Power fade and capacity fade resulting from cycle-life testing of Advanced Technology Development Program lithium-ion batteries

Abstract

This paper presents the test results and analysis of the power and capacity fade resulting from the cycle-life testing using PNGV (now referred to as FreedomCAR) test protocols at 25 and 45 °C of 18650-size Li-ion batteries developed by the US Department of Energy sponsored Advanced Technology Development (ATD) Program. Two cell chemistries were studied, a Baseline chemistry that had a cathode composition of LiNi 0.8Co 0.15Al 0.05O 2 with binders, that was cycle-life tested at 25 and 45 °C, and a Variant C chemistry with a cathode composition of LiNi 0.8Co 0.10Al 0.10O 2 with binders, that was tested only at 45 °C. The 300 Wh power, and % power fade were determined as a function of test time, i.e. the number of test cycles for up to 44 weeks (369,600 test cycles) for the Baseline cells, and for 24 weeks (201,600 test cycles) for the Variant C cells. The C/1 and C/25 discharge capacity and capacity fade were also determined during the course of these studies. The results of this study indicate that the 300 Wh power for the Baseline cells tested at 25 °C (up to 44 weeks of testing) decreased as a linear function of test time. The % power fade for these cells increased as a linear function of test time. The Baseline cells tested at 45 °C (up to 44 weeks of testing) displayed a decrease in their power proportional to the square root of the test time, with a faster rate of decrease of the power occurring at ∼28 weeks of testing. The % power fade for these cells also increased as the square root of the test time, and exhibited an increase in the % power fade rate at ∼28 weeks of testing. The 45 °C tested Baseline cells’ power decreased, and their % power fade increased at a greater rate than the 25 °C tested Baseline cells. The power fade was greater for the Variant C cells. The power of the Variant C cells (tested at 45 °C) decreased as the square root of the test time, and their % power fade was also found to be a function of the square root of the test time (up to 24 weeks of testing), i.e. the rate of decrease in the power and the increase in the % power fade rate was greater for the Variant C cells than for the Baseline cells also tested at 45 °C. The C/1 and C/25 Ah capacities of the Baseline cells tested at 25 and 45 °C were determined to be a function of the square root of the cycle time (i.e. number of test cycles) for test times up to 44 weeks. The capacity fade was greater at 45 °C than at 25 °C. Similarly, the C/1 and C/25 charge capacities of the Variant C cells were found to be a function of the square root of the test time (up to 24 weeks of testing). The C/1 and C/25 charge capacities decreased as a function of test time and the rate of decrease was smaller for the Variant C cells as compared to the Baseline cells over comparable test times (24 weeks).