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. (C) 2004 Elsevier Ltd. All rights reserved.