Abstract
AbstractThis work evolves a universal strategy, toward simultaneously addressing the air/water‐instability and structural‐cum‐electrochemical instability of “layered” Na–transition‐metal (TM)–oxide‐based cathode materials for Na‐ion batteries, by way of varying the “interslab” spacing via tuning the TMO bond covalency. In this regard, model O3‐structured NaTMoxides, with varied “charge‐to‐size” ratio of the cation‐combination (viz., TM‐ + non‐TM‐ions) in the TM‐layer [i.e., (C:S)TM], 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)TM => lower TMO covalency => higher effective negative charge on O‐ion (which gets shared by both TM‐ and Na‐ions) => stronger‐cum‐shorter NaO 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(ONaO)/d(OTMO) of ≈1.38 for the O3 structure, corresponding to the initiation of O3 → P3 transformation during desodiation/charging is revealed. NaTMO2s having higher initial (C:S)TMs reach this critical d(ONaO)/d(OTMO) earlier (i.e., upon minimized Na‐removal) and, thus, suffer from more transformations during continued desodiation/charge, resulting in structural‐cum‐electrochemical instability.
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