Solvent co-intercalation in layered cathode active materials for sodium-ion batteries

Solvent co-intercalation, i.e. the combined intercalation of ions and solvent molecules into electrode materials, is an additional but much less explored lever for modifying the properties of electrodes in metal-ion batteries (metal= Li, Na, Mg, …). So far, the knowledge on solvent co-intercalation is relatively scarce and largely limited to graphite anodes, for which in Na-ion batteries, the co-intercalation of glyme molecules is found to be very fast and highly reversible over many cycles. The use of co-intercalation in metal-ion batteries, however, still lacks a scientific breakthrough with respect to cathode active materials (CAMs). Here, we show for a series of Na-layered sulfide CAMs (Na_xMS₂, M = Ti, V, Cr, and mixtures) by experimental methods and theory, under which conditions solvent co-intercalation occurs and how this process impacts the phase behavior, electrode breathing, redox potential, and cycle life compared to “Na⁺-only” intercalation. Overall, co-intercalation provides an alternative way for designing CAMs.