Electrochemical Performance Of Flame-Retardant Electrolyte For Room-Temperature Sodium-Sulfur Battery

Room temperature sodium sulfur?RT Na-S?battery is expected to be used in the large-scale energy storage equipment due to its high specific capacity and low cost,so it has received extensive attention.Despite the excellent performance of RT Na-S batteries with carbonate-based electrolytes,safety issues still exist.A lot of heat,which produced by over-charge/over-discharge,over-loading and short-circuit,will lead to the decomposition of electrolyte and active substances and in batteries.As a result,gases are generated and the internal pressure of batteries increases,which eventually leads to burning and explosion.Thus,it is particularly important to develop electrolytes that have a balance between safety and electrochemical performance.A straightforward strategy is to introduce flame-retardant?FR?additives into existing electrolytes.These FR additives terminate combustion process through‘free radical capture mechanism'—FR additives readily release free radical[P]·to capture the radicals H·and HO·generated in combustion chain reactions.Here,YP50F/S composites were prepared by thermal melting method;flame-retardant electrolytes were prepared by adding trimethyl phosphate?TMP?and dimethyl methylphosphonate?DMMP?FR additives into 1.0 M NaClO₄-EC/PC?v/v=1/1?reference electrolyte.The electrochemical reaction mechanism and cycle performance of Na-YP50F/S batteries in 0.5-3.0 V and0.1-3.0 V were analyzed by cyclic voltammetry?CV?and constant current charge-discharge tests.The effect of FR additives on flame resistance,thermal stability and electrical conductivity were discussed,and the electrochemical compatibility between electrolytes and sodium metal were investigated.The electrochemical properties,such as electrochemical reaction,reversible capacity,rate performance of Na-YP50F/S batteries in different electrolytes were investigated by CV and charge-discharge test.The essential reasons for the effect of FR additives on the electrochemical properties were studied by surface morphology and elemental composition.The main results of this paper are as follows:?1?In 0.5-3.0 V and 0.1-3.0 V,the electrochemically reaction mechanism of Na-YP50F/S battery are same basically.After the first cycle,there are two electrochemical reduction reactions:S₈?Na₂S_x?4?x?8?and Na₂S_x?4?x?8??Na₂S_x?1?x<4?,the former is irreversible during the subsequent charge-discharge cycle.During subsequent cycling,only the conversion reaction between Na₂S_x?4?x?8?and Na₂S_x?1?x<4?occurs.In 0.5-3.0 V,the cell only can exhibit a reversible capacity of 434 mAh/g and poor cycle stability.In 0.1-3.0 V,cell exhibits a higher reversible capacity?899 mAh/g?and good cycle stability,so0.1-3.0 V is a more suitable voltage range for our battery system.?2?Adding 15 wt.%or more TMP into the 1.0 M NaClO₄-EC/PC?v/v=1/1?electrolyte can obtain flame retardant electrolyte,with high thermal stability,high conductivity and excellent electrochemical compatibility with Na metal.Using 15 wt.%TMP-contained electrolyte,the battery exhibits excellent electrochemical properties,which is approached to the electrochemical properties of cell using reference electrolyte.The electrochemical performance decreases with the increase of TMP.When the content of TMP increases to 25wt.%,the capacity fades significantly.Therefore,the optimized content of TMP in the system is 15 wt.%.?3?Adding 10 wt.%or more DMMP into the 1.0 M NaClO₄-EC/PC?v/v=1/1?electrolyte can obtain flame-retardant electrolyte,with high thermal stability,high conductivity and excellent electrochemical compatibility with Na metal.Using 10 wt.%DMMP-contained electrolyte,the battery exhibits excellent electrochemical properties,which is approached to the electrochemical properties of cell using reference electrolyte.The electrochemical performance decreases with the increase of DMMP.When the content of TMP increases to 20wt.%,the capacity fades significantly,therefore,the optimized content of DMMP in the system is 10 wt.%.?4?25 wt.%TMP and 20 wt.%DMMP impair the electrochemical performance by restraining the reduction of EC/PC and harming the stability of SEI during the initial discharge,which results an irreversible adverse contact between active substance and electrolyte solution.Eventually,the serious shuttle effect becomes too severe so that electrochemical performance deteriorates.In summary,we first developed a kind of non-flammable electrolyte for RT Na-S batteries by adding FR additives to the original electrolyte.This non-flammable electrolytes have a good balance between safety and electrochemical performance,which will contribute to the further application of RT Na-S batteries in practical applications.