Research On Mechanism For Synergistic And Selective Recovery Of Different Metals From Spent Li-ion Batteries

With the widespread use of lithium-ion batteries（LIBs）in portable electronic devices and electric vehicles,etc.,immense amount of spent LIBs has increased dramatically.Without proper treatment,residual electricity and toxic substances in spent LIBs may cause explosions,fires and environmental pollution,resulting in serious waste of scarce metal resources such as Co and Li.Therefore,extracting valuable metals from spent LIBs through efficient and green processes has significant economic and environmental benefits,and also is a key part of the resource recovery of spent LIBs.In conventional technologies,there is the defect of overly relying on additional reductants and oxidants during recovering lithium cobaltate（LiCoO₂,LCO）and lithium iron phosphate（LiFePO₄,LFP）.Therefore,based on their own redox properties of LiCoO₂ and LiFePO₄,we designed the redox reactions of LiCoO₂ and LiFePO₄ materials obtained from spent LIBs under mild liquid-phase environment and solid-phase mechanochemical induction,respectively,to construct two routes to achieve selective and synergistic recovery of valuable metals,and also explore the transformation of pivotal elements and mechanochemical induction mechanisms during the redox processes.In the liquid-phase synergistic leaching of LiCoO₂ and LiFePO₄,the optimal conditions were determined by single-factor experiments:H₂SO₄concentration of 0.50 mol/L,leaching time of 20 min,solid-liquid ratio of30 g/L,molar ratio（LCO:LFP）of 1:1,the leaching efficiency of Li,Fe and P were over 99%with the 92.4%of Co.The leaching kinetics of Co and Li in LiCoO₂ with and without LiFePO₄ was analyzed,indicating that LiFePO₄ greatly reduced the apparent energies of Co and Li,and significantly increased the leaching rate and efficiency of LiCoO₂ without influence to its leaching behavior.The thermodynamic analysis of the leaching process reflected that it is feasible to convert LiFePO₄ into Fe PO₄·2H₂O in acid solution,and the intermediate Co₃O₄ will appear in the leaching of LiCoO₂.Combined thermodynamic ion species simulation with practical experiments of the lixivium,it can be discovered that more than 99%of Fe³⁺can be precipitated as iron phosphate without any coprecipitation of other metals under the p H of 2.5,and the product was verified to be Fe PO₄ with a purity of 99.22%by XRD,XPS and ICP-MS.The chemical transformation of Fe in the whole process is LiFePO₄→Fe²⁺→Fe³⁺→Fe PO₄,which is the key to promote efficient leaching of Co and Li and selective separation of Fe.The solid-phase redox of LiCoO₂ and LiFePO₄ was induced by mechanochemical reaction（MR）.The physicochemical properties,crystal structure and valence state of the elements of the products after MR were analyzed by XRD,HRTEM and XPS,etc,and the results proved that the materials size was reduced to nanometer with lattice distortion and amorphization.The lattice oxygen of LiCoO₂ was decreased,and Co（Ⅱ）and Fe（Ⅲ）synchronously increased with the stable existance of PO₄structure from LiFePO₄.After MR,53%of Li was extracted by water leaching.Acid leaching was applied to extract Co and Li simultaneously,and the optimal ball milling conditions were determined:molar ratio（LCO:LFP）of 1:1,milling time of 5 h,rotation speed of 650 rpm,ball-powder ratio of 50:1,and 89.16%,99.99%,1.13%,and 9.64%of Co,Li,Fe,and P,respectively,were leached at the stoichiometric acid leaching of 0.15 mol/L H₂SO₄.Li and Co were selectively dissolved in the solution,and Fe and P were converted to the insoluble Fe PO₄ which were separated as leaching residue in the solid phase.The mechanism is the redox reaction between Co（Ⅲ）of LiCoO₂ and Fe（Ⅱ）of LiFePO₄induced by mechanochemistry.Combined with the analysis of all leaching phenomenon,the products after MR can be deduced as Co O,Li₂O and Fe PO₄,and DFT calculation also support above process.The strategy further reduces acid consumption compared with acid leaching to achieve selective and efficient recovery of Co and Li.