New insight into the dual-dissociation effect of POSS fillers with suitable aperture for solid-state polymer electrolytes

The dissociation degree of lithium electrolyte salts greatly affects the performance of solid-state polymer electrolytes (SSPE) in solid-state lithium-metal batteries (SSLBs). The previous synthetic tactics are generally based on single dissociation of electrolyte salt, for example, solely tethering the anions or capturing lithium cations. Herein, we present a dual-dissociation strategy to synthesize SSPE by simultaneously dissociate both the anions and cations among electrolyte salts. This is achieved by integrating the oligomeric silsesquioxanes (POSS) nanoparticles with suitable aperture in a poly(propylene carbonate) (PPC) gel matrix, where the POSS filler with suitable aperture interacts with both the TFSI anions and Li+ cations of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. However, the Li+ cations are movable under an external electric field and can migrate between cathode and anode to achieve charge transportations. The room temperature ionic conductivity of SSPE filled with POSS nanoparticles was found to be superior to that of SSPE with SiO2 fillers, with a conductivity value of 2.04 x 10-4 S cm-1 and a higher Li+ ion transfer number of 0.71. This improvement is attributed to the utilization of a dual-dissociation strategy of lithium salt. The theoretical computations with nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) manifest that the efficient dissociation of electrolyte salt is ascribed to the coordination effect of Li+ ions with four oxygen atoms among POSS cage as well as the tethering function of POSS filler for TFSI- anions. This work provides an innovative dual-dissociation strategy to synthesize SSPE, which would benefit the practical use of polymer electrolytes in solid-state lithium-metal batteries.