Layered-to-Rock-Salt Transformation in Desodiated NaCrO2

Abstract

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 NaxCrO2 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 NaxCrO2 (0 < x < 1) remains in the layered structure framework without Cr migration up to a composition of Na0.4CrO2. Further removal of Na beyond this composition will trigger a layered to rock-salt transformation converting the P'3-Na0.4CrO2 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.