Abstract
Mixed transition-metal ions NASICON-structured compounds are enabled simultaneously as cathode and anode for aqueous Na-ion batteries (ANIBs), simplifying the fabrication process and reducing manufacturing costs in practical applications. In this study, a mixed V³⁺/Ti⁴⁺ NASICON-structured Na₂VTi(PO₄)₃@C composite acts as a bifunctional electrode material of ANIBs, delivering excellent sodium storage performance including high reversible capacity (cathode: 52.4 mAh g–¹ at 1C, anode: 59.8 mAh g–¹ at 1C, 1C = 62 mA g–¹) and remarkable rate capability (cathode: 44.8 mAh g–¹ at 20C, anode: 52.9 mAh g–¹ at 20C), as well as long cycle life. Meanwhile, ex situ X-ray photoelectron spectra (XPS) measurements reveal that V⁴⁺/V³⁺ and Ti⁴⁺/Ti³⁺ oxidation/reduction couples are redox-active sites of NVTP@C cathode and anode, respectively. When NVTP@C assembled into symmetric ANIBs, a full cell achieves ultrahigh stability of over 2500 cycles at 10C, showing broad prospects in large-scale electric energy storage applications.
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