Deciphering the functions of rubidium in structural stability and ionic conductivity of KAg4I5

Abstract

Solid electrolytes, a key component of all solid-state ion batteries, have gained considerable attention for their high safety and chemical stability. Two key metrics for solid electrolytes are ionic conductivity and structural stability. Herein, we found that both metrics can be improved by the collaborative effects of rubidium substitution in KAg₄I₅ solid electrolytes. K_0.8Rb_0.2Ag₄I₅ was obtained through a melting method, achieving an ionic conductivity of 0.15 S cm⁻¹ and a low activation energy of 19 meV, because K⁺ (1.38 Å) was partially replaced by Rb⁺ with a larger ionic radius of 1.52 Å, resulting in a larger and wider silver ion migration channel. The three-dimensional silver ion diffusion pathway in KAg₄I₅ was determined by the maximum entropy method analysis experimentally. Rb⁺ doping changes the local chemical environment of KAg₄I₅, which increases the energy barrier required for phase transition and decomposition, enhancing the structural stability. The resulting performance suggests that the designed electrolyte K_0.8Rb_0.2Ag₄I₅ has a promising application potential for silver-ion solid batteries.