(Invited) Potassium (co)Intercalation in Graphite: How Electrolyte Concentration May Trigger Different Cation Insertion Mechanisms

Abstract

Recently, potassium-ion batteries (KIB) have been considered as a potential alternative of Li-ion batteries due to the cost effectiveness and the variety of electrode materials. Graphite – the well-known anode in LIB – has gained numerous attention from KIB scientists owing to its ability of intercalating K+ ion to form KC8 with high theoretical capacity (278 mAh/g) and good cycling retention.KFSI in DME has been considered as a promising electrolyte for KIB due to its compatibility with not only K metal but also the electrode materials. However, the electrolyte concentration strongly affects the electrochemistry of graphite. A change in the intercalating species as increasing the salt concentration, going from the cointercalation of [K(DME)x]+ (observed at 1M) to the simple intercalation of bare K+ ions (at 5M), was confirmed by galvanostatic profiles and operando XRD.By a complementary theoretical-experimental approach combining Raman spectroscopy with quantum chemistry, we provide here an insightful and coherent explanation of the occurring of the two different mechanisms, showing that they depend upon the solvation of K+ ions in DME rather than on the different SEI formed, as usually observed in lithium batteries.In some specific conditions, it was also possible to observe the co-existence of the two mechanisms, with the partial formation of staged intercalated-cointercalated layers, proving without doubt that the nature of the solvated species can be a major driving force driving specific ion insertion mechanisms.