Cobalt Cyclotetraphosphate (Co2P4O12): A New High-Performance Electrode Material for Supercapacitors

Recently, transition metal phosphates (TMPs) are demonstrated as promising candidates for supercapacitors due to their good conductivity and long-term stability. Herein, we have inrtoduced cobalt cyclotetraphosphate (Co2P4O12) as novel supercapacitive electrode material, which deliver a capacitance of 437 F/g with good stability over 3000 cycles (around 90%). To display the practical relevance, we have fabricated asymmetric device using activated carbon and Co2P4O12 as negative and positive electrodes, respecitvely, which can be operated up to 1.4 V. The introduction of redox-active moieties (such as potassium iodide (KI)) in parent KOH solution enhances the capacitance from 120 F/g to 156 F/g for Co2P4O12//AC cell with extended voltage window (1.8 V). Owing to the additional voltage and advancement in the capacitance, the cell delivers battery-level energy of 70 Wh/kg at power density of 2.3 kW/kg. It should be emphasized that even at high power (7.6 kW/kg), the cell maintained very good specific energy of 48 Wh/kg, suggesting excellent rate performance. The asymmetric device further showed long cycling stability with negligible loss and excellent Coulombic efficiency over 5000 cycles. Thus, metal cyclotetraphosphate can be employed as a promising electrode material in energy storage systems.