Ion Transport in Li-Doped Triethyl(methyl)phosphonium Tetrafluoroborate (Li-[P1222][BF4]) Impregnated with PVDF Nanoparticles

Abstract

Organic ionic plastic crystals (OIPCs) combine some advantages of liquid electrolytes and ion-conducting ceramic electrolytes in the form of nonvolatile, nonflammable, and mechanically stable electrolytes that improve electrolyte/electrode interfacial contact and have good thermal stability. Investigations of the interactions (e.g., between cation/anion pairs or between OIPC/second phases) and ion transport behaviors are crucial to understand the physical chemistry nature of their interactions, which benefits the development of new electrolyte systems. In this report, we first analyzed the thermal phase behavior and ion transport of a new OIPC, triethyl(methyl)phosphonium tetrafluoroborate [P1222][BF4]. We further discussed how the interactions between poly(vinylidene difluoride) (PVDF) and the OIPC, as well as the Li-doped [P1222][BF4], improve OIPC properties such as crystalline behavior, conductivity, ion mobility, and electrochemical behaviors. We found that less conductive secondary phases are formed after doping with LiBF4; however, the crystallization of the secondary phase is suppressed in the presence of PVDF nanoparticles. Lithium symmetric cell cycling shows that [P1222][BF4]-based composites show stable Li plating/stripping behaviors at a high temperature of 100 °C, with a current density up to 0.2 mA cm–2. This work demonstrates the potential of [P1222][BF4] composites as solid electrolytes in next-generation solid-state secondary batteries.