J.H.W. De Wit

Bounded diffusion in solid solution electrode powder compacts. Part II. The simultaneous measurement of the chemical diffusion coefficient and the thermodynamic factor in LixTiS2 and LixCoO2

Abstract Two electrochemical methods - involving the application of a long-time galvanostatic current pulse and a small potentiostatic voltage step to a M/M x SSE cell - are presented. From the overvoltage, respectively current response the chemical diffusion coefficient ( D M + ) and the thermodynamic factor (∂ ln a /∂ ln c ) are obtained. The methods have been applied to the cells: Li/1M·LiClO 4 in propylenecarbonate/Li x Ti 1.03 S 2 0.05 x T = 20°C; and Li x CoO 2 0.10 T = 20°C. From the application of the current pulse/voltage decay method it followed: D Li + ( Li x Ti 1.03 S 2 ) = 1−4 × 10 −8 cm 2 s −1 , with a slight tendency to increase with decreasing x ; D Li C ( Li x CoO 2 ) = 2−40 × 10 −9 cm 2 s −1 , decreasing with decreasing x . These values are among the highest found for solid state Li + -ion diffusion, and will be closely evaluated and compared with data reported by other workers. The x -dependence of the thermodynamic factor, determined from kinetic data, for Li x Ti 1.03 S 2 ( x = 0.05-0.95) and Li x CoO 2 ( x = 0.60-1.00) is in accordance with a simple thermodynamic model. Unlike for Li x Ti 1.03 S 2 , the thermodynamic factor for Li x CoO 2 , determined from the EMF- x relation, cannot be accounted for by this model. Furthermore, a fast, but crude method to determine the average chemical diffusion coefficient in Li x Ti 1.03 S 2 and Li x CoO 2 is discussed.

The high temperature behavior of In2O3

Abstract The electrical conductivity of In2O3 has been measured up to 1400°C in air. The temperature dependence of the conductivity at high temperatures yields an activation energy of 1.5 ± 0.1 eV. This activation energy is interpreted in terms of a nonstoichiometric decomposition of the compound. This interpretation is sustained by thermogravimetric analysis in combination with a gas mass analyser. Hall experiments on quenched samples are not in contradiction with this interpretation.

Electrical properties of In2O3

Abstract Conductivity data for In2O3 both from literature and from new measurements are critically compared. They are correlated with atmospheric conditions and temperature. The conductivity data and structural considerations lead to the conclusion that non-stoichiometric In2O3 is an n-type semiconductor. Interstitial indium ions are probably the predominant defects.

Electron concentration and mobility in In2O3

Abstract The electron concentration and mobility in polycrystalline In 2 O 3 have been measured as a function of temperature and partial oxygen pressure, in the temperature range from 25 to 700°C. These experimental data are critically compared with literature data. A conduction model is proposed. Theoretical values of the electron concentration as a function of the partial oxygen pressure are reported for temperatures from 700 to 1400°C. An estimate is given for the minimum room temperature “intrinsic” electron concentration in In 2 O 3 after a high temperature annealing experiment. It is also shown that interpretations of conductivity values for porous material in terms of carrier concentration only, can be very misguiding.

Structural aspects and defect chemistry in In2O3

Abstract The C -type rare earth structure of In 2 O 3 is compared with the fluorite structure. The stability of the structure is discussed based on DTA and X-ray work and electron microscopy. Ordering of defects does not take place, so that defect chemistry can be formulated for isolated defects in nonstoichiometric In 2 O 3 . The influence of the incorporation of aliovalent cations into the In 2 O 3 host lattice is discussed and a defect model is suggested.

The thermodynamic and thermoelectric properties of LixTiS2 and LixCoO2

Abstract The partial thermodynamic functions Δ H Li and Δ S Li for Li x Ti 1.03 S 2 (0.13 ⩽ x ⩽ 0.97) and Li 0.95 CoO 2 were obtained from EMF-temperature measurements ( T =-30−20° C ). For Li x Ti 1.03 S 2 , the x -dependence of these quantities is discussed in relation to a semiempirical expression for the EMF- x relation. The electronic component of the thermoelectric power in Li x Ti 1.03 S 2 (0 ⩽ × ⩽ 0.97, T = 50−200° C ) and Li x CoO 2 (0.20 ⩽ x ⩽ 1.00, T = 30−400° C ) was determined. From the sign of the (electronic) Seebeck coefficient it followed that Li x Ti 1.03 S 2 is a n -type and Li x CoO 2 a p - type electronic conductor. The influence of the amount of inserted lithium and temperature dependence on the Seebeck coefficient is discussed. A new method to determine the ionic heat of transport directly from the ionic Seebeck co-efficient was developed. This method was applied to Li x Ti 1.03 S 2 (0.61 ⩽ x ⩽ 0.97, T = -30−30° C ). The heat of transport is uch smaller than the activation enthalpy for Li + -conduction, indicating a high ionic polaron binding energy. Thermogravimetric analysis indicates that Li x CoO 2 with x 1 CoO 2 and Co 2 O 3 at temperatures higher than 80°C. This is sustained by the data for the electronic Seebeck coefficient. Also the thermodynamic, thermoelectric and kinetic data of Li x Ti 1.03 S 2 are critically compared with those of Ag x TiS 2 .

Surface instability and nonstoichiometry of α-Fe2O3

Abstract The thermodynamic stability region of α-Fe 2 O 3 is investigated by thermogravimetric measurements. By means of electron microscopy, the surface of the grains of sintered compacts is shown to be already reduced well within the α-Fe 2 O 3 stability region. Based on this information, an electrical conductivity model is presented, in which the inhomogeneous character of the nonstoichiometry of the grains is emphasized.

Preparation of In2O3 single-crystals via chemical transport reaction

Octahedral In2O3-crystals, with a volume of 1 mm3 have been grown from the vapour phase, using HCl as a transporting agent. The HCl pressure must be above a certain level; with low HCl pressures the crystal habit is changed.

The oxygen evolution reaction on cobalt: Part II. Transient measurements

Abstract Open-circuit overpotential decays on an aged cobalt electrode in the oxygen evolution range in 6 M KOH show different slopes for two overpotential regions. These slopes are lower than the Tafel slope in the same region: Tafel slopes of ∼100 and ∼40 mV/dec, at high and low overpotentials, respectively. compared to decay slopes of ∼−60 and ∼−20 to −30 mV/dec. For a fresh cobalt electrode a decay slope of ∼−40 mV/dec is found at high overpotentials. From impedance measurements during a decay it is concluded that the electrode capacitance cannot account for the decay curves obsered. By means of steady-state potentiostatic impedance measurements (with stabilization times > 24 h) it is found that the differential Tafel slope remains constant at ∼40–50 mV/dec and differs considerably from the Tafel slope at high overpotentials, ∼100 mV/dec. Galvanostatic pulse experiments give evidence of the presence of CoO 2 in the oxide layer. Two models which may explain the observed experimental results are analysed. Both include a potential-dependent (extra) process which is fixed by the amount of CoO 2 at the surface. In one model, CoO 2 is responsible for partial surface blockage (parallel process); in the other model, CoO 2 controls the conductivity of the top layer of the oxide layer on the electrode.

Association of defects in lead chloride and lead bromide: Ionic conductivity and dielectric loss measurements

The ionic conductivity data of pure and doped lead bromide without associated defects are used in order to explain the anomalous conductivity behaviour of copper (I) bromide and lead oxide-doped lead-bromide crystals. In these crystals precipitated dopant and associated defects are present. The association enthalpy, and enthalpies| of solution for free defects and for associated defects in lead bromide are calculated from the concentration and temperature dependence of the ionic conductivity, respectively. A detailed comparison has been made between the ionic conductivity data and dielectric loss measurements on both lead chloride and lead bromide. Associated defects of the type (MePb·VX)× and (Ox·Vx)×, (X = Cl, Br) as found from the conductivity data, are confirmed by the dielectric loss measurements. For the copper (I) bromide-doped lead-bromide crystals a relaxation process is proposed because of hopping electrons.