Revealing dual capacitive mechanism of carbon cathode toward ultrafast quasi-solid-state lithium ion capacitors

Abstract

The appropriate pore size and C=O group are vital factors for improving absorption/desorption process of PF 6 − ions, greatly boosting electrochemical performances of carbon cathode and quasi-solid-state lithium ion capacitor. High-performance lithium ion capacitors (LICs) have been seriously hindered by the very low capacity and unclear capacitive mechanism of carbon cathode. Herein, after the combination of experimental results and theoretical calculations, it is found that the critical pore size of 0.8 nm for PF 6 − ion adsorption decreases strong interactive repulsion of electrons and largely reduces adsorption energy barrier, which greatly improves the charge accommodation capacity in electrical double-layer behavior. Most importantly, the chemical-bond evolution process of C=O group has been firstly revealed by X-ray photoelectron spectroscopy (XPS), indicating that the introduction of C=O group can provide abundant redox active sites for PF 6 − ion adsorption accompanied with enhanced pseudocapacitive capacity. Attributed to the synergistic effect of dual capacitive mechanism, porous carbon sheet (PCS) cathode shows a reversible specific capacity of 53.6 mAh g −1 even at a high current density of 50 A g −1 . Significantly, the quasi-solid-state LIC manifests state-of-the-art electrochemical performances with an integrated maximum energy density of 163 Wh kg −1 and an outstanding power density of 15,000 W kg −1 . This elaborate work promotes better fundamental understanding about capacitive mechanism of PF 6 − ion and offers a rational dual-capacitive strategy for the design of advanced carbon cathodes.