Elucidating Sodium-Ion Storage Mechanisms in Bowl-Shaped Hierarchical Porous Carbon Anodes: A Combined Solid-State NMR and EPR Approach

Understanding sodium-ion storage in carbon anodes is essential for fabricating efficient batteries via the design of carbon architectures with optimized pores, defects, and surface area. Previous mechanistic studies have mainly focused on hard carbon anodes with poorly defined structures, leaving the structural correlation with sodium storage unexplored. This work presents an investigation of the Na+ ion storage mechanisms in bowl-shaped hierarchical porous carbon anodes using a combination of nuclear magnetic and electron paramagnetic resonance spectroscopies. The material exhibits hierarchical porosity, a surface area of 917.5 m2/g, and delivers a reversible capacity of 250 mAh/g at 0.1 A/g in 1 M aqueous NaClO4. Solid-state magic-angle spinning nuclear magnetic resonance is used to identify four distinct Na+ ion environments in the carbon anode material. In addition, temperature-dependent continuous-wave electron paramagnetic resonance spectroscopy enables the differentiation between weakly adsorbed Na+ ions and more strongly interacting Na+ ions by analyzing line widths and g values as the cell voltage is varied. Our results reveal the correlation of the sodium storage processes: adsorption, insertion, and intercalation, to the potential of the electrochemical cell. This work establishes a clear relationship between hierarchical carbon architecture and Na+ storage pathways as a function of potentials. Providing mechanistic insight will help design advanced carbonaceous anodes for efficient sodium batteries.