Design and Performance of Rechargeable Sodium Ion Batteries, and Symmetrical Li‐Ion Batteries with Supercapacitor‐Like Power Density Based upon Polyoxovanadates

The polyanion Li 7 V 15 O 36 (CO 3 ) is a nanosized molecular cluster (≈1 nm in size), that has the potential to form an open host framework with a higher surface‐to‐bulk ratio than conventional transition metal oxide electrode materials. Herein, practical rechargeable Na‐ion batteries and symmetric Li‐ion batteries are demonstrated based on the polyoxovanadate Li 7 V 15 O 36 (CO 3 ). The vanadium centers in {V 15 O 36 (CO 3 )} do not all have the same V IV/V redox potentials, which permits symmetric devices to be created from this material that exhibit battery‐like energy density and supercapacitor‐like power density. An ultrahigh specific power of 51.5 kW kg −1 at 100 A g −1 and a specific energy of 125 W h kg −1 can be achieved, along with a long cycling life (>500 cycles). Moreover, electrochemical and theoretical studies reveal that {V 15 O 36 (CO 3 )} also allows the transport of large cations, like Na + , and that it can serve as the cathode material for rechargeable Na‐ion batteries with a high specific capacity of 240 mA h g −1 and a specific energy of 390 W h kg −1 for the full Na‐ion battery. Finally, the polyoxometalate material from these electrochemical energy storage devices can be easily extracted from spent electrodes by simple treatment with water, providing a potential route to recycling of the redox active material.