Electrolyte Design And Performance Study For Highly Safe And Long Lifespan Lithium Sulfur Batteries

Lithium-sulfur batteries are considered to be one of the most promising candidates for the next-generation rechargeable lithium batteries owing to their high theoretical energy density and high sulfur abundance.However,its large-scale adpotion is limited by fast capacity fading and safety hazards.Firstly,constant interfacial reaction between lithium metal and electrolyte results in lower Coulombic efficiency and dendritic lithium formation;secondly,common electrolytes for lithium sulfur batteries have low flash point and prone to ignite under extreme condition,posing potential safety concern during the battery operation;finally,electrode interphases instability limit the electrochemical behavior.In this work,sulfurized pyrolyzed poly(acrylonitrile)(S@p PAN)was employed as the cathode.This work presents advanced electrolytes to tackle with problems of safety concerns and poor cyclic performance.Related results are summarized as follows:(1)Triethyl phosphate(TEP),lithium bis(oxalate)borate(Li BOB)and co-solvent fluoroethylene carbonate(FEC)were optimized to obtain a non-flammable safe electrolyte.When compared with the previous reports using flame retardant additives,main solvent of TEP can ensure the battery safety in the whole operation.When coupled with S@p PAN cathode,it maintains capacity retention of 91.3% after 500 cycles at 1C rate,exhibiting good electrochemical stability.(2)A local high salt concentration electrolyte system was designed for Li/S@p PAN battery,and electrochemical performances under extreme conditions(high temperature and high sulfur content cathode)were tested.The existence of TTE not only reduces electrolyte viscosity,but also dilutes the high salt concentration system without destroying its internal solvation structure.The advantages of the intrinsical safe electrolyte can be summarized as follows: firstly,all the solvents are flame retardant with high flash point,which can ensure the safety of the battery;secondly,it delivered high deposition/dissolution efficiency(>99%)and micro-sized deposition morphology;thirdly,it ensured stable cycling with reversible capacity of 840.1 m Ah g-1 and sulfur utilization rate of 95.6%.In conclusion,this electrolyte system makes full use of the advantages of each component of electrolyte and exhibits excellent electrochemical properties.(3)A compatible electrolyte for Li/S@p PAN battery was designed and electrochemical performance with high loading cathode was tested.FEC can form a stable interfacial film on both on Li anode and S@p PAN cathode.The symmetric cell has a normal over-potential profile at current density of 1 m A cm-2 over 2000 hours.The SEI interphase is dominated by poly(carbonate)and Li F,which guides uniform lithium deposition.Moreover,it can accommodate a certain volume expansion,thereby reducing the contact between sulfur and electrolyte.This compatible electrolyte enables stable cycling over 4000 cycles at 6C with capacity retention of 91%.In high-loading lithium-sulfur batteries,the S@p PAN cathode still exhibits good compatibility,showing a reversible areal capacity of 7.7 m Ah cm-2,and uniform dendrite-free lithium deposition.