The inferior cycling stability critically impedes the development of potassium-ion batteries (KIBs). The solid electrolyte interface (SEI) for nitrogen doped graphite foam (NGF) was studied in both KPF₆ and KN(SO₂F)₂ (KFSI)-based organic electrolytes, aiming to unravel the SEI effect on K⁺ ion storage mechanism. Electrochemical characterizations disclose that the KN(SO₂F)₂-based cells deliver improved electrochemical performance in terms of reversibility and cycling stability, compared to KPF₆ based cells. Experimental results including depth-profiling XPS and FTIR spectra, together with the theoretical calculations, reveal that (CH₂OCO₂K)₂, C₂H₅OCO₂K, KF and K₂CO₃ are the dominant components of SEI layers in both electrolytes. Particularly, the amount of K₂CO₃ in KPF₆-based electrolyte is much more than that in KFSI-based electrolyte, resulting in inferior stability. Moreover, the depth-profile XPS results indicate that the SEI formed in KFSI based electrolyte is much more stable, compact and thinner than that in KPF₆ based electrolyte. All these features, together, ensure good stability and high reversibility in KFSI-based electrolyte.

The nitrogen doping effect on K⁺ ion storage was also explored. Enhanced electrochemical performance was identified upon increasing the nitrogen content, due to i) the enrichment of active sites for K⁺ ion storage and ii) the improved electronic conductivity. Moreover, the electrochemical performance is strongly dependent on N-doping types. Specifically, the pyridinic nitrogen dominates the reversible capacity, as identified by the presence of critical point at 4.29 at. %. In which, the atomic ratio of pyridinic N in NGF is 2.53 at. %, which is higher than that of 7.03 at. %, with only 2.16 at. % pyridinic N. The result is consistent with the theoretical study, which verifies that the charge transferred from K ions to NGF increases with the pyridinic nitrogen doping. Our results promote better understanding of K⁺ ion storage mechanism in graphite and provide invaluable guidance for optimized carbon-based electrode design for high-performance KIBs.