Publications

Chromium oxides and lithiated chromium oxides were synthesized by thermal decomposition of chromium trioxide (CrO3) at high temperatures and oxygen pressures. Synthesis temperature and pressure markedly affect the performance of these cathode materials. Higher pressures lead to a higher O/Cr ratio and fewer impurities in the final product. These materials are stable intercalation hosts for lithium, and exhibit a higher capacity than any of the prominent positive electrodes used in secondary lithium batteries. m-CrOx has a capacity of 255 mAh/g, while m-LiCrOx has a capacity of 210 mAh/g, during the first discharge. The average voltage of these cells is 3.0 V vs. Li/Li+ that gives an average energy density of approximately 650 Wh/Kg. © 1998 The Electrochemical Society. S1099-0062(98)07-090-4. All rights reserved.

LaNi 4.17 Sn 0.24 allov was microencapsulated with cobalt by electroless deposition from an alkaline hypophosphite bath. Discharge curves of the encapsulated alloy indicate an additional contribution to the capacity arising from the cobalt on the surface. Studies on cobalt thin films reveal the presence of adsorbed hydrogen in cobalt. The amount of hydrogen adsorbed was observed to increase with time of cathodic polarization and to reach a maximum. Polarization techniques have been used to characterize the cobalt-plated alloy as a function of state of charge. The equilibrium potential of the microencapsulated electrode at low hydrogen concentration is determined by the potential of the cobalt coating on the surface.

A mathematical model was developed for the characterization of hydrogen permeation into metals and alloys under corroding conditions. The model requires a knowledge of the corrosion rate of the given metal or alloy in solutions of various H + concentrations. It provides quantitative values of rate constants of the hydrogen discharge, recombination, and adsorption reactions. This model can serve as a diagnostic criterion for determining the effectiveness of various metals and alloys as hydrogen permeation inhibitors and can be used for evaluation of hydrogen entry inhibition efficiency. Experiments were carried out to study the effectiveness of thin layers of electrodeposited Zn-Ni alloys to inhibit hydrogen permeation into iron. The experimental permeation data obtained for Zn-Ni-coated iron were analyzed using the model and the results were compared with the hydrogen permeation characteristics of bare iron.

A theoretical model for the metal hydride electrode has been developed assuming that hydrogen diffusion in the alloy and charge-transfer at the surface control the discharge process. Theoretical equations for the dependence of equilibrium potential and exchange current density on the surface hydrogen concentration have been derived. These parameters have been used to correlate experimental data with the theoretical electrode discharge model. Analysis of both the experimental and theoretical discharge curves reveals a potential plateau determined by the magnitude of the interactions between the hydrogen in the alloy and the unhydrided metal. Neglecting these hydrogen-metal site interactions results in simulations predicting the electrode potential varying over the entire duration of discharge. The results also indicate that utilization of the electrode is controlled by the rate of hydrogen diffusion in the electrode and by the alloy particle size. Kinetic resistance at the surface is a determining factor of the polarization losses of the electrode. The variation of equilibrium potential and exchange current density with the state of charge has been characterized experimentally. These results are compared with the model predictions, and good agreement is seen.

Cement stabilization of backfill has been used for some time in mechanically stabilized earth type retaining walls. However, there has been no data on the corrosion life of galvanized steel reinforcement in this environment, which is intermediate in pH between normal soil and pure concrete. Field observations had indicated a potential corrosion problem at a particular site in Deer Park, Texas. Cement addition to backfill in the usual quantities (i.e., 7 percent or more) raised the pH environment to values close to that of normal concrete. At these levels corrosion rates of zinc coatings were not significantly accelerated - if anything, corrosion rates were less than for unstabilized fill. Very small amounts of cement addition, in the order of 1 to 4 percent producing pH values significantly less than 12, could cause limited acceleration of corrosion. It is, therefore, advisable to control minimum cement levels and to encourage efficient mixing. The use of crushed concrete as backfill did not accelerate corrosion. This material, therefore, appears to be acceptable for this application.

Solution equilibrium characteristics of two electroless copper baths containing EDTA and tartrate as the complexing agents were studied as functions of pH. Equilibrium diagrams were constructed for both Cu-tartrate and Cu-EDTA systems. It was determined that copper is chiefly complexed as Cu2L2 in acidic conditions and as Cu(OH)2L2-4 in alkaline conditions in the tartrate bath, and as CuA-2 in the EDTA bath, where L and A are the complexing tartrate and EDTA ligands, respectively. Electroless copper deposition rates were studied from a tartrate bath on thermally activated palladium-catalysed polyimide substrates as functions of copper and formaldehyde concentrations, and pH. © 1998 Chapman & Hall.

Electrochemical investigations were carried out on electrodes made from both bare and Pd-coated LaNi4.25Al0.75 particles. Experimental results showed that the Pd-coating significantly decreases the electrode resistance and increases the electrode discharge capacity. The electrochemical impedance spectroscopy technique was used to determine various resistive components in the electrodes and electrolyte by fitting an equivalent circuit to the experimental data. The results indicated that an electrode made from the Pd-coated alloy has much less ohmic resistance (particle-to-particle contact resistance and current collector to electrode pellet contact resistance) compared to the electrodes made from bare alloy.

We investigate the effect of particle size on the discharge performance of a nickel-metal hydride cell with a mathematical model. Electrodes with uniform as well as with nonuniform particle sizes are studied. With uniform particle size, the dependence of the particle-to-particle resistance on the particle size is taken into account. The optimal particle size depends on the discharge rate. Moreover, we show that under certain conditions it is advantageous to use a nonuniform particle size. In general, the higher the discharge current density, the more the particle size affects the electrode performance.

The capacity of a lithium-ion battery decreases during cycling. This capacity loss or fade occurs due to several dif- ferent mechanisms which are due to or are associated with unwanted side reactions that occur in these batteries. These reactions occur during overcharge or overdischarge and cause electrolyte decomposition, passive film formation, active material dissolution, and other phenomena. These capacity loss mechanisms are not included in the present lithium-ion battery mathematical models available in the open literature. Consequently, these models cannot be used to predict cell performance during cycling and under abuse conditions. This article presents a review of the current literature on capac- ity fade mechanisms and attempts to describe the information needed and the directions that may be taken to include these mechanisms in advanced lithium-ion battery models.

Cobalt coatings on metal hydrides give rise to an additional capacity due to the cobalt on the surface of the alloy. In such a case, the galvanostatic discharge technique cannot be used to measure the diffusion coefficient of hydrogen in the alloy. We present here an analytical impedance model of the metal hydride electrode. This simple model is used to calculate the diffusion coefficient of hydrogen in the metal alloy. The impedance response of cobalt microencapsulated LaNi4.27Sn0.24 electrode was measured at different hydrogen contents. From the slope of the Nyquist plot in the transition region the diffusion coefficient of hydrogen was calculated at various states of charge (SOC). It is seen that the diffusion coefficient increases with the hydrogen content in the alloy. The technique applied for cobalt encapsulated LaNi4.27Sn0.24 could however be used for any hydrogen storage alloy. © 1998 Elsevier Science S.A. All rights reserved.