Protic Poly(ionic liquid)s with High Ionic Conductivity and Li+ Transport Number as Moderate Temperature Solid Electrolytes

Abstract

Despite the advantages over traditional liquid electrolytes, solid-state polymer electrolytes present several challenges to be addressed for their widespread application in solid-state lithium metal batteries. In this work, we report a novel class of solid polymer electrolytes based on protic poly(diallylmethylammonium) bis(fluorosulfonyl)imide (poly(DAMAH)FSI) with high lithium bis(fluorosulfonyl)imide (LiFSI) salt content. Protic polymer electrolytes with different compositions show improved properties over their aprotic counterpart. We postulate that the protic nature of the polymer backbone positively affects the coordination of the FSI anions leading to a Li-FSI-polycation cocoordination environment with resultant high Li diffusivity of 4.1 × 10^–11 m² s^–1 and ionic conductivity of 6.4 × 10^–4 S cm^–1 at 80 °C which is a factor almost 10 times higher than the equivalent aprotic systems (0.7 × 10^–4 S cm^–1 at 80 °C). The apparent superior salt dissociation ability leads to homogeneous mixtures with a 1:2 mol ratio of Poly(DAMAH)FSI:LiFSI characterized by a Li⁺ ion transport number of 0.67. The cyclic voltammetry of the polymer materials on a Cu working electrode indicates the stability of the N–H proton in these high LiFSI-containing electrolytes, allowing successful plating/stripping analysis using a protic polymer-based electrolyte in a Li/Li cell. We thus demonstrate for the first time, the potential of these solvent-free protic poly(ionic liquid)-based solid polymer electrolytes in Li/Li metal cells at a moderate temperature of 50 °C, paving the way for future investigation and implementation in Li metal and Li-ion cells.