Effect of Crystallinity of Synthetic Graphite on Electrochemical Potassium Intercalation into Graphite
Daisuke Igarashi, Kei Kubota, Tomooki Hosaka, Ryoichi Tatara, Tsuyoshi Inose, Yuji Ito, Hirofumi Inoue, Masataka Takeuchi, Shinichi Komaba
Abstract
An effect of crystallinity of graphite on formation of graphite intercalation compounds (GICs) and reversibility in K cells was studied by comparing that in Li cells. Though high reversible capacities and coulombic efficiencies of graphite electrodes in K cells were achieved during initial cycles regardless of the crystallinity, high crystallinity graphite demonstrated less potential-hysteresis and superior capacity retention to low crystallinity graphite. Operando XRD measurement confirmed similar staging process of K-GICs for both graphite, however, high crystallinity graphite transformed into higher crystallinity of K-GIC as well as higher reversibility of potassium de-/intercalation than low crystallinity graphite. A turbostratic disorder in low crystallinity graphite led to redox-potential split and lower crystalline K-GIC and potassium-extracted graphite. Thus, the crystallinity of graphite, which includes coherence length and the degree of random stacking, is found to be a predominant factor for highly reversible potassium intercalation, which differs from the lithium case. We concluded that the high crystallinity is of importance for the application of graphite to long-life potassium-ion batteries.