Surface characterization of electrodes from high power lithium-ion batteries

X-ray photoelectron spectroscopy and scanning electron microscopy were used to study electrode samples obtained from 18650-type lithium-ion cells subjected to accelerated calendar-life testing at temperatures ranging from 25 to 70degreesC and at states-of-charge from 40 to 80%. The cells contained LiNi0.8Co0.2O2-based positive electrodes (cathodes), graphite-based negative electrodes (anodes), and a 1 M LiPF6 ethylene carbonate: diethyl carbonate (1: 1) electrolyte. The results from electrochemically treated samples showed surface film formation on both electrodes. The positive electrode laminate surfaces contained a mixture of organic species that included polycarbonates, and LiF, LixPFy-type and LixPFy O-z-type compounds. The same surface compounds were observed regardless of test temperature, test duration, and state-of-charge. On the negative electrode laminates lithium alkyl carbonates (ROCO2Li) and Li2CO3 were found in addition to the above-mentioned compounds. Decomposition of lithium alkyl carbonates to Li2CO3 occurred on negative electrodes stored at elevated temperature. Initial depth-profiling results suggest that the surface layer thickness is greater on positive electrode samples from cells stored at high temperature than on samples from cells stored at room temperature. This observation is significant because positive electrode impedance, and more specifically, charge-transfer resistance at the electrode/electrolyte interface, has been shown to be the main contributor to impedance rise in these cells. (C) 2002 The Electrochemical Society.