High performance hybrid sodium-ion capacitor with tin phosphide used as battery-type negative electrode

A novel hybrid Na-ion capacitor (NIC), in which Sn4P3 is implemented as battery-type negative electrode together with activated carbon as positive electrical double-layer electrode, is disclosed. Sn4P3 was formed by high-energy ball milling in Ar atmosphere, which allows the Sn4P3-based electrodes to display the lowest irreversible capacity (80 mAh g(-1)) today reported. The NIC demonstrates stable performance in the voltage range from 2.2 V to 3.8 V, with 94% capacitance retention after 6500 galvanostatic cycles at 0.2 A g(-1) (per total mass of electrodes). At specific power of 1 kWkg(-1), it displays a specific energy of 39 Wh kg(-1), against only 7Wh kg(-1) for the EDLC based on the same activated carbon electrodes. These metrics are superior to any of the values reported for NICs in the literature. Hence, the realized AC//Na-Sn4P3 NIC shows the possibility of developing a next-generation of low cost and safe high-power and energy storage devices.