Abstract
The sodium cobalt pyrophosphate (Na2CoP2O7) forms a diphosphate (PO4–PO4) compound, which exhibits allotropic evolution from the rose phase of a triclinic structure to the blue phase of a tetragonal crystal structure. Enhanced electrochemical performance of the two allotropic forms arises from changes in the orientation of the surface oxide layer and morphology. The triclinic rose phase structure of Co6−4 and PO4−3, features a staggered conformation with tunnels. Sodium ions are housed in these tunnels for ion exchange. Ionic intercalation/deintercalation occurs between the oxide layers, facilitating reversible charge storage processes. In this work, we adopted this mechanism for energy storage hybrid devices with an asymmetric configuration. In the proposed hybrid device, the battery charge storage mechanism from the Na2CoP2O7 cathode is coupled with a capacitive-type activated carbon anode to enhance device performance. The hybrid electrochemical cell demonstrates an excellent energy density of 50.2 Wh kg−1 at a maximum power density of 662.8 W kg−1, with long cycle retention of up to 10,000 cycles for the rose allotropic form of sodium cobalt pyrophosphate. The key parameters obtained from the hybrid device have been used for simulation to understand the interplay between kinetics and electrochemistry in the electrodes. The electrode-electrolyte interfacial layers show a wide distribution of ions in the hybrid device.