Mechanism of Sodium Storage in Hard Carbon: An X-Ray Scattering Analysis

Hard carbon is a standard anode material for Na-ion batteries. However, its low crystallinity and diverse microstructures make obtaining a full understanding of the sodium storage mechanism challenging. Here, the results of a systematic ex situ small and wide angle X-ray scattering study of a series of nanostructured hard carbons, which reveal clear evidence of sodium storage in the graphene-graphene interlayers and nanopores, are presented. Particularly, an emergence of a broad peak around q approximate to 2.0-2.1 angstrom(-1) in the low voltage region is suggested to be an indicator that sodium is densely confined in the nanopores. Thus, classical X-ray scattering techniques are demonstrated to be effective in elucidating the overall reaction scheme of Na insertion into hard carbon.