In-situ Preparation And Performance Study Of Poly(1,3-dioxolane)gel Electrolytes For Lithium-sulfur Batteries

Lithium-sulfur batteries have the advantage of high energy density(2560 Wh kg^-1)and are the main research target of next-generation secondary batteries.However,the electrochemical performance and industrial application of lithium-sulfur batteries are severely limited by the"shuttle effect"of polysulfide ions and the dendritic problem of lithium cathode,which are rooted in the diffusion of soluble polysulfide ions in the electrolyte and the inhomogeneous deposition of lithium ions.In this study,a new gel electrolyte system with high ionic conductivity and lithium-ion mobility was designed and constructed based on the ring-opening polymerization of 1,3-dioxolane（DOL）to effectively suppress the"shuttle effect"and lithium dendrite growth,thus improving the electrochemical performance of lithium-sulfur batteries.The primary research work includes:1.The feasibility of using aluminum trifluoromethanesulfonate Al（OTf）₃ as an initiator to initiate in situ polymerization of DOL to prepare gel electrolytes was investigated.The effects of initiator concentration on the polymerization reaction rate and electrode wettability were explored,the composition and structure of in-situ PDOL gel electrolytes were elucidated,and the effects of PDOL gel electrolytes on the electrochemical performance of lithium-sulfur batteries were examined.The results show that the in situ preparation of gel electrolytes with Al（OTf）₃ as initiator added to the ether-based electrolyte system(2 mol L^-1 Li TFSI DOL/DME,v:v=1:1)at room temperature is feasible and exhibits a suitable gelation rate and good electrode wettability at an initiator concentration of 2 mmol L^-1,and the prepared gel electrolytes have an ionic conductivity of 2.3×10^-3 S cm^-1 and a lithium-ion mobility number of 0.46.The gel electrolyte was applied to a lithium-lithium symmetric cell and showed an overpotential of less than 35 m V after 1200 h of cycling at 0.5 m A cm^-2,which proved that the electrolyte has good ionic conductivity and interfacial stability to lithium.The lithium-sulfur pouch cells contained PDOL electrolyte achieved a significant improvement in capacity and cycling stability:after40 cycles at 0.2 C multiplier under the condition of electrolyte/sulfur mass ratio（E/S）=3.5 g g^-1,the specific capacity of cathode material discharge was 691.8 m Ah g^-1（in terms of sulfur）with a capacity retention rate of71.2%,which was 36.6%higher than that of the liquid electrolyte lithium-sulfur battery.2.1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether（TTE）was introduced into the PDOL system to enhance the ionic conductivity of the gel electrolyte and further stabilize the lithium cathode interface.The introduction of TTE,on the one hand,adjusts the microligand environment of the polymer and improves the ionic conductivity of the electrolyte at room temperature;on the other hand,TTE is involved in the generation of stable SEI films on the lithium cathode surface and enhances the inhibition of lithium dendrite.The results showed that the introduction of TTE improved the lithium-ion mobility and ionic conductivity of the gel electrolyte to 0.68 and 3.7×10^-3 S cm^-1,respectively,and the overpotential of the lithium-lithium symmetric cell containing TTE electrolyte was lower than 17 m V after 1200 h of cycling at 0.5 m A cm^-2,indicating that the interfacial stabilization effect of PDOL@TTE gel electrolyte on the lithium cathode was significantly enhanced.The specific capacity of the lithium-sulfur pouch cells containing this gel electrolyte remained 831.8 m Ah g^-1after 40 cycles under E/S=3.5 g g^-1 and multiplicity 0.2 C,which represents a substantial improvement in performance over the PDOL gel electrolyte lithium-sulfur battery.The study on the in-situ preparation of gel electrolytes provide a new idea for high-performance lithium-sulfur batteries.