For a wider adoption of electromobility, the market calls for fast-charging, safe, long-lasting batteries with sufficient performance. This drives the exploration of new energy storage materials, and also promotes fundamental investigations of materials already widely used. At the moment, renewed interest in anode materials is observed -- with a particular focus on graphite electrodes for lithium-ion batteries. Here, we focus on the upper limit of lithium intercalation in the morphologically quasi-ideal Highly Oriented Pyrolytic Graphite (HOPG). The state and long-term stability of a sample prepared by immersion of an HOPG crystal in liquid lithium metal at ambient pressure is investigated by static $^7$Li nuclear magnetic resonance (NMR). We resolved signatures of superdense intercalation compounds, LiC$_{6-x}$ with $x>0$, which are monitored upon calendaric ageing. {em Ab initio} thermodynamics and {em ab initio} molecular dynamics reveal the relative stabilities and kinetics of different superdense configurations, providing leads for the interpretation of the NMR results. Including these superdense structures in the conceptual design of high-energy, fast-charge electrodes might provide further insights on the failure mechanisms and performance of Li-ion batteries.
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