Publications

A one-dimensional model for predicting the performance of a battery/electrochemical capacitor-hybrid system has been developed. Simulation results are presented for a LiCoO 2 |LiPF 6 ethylene carbonate/dimethyl carbonate|carbon battery system and a Maxwell PC 10F carbon double-layer electrochemical capacitor. The current shared between the battery and the electrochemical capacitor at very short times depends on the ohmic resistances of the battery and the capacitor. As the discharge proceeds, the operating conditions such as frequency, duty ratio, and peak pulse discharge current control the current shared among parallel circuits. These parameters also determine the extent of the run time increase of the hybrid system as compared to the battery system. The inclusion of a number of identical series/parallel capacitors is considered in the present model by introducing the parameter, capacitor configuration index. Ragone plots are simulated for a battery-alone and a hybrid system. A substantial improvement in the available energy density is observed while operating hybrid systems under high power densities. Finally, a general optimization approach is presented. © 2005 The Electrochemical Society. All rights reserved.

An accelerated method was used to determine the effect of temperature, end-of-charge voltage and the type of storage condition over the performance pouch lithium-ion cells. The cells were studied for 4.0 V and 4.2 V end-of-charge voltages (EOCV) both at 5 °C and 35 °C. The irreversible capacity loss of the cell was analyzed every month using a capacity measurement protocol. The results indicated that higher temperature and voltage accelerates the degradation of the cells. The open circuit voltage (OCV) decay of the cells stored under open circuit conditions was also analyzed. The reasons for the irreversible capacity loss, energy loss, OCV decay and the increase in the internal resistance of the cell are discussed in detail. The most detrimental storage condition and the most mild storage condition are identified and discussed in detail. © 2004 Elsevier B.V. All rights reserved.

An analytical expression is presented for the voltage response including the transient voltage for a simple (i.e., no concentration gradients) porous electrode model subject to a sinusoidal input current density. The transient voltage response as a function of the frequency, exchange current density, and double layer capacitance is studied independent of the periodic (steady state) voltage response. The change in the voltage response in the transient region is compared to that of the periodic voltage response with respect to the parameters. The physical properties of the porous electrode can be estimated using the voltage response in the transient region is presented. The methodology for doing this is described.

The virtual test bed (VTB) computation environment provides an easy-to-use way for electric system modeling and simulation. Resistive companion (RC) modeling of batteries in VTB is presented in the paper. Native RC battery modeling approach and the one using current controlled voltage source (CCVS) interface are presented and compared through two battery models with different degrees of complexity. The RC battery models are validated by comparing VTB simulation data to those calculated directly through stand-alone Fortran codes. It is concluded that using CCVS interface is currently the first choice in modeling batteries in VTB. Simulations using the RC battery models in VTB are also presented and analyzed. It is shown that RC modeling provides a powerful way for the simulation of battery systems in VTB. © 2004 Elsevier B.V. All rights reserved.

A cubic spline regression model was used to fit the experimental open-circuit potential (OCP) curves of two intercalation electrodes of a lithium-ion battery. All the details of in OCP curve were accurately predicted by the resulting model. The number of regression intervals used to fit an OCP curve was determined in a way such that in each regression interval the OCP exhibits a profile predictable by a third-order polynomial. The locations of the data points used to separate regression intervals were optimized. Compared to a polynomial model with the same number of fitting parameters, the cubic spline regression model is more accurate. The cubic spline regression model presented here can be used conveniently to fit complicated profiles such as the OCP curves of lithium-ion battery electrodes. © 2004 The Electrochemical Society. All rights reserved.

Two parameter estimation methods are presented for online determination of parameter values using a simple charge/discharge model of a Sony 18650 lithium ion battery. Loss of capacity and resistance increase are both included in the model. The first method is a hybrid combination of batch data reconciliation and moving-horizon parameter estimation. A discussion on the selection of tuning parameters for this method based on confidence intervals is included. The second method uses batch data reconciliation followed by application of discrete filtering of the resulting parameters. These methods are demonstrated using cycling data from an experimental cell with over 1600 charge-discharge cycles. © 2005 Elsevier B.V. All rights reserved.

A series solution to the transient convective diffusion equation for the rotating disc electrode system is presented and compared to previously reported solutions. The solution presented here is for the entire time domain and agrees well with both the short and long time solutions presented earlier in the literature. © 2004 The Electrochemical Society. All rights reserved.

A boundary condition for electrolyte concentration at the porous electrode/separator interface is developed. This boundary condition helps predict the electrolyte concentration profile in the porous electrode without having to solve for the concentration profile in the separator. © 2004 The Electrochemical Society All rights reserved.

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. © 2004 The Electrochemical Society. All rights reserved.

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 performance during initial cycling as well as long-term operation. 2004 The Electrochemical Society. All rights reserved.