X-ray photoelectron spectroscopy of negative electrodes from high-power lithium-ion cells showing various levels of power fade

The current generation of high-power lithium-ion cells for transportation applications, being developed and studied at Argonne National Laboratory, contain LiNi0.8Co0.15Al0.05O2-based cathodes, graphite-based anodes, and LiPF6-based electrolytes. These cells show loss of capacity and power during accelerated testing at elevated temperatures. X-ray photoelectron spectroscopy (XPS) examination of negative electrode samples harvested from some cells that showed varying degrees of power and capacity fade revealed a surface film on the graphite. Samples from cells that showed higher fade exhibited a thicker film. Furthermore, solvent-based compounds were dominant on samples from low power fade cells, whereas LiPF6-based products were dominant on samples from high-power fade cells. The effects of sample rinsing and air exposure are explored. Mechanisms are proposed to explain the formation of compounds suggested by the XPS data.