Ionic conduction in Li₃Na(NH₂)₄: Study of the material design for the enhancement of ion conductivity in double-cation complex hydrides

Complex hydrides have collected recent attention as a new class of solid electrolytes with potential applications in all-solid-state batteries. To improve ionic conduction in the complex hydrides, multi-cation crystal structure can be attractive. It will allow tuning the cation dynamics via structure modification depending on types and number of additional cations. However, multi-cation crystal structure struggles with the inter-cation scattering among different cations. To address this issue, understanding the conduction mechanisms in the multi-cationic crystals is indispensable. Here, we study cationic conduction in a double-cation (Li and Na) complex hydride Li₃Na(NH₂)₄, which is formed by replacing Li (with Na) from specific lattice site of LiNH₂ without altering the crystal symmetry. The nuclear magnetic resonance (NMR) measurements found that Li₃Na(NH₂)₄ is a Li-ion conductor with negligibly small Na-ion conduction. This finding is critically important to elucidate Li-ion conduction mechanism in Li₃Na(NH₂)₄. Enhanced Li-ion conduction in Li₃Na(NH₂)₄ is achieved by (a) suppressing diffusion of Na cation trapped at the strategically located 2c lattice sites under deep potential well; and (b) by increasing the Li defect concentration influenced by the larger volume of the Li metastable sites due to Na substitution into LiNH₂. Our study will provide the design principle for multi-cation complex hydrides, and accelerate development of superior solid electrolytes for all-solid-state batteries.