Effects of ion insertion on cycling performance of miniaturized electrochemical capacitor of carbon nanotubes array

Abstract

Capacity degradation and ion insertion of a miniaturized electrochemical capacitor are studied using ionic liquid [EMI] [TFSI] as the electrolyte. This capacitor is featured with two comb-like electrodes of vertical carbon nanotubes, ∼70 μm in height and 20 μm in interelectrode gap. We quantify the levels of ion insertion damage with Raman spectroscopy after the electrode experiences 120 consecutive voltammetric cycles to various potential limits. Distinct structural damage emerges due to [EMI] when the negative potential reaches −1.7 V, and those due to [TFSI] arise when the positive potential reaches 1.7 V vs. RHE. Judging from the peak broadenings, [EMI] is more detrimental than [TFSI]. When the voltage window ΔU is set as less than or equal to 2.8 V, both electrode potentials are within the two intercalation limits, little or no decay is observed in 104 charge/discharge cycles. When ΔU is 3.4 V, the positive potential exceeds the upper limit, but the negative potential stays within the lower limit, the cell capacitance decreases moderately. When ΔU increases to 3.8 V, both electrodes suffer from damages because of exceeding the intercalation limits. And the cell capacitance decreases substantially, even leading to a premature failure.