In recent years lithium-ion (Li-ion) batteries with a lithium iron phosphate (LFP) cathode have become popular for grid storage and electric vehicle applications. To address uncertainty in their performance and lifetime, we initiated a multi-year cycle and calendar aging study of commercial LFP cells under systematically varied conditions. Here, we present an update of cycling data from these cells at their current state where most have reached 80% capacity retention. Cells were cycled at different state of charge (SOC) ranges (0-100%, 20-80%, and 40-60%), temperatures (15, 25, and 35 °C) and discharge rates (0.5C, 1C, 2C, and 3C). Additionally, we present data from a concurrent calendar aging study at different temperatures (15, 25, and 35 °C) and SOCs (25, 50, and 90%).

To date, higher ambient temperatures have most significantly reduced the cycle and calendar life of LFP cells. A higher SOC range or single SOC value during cycle and calendar aging, respectively, also increased the capacity fade rate, but to a lesser extent. The rate of discharge during cycling showed a mixed influence on capacity fade, with the highest and lowest rates producing the most rapid fade. This data provides a foundation to identify the distinct contributions of calendar and cycle aging. We will also discuss changes in other metrics that are of interest to system integrators, such as round-trip efficiency, cell skin temperature during cycling and discharge energy throughput. Finally, we will discuss future work that will center around materials cycling of cells disassembled at 80% capacity and cycling the remaining cells down to an end of life of 40% capacity, at which point materials characterization will be conducted again. This will help determine why capacity fade varies in the cells and what is their useful life under these aging conditions.

This work was funded by the DOE Office of Electricity under the direction of Dr. Imre Gyuk.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. SAND2021-15833 A