High-rate NaMo_0.05Ti_1.95(PO_4)_3 for aqueous sodium-ion battery anode material

Abstract

Aqueous sodium-ion batteries (ASIBs) are currently being developed as low-cost candidates for large-scale energy storage of green energy. Na superionic conductor-type NaTi_2(PO_4)_3 is a promising anode material for ASIBs owing to its excellent theoretical capacity, open three-dimensional framework, and sufficiently low-redox potential. However, its retention rate is restricted by its poor electronic conductivity. In this study, Mo-doped NTPs, NaMo_ x Ti_2− x (PO_4)_3 ( x  = 0, 0.01, 0.03, 0.05, 0.07), are synthesized using a facile sol–gel method to enhance its electronic conductivity. X-ray diffraction analysis reveals that composites doped with high-valence Mo retain rhombohedral crystal structure. Owing to the improved electronic conductivity and sodium-ion kinetics, NaMo_0.05Ti_1.95(PO_4)_3 exhibits superior capacity of 100.9 mAh g^−1 at 50 mA g^−1 and excellent rate performance of 71.9 mAh g^−1 at 10 A g^−1. Moreover, Mo-doped composites retain 82.7% of their original capacity after 500 cycles at 1 A g^−1, indicating the excellent cycling stability of NaMo_0.05Ti_1.95(PO_4)_3. Full cell with Mg-doped Na_3V_1.95Mg_0.05(PO_4)_2F_3/C cathode exhibits a high voltage window of 1.5 V and a sustained high energy density of 28.7 Wh kg^−1 at 512.7 W kg^−1 and 22.1 Wh kg^−1 at 2405.1 W kg^−1. These results demonstrate that NaMo_0.05Ti_1.95(PO_4)_3 exhibits high rate capability and long cycle life, making it a promising ASIB anode material for grid-scale energy storage. Graphical abstract