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

Journal article, 2018

Carbon fibres (CFs), originally made for use in structural composites, have also been demonstrated as

high capacity Li-ion battery negative electrodes. Consequently, CFs can be used as structural

electrodes; simultaneously carrying mechanical load and storing electrical energy in multifunctional

structural batteries. To date, all CF microstructural designs have been generated to realise a targeted

mechanical property, e.g. high strength or stiffness, based on a profound understanding of the

relationship between the graphitic microstructure and the mechanical performance. Here we further

advance this understanding by linking CF microstructure to the lithium insertion mechanism and the

resulting electrochemical capacity. Different PAN-based CFs ranging from intermediate- to highmodulus

types with distinct differences in microstructure are characterised in detail by SEM and HRTEMand

electrochemical methods. Furthermore, the mechanism of Li-ion intercalation during

charge/discharge is studied by in situ confocal Raman spectroscopy on individual CFs. RamanGband

analysis reveals a Li-ion intercalation mechanism in the high-modulus fibre reminiscent of that in

crystalline graphite. Also, the combination of a relatively low capacity of the high-modulus

CFs (ca. 150 mAh g−1) is shown to be due to that the formation of a staged structure is frustrated by an

obstructive turbostratic disorder. In contrast, intermediate-modulus CFs, which have significantly

higher capacities (ca. 300 mAh g−1), have Raman spectra indicating a Li-ion insertion mechanism

closer to that of partly disordered carbons. Based on these findings, CFs with improved multifunctional

performance can be realised by tailoring the graphitic order and crystallite sizes.