An Intrinsic Stable Layered Oxide Cathode for Practical Sodium‐Ion Battery: Solid Solution Reaction, Near‐Zero‐Strain and Marvelous Water Stability

Non‐aqueous solvents, in particular N,N‐dimethylaniline (NMP), are widely applied for electrode fabrication since most sodium layered oxide cathode materials are readily damaged by water molecules. However, the expensive price and poisonousness of NMP unquestionably increase the cost of preparation and post‐processing. Therefore, developing an intrinsically stable cathode material that can implement the water‐soluble binder to fabricate an electrode is urgent. Herein, a stable nanosheet‐like Mn‐based cathode material is synthesized as a prototype to verify its practical applicability in sodium‐ion batteries (SIBs). The as‐prepared material displays excellent electrochemical performance and remarkable water stability, and it still maintains a satisfactory performance of 79.6% capacity retention after 500 cycles even after water treatment. The in situ X‐ray diffraction (XRD) demonstrates that the synthesized material shows an absolute solid‐solution reaction mechanism and near‐zero‐strain. Moreover, the electrochemical performance of the electrode fabricated with a water‐soluble binder shows excellent long‐cycling stability (67.9% capacity retention after 500 cycles). This work may offer new insights into the rational design of marvelous water stability cathode materials for practical SIBs. An intrinsic stable layered oxide cathode material with absolute solid‐solution reaction, near‐zero‐strain, and marvelous water stability is designed for demonstrating the feasibility of fabricating an electrode with a water‐soluble binder. The electrode using a water‐soluble binder exhibits comparable electrochemical performance to that fabricated with an organic‐solution binder. This work will inspire the exploration of highly water‐stable sodium layered oxide cathode materials.