Unveiling the crucial morphological effect of non-conducting polymer binders on inorganic-rich hybrid electrolytes

Abstract

The limited fundamental knowledge about polymer binder selection in inorganic-rich hybrid electrolytes (HSEs) hinders their optimization and translation to an industrial scale. Herein, we investigate, for the first time, the crucial morphological effect of non-conducting polymer binders in HSEs based on a halide electrolyte (Li₃InCl₆). We compared the effect of the ordered nanostructured styrene–ethylene–butylene–styrene (SEBS) block copolymer and disordered polyisobutylene (PIB) homopolymer binders. This work was aimed at understanding ionic conduction transport across halide-based inorganic-rich HSEs with different polymer morphologies to optimize their design. We investigated ionic conductivity via EIS and Li⁺ diffusion using ⁷Li PFG-NMR, and the obtained results were supported by 3D reconstruction from cryo-plasma FIB-SEM images, which were further correlated with rheology measurements. PIB HSE presented higher transport properties than SEBS HSE with higher ionic conductivity (0.39 × 10⁻⁴ and 0.23 × 10⁻⁴ S cm⁻¹ at 30 °C, respectively) and ⁷Li diffusion (6.3 × 10⁻¹³ and 4.3 × 10⁻¹³ m² s⁻¹ at 30 °C, respectively) owing to a less tortuous percolated inorganic network observed via the 3D reconstruction of cryo-plasma FIB-SEM images. Moreover, rheology measurements indicated that HSEs composed of ordered block copolymers should be processed in the disordered state (T > T_ODT) (in which the ordered microdomains disappear), reaching the terminal flow zone as disordered homopolymers to improve the percolated inorganic network and thereby achieving high transport properties in HSEs. Finally, Li plating/stripping demonstrated a more stable electrochemical performance of PIB HSE and higher critical current density (400 μA cm⁻²) compared with SEBS HSE (25 μA cm⁻²), which was in good agreement with the obtained transport and morphological properties.