Composite solid-state electrolytes for all solid-state lithium batteries: progress, challenges and outlook

Composite solid-state electrolytes (CSEs) with multiple phases offer greater flexibility to customize and combine the advantages of single-phase electrolytes, making them promising candidates for commercial all-solid-state batteries (ASSBs). Based on existing investigations, this review provides a comprehensive overview of the recent progress in CSEs. First, we introduce the historical development of solid-state ionic conductors, and then summarize the fundamentals including key materials and mechanisms of lithium-ion transport. Three main types of advanced structures for CSEs are classified and highlighted according to the recent progress, namely composite solid electrolytes with passive fillers, composite solid electrolytes with active fillers, and 3D framework composite solid electrolytes. Finally, the challenges and perspectives of the composite solid-state electrolytes are discussed.