O3 layered sodium transition metal oxides (i.e., NaMO₂, M = Ti, V, Cr, Mn, Fe, Co or Ni) are a promising class of cathode materials for Na-ion battery applications. These materials, however, all suffer from severe capacity decay when a large amount of Na is extracted from the hosts. Understanding the causes of this capacity decay is the key to fully unlock the potential of these materials for battery applications.

In this work, we elaborate the capacity fading mechanism for one of the compounds in this class, i.e., NaCrO₂. The (de)sodiation processes of NaCrO₂ were first characterized by in situ XRD. Using a slow cycling rate (C/50), a phase diagram of Na_xCrO₂ close to thermodynamic equilibrium could be constructed. Ex situ synchrotron XRD and electron diffraction were then performed on samples that were charged to selected stages of charge. Through these and additional ab initio computation, we demonstrate that Na_xCrO₂ (0 < x < 1) remains in the layered structure framework without Cr migration up to a composition of Na_0.4CrO₂. Further removal of Na beyond this composition will trigger a layered to rock-salt transformation converting the P'3-Na_0.4CrO₂ to a rock-salt CrO­₂ phase, which is responsible for the capacity fade of NaCrO₂. This structural transformation proceeds via the formation of an intermediate O3-CrO₂ phase that contains Cr in both Na and Cr slabs. It is intriguing to note that intercalation of alkaline ions (i.e., Na⁺ and Li⁺) into the rock-salt CrO₂ is actually possible, albeit in a limited amount (~0.2 per formula).

Preventing the layered to rock-salt transformation is of uttermost importance to improve the cyclibility of NaCrO₂. Possible strategies for circumventing this detrimental phase transition will be proposed. We believe insights obtained in this work can be potentially applied to other O3 type of Na and Li transition metal oxides, and can serve as strong basis for future materials design of NaCrO₂ based cathodes.