Publications

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.

Staging is a characteristic phenomena observed in intercalation electrodes. During staging process the equilibrium potential of the electrode exhibits weak dependence on the solid phase Li concentration and does not follow the classical Nernst behavior. The coexistence of structurally different solid phases results in multiple plateaus in the equilibrium potential curve. Such complexities make the thermodynamic description of equilibrium potential as a function of concentration difficult, and so it is usually represented through an empirical expression. The objective of this work is to develop a frame work based on thermodynamic principles to describe the equilibrium potential of intercalation electrodes. Redlich-Kister thermodynamic equation was used to describe the excess Gibbs free energy, which in turn was used to evaluate the equilibrium potential as a function of concentration. The equilibrium potential expression for different lithium intercalation electrodes such as LiCoO2, LiNi0.8Co0.15Al0.05O2, graphite and hard carbon were developed based on Redlich-Kister equation. The thermodynamic model was also used to estimate the activity of species directly from excess Gibbs free energy. The developed thermodynamic expressions along with the activity correction are incorporated into a single particle diffusion model for a Li-ion cell consisting of a graphite and LiCoO2 electrode. The interactions between the Li-ions during intercalation/deintercalation process were incorporated into the present model by considering the chemical potential gradient corrected for activity as the driving force. The effect of inclusion of activity correction in the single particle model was studied for different discharge rates. It was observed that the activity correction term yielded increased capacity especially at higher rates. The effect of activity correction term was also found to be more significant in the LiCoO2 electrode compared to the carbon electrode. © 2008 Elsevier B.V. All rights reserved.

The velocity and pressure profiles in the electrolyte due to a rotating disk electrode are determined by solving the two-dimensional (2D) Navier-Stokes equations employing axial symmetry. Most applications in the literature employ a one-term approximation of the series solution to von K\ arm\ an s one-dimensional (1D) model for the rotating disk electrode. In this work, the finite-element method is used to solve the model equations rigorously within an electrochemical cell of practical dimensions, and the results are compared with the one-term approximate solution and a complete solution to the 1D model. The different hydrodynamic models are coupled with a mass-transport model for oxygen reduction reaction at the surface of the rotating ring disk electrode. The complete series solution is accurate to within four digits when compared to the 2D model, whereas the one-term approximation gives rise to an error as high as 4% in the limiting current values. Similar calculations on a ring disk electrode show that the one-term approximation underestimates the collection efficiency and the ratio of the ring and the disk currents of a sectioned electrode by 1-4%. © 2008 The Electrochemical Society.

The experimental charge and discharge profiles of a LiCoO2 electrode show that the overpotential of the electrode does not change much during galvanostatic charge, but changes significantly during galvanostatic discharge. Semi-empirical porous electrode models are presented to simulate the charge and discharge profiles of the LiCoO2 electrode. The symmetry factor is empirically assumed to decrease with the state of discharge of the electrode to enable the model predictions to agree well with the experimental discharge profiles. © 2006 Elsevier B.V. All rights reserved.

Approximation methods are often used in porous electrode models to eliminate the need to solve the local solid phase diffusion equation. These methods include Duhamel s superposition method, a diffusion length method and a polynomial approximation method which have long been used in the literature. The pseudo steady state (PSS) method is a method that has been used recently to develop a solution to the diffusion equation in a spherical particle with time dependent boundary conditions, but the PSS method has not been used in a porous electrode model. These methods are compared to each other in a dimensionless analysis study, and they are used in a porous electrode model to predict the discharge curves for a LiCoO2 electrode. Simulation results presented here indicate that the PSS method or the high order polynomial method should be used in a porous electrode model to obtain accuracy and save computation time. © 2007 Elsevier B.V. All rights reserved.

A calendar life study was conducted on lithium ion pouch cells which were stored under float charge condition at five temperatures. The half cell study showed that the anode experienced severe loss of the active material, especially at high temperatures. The capacity fade mechanisms were then proposed. The capacity fade at low temperatures could mostly be caused by the loss of lithium inventory to side reactions and impedance increase. The capacity fade at high temperatures demonstrated a two-regime pattern. The fading mechanisms in the first regime could be similar to those at low temperatures. The capacity fade in the second regime could be dominated by the severe loss of active carbon material. The impedance rise plays a minor role in the second capacity fade regime. © 2007 Elsevier B.V. All rights reserved.

A short-time transient analysis is presented for a sinusoidal input potential for a spherical particle. The objective of this work was to extract accurate values of the parameters associated with an intercalation into a spherical particle. These parameters are exchange current density, double-layer capacitance, and diffusion coefficient. The effects of these parameters on the response were examined using a sensitivity analysis, which indicated that optimum frequency values of the input perturbation exist for estimation of these parameters. A procedure is presented to obtain all these parameters using the short-time response. The results show that the short-time analysis is a useful method for estimating rapidly the values of these parameters of a system. (c) 2007 The Electrochemical Society.

Two different models were used to obtain transport and kinetic parameters using nonlinear regression from experimental charge/discharge curves of a lithium-ion cell measured at 35°C under four rates, C/5, C/2, 1C, and 2C, where the C rate is 1.656 A. The Levenberg-Marquardt method was used to estimate parameters in the models such as the diffusion of lithium ions in the positive electrode. A confidence interval for each parameter was also presented. The parameter values lie within their confidence intervals. The use of statistical weights to correct for the scatter in experimental data as well as to treat one set of data in preference to other is illustrated. An F-test was performed to discriminate between the goodness of fit obtained from the two models. © 2007 The Electrochemical Society.

A cylindrical two-dimensional model based on the Nernst-Planck equations, the Navier-Stokes equation, and the continuity equation is used to simulate the oxygen reduction reaction in 0.5 M H2 SO4 at a rotating ring disk electrode. Concentration distributions and a potential profile are obtained as a function of the axial and radial distances from the center of the electrode surface. Polarization curves are simulated to interpret experimental results by studying various reaction mechanisms, i.e., the four-electron-transfer reduction of oxygen, the two-electron-transfer reduction of oxygen, a combination of the above two reactions, mechanisms with reduction of peroxide to water, and/or the heterogeneous chemical decomposition of peroxide. Special attention is devoted to the effect of peroxide. © 2007 The Electrochemical Society.

A moving boundary model in a spherical LiCoO2 particle is presented to account for the diffusion controlled phase transition in LiCoO2 solid particles, and this model is incorporated into a porous electrode model for the LiCoO2 electrode. The simulation results agree well with the experimental data of a LiCoO2 electrode. A study of the flux distribution in the porous electrode shows that the phase transition phenomenon in the LiCoO2 particles has a significant effect on the flux distribution by changing the solid phase diffusion resistance in the particles.