High‐Entropy Co‐Free O3‐Type Layered Oxyfluoride: A Promising Air‐Stable Cathode for Sodium‐Ion Batteries

Abstract

AbstractSodium‐ion batteries have recently emerged as a promising alternative to lithium‐based batteries, driven by an ever‐growing demand for electricity storage systems. The present workproposes a cobalt‐free high‐capacity cathode for sodium‐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, lithium is introduced in the composition to obtain high configurational entropy and sodium 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 successfully suppresses 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 sodium‐ion batteries.