Pulse Polarization for Li-Ion Battery under Constant State of Charge: Part I. Pulse Discharge Experiments

Abstract

Given the increased dependence on battery-powered devices, it is necessary to develop new and robust methods to evaluate and predict battery performance and state of charge. Batteries, including Li-Ion batteries, are unsteady state systems and a need exists to evaluate performance under constant state of charge (SOC). The purpose of this work was to develop pulse polarization curves (PPC) for a Li-Ion battery under constant SOC in order to identify individual overvoltages, such as charge transfer kinetics and mass transport, and their SOC dependence. Consequently, we elected to conduct pulse discharges at various duration and discharge current densities, under well maintained SOCs. Based on the results we were able to construct pulse polarization curves for constant SOCs of 10, 40 and 70% at various pulse durations of 2, 10 and 30 seconds. The experimentally obtained pulse polarizations suggest that the kinetic overpotential is captured in the 2 second pulse, while electrolyte Li+ concentration and Li solid state diffusion gradients are established in the 10 and 30 second pulse times, respectively. This allowed us to identify the individual overvoltages experimentally. An important consideration was that as the battery's SOC changes, the open circuit voltage (OCV) also changes. Therefore, the pulse discharge method we used needed to end at the same SOC in all cases to enable the construction of a PPC with the same OCV. To ensure that this method applied to various sizes and chemistries of batteries reliably, we ran pulse discharge experiments on a high Amp hour (15 Ah) large Li-Ion pouch battery (power) that used a NMC LMO cathode, as well as on a low Amp hour (0.04 Ah) small electrode coin battery (energy) with a CoO 2 cathode. In a subsequent paper we employed the Newman method to model the pulse discharges, allowing prediction of the performance of Li-Ion batteries and constructing their PPCs under constant SOC.

Pulse Polarization for Li-Ion Battery under Constant State-of-Charge: Part II. Modeling of Individual Voltage Losses and SOC Prediction

“…This makes sense since the kinetics operate on millisecond time scales. 1 While we attempted to capture the voltage loss due to kinetics alone with the 2 second pulse discharge, our modeling analysis in Figure 8 shows that we were not successful in completely eliminating other voltage losses. Even for a 2 second pulse discharge, losses attributable to the separator and mass transport are present in the positive and negative electrodes.…”

“…Construction of pulse polarization curves from pulse discharge simulations.-Utilizing Newman's model in Comsol Multiphysics 3.5a [2][3][4][5][11][12][13][14][15][16][17][18][19][20] and fitting it to the experimental data 1,26 based on the four previously described factors (diffusivity 17 and kinetic rate constant [13][14][15][16]21 for each electrode), we constructed PPCs. We also created PPCs that were predicted by the model but were not experimentally validated.…”

“…1 The equations for the simulation are presented below. The electronic current density balance is a modified Ohm's Law for the solid phase electronic potential, φ 1 , denoted by subscript 1 and is given by: [1] k ef f 1 is the electronic conductivity, i loc is the local charge transfer current density, S a,i is the specific surface area of the electrode, and L i is the width of each respective electrode to make the equation dimensionless. This equation is only applied in the electrode domain, not the separator domain since no solid electronic phase exists in the separator domain.…”

“…As mentioned 1 studying battery voltage vs current density [1][2][3][4][5][6] remains difficult, in part since the SOC changes as the battery is used. Previous work experimentally evaluates a voltage vs current density relationship at a constant SOC.…”

“…Previous work experimentally evaluates a voltage vs current density relationship at a constant SOC. 1 In this paper, we use the Newman model 2,3 and COMSOL Multiphysics 4,5 to expand predictions about PPC behavior as pulse times increase reaching pseudo steady state polarization. Bernardi and Go 6 also utilized a pulse discharge method to gather data and a similar model to analyze the performance and voltage evolution, and the modeled results matched very well with their experimental data.…”