Crystal Structures and Electrochemical Performance of Air-Stable Na_2/3Ni_{1/3–<i>x</i>}Cu_<i>x</i>Mn_2/3O₂ in Sodium Cells

Abstract

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–Na 2/3 Ni 1/3 Mn 2/3 O 2 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 Ni 2+ with other 2+ cations, such as Zn 2+ or Mg 2+, which improved capacity retention but significantly decreases reversible capacity, since these ions were not electrochemically active. Since Cu 2+ has been shown to be electrochemically active, we replaced Ni 2+ with Cu 2+, resulting in air-stable Na 2/3 Ni 1/3– x Cu x Mn 2/3 O 2 (0 ≤ x ≤ 1/3). Both Ni 2+ /Ni 4+ and Cu 2+ /Cu 3+ 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.