Ionic Liquid Electrochemistry And Its Application In Lithium Metal Deposition

Solid electrolyte interphase(SEI)formation and dendrite growth in lithium metal electrode cause safety problems and lower cycle efficiency which have seriously hindered the application of lithium metal batteries.The growth of dendrites is directly related to the deposition behavior of lithium metal.By studying the fundamental scientific problem of nucleation and growth mechanism in the early stage of lithium plating,a more comprehensive understanding of the lithium deposition process can be obtained.However,the presence of SEI increases the difficulty of studying lithium nucleation and growth mechanism.Ionic liquids are a new type of green solvent,which have the advantages of good thermal stability,non-flammability,non-volatility,high ionic conductivity and wide electrochemical window.It can be used as an electrolyte in lithium batteries and is expected to solve the safety issues of batteries.But,there is a lack of in-depth research on lithium deposition behavior in ionic liquid electrolytes,especially the nucleation and growth behavior of initial lithium deposition.Electrochemical investigations of ionic liquids can provide a basis for their applications in lithium deposition.By employing traditional electrochemical techniques,combined with advanced characterization tools,the electrochemical behaviors and film formation process in imidazolium and pyrrolidinium based ionic liquids are studied and the role of lithium salts in ionic liquids that composed of imidazolium and pyrrolidinium cations,bis(trifluoromethylsulfonyl)imide(TFSI-)and bis(fluorosulfonyl)imidc anion(FSI-)are discussed.Chronoamperometry was used to investigate the nucleation and growth behavior of lithium in mixed solutions of Py14TFSI and Py14FSI in different proportions,revealed the effect of overpotential and FSI-content on lithium nucleation and growth;lithium nucleation and growth model(3D-SEI model)considering electrolyte decomposition was used to obtain kinetic parameters such as nucleation density(N0)and electrolyte decomposition rate constant(kSEI)associated with the SEI formation.Similar studies are extended to sodium and potassium nucleation and growth.The main contents and results are as follows:1.The electrochemical behavior and film formation process of imidazolium and pyrrolidinium ionic liquids in the absence and presence of lithium salts.The results show that cathodic reductive decomposition potential of pyrrolidinium ionic liquid(Py14TFSI)is more negative than that of imidazolium ionic liquids;after adding lithium salt,the cathodic potential window of ionic liquids shift more negatively.Both ionic liquids undergo a film formation process with anion reductive decomposition at 1.0～1.5 V vs.Li/Li+ The presence of lithium salts contributes to the formation of the SEI film in these ionic liquids;reversible lithium deposition/dissolution could not be observed in the imidazolium ionic liquids(EMITFSI and BMITFSI)with LiTFSI.The phenomenon is probably due to that the SEI formed in LiTFSI/EMITFSI not only conduction lithium ions but also conduction EMI+ cations which could interact with the deposited Li metal.2.Lithium nucleation and growth behavior in pyrrolidinium ionic liquids.Chronoamperometry was used to study the nucleation and growth behavior of lithium in pyrrolidinium ionic liquids with different FSI-content.It has been found that as the deposition overpotential and FSI-content increase,imax on the current-time transients increases and tmax decreases.By using traditional nucleation theory of metal electrodeposition to fit experimental data,the nucleation and growth of lithium in all systems is found to be close to 3D instantaneous nucleation and growth;however,as lithium nucleation and growth is accompanied by the electrolyte reductive decomposition,there is a large deviation between the experimental curve and the fitting curve.A lithium nucleation model(3D-SEI model)that considered electrolyte decomposition was further used to perform nonlinear fitting,and the current of lithium nucleation(J3D-DC)and electrolyte decomposition(JSEI)individually,as well as relevant kinetic parameters could be obtained.The results show that except the ionic liquids which only contain FSI-,the nucleation density(N0)and electrolyte decomposition rate constant(kSEI)in the remaining four ionic liquids generally presented the trend of increasing with the overpotential and FSI-content,indicating that the electrolyte decomposition related to the formation of SEI is more serious under the larger overpotential.After removing the current contribution of electrolyte decomposition,the traditional nucleation theory was used for analysis again.No obvious deviation was observed.Lithium nucleation and growth mechanisms all show a transition from 3D progressive nucleation to 3D instantaneous nucleation as the overpotential increases.The work was extended to the electrodeposition of sodium and potassium,and 3D instantaneous nucleation and growth characteristics were also observed.