Enabling High Performance Potassium-Based Dual-Graphite Battery Cells by Highly Concentrated Electrolytes

This work studies an advanced potassium dual-graphite battery (DGB) cell system based on a highly concentrated electrolyte (HCE), i. e., potassium bis(fluorosulfonylimide) (KFSI) in ethyl methyl carbonate (EMC). Structural and electrochemical properties of the designated KFSI/EMC electrolyte are investigated and discussed at different concentrations (1.0 M-4.0 M). Ionic aggregation at high salt concentrations leads to enhanced electrochemical stability and pushes the oxidative stability limit beyond 5 V vs. K|K+. Based on those results, the electrochemical performance with graphite as positive and negative electrode active material in graphite||K metal cells is presented. For potassium intercalation into graphite, an impressive capacity retention and rate capability is found for the EC-free HCEs (EC=ethylene carbonate), outperforming potassium electrolytes used in the literature. New insights into the formation of the solid electrolyte interphase (SEI) are presented and confirm improved electrochemical performance. Additionally, high salt concentrations in the electrolyte stabilize the aluminium (Al) current collector and enable reversible intercalation of FSI anions into the graphite positive electrode. Furthermore, reversible cycling in DGB cells is shown and a capacity fading mechanism based on parasitic side reactions causing K+ ion accumulation in the negative electrode, followed by K metal plating, is comprehensively evaluated.