A. V. Churikov

On the determination of kinetic characteristics of lithium intercalation into carbon

Abstract The diffusional processes proceeding in the intercalation electrode on applying a potential step are theoretically treated. The theory of chronovoltammetry is shown to require the necessary account of the contribution of the surface solid-state film to the overall diffusion resistance of the electrode. For the case of small deviations from equilibrium, analytical solutions of the diffusion problem have been obtained. Ignoring the retarding contribution of the surface film is shown to bring about an error into the diffusion coefficient of lithium in its alloys and intercalates. The theory has been experimentally verified with LixC6 electrodes of various compositions made of pyrocarbon films on a nickel backing as examples. The diffusion coefficients are 10−11–10−9 cm2 s−1, depending on composition.

Application of pulse methods to the determination of the electrochemical characteristics of lithium intercalates

Abstract The transfer processes proceeding in insertion electrodes with surface control on the application of a potential or current step are considered theoretically. The theoretical relationships have been verified by the determination of the kinetic and diffusion parameters of electrochemical lithium intercalation into thin carbon films. The overall electrode polarization is divided, both theoretically and experimentally, into the kinetic component, related to hindered ion transfer in the passive surface layer, and the diffusion one, related to decelerated lithium diffusion in the carbon matrix. The polarization dependence of kinetic current is shown to obey the same regularities that the current–potential function of the lithium electrode. The concentration dependences of the surface layer parameters and the diffusion coefficient of lithium in carbon have been determined.

Transfer mechanism in solid-electrolyte layers on lithium: influence of temperature and polarization

The kinetics of lithium electrochemical systems is governed by the transport processes in the solid electrolyte interphase (SEI) coating the lithium anode. The present work studies the temperature effect on the electrochemical kinetics of a metallic lithium electrode immersed into a LiClO4 solution in propylene carbonate in a wide polarization range. A series of polarization curves of the Li electrode within a temperature range of −35 to +70°C were recorded using the pulse voltammetry method. Any of these symmetrical anodic and cathodic polarization curves looks as a segment of a straight line (the Ohmic current jΩ caused by the intrinsic ionic conductivity of SEI) shading, as the overpotential η rises, into a power curve jinj∝ηn (jinj being the injection current) with a temperature-dependent exponent n≥2. Similar polarization curves were recorded for the Li electrode in LiClO4 and LiBF4 solutions in γ-butyrolactone as well. The cause of such a j(η, T) dependence is assumed to be structural disordering of the SEI material resulting in the appearance of a distribution of jump distances and energy barrier height for charge carriers. The stochastic transport of carriers in a disordered solid with a wide distribution of site-to-site jump times leads, by calculation of the current-voltage dependence, to the above power function jinj∝ηn with an exponent depending on the absolute temperature T as n=1+(a1−b1/T)−2. Our experimental data are in good agreement with this model. Comparing the experimental j−η curves with the theoretical equations, one could estimate a set of the microscopic parameters of transfer, including the mean jump distance, the effective radius of charge localization, the jumping attempt frequency, and the mean height of energy barriers.

Diffusion aspects of lithium intercalation as applied to the development of electrode materials for lithium-ion batteries

The creation of new electrode materials and the modification of existing ones are important trends in the development of lithium-ion batteries. Of special significance is to evaluate their diffusivity, i.e., the ability of providing transfer of the electroactive component. Such electrochemical techniques as cyclic voltammetry, electrochemical impedance spectroscopy, potentiostatic intermittent titration technique, and galvanostatic intermittent titration technique are used for this purpose. The values of chemical diffusion coefficient D estimated in similar electrode materials are shown to scatter by several orders of magnitude. Principal causes of this rather considerable scattering are discussed, including the uncertainty of diffusion area estimations and the use of various approaches to deriving equations to calculate D. Our conclusions are illustrated by examples of D estimations in the electrode materials LixC6, LixSn, LixTiO2, LixWO3, LiMyMn2−yO4, and LiFePO4.

Impedance of LiSn, LiCd and LiSnCd alloys in propylene carbonate solution

Abstract Electric properties of passivating films forming on the surface of LiSn, LiCd and LiSnCd alloy electrodes in propylene carbonate-based solutions have been investigated using the electrode impedance spectroscopy technique. Within the range of high and medium frequencies, the impedance spectrum has been shown to be described by the equivalent circuit represented by geometric capacitance and ionic resistance of the film, as well as capacitance of ionic space charge in the film and Warburg impedance assigned to lithium cation diffusion. The change of impedance spectra in the course of the electrode storage in the solutions has been discussed.

Kinetics of electrochemical lithium intercalation into thin tungsten (VI) oxide layers

The methods of galvanostatic intermittent titration, cyclic voltammetry, and electrode impedance spectroscopy are used to study the behavior of tungsten (VI) oxide film electrodes free of binding and conducting additives in the course of reversible lithium intercalation from nonaqueous electrolyte at 25°C. The studies are performed for electrodes with different degrees of crystallinity at the variation of the lithium concentration in intercalate from zero to 0.017 mol/cm3. Lithium diffusion coefficient is in the range of 10−11–10−16 cm2/s. The concentration dependences of the intercalation-layer transport parameters are analyzed, the equivalent circuit versions are discussed, and results obtained by different methods are compared.

Electrochemical behaviour of LiSn, LiCd and LiSnCd alloys in propylene carbonate solution

Abstract Transport properties and composition of passivating films, forming on the surface of LiSn, LiCd and LiSnCd alloy electrodes in propylene carbonate-based solutions, have been investigated using electrochemical and secondary-ion mass spectrometry techniques. The profile of polarization curves obtained by using the single-pulse current technique shows that the transport rate of mobile Li + ions through a passivating film is limited by the space charge of ions injected in the latter from the electrode or from the solution for anodic and cathodic currents, respectively. The values of mobility and concentration for mobile Li + ions in passivating films have been determined. It has been stated that introduction of MoCl 5 additions in solution leads to a modification of the passivating film composition and to an improvement of its transport properties on account of an increase in the Li + ion mobility.

Impedance spectroscopy of lithium-carbon electrodes

The methods of coulometric titration and electrode impedance spectroscopy are used in studying the behavior of carbon film electrodes free of binding and conducting additives in the course of reversible lithium intercalation from nonaqueous electrolytes. The electrodes with the high and low degrees of graphitization are studied. The measurements are performed in the frequency range from 105 to 10−2 Hz with the lithium concentration in intercalate varied from 0.025 mol/cm3 (corresponds to LiC6) to a state free of lithium. The factors responsible for the hysteresis in charge-discharge curves, the versions of equivalent circuits (EC) suitable for modeling the impedance spectra of LixC6 electrodes, the dependence of EC parameters and the lithium diffusion coefficient on the concentration are discussed. It is shown that all experimental impedance spectra can be adequately modeled by a common general EC. The concentration dependences are consistent with the earlier data of pulse methods. The diffusion coefficient varies approximately from 10−12 to 10−13 cm2/s.

Modelling of electrochemically stimulated ionic transport in lithium intercalation compounds

To establish the mechanism and to determine lithium transport parameters in the intercalation electrodes based on solid lithium-accumulating compounds, the kinetic models were used, allowing performing a joint analysis of data obtained via the application of the following methods: electrochemical impedance spectroscopy, cyclic voltammetry, pulse chronoamperometry, and chronopotentiometry. The models describe sequential steps of lithium transport into the surface layer and the bulk of the electrode material particles, including the accumulation of lithium in the bulk. Stages of lithium transport in the surface layer and bulk of intercalation material particles are of diffusion nature and differ from each other by characteristic time and diffusion coefficient D. Taking into account of this peculiarity, as well as an adequate assessment of the geometrical configuration of the intercalation system, allows to determine correctly the diffusion parameters of lithium transport.Graphical abstract

Impedance spectroscopy of lithium-tin film electrodes

A method of electrochemical impedance spectroscopy was used to study the reversible lithium intercalation from nonaqueous electrolyte into tin films with the thickness of 0.1–1 μm. The impedance spectra of lithium-tin (LixSn) electrodes have a complicated shape depending on the electrode state and prehistory; they reflect the occurrence of several consecutive and parallel processes, including the lithium migration, diffusion, and accumulation. The formation of a solid-electrolyte layer on the surface at Li intercalation into Sn is observed. Equivalent circuits are proposed that adequately model the experimental data on the LixSn electrodes both freshly prepared and after prolonged cycling. Problems associated with the choice of equivalent circuits and determination of their parameters, the accuracy of the diffusion coefficient determination, the trends in the parameters’ variation with electrode potential (composition) are discussed.