High-Entropy Co-Free O3-type Layered Oxyfluoride: A Promising Air-Stable Cathode for Sodium-Ion Batteries.

Na-ion batteries have recently emerged as a promising alternative to Li-based batteries, driven by an ever-growing demand for electricity storage systems. In the present work, we propose a cobalt-free high-capacity cathode for Na-ion batteries, synthesized using a high-entropy approach. The high-entropy approach entails mixing more than five elements in a single phase; hence, obtaining the desired properties is a challenge since this involves the interplay between different elements. Here, instead of oxide, oxyfluoride is chosen to suppress oxygen loss during long-term cycling. Supplement to this, Li was introduced in the composition to obtain high configurational entropy and Na vacant sites, thus stabilizing the crystal structure, accelerating the kinetics of intercalation/deintercalation, and improving the air stability of the material. With the optimization of the cathode composition, a reversible capacity of 109 mAh g -1 (2-4 V) and 144 mAh g -1 (2-4.3 V) is observed in the first few cycles, along with a significant improvement in stability during prolonged cycling. Furthermore, in-situ and ex-situ diffraction studies during charging/discharging reveal that the high-entropy strategy is successful in suppressing the complex phase transition. The impressive outcomes of the present work strongly motivate the pursuit of the high-entropy approach to develop efficient cathodes for Na-ion batteries. This article is protected by copyright. All rights reserved.