The well-known structural properties of PAN-based carbon fibres (CFs) have previously been combined with electrochemical functionalities such as energy storage, solid-state actuation, and strain sensing. These properties have been created through lithium insertion/intercalation in the turbostratic microstructure of the CFs, with the goal of making truly multifunctional structures with potentially large mass and volume savings.

The presented work investigates the insertion of sodium ions into CFs, and whether these multifunctional effects are still observable and exploitable for applications in future multifunctional structures. The motivation for this work comes from the fact that sodium's ionic radius is larger than that of lithium's, and hence a more pronounced multifunctional effect is hypothesised. A secondary motivation comes from a general increase in interest in sodium-ion batteries (SIBs) as a more environmentally friendly alternative to lithium-ion batteries (LIBs).

In this study it is found that sodium-ion capacity in commercial CFs is considerably less than that of lithium-intercalated CFs, with a maximum first cycle capacity of 180mAh/g in comparison with 327mAh/g for lithium insertion. The cycling stability is found to be less consistent, and the first cycle losses found to be larger when compared with lithium-intercalated CFs in a half-cell set-up.

Using in-situ strain measurements during sodiation and desodiation it is found that sodium insertion creates a lower axial expansion in CFs in comparison with lithium intercalation, even when normalised for capacity. Despite this, the mechanical work available is still significantly larger than that of traditional piezoelectric (PZT) ceramics that are used today for solid-state actuation.

The piezo-electrochemical (PECT) effect, observed in lithium-intercalated CFs - in which mechanical strains produce linearly proportional voltage responses - is also observed for sodiated CFs. The coupling factor k (V/unit strain) is found to be considerably lower in sodiated CFs than in lithium-intercalated CFs for a corresponding state of charge. There is still potential for this effect to be exploited to create self-strain sensing structures, although the lower coupling factor makes sodiated CFs less favourable than lithium-intercalated CFs for this application.

The present work concludes that sodiated CFs have a lower potential for use in multifunctional structures than lithium-intercalated CFs for energy storage, solid-state actuation, and strain sensing. It is acknowledged, however, that continued improvements in electrolytes, and cell design may contribute to an increase in this potential in future.