A pre-oxidation strategy applied to lithium iron phosphate regeneration

Abstract

The application provides a pre-oxidation strategy applied to lithium iron phosphate regeneration. The lithium iron phosphate is regenerated from positive and negative mixed active substances, most of impurity metal is removed through centrifugation, the impurity metal ions in the mixture are fully oxidized through calcination in an air atmosphere, and finally the residual impurity metal ions are removed through organic weak acid acetic acid. The proportion of Li, Fe and P is adjusted, and the active substance graphite of the original negative electrode and the conductive carbon of the positive electrode are used as reducing agents, and the oxidized lithium iron phosphate is used as a precursor to prepare lithium iron phosphate material. The method for regenerating lithium iron phosphate from positive and negative mixed active substances provides a lithium iron phosphate battery recycling method suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of recycling and resource utilization of retired lithium-ion batteries, and particularly to the recycling and regeneration of cathode materials for lithium iron phosphate batteries. Background Technology

[0002] The overexploitation and use of fossil fuels has triggered an energy crisis and caused significant environmental problems. Consequently, lithium-ion batteries, as one of the most important renewable energy storage technologies, have experienced rapid development. However, with the end of the lifespan of lithium-ion batteries, the recycling and resource utilization of lithium-ion battery materials are crucial to prevent environmental pollution from discarded batteries and to extract valuable metal materials from them.

[0003] Since the 1990s, lithium-ion batteries, as a representative technology for renewable energy storage, have dominated the market due to their high energy density, large energy density, and strong energy storage capacity. Lithium iron phosphate batteries have a wide range of applications, including medical, electronic, and small electrical appliances, but primarily serve the electric vehicle sector. Commercial vehicle batteries typically have a lifespan of about 5 years, while passenger vehicle batteries last about 8 years, resulting in a large number of batteries being retired and scrapped in the future. Given the dwindling resources of cobalt and lithium, and the serious environmental damage caused by discarded lithium batteries, the recycling of used lithium batteries is urgently needed. How to efficiently, cleanly, and cost-effectively recycle lithium-ion batteries, especially their cathode materials, has become one of the most important energy, resource, and environmental issues facing human society. Currently, scholars have conducted extensive research on the recycling of retired lithium-ion batteries. However, there are still many problems with the current technology for recycling cathode materials of retired lithium-ion batteries: (1) Dry recycling of lithium-ion batteries has high energy consumption, high equipment investment, low returns, and is prone to generating a large amount of toxic and harmful gases; (2) Wet recycling process consumes a large amount of inorganic acid or alkali, which may generate volatile toxic gases, which are detrimental to human health and also pollute the environment, and there is also a risk of damaging equipment; (3) Although biological method has high recycling efficiency and good separation effect, the biological community cultivation process is complicated and it is difficult to achieve industrialization with the current level of technology; (4) Direct repair method is difficult to remove metal impurities in the separated cathode material powder in advance; (5) When regenerating lithium-ion battery cathode materials by carbothermal reduction method, it is generally necessary to add carbon sources such as organic reducing sugars, which increases the cost. On the other hand, the graphite in the lithium-ion battery anode material is directly discarded. In view of the above problems, the key to solving the problem is to rationally design a green and economical technology for recycling cathode materials of retired lithium-ion batteries, which is an important step in the resource utilization of retired lithium-ion batteries, and is also the main focus of this patent research. Summary of the Invention

[0004] To address the aforementioned technical problems, the first objective of this invention is to provide a method for discharging retired lithium-ion batteries using Na2SO4 solution. The key characteristics of the material synthesized using this method are:

[0005] Chemical discharge methods have a wider range of applications. Furthermore, the large amount of heat generated by a battery short circuit can be absorbed by the solution, which also improves the safety of the process.

[0006] Commonly used chemical discharge solutions are generally corrosive, but using Na2SO4 solution as the discharge solution avoids this risk.

[0007] Discharging with a 20% Na2SO4 solution can bring retired lithium-ion batteries below a safe voltage after 15 hours.

[0008] A second objective of this invention is to provide a method for separating metallic impurities in powder based on differences in powder specific gravity, comprising the following steps:

[0009] (1) Place the calcined powder into a centrifuge tube, add water as a dispersant, centrifuge in the centrifuge tube, and take the top layer of centrifuged product to dry and grind into powder.

[0010] (2) The treated powder is oxidized and roasted in a muffle furnace so that the aluminum foil and copper foil remaining in the powder are fully oxidized.

[0011] (3) The oxidized and roasted sample was treated with a mixed solution of acetic acid and NaCl under ultrasonic conditions. Halogen anions replace O 2- With Al 3+ They combine to form coordinating complex ions and dissolve, promoting the dissolution of the oxide film on the aluminum surface.

[0012] In step (1), the mass ratio of powder to water in the centrifugal separation should be 1:10-1:20, and the centrifugal speed should be 8000-10000 r / min.

[0013] The oxidation roasting temperature in the muffle furnace in step (2) should be 500-550℃, and the roasting time should be 1-2h.

[0014] In step (3), the acetic acid concentration should be 4%-10%, the NaCl concentration should be 3%, and the ultrasonic time should be 2-4 hours.

[0015] Ideally, the acetic acid concentration should be 6% and the reaction time should be 4 hours.

[0016] The third objective of this invention is to provide a method for preparing lithium iron phosphate cathode materials using inorganic carbon as a carbon source for carbothermal reduction:

[0017] (1) Inorganic carbon (graphite / acetylene black) is used as the carbon source for carbothermal reduction, making the raw material source for carbothermal reduction more abundant.

[0018] (2) Graphite from the negative electrode of retired lithium-ion batteries and conductive carbon from the positive electrode were used for carbothermal reduction. When generating lithium iron phosphate material, the lithium iron phosphate material was also carbon coated.

[0019] (3) The lithium iron phosphate obtained by carbothermic reduction of graphite from the negative electrode and conductive carbon from the positive electrode of retired lithium-ion batteries has a thicker carbon coating layer than that of lithium iron phosphate obtained by carbothermic reduction of traditional organic reducing sugars, which results in lower impedance of the regenerated lithium iron phosphate positive electrode material and effectively improves the conductivity of the lithium iron phosphate positive electrode material. Attached Figure Description

[0020] Figure 1 This is a transmission electron microscope image of the regenerated lithium iron phosphate cathode material of this invention;

[0021] Figure 2 This is an X-ray diffraction pattern of the regenerated lithium iron phosphate cathode material of the present invention;

[0022] Figure 3 The cycling performance and coulombic efficiency of the regenerated lithium iron phosphate cathode material of this invention at a rate of 0.5C are shown. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The specific embodiments described herein are only for explaining this application and are not intended to limit this application.

[0024] One embodiment of the present invention is as follows: (1) Pretreatment: First, the waste lithium iron phosphate battery is placed in a Na2SO4 solution of a certain concentration and left to stand in order to release the remaining charge. The discharged battery is mechanically disassembled in a fume hood, and the positive electrode and negative electrode are taken out. The positive electrode and negative electrode are crushed and screened to obtain small particle powder.

[0025] (2) The sieved powder is calcined in a tube furnace under a nitrogen atmosphere to pyrolyze the binder (PVDF) and residual electrolyte. This separates the aluminum and copper foils used as current collectors from the lithium iron phosphate and graphite used as electrode materials. The calcined powder is placed in a centrifuge tube, water is added as a dispersant, and the mixture is centrifuged. The top layer of the centrifuged product is then dried and ground into powder. Centrifugation removes most of the aluminum and copper impurities from the powder.

[0026] (3) The treated powder is oxidized and roasted in a muffle furnace so that the aluminum foil and copper foil remaining in the powder are fully oxidized into Al2O3 and CuO. The original negative electrode active material graphite and the positive electrode conductive carbon are also partially oxidized. At the same time, the original positive electrode active material lithium iron phosphate is oxidized into LiFePO4+O2→Li3Fe2(PO4)3+Fe2O3. The oxidized and roasted sample is treated with a mixed solution of acetic acid and NaCl under ultrasonic conditions. The concentration of acetic acid and NaCl and the ultrasonic time are controlled so that the content of impurity metals Al and Cu in the active material is below 1‰.

[0027] (4) The leachate is rich in Li, which may be from the electrolyte products after pyrolysis, the Li elements remaining at the negative electrode of the battery during battery cycling, and trace amounts of Li3Fe2(PO4)3 being leached by acid. Na2CO3 is added to the acid leachate, and NaOH is added to adjust its pH, so that the Li elements in the acid leachate are precipitated out in the form of Li2CO3.

[0028] (5) The filtered sample was dried, and after replenishing the Li, Fe and P sources, it was ball-milled to make the mixture uniform. In a tube furnace under an argon atmosphere, the residual graphite and conductive carbon were used as reducing agents for carbothermic reduction calcination to obtain regenerated LiFePO4.

[0029] The lithium iron phosphate carbon coating obtained by this invention is thicker than that obtained by the traditional carbothermal reduction of organic reducing sugars, such as... Figure 1 This effectively improves the conductivity of lithium iron phosphate cathode materials. (See also...) Figure 2 The X-ray diffraction pattern of the lithium iron phosphate material obtained by this invention matches well with the standard card, proving that lithium iron phosphate was successfully recycled. See also Figure 3 As shown, again

[0030] The obtained lithium iron phosphate cathode material has a discharge specific capacity of 152.1 mAh g at a 0.5C rate. -1 It still maintains 145.31 mAh g after 150 cycles. -1 The discharge specific capacity is equivalent to a capacity retention rate of 95.5%, which indicates the material's superior reversible capacity.

Claims

1. A method for regenerating lithium iron phosphate, comprising the following steps: (1) Pretreatment: First, the waste lithium iron phosphate batteries are placed in a Na2SO4 solution of a certain concentration and left to stand in order to release the remaining charge; the discharged batteries are mechanically disassembled in a fume hood, and the positive and negative electrode plates are taken out. The positive and negative electrode plates are then roasted in a tube furnace under a nitrogen atmosphere to pyrolyze the binder and residual electrolyte; so that the aluminum foil and copper foil used as current collectors are separated from the lithium iron phosphate and graphite used as electrode materials; the concentration of Na2SO4 solution is 20%, the discharge temperature is room temperature, the discharge time should be 15h, the roasting temperature under a nitrogen atmosphere is 400-500℃, and the roasting time is 2-3h. (2) The separated electrode sheets are crushed and sieved. The sieved powder is placed in a centrifuge tube, water is added as a dispersant, and the product is centrifuged in the centrifuge tube. The top centrifuged product is dried and ground into powder. Most of the aluminum and copper impurities in the powder are removed by centrifugation. The mesh size is 0.038 mm, the mass ratio of powder to water in the centrifugation is 1:10-1:20, and the centrifugation speed is 8000-10000 r / min. (3) The treated powder is oxidized and roasted in a muffle furnace so that the aluminum foil and copper foil remaining in the powder are fully oxidized into Al2O3 and CuO. The original negative electrode active material graphite and the positive electrode conductive carbon are also partially oxidized. At the same time, the original positive electrode active material lithium iron phosphate is oxidized into LiFePO4+O2→Li3Fe2(PO4)3+Fe2O3. The oxidized and roasted sample is treated with a mixed solution of acetic acid and NaCl under ultrasonic conditions. The concentration of acetic acid and NaCl and the ultrasonic time are controlled so that the content of impurity metals Al and Cu in the active material is below 1‰. The roasting temperature in the muffle furnace is 500-550℃, the roasting time is 1-2h, the acetic acid concentration is 4%-10%, the NaCl concentration is 3%, and the ultrasonic time is 2-4h. (4) Add Na2CO3 to the acid leaching solution and add NaOH to adjust its pH so that the Li element in the acid leaching solution is precipitated out in the form of Li2CO3; (5) The filtered sample is dried, and after replenishing the Li source, Fe source and P source, it is ball-milled to make the mixture uniform. In the argon atmosphere in the tube furnace, the residual graphite and conductive carbon are used as reducing agents for carbothermal reduction calcination to obtain regenerated LiFePO4. The final ratio of Li source, Fe source and P source should be adjusted to Li:Fe:P=1:1:1, the carbothermal reduction calcination temperature is 700-800℃, and the calcination time is 2-4h.

Images