Resolving the Sodiation Process in Hard Carbon Anodes with Nanostructure Specific X-Ray Imaging.

Hard carbons show significant promise as anode materials for sodium-ion batteries. However, monitoring the sodiation process in the hard carbon electrode during cycling and understanding the sodiation mechanism remain challenging. This article reports on operando 2D scanning small- and wide-angle X-ray scattering (SWAXS) and ex situ 3D SAXS tomography of hard carbon electrodes during the sodiation process. Structural changes are monitored with spatial and temporal resolution during the electrochemical process and shows that sodiation through micropore filling is the more dominating mechanism in the later stages of sodiation, i.e. in the plateau region of the voltage profile, while intercalation occurs continuously. Spatial inhomogeneities are resolved over the electrode and reveal an increased level of inhomogeneity at higher degree of sodiation with regions of different degrees of micropore filling. Resolving the processes spatially shows that plating starts at the interface between the electrode and the current collector where also a high degree of micropore filling and formation of pseudo-metallic sodium is found. The work demonstrates how SWAXS imaging can contribute to understanding the sodiation of hard carbon anodes, not only by spatially resolved analysis, but as a method to decouple contributions from different components in a cell, enabling more accurate scattering analysis in in situ environments.