A new formula for the faradaic fraction used to estimate the coulombic hysteresis in the charge/discharge profiles of electrochemical energy storage systems

Abstract

The present work is concerned with the charge hysteresis frequently observed in the charge/discharge profiles of electrochemical capacitors (ECs) at the full charge voltage. In energy storage applications, the concept of operating potential window (OPW), envisioned as the potential window in which ECs are not deteriorated, is a key parameter for addressing the problems of both the life cycle and the energy density, but its determination remains arbitrary in nature due to its experimental determination based on kinetic parameters. For ECs, it was proposed to define the OPW as the closed gap in potential limited by a preset faradaic fraction at both ends of the stability window to ensure that the faradaic degradation of the electrode/electrolyte interface is commensurate with the charge stored in the electrochemical double layer. By implication, the potential limits so obtained are closely related to the potential range covered to reach a preset value of the faradaic fraction, so that the initial potential from which the stability is evaluated is expected to have an impact on the OPW. Here, this source of error was explored in detail and a voltage-correction is proposed to make the faradaic fraction less sensitive to the initial potential.