Disrupting Sodium Ordering and Phase Transitions in a Layered Oxide Cathode

Layered Na_xMO₂cathodes are of immense interest as rechargeable sodium batteries further their development as a lithium-ion battery alternative. However, two primary intrinsic structural issues hinder their practicality-sodium ordering and transition-metal layer gliding during cycling. These phenomena plague the electrochemical profiles of these materials with several unwanted voltage plateaus. A Na⁺and Fe3⁺substitution for Ni2⁺strategy is employed here to obtain a series of Na_3+xNi_2-2xFe_xSbO₆(0 ≤ x ≤ 0.5) materials to suppress the structural phenomena that are apparent in O 3-layered Na3Ni2SbO6 cathode material. This strategy is successful in obtaining a sloping voltage curve without distinct plateaus-an indication of suppression of the underlying structural phenomena that cause them-at doping concentrations of x ≥ 0.3. The first-cycle coulombic efficiency of the doped compounds is much greater than the starting compound, presumably owing to a kinetic barrier to reforming the full O'3-layered starting materials within the voltage range of 2.5 4.3 V vs Na⁺/Na. Sodium doping into the MO₂layer thus remains a promising strategy for enabling commercial Na_xMO₂cathodes, but further development is required to lower the kinetic barrier for sodium reinsertion into these materials in a useful voltage range to maximize their reversible capacity.