Crystal Structures and Electrochemical Performance of Air-Stable Na2/3Ni1/3–xCuxMn2/3O₂ in Sodium Cells

Sodium ion batteries have garnered significant research attention in recent years due to the rising demand for large-scale energy storage solutions as well as the high abundance of sodium. P2-type layered oxide materials have been identified as promising positive electrode materials for sodium ion batteries. Previously, P2–Na2/3Ni1/3Mn2/3O2 was shown to have a high operating voltage and high capacity but suffers from a step-like voltage curve and capacity loss during cycling, potentially due to its P2–O2 transition at high voltages. One strategy to improve cycling performance has been to dope Ni2+ with other 2+ cations, such as Zn2+ or Mg2+, which improved capacity retention but significantly decreases reversible capacity, since these ions were not electrochemically active. Since Cu2+ has been shown to be electrochemically active, we replaced Ni2+ with Cu2+, resulting in air-stable Na2/3Ni1/3–xCuxMn2/3O2 (0 ≤ x ≤ 1/3). Both Ni2+/Ni4+ and Cu2+/Cu3+ participate in the redox reaction during cycling, capacity retention was greatly improved, and phase changes were suppressed during cycling without sacrificing much capacity. The material retains a P2/OP4 structure even when cycled to high voltages. The doping strategy is a promising approach for the future development of positive electrode materials for sodium ion batteries.