Publications

Computer simulations are compared with experimental data for Bellcore PLION cells using the graphite/1 M LiPF6 in EC:DMC(2:1)/LiMn2O4 system. The motivation is to model lithium-ion polymer cells having higher active material loadings and competitive energy densities and specific energies to liquid lithium-ion batteries. Cells with different electrode thickness, initial salt concentrations, and higher active material loadings were examined using the mathematical model to understand better the transport processes in the plasticized polymer electrolyte system. A better description of the ionic conductivity is employed based on new conductivity data. Improvements in the agreement between the simulations and experimental data are obtained by using the contact resistance at the current collector/electrode interface as an adjustable parameter for different cells, whose values vary from 20 to 35 $Ømega$ cm2 (based on separator area). The contact resistance is believed to originate at the mesh current collector interfaces. Reducing the salt diffusion coefficient by a factor of two or more at the higher discharge rates was necessary to obtain better agreement with the experimental data. Based on the experimental data and model predictions from this study, it can be concluded that the solution-phase diffusion limitations are the major limiting factor during high-rate discharges.

A mathematical model of an electrochemical capacitor with hydrous ruthenium oxide (RuO 2 ·xH 2 O) electrodes including both double-layer and surface faradaic processes is developed to predict the behavior of the capacitor under conditions of galvanostatic charge and discharge. The effect of RuO 2 ·xH 2 O particle size is studied and shows that the smaller the particles the better the performance because of the increased surface area per unit volume or mass. The model also predicts that the faradaic process increases significantly the energy per unit volume of the capacitor for power densities of 100 kW/L or less.

The processes that lead to capacity fading affect severely the cycle life and rate behavior of lithium-ion cells. One such process is the overcharge of the negative electrode causing lithium deposition, which can lead to capacity losses including a loss of active lithium and electrolyte and represents a potential safety hazard. A mathematical model is presented to predict lithium deposition on the negative electrode under a variety of operating conditions. The LixC6 |1 M LiPF6, 2:1 ethylene carbonate/dimethyl carbon- ate, poly(vinylidene fluoride-hexalfuoropropylene)|LiMn2O4 cell is simulated to investigate the influence of lithium deposition on the charging behavior of intercalation electrodes. The model is used to study the effect of key design parameters (particle size, elec- trode thickness, and mass ratio) on the lithium deposition overcharge reaction. The model predictions are compared for coke and graphite-based negative electrodes. The cycling behavior of these cells is simulated before and after overcharge to understand the effect of overcharge on extended cycling. These results can be used to establish operational and design limits within which safety hazards and capacity fade problems, inherent in these cells, can be minimized

LaNi4.27Sn0.24 electrodes were characterized using electrochemical techniques at different alloy weights and binder contents. For a given alloy weight, the polarization resistance (Rp) increases with the state of charge (SOC). This arises due to changes from $\alpha$ to $\beta$ phase at the alloy surface. The electroactive surface area for the hydrogen adsorption/desorption reaction changes with SOC and this also contributes to the variation of Rp. Since the interfacial area increases with alloy content, the polarization resistance decreases with increase in the alloy weight. An increase in the alloy weight reduces Rp and lowers the total resistance. The electrode utilization decreases by increasing the binder content and the electrode weight. A theoretical model is presented to study the effect of alloy weight and particle size on the electrode performance. The model simulations predict lowering of the utilization with increase in the electrode weight. The effect of particle size on the energy and power density of the electrode was also studied.

LaNi4.27Sn0.24 alloy was microencapsulated with cobalt by electroless deposition. The coated material has a higher capacity compared to the bare alloy due to the faradaic reaction of cobalt during discharge. This additional capacity has been studied using various material and electrochemical characterization techniques. The capacity due to cobalt varies depending on the amount of active material available for reaction. An increase in utilization is seen with decrease in thickness of the coating. Active surface area and the transport process within the film control the amount of cobalt utilized. Finally, cobalt coated alloys are seen to cycle ten times more than bare LaNi4.24Sn0.27 with constant capacity. © 1999 Elsevier Science S.A. All rights reserved.

Cobalt doped chromium oxides were synthesized and characterized as cathode materials for secondary lithium batteries. A small amount of Co as dopant in the chromium oxides provides greater stability to the structure of CrOx and helps in improving the high-rate behavior of these oxides. Optimized Co 0.2CrOx exhibited an initial capacity of 290 mAh/g with an average discharge voltage of 3.0 V vs. Li/Li+. The lithiated Co 0.2CrOx exhibited an initial capacity of 230 mAh/g. Both lithiated and non-lithated Co0.2CrOx were found to be reversible in the entire intercalation range (2.0 - 4.2 V vs. Li/Li +). Compared with pure CrOx, Co doped CrOx is characterized by better high-rate behavior. (\textgreater 85% capacity for Co 0.2CrOx and \textgreater 75% capacity for LiCo 0.2CrOx at 0.65 C discharge rate). Copyright © 1999 Society of Automotive Engineers, Inc.

A novel electrochemical detection technique was developed for selective detection of nerve gases based on double-potential-step chronoamperometry (DPSCA) and the oxidation of nerve gas by electrochemically generated iodine. Diethyl cyanophosphonate (DECP) was used as a nerve gas mimic. The double-potential-step chronoamperometry has a detection limit of 2 × 10-6 M for DECP and is more sensitive than the currently available electrochemical techniques for detection of nerve gases. The DPSCA technique, which uses the ratio of anodic to cathodic currents as a measuring parameter, was found to be more sensitive than the single-potential-step technique. Changes in the area and activity of the electrode and the concentration of the catalyst do not influence the current ratio, which contributes to the high reproducibility of the DPSCA technique.

This paper presents a method for obtaining series solutions for boundary value problems (BVPs). The technique consists of converting the given two point BVP into an initial value problem (IVP). This IVP is then solved using the successive substitution method (SSM) with the boundary condition at the other endpoint as an additional constraint. The series solutions obtained by this process depend on both the independent variable and the parameters (such as reaction rate constants) that appear in the governing equations. The method is illustrated for both linear and nonlinear problems.

A method is introduced to allow the calculation of differential diffusion coefficients from integral diffusion coefficient data collected with diaphragm cells below 0 degrees C. The method is demonstrated with concentrated potassium hydroxide diaphragm cell data at -15 degrees C and with published data from Stokes. Differential diffusion coefficients, calculated using Stokes method and the proposed method, are compared for the HCl-water system at 25 degrees C. (C) 1999 The Electrochemical Society. S0013-4651(98)05-008-3. All rights reserved.