Highly Concentrated and Nonflammable Electrolyte for High Energy Density K-Based Dual-Ion Battery

K-based dual-ion batteries (K-DIBs) show the advantages of being cost-effective, high-voltage, and environmentally friendly; however, their energy density is restricted by limited intercalation capacity of anions at the graphite cathode and low electrolyte concentration. Herein, we developed a highly concentrated electrolyte system by dissolving 6.6 m potassium bis(fluorosulfonyl)imide (KFSI) into nonflammable trimethyl phosphate (TMP). Advantages exhibited by this concentrated KFSI/TMP electrolyte include (1) high oxidation potential (over 5.4 V) that improves the intercalation reversibility and capacity of FSI– anions at the graphite cathode side; (2) enhanced cycling performance of tin (Sn) foil anode; and (3) remarkably improved energy density of K-DIBs. With the concentrated electrolyte system, a proof-of-concept K-DIB constructed with a graphite cathode and a Sn foil anode displays a high specific discharge capacity of 93.6 mA h g–1 at 300 mA g–1 and energy density of ∼144 W h kg–1 (including electrode active materials and electrolytes), which are among the best results compared with previously reported K-DIBs.