Mg/Ti co-doping induced P2/O3 biphasic modulation strategies enhancing structural stability for sodium-ion battery cathodes

Abstract

P2-type Na-NiMn oxides have garnered significant attention for sodium-ion batteries owing to their excellent rate capability. However, they are adversely affected by the harmful P2–O2 phase transitions, which cause substantial capacity degradation and poor cycling performance. In this work, a unique P2(84%)/O3(16%) biphasic layered oxide, Na_0.76Ni_0.22Mg_0.11Mn_0.57Ti_0.1O₂ (NNMMT), is successfully designed and synthesized by a Mg/Ti co-doping strategy. The doping of Mg and Ti can form stable O–TM–O bonds, while the biphasic structure generates distinctive interlocked interfaces at the boundaries. The harmful phase transition caused by the interlayer slip and structural strain is well suppressed under the synergistic effect of these two factors. As a result, NNMMT demonstrates exceptional capacity and cycling stability (reversible capacity of 109.8 mAh g⁻¹ at 1 C and capacity retention of 97.35% after 100 cycles). Meanwhile, the higher proportion of the P2 phase enables NNMMT to maintain excellent rate capability (initial discharge capacity of 88.9 mAh g⁻¹ at 5 C). Additionally, ex situ X-ray diffraction analyses before and after cycling demonstrated a significant suppression of the detrimental P2–O2 phase transition in NNMMT during the charge–discharge process, leading to a notable enhancement in structural stability. This work provides an innovative and efficient strategy for designing highly stable layered oxide cathodes.