Research On The Recovery And Mechanism Of Cobalt In Waste Lithium Cobalt Oxide Materials

Lithium ion batteries?LIBs?are widely used in the fields of consumption,power,energy storage,etc.because of their excellent performance.Large quantities of spent LIBs are discarded after limited service lifetime.These spent LIBs cannot be disposed into landfill because it contains hazardous materials,such as organic solvents,heavy metals.While,valuable metals in spent LIBs have been identified as attractive secondary source.Hence,recycling of spent LIBs has become an important issue for research and application.The hydrometallurgy has become the main method for recycling of spent LIBs,which is composed of acid and reductant of hydrogen peroxide.However,reductant of H₂O₂ cannot be used because of its strong corrosiveness,acidic decomposition and poor stability.The development of green and mild reductant is the key for green and efficient leaching of spent LIBs.Based on thermodynamics and kinetics theory,spent Li CoO₂ material is recovered by green recycling process,which including leaching enhanced by reduction,precipitation of cobalt with oxalate,preparation of Co₃O₄ by oxidation and regeneration of LiCoO₂.The control steps and reaction mechanism of leaching enhanced by green reductants of glucose and grape seed and electrochemical reduction are studied,which is conducive to the efficient recovery of spent LIBs.The leaching of spent LiCoO₂ material is closely related to its thermodynamic equilibrium.The potential E-pH diagram of Co-H₂O system is plotted under different ionic concentrations.Based on the relative potential and possible leaching process of LiCoO₂ and Co²⁺,LiCoO₂ material is more easily transformed into Co²⁺at low potential values.Reduction is beneficial for leaching of LiCoO₂.Instead of commonly used chemical reductant of hydrogen peroxide,the leaching process of spent LiCoO₂materials is studied by using glucose as green and mild reductant.The reduction reaction of glucose mainly includes oxidation of aldehyde group and oxidation of carboxyl group during leaching process.The intermediate product of the reaction is single organic acid,which can increase the acid concentration of the leaching system.The leaching of cobalt increases obviously and lithium increases faintly with help of glucose reduction.The leaching process of spent LiCoO₂ material is novelly studied by using natural grape seed as reductant.The oxidation of functional group in grape seed could provide reduction during acid leaching process,which is related to oxidation of carbon carbon double bond,aldehyde group and hydroxyl group happened in the components of EC,EGCG,CEC contained in grape seed.The leaching interface reaction includes leaching of LiCoO₂ material or by-product with H⁺and the converter of Co³⁺to Co²⁺because the proportion of Co³⁺decreases with the increases of Co²⁺in the leaching solution and the content of hydroxyl oxygen increases with unchanged of the cobalt valence state on the surface of LiCoO₂ particles.The leaching process is better described by avrami equation model.The leaching process with activation energy of 11.96 kJ/mol is controlled by the diffusion step of reactants to the interface.The increase of the amount of grape seed is beneficial to promoting leaching of cobalt.Based on the electrochemistry principle,the electrochemical reduction method is studied for the leaching of spent LiCoO₂ materials.Based on the classical unreacted nuclear shrinkage model,the leaching process of cobalt is divided into two steps,and the activation energy are 25.47 kJ/mol and 11.40 kJ/mol,respectively.The control of leaching process is converted from mixture control to diffusion control.The EIS analysis of leaching process is better described by model of R?QR?.The charge transfer impedance?Rct?decreases at beginning and become stable later,and the solution impedance?Rs?is relatively small during the leaching process.The interfacial chemical reaction is the main step of the mixed control.The edge of LiCoO₂ material particles is more vulnerable to be destroyed during the leaching process.Based on the dominant region of cobalt oxalate in ion morphologic distribution,the spherical cobalt oxalate with crystalline perfection and fluffy structure is obtained by using ammonium oxalate as precipitator.The precipitation reaction is belonged to two-order reaction with apparent activation energy of 19.68 kJ/mol,which is controlled by diffusion step.Chemical reaction is relatively fast in the process of cobalt ion precipitation.The Co₃O₄ precursor is papered by cobalt oxalate through oxidation in the air atmosphere at temperature.Based on the unity between constant temperature model of g???function and free model of Z???function during solid thermal decomposition process,the oxidation process is according with model of g???=[-ln?1-?]^1/3,which is related to random nucleation and growth process.The columnar morphology of the oxidation product Co₃O₄ is coincide with the model mechanism.The purity of Co₃O₄ reaches to 99%with a crystalline perfection.The regeneration LiCoO₂material has high crystallinity and good electrochemical performance with Co₃O₄ as precursor.