Pseudocapacitive Graphene-Wrapped Porous VO ₂ Microspheres for Ultrastable and Ultrahigh-Rate Sodium-Ion Storage

The exploration of anode materials with enhanced electronic/ionic conductivity and structural stability is beneficial for the development of sodium-ion batteries. Herein, a simple solution-derived method is demonstrated to fabricate porous VO ₂ microsphere composite with a graphene-wrapped structure (VO ₂ /G). When used as the anode material for sodium-ion batteries, the VO ₂ /G electrode delivers a high reversible specific capacity (373.0 mAh g ⁻¹ ), great rate capability (138.8 mA h g ⁻¹ at 24.0 A g ⁻¹ , ≈21 s per charge/discharge), and excellent long-cycling performance (95.9 % capacity retention for 3600 cycles at 2.0 A g ⁻¹ ). The outstanding electrochemical property of VO ₂ /G is mainly attributed to its unique graphene-wrapped porous structure and the pseudocapacitive-dominated feature. In addition, the sodium-ion storage mechanism of VO ₂ /G is investigated by various ex-situ characterization techniques. During the first sodiation process, the sodium-ion appears to partially reduce VO ₂ /G and form metallic vanadium, sodium oxide, and amorphous sodium vanadium. This work provides new fundamental information for the design and application of vanadium oxides for energy storage system.