Insights on the Mechanism of Na-Ion Storage in Soft Carbon Anode

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is the commercial anode for lithium-ion batteries; however,
it fails to extend its success to sodium-ion batteries. Recently,
we demonstrated that a low-cost amorphous carbonsoft carbon
exhibits remarkable rate performance and stable cycling life of Na-ion
storage. However, its Na-ion storage mechanism has remained elusive,
which has plagued further development of such carbon anodes. Here,
we remedy this shortfall by presenting the results from an integrated
set of experimental and computational studies that, for the first
time, reveal the storage mechanism for soft carbon. We find that sodium
ions intercalate into graphenic layers, leading to an irreversible
quasi-plateau at ∼0.5 V versus Na+/Na as well as
an irreversible expansion seen by in situ transmission electron microscopy
(TEM) and X-ray diffraction (XRD). Such a high-potential plateau is
correlated to the defective local structure inside the turbostratic
stacking of soft carbon and the associated high-binding energies with
Na ions, suggesting a trapping mechanism. On the other hand, soft
carbon exhibits long sloping regions above and below the quasi-plateau
during the first sodiation, where the sloping regions present highly
reversible behavior. It is attributed to the more defects contained
by soft carbon revealed by neutron total scattering and the associated
pair distribution function studies.