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

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480–2800 °C and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 °C, while delocalised electrons were found to contribute to the EPR profile of cokes treated between 1400 and 2800 °C. 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 with 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 °C, where a higher hysteresis in the voltage versus composition curve is observed.