Sodium ion intercalation and multi redox behavior of a Keggin type polyoxometalate during [PMo10V2O40]5- to [PMo10V2O40]27- as a cathode material for Na-ion rechargeable batteries

Recently, the development of cathode materials for Na-ion batteries has gained much attention due to the abundance, low cost, and easy availability of resources. Apart from the usual metal oxides, multi-electron redox materials grabbed attention due to their high energy density and practical capacity with long cycle life. Polyoxometalates (POMs) are inorganic clusters of higher valent metals, and act as electron sponges with multi-electron redox properties. Herein we report a Keggin-type polyoxometalate [PMo10V2O40](5-) with Na+ and H+ counter cations as a cathode material for Na-ion batteries. Further the formation of POM is evidenced by PXRD, FT-IR, flame photometry and XPS studies. In Na-POM, Na+ ions in the intercluster cavities provide a better pathway and easy diffusion during the charge/discharge process, and contribute to better electrochemical properties than H-POM. The DFT studies further explore the detailed mechanistic pathway of Na+ ions around the clusters in the normal and super-reduced states. Na-POM enables better cycling stability and capacity retention with a specific discharge capacity of 123 mA h g(-1) at 0.1C rate at room temperature.