Crystal Structures and Electrochemical Performance of Air-Stable Na_2/3Ni_1/3–xCu_xMn_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.

Na + /vacancy disordering promises high-rate Na-ion batteries

“…Two superstructure peaks at ~27.2° and ~28.3° are clearly observed (fig. S1), which originate from the in-plane Na-vacancy ordering in P2-NaNM ( 26 ) and cannot be resolved using the classical refinement model. These superstructure peaks vanished in this region for P2-NaNMT, suggesting successful suppression of Na + /vacancy ordering by Ni/Mn/Ti mixing.…”

Realizing Complete Solid‐Solution Reaction in High Sodium Content P2‐Type Cathode for High‐Performance Sodium‐Ion Batteries

“…The outstanding air/water stability of P2‐NLNMO greatly increase its application prospect. We also compared the electrochemical performance of P2‐NLNMO with typical Ni/Mn‐based P2‐type Na layered oxides in the literature . Because of the solid‐solution reaction mechanism, Na + /vacancy disordering, and high Na content, the electrochemical performance of P2‐NLNMO outperforms almost all the previously reported Ni/Mn‐based sodium layered oxides (see Table S4) and typical cathodes (see Table S5) for SIBs.…”

Electrochemistry and Solid‐State Chemistry of NaMeO2 (Me = 3d Transition Metals)

“…Recently, addition of sacrificial Na sources such as NaN 3 and Na 2 CO 3 into the P2 type electrode is proposed to compensate Na in the full cells without Na metal . Third, as discussed in hygroscopic property of O3‐type materials above, P2 type materials are also generally hygroscopic except for P2‐Na 2/3 Ni 1/3− x Cu x Mn 2/3 O 2 (0 ≤ x ≤ 1/3) . Air stable P2‐Na 2/3 [Mn, Fe]O 2 is achieved by Cu doping .…”