A flame retardant benzimidazole-linked covalent organic framework as an organic solution sponge for acceleration of Li⁺-ion migration in solid-state electrolytes

Solid-state electrolytes have demonstrated significant potential in achieving safer Li-metal batteries with minimal negative impact on battery performance, and in some cases, even enhancing electrochemical performance. Nonetheless, a primary obstacle lies in the advancement of cutting-edge solid-state electrolytes that exhibit rapid Li⁺-ion conduction, high electrochemical stability, and enhanced safety. Covalent organic frameworks (COFs) have excellent organophilicity and open channels, providing a unique feature of being an ‘organic solution sponge’ that allows for the absorption and release of liquid organic electrolytes, offering a useful platform for developing fast ion-conducting solid-state electrolytes. This study focuses on a benzimidazole-linked COF (PBI-COF), which acts as an organic liquid electrolyte sponge, by encapsulating propylene carbonate (PC) solution of LiBF₄ into PBI-COF channels, to produce a superionic conducting solid-state electrolyte (LiBF4/PC@PBI-COF) with an ionic conductivity above 10⁻³ S cm⁻¹ and a Li⁺-ion transference number of 0.58 at ambient temperature, and excellent flame retardancy. Our study presents a novel approach to design safe solid-state alkaline-metal ion electrolytes for use in all-solid-state alkaline-metal batteries.