Insight into Sodium Insertion and the Storage Mechanism in Hard Carbon

While the technological importance of carbon-based anodes for sodium-ion batteries is undebated, the underlying mechanism for sodium insertion and storage is still strongly disputed. Here, we present a joint experimental and theoretical study that allows us to provide detailed insights into the process of Na insertion in nongraphitizable (hard) carbon. For this purpose, we combine data from in situ Raman scattering of Na insertion in hard carbon with density functional theory-based lattice dynamics and band structure calculations for Na insertion in graphitic model structures used for a local description of graphitic domains in hard carbon. The agreement of experimental results and computational findings yields a clear picture of the Na insertion mechanism, which can be described by four different stages that are dominated by surface morphology, defect concentration, bulk structure, and nanoporosity. On the basis of the resulting model for sodium insertion, we suggest design strategies to maximize the capacity of hard carbon.