The Debate over Hard Carbon and Alloy Anodes Continues for Solid-State Sodium Batteries

Although hard carbon anodes are known to outperform alloys in conventional sodium-ion batteries, this trend is reversed in solid-state sodium batteries due to the different underlying sodiation processes. Whereas the sodiation of hard carbon is triggered by Na cation (Na+) adsorption onto electrochemically active sites, that of alloys is driven by solid-state diffusion of Na+ and successive Na–alloy phase transformations. Thus, sodiation processes critically depend upon the chemical nature of Na+, which is solvated in liquids but is bonded at lattice sites in solid electrolytes. In addition, elucidating charge-transport and charge-transfer processes as well as electromechanical coupling at solid/solid interfaces (for solid-state batteries) remains an unresolved challenge. The transition in knowledge from well-investigated solid/liquid interfaces to solid-state sodium batteries is not straightforward. The exploration of hard carbon, alloys, and their composites requires further advancement. This perspective aids in streamlining the research efforts of battery communities, thereby accelerating the development of solid-state sodium batteries.