Lithium iron phosphate positive electrode material with low interface impedance as well as preparation method and application of lithium iron phosphate positive electrode material

Abstract

The invention relates to the field of lithium iron phosphate positive electrode materials, in particular to a lithium iron phosphate positive electrode material with low interface impedance as well as a preparation method and application thereof, which are used for solving the problems of low electrochemical performance, poor charge-discharge efficiency and short cycle life of the existing lithium iron manganese phosphate positive electrode material. According to the positive electrode material, oxygen is doped in a Li2ZrCl6 crystal structure, rapid transmission of lithium ions on an interface layer is promoted, the conductivity is increased, neodymium oxide and iridium are loaded on expanded graphite to form a conductive path, the loaded expanded graphite and polyphenylene sulfide are blended to form a conductive network, and the conductivity of the positive electrode material is improved through a synergistic effect; the high cycle stability and safety of the positive electrode material can reduce the full life cycle cost of an energy storage system, the high rate capability can realize rapid charging of electronic equipment, and the positive electrode material can be widely applied to the fields of portable electronic equipment, energy storage systems and the like.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode materials, specifically to lithium iron phosphate cathode materials with low interfacial impedance, their preparation methods, and applications. Background Technology

[0002] In the current field of battery technology, solid-state batteries have become a research hotspot due to their superior safety and energy density. In traditional solid-state batteries, poor interfacial contact between the solid electrolyte and the cathode material often leads to problems such as unstable electrical contact and obstructed ion transport, which greatly limits the performance of the battery. Lithium iron phosphate (LFP) has received widespread attention as a cathode material due to its low cost and good safety. However, in solid-state battery applications, the interfacial characteristics between LFP cathode material and solid electrolyte directly affect the performance of the battery. Traditional LFP cathode materials are prone to side reactions at the solid-state battery interface, resulting in the degradation of electrochemical performance. In addition, LFP cathode material itself has low ionic conductivity, which further exacerbates the challenges of applying the material in solid-state batteries. This invention provides a LFP cathode material with low interfacial impedance to improve the performance of lithium-ion batteries. Summary of the Invention

[0003] In order to overcome the above-mentioned technical problems, the present invention aims to provide a lithium iron phosphate cathode material with low interface impedance, which solves the problems of low electrochemical performance, high interface impedance, poor charge and discharge efficiency and short cycle life of existing lithium manganese iron phosphate cathode materials.

[0004] The objective of this invention can be achieved through the following technical solutions: In a first aspect, the present invention provides a lithium iron phosphate cathode material with low interfacial impedance, comprising the following components by weight: 50-100 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 5-10 parts of oxygen-doped Li2ZrCl6 powder. The oxygen-doped Li₂ZrCl₆ powder is prepared by the following steps: Step A1: Zirconia, lithium oxide, and lithium chloride are added to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide, and lithium chloride to the mass of the grinding balls is 1:20. The mixture is premixed at 110 r / min for 1-2 h, and then ball-milled at 550 r / min for 14-16 h. The resulting powder is placed in a quartz tube, vacuum-pressed and sealed, calcined at 490℃ for 1-2 h, cooled, and ground to obtain oxygen-doped Li2ZrCl6 powder. The ratio of zirconium oxide, lithium oxide, and lithium chloride used in step A1 is 2-4g: 2-4g: 6-12g.

[0005] As a further aspect of the present invention: the polyphenylene sulfide blend-modified expanded graphite-coated lithium iron phosphate powder is prepared by the following steps: Step B1: Add ferrous sulfate heptahydrate, ascorbic acid, and anhydrous ethanol to a three-necked flask equipped with a stirrer and thermometer. Stir for 5 min at 50-70℃ and a stirring rate of 300-400 r / min. Then add phosphoric acid and stir for 1-2 h at a stirring rate of 500-1000 r / min. Next, add lithium hydroxide monohydrate and stir for 1-2 h. After the reaction, place the suspension in a constant temperature water bath and stir for 6-7 h at 60-80℃. Filter the suspension obtained from the reaction. Centrifuge the filter cake three times and place it in a drying oven to vacuum dry at 70-90℃ for 4-6 h to obtain the lithium iron phosphate precursor. Step B2: Add graphite powder, sodium nitrate, and concentrated sulfuric acid to a three-necked flask equipped with a thermometer. Place the flask in an ice-water bath and stir with a magnetic stir bar for 5-10 minutes. Add potassium permanganate and stir for 5-10 minutes. Then transfer the flask to an oil bath at 30-35°C and stir for 1-2 hours. Add deionized water and stir for 5-10 minutes. Then raise the oil bath temperature to 98-100°C and stir for 20-30 minutes. Add distilled water at 50-60°C and add hydrogen peroxide dropwise. After cooling, centrifuge at 800-10000 r / min. Wash the lower layer solution with water 3-5 times. Freeze-dry and then heat in a tube furnace at 600°C to expand, obtaining expanded graphite. Step B3: Add expanded graphite and deionized water to a flask equipped with a stirrer, sonicate for 8-10 min, add neodymium nitrate solution dropwise, stir at 800-900 r / min for 15 min, add sodium hydroxide solution dropwise to react, control the pH value to 7-9, let stand for 30 min, filter, wash 3 times with anhydrous ethanol, place in a drying oven and vacuum dry at 70-80℃ for 30-40 min to obtain neodymium hydroxide-expanded graphite composite powder; Step B4: Place the neodymium hydroxide-expanded graphite composite powder into a tube furnace, purge it with argon gas, and calcine it at 800°C for 3 hours to obtain neodymium oxide-expanded graphite composite powder. Step B5: Add neodymium oxide-expanded graphite composite powder and deionized water to a single-necked flask, sonicate for 20-30 min, add iridium chloride, sonicate for 20-30 min, transfer the single-necked flask to a heating platform, add ethylene glycol, stir at 230-240℃ for 5-10 min, cool to 24-26℃, centrifuge, wash with deionized water, freeze dry to obtain expanded graphite-supported neodymium oxide-iridium powder; Step B6: Add expanded graphite-supported neodymium oxide-iridium powder and polyphenylene sulfide to a drying oven and dry at 70-80℃ for 7-8 hours. Add the dried expanded graphite-supported neodymium oxide-iridium powder and polyphenylene sulfide to a high-speed mixer and mix evenly. After mixing, add the mixture to a micro twin-screw extruder, add tert-butylhydroquinone, and extrude and blend at a screw speed of 50 r / min. After extrusion, granulate the extrudate and put it into a drying oven and dry at 100℃ for 10 hours to obtain polyphenylene sulfide-modified expanded graphite. Step B7: Add polyphenylene sulfide-modified expanded graphite and tetrahydrofuran to a three-necked flask, sonicate for 20-30 min, add lithium iron phosphate precursor, sonicate for 1-2 h, stir for 6-8 h, purge with nitrogen, and heat treat at 600-700℃ for 4-6 h to obtain polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder.

[0006] As a further aspect of the present invention: the ratio of ferrous sulfate heptahydrate, ascorbic acid, anhydrous ethanol, phosphoric acid and lithium hydroxide monohydrate used in step B1 is 10-15 mmol: 10-20 mmol: 70-80 mL: 10-15 mmol: 10.5-15.75 mmol.

[0007] As a further aspect of the present invention: the ratio of graphite powder, sodium nitrate, concentrated sulfuric acid, potassium permanganate, deionized water, distilled water and hydrogen peroxide used in step B2 is 1-2g: 0.5-1g: 23-46mL: 3-6g: 46-92mL: 140-280mL: 20-40mL.

[0008] As a further aspect of the present invention: the mass fraction of hydrogen peroxide in step B2 is 27.5%.

[0009] As a further aspect of the present invention: the molar concentration of the concentrated sulfuric acid in step B2 is 18.4 mol / L.

[0010] As a further aspect of the present invention: the ratio of expanded graphite, deionized water and neodymium nitrate solution used in step B3 is 1-2g: 500-1000mL: 250-500mL.

[0011] As a further aspect of the present invention: the molar concentration of the neodymium nitrate solution in step B3 is 0.1 mol / L.

[0012] As a further aspect of the present invention: the molar concentration of the sodium hydroxide solution in step B3 is 0.5 mol / L.

[0013] As a further aspect of the present invention: the ratio of neodymium oxide-expanded graphite composite powder, deionized water, iridium chloride and ethylene glycol in step B5 is 1-2g: 40-80mL: 0.24-0.48mmol: 50-100mL.

[0014] As a further aspect of the present invention: the ratio of expanded graphite-supported neodymium-iridium oxide powder, polyphenylene sulfide, and tert-butylhydroquinone in step B6 is 1-2g: 5.67-11.34g: 2-3g.

[0015] As a further aspect of the present invention: the polyphenylene sulfide mentioned in step B6 is of type PPS 6345A4.

[0016] As a further aspect of the present invention: the ratio of polyphenylene sulfide blended modified expanded graphite, tetrahydrofuran and lithium iron phosphate precursor in step B7 is 1-2g: 30-50mL: 5-10g.

[0017] Secondly, this invention provides a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, comprising the following steps: Step 1: Weigh out 50-100 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 5-10 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step 2: Mix polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain lithium iron phosphate cathode material with low interfacial impedance.

[0018] Thirdly, this invention provides a method for preparing lithium iron phosphate cathode materials with low interfacial impedance, and the application of the prepared lithium manganese iron phosphate cathode materials in lithium-ion batteries.

[0019] The beneficial effects of this invention are: The low interfacial impedance lithium iron phosphate cathode material of the present invention improves conductivity by doping Li2ZrCl6 with oxygen, loads neodymium oxide and iridium on expanded graphite, and blends them with polyphenylene sulfide to form a conductive three-dimensional network coating the surface of lithium iron phosphate. After mixing with oxygen-doped Li2ZrCl6 powder, an interfacial layer is formed between the polyphenylene sulfide-modified expanded graphite coating lithium iron phosphate powder and the solid electrolyte, thereby improving the conductivity of the lithium iron phosphate cathode material, reducing the interfacial impedance, and effectively improving the overall performance and service life of the solid-state battery.

[0020] To prepare lithium iron phosphate cathode materials with low interfacial impedance, oxygen-doped Li₂ZrCl₆ powder was first prepared. This powder was obtained by ball milling zirconium oxide, lithium oxide, and lithium chloride. Oxygen doping alters the crystal structure of Li₂ZrCl₆, increasing its ionic conductivity and promoting rapid lithium-ion transport at the interface layer. Furthermore, the high oxidation potential of Li₂ZrCl₆ itself is well-matched with the lithium iron phosphate cathode material. The presence of the interface layer effectively reduces the interfacial resistance between the cathode material and the solid electrolyte, enhancing interfacial compatibility and reducing side reactions. Neodymium hydroxide was obtained by reacting sodium hydroxide solution with neodymium nitrate solution, and then loaded onto expanded graphite. The neodymium hydroxide was then converted to neodymium oxide at high temperature, yielding a neodymium oxide-expanded graphite composite powder. This neodymium oxide-expanded graphite composite powder was then reacted with iridium chloride... Expanded graphite-supported neodymium oxide-iridium powder is developed. Expanded graphite, made from natural graphite through an expansion process, retains the layered structure of graphite and possesses excellent conductivity, providing a conductive path for neodymium oxide and iridium. Neodymium oxide, due to the presence of electron holes on its surface, forms conductive channels, while iridium exhibits good conductivity. The expanded graphite-supported neodymium oxide-iridium powder is then blended with polyphenylene sulfide (PPS). PPS is uniformly mixed with the expanded graphite-supported neodymium oxide-iridium powder through melt blending, forming a conductive network. The conductivity of the expanded graphite-supported neodymium oxide-iridium powder and the conductive network of PPS can significantly reduce the charge transfer resistance of lithium iron phosphate (LFP) batteries, improving the conductivity of the LFP cathode material. This LFP cathode material can significantly improve the energy density and cycle life of solid-state batteries, meeting the requirements of long range, high safety, and fast charging for electric vehicles. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1: This embodiment describes a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, including the following steps: Step S1: Add 2g of zirconium oxide, 2g of lithium oxide and 6g of lithium chloride to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:20. Premix at 110 r / min for 1 h, ball mill at 550 r / min for 14 h, put the obtained powder into a quartz tube, vacuum press and seal, calcine at 490℃ for 1 h, cool, and grind to obtain oxygen-doped Li2ZrCl6 powder. Step S2: Add 10 mmol of ferrous sulfate heptahydrate, 10 mmol of ascorbic acid and 70 mL of anhydrous ethanol to a three-necked flask equipped with a stirrer and thermometer. Stir for 5 min at 50 °C and a stirring rate of 300 r / min. Then add 10 mmol of phosphoric acid and stir for 1 h at a stirring rate of 500 r / min. Then add 10.5 mmol of lithium hydroxide monohydrate and stir for 1 h. After the reaction, place the suspension in a constant temperature water bath and stir for 6 h at 60 °C. Filter the suspension obtained from the reaction. Centrifuge the filter cake three times and place it in a drying oven to vacuum dry at 70 °C for 4 h to obtain the lithium iron phosphate precursor. Step S3: Add 1g of graphite powder, 0.5g of sodium nitrate, and 23mL of concentrated sulfuric acid to a three-necked flask equipped with a thermometer. Place the flask in an ice-water bath and stir with a magnetic stir bar for 5 minutes. Add 3g of potassium permanganate and stir for 5 minutes. Then transfer the flask to a 30℃ oil bath and stir for 1 hour. Add 4mL of deionized water and stir for 5 minutes. Then raise the oil bath temperature to 98℃ and stir for 20 minutes. Add 140mL of 50℃ distilled water and add 20mL of hydrogen peroxide dropwise. After cooling, centrifuge at 800r / min. Wash the lower layer solution with water three times. Freeze-dry the solution and heat it in a 600℃ tube furnace to expand it, thus obtaining expanded graphite. Step S4: Add 1g of expanded graphite and 500mL of deionized water to a flask equipped with a stirrer, sonicate for 8min, add 250mL of neodymium nitrate solution dropwise, stir at 800r / min for 15min, add sodium hydroxide solution dropwise to react, control the pH value to 7, let stand for 30min, filter, wash 3 times with anhydrous ethanol, place in a drying oven and vacuum dry at 70℃ for 30min to obtain neodymium hydroxide-expanded graphite composite powder; Step S5: Place the neodymium hydroxide-expanded graphite composite powder into a tube furnace, purge it with argon gas, and calcine it at 800°C for 3 hours to obtain neodymium oxide-expanded graphite composite powder. Step S6: Add 1g of neodymium oxide-expanded graphite composite powder and 40mL of deionized water to a single-necked flask, sonicate for 20min, add 0.24mmol of iridium chloride, sonicate for 20min, transfer the single-necked flask to a heating platform, add 50mL of ethylene glycol, stir at 230℃ for 5min, cool to 24℃, centrifuge, wash with deionized water, freeze dry to obtain expanded graphite-supported neodymium oxide-iridium powder; Step S7: Add 1g of expanded graphite supported neodymium-iridium oxide powder and 5.67g of polyphenylene sulfide PPS 6345A4 to a drying oven and dry at 70℃ for 7h. Add the dried expanded graphite supported neodymium-iridium oxide powder and polyphenylene sulfide to a high-speed mixer and mix evenly. After mixing, add the mixture to a micro twin-screw extruder and add 2g of tert-butylhydroquinone. Extrude and blend at a screw speed of 50r / min. After extrusion, granulate the extrudate and put it into a drying oven and dry at 100℃ for 10h to obtain polyphenylene sulfide blended modified expanded graphite. Step S8: Add 1g of polyphenylene sulfide blended modified expanded graphite and 30mL of tetrahydrofuran to a three-necked flask, sonicate for 20min, add 5g of lithium iron phosphate precursor, sonicate for 1h, stir for 6h, purge with nitrogen, and heat treat at 600℃ for 4h to obtain polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder. Step S9: Weigh out 50 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 5 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step S10: Mix polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain a lithium iron phosphate cathode material with low interfacial impedance.

[0023] Example 2: This embodiment describes a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, including the following steps: Step S1: Add 3g of zirconium oxide, 3g of lithium oxide and 9g of lithium chloride to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:20. Premix at 110 r / min for 1.5 h, ball mill at 550 r / min for 15 h, put the obtained powder into a quartz tube, vacuum press and seal, calcine at 490℃ for 1.5 h, cool, and grind to obtain oxygen-doped Li2ZrCl6 powder. Step S2: 12.5 mmol ferrous sulfate heptahydrate, 15 mmol ascorbic acid, and 75 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 5 min at 60 °C and a stirring rate of 350 r / min. Then, 12.5 mmol phosphoric acid was added, and the mixture was stirred for 1.5 h at a stirring rate of 750 r / min. After that, 13.13 mmol lithium hydroxide monohydrate was added, and the mixture was stirred for 1.5 h. After the reaction, the suspension was placed in a constant temperature water bath and stirred for 6.5 h at 70 °C. The suspension obtained from the reaction was filtered, and the filter cake was centrifuged three times and then placed in a drying oven and vacuum dried at 80 °C for 5 h to obtain the lithium iron phosphate precursor. Step S3: Add 1.5g graphite powder, 0.75g sodium nitrate, and 34.5mL concentrated sulfuric acid to a three-necked flask equipped with a thermometer. Place the flask in an ice-water bath and stir with a magnetic stir bar for 7 minutes. Add 4.5g potassium permanganate and stir for 7 minutes. Then transfer the flask to an oil bath at 33℃ and stir for 1.5 hours. Add 69mL deionized water and stir for 7 minutes. Then raise the oil bath temperature to 99℃ and stir for 25 minutes. Add 210mL distilled water at 55℃ and add 30mL hydrogen peroxide dropwise. After cooling, centrifuge at 900r / min. Wash the lower layer solution with water four times. Freeze-dry the solution and heat it in a tube furnace at 600℃ to expand it, thus obtaining expanded graphite. Step S4: Add 1.5g of expanded graphite and 750mL of deionized water to a flask equipped with a stirrer, sonicate for 9min, add 375mL of neodymium nitrate solution dropwise, stir at 850r / min for 15min, add sodium hydroxide solution dropwise to react, control the pH value to 8, let stand for 30min, filter, wash 3 times with anhydrous ethanol, place in a drying oven and vacuum dry at 75℃ for 35min to obtain neodymium hydroxide-expanded graphite composite powder; Step S5: Place the neodymium hydroxide-expanded graphite composite powder into a tube furnace, purge it with argon gas, and calcine it at 800°C for 3 hours to obtain neodymium oxide-expanded graphite composite powder. Step S6: Add 1.5g of neodymium oxide-expanded graphite composite powder and 60mL of deionized water to a single-necked flask, sonicate for 25min, add 0.36mmol of iridium chloride, sonicate for 25min, transfer the single-necked flask to a heating platform, add 75mL of ethylene glycol, stir at 235℃ for 7min, cool to 25℃, centrifuge, wash with deionized water, freeze dry to obtain expanded graphite-supported neodymium oxide-iridium powder; Step S7: Add 1.5g of expanded graphite supported Nd:iridium oxide powder and 8.51g of polyphenylene sulfide (PPS) 6345A4 to a drying oven and dry at 75°C for 7.5h. Add the dried expanded graphite supported Nd:iridium oxide powder and polyphenylene sulfide to a high-speed mixer and mix evenly. Then add the mixture to a micro twin-screw extruder and add 2.5g of tert-butylhydroquinone. Extrude and blend at a screw speed of 50r / min. After extrusion, granulate the extrudate and dry it in a drying oven at 100°C for 10h to obtain polyphenylene sulfide blended modified expanded graphite. Step S8: Add 1.5g of polyphenylene sulfide-modified expanded graphite and 40mL of tetrahydrofuran to a three-necked flask, sonicate for 25min, add 7.5g of lithium iron phosphate precursor, sonicate for 1.5h, stir for 7h, purge with nitrogen, and heat treat at 650℃ for 5h to obtain polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder. Step S9: Weigh out 75 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 7.5 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step S10: Mix polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain a lithium iron phosphate cathode material with low interfacial impedance.

[0024] Example 3: This embodiment describes a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, including the following steps: Step S1: Add 4g of zirconium oxide, 4g of lithium oxide and 12g of lithium chloride to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:20. Premix at 110 r / min for 2 h, ball mill at 550 r / min for 16 h, put the obtained powder into a quartz tube, vacuum press and seal, calcine at 490℃ for 2 h, cool, and grind to obtain oxygen-doped Li2ZrCl6 powder. Step S2: 15 mmol ferrous sulfate heptahydrate, 20 mmol ascorbic acid and 80 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 5 min at 70 °C and a stirring rate of 400 r / min. Then 15 mmol phosphoric acid was added and the mixture was stirred for 2 h at a stirring rate of 1000 r / min. Then 15.75 mmol lithium hydroxide monohydrate was added and stirred for 2 h. After the reaction, the suspension was placed in a constant temperature water bath and stirred for 7 h at 80 °C. The suspension obtained from the reaction was filtered, and the filter cake was centrifuged three times and then placed in a drying oven and vacuum dried at 90 °C for 6 h to obtain the lithium iron phosphate precursor. Step S3: Add 2g of graphite powder, 1g of sodium nitrate, and 46mL of concentrated sulfuric acid to a three-necked flask equipped with a thermometer. Place the flask in an ice-water bath and stir with a magnetic stir bar for 10 minutes. Add 6g of potassium permanganate and stir for 10 minutes. Then transfer the flask to a 35℃ oil bath and stir for 2 hours. Add 92mL of deionized water and stir for 10 minutes. Then raise the oil bath temperature to 100℃ and stir for 30 minutes. Add 280mL of 60℃ distilled water and add 40mL of hydrogen peroxide dropwise. After cooling, centrifuge at 1000r / min. Wash the lower layer solution with water 5 times. Freeze-dry the solution and heat it in a 600℃ tube furnace to expand it, thus obtaining expanded graphite. Step S4: Add 2g of expanded graphite and 1000mL of deionized water to a flask equipped with a stirrer, sonicate for 10min, add 500mL of neodymium nitrate solution dropwise, stir at 900r / min for 15min, add sodium hydroxide solution dropwise to react, control the pH value to 8, let stand for 30min, filter, wash 3 times with anhydrous ethanol, place in a drying oven and vacuum dry at 80℃ for 40min to obtain neodymium hydroxide-expanded graphite composite powder; Step S5: Place the neodymium hydroxide-expanded graphite composite powder into a tube furnace, purge it with argon gas, and calcine it at 800°C for 3 hours to obtain neodymium oxide-expanded graphite composite powder. Step S6: Add 2g of neodymium oxide-expanded graphite composite powder and 80mL of deionized water to a single-necked flask, sonicate for 30min, add 0.48mmol of iridium chloride, sonicate for 30min, transfer the single-necked flask to a heating platform, add 100mL of ethylene glycol, stir at 240℃ for 10min, cool to 26℃, centrifuge, wash with deionized water, freeze dry to obtain expanded graphite-supported neodymium oxide-iridium powder; Step S7: Add 2g of expanded graphite supported neodymium-iridium oxide powder and 11.34g of polyphenylene sulfide PPS 6345A4 to a drying oven and dry at 80℃ for 8h. Add the dried expanded graphite supported neodymium-iridium oxide powder and polyphenylene sulfide to a high-speed mixer and mix evenly. After mixing, add the mixture to a micro twin-screw extruder and add 3g of tert-butylhydroquinone. Extrude and blend at a screw speed of 50r / min. After extrusion, granulate the extrudate and put it into a drying oven and dry at 100℃ for 10h to obtain polyphenylene sulfide blended modified expanded graphite. Step S8: Add 2g of polyphenylene sulfide blended modified expanded graphite and 50mL of tetrahydrofuran to a three-necked flask, sonicate for 30min, add 10g of lithium iron phosphate precursor, sonicate for 2h, stir for 8h, purge with nitrogen, and heat treat at 700℃ for 6h to obtain polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder. Step S9: Weigh out 100 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 10 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step S10: Mix polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain a lithium iron phosphate cathode material with low interfacial impedance.

[0025] Comparative Example 1: This comparative example demonstrates a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, comprising the following steps: Step S1: Add 4g of zirconium oxide, 4g of lithium oxide and 12g of lithium chloride to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:20. Premix at 110 r / min for 2 h, ball mill at 550 r / min for 16 h, put the obtained powder into a quartz tube, vacuum press and seal, calcine at 490℃ for 2 h, cool, and grind to obtain oxygen-doped Li2ZrCl6 powder. Step S2: 15 mmol ferrous sulfate heptahydrate, 20 mmol ascorbic acid and 80 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 5 min at 70 °C and a stirring rate of 400 r / min. Then 15 mmol phosphoric acid was added and the mixture was stirred for 2 h at a stirring rate of 1000 r / min. Then 15.75 mmol lithium hydroxide monohydrate was added and stirred for 2 h. After the reaction, the suspension was placed in a constant temperature water bath and stirred for 7 h at 80 °C. The suspension obtained from the reaction was filtered, and the filter cake was centrifuged three times and then placed in a drying oven and vacuum dried at 90 °C for 6 h to obtain the lithium iron phosphate precursor. Step S3: Add 2g of graphite powder, 1g of sodium nitrate, and 46mL of concentrated sulfuric acid to a three-necked flask equipped with a thermometer. Place the flask in an ice-water bath and stir with a magnetic stir bar for 10 minutes. Add 6g of potassium permanganate and stir for 10 minutes. Then transfer the flask to a 35℃ oil bath and stir for 2 hours. Add 92mL of deionized water and stir for 10 minutes. Then raise the oil bath temperature to 100℃ and stir for 30 minutes. Add 280mL of 60℃ distilled water and add 40mL of hydrogen peroxide dropwise. After cooling, centrifuge at 1000r / min. Wash the lower layer solution with water 5 times. Freeze-dry the solution and heat it in a 600℃ tube furnace to expand it, thus obtaining expanded graphite. Step S4: Add 2g of expanded graphite and 1000mL of deionized water to a flask equipped with a stirrer, sonicate for 10min, add 500mL of neodymium nitrate solution dropwise, stir at 900r / min for 15min, add sodium hydroxide solution dropwise to react, control the pH value to 8, let stand for 30min, filter, wash 3 times with anhydrous ethanol, place in a drying oven and vacuum dry at 80℃ for 40min to obtain neodymium hydroxide-expanded graphite composite powder; Step S5: Place the neodymium hydroxide-expanded graphite composite powder into a tube furnace, purge it with argon gas, and calcine it at 800°C for 3 hours to obtain neodymium oxide-expanded graphite composite powder. Step S6: Add 2g of neodymium oxide-expanded graphite composite powder and 80mL of deionized water to a single-necked flask, sonicate for 30min, add 0.48mmol of iridium chloride, sonicate for 30min, transfer the single-necked flask to a heating platform, add 100mL of ethylene glycol, stir at 240℃ for 10min, cool to 26℃, centrifuge, wash with deionized water, freeze dry to obtain expanded graphite-supported neodymium oxide-iridium powder; Step S7: Add 2g of polyphenylene sulfide (PPS) 6345A4 blended modified expanded graphite and 50mL of tetrahydrofuran to a three-necked flask, sonicate for 30min, add 10g of lithium iron phosphate precursor, sonicate for 2h, stir for 8h, purge with nitrogen, and heat treat at 700℃ for 6h to obtain modified expanded graphite coated lithium iron phosphate powder. Step S8: Weigh out 100 parts of modified expanded graphite-coated lithium iron phosphate powder and 10 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step S9: Mix modified expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain lithium iron phosphate cathode material with low interfacial impedance.

[0026] Comparative Example 2: This comparative example demonstrates a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, comprising the following steps: Step S1: Add 4g of zirconium oxide, 4g of lithium oxide and 12g of lithium chloride to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:20. Premix at 110 r / min for 2 h, ball mill at 550 r / min for 16 h, put the obtained powder into a quartz tube, vacuum press and seal, calcine at 490℃ for 2 h, cool, and grind to obtain oxygen-doped Li2ZrCl6 powder. Step S2: 15 mmol ferrous sulfate heptahydrate, 20 mmol ascorbic acid and 80 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 5 min at 70 °C and a stirring rate of 400 r / min. Then 15 mmol phosphoric acid was added and the mixture was stirred for 2 h at a stirring rate of 1000 r / min. Then 15.75 mmol lithium hydroxide monohydrate was added and stirred for 2 h. After the reaction, the suspension was placed in a constant temperature water bath and stirred for 7 h at 80 °C. The suspension obtained from the reaction was filtered, and the filter cake was centrifuged three times and then placed in a drying oven and vacuum dried at 90 °C for 6 h to obtain the lithium iron phosphate precursor. Step S3: Add 2g of polyphenylene sulfide (PPS) 6345A4 and 50mL of tetrahydrofuran to a three-necked flask, sonicate for 30min, add 10g of lithium iron phosphate precursor, sonicate for 2h, stir for 8h, purge with nitrogen, and heat treat at 700℃ for 6h to obtain polyphenylene sulfide-coated lithium iron phosphate powder. Step S4: Weigh out 100 parts of polyphenylene sulfide-coated lithium iron phosphate powder and 10 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step S5: Mix polyphenylene sulfide-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain lithium iron phosphate cathode material with low interfacial impedance.

[0027] Comparative Example 3: This comparative example demonstrates a method for preparing a lithium iron phosphate cathode material with low interfacial impedance, comprising the following steps: Step S1: Add 4g of zirconium oxide, 4g of lithium oxide and 12g of lithium chloride to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:20. Premix at 110 r / min for 2 h, ball mill at 550 r / min for 16 h, put the obtained powder into a quartz tube, vacuum press and seal, calcine at 490℃ for 2 h, cool, and grind to obtain oxygen-doped Li2ZrCl6 powder. Step S2: 15 mmol ferrous sulfate heptahydrate, 20 mmol ascorbic acid and 80 mL anhydrous ethanol were added to a three-necked flask equipped with a stirrer and thermometer. The mixture was stirred for 5 min at 70 °C and a stirring rate of 400 r / min. Then 15 mmol phosphoric acid was added and the mixture was stirred for 2 h at a stirring rate of 1000 r / min. Then 15.75 mmol lithium hydroxide monohydrate was added and stirred for 2 h. After the reaction, the suspension was placed in a constant temperature water bath and stirred for 7 h at 80 °C. The suspension obtained from the reaction was filtered, and the filter cake was centrifuged three times and then placed in a drying oven and vacuum dried at 90 °C for 6 h to obtain the lithium iron phosphate precursor. Step S3: Add 2g of graphite powder, 1g of sodium nitrate, and 46mL of concentrated sulfuric acid to a three-necked flask equipped with a thermometer. Place the flask in an ice-water bath and stir with a magnetic stir bar for 10 minutes. Add 6g of potassium permanganate and stir for 10 minutes. Then transfer the flask to a 35℃ oil bath and stir for 2 hours. Add 92mL of deionized water and stir for 10 minutes. Then raise the oil bath temperature to 100℃ and stir for 30 minutes. Add 280mL of 60℃ distilled water and add 40mL of hydrogen peroxide dropwise. After cooling, centrifuge at 1000r / min. Wash the lower layer solution with water 5 times. Freeze-dry the solution and heat it in a 600℃ tube furnace to expand it, thus obtaining expanded graphite. Step S4: Add 2g of expanded graphite and 11.34g of polyphenylene sulfide (PPS) 6345A4 to a drying oven and dry at 80℃ for 8 hours. Add the dried expanded graphite and polyphenylene sulfide to a high-speed mixer and mix evenly. Then add the mixture to a micro twin-screw extruder and add 3g of tert-butylhydroquinone. Extrude and blend at a screw speed of 50r / min. After extrusion, granulate the extrudate and put it into a drying oven to dry at 100℃ for 10 hours to obtain polyphenylene sulfide blended expanded graphite. Step S5: Add 2g of polyphenylene sulfide blended expanded graphite and 50mL of tetrahydrofuran to a three-necked flask, sonicate for 30min, add 10g of lithium iron phosphate precursor, sonicate for 2h, stir for 8h, purge with nitrogen, and heat treat at 700℃ for 6h to obtain polyphenylene sulfide blended expanded graphite coated lithium iron phosphate powder. Step S6: Weigh out 100 parts of polyphenylene sulfide blended expanded graphite coated lithium iron phosphate powder and 10 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step S7: Mix polyphenylene sulfide-coated expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain a lithium iron phosphate cathode material with low interfacial impedance.

[0028] Performance testing The lithium iron phosphate cathode material, conductive carbon black, and polyvinylidene fluoride HR460 of Examples 1-3 and Comparative Examples 1-3 were ground. Then, N-methylpyrrolidone was added to adjust the solid content to 55% and grinding continued. After grinding, the material was coated on aluminum foil current collector and dried in a constant temperature vacuum drying oven for 10 hours. It was then compacted in a roller press and finally punched into a circular electrode using a slicing machine to obtain the cathode. The lithium sheet was used as the anode, and the Celgard2400 battery separator was used as the separator. The electrolyte was 1M LiPF6 / EC+DEC+EMC (1:1:1, v / v / v). The battery was assembled using a CR2032 button cell casing. After injecting the electrolyte, the battery was sealed with 8-12kN pressure using a packaging machine to ensure that the sealing ring was fully pressed together, thus obtaining the lithium battery.

[0029] The sample batteries were subjected to 0.2C cycle performance testing at 25℃ using a Blue Electric Tester. The prepared sample batteries were placed in a constant temperature chamber at 25℃ for charge-discharge testing with a voltage range of 2.0V-3.75V. The batteries were first activated by 0.1C charge-discharge for one cycle, and then charged and discharged at 0.2C current (constant current charging was used) for 100 cycles. The relevant data of parameters such as the first discharge specific capacity and the capacity retention rate after 100 cycles were obtained.

[0030] This experiment used a CHI660C electrochemical workstation for the button cell. The test conditions were: AC voltage amplitude of 5mV, frequency range of 1Hz to 100kHz, and test temperature of 25℃, to obtain the interfacial impedance value.

[0031] The test results are shown in Table 1 below: Table 1. Initial discharge specific capacity, capacity retention at 100th cycle, conductivity, and interfacial impedance.

[0032] Referring to the table above, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that the cathode material obtained by mixing polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder has high electronic conductivity.

[0033] Based on the comparison between Example 3 and Comparative Example 1, it can be seen that the conductivity of the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder is higher than that of the cathode material obtained by mixing modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder. This indicates that the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder has excellent conductivity. Based on the comparison between Example 3 and Comparative Example 2, it can be seen that the conductivity of the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder is higher than that of the cathode material obtained by mixing polyphenylene sulfide-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder. This indicates that the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder has excellent conductivity. Based on the comparison between Example 3 and Comparative Example 3, it can be seen that the conductivity of the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder is higher than that of the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder. This indicates that the cathode material obtained by mixing polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder with oxygen-doped Li2ZrCl6 powder has excellent conductivity.

[0034] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0035] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A lithium iron phosphate cathode material with low interfacial impedance, characterized in that, Includes the following mass fractions: 50-100 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 5-10 parts of oxygen-doped Li2ZrCl6 powder. The oxygen-doped Li₂ZrCl₆ powder is prepared by the following steps: Step A1: Zirconia, lithium oxide and lithium chloride are added to a high-energy ball mill. The total mass ratio of zirconium oxide, lithium oxide and lithium chloride to the mass of the grinding balls is 1:

20. After premixing, the mixture is ball-milled. The resulting powder is placed in a quartz tube, vacuum-pressed and sealed, calcined and cooled, and then ground to obtain oxygen-doped Li2ZrCl6 powder. The ratio of zirconium oxide, lithium oxide, and lithium chloride used is 2-4g: 2-4g: 6-12g.

2. The lithium iron phosphate cathode material with low interfacial impedance according to claim 1, characterized in that, The polyphenylene sulfide blend modified expanded graphite coated lithium iron phosphate powder is prepared by the following steps: Step B1: Stir ferrous sulfate heptahydrate, ascorbic acid and anhydrous ethanol, add phosphoric acid and stir, add lithium hydroxide monohydrate and stir, after reaction, put it in a constant temperature water bath and stir, filter, centrifuge the filter cake and vacuum dry to obtain lithium iron phosphate precursor. Step B2: Add graphite powder, sodium nitrate, and concentrated sulfuric acid to a three-necked flask, stir in an ice-water bath, add potassium permanganate, stir in an oil bath, add deionized water, heat and stir, add distilled water, add hydrogen peroxide, centrifuge and wash, freeze dry and expand to obtain expanded graphite. Step B3: Ultrasonically mix expanded graphite and deionized water, add neodymium nitrate solution dropwise while stirring, add sodium hydroxide solution dropwise while reacting, control the pH value, let stand, filter, wash, and dry to obtain neodymium hydroxide-expanded graphite composite powder; Step B4: Place the neodymium hydroxide-expanded graphite composite powder into a tube furnace and calcine it with argon gas to obtain neodymium oxide-expanded graphite composite powder; Step B5: Disperse the neodymium oxide-expanded graphite composite powder with deionized water using ultrasound, add iridium chloride and disperse using ultrasound, heat and add ethylene glycol while stirring, cool and centrifuge, wash with deionized water, freeze dry to obtain expanded graphite-supported neodymium oxide-iridium powder. Step B6: After drying the expanded graphite-supported neodymium-iridium oxide powder and polyphenylene sulfide separately, add them to a high-speed mixer, mix them, add them to a micro twin-screw extruder, add tert-butylhydroquinone, extrude and blend, granulate the extrudate and dry it to obtain polyphenylene sulfide blended modified expanded graphite. Step B7: Polyphenylene sulfide-modified expanded graphite and tetrahydrofuran are ultrasonically treated, lithium iron phosphate precursor is added and ultrasonically treated, stirred, and heat-treated under nitrogen protection to obtain polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder.

3. The lithium iron phosphate cathode material with low interfacial impedance according to claim 2, characterized in that, The ratio of ferrous sulfate heptahydrate, ascorbic acid, anhydrous ethanol, phosphoric acid, and lithium hydroxide monohydrate used in step B1 is 10-15 mmol: 10-20 mmol: 70-80 mL: 10-15 mmol: 10.5-15.75 mmol.

4. The lithium iron phosphate cathode material with low interfacial impedance according to claim 2, characterized in that, In step B2, the ratio of graphite powder, sodium nitrate, concentrated sulfuric acid, potassium permanganate, deionized water, distilled water, and hydrogen peroxide is 1-2g:0.5-1g:23-46mL:3-6g:46-92mL:140-280mL:20-40mL; the mass fraction of the hydrogen peroxide is 27.5%; and the molar concentration of the concentrated sulfuric acid is 18.4mol / L.

5. The lithium iron phosphate cathode material with low interfacial impedance according to claim 2, characterized in that, In step B3, the ratio of expanded graphite, deionized water, and neodymium nitrate solution is 1-2g: 500-1000mL: 250-500mL; the molar concentration of the neodymium nitrate solution is 0.1mol / L; and the molar concentration of the sodium hydroxide solution is 0.5mol / L.

6. The lithium iron phosphate cathode material with low interfacial impedance according to claim 2, characterized in that, The ratio of neodymium oxide-expanded graphite composite powder, deionized water, iridium chloride, and ethylene glycol in step B5 is 1-2g: 40-80mL: 0.24-0.48mmol: 50-100mL.

7. The lithium iron phosphate cathode material with low interfacial impedance according to claim 2, characterized in that, In step B6, the ratio of expanded graphite-supported neodymium-iridium oxide powder, polyphenylene sulfide, and tert-butylhydroquinone is 1-2g: 5.67-11.34g: 2-3g.

8. The lithium iron phosphate cathode material with low interfacial impedance according to claim 2, characterized in that, In step B7, the ratio of polyphenylene sulfide blended modified expanded graphite, tetrahydrofuran, and lithium iron phosphate precursor is 1-2g: 30-50mL: 5-10g.

9. A method for preparing lithium iron phosphate cathode material with low interfacial impedance, characterized in that, The method for preparing the lithium iron phosphate cathode material with low interfacial impedance as described in any one of claims 1-8 includes the following steps: Step 1: Weigh out 50-100 parts of polyphenylene sulfide blended modified expanded graphite coated lithium iron phosphate powder and 5-10 parts of oxygen-doped Li2ZrCl6 powder according to the mass ratio, and set aside. Step 2: Mix polyphenylene sulfide-modified expanded graphite-coated lithium iron phosphate powder and oxygen-doped Li2ZrCl6 powder to obtain lithium iron phosphate cathode material with low interfacial impedance.

10. The application of a lithium iron phosphate cathode material with low interfacial impedance according to any one of claims 1-8, or a lithium iron phosphate cathode material with low interfacial impedance prepared by the preparation method of the lithium iron phosphate cathode material with low interfacial impedance according to claim 9, characterized in that, The lithium iron phosphate cathode material with low interfacial impedance is used in the preparation of lithium-ion batteries.

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