Inductance-Dominant Impedance Discovered in Lithium-Ion Polymer Electrolyte

Inductance and inductance-dominant impedance were discovered in a coil-less ionic conductor, as relevant to electrolytes, which are needed for electrochemical devices. The electrolyte is a gel polymer electrolyte, namely, poly(ethylene oxide) (PEO) with dissolved Li⁺ions, studied as a coating on glass. The studied electrical behavior is relevant to electrical self-sensing (without sensor incorporation). The results are also useful for advancing the basic understanding of electrochemical device performance. The impedance Z and angle θ between Z and its real part (resistance R) are measured by using an LCR meter under apparent impedance amplification. The Z increases monotonically with increasing Li content (decreasing EO:Li molar ratio from 13.0 to 7.6), being dominated by the reactance X rather than R; the degree of X dominance increases monotonically with increasing thickness (30–120 μm) and interelectrode distance (20–50 mm). Both Z and X increase linearly with frequency (0.5–5.0 kHz) and increase monotonically with increasing thickness. The X is vastly dominated by L (not the capacitance). Increasing the Li⁺ion content enhances L monotonically, indicating that the Li⁺ions contribute to L. The particle nature of the ions causes the ions to exhibit sharper directional changes than electrons, thereby promoting the inductance. The increase in thickness provides more room for ion path tortuosity, thereby enhancing L, which hinges on the tortuosity, so the model of inductors in parallel is not obeyed at all. The R does not require tortuosity, so the model of resistors in parallel is substantially obeyed. The ions influence L more than does R. Both L and R increase monotonically with increasing interelectrode distance, though the model of inductors or resistors in series is not obeyed exactly. The θ increases monotonically with increasing Li content, frequency, thickness, and interelectrode distance. The L has a less stringent requirement on the electrical contact quality than R.