Cation disordered rock salt cathode materials have gathered increased research interest over the last couple of years due to their high specific capacity and wide array of element combinations. It is still unclear whether the capacity fading observed for this type of material is solely due to the occurrence of anionic redox reactions and consequent material degradation or also due to the side reactions between the cathode material and the carbonate-based electrolyte. In order to address it, this study compares the differences in electrochemical performance of a rock salt Li 1.25 Fe 0.5 Nb 0.25 O 2 cathode and cathode electrolyte interphase (CEI) formation in both lithium metal and lithium ion cells by using a conventional carbonate-based electrolyte and an ionic liquidbased electrolyte. Thereby, the ionic liquid electrolyte promotes capacity retention, whereas the organic carbonate-based electrolyte leads to increased capacity fading and ineffective CEI formation. Severe side reactions between the carbonate-based electrolyte and the cathode material are characterized by poor Coulombic efficiency and result in continuous inner resistance growth, ongoing gas evolution, and the coverage of the cathode surface with electrolyte degradation products like LiF and Li 2 CO 3 . This study shows the mismatch of carbonate-based electrolytes with the Li 1.25 Fe 0.5 Nb 0.25 O 2 cathode and offers a strategy that can be also applied for the improvement of performance of other disordered rock salt cathode materials.