Research On Subcritical-supercritical CO₂ Recycling Of Electrolyte And Cathode Material From Spent Lithium Ion Battery

Recycling spent lithium-ion batteries is an important measure to solve the contradiction between energy demand and resource consumption.At present,two basic recycling technical routes of pyrometallurgy and hydrometallurgy have been basically formed in the industrial application,but there are still key technical problems to be solved,such as environmental risks of mishandled electrolyte,electrode material dissociation difficulty,the impurity ion mixing.In view of the above problems,the green chemical means of subcritical-supercritical CO₂ is applied to realize the harmless treatment of electrolyte and the dissociation of cathode materials from the foil,so as to the efficient recovery of electrolyte and cathode materials.The principle of molecular dynamics,component changes and extraction behavior in the process are studied and analyzed,and the function and application value of supercritical CO₂ extraction on the interface separation of cathode material/foil are clarified.CO₂ fluid with high mass transfer coefficient and dissolution characteristics are used to investigate the extraction rule of the electrolyte organic components under different temperature and pressure combinations.The study shows that the extraction efficiency of organic solvents under subcritical CO₂（30℃,8 MPa）is 95.02%,while that of supercritical CO₂（40℃,23 MPa）is 86.73%.Box-behnken experimental design is used to analyze the significant relationship between the influence degree of each factor on extraction rate according to the response surface equation of ANOVA.The main effect relationship of each factor is extraction pressure>extraction temperature>extraction time.The quadratic polynomial model between temperature X₁,pressure X₂,time X₃ and extraction efficiency Y is established as Y=95.18-10.43*X₁+16.89*X₂-2.26*X₃+8.60*X₁*X₂-2.98*X₁*X₃+3.37*X₂*X₃-8.39*X₁²-11.27*X₂²-10.15*X₃²;the optimal conditions of the electrolyte subcritical CO₂extraction are pressure 8.84 MPa,temperature 28.86℃,and dynamic extraction time9.77 min.Under the optimized conditions,the actual extraction efficiency of the electrolyte reaches 96.72%.Molecular dynamics simulation shows that the cohesive energy density and solubility parameters of DMC/CO₂ and EMC/CO₂ binary systems are close to CO₂,and the interaction between organic carbonate is weakened.The interaction energies between the two molecules and CO₂ are 535.07 and 547.89k J/mol,respectively.It indicates that the chain carbonate interacts more strongly with subcritical CO₂.The component changes and extraction behavior of the commercial electrolyte were studied under subcritical CO₂ conditions.According to the characteristic proton ion peak of GC-MS,the carbonate solvent in the electrolyte did not change before and after the subcritical CO2 extraction,while Li PF₆ salt decomposes a little,and the concentration decreases from 0.8963 mol·L-1 to 0.6746 mol·L-1,and the extraction efficiency of Li PF₆ is about 75.26%.The subcritical CO₂ fluid has a stronger selectivity for DMC,which has a significant advantage in the system of 28℃and 6MPa.However,with the increase of pressure,the concentration of DMC with low melting point and high volatile components decreases,and the proportion of organic components also decreases from 40%to 38%.According to the linear relationship between the area and intensity of absorption peak and the number of fluorine nuclei in NMR spectra,the Li PF₆ salt is further hydrolyzed during the extraction process.With the increase of the battery cycling,the content of decomposition products PO₂F₂^-in the aging electrolyte was high,while the content of PO₃F^2-and HF is relatively low,indicating that the decomposition reaction is deepened with the increase of the battery cycling.A single peak of H₃PO₄ atδp=0.1 ppm indicates that Li PF₆ has been largely decomposed.In order to realize the separation of the positive electrode material and the foil,the temperature and pressure of the system are gradually increased to the supercritical state.With the removal of organic binder and the insertion of CO₂ fluid,the interface adhesion between the cathode electrode material and aluminum foil is weakened,and the bonding force is insufficient to resist the stress.Finally,the separation of the positive electrode material and aluminum foil is realized.When the pressure is 10MPa,the temperature 38℃,and the time 15 min,the optimum process parameters are obtained,and the stripping efficiency of cathode electrode material and aluminum foil reaches 98.86%.Taking heat treatment and chemical dissolution as the control groups,the differences of recycled materials in crystal structure,microstructure and dissociation degree are comparatively analyzed.The results show that chemical dissolving introduces more aluminum impurities,high-temperature calcination causes lattice defects and the loss of lithium,even accompanied by new impurity phase.Under the same aging condition,the reclaimed materials by high-temperature calcination and supercritical CO₂ extraction are assembled into cells to test electrochemical performances.The results show that the recovered LCO materials by supercritical CO₂ show longer charging platform,higher discharge specific capacity and initial coulombic efficiency.The capacity retention rate after 100 cycles is 77.1%,and the good cycling stability indicates that the overall structural framework of the material is still stable.In addition,electrochemical impedance spectroscopy and DQ/DV curves also confirm that supercritical CO₂ treatment is beneficial to charge conduction and rapid diffusion of lithium ions,and the redox reaction polarization is small in the cycle process,making recovered materials the potential of direct repair.