Lithium sulfur cells are the most promising candidate for the post lithium-ion battery era. Their major drawback is rapid capacity fading attributed to the complex electrochemical processes during charge and discharge which are not known precisely. Here we present for the first time a multidimensional operando measurement by combining X-ray radiography with impedance spectroscopy while galvanostatically charging and discharging a lithium sulfur cell. The formation of macroscopic sulfur crystals at the end of charge can be seen directly by X-ray radiography. These crystals can be assigned to stable α-sulfur (rhombic) and metastable β-sulfur (monoclinic) by their characteristic crystal habit. These crystal structures with a length of more than 1 mm form and dissolve rapidly during cycling. Their appearance is accompanied by characteristic signals in impedance spectroscopy. Macroscopic crystals of Li₂S cannot be observed in full agreement with earlier studies by operando X-ray diffraction. In addition, X-ray radiography reveals non-wetted areas on the carbon cathode. These regions grow during discharge and are reduced during charge. The area of these electrochemically inactive spots is inversely proportional to discharge capacity.

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 _x MS₂, 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.

The quantitative evolution of dissolved polysulfides at both electrode sides of lithium–sulfur batteries is determined with operando X-ray spectrometry.