Sodium ion batteries (SIBs) are emerging as an alternative battery technology to lithium ion batteries because they have the potential of having a similar energy density and the advantage of sodium being more environmentally friendly than lithium. Hard carbon has been shown to be one of the best candidates as anode material for SIBs. However, several challenges need to be solved before commercializing SIBs such as finding cheaper and more efficient precursors to produce hard carbon and increasing the stability of hard carbon electrodes with the electrolyte. Herein, we report a new bio-based free standing electrode made from lignin based electrospun carbon fibers (LCFs) with a high specific capacity of 310 mAh.g−1 and a first coulombic efficiency of 89%. By using high precision coulometry on the LCFs at different carbonization temperatures, it was found that the cycling stability was dependent on the carbonization temperature. The results show that LCFs are a viable and renewable source to be used as anodes in future SIBs.

Hard carbon is one the most promising negative electrode materials for potassium-ion batteries (KIBs). However, the structure of hard carbon is complex, made of curved graphene like structures containing defects, heteroatoms, and open and closed pores, and a complete structure model is still lacking. As a result, the potassium-ion insertion mechanism into hard carbon is still under debate. In this work, we analyze the electrochemical behavior of lignin-based carbon fibres (LCFs) electrodes manufactured at different carbonization temperatures. By analyzing the open-circuit voltage curves and entropy change of the insertion reaction, it is found that hard carbon electrodes behave like a blended electrode system in which the capacity contribution of the slope and the plateau regions of the galvanostatic profile can be considered as two separate domains, with different properties. To analyze the voltage relaxation curves, we compare results from both an analytical galvanostatic intermittent titration (GITT) model and an extended finite-element based potassium insertion model in the fibers. In some regions, the model exhibits poor fitting to our experimental results, which indicate shortcomings of using an analytical GITT model for analyzing potassium ion insertion into hard carbon. We propose that, to better predict the voltage relaxation behavior of the system, blended electrode characteristics with at least two domains should be incorporated in future modeling work

We investigate hard carbon fibers in different states of charge by a combination of 7Li-NMR and 2D-XRD. In particular, we record the quadrupole-split 7Li-NMR spectra and 7Li longitudinal relaxation over a wide temperature range, and determine lithium self-diffusion both parallel and perpendicular to the fiber axis. Recording the temperature dependence permits us to interpret the presence of motional averaging of spin couplings for mobile Li. The joint analysis shows that at low Li content, Li occupies sites that lack ordered coordination and delocalized electrons and are collected in disordered spatial domains. Upon increasing the Li content, ordered sites collected in ordered domains become populated. Both disordered and ordered domains have a high inherent heterogeneity with a typical spatial extension of a few nanometers. The disordered domains exhibit a continuous topology that permits unhindered diffusion within it. At high Li content we also observe the presence of very small (∼nm) particles of metallic lithium. The joint analysis of XRD in combination with diffusion anisotropy, and anisotropy from the 7Li-NMR spectrum (with samples oriented differently with regard to the applied magnetic field), shows that the mesoscopic structure is made by ordered domains arranged in a cylindrically rolled-up manner with the mesoscopic axis parallel to the fiber axis. 

An investigation is conducted into the potential for sodiated PAN-based carbon fibers (CFs) to be used in multifunctional actuation, sensing, and energy harvesting. Axial CF expansion/contraction is measured during sodiation/desodiation using operando strain measurements. The reversible expansion/contraction is found to be 0.1% - which is lower than that of lithiated CFs. The axial sodiation expansion occurs in two well-defined stages, corresponding to the sloping and plateau regions of the galvanostatic cycling curve. The results indicate that the sloping region most likely corresponds to sodium insertion between graphitic sheets, while the plateau region corresponds to sodium insertion in micropores. A voltage-strain coupling is found for the CFs, with a maximum coupling factor of 0.15 +/- 0.01 V/unit strain, which could be used for strain sensing in multifunctional structures. This voltage-strain coupling is too small to be exploited for harvesting mechanical energy. The measured axial expansion is further used to estimate the capacity loss due to solid electrolyte interphase (SEI) formation, as well as capacity loss due to sodium trapped in the CF microstructure. The outcomes of this research suggest that sodiated CFs show some potential for use as actuators and sensors in future multifunctional structures, but that lithiated CFs show more promise.