Research On Lithium-Sulfur Battery Electrolyte Based On Low Polarity Solvent Combination

The theoretical energy density of lithium-sulfur batteries composed of lithium metal and sulfur positive electrode can reach up to 2600 Wh/kg,which is much higher than that of commercialized lithium-ion batteries.Therefore,lithium-sulfur batteries are considered as a secondary battery system with development potential.However,current lithium-sulfur batteries generally suffer from the shuttle effect caused by the dissolution of lithium polysulfide intermediate products,resulting in low battery charging and discharging efficiency and significant self-discharge effect.In addition,dissolved lithium polysulfide can cause severe corrosion of the lithium metal electrode,reducing the cycling efficiency of the lithium negative electrode and ultimately greatly shortening the battery life.To address the current issues of lithium-sulfur batteries,this study focuses on the electrolyte,and carries out research and optimization work on the electrolyte from the perspectives of inhibiting lithium polysulfide dissolution and protecting lithium metal,with the specific contents as follows:(1)The first part of the work prepared a medium-concentration electrolyte based on a weakly polar ether solvent system.This part of the work used tetrahydrofuran-normal propyl ether as a new mixed solvent to effectively inhibit the dissolution of lithium polysulfide through the same ion effect caused by medium concentration(2.5 M)lithium salt,thus avoiding the sulfur shuttle effect.The work also improved the conductivity of the liquid electrolyte and its compatibility with the lithium metal negative electrode by regulating the ratio of lithium bis(trifluoromethanesulfonyl)imide(Li TFSI)and lithium bis(fluorosulfonyl)imide(Li FSI).The L1L2TE-1382 electrolyte achieved a lithium deposition and stripping efficiency of 99.3%,and the corresponding lithium-sulfur battery had a capacity retention rate of 94% after 500 charge-discharge cycles.(2)The second part of the work replaced the expensive propylene carbonate solvent in the electrolyte described in the first part of the work with a cheaper isopropyl ether solvent,thereby ensuring a reduction in the cost of the liquid electrolyte or even the battery without compromising the overall performance of the electrolyte.Theoretical calculations were used to simulate the effective coordination number of different types of ether solvent molecules with lithium ions and their corresponding binding energies to determine the solvation strength of lithium ions,providing a theoretical basis for the applicability of new solvent combinations in lithium-sulfur batteries.In addition,the superiority of using Li TFSI and Li FSI as mixed lithium salts in the same proportion was verified again.The L1L2TD-1382 electrolyte prepared in this part of the work achieved a cost reduction of about 20% while achieving almost the same performance as the L1L2TE-1382 electrolyte.(3)The third part of the work was to add fluorinated ether diluents to the electrolyte described in the second part of the work to reduce the lithium salt concentration and improve the cycling utilization rate of the negative lithium metal electrode.Specifically,a highly fluorinated ether solvent(1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether)was introduced into the L1L2TD-1382 electrolyte in the above work to form a new electrolyte.This type of diluent can significantly reduce the concentration of lithium salt and other ether solvents,thereby further weakening the electrolyte’s ability to dissolve lithium polysulfide.In addition,fluorinated ether will also participate in the passivation process of the lithium metal electrode surface and form a more stable passivation layer rich in lithium fluoride.The DMCE-2.0M electrolyte prepared in this part of the work achieved a lithium deposition and stripping efficiency of 99.6%,and the corresponding lithium-sulfur battery had a capacity retention rate of 93% after 500 charge-discharge cycles.(4)The fourth part of the work was based on the electrolyte described in the third part of the work,and the way of changing the solvent was used to alleviate the problem of low conductivity of the diluted electrolyte and the increased cost and environmental unfriendliness caused by the large use of fluorinated ether.Specifically,the tetrahydrofuranisopropyl ether-fluorinated ether solvent system in the DMCE-2.0M solution was replaced with the ethylene glycol diethyl ether-butyl ether-fluorinated ether solvent system.The replacement of tetrahydrofuran with ethylene glycol diethyl ether was to improve the ionic conductivity of the electrolyte,while butyl ether has excellent reducibility and,as a common diluent with fluorinated ether,can reduce the cost of the electrolyte and improve its compatibility with the lithium metal electrode.The BFDMCE1 electrolyte(2.0 M)prepared in this part of the work achieved a lithium deposition and stripping efficiency of 99.6%while reducing the cost by at least 20%,and the corresponding lithium-sulfur battery had a capacity retention rate of 93% after 500 charge-discharge cycles.