All measurements were carried out in the David Cockayne centre for Electron Microscopy, University of Oxford, using a Zeiss EVO MA10 equipped with an Oxford Instruments X-act energy dispersive X-ray spectroscopy (EDS) detector. Powder samples were mounted on adhesive carbon tape and then surfaces of the samples were coated with 6 nm carbon layers using a Leica ACE600 Coater. Specimens were prepared under inert atmosphere and stored in a polythene bag until the moment of their use, but brief exposure of samples to air was inevitable during quick transfer of specimen to the carbon coater and to the sample chamber of the SEM, resulting in possible degradation of air-sensitive samples, in particular those obtained after chemical lithiation. For imaging, secondary electron (SE) images were acquired and combined with EDS mapping at an operating voltage of 20 kV (sulphide systems) or 10 kV (selenide systems) with a probe current of 1.0 nA. Backscattered electron (BSE) images were equally closely examined but no further information about compositional heterogeneity was obtained. EDS maps were processed using the Oxford Instruments Aztec software for initial assessments of spatial distribution of each element as well as presence of impurity phases. This was followed by EDS measurements at individual points or at series of points on predefined lines (using "linescan" mode) in order to obtain quantitative views of their compositions.