In Situ Investigation of Layered Oxides with Mixed Structures for Sodium‐Ion Batteries

Abstract Experimentalists and theoreticians commonly prefer single‐phase materials for their studies, since this allows for a direct correlation of the findings obtained and the compound studied. For the design of high‐performance materials for energy applications, however, mixtures of different phases frequently reveal an advanced set of desired properties. Recently, it has been shown that a combination of different phases, that is, P2/P3/O3‐ Na x Mn 0.5 Ni 0.3 Fe 0.1 Mg 0.1 O 2 , allows for higher capacities and enhanced cycling stability when employed as sodium‐on cathode compared to pure P2‐Na x Mn 0.7 Ni 0.1 Fe 0.1 Mg 0.1 O 2 or O3‐type Na x Mn 0.5 Ni 0.3 Fe 0.1 Mg 0.1 O 2 . Herein, the in‐depth comparative investigation of these three materials is presented via in situ X‐ray diffraction and X‐ray absorption spectroscopy coupled with electrochemical techniques to fully elucidate the origin of this superior performance. In fact, it appears that the redox activity or inactivity of the manganese cation plays a decisive role for the reversibility of the sodium‐ion uptake and release.