Influence of KPF6 and KFSI on the Performance of Anode Materials for Potassium-Ion Batteries: A Case Study of MoS2

Recently, nonaqueous potassium-ion batteries (KIBs) are attracting because of increasing interest due to the abundance of potassium resources, but the systematic study about the effects of electrolyte's salt on the electrochemical performance of electrode materials is still insufficient. Here, it is shown that the capacity retention and Coulombic efficiency of commercial micrometric MoS2 can be remarkably improved by simply using potassium bis(fluorosulfonyl)imide (KFSI) over potassium hexafluorophosphate (KPF6) dissolved in ethylene carbonate/diethyl carbonate as the electrolyte. By constructing various cell configurations, it is discovered that the degradation of MoS2 parallel to K half-cells in KPF6-containing electrolyte originates from the failure of the MoS2 electrode. The solid electrolyte interphase (SEI) layer formed on MoS2 during cycling was systematically investigated by using a series of characterizations. It is found that a stable, protective, and KF-rich SEI layer is formed on MoS2 in the KFSI-containing electrolyte, while an unstable, KF-deficient, and organic species-rich SEI layer is formed in the KPF6-containing electrolyte. Finally, the origins of such differences are discussed, which will provide new insights into further exploration of novel electrolytes for KIBs.