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

Solvent co-intercalation, that is, the combined intercalation of ions and solvent molecules into electrode materials, is an additional but much less explored lever for modifying the properties of metal-ion battery electrodes (metal = Li, Na, Mg, etc.). Knowledge on solvent co-intercalation is relatively scarce and largely limited to graphite anodes, for which in sodium-ion batteries, the co-intercalation of glyme molecules is fast and highly reversible. The use of co-intercalation for cathode active materials (CAMs) remains much less explored. Here we investigate for a series of sodium-layered sulfide CAMs (Na_xMS₂, M = Ti, V, Cr and mixtures) under which conditions solvent co-intercalation occurs and how this process impacts the phase behaviour, electrode breathing, redox potential and cycle life compared to 'Na⁺-only' intercalation. Co-intercalation is a complex process that can, for example, cause opposing fluxes, meaning that solvents intercalate into the CAMs while sodium ions simultaneously deintercalate. Co-intercalation leads to layered structures that can include different amounts of confined solvated ions, ions and unbound solvent molecules. It is an approach to designing structurally diverse, layered materials with potential applications for batteries and beyond.