Evaluating Electrochemical Properties of Layered NaxMn0.5Co0.5O2 Obtained at Different Calcined Temperatures

Abstract

P-type layered oxides recently became promising candidates for Sodium-ion batteries (NIBs) for their high specific capacity and rate capability. This work elucidated the structure and electrochemical performance of the layered cathode material NaxMn0.5Co0.5O2 (NMC) with x~1 calcined at 650, 800 and 900 °C. XRD diffraction indicated that the NMC material possessed a phase transition from P3- to P2-type layered structure with bi-phasic P3/P2 at medium temperature. The sodium storage behavior of different phases was evaluated. The results showed that the increased temperature improved the specific capacity and cycling stability. P2-NMC exhibited the highest initial capacity of 156.9 mAh·g−1 with capacity retention of 76.2% after 100 cycles, which was superior to the initial discharge capacity of only 149.3 mAh·g−1 and severe capacity fading per cycle of P3-NMC, indicating high robust structure stability by applying higher calcination temperature. The less stable structure also contributed to the fast degradation of the P3 phase at high current density. Thus, the high temperature P2 phase was still the best in sodium storage performance. Additionally, the sodium diffusion coefficient was calculated by cyclic voltammetry (CV) and demonstrated that the synergic effect of the two phases facile the sodium ion migration. Hard carbon||P2-NMC delivered a capacity of 80.9 mAh·g−1 and 63.3% capacity retention after 25 cycles.