Publications

In this work, the role of carbon in alkaline oxygen electrocatalysis was investigated using an oxygen-deficient, perovskite-structured oxide catalyst, La0.6Sr0.4CoO3−$δ$ (LSCO), mixed in different mass ratios with Vulcan carbon (XC-72R, referred to as XC-72) to observe the effect of the LSCO-to-XC-72 ratio (LSCO/XC-72) on the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics. Six different LSCO/XC-72 mass ratios were designed for the study as follows: 1:0 (pure LSCO), 10:1, 7:1, 5:1, 2:1 and 1:1. A thin-film oxygen electrode consisting of the LSCO/XC-72 catalysts was characterized in 0.1 M KOH solution using cyclic voltammetry (CV), rotating disk electrode (RDE) based linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). A physics-based, generalized electrochemical model was used to establish the charge-transfer mechanism and to obtain the electrode kinetic transfer coefficients and exchange current densities for the electrochemical reactions considered. The results show that the oxygen electrocatalysis process depends quite heavily on the carbon content in the catalyst, and there appears to be a synergistic effect between the perovskite-oxide and the XC-72, with a transition from the two-step, 2e− pathway to the direct 4e− transfer pathway as carbon content increases.

A mixed mode solid electrolyte interphase (SEI) growth model is presented that includes both the influence of the kinetics of the solvent reduction reaction at the SEI/electrode interface and diffusion of the solvent through the SEI layer. The governing equations are solved numerically to predict the solvent concentration profile, the SEI layer thickness, and the capacity loss. Capacity loss predictions are fitted to two sets of experimental data from the literature. The high quality of the fits demonstrates that the mixed mode model provides a useful description of the capacity loss in the cell due to the growth of the SEI layer on the anode under constant voltage storage condition. Also, analytical expressions are presented for SEI growth for both the combined kinetics/diffusion mode (mixed mode model) and the limit of a fast solvent reduction reaction (diffusion limited mode model) as a function of cell characteristics.

Six experiments are presented to validate the testing methods given in TR-propagation regulation (TF5) being developed by The Electrical Vehicle Safety - Global Technical Regulation (EVS-GTR). The novel testing methods in TF5 draft are present here and used on six different battery packs exposed to thermal runaway. The characteristics of the thermal runaway propagation of the EV battery packs with different designs are investigated. Some of the experiments use the same sample with different heater powers. The result indicated that heater power does not affect the maximum temperature very much, but rather the propagation time. The feasibility of the test approach is demonstrated by the test results. However, the repeatability of the test approach requires improvement.

Alkaline-based metal-air batteries require a low-resistance, bifunctional oxygen electrode to perform fast oxygen reduction and oxygen evolution reactions (ORR/OER) during discharging and charging cycles, respectively. However, achieving good ORR/OER bifunctionality with single materials has proven challenging. Here we report a composite material as a bifunctional oxygen electrode for ORR and OER. The composite oxygen electrode consists of an ORR-active 20 wt% Pt/C and an OER-active perovskite La0.6Sr0.4CoO3-$δ$ (LSCO). The study focuses on identifying the optimal LSCO:Pt/C ratio through electrochemical DC voltammetry techniques. The results show that the addition of Pt/C into the LSCO catalyst greatly enhances the ORR activity, due to Pt/C s superior ORR capabilities, while LSCO retains good OER performance, which is known to be poor for Pt/C. The optimal LSCO:Pt/C ratio among the seven compositions studied is found to be 60:40 (wt%) and is tested for stability through using a square-wave potentiostatic method. Overall, this study demonstrates a synergetic effect between LSCO and Pt/C, with each one contributing towards one of the electrode reactions in a positive manner.

In this work, a perovskite-structured and oxygen-deficient oxide, La0.6Sr0.4CoO3-$δ$ (LSCO), has been investigated as a model bifunctional thin-film oxygen electrode for alkaline metal-air cells. The rotating disk electrode (RDE) configuration in combination with common electrochemical techniques such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were applied to characterize the behavior of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) taking place on LSCO in 0.1 M KOH solution. The results show that the oxygen electrocatalysis process in LSCO follows a multistep charge-transfer pathway. A physics-based, generalized electrochemical model, encompassing two sequential 2e- steps with HO2- as an intermediate species and one parallel 4e- step, has been established to account for the multistep charge-transfer behavior with very satisfactory results, yielding a series of important electrode kinetic transfer coefficients and exchange current densities for the elementary electrochemical reactions considered. Finally, LSCO is found to be a better oxygen electrode for OER than ORR.

Oriented one-dimensional nanostructures have been of substantial interest as electrodes for lithium-ion batteries due to the better performance both in terms of initial capacity and lower capacity fade compared to powder pressed electrodes. This paper focuses on a model driven approach to understanding the relationship between the morphology of these oriented nanostructures to the performance of the battery. The Newman-type P2D modeling technique is applied to a porous electrode made up with solid continuous cylinders that extends from the current collectors to separator. TiO2 columnar nanostructures of varying heights were synthesized using the aerosol chemical vapor deposition (ACVD) and their performance as electrodes in a lithium-ion battery was measured. This electrochemical transport model was validated with the experimental data. This model was used to understand the role of transport parameters, including the diffusivity of lithium in the TiO2 and the electronic conductivity of the TiO2 columns, and structural parameters, including the height of the columns and the porosity of the electrode, on the areal capacity of a lithium ion battery at different rates of discharge. The model enables for the prediction of optimized structural parameters of one-dimensional electrodes tailored to the desired application of lithium and sodium-ion batteries.

The finite element method (FEM) was used in our nonlinear state-variable method (NSVM) presented recently (J. Electrochem. Soc., 164, E3001 (2017)). The details of the application of the FEM to solve the lithium ion pseudo-2D (P2D) model equations using the NSVM are presented here for several control modes (constant current, voltage, power, or load). Validation of the method was performed by comparison to rigorous full-ordermodels and experimental data. The FEM based NSVM shows excellent performance, and the estimated cell parameters are determined with a high confidence level. (C) The Author(s) 2017. Published by ECS. All rights reserved.

This paper presents a numerical model used for analyzing heat propagation as a safety feature in a custom-made battery pack. The pack uses a novel technology consisting of an internal short circuit device implanted in a cell to trigger thermal runaway. The goal of the study is to investigate the importance of wrapping cylindrical battery cells (18650 type) in a thermally and electrically insulating mica sleeve, to fix the cells in a thermally conductive aluminum heat sink. By modeling the full-scale pack using a 2D model and coupling the thermal model with an electrochemical model, good agreement with a 3D model and experimental data was found (less than 6%). The 2D modeling approach also reduces the computation time considerably (from 11 h to 25 min) compared to using a 3D model. The results showed that the air trapped between the cell and the boreholes of the heat sink provides a good insulation which reduces the temperature of the adjacent cells during thermal runaway. At the same time, a highly conductive matrix dissipates the heat throughout its thermal mass, reducing the temperature even further. It was found that for designing a safe battery pack which mitigates thermal runaway propagation, a combination of small insulating layers wrapped around the cells, and a conductive heat sink is beneficial.

In this work, a reduced-order-modeling(ROM) approach is introduced to simulate a battery system with multiple cells. The ROM is described by only two state variable equations and several explicit expressions.While running a stand-alone pseudo-2D (P2D) model by the nonlinear state variable modeling (NSVM) algorithm at a specified current, several parameters can be estimated through the NSVM procedure; and with these parameters, the cell voltage under additional current perturbations can be computed approximately by the ROM. This modeling scheme was tested for a 24P battery module in a co-simulation process: the stand-alone P2D model is solved at the battery-average current to estimate certain parameters to share with its ROM, then the ROM is implemented to evaluate the voltage response of each cell at its respective cell current. The ROM shows excellent accuracy and a much higher speed than the full-order-model with P2D models used for each cell. © The Author(s) 2017. Published by ECS.