Ultra-high-rate pseudocapacitive energy storage in twodimensional transition metal carbides. (2017) Nature Energy, 2 (8). 1-12.

Slow scan rate cyclic voltammetry (CV) and highly resolved (with respect to potential) electrochemical impedance spectroscopy (EIS) have been applied for lithiated graphite electrodes of different thicknesses. The impedance spectra have been successfully modeled for the whole range of intercalation potentials, using a combination of a Voigt-type equivalent circuit analog and the Frumkin and Melik-Gaykazyan (FMG) model. The Voigt-type analog, which is a series combination of R||C circuits, models the Li ion migration through the surface films covering the graphite particles. The FMG model combines a finite-length Warburg element, which reflects solid state Li diffusion in the graphite particles in series with capacitance that reflects the bulk capacity of the graphite particles (doped with intercalated lithium). The highly anisotropic nature of the graphite particles predetermines different extensive properties, including their charge transfer resistance and the parameters of the finite-length Warburg. The application of small-amplitude EIS and slow scan rate CV to very thin graphite electrodes (micronic and submicronic thicknesses) enabled us to obtain a good separation of the various processes which take place along the intercalation reaction path (e.g., Li + migration through the passivating surface films, solid state diffusion of Li ion in graphite, electron migration across the boundaries of the graphite particles partly covered by the passivating films, interfacial charge transfer, accumulationconsumption of Li into graphite, and phase transition). The application of an electroanalytical model based on a Frumkin-type adsorption isotherm complicated with a slow charge transfer for the voltammetric behavior of these electrodes at slow scan rates is discussed.

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The electroanalytical behavior of thin Li1−xCoO2 electrodes is elucidated by the simultaneous application of three electroanalytical techniques: slow‐scan‐rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy. The data were treated within the framework of a simple model expressed by a Frumkin‐type sorption isotherm. The experimental SSCV curves were well described by an equation combining such an isotherm with the Butler‐Volmer equation for slow interfacial Li‐ion transfer. The apparent attraction constant was −4.2, which is characteristic of a quasi‐equilibrium, first‐order phase transition. Impedance spectra reflected a process with the following steps: Li+ ion migration in solution, Li+ ion migration through surface films, strongly potential‐dependent charge‐transfer resistance, solid‐state Li+ diffusion, and accumulation of the intercalants into the host materials. An excellent fit was found between these spectra and an equivalent circuit, including a Voigt‐type analog ( Li+ migration through multilayer surface films and charge transfer) in series with a finite‐length Warburg‐type element ( Li+ solid‐state diffusion), and a capacitor (Li accumulation). In this paper, we compare the solid‐state diffusion time constants and the differential intercalation capacities obtained by the three electroanalytical techniques. © 1999 The Electrochemical Society. All rights reserved.

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