Sodium-ion batteries (SIBs) are currently being commercialized and are seen as a promising energy storage technology based on more abundant, non-critical elements while achieving energy densities close to those of Li-ion batteries (LIBs). A straightforward approach for increasing the energy density of SIBs with layered oxides as cathode active materials (CAMs) is to increase the charging cut-off voltage. Extending the upper cutoff potential beyond 4 V vs. Na + /Na will therefore in most cases result in a significantly higher storage capacity and hence higher energy density. However, a too high charging voltage causes poor cycle life due to degradation of the electrode materials and/or electrolyte decomposition. To overcome these issues, a fundamental understanding of the degradation mechanism of cathode materials at high voltages (above 4.0 V vs Na + /Na) is essential.