Separation Of Cobalt And Lithium From Spent LiCoO₂ Cathode Material During Citric Acid Leaching-electrochemical Reduction Process

Lithium batteries have become the earliest commercially used batteries due to their many advantages,and their cathode materials have also become one of the used of anode materials in most wide because of its high density of the energy and great safety.After lots of lithium batteries are disputed,cobalt and lithium metals in the cathode material are also discarded.Therefore,recycling cobalt and lithium metals from waste lithium batteries and exploring their applications in other fields have become a hot issue in society.In this thesis,waste lithium cobalt acid batteries are used as raw materials,and LiCoO₂ cathode materials are separated and physically disassembled.The system of citric acid and sodium thiosulfate is used for leaching to enrich Co and Li in the leaching solution.Then,a cobalt coating is deposited on stainless steel plates by electrochemical reduction method,achieving preliminary separation of cobalt and lithium while recovering cobalt.Through acid leaching and electrodeposition experiments,with the help of an auto lab electrochemical workstation,combined with analysis and the methods of testing like XRD,SEM,EDS,and so on.,the leaching action of LiCoO₂ positive electrode material in citric acid system and the mechanism of Co²⁺electrodeposition under different p H conditions were studied,and the performance of cobalt coating was analyzed.The research results are as follows:（1）High purity LiCoO₂ powder with 63 wt%Co and 7 wt%Li content can be obtained after the waste lithium cobalt oxide battery is fully discharged in 10%Na Cl solution by disassembly,heat treatment and stripping.The particle size distribution of LiCoO₂ powder is uniform.（2）The optimal process for the citric acid sodium thiosulfate system leaching experiment is as follows:1.2 mol/L citric acid and 0.35 mol/L sodium thiosulfate can be used for leaching experiments at a temperature of 80℃,with a solid-liquid ratio set at 20 g/L,the leaching time is 70 minutes to optimize the leaching process.and the dissolution rate of LiCoO₂ powder reaches 99%.The Co²⁺leaching rate is 98%,and the Li⁺leaching rate is 97.8%.The concentrations of Co²⁺and Li⁺in the leaching solution are about 2.445 g/L and 0.271 g/L,respectively.The Arrhenius equation was used as the leaching reaction rate model.According to kinetic calculations,the activation energies of cobalt and lithium leaching reactions are31.53 k J/mol and 46.98 k J/mol,respectively.The surface reaction and diffusion process of the leaching reaction control process constitute the mixed control of cobalt and lithium metal ions.（3）The p H value has a significant impact on the reduction reaction of Co²⁺,which can be concluded from the results of LSV testing（linear voltammetry）,CV testing（cyclic voltammetry）,and EIS testing（AC impedance）.When p H≤3,there is only one step reduction process of free Co²⁺in the solution.The free Co²⁺is directly reduced to cobalt atoms,and the reduction potential is about-0.6 V/SCE;When p H≥4,the reduction of Co²⁺includes two steps,from Co（C₆H₅O₇）^-to Co（C₆H₆O₇）,and then to cobalt atoms.At this time,the reduction potential is more negative about-1.40 V/SCE than that of the first step;When p H>6,the solution environment is alkaline,the complex of Co²⁺and citric acid is more stable,and the Co²⁺reduction potential is negative to the hydrogen evolution potential,so the reduction becomes difficult.（4）The optimal process parameters for electrodepositing cobalt from leaching solution to prepare cobalt coatings are:cobalt mass concentration 42 g/L,sodium sulfate mass concentration 145 g/L,electrodeposition temperature 63℃,current density 450 A/m²,leaching solution p H=4,electrodeposition time 180 min.The cobalt grains in the cobalt coating obtained under this condition exhibit a sharp pyramid shaped arrangement,and the cone tips grow outward,closely arranged on the surface of the substrate,forming a fine and compact metal cobalt coating.The current efficiency of electrodeposition is 90%.After three electrodeposition cycles of citric acid leaching solution,The minimum concentration of cobalt ions in the leaching solution is 0.163 g/L,while the minimum concentration of lithium ions is0.253 g/L.which is basically not consumed.The green preliminary separation of cobalt and lithium metals has been achieved.