Characterisation of the ambient and elevated temperature performance of a graphite electrode

Thermal stability of the SEI layer on graphite in < Li(liquid electrolyte)graphite > half-cells has been investigated. DSC measurements reveal a two-stage exothermal reaction. The first, corresponding to a breakdown of the SEI layer, begins at 58 degrees C for a 1 M LiBF4 in EC/DMC 2:1 electrolyte. The second, starting at similar to 80 degrees C, corresponds to lithium deintercalation, followed by some irreversible chemical reaction; the heat evolved in the second stage increases for increasing lithium content in the graphite. Precycling at RT to generate the Solid Electrolyte Interface (SEI) layer, followed by storage for 1 week at different temperatures and then continued cycling, brings about a rapid decline in capacity for cells stored above 50 degrees C. XRD could also show that graphite electrodes are partially blocked for subsequent lithium-ion insertion after such treatment. ESCA (XPS) characterisation of the surface of fresh graphite electrodes compared with that of graphite electrodes extracted from these RT precycled/stored cells gives evidence of the formation of a thicker macroscopic layer on the electrode surface of cells scored at 60 degrees C. This layer is not found for half-cells stored at lower temperatures. (C) 1999 Elsevier Science S.A. All rights reserved.