Redox Chemistry of Ternary Graphite Intercalation Compounds

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 for form a binary GIC with the stoichiometry LiC6. Well known are also binary GICs of other alkali metals such as potassium or rubidium. Surprisingly, sodium is a notable exception as it hardly forms binary GICs with graphite. In preliminary studies, however, it was shown that sodium ions do intercalate 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 compositions 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 favoured (low overpotentials are found). The voltage profile during intercalation/deintercalation shows complex features indicating the existence of several intermediate phases. The results from these preliminary studies are the starting point for this research proposal. So far, the reversible redox chemistry of ternary intercalation compounds has been hardly studied. Therefore, the overall aim of this project is to unravel and to understand the underlying principles of this type of reaction. To do so, a combination of experiment and theory is necessary and therefore this research proposal is a combined proposal from two groups (AG Adelhelm: experiment; AG Mollenhauer: theory). Important sub-goals are (1) Explaining and predicting the formation of ternary GICs by means of theoretical calculations and experiments: structure, thermodynamics and kinetics. Validation and assessment of different theoretical approaches. Comparison with binary GICs. (2) Explaining the special role of sodium within the alkali metals. (3) Determining the influence of solvent, salt concentration, ion species and surface chemistry on the formation of ternary GICs. Thereto, on the experimental side, a number of systematical studies are planned for which several electrochemical methods will be used. Structural analysis will be done by means of x-ray diffraction and microscopy, for example. In situ measurements by combining electrochemical methods with dilatometry will be used to gain a deeper understanding of the structural changes occurring during the redox reaction. On the theoretical side density functional theory will be utilized to explain and predict the formation and properties of binary and ternary GICs. For selected systems first principle molecular dynamics simulations will be employed.