Mark W. Verbrugge

Aging Mechanisms of LiFePO4 Batteries Deduced by Electrochemical and Structural Analyses

The performance loss of lithium-ion batteries with lithium iron phosphate positive chemistry was analyzed using electrochemical characterization techniques such as galvanostatic charge-discharge at different rates, ac impedance, and hybrid pulse power characterization measurements. Differentiation analysis of the discharge profiles as well as in situ reference electrode measurement revealed loss of lithium as well as degradation of the carbon negative; the cell capacity, however, was limited by the amount of active lithium. Destructive physical analyses and ex situ electrochemical analyses were performed at test completion on selected cells. While no change in positive morphology and performance was detected, significant cracking and delamination of the carbon negative was observed. In addition, X-ray diffraction analysis confirmed the changes in the crystal structure of the graphite during cycling. The degradation of the carbon negative is consistent with the observations from the electrochemical analysis. Ex situ electrochemical analysis confirmed that active lithium controlled cell capacity and its loss with cycling directly correlated with cell degradation. The relationship between carbon negative degradation and loss of active lithium is discussed in the context of a consistent overall mechanism.

The influence of surface mechanics on diffusion induced stresses within spherical nanoparticles

We examine the effects of surface tension and surface modulus on diffusion-induced stresses within spherical nanoparticles. We show that both the magnitude and distribution of stresses can be significantly affected by surface mechanics if the particle diameter is in the nanometer range. In particular, a tensile state of stress may be significantly reduced in magnitude or even be reverted to a state of compressive stress with decreasing particle radius. This reduction in tensile stress may be responsible for the observed resilience to fracture and decrepitation of nanoparticles used in various industrial applications.

Diffusion-Induced Stress, Interfacial Charge Transfer, and Criteria for Avoiding Crack Initiation of Electrode Particles

Most lithium-ion battery electrodes experience large volume changes caused by concentration changes within the host particles during charging and discharging. Electrode failure, in the form of fracture or decrepitation, can occur as a result of repeated volume changes. In this work, we first develop analytic solutions for the evolution of concentration and stresses within a spherical electrode element under charging-discharging conditions when the system thermodynamics are ideal (e.g., no repulsion forces are significant between intercalate species). Both interfacial (electrochemical) kinetics and intercalate diffusion are comprehended. Based on the analytic solutions, we propose tensile stress-based criteria for the initiation of cracks within a spherical insertion electrode. These criteria may help guide the development of new materials for lithium-ion batteries with enhanced mechanical durability and identify battery operating conditions that, when maintained, keep the mechanical stresses below acceptable values, thereby increasing cell life.

Electrochemical and Thermal Characterization of Battery Modules Commensurate with Electric Vehicle Integration

A relatively simple mathematical representation of a nickel metal hydride traction battery is developed and implemented. The approach is based in part on an equivalent circuit comprising a resistor element in series with a parallel resistor-capacitor combination. Additional features include self-discharge, current inefficiency, temperature and state-of-charge (SOC) dependencies, and mass-transport limitations. An energy balance is coupled to the electrochemical problem: the energy balance incorporates transient heat-transfer to the battery surroundings as well as reversible and irreversible heat generation. The high-frequency and pseudo-steady-state impedance exhibit an Arrhenius temperature dependence. All the model parameters arc constants or are described by continuous functions of temperature and SOC. Calculated results from the coupled electrochemical and thermal model are compared with charge and discharge experiments conducted over the time scales and current magnitudes of interest for electric-vehicle applications. The paper closes with a brief summary and a discussion of open questions.

Stress and Strain-Energy Distributions within Diffusion-Controlled Insertion-Electrode Particles Subjected to Periodic Potential Excitations

We derive and implement analytic solutions for the description of insertion particles subject to cyclic surface concentration variations consistent with a periodic voltage excitation source applied to an insertion electrode wherein the overall resistance is dominated by that of solid-state diffusion within the electrode particles. The form of the analytic solution allows for a direct analogy to cyclic fatigue phenomena that have been examined in detail for structural materials over the past two centuries. We utilize the strain-energy density to assess the potential for crack nucleation, and we show that while the shear stress is independent of the surface tension and surface modulus, the strain-energy density, which drives particle fracture, is sensitive to the surface mechanics and therefore the particle radii. Specifically, the analysis implies that smaller particles are more stable relative to diffusion-induced decrepitation and cracking, consistent with experimental observations.

Analysis of Promising Perfluorosulfonic Acid Membranes for Fuel‐Cell Electrolytes

Fuel cells offer means by which quiet, energy-efficient, high-power, high-power-density processes can be achieved without the generation of unwanted emissions. The major obstacles to the realization of economical fuel cells are the high capital costs for the inert perfluorosulfonic acid (PSA) membranes and the noble-metal catalysts. New PSA membranes have been developed and subsequently used to obtain higher fuel-cell current and power densities. In this paper, the authors investigate the equilibrium and transport characteristics of promising PSA fuel-cell electrolytes; electroanalytical and radiotracer techniques are employed. particular emphasis is devoted to proposing membrane structural characteristics that can be used explain our experimental results and shed light on fuel-cell-performance data obtained with the different polymers.

Modeling Lithium Intercalation of Single‐Fiber Carbon Microelectrodes

To clarify the electrochemical processes governing the performance of lithiated carbon electrodes and obtain appropriate physicochemical properties, experiments conducted with a single-fiber carbon microelectrode (3.5 {micro}m radius, 1 cm length) are mathematically simulated. Equilibrium-potential data are used to determine the activity coefficient of the lithium intercalate and associated hot sites. Transport within the carbon fiber is influenced significantly by activity-coefficient variations; the use of the guest chemical-potential gradient as the driving force for transport phenomena is shown to yield constant physicochemical properties that are independent of the degree of intercalation. The theoretical calculations display good agreement with several different experimental data sets. The diffusion coefficient of lithium in partially graphitic carbon is obtained along with rate constants (i.e., the exchange current density) associated with the electrochemical reaction that takes place on the fiber surface.

Electrochemical Analysis of Lithiated Graphite Anodes

Theoretical calculations are compared with well-controlled experiments conducted on a porous, graphite-based, lithiated-carbon electrode. The interpretation of the electrode behavior is facilitated by the use of a reference electrode in a cell that maintains a substantially uniform current distribution. A solvent-casting procedure for constructing graphite anodes, employing a hydrocarbon [poly(ethylene + propylene + norborene)] binder is implemented. Nonlinear diffusion of lithium intercalate within the host carbon particles is considered; previously published concentration-dependent diffusion-coefficient data are employed for the lithium intercalate species. Other important resistances result from ionic diffusion and migration within the solvent phase, interfacial reaction at the surface of the carbon particles, and electron transport within the solid phase. Calculations are used to assess the impact of particle shape and the nature of the carbon precursor. The overall analysis indicates that interfacial resistance plays a dominant role in limiting the available capacity at high rates of current passage.

Diffusion Induced Stresses and Strain Energy in a Phase-Transforming Spherical Electrode Particle

Lithium insertion and removal in lithium ion battery electrodes can result in diffusion induced stresses (DISs) which may cause fracture and decrepitation of electrodes. Many lithium ion electrode materials undergo formation of two or more phases during lithium insertion or removal. In this work, we mathematically investigate the DISs in phase transforming electrodes using a coreshell structural model. We examine the concentration jumps at phase boundaries that result in stress discontinuities, which in turn can cause cracking. The influence of the mechanical properties of the two phases on stress evolution, stress discontinuity, and strain energy are clarified. The trends obtained with the model may be used to help tune electrode materials with appropriate interfacial and bulk properties so as to increase the durability of battery electrodes.

Generalized Recursive Algorithm for Adaptive Multiparameter Regression Application to Lead Acid, Nickel Metal Hydride, and Lithium-Ion Batteries

We derive and implement an algorithm that can accommodate an arbitrary number of model parameters, thereby allowing for more complicated battery models to be employed in formulating model reference adaptive systems as part of an energy management scheme for systems employing batteries. We employ the (controls) methodology of weighted recursive least squares with exponential forgetting. The output from the adaptive algorithm is the battery state of charge (remaining energy), state of health (relative to the batterys nominal rating), and power capability. The adaptive characterization of lead acid, nickel metal hydride, and lithium-ion batteries is investigated with the algorithm. The algorithm works well for lithium-ion and lead-acid batteries; more work is needed on nickel metal hydride batteries.