Ca²⁺-Based Dual-Carbon Batteries in Ternary Ionic Liquid Electrolytes

Herein, we propose Ca²⁺-based dual-carbon batteries (DCBs) that undergo a simultaneous occurrence of reversible accommodations of Ca²⁺ in a graphite anode (mesocarbon microbeads) and of bis(trifluoromethanesulfonyl)imide (TFSI^-) in a graphite cathode (KS6L). For this purpose, we precisely tune electrolytes composed of Ca²⁺ complexed with a single tetraglyme molecule ([Ca:G₄]) in N-butyl-N-methylpyrrolidinium TFSI (Pyr₁₄TFSI) ionic liquid (IL). This ternary electrolyte is required for the enhancement of anodic stability that is needed to accomplish maximal TFSI^- intercalation into KS6L at a high potential. A solution of 0.5 M [Ca:G₄] in IL ([Ca:G₄]/IL) is found to be optimal for DCBs. First, the electrochemical properties and the structural evolution of each graphite in a half-cell configuration are described to demonstrate excellent electrochemical performance. Second, the negligible intercalation of Pyr₁₄⁺ into an MCMB anode is ascertained in 0.5 M [Ca:G₄]/IL. Finally, DCBs are constructed by coupling two electrodes to show high capacity (54.0 mA h g^-1 at 200 mA g^-1) and reasonable cyclability (capacity fading of 0.022 mA h g^-1 cycle^-1 at 200 mA g^-1 during 300 charge/discharge cycles). This work is the first to examine DCBs based on Ca²⁺ intercalation and helps pave the way for the development of a new type of next-generation batteries.