Pore-structural descriptors of hard carbon anodes towards tuning their interface stability for sodium storage

Abstract

The sodium storage behavior of hard carbon anodes depends on the thermodynamics and dynamics of electrolyte filling in nanopores, which is governed by pore throats controlling solvent sieving and pore cavities that provide storage space. These two types of structural elements are tightly coupled, and their size ratio directly determines the capillary effect during electrolyte filling. Herein, a series of hard carbon anodes with tunable pore throats and cavity sizes are prepared to study the effect of size ratio on the electrolyte filling behavior. Importantly, for the first time, λ (throat diameter/cavity diameter) is proposed as a descriptor that quantifies the constraints of pore structures on electrolyte filling and sodium storage. A low λ value means a narrow throat and a large cavity, which can effectively sieve solvent molecules, provide sufficient sodium storage space, and increase capillary resistance, thereby synergistically creating a slow and controllable ion transport environment. Guided by this descriptor, the optimized hard carbon anode achieves a high initial Coulombic efficiency of 90.4% and a reversible capacity of 397.4 mAh g−1 at 0.05 A g−1. This work further provides an understanding of the mechanism underlying the interaction between pore structural elements and battery performance by elucidating the coupling between osmotic pressure and capillary force.