Despite them missing a layered structure, cation disordered rocksalt (DRX) cathode materials have risen in research interest in the last couple of years, not only because of their high specific capacity but also for the possibility to lift the limitation on specific transition metals like Ni, Co and Mn.(1-3) Many of them involve the combination of a redox inactive d0 stabilizing element like Nb.(4, 5) The utilization of anionic redox reaction in this material class is believed to be the cause of the materials poor electrochemical performance, while possible other factors like unfavourable side reactions with commonly used carbonate-based electrolytes or other cell components are less investigated.(6, 7)
In order to address this fact, the electrochemical performance as well as the formation of the cathode electrolyte interphase (CEI) of a Li1.25Fe0.5Nb0.25O2 DRX cathode was investigated in Lithium metal and Lithium ion cells. Thereby, the use of a carbonate-based electrolyte and an ionic liquid-based electrolyte were compared. Severe side reactions occur between the cathode material and the carbonate-based electrolyte which leads to a strong capacity fading and poor Coulombic efficiency. An ongoing decomposition of electrolyte components on the cathode materials surface during charge and discharge cycling covers the material with degradation products like Li2CO3 and LiF which causes resistance growth and is accompanied by dangerous gassing in the cells. On the other hand, the ionic liquid electrolyte shows only negligible degradation and can promote capacity retention.(8)
The mismatch of carbonate based electrolytes with the Li1.25Fe0.5Nb0.25O2 cathode shown here can be applied to other DRX materials which opens new approaches for improvements in their performance.(8)
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