External-pressure-electrochemistry coupling in solid-state lithium metal batteries

Solid-state lithium metal batteries (SSLBs) using inorganic solid-state electrolytes (SSEs) have attracted extensive scientific and commercial interest owing to their potential to provide higher energy density and safety than conventional Li-ion batteries. These batteries are subject to external pressure during both their manufacturing processes (fabrication pressure) and their operation (stack pressure). This pressure not only affects the intrinsic properties of both the electrolytes (such as ionic conductivity and electrochemical voltage window) and the electrodes (such as ion transport and structural variation) but also determines the cyclability and safety of the whole battery. Hence, understanding the effect of pressure is essential when designing high-performance SSLBs. This Review aims to elucidate the coupling between external pressure and electrochemistry in these batteries. We summarize the effects of external pressure on SSEs and electrodes, and on the interfaces between the components. We analyse the overall electrochemical performance and safety of the batteries under external pressure. Finally, we clarify the dominant challenges in achieving pressure-proof and low-pressure SSLBs, laying out a perspective for future breakthroughs. Solid-state lithium metal batteries have the potential to meet energy density and safety requirements that current commercial Li-ion batteries cannot. Given their solid-state components, these batteries are subject to - and strongly affected by - external pressure during their manufacturing and operation. This Review examines the relationship between external pressure and electrochemical behaviour in these batteries.