Chronoamperometric studies of the stage 2 (LiC12) to stage 1 (LiC6) transition during electrochemical insertion of Li ions into graphite electrodes showed 5 distinct domains in their current vs. time curves. A linear increase of current at the beginning of domain II and a decrease in current with time within domain IY were identified with two-dimensional instantaneous nucleation of cylindrical grains of stage 1 into stage 2, their growth and further coalescence with the formation of the continuous moving boundary. We could calculate the following characteristic parameters: the nucleation time constant, τn=990s; the density of nucleation sites No=2.5×108cm−2, the velocity of the lateral growth of cylindrical nuclei, a=3.6×10−8cms−1 and the thickness of the nucleation layer, d=0.36μm. After nucleation stage, the interphase propagates initially linearly with time and then proportionally to the square root of time (domain IV). At 30°C the speed of propagation of the moving boundary between stage 1 and stage 2, dξ/dt′, is around 1.6×10−7cms−1. The activation energies of a, dξ/dt′ and D1 obtained from the related Arrhenius plots were found to be 0.13, 0.19 and 0.23eV, respectively.

We propose a new method for the kinetic treatment of self-discharge of intercalation electrodes based on a combination of cyclic voltammetry (CV) and chronopotentiometry under net zero current conditions. This was applied to composite graphite electrodes in 1 M LiPF6/ethylene carbonate + dimethyl carbonate (1:1) + 5% dimethyl polycarbonate at four different temperatures from 25 to 80degreesC, and was quantitatively proved for small degrees of self-discharge. The degree of self-discharge and the self-discharge current, I-sd, were shown to substantially increase with the increase in temperature. Self-discharge experiments, which continued for 50 h, were complemented by detailed impedance characterizations of the gradually discharged electrodes, which show an increase in the diameter of the high-frequency semicircles (HFS) with open-circuit time at different temperatures. A comparison between Arrhenius plots for the HFS, the chemical diffusion coefficient, and the self-discharge current may create a basis for a new method of "electrochemical temperature spectroscopy'' for identification of the nature of the rate-determining step of self-discharge for different temperature ranges. Our results are discussed in light of studies by others of self-discharge phenomena and mechanisms (Li-graphite electrodes). (C) 2004 The Electrochemical Society.

The addition of small concentrations of dimethyl pyrocarbonate (DMPC), about 5% by volume, to a standard solution of 1 M LiPF6/ethylene carbonate 1 dimethyl carbonate (1:1) was shown to improve substantially the cycling behavior of graphite electrodes. A combination of fast and slow scan rate cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Fourier transform infrared, H-1, and C-13 nuclear magnetic resonance spectroscopy were applied to understand the nature of the stabilizing effect caused by the presence of DMPC. DMPC was shown to increase impregnation of the active mass of the porous electrodes with solution (thus increasing the specific electrode capacity), and facilitates the rate of Li-ion migration across the surface films around the graphite particles and across the surface film/particle interface. Based on a combination of fast- and slow-scan rate CV and EIS, a convenient procedure allowing for an intermittent monitoring of the kinetic and thermodynamic characteristics of the Li-insertion process into graphite in the course of long-term electrode cycling was elaborated. We show that a simple procedure consisting of the application of a series of consecutive fast- and slow-scan rate CVs can be conveniently used for a systematic search and optimization of electrolyte solutions suitable for long-term cycling of graphite electrodes in lithium-ion cells. (C) 2004 The Electrochemical Society.

A combination of three major electroanalytical techniques (slow-scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT) and electrochemical impedance spectroscopy (EIS)) was applied for comprehensive analysis of temperature dependence of the most important equilibrium and kinetic parameters related to the Li-insertion process. These parameters include the differential intercalation capacity, Cint and the chemical diffusion coefficient, Dchem. We have analyzed the complicated, ambivalent influence of the temperature on the shape of the cyclic voltammety curves, probing the rate of separate steps of the intercalation process close to equilibrium. Good agreement had been found between the SSCV data measured over a wide range of temperatures, and the classical staging phase diagram of LixC6 (this diagram is presented as a plot of the absolute temperature, T versus intercalation level, x).

We report on the potential and temperature dependences of the differential intercalation capacitance, Cdif, and the chemical diffusion coefficient, D, during Li intercalation into a graphite anode by a combined application of slow-scan rate cyclic voltammetry (SSCV) and the potentiostatic intermittent titration technique (PITT). Drastically different behavior was observed within the short-time ranges of the PITT response measured for a two-phase coexistence domain and a solid solution of phases 4 and 3. Electroanalytical evidence for small droplet formation (nucleation) of a new phase in the bulk of the old one was found for the former domain, showing good correlation with in situ XRD studies. SSCV data obtained in the 25–80 °C temperature range were in excellent agreement with the published temperature–concentration phase diagrams built on the basis of detailed XRD characterizations. The simultaneous appearance of maxima on Cdif versus E plots and minima on the related log D versus E plots in the two-phase domains was rationalized in terms of a lattice gas model with single site energy, and highly attractive interactions between the intercalated guest atoms. The electroanalytical behavior of graphite within the solid-solution domain (a mixture of phases 4 and 3) was interpreted semi-quantitatively on the basis of a model that took into account the presence of two sub-lattices of different energy (a model of “energetic heterogeneity”) for Li accommodation, and attractive interactions between the guest atoms on each sub-lattice, or, alternatively, due to in-plane order–disorder transitions because of short-range repulsive interactions between the intercalated guest species.

A careful study of electrochemical lithiation of a composite graphite electrode at elevated temperatures was performed using a potentiostatic intermittent titration technique. Special emphasis was placed on the stage 4 to stage 3 phase transition, occurring in the range of x (in Li x C6) between 0.12 and 0.22 (this corresponds to the partial dimensionless intercalation level θ ranging from 0 to 1). An abrupt increase in the Li chemical diffusion coefficient, D chem, when approaching the relative coverage level θ=1/3, and a specific non-Arrhenius shape of the plot of D chem vs. the inverse of the absolute temperature were observed. These two features of the phase transition seem to be in agreement with a recent thermodynamic model for adatoms diffusing on a surface with two considerably different barrier energy sites.