Revisiting potassium intercalation in graphite: an <i>operando</i> characterisation and computational approach

Potassium-ion batteries (KIBs) with graphite anodes are emerging as a highly promising "beyond lithium" technology driven by battery demands, potassium's abundant reserves and the inherent similarities in intercalation chemistry to lithium-ion systems. Despite this potential, a understanding of potassium intercalation into graphite, particularly concerning early intercalation stages and the in-plane ordering of K⁺ within graphite intercalation compounds (GICs), lacks sufficient elucidation. Herein, we employed a multi-modal, <i>operando</i> characterisation approach to elucidate the correlation of electrochemical potassiation and structural evolution in graphite, hence unravelling the specific mechanisms of K-ion storage. <i>Operando</i> electrochemical dilatometry precisely quantifies the macroscopic volume expansion of a graphite electrode during potassiation. Meanwhile, <i>operando</i> synchrotron X-ray diffraction (XRD) records ordered phase transitions during early-stage intercalation, detailing the formation of distinct GIC phases. Furthermore, Raman spectroscopy and density-functional theory (DFT) reveal the in-plane ordering of K⁺ within the graphite gallery and stacking modes. <i>Operando</i> optical microscope and UV-vis spectroscopy together provide insights into the changing optical properties, linking these changes to different GICs and electronic structural changes. This comprehensive study offers fundamental mechanistic insights into K-ion storage in graphite, paving the way for the rational design of high-performance KIB anodes.