Redox chemistry of ternary graphite intercalation compounds: Theory and Experiment

• Applicants: Professor Dr. Philipp Adelhelm; Professorin Dr. Doreen Mollenhauer
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 298787956

Graphite intercalations compounds (GICs) constitute a class of materials with a large chemical variety. In the field of electrochemistry, the lithium-graphite system is the by far most studied one, owing to the technological relevance for lithium-ion batteries. Here, lithium ions intercalate between the layers of the graphite host to form a binary GIC with the stoichiometry LiC6. Surprisingly, sodium is a notable exception as it hardly forms binary GICs with graphite. In preliminary studies, however, it was shown that sodium ions intercalate in graphite electrochemically as long as suitable electrolytes are used. Using ethers as solvents, co-intercalation of the solvation shell takes place leading to the formation of a ternary GIC with the proposed composition Na(diglyme)2C20. Intriguingly, the formation of this compound appears to be highly reversible (more than 1000 cycles without significant loss in capacity) and is kinetically very much favored (low overpotentials are found). Furthermore, little evidence of solid electrolyte interphase forming or decomposing was found. Further investigations revealed ternary graphite intercalation compounds with different ether solvents, which show temperature dependent effects during intercalation. In addition, a first quaternary GIC was prepared electrochemically by combined co-intercalation of two different solvents.The aim of the project is the continuation of the study of the redox chemistry of ternary GICs, which has so far been little studied, and the investigation of new, highly original aspects that will lead to an extension of the family of ternary and quaternary GICs. This will be achieved mainly through a combined approach of experimental and theoretical modelling.Important sub-objectives are (1) to elucidate the geometrical arrangement of solvent molecules and ions in the graphite structure and their influence on the ion mobility. Consideration will also be given to temperature and concentration dependence. (2) To investigate the changes when monovalent ions are replaced by divalent ions such as Mg2+. (3) To test the transferability of the t-GIC concept to mixed cationic systems. In this case, the combined intercalation of different cations, made possible by the co-intercalation of solvent molecules, lead to new quaternary GICs.On the experimental side, electrochemical investigations in particular are to be carried out for this purpose, supplemented by various analytical methods (if possible, under in situ or operando conditions). On the theoretical side density functional theory and molecular dynamics simulation will be used to provide explanations and predictions for the formation and properties of ternary and quaternary GICs.

• DFG Programme: Research Grants