Insight into the structural evolution mechanism and potassium storage performance of expanded graphitic onion-like carbon

Abstract

Although expanded graphite prepared from flake graphite has been well developed, the expanded mechanism of graphite with a confined structure has not been well researched. Herein, we systematically reveal how confined graphite is transformed into bamboo shoot-like graphite by a modified Hummers’ method using microsized graphitic onion-like carbon (GOC). Unlike the parallel stacked open structure of natural flake graphite (NFG), GOC covered by carbon basal planes presents a restricted polyhedral structure, which makes GOC present a different expansion pattern from the longitudinal extension of NFG, but dissociates into bamboo shoot-like expanded graphite along different directions of the polyhedron. Notably, different from the planar structure of expanded natural flake graphite (ENFG), expanded graphitic onion-like carbon (EGOC) exhibits a helical structure with plenty of wavy wrinkles, and presents a higher structural stability than ENFG. EGOC exhibits superior potassium-ion storage performance with high reversible capacity (404 mA h g⁻¹ at 0.2C), outstanding rate capability (218 mA h g⁻¹ at 5C) and superior cyclability (275 mA h g⁻¹ after 1000 cycles at 2C). Furthermore, EGOC is used as the anode to assemble potassium-ion capacitors that deliver an ultrahigh energy/power density (113.8 W h kg⁻¹ and 12.92 kW kg⁻¹, respectively). Our work further perfects the thermal expansion mechanism of spherical graphite and provides new insights for the design and development of competitive expanded graphite from GOC.