From structure to electrochemistry: the influence of transition metal ordering on Na+/vacancy orderings in P2-type NaxMO2 cathode materials for sodium-ion batteries

Abstract

P2-type layered oxides are attractive cathode active materials for sodium-ion batteries, however, these materials typically suffer from detrimental Na⁺/vacancy orderings. In this work, we investigate the origin as well as the influence of the transition metal ratio on Na⁺/vacancy orderings in P2-type cathode materials. A combination of X-ray diffraction (XRD), neutron diffraction, advanced electrochemical methods, operando XRD and DFT calculations is applied to study Na⁺/vacancy orderings in P2-Na_xNi_1/3Mn_2/3O₂ and P2-Na_xMn_3/4Ni_1/4O₂. In P2-Na_xNi_1/3Mn_2/3O₂, a honeycomb Ni/Mn superstructure leads to charge ordering within the transition metal slab and pronounced Na⁺/vacancy orderings, causing distinct voltage jumps at specific sodium contents (x = 2/3, 1/2 and 1/3). For P2-Na_0.60Mn_3/4Ni_1/4O₂, the Ni/Mn superstructure is disrupted, resulting in more complex charge orderings within the transition metal slab, partially suppressed Na⁺/vacancy orderings and an overall smoother potential profile. Based on our findings, guidelines to suppress Na⁺/vacancy orderings in P2-type cathode materials for sodium-ion batteries are postulated and discussed with respect to electrochemical measurements of various transition metal compositions. These guidelines can serve to predict the tendency towards Na⁺/vacancy orderings for a given cathode composition or to design new cathode compositions for enhanced cycle life based on the absence of Na⁺/vacancy orderings.