The Research on Recycling of Cathode Materials from Spent Lithium-ion Batteries by Leaching with Organic Acid Reducing Systems
Author:Gao Gui Lan
As one of the most popular rechargeable batteries,lithium-ion battery(LIB)has been widely used in portable electronic devices(eg.cellular phones,laptops,video cameras,etc.),electric vehicles,medical equipments and military/aerospace applications.At present,China has become the major countries of production,consumption and export for lithium-ion battery.With the rapid increase in LIB production,we face the problem of dealing with the mountains of end-of-life batteries.Recycling the spent LIBs,especially the cathode materials,is of great significance not only to protect the environment and avoid the risk of human health,but also to bring enormous economic value,which will promote the sustainable development of LIBs industry.In this paper,an in-depth study on the leaching of valuable metals from cathode active materials in two kinds of LIBs used in mobile phones was carried out by easily degradable organic acid reductive systems.Some exploratory works have been done on the mechanochemical method to strengthen the leaching process of organic acids.Three kinds of simple,effective and environmentally friendly leaching methods for the recycling of valuable metals were raised.The main contents of the study are as follows:(1)The pretreatment process of LIBs for waste mobile phone was studied systematically,including discharge,disassembly,separation of active material from collecting foils.The spent LIBs were first discharged completely by using a NaCl solution of 20g/L for 6 hours to remove any remaining electrical potential capacity,and then dismantled to collect the complete cathodes.The cathodes were thermally treated to separate the cathode materials from the aluminum foils at 450℃for 30 min,and then the polyvinylidene fluoride(PVDF)binder and acetylene black were removed from the cathode active materials by thermal treatment at 600℃for 5 h.The relatively pure lithium cobalt oxide powders were then obtained as raw materials.(2)In the present study,we designed an efficient sulfuric acid-vitamin C leaching system for the extraction of Cobalt(Co)from spent LIBs.We followed the effects of operational parameters,such as concentrations of sulfuric acid and vitamin C,leaching time,temperature,and solid-liquid ratio,on the efficiency of leaching.The final optimized experimental conditions were determined with the assistance of Design-Expert 220.127.116.11 software.Optimal reaction conditions,under which 95.46%of Co was recovered,contained 3mol·L-1 sulfuric acid and 0.28 mol·L-1 vitamin C with S/L ratio at 15 g·L-1 and leaching temperature of 70℃.The leaching kinetics analysis showed that the rate equation of the product layer diffusion control can describe the leaching kinetics of the valuable metal Co well.The apparent activation energy(Ea)was determined to be 32.7KJ/mol,indicating that the leaching process was controlled by surface chemical reaction and mixed diffusion in the product layer,the dominant rate-controlling step.The present research explored some details of a promising environmentally friendly technology for the recovery of valuable metal Co from spent LIBs.(3)In order to avoid the problem of using inorganic acids,an efficient hydrometallurgical leaching of Cobalt(Co)and lithium(Li)from cathode materials of spent LIBs using a citric acid/sodium thiosulfate(Na2S2O3)system was explored.The effects of citric acid and Na2S2O3 concentrations,leaching time,temperature and the solid/liquid(S/L)ratio on the leaching process were also examined.With the exception of the S/L ratio,increases in citric acid concentration,Na2S2O3 concentration,leaching time and temperature all had positive effects on the leaching of Co and Li.Ultimately,this citric acid/sodium thiosulfate system resulted in recoveries of approximately 99%of Co and 100%of Li from the spent LIBs.Further,pure sulfur was obtained as a by-product during the leaching process.The data from the leaching processes fit the Avrami equation well and model equations were advanced to describe the leaching of Co and Li.The apparent activation energies of Co and Li were determined.The results showed that the surface chemical reaction was the rate-controlling step during the leaching process.The relationships between the leaching rate of cobalt or lithium and the reaction temperature and reaction time were obtained.The present study shows promise for hydrometallurgical recovery of valuable metals from spent LIBs.(4)Co and Li were extracted with ascorbic acid at room temperature by mechanochemical process.The activated conditions and the concentration of ascorbic acid in the leaching process were investigated.The results showed that 98.32%Co and 100%Li were respectively leached under room temperature with the conditions:rotary speed of 650 rpm,milling time of 90 min,ball-to-powder mass ratio of 60:1.and the concentration of ascorbic acid of 0.26 mol.L-1.From X-ray diffraction(XRD)and scanning electron microscope(SEM)characterization,it was found that the layer structure and morphology of LiCoO2 were destroyed,probably leading to enhanced leaching of Co and Li.Kinetic analysis was conducted to better understand the leaching process.The results showed that the internal diffusion model equation 1-2/3x-(1-x)2/3 can well describe the leaching kinetics of mechanical activated vitamin C leaching reaction,which is consistent with sulfuric acid-vitamin C system.The corresponding apparent activation energies of Co and Li were calculated and the leaching process was controlled by the internal diffusion reaction.The relationships between the leaching rate of cobalt or lithium and the reaction temperature and reaction time were obtained.