Design guidelines for a high-performance hard carbon anode in sodium ion batteries

Unclear information on materials design significantly hinders the construction of enhanced hard carbon anodes with high sodium plateau capacities (SPCs). The pore volume ratio of hard carbon imposes thermodynamic limitations on the theoretical sodium plateau capacities (T-SPCs); however, relying solely on its pore structures is not sufficient to predict the practicable SPCs. This study entailed an investigation of a key kinetic parameter of hard carbons that mainly affects the coefficient of capacity utilization (CCU) of SPCs by using a series of polymeric hard carbons (PHCs) with different microstructures. A systematic study revealed a close relationship between the 2D to G band intensity ratio (I2D/IG) in the Raman spectrum and the internal kinetic barrier for sodium-ion transfer. On the basis of the thermodynamic and kinetic parameters, the structural indicator referred to as the SPC factor was devised to characterize the CCU for SPCs. The SPC factor clearly describes an optimal hard carbon anode as one that possesses a high closed pore volume ratio and low I2D/IG value. The highest SPC of similar to 400 mA h g-1 was achieved through simple microstructural tuning of the PHCs, demonstrating the feasibility of the proposed design guidelines for a high-performance hard carbon anode for sodium-ion batteries. A systematic investigation established a significant correlation between the 2D to G band intensity ratio (I2D/IG) in the Raman spectrum and the internal kinetic barrier for sodium-ion transfer, achieving the highest sodium plateau capacity of similar to 400 mA h g-1 (A30 sample).