Study On Ionic Liquids Electrolytes And Additives For Lithium-Sulfur Batteries

Li-S battery has spurred worldwide attentions and hold great potential to serve as next-generation energy storage device due to high theoretical specific energy(1675 mAh g-1)and energy density(2600 Wh kg-1),low material cost and good environmental friendliness.However,there are still some serious challenges hindering its commercialization.The major one is the polysulfides shuttle effect,which leads to a series of problems such as Li corrosion,capacity fading and self-discharge etc.For the main problems,a new functional electrolyte system of lithium-sulfur batteries has been proposed by optimizing of electrolyte solvent,which based on ionic liquids and fluorinated ether binary solvents and lithium salt additives.The specific research content and main conclusions obtained are as follows:1.Selection of functional solvents.The structure effect of solvents was analyzed in this chapter,the selection principle of fluorinated ether and ionic liquids functional solvents were put forward.The effects of fluorine substitution positions and quantities on the properties of fluorinated ether solvents were mainly studied.The overall trend is that the higher degree of fluorine substitution,the stronger steric barrier ability to etheroxy groups,and the poorer ability to dissolve polysulfides;the smaller quantity of fluorine groups,the lower viscosity,and the higher conductivity of the electrolyte prepared by the same ratio.As a whole,ETFE has suitable ionic conductivity and remarkable inhibition of polysulfides dissolution.For ionic liquids,a wide potential window,excellent thermal stability and outstanding inhibition of polysulfides dissolution were found in PP13TFSI,but the viscosity is too high and ion conductivity is much low.Higher conductivity and relatively lowest viscosity,but reduced reduction stability were obtained in EMITFSI.The acceptable viscosity,ionic conductivity and good electrochemical stability were discovered in two etherified ionic liquids(DEMETFSI and P1,2O1TFSI).However,in the polysulfide dissolution experiment,the best ability of inhibiting polysulfides dissolution was found in P1,2O1TFSI,but DEMETFSI showed a large difference compared with it.The lithium plating-stripping tests further demonstrated that the better compatibility with lithium metal was also proved in P1,2O1TFSI.2.Preparation and optimization of electrolyte solvent components.It is found that the novel ternary functional solvents electrolyte owns acceptable ion conductivity,moderate polysulfide solubility and good Li compatibility.The cell in the hybrid system with a suitable content of fluorinated ether exhibits excellent cycle and rate performances.The capacity retention up to 80%after 100 cycles accompanied by an acceptable coulomb efficiency(CE)level is demonstrated.The synergistic function of the two support solvent components(DOL and ETFE)not only improves the ion transport and Li+coordination environment in the IL-based electrolytes,but also contributes to formation of stable interface film with low resistance on the Li surface.Based on this,the design mechanism of polysulfide-suppressed balance in electrolyte is proposed,that is to establish a benign force balance between the dissolution of polysulfide and lithium surface protection,and achieve the linkage control of shuttle effect,which is conducive to ensuring the comprehensive performance of the battery.3.Research on film-forming additives in electrolyte.In PDE ternary solvent electrolyte system,the greatly increased first discharge specific capacity of lithium-sulfur battery and improved initial utilization of the active material were obtained by addition of additives;especially addition of LiFSI not only improved the first discharge specific capacity,but also increased the reversible capacity,cycle retention rate and CE values;the addition of film-forming additives can increase ionic conductivity,thereby facilitating ion transmission and improving the utilization rate of active materials.For optimization the content of LiFSI additive,the excellent electrochemical stability and best cycle stability were found in the electrolyte system containing 1%LiFSI;however,the cycle and rate performance had not been significantly improved in the ternary solvent electrolyte PDE system.In binary solvent electrolyte system,the fluorinated ether ETFE and the additive LiFSI participate in film formation on the electrode surface,forming a dense and uniform passivation protective film,which effectively prevents the direct contact between polysulfide and lithium anode,and prevents continuous corrosion and reversible shuttle effect;therefore,the improved cycling and rate performance of Li-S battery were achieved.