Study On Hydrogen Reduction Of Cathode Materials From Spent Lithium-ion Batteries

Spent lithium-ion batteries are an important strategic secondary resource.Recycling spent lithium-ion batteries is crucial to alleviate mineral resource pressure,reduce the cost of lithium-ion battery production,mitigate environmental pollution,and promote the sustainable development of the lithium-ion battery industry.The current recycling methods generally have the limitations of vast discharge of wastewater and slag,severe secondary pollution,high cost,and complex process.Under the mounting pressure to reduce carbon emissions and guarantee environmental protection,developing a clean,short,low-cost,and efficient recycling process is vital to address the current recycling challenges.In this study,a hydrogen reduction-water leaching-wet magnetic separation process was proposed to recover valuable metals in cathode materials of spent lithium-ion batteries.Kinetic analysis of the hydrogen reduction and a detailed study of the hydrogen reduction mechanism was studied carried out.The effects of hydrogen reduction temperature,reduction time,ball milling time and liquid-solid ratio during water immersion on metal recovery were discussed,and high-purity LiCl products were prepared.The following findings can be drawn based on the experimental results:（1）The hydrogen reduction mechanism of the spent lithium-ion battery cathode material was studied.The reduction of the cathode material mainly occurred at 195～475℃;the reduction of nickel and cobalt oxides mainly occurred at 475～685℃;and the reduction of LiMnO₂ and residual nickel and cobalt oxides mainly occurred at 685～880℃.During the isothermal reduction process,the reaction conformed to the 3D diffusion model at 500-700℃,and the apparent activation energy was 84.86 k J/mol,and the reaction is controlled from the diffusion rate of hydrogen in the material to the control of the diffusion rate of hydrogen in the alloy.The reaction conformed to both the 3D diffusion model and the R2 contracting area model at 700℃.At 800～900℃,the reaction conforms to the R2contracting area model,and the apparent activation energy is 51.82k J/mol,and the reaction is mainly controlled by the rate of chemical reaction at the interface between the alloy and the cathode material.（2）The hydrogen reduction temperature and reduction time of cathode material for spent lithium ion battery were optimized.Under the optimal experimental conditions of reduction temperature of 800℃ and reduction time of 90 min,the Li,Ni,Co,and Mn recovery rates under these conditions reached 96.79%,99.81%,99.41%,and 90.31%,respectively.The leaching rate of Al element reached 96.49%.In the range of 600-800℃,the increase in temperature was beneficial in improving the separation efficiency of nickel/cobalt elements and manganese/oxygen elements.However,when the temperature exceeds 800℃,a continuous increase in temperature deters the magnetic recovery rate of the nickel and cobalt and the purity of the product.Moreover,it also resulted in a dramatic decline in the lithium recovery rate.Contrarily,prolonging the reduction time is conducive to the enrichment of nickel and cobalt elements and the growth of nickel-cobalt alloy particles,which enhances the magnetic separation efficiency.（3）The ball milling time of reduction product and liquid-solid ratio during leaching process were optimized,and LiCl products with high purity were prepared.Under the optimal experimental conditions with a ball milling time of 180s and a liquid-solid ratio of 10:1（mL/g）,the recovery rate of Li,Ni,Co,and Mn under these conditions reached 94.89%,99.86%,99.64%and 91.12%,respectively,and the leaching rate of Al is91.44%.The pH was adjusted to around 7.The aluminum removal rate from the water leaching solution was 97.95%,and only 0.27%lithium was lost during precipitation.The resultant product was high purity LiCl（99.98%purity）.