Chester G. Motloch

An accelerated calendar and cycle life study of Li-ion cells

Abstract The accelerated calendar and cycle life of lithium-ion cells was studied. Useful cell life was strongly affected by temperature, time, state-of-charge (SOC) and change in state-of-charge (ΔSOC). In calendar life experiments, useful cell life was strongly affected by temperature and time. Temperature accelerated cell performance degradation. The rates of area specific impedance (ASI) increase and power fade followed simple laws based on a power of time and Arrhenius kinetics. The data have been modeled using these two concepts and the calculated data agree well with the experimental values. The calendar life ASI increase and power fade data follow (time) 1/2 kinetics. This behavior may be due to solid electrolyte interface layer growth. From the cycle life experiments, the ASI increase data follow (time) 1/2 kinetics also, but there is an apparent change in overall power fade mechanism when going from 3 to 6% ΔSOC. Here, the power of time drops to below 1/2, which indicates that the power fade mechanism is more complex than layer growth.

Effect of cathode composition on capacity fade, impedance rise and power fade in high-power, lithium-ion cells ☆

We tested the effect of Al concentration on the performance of lithium-ion cells. One set of cells contained a LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} cathode and the other, LiNi{sub 0.8}Co{sub 0.10}Al{sub 0.10}O{sub 2}. The cells were calendar- and cycle-life tested at several temperatures, with periodic interruptions for reference performance tests that were used to gauge capacity and power fade as a function of time. The C{sub 1}/25 capacity fade in the cells displayed t{sup 1/2} dependence. The capacity fade of the 10% Al-doped cells tested at 45 {sup o}C was similar to that of the 5% Al-doped cells at 25 {sup o}C. The impedance rise and power fade were also sensitive to the Al concentration. For the one common temperature investigated (i.e., 45 {sup o}C), the 10% Al-doped cells displayed higher impedance rise and power fade than the 5% Al-doped cells. Additionally, the time dependence of the impedance rise displayed two distinct kinetic regimes; the initial portion depended on t{sup 1/2} and the final, on t. On the other hand, the 10% Al-doped cells depended on t{sup 1/2}2 only.

A capacity and power fade study of Li-ion cells during life cycle testing

We tested three lithium-ion cells to evaluate capacity and power fade during cycle life testing of a hybrid electric vehicle (HEV) cell with varying state of charge (ΔSOC). Test results showed that the cells had sufficient power and energy capability to meet the Partnership for a New Generation of Vehicles (PNGV), now called FreedomCAR, goals for Power Assist at the beginning of life and after 120,000 life cycles using 48 cells. The initial static capacity tests showed that the capacity of the cells stabilized after three discharges at an average of 14.67 Ah. Capacity faded as expected over the course of 120,000 life cycles. However, capacity fade did not vary with ΔSOC. The hybrid pulse power characterization (HPPC) tests indicated that the cells were able to meet the power and energy goals at the beginning of testing and after 120,000 life cycles. The rate of power fade of the lithium-ion cells during cycle life testing increased with increasing ΔSOC. Capacity fade is believed to be due to lithium corrosion at the anode, and power fade suggested a buildup of the SEI layer or a decrepitation of the active material.

Effects of Reference Performance Testing during Aging Using Commercial Lithium-Ion Cells

The Advanced Technology Development Program, under the oversight of the U.S. Department of Energy’s FreedomCAR and Vehicle Technologies Program, is investigating lithium-ion batteries for hybrid-electric vehicle applications. Cells are aged under various test conditions, including temperatures and states-of-charge. Life testing is interrupted at regular intervals to conduct reference performance tests (RPTs), which are used to measure changes in the electrical performance of the cells and then to determine cell degradation as a function of test time. Although designed to be unobtrusive, data from the Advanced Technology Development Gen 2 cells indicated that RPTs actually contributed to cell degradation and failure. A study was performed at the Idaho National Laboratory using commercially available lithium-ion cells to determine the impact of RPTs on life. A series of partial RPTs were performed at regular intervals during life testing and compared to a control group that was life tested without RPT interruption. It was determined that certain components of the RPT were detrimental, while others appeared to improve cell performance. Consequently, a new “mini” RPT was designed as an unobtrusive alternative. Initial testing with commercial cells indicates that the impact of the mini RPT is significantly less than the Gen 2 cell RPT.

Cycle Life Studies of Advanced Technology Development Program Gen 1 Lithium Ion Batteries

This report presents the test results of a special calendar-life test conducted on 18650-size, prototype, lithium-ion battery cells developed to establish a baseline chemistry and performance for the Advanced Technology Development Program. As part of electrical performance testing, a new calendar-life test protocol was used. The test consisted of a once-per-day discharge and charge pulse designed to have minimal impact on the cell yet establish the performance of the cell over a period of time such that the calendar life of the cell could be determined. The calendar life test matrix included two states of charge (i.e., 60 and 80%) and four temperatures (40, 50, 60, and 70°C). Discharge and regen resistances were calculated from the test data. Results indicate that both discharge and regen resistance increased nonlinearly as a function of the test time. The magnitude of the discharge and regen resistance depended on the temperature and state of charge at which the test was conducted. The calculated discharge and regen resistances were then used to develop empirical models that may be useful to predict the calendar life or the cells.

Crosstalk compensation for a rapid, higher-resolution impedance spectrum measurement

Crosstalk Compensation is an approach that enables rapid, higher-resolution impedance spectra measurements of energy storage devices. The input signal consists of a sum-of-sines excitation current that has a known frequency spread. The advantage of Crosstalk Compensation is that high resolution spectra can be acquired within one period of the lowest frequency while also including non-harmonic frequencies. The crosstalk interference at a given frequency can be pre-determined and assigned to an error matrix. The real and imaginary impedance can then be calculated based on the inverse of the error matrix and captured response. Analytical validation of Crosstalk Compensation was performed using a battery equivalent circuit model. Two different frequency ranges were simulated, and both indicated that a minimum step factor between frequencies should be 1.25 to reduce the error in compensating the captured response signal. For a frequency range of 1638.4-0.1 Hz, for example, a maximum of 45 frequencies should be included within the excitation signal to accurately acquire the impedance spectra at high resolution. A simplified derivation of Crosstalk Compensation and its corresponding analytical validation studies are discussed.

Long-Term Validation of Rapid Impedance Spectrum Measurements as a Battery State-of-Health Assessment Technique

CITATION: SAE Int. J. Alt. Power. ____________________________________ EXPERIMENTAL Christophersen et al / SAE Int. J. Alt. Power. / Volume 6, Issue 1(May 2013)