A new perspective for potassium intercalation chemistry in graphitic carbon materials

Abstract

Potassium intercalation mechanism in graphitic carbon materials is known to be a staging reaction similar to that of lithium, despite their antithetic intercalation trend in turbostratic carbon (TBC) materials. This study clarified the distinctive potassium intercalation behavior of graphitic carbon materials with different local microstructures through a systematic comparative investigation. In contrast to the monotonic stacking sequence of lithium-intercalated graphitic carbon materials, multiple potassium-intercalated graphitic configurations can be formed by potassiation at an energy level similar to the theoretical KC8 formation. Accordingly, potassium intercalation in TBC materials can result in a theoretical low-voltage plateau capacity without long-range ordering. In contrast, the high energy cost for the threshold number of initial potassium insertions significantly hinders two-phase potassium intercalation in well-ordered graphite-like carbon materials, leading to a poor plateau capacity. Hence, potassium intercalation is more favorable in a turbostratic microstructure that includes intrinsic defects, whereas the presence of too many intrinsic defects reduces the polyhexagonal carbon plane for turbostratic intercalation, leading mainly to solid-solution potassium intercalation. Based on these results, the potassium intercalation mechanism in graphitic carbon materials can be classified into three different types such as 1) solid-solution intercalation, 2) turbostratic intercalation, and 3) two-phase intercalation.