Low-temperature Electrochemical Extraction Of Li And Its Alloys From Amide-based Electrolyte Systems

Lithium metal,with low density,high specific heat capacity and high metal activity,is widely used in high-energy power batteries,light metal alloys,controlled nuclear fusion and military fuels.As the global new energy industry layout accelerates,the application value of lithium has further increased and lithium is in short supply,thus its preparation methods have received attention.Lithium metal is produced industrially by high-temperature molten salt electrolysis or vacuum thermal reduction methods,which inherently have complex processes,low energy efficiency and difficult production control.If lithium metal can be electrolyzed at room temperature,the process is simple and energy consumption is significantly reduced.Preferring a simple and practical solvent to dissolve lithium chloride is the key to achieve low carbon short process smelting,but traditional ionic liquids have the disadvantages of complex synthesis,water sensitivity and high cost.Therefore,in this study,we carried out a study related to the low-temperature electrochemical extraction of lithium metal and its alloys using inexpensive amide-based electrolyte systems at different cathodes to provide technological reserves and support for the green lithium smelting industry.Details are as follows.A binary DMAc-LiCl electrolyte system was designed using the nonprotonic polar N,N-dimethylacetamide（DMAc）as the solvent and LiCl as the raw material.The conductivity and viscosity of the electrolyte systems with different LiCl concentrations were measured in the temperature range of 298 K～368 K.The optimal LiCl electrolytic concentration was derived as 0.7 M（mol/L）by combining the cyclic voltammetry technique.According to the Arrhenius formula,the system conductivity activation energy and viscous flow activation energy were calculated to be 6.17 and 6.20 k J/mol,respectively.Infrared spectroscopy combined with theoretical calculations confirmed the formation of[Li（DMAc）_n]⁺from Li⁺and DMAc molecules through the solventization of Li-O bonds.The electrochemical behavior of lithium-containing ionophores was studied using cyclic voltammetry,with lithium reduction as a one-step electron gain process in the reaction of the follow equation:[Li（DMAc）_n]⁺+e^–=Li（0）+n DMAc.And the nucleation growth process of lithium metal was analyzed.Constant potential deposition（–3 V vs.Ag/Ag⁺,1800 s）on aluminum cathode substrates produces uniform,flat and dense Li Al alloy film layers as a result of solid-state cathode alloying,and SEM images show that the alloy exists as an array of nanosphere structures.Lithium metal plating was also deposited on the tin substrate,and the XRD results showed that the active metal Li combined with the Sn substrate to form Li Sn alloy,and Li₂Sn O₃ could be obtained after monolithic melting.The physical and chemical properties of the binary DMF-LiCl electrolyte system were investigated for the electrochemical extraction of lithium metal using N,N-dimethylformamide（DMF）as the solvent.The conductivity and viscosity of DMF-LiCl and DMAc-LiCl systems were compared and analyzed,and the electrolyte system was optimized.DMF has better dissolved salt performance,more stable electrochemistry and cheaper than DMAc.The presence morphology of lithium ion clusters in the binary system was analyzed using ⁷Li NMR,IR spectroscopy and computational simulations,and the dissolution reaction of LiCl was derived as:x LiCl+y DMF（?）x[Li（DMF）_{（y–n）/x}]⁺+x[Cl（DMF）_n/x]^–.The redox process of the lithium-containing species was investigated using cyclic voltammetry with an electrochemical behavior consistent with that of the DMAc-LiCl system,and the diffusion coefficient of the lithium-containing solvated ions was calculated to be 3.31×10^-7 cm² s^-1.The changing states of the chlorine-containing species before and after electrolysis were analyzed using UV-Vis absorption spectroscopy and the anodic reaction can be divided into two steps as follows:2[Cl（DMF）_n/x]^––e^–=Cl₂+2n/x DMF,Cl^–+Cl₂=Cl₃^–.Finally,lithium metal layers were deposited on high-purity indium and tin substrates at constant potential（–3 V vs.Ag/Ag⁺,t=3600 s）,and XPS and XRD techniques demonstrated and the presence of lithium metal in the deposited layer its phase as Li In alloy,indicating that the binary DMF-LiCl electrolyte system can be successfully used for electrochemical extraction of lithium metal.The deposited lithium metal is highly active,and in order to stabilize the lithium metal,it is proposed to further melt the deposit and the substrate material directly to finally form a stable bulk lithium-based alloy,so this strategy can realize the transformation from active lithium film to stable bulk alloy.The study further explored the co-deposition of Mg-Li alloy from the ternary amide-based electrolyte system DMF-Li NO₃（0.3 M）-Mg Cl₂（0.15 M）,and found that the binary DMF-Mg Cl₂ system could not deposit Mg metal,but the addition of Li NO₃ could achieve the deposition of Mg metal,probably because the strong solvation of Mg（II）ions became weaker under the action of Li NO₃,and it was easier to achieve one-step reduction of magnesium metal.The fugitive forms of Li（I）-containing and Mg（I）-containing ionophores were analyzed by infrared spectroscopy,their discharge reaction mechanism was investigated by cyclic voltammetry,and Li-Mg alloy layers were successfully deposited on aluminum substrates,and SEM showed that the alloy layers existed in elongated ellipses.The related basic research provides theoretical and technological support for low-carbon and low-cost extraction of bulk lithium metal and its alloys.