Electrochemical Intercalation of Potassium into Graphite

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 8103 wileyonlinelibrary.com High performance cathodes, including metal oxides,[11–14] phosphates,[15–18] hexacyanometalates,[19–21] sulfur,[22,23] metal sulfides,[24,25] and organic materials,[26–28] have been reported, and porous carbonaceous materials and metallic Sb and Sn anodes also show high capacity and long cycling stability.[29–34] In contrast to Li-ion and Na-ion batteries, very few studies on K-ion batteries have been reported.[6–10] Graphite, a standard anode in commercial Li-ion batteries through electrochemical formation of graphite intercalation compounds (GICs), has low capacity with fast capacity decay to potassium,[7] and is electrochemically inactive to sodium ions.[35,36] GICs have been extensively studied in last century because of its appealing physical, chemical, and electronic characteristics in superconductors, catalysts, and Li-ion batteries.[35] GICs are produced by inserting different chemical species into carbon layers of graphite without destroying the layered structure of the host graphite. Many metal atoms have been reported to be able to intercalate into graphite to form layered structures, particularly lithium GICs have been successfully employed in batteries. Although potassium GICs have been thoroughly studied in structures and chemical/physical properties last century,[35] little attention has been given to its application in batteries. Actually, stage phenomena of ternary potassium-dimethyl sulfoxide-GICs have been studied by electrochemically intercalating solvated potassium into graphite to different intercalation levels.[37–39] The electrochemically reversible intercalation/ deintercalation of alkali metal in graphite has been observed during the study of electrochromic effects of GICs as early as 1970s.[40,41] Recently, the electrochemically reversible intercalation of potassium into graphite was also proved in molten salt of KF at 1163 K, in which the electrochemical reversibility was evidenced by cyclic voltammetric measurement and X-ray diffraction (XRD) analysis.[42,43] Unfortunately, the high temperature, the instability of the resulting K-GICs in molten KF, and severe structure damage of graphite during the insertion/deinsertion processes make this system unsuitable for energy storage. In 1997, KC8, a fully intercalated K-GIC, prepared by two-zone vapor transport method was used as anode in Li-ion batteries and the successful extraction of potassium was identified by recovered graphite structure after releasing potassium.[44] Very recently, Ji group reported room temperature electrochemical performance of potassium GICs using graphite, where the electrochemically reversible formation of stage phases of potassium GICs was confirmed by ex situ XRD measurement.[7] However, mechanism behind the poor cycling stability (capacity loss of 0.98% per cycle) and slow kinetics of electrochemical intercalation of K into graphite needs to be explored. Electrochemical Intercalation of Potassium into Graphite