Research On SEI Film Regulation In The Carbonate-based Electrolyte For Li-S Batteries

With the increase of the world’s population and the massive consumption of oil energy,the whole society urgently needs a new type of clean and renewable energy source to satisfy our modern life and our growing energy needs.The clean energy sources of solar,wind and tidal power are gradually entering our society,but these renewable energy sources cannot be supplied sustainably.As an advanced energy storage technology,lithium-ion battery has developed rapidly at the end of the last century.Until now,a number of excellent lithium-ion cathode materials such as lithium iron phosphate and ternary positive material have been successfully applied to realize the comprehensive commercialization of electric vehicles.However,the rapid development of science and technology makes the commercialized cathode materials unable to satisfy the energy demand of the future society,so it is urgent to find a cathode material with high energy density to replace the existing cathode materials.Lithium-sulfur battery stands out among many materials with its energy density(2600Wh/kg),low price and environmental friendliness.Due to the irreversible nucleophilic reaction between the traditional carbonate electrolyte and polysulfide in lithium-sulfur battery,it is difficult to work normally for the sulfur cathode material in the traditional carbonate electrolyte.Recent studies have shown that the irreversible side reactions can be effectively inhibited by constructing a stable SEI film on the surface of the sulfur cathode,so as to realize the application of lithium-sulfur batteries in ester electrolytes.Our work mainly focus on the perspective of electrolyte,taking carbon-sulfur composites as the research object.Through the theoretical calculation and experiment matching verification,optimize the types of lithium salt and additives.The evolution rule of SEI film composition and structure was investigated by various characterization measurement.The stable SEI film was constructed to improve the electrochemical performance of lithium-sulfur batteries.Relevant specific studies are as follows:(1)In the high concentration carbonate electrolyte,the range of carbon host has been extended from microporous carbon to mesoporous carbon in lithium sulfur battery,and the range of application of carbon host needs to be further clarified.Hence,we designed three various carbon host(nonporous carbon,microporous carbon,mesoporous carbon)to prepare carbon-sulfur composites,the correlation between the concentration of electrolyte and the internal solvation structure was studied by the simulation of molecular dynamics theory,and then verified by Raman spectroscopy and fourier infrared spectroscopy.Finally,the structure-activity relationship between structure and property was realized by studying the relevant electrochemical behavior and the depth information of SEI structure composition,so as to obtain better lithium-sulfur batteries.Moreover,due to the characteristics of low ionic conductivity and high viscosity of the high concentration electrolyte itself,we explored the relevant electrochemical behavior of the lithium-sulfur battery at55℃,indicating that high temperature can reduce the energy barrier of electrode reaction and achieve further improvement of performance.(2)Combined with the previous work of high concentration electrolyte,we conclude that the lithium halide in SEI film is conducive to the formation of stable SEI film.Furthermore,high concentration electrolyte also brings the shortcomings of low ionic conductivity,high viscosity and high price.In this work,we introduced amyl triphenyl phosphorus bromide(ATPPB)as the electrolyte additive and the microporous carbon-sulfur complex as the research object.The molecular dynamics calculation showed that the additive was more easily decomposed to form SEI film.The characterization methods such as X-ray photoelectron spectroscopy,scanning electron microscopy and EDS mapping showed that the additive of amyl triphenyl phosphorus bromide(ATPPB)in the electrolyte could effectively change the composition and structure of SEI and construct a stable SEI film.Thus,the capacity and cycle stability of lithium sulfur battery can be improved.