Study On Recycling Of Cathod Material From Waste Lithium Ion Batteries

Lithium ion batteries（LIBs） are widely used as electrochemical power sources in electronic equipment and electric vehicles（EV）, and the market share increased year by year. Then large amount of waste LIBs was occurred for a limited cycling life of LIBs. If waste LIBs are disposed of in landfill sites, soil contamination will ensue from leakage of the organic electrolyte, and the heavy metals contained in the batteries would pose a threat to the environment. Waste LIBs is also a scare resource for containing the metal ions in it. So, finding a proper way to recycle waste LIBs is an urgent problem. Most researches had carried out research on recyling the cobalt and lithium from waste LIBs with LiCoO₂ as cathode material with strong inorganic acid. The techonology is complex and would pose second pollution to the environment. Up to now, The literature about recyling the waste LIBs with LiNi_1/3Co_1/3Mn_1/3O₂ or LiMn₂O₄ is very few.Now, the main industrial cathode materials for lithium ion battery are LiCoO₂, LiMn₂O₄, LiNi_1/3Co_1/3Mn_1/3O₂ and LiFePO4. We used waste lithium ion batteries with different cathode material as the raw material, using organic acid dissolution method to recycle metal resource contained in the waste LIBs. With the leaching solution, we synthesized the corresponding materials for lithium ion batteries through sol-gel methode and the coupled method of sol-gel and hydrothermal method. The re-synthesized materials were well characterized with the aid of X-ray diffraction（XRD） patterns, field emission scanning electron microscope（FESEM）, high resolution transmission electron microscope（HRTEM）, Energy dispersive spectrometer（EDS）, N₂ absorption-adsorption spectrometer（BET）, Fourier transform infrared spectroscopy（FT-IR）, X-ray photoelectron spectroscopy（XPS）, UV-Vis absorption spectroscopy（UV-Vis）. The electrochemical properties were evaluated by cyclic voltammetry（CV）、electrochemical impedance spectroscopy（EIS） and testing the half cells. This thesis mainly encompasses the following five sections.（1） Adopting orthogonal experiment and single factor experiment to optimize the proper leaching condition. The influence on leaching efficiency of different citric acid concentration, leaching temperature, leaching time, H2O₂ concentration and the solid-to liquid ratio were discussed. The leaching mechanism was explained with the electrode potential theory. With the leaching solution, new LiMn₂O₄ was synthesized under sol-gel process without new sol reagent. The discharge capacity of the resynthesized LiMn₂O₄ calcined at proper temperature can reach 136 mAh/g. In the thesis, glucose was first used as reductive reagent in the leaching process, and new LiMn₂O₄ was re-synthesized with the leaching solution. The charge and discharge capacity of resynthesized LiMn₂O₄ can reach 130 mAh/g, and the capacity retention kept above 90 %.（2） Organic acid citric acid and D, L-malic acid were used both as leaching reagent and chelating reagent, and new LiNi_1/3Co_1/3Mn_1/3O₂ was resynthesized through sol-gel process. The dissolution process follows a reduction-complex mechanism, and it was also a surface reaction process. The optimum leaching condition for citric acid is as follows: citric acid concentration with 1.0 mol/L,H2O₂ concentration with 12 vol%, leaching temperature with 60℃,leaching time with 50 min,solid-to liquid concentration with 80 g/L. The optimum leaching condition for D,L-malic acid is as follows: malic acid concentration with 1.25 mol/L,H2O₂ concentration with 8 vol%,leaching temperature with 50℃, leaching time with 30 min,solid to liquid ratio with 60 g/L。With the leaching solution, new LiNi_1/3Co_1/3Mn_1/3O₂ with good electrochemical property was synthesized. The discharge capacity of resynthesized LiNi_1/3Co_1/3Mn_1/3O₂ with citric acid as the leaching and chelating agent can reach 147 mAh/g, while the LiNi_1/3Co_1/3Mn_1/3O₂ with D,L-malic acid with 147.2 mAh/g. The electrochemical properties were almost as good as the LiNi_1/3Co_1/3Mn_1/3O₂ synthesized with analytical metal salts as raw material. With citric acid as leaching and chelating reagent, regular shape and well dispersed LiNi_1/3Co_1/3Mn_1/3O₂ was obtained after calcined at 750℃ for 2 h, while it need to be calcined at 850℃ for 6 h when use D, L-malic acid as leaching and chelating reagent. The different complexing constant between organic acid and the metal ions may be the main influencing factor.（3） Metal ions were recycled from waste lithium ion batteries with LiCoO₂ as cathode material under an acid dissolution-base precipitation process. With the leaching solution, durian like cobalt ferrite was synthesized through a sol-gel-hydrothermal process. Cobalt ferrite is an important magnetic material and lithium ion battery anode material. In the thesis, the magnetic property and electrochemical properties were both studied. The formation of the durian-like microsphere structure might involve two important steps: Ostwald ripening and self-assembly. The new cobalt processed large specific surface area, which is helpful for the lithium ion transition and the lithium ion storage. During the lithiation process, volume expansion is often commonly observed. Compared with the bulk sphere structure, durian-like structure with high surface area can decrease the anode expansion, which is helpful for the cycling property. As a magnetic material, when calcined at 1450℃, the cobalt ferrite possessed the best magnetostriction properties.