Application Of Bi-functional Electrolyte Additive In Li-O₂ Battery

Lithium-oxygen batteries have received increasing attentions because of their super-high theoretical energy density.They show great application potentials in the fields of electric vehicles and believed to be alternatives to traditional to lithium-ion batteries and act as the next-generation energy storage system.However,at present,compared with lithium-ion batteries,the development of lithium-oxygen batteries is still at its infant stage and lots of problems need to be resolved.For instance,the discharging product of lithium-oxygen batteries,namely,Li₂O₂,is not conductive and hence a high potential is needed for its decomposition during the charging process.Such high over-potential will lead to the destruction of the carbon cathode materials and the decomposition of the electrolyte.Besides,since pure lithium foil has been widely employed as the anode,the growth of lithium dendrites and the expansion of lithium metal during the charging and discharging processes may have a significant effect on the safety of the batteries.Regarding on these problems,introduction of solid catalysts into the cathodes was considered as one of the most effective solutions.Although many solid catalysts have superior catalytic activity intrinsically,their lack of direct contact with the discharging products has restricted their actual catalytic efficiency.Additionally,solid catalysts could not suppress the formation of lithium dendrites and therefore their improvement for the battery performance is quite limited.To decrease the over-potentials and hinder the formation of lithium dendrites,from the point view of electrolyte additives,we utilized N-methyl-N-propyl pyrrolidine bromide?MPPBr?as the redox mediator?RM?and introduced it into the electrolytes of lithium-oxygen batteries and investigated its stability during the charging and discharging processes.?1?Evaluate the stability and feasibility of MPPBr as an electrolyte additive in the process of battery charging and discharging.By comparing and analyzing the CV curves of lithium-oxygen batteries,coupled with the characterization results of the carbon cathodes including XRD,FTIR,and SEM and the chemical shifts of ¹H NMR of the electrolytes before and after discharging,it turned out that MPPBr was stable.?2?Investigate the effect of MPPBr on the over-potential by analyzing the CV curves,deep charging and discharging curves,charging and discharging cycling curves of lithium-oxygen batteries and the electrochemical performances of the lithium symmetric batteries.The results indicated that the presence of MPPBr in the electrolytes could significantly reduce the over-potential.This was mainly because the working mechanism of Br^-/Br₃^-was similar to that of LiBr.Briefly,Br₃^-was able to oxidize the discharging product of Li₂O₂.As a result,the charging potential could be greatly decreased and so did the over-potential.?3?Investigate the effect of MPPBr on the lithium metal anodes of lithium-oxygen batteries based on the electrochemical performances of the lithium symmetric batteries and the results of SEM and XPS of the lithium electrodes before and after discharging.The results indicated that MPP⁺dissociated from MPPBr in the electrolyte could promote the formation of stable solid electrolyte interphase?SEI?membranes with abundant organic species.They could effectively reduce the direct contact of lithium metal with electrolyte,inhibit the growth of lithium dendrites and avoid the production of dead lithium and finally realized the goal of protecting anodes and improving the cycling stability of the batteries.