A method for recycling used lithium iron phosphate

Abstract

The present invention belongs to the technical field of cathode materials for lithium-ion batteries, and specifically relates to a method for recycling waste lithium iron phosphate. The method includes first calcining waste high-carbon lithium iron phosphate powder or lithium iron phosphate electrode sheets to obtain oxidized lithium iron phosphate; then performing two times of coating-sintering on the oxidized lithium iron phosphate and carbon source substances to obtain a regenerated lithium iron phosphate / carbon composite material; wherein during the first coating-sintering, the carbon source substances include a lithium source, an iron source, a phosphorus source and a carbon source 1, and the carbon source substances in the second coating-sintering include a carbon source 2. The method of the present invention does not require acid leaching of waste materials, no waste water or waste acid is generated during the entire recycling process, it is more environmentally friendly, does not require high temperature and high pressure, has low cost, and is a method for recycling waste lithium iron phosphate suitable for large-scale production.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for lithium-ion batteries, and particularly relates to a method for recycling waste lithium iron phosphate. Background Art

[0002] Lithium iron phosphate has been favored by the power and energy storage markets due to its advantages such as stable structure, low price, relatively high theoretical capacity (170 mAh / g), stable working voltage, non-toxic and environmentally friendly, stable structure, good safety and thermal stability, and long cycle life.

[0003] With the rapid development of the electric vehicle and energy storage markets, the output of lithium iron phosphate is also increasing rapidly. However, during the production of lithium iron phosphate materials and the preparation of lithium iron phosphate batteries, a large amount of unqualified lithium iron phosphate waste (such as excessive carbon content, unqualified electrochemical performance, too high specific surface area, too low tap density, etc.), unqualified waste electrode sheets of lithium iron phosphate batteries (such as unqualified surface density and tap density), and a large number of scrapped lithium iron phosphate batteries are generated. How to reuse these lithium iron phosphate wastes, waste electrode sheets, and waste batteries has gradually become a research hotspot.

[0004] Traditional recycling methods for waste lithium iron phosphate materials mainly use roasting or acid leaching methods. However, the recycling process is complex, the process is long, the equipment investment is large, and new by-products are easily brought during the acid leaching process. At the same time, a large amount of waste acid and waste water are generated, which is not conducive to environmental protection. For example, Chinese Patent CN108996484 A discloses a method for preparing lithium iron phosphate from a lithium iron phosphate positive electrode sheet. The patent first dissolves the lithium iron phosphate positive electrode sheet with acid, then adjusts the pH value by adding ammonia water to obtain a lithium and aluminum filtrate and a ferric phosphate filter residue. Then, ammonia water is added to the lithium and aluminum filtrate to separate aluminum hydroxide. Next, an alkali and a phosphorus compound are added to the lithium solution to obtain lithium phosphate. Then, the obtained lithium phosphate, ferric phosphate, phosphorus source, and carbon source are mixed to finally obtain a lithium iron phosphate material.

[0005] In order to avoid the extensive use of acids and alkalis in the liquid-phase recycling process, people have improved on the traditional roasting method (the purpose of roasting is to remove organic substances such as carbon and PVDF in waste lithium iron phosphate, especially for high-carbon waste lithium iron phosphate, and the residual carbon will affect the coating effect of recycled carbon in the later stage). By sintering the recycled lithium iron phosphate cathode powder in air, oxidized lithium iron phosphate is obtained (the main oxidation products are Li3Fe2(PO4)3 and Fe2O3). Then, by remixing a small amount of lithium source, oxidized lithium iron phosphate, and carbon source, recycled lithium iron phosphate is obtained after one-time mixing and sintering, such as in Chinese Patent Applications CN 109346789 A and CN 112142029 A. However, the lithium iron phosphate prepared by this process has a low tap density, a high impurity content, and although its electrochemical performance is improved compared to the recycled lithium iron phosphate, its capacity only reaches 90-95% of that of newly produced lithium iron phosphate, making it difficult to meet the requirements of the power market. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the present invention provides a method for recycling waste lithium iron phosphate. The recycled phosphate cathode material sintered by this method has excellent performance, a high tap density, few impurity phases, and a high capacitance.

[0007] The object of the present invention is achieved through the following technical solutions:

[0008] A method for recycling waste lithium iron phosphate, comprising the following steps:

[0009] (1) Calcining waste high-carbon lithium iron phosphate powder or lithium iron phosphate electrode sheet to obtain oxidized lithium iron phosphate;

[0010] (2) Conducting two-stage coating-sintering on the oxidized lithium iron phosphate with carbon source substances to obtain a recycled lithium iron phosphate / carbon composite material;

[0011] Among them, the first-stage coating-sintering obtains a low-carbon lithium iron phosphate precursor, and the carbon source substances include a lithium source, an iron source, a phosphorus source, and a carbon source 1. When conducting the second-stage coating-sintering, the carbon source substances include a carbon source 2.

[0012] Preferably, in step (1), the calcining is carried out by sintering in an air or oxygen atmosphere, the temperature of the calcining is 300-450 °C, and the time of the calcining is 3-6 h.

[0013] Preferably, the carbon content of lithium iron phosphate in the waste lithium iron phosphate powder or lithium iron phosphate electrode sheet in step (1) is 2.6-6%.

[0014] Preferably, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, and lithium acetate.

[0015] Preferably, the iron source is one or more of iron phosphate, magnetite, iron hydroxide, iron oxide, and trivalent iron source of iron oxyhydroxide.

[0016] Preferably, the phosphorus source is one or more of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and lithium phosphate.

[0017] Preferably, the carbon source 1 is selected from one or more of glucose, rock sugar, sucrose, fructose, cyclodextrin, and starch.

[0018] Preferably, the carbon source 2 is selected from one or more of glucose, rock sugar, sucrose, fructose, cyclodextrin, starch, polyethylene glycol, polyvinyl alcohol, ascorbic acid, citric acid, and amino acids.

[0019] Preferably, the mass of the lithium iron phosphate after oxidation is 30-60% of the mass of the low-carbon lithium iron phosphate precursor.

[0020] Preferably, the mass of the carbon source 1 accounts for 5-6% of the mass of the low-carbon lithium iron phosphate precursor.

[0021] Preferably, the process of the first coating-sintering includes: mixing the oxidized lithium iron phosphate powder, lithium source, iron source, phosphorus source, and carbon source 1, grinding, drying, and sintering to obtain a low-carbon lithium iron phosphate precursor.

[0022] Preferably, the sintering temperature is 650-730 °C, and the sintering time is 4-6 h.

[0023] Preferably, the mass fraction of carbon in the low-carbon lithium iron phosphate precursor is 0.1-0.4%.

[0024] Preferably, the process of the second coating-sintering includes: mixing the low-carbon lithium iron phosphate precursor and carbon source 2, grinding, drying, and sintering to obtain a regenerated lithium iron phosphate / carbon composite material.

[0025] Preferably, the sintering temperature is 700-780 °C, and the sintering time is 8-10 h.

[0026] Preferably, the first or second coating-sintering further includes adding an additive, and the additive is one or more of titanium dioxide, tetrabutyl titanate, magnesium acetate, magnesium hydroxide, magnesium oxide, zirconium oxide, zirconium hydroxide, niobium pentoxide, niobium hydroxide, nickel acetate, manganese acetate, aluminum oxide, molybdenum oxide, and ammonium molybdate.

[0027] Preferably, the additive added during the first or second coating-sintering accounts for 0-0.5% of the mass of the lithium iron phosphate precursor.

[0028] Preferably, during the first or second coating-sintering, a solvent is added during the mixing process, and the solvent is selected from one or more of water, methanol, ethanol, acetone, and NMP.

[0029] Preferably, the grinding means that the mixed slurry is first coarsely ground, and when the particle size of the slurry is D 50 <1 μm, then it is finely ground, and when the particle size of the slurry is ground to 350 - 500 nm, it is okay.

[0030] Preferably, the drying is static drying or spray drying.

[0031] Preferably, the sintering is carried out in an inert atmosphere.

[0032] Preferably, the inert atmosphere is one or more of nitrogen, argon, helium, and carbon dioxide.

[0033] Preferably, the mass fraction of carbon in the recycled lithium iron phosphate / carbon composite material is 1.5 - 3.5%.

[0034] Preferably, the molar ratio of lithium, iron, and phosphorus elements in the recycled lithium iron phosphate / carbon composite material is 1 - 1.1:0.9 - 1.05:1 - 1.08.

[0035] Another object of the present invention is to provide the application of the above method in the preparation of lithium iron phosphate cathode materials or the recycling preparation process of waste lithium iron phosphate.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] (1) The method provided by the present invention has the characteristics of low cost, simple process, and can be mass-produced. During the production process, waste materials do not need acid leaching and high temperature and high pressure. No waste water or waste acid is generated during the entire recycling process, which is environmentally friendly. Especially for high-carbon waste lithium iron phosphate, it has a good recycling effect.

[0038] (2) Through two carbon coating and two sintering processes, the present invention is beneficial to the growth of the lithium iron phosphate crystal form, the uniformity of carbon coating, and the avoidance of the generation of trivalent iron impurity phases during the preparation of recycled lithium iron phosphate. Among them, the first carbon coating mainly reduces the trivalent iron in the oxidized lithium iron phosphate and the trivalent iron in the iron source raw materials to divalent iron through carbothermal reduction. And the first sintering process is mainly used to prepare a low-carbon lithium iron phosphate precursor, making the Li / Fe / P elements sinter and fuse evenly, and at the same time, it is also beneficial to the growth of lithium iron phosphate particles, the surface smoothness, and the attachment of the secondary carbon source during the later second sintering process.

[0039] (3) The recycled lithium iron phosphate / carbon composite electrode material of the present invention is used in the preparation of power batteries and energy storage markets, and the obtained batteries have excellent electrochemical performance, high tap density, and good processing performance. Description of the Drawings

[0040] Figure 1SEM photograph of the lithium iron phosphate precursor prepared in Example 1;

[0041] Figure 2 Electrochemical performance of the lithium iron phosphate / carbon composite material prepared in Example 1. Detailed implementation manners

[0042] The present invention will be further described below in conjunction with the detailed implementation manners.

[0043] Example 1

[0044] The preparation method of the recycled lithium iron phosphate / carbon composite material in this example is as follows:

[0045] (1) Waste lithium iron phosphate with a carbon content of 3.0 wt% is placed in an atmosphere furnace with air being blown in, and is heated at a constant temperature of 300 °C for 4 hours to obtain oxidized lithium iron phosphate powder, which is crushed for later use.

[0046] (2) Preparation of the lithium iron phosphate precursor

[0047] 38.69 g of lithium carbonate (99.5 wt%), 150 g of iron phosphate, 150 g of oxidized lithium iron phosphate, and 16 g of glucose are successively added to a 2 L Retsch mill containing 1200 mL of absolute ethanol, and coarse grinding is started at a rotation speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1um, it is transferred to a sand mill for fine grinding. After the particle size of the slurry is controlled at 400 nm, static drying is carried out. The obtained dried powder is placed in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature is 650 °C, and the constant temperature time is 4 h. After the tube furnace naturally cools down to 80 °C, the material is taken out and crushed to obtain 300 g of lithium iron phosphate precursor with a carbon content of 0.2 wt%.

[0048] (3) Preparation of the lithium iron phosphate / carbon composite material

[0049] 300 g of lithium iron phosphate precursor, 20 g of glucose, 4 g of cyclodextrin, and 1.5 g of titanium dioxide are added to a 2 L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, coarse grinding is started at a rotation speed of 2000 r / min. After grinding for 35 minutes, when the particle size D of the slurry 50 <1um, it is transferred to a sand mill for fine grinding. After the particle size of the slurry is controlled at 450 nm, static drying is carried out. The obtained dried powder is placed in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature is 740 °C, and the constant temperature time is 10 h. After the tube furnace naturally cools down to 80 °C, the material is taken out and fractionally crushed to obtain a recycled lithium iron phosphate / carbon composite material with a carbon content of 1.5 wt%.

[0050] After analysis, the molar ratio of lithium, iron, and phosphorus in the regenerated lithium iron phosphate is Li:Fe:P = 1.05:1:1.03, and the powder compaction is 2.58 g / cm 3 .

[0051] The regenerated lithium iron phosphate / carbon composite material obtained was observed by scanning electron microscopy, and the results are as follows Figure 1 . From Figure 1 , it can be seen that the size range of the prepared primary particles is about 0.5 - 5 μm, and most of the particles are mainly concentrated at about 0.5 μm.

[0052] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, an electrode sheet was made. Using metallic lithium as the negative electrode, a simulated button battery was assembled. At 2 - 3.75 V and room temperature, tests were carried out under different charge-discharge current conditions. The initial reversible capacity during charge-discharge at 0.1C was 159.5 mAh / g, the initial reversible capacity during charge-discharge at 0.2C was 157.7 mAh / g, and the initial reversible capacity during charge-discharge at 1C was 144.5 mAh / g (see Figure 2 ).

[0053] Example 2

[0054] The preparation method of the regenerated lithium iron phosphate / carbon composite material in this example is as follows

[0055] (1) A certain amount of waste lithium iron phosphate electrode sheets were placed in an atmosphere furnace with oxygen being introduced, and heated at a constant temperature of 400 °C for 4 hours to obtain peeled aluminum sheets and oxidized lithium iron phosphate. The oxidized lithium iron phosphate was crushed for future use.

[0056] (2) Preparation of lithium iron phosphate precursor: 46.95 g of lithium carbonate (99.5 wt%), 180 g of iron phosphate, 120 g of oxidized lithium iron phosphate, and 16 g of rock sugar were added to a 2 L planetary mill containing 1200 mL of anhydrous ethanol. After adding the materials, coarse grinding was started at a speed of 2000 r / min. After several minutes of coarse grinding, when the particle size D 50 <1 μm, it was transferred to a sand mill for fine grinding. After the particle size of the slurry was controlled at 400 nm, static drying was carried out. The obtained dried powder was placed in a tubular furnace under a nitrogen atmosphere for sintering. The sintering temperature was 650 °C, and the constant temperature time was 4 h. After the tubular furnace naturally cooled to 80 °C, the material was taken out and crushed to obtain 300 g of lithium iron phosphate precursor with a carbon content of 0.3 wt%.

[0057] (3) Preparation of lithium iron phosphate / carbon composite material

[0058] 300 g of lithium iron phosphate precursor, 30 g of glucose, and 1.2 g of magnesium hydroxide were successively added to a 2 L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, rough grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1um, it was transferred to a sand mill for fine grinding. After the particle size of the slurry was controlled at 400 nm, static drying was carried out. The obtained dried powder was placed in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature was 780 °C, and the constant temperature time was 10 h. After the tube furnace naturally cooled to 80 °C, the material was taken out, crushed, and passed through a 200-mesh sieve to obtain regenerated lithium iron phosphate with a carbon content of 1.8 wt%.

[0059] After analysis, the molar ratio of lithium, iron, and phosphorus in the regenerated lithium iron phosphate was Li:Fe:P = 1.05:1:1.035, and the powder compaction was 2.55 g / cm 3 .

[0060] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, an electrode sheet was made. Using metallic lithium as the negative electrode, a simulated button battery was assembled. At 2 - 3.75 V and room temperature, under different charge and discharge current conditions, the initial reversible capacity was 158.5 mAh / g when charging and discharging at 0.1C, 156.5 mAh / g when charging and discharging at 0.2C, and 143.2 mAh / g when charging and discharging at 1C (see Table 1).

[0061] Example 3

[0062] The preparation method of the regenerated lithium iron phosphate / carbon composite material in this example is as follows:

[0063] (1) A certain amount of waste lithium iron phosphate with a carbon content of 3.0 wt% was placed in an atmosphere furnace with air injection and heated at a constant temperature of 350 °C for 4 hours to obtain an oxidized lithium iron phosphate powder, which was crushed for future use.

[0064] (2) Preparation of lithium iron phosphate precursor

[0065] 39.72 g of lithium carbonate (99.5 wt%), 89.74 g of iron oxide hydroxide (99 wt%), 117.12 g of diammonium hydrogen phosphate (99.5 wt%), 100 g of the oxidized lithium iron phosphate powder, and 15 g of glucose were successively added to a 2 L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, rough grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50Transfer <1um of it> to a sand mill for fine grinding. After controlling the particle size of the slurry at 450 nm, perform spray drying. Place the obtained dried powder in a tubular furnace under a nitrogen atmosphere for sintering. The sintering temperature is 700 °C, and the constant temperature time is 5 h. Wait for the tubular furnace to cool naturally to 80 °C, take out the material and crush it to obtain 250 g of lithium iron phosphate precursor with a carbon content of 0.3 wt%.

[0066] (3) Preparation of lithium iron phosphate / carbon composite

[0067] Add 250 g of lithium iron phosphate precursor, 17.64 g of rock sugar, 6 g of polyethylene glycol 20000, and 1 g of niobium pentoxide to a 2 L basket mill containing 1200 mL of deionized water in sequence. After adding the materials, start coarse grinding at a speed of 2000 r / min. After grinding for several minutes, when the particle size of the slurry D 50 Transfer <1um of it> to a sand mill for fine grinding. After controlling the particle size of the slurry at 450 nm, perform spray drying. Place the obtained dried powder in a tubular furnace under an argon atmosphere for sintering. The sintering temperature is 760 °C, and the constant temperature time is 10 h. Wait for the tubular furnace to cool naturally to 80 °C, take out the material and perform classification and crushing to obtain recycled lithium iron phosphate with a carbon content of 1.6 wt%.

[0068] After analysis, the molar ratio of lithium, iron, and phosphorus in the recycled lithium iron phosphate is Li:Fe:P = 1.045:1:1.03, and the powder compaction is 2.58 g / cm 3 .

[0069] Using the prepared lithium iron phosphate / carbon composite for lithium-ion battery cathodes as the cathode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, make an electrode sheet. Use metallic lithium as the anode to assemble a simulated button battery. At 2 - 3.75 V and room temperature, test under different charge-discharge current conditions. The initial reversible capacity for charge-discharge at 0.1C is 157.8 mAh / g, the initial reversible capacity for charge-discharge at 0.2C is 156.3 mAh / g, and the initial reversible capacity for charge-discharge at 1C is 143.5 mAh / g (see Table 1).

[0070] Example 4

[0071] The preparation method of the recycled lithium iron phosphate / carbon composite in this example is as follows:

[0072] (1) Place a certain amount of waste lithium iron phosphate electrode sheets in an atmosphere furnace with oxygen introduced, heat at a constant temperature of 400 °C for 4 hours to obtain peeled aluminum sheets and oxidized lithium iron phosphate. Crush the oxidized lithium iron phosphate for later use.

[0073] (2) Preparation of lithium iron phosphate precursor

[0074] 59.03 g of lithium carbonate (99.5 wt%), 122.45 g of iron oxide (98 wt%), 180 g of diammonium hydrogen phosphate (99.5 wt%), 100 g of oxidized lithium iron phosphate powder, 18 g of sucrose, and 0.6 g of magnesium oxide were successively added to a 2-L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, coarse grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1um, it was transferred to a sand mill for fine grinding. After the particle size of the slurry was controlled to about 400 nm, spray drying was carried out. The obtained dried powder was placed in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature was 700 °C, and the constant temperature time was 4 h. After the tube furnace naturally cooled to 80 °C, the material was taken out and crushed to obtain 330 g of lithium iron phosphate precursor with a carbon content of 0.25 wt%.

[0075] (3) Preparation of lithium iron phosphate / carbon composite material

[0076] 330 g of lithium iron phosphate precursor, 30 g of fructose, 5 g of polyethylene glycol 20000, 1.2 g of titanium dioxide, and 0.3 g of niobium pentoxide were successively added to a 2-L Retsch mill containing 1200 mL of deionized water. After adding the materials, coarse grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1um, it was transferred to a sand mill for fine grinding. After the particle size of the slurry was controlled to about 350 nm, spray drying was carried out. The obtained dried powder was placed in a tube furnace under an argon atmosphere for sintering. The sintering temperature was 780 °C, and the constant temperature time was 8 h. After the tube furnace naturally cooled to 80 °C, the material was taken out and crushed in stages to obtain recycled lithium iron phosphate with a carbon content of 1.6 wt%. After analysis, the molar ratio of lithium, iron, and phosphorus in the recycled lithium iron phosphate was Li:Fe:P = 1.055:1:1.036, and the powder compaction was 2.54 g / cm 3 .

[0077] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, an electrode sheet was made. Using metallic lithium as the negative electrode, a simulated button battery was assembled. At 2 - 3.75 V and room temperature, under different charge and discharge current conditions, the initial reversible capacity was 158.9 mAh / g when charging and discharging at 0.1 C, 157.5 mAh / g when charging and discharging at 0.2 C, and 144.1 mAh / g when charging and discharging at 1 C (see Table 1).

[0078] Comparative Example 1

[0079] The difference between this comparative example and Example 1 is that the preparation method uses a carbon source substance for primary coating - sintering. The specific preparation method is as follows:

[0080] The waste lithium iron phosphate with a carbon content of 3.0 wt% is placed in an atmosphere furnace with air being blown in, and is heated at a constant temperature of 300 °C for 4 hours to obtain the oxidized lithium iron phosphate powder, which is then crushed for standby use.

[0081] Next, 38.69 g of lithium carbonate (99.5 wt%), 150 g of iron phosphate, 150 g of the oxidized lithium iron phosphate, 36 g of glucose, 4 g of cyclodextrin, and 1.5 g of titanium dioxide are successively added to a 2 L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, coarse grinding is started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1um is transferred to a sand mill for fine grinding. After the particle size of the slurry is controlled at 450 nm, static drying is carried out. The obtained dried powder is placed in a tubular furnace under a nitrogen atmosphere for sintering. The sintering temperature is 740 °C and the constant temperature time is 10 h. After the tubular furnace naturally cools down to 80 °C, the material is taken out and crushed in stages to obtain the regenerated lithium iron phosphate / carbon composite material with a carbon content of 1.5 wt% and a powder compaction of 2.40 g / cm 3 of.

[0082] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, an electrode sheet is made. Using metallic lithium as the negative electrode, a simulated button battery is assembled. At 2 - 3.75 V and at room temperature, tests are carried out under different charge and discharge current conditions. The initial reversible capacity during charge and discharge at 0.1C is 154.6 mAh / g, the initial reversible capacity during charge and discharge at 0.2C is 153.1 mAh / g, and the initial reversible capacity during charge and discharge at 1C is 139.5 mAh / g (see Table 1).

[0083] Comparative Example 2

[0084] The difference between this comparative example and Example 2 is that the preparation method uses a carbon source substance for primary coating - sintering. The specific preparation method is as follows:

[0085] A certain amount of waste lithium iron phosphate electrode sheets are placed in an atmosphere furnace with oxygen being blown in, and are heated at a constant temperature of 400 °C for 4 hours to obtain the peeled aluminum sheets and the oxidized lithium iron phosphate. The oxidized lithium iron phosphate is crushed for standby use.

[0086] 46.95 g of lithium carbonate (99.5 wt%), 180 g of iron phosphate, 120 g of the oxidized lithium iron phosphate, 16 g of rock sugar, 30 g of glucose, and 1.2 g of magnesium hydroxide are added to a 2 L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, coarse grinding is started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50Transfer <1um of it> to a sand mill for fine grinding. After controlling the particle size of the slurry at 400 nm, conduct static drying. Place the obtained dried powder in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature is 780 °C, and the constant temperature time is 10 h. Wait for the tube furnace to cool naturally to 80 °C, take out the material, and perform classification and crushing to obtain a regenerated lithium iron phosphate / carbon composite material with a carbon content of 1.8 wt% and a powder compaction of 2.42 g / cm 3

[0087] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, make an electrode sheet. Use metallic lithium as the negative electrode and assemble a simulated button battery. At 2 - 3.75 V and room temperature, test under different charge-discharge current conditions. The initial reversible capacity during charge-discharge at 0.1 C is 153.3 mAh / g, the initial reversible capacity during charge-discharge at 0.2 C is 152.5 mAh / g, and the initial reversible capacity during charge-discharge at 1 C is 138.2 mAh / g (see Table 1).

[0088] Comparative Example 3

[0089] The difference between this comparative example and Example 3 is that the preparation method uses a carbon source substance for primary coating-sintering. The specific preparation method is as follows:

[0090] Place a certain amount of waste lithium iron phosphate electrode sheets in an atmosphere furnace with oxygen being introduced, and heat at 400 °C for 4 hours under constant temperature to obtain peeled aluminum sheets and oxidized lithium iron phosphate. Crush the oxidized lithium iron phosphate for future use.

[0091] Sequentially add 59.03 g of lithium carbonate (99.5 wt%), 122.45 g of iron oxide (98 wt%), 180 g of diammonium hydrogen phosphate (99.5 wt%), 100 g of the oxidized lithium iron phosphate powder, 18 g of sucrose, 30 g of fructose, 5 g of polyethylene glycol 20000, 1.2 g of titanium dioxide, and 0.3 g of niobium oxide into a 2 L Retsch mill containing 1200 mL of deionized water. After adding all the materials, start rough grinding at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 Transfer <1um of it> to a sand mill for fine grinding. After controlling the particle size of the slurry at about 350 nm, conduct spray drying. Place the obtained dried powder in a tube furnace under an argon atmosphere for sintering. The sintering temperature is 780 °C, and the constant temperature time is 8 h. Wait for the tube furnace to cool naturally to 80 °C, take out the material, and perform classification and crushing to obtain a regenerated lithium iron phosphate / carbon composite material with a carbon content of 1.6 wt% and a powder compaction of 2.36 g / cm 3

[0092] ​​Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, an electrode sheet was made. Using metallic lithium as the negative electrode, a simulated button battery was assembled. At 2 - 3.75V and room temperature, tests were carried out under different charge-discharge current conditions. The initial reversible capacity during charge-discharge at 0.1C was 153.1 mAh / g, the initial reversible capacity during charge-discharge at 0.2C was 151.8 mAh / g, and the initial reversible capacity during charge-discharge at 1C was 137.5 mAh / g (see Table 1).

[0093] Comparative Example 4

[0094] The difference between this comparative example and Example 1 is that the addition amounts of the carbon sources for the first and second coating-sintering are different, but the total mass of the carbon sources is the same, and the others are the same as in Example 1.

[0095] (1) Waste lithium iron phosphate with a carbon content of 3.0 wt% was placed in an atmosphere furnace with air being blown in, and heated at a constant temperature of 300 °C for 4 hours to obtain oxidized lithium iron phosphate powder, which was crushed and prepared for use.

[0096] (2) Preparation of lithium iron phosphate precursor

[0097] 38.69 g of lithium carbonate (99.5 wt%), 150 g of iron phosphate, 150 g of oxidized lithium iron phosphate, and 28 g of glucose were successively added to a 2 L Retsch mill containing 1200 mL of absolute ethanol, and coarse grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1um it was transferred to a sand mill for fine grinding. After controlling the particle size of the slurry to 400 nm, static drying was carried out. The obtained dried powder was placed in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature was 600 °C, and the constant temperature time was 4 h. After the tube furnace naturally cooled to 80 °C, the material was taken out and crushed to obtain 300 g of lithium iron phosphate precursor with a carbon content of 0.7 wt%.

[0098] (3) Preparation of lithium iron phosphate / carbon composite material

[0099] 300 g of lithium iron phosphate precursor, 8 g of glucose, 4 g of cyclodextrin, and 1.5 g of titanium dioxide were added to a 2 L Retsch mill containing 1200 mL of absolute ethanol. After adding the materials, coarse grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50Transfer it to a sand mill for fine grinding. After controlling the particle size of the slurry at 450 nm, perform static drying. Place the obtained dried powder in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature is 740 °C, and the constant temperature time is 10 h. Wait for the tube furnace to cool naturally to 80 °C, take out the material, classify and crush it to obtain a regenerated lithium iron phosphate / carbon composite material with a carbon content of 1.5 wt% and a powder compaction of 2.38 g / cm 3 of.

[0100] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, make an electrode sheet. Use metallic lithium as the negative electrode and assemble a simulated button battery. At 2 - 3.75 V and room temperature, test under different charge-discharge current conditions. The initial reversible capacity during charge-discharge at 0.1 C is 154.3 mAh / g, the initial reversible capacity during charge-discharge at 0.2 C is 150.2 mAh / g, and the initial reversible capacity during charge-discharge at 1 C is 138.7 mAh / g.

[0101] Comparative Example 5

[0102] The difference between this comparative example and Example 1 is that the sintering temperatures during the first and second coating-sintering are different. Specifically: the first is 500 °C, and the second is 790 °C. The rest is the same as in Example 1.

[0103] 1) Place the waste lithium iron phosphate with a carbon content of 3.0 wt% in an atmosphere furnace with air blown in, and heat it at a constant temperature of 300 °C for 4 hours to obtain the oxidized lithium iron phosphate powder body, and perform crushing for later use.

[0104] (2) Preparation of lithium iron phosphate precursor

[0105] Sequentially add 38.69 g of lithium carbonate (99.5 wt%), 150 g of iron phosphate, 150 g of the oxidized lithium iron phosphate, and 16 g of glucose to a 2 L Retsch mill containing 1200 mL of absolute ethanol and start coarse grinding at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 Transfer it to a sand mill for fine grinding. After controlling the particle size of the slurry at 400 nm, perform static drying. Place the obtained dried powder in a tube furnace under a nitrogen atmosphere for sintering. The sintering temperature is 500 °C, and the constant temperature time is 4 h. Wait for the tube furnace to cool naturally to 80 °C, take out the material and crush it to obtain 300 g of lithium iron phosphate precursor with a carbon content of 0.1 wt%.

[0106] (3) Preparation of lithium iron phosphate / carbon composite material

[0107] 300 g of lithium iron phosphate precursor, 20 g of glucose, 4 g of cyclodextrin, and 1.5 g of titanium dioxide were added to a 2 L planetary ball mill containing 1200 mL of absolute ethanol. After adding the materials, coarse grinding was started at a speed of 2000 r / min. After grinding for several minutes, when the particle size D of the slurry 50 <1 um, it was transferred to a sand mill for fine grinding. After the particle size of the slurry was controlled at 450 nm, static drying was carried out. The obtained dried powder was placed in a tubular furnace under a nitrogen atmosphere for sintering. The sintering temperature was 790 °C, and the constant temperature time was 10 h. After the tubular furnace naturally cooled to 80 °C, the material was taken out and classified and crushed to obtain a regenerated lithium iron phosphate / carbon composite material with a carbon content of 1.5 wt% and a powder compaction of 2.45 g / cm 3 .

[0108] Using the prepared lithium iron phosphate / carbon composite material for the positive electrode of a lithium-ion battery as the positive electrode material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, an electrode sheet was made. Using metallic lithium as the negative electrode, a simulated button battery was assembled. At 2 - 3.75 V and room temperature, tests were carried out under different charge and discharge current conditions. The initial reversible capacity during charge and discharge at 0.1 C was 151.2 mAh / g, the initial reversible capacity during charge and discharge at 0.2 C was 148.5 mAh / g, and the initial reversible capacity during charge and discharge at 1 C was 133.2 mAh / g.

[0109] Table 1 Comparison table of chemical properties and powder compaction between examples and comparative examples

[0110]

[0111] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification without departing from the present invention shall be included within the scope of the technical solution of the present invention.

Claims

1. A method for recycling used lithium iron phosphate, characterized in that It includes the following steps: (1) Calcining waste high-carbon lithium iron phosphate powder or lithium iron phosphate electrode sheets to obtain oxidized lithium iron phosphate; (2) Conducting two times of coating-sintering on the oxidized lithium iron phosphate with carbon source substances to obtain a regenerated lithium iron phosphate / carbon composite material; Among them, the first coating-sintering obtains a low-carbon lithium iron phosphate precursor, and the carbon source substances include a lithium source, an iron source, a phosphorus source, and a carbon source 1. When conducting the second coating-sintering, the carbon source substances include a carbon source 2; The mass of the carbon source 1 accounts for 5-6% of the mass of the low-carbon lithium iron phosphate precursor; In the process of the first coating-sintering, the sintering temperature is 650-730 °C, and the sintering time is 4-6 h; In the process of the second coating-sintering, the sintering temperature is 700-780 °C, and the sintering time is 8-10 h.

2. The method according to claim 1, characterized in that, In step (1), the calcining is carried out by sintering in an air or oxygen atmosphere. The calcining temperature is 300-450 °C, and the calcining time is 3-6 h.

3. The method according to claim 1, wherein In step (1), the carbon content of lithium iron phosphate in the waste high-carbon lithium iron phosphate powder or lithium iron phosphate electrode sheets is 2.6-6%.

4. The method according to claim 1, wherein The lithium source is one or several of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, and lithium acetate. The iron source is one or several of iron phosphate, ferric tetroxide, iron hydroxide, iron oxide, and ferric hydroxyoxide. The phosphorus source is one or several of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and lithium phosphate. The carbon source 1 is selected from one or more of glucose, rock sugar, sucrose, fructose, cyclodextrin, and starch. The carbon source 2 is selected from one or several of glucose, rock sugar, sucrose, fructose, cyclodextrin, starch, polyethylene glycol, polyvinyl alcohol, ascorbic acid, citric acid, and amino acids.

5. The method according to claim 1, wherein The mass of the oxidized lithium iron phosphate is 30-60% of the mass of the low-carbon lithium iron phosphate precursor.

6. The method according to claim 1, characterized in that The process of the first coating-sintering includes: mixing the oxidized lithium iron phosphate powder, the lithium source, the iron source, the phosphorus source, and the carbon source 1, grinding, drying, and sintering to obtain a low-carbon lithium iron phosphate precursor; the mass fraction of carbon in the low-carbon lithium iron phosphate precursor is 0.1-0.4%.

7. The method according to claim 1, wherein The process of the second coating-sintering includes: mixing the low-carbon lithium iron phosphate precursor and the carbon source 2, grinding, drying, and sintering to obtain a regenerated lithium iron phosphate / carbon composite material.

8. The method according to claim 1, wherein When conducting the first or second coating-sintering, an additive is also included. The additive is one or several of titanium dioxide, tetrabutyl titanate, magnesium acetate, magnesium hydroxide, magnesium oxide, zirconium oxide, zirconium hydroxide, niobium pentoxide, niobium hydroxide, nickel acetate, manganese acetate, aluminum oxide, molybdenum oxide, and ammonium molybdate. The additive added during the first or second coating-sintering accounts for 0-0.5% of the mass of the lithium iron phosphate precursor.

9. The method according to claim 6 or 7, characterized in that, A solvent is added during the mixing process, and the solvent is selected from one or more of water, methanol, ethanol, acetone, and NMP. The grinding means that the mixed slurry is first coarsely ground, and when the particle size of the slurry is D 50 <1um, then it is finely ground, and when the particle size of the slurry is ground to 350-500nm, it is ready.

10. The method according to any one of claims 1-8, characterized in that, The mass fraction of carbon in the regenerated lithium iron phosphate / carbon composite material is 1.5-3.5%; the molar ratio of lithium, iron, and phosphorus elements in the regenerated lithium iron phosphate / carbon composite material is 1-1.1:0.9-1.05:1-1.

08.

11. Application of the method according to any one of claims 1-10 in the preparation of a lithium iron phosphate cathode material or the recycling preparation process of waste lithium iron phosphate.

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