Electrochemical conversion from hydroxyl to carbonyl groups for improved performance of dual-carbon lithium ion capacitors

Pseudocapacitance from oxygen functional groups of carbon materials have been taken advantage to improve the capacity of lithium ion capacitors (LICs). However, the Li+ adsorption of hydroxyl groups and the Li+ desorption from lithiated hydroxyl groups remain poorly understood. Herein, density functional theory (DFT) calculations in combination with experimental characterizations of hydroxyl-rich graphene (HRG) cathodes at different lithiated states are performed. The calculation results predict that fully lithiated hydroxyl groups undergo Li+ desorption at a high potential of 4.45 V (vs. Li+/Li), and consequent conversion into carbonyl groups instead of return to hydroxyl groups. Experimental investigations through galvanostatic charge/discharge tests and ex-situ X-ray photoelectron spectra (XPS) analyses of HRG verify the predicted Li+ desorption potential and the conversion. This electrochemical conversion is effective to obtain carbonyl-rich graphene (CRG) with significantly improved specific capacities. The conversion process can be completed in LIC full cells assembled with pre-lithiated HRG cathodes, through which an HRG//N-doped porous carbon (NPC) LIC transforms into a CRG//NPC LIC with improved performance. The dual-carbon CRG//NPC LIC delivers an energy density of 206.2 Wh kg−1 at a power density of 169.5 W kg−1, and superior cycling stability.