Novel P2-Type Layered Medium-Entropy Ceramics Oxide as Cathode Material for Sodium Ion Batteries

Abstract Similar to high-entropy oxides (HEOs), medium-entropy oxides (MEOs) are a new type of single-phase solid solution material. As a positive electrode material for sodium ion batteries (SIBs), it has been rarely reported. Here, we first proposed the concept of the application of MEOs in SIBs. P2-type 3-cation oxide Na2/3Ni1/3Mn1/3Fe1/3O2 (NaNMF) and 4-cation oxide Na2/3Ni1/3Mn1/3Fe1/3−xAlxO2 (NaNMFA) were prepared by solid-state method, rather than the doping technology. In addition, the importance of the concept of entropy stabilization in material performance and battery cycling was demonstrated by testing 3-cation oxide (NaNMF) and 4-cation oxide in (NaNMFA) the same system. As a result, NaNMFA can provide a reversible capacity of about 125.6 mAh g–1 in the voltage range of 2-4.2 V, and has enhanced cycle stability. The capacity and decay law of the mid-entropy oxide battery indicate that the configuration entropy (1.28R (NaNMFA) > 1.10R (NaNMF)) of the cationic system is the main factor affecting the structural stability and cycle stability of the electrode material. This work emphasizes that the rational design of MEOs with novel structures and different electrochemically active elements may be an effective strategy for exploring high-performance SIBs cathode materials in next-generation energy storage devices.