Suppressing multiphase transitions of an O3-NaNi0.5Mn0.5O2 cathode by iron and magnesium co-doping towards sodium-ion batteries

Abstract

Sodium-rich O3-type sodium-layered oxides have been well recognized as one of the most promising cathode materials for sodium-ion batteries (SIBs) owing to their high capacity and ease of synthesis; however, they suffer from the decay in electrochemical performances due to the complicated phase transformations. Herein, we demonstrate the preparation of an Fe/Mg co-doped O3-NaNi_0.35Fe_0.2Mg_0.05Mn_0.4O₂ (O3-NFMM) cathode material for SIBs via a solid-state reaction method. Based on Ni²⁺/Ni³⁺ and Fe³⁺/Fe⁴⁺ redox couples, the O3-NFMM cathode delivers a high reversible capacity of 129.4 mA h g⁻¹ at 0.1 C, a capacity retention of 86% after 150 cycles at 1 C, and a good rate capability (57% of the initial capacity at 8 C) compared with those of the pristine O3-NaNi_0.5Mn_0.5O₂ and two mono-doped counterparts. In situ XRD characterizations demonstrate that the O3_hex-O3′_mon-P3_hex-P3′_mon-P3′′_hex complex phase transition of the O3-NaNi_0.5Mn_0.5O₂ material is suppressed, and the highly reversible O3-P3 phase transition is achieved by Fe/Mg co-doping, thus facilitating the fast Na⁺ migration of O3-NFMM. Furthermore, the sodium-ion full battery paired with a hard-carbon anode exhibits a high reversible capacity of 132.7 mA h g⁻¹ at 0.1 C and a cycling stability of 80% after 200 cycles at 1 C. These results show that the Fe/Mg co-doping strategy can provide a facile avenue for designing high-performance O3-type layered cathode materials for sodium-ion batteries.