Al-doped walnut-shell-like P2-type Na2/3Ni1/3Co(1/3-x)Mn1/3AlxO2 as advanced sodium ion battery cathode materials with enhanced rate and cycling performance

Sodium layered oxides are considered to be promising candidates as sodium ion battery cathode materials due to their layered structure which is beneficial for fast Na-ion diffusion. However, their practical application is hampered by the rapid capacity decay as a result of the multiple irreversible phase transitions and the volume expansion during operation. Herein, by regulating morphology through the surfactant of tween 80 and partially substituting Co3+ with Al3+ ions, we successfully prepared Al-doped P2-type Na2/3Ni1/3Co(1/3-x)Mn1/3AlxO2 with uniform walnut-like porous and hollow structure. As the result of the synergy of Al-doping and morphology design, the resulting material exhibits good accommodation of volume change and suppressed phase transitions during charge/discharge processes. Moreover, the optimization on the Al-doping content is carefully investigated and the results demonstrate that the Na2/3Ni1/3Co(1/3-x)Mn1/3AlxO2 with x = 0.03 exhibits the best performance with a capacity of 74.7 mA h g(-1) delivered at 10C and a capacity retention of 76.6% achieved after 150 cycles. Further study reveals that the performance of the P2-type Na2/3Ni1/3Co(1/3-x)Mn1/3AlxO2 is closely related to the limiting effect of the Al-doping on the phase transitions as well as the charge transfer resistance. Ex-situ scanning electron microscope and X-ray powder diffraction results uncover the morphological stability and the electrochemical reversibility of the Na2/3Ni1/3Co(1/3-x)Mn1/3AlxO2 with x = 0.03. This work introduces the synergistic combination of doping and morphology design as an efficient method to enhance the Na-ion storage performance, which may provide a promising strategy for developing advanced sodium ion battery cathode materials. (c) 2020 Elsevier Ltd. All rights reserved.