Revisiting the Storage Capacity Limit of Graphite Battery Anodes: Spontaneous Lithium Overintercalation at Ambient Pressure

Abstract

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 materials already widely used. Presently, renewed interest in anode materials is observed - primarily 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, with a LiC6 stoichiometry corresponding to nominally 100% state of charge. We prepare a sample by immersion in liquid lithium at ambient pressure and investigate it by static 7Li nuclear magnetic resonance (NMR). We resolve unexpected signatures of superdense intercalation compounds, LiC6-x. These have been 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 insignificant under ambient conditions. We monitor the sample upon calendaric ageing and employ ab initio calculations to rationalize the NMR results. 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 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.