Lithiated carbon fibres for structural batteries characterised with Auger electron spectroscopy

Structural batteries are multifunctional devices that store energy and carry mechanical load, simultaneously. The pivotal constituent is the carbon fibre, which acts as not only structural reinforcement, but also as electrode by reversibly hosting Li ions. Still, little is known about how Li and carbon fibres interact. Here we map Li inserted in polyacrylonitrile based carbon fibres with Auger electron spectroscopy (AES). We show that with slow charge/discharge rates, Li distributes uniformly in the transverse and longitudinal direction of the fibre, and when fully discharged, all Li is virtually expelled. With fast rates, Li tends to be trapped in the core of the fibre. In some fibres, Li plating is found between the solid electrolyte interphase (SEI) and fibre surface. Our findings can guide AES analysis on other carbonaceous electrode materials for Li-ion batteries and be used to improve the performance of structural batteries.