Insights into pseudographite-structured hard carbon with stabilized performance for high energy K-ion storage

Pseudo-graphite structured hard carbon (HC) recently attracted increased interest as an alternative to graphite for better rate performance K-ion batteries (KIBs). Here, we optimized the synthesis temperature of HC and found that optimum Sichuan pepper hard carbon (SP-HC) 1200 delivers a high reversible capacity (284 mA h·g−1 at a current density of 0.1 C), an outstanding rate property (167 mA h·g−1 at 1 C), and an excellent stable cycling performance (71% capacity retention over 300 cycles at 0.5 C). Furthermore, in-situ XRD analysis in combination with ex-situ characterizations and kinetics analysis provide an insightful understanding of potassiation mechanism, revealing that K-ions are absorbed into the gaps of the micro-graphitic structure (HC) while intercalating micro-graphitic layers, which corresponds to a sloping voltage profile. When the gap is almost filled with K-ions, the intercalation plays dominant role, resulting to a gently plateauing voltage profile. In addition, an asymmetrical K-based dual-ion battery (AK-DIB) based on SP-HC 1200 delivers 67 mA h·g−1 of the initial capacity at 1.3 C, and a superior cycling stability of 90% retention of capacity after 300 cycles. The present findings demonstrated that hard carbon possesses promising properties able to meet the level required for practical applications. [Display omitted] •Evolutional structures of hard carbons are derived from gradient temperatures.•Evolutional micro-graphite structures own different K-storage behavior.•In-situ XRD and ex-situ tests reveal K-storage mechanism in hard carbon.•A K-double ion battery based on the hard carbon and graphite electrode is fabricated.