EPR study on petroleum cokes annealed at different temperatures and used in lithium and sodium batteries

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480-2800 degreesC and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 degreesC. while delocalised electrons,were found to contribute to the EPR profile of cokes treated between 1400 and 2800 degreesC. After electrochemical interaction of cokes with lithium. the EPR signal due to delocalised electrons undergoes a strong line narrowing combined with an increase in signal intensity, Localised paramagnetic centres interact,vith Li and Na, as a result of which there is a line narrowing and decrease in the signal intensity. A new narrow signal is detected for low-temperature cokes after reaction with lithium. The origin of this signal could be associated with 'near-metallic' lithium. which is accommodated in non-ordinary carbon sites such as microcavities. The intensity of this signal is higher for samples treated at 600 degreesC. where a higher hysteresis in the voltage versus composition curve is observed. (C) 2002 Elsevier Science Ltd. All rights reserved.