Understanding Na/vacancy Re-arrangement upon Ion Transport in Sodium Layered Oxide Cathodes

Sodium layered oxide cathode materials attract massive interests owing to the low price. However, this class of materials always suffers from complicated and deleterious phase transformations upon Na extraction and re-insertion, leading to stepwise electrochemical curves along with inferior rate capability and serious capacity degradation. Na ⁺/vacancy re-arrangement is considered to play a significant role in the structural evolution. When Na is extracted and re-inserted, the structural transition induced by lattice distortion could be viewed as a valid evidence for Na ⁺/vacancy ordering re-arrangement. Unfortunately, the characterization of Na ⁺/vacancy ordering re-arrangement caused by structural distortion could be confused by the lattice distortion in TM layers because Na ions diffuse between transition metal layers in the electrochemical process. The Na ⁺/vacancy re-arrangement is not only determined by the sodium content or the redox of TM cations in the structure, but also significantly influenced by the manner how Na ions are removed from the structure. Herein, we systematically study the structural transition processes of O3-type TM oxide cathode material upon sluggish and fast Na ion migration, which is viewed to be strongly associated with Na ⁺/vacancy re-arrangement. The O3-type material experiences different phase transformations in the electrochemical process under different diffusion rates. The emergence and disappearance of monoclinic O3’ phase under different current densities indicate that the O3-type material experiences different Na ⁺/vacancy re-arrangement upon sluggish and fast Na ion migration. The results provide an effective structural analysis method to clarify the Na ⁺/vacancy re-arrangement.