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

Abstract

This 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 TMO bond covalency. In this regard, model O3‐structured NaTMoxides, 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 TMO covalency => higher effective negative charge on O‐ion (which gets shared by both TM‐ 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. NaTMO2s having higher initial (C:S)TMs 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.

Practical Cathodes for Sodium‐Ion Batteries: Who Will Take The Crown?

“…[160,163] Therefore, the optimized cathode NaNi 0.5 Mn 0.2 Ti 0.3 O 2 exhibited long cycling stability (85% capacity retention after 200 cycles at 1C) and fast ionic diffusion (3.75 × 10 −12 cm 2 s −1 ) over the voltage window of 2.0-4.0 V. [166] More importantly, the air stability of layered oxides can be effectively improved by introducing dopants with large ionic radii and different Fermi levels to weaken the hybridization between O(2p) and TM orbitals and enhance the Na-O binding energy (reducing the Na layer spacing). [163,167] Therefore, NaNi 0.5 Mn 0.25 Ti 0.25 O 2 [163] and NaNi 0.45 Cu 0.05 Mn 0.4 Ti 0.1 O 2 [164] exhibit excellent air stability and do not undergo structural changes, even when soaked in water. It should be noted that due to the structural similarity between the O-and P-type phases, some studies mistakenly attribute the O-phase structure in the highvoltage region to the P-type phase, thereby misidentifying the solid-solution reaction zone.…”

Facile and Scalable Development of High-Performance Carbon-Free Tin-Based Anodes for Sodium-Ion Batteries

“…This provided real-time information pertaining to the phase changes/transformations during the electrochemical sodiation/desodiation process. A similar setup, conditions, and source have also been used in some of our previous studies. ,− …”

Structural and electrochemical progress of O3-type layered oxide cathodes for Na-ion batteries

“…Hu et al 16 introduced a “cation potential” to forecast the stacking structure, which offers a practical remedy for the construction of layered oxides of alkali metals. Mukhopadhyay et al 25 have recently proposed that the design of layered cathode materials can be guided by adjusting the “charge to size” (C : S) of the cations. A comparative study of different NaTMO 2 models has shown that a lower (C : S) TM can greatly improve air/water stability and suppress irreversible phase transitions during charging.…”

“…1, the development of O3-type layered oxide cathodes for SIBs has a long history and they have been widely studied in recent years. 21–25 For example, it was first discovered in 1980 that the layered oxide NaTMO 2 could be used as a cathode material for sodium-ion batteries, and in 2020 our scholars introduced a method for predicting the conformation of layered oxides to sodium ions, to this year it was proposed that tuning the TM–O bond covalency could improve air/water stability.…”