The market quest for fast-charging, safe, long-lasting, and performant batteries drives the exploration of new energy storage materials, but also promotes fundamental investigations of those already widely used. Presently, revamped interest in understanding fundamental mechanisms governing anode materials is observed – primarily graphite electrodes for Li-ion batteries. Many aspects of the charging and discharging mechanism, as well as the onset of lithium plating, are to this day poorly understood at the atomistic level.

We focus on the upper intercalation limit in the morphologically quasi-ideal highly oriented pyrolytic graphite (HOPG), with a LiC₆ stoichiometry corresponding to 100% state of charge (SOC). HOPG represents a convenient model system for graphitic anodes to investigate lithium mobility at the upper SOC as it allows only few, well-defined fundamental diffusion processes.

We prepared a sample by immersion in liquid lithium at ambient pressure. Investigation by static ⁷Li nuclear magnetic resonance (NMR) resolves unexpected signatures of so-called superdense intercalation compounds, LiC_6-x. These were ruled out for decades, since the highest geometrically accessible composition, LiC₂, can only be prepared under high pressure.

We thus challenge the widespread notion that any additional intercalation beyond LiC₆ is not possible under ambient conditions. We monitored the sample upon calendaric aging and employed ab initio calculations to rationalise the NMR results. The computed relative stabilities of different superdense configurations reveal that non-negligible overintercalation does proceed spontaneously beyond the currently accepted capacity limit.

The associated capacity gain (of the order of few percent) is not worth pushing graphitic battery anodes beyond the LiC₆ limit in practical applications; rather these findings carry more fundamental implications. Neglecting overintercalation may hinder the correct interpretation of experimental observations, as well as the correct design of computational models, in investigations of performance-critical phenomena, as it is likely to play a crucial role in the onset mechanism of lithium plating and dendrite formation in real battery materials.

Chiara Panosetti

Appearances