Interfacial Reaction\nMechanisms on Graphite Anodes\nfor K‑Ion Batteries

Abstract

Potassium-ion (K-ion) batteries (KIBs) potentially offer\nnumerous\nadvantages over conventional lithium-ion batteries as a result of\nthe high natural abundance of potassium and its lower positive charge\ndensity compared with lithium. This introduces the possibility of\nusing K-ion in fast charging applications, in which cost effectiveness\nis also a major factor. Unlike in sodium-ion batteries, graphite can\nbe used as an anode in K-ion cells, for which an extensive supply\nchain, electrode manufacturing infrastructure, and knowledge already\nexist. However, the performance of graphite anodes in K-ion cells\ndoes not meet expectations, with rapid capacity fading and poor first\ncycle irreversible capacities often reported. Here, we investigate\nthe formation and composition of the solid electrolyte interphase\n(SEI) as well as K⁺ insertion in graphite anodes in KIBs.\nThrough the use of energy-tuned synchrotron-based X-ray photoelectron\nspectroscopy, we make a detailed analysis at three probing depths\nup to ∼50 nm of graphite anodes cycled to various potentials\non the first discharge–charge cycle. Extensive SEI formation\nfrom a KPF₆/DEC/EC electrolyte system is found to occur\nat low potentials during the insertion of potassium ions into graphite.\nDuring the subsequent removal of potassium ions from the structure,\nthe thick SEI is partially stripped from the electrode, demonstrating\nthat the SEI layer is unstable and contributes to a significant proportion\nof the capacity upon both discharge and charge. With this in mind,\nfurther work is required to develop an electrolyte system with stable\nSEI layer formation on graphite in order to advance the KIB technology.