P2-Na_2/3Ni_2/3Te_1/3O₂ Cathode for Na-ion Batteries with High Voltage and Excellent Stability

Air-stable layered structured cathodes with high voltage and good cycling stability are highly desired for the practical application of Na-ion batteries. Herein, we report a P2-Na_2/3Ni_2/3Te_1/3O₂ cathode that is stable in ambient air with an average operating voltage of ~3.8 V, demonstrating excellent cycling stability with a capacity retention of more than 92.7% after 500 cycles at 20 mA g⁻¹ and good rate capability with 91.9% capacity utilization at 500 mA g⁻¹ with respect to capacity at 5 mA g⁻¹ between 2.0 and 4.0 V. When the upper cutoff voltage is increased to 4.4 V, P2-Na_2/3Ni_2/3Te_1/3O₂ delivers a reversible capacity of 71.9 mAh g⁻¹ and retains 91.8% of the capacity after 100 cycles at 20 mA g⁻¹. The charge compensation during charge/discharge is mainly due to the redox couple of Ni²⁺/Ni³⁺ in the host with a small amount of contribution from oxygen. The stable structure of the material without phase transformation and with small volume change during charge-discharge allows it to give excellent cycle performance especially when the upper cutoff voltage is not higher than 4.2 V.