Tailoring performance of hybrid supercapacitors by fluorine-rich block copolymer-derived carbon coated mixed-phase TiO2 nanoparticles

Abstract

Hybrid supercapacitors can function as both batteries and supercapacitors owing to their high specific energy and capacitive power, respectively. Transition metal oxide-based electrodes exhibit a high theoretical specific capacitance, but their large-scale application in charge storage devices is limited by their low conductivity and electrical stability. To address this problem, we introduced a highly conductive carbon coating over mixed-phase titanium dioxide (C/TiO2) using a novel carbon-rich polyacrylonitrile block copolymer containing an active pentafluorophenyl acrylate ester block (PAN-b-PFPA). The LiFePO4 (LFP) positrode and C/TiO2 negatrode assembled hybrid LFP║C/TiO2 @ 800 supercapacitor exhibited a high specific capacitance of 227 F/g (current density of 1 A/g), power density of 500 W/kg, and energy density of 32 W h/kg. Importantly, higher specific capacitance (∼84%) and Coulombic efficiency (∼96%) were maintained over 5000 charge–discharge cycles. The improved performance of hybrid LFP║C/TiO2 @ 800 supercapacitor is attributable to (1) selectively mixed anatase–rutile TiO2 phases that led to additional Ti3+ oxidation state formation, (2) firm porous carbon coating resulting from the surface anchoring of PAN-b-PFPA copolymer, and (3) fluorine/sulfur impurities from pyrolysis residues. The carbon coating of transition metal oxides from the pyrolysis of PAN-b-PFPA copolymers can facilitate the large-scale development of energy storage materials.