I. A. Ivanishcheva

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.

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.

Electrochemical properties of LiMn2−yMeyO4 (Me = Cr, Co, Ni) spinels as cathodic materials for lithium-ion batteries

A number of spinel phases with a general formula LiMn2−yMeyO (Me = Cr, Co, Ni) was synthesized using the melt-impregnation and sol-gel methods. All synthesized materials were subjected to electrochemical testing of their suitability as cathodes in lithium-ion batteries. Cyclic voltammograms were used in the testing. The cathode materials prepared using the melt-impregnation method showed the highest initial discharge capacity (up to 120 mA h/g) and stable operation during the cycling. The partial substitution of chromium and cobalt atoms for manganese gives positive effect: the spinel structure is stabilized during the cycling. The double doping of the Li-Mn-O system with small amounts of Co and Ni results in the stabilizing of the discharge capacity. An overstoichiometry excess of lithium in Co- or Cr-doped spinels also favors the increasing of the discharge capacity and slows down its decaying during the cycling.

Models of lithium transport as applied to determination of diffusion characteristics of intercalation electrodes

In order to elucidate the mechanism of lithium transport in intercalation electrodes based on solid lithium-accumulating compounds and determine its parameters, the kinetic models are used which allow the combined analysis of electrode impedance spectroscopy, cyclic voltammetry, pulse chronoampero- and chronopotentiometry data to be carried out. The models describe the stages of consecutive lithium transport in the surface layer and bulk of electrode-material particles, including the accumulation of species in the bulk. The lithium transport stages that occur in the surface layer of an intercalation-material particle and in its bulk are both of the diffusion nature but substantially differ as regards their characteristic times and diffusion coefficients D. Taking account of this peculiarity and assessing adequately the geometrical configuration of intercalation system allow the diffusion parameters of lithium transport to be correctly determined.

The synthesis, structure, and electrochemical properties of Li2FeSiO4-based lithium-accumulating electrode material

Different approaches to synthesis of Li2FeSiO4-based electrode materials for lithium intercalation, using low-cost and abundant Li-, Si-, and Fe-containing parent substances, are discussed. XRD, SEM, and a laser-diffraction analyzer of particle size were used for structure and morphology characterization of the composite electrode materials. Li2FeSiO4 was shown to be the main lithium-accumulating crystalline phase; minor LiFeO2 and Li2SiO3 admixtures are also present. The material microparticles’ average size was shown to vary from tenths of micrometer to 1 μm. Larger objects sized ca. 2–4 μm are the microparticles’ agglomerates. The material electrochemical properties were studied by dc chronopotentiometry (galvanostatic charging–discharging) and cyclic voltammetry with potential linear sweeping. The initial reversible cycled capacity of the best samples is 170 mA h/g. The anodic and cathodic processes manifest obvious hysteresis caused by the presence of several different lithium ion energy states in the material; the transition between the states is kinetically hindered. The dependences of the specific capacity and its stability under cycling on the current load and the conductive carbon component content in the composite were elucidated.