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Moye, D. G. (2019). A Predictive Model of Design Performance for Lithium-Ion Capacitors. Retrieved from http://purl.flvc.org/fsu/fd/2019_Fall_Moye_fsu_0071E_15490

Recent years have seen developments in lithium-ion capacitor (LIC) technology. However, there has been scant work in mathematical models to predict the performance of LICs. Existing models have focused upon cell degradation over time or simple Randles circuits to describe laboratory work. Experimentalists have determined that LICs’ energy storage capabilities are inversely proportional to their charge or discharge (dis/charge) current. But this phenomenon is not well understood, a serious barrier to LIC market entry as designers struggle to predict the energy storage of LICs under design. This study begins with an experiment on applying lithium to LICs. Then it discusses a study on the cycle life of LICs. Earlier studies had found the degradation of LICs cycled at constant current but different temperatures can be described by an Arrhenius equation. This cycle life study found that when an LIC is cycled at a constant temperature but different cycle currents the results can also be described by an Arrhenius equation, which can be derived from the Arrhenius equation describing degradation as a function of current. The Butler-Volmer equation anticipates these results because it predicts direct proportionality between a LIC’s cycle current and temperature. The predictive model, which comprises the bulk of this research, began with electrochemical impedance spectroscopy experiments (EIS) on an LIC to build a Randles equivalent circuit model (ECM) describing the same LIC. This LIC was then charged at several constant powers, recording the stored energy. Using the Randles ECM, simulated LICs were charged at these same constant powers, yielding high fidelity at low power but significant error at high power. In order to solve this high power inaccuracy, all of the experimentally-derived values, except for the series resistance and Warburg resistance, were replaced by physics equations that would compute these values. The Butler-Volmer equation was manipulated to express dis/charge current in terms of temperature, which computed the Warburg capacitance and dis/charge transfer resistance, key elements in a Randles ECM. This change yielded an accurate energy computation, but not an accurate voltage computation. Furthermore, the temperature values computed by the Butler-Volmer equation were unrealistic and could not be reconciled mathematically. The next iteration of the study involved in situ temperature measurements of LICs during dis/charge cycles. These in situ measurements indicated that temperature change is only observed during low power dis/charges, and even then it is <1% of absolute temperature. This indicates that although the Butler-Volmer equation can express LIC temperature in terms of dis/charge current, temperature can be treated as a constant without much loss in model fidelity. Warburg impedance was observed to be computed in terms of temperature and is almost constant, like temperature. Series resistance negligibly affects voltage change or energy stored in an LIC. This is the first known physics-based model to predict an LIC’s energy storage as a function of its dis/charge current and constituent components. This model also explains the current-energy inverse relationship observed in LICs using a new derivation of the Butler-Volmer equation. This model may have significant commercial value for LIC designers in the future as it eliminates a significant barrier to market entry.

A Dissertation submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

Bibliography Note

Includes bibliographical references.

Advisory Committee

Simon Foo, Professor Directing Dissertation; Anke Meyer-Baese, University Representative; Petru Andrei, Committee Member; Rodney Roberts, Committee Member; Zhibin Yu, Committee Member.

Publisher

Florida State University

Identifier

2019_Fall_Moye_fsu_0071E_15490

Moye, D. G. (2019). A Predictive Model of Design Performance for Lithium-Ion Capacitors. Retrieved from http://purl.flvc.org/fsu/fd/2019_Fall_Moye_fsu_0071E_15490