Graphitic microstructure and performance of carbon fibre Li-ion structural battery electrodes

Abstract

Carbon fibres (CFs), originally made for use in structural composites, have also been demonstrated ashigh capacity Li-ion battery negative electrodes. Consequently, CFs can be used as structuralelectrodes; simultaneously carrying mechanical load and storing electrical energy in multifunctionalstructural batteries. To date, all CF microstructural designs have been generated to realise a targetedmechanical property, e.g. high strength or stiffness, based on a profound understanding of therelationship between the graphitic microstructure and the mechanical performance. Here we furtheradvance this understanding by linking CF microstructure to the lithium insertion mechanism and theresulting electrochemical capacity. Different PAN-based CFs ranging from intermediate- to highmodulustypes with distinct differences in microstructure are characterised in detail by SEM and HRTEMandelectrochemical methods. Furthermore, the mechanism of Li-ion intercalation duringcharge/discharge is studied by in situ confocal Raman spectroscopy on individual CFs. RamanGbandanalysis reveals a Li-ion intercalation mechanism in the high-modulus fibre reminiscent of that incrystalline graphite. Also, the combination of a relatively low capacity of the high-modulusCFs (ca. 150 mAh g−1) is shown to be due to that the formation of a staged structure is frustrated by anobstructive turbostratic disorder. In contrast, intermediate-modulus CFs, which have significantlyhigher capacities (ca. 300 mAh g−1), have Raman spectra indicating a Li-ion insertion mechanismcloser to that of partly disordered carbons. Based on these findings, CFs with improved multifunctionalperformance can be realised by tailoring the graphitic order and crystallite sizes.