Cation–Oxygen Bond Covalency: A Common Thread and a Major Influence toward Air/Water‐Stability and Electrochemical Behavior of “Layered” Na–Transition‐Metal‐Oxide‐Based Cathode Materials

This work evolves a universal strategy, toward simultaneously addressing the air/water‐instability and structural‐cum‐electrochemical instability of “layered” Na–transition‐metal (T M )–oxide‐based cathode materials for Na‐ion batteries, by way of varying the “interslab” spacing via tuning the T M O bond covalency. In this regard, model O3‐structured NaT M oxides, with varied “charge‐to‐size” ratio of the cation‐combination (viz., T M ‐ + non‐T M ‐ions) in the T M ‐layer [i.e., ( C : S )T M ], are designed and subjected to structural characterizations, density‐functional‐theory‐based studies, air/water‐exposure studies, electrochemical cycling, and operando investigations. Such studies have yielded a clear correlation‐cum‐trend concerning lower ( C : S )T M => lower T M O covalency => higher effective negative charge on O‐ion (which gets shared by both T M ‐ and Na‐ions) => stronger‐cum‐shorter NaO bond => reduced “interslab” spacing => lower Na‐transport kinetics => suppressed spontaneous Na‐extraction upon air/water‐exposure => concomitant vastly improved air/water‐stability => suppressed/delayed O3 → P3 transformation during electrochemical Na‐extraction (i.e., charging) => concomitant vastly improved electrochemical cyclic stability. Furthermore, a critical d (ONaO) / d (OTMO) of ≈1.38 for the O3 structure, corresponding to the initiation of O3 → P3 transformation during desodiation/charging is revealed. NaT M O 2 s having higher initial ( C : S )T M s reach this critical d (ONaO) / d (OTMO) earlier (i.e., upon minimized Na‐removal) and, thus, suffer from more transformations during continued desodiation/charge, resulting in structural‐cum‐electrochemical instability.