A method for repairing lithium iron phosphate cathode materials, a preparation method and an application thereof

Abstract

The present invention discloses a method for repairing lithium iron phosphate cathode materials, a preparation method and an application thereof. The preparation method includes: crushing the lithium iron phosphate cathode sheet to be treated to a particle size not greater than 50 mm; drying the crushed lithium iron phosphate cathode sheet, adding a cracking auxiliary material thereto and performing anaerobic cracking on it, separating lithium iron phosphate, the cracking auxiliary material and carbon in the cracked mixture to obtain lithium iron phosphate powder and performing impurity removal on it to obtain high-purity lithium iron phosphate material, adding a carbon source, a lithium source and an iron source thereto and performing ball milling on it, and then performing high-temperature sintering on the ball-milled mixture to obtain the repaired lithium iron phosphate cathode material. The present invention adopts anaerobic cracking technology to avoid corrosion and oxidation, large-size crushing avoids the introduction of aluminum impurities, carbon is removed separately, the removal efficiency is high, and the energy consumption is lower. The prepared repaired lithium iron phosphate cathode material has high purity, low impurity content and small particle size, and its first discharge specific capacity can reach up to 161 mAh / g at most, and the first discharge efficiency can reach up to 95.5% at most.

Description

Technical Field

[0001] The present invention relates to the field of lithium iron phosphate cathode materials, and particularly to a method for repairing lithium iron phosphate cathode materials, a preparation method and an application thereof. Background Art

[0002] With the continuous expansion of the new energy power vehicle market, the demand for power lithium batteries is also increasing day by day. At present, the main power lithium batteries are nickel-cobalt-manganese ternary lithium batteries and lithium iron phosphate batteries. With the gradual progress of battery module and battery PACK technologies, the performance of lithium iron phosphate batteries has been further improved. Compared with nickel-cobalt-manganese ternary lithium batteries, the installed capacity of lithium iron phosphate batteries is also showing a catching-up trend. As lithium iron phosphate batteries enter the large-scale retirement period in the future, the treatment of waste lithium iron phosphate batteries has become a major problem. Repairing and recycling the lithium iron phosphate in waste lithium iron phosphate batteries can not only solve environmental protection and resource waste problems, but also recycle resources and generate certain economic benefits.

[0003] At present, the processes for directly repairing and recycling waste lithium iron phosphate can generally be divided into the following steps: battery discharging, disassembly and electrode sheet sorting, crushing, primary sintering for degumming, and secondary sintering for composition regulation.

[0004] For example, the patent with publication number CN 112390239A provides a method for repairing crystal structure defects of lithium iron phosphate materials in waste batteries. The steps include high-temperature heat treatment to remove organic substances and carbon components, element content detection, composition regulation and nanoscale ball milling, drying and powder making, and high-temperature calcination for material recrystallization to finally obtain the repaired lithium iron phosphate material. However, while oxidizing and removing organic substances and carbon components, this process will oxidize divalent iron in lithium iron phosphate, and it is difficult to reduce it during the subsequent sintering process.

[0005] Therefore, based on the above problems, a method for repairing lithium iron phosphate combined with an anaerobic cracking process is designed, which can simplify the process and be highly efficient while removing impurities in waste lithium iron phosphate cathode sheets to the greatest extent. Summary of the Invention

[0006] The purpose of the present invention is to provide a solution for removing impurities in waste lithium iron phosphate cathode sheets to the greatest extent, then repairing lithium iron phosphate, and finally realizing the reuse of waste lithium iron phosphate cathode materials. The specific solution is as follows:

[0007] On the one hand, the present invention provides a preparation method for repairing lithium iron phosphate cathode materials, including the following steps:

[0008] S1: Crush the lithium iron phosphate cathode sheet to be treated to a particle size not greater than 50 mm;

[0009] S2: Dry the crushed lithium iron phosphate cathode sheets, and the moisture content of the dried lithium iron phosphate cathode sheets is < 1%;

[0010] S3: Add pyrolysis auxiliary materials to the dried lithium iron phosphate cathode sheets, and put the dried lithium iron phosphate cathode sheets with pyrolysis auxiliary materials into a pyrolysis furnace for anaerobic pyrolysis. The oxygen content in the anaerobic pyrolysis atmosphere is ≤ 0.2%. Pyrolyze at a pyrolysis temperature of 400 - 600 °C for 0.5 - 2 h, and control the residence time of the pyrolysis-generated gas in the pyrolysis furnace to be less than 10 s to obtain a mixture;

[0011] S4: Separate the lithium iron phosphate, pyrolysis auxiliary materials, and carbon in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF pyrolysis residual carbon, and carbon coated on the surface of lithium iron phosphate;

[0012] S5: Remove impurities from the lithium iron phosphate powder. The main impurities removed from the lithium iron phosphate powder are aluminum, copper, and iron oxide. The impurity removal methods include color sorting, magnetic separation, gravity separation, and flotation to obtain high-purity lithium iron phosphate material;

[0013] S6: Add a lithium source, a carbon source, and an iron source to the high-purity lithium iron phosphate material, and ball-mill the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source;

[0014] S7: Sinter the mixture after ball-milling in step S6 at a high temperature of 600 - 800 °C in an inert atmosphere to obtain the repaired lithium iron phosphate cathode material.

[0015] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate cathode material described in this patent, in step S3, the pyrolysis auxiliary materials include one or more of calcium oxide, potassium oxide, and sodium oxide;

[0016] In step S4, the separation method includes gravity separation and eddy current separation.

[0017] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate cathode material described in this patent, the particle size of the pyrolysis auxiliary materials is greater than 100 mesh and the Mohs hardness is greater than 6.0.

[0018] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate cathode material described in this patent, in step S1, the particle size of the crushed lithium iron phosphate cathode sheets is not greater than 30 mm;

[0019] In step S6, the ball-milling is planetary ball-milling.

[0020] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate cathode material described in this patent, in step S2, the moisture content of the dried lithium iron phosphate cathode sheets is < 0.5%;

[0021] In step S1, the lithium iron phosphate positive electrode sheets to be processed include lithium iron phosphate positive electrode sheet scraps, defective lithium iron phosphate batteries from battery factories, batteries that are scrapped after being recycled and reused, and lithium iron phosphate positive electrode sheets in waste lithium iron phosphate batteries that have been poorly maintained and stored for a long time.

[0022] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate positive electrode material described in this patent, in step S3, the residence time of the gas generated by pyrolysis in the pyrolysis furnace is controlled to be less than 3 s.

[0023] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate positive electrode material described in this patent, in step S5, the mass ratio of the pyrolysis auxiliary material to the dried lithium iron phosphate positive electrode sheet is 1:(25 - 50).

[0024] As a preferred embodiment of the preparation method of the repaired lithium iron phosphate positive electrode material described in this patent, in step S7, the high-temperature sintering temperature is 700 - 750 °C, and the sintering atmosphere includes nitrogen, helium or argon.

[0025] On the other hand, the present invention also provides a repaired lithium iron phosphate positive electrode material, which is obtained by the preparation method of the repaired lithium iron phosphate positive electrode material according to any of the above embodiments.

[0026] On the other hand, the repaired lithium iron phosphate positive electrode material prepared above is used alone for the preparation of lithium iron phosphate batteries; or

[0027] The repaired lithium iron phosphate positive electrode material prepared above is compounded with a commercial lithium iron phosphate positive electrode material for the preparation of lithium iron phosphate batteries, and the commercial lithium iron phosphate positive electrode material refers to the lithium iron phosphate positive electrode material purchased from the market.

[0028] Compared with the prior art, the present invention has at least one or more of the following beneficial effects:

[0029] 1. By simultaneously controlling the moisture content, oxygen content, and the residence time of the gas generated by pyrolysis in the pyrolysis furnace, this method avoids the corrosion of lithium iron phosphate by fluorides generated by the thermal decomposition of PVDF (polyvinylidene fluoride) during anaerobic pyrolysis, and at the same time avoids the oxidation of iron in lithium iron phosphate. The control conditions of these three factors, namely the moisture content, oxygen content, and residence time of the pyrolysis gas in the pyrolysis furnace, cooperate with each other, work synergistically, and are indispensable. In traditional oxidative atmosphere sintering, while removing the conductive agent carbon and PVDF in the lithium iron phosphate positive electrode sheet, the iron in lithium iron phosphate is also oxidized to form iron phosphate or iron oxide, etc., increasing the difficulty of subsequent process repair. Through anaerobic pyrolysis treatment, not only can the adhesiveness of PVDF in the lithium iron phosphate positive electrode sheet be invalidated, but also no new impurities are introduced into the lithium iron phosphate positive electrode sheet;

[0030] 2. By controlling the introduction of impurities during the cracking process, carbon and other impurities are removed separately in the subsequent process, with high removal efficiency, high purity and lower energy consumption. At the same time, no new impurities will be introduced during the treatment process, which reduces the pressure of impurity removal at the back end and ensures the purity of high-purity lithium iron phosphate materials;

[0031] 3. The repaired lithium iron phosphate positive electrode material obtained by the present invention has the characteristics of high purity, low impurity content, and small particle size (nanoscale). Its first discharge specific capacity can reach up to 161 mAh / g, and its first discharge efficiency can reach up to 95.5%;

[0032] 4. Through large-size crushing, the introduction of aluminum powder into the lithium iron phosphate positive electrode sheet to be processed can be avoided, and impurities are reduced in the pre-treatment stage;

[0033] 5. The impurity removal treatment after anaerobic pyrolysis is targeted impurity removal, which removes impurities such as aluminum, copper and iron oxide in lithium iron phosphate powder to obtain high-purity lithium iron phosphate material;

[0034] 6. Ball milling refines the lithium iron phosphate particles in the high-purity lithium iron phosphate material after adding lithium source, carbon source and iron source, so that the lithium iron phosphate particles reach nanometer level, which helps to improve the electrochemical performance. At the same time, it helps to combine the lithium source, carbon source and iron source with the high-purity lithium iron phosphate material, and helps to control the content of lithium iron phosphate in the ball-milled mixture during the high-temperature sintering process in an inert atmosphere;

[0035] 7. Introduce cracking auxiliary materials to completely crack PVDF into carbon rather than fluorinated biphenyls. After PVDF is thermally decomposed, hydrogen fluoride, vinylidene fluoride monomer, and fluorine-substituted benzene substances (biphenyls) will be produced; among these substances, hydrogen fluoride, vinylidene fluoride monomer, and fluorine-substituted benzene will be removed in the form of gas, but the fluorine-substituted substances in the cracking residue are not easy to remove completely and will adhere to the lithium iron phosphate powder, thereby affecting the initial charge and discharge capacity, causing particle adhesion and affecting the purity of the electrolyte; in general, the cracking process needs to ensure that PVDF is completely decomposed. To achieve this goal, it is necessary to introduce cracking auxiliary materials to ensure that under the process conditions, PVDF is completely cracked into carbon rather than fluorinated biphenyls;

[0036] 8. Remove impurities such as aluminum, copper and iron oxide from lithium iron phosphate powder. When discharging and finely disassembling defective lithium iron phosphate batteries from battery factories, batteries that are scrapped after cascade utilization, and waste lithium iron phosphate batteries that are poorly maintained and stored for a long time, the aluminum of the aluminum foil on the lithium iron phosphate positive electrode sheet and the copper on the negative electrode sheet are easily mixed into the lithium iron phosphate. In addition, the long-term storage of the above-mentioned batteries easily oxidizes the iron in the lithium iron phosphate into iron oxide. The introduction of these impurities not only affects the purity of the repaired lithium iron phosphate positive electrode material, but also makes the performance unstable and the cycle performance deteriorates during subsequent use. Therefore, it is necessary to remove impurities from the lithium iron phosphate powder to obtain high-purity repaired lithium iron phosphate positive electrode material.

[0037] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0039] Figure 1 It is a process flow chart of the preparation method of the repaired lithium iron phosphate cathode material described in this patent;

[0040] Figure 2 It is the first charge-discharge curve graph of the repaired lithium iron phosphate cathode material in Example 1 of this patent. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] The following will describe the embodiments of the present invention in detail, and clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0042] The anoxia in this application means that the oxygen content approaches 0 (<0.5%) infinitely.

[0043] The present invention provides a repaired lithium iron phosphate cathode material, a preparation method and an application, which can remove impurities in waste lithium iron phosphate cathode sheets to the greatest extent, then repair high-purity lithium iron phosphate materials, and finally realize the reuse of waste lithium iron phosphate cathode materials. First, through discharging and fine disassembly, the lithium iron phosphate cathode sheets to be processed are obtained. After adding cracking auxiliary materials, the residual electrolyte and impurities such as PVDF on the surface of the lithium iron phosphate cathode sheets are removed by an anoxic cracking process, so that the lithium iron phosphate in the lithium iron phosphate cathode sheets is exposed and detached from the aluminum sheets, while avoiding the oxidation of lithium iron phosphate; in the subsequent process, the purpose of repairing waste lithium iron phosphate cathode materials is achieved through steps such as separation, impurity removal, component content regulation, and sintering, and high-performance and high-purity repaired lithium iron phosphate cathode materials that can be reused in the market are obtained.

[0044] The present invention provides an efficient repair and regeneration method for raw materials such as lithium iron phosphate cathode scraps, defective lithium iron phosphate batteries in battery factories, batteries that are scrapped after being recycled in echelon use, and waste lithium iron phosphate batteries that have been poorly maintained and stored for a long time. The raw materials are subjected to front-end discharging and refined disassembly to obtain the lithium iron phosphate cathode sheets to be processed. The method includes the following steps:

[0045] S1: Crush the lithium iron phosphate cathode sheets, and the particle size of the crushed lithium iron phosphate cathode sheets is not greater than 50 mm; the size of the lithium iron phosphate cathode sheets to be processed is relatively large when crushed to control the introduction of aluminum impurities;

[0046] S2: Dry the crushed lithium iron phosphate cathode sheets, and the moisture content of the dried lithium iron phosphate cathode sheets < 1%; preferably, the moisture content of the dried lithium iron phosphate cathode sheets < 0.5%; wherein, the moisture content of the dried lithium iron phosphate cathode sheets is detected by an on-line testing instrument;

[0047] S3: Add a cracking auxiliary material with a particle size greater than 100 mesh and a Mohs hardness greater than 6.0 to the dried lithium iron phosphate cathode sheets, and put the dried lithium iron phosphate cathode sheets added with the cracking auxiliary material into a cracking furnace for anaerobic cracking. The oxygen content in the anaerobic cracking atmosphere ≤ 0.2%, crack at a cracking temperature of 400 - 600 °C for 0.5 - 2 h, and control the residence time of the gas generated by cracking (i.e., cracking gas) in the cracking furnace to be less than 10 s to obtain a mixture; the residence time of the cracking gas is controlled by controlling the gas flow rate of the cracking furnace; preferably, control the residence time of the gas generated by cracking in the cracking furnace to be less than 3 s; the cracking auxiliary material includes one or more of calcium oxide, potassium oxide, and sodium oxide. Preferably, the cracking auxiliary material has a particle size greater than 100 mesh and a Mohs hardness greater than 6.0, which is convenient for separation after pyrolysis; the mass ratio of the cracking auxiliary material to the dried lithium iron phosphate cathode sheets is 1:(25 - 50). Adding the cracking auxiliary material completely cracks the PVDF in the lithium iron phosphate cathode sheets to be regenerated into solid carbon and gaseous fluorides, avoiding the influence of residual fluorides on the subsequent application of the lithium iron phosphate cathode sheets. After adding the cracking auxiliary material, it is detected that the residual fluorine content in the lithium iron phosphate powder after anaerobic cracking < 100 ppm, while if the cracking auxiliary material is not added, the residual fluorine content in the lithium iron phosphate powder usually reaches 1000 ppm;

[0048] S4: Separate the lithium iron phosphate, cracking auxiliary material, and carbon in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF cracking residual carbon, and carbon coated on the surface of lithium iron phosphate, etc. The separation method includes but is not limited to gravity separation and eddy current separation;

[0049] S5: Impurities are removed from the lithium iron phosphate powder after carbon removal to obtain high-purity lithium iron phosphate material; the impurities removed mainly include aluminum, copper, iron oxide, etc. in the lithium iron phosphate powder, and the removal methods include but are not limited to color sorting, magnetic separation, gravity separation, flotation, etc.

[0050] S6: A carbon source, a lithium source, and an iron source are added to the high-purity lithium iron phosphate material, and the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source is ball-milled; the ball-milling is preferably planetary ball-milling.

[0051] S7: The ball-milled mixture obtained in step S6 is sintered at a high temperature of 600 - 800 °C in an inert atmosphere to perform carbon coating, lithium supplementation, and iron supplementation on the ball-milled lithium iron phosphate, so as to obtain a high-purity lithium iron phosphate cathode material, that is, to repair the lithium iron phosphate cathode material; the high-temperature sintering temperature is preferably 700 - 750 °C, and the sintering atmosphere includes but is not limited to nitrogen, helium, or argon.

[0052] The prepared repaired lithium iron phosphate cathode material is mainly used in the preparation of lithium iron phosphate batteries. The repaired lithium iron phosphate cathode material is used alone or in combination with a commercial lithium iron phosphate cathode material for the preparation of lithium iron phosphate batteries. The commercial lithium iron phosphate cathode material refers to the lithium iron phosphate cathode material purchased from the market.

[0053] On the other hand, the present invention also provides a repaired lithium iron phosphate cathode material, which is prepared according to the above-mentioned preparation method of the repaired lithium iron phosphate cathode material.

[0054] Example 1

[0055] This example provides a preparation method of a repaired lithium iron phosphate cathode material, and the specific steps are as follows:

[0056] 1. The raw materials are the waste materials of lithium iron phosphate cathode sheets, defective lithium iron phosphate batteries from battery factories, batteries that are scrapped after being used in a second life, and waste lithium iron phosphate batteries that have been poorly maintained and stored for a long time. The defective lithium iron phosphate batteries from battery factories, batteries that are scrapped after being used in a second life, and waste lithium iron phosphate batteries that have been poorly maintained and stored for a long time are discharged and finely disassembled to obtain the lithium iron phosphate cathode sheets to be processed, and the waste materials of lithium iron phosphate cathode sheets are directly used as the lithium iron phosphate cathode sheets to be processed.

[0057] 2. The lithium iron phosphate cathode sheets to be processed are crushed and sheared, and the lithium iron phosphate cathode sheets to be processed are crushed and sheared into large pieces with a particle size not greater than 50 mm. The size of the crushed lithium iron phosphate cathode sheets to be processed is relatively large to control the introduction of aluminum impurities.

[0058] 3. The crushed lithium iron phosphate cathode sheets are dried to ensure that the moisture content of the dried lithium iron phosphate cathode sheets is lower than 0.5%; among them, the moisture content of the dried lithium iron phosphate cathode sheets is detected by an on-line testing instrument.

[0059] 4. Add pyrolysis auxiliary materials with a particle size greater than 100 mesh and a Mohs hardness greater than 6.0 to the dried lithium iron phosphate cathode sheet, and then put the dried lithium iron phosphate cathode sheet with pyrolysis auxiliary materials into a pyrolysis furnace for anaerobic pyrolysis. The oxygen content in the anaerobic pyrolysis atmosphere is ≤0.2%, pyrolyze at a pyrolysis temperature of 500 °C for 1 h, and control the residence time of the gas generated by pyrolysis (i.e., pyrolysis gas) in the pyrolysis furnace to be less than 3 s to obtain a mixture; the residence time of the pyrolysis gas is controlled by controlling the gas flow rate of the pyrolysis furnace; separate the carbon, pyrolysis auxiliary materials and lithium iron phosphate in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF pyrolysis residual carbon and carbon coated on the surface of lithium iron phosphate, etc. The separation methods include but are not limited to gravity separation and eddy current separation; the pyrolysis auxiliary materials include one or more of calcium oxide, potassium oxide and sodium oxide. Preferably, the pyrolysis auxiliary materials have a particle size greater than 100 mesh and a Mohs hardness greater than 6.0, which is convenient for separation after pyrolysis; the mass ratio of the pyrolysis auxiliary materials to the dried lithium iron phosphate cathode sheet is 1:(25 - 50). Adding pyrolysis auxiliary materials can completely pyrolyze the PVDF in the lithium iron phosphate cathode sheet to be regenerated into solid carbon and gaseous fluorides, avoiding the influence of residual fluorides on the subsequent application of the lithium iron phosphate cathode sheet. After adding pyrolysis auxiliary materials, the residual fluoride content in the lithium iron phosphate powder after anaerobic pyrolysis is detected to be <100 ppm; further, impurities such as aluminum, copper and iron oxide remaining in the lithium iron phosphate powder are removed to obtain high-purity lithium iron phosphate material, and the removal methods include but are not limited to color sorting, magnetic separation, gravity separation and flotation, etc.;

[0060] 5. Detect the Li (lithium) and Fe (iron) contents in the high-purity lithium iron phosphate material by ICP and determine the C (carbon) content by a carbon element determination method, and quantitatively add the above elements to the high-purity lithium iron phosphate material according to the results, so that the molar ratio of lithium:iron:phosphorus in the high-purity lithium iron phosphate material after adding lithium source, carbon source and iron source is (1 - 1.05):1:1. Mix the high-purity lithium iron phosphate material after adding lithium source, carbon source and iron source evenly by ball milling, and refine the lithium iron phosphate particles in the high-purity lithium iron phosphate material after adding lithium source, carbon source and iron source to the nanoscale, and finally sinter in an inert atmosphere at 750 °C to obtain a repaired lithium iron phosphate cathode material.

[0061] Perform performance testing on the prepared repaired lithium iron phosphate cathode material. First, the repaired lithium iron phosphate cathode material needs to be prepared into a button cell. The steps are as follows:

[0062] 1. Prepare the repaired lithium iron phosphate cathode material, acetylene black, and polyvinylidene fluoride (PVDF) obtained by the above method in a mass ratio of 80:10:10. First, dissolve PVDF in an appropriate amount of N-methylpyrrolidone (NMP) and stir magnetically for 1 h until the solution becomes transparent. Then, add acetylene black and the repaired lithium iron phosphate cathode material to the above solution and stir for 8 h for standby. During the mixing process, remove the material sticking to the wall and mix it into the slurry;

[0063] 2. Coat the above-mentioned uniformly mixed slurry on a smooth aluminum sheet. Then, place the coated aluminum sheet in a vacuum drying oven at 80 °C and bake for 12 h, and then take it out. Cut the baked aluminum sheet into circular pieces with a diameter of 14 mm and press them under a pressure of 2 MPa to be used as the positive electrode plate of the button battery;

[0064] Assemble the button battery in a glove box filled with dry argon gas, using a lithium metal sheet as the negative electrode, Celgard 2400 as the separator, and the electrolyte as 1.0 mol / L LiPF6 / EC + DMC + EMC, where the volume ratio of LiPF6 / EC, DMC, and EMC is 1:1:1. Assemble the button battery and let it stand for 12 h before testing; where LiPF6 is lithium hexafluorophosphate, EC is ethylene carbonate, DMC is dimethyl carbonate, and EMC is ethyl methyl carbonate.

[0065] Perform charge-discharge tests on the prepared button battery:

[0066] The button battery is charged at a constant current of 0.2C to 3.75V, and then discharged at a constant current of 0.2C to 2.7V, with cyclic charge and discharge. Calculate the gram capacity of the active material in the positive electrode of the button battery. As Figure 2 shown, according to Figure 2 it can be obtained that the first discharge specific capacity of the repaired lithium iron phosphate cathode material can reach 161 mAh / g, and the first discharge efficiency can reach 95.5%.

[0067] Example 2

[0068] This example provides a preparation method for a repaired lithium iron phosphate cathode material. The lithium iron phosphate cathode sheet to be treated is obtained by discharging and finely disassembling the battery after secondary utilization and then scrapping. The preparation method is as follows:

[0069] 1. Crush and shear the lithium iron phosphate cathode sheet to be treated until the particle size is not greater than 50 mm. The size of the crushed lithium iron phosphate cathode sheet to be treated is relatively large to control the introduction of aluminum impurities;

[0070] 2. Dry the crushed lithium iron phosphate cathode sheet to ensure that the moisture content of the dried lithium iron phosphate cathode sheet is lower than 0.5%; among them, the moisture content of the dried lithium iron phosphate cathode sheet is detected by an on-line testing instrument;

[0071] 3. Add pyrolysis auxiliary materials with a particle size greater than 100 mesh and a Mohs hardness greater than 6.0 to the dried lithium iron phosphate cathode sheet, and then place the dried lithium iron phosphate cathode sheet with pyrolysis auxiliary materials into a pyrolysis furnace for anaerobic pyrolysis. The oxygen content in the anaerobic pyrolysis atmosphere is ≤0.2%, pyrolyze at 500°C for 1 h, and control the residence time of the gas generated by pyrolysis (i.e., pyrolysis gas) in the pyrolysis furnace to be less than 3 s to obtain a mixture; the residence time of the pyrolysis gas is controlled by controlling the gas flow rate of the pyrolysis furnace; the pyrolysis auxiliary materials include one or more of calcium oxide, potassium oxide, and sodium oxide. Preferably, the particle size of the pyrolysis auxiliary materials is greater than 100 mesh for easy separation after pyrolysis; the mass ratio of the pyrolysis auxiliary materials to the dried lithium iron phosphate cathode sheet is 1:(25 - 50). Adding pyrolysis auxiliary materials completely pyrolyzes the PVDF in the lithium iron phosphate cathode sheet to be regenerated into solid carbon and gaseous fluorides, avoiding the influence of residual fluorides on the subsequent application of the lithium iron phosphate cathode sheet. After adding pyrolysis auxiliary materials, the residual fluorine content in the lithium iron phosphate powder after anaerobic pyrolysis is detected to be <100 ppm;

[0072] 4. Separate the carbon, pyrolysis auxiliary materials, and lithium iron phosphate in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF pyrolysis residual carbon, and carbon coated on the surface of lithium iron phosphate, etc. The separation methods include but are not limited to gravity separation and eddy current separation;

[0073] 5. Remove impurities from the lithium iron phosphate powder after carbon removal, generally removing impurities such as residual aluminum, copper, and iron oxide in the lithium iron phosphate powder to obtain high-purity lithium iron phosphate material; the impurity removal methods include but are not limited to color sorting, magnetic separation, gravity separation, and flotation, etc.;

[0074] 6. Detect the Li (lithium) and Fe (iron) contents in the high-purity lithium iron phosphate material by ICP and determine the C (carbon) content by a carbon element determination method, and add a certain amount of lithium source, carbon source, and iron source to the high-purity lithium iron phosphate material according to the results, so that the molar ratio of lithium:iron:phosphorus in the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source is (1 - 1.05):1:1, and ball-mill the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source;

[0075] 7. Sinter the ball-milled mixture obtained in step 6 at 800°C in an inert atmosphere to obtain a repaired lithium iron phosphate cathode material.

[0076] Prepare a button cell from the obtained repaired lithium iron phosphate cathode material according to the method for preparing a button cell in Example 1, and perform charge and discharge tests on the prepared button cell:

[0077] The coin cell is charged at a constant current of 0.2C to 3.75V, and then discharged at a constant current of 0.2C to 2.7V for cyclic charge and discharge. The gram capacity of the active material in the positive electrode of the coin cell is calculated. It is tested that the first discharge specific capacity of the repaired lithium iron phosphate positive electrode material can reach 152 mAh / g, and the first discharge efficiency can reach 93%.

[0078] Example 3

[0079] This example provides a preparation method for repairing lithium iron phosphate positive electrode materials. The used lithium iron phosphate batteries that have been poorly maintained and stored for a long time are discharged and finely disassembled to obtain the lithium iron phosphate positive electrode sheets to be processed. The preparation method is as follows:

[0080] 1. Crush and shear the lithium iron phosphate positive electrode sheets to be processed until the particle size is not greater than 50 mm. The size of the crushed lithium iron phosphate positive electrode sheets to be processed is relatively large to control the introduction of aluminum impurities;

[0081] 2. Dry the crushed lithium iron phosphate positive electrode sheets to ensure that the moisture content of the dried lithium iron phosphate positive electrode sheets is lower than 0.5%; among them, the moisture content of the dried lithium iron phosphate positive electrode sheets is detected by an on-line tester;

[0082] 3. Add cracking auxiliary materials with a particle size greater than 100 mesh and a Mohs hardness greater than 6.0 to the dried lithium iron phosphate positive electrode sheets, and then put the dried lithium iron phosphate positive electrode sheets with cracking auxiliary materials into a cracking furnace for anaerobic cracking. The oxygen content in the anaerobic cracking atmosphere is ≤0.2%. Cracking is carried out at 400°C for 2 h, and the residence time of the gas generated by cracking (i.e., cracking gas) in the cracking furnace is controlled to be less than 3 s to obtain a mixture; the residence time of the cracking gas is controlled by controlling the gas flow rate of the cracking furnace; the cracking auxiliary materials include one or more of calcium oxide, potassium oxide, and sodium oxide. Preferably, the cracking auxiliary materials have a particle size greater than 100 mesh and a Mohs hardness greater than 6.0, which is convenient for separation after pyrolysis; the mass ratio of the cracking auxiliary materials to the dried lithium iron phosphate positive electrode sheets is 1:(25 - 50). Adding cracking auxiliary materials completely cracks the PVDF in the lithium iron phosphate positive electrode sheets to be regenerated into solid carbon and gaseous fluorides, avoiding the influence of residual fluorides on the subsequent application of the lithium iron phosphate positive electrode sheets. After adding the cracking auxiliary materials, the residual fluorine content in the lithium iron phosphate powder after anaerobic cracking is detected to be <100 ppm;

[0083] 4. Separate the carbon, cracking auxiliary materials, and lithium iron phosphate in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF cracking residual carbon, and carbon coated on the surface of lithium iron phosphate, etc. The separation method includes but is not limited to gravity separation and eddy current separation;

[0084] 5. The lithium iron phosphate powder after carbon removal is purified, generally removing impurities such as residual aluminum, copper, and iron oxide in the lithium iron phosphate powder to obtain high-purity lithium iron phosphate material; the purification methods include but are not limited to color sorting, magnetic separation, gravity separation, and flotation, etc.

[0085] 6. The contents of Li (lithium) and Fe (iron) in the high-purity lithium iron phosphate material are detected by ICP, and the content of C (carbon) is determined by a carbon element determination method. Then, a certain amount of lithium source, carbon source, and iron source are added to the high-purity lithium iron phosphate material according to the results, so that the molar ratio of lithium: iron: phosphorus in the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source is (1 - 1.05):1:1, and the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source is ball-milled.

[0086] 7. The ball-milled mixture obtained in step 6 is sintered at a high temperature of 750 °C in an inert atmosphere to obtain a repaired lithium iron phosphate cathode material.

[0087] The obtained repaired lithium iron phosphate cathode material is made into a button cell according to the method of preparing a button cell in Example 1, and the prepared button cell is subjected to charge and discharge tests:

[0088] The button cell is charged at a constant current of 0.2C to 3.75V, and then discharged at a constant current of 0.2C to 2.7V, and the charge and discharge are cycled. The gram capacity of the active substance in the cathode of the button cell is calculated. It is tested that the first discharge specific capacity of the repaired lithium iron phosphate cathode material can reach 150 mAh / g, and the first discharge efficiency can reach 90%.

[0089] Example 4

[0090] This example provides a preparation method of a repaired lithium iron phosphate cathode material. The defective lithium iron phosphate battery in the battery factory is discharged and finely disassembled to obtain the lithium iron phosphate cathode sheet to be processed. The preparation method is as follows:

[0091] 1. The lithium iron phosphate cathode sheet to be processed is crushed and sheared. The lithium iron phosphate cathode sheet to be processed is crushed and sheared to a particle size not greater than 50 mm. The size of the crushed lithium iron phosphate cathode sheet to be processed is relatively large to control the introduction of aluminum impurities.

[0092] 2. The crushed lithium iron phosphate cathode sheet is dried to ensure that the moisture content of the dried lithium iron phosphate cathode sheet is lower than 0.5%; among them, the moisture content of the dried lithium iron phosphate cathode sheet is detected by an on-line testing instrument.

[0093] 3. Add cracking auxiliary materials with a particle size greater than 100 mesh and a Mohs hardness greater than 6.0 to the dried lithium iron phosphate cathode sheet. Then, place the dried lithium iron phosphate cathode sheet with the cracking auxiliary materials into a cracking furnace for anaerobic cracking. The oxygen content in the anaerobic cracking atmosphere is ≤0.2%, and crack for 1 h at 600 °C. Control the residence time of the gas generated by cracking in the cracking furnace to be less than 3 s to obtain a mixture; the residence time of the cracking gas is controlled by controlling the gas flow rate of the cracking furnace; the cracking auxiliary materials include one or more of calcium oxide, potassium oxide, and sodium oxide. Preferably, the cracking auxiliary materials have a particle size greater than 100 mesh and a Mohs hardness greater than 6.0, which is convenient for separation after pyrolysis; the mass ratio of the cracking auxiliary materials to the dried lithium iron phosphate cathode sheet is 1:(25 - 50). Adding the cracking auxiliary materials completely cracks the PVDF in the lithium iron phosphate cathode sheet to be regenerated into solid carbon and gaseous fluorides, avoiding the influence of residual fluorides on the subsequent application of the lithium iron phosphate cathode sheet. After adding the cracking auxiliary materials, the residual fluorine content in the lithium iron phosphate powder after anaerobic cracking is detected to be <100 ppm;

[0094] 4. Separate the carbon, cracking auxiliary materials, and lithium iron phosphate in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF cracking residual carbon, and carbon coated on the surface of lithium iron phosphate, etc. The separation methods include but are not limited to gravity separation and eddy current separation;

[0095] 5. Remove impurities from the lithium iron phosphate powder after carbon removal, generally removing impurities such as residual aluminum, copper, and iron oxide in the lithium iron phosphate powder to obtain high-purity lithium iron phosphate material; the impurity removal methods include but are not limited to color sorting, magnetic separation, gravity separation, and flotation, etc.;

[0096] 6. Detect the Li (lithium) and Fe (iron) contents in the lithium iron phosphate powder by ICP and determine the C (carbon) content by a carbon element determination method. And according to the results, add a certain amount of lithium source, carbon source, and iron source to the high-purity lithium iron phosphate powder so that the molar ratio of lithium:iron:phosphorus in the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source is (1 - 1.05):1:1, and ball mill the high-purity lithium iron phosphate material after adding the lithium source, carbon source, and iron source;

[0097] 7. Sinter the ball-milled mixture obtained in step 6 at a high temperature of 700 °C in an inert atmosphere to obtain a repaired lithium iron phosphate cathode material.

[0098] Prepare the obtained repaired lithium iron phosphate cathode material into a button cell according to the method for preparing a button cell in Example 1, and perform charge and discharge tests on the prepared button cell:

[0099] The coin-type battery is charged at a constant current of 0.2C to 3.75V, and then discharged at a constant current of 0.2C to 2.7V for cyclic charge and discharge. The gram capacity of the active material in the positive electrode of the coin-type battery is calculated. It is tested that the first discharge specific capacity of the repaired lithium iron phosphate positive electrode material can reach 155 mAh / g, and the first discharge efficiency can reach 93%.

[0100] Example 5

[0101] This example provides a preparation method of a repaired lithium iron phosphate positive electrode material, using the lithium iron phosphate positive electrode sheet scraps as the lithium iron phosphate positive electrode sheet to be treated. The preparation method is as follows:

[0102] 1. Crush and shear the lithium iron phosphate positive electrode sheet to be treated, and crush the lithium iron phosphate positive electrode sheet to be treated to a particle size not greater than 30 mm;

[0103] 2. Dry the crushed lithium iron phosphate positive electrode sheet to ensure that the moisture content of the dried lithium iron phosphate positive electrode sheet is lower than 0.5%; among them, the moisture content of the dried lithium iron phosphate positive electrode sheet is detected by an on-line tester;

[0104] 3. Add a cracking auxiliary material with a particle size greater than 100 mesh and a Mohs hardness greater than 6.0 to the dried lithium iron phosphate positive electrode sheet, and then put the dried lithium iron phosphate positive electrode sheet with the cracking auxiliary material into a cracking furnace for anaerobic cracking. The oxygen content in the anaerobic cracking atmosphere is ≤0.2%, and it is cracked at 500°C for 1 h. Control the residence time of the gas generated by cracking (i.e., cracking gas) in the cracking furnace to be less than 3 s to obtain a mixture; the residence time of the cracking gas is controlled by controlling the gas flow rate of the cracking furnace; the cracking auxiliary material includes one or more of calcium oxide, potassium oxide and sodium oxide. Preferably, the cracking auxiliary material has a particle size greater than 100 mesh and a Mohs hardness greater than 6.0, which is convenient for separation after pyrolysis; the mass ratio of the cracking auxiliary material to the dried lithium iron phosphate positive electrode sheet is 1:(25 - 50). Adding the cracking auxiliary material completely cracks the PVDF in the lithium iron phosphate positive electrode sheet to be regenerated into solid carbon and gaseous fluoride, avoiding the influence of residual fluoride on the subsequent application of the lithium iron phosphate positive electrode sheet. After adding the cracking auxiliary material, the residual fluoride content in the lithium iron phosphate powder after anaerobic cracking is detected to be <100 ppm;

[0105] 4. Separate the carbon, cracking auxiliary material and lithium iron phosphate in the mixture to obtain lithium iron phosphate powder. The carbon includes conductive agent carbon, PVDF cracking residual carbon, lithium iron phosphate surface-coated carbon, etc. The separation method includes but is not limited to gravity separation and eddy current separation;

[0106] 5. Remove impurities from the carbon-removed lithium iron phosphate powder, generally removing impurities such as residual aluminum, copper and iron oxide in the lithium iron phosphate powder to obtain high-purity lithium iron phosphate material; the impurity removal methods include but are not limited to color sorting, magnetic separation, gravity separation and flotation, etc.;

[0107] 6. Detect the Li (lithium) and Fe (iron) contents in the lithium iron phosphate powder by ICP, and determine the C (carbon) content by a carbon element determination method. Then, add a certain amount of lithium source, carbon source, and iron source to the high-purity lithium iron phosphate powder according to the results, so that the molar ratio of lithium:iron:phosphorus in the high-purity lithium iron phosphate powder after adding the lithium source, carbon source, and iron source is (1 - 1.05):1:1, and ball-mill the high-purity lithium iron phosphate powder after adding the lithium source, carbon source, and iron source;

[0108] 7. Sinter the ball-milled mixture obtained in step 6 at a high temperature of 600 °C in an inert atmosphere to obtain a repaired lithium iron phosphate cathode material.

[0109] Prepare a button cell with the obtained repaired lithium iron phosphate cathode material according to the method for preparing a button cell in Example 1, and perform charge-discharge tests on the prepared button cell:

[0110] The button cell is charged at a constant current of 0.2C to 3.75V, and then discharged at a constant current of 0.2C to 2.7V, and the charge-discharge cycle is carried out. Calculate the gram capacity of the active material in the positive electrode of the button cell. The first discharge specific capacity of the repaired lithium iron phosphate cathode material is measured to reach 147 mAh / g, and the first discharge efficiency can reach 90%.

[0111] The beneficial effects of the present invention are as follows:

[0112] 1. By simultaneously controlling the moisture content, oxygen content, and residence time of the gas generated by pyrolysis in the pyrolysis furnace, the corrosion of lithium iron phosphate by fluorides generated by the pyrolysis of PVDF (polyvinylidene fluoride) during anaerobic pyrolysis is avoided, and at the same time, the oxidation of iron in lithium iron phosphate is avoided. The control conditions of the moisture content, oxygen content, and residence time of the pyrolysis gas cooperate with each other, work synergistically, and are indispensable. While traditional oxidative atmosphere sintering removes the conductive agent carbon and PVDF in the lithium iron phosphate cathode sheet, it also causes the iron in lithium iron phosphate to oxidize, generating iron phosphate or iron oxide, etc., increasing the difficulty of subsequent process repair. Through anaerobic pyrolysis treatment, not only can the adhesiveness of PVDF in the lithium iron phosphate cathode sheet be invalidated, but also no new impurities can be introduced into the lithium iron phosphate cathode sheet;

[0113] 2. By controlling the introduction of impurities during the pyrolysis process, subsequent removal of carbon and other impurities is carried out separately, with high removal efficiency, high purity, lower energy consumption, and no new impurities are introduced during the treatment process, reducing the pressure of impurity removal at the backend and ensuring the purity of the high-purity lithium iron phosphate powder;

[0114] 3. The repaired lithium iron phosphate cathode material obtained by the present invention has the characteristics of high purity, low impurity content, and small particle size (nanoscale). Its first discharge specific capacity can reach up to 161 mAh / g at most, and the first discharge efficiency can reach up to 95% at most;

[0115] 4. Through large-size crushing, the introduction of aluminum powder into the lithium iron phosphate positive electrode sheet to be processed can be avoided, and impurities are reduced in the pre-treatment stage;

[0116] 5. The impurity removal treatment after anaerobic pyrolysis is targeted impurity removal, which removes impurities such as aluminum, copper and iron oxide in lithium iron phosphate powder to obtain high-purity lithium iron phosphate material;

[0117] 6. Ball milling refines the lithium iron phosphate particles in the high-purity lithium iron phosphate material after adding lithium source, carbon source and iron source, so that the lithium iron phosphate particles reach nanometer level, which helps to improve the electrochemical performance. At the same time, it helps to combine the lithium source, carbon source and iron source with the high-purity lithium iron phosphate material, and helps to control the content of lithium iron phosphate in the ball-milled mixture during the high-temperature sintering process in an inert atmosphere;

[0118] 7. Introduce cracking auxiliary materials to completely crack PVDF into carbon rather than fluorinated biphenyls. After PVDF is thermally decomposed, hydrogen fluoride, vinylidene fluoride monomer, and fluorine-substituted benzene substances (biphenyls) will be produced; among these substances, hydrogen fluoride, vinylidene fluoride monomer, and fluorine-substituted benzene will be removed in the form of gas, but the fluorine-substituted substances in the cracking residue are not easy to remove completely and will adhere to the lithium iron phosphate powder, thereby affecting the initial charge and discharge capacity, causing particle adhesion and affecting the purity of the electrolyte; in general, the cracking process needs to ensure that PVDF is completely decomposed. To achieve this goal, it is necessary to introduce cracking auxiliary materials to ensure that under the process conditions, PVDF is completely cracked into carbon rather than fluorinated biphenyls;

[0119] 8. Remove impurities such as aluminum, copper and iron oxide from lithium iron phosphate powder. When discharging and finely disassembling defective lithium iron phosphate batteries from battery factories, batteries that are scrapped after cascade utilization, and waste lithium iron phosphate batteries that are poorly maintained and stored for a long time, the aluminum of the aluminum foil on the lithium iron phosphate positive electrode sheet and the copper on the negative electrode sheet are easily mixed into the lithium iron phosphate. In addition, the long-term storage of the above-mentioned batteries easily oxidizes the iron in the lithium iron phosphate into iron oxide. The introduction of these impurities not only affects the purity of the repaired lithium iron phosphate positive electrode material, but also makes the performance unstable and the cycle performance deteriorates during subsequent use. Therefore, it is necessary to remove impurities from the lithium iron phosphate powder to obtain high-purity repaired lithium iron phosphate positive electrode material.

[0120] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A preparation method of a repaired lithium iron phosphate cathode material, characterized in that, It includes the following steps: S1: Crush the lithium iron phosphate cathode sheet to be processed to a particle size not greater than 50 mm; S2: Dry the crushed lithium iron phosphate cathode sheet, and the moisture content of the dried lithium iron phosphate cathode sheet < 1%; S3: Add cracking auxiliary materials to the dried lithium iron phosphate cathode sheet, and put the dried lithium iron phosphate cathode sheet with cracking auxiliary materials into a cracking furnace for anaerobic cracking. The oxygen content in the anaerobic cracking atmosphere ≤ 0.2%, crack at a cracking temperature of 400 - 500 °C for 0.5 - 2 h, and control the residence time of the gas generated by cracking in the cracking furnace to be less than 3 s to obtain a mixture; wherein, the mass ratio of the cracking auxiliary materials to the dried lithium iron phosphate cathode sheet is 1:(25 - 50); the residual fluorine content in the lithium iron phosphate powder after anaerobic cracking < 100 ppm; S4: Separate the lithium iron phosphate, cracking auxiliary materials and carbon in the mixture to obtain lithium iron phosphate powder, and the carbon includes conductive agent carbon, PVDF cracking residual carbon and carbon coated on the surface of lithium iron phosphate; S5: Remove impurities from the lithium iron phosphate powder. The main impurities removed from the lithium iron phosphate powder are aluminum, copper and iron oxide. The impurity removal methods include color sorting, magnetic separation, gravity separation and flotation to obtain high-purity lithium iron phosphate material; S6: Add a lithium source, a carbon source and an iron source to the high-purity lithium iron phosphate material, and ball-mill the high-purity lithium iron phosphate material after adding the lithium source, the carbon source and the iron source; the molar ratio of lithium:iron:phosphorus in the high-purity lithium iron phosphate material after adding the lithium source, the carbon source and the iron source is (1 - 1.05):1:1; S7: Sinter the ball-milled mixture obtained in step S6 at a high temperature of 700 - 750 °C in an inert atmosphere to obtain a repaired lithium iron phosphate cathode material.

2. The preparation method of the repaired lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the cracking auxiliary materials include one or more of calcium oxide, potassium oxide and sodium oxide; In step S4, the separation method includes gravity separation and eddy current separation.

3. The preparation method of the repaired lithium iron phosphate cathode material according to claim 2, wherein The particle size of the cracking auxiliary materials is greater than 100 mesh, and the Mohs hardness is greater than 6.

0.

4. The preparation method of the repaired lithium iron phosphate cathode material according to claim 1, wherein, In step S1, the particle size of the crushed lithium iron phosphate cathode sheet is not greater than 30 mm; In step S6, the ball-milling is planetary ball-milling.

5. The preparation method of the repaired lithium iron phosphate cathode material according to claim 1, wherein, In step S2, the moisture content of the dried lithium iron phosphate cathode sheet < 0.5%; In step S1, the lithium iron phosphate cathode sheet to be processed includes lithium iron phosphate cathode sheet scraps, defective lithium iron phosphate batteries from battery factories, batteries that are scrapped after being recycled and reused, and lithium iron phosphate cathode sheets in waste lithium iron phosphate batteries that have been poorly maintained and stored for a long time.

6. The preparation method of the repaired lithium iron phosphate cathode material according to claim 1, characterized in that, In step S7, the sintering atmosphere includes nitrogen, helium or argon.

7. A lithium iron phosphate cathode material for repair, characterized in that, The repaired lithium iron phosphate cathode material prepared by the preparation method of the repaired lithium iron phosphate cathode material according to any one of claims 1 - 6.

8. Application of a method for repairing a lithium iron phosphate cathode material in preparing a lithium iron phosphate battery, characterized in that, The repaired lithium iron phosphate cathode material according to claim 7 is used alone for the preparation of lithium iron phosphate batteries; or The repaired lithium iron phosphate cathode material according to claim 7 is compounded with a commercial lithium iron phosphate cathode material for the preparation of lithium iron phosphate batteries.

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