Confinement-Induced Stability Evolution of Na Clusters in Closed Pores of Hard Carbon: A DFT and AIMD Study

Hard carbon (HC) is considered as the most promising anode material for sodium-ion batteries due to its disordered structure, high sodium storage capacity, and low cost. However, within the domain of low-voltage plateaus, the thermodynamic stability and kinetic evolution of Na clusters confined within microporous structures have remained inadequately characterized. In this work, the nucleation mechanisms as well as the thermodynamic stability of Na clusters are thoroughly investigated by density functional theory (DFT) and ab initio molecular dynamics (AIMD) methods based on a well-built disordered HC structure model. A new method to construct the HC model is applied by applying strain to achieve contraction of the carbon skeleton through DFT calculations to obtain the curved HC model. On the basis of the HC model, we calculated the confinement effect of the closed pore on the cluster by DFT calculations. We find that the bent carbon layer leads to an elevated electrostatic potential, which makes it easy to attract clusters, as well as a change in the configuration of the clusters, thus affecting the electron distribution within the clusters, leading to different cohesive energies and affecting the reversibility of the clusters. During the evolution of cluster dynamics, Na⁺ tends to form clusters in larger closed pores while it tends to fill the pore walls in smaller closed pores. This work provides theoretical guidance for improving the plateau capacity and initial Coulombic efficiency of sodium-ion batteries.