Unexpected effects of zirconium-doping in the high performance sodium manganese-based layer-tunnel cathode

Abstract

Hybrid electrode materials, which could integrate the respective advantages and overcome the drawbacks of each identical component, have caught vast attention and displayed promising applications. Recently, manganese (Mn)-based layer-tunnel hybrid cathodes (Na_xMnO₂) have shown enhanced performance based on the amalgamation of high capacity (layer structure), excellent stability, and high rate capability (tunnel structure). Ion-doping is usually carried out to determine the underlying performance. Moreover, ion-doping would tune the component ratio and adjust the crystal structure at the same time, which may affect the balanced synergistic effect of the pristine sample. With the help of density functional theory (DFT), we predicted that zirconium ions (Zr⁴⁺) could fulfill an equal-tendency doping, which could almost maintain the original hybrid structure component ratio and focus on the crystal structure adjustment. The Zr⁴⁺ doping effects were systematically investigated for both structure properties and sodium storage behavior. The stronger bond energy of Zr–O and larger ionic radius of Zr⁴⁺ could weaken the charge order and significantly enhance cycling stability and rate performance. The structure evolution and ion transportation kinetics were carefully tracked during the charge–discharge process. The designed material, Na_0.6Mn_0.98Zr_0.02O₂, can deliver a high capacity of 81 mA h g⁻¹ at 2 A g⁻¹ with 75% retention after 1000 cycles. Excellent full cell performance evidently proves the application potential. This study may suggest a possible new scope for the design of high performance hybrid electrodes.