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 70°C 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 LixPFyOz -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 false(ROCO2normalLifalse) 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. © 2002 The Electrochemical Society. All rights reserved.

The relationship between the elevated temperature performance of Li/graphite half-cells and the composition and morphology of the solid electrolyte interphase (SEI) formed on the graphite surface has been investigated for two electrolyte systems: 1MLiPF6 in ethylene carbonate/dimethyl carbonate EC/DMC (2:1) and 1MLiBF4 in EC/DMC (2:1). Precycled cells were stored at different temperatures up to 80°C, and the graphite electrodes were analyzed chemically (by X-ray photoelectron spectroscopy) and electrochemically under continued cycling. Loss of charge (for both salts) and of intercalation capacity (for LiBF4false) occurred after elevated temperature storage. The charge loss could be coupled to disappearance of the R­OCO2normalLi phase from the surface, with subsequent exposure of the graphite surface. The amount of LiF increased with increased storage temperature, but the LiF morphology differed between the two electrolyte systems. A model for the morphological changes of the SEI layer on storage at elevated temperature is proposed. © 2001 The Electrochemical Society. All rights reserved.

The temperature dependence of surface layer formation on Li x Mn2O4 electrodes in carbonate-based electrolytes has been studied. No significant differences were observed in the elemental composition of the surface film for cycled and stored samples. This argues against an electrochemical contribution to the surface film formation at elevated temperature. A surface film is formed at higher temperatures containing poly(oxyethylene)/polycarbonate, LiF, Li x PO y F z , and phosphorus oxides (or Li x BO y F z and boron oxides, depending on the electrolyte salt). The thickness and coverage increase at higher temperatures. No onset temperature could be found for the formation process, suggesting a general increase in reaction kinetics with temperature. A model is presented for the surface layer formed on Li x Mn2O4 (0 ≤ x ≤ 1) electrodes in contact with carbonate-based electrolytes. Polymeric compounds were found to precipitate closest to the electrode surface, with an intermediate layer of LiF and a phosphorus-rich layer outermost.