Disorder and entropy engineering in solid-state electrolytes for fast Li⁺ and Na⁺ transport

All-solid-state batteries promise significant improvements in safety and energy density, but their realization depends on solid electrolytes capable of delivering liquid-like ion mobility while maintaining chemical and mechanical stability. Increasing evidence indicates that optimal ion transport rarely occurs in perfectly ordered lattices. Instead, controlled structural disorder and configurational entropy reshape the migration energy landscape, broaden site-energy distributions, and create percolating diffusion pathways that enable fast ion transport. This review examines how such disorder-mediated mechanisms emerge across major classes of solid electrolytes, including halides, hydroborates, sulfides, oxides, and amorphous or glass-ceramic systems. Particular emphasis is placed on the interplay between lattice softness, defect chemistry, site-energy overlap, and pathway connectivity in determining ionic conductivity. A comparative analysis of Li + and Na + electrolytes highlights distinct transport responses to disorder, where lithium systems primarily benefit from energy-landscape flattening, whereas sodium conductors require preservation of diffusion bottlenecks and network continuity. The review further discusses how entropy-stabilized and dynamically disordered frameworks can sustain fast transport while maintaining structural integrity. Emerging experimental and computational approaches that resolve correlated disorder across multiple length scales are also highlighted. Finally, design guidelines are proposed to translate controlled disorder and entropy engineering into scalable solid electrolytes suitable for next-generation Li- and Na-all-solid-state batteries.