Enhanced structural and cycling stability of O3-type NaNi1/3Mn1/3Fe1/3O2 cathode by Al replacement of Mn studied in half cell/full sodium-ion batteries

This study uses ball-milling spray drying to prepare a hollow spherical cathode material by doping non-electrochemically active Al into O 3 -NaNi 1/3 Mn 1/3 Fe 1/3 O 2 (NMF) cathode material. The effects of modification on the material's properties were analysed, and the optimum-doped ratio of NaNi 1/3 Mn (1/3−x) Fe 1/3 Al x O 2 (x = 0.005, NMFA 0.005) was determined. For half-batteries, NMFA 0.005 exhibits a good initial discharge capacity of 115.1 mAh g⁻ 1 at 1 C and capacity retention of 87.8% after 200 cycles, which is 16.1% higher than that of NMF samples. Furthermore, at a high rate (5 C), the initial capacity was 92.3 mAh g −1 and the capacity decline rate was 16.14%. However, the initial discharge capacity at high voltage was still 136.9 mAh g⁻ 1 . For full batteries, NMFA 0.005 delivered a discharge capacity of 81.1 mAh g⁻ 1 with a capacity retention of 82.2% after 100 cycles. Capacity retention was reinforced by approximately 37% compared with the matrix. Ex situ X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) failure analysis results put together show that an appropriate amount of Al doping could successfully alleviate the Jahn–Teller effect caused by Mn 3+ , decrease the oxygen loss and increase the stability of the layered structure, all of which result in an excellent electrochemical performance. NaNi 1/3 Mn (1/3-0.005) Fe 1/3 Al 0.005 O 2 cathode material demonstrated excellent electrochemical performance in half and full cells, which provided reference ideas for the practical applications of O3 type NaNi 1/3 Mn 1/3 Fe 1/3 O 2 materials. • Suitable Al doping can broaden the interlayer spacing of the sodium layer in NMF samples. • NMFA 0.005 samples exhibit excellent rate capability and cycle performance in half/full batteries. • Al doping can effectively inhibit Jahn-Teller effect and enhance the structural stability of materials. • The modification mechanism of Al doping under high voltage cycling was further elucidated by ex-situ XRD and EDS spectra.