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.