A method for recovering lithium salts from spent lithium-ion batteries and preparing lithium manganese iron phosphate cathode materials.

Abstract

This invention belongs to the field of lithium-ion battery recycling technology, and more specifically, relates to a method for recovering lithium salts from spent lithium-ion batteries and preparing lithium manganese iron phosphate cathode materials. This invention utilizes oxalic acid or persulfate to selectively recover metal salts from spent lithium-ion battery cathode materials. Under hydrothermal conditions, the crystal structure of the cathode material is destroyed, leaching valuable metals from the material. Simultaneously, the reducing and complexing properties of oxalate ions are used to reduce high-valence metal ions to low-valence metal ions, which then form a precipitate through complexation between the oxalate and the metal ions. Persulfate, with its strong oxidizing properties, can oxidize the leached metal ions into stable high-valence solid oxides. Both methods can achieve the separation of valuable metals from spent lithium-ion battery cathode materials. This invention employs a green and efficient method, avoiding secondary environmental pollution, and has advantages such as low cost, simple process, readily available raw materials, easy process scaling, and industrialization.

Description

Technical Field

[0001] This invention belongs to the field of electronic waste lithium-ion battery recycling technology, and more specifically, relates to a method for recovering lithium salts from waste lithium-ion batteries and preparing lithium manganese iron phosphate cathode materials. Background Technology

[0002] Lithium-ion batteries, with their unique advantages such as high energy density, high operating voltage, long cycle life, no memory effect, low self-discharge rate, wide operating temperature range, and rapid charge / discharge capability, have become one of the main power sources for electric vehicles on the market. With the rapid development of electric vehicles globally and the increasing demand, lithium-ion battery production and shipments are also increasing year by year. The average lifespan of power batteries is 5 to 8 years, which has led to a rapid increase in the amount of used lithium-ion batteries. Among the many types of lithium-ion batteries, lithium iron phosphate (LiFePO4) batteries and lithium manganese oxide (LiMn2O4) batteries have become strong competitors in the fields of power batteries and large-scale energy storage batteries due to their reliability and low cost. Spinel-type LiMn2O4 has high structural stability, a discharge voltage platform above 4.0V, and advantages such as low price, safety, and low pollution. However, LiMn2O4 also has many disadvantages: poor cycle stability, large capacity decay rate at high temperatures, short lifespan; and the actual specific capacity is far lower than the theoretical capacity, only 120-130 mAh / g (the theoretical capacity is 148 mAh / g). LiFePO4 has an olivine structure, which has good stability, safety, excellent cycle performance, and long service life. However, it also has obvious disadvantages: low conductivity, small lithium-ion diffusion coefficient, and a discharge voltage plateau of only 3.4V, resulting in a low energy density.

[0003] Currently, the mainstream recycling processes for waste lithium-ion battery cathode materials mainly include direct regeneration, pyrometallurgical processes, and hydrometallurgical processes. Hydrometallurgical recycling of waste lithium-ion battery cathode materials is the most widely studied and most effective method. It uses suitable leaching agents to leach valuable metals from the waste lithium-ion battery cathode materials into a solution, thereby recovering metal salts or regenerating the cathode materials. Although traditional hydrometallurgical leaching can achieve full recovery of valuable metal elements, and the recovered products meet industrial-grade application requirements, the added value of the recovered products is relatively low. Furthermore, the excessive use of acid and alkali solutions inevitably generates a large amount of waste liquid, increasing processing costs and reducing economic efficiency. Therefore, comprehensive multi-element recovery and how to improve the added value of recovered products still require further in-depth research.

[0004] In summary, the recycling of spent lithium iron phosphate and lithium manganese oxide batteries has gradually become a priority, and various recycling methods have emerged. However, they essentially fall into three categories: direct regeneration, pyrometallurgy, and hydrometallurgy. Other recycling methods are extensions and expansions of these three. Direct regeneration is simple but produces low-purity recycled products; pyrometallurgy is easy to operate and suitable for large-scale production, but has high recycling costs and cannot effectively recover valuable lithium metal; hydrometallurgy can achieve comprehensive recovery of valuable metal elements, but generates a large amount of waste liquid, increasing processing costs. Therefore, current efforts should focus on developing newer, greener, and more efficient recycling solutions to promote the battery recycling industry towards greater environmental friendliness and economic feasibility. Furthermore, high energy density, stable cycle performance, and excellent safety performance are the research directions and key areas of focus for modern commercial lithium-ion batteries. Lithium manganese iron phosphate (LFP) with an olivine structure exhibits excellent structural stability due to its tetrahedral phosphate framework. Furthermore, compared to lithium iron phosphate (LFP), the introduction of manganese ions results in a discharge voltage plateau of 4.0V, significantly higher than LFP's 3.4V. Therefore, LFP cathode materials can be considered a key area for research and development in next-generation lithium-ion battery cathode materials.

[0005] Currently, reported methods for preparing lithium manganese iron phosphate cathode materials from spent lithium-ion batteries mainly include mechanochemical methods and wet processes. For example, Chinese patent CN106997975A uses an inorganic acid combination wet process to recover manganese iron elements from spent lithium iron phosphate and lithium manganese oxide batteries to prepare lithium manganese iron phosphate. The entire process requires a large amount of acid reagents for leaching, and subsequently requires the consumption of alkaline solution to adjust the pH of the leaching solution to obtain the precursor. This generates a large amount of wastewater and cannot effectively recover lithium ions. In the process of preparing the cathode material, a large amount of lithium source needs to be added. The overall process is complex and costly, and the electrochemical performance of the prepared lithium manganese iron phosphate is poor, requiring further improvement. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for selectively recovering lithium salts from waste lithium-ion batteries and preparing lithium manganese iron phosphate cathode materials, so as to solve the technical problems of existing technologies for recovering cathode materials from waste lithium-ion batteries, such as the large number of reagents required, high resource consumption, and low added value of recycled materials.

[0007] To achieve the above objectives, the present invention provides a method for selectively recovering lithium salts from manganese- and iron-containing cathode materials of spent lithium-ion batteries, comprising the following steps:

[0008] (1) Waste lithium iron phosphate cathode material and waste lithium manganese oxide cathode material were separated from waste lithium-ion batteries with lithium iron phosphate and lithium manganese oxide as cathode active materials, respectively.

[0009] (2) The waste lithium iron phosphate cathode material and waste lithium manganese oxide cathode material obtained in step (1) are mixed with the leaching agent, deionized water is added and hydrothermal reaction is carried out under closed conditions to obtain the reaction solution; the leaching agent is oxalic acid or persulfate.

[0010] (3) The reaction solution described in step (2) is subjected to solid-liquid separation. The liquid phase obtained is a leachate containing lithium metal ions, and the solid phase is an organic acid complex precipitate containing manganese and iron, or a solid oxide of manganese and iron.

[0011] When the leaching agent is oxalic acid, during the hydrothermal reaction, the H+ ions generated by the oxalic acid ionization... + The crystal structure of waste lithium iron phosphate and waste lithium manganese oxide cathode materials is destroyed, releasing valuable metal ions. The strong reducing property of oxalic acid is used to reduce the high-valence metal ions to low-valence states. At the same time, the ionized oxalate ions form complexes with manganese and iron ions to generate manganese oxalate and ferrous oxalate precipitates, which enter the solid phase. Meanwhile, lithium ions and phosphate ions exist in the liquid phase, thus achieving selective leaching of lithium ions and recovery of manganese and iron ions from waste lithium-ion battery cathode materials.

[0012] When the leaching agent is persulfate, during the hydrothermal reaction, the persulfate, through the strong oxidizing property of persulfate, oxidizes the leached manganese and iron into manganese dioxide and iron phosphate, while lithium ions and sulfate ions exist in the liquid phase, thereby achieving selective leaching of lithium ions and recovery of manganese and iron ions from waste lithium iron phosphate cathode materials and waste lithium manganese oxide cathode materials.

[0013] Preferably, the persulfate is one or a combination of potassium persulfate, sodium persulfate, and ammonium persulfate.

[0014] Preferably, the total mass ratio of the waste lithium iron phosphate cathode material and the waste lithium manganese oxide cathode material to the leaching agent is 1:(2-5), more preferably 1:3.5 to 1:4.5; the solid-liquid mass ratio in the hydrothermal reaction system is 20-50 g / L, more preferably 35-45 g / L.

[0015] Preferably, the hydrothermal temperature in step (2) is 110–170°C; more preferably, it is 150–170°C; and the hydrothermal time is 60–240 min, more preferably 180–210 min.

[0016] According to another aspect of the present invention, a method for preparing lithium manganese iron phosphate cathode material based on the method is provided, further comprising the following steps:

[0017] (4) A lithium salt is obtained from the leachate containing lithium metal ions; the lithium salt is lithium phosphate or lithium carbonate;

[0018] (5) Determine the content of manganese and iron metal elements in the solid phase, and add appropriate amounts of manganese compound and iron compound to the target precursor ratio according to the element ratio in the lithium manganese iron phosphate cathode material.

[0019] (6) The lithium salt obtained in step (4), the manganese source and iron source added in step (5) to the target precursor ratio, and the phosphorus source and carbon source required according to the element ratio of lithium manganese iron phosphate cathode material are mixed evenly and sintered in an inert atmosphere to obtain lithium manganese iron phosphate cathode material.

[0020] Preferably, in step (4), the reaction solution described in step (3) is subjected to solid-liquid separation. The liquid phase obtained by using oxalic acid as a leaching agent is a leaching solution containing lithium ions, and the solid phase is a precipitate of manganese oxalate and ferrous oxalate complex. Lithium phosphate is obtained by concentrating, evaporating, crystallizing and washing the lithium-rich and phosphorus-rich leaching solution after solid-liquid separation.

[0021] The liquid phase obtained by using persulfate as a leaching agent is a leaching solution containing lithium ions, and the solid phase is a mixture of manganese dioxide and iron phosphate. Carbonate is added to the lithium-rich leaching solution after solid-liquid separation, and lithium carbonate is obtained after concentration, evaporation, crystallization and washing.

[0022] Preferably, the manganese compound added in step (5) is at least one of manganese oxalate, manganese dioxide, and manganese carbonate, and the iron compound added is at least one of ferrous oxalate, ferric phosphate, and ferrous carbonate; an appropriate amount of manganese compound and iron compound are added to adjust their ratio in the precursor so that the ratio of metallic manganese to iron in the precursor is 5:5, 6:4, 7:3, or 8:2.

[0023] Preferably, the phosphorus source added in step (6) is at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and phosphoric acid; the carbon source is at least one of sucrose, sorbitol, and polyethylene glycol.

[0024] The total molar ratio of the manganese and iron sources to the lithium in the lithium salt is 1:1.05 to 1:1.2, and the amount of carbon source added is 6 to 12 wt%. That is, the mass of the carbon source accounts for 6 to 12 wt% of the total mass of the manganese, iron, phosphorus, carbon source, and lithium salt.

[0025] Preferably, step (6) includes the following sub-steps:

[0026] (6-1) The manganese source, iron source, lithium salt, phosphorus source and part of the carbon source are placed in a high-energy ball mill, and after ball milling and mixing, they are pre-calcined under a nitrogen atmosphere to obtain the pre-calcined product.

[0027] (6-2) The pre-calcined product obtained in step (6-1) is added to the remaining carbon source and placed in a high-energy ball mill. After ball milling and mixing, it is calcined again under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material.

[0028] Preferably, the pre-calcination temperature in step (6-1) is 300-500℃, the pre-calcination time is 2-6h, and the amount of carbon source added is 40-60% of the total amount of carbon source added; the calcination temperature in step (6-2) is 600-800℃, the calcination time is 6-12h, and the amount of carbon source added is 40-60% of the total amount of carbon source added.

[0029] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0030] (1) This invention proposes a method for recycling manganese and iron-containing cathode materials from waste lithium-ion batteries, solving the problems of existing recycling technologies such as the large variety of additives, low metal ion recovery rate, and low added value of recycled products. This method first separates waste lithium iron phosphate cathode materials and waste lithium manganese oxide cathode materials from waste lithium-ion batteries using lithium iron phosphate and lithium manganese oxide as cathode active materials, respectively. Then, oxalic acid or persulfate is used under hydrothermal conditions to strongly reduce or oxidize the structure of the waste cathode materials, leaching out the lithium, manganese, and iron ions. Simultaneously, the strong reducing property of oxalic acid reduces high-valence manganese and iron ions to low-valence states, forming complex precipitates with oxalic acid. Persulfate, through its strong oxidizing property, oxidizes the leached manganese and iron ions into stable high-valence solid oxides, achieving efficient leaching and separation of metal ions. This method does not require additional co-precipitants to separate mixed waste lithium-ion batteries and achieve selective lithium ion recovery. This method has the advantages of high efficiency, environmental friendliness, low cost, simple process, and high-quality recycled products.

[0031] (2) This invention first pre-treats the waste lithium-ion batteries (waste lithium iron phosphate batteries and waste lithium manganese oxide batteries) by dismantling, crushing, alkali treatment to remove aluminum foil, acetone ball milling, and high-temperature pyrolysis to remove binders and conductive carbon black. Then, oxalic acid or persulfate is used to leach lithium, manganese, and iron ions from the waste cathode materials under hydrothermal conditions. The precipitate formed by the complexation of manganese and iron by oxalic acid and the recovered lithium phosphate are used as raw materials to prepare lithium manganese iron phosphate cathode materials. Persulfate oxidizes manganese and iron ions into manganese dioxide and iron phosphate through strong oxidation, and then uses the recovered lithium carbonate as raw materials to prepare lithium manganese iron phosphate cathode materials. The steps work together to achieve a short-process green treatment of waste lithium-ion battery cathode materials. The process is simple and consumes little reagents and energy. This invention recovers iron and lithium from waste lithium iron phosphate batteries and manganese and lithium from waste lithium manganese oxide batteries. In this way, most of the manganese and iron come from waste batteries, achieving full resource utilization of waste.

[0032] (3) This invention combines selective lithium recovery with the preparation of lithium iron phosphate cathode materials. It achieves full recovery of all elements in mixed waste lithium-ion battery cathode materials and, through ball milling nanotechnology and carbon coating technology, prepares high-performance lithium iron phosphate cathode materials from the recovered manganese, iron, and lithium salts via a two-stage solid-state sintering method. In a preferred embodiment of this invention, the lithium iron phosphate cathode material prepared and assembled into a button cell shows an initial discharge specific capacity of 166.8 mAh / g at a rate of 0.2C (1C = 170 mAh / g), a first-cycle coulombic efficiency of 96.41%, and a capacity retention rate of over 93% after 500 charge-discharge cycles at 1C. This invention simultaneously achieves highly selective lithium recovery from waste cathode materials and the preparation of high-performance electrochemical lithium iron phosphate cathode materials, realizing the full recovery and high-value utilization of waste lithium-ion battery cathode materials.

[0033] (4) This invention uses hydrothermal leaching, which has a wide operating temperature range. In addition to recovering the precious metal lithium, it also recovers the non-precious metals manganese and iron, which are not widely paid attention to in the current recycling process, and makes them valuable.

[0034] (5) The lithium recycling and lithium manganese iron phosphate cathode material preparation method proposed in this invention has a wide range of applications, simple process, no secondary pollution, low cost, good product quality and high economic value, and has good promotion prospects. It has considerable potential in the field of recycling and regeneration of waste lithium-ion batteries, especially mixed waste lithium-ion batteries. Attached Figure Description

[0035] Figure 1 The flowchart provided by the present invention describes a method for selectively recovering lithium salts from waste lithium-ion battery cathode materials and preparing lithium manganese iron phosphate cathode materials.

[0036] Figure 2 The images show X-ray diffraction (XRD) patterns of the precipitates after leaching of waste lithium-ion battery cathode materials from Examples 1 and 2.

[0037] Figure 3 The images show the XRD patterns of lithium phosphate and lithium carbonate recovered in Examples 1 and 2, respectively.

[0038] Figure 4 The images are scanning electron microscope (SEM) images of the precipitates after leaching of waste lithium-ion battery cathode materials in Examples 1 and 2.

[0039] Figure 5 The figures show a comparison of the leaching rates of lithium, manganese, iron, and phosphorus and the selectivity of lithium ions under different reaction conditions in Examples 1-5.

[0040] Figure 6The image shows the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of lithium, manganese, iron, and phosphorus in Example 1.

[0041] Figure 7 The images show the FTIR spectra of the original material and the leached precipitate before and after the hydrothermal reaction of the cathode material from the waste lithium-ion battery in Example 1.

[0042] Figure 8 X-ray diffraction (XRD) patterns of the lithium manganese iron phosphate cathode materials prepared in Examples 1 and 2.

[0043] Figure 9 The images are scanning electron microscope (SEM) images of the lithium manganese iron phosphate cathode materials prepared in Examples 1 and 2.

[0044] Figure 10 Example 1, Comparative Examples 1-4: Leaching rates of lithium, manganese, iron, and phosphorus, and lithium ion selectivity under different reaction conditions.

[0045] Figure 11 This is a comparison chart of the leaching rates of lithium, manganese, iron, and phosphorus elements and the lithium selectivity under different reaction systems in Example 1 and Comparative Example 5.

[0046] Figure 12 The graphs show the specific capacity curves of the lithium manganese iron phosphate cathode materials prepared in Examples 1-3 at an initial charge-discharge rate of 0.2C.

[0047] Figure 13 The discharge specific capacity curves of the lithium manganese iron phosphate cathode materials prepared in Examples 1-3 after 500 cycles at 1C rate are shown. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0049] In the description of this invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0050] Furthermore, throughout this specification, references to "an embodiment"; "an embodiment," "an example," or similar language indicate that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Therefore, the appearance of the phrase "in one embodiment;" throughout this specification, and similar language, may, but not necessarily, refer to the same embodiment.

[0051] This invention provides a method for selectively recovering lithium salts from manganese- and iron-containing cathode materials of spent lithium-ion batteries, comprising the following steps:

[0052] (1) Waste lithium iron phosphate cathode material and waste lithium manganese oxide cathode material were separated from waste lithium-ion batteries with lithium iron phosphate and lithium manganese oxide as cathode active materials, respectively.

[0053] (2) The waste lithium iron phosphate cathode material and waste lithium manganese oxide cathode material obtained in step (1) are mixed with the leaching agent, deionized water is added and hydrothermal reaction is carried out under closed conditions to obtain the reaction solution; the leaching agent is oxalic acid or persulfate.

[0054] (3) The reaction solution described in step (2) is subjected to solid-liquid separation. The liquid phase obtained is a leachate containing lithium metal ions, and the solid phase is an organic acid complex precipitate containing manganese and iron, or a solid oxide of manganese and iron.

[0055] When the leaching agent is oxalic acid, during the hydrothermal reaction, the H+ ions generated by the oxalic acid ionization... + The crystal structure of waste lithium iron phosphate and waste lithium manganese oxide cathode materials is destroyed, releasing valuable metal ions. The strong reducing property of oxalic acid is used to reduce the high-valence metal ions to low-valence states. At the same time, the ionized oxalate ions form complexes with manganese and iron ions to generate manganese oxalate and ferrous oxalate precipitates, which enter the solid phase. Meanwhile, lithium ions and phosphate ions exist in the liquid phase, thus achieving selective leaching of lithium ions and recovery of manganese and iron ions from waste lithium-ion battery cathode materials.

[0056] When the leaching agent is persulfate, during the hydrothermal reaction, the persulfate, through the strong oxidizing property of persulfate, oxidizes the leached manganese and iron into manganese dioxide and iron phosphate, while lithium ions and sulfate ions exist in the liquid phase, thereby achieving selective leaching of lithium ions and recovery of manganese and iron ions from waste lithium iron phosphate cathode materials and waste lithium manganese oxide cathode materials.

[0057] In some embodiments, step (1) of separating and recycling waste cathode materials from waste lithium iron phosphate batteries and waste lithium manganese oxide batteries specifically includes the following steps: discharging the waste lithium-ion batteries, then disassembling and removing the parts other than the lithium-ion battery cathode materials to obtain waste cathode materials.

[0058] In some embodiments, dismantling can be pre-processed by manual dismantling or by integrated industrial equipment. Pre-processed manual dismantling generally includes steps such as manual dismantling, crushing, alkali treatment, and calcination; pre-processed integrated industrial equipment generally includes steps such as mechanical crushing, magnetic separation, air separation, and calcination.

[0059] In some embodiments, the manual pretreatment disassembly specifically involves: connecting the waste lithium-ion battery to a discharger for discharge; after discharge, manually cutting open the battery casing, removing the positive and negative electrode tabs, and then separately collecting the positive electrode sheet, negative electrode sheet, and separator. Crushing involves: using scissors to cut the positive electrode sheet containing aluminum foil into approximately 1cm × 1cm fragments; mixing the crushed positive electrode sheet with 2mol / L NaOH or KOH and stirring until no more bubbles are produced in the reaction solution, indicating that the aluminum foil has basically dissolved; filtering the solution and washing it multiple times with deionized water; and drying the resulting black solid powder in a 60℃ oven. Ball milling to remove binder and conductive carbon black specifically involves: adding the black solid powder to acetone at a 1:5 mass ratio, ball milling the positive electrode sheet at 200–500 r / min for 4–6 hours using a high-energy ball mill, and calcining it at 550–650℃ for 2–4 hours under a nitrogen atmosphere to remove the binder; finally, the waste positive electrode material is obtained.

[0060] In other embodiments, waste cathode materials are also obtained through large-scale comprehensive physical sorting using equipment such as mechanical crushing, air separation, and magnetic separation. Specifically, waste lithium-ion batteries are connected to a discharger for discharge. After discharge, they are fed into an integrated lithium battery crushing and sorting device. The batteries are coarsely crushed and sorted to obtain a mixture containing cathode materials, negative electrode graphite, iron sheets, aluminum foil, and copper foil. The mixture is then passed through an air separation device to separate non-metallic materials such as negative electrode graphite. Copper and aluminum are separated by a heating device, resulting in a black solid powder. Finally, the black solid powder is placed in a vacuum tube furnace and calcined at 550–650°C for 3–5 hours to obtain waste lithium-ion cathode materials.

[0061] In some embodiments, the persulfate is one or more of potassium persulfate, sodium persulfate, and ammonium persulfate, preferably sodium persulfate.

[0062] In some embodiments, the total mass ratio of the waste lithium iron phosphate cathode material and the waste lithium manganese oxide cathode material to the leaching agent is 1:(2-5), preferably 1:3.5 to 1:4.5; the solid-liquid mass ratio in the hydrothermal reaction system is 20-50 g / L, preferably 35-45 g / L.

[0063] The ratio of the waste lithium iron phosphate cathode material to the waste lithium manganese oxide cathode material is feasible within a wide range. When finally preparing the lithium manganese iron phosphate cathode material, it can be supplemented according to the respective contents of the solid and liquid phases after solid-liquid separation. This does not affect the recovery of lithium ions, iron salts, and manganese salts, nor does it affect the preparation of the lithium manganese iron phosphate cathode material. Usually, the mass ratio of the two is set to 1-2:1.

[0064] In some embodiments, the hydrothermal temperature in steps (1) and (2) is 110–170°C; preferably 150–170°C; and the hydrothermal time is 60–240 min, preferably 180–210 min.

[0065] In some embodiments, the solid-liquid separation in step (2) is a conventional solid-liquid separation method such as atmospheric pressure filtration.

[0066] This invention also provides a method for preparing lithium manganese iron phosphate cathode material based on the above method, which further includes the following steps:

[0067] (4) A lithium salt is obtained from the leachate containing lithium metal ions; the lithium salt is lithium phosphate or lithium carbonate;

[0068] (5) Determine the content of manganese and iron metal elements in the solid phase, and add appropriate amounts of manganese compound and iron compound to the target precursor ratio according to the element ratio in the lithium manganese iron phosphate cathode material.

[0069] (6) The lithium salt obtained in step (4), the manganese source and iron source added in step (5) to the target precursor ratio, and the phosphorus source and carbon source required according to the element ratio of lithium manganese iron phosphate cathode material are mixed evenly and sintered in an inert atmosphere to obtain lithium manganese iron phosphate cathode material.

[0070] In some embodiments, step (4) involves solid-liquid separation of the reaction solution described in step (3). The liquid phase obtained by using oxalic acid as a leaching agent is a leaching solution containing lithium ions, and the solid phase is a precipitate of manganese oxalate and ferrous oxalate complex. The lithium- and phosphorus-rich leaching solution after solid-liquid separation is concentrated (pH adjusted to 9-10 with ammonia and dilute phosphoric acid), evaporated, crystallized, and washed to obtain lithium phosphate.

[0071] The liquid phase obtained by using persulfate as a leaching agent is a leaching solution containing lithium ions, and the solid phase is a mixture of manganese dioxide and iron phosphate. Carbonate is added to the lithium-rich leaching solution after solid-liquid separation, and lithium carbonate is obtained after concentration (adjusting the pH to 8-10 with sodium bicarbonate and acetic acid), evaporation, crystallization, and washing.

[0072] In the hydrothermal reaction process of this invention, oxalic acid or persulfate, under hydrothermal conditions, strongly reduces or oxidizes the structure of the waste cathode material, leaching out metallic lithium, manganese, and iron ions. Simultaneously, it is speculated that the strong reducing property of oxalic acid reduces high-valence manganese and iron ions to low-valence states, forming complex precipitates with oxalic acid radicals; while persulfate, through its strong oxidizing property, oxidizes the leached manganese and iron ions into stable high-valence solid oxides, achieving efficient leaching and separation of metal ions. This method does not require additional co-precipitants to separate mixed waste lithium-ion batteries and achieve selective lithium-ion recovery.

[0073] In some embodiments, the manganese compound added in step (5) is at least one of manganese oxalate, manganese dioxide, and manganese carbonate, and the iron compound added is at least one of ferrous oxalate, ferric phosphate, and ferrous carbonate; an appropriate amount of manganese and iron compounds are added to adjust their ratio in the precursor so that the ratio of metallic manganese to iron in the precursor is 5:5, 6:4, 7:3, or 8:2.

[0074] In some embodiments, the phosphorus source added in step (6) is at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and phosphoric acid; the carbon source is at least one of sucrose, sorbitol, and polyethylene glycol.

[0075] The total molar ratio of the manganese and iron sources to the lithium in the lithium salt is 1:1.05 to 1:1.2, and the amount of carbon source added is 6 to 12 wt%. That is, the mass of the carbon source accounts for 6 to 12 wt% of the total mass of the manganese, iron, phosphorus, carbon source, and lithium salt.

[0076] In some embodiments, step (6) includes the following sub-steps:

[0077] (6-1) The manganese source, iron source, lithium salt, phosphorus source and part of the carbon source are placed in a high-energy ball mill, and after ball milling and mixing, they are pre-calcined under a nitrogen atmosphere to obtain the pre-calcined product.

[0078] (6-2) The pre-calcined product obtained in step (6-1) is added to the remaining carbon source and placed in a high-energy ball mill. After ball milling and mixing, it is calcined again under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material.

[0079] In some embodiments, the pre-calcination temperature in step (6-1) is 300–500°C, and the pre-calcination time is 2–6 h; the amount of carbon source added is 40–60% of the total carbon source added; in step (6-2), the calcination temperature is 600–800°C, the calcination time is 6–12 h, and the amount of carbon source added is 40–60% of the total carbon source added.

[0080] This invention utilizes oxalic acid or persulfate under hydrothermal conditions to break down the structure of waste cathode materials, leaching lithium-ion metal from the cathode materials. Simultaneously, the reducing properties of oxalic acid are used to reduce high-valence manganese and iron metal ions to low-valence states, forming complex precipitates with oxalate ions. Persulfate, through the strong oxidizing properties of persulfate ions, oxidizes the leached manganese and iron ions into stable solid oxides, thus achieving efficient leaching and separation of metal elements. The leachate is then concentrated, crystallized, evaporated, and washed to obtain lithium phosphate or lithium carbonate, which is then ball-milled and mixed with filtered manganese and iron salts before solid-state sintering to obtain lithium manganese iron phosphate cathode materials. The advantage of this method is that it eliminates the need for additional co-precipitants, achieving the separation of mixed waste lithium-ion batteries and selective recovery of lithium-ion metals. This reduces production costs and solves the problems of existing technologies for recycling waste lithium-ion battery cathode materials, such as the need for numerous additives, difficulties in separating different types of batteries, and low added value of recycled cathode materials. This invention also proposes a novel method for selectively recovering lithium salts from spent lithium-ion batteries containing mixed manganese and iron, and preparing lithium manganese iron phosphate cathode materials. This method enables the efficient recovery of manganese and iron from the cathode materials of mixed spent lithium-ion batteries, reducing resource waste and improving the overall recycling rate of spent lithium-ion batteries. The method features a simple process flow, convenient operation, and high economic benefits and environmental friendliness.

[0081] The following is an example:

[0082] Example 1

[0083] This embodiment utilizes a method for the hydrothermal selective recovery of lithium salts from oxalic acid and the preparation of lithium iron manganese phosphate cathode materials, such as... Figure 1 As shown, it includes the following steps:

[0084] (1) Discharge the waste lithium iron phosphate lithium-ion battery to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheet. Cut the positive electrode sheet into fragments of about 1cm×1cm. Mix the broken positive electrode sheet with 2mol / L NaOH and stir until no bubbles are produced. Filter the solution and wash it several times with deionized water. Place the obtained black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5. Use a ball mill to ball mill the positive electrode sheet at a speed of 400r / min for 4h. Then calcine it at 650℃ for 4h under a nitrogen atmosphere to remove the binder. Finally, the waste lithium iron phosphate positive electrode material is obtained.

[0085] (2) Process the waste lithium manganese oxide batteries in the same way as in step (1) to obtain waste lithium manganese oxide cathode material.

[0086] (3) Place 22.5 ml of deionized water in a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of oxalic acid dihydrate. The hydrothermal reactor is subjected to hydrothermal reaction at 160 °C for 180 min. After cooling, filter to obtain leachate and leachate precipitate. Then concentrate (adjust pH to 9.5 with ammonia and dilute phosphoric acid), evaporate, crystallize and wash to obtain lithium phosphate.

[0087] (4) The leaching rate of metal elements was determined by ICP-OES, and the contents of manganese and iron metal elements in the complex precipitate were obtained. Appropriate amounts of manganese oxalate and ferrous oxalate were added to make the manganese-iron ratio in the precursor 6:4.

[0088] (5) Lithium phosphate, manganese oxalate, ferrous oxalate and ammonium dihydrogen phosphate were ball-milled and mixed at a molar ratio of lithium manganese iron phosphorus 1.05:0.6:0.4:1, and 6 wt% sucrose was added as a carbon source. The mixture was then calcined at 450°C for 4 h to obtain a pre-calcined product. 6 wt% sucrose was added to the pre-calcined product again and ball-milled and mixed. The mixture was then calcined at 700°C for 10 h to obtain lithium manganese iron phosphorus cathode material.

[0089] (6) Using lithium sheet as negative electrode, the positive electrode material is assembled into a button half cell. The initial discharge specific capacity at a rate of 0.2C (1C = 170mAh / g) is 166.8mAh / g, the first-cycle coulombic efficiency reaches 96.41%, and the capacity retention rate can reach more than 93% after 500 charge-discharge cycles at 1C.

[0090] Figure 2 Example 1 shows the XRD pattern of the precipitate after hydrothermal selective leaching of metal elements using oxalic acid as the leaching agent. As can be seen from the figure, the main components of the precipitate are MnC₂O₄·2H₂O and FeC₂O₄·2H₂O.

[0091] Figure 3 Example 1 describes the lithium phosphate product obtained after hydrothermal selective leaching using oxalic acid as the leaching agent, followed by concentration, evaporation, crystallization, and washing of the leachate. The diffraction peak positions and intensities of the product are consistent with those of the standard card, indicating that the recovered lithium phosphate has low impurity content and high purity.

[0092] Figure 4 Example 1 shows the SEM image of the precipitate after hydrothermal selective leaching of metal elements using oxalic acid as the leaching agent. The image shows that the precipitate particle size is between 10 and 20 μm, with rod-shaped particles being FeC₂O₄·2H₂O and irregular ellipsoidal particles being MnC₂O₄·2H₂O.

[0093] Figure 5It can be seen that the leaching rates of manganese and iron ions in the leachate of this embodiment are low, both below 5%, while lithium ions have a leaching rate of 98% and phosphorus ions have a leaching rate of 100%, exhibiting high selectivity. The manganese and iron ions in the cathode material of waste lithium-ion batteries are selectively separated from lithium-ion metals in the precipitate by the method of this embodiment.

[0094] Figure 6 The figures show XPS high-resolution energy dispersive spectroscopy (EDS) spectra of lithium, manganese, iron, and phosphorus in the precipitate and the original material obtained by hydrothermal selective leaching of metal ions using oxalic acid as the leaching agent in Example 1. As can be seen from the figures, the diffraction peak intensities of lithium and phosphorus are low after leaching, indicating that they are essentially absent in the precipitate and have largely entered the liquid phase. Manganese, which has +3 and +4 valences in the original material, has a +2 valence in the precipitate. Similarly, iron, which has +2 and +3 valences in the original material, has a +2 valence in the precipitate. This indicates that during the leaching process, the high-valence metal ions were reduced to lower valence states by oxalate and formed complex precipitates.

[0095] Figure 7 The figures show the FTIR spectra of the precipitate and the original material obtained by hydrothermal selective leaching of metal ions using oxalic acid as the leaching agent in Example 1. The figures show that the 1640 cm⁻¹ of the raw material... -1 The C=O antisymmetric tensile vibration absorption peak at 470-1130 cm⁻¹ may be due to incompletely removed carbonaceous material. -1 All are PO4 3- Absorption peak, 960 cm⁻¹ -1 PO4 3- Symmetrical stretching vibration, 630cm -1 Corresponding to PO4 3- Asymmetric bending vibration, 568cm -1 502cm -1 469cm -1 Corresponding to PO4 3- Bending vibrations correspond to the characteristic groups of phosphate ions in waste lithium iron phosphate cathode materials, while the leached precipitate contains phosphate groups at 1360 cm⁻¹. -1 1306cm -1 The point is a CO tensile vibration, 774 cm. -1 733cm -1 Corresponding to CO bending vibration, 810 cm -1 Corresponding to OH…O, 498cm -1 The corresponding plane rocking vibration peak of COOH confirms the presence of oxalate and also indicates that all phosphorus has leached into the liquid phase.

[0096] Example 2

[0097] This embodiment utilizes a method for thermally selectively recovering lithium salts from persulfate brine and preparing lithium iron manganese phosphate cathode materials, comprising the following steps:

[0098] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheet. Cut the positive electrode sheet into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheet with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheet at 400r / min for 6h. Calcine it at 600℃ for 4h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0099] (2) Place 22.5 ml of deionized water in a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of sodium persulfate. The hydrothermal reactor is subjected to hydrothermal reaction at 160 °C for 180 min. After cooling, filter to obtain leachate and leachate precipitate. Add sodium carbonate to the lithium-rich leachate after solid-liquid separation, concentrate (adjust pH to 9.5 with sodium bicarbonate and acetic acid), evaporate, crystallize and wash to obtain lithium carbonate.

[0100] (3) The leaching rate of metal elements was determined by ICP-OES, and the contents of manganese and iron metal elements in the complex precipitate were obtained. Appropriate amounts of manganese dioxide and iron phosphate were added to make the manganese-iron ratio in the precursor 7:3.

[0101] (4) Lithium carbonate, manganese dioxide, iron phosphate and ammonium dihydrogen phosphate were mixed by ball milling at a molar ratio of lithium manganese iron phosphorus 1.05:0.7:0.3:1, and 6 wt% sucrose and polyethylene glycol were added as carbon sources. The mixture was then calcined at 350°C for 4 h to obtain a pre-calcined product. The pre-calcined product was then mixed again with 6 wt% sucrose and polyethylene glycol, and then calcined at 650°C for 8 h to obtain lithium manganese iron phosphate cathode material.

[0102] (5) Using lithium sheet as negative electrode, the positive electrode material is assembled into a button half cell. The initial discharge specific capacity at a rate of 0.2C (1C = 170mAh / g) is 152.5mAh / g, the first-cycle coulombic efficiency reaches 96.6%, and the capacity retention rate can reach more than 91.99% after 500 charge-discharge cycles at 1C.

[0103] Figure 2Example 2 shows the XRD pattern of the precipitate after hydrothermal selective leaching of metal elements using sodium persulfate as the leaching agent. As can be seen from the figure, the main components of the precipitate are MnO2 and FePO4.

[0104] Figure 3 Example 2 describes a lithium carbonate product obtained by hydrothermal selective leaching using sodium persulfate as the leaching agent, followed by the addition of sodium carbonate as a precipitant, concentration, evaporation, crystallization, and washing. The diffraction peak positions and intensities of the product are consistent with those of the standard card, indicating that the recovered lithium carbonate has low impurity content and high purity.

[0105] Figure 4 Example 2 shows the SEM image of the precipitate after hydrothermal selective leaching of metal elements using sodium persulfate as the leaching agent. The image shows that the precipitate particle size is between 5 and 10 μm, with spherical particles being MnO2 nanoflowers and some irregular particles being FePO4.

[0106] Figure 5 It can be seen that the leaching rates of manganese and iron ions in the leachate of this embodiment are relatively low, with a leaching rate of 4.82% for manganese, 1.77% for iron, and 15.98% for phosphorus. In contrast, lithium ions have a leaching rate of 98%, exhibiting high selectivity. The manganese and iron ions in the cathode material of waste lithium-ion batteries are selectively separated from lithium-ion metals in the precipitate by the method of this embodiment.

[0107] Figure 8 The XRD patterns of lithium manganese iron phosphate cathode materials prepared in Examples 1 and 2 are shown. The diffraction peak (200) / (020) values ​​in the figures are relatively high, indicating that the material has good crystallinity. The XRD patterns are consistent with the standard card, indicating that the prepared lithium manganese iron phosphate cathode material has a good structure and low impurities.

[0108] Figure 9 The images show scanning electron microscope (SEM) images of lithium manganese iron phosphate cathode materials prepared in Examples 1 and 2. As can be seen from the images, the prepared lithium manganese iron phosphate materials are uniform in size, with a particle size between 300 and 500 nm, and the particles do not show obvious agglomeration, indicating that the material has good physical properties.

[0109] Example 3

[0110] This embodiment utilizes a method for the hydrothermal selective recovery of lithium salts from oxalic acid and the preparation of lithium iron manganese phosphate cathode materials, comprising the following steps:

[0111] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheets. Cut the positive electrode sheets into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheets with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheets at 500r / min for 4h. Calcine the positive electrode sheets at 650℃ for 2h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0112] (2) Place 22.5 ml of deionized water in a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.8 g of oxalic acid dihydrate. The hydrothermal reactor is subjected to hydrothermal reaction at 170 °C for 180 min. After cooling, filter to obtain leachate and leachate precipitate. Then concentrate (adjust pH to 9 with ammonia and dilute phosphoric acid) the separated lithium-rich and phosphorus-rich leachate, evaporate, crystallize and wash to obtain lithium phosphate.

[0113] (3) The leaching rate of metal elements was determined by ICP-OES, and the contents of manganese and iron metal elements in the complex precipitate were obtained. Appropriate amounts of manganese carbonate and ferrous carbonate were added to make the manganese-iron ratio in the precursor 6:4.

[0114] (4) Lithium phosphate, manganese source (manganese carbonate and manganese oxalate), iron source (ferrous carbonate and ferrous oxalate) and phosphoric acid were ball-milled and mixed at a molar ratio of lithium manganese iron phosphorus 1.05:0.6:0.4:1, and 6 wt% sorbitol was added as a carbon source. The mixture was then calcined at 450°C for 4 h to obtain a pre-calcined product. 6 wt% sorbitol was added to the pre-calcined product again and ball-milled and mixed. The mixture was then calcined at 750°C for 10 h to obtain lithium manganese iron phosphorus cathode material.

[0115] (5) Using lithium sheet as negative electrode, the positive electrode material is assembled into a button half cell. The initial discharge specific capacity at a rate of 0.2C (1C = 170mAh / g) is 148mAh / g, the first-cycle coulombic efficiency reaches 93.6%, and the capacity retention rate can reach more than 85.79% after 500 charge-discharge cycles at 1C.

[0116] Example 4

[0117] This embodiment utilizes a method for the hydrothermal selective recovery of lithium salts from oxalic acid and the preparation of lithium iron manganese phosphate cathode materials, comprising the following steps:

[0118] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, and then put them into an integrated lithium battery crushing and sorting equipment. The batteries are coarsely crushed and sorted to obtain a mixture containing positive electrode material, negative electrode graphite, iron sheets, aluminum foil, and copper foil. The non-metallic materials such as negative electrode graphite are separated by an air classifier, and copper and aluminum are separated by a heating device to obtain a black solid powder. The black solid powder is then added to acetone at a mass ratio of 1:5, and the positive electrode sheet is ball-milled at 500 r / min for 5 h. The positive electrode sheet is then calcined at 600℃ for 4 h in a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0119] (2) Place 22.5 ml of deionized water in a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of oxalic acid dihydrate. The hydrothermal reactor is subjected to hydrothermal reaction at 160 °C for 150 min. After cooling, filter to obtain leachate and leachate precipitate. Then concentrate (adjust pH to 9.5 with ammonia and dilute phosphoric acid), evaporate, crystallize and wash to obtain lithium phosphate.

[0120] (3) The leaching rate of metal elements was determined by ICP-OES, and the contents of manganese and iron metal elements in the complex precipitate were obtained. Appropriate amounts of manganese oxalate and ferrous oxalate were added to make the manganese-iron ratio in the precursor 6:4.

[0121] (4) Lithium phosphate, manganese oxalate, ferrous oxalate and ammonium hydrogen phosphate were ball-milled and mixed at a molar ratio of lithium manganese iron phosphorus 1.2:0.6:0.4:1, and 6 wt% sucrose and sorbitol were added as carbon sources. The mixture was then calcined at 450°C for 4 h to obtain a pre-calcined product. 6 wt% sucrose and sorbitol were added to the pre-calcined product again and ball-milled and mixed. The mixture was then calcined at 700°C for 10 h to obtain lithium manganese iron phosphorus cathode material.

[0122] like Figure 5 As shown, this embodiment uses industrial equipment for dismantling and sorting. The manganese content in the leachate is 6.16 wt%, and the iron content is approximately 7.2 wt%. It can be seen that whether pre-treatment is performed manually or using industrial equipment, as long as the waste lithium-ion batteries are pre-treated sequentially to obtain waste lithium-ion cathode materials, the leaching rate is basically the same when using the method of this invention for leaching.

[0123] Example 5

[0124] This embodiment utilizes a method for thermally selectively recovering lithium salts from persulfate brine and preparing lithium iron manganese phosphate cathode materials, comprising the following steps:

[0125] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheet. Cut the positive electrode sheet into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheet with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheet at 500r / min for 4h. Calcine it at 650℃ for 4h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0126] (2) Place 22.5 ml of deionized water in a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of sodium persulfate and ammonium persulfate. The hydrothermal reactor is subjected to hydrothermal reaction at 150 °C for 180 min. After cooling, filter to obtain leachate and leachate precipitate. Add sodium carbonate to the lithium-rich leachate after solid-liquid separation, concentrate (adjust pH to 9 with ammonium bicarbonate and acetic acid), evaporate, crystallize and wash to obtain lithium carbonate.

[0127] (3) The leaching rate of metal elements was determined by ICP-OES, and the contents of manganese and iron metal elements in the complex precipitate were obtained. Appropriate amounts of manganese dioxide and iron phosphate were added to make the manganese-iron ratio in the precursor 6:4.

[0128] (4) Lithium carbonate, manganese dioxide, iron phosphate and ammonium dihydrogen phosphate were ball-milled and mixed at a molar ratio of lithium manganese iron phosphorus 1.2:0.6:0.4:1, and 5 wt% sucrose was added as a carbon source. The mixture was then calcined at 400°C for 4 h to obtain a pre-calcined product. 5 wt% sucrose was added to the pre-calcined product again and ball-milled and mixed. The mixture was then calcined at 700°C for 8 h to obtain lithium manganese iron phosphate cathode material.

[0129] To facilitate comparison of the necessity of the method described in this invention, the advantages of this invention will be explained below with reference to comparative examples.

[0130] Comparative Example 1

[0131] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheets. Cut the positive electrode sheets into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheets with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheets at 500r / min for 4h. Calcine the positive electrode sheets at 650℃ for 2h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0132] (2) Place 22.5 ml of deionized water in a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of oxalic acid dihydrate, and perform hydrothermal reaction at 160 °C for 60 min. After cooling, filter to obtain leachate and leachate precipitate.

[0133] The conditions for this comparative example are the same as in Example 1, except for the hydrothermal reaction time. In Example 1, the reaction time was 180 minutes, while in this comparative example it was only 60 minutes. Figure 10 It can be seen that the leaching rates of manganese and iron ions in the comparative example leachate were 25.05% and 43.79%, respectively, which were about 20% and 40% higher than those in Example 1. This comparative example illustrates that during the hydrothermal reaction of oxalic acid, oxalic acid first ionizes to release H+. + The process involves disrupting the crystal structure of the material, releasing the metal ions, further reducing the high-valence metal ions, and finally complexing them. In this comparative example, the hydrothermal time was relatively short. Although most of the metal ions could be leached out, the reduction and complexation reactions could not be completely completed. Therefore, a suitable hydrothermal time has a strong effect on the stable operation of the system.

[0134] Comparative Example 2

[0135] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheets. Cut the positive electrode sheets into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheets with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheets at 500r / min for 4h. Calcine the positive electrode sheets at 650℃ for 2h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0136] (2) 22.5 ml of deionized water was placed in a hydrothermal reactor, and 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of citric acid monohydrate were added. The hydrothermal reactor was subjected to hydrothermal reaction at 160 °C for 180 min. After cooling, the leachate and leachate precipitate were obtained by filtration. The leaching rate of metal elements was determined by ICP-OES.

[0137] from Figure 10 It can be seen that the leaching rates of manganese and iron in this comparative example are relatively high, reaching 91.93% and 62.17% respectively, both higher than the leaching rates of manganese and iron in the waste lithium-ion cathode material in Example 1. This indicates that citrate has a weaker complexing ability for manganese and iron ions than oxalate, and the corresponding selectivity for lithium is also lower.

[0138] In addition, citric acid was replaced with tartaric acid and malic acid using the same method. However, the experiment found that tartaric acid and malic acid, as organic acids, have poor complexing ability of their organic acid anions for manganese and iron ions, and correspondingly, their lithium selectivity is also low.

[0139] Comparative Example 3

[0140] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheets. Cut the positive electrode sheets into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheets with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheets at 500r / min for 4h. Calcine the positive electrode sheets at 650℃ for 2h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0141] (2) Place a 22.5 ml sulfuric acid solution with a concentration of 2 mol / L into a hydrothermal reactor, add 0.1 g of waste lithium iron phosphate cathode material and 0.1 g of waste lithium manganese oxide cathode material, and perform hydrothermal reaction at 160 °C for 180 min. After cooling, filter to obtain leachate and leachate precipitate.

[0142] The difference between this comparative example and Example 2 is that it uses the inorganic acid sulfuric acid as the leaching agent, from... Figure 10 It can be seen that the leaching rates of manganese and iron ions in the comparative example leachate were 51.42% and 95.96%, respectively, the leaching rate of phosphorus was 82.99%, and the leaching rate of lithium was only 84.48%. The selectivity for lithium was poor, indicating that under certain conditions, the use of inorganic acid sulfuric acid cannot achieve a good selective leaching effect for lithium from waste lithium manganese oxide and lithium iron phosphate cathode materials.

[0143] Comparative Example 4

[0144] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheets. Cut the positive electrode sheets into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheets with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheets at 500r / min for 4h. Calcine the positive electrode sheets at 650℃ for 2h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0145] (2) A 20 ml solution of phosphoric acid with a concentration of 2 mol / L and 2.5 ml of 30 wt% hydrogen peroxide solution were placed in a hydrothermal reactor. 0.1 g of waste lithium iron phosphate cathode material and 0.1 g of waste lithium manganese oxide cathode material were added. The hydrothermal reactor was subjected to hydrothermal reaction at 160 °C for 180 min. After cooling, the solution and leaching precipitate were obtained by filtration.

[0146] The difference between this comparative example and Example 1 is that it uses an inorganic acid phosphoric acid leaching agent and hydrogen peroxide as a reducing agent, from Figure 10 It can be seen that the leaching rates of manganese and iron ions in the comparative example leachate were 95.33% and 94.01%, respectively, while the lithium leaching rate was 91.81%. The selectivity for lithium was poor, indicating that under certain conditions, using inorganic acid as a leaching agent and hydrogen peroxide as a reducing agent cannot achieve a good lithium selectivity effect for leaching waste lithium manganese oxide and lithium iron phosphate cathode materials.

[0147] Table 1 compares the leaching rates of lithium, manganese, iron, and phosphorus metal ions under different reaction conditions in Examples 1, 2, and Comparative Examples 1-4. It can be seen that the one-step hydrothermal leaching using oxalic acid and sodium persulfate in the examples of this invention, except for oxalic acid, shows that the selectivity of the other organic acids for lithium ions is not very high. This may be related to the different acidities of the organic acids and their complexation with manganese and iron ions. The selectivity of the other inorganic acid oxidants and reducing agents for lithium ions is also poor. Experiments showed that the leaching rates of lithium ions in these comparative examples were lower than those in the examples to varying degrees, but the leaching rates of manganese and iron ions were higher than those in the examples to varying degrees. These differences are not conducive to the separation of manganese and iron ions from lithium ions, nor to the selective recovery of lithium ions.

[0148] Table 1. Comparison of leaching rates of lithium, manganese, iron ions and phosphorus under different reaction conditions in Examples 1, 2, and Comparative Examples 1-4.

[0149]

[0150] Comparative Example 5

[0151] (1) Discharge the waste lithium iron phosphate batteries to a voltage ≤2V using a discharger, then manually disassemble and separate the positive electrode sheets. Cut the positive electrode sheets into fragments of approximately 1cm × 1cm. Mix the broken positive electrode sheets with 2mol / L NaOH and stir until no more bubbles are produced. Filter the solution and wash it several times with deionized water. Place the resulting black solid powder in a 60℃ oven to dry. Then add acetone to the black solid powder at a mass ratio of 1:5 and ball mill the positive electrode sheets at 500r / min for 6h. Calcine the positive electrode sheets at 650℃ for 4h under a nitrogen atmosphere to remove the binder. Finally, waste lithium iron phosphate positive electrode material is obtained. The same method is used to process waste lithium manganese oxide batteries to obtain waste lithium manganese oxide positive electrode material.

[0152] (2) Place 22.5 ml of deionized water in a zirconia ball mill jar, add 0.1 g of waste lithium iron phosphate cathode material, 0.1 g of waste lithium manganese oxide cathode material and 0.7 g of oxalic acid dihydrate, and add zirconia balls with a particle size of 3 mm / 2 mm / 1 mm in a ball-to-material ratio of 10:1. After mixing, ball mill at 400 rpm for 180 min in a high-energy ball mill. After ball milling, rinse the ball mill jar with deionized water, filter to obtain leachate and leachate residue, and use ICP-OES to determine the leaching rate of metal elements.

[0153] In Comparative Example 5, Example 1, the hydrothermal method was changed to a mechanochemical method. Figure 11 The graph shows a comparison of the metal element leaching rates of Example 1 and Comparative Example 5. It can be seen from the graph that, under the same conditions, the hydrothermal process achieves a higher lithium-ion leaching rate and better leaching selectivity compared to the mechanochemical method. This may be because the high temperature and high pressure conditions provided by the hydrothermal process are conducive to H... + It disrupts the crystal structure of the cathode material, but at the same time it helps to enhance the complexation of organic acid radicals with manganese and iron ions.

[0154] Figure 12 The graphs show the initial charge-discharge specific capacity curves of the lithium manganese iron phosphate cathode materials prepared in Examples 1-3 at 0.2C. As can be seen from the graphs, the initial discharge specific capacity and first-cycle coulombic efficiency of the battery in Example 1 are higher than those in Examples 2 and 3, followed by Example 2, and the initial discharge specific capacity of Example 3 is the lowest. Figure 13 The data presented are the charge-discharge cycle data of the batteries in Examples 1 to 3. After 500 cycles at 1C, the battery in Example 1 has a higher capacity retention rate, indicating that different types of lithium salt, manganese source, iron source, phosphorus source, and carbon source, different manganese-iron ratios in the precursor, and different calcination temperatures have a significant impact on the electrochemical performance of the electrode materials.

[0155] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for selectively recovering lithium salts from a manganese, iron cathode material of a spent lithium-ion battery, characterized in that, Includes the following steps: (1) Waste lithium iron phosphate cathode material and waste lithium manganese oxide cathode material were separated from waste lithium-ion batteries with lithium iron phosphate and lithium manganese oxide as cathode active materials, respectively. (2) The waste lithium iron phosphate cathode material and waste lithium manganese oxide cathode material obtained in step (1) are mixed with the leaching agent, deionized water is added and hydrothermal reaction is carried out under closed conditions to obtain the reaction solution; the leaching agent is oxalic acid or persulfate. (3) The reaction solution described in step (2) is subjected to solid-liquid separation. The liquid phase obtained is a leachate containing lithium metal ions, and the solid phase is an organic acid complex precipitate containing manganese and iron, or a solid oxide of manganese and iron.

2. The method of claim 1, wherein, The persulfate is one or more of potassium persulfate, sodium persulfate, and ammonium persulfate.

3. The method of claim 1, wherein, The total mass ratio of the waste lithium iron phosphate cathode material and the waste lithium manganese oxide cathode material to the leaching agent is 1:(2-5); the solid-liquid ratio in the hydrothermal reaction system is 20-50 g / L.

4. The method of claim 1, wherein, The hydrothermal temperature in step (2) is 110–170°C; the hydrothermal time is 60–240 min.

5. A method for producing a lithium iron manganese phosphate cathode material based on the method according to any one of claims 1 to 4, characterized in that It also includes the following steps: (4) A lithium salt is obtained from the leachate containing lithium metal ions; the lithium salt is lithium phosphate or lithium carbonate; (5) Determine the content of manganese and iron metal elements in the solid phase, and add appropriate amounts of manganese compound and iron compound to the target precursor ratio according to the element ratio in the lithium manganese iron phosphate cathode material. (6) The lithium salt obtained in step (4), the manganese source and iron source added in step (5) to the target precursor ratio, and the phosphorus source and carbon source required according to the element ratio of lithium manganese iron phosphate cathode material are mixed evenly and sintered in an inert atmosphere to obtain lithium manganese iron phosphate cathode material.

6. The method of claim 5, wherein, Step (4) The reaction solution described in step (3) is subjected to solid-liquid separation. The liquid phase obtained by using oxalic acid as the leaching agent is a leaching solution containing lithium ions, and the solid phase is a precipitate of manganese oxalate and ferrous oxalate complex. After solid-liquid separation, lithium phosphate is obtained by concentration, evaporation, crystallization and washing. The liquid phase obtained by using persulfate as a leaching agent is a leaching solution containing lithium ions, and the solid phase is a mixture of manganese dioxide and iron phosphate. Carbonate is added to the lithium-rich leaching solution after solid-liquid separation, and lithium carbonate is obtained after concentration, evaporation, crystallization and washing.

7. The method as described in claim 5, characterized in that, The manganese compound added in step (5) is at least one of manganese oxalate, manganese dioxide, and manganese carbonate, and the iron compound added is at least one of ferrous oxalate, ferric phosphate, and ferrous carbonate; an appropriate amount of manganese compound and iron compound are added to adjust their ratio in the precursor so that the ratio of metallic manganese to iron in the precursor is 5:5, 6:4, 7:3, or 8:

2.

8. The method as described in claim 5, characterized in that, The phosphorus source added in step (6) is at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and phosphoric acid; the carbon source is at least one of sucrose, sorbitol, and polyethylene glycol. The total molar amount of manganese and iron sources is in a molar ratio of 1:1.05 to 1:1.2 to lithium in the lithium salt, and the amount of carbon source added is 6 to 12 wt%.

9. The method as described in claim 5, characterized in that, Step (6) includes the following sub-steps: (6-1) The manganese source, iron source, lithium salt, phosphorus source and part of the carbon source are placed in a high-energy ball mill, and after ball milling and mixing, they are pre-calcined under a nitrogen atmosphere to obtain the pre-calcined product. (6-2) The pre-calcined product obtained in step (6-1) is added to the remaining carbon source and placed in a high-energy ball mill. After ball milling and mixing, it is calcined again under a nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material.

10. The method as described in claim 9, characterized in that, In step (6-1), the pre-calcination temperature is 300–500℃ and the pre-calcination time is 2–6 h; the amount of carbon source added is 40–60% of the total amount of carbon source added. In step (6-2), the calcination temperature is 600–800℃ and the calcination time is 6–12 h; the amount of carbon source added is 40–60% of the total amount of carbon source added.

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