Needle coke anodes for potassium-ion batteries: Storage mechanism and interfacial evolution in soft carbon

Needle coke has attracted much attention as soft carbon anode material for potassium-ion batteries (PIBs) owing to its high crystallinity and conductivity. However, limited research has been conducted on the storage mechanism and the functional and structural evolution of the solid electrolyte interphase (SEI) in ester and ether based electrolytes. This work investigates the potassium ion storage mechanisms of soft carbon anodes in three electrolytes: KPF6 EC/DEC, KPF6 DME, and KFSI DME, and reveals that K+ stored in the KPF6 EC/DEC electrolyte could be through an "adsorption-intercalation" mechanism. Whereas K+ stored in the DME-based electrolytes is believed to undergo a "co-intercalation/adsorption-intercalation" process. Interfacial analyses indicate that the SEI formed in the EC/DEC electrolyte inhibits co-intercalation of potassium ion and solvent, which preserves sufficient space to accommodate potassium ion. Conversely, the [K-DME]+ solvation co-intercalation in DME-based electrolytes compromises the structural integrity, and results in a persistent capacity decline. This work offers novel insights into potassium ion storage in ester and ether-based electrolytes, and provides theoretical support for the fabrication of high-performance soft carbon anodes.