Research On Recycling And Reusing Of Carbonate Based Electrolyte For Lithium Ion Batteries By Supercritical CO₂

Li-ion battery has been successfully and widely used in digital electronic products,and in the future,will flourish in the fields of new energy and energy storage because of its high energy density.Used Li-ion batteries must be treated to be harmless and be utilized comprehensively after projected life-span because it contains valuable metals and fluoride electrolyte.In this research,the spent Li-ion battery electrolyte was recycled using the supercritical CO₂ extraction technique.The Box-Behnken design of response surface methodology was adopted to optimize the supercritical CO₂ extraction operational conditions.The extraction behavior of electrolyte’s main ingredients and the influence of cosolvent on electrolyte extraction yield in supercritical CO₂ were studied through establishing GC-FID and NMR quantitative method.The reclycling was ultimately achieved for resource utilization.The reasons of low initial coulombic efficiency for Li/Li Co O₂ battery using reclaimed electrolyte was discussed.The Box-Behnken design of response surface methodology was adopted to optimize the supercritical CO₂ extraction operational conditions based the single-factor test.Quadric polynomial model on extraction pressure P,extraction temperature T,extraction time t and extraction yield η was established by stepwise regression method: η =+87.61+2.11P+0.81T+0.92t-0.23P·t+0.31T·t-2.22P2-0.52t2.The influence extent of factors on extraction yield is P>t>T.The optimized conditions were determined: P=23 MPa,T=40 °C and t=45 min.Under these optimum conditions,the actual extraction yield was 85.1%.The contents of extract was analyzed by ICP,FT-IR,GC-MS and NMR.The results show that contents largely remained unchanged,and LiPF₆ concentration drops from 0.89 to 0.66 mol·L_-1.Supercritical CO₂ extraction is proved to be an efficient method for recovery of organic carbonate-based electrolytes from spent lithium-ion batteries.Quantitative analysis method were established for GC-FID for organic solvent and NMR for LiPF₆ of electrolyte.Both methods have good precision,repeatability and stability in 0.2⁰.45 mg·m L_-1 and 0.167⁰.99 mol·L_-1 concentration range.The extraction behavior of main ingredients in supercritical CO₂ was studied using the quantitative analysis method.The results reveal that linear carbonate and cyclic carbonate have different extraction behaviors in supercritical CO₂.The nonpolar linear carbonate DMC and EMC could be executed using relatively non-or weak polar extraction medium,and the extraction yield are 87.4% and 85.7%.Polar cyclic carbonate and LiPF₆ could be extracted using polar extraction medium,and the extraction yield are 98.1% and 68.4%.Extraction yield of low melting point and high volatile organic carbonates（DMC and EMC）decreased after 45 extraction time.LiPF₆ was also highly decomposed after 45 extraction time.Two quantitative methods were used to study the influence of the variety and the amount of entrainers（such as methanol,ethanol and PC,BC）on supercritical CO 2 extraction efficiency of the main electrolyte components.It is shown that under the same extraction condition,the optimal dosage of light alcohols entrainer was 6% with the electrolyte extraction efficiency increased from 85.1% to 89.8% by methanol and 87.5% by ethanol;the optimal amount of entrainer of cyclic carbonate was 9% with the extraction efficiency increased from 85.1% to 89.5% by PC and 86.4% by BC,respectively.Cyclic carbonate entrained agent contributed considerably to the extraction efficiency of both organic solvent composition and lithium salt,among which the PC modifier performed better in terms of the extraction efficiency.Entrainer used for supercritical CO₂ electrolyte extraction could chosed from organic carbonate solvents with medium polarity and low viscosity,which facilitates the dissolution and diffusion of the electrolyte with strong polar components.Molecular sieve and weak-base anion exchange resin were used to remove the moisture and hydrofluoric acid in the electrolytes.The electrolyte was mixed according to the formula of the commercial electrolyte.In reclaimed electrolyte,HF content was 37 ppm,moisture content was 18 ppm,ionic conductivity was 0.19 m S·cm^-1 at 20 °C,Li-ion transference number was 0.76 at 25 °C,electrochemical window was 5.4 V（vs.Li/Li+）.The initial discharge specific capacity at 0.2 C ratio assembled Li Co O₂ cell with reclaimed electrolyte was 115 m Ah·g^-1,and the discharge specific capacity kept over 77 m Ah·g^-1 after 100 times cycle with the capacity retention ratio of 66% and the coulomb efficiency of 96.2%.The recycling of products was achieved.EIS,SEM and XPS were used to study influence of reclaimed electrolyte on the interface behavior of Li Co O₂ electrode.The results show that CEI films corroded in HF because of POy Fz x-hydrolysis in reclaimed electrolyte.Therefore,the coulombic efficiency of Li Co O₂ cell with reclaimed electrolyte increased gradually during the initial charge-discharge cycles.