• Energy Research

AbstractUntangling the relationship between reactions, mass transfer, and temperature within lithium-ion batteries enables approaches to mitigate thermal hot spots and slow degradation. Here, we develop an efficient physics-based three-dimensional model to simulate lock-in thermography experiments, which synchronously record the applied current, cell voltage, and surface-temperature distribution from commercial lithium iron phosphate pouch cells. We extend an earlier streamlined model based on the popular Doyle–Fuller–Newman theory, augmented by a local heat balance. The experimental data reveal significant in-plane temperature non-uniformity during battery charging and discharging, which we rationalize with a multiscale coupling between heat flow and solid-state diffusion, in particular microscopic lithium intercalation within the electrodes. Simulations are exploited to quantify properties, which we validate against a fast full-discharge experiment. Our work suggests the possibility that non-uniform thermal states could offer a window into—and a diagnostic tool for—the microscopic processes underlying battery performance and cycle life.

Abstract The performance and thermal response of large-scale GS-Yuasa LEV50 50-Ah NMC automotive battery cells were investigated via simulation. To evaluate local transient temperature distributions, the Dualfoil model was coupled to local energy-balance equations. At similar C rates the difference between maximum and minimum temperature in the LEV50 was found to be higher than that in an 18650 cell with identical chemistry. Unlike thinner prismatic lithium ion batteries, the temperature variation through the cell thickness in the large-format cell was not negligible (∼5 °C at 4C discharge). Because of the non-uniform temperature distribution within the jellyroll, the risk of lithium plating at high charging rates and low ambient temperatures may be greater toward the jellyroll exterior. Simulations of thermal abuse (oven test) of the large cell showed a delayed thermal response relative to the 18650, but also indicated a lower onset temperature for thermal runaway.

An isothermal porous-electrode model of a discharging lead-acid battery is presented, which includes an extension of concentrated-solution theory that accounts for excluded-volume effects, local pressure variation, and a detailed microscopic water balance. The approach accounts for three typically neglected physical phenomena: convection, pressure diffusion, and variation of liquid volume with state of charge. Rescaling of the governing equations uncovers a set of fundamental dimensionless parameters that control the battery's response. Total volume change during discharge and nonuniform pressure prove to be higher-order effects in cells where variations occur in just one spatial dimension. A numerical solution is developed and exploited to predict transient cell voltages and internal concentration profiles in response to a range of C-rates. The dependence of discharge capacity on C-rate deviates substantially from Peukert's simple power law: charge capacity is concentration-limited at low C-rates, and voltage-limited at high C-rates. The model is fit to experimental data, showing good agreement. Submitted to Journal of the Electrochemical Society. First part of a two-part paper. Part II: "Faster Lead-Acid Battery Simulations from Porous-Electrode Theory: II. Asymptotic Analysis"

Efforts to avoid dendrites by increasing the interfacial surface area to lower local current densities are limited by significant local pressure accumulation associated with the topography of any surface contouring.

Abstract Swelling of a commercial 5 Ah lithium-ion cell with a nickel/manganese/cobalt-oxide cathode is investigated as a function of the charge state and the charge/discharge rate. In combination with sensitive displacement measurements, knowledge of the electrode configuration within this prismatic cell's interior allows macroscopic deformations of the casing to be correlated to electrochemical and mechanical transformations in individual anode/separator/cathode layers. Thermal expansion and interior charge state are both found to cause significant swelling. At low rates, where thermal expansion is negligible, the electrode sandwich dilates by as much as 1.5% as the charge state swings from 0% to 100% because of lithium-ion intercalation. At high rates a comparably large residual swelling was observed at the end of discharge. Thermal expansion caused by joule heating at high discharge rate results in battery swelling. The changes in displacement with respect to capacity at low rate correlate well with the potential changes known to accompany phase transitions in the electrode materials. Although the potential response changes minimally with the C-rate, the extent of swelling varies significantly, suggesting that measurements of swelling may provide a sensitive gauge for characterizing dynamic operating states.

Abstract The theoretical foundation of potentiometric diffusivity and transference-number measurements is revisited, through an analysis of the diffusion-potential relaxation a concentrated binary electrolyte exhibits after being subjected to a current pulse in a planar electrolytic cell. Earlier theory is extended to include solute-volume effects, as well as being modified to incorporate a particle-fraction basis for composition and to account for the nonlinear relationship between composition differences and cell voltage. The new theory provides significant corrections when concentration polarization is very large or when electrolytes are moderately concentrated, rationalizing the unexpected voltage responses seen during some previous transport measurements. Guidelines are developed to aid the design of galvanostatic-polarization experiments involving non-aqueous electrolytes similar to those used in lithium batteries. Complete property sets are provided for 0.85 M LiPF 6 in propylene carbonate and for 2.24 M LiPF 6 in a mixed-carbonate solvent.

Liquid lithium-battery electrolytes universally incorporate at least two solvents to balance conductivity and viscosity. Almost all continuum models treat cosolvent systems such as ethylene carbonate:ethyl-methyl carbonate (EC:EMC) as single entities whose constituents travel with identical velocities. We test this “single-solvent approximation” by subjecting LiPF6:EC:EMC blends to constant-current polarization in Hittorf experiments. A Gaussian process regression model trained on physicochemical properties quantifies changes in composition across the Hittorf cell. EC and EMC are found to migrate at noticeably different rates under applied current, demonstrating conclusively that the single-solvent approximation is violated and that polarization of salt concentration is anticorrelated with that of EC. Simulations show extreme solvent segregation near electrode/liquid interfaces: a 5% change in EC:EMC ratio, post-Hittorf polarization, implies more than a 50% change adjacent to the interface during the current pulse. Understanding how lithium-ion flux induces local cosolvent or additive imbalances suggests new approaches to electrolyte design.

Abstract Flux-explicit transport laws based on Newman's concentrated-solution theory are developed for application to phases with domains of imbalanced charge. General procedures are provided to create flux laws and a current–voltage relation that describe diffusion and migration in isothermal, isobaric, non-neutral multicomponent electrolytes. To retain thermodynamic consistency within the non-neutral concentrated-solution theory, driving forces for diffusion are based on the chemical potentials of neutral combinations of species, and an excess current density is used as a driving force for migration. Procedures are developed for identifying the solution conductivity and Hittorf transference numbers in non-neutral electrolytes comprising three or more species. Flux laws for non-neutral binary electrolytic solutions involving the thermodynamic diffusion coefficient, cation transference number, and ionic conductivity are presented. When local electroneutrality is assumed, the new transport equations reduce to the familiar flux laws for binary electrolytes.