Open or Closed? Elucidating the Correlation between Micropore Nature and Sodium Storage Mechanisms in Hard Carbon

The sodium storage mechanism related to a high voltage slope region and a low voltage plateau region observed commonly in hard carbon (HC) is still a topic of debate. The difference in physicochemical properties of controllably synthesized carbon reflects the ambiguity in explaining their charge storage mechanism. Herein, we attempt to unravel the sodium storage mechanism using HCs with controlled 'closed' and 'open' porosities. Opening the 'closed' pores diminishes the plateau region but does not affect the slope region. Electrochemical measurements coupled with N-2 and CO2 gas adsorption studies reveal a strong correlation between closed pores with a diameter (d), 0.4 < d <1.5 nm and capacity contribution from the plateau region, supporting the adsorption/intercalation-pore filling model. Solid-state NMR measurements confirm the near metallic state of sodium in the 'closed' micropores of HCs at potentials close to 0 V (vs. Na).