The potassium storage performance of carbon nanosheets derived from heavy oils

• Heavy oil composition critically influences the structure and potassium storage performance of derived carbon nanosheets. • FCCS-derived carbon nanosheets exhibit optimal properties: smaller layer thickness, larger interlayer spacing (0.372 nm), and more folds. • CNS-FCCS delivers high capacity (248.7 mAh g−1 at 0.1 A g−1) and excellent cycling stability (190.8 mAh g−1 after 800 cycles at 1.0 A g−1). • The molten salt method effectively converts heavy oils into high-performance carbon anodes for potassium-ion batteries.