| Nowadays,as science and technology continue to advance,people’s quality of life is improving.However,the consumption of non-renewable energy sources,such as fossil fuels,is also increasing to meet the demands of modern living.The continuous use of fossil fuels has resulted in rising emissions of carbon dioxide and other harmful gases,leading to significant climate issues such as the greenhouse effect and acid rain.Therefore,the development of new energy systems has emerged,and the development and use of low-pollution energy systems is the key to solving climate problems and environmental pollution problems.However,most low-pollution energy sources face the dilemma of discontinuity,and there is an urgent need for energy storage systems that can store critical energy needs for a long time and efficiently in order to increase the possibility of being utilized.Above all,developing low-cost,high-energy density,and long-cycle life electrochemical energy storage devices is a crucial and efficient area of research.Lithium-ion and sodium-ion batteries which are well known as mature energy storage systems are widely used in portable devices such as electric vehicles,cell phones and laptops.However,lithium-ion batteries have some serious problems,such as increased electrode cost and environmental pollution due to the presence of precious metals（nickel,cobalt）,and lower energy density and specific capacity due to the limitations of the intercalation mechanism.These problems hinder the development of energy storage devices with long cycle life and low cost.Currently,lithium-sulfur batteries are widely studied due to their high energy density and low-cost sulfur（S）materials.However,due to the special“solid-liquid-solid”reaction mechanism of lithium-sulfur batteries,soluble polysulfides are generated in the sulfur cathode inside their cells,which shuttle to the lithium anode,leading to the corrosion of the lithium anode and eventually to the failure of the battery.Sodium is also of great interest to the research and business community because of its abundant reserves and environmentally friendly properties,room-temperature sodium-sulfur batteries have the same electrochemical reaction mechanism as lithium-sulfur batteries and are being explored,and the same problems need to be solved for room-temperature sodium-sulfur batteries as well.As the modification of battery electrolyte,it also becomes the key of research to suppress the generation of polysulfides and the protection of lithium/sodium metal anodes.Therefore,this paper introduces an excellent fluorine-containing additive for lithium/sodium-sulfur batteries,namely:3,3,3-trifluoromethyl propylene carbonate（TFPC）,which can form a bifunctional solid electrolyte interface（known as SEI film）with high mechanical strength and high ionic conductivity on the surface of lithium/sodium anode due to its high electrochemical reactivity.This bifunctional SEI film can not only mitigate the volume expansion of lithium metal anodes during cycling,but also inhibit the corrosion of lithium anodes by polysulfides,thus improving performance.Not only that,it can also make the polysulfide react with the highly polar TFPC to form a solid through the intermolecular interaction force,which can adsorb on the sulfur cathode and alleviate the shuttle effect of polysulfide.In this paper,the chemical properties of the electrolyte additive TFPC are investigated to expose its reaction mechanism and potential reasons for the improvement of electrochemical performance in lithium/sodium-sulfur batteries.The practical applicability of the TFPC additive in lithium/sodium-sulfur batteries is demonstrated through the assembly and testing of full-batteries and symmetric cells.The main work can be summarized as follows.（1）The LUMO energy and electron affinity of TFPC were calculated by computational density flooding theory（DFT）to prove its high reactivity,and the reaction process of TFPC additive and lithium was accurately exposed by calculating the electrochemical reaction path and ab initio molecular dynamics（AIMD）simulation.Meanwhile,the decomposition voltage of TFPC in lithium-sulfur batteries was calculated using DFT and verified with each other by experimental tests.Since the decomposition voltage of TFPC additive was lower than the conventional operating voltage of lithium-sulfur battery（1.7-2.8 V）,the present work cleverly designed a pre-activation strategy to over-discharge the lithium-sulfur battery,and finally the TFPC additive was fully decomposed and formed a SEI film with high ionic conductivity and high mechanical strength on the lithium metal surface during the pre-activation process.Based on the above advantages,the long cycle time of Li symmetric battery was over1200 h and the stable Coulombic efficiency of Li-Cu half cell was over 250 cycles;the overall improvement of rate performance and long cycle performance in lithium-sulfur full battery was obtained.（2）Based on the TFPC additive in work 1,we found that it could react with liquid polysulfide to produce precipitation,so we conducted a series of tests on it.Since TFPC was a cyclic ester organic substance,it could partially nucleophilize with polysulfide because of its high polarity,and make full use of the spontaneity of nucleophilic reaction,so that it could get solidified polysulfide and generate solid precipitate after polysulfide formation,so it could alleviate the shuttle effect of polysulfide.Also,because of the spontaneous reaction,it could react quickly with polysulfides in various states,which in turn could lead to the rapid conversion of solid sulfur materials with poor electrical conductivity and Li2S,and promote their conversion rates.Based on the above mechanism,the electrolyte additive enhanced the rate performance and cycling performance of the battery.（3）Based on the first work on TFPC additives,we extended it to sodium-sulfur battery applications.Sodium was far more abundant than lithium in the earth,and it had the advantage of lower cost and environmental friendliness.Sodium-sulfur batteries had the same theoretical specific capacity and impressive energy density as lithium-sulfur batteries.Therefore,we introduced TFPC as an additive into sodium-sulfur batteries,and after a series of mechanism and performance tests,we found that it has certain sodium metal protection performance,which could form a SEI film on the surface of sodium metal anode to protect sodium metal from corrosion,thus improving the performance of sodium-sulfur batteries. |
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