Intercalation Pseudocapacitance in Ultrathin VOPO4 Nanosheets: Toward High-Rate Alkali-Ion-Based Electrochemical Energy Storage.

There is a growing need for energy storage devices in numerous applications where a large amount of energy needs to be either stored or delivered quickly. The present paper details the study of alkali-ion intercalation pseudocapacitance in ultrathin VOPO4 nanosheets, which hold promise in high-rate alkali-ion based electrochemical energy storage. Starting from bulk VOPO4·2H2O chunks, VOPO4 nanosheets were obtained through simple ultrasonication in 2-propanol. These nanosheets as the cathode exhibit a specific capacity of 154 and 136 mAh/g (close to theoretical value 166 mAh/g) for lithium and sodium storage devices at 0.1 C and 100 and ∼70 mAh/g at 5 C, demonstrating their high rate capability. Moreover, the capacity retention is maintained at 90% for lithium ion storage and 73% for sodium ion storage after 500 cycles, showing their reasonable stability. The demonstrated alkali-ion intercalation pseudocapacitance represents a promising direction for developing battery materials with promising high rate capability.