Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The application discloses a lithium iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of lithium battery materials. A chelating dispersant, titanium dioxide and part of water are mixed and stirred for 2-3 hours, then a phosphorus source, a lithium source, a carbon source for coating and the rest of the water are added, and stirring is continued for 0.5-1 hour to obtain a mixed slurry; the mixed slurry is ground at 20-50 DEG C, and then spray drying is carried out to obtain a lithium iron phosphate precursor; the lithium iron phosphate precursor is heated to 650-750 DEG C under nitrogen protection for 4-6 hours, is calcined at the temperature range for 8-12 hours, and is cooled and crushed to obtain the lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material prepared by the method; titanium doped by the method is chelated by a long-chain polymer, forms linear order, can be exposed on the surface of an iron lithium atomic layer in the preparation process, and improves the electron conductivity; the long-chain structure of the polymer can promote the ordered arrangement of the iron lithium atomic layer, and further improves the cycle stability and rate performance of the positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery material technology, and specifically relates to a lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries, as a novel energy carrier, are widely used in new energy vehicles, electronic products, energy storage devices, and other fields. Currently, efforts to improve the performance of commercially available lithium-ion batteries mainly focus on the modification of battery electrode materials, especially the modification of cathode materials.

[0003] The positive electrode material of a lithium-ion battery serves as a lithium source during charging and discharging, providing the lithium needed for the repeated intercalation and deintercalation between the positive and negative electrode materials within the battery. It also absorbs the lithium consumed in forming the solid-liquid interface film (SEI film) on the surface of the negative electrode material. Therefore, modification of the positive electrode material is particularly effective. The main methods for modifying positive electrode materials are doping, coating, and material nanostructuring. Industrial production utilizes doping and coating methods, which can be combined: various metal ions are doped into lithium iron phosphate, then mixed with carbon sources in a specific ratio, and calcined under a high-temperature reducing atmosphere. This inhibits the growth of lithium iron phosphate grains, increases the specific surface area, and improves the specific capacity. Studies have found that titanium doping significantly improves the performance of lithium-ion battery cathode materials. For example, patent CN118598109B discloses a modified lithium iron phosphate cathode material and its preparation method. The method involves sequentially ball-milling, drying, calcining, and grinding lithium source, anhydrous iron phosphate, carbon source, titanium dioxide powder, phosphoric acid, manganese carbonate, and anhydrous ethanol, followed by adding a nitrogen source and secondary calcination to obtain the modified lithium iron phosphate cathode material. Patent CN111740101B discloses a method for preparing lithium iron phosphate, where iron phosphate, lithium source, coated carbon source, and titanium / vanadium doped ion source are mixed into a slurry, then spray-dried and calcined to obtain the lithium iron phosphate cathode material. Patent CN107611413B discloses a method for preparing titanium-doped lithium iron phosphate cathode material, where lithium source, phosphorus source, iron source, and metallic titanium are directly mixed and melted at high temperature in a furnace, water-quenched into particles, then spray-dried with a carbon source to form powder, and calcined and cooled to obtain the lithium iron phosphate cathode material.

[0004] In summary, doping with titanium powder or titanium dioxide can improve the electronic conductivity of lithium iron phosphate cathode materials and increase the migration channels for lithium ions, effectively enhancing the cycle stability and rate performance of the cathode materials. However, due to the disordered doping of titanium ions, they cannot be well exposed on the surface of the migration channels, so there is still considerable room for improvement in the cycle stability and rate performance of cathode materials through titanium doping. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a lithium iron phosphate cathode material and its preparation method. By adding a macromolecular polymer chelating agent (organic acid polymer) and reacting with doped titanium ions, the lithium iron phosphate cathode material is prepared, thereby improving the cycle stability and rate performance of the lithium battery cathode material. This can be achieved through the following technical solutions: A method for preparing a lithium iron phosphate cathode material includes the following steps: (1) Mix the chelating dispersant, titanium dioxide and some water and stir for 2-3 hours. Then add the phosphorus source, lithium source, coated carbon source and remaining water, and continue stirring for 0.5-1 hours to obtain the mixed slurry. (2) Grind the mixed slurry from step (1) at 20~50℃, and then spray dry it to obtain the lithium iron phosphate precursor; (3) In step (2), the lithium iron phosphate precursor is heated to 650-750℃ in 4-6 hours under nitrogen protection, and calcined in this temperature range for 8-12 hours. After cooling and crushing, lithium iron phosphate cathode material is obtained.

[0006] Furthermore, in step (1), the chelating dispersant is an organic acid polymer, the phosphorus source is iron phosphate, the lithium source is lithium carbonate, the coating carbon source is a mixture of glucose and soluble starch, and the titanium dioxide particle size D50 ≤ 0.4 μm.

[0007] Further, the organic acid polymer mentioned in step (1) is at least one of polyacrylic acid (PAA), polymaleic acid (HPMA), acrylic acid-2-acrylamide-2-methylpropanesulfonic acid copolymer (AA-AMPS) or acrylic acid-2-acrylamide-2-methylpropanesulfonic acid-acrylamide terpolymer (AA-AMPS-AM); the glucose and soluble starch are in a mass ratio of 1:0.8~1.2.

[0008] Furthermore, the weight-average molecular weight of the organic acid polymer is 1000~10000.

[0009] Further, in step (1), the soluble starch is any one of corn starch, potato starch or sweet potato starch; the molar ratio of iron phosphate and lithium carbonate is 1:1.03~1.06; and the amount of glucose added is 5~8% of the mass of iron phosphate.

[0010] Further, in step (1), the amount of titanium dioxide added is 0.5 to 1% of the mass of ferric phosphate; the amount of pure water used is 1 to 1.5 times the mass of ferric phosphate; and the amount of chelating dispersant added is 5 to 10 times the mass of titanium dioxide.

[0011] Further, the mixed slurry in step (2) is ground at 20~50℃ until the particle size D50≤0.6μm, and then spray-dried to obtain a lithium iron phosphate precursor with a particle size D50≤30μm and a water content ≤1%.

[0012] Furthermore, the spray pressure of the spray dryer in step (3) is 0.3~0.4MPa, the inlet temperature is 200~220℃, and the outlet temperature is 120~130℃.

[0013] This invention also provides a lithium iron phosphate cathode material prepared by the above method.

[0014] Beneficial effects of this invention: The method for preparing lithium iron phosphate cathode material provided by this invention involves first chelating a polymer chelating agent with titanium, then adding other raw materials and mixing them. The mixture is then ground, dried, calcined, and pulverized to obtain the lithium iron phosphate cathode material. The titanium doped in this method forms a linear order through chelation with long-chain polymers, which can be exposed on the surface of the lithium iron phosphate atomic layer during the preparation process, thereby improving electronic conductivity. The long-chain structure of the polymer can promote the orderly arrangement of the lithium iron phosphate atomic layer, further improving the cycle stability and rate performance of the cathode material. Attached Figure Description

[0015] Figure 1 SEM image of the lithium iron phosphate cathode material prepared in Example 1; Figure 2 The image shows a SEM image of the lithium iron phosphate cathode material prepared in Comparative Example 1. Detailed Implementation

[0016] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below. Based on the embodiments in this application, other similar embodiments obtained by those skilled in the art without creative effort should all fall within the scope of protection of this application.

[0017] The titanium dioxide used in the embodiments and comparative examples of this invention was pretreated and ground to a particle size D50 = 0.35 μm.

[0018] Example 1

[0019] (1) Mix 25g polyacrylic acid (weight average molecular weight of 4000), 5g titanium dioxide and 200g water and stir for 2h. Then add 800g pure water, 1000g iron phosphate, 252.3g lithium carbonate, 50g glucose and 40g corn starch and stir for 0.5h to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred to a ball mill, the ball milling temperature is controlled to be below 50℃, and the ball milling is performed for 1 hour. Then it is transferred to a sand mill, the sand milling temperature is controlled to be below 50℃, and the sand milling is performed for 1 hour to obtain a slurry with a particle size D50=0.52μm. The slurry is then spray-dried with a spray pressure of 0.3MPa. The inlet temperature of the spray dryer is set to 200℃ and the outlet temperature is set to 120℃. The spray granulation yields a lithium iron phosphate precursor with a D50=28.56μm and a particle moisture content of 0.85%. (3) Under nitrogen protection, the lithium iron phosphate precursor in step (2) was heated to 650°C in a calcining furnace for 4 hours, then held at 650°C for 8 hours, and finally cooled naturally to 30°C before being removed from the furnace. The calcined solid was then pulverized by airflow to obtain the finished lithium iron phosphate cathode material with a particle size D50 = 1.12 μm. The SEM image of the lithium iron phosphate cathode material prepared in Example 1 is shown below. Figure 1 As shown.

[0020] Example 2

[0021] (1) Mix 100g of polyacrylic acid (weight average molecular weight of 4000), 10g of titanium dioxide and 300g of water and stir for 3h. Then add 1200g of pure water, 1000g of iron phosphate, 259.7g of lithium carbonate, 80g of glucose and 96g of corn starch and stir for 1h to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred to a ball mill, the ball milling temperature is controlled to be below 50℃, and the ball milling is performed for 1 hour. Then it is transferred to a sand mill, the sand milling temperature is controlled to be below 50℃, and the sand milling is performed for 1 hour to obtain a slurry with a particle size D50=0.54μm. The slurry is then spray-dried with a spray pressure of 0.4MPa. The inlet temperature of the spray dryer is set to 220℃ and the outlet temperature is set to 130℃. The spray granulation yields a lithium iron phosphate precursor with a D50=29.17μm and a water content of 0.92%. (3) Under nitrogen protection, the above particles are heated to 750°C in a calcining furnace for 6 hours, then kept at 750°C for 12 hours, and finally cooled to 30°C before being removed from the furnace. The solid after calcination is pulverized by airflow to obtain the finished lithium iron phosphate cathode material with a particle size D50=1.10μm.

[0022] Example 3

[0023] (1) Mix 25g of polymaleic acid (weight average molecular weight of 1000), 5g of titanium dioxide and 200g of water and stir for 2 hours. Then add 800g of pure water, 1000g of iron phosphate, 252.3g of lithium carbonate, 50g of glucose and 40g of corn starch and stir for 0.5 hours to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred into a ball mill, and the ball milling temperature is controlled at 20~50℃. After ball milling for 1 hour, it is transferred into a sand mill, and the sand milling temperature is controlled at 50℃. After sand milling for 1 hour, a slurry with a particle size D50=0.55μm is obtained. The slurry is then spray-dried with a spray pressure of 0.3MPa. The inlet temperature of the spray dryer is set to 200℃ and the outlet temperature is set to 120℃. The spray granulation yields a lithium iron phosphate precursor with a D50=28.98μm and a water content of 0.83%. (3) Under nitrogen protection, the lithium iron phosphate precursor in step (2) is heated to 650°C in a calcining furnace for 4 hours, then kept at 650°C for 8 hours, and finally cooled to 30°C before being taken out of the furnace. The solid after calcination is pulverized by airflow to obtain the finished lithium iron phosphate cathode material with a particle size D50=1.20μm.

[0024] Example 4

[0025] (1) Mix 25g of acrylic acid-2-acrylamide-2-methylpropanesulfonic acid copolymer (weight average molecular weight of 5000), 5g of titanium dioxide and 200g of water and stir for 2h. Then add 800g of pure water, 1000g of iron phosphate, 252.3g of lithium carbonate, 50g of glucose and 40g of corn starch and stir for 0.5h to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred to a ball mill, the ball milling temperature is controlled to be below 50℃, and the ball milling is performed for 1 hour. Then it is transferred to a sand mill, the sand milling temperature is controlled to be below 50℃, and the sand milling is performed for 1 hour to obtain a slurry with a particle size D50=0.53μm. The slurry is then spray-dried with a spray pressure of 0.3MPa. The inlet temperature of the spray dryer is set to 200℃ and the outlet temperature is set to 120℃. The spray granulation yields a lithium iron phosphate precursor with a D50=28.31μm and a water content of 0.82%. (3) Under nitrogen protection, the lithium iron phosphate precursor in step (2) is heated to 650°C in a calcining furnace for 4 hours, then kept at 650°C for 8 hours, and finally cooled to 30°C before being taken out of the furnace. The solid after calcination is pulverized by airflow to obtain the finished lithium iron phosphate cathode material with a particle size D50=1.13μm.

[0026] Comparative Example 1 Polyethylene glycol was used instead of polyacrylic acid, and all other conditions were the same as in Example 1, specifically: (1) Mix 25g polyethylene glycol (weight average molecular weight of 2000), 5g titanium dioxide and 200g water and stir for 2h. Then add 800g pure water, 1000g iron phosphate, 252.3g lithium carbonate, 50g glucose and 40g corn starch and stir for 0.5h to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred to a ball mill, the ball milling temperature is controlled to be below 50℃, and the ball milling is performed for 1 hour. Then it is transferred to a sand mill, the sand milling temperature is controlled to be below 50℃, and the sand milling is performed for 1 hour to obtain a slurry with a particle size D50=0.57μm. The slurry is then spray-dried with a spray pressure of 0.3MPa. The inlet temperature of the spray dryer is set to 200℃ and the outlet temperature is set to 120℃. The spray granulation yields a lithium iron phosphate precursor with a D50=29.26μm and a water content of 0.86%. (3) Under nitrogen protection, the lithium iron phosphate precursor in step (2) was heated to 650°C in a calcining furnace for 4 hours, then held at 650°C for 8 hours, and finally cooled naturally to 30°C before being removed from the furnace. The calcined solid was then pulverized by airflow to obtain the finished lithium iron phosphate with a particle size D50 = 1.15 μm. The SEM image of the lithium iron phosphate cathode material prepared in Comparative Example 1 is shown below. Figure 2 As shown.

[0027] Comparative Example 2 Using aminotrimethylphosphonic acid instead of polyacrylic acid, and with other conditions the same as in Example 1, specifically: (1) Mix 25g aminotrimethylphosphonic acid, 5g titanium dioxide and 200g water and stir for 2h. Then add 800g pure water, 1000g iron phosphate, 252.3g lithium carbonate, 50g glucose and 40g corn starch and stir for 0.5h to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred to a ball mill, the ball milling temperature is controlled to be below 50℃, and the ball milling is performed for 1 hour. Then it is transferred to a sand mill, the sand milling temperature is controlled to be below 50℃, and the sand milling is performed for 1 hour to obtain a slurry with a particle size D50=0.55μm. The slurry is then spray-dried with a spray pressure of 0.3MPa. The inlet temperature of the spray dryer is set to 200℃ and the outlet temperature is set to 120℃. The spray granulation yields a lithium iron phosphate precursor with a D50=29.10μm and a water content of 0.84%. (3) Under nitrogen protection, the lithium iron phosphate precursor in step (2) is heated to 650°C in a roasting furnace for 4 hours, then kept at 650°C for 8 hours, and finally cooled to 30°C before being taken out of the furnace. The solid after roasting is pulverized by airflow to obtain the finished lithium iron phosphate with a particle size D50=1.21μm.

[0028] Comparative Example 3 The reaction principle involves a single mixing process to prepare a mixed slurry, with other conditions identical to those in Example 1: (1) Mix 25g polyacrylic acid (weight average molecular weight of 4000), 5g titanium dioxide, 1000g water, 1000g iron phosphate, 252.5g lithium carbonate, 50g glucose and 40g corn starch, and stir for 1 hour to obtain a mixed slurry. (2) The mixed slurry in step (1) is transferred to a ball mill, the ball milling temperature is controlled to be below 50℃, and the ball milling is performed for 1 hour. Then it is transferred to a sand mill, the sand milling temperature is controlled to be below 50℃, and the sand milling is performed for 1 hour to obtain a slurry with a particle size D50=0.55μm. The slurry is then spray-dried with a spray pressure of 0.3MPa. The inlet temperature of the spray dryer is set to 200℃ and the outlet temperature is set to 120℃. The spray granulation yields a lithium iron phosphate precursor with a D50=29.11μm and a particle moisture content of 0.88%. (3) Under nitrogen protection, the lithium iron phosphate precursor in step (2) is heated to 650°C in a roasting furnace for 4 hours, then kept at 650°C for 8 hours, and finally cooled to 30°C before being taken out of the furnace. The solid after roasting is pulverized by airflow to obtain the finished lithium iron phosphate with a particle size D50=1.14μm.

[0029] According to the national standard GB / T42161-2022 "Test Method for First Discharge Specific Capacity and First Charge-Discharge Efficiency of Lithium Iron Phosphate Electrochemical Performance", coin cells were made from lithium iron phosphate obtained in the examples and comparative examples, and their electrochemical performance was tested. The results are shown in Table 1 below: Table 1 0.1C initial efficiency (%) 99.44 99.35 99.54 99.34 99.14 99.01 99.05 98.62 0.1C discharge specific capacity (mAh / g) 160.67 161.04 160.83 161.75 160.55 158.30 157.69 157.13 1C First-time efficiency (%) 94.35 94.31 94.38 94.33 92.22 91.31 91.22 91.25 1C discharge specific capacity (mAh / g) 152.82 152.67 153.93 152.67 150.28 144.36 142.96 142.67 2C First-time efficiency (%) 93.45 93.28 93.80 93.72 91.85 89.44 90.06 89.52 2C discharge specific capacity (mAh / g) 148.42 148.12 150.04 149.53 145.21 140.11 139.47 138.41 The test results from the examples and comparative examples show that the battery performance of the examples is significantly improved compared with commercially available products. Compared with Comparative Example 1, the battery capacity and coulombic efficiency decay more slowly with the increase of discharge rate, and the battery performance is well maintained at high discharge rates. Compared with commercially available products, Comparative Example 2 does not show a significant improvement in performance. However, compared with lithium iron titanate prepared by direct mixing, the battery prepared by the method of this application shows a significant improvement in performance.

[0030] This indicates that using organic acid polymer chelating agents significantly improves battery performance compared to using organophosphorus chelating agents; and that using organic acid polymer chelating agents, compared to using polyethylene glycol, provides a more significant performance improvement at high discharge rates.

[0031] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: (1) Mix and stir the chelating dispersant, titanium dioxide and some water for 2-3 hours. The chelating dispersant is an organic acid polymer, which is at least one of polyacrylic acid, polymaleic acid, acrylic acid-2-acrylamide-2-methylpropanesulfonic acid copolymer or acrylic acid-2-acrylamide-2-methylpropanesulfonic acid-acrylamide terpolymer. Then add phosphorus source, lithium source, coated carbon source and remaining water, and continue stirring for 0.5-1 hours to obtain a mixed slurry. (2) Grind the mixed slurry from step (1) at 20~50℃, and then spray dry it to obtain the lithium iron phosphate precursor; (3) In step (2), the lithium iron phosphate precursor is heated to 650-750℃ in 4-6 hours under nitrogen protection, and calcined in this temperature range for 8-12 hours. After cooling and crushing, lithium iron phosphate cathode material is obtained.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step (1), the phosphorus source is iron phosphate, the lithium source is lithium carbonate, the coating carbon source is a mixture of glucose and soluble starch, and the titanium dioxide particle size D50 ≤ 0.4 μm.

3. The method for preparing lithium iron phosphate cathode material according to claim 2, characterized in that, The glucose and soluble starch mentioned in step (1) are in a mass ratio of 1:0.8~1.

2.

4. The method for preparing the lithium iron phosphate cathode material according to claim 2, characterized in that, The weight-average molecular weight of the organic acid polymer is 1000~10000.

5. The method for preparing the lithium iron phosphate cathode material according to claim 2, characterized in that, In step (1), the soluble starch is any one of corn starch, potato starch or sweet potato starch; the molar ratio of iron phosphate and lithium carbonate is 1:1.03~1.06; and the amount of glucose added is 5~8% of the mass of iron phosphate.

6. The method for preparing lithium iron phosphate cathode material according to claim 2, characterized in that, In step (1), the amount of titanium dioxide added is 0.5 to 1% of the mass of ferric phosphate; the amount of water used is 1 to 1.5 times the mass of ferric phosphate; and the amount of chelating dispersant added is 5 to 10 times the mass of titanium dioxide.

7. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The mixed slurry in step (2) is ground at 20~50℃ until the particle size D50≤0.6μm, and then spray-dried to obtain a lithium iron phosphate precursor with a particle size D50≤30μm and a water content ≤1%.

8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The spray pressure of the spray dryer in step (3) is 0.3~0.4MPa, the inlet temperature is 200~220℃, and the outlet temperature is 120~130℃.

9. A lithium iron phosphate cathode material prepared by the method according to any one of claims 1 to 8.

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