Electrochemical impedance spectroscopy study of lithium-ion capacitors: Modeling and capacity fading mechanism

Abstract As important electrochemical power storage technology, lithium-ion capacitors (LICs) combine the advantages of both electric double layer capacitors (EDLCs) and lithium-ion batteries (LIBs). The impedance performances of LICs have been analyzed using electrochemical impedance spectroscopy (EIS), and the impedance equivalent circuit model (ECM) of LICs is established. Based on the EIS results, the ECM is established to describe the impedance behavior of LICs. The impedance characteristic variations (the ohmic resistance, charge transfer resistance and porous diffusion resistance) of LICs are analyzed with various open circuit voltage and different cycle numbers. The charging-discharging cut-off voltage and the cycle numbers greatly influence the LICs impedance and thus the capacity retention rate. It is shown that the capacity retention ratio is 73.8% after 80,000 cycle numbers when charging-discharging cut-off voltage is set to 2.0–4.0 V. When charging-discharging cut-off voltage is set to 2.2–3.8 V, the capacity retention ratio is 94.5% of the initial value after 200,000 cycle numbers. It provides useful guidance for setting the charging-discharging cut-off voltage of LICs.