A composite electrode of tin dioxide and carbon nanotubes and its role as negative electrode in lithium ion hybrid capacitor

A nanostructured electrode, denoted as SnO2/Cu/CNT, is prepared with tin dioxide and copper coated on the carbon nanotubes (CNT) backbone. Before coating of tin and copper, the CNT surface has been electrochemically oxidized and examined with the C1s photoelectron spectrum to ensure a minimum oxidation level while sufficient for electroless plating. With the prelithiated SnO2/Cu/CNT electrode, matching with an activated carbon (AC) electrode, this lithium ion hybrid capacitor taps into the lithium storage aptitude of tin dioxide for reversible capacity. Thus three capacitors, of the same AC loading, are assembled and studied with the AC:SnO2/Cu/CNT mass ratio 1:1, 1:0.67, 1:0.33. Their energy storage capacities are similar in magnitude at 0.1 A g−1, around 90 Wh kg−1 in a 3.8 V window, but the 1:0.67 cell surpasses the other two cells when the charging current increases. Superiority of the 1:0.67 cell on energy and electric charge storage is discussed, considering the SnO2/Cu/CNT role in capacitor. The discussion reveals the prelithiated SnO2/Cu/CNT has kept the potential at 0% state of charge (U0%SOC) low to constrict the maximum positive potential. But U0%SOC should avoid forcing the negative potential descends lower than 0.15 V (vs. Li/Li+), where the kinetic barrier of SnO2/Cu/CNT becomes problematic at high rates.