The Electrolyte Design And Electrochemical Performance For Lithium Sulfur Batteries

Sulfur cathode has a theoretical capacity of 1675 m Ah/g,while the theoretical energy density is as high as 2600 Wh/kg with the lithium anode for Li-S batteries,far beyond current lithium ion batteries.Besides,sulfur has advantages of abundance,low cost and being environmentally-friendly.Among various promising batteries,Li-S batteries have become one of the most attractive candidates for the next generation power sources.However,the practical use is restricted by avariety of problems such as fast capacity fading and poor cycle life,which is caused by the high solubility of polysulfides in electrolytes and shuttle effect.Furthermore,the insulating charge/discharge products also lead to low utilization of active sulfur.In this dissertation,different strategies,including the modification of liquid electrolyte and the design of electrolyte structure,have been devoted to address these problems mentioned above.1.Modification of electrolyte solvent.After the addition of FDE solvent,capacity performance,coulombic efficiency and self-discharge of Li-S batteries can be greatly improved.With the highly fluorinated structure,the donor-ability of FDE is weakened causing low ability of dissociation and dissolution to polysulfides.The utilization of active sulfur is improved.Moreover,parasitic reactions between lithium and polysulfides are also suppressed owing to the negligible solubility in FDE electrolytes.The discharge capacity and coulombic efficiency can be increased to 910 m Ah/g and 99% at 0.2C after 40 cycles compared with 340 m Ah/g and 80% of basic electrolyte,respectively.Meanwhile,the self discharge efficiency is also improved to 16% from 41%.2.Employment of functional additives.Two kinds of sulfides additives are investigated:(a)Research on organic polysulfides DMTS based on-S-S-S-structure as additives for Li-S batteries.DMTS is composed of the terminal CH3-and bridge chain –Sn-,which can significantly improve the discharge capacity of Li-S batteries.The extra capacity is linked to the synergistic effect of DMTS,and the electrochemical active products between DMTS and elemental sulfur.Corrosion reactions on lithium anode still happen because of the presence of-Sn-in DMTS.(b)Research on carbon disulfide CS2 as additives for Li-S batteries.The function of CS2 in electrolyte is two-fold:(1)CS2 reacts with polysulfides forming insoluble complexes,and suppresses the migration and shuttle of polysulfides to anode.The solid phase reaction of sulfur cathode is also lagged during discharge.(2)CS2 passivates electrode by inducing the formation of protective SEI layers.The Li-S batteries with CS2 exhibit long cycle stability and high Coulombic efficiency.3.Design of electrolyte structure.Hybrid electrolytes prepared by NASICONtype LAGP(or Garnet-type LLZTO)oxide ceramic and fluorinated liquid electrolytes are used in Li-S batteries.Oxide ceramic with single Li+ conductivity can eliminate the shuttle effect caused by soluble polysulfides.Fluorinated liquid electrolytes not only wet the interface between ceramic and electrode to contribute a low contact resistance,but also suppress the polysulfides dissolution and migration in cathode to improve the utilization of active sulfur.The Li-S batteries with hybrid electrolytes exhibit superior cycle stability and low-temperature performance.