2, Technical University of Denmark, Roskilde, , Denmark
The interest in new battery materials, which are able to “follow in the footstep” of the superior lithium ion batteries (LIBs), has increased the recent years. As a suitable candidate for the replacement of LIB employing graphite as the negative electrode, sodium-ion batteries (SIB) have long been investigated due to their intrinsic similarity to LIB, e.g. comparable electrode potentials to that of LIBs. However, due to unfavorable thermodynamics, sodium ions have been found unable to intercalate into graphite unless electrolyte solvents are co-intercalated. Fortunately, hard carbons, or non-graphitic carbons, have been successfully used as anodes. Previous studies proposed the sodiation mechanism to follow a three-stage mechanism characterized by absorption of sodium ions onto pore surfaces, at defect sites and between expanded layers of graphene [1, 2]. However, the continuous, dynamical changes have not yet been addressed and are essential to fully understand the sodiation mechanism.
Using operando X-ray total scattering and Pair Distribution Function (PDF) analysis utilizing a micro-capillary battery cell, the mechanism of hard carbon sodiation has been investigated to obtain information about the local structural changes of the disordered material, which are difficult to address using conventional crystallographic methods due to the amorphous state of the materials. The PDF data reveals that inter-layer and C-C bond distance distortions in the graphene sheets appear upon sodiation. This behavior corresponds to a reversible charge transfer between sodium and the antibonding orbitals in the upper π band of the graphene sheet resulting in in-plane elongation and contraction upon discharge and charge, respectively. However, the data also reveals that the hard carbon structure becomes increasingly disordered upon discharge, in which the initial structure is never fully recovered.
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 Stratford, J. M, Allan, P. K., Pecher, O., Chater, P. A. & Grey, C. P., Chem. Commun., 52, 12430-12433 (2016).