| Currently,the fast-growing market of consumer electronics,electric vehicles and stationary energy storage,has spurred a surge demand for lithium-ion battery?LIB?,which occupies the largest market share of batteries.It is predicted that there will be a huge growth for LIB industry in the future,particularly stimulated by the burst of electric vehicles for green transportation.In view of the significant quantity,environmental preservation and valuable resources conservation,the recycling of spent LIBs is highly desirable and has become a research hotspot around the world.Conventional recycling processes using strong inorganic acids bring about environmental pollution problems like toxic gases and waste acid solution.For these problems,green and simple recycling processes of spent LIBs have been systematically studied in this paper.The main obtained achievements and progress are listed below:LiNi1/3Co1/3Mn1/3O2 electrode scraps from industrial production of LIBs were regenerated as new cathodes by a simple and green method,which includes separation and heat treatment processes.A comprehensive and detailed investigation was conducted to find the optimal regeneration conditions and understand the underlying reasons.After regeneration,the cathodes exhibit satisfied electrochemical performances comparable with commercial LiNi1/3Co1/3Mn1/3O2 cathodes.Both the separation methods and calcination temperatures play an important role in the electrochemical performances.The profound reason lies in the content of LiF from PVDF decomposition and the morphology change of particles during the separation and calcination processes.Specifically,scraps separated by N-methylpyrrolidone?NMP?solvent dissolution and calcination at800°C deliver the highest discharge capacity of 150.2 mA h g-1 at 0.2C after 100 cycles,with the capacity retention of 95.1%.In order to reduce secondary pollution and deal with various cathodes,easily degradable citric acid was used as leaching acid to leach mixed waste cathode materials?including LiCoO2,LiMn2O4 and LiNi1/3Co1/3Mn1/3O2?.By varying the temperature,concentration of citric acid and hydrogen peroxide,solid-liquid ratio and leaching time,orthogonal test and single factor experiments were designed to analyze influences of each condition on the leaching of metal ions.Kinetic analysis was conducted to better understand the leaching process.The results show that surface chemical reaction is the rate-controlling step during the leaching process.The corresponding apparent activation energies of Li,Ni,Co and Mn are calculated.The best leaching conditions are:90 oC,60 min,20 g L-1,citric acid 0.5 M and H2O2 1.5 vol.%.The leaching efficiencies of all the metal ions can exceed 95%.After citric acid leaching process,sol-gel method was used to synthesize new cathode material LiNi1/3Co1/3Mn1/3O2 by adjusting the ratio of four metal ions in the leaching solution.The similar property of Ni,Co and Mn makes the sepatation process difficult and complex.So,the re-synthesis of new electrode materials can save this step and high-value product can be obtained.The citric acid in the leaching process can act as chelating agent in the sol-gel synthesis.Compared with materials synthesized using chemical regents,the re-synthesized materials using leaching solution exhibit better electrochemical performances.The reason lies in the trace Al doping in the re-synthesized materials,which has stronger covalent interaction with oxygen and can stablize the structure.Oxalate acid with strong acidity and reducibility was used to leach spent cathode LiNi1/3Co1/3Mn1/3O2.Due to the different solubility of lithium oxalate and other transition metal oxalates,lithium will dissolve in the leaching solution and cobalt oxalate,nickel oxalate and manganese oxalate will precipitate.Then,lithium and transition metal ions can be separated with oxalate acid leaching.By designing orthogonal test and single factor experiments,the temperature,concentration of oxalate acid,solid-liquid ratio and leaching time were optimized to obtain the best leaching of Li with more than 85%leaching efficiency.The corresponding conditions are 70 oC,60 min,20 g L-1 and oxalate acid 0.6 M.Kinetic analysis results show that unlike citric acid leaching process,diffusion process is the rate-controlling step during oxalate acid leaching process.The apparent activation energies of Li is calculated to be 7.34 kJ mol-1.In the leaching residues,transition metal oxalates were obtained.The oxalate precipitates can be mixed with certain amount of Li2CO3 to re-synthesize new cathode materials,which exhibite satisfied electrochemical performances comparable to commercial cathodes. |
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