Ion-insertion Electrodes Research Articles

Comparison between Cottrell diffusion and moving boundary models for determination of the chemical diffusion coefficients in ion-insertion electrodes

Using PITT technique, the various shapes of the Cottrellian diffusion plots, It 1/2 versus log t, were studied as a function of the electrode potential for the Li ions insertion into graphite electrodes. Single maximum appeared on the It 1/2 versus log t plots for stoichiometries close to pure staged phases, whereas two maxima were always observed in the two-phase coexistence domains. Slow nucleation rate during phase 2/phase 1 transition was revealed by the characteristic shape of the initial time domains of both chronoamperometric and chronovoltammetric curves. Corrected for nucleation, the longer time domain of the response was treated chronocoulometrically in the framework of two alternative approaches using the moving boundary and the classical Cottrellian models. The alternative approaches resulted in virtually the same value of D. The diffusion of Li ions during phase 2/phase 1 transition is largerly affected by slow interfacial charge transfer. In this case, the spinodal domain in the free-energy curve is effectively degenerated even at small overvoltages. This substantiates the formal application of the Cottrellian diffusion approach to determine D during the processes of intercalation and deintercalation, accompanied by first-order transitions; however, involvement of the slow interfacial kinetics in the measured response should be adequatly taken into account.

Comparison between potentiostatic and galvanostatic intermittent titration techniques for determination of chemical diffusion coefficients in ion-insertion electrodes

Using well-cycled, thin composite graphite electrodes we analyze carefully the limitations of potentiostatic and galvanostatic intermittent titration techniques (PITT and GITT, respectively) for determination of the differential (incremental) intercalation capacitance, C dif, and the chemical diffusion coefficient, D, of Li ions in these ion-insertion electrodes (IIEs). We demonstrate the superiority of the GITT over PITT to determine these quantities as the former technique allows for a more accurate determination of C dif and hence D which closely approach to the spinodal domain related to the first-order phase transition during ion-insertion. We show that GITT is also more effective in eliminating the parasitic contributions of background currents to the total measured response. A pronounced difference in the initial, intrinsic kinetics of formation of a new phase in the bulk of the old one has been observed depending on the direction of titration (phases less saturated with Li are formed faster during deintercalation than the Li-rich phases in the course of intercalation).

Pulse Voltammetric and ac Impedance Spectroscopic Studies on Lithium Ion Transfer at an Electrolyte/Li4/3Ti5/3O4 Electrode Interface

Pulse voltammetry and ac impedance spectroscopy were used to study the lithium ion kinetics at a lithium ion insertion electrode consisting of Li4/3Ti5/3O4 thin films in an organic electrolyte. In the cyclic voltammogram, two redox peaks appeared at around 1.56 V vs Li/Li+ due to the insertion and extraction of lithium ion at the electrode. Differential pulse voltammetry gave a large reduction current at approximately 1.56 V during a cathodic scan due to lithium ion insertion into the electrode. From the peak current and potential, the charge-transfer resistance was evaluated by quantitative analysis using approximate equations for irreversible reactions. In the Nyquist plot, one semicircle was observed at 1.56 V, which was assigned to the charge-transfer resistance due to lithium ion transfer at the electrode/electrolyte interface. The value of the charge-transfer resistance at 1.56 V was almost identical to that evaluated by differential pulse voltammetry with an identical characteristic relaxation time. This result shows that both dc differential pulse voltammetry and ac impedance spectroscopy are useful for elucidating the phase transfer kinetics of lithium ion at insertion electrodes.

Distinction between Energetic Inhomogeneity and Geometric Non-Uniformity of Ion Insertion Electrodes Based on Complex Impedance and Complex Capacitance Analysis

Electrochemical insertion of Mg ions into Mo6S8 Chevrel phase is a unique, model system for studying the nature of the energetic inhomogeneity of the host sites suitable for ions accommodation. We show that the two energy state model can be successfully used for describing the very specific Mg ions insertion kinetics into the host, in particular, as relates to a drastic increase of Mg ions mobility in the vicinity of the critical potential of 1.25 V (vs Mg). This is accompanied by very pronounced changes of the impedance spectra. On the other hand, similar behavior of impedance spectra could be obtained for geometrically nonhomogeneous intercalation electrodes, comprising a distribution of thicknesses. One can frequently meet both these cases in practice for a vast variety of intercalation electrodes (e.g., for lithiated graphite, composite Li(x)MO2, M = Mn, Ni, Co, etc.). In this paper, we developed a methodology aimed at a reliable distinction between the two alternatives, based on complex impedance and complex capacitance analysis.

Discussion of the potential step method for the determination of the diffusion coefficients of guest species in host materials: Part I. Influence of charge transfer kinetics and ohmic potential drop

Theoretical expressions are given for the output response of ion-insertion electrodes to a potential step assuming linear diffusion, restricted (blocking) diffusion conditions and possible limitations by insertion reaction kinetics. The effects of ohmic potential drop are also investigated. It is shown that slow interfacial charge transfer cannot be distinguished from ohmic drop effects, in contrast to impedance diagrams where ohmic drop and charge transfer effects can be separated. The influence of potential step amplitude is discussed. Chronocoulometric analysis is dealt with considering diffusion controlled processes as well as mixed control conditions. The error in the determination of the chemical diffusion coefficient of a guest species from chronoamperometric data, when using the limiting Cottrell equation in the short-time range or the exponential decay of current in the long-time domain, is evaluated in relation to insertion reaction kinetics and ohmic potential drop. Determination of the diffusion coefficients by curve fitting is also envisaged using the current versus time and charge versus time relationships. Finally, previous results in the electrochemical literature are discussed in the light of the theoretical derivations proposed in this paper.

Impedance analysis of the electrochemical doping of poly(3-methyl-thiophene) from aqueous solutions of KClO 4, KCl and K 2SO 4

Impedance spectra of poly(3-methyl-thiophene) (P3MeT) films on Pt electrodes in aqueous solutions of 0.1 M KClO 4, 0.1 M KCl and 0.1 M K 2SO 4 show that, in the undoped form, the polymer acts as an ion-insertion electrode. In the doped state, the behaviour in 0.1 M KClO 4 is similar to that of porous metal electrodes with finite-length pores, although the low frequency capacitance is too high to be explained as an ordinary double-layer capacitance at the polymer-solution interface. This capacitance is therefore interpreted as a redox capacity connected to the doping of the polymer. In KClO 4, the doping threshold found by stepwise coulometry is at least 250 mV lower than the transition to porous electrode behaviour seen in the impedance data. In 0.1 M KCl(aq) and 0.1 M K 2SO 4(aq) this doping threshold is ca. 200 mV higher than in 0.1 M KClO 4, paralleling voltammetric data. In the KCl and K 2SO 4 solutions, the same equivalent circuit can be used both above and below the onset potential for doping. However, a Warburg element present in this circuit may be interpreted as due to diffusion below the metal-insulator transition and as capacitive charging of electrode pores of apparent infinite depth above this transition. The impedance data indicate that the pore structure of P3MeT varies with the contacting electrolyte.

Impedance analysis of the electrochemical doping of poly(3-methyl-thiophene) from aqueous nitrate solutions

Electrochemical impedance data for poly(3-methyl-thiophene) (P3MeT) films deposited on platinum electrodes were collected in aqueous solutions of KNO 3 and HNO 3. The impedance results show that undoped P3MeT (0–400 mV vs. Ag/AgCl/3.5 M KCl) can be described as an electroactive electrode film into which ions are inserted upon oxidation. A space-charge layer develops in the polymer, probably at the polymer-solution interface. This space-charge layer can be adequately described as a diffuse double layer consisting of holes and counter-ions. At 600 mV a transitional behaviour is seen which can be described by a Randles equivalent circuit, and the response is similar to that of an ion-insertion electrode. This transition reflects the onset of doping, which is also seen as a break in the potential versus doping charge curve recorded by electrochemical voltage spectroscopy. Apparent diffusion coefficients are found to decrease from ca. 10 −7 cm 2/s at 0 mV to ca. 10 −10 cm 2/s at 600 mV. This decrease may be due to an enhancement of ionic transport by electrostatic coupling of the moving charged species (counter-ions and holes) below the insulator-metal transition. At 800 mV the thickness and electrolyte concentration dependences of the impedance spectra indicate that P3MeT is best described as a porous metal electrode. The experimentally determined values of capacitance per surface area are not consistent with typical metal/electrolyte double layers, unless unreasonably large surface-to-volume ratios are assumed for the porous electrode. Therefore it is suggested that the capacitance of the porous polymer electrode is due to doping (redox capacity). Replacing K + ions by protons had only a negligible effect on the electrochemical response of the film. Polarization at 1 V deactivates the film, and a new equivalent circuit is required to describe the degraded polymer. P3MeT is found to exhibit a doping-dependent wettability.