Probing the Energy Storage Mechanism of Quasi-Metallic Na in Hard Carbon for Sodium-Ion Batteries

Hard carbon (HC) is the most promising anode material for sodium-ion batteries (SIBs), nevertheless, the understanding of sodium storage mechanism in HC is very limited. As an important aspect of storage mechanism, the steady state of sodium stored in HC has not been revealed clearly to date. Herein, the formation mechanism of quasi-metallic sodium and the quasi-ionic bond between sodium and carbon within the electrochemical reaction on the basis of theoretical calculations are disclosed. The presence of quasi-metallic sodium is further confirmed with the assistance of a specific reaction between the sodiated HC electrode and ethanol, by analyzing the reaction products with Fourier-transform infrared spectroscopy, gas chromatography, and nuclear magnetic resonance. Moreover, based on the specific chemical reaction, the composition of fully sodiated HC is estimated to be NaC_6.7, and the corresponding capacity of sucrose-derived HC is calculated to be 333.4 mAh g⁻¹ in SIBs, matching well with the experimental result. This work helps to reveal the steady state of sodium and improve the understanding of sodium storage behavior in HC from the aspect of charge transfer. In addition, the proposed method is also expected to pave the way to investigate sodium storage mechanisms in other electrode systems.