There are several questions and controversies regarding
the Na
storage mechanism in hard carbon. This springs from the difficulty
of probing the vast diversity of possible configurational environments
for Na storage, including surface and defect sites, edges, pores,
and intercalation morphologies. In the effort to explain the observed
voltage profile, typically existing of a voltage slope section and
a low-voltage plateau, several experimental and computational studies
have provided a variety of contradicting results. This work employs
density functional theory to thoroughly examine Na storage in hard
carbon in combination with electrochemical experiments. Our calculation
scheme disentangles the possible interactions by evaluating the enthalpies
of formation, shedding light on the storage mechanisms. Parallel evaluation
of the Li and K storage, and comparison with experiments, put forward
a unified reaction mechanism for the three alkali metals. The results
underline the importance of exposed metal surfaces and metal–carbon
interfaces for the stability of the pore-filling mechanism responsible
for the low-voltage plateau, in excellent agreement with the experimental
voltage profiles. This generalized understanding provides insights
into hard carbons as negative electrodes and their optimized properties.