<p>The formation of surface films on electrodes in contact with non-aqueous electrolytes in lithium-ion batteries has a vital impact on battery performance. A basic understanding of such films is essential to the development of next-generation power sources. The surface chemistry, morphology and thermal stability of two typical anode and cathode materials, graphite and LiNi<sub>0.8</sub>Co<sub>0.2</sub>O<sub>2</sub>, have here been evaluated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, scanning electron microscopy and differential scanning calorimetry, and placed in relation to the electrochemical performance of the electrodes. </p><p>Chemical and morphological information on electrochemically formed graphite surface films has been obtained accurately by combining XPS measurements with Ar<sup>+</sup> ion etching. An improved picture of the spatial organisation, including thickness determination of the surface film and characterisation of individual component species, has been established by a novel sputtering calibration procedure. The stability of the surface films has been shown to depend strongly on temperature and choice of lithium salt. Decomposition products from elevated-temperature storage in different electrolyte systems were identified and coupled to effects such as capacity loss and increase in electrode resistance. Different decomposition mechanisms are proposed for surface films formed in electrolytes containing LiBF<sub>4</sub>, LiPF<sub>6</sub>, LiN(SO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub> and LiCF<sub>3</sub>SO<sub>3</sub> salts.</p><p>Surface film formation due to electrolyte decomposition has been confirmed on LiNi<sub>0.8</sub>Co<sub>0.2</sub>O<sub>2</sub> positive electrodes. An overall surface-layer increase with temperature has been identified and provides an explanation for the impedance increase the material experiences on elevated-temperature storage. </p><p>Surface phenomena are clearly major factors to consider in selecting materials for practical Li-ion batteries.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:uu-1381 |
| Date | January 2001 |
| Creators | Andersson, Anna |
| Publisher | Uppsala University, Department of Materials Chemistry, Uppsala : Acta Universitatis Upsaliensis |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
| Relation | Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 1104-232X ; 656 |
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