Lithium iron phosphate coated lithium-rich manganese-based material and preparation method thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate coated lithium-rich manganese-based material and a preparation method thereof.The preparation method comprises the following steps that firstly, ferrous sulfate, oxalic acid and an organic solvent form a first mixed solution, an ammonium dihydrogen phosphate solution is added to form a second mixed solution, lithium hydroxide is dropped to form a third mixed solution, and the first mixed solution and the second mixed solution are mixed; then heating and reacting the third mixed solution to obtain a coated precursor; grinding and calcining the coated precursor to obtain a lithium iron phosphate coated lithium-rich manganese-based material; the organic solvent is a mixture of ethylene glycol and glycerol. The preparation method comprises the following steps: firstly, generating nanoscale ferrous oxalate from ferrous sulfate and oxalic acid, then adding ammonium dihydrogen phosphate and lithium hydroxide, heating and calcining to generate lithium iron phosphate, and coating the surface of a lithium-rich manganese-based substrate with the lithium iron phosphate; the battery prepared by coating the lithium-rich manganese-based material with the lithium iron phosphate has beneficial cycle performance and rate capability.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, specifically to a lithium iron phosphate coated lithium-rich manganese-based material and its preparation method. Background Technology

[0002] Developing high-performance, low-cost novel cathode materials has always been a hot topic and focus in the field of lithium-ion battery research. Compared with lithium cobalt oxide, lithium iron phosphate, and high-nickel ternary cathode materials, lithium-rich manganese-based cathode materials are expected to become the cathode materials for next-generation commercial high-energy-density lithium-ion batteries due to their advantages such as high specific capacity, low price, and good thermal stability. However, lithium-rich manganese-based cathode materials suffer from severe voltage hysteresis and capacity / voltage decay, resulting in limited cycle life, which urgently needs to be improved. Summary of the Invention

[0003] To address the severe voltage hysteresis and capacity / voltage decay issues of lithium-rich manganese-based cathode materials, this application provides a method for preparing lithium iron phosphate-coated lithium-rich manganese-based materials. First, ferrous sulfate and oxalic acid are reacted to generate nano-sized ferrous oxalate. Then, ammonium dihydrogen phosphate and lithium hydroxide are added, followed by heating and calcination to generate lithium iron phosphate coating on the surface of the lithium-rich manganese-based substrate. This results in batteries prepared using lithium iron phosphate-coated lithium-rich manganese-based materials exhibiting beneficial cycle performance and rate capability.

[0004] In a first aspect, this application provides a method for preparing lithium iron phosphate coated lithium-rich manganese-based materials, employing the following technical solution: A method for preparing lithium iron phosphate coated lithium-rich manganese-based materials includes the following steps: Ferrous sulfate was dissolved in an organic solvent, then lithium-rich manganese-based substrate was added, and oxalic acid was added and stirred to obtain the first mixed solution. Ammonium dihydrogen phosphate is dissolved in an organic solvent and then added to the first mixed solution and stirred to obtain a second mixed solution; After dissolving lithium hydroxide in an organic solvent, it is added dropwise to the second mixed solution and stirred to obtain a third mixed solution; The third mixed solution was heated to react, and after the reaction was completed, it was cooled and filtered. The filter residue was collected, washed, and dried to obtain the coated precursor. The coated precursor was ground and then calcined to obtain lithium iron phosphate coated lithium-rich manganese-based material; The organic solvent is a mixture of ethylene glycol and glycerol.

[0005] By employing the above technical solution, ferrous sulfate and oxalic acid are first added to react and generate nano-sized ferrous oxalate, which then adheres directionally to the surface of a lithium-rich manganese-based substrate. Next, ammonium dihydrogen phosphate is added to introduce phosphate groups, and finally, lithium hydroxide is added dropwise to introduce lithium. Through heating and calcination, lithium iron phosphate is generated and coated onto the surface of the lithium-rich manganese-based substrate, thus obtaining a lithium iron phosphate-coated lithium-rich manganese-based material.

[0006] Lithium-rich manganese-based materials are prone to undergoing a transformation from a layered structure to a spinel phase during cycling, leading to a decrease in voltage plateau (voltage decay) and capacity loss. The lithium iron phosphate coating acts as a physical barrier, reducing direct contact between the material surface and the electrolyte, inhibiting the migration and irreversible dissolution of transition metal ions (Mn, Ni, etc.), thereby delaying the phase transition process. Under high voltages (>4.5 V), lithium-rich manganese-based materials are susceptible to electrolyte oxidative decomposition, generating gas and forming a thick CEI film. The lithium iron phosphate coating possesses electrochemical inertness, blocking direct contact between the electrolyte and active materials, inhibiting side reactions (such as HF corrosion and lattice oxygen release), and improving interfacial stability.

[0007] In addition, the organic solvent used in this application is a mixed solution of ethylene glycol and glycerol, mainly because ethylene glycol can dominate the dissolution, and the glycerol hydroxyl group of glycerol can provide protons to inhibit the ionization of lithium hydroxide, maintain the low pH of the system, and at the same time achieve the slow release of lithium ions.

[0008] This application first generates nano-sized ferrous oxalate by reacting ferrous sulfate with oxalic acid, then adds ammonium dihydrogen phosphate and lithium hydroxide, and finally generates lithium iron phosphate by heating and calcination, which coats the surface of a lithium-rich manganese-based substrate. The lithium iron phosphate coating acts as a physical barrier, which can inhibit the migration and irreversible dissolution of transition metal ions (Mn, Ni, etc.), thereby delaying the phase transition process and improving interface stability. Therefore, the battery prepared by coating lithium-rich manganese-based materials with lithium iron phosphate has beneficial cycle performance and rate performance.

[0009] Preferably, when adding oxalic acid and stirring, the mixing time is greater than 30 minutes.

[0010] By adopting the above technical solution, the mixing time must be greater than 30 minutes to allow ferrous sulfate and oxalic acid to form a stable ferrous oxalate complex, which can then better adhere to the lithium-rich manganese-based substrate.

[0011] Preferably, when adding oxalic acid and stirring, ascorbate palmitate can also be added and stirred to obtain a first mixed solution.

[0012] By employing the above technical solution, the addition of ascorbate palmitate effectively inhibits the oxidation of ferrous ions to ferric ions, and its long palmitate chain effectively improves the dispersibility of the coating layer. Furthermore, ascorbate palmitate needs to be added together with oxalic acid; in this acidic environment, the enol group (-OH) of ascorbate palmitate is protonated, forming a stable oxonium ion structure. If ascorbate palmitate is added together with ammonium dihydrogen phosphate, it easily chelates with phosphate groups, potentially blocking the growth channels of the coating layer. If ascorbate palmitate is added together with lithium hydroxide, it is easily affected by localized lithium hydroxide, leading to hydrolysis and reduced effectiveness.

[0013] Preferably, the mass ratio of the lithium-rich manganese-based substrate to the ascorbate palmitate is 1000:3-5.

[0014] Preferably, sucrose is added after obtaining the third mixed solution.

[0015] By adopting the above technical solution, the addition of sucrose to the system can play a reducing role during the heating stage, ensuring that ferrous ions are not oxidized to ferric ions. Sucrose can also generate an amorphous conductive carbon layer during calcination, and the shrinkage force of sucrose caramelization can refine the lithium iron phosphate grains.

[0016] If sucrose is added before obtaining the first mixed solution, oxalic acid has not yet reacted with ferrous sulfate to form stable ferrous oxalate, making it easy for sucrose to chelate with ferrous ions and block the nucleation of lithium iron phosphate. If sucrose is added after obtaining the first mixed solution, it will react with the later-added ammonium dihydrogen phosphate to form sucrose phosphate, resulting in a porous coating layer. If sucrose is added after obtaining the second mixed solution, the addition of lithium hydroxide may create localized high alkalinity, triggering the Maillard reaction of sucrose and leading to browning. Therefore, it is best to add sucrose after obtaining the third mixed solution.

[0017] Preferably, the mass ratio of the lithium-rich manganese-based substrate to the sucrose is 1000:12-18.

[0018] By adopting the above technical solution, when the sucrose content is too high, it is easy to cause the coating layer to be too thick, which leads to the extension of the lithium diffusion path and reduces the material performance. Therefore, after a lot of research and experimental verification, the applicant finally determined that the mass ratio of lithium-rich manganese-based substrate to sucrose in this application should be as described above.

[0019] Preferably, when the third mixed solution undergoes a heating reaction, the heating temperature range of the heating reaction is 180-220℃.

[0020] Preferably, when the coated precursor is ground and then calcined, the calcination temperature range is 500-550℃.

[0021] Secondly, this application provides a lithium iron phosphate coated lithium-rich manganese-based material, employing the following technical solution: A lithium iron phosphate coated lithium-rich manganese-based material is prepared by the above-mentioned preparation method of lithium iron phosphate coated lithium-rich manganese-based material.

[0022] In summary, this application has the following beneficial effects: 1. In this application, ferrous sulfate is first reacted with oxalic acid to generate nano-sized ferrous oxalate, then ammonium dihydrogen phosphate and lithium hydroxide are added, followed by heating and calcination to generate lithium iron phosphate coating on the surface of a lithium-rich manganese-based substrate. The lithium iron phosphate coating layer acts as a physical barrier, which can inhibit the migration and irreversible dissolution of transition metal ions (Mn, Ni, etc.), thereby delaying the phase transition process and improving interface stability. Therefore, the battery prepared by coating lithium-rich manganese-based materials with lithium iron phosphate has beneficial cycle performance and rate performance. 2. By adding ascorbate palmitate when adding oxalic acid, this application can effectively inhibit the oxidation of ferrous ions to ferric ions, and its long palmitate chain can effectively improve the dispersibility of the coating layer; and when ascorbate palmitate is added together with oxalic acid, the system is in an acidic environment, and the enol group (-OH) of ascorbate palmitate is protonated to form a stable oxonium ion structure; 3. After obtaining the third mixed solution, sucrose is added, which can play a reducing role during the heating stage to ensure that ferrous ions are not oxidized to ferric ions. Sucrose can also generate an amorphous conductive carbon layer during calcination, and the shrinkage force of sucrose caramelization can refine the lithium iron phosphate grains. Detailed Implementation

[0023] The raw materials in this application include the following: Ascorbyl palmitate: Uses commercially available product with CAS number 137-66-6; The present application will be further described in detail below with reference to embodiments and comparative examples.

[0024] Example 1 A method for preparing lithium iron phosphate coated lithium-rich manganese-based materials includes the following steps: 13.8g of ferrous sulfate was dissolved in 100mL of organic solvent, then 100g of lithium-rich manganese-based substrate was added, and then 6.8g of oxalic acid was added and stirred for 40min to obtain the first mixed solution. Dissolve 3.8g of ammonium dihydrogen phosphate in 100mL of organic solvent and add it to the first mixed solution. Stir for 35min to obtain the second mixed solution. 1.4 g of lithium hydroxide was dissolved in 100 mL of organic solvent and then added dropwise to the second mixed solution at a rate of 1.0 mL / min while stirring continuously to obtain the third mixed solution. The third mixed solution was heated to react at a temperature of 200°C for 4 hours. After the reaction was completed, the mixture was cooled, filtered, and the filter residue was collected, washed, and dried to obtain the coated precursor. After grinding the coating precursor, it was calcined in an argon atmosphere at a temperature of 525℃ for 4 hours to obtain lithium iron phosphate coated lithium-rich manganese-based material. The organic solvent is a mixture of ethylene glycol and glycerol in a volume ratio of 3:1.

[0025] Example 2-3 Examples 2-3 are based on the preparation method of Example 1, but the mixing time when adding oxalic acid and stirring is adjusted. The specific adjustments are shown in Table 1.

[0026] Comparative Examples 1-4 Comparative Example 1, based on the preparation method of Example 1, did not add oxalic acid when preparing the first mixed solution. Instead, it dissolved ammonium dihydrogen phosphate in an organic solvent and added it to the first mixed solution, and then added 6.8g of oxalic acid and stirred for 40min to obtain the second mixed solution.

[0027] Comparative Example 2, based on the preparation method of Example 1, did not add oxalic acid when preparing the first mixed solution. Instead, lithium hydroxide was dissolved in an organic solvent and completely added dropwise to the second mixed solution. Then, 6.8 g of oxalic acid was added and stirred for 40 min to obtain the third mixed solution.

[0028] Comparative Example 3 is based on the preparation method of Example 1, except that the organic solvent is ethylene glycol.

[0029] Comparative Example 4 is based on the preparation method of Example 1, except that the organic solvent is glycerol.

[0030] Performance testing The lithium iron phosphate-coated lithium-rich manganese-based materials of Examples 1-3 and Comparative Examples 1-4 were subjected to the following performance tests, and the test results are shown in Table 1.

[0031] 1. Capacity retention rate The volume retention rate was determined according to GB / T 18287-2013.

[0032] 2. Voltage holding rate Voltage retention rate was determined according to GB / T 33827-2017.

[0033] 3. Magnification The magnification is determined according to IEC 61960.

[0034] Table 1. Mixing time, type of organic solvent, and performance test results after adding oxalic acid in Examples 1-3 and Comparative Examples 1-4. Referring to Table 1, and comparing Examples 1-3 and Comparative Examples 1-4, it can be seen that this application generates nano-sized ferrous oxalate through ferrous sulfate and oxalic acid. The ferrous oxalate adheres to the surface of a lithium-rich manganese-based substrate. Then, ammonium dihydrogen phosphate and lithium hydroxide are added, and after reaction and calcination, lithium iron phosphate is generated to coat the lithium-rich manganese-based substrate. When oxalic acid is added, the mixing time is relatively short, resulting in insufficient stability of the generated ferrous oxalate complex, which cannot adhere well to the lithium-rich manganese-based substrate. Therefore, when oxalic acid is added and mixing is performed, the mixing time should be greater than 30 minutes to ensure that the lithium iron phosphate-coated lithium-rich manganese-based substrate exhibits good performance.

[0035] Furthermore, since oxalic acid is added to form nano-sized ferrous oxalate with ferrous sulfate, it must be added after ferrous sulfate. If oxalic acid is added after ammonium dihydrogen phosphate, the phosphate groups will compete with the oxalic acid groups for ferrous ions, easily forming ferric phosphate impurities. If oxalic acid is added after lithium hydroxide, hydroxide ions may combine with ferrous ions first, forming ferrous hydroxide colloids. Incorrect order of oxalic acid addition will affect the formation of nano-sized ferrous oxalate, thus affecting the performance of the final lithium iron phosphate-coated lithium-rich manganese-based material.

[0036] When ethylene glycol is used alone as an organic solvent, the nucleation rate is fast, but the grains are coarse. When glycerol is used alone, nucleation is easily hindered, resulting in discontinuous coating. Therefore, a mixture of ethylene glycol and glycerol is required as an organic solvent to fully utilize their synergistic effect, effectively promoting the reaction and thus preparing high-performance lithium iron phosphate-coated lithium-rich manganese-based materials.

[0037] Comparing Examples 1-3, Example 1 showed the best performance; therefore, Example 1 is preferred.

[0038] Examples 4-6 In Example 4, based on the preparation method of Example 1, 0.5g of ascorbate palmitate was added and stirred while adding oxalic acid to obtain a first mixed solution.

[0039] In Example 5, based on the preparation method of Example 1, after dissolving ammonium dihydrogen phosphate in an organic solvent and adding it to the first mixed solution, 0.5 g of ascorbate palmitate was also added and stirred to obtain a second mixed solution.

[0040] In Example 6, based on the preparation method of Example 1, lithium hydroxide was dissolved in an organic solvent and then added dropwise to the second mixed solution while stirring. At the same time, 0.5 g of ascorbate palmitate was also added and stirred to obtain a third mixed solution.

[0041] The lithium iron phosphate-coated lithium-rich manganese-based materials of Examples 4-6 were subjected to the above performance tests, and the test results are shown in Table 2.

[0042] Table 2. Timing of Ascorbate Palmitate Addition and Performance Testing in Examples 1 and 4-6 Referring to Table 2, a comparison of Examples 1 and 4-6 shows that the addition of ascorbate palmitate along with oxalic acid effectively improves the performance of lithium iron phosphate-coated lithium-rich manganese-based materials. This is because the addition of ascorbate palmitate effectively inhibits the oxidation of ferrous ions to ferric ions, and its long palmitate chain effectively improves the dispersibility of the coating layer.

[0043] Furthermore, ascorbate palmitate needs to be added together with oxalic acid. In this acidic environment, the enol group (-OH) of ascorbate palmitate is protonated, forming a stable oxonium ion structure. If ascorbate palmitate is added together with ammonium dihydrogen phosphate, it easily chelates with the phosphate group, potentially blocking the growth channels of the coating layer. If ascorbate palmitate is added together with lithium hydroxide, it is susceptible to localized effects from the lithium hydroxide, leading to hydrolysis and reduced effectiveness.

[0044] Examples 7-9 Examples 7-9 are based on the preparation method of Example 4, but the amount of ascorbate palmitate added is adjusted. The specific adjustments are shown in Table 3.

[0045] The lithium iron phosphate-coated lithium-rich manganese-based materials of Examples 7-9 were subjected to the above performance tests, and the test results are shown in Table 3.

[0046] Table 3. Ascorbate palmitate addition amount and performance test results for Examples 1, 4 and 7-9 Referring to Table 3, a comparison of Examples 1, 4, and 7-9 shows that as the amount of ascorbate palmitate added increases, the performance of the prepared lithium iron phosphate coated lithium-rich manganese-based material improves until it stabilizes.

[0047] Examples 10-13 In Example 10, based on the preparation method of Example 1, 1.5g of sucrose was added after obtaining the third mixed solution.

[0048] In Example 11, based on the preparation method of Example 1, 1.5g of sucrose was added before obtaining the first mixed solution.

[0049] In Example 12, based on the preparation method of Example 1, 1.5g of sucrose was added after obtaining the first mixed solution.

[0050] In Example 13, based on the preparation method of Example 1, 1.5g of sucrose was added after obtaining the second mixed solution.

[0051] The lithium iron phosphate-coated lithium-rich manganese-based materials of Examples 10-13 were subjected to the above performance tests, and the test results are shown in Table 4.

[0052] Table 4. Timing of sucrose addition and performance testing in Examples 1 and 10-13 Referring to Table 4, a comparison of Examples 1 and 10-13 shows that adding sucrose to the system after obtaining the third mixed solution effectively improves the performance of lithium iron phosphate-coated lithium-rich manganese-based materials. This is because the addition of sucrose allows it to play a reducing role during the heating stage, ensuring that ferrous ions are not oxidized to ferric ions. Sucrose also generates an amorphous conductive carbon layer during calcination, and the shrinkage force of sucrose caramelization refines the lithium iron phosphate grains.

[0053] If sucrose is added before obtaining the first mixed solution, oxalic acid has not yet reacted with ferrous sulfate to form stable ferrous oxalate, making it easy for sucrose to chelate with ferrous ions and block the nucleation of lithium iron phosphate. If sucrose is added after obtaining the first mixed solution, it will react with the later-added ammonium dihydrogen phosphate to form sucrose phosphate, resulting in a porous coating layer. If sucrose is added after obtaining the second mixed solution, the addition of lithium hydroxide may create localized high alkalinity, triggering the Maillard reaction of sucrose and leading to browning. Therefore, it is best to add sucrose after obtaining the third mixed solution.

[0054] Examples 14-15 Examples 14-15 are based on the preparation method of Example 10, but the amount of sucrose added is adjusted. The specific adjustments are shown in Table 5.

[0055] The lithium iron phosphate-coated lithium-rich manganese-based materials of Examples 14-15 were subjected to the above performance tests, and the test results are shown in Table 5.

[0056] Table 5. Sucrose addition amount and performance test results for Examples 1, 10, and 14-15. Referring to Table 5, a comparison of Examples 1, 10, and 14-15 shows that as the amount of sucrose added increases, the performance of the prepared lithium iron phosphate-coated lithium-rich manganese-based material exhibits a trend of first increasing and then decreasing. This is because when the sucrose content is too high, it easily leads to an excessively thick coating layer, resulting in a prolonged lithium diffusion path and consequently reducing material performance.

[0057] Examples 16-19 Examples 16-17 are based on the preparation method of Example 1, but the heating temperature of the heating reaction is adjusted.

[0058] Examples 18-19 are based on the preparation method of Example 1, but the calcination temperature is adjusted.

[0059] The lithium iron phosphate-coated lithium-rich manganese-based materials of Examples 16-19 were subjected to the above performance tests, and the test results are shown in Table 6.

[0060] Table 6. Heating temperature, calcination temperature, and performance test results for Examples 1 and 16-19. Referring to Table 5, a comparison of Example 1 and Examples 16-19 shows that heating temperatures of 180-220℃ and calcination temperatures of 500-550℃ both meet the requirements of this application.

[0061] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for preparing lithium iron phosphate coated lithium-rich manganese-based materials, characterized in that, Includes the following steps: Ferrous sulfate was dissolved in an organic solvent, then lithium-rich manganese-based substrate was added, and oxalic acid was added and stirred to obtain the first mixed solution. Ammonium dihydrogen phosphate is dissolved in an organic solvent and then added to the first mixed solution and stirred to obtain a second mixed solution; After dissolving lithium hydroxide in an organic solvent, it is added dropwise to the second mixed solution and stirred to obtain a third mixed solution; The third mixed solution was heated to react, and after the reaction was completed, it was cooled and filtered. The filter residue was collected, washed, and dried to obtain the coated precursor. The coated precursor was ground and then calcined to obtain lithium iron phosphate coated lithium-rich manganese-based material; The organic solvent is a mixture of ethylene glycol and glycerol.

2. The method for preparing lithium iron phosphate-coated lithium-rich manganese-based materials according to claim 1, characterized in that: When adding oxalic acid and stirring, the mixing time should be greater than 30 minutes.

3. The method for preparing lithium iron phosphate-coated lithium-rich manganese-based materials according to claim 1, characterized in that: When adding oxalic acid and stirring, ascorbate palmitate can also be added and stirred to obtain a first mixed solution.

4. The method for preparing lithium iron phosphate coated lithium-rich manganese-based materials according to claim 3, characterized in that: The mass ratio of the lithium-rich manganese-based substrate to the ascorbate palmitate is 1000:3-5.

5. The method for preparing lithium iron phosphate coated lithium-rich manganese-based materials according to claim 1, characterized in that: After obtaining the third mixed solution, sucrose is added.

6. The method for preparing lithium iron phosphate coated lithium-rich manganese-based materials according to claim 5, characterized in that: The mass ratio of the lithium-rich manganese-based substrate to the sucrose is 1000:12-18.

7. The method for preparing lithium iron phosphate coated lithium-rich manganese-based materials according to claim 1, characterized in that: When the third mixed solution is heated, the heating temperature range of the heating reaction is 180-220℃.

8. The method for preparing lithium iron phosphate coated lithium-rich manganese-based materials according to claim 1, characterized in that: When the coated precursor is ground and then calcined, the calcination temperature range is 500-550℃.

9. A lithium iron phosphate-coated lithium-rich manganese-based material, characterized in that: It is prepared by the method of any one of claims 1-8 for preparing lithium iron phosphate coated lithium-rich manganese-based materials.

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