Enhancing The Stability Of Lithium Anode In Lithium-sulfur Battery By Inorgainc Polymer

Today's society is in the era of mobile internet.Mobile electronic devices?wearable devices,mobile phones,mobile balancing vehicles,consumer-grade drones?are eagerly awaiting the development of higher-energy-density battery systems.Due to the depletion of petroleum resources,atmospheric pollution and the greenhouse effect seriously destroying the ecological environment,the electrification and internetization of the automobile industry have become a major trend.However,the popularization of new energy vehicles requires a battery system with lower cost and higher mileage.Lithium sulfur batteries has a high theoretical specific capacity(2600 Wh kg^-1)due to its electrode materials are all have high specific capacity.Therefore,the lithium sulfur battery is considered to be representative of the next-generation lithium-ion batteries and received extensive research.However,the volume expansion and the uncontrollable morphology of the lithium metal anode during electrodeposition result in cracking of the anode solid electrolyte layer.Lithium ions preferentially deposit in these cracked places and grow into lithium dendrites.Lithium dendrites can pass through the porous membrane and cause short-circuiting of the cell,causing thermal runaway.Lithium dendrite"roots"are preferentially oxidized to lithium ions during the discharge process.Thereby the dendrite is detached from the anode,leaving"dead lithium"and causing the battery capacity to decay.In addition,lithium metal is chemically active and easily reacts with liquid electrolytes.The formation of lithium dendrites continuously consumes the electrolyte and the lithium anode,causing the increase of internal resistance and capacity loss of the battery.SEI play a vital role in suppressing the growth of dendrite lithium.Based on the above-mentioned problems,we enhance the stability of lithium anode by in-situ modifying SEI.This research work systematically studied the methods to improve the stability of lithium metal anode through three aspects.The research work includes in-situ forming inorganic/organic hybrid SEI,inorganic polymer hexafluorophosphate-propped SEIand trace amount of LiPF₆ as additive to induce solvent polymerization.And these methods exhibits very effectively to enhance the mechanical stability of SEI.Through the three strategies,the stability of SEI are improved obviously.And it deepens the understanding of the SEI characteristics of the Li metal anode.In the first part,we have synthetized of ethyl?-iodoacrylate through catalytic replace?-hydrogen of olefin by iodine,ethyl acrylate and iodine as raw materials.Subsequently,0.25wt%of ethyl?-iodoacrylate was added to the Li-S electrolyte to in-situ generate the inorganic/organic hybrid SEI.The lithium iodide rich SEI can guarantee the lithium ion conductivity of the interface,and the olefin function group can be polymerized to form organic oligomer,thus improving the mechanical stability of the SEI.Based on this strategy,the initial discharge capacity and capacity retention rate of lithium-sulfur batteries have been greatly improved,and the morphology of lithium deposition has also been improved effectively.Lithium anode surface shows relatively flat topography after 500 cycles at 1C in Lithium-sulfur batteries,moreover,voltage curve can be stable around at 500 h at a current density of 1 mA cm^-2 and deposit capacity is 1 mAh cm^-2 in lithium symmetric batteries.In the second part,by adding 0.5 wt%of the inorganic polymer hexafluorocyclotriphosphazene to the electrolyte,the anode SEI exhibits a lower interface resistance and guides the uniform deposition of lithium ions more effectively.The alone pair of electrons in the nitrogen atom of hexafluorocyclotriphosphazene has the effect of guiding the uniform diffusion of lithium ions.At the same time,the additive also enhances the mechanical stability of the SEI and inhibits dendrite formation.Lithium anode surface shows extremely flat topography after 500 cycles at 1C in lithium-sulfur batteries,moreover,voltage curve can be stable around at 600 h at a current density of 1 mA cm^-2 and deposit capacity is 1 mAh cm^-2 in lithium symmetric batteries.In the third part,we added a small amount of LiPF₆ to the electrolyte system commonly used in lithium-sulfur batteries,and studied whether it can also induce the ring-opening polymerization of 1,3-dioxolane.It has been known that LiPF₆ can decompose itself into a strong Lewis acid PF₅and POF₃,just like in a carbonate electrolyte,thereby improving the mechanical stability of the SEI.