppm, CO2 content below 500 ppm, and humidity ≤2% during the holding period by adjusting the nitrogen flow rate. After calcination, the material is cooled to a temperature ≤100 °C before being discharged. The discharged material is sieved using a 100-mesh ultrasonic vibrating screen. Iron removal and packaging are carried out in a constant temperature and humidity chamber, controlling the moisture content of the final product to ≤600 ppm, thus obtaining the cathode material.

[0116] 7. After the ion exchange resin adsorbs lithium, when the lithium content in the ion exchange resin reaches 4 g / L, 4 mol / L sulfuric acid is added for elution. The lithium is recovered by precipitating the eluent, and the regenerated ion exchange resin is then reused.

[0117] The obtained cathode material was subjected to XRD testing, and the results are shown in Figure 2. Analysis of the obtained spectra reveals that the cathode material includes materials with the molecular formula Na. 5.02 Fe3(PO4) 2.0 (P2O7) 1.01 F 1.01 The cathode material contains sodium iron phosphate with the molecular formula NaFePO4 and sodium iron phosphate as impurities. The mass fraction of sodium iron phosphate impurities in the cathode material is 0.23%.

[0118] Meanwhile, in the XRD pattern of the cathode material, the main peak height H1 of sodium fluoropyrophosphate and the main peak height H2 of sodium iron phosphate impurity satisfy H2 / H1 = 0.4% (the peak position of the main peak height H2 of sodium iron phosphate impurity is shown by the dashed line, while the main peak height H1 of sodium fluoropyrophosphate is marked).

[0119] Example 3

[0120] Preparation method based on waste lithium iron phosphate cathode material:

[0121] 1. Waste lithium iron phosphate material is dissolved in an alkaline solution (alkali agent), then filtered to obtain pretreated filtrate and pretreated filter residue. The mass ratio of waste lithium iron phosphate material to alkaline solution is 1:5. The alkaline solution is sodium hydroxide solution with a concentration of 5 mol / L. The reaction is stopped when the aluminum content in the filter residue is less than 0.1%. Carbon dioxide is passed through the pretreated filtrate to obtain aluminum hydroxide precipitate and filtrate.

[0122] 2. Add 3 mol / L pyrophosphate to the pretreated filter residue to carry out the first reaction. The first reaction temperature is 90℃ and the first reaction time is 1 h. Filter to obtain the first filtrate and the first filter residue. The molar ratio of phosphate to pyrophosphate in the first filtrate is 1:0.49.

[0123] 3. Add sodium sulfide (precipitant) to the first filtrate to carry out the second reaction. The amount of sodium sulfide added to the first filtrate is 1.5 times the total amount of nickel, cobalt, copper, and zinc ions in the first filtrate. The second reaction temperature is 40℃, the second reaction time is 30 min, and the pH value is maintained at 1.8 during the reaction. After the heavy metals are precipitated by the reaction, filter to obtain the second filtrate and the second filter residue.

[0124] 4. Add sodium hydroxide (pH adjuster), ferrous acetate (ferrous source), and oxidant to the obtained second filtrate to carry out the third reaction. The molar ratio of ferrous acetate to pyrophosphate in the second filtrate is 0.98:1. Add sodium hydroxide to adjust the pH value of the solution. The concentration of sodium hydroxide solution is 10 mol / L, and the concentration of ferrous acetate solution is 2.5 mol / L. Stir the reaction until the endpoint pH value is 3.2, and continue the third reaction for 30 minutes to obtain the third filtrate and the third filter residue.

[0125] 5. The third filtrate is added to HPL900 weakly acidic cation exchange resin for adsorption until the lithium ion concentration is reduced to 100 pm, yielding an adsorption residue. The adsorption residue and the third filter residue are mixed and slurried to obtain a slurry. The concentrations of Na, Fe, P, and F in the slurry are sampled and tested. Sodium gluconate (sodium source), ammonium dihydrogen phosphate (phosphorus source), and hydrogen fluoride (fluorine source) are added to the slurry to obtain a mixed slurry, maintaining the molar ratio of Na, Fe, P, and F in the mixed slurry at 5.05:3:4.05:1.05.

[0126] 6. The above-mentioned mixed slurry is ground until the particle size of the solid components is 200 nm. A dispersant, sodium hexadecylbenzenesulfonate, is added during the grinding process at 3% of the mass of the mixed slurry. The ground mixed slurry is then spray-dried under nitrogen atmosphere protection. The resulting spray-dried material has a particle size of 8 μm and a moisture content of less than 0.5%. The spray-dried material is calcined in a roller furnace at a heating rate of 120 °C / h, reaching 650 °C and holding for 10 hours. The oxygen content is maintained below 2 ppm, CO content below 100 ppm, CO2 content below 500 ppm, and humidity ≤2% by adjusting the nitrogen supply during the holding period. After calcination, the material is cooled to a temperature ≤100 °C before being discharged. The discharged material is screened using a 200-mesh ultrasonic vibrating screen. Iron removal and packaging are carried out in a constant temperature and humidity chamber, controlling the moisture content of the final product to ≤600 ppm, thus obtaining the cathode material.

[0127] 7. After the ion exchange resin adsorbs lithium, when the lithium content in the ion exchange resin reaches 6 g / L, 4 mol / L sulfuric acid is added for elution. The lithium is recovered by precipitating the eluent, and the regenerated ion exchange resin is then reused.

[0128] The obtained cathode material was subjected to XRD testing, and the results are shown in Figure 2. Analysis of the obtained spectra reveals that the cathode material includes materials with the molecular formula Na. 5.05 Fe3(PO4) 2.01 (P2O7) 1.01 F 1.05 The cathode material contains sodium iron phosphate with the molecular formula NaFePO4 and sodium iron phosphate as impurities. The mass fraction of sodium iron phosphate impurities in the cathode material is 0.21%.

[0129] Meanwhile, in the XRD pattern of the cathode material, the main peak height H1 of sodium fluoropyrophosphate and the main peak height H2 of sodium iron phosphate impurity satisfy H2 / H1 = 0.3% (the peak position of the main peak height H2 of sodium iron phosphate impurity is shown by the dashed line, while the main peak height H1 of sodium fluoropyrophosphate is marked).

[0130] Example 4

[0131] Preparation method based on waste lithium iron phosphate cathode material:

[0132] The difference between this embodiment and Embodiment 1 lies only in steps 1 and 2, specifically:

[0133] 1. Dissolve the waste lithium iron phosphate material in an alkaline solution (alkali agent). The mass ratio of waste lithium iron phosphate material to alkaline solution is 1:2. The alkaline solution is sodium hydroxide solution with a concentration of 4 mol / L. Stop the reaction when the aluminum content in the filter residue is less than 0.1%. Filter to obtain pretreated filtrate and pretreated filter residue.

[0134] 2. Add 2 mol / L pyrophosphate to the pretreated filter residue to carry out the first reaction. The first reaction temperature is 100℃ and the first reaction time is 0.5h. The first filtrate and the first filter residue are obtained. The molar ratio of phosphate to pyrophosphate in the first filtrate is 1:0.5.

[0135] The remaining steps are exactly the same as in Example 1.

[0136] Example 5

[0137] Preparation method based on waste lithium iron phosphate cathode material:

[0138] The difference between this embodiment and Embodiment 1 lies only in steps 1 and 2, specifically:

[0139] 1. Dissolve the waste lithium iron phosphate material in an alkaline solution (alkali agent). The mass ratio of waste lithium iron phosphate material to alkaline solution is 1:6. The alkaline solution is sodium hydroxide solution with a concentration of 4 mol / L. Stop the reaction when the aluminum content in the filter residue is less than 0.1%. Filter to obtain pretreated filtrate and pretreated filter residue.

[0140] 2. Add 6 mol / L pyrophosphoric acid to the pretreated filter residue to carry out the first reaction. The first reaction temperature is 60℃ and the first reaction time is 2.5h. After filtration, the first filtrate and the first filter residue are obtained. The molar ratio of phosphate to pyrophosphoric acid in the first filtrate is 1:0.4.

[0141] The remaining steps are exactly the same as in Example 1.

[0142] Example 6

[0143] Preparation method based on waste lithium iron phosphate cathode material:

[0144] The only difference between this embodiment and Embodiment 1 is step 3, specifically:

[0145] 3. Add sodium sulfide (precipitant) to the first filtrate to carry out the second reaction. The amount of sodium sulfide added to the first filtrate is 1 times the total amount of nickel, cobalt, copper, and zinc ions in the first filtrate. The second reaction temperature is 65℃, the second reaction time is 25min, and the pH value is maintained at 1.5 during the reaction. After the heavy metals are precipitated by the reaction, filter to obtain the second filtrate and the second filter residue.

[0146] The remaining steps are exactly the same as in Example 1.

[0147] Example 7

[0148] Preparation method based on waste lithium iron phosphate cathode material:

[0149] The only difference between this embodiment and Embodiment 1 is step 3, specifically:

[0150] 3. Add sodium sulfide (precipitant) to the first filtrate to carry out the second reaction. The amount of sodium sulfide added to the first filtrate is twice the total amount of nickel, cobalt, copper, and zinc ions in the first filtrate. The second reaction temperature is 35℃, the second reaction time is 65min, and the pH value is maintained at 3.0 during the reaction. After the heavy metals are precipitated by the reaction, filter to obtain the second filtrate and the second filter residue.

[0151] The remaining steps are exactly the same as in Example 1.

[0152] Example 8

[0153] Preparation method based on waste lithium iron phosphate cathode material:

[0154] The difference between this embodiment and Embodiment 1 lies only in steps 5 and 6, specifically:

[0155] 5. The third filtrate was added to an HPL900 weakly acidic cation exchange resin for adsorption, with a saturated adsorption capacity of 5.5 g / L (lithium ion content adsorbed by 1 L of ion exchange resin). Before treatment with the ion exchange resin, sodium hydroxide was added to adjust the pH of the third filtrate to 6.2. Then, the temperature of the pH-adjusted third filtrate was raised to 60℃. The lithium content before entering the ion exchange resin was 12.24 g / L, and the adsorption temperature was 60℃. After passing through five consecutive ion exchange resin columns, the lithium ion concentration was reduced to 67 ppm, yielding the adsorption residue. The adsorption residue and the third filter residue were mixed and slurried to obtain a slurry. The concentrations of Na, Fe, P and F in the slurry were sampled and tested. A sodium source (a mixture of sodium acetate and sodium citrate in a mass ratio of 5:5), a phosphorus source (ammonium dihydrogen phosphate) and a fluorine source (sodium fluoride) were added to the slurry to obtain a mixed slurry, and the molar ratio of Na, Fe, P and F in the mixed slurry was maintained at 5.00:3:4.035:1.03.

[0156] 6. The above-mentioned mixed slurry is ground until the particle size of the solid component is 130 nm. A dispersant, sodium hexadecylbenzenesulfonate, is added during the grinding process at 2.1% of the mass of the mixed slurry. The ground mixed slurry is then spray-dried under nitrogen atmosphere protection. The resulting spray-dried material has a particle size of 6.2 μm and a moisture content of less than 0.5%. The spray-dried material is calcined in a roller furnace at a heating rate of 50 °C / h, reaching 500 °C and holding for 14 hours. The oxygen content, CO content, and CO2 content are maintained below 2 ppm, 100 ppm, and 500 ppm respectively, and the humidity is ≤2%, by adjusting the nitrogen introduction rate during the holding period. After calcination, the material is cooled to a temperature ≤100 °C before being discharged. The discharged material is screened using a 100-mesh ultrasonic vibrating screen. Iron removal and packaging are carried out in a constant temperature and humidity chamber, controlling the moisture content of the final product to ≤600 ppm, thus obtaining the cathode material.

[0157] The remaining steps are exactly the same as in Example 1.

[0158] Example 9

[0159] Preparation method based on waste lithium iron phosphate cathode material:

[0160] The difference between this embodiment and Embodiment 1 lies only in steps 5 and 6, specifically:

[0161] 5. The third filtrate was added to an HPL900 weakly acidic cation exchange resin for adsorption, with a saturated adsorption capacity of 5.5 g / L (lithium ion content adsorbed by 1 L of ion exchange resin). Before treatment with the ion exchange resin, sodium hydroxide was added to adjust the pH of the third filtrate to 6.2. Then, the temperature of the pH-adjusted third filtrate was raised to 60℃. The lithium content before entering the ion exchange resin was 12.24 g / L, and the adsorption temperature was 60℃. After passing through five consecutive ion exchange resin columns, the lithium ion concentration was reduced to 67 ppm, yielding the adsorption residue. The adsorption residue and the third filter residue were mixed and slurried to obtain a slurry. The concentrations of Na, Fe, P and F in the slurry were sampled and tested. A sodium source (a mixture of sodium acetate and sodium citrate in a mass ratio of 5:5), a phosphorus source (ammonium dihydrogen phosphate) and a fluorine source (sodium fluoride) were added to the slurry to obtain a mixed slurry, and the molar ratio of Na, Fe, P and F in the mixed slurry was maintained at 5.035:3:4.00:1.03.

[0162] 6. The above-mentioned mixed slurry is ground until the particle size of the solid components is 130 nm. A dispersant, sodium hexadecylbenzenesulfonate, is added during the grinding process at 2.1% of the mass of the mixed slurry. The ground mixed slurry is then spray-dried under nitrogen atmosphere protection. The resulting spray-dried material has a particle size of 6.2 μm and a moisture content of less than 0.5%. The spray-dried material is then calcined in a roller furnace at a heating rate of 150 °C / h, reaching 700 °C and holding for 9 hours. The oxygen content is maintained below 2 ppm, CO content below 100 ppm, CO2 content below 500 ppm, and humidity ≤2% during the holding period by adjusting the nitrogen supply. After calcination, the material is cooled to a temperature ≤100 °C before being discharged. The discharged material is screened using a 100-mesh ultrasonic vibrating screen. Iron removal and packaging are carried out in a constant temperature and humidity chamber, controlling the moisture content of the final product to ≤600 ppm, thus obtaining the cathode material.

[0163] The remaining steps are exactly the same as in Example 1.

[0164] Comparative Example 1

[0165] Preparation method based on waste lithium iron phosphate cathode material:

[0166] The only difference between this comparative example and Example 1 is that sodium fluoride was not added to the slurry obtained by mixing and slurrying the adsorption residue and the third filter residue. Instead, a sodium source (a mixture of sodium acetate and sodium citrate in a mass ratio of 5:5) and a phosphorus source (ammonium dihydrogen phosphate) were added to the slurry to obtain a mixed slurry, and the molar ratio of Na, Fe, and P in the mixed slurry was maintained at 4.03:3:4.04. The resulting cathode material includes materials with the molecular formula Na. 4.03 Fe3(PO4) 1.95 (P2O7) 1.045 Sodium iron phosphate pyrophosphate.

[0167] II. Testing Methods

[0168] 1. Testing of the physicochemical properties of cathode materials

[0169] Iron dissolution test: Add 1g of the test sample to 100mL of 0.1mol / L hydrogen fluoride-ethanol solution, stir and dissolve at 45℃ for 30min, then filter, and measure the iron content in the filtrate, which is the amount of iron dissolution.

[0170] Free sodium content: determined by potentiometric titration.

[0171] Magnetic material content: Collected using a magnet, then dissolved in aqua regia, and measured using an atomic absorption spectrometer.

[0172] Sphericity: Tested using a sphericity analyzer and SEM.

[0173] Powder resistivity: The four-probe method was used to test the resistivity at a pressure of 10 MPa.

[0174] Particle size (D50 particle size): GB / T 19077, Particle size analysis by laser diffraction.

[0175] Specific surface area (BET): The specific surface area of ​​solid substances is determined by the gas adsorption BET method.

[0176] Tap density: GB / T 5162-2021, Determination of tap density of powder.

[0177] Compacted density: GB / T 24533, Determination of compacted density of powder.

[0178] 2. Button cell battery performance test

[0179] The positive electrode materials obtained from each embodiment and comparative example were mixed with polyvinylidene fluoride and conductive carbon black in a mass ratio of 85:7:8, coated onto aluminum foil, and then prepared into electrode sheets with a compaction density of 2.2 g / mL. The negative electrode was a sodium sheet, and the electrolyte was a 1 mol / L sodium perchlorate solution. The electrode sheets were assembled into coin cells, and the specific capacity of the first charge, the specific capacity of the first discharge, and the median voltage of the first discharge at 0.2C rate were tested; the specific capacity of the first discharge at 1C rate; and the capacity retention rate after 500 cycles at 1C rate.

[0180] The test results for the above tests are shown in Tables 2, 3 and 4, respectively.

[0181] Table 2

[0182] Table 3

[0183] Table 4

[0184] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0185] As shown in Table 1 above, this application obtained high-purity aluminum hydroxide by further precipitation treatment of the pretreated filtrate obtained from the alkali dissolution of waste lithium iron phosphate cathode material. This aluminum hydroxide has a large BTE specific surface area and can be used as a catalyst support for recovery and application in the catalytic field. In addition, the remaining filtrate after the precipitation treatment of the pretreated filtrate to obtain aluminum hydroxide precipitation is mainly composed of sodium salts and can be mixed with the third filter residue to supplement sodium elements.

[0186] As can be seen from the results in Tables 2 to 4 above, the embodiments of this application have achieved the preparation of sodium-ion battery cathode materials with low impurity content, high purity, and high sphericity. Specifically:

[0187] Comparing Examples 1 to 9 with Comparative Example 1, it can be seen that the preparation method used in this application introduces fluoride ions, resulting in a cathode material with sodium iron phosphate containing fluoropyrophosphate. Its various properties are superior. After further preparation of a sodium-ion battery, the corresponding sodium-ion battery exhibits higher initial charge specific capacity, initial discharge specific capacity, initial discharge median voltage, and long-term cycle stability, indicating that the cathode material provided in this application has higher energy density, lower polarization, and stronger stability.

[0188] Comparing Examples 1 to 3 with Examples 4 and 5, it can be seen that during the alkaline dissolution process, the preferred weight ratio of alkaline solution to waste lithium iron phosphate cathode material is (3-5):1, and the concentration of alkaline solution is 3 mol / L to 5 mol / L. In the first reaction process, the preferred concentration of pyrophosphate solution is 3 mol / L to 5 mol / L, the molar ratio of phosphate to pyrophosphate is 1:(0.45-0.49), the first reaction temperature is controlled at 70℃ to 90℃, and the first reaction time is controlled at 1h to 2h. This can effectively reduce the formation of sodium iron phosphate impurity phase, improve the sphericity of the obtained cathode material, and improve the electrochemical performance of the cathode material, especially the cycle stability of the prepared sodium-ion battery.

[0189] Comparing Examples 1 to 3 with Examples 6 and 7, it can be seen that in the process of removing heavy metal ions in the second reaction, by preferably using a ratio of sodium sulfide (precipitant) to the total amount of nickel ions, cobalt ions, copper ions, and zinc ions in the first filtrate of (1.2 to 1.5):1, setting the second reaction temperature to 40°C to 60°C, the second reaction time to 30 min to 60 min, and controlling the pH value during the second reaction process to 1.8 to 2.5, it is beneficial to significantly improve the purity of the obtained cathode material, so that the corresponding sodium-ion battery exhibits higher initial charge specific capacity, initial discharge specific capacity, initial discharge median voltage, and cycle stability.

[0190] Comparing Examples 1 to 3 with Examples 8 and 9, it can be seen that the preferred molar ratio of Na, Fe, P and F in the mixed slurry is (5.02~5.05):3:(4.02~4.05):(1.01~1.05), and the calcination temperature during the calcination process is controlled at 550℃~650℃ and the holding time is controlled at 10h~13h. This is beneficial to effectively optimize the microstructure and electrochemical performance of the obtained cathode material, and further improve the charging specific capacity, discharging specific capacity and cycle stability of the corresponding sodium-ion battery.

[0191] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode material, characterized in that, The positive electrode material includes sodium ferric fluoropyrophosphate, and the molecular formula of sodium ferric fluoropyrophosphate is Na. m Fe3(PO4) p (P2O7) q F w , where 5.02≤m≤5.05, 4.02≤p+2q≤4.05, 0.5p≤q≤0.55p, 1.01≤w≤1.05, and 1.87≤p≤2.

05.

2. The cathode material according to claim 1, characterized in that, The cathode material further includes sodium iron phosphate, and the mass fraction of sodium iron phosphate in the cathode material is ≤1.1%; In the XRD pattern of the cathode material, the main peak height H1 of sodium fluoropyrophosphate and the main peak height H2 of sodium ferric phosphate impurity satisfy: H2 / H1≤1.5%.

3. The cathode material according to claim 1 or 2, characterized in that, The sphericity of the positive electrode material is 0.94 to 0.

99.

4. A method for preparing a positive electrode material, characterized in that, The method for preparing the cathode material includes: Provide waste lithium iron phosphate cathode materials; Dissolve the waste lithium iron phosphate cathode material to obtain pretreated filter residue; The pretreated filter residue is mixed with pyrophosphate solution to obtain the first filtrate; The first filtrate is mixed with a precipitant to obtain a second filtrate; The second filtrate is mixed with a pH adjuster, a ferrous source, and an oxidant to obtain a third filtrate and a third filter residue; The third filtrate is subjected to adsorption treatment to obtain an adsorption residue. The adsorption residue, the third filter residue, the sodium source, the phosphorus source, and the fluorine source are mixed to obtain a mixed slurry; The mixed slurry is successively ground, dried and calcined to obtain the positive electrode material.

5. The method for preparing the cathode material according to claim 4, characterized in that, In the step of dissolving the waste lithium iron phosphate cathode material to obtain the pretreated filter residue, an alkaline agent is used to dissolve the waste lithium iron phosphate cathode material. And / or, The step of mixing the pretreated filter residue with the pyrophosphate solution to obtain the first filtrate includes: mixing the pretreated filter residue with the pyrophosphate solution, subjecting the mixture to a first reaction temperature and a first reaction time, followed by a first solid-liquid separation treatment to obtain the first filtrate; and / or, The step of mixing the first filtrate with the precipitant to obtain the second filtrate includes: mixing the first filtrate with the precipitant, subjecting the mixture to a second reaction temperature and a second reaction time, followed by a second solid-liquid separation treatment to obtain the second filtrate; and / or, The step of mixing the second filtrate with the pH adjuster, the ferrous source, and the oxidant to obtain the third filtrate and the third filter residue includes: mixing the second filtrate with the pH adjuster, the ferrous source, and the oxidant; after reaching the third reaction endpoint pH value, undergoing a third reaction time; and then undergoing a third solid-liquid separation treatment to obtain the third filtrate and the third filter residue; and / or, The adsorption treatment step includes: using an ion exchange resin to adsorb and treat the third filtrate.

6. The method for preparing the cathode material according to claim 5, characterized in that, The mass ratio of the alkaline agent to the waste lithium iron phosphate cathode material is (3-5):1; In the first filtrate, the molar ratio of phosphate to pyrophosphate is 1:(0.45-0.49). The ratio of the amount of the precipitant to the total amount of nickel, cobalt, copper, and zinc ions in the first filtrate is (1.2–1.5):

1. The ratio of the amount of iron in the ferrous source to the amount of pyrophosphate in the second filtrate is (0.95-0.98):

1. In the mixed slurry, the molar ratio of Na, Fe, P and F is (5.02~5.05):3:(4.02~4.05):(1.01~1.05).

7. The method for preparing the cathode material according to claim 6, characterized in that, The concentration of the alkali agent is 3 mol / L to 5 mol / L; The concentration of the pyrophosphate solution is 3 mol / L to 5 mol / L; The mass fraction of Al in the pretreated filter residue is <0.1%; Li in the adsorption residue + Concentration < 100 ppm.

8. The method for preparing the cathode material according to any one of claims 5 to 7, characterized in that, The first reaction temperature is 70℃~90℃, and the first reaction time is 1h~2h; The second reaction temperature is 40℃~60℃, and the second reaction time is 30min~60min; The third reaction endpoint pH value is 2.8–3.2, and the third reaction time is 30 min–60 min; The calcination temperature is 550℃~650℃, and the calcination holding time is 10h~13h.

9. The method for preparing the cathode material according to any one of claims 5 to 7, characterized in that, The alkaline agent is a sodium hydroxide solution; The precipitant is selected from one or more of sodium sulfide, sodium hydrogen sulfide and sodium N,N-dimethyldithiocarbamate. The pH adjuster is selected from one or more of sodium hydroxide, ammonia, ammonium bicarbonate, and ammonium carbonate; The ferrous source is selected from one or more of ferrous acetate, ferrous citrate and ferrous gluconate; The oxidant is selected from one or more of hydrogen peroxide, oxygen, and ozone.

10. A positive electrode plate, characterized in that, The positive electrode sheet comprises the positive electrode material according to any one of claims 1 to 3, or the positive electrode material prepared by the method of preparing the positive electrode material according to any one of claims 4 to 9.

11. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode sheet as described in claim 10.

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