The roles of electrolyte chemistry in hard carbon anode for potassium-ion batteries

Potassium-ion battery (PIB) is a rising star in the rechargeable battery field due to its potential low cost and high energy for large-scale applications. Hard carbon (HC) is one of the most popular anodes for practical PIBs due to its high K-ion storage and relatively low material cost. However, the role of electrolytes in determining the HC performance and K-ion storage mechanism has been rarely investigated. Herein, we systematically studied the influence of four electrolyte systems, i.e., two K salts (i.e., KPF6 and KFSI) in carbonate ester and ether solvents, on the K-ion mobility, cycling stability, and charge transfer kinetics of the HC anode in PIBs. It is found that HC anode achieved the best cycling stability and kinetics performance in the KFSI EC/DEC electrolyte. Mechanismic study disclosed that the improved performance could be ascribed to the formation of robust KF-rich SEI resulting from FSI- decomposition, which effectively prevented irreversible side reactions and severe structural decay (e.g., exfoliation and pulverization). The degradation mechanisms of other electrolyte systems are also explained from the viewpoints of SEI formation and solvation/desolvation effect. It is expected that this work will provide guidance on the anode and electrolyte selection and design for PIBs in the near future.