Elucidation of the Sodium-Ion Storage Behaviors in Hard Carbon Anodes through Pore Architecture Engineering

Hard carbon stands out as an auspicious anode material for commercial sodium-ion batteries, yet the correlation between plateau-potential capacity and its pore architecture remains poorly understood. In this study, we systematically investigated the sodium-ion storage behavior in hard carbons with tailored pore architecture. The plateau-potential capacity of hard carbon is attributed to the filling of sodium clusters within closed nanopores and open nanopores that are impervious to the solvent molecules of the electrolyte. Small-angle X-ray scattering (SAXS) has been shown to be an effective method for estimating the volume of nanopores that can store sodium clusters. A rapid and user-friendly butanol pycnometry technique is designed to assess the volume of nanopores available for sodium-ion storage. This method has established a linear correlation between the nanopore volume detected and the plateau-potential capacity measured experimentally. We identified two scenarios where the plateau-potential capacity deviates from the congruence linear relationship established by SAXS and butanol pycnometry techniques. First, sodium clusters are unable to fill nanopores larger than 4 nm and could only partially fill those larger than 2 nm. Second, the diffusion of Na⁺ ions is impeded in graphene nanodomains with tight interlayer spacing and extended crystalline planes.