Differentiating Double-Layer, Psuedocapacitance, and Battery-like Mechanisms by Analyzing Impedance Measurements in Three Dimensions.

Electrochemical energy storage arises from processes that are broadly categorized as capacitive, pseudocapacitive, or battery-like. Advanced charge-storing materials that are designed to deliver high capacity at a high rate often exhibit a multiplicity of such mechanisms, which complicates the understanding of their charge-storage behavior. Herein, we apply a "3D Bode analysis" technique to identify key descriptors for fast Li-ion storage processes, where AC impedance data, such as the real capacitance (C') or phase angle (ϕ), are represented versus the frequency (f) and a third independent variable, the applied DC cell voltage. For double-layer processes, a near-constant C' or ϕ is supported across the entire voltage range, and the decrease in these values shows a near-linear decrease at higher f. For pseudocapacitance, an increase in C' is delivered, accompanied by high C' retention at higher f compared to double-layer processes. Interestingly, the lower ϕ values, where C' is highest, suggest that this is a key descriptor for pseudocapacitance, where high-rate charge storage is still facilitated within a kinetically limited regime. For battery-like processes, a high C' is only observed at the voltage at which the material stores charge, while outside that voltage, C' is negligible. The three-dimensional (3D) Bode analysis allows charge-storage dynamics to be mapped out in great detail with more delineation between mechanisms compared to the more frequently deployed kinetic analyses derived from cyclic voltammetry.