Size-Mismatched Dual-Cation Alloying for Coexistence H ⁺ /Li ⁺ Conduction in Lead Bromide Hybrids

Mixed ionic conductors that enable the cooperative transport of multiple charge carriers are essential for advanced solid-state electrochemical devices but remain challenging to realize in crystalline materials. Size-mismatched substitution provides an effective design strategy to promote ionic transport by expanding the lattice and generating an additional interstitial free volume. Guided by this principle, a dual-cation alloying approach is employed to activate coexistence H + /Li + conduction in a one-dimensional (1D) lead bromide hybrid derived from TBA 0.8 (H 3 O) 0.2 PbBr 3 (TBA + = tetrabutylammonium), a framework intrinsically containing cation vacancies. Partial substitution of Li + and Mn 2+ at the A- and B-sites induces a pronounced size mismatch within the soft hybrid lattice, collectively facilitating ion migration. As a result, the optimized composition ( x = 0.134) exhibits a high ionic conductivity of 3.75 × 10 –3 S cm –1 at 373 K. This work establishes size-mismatched dual-cation alloying as a general design principle for high-performance mixed ionic conductors in hybrid halides.