4V-class Magnesium-ion pseudocapacitors fabricated using an in situ inverse-charging process

Pseudocapacitors can deliver much more improved energy densities than those (<4% of typical lithium-ion batteries) of the electrochemical double layer (EDL) capacitors. Nevertheless, surface-limited redox behaviors based on typical monovalent-ion charge carriers exhibit insufficient energy densities, necessitating a new high-performance electrochemical system based on a feasible cell configuration. In this study, 4 V-class multivalent magnesium-ion pseudocapacitors (MIPs) were fabricated from mass-producible nanocarbon electrodes and a glyme-based electrolyte system via an in situ electrochemical oxidation process. A redox-free nanocarbon electrode was electrochemically tuned into a pseudocapacitive nanocarbon anode (PNA) using a well-controlled oxidation process, showing an approximately four times higher specific capacitance value (∼196F g⁻¹) compared with its initial EDL capacitance. The dual experimental and theoretical analysis results elucidate that the pseudocapacitance originates from the strong chemisorption ability with divalent magnesium-ions by the concerted effect of surface carbonyl functional groups and topological carbon defects. The high-capacitance PNA can work in a wide voltage range of 4 V. Therefore, the PNA-based MIP showed a high specific energy density of 167 Wh kg⁻¹, which is much higher than those (46 ∼ 145 Wh kg⁻¹) of previously reported alkali-ion capacitors. Additionally, a high cycling performance of the MIP full cell was achieved over 5,000 cycles.