Publications

A mathematical model to simulate the generation of mechanical stress during the discharge process in a dual porous insertion electrode cell sandwich comprised of lithium cobalt oxide and carbon is presented. The model attributes stress buildup within intercalation electrodes to two different aspects: changes in the lattice volume due to intercalation and phase transformation during the charge/discharge process. The model is used to predict the influence of cell design parameters such as thickness, porosity, and particle size of the electrodes on the magnitude of stress generation. The model developed in this study can be used to understand the mechanical degradation in a porous electrode during an intercalation/deintercalation process, and the use of this model results in an improved design for battery electrodes that are mechanically durable over an extended period of operation. © 2009 The Electrochemical Society.

The proper orthogonal decomposition method was applied to develop an efficient, reduced order electrochemical–thermal model for a lithium-ion cell. This model was validated for discharge simulations over a wide range of C rates and various cooling conditions of the cell. The reduced order model agrees well with the COMSOL model, a commercial finite element method solver, and requires times less computation time than the COMSOL model. The model predictions indicate that the discharge time or percent of capacity removed from the cell at an end of discharge voltage of 3.0 V depends on the rate of the discharge and heat transfer rate away from the cell. Also, the heat transfer rate determines whether the capacity removed is limited by mass transfer in the solid phase or mass transfer in the electrolyte.

High power lithium ion batteries are increasingly used in power tools, hybrid electric vehicles and military applications, as a transient power source capable of delivering instant energy, around a relatively fixed state of charge (SOC). Maintaining the battery within pre-specified limits for SOC is important, since lithium ion batteries are prone to safety and/or performance issues during overcharge or rapid discharge below the cut-off voltages. With an increase in the number of cells used in the battery, SOC has a crucial role in cell balancing and optimization of the pack performance. Several techniques have been proposed for the SOC estimation. Most of the existing literature supports an empirical model based on either an electric circuit, arbitrary pole placement or an analytical expression with an arbitrary set of parameters. In spite of their simplicity, the empirical battery models do not provide information on the physical cell limitations. Alternatively, a rigorous electrochemical cell model, aimed at incorporating transport, kinetic and thermodynamic limitations, can be used to estimate parameters that hold a physical significance and hence provide an accurate measure of the cell performance. However, the demand for onboard estimation devices requires estimation techniques that are computationally efficient. In this work, estimation of the SOC of a lithium ion cell using an unscented filtering algorithm is illustrated. The relative advantages and disadvantages of the proposed methodology are briefly discussed. Copyright © 2009 John Wiley & Sons, Ltd.

Areduced-order model ROM is developed using proper orthogonal decomposition POD for a physics-based lithium-ion battery model. The methodology to obtain the proper orthogonal modes and to analyze their optimality is included. The POD-based ROM for a lithium-ion battery is used to simulate a charge/discharge process and the behavior of a battery pack. Compared to the physics-based model, the computational time to solve the ROM is significantly less, and the two models show excellent agreement.

A model is presented for the reduction of hexavalent chromium in a parallel-plate electrochemical reactor via a homogenous reaction between Cr(VI) and Fe(II) generated at the iron anode. The effects of the space velocity of the feed solution, the concentration of supporting electrolyte, the distance between the electrodes, and the cell potential on conversion of Cr(VI) to Cr(III), are discussed. This study indicates that for reduction of Cr(VI) using Fe(II), the space velocity must be maintained below 0.02 s-1 or the system becomes limited by the rate of reduction of Cr(VI) by Fe(II). Increasing the current density by increasing the cell potential, increasing the amount of supporting electrolyte, and decreasing the distance between the electrodes increases single pass conversion of Cr(VI) to Cr(III); however, increasing the current density also increases the specific energy required by the system. © 2009 The Electrochemical Society.

Lithium-ion pouch cells were cycled at five different temperatures (5, 15, 25, 35, and 45°C), and rate capability studies were performed after every hundred cycles. The data were used with a simple physics-based model to estimate parameters that capture the capacity fade in the cell, with cycling. The weight of active material within each electrode was estimated as a function of time, using rate capability data at the C/33 rate. The C-rate for these cells is 1.656 A. The capacity fade due to the loss of active material and that due to the loss of cyclable lithium were quantified. It was found that while the loss of cyclable lithium is the limiting cause for the capacity decay of the cell during the first 200 cycles, the loss of active carbon, which is the anode material, becomes limiting for these cells. The loss of active material leads to a drastic decrease in cell capacity at higher temperatures. © 2008 The Electrochemical Society.

A thermal model for a lithium-ion cell is presented and used to predict discharge performance at different operating temperatures. The results from the simulations are compared to experimental data obtained from lithium-ion pouch cells. The model includes a set of parameters (and their concentration and temperature dependencies) that has been obtained for a lithium-ion cell composed of a mesocarbon microbead anode, LiCoO2 cathode in 1M LiPF6 salt, in a mixture of ethylene carbonate, propylene carbonate, ethyl-methyl carbonate, and diethyl carbonate electrolyte. The parameter set was obtained by comparing the model predictions to the experimental discharge profiles obtained at various temperatures and rates. The concentration and temperature dependence of the extracted parameters were correlated through empirical expressions. Also, the effect of including the thermal dependence of various parameters in the model on the simulated discharge profiles is discussed.

A mathematical model is presented for a complete lithium-sulfur cell. The model includes various electrochemical and chemical (precipitation) reactions, multicomponent transport phenomena in the electrolyte, and the charge transfer within and between solid and liquid phases. A change in the porosity of the porous cathode and separator due to precipitation reactions is also included in the model. The model is used to explain the physical reasons for the two-stage discharge profiles that are typically obtained for lithium-sulfur cells. © 2008 The Electrochemical Society.

A physics-based single particle model was used to simulate the life cycling data of a lithium ion cell. The simulation indicates that there are probably three stages of capacity fade in a lithium ion cell used at low rates. In the first stage, lithium ions are lost to a film formation reaction (e.g. SEI formation) and, consequently, the cathode becomes less intercalated during cycling. In the second stage, the loss of active cathode material outpaces the loss of lithium ions and the cathode gradually becomes more intercalated at the end of discharge. The anode is the limiting electrode in stages one and two and the change in the anode voltage causes the cell to reach end of discharge voltage. In the third stage, the limiting electrode shifts from the anode to the cathode, and the anode becomes increasingly less discharged at the end of discharge. Thus, more and more "cyclable" lithium ions are left inside the anode, which causes additional capacity fade. © 2008 Elsevier B.V. All rights reserved.

The low rate (C/33) discharge data obtained from a calendar life study were numerically analyzed with a single particle model. The simulation showed that the stoichiometric window for the cathode shrank with capacity fade. The change of the stoichiometric window for the anode was more complicated. The aged anode became less charged when the capacity fade was caused mostly by the loss of cyclable lithium ions. The anode would be charged to a higher stoichiometric number (or state of charge, SOC) when the capacity fade became controlled by the loss of active material in the anode. © 2007 Elsevier B.V. All rights reserved.