Preparation method of lithium iron phosphate anode material with super-long cycle life

Abstract

The application relates to a preparation method of a lithium iron phosphate positive electrode material with super-long cycle life, and belongs to the technical field of lithium batteries. Under the protection of an inert atmosphere, a ferrous sulfate aqueous solution and a phosphate aqueous solution are added into a reactor with stirring to carry out a coprecipitation reaction, aging, filtration, washing, vacuum drying, and a ferrous phosphate precursor is obtained; the ferrous phosphate precursor, lithium phosphate and a carbon source are uniformly mixed and pre-sintered, a pre-sintered product is configured into a slurry with a solvent, and after grinding, spray drying is carried out; finally, high-temperature sintering is carried out, and a lithium iron phosphate positive electrode material with super-long cycle life is obtained. The lithium iron phosphate positive electrode material is applied to a lithium battery, and has super-long cycle life and excellent fast-charging performance.

Description

Technical Field

[0001] This application relates to a method for preparing an ultra-long cycle life lithium iron phosphate cathode material, belonging to the field of lithium battery technology. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is widely used as a cathode material for lithium-ion batteries due to its high safety, long cycle life, and wide availability of raw materials. However, existing technologies face numerous challenges in the production of lithium iron phosphate materials:

[0003] 1. Low compaction density

[0004] Many lithium iron phosphate materials prepared by traditional solid-state or conventional liquid-state methods have irregular particle morphology, wide particle size distribution, or hard agglomeration, resulting in low tap density and compaction density (typically below 2.55 g / cm³). This directly limits the improvement of battery volumetric energy density and cannot meet the current market demand for long-range batteries.

[0005] 2. Poor electrochemical performance

[0006] 1) Poor fast charging performance: The material has low intrinsic conductivity, and if the particles are too large or the lithium-ion diffusion channels are not smooth, it will cause severe polarization during high-current charging, resulting in a sharp drop in capacity and failing to meet the fast charging requirements.

[0007] 2) Poor low-temperature performance: At low temperatures, the migration rate of lithium ions inside the material and in the electrolyte slows down. If the material structure is not ideal, it will exacerbate the performance degradation at low temperatures, resulting in low discharge capacity.

[0008] 3. Poor process controllability and low product consistency

[0009] In particular, solid-state methods that use direct iron sources (such as ferrous oxalate) mixed with lithium and phosphorus sources for sintering are prone to component segregation and poor mixing uniformity, which may result in the presence of impurity phases (such as Fe2O3) in the final product, affecting the structural stability and batch consistency of the material.

[0010] 4. Unstable precursor quality

[0011] If purchased ferric phosphate is used as a precursor, its quality fluctuates greatly and its cost is high. On the other hand, the method of self-synthesizing ferrous phosphate may lead to the formation of amorphous or crystalline products due to improper control of reaction conditions, which in turn leads to uneven reaction activity during subsequent mixing and sintering with lithium source, affecting the crystallinity and performance of the final product. Summary of the Invention

[0012] In view of this, this application provides a method for preparing lithium iron phosphate cathode materials with ultra-long cycle life, which not only endows the materials with excellent fast charging performance and ultra-long cycle life, but also endows the materials with narrow particle size distribution and high compaction density.

[0013] A method for preparing an ultra-long cycle life lithium iron phosphate cathode material, comprising the following steps:

[0014] Step 1: Under an inert atmosphere, aqueous solutions of ferrous sulfate and phosphate are added concurrently to a stirred reactor. The reaction temperature is controlled at 40–80°C and the pH at 5.5–7.5 for co-precipitation. After co-precipitation, the mixture is aged, filtered, washed, and vacuum dried to obtain the ferrous phosphate precursor.

[0015] The concentration of the ferrous sulfate aqueous solution is 0.5–2 mol / L, and the concentration of the phosphate aqueous solution is 0.5–2.5 mol / L.

[0016] Step two: Following a lithium:iron:phosphorus molar ratio of 1.00–1.05:1:1, the ferrous phosphate precursor, lithium phosphate, and carbon source are mixed uniformly and then pre-sintered under an inert atmosphere. The amount of carbon source added is 5–15 wt% of the theoretical mass of the lithium iron phosphate cathode material.

[0017] Step 3: The pre-sintered product is mixed with a solvent to form a slurry, which is then ground and dried to obtain a microspherical precursor. The solid content of the slurry is 30-50 wt%.

[0018] Step four: The microspherical precursor is sintered at a high temperature of 600-750℃ under an inert atmosphere, then cooled to room temperature, crushed and depolymerized, and sieved to obtain lithium iron phosphate cathode material with ultra-long cycle life.

[0019] Furthermore, as a preferred option:

[0020] In step one,

[0021] The ferrous phosphate is ferrous phosphate heptahydrate.

[0022] The phosphate is sodium phosphate or ammonium phosphate.

[0023] The vacuum drying temperature is 80–120°C, and the duration is 4–12 hours.

[0024] In step two,

[0025] The carbon source is any one of sucrose, glucose, phenolic resin, or asphalt.

[0026] The pre-sintering temperature is 300–500℃, and the duration is 2–6 hours.

[0027] In step three,

[0028] The solvent is deionized water or ethanol.

[0029] The grinding speed is 1000-2000 rpm, and the grinding time is 1-3 hours.

[0030] The drying process employs vacuum drying or spray drying. More preferably, the inlet temperature of the spray dryer is 180–220°C, and the outlet temperature is 80–110°C.

[0031] In step four,

[0032] The high-temperature sintering is carried out in a tube furnace or roller kiln, and the heating rate from room temperature to the high-temperature sintering temperature is 2 to 5 °C / min.

[0033] The mesh size of the sieve is 300-500.

[0034] This application has the following beneficial effects:

[0035] 1) Uniform structure, significantly improving compaction density. A ferrous phosphate precursor with regular morphology and uniform particle size is pre-synthesized via a liquid-phase homogeneous precipitation method. Combined with efficient grinding and sintering processes, this results in a product exhibiting a regular spherical or near-spherical morphology, narrow particle size distribution, and good dispersibility, with a stable compaction density of 2.65 g / cm³. 3 The highest value can reach 2.75 g / cm³. 3 This greatly improves the volumetric energy density of the battery.

[0036] 2) Excellent electrochemical performance. The uniform precursor and grinding process ensure smooth lithium-ion diffusion channels. With a specific addition ratio of ferrous phosphate precursor, lithium phosphate, and carbon source, the carbon coating effectively improves electronic conductivity, endowing the material with excellent high-rate performance. Furthermore, its highly crystalline, impurity-free pure crystal structure, combined with the carbon coating structure, ensures rapid lithium-ion migration at low temperatures, exhibiting excellent low-temperature discharge capacity retention. The resulting lithium iron phosphate product possesses a stable olivine structure and uniform carbon layer protection, effectively suppressing structural collapse and iron dissolution during cycling, giving the material excellent structural stability and cycling performance. After 3000 cycles at 1C, its capacity retention remains above 85%.

[0037] 3) Strong process controllability and good product consistency. This application uses a liquid-phase homogeneous precipitation method to synthesize the precursor, achieving uniform mixing at the molecular / ionic level. This ensures the atomic-level uniformity of iron and phosphorus elements from the source. Further homogenization is achieved when mixing with the lithium source, ensuring high purity and batch-to-batch consistency of the final product. Using ferrous phosphate and sodium phosphate / amine as raw materials provides readily available and low-cost materials. The carbon source is a variety of organic substances such as resins, making it widely available and allowing for flexible adjustments based on the target product performance. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the preparation process of this application. Detailed Implementation Example 1

[0039] This embodiment provides a method for preparing an ultra-long cycle life lithium iron phosphate cathode material, combined with... Figure 1 The preparation process is described as follows:

[0040] (1) Synthesis of ferrous phosphate precursor

[0041] S1. Dissolve 1000g of ferrous sulfate heptahydrate (FeSO4·7H2O, analytical grade) in 2L of deionized water to prepare solution A of about 1.8mol / L.

[0042] S2, dissolve 265g of ammonium dihydrogen phosphate (NH4H2PO4, analytical grade) in 2L of deionized water to prepare a solution B of about 1.0 mol / L.

[0043] S3, under nitrogen protection, solutions A and B are pumped in parallel into a 5L jacketed reactor (material: 316L stainless steel), the reaction temperature is controlled at 60℃, and the pH is maintained at 6.5 by adding ammonia dropwise.

[0044] S4, after the reaction is complete, age for 2 hours, filter, wash 3 times each with deionized water and anhydrous ethanol, and dry in a vacuum drying oven at 100℃ for 10 hours to obtain the ferrous phosphate precursor.

[0045] (2) Mixing and pre-sintering

[0046] Weigh out 130 g of the vacuum-dried ferrous phosphate precursor (approximately 1.0 mol Fe), 58.5 g of lithium phosphate (Li3PO4, battery grade) (approximately 0.5 mol, Li / Fe molar ratio = 1.03), and 15 g of sucrose (analytical grade) (approximately 10% of the theoretical product mass). Mix using a Henschel high-speed mixer for 2 hours. Transfer the mixture to an atmosphere sintering furnace and pre-sinter at 400°C under nitrogen flow at a rate of 3°C / min for 4 hours.

[0047] (3) Grinding and drying

[0048] The pre-sintered product was mixed with deionized water at a solid content of 40% to form a slurry, which was then poured into a three-cylinder vertical sand mill (using zirconia beads as the grinding media) and ground at 1500 rpm for 2 hours. The slurry was then conveyed to a centrifugal spray drying tower (Wuxi Linde Engineering Equipment Co., Ltd., model LPG-50) for drying, with an inlet temperature of 200℃ and an outlet temperature of 90℃.

[0049] (4) High-temperature sintering and pulverization

[0050] The spray-dried powder was placed in a crucible and then in a box-type atmosphere sintering furnace (Hefei Kejing Materials Technology Co., Ltd., KSL-1700X). High-purity nitrogen was introduced, and the temperature was increased to 680℃ at a rate of 3℃ / min and held for 12 hours. After natural cooling, the powder was pulverized using an air jet mill (Weifang Zhengyuan Powder Engineering Equipment Co., Ltd., QLM-100) and passed through a 400-mesh sieve to obtain the final product, denoted as Material 1.

[0051] Example 2

[0052] This embodiment provides a method for preparing an ultra-long cycle life lithium iron phosphate cathode material, combined with... Figure 1 The preparation process is described as follows:

[0053] (1) Synthesis of ferrous phosphate precursor

[0054] S1. Dissolve 1000g of ferrous sulfate heptahydrate (FeSO4·7H2O, analytical grade) in 2L of deionized water to prepare solution A of about 1.8mol / L.

[0055] S2, dissolve 265g of ammonium dihydrogen phosphate (NH4H2PO4, analytical grade) in 2L of deionized water to prepare a solution B of about 1.0 mol / L.

[0056] S3, under nitrogen protection, solutions A and B are pumped in parallel into a 5L jacketed reactor (material: 316L stainless steel), the reaction temperature is controlled at 60℃, and the pH is maintained at 6.5 by adding ammonia dropwise.

[0057] S4, after the reaction is complete, age for 2 hours, filter, wash 3 times each with deionized water and anhydrous ethanol, and dry in a vacuum drying oven at 100℃ for 10 hours to obtain the ferrous phosphate precursor.

[0058] (2) Mixing and pre-sintering

[0059] Weigh out 130g of the vacuum-dried ferrous phosphate precursor (approximately 1.0 mol Fe), 58.0g of lithium phosphate (Li3PO4, battery grade) (Li / Fe molar ratio = 1.02), and 18g of glucose (analytical grade) (approximately 12% of the theoretical product mass). After mixing for 3 hours using a three-dimensional motion mixer, transfer the mixture to an atmosphere sintering furnace and hold at 350℃ for 5 hours under nitrogen flow to complete the pre-sintering.

[0060] (3) Grinding and drying

[0061] The pre-sintered product was mixed with deionized water at a solid content of 35% to form a slurry, which was then poured into a basket mill (using zirconia beads as the grinding media) and ground at 1200 rpm for 2.5 hours. The slurry was then conveyed to a centrifugal spray drying tower (Wuxi Linde Engineering Equipment Co., Ltd., model LPG-50) for drying, with an inlet temperature of 190℃ and an outlet temperature of 85℃.

[0062] (4) High-temperature sintering and pulverization

[0063] The spray-dried powder was placed in a crucible and then in a box-type atmosphere sintering furnace (Hefei Kejing Materials Technology Co., Ltd., KSL-1700X). High-purity nitrogen gas was introduced, and the temperature was raised to 700℃ at a rate of 4℃ / min and held for 10 hours. After natural cooling, the powder was pulverized using an air jet mill (Weifang Zhengyuan Powder Engineering Equipment Co., Ltd., QLM-100) and passed through a 400-mesh sieve to obtain the final product, denoted as Material 2.

[0064] Example 3

[0065] This embodiment provides a method for preparing an ultra-long cycle life lithium iron phosphate cathode material, combined with... Figure 1 The preparation process is described as follows:

[0066] (1) Synthesis of ferrous phosphate precursor

[0067] S1. Dissolve 1000g of ferrous sulfate heptahydrate (FeSO4·7H2O, analytical grade) in 2L of deionized water to prepare solution A of about 1.8mol / L.

[0068] S2, dissolve 265g of ammonium dihydrogen phosphate (NH4H2PO4, analytical grade) in 2L of deionized water to prepare a solution B of about 1.0 mol / L.

[0069] S3, under nitrogen protection, solutions A and B are pumped in parallel into a 5L jacketed reactor (material: 316L stainless steel), the reaction temperature is controlled at 50℃, and the pH is maintained at 7.0 by adding ammonia dropwise.

[0070] S4, after the reaction is complete, age for 2 hours, filter, wash 3 times each with deionized water and anhydrous ethanol, and dry in a vacuum drying oven at 100℃ for 10 hours to obtain the ferrous phosphate precursor.

[0071] (2) Mixing and pre-sintering

[0072] Weigh out 130g of the vacuum-dried ferrous phosphate precursor (approximately 1.0 mol Fe), 59g of lithium phosphate (Li3PO4, battery grade) (Li / Fe molar ratio = 1.04), and 12g of thermosetting phenolic resin (approximately 8% of the theoretical product mass). Mix using a Henschel high-speed mixer for 2 hours. Transfer the mixture to an atmosphere sintering furnace and pre-sinter at 400°C under nitrogen flow at a rate of 3°C / min for 4 hours.

[0073] (3) Grinding and drying

[0074] The pre-sintered product was mixed with anhydrous ethanol at a solid content of 45% to form a slurry, which was then poured into a three-bar vertical sand mill (using zirconia beads as the grinding media) and ground at 1800 rpm for 1.5 hours. Then it was vacuum dried at 80°C for 8 hours.

[0075] (4) High-temperature sintering and pulverization

[0076] The spray-dried powder was placed in a crucible and then placed in a box-type atmosphere sintering furnace (Hefei Kejing Materials Technology Co., Ltd., KSL-1700X). High-purity nitrogen gas was introduced, and the temperature was raised to 650℃ at a rate of 2℃ / min and held for 15 hours. After natural cooling, the powder was pulverized using an air jet mill (Weifang Zhengyuan Powder Engineering Equipment Co., Ltd., QLM-100) and passed through a 400-mesh sieve to obtain the final product, denoted as Material 3.

[0077] Comparative Example 1

[0078] Example 1 of CN120208180A is used as Comparative Example 1. A ferrous source and an aqueous solvent are mixed and ground to obtain a first slurry; a lithium source, a carbon source, and a titanium hydrolysis precursor are added to the first slurry and blended to obtain a second slurry; an aqueous carbonate solution is mixed with the second slurry and a precipitation reaction is carried out to obtain a third slurry; the third slurry is calcined to obtain lithium iron phosphate material.

[0079] Comparative Example 2

[0080] Example 1 of CN119143101A is used as Comparative Example 1. Carbon nanotubes were pretreated and used as a carbon source. A mixture of lithium, manganese, iron, and phosphorus sources was added, followed by ball milling, drying, and pre-sintering. The carbon source was then added again, and the process was repeated with ball milling, drying, and secondary sintering to obtain a lithium manganese iron phosphate cathode material with high specific capacity, high rate performance, and high compaction density.

[0081] Table 1: Performance Comparison of Different Solutions .

[0082] As shown in Table 1, although Comparative Examples 1 and 2 can achieve relatively ideal initial discharge specific capacity and compaction density, and maintain good capacity retention within 100 cycles, their capacity retention decays rapidly during continuous cycling, and can only maintain normal cycling within 1000 cycles. However, this application can still maintain a capacity retention of more than 89% after 3000 cycles.

Claims

1. A method for preparing an ultra-long cycle life lithium iron phosphate cathode material, characterized in that, The steps are as follows: Step 1: Under an inert atmosphere, aqueous solutions of ferrous sulfate and phosphate are added concurrently to a stirred reactor. The reaction temperature is controlled at 40–80°C and the pH at 5.5–7.5 for co-precipitation. After co-precipitation, the mixture is aged, filtered, washed, and vacuum dried to obtain the ferrous phosphate precursor. The concentration of the ferrous sulfate aqueous solution is 0.5–2 mol / L, and the concentration of the phosphate aqueous solution is 0.5–2.5 mol / L. Step two: Following a lithium:iron:phosphorus molar ratio of 1.00–1.05:1:1, the ferrous phosphate precursor, lithium phosphate, and carbon source are mixed uniformly and then pre-sintered under an inert atmosphere. The amount of carbon source added is 5–15 wt% of the theoretical mass of the lithium iron phosphate cathode material. Step 3: The pre-sintered product is mixed with a solvent to form a slurry, which is then ground and dried to obtain a microspherical precursor. The solid content of the slurry is 30-50 wt%. Step four: The microspherical precursor is sintered at a high temperature of 600-750℃ under an inert atmosphere, then cooled to room temperature, crushed and depolymerized, and sieved to obtain lithium iron phosphate cathode material with ultra-long cycle life.

2. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The ferrous phosphate is ferrous phosphate heptahydrate.

3. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The phosphate is sodium phosphate or ammonium phosphate.

4. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The vacuum drying temperature is 80–120°C, and the duration is 4–12 hours.

5. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The carbon source is any one of sucrose, glucose, phenolic resin, or asphalt.

6. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The pre-sintering temperature is 300–500℃, and the duration is 2–6 hours.

7. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The grinding speed is 1000-2000 rpm, and the grinding time is 1-3 hours.

8. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: In step three, the drying process employs spray drying, with an inlet temperature of 180–220°C and an outlet temperature of 80–110°C.

9. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The high-temperature sintering is carried out in a tube furnace or roller kiln, and the heating rate from room temperature to the high-temperature sintering temperature is 2 to 5 °C / min.

10. The method for preparing an ultra-long cycle life lithium iron phosphate cathode material according to claim 1, characterized in that: The mesh size of the sieve is 300-500.

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