Disrupting Sodium Ordering and Phase Transitions in a Layered Oxide Cathode

Abstract

Layered Na x M O 2 cathodes 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 Fe 3+ substitution for Ni 2+ strategy is employed here to obtain a series of Na 3+ x Ni 2–2 x Fe x SbO 6 (0 ≤ x ≤ 0.5) materials to suppress the structural phenomena that are apparent in O’3-layered Na 3 Ni 2 SbO 6 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 M O 2 layer thus remains a promising strategy for enabling commercial Na x M O 2 cathodes, 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.