Insight into Sodium Storage Behaviors in Hard Carbon by ReaxFF Molecular Dynamics Simulation

The uncertainty in the Na-storage behaviors has seriously prevented the optimization of hard carbon electrodes, while molecular simulations can provide some unique research perspectives. In this work, a large-scale molecular model with over 65,000 carbon atoms was constructed for commercial hard carbon by combining experimental characterization and theoretical modeling, which well reproduces some key structural features. Based on such an experimentally determined hard carbon model, reactive molecular dynamics simulations were performed to obtain more detailed insights into the nature of Na storage at the atomic scale. It is found that the computational findings are basically in agreement with the electrochemical results. The final structure–capacity–potential relationship not only verifies the controversial "card-house" theory but also unveils a new mechanism involving small sodium clusters on defective sites in the high-potential sloping region. On this basis, it yielded a clear picture of the Na-storage behaviors in hard carbon, which consists of sodium adsorption, intercalation, and pore filling. Besides, several design strategies were proposed to maximize the capacity of hard carbon. Such fundamental research based on ReaxFF simulation can open new studying routes for elucidation of the Na-storage mechanism and may pave the way for designing high-performance hard-carbon-based anodes.