Chinh D. Ho

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.