Ralph E. White

Modern Aspects of Electrochemistry

This book focuses on topics at the forefront of electrochemical research. Splitting water by electrolysis; splitting water by visible light; the recent development of lithium batteries; theoretical approaches to intercalation; and fundamental concepts of electrode kinetics, particularly as applied to semiconductors are discussed. It is recommended for electrochemists, physical chemists, corrosion scientists, and those working in the fields of analytical chemistry, surface and colloid science, materials science, electrical engineering, and chemical engineering.

A Water and Heat Management Model for Proton‐Exchange‐Membrane Fuel Cells

Proper water and heat management are essential for obtaining high-power-density performance at high energy efficiency for proton-exchange-membrane fuel cells. A water and heat management model was developed and used to investigate the effectiveness of various humidification designs. The model accounts for water transport across the membrane by electro-osmosis and diffusion, heat transfer from the solid phase to the gas phase and latent heat associated with water evaporation and condensation in the flow channels. Results from the model showed that at high current (> 1A/cm[sup 2]) ohmic loss in the membrane accounts for a large fraction of the voltage loss in the cell and back diffusion of water from the cathode side of the membrane is insufficient to keep the membrane hydrated (i.e., conductive). Consequently, to minimize this ohmic loss the anode stream must be humidified, and when air is used instead of pure oxygen the cathode stream must also be humidified.

Capacity Fade Mechanisms and Side Reactions in Lithium‐Ion Batteries

The capacity of a lithium-ion battery decreases during cycling. This capacity loss or fade occurs due to several different 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 capacity 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.

Development of First Principles Capacity Fade Model for Li-Ion Cells

A first principles-based model has been developed to simulate the capacity fade of Li-ion batteries. Incorporation of a continuous occurrence of the solvent reduction reaction during constant current and constant voltage (CC-CV) charging explains the capacity fade of the battery. The effect of parameters such as end of charge voltage and depth of discharge, the film resistance, the exchange current density, and the over voltage of the parasitic reaction on the capacity fade and battery performance were studied qualitatively. The parameters that were updated for every cycle as a result of the side reaction were state-of-charge of the electrode materials and the film resistance, both estimated at the end of CC-CV charging. The effect of rate of solvent reduction reaction and the conductivity of the film formed were also studied.

Power and life extension of battery-ultracapacitor hybrids

The performance of a battery-ultracapacitor hybrid power source under pulsed load conditions is analytically described using simplified models. We show that peak power can be greatly enhanced, internal losses can be considerably reduced, and that discharge life of the battery is extended. Greatest benefits are seen when the load pulse rate is higher than the system eigenfrequency and when the pulse duty is small. Actual benefits are substantial; adding a 23 F ultracapacitor bank (3 /spl times/ 7 PC10 ultracapacitors) in parallel with a typical Li-ion battery of 7.2 V and 1.35 A hr capacity can boost the peak power capacity by 5 times and reduce the power loss by 74%, while minimally impacting system volume and weight, for pulsed loads of 5 A, 1 Hz repetition rate, and 10% duty.

Mathematical modeling of the capacity fade of Li-ion cells

A capacity fade prediction model has been developed for Li-ion cells based on a semi-empirical approach. Correlations for variation of capacity fade parameters with cycling were obtained with two different approaches. The first approach takes into account only the active material loss, while the second approach includes rate capability losses too. Both methods use correlations for variation of the film resistance with cycling. The state of charge (SOC) of the limiting electrode accounts for the active material loss. The diffusion coefficient of the limiting electrode was the parameter to account for the rate capability losses during cycling.

Mathematical modeling of lithium-ion and nickel battery systems

A review of mathematical models of lithium and nickel battery systems developed at the University of South Carolina is presented. Models of Li/Li-ion batteries are reviewed that simulated the behavior of single electrode particles, single electrodes, full cells and batteries (sets of full cells) under a variety of operating conditions (e.g. constant current discharge, pulse discharge, impedance and cyclic voltammetry). Models of nickel battery systems are reviewed that simulate the performance of full cells, as well as the behavior of the nickel hydroxide active material. The ability of these models to predict reality is demonstrated by frequent comparisons with experimental data.

Studies on Capacity Fade of Lithium-Ion Batteries

The capacity fade of Sony 18650S Li-ion cells has been analyzed using cyclic voltammetry, impedance spectroscopy and electron . . . probe microscopic analysis EPMA . The surface resistance at both the positive LiCoO and negative carbon electrodes were found to 2 increase with cycling. This increase in resistance contributes to decreased capacity. Impedance data reveal that the interfacial resistance at . LiCoO electrode is larger than that at the carbon electrode. The impedance of the positive electrode LiCoO dominates the total cell 2 2 resistance initially and also after 800 charge-discharge cycles. EPMA analysis on carbon electrodes taken from the fresh and cycled cell show the presence of oxidation products in the case of cycled cells. No change in the electrolyte resistance is seen with cycling. q 2000 Elsevier Science S.A. All rights reserved.

Solvent Diffusion Model for Aging of Lithium-Ion Battery Cells

This work presents a rigorous continuum mechanics model of solvent diffusion describing the growth of solid-electrolyte interfaces ~SEIs! in Li-ion cells incorporating carbon anodes. The model assumes that a reactive solvent component diffuses through the SEI and undergoes two-electron reduction at the carbon-SEI interface. Solvent reduction produces an insoluble product, resulting in increasing SEI thickness. The model predicts that the SEI thickness increases linearly with the square root of time. Experimental data from the literature for capacity loss in two types of prototype Li-ion cells validates the solvent diffusion model. We use the model to estimate SEI thickness and extract solvent diffusivity values from the capacity loss data. Solvent diffusivity values have an Arrhenius temperature dependence consistent with solvent diffusion through a solid SEI. The magnitudes of the diffusivities and activation energies are comparable to literature values for hydrocarbon diffusion in carbon molecular sieves and zeolites. These findings, viewed in the context of recent SEI morphology studies, suggest that the SEI may be viewed as a single layer with both micro- and macroporosity that controls the ingress of electrolyte, anode passivation by the SEI, and cell perfor

Comparison between computer simulations and experimental data for high-rate discharges of plastic lithium-ion batteries

Abstract 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 Ω 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.