Exploring the effect of interlayer distance of expanded graphite for sodium ion storage using first principles calculations

Expanded graphite (EG) has been shown to be able to store a significant amount of sodium ions. Understanding the alkali metal ion storage in EG is of importance for improving EG electrode performance. In this work, the effect of interlayer distance of pure EG on sodium ion storage was investigated using the density functional theory calculation method. EG structure models with interlayer distances ranging from 3.4 angstrom to 10.0 angstrom were simulated. It was found that EG can store a fairly large amount of sodium ions through an intercalation mechanism without any contributions from the co-intercalation mechanism or adsorption mechanism if the interlayer distance is larger than 4.4 angstrom and smaller than 6.0 angstrom. It was also found that an interlayer distance of 6.0 angstrom gives strong binding energy of sodium ions with EG forming thermodynamically stable sodium-graphite intercalation compound (Na-GIC). However, when the interlayer distance becomes larger than 6.0 angstrom, the binding energy between sodium ions and EG becomes weaker. Computational results have also shown that the enthalpy of formation of the Na-GIC of EG is energetically more favourable when the interlayer distance is increased. An optimal d-spacing of EG for sodium ion storage was identified in this work. These findings provide atomistic insights into sodium ion storage in EG, providing guidelines for the design of graphite-based anode materials for sodium-ion batteries.

Automated summary

By using density functional theory calculation method, the effect of interlayer distance on sodium ion storage in pure EG was investigated. The results showed that within the range of 4.4 angstrom to 6.0 angstrom, EG can store a significant amount of sodium ions through an intercalation mechanism, forming a stable sodium-graphite intercalation compound.