Nanophase-Separated, Elastic Epoxy Composite Thin Film as an Electrolyte for Stable Lithium Metal Batteries

The design of solid polymer electrolytes (SPE) with high ionic conductivity and excellent mechanical properties is challenging because these two properties are often conflicting. To achieve both, a reaction-controlled strategy is proposed based on the nanophase separation of an ionic transport pathway and a supporting matrix to balance ionic mobility and mechanical properties. Specifically, an elastic epoxy polymer electrolyte (eEPE), synthesized via two-step polymerization, combines outstanding mechanical strength (toughness of 3.4 MJ m^-3) and high ionic conductivity (3.5 × 10^-4 S cm^-1 at 25 °C). The nanostructured eEPE is both tough and flexible, therefore promotes uniform deposition of Li even under a high current density (2 mA cm^-2 and 2 mAh cm^-2). Importantly, eEPE composite films greatly improve the safety performance of the LiFePO₄/Li pouch cells: safe operations are achieved under several abusive conditions. This work highlights an alternative route for high-safety solid-state lithium metal batteries of the next generation.