Brian J. Koch

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.

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.

Li-ion battery parameter estimation for state of charge

Battery state of charge (SOC) is a critical parameter for the control of propulsion systems in plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV). As SOC is not measureable during vehicle operation, an onboard adaptive algorithm is developed in this paper. The algorithm estimates six electrical parameters for Li-ion batteries and provides a reliable SOC based on one of the estimated battery parameters, i.e. open circuit voltage (OCV). Simulation and vehicle validation results show good robustness and adaptation of the algorithm with high computational efficiency and low implementation cost.

Electrochemistry of Intercalation Materials Charge‐Transfer Reaction and Intercalate Diffusion in Porous Electrodes

For the research, development, and eventual manufacture of lithium‐ion batteries, it is necessary to understand the phenomena limting cell performance and incorporate this knowledge into battery design. While methods exist for the determination of electrolyte‐phase properties, procedures for the assessment of analogous solid‐state properties in battery electrode materials are more difficult to implement. The perturbation solution derived in this work is implemented to determine the intercalate diffusion coefficient in a solvent‐cast porous electrode containing carbonized poly(acrylonitrile) fibers suitable for a lithium‐ion battery. It is shown that both intercalate and vacant site contributions to the excess free energy of the solid phase influence significantly the nonequilibrium behavior of the electrode. In addition, the analysis is shown to provide insight into the processes that govern the pulse‐power performance of batteries based on insertion electrodes (e.g., lithium‐ion and metal‐hydride batteries).

Adaptive Energy Management of Electric and Hybrid Electric Vehicles

An adaptive algorithm based on weighted recursive least squares is derived and implemented. The generality of the approach is underscored by the application of the algorithm to a 42 V lead acid and a high-voltage (375 V) nickel metal hydride battery system. The algorithm is fully recursive in that the only variables required for on-line regression are those of the previous time step and the current time step. A time-weighting technique often referred to as exponential forgetting is employed to damp exponentially the influence of older data on the regression analysis. 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. Such algorithms are likely to play a critical role in optimal operation of hybrid electric vehicles and on-board diagnostics. The behavior of the algorithm in terms of convergence, accuracy, and robustness is examined.

Lithium Intercalation of Carbon‐Fiber Microelectrodes

Individual carbon-fiber electrodes are used to isolate characteristics intrinsic to the lithiated-carbon fiber. The results are not complicated by the influence of binders, electronically conductive additives, current collectors, or other components necessary for the fabrication of porous carbon electrodes. Central to the technique is the use of microelectrode instrumentation and microscopic carbon fibers. Analytic expressions are derived to represent qualitatively the electrochemical data. The results of this study allow one to determine the performance limitations imposed on a porous carbon anode by the active carbon fibers.

Modeling and estimation of Nickel Metal Hydride battery hysteresis for SOC estimation

Accurate estimation of battery state of charge (SOC) is essential for battery control and effective energy management of hybrid electric vehicles (HEV). Battery hysteresis poses a big challenge to the estimation of SOC. In the present battery state estimator (BSE), a rule-based hysteresis estimation method for nickel metal hydride (NiMH) batteries is used due to the lack of physical hysteresis models. The rule-based hysteresis estimation method gives only a coarse result with limited accuracy. To improve estimation accuracy and algorithm robustness, a new on-board algorithm, based on the Preisach operator, is developed to estimate battery hysteresis. The enhanced model-based algorithm is then integrated into the BSE algorithms for evaluation. Simulation and preliminary evaluation results for NiMH batteries show significant improvement in the accuracy and robustness of SOC estimation.

Microelectrode Study of the Lithium/Propylene Carbonate Interface: Temperature and Concentration Dependence of Physicochemical Parameters

Selected physicochemical parameters useful for characterizing the metallic lithium electrode exposed to a lithium perchlorate salt in a propylene carbonate solvent are reported as functions of temperature and salt composition. The advantages of using microelectrodes for the measurements are elucidated. Transport parameters governing the electrolyte‐phase limiting current density are found to be independent of salt composition over a concentration range of interest for lithium batteries. The Li‐Li+ reaction is shown to correspond to classic electron‐transfer theory, with a symmetry factor of one‐half, as would be expected for a strongly solvated ionic reactant. Simplified equations are derived that allow one to focus on specific aspects of a lithium thin‐film battery, demonstrating the utility of the measured parameters for engineering design studies.

Mathematical modeling of high-power-density insertion electrodes for lithium ion batteries

Theoretical calculations are compared with well-controlled experiments conducted on a high-surface area, small diameter lithiated-carbon electrodes. The electrodes are shown to yield very high current densities and exhibit little interfacial kinetics resistance or intercalate diffusion resistance. The mathematical treatment describes quantitatively a wide range of electrochemical experiments. The application of the model to the experimental data is facilitated by the use of a reference electrode. Initial cycling behavior of the high-surface-area electrode is elucidated, including clarification of the first-cycle coulombic inefficiency. Nitrogen absorbtion and scanning electron micrographs are utilized to ascertain the microstructural characteristics that distinguish the active electrode material. An asymptotic analysis is used to indicate when diffusion resistance within host particles is negligible; this fact simplifies model calculations and contributes to our overall understanding of insertion processes associated with host particles of very small dimensions.