A rechargeable Zn/graphite dual-ion battery with an ionic liquid-based electrolyte

This work presents a Zn/graphite dual-ion battery using natural graphite as the cathode and metallic zinc as the anode, with ionic liquid-based electrolyte. Upon charge, the Zn2+ cations deposit on the zinc anode, and the trifluoromethanesulfonate (TfO−) anions simultaneously intercalate into the graphite cathode; upon discharge, both the ions are released back into the electrolyte. The 0.2 M Zn(TfO)2/EMImTfO ionic liquid-based electrolyte exhibits a high electrochemical window of 2.8 V (vs. Zn2+/Zn) as well as a high conductivity of 7.3 ms cm−1. The deposition/stripping of zinc on a copper working electrode is systematically studied, which reveals that Zn2+ cations are mobile in the ionic liquid electrolyte, and zinc can be deposited and stripped in this electrolyte. The insertion/extraction of TfO− into/from graphite is also investigated, demonstrating a reversible process. At a current of 0.2 mA cm−2 and within the voltage of 0.8–2.8 V, the Zn/graphite dual-ion cells exhibit a high sloping discharge plateau within 1.8–2.4 V (corresponding to a medium voltage of about 2.0 V), a discharge capacity of 33.7 mAh g−1, and an energy density of 65.1 Wh kg−1; cells deliver a cyclability of 93.5% Coulombic efficiency for 100 cycles. The SEM image reveals that the zinc deposits are compact and dense, and uniformly distribute on anodes with an average grain size of about 200 nm, without the formation of dendrites. The use of high-safety electrolyte and low-cost electrode materials may enable this cell configuration to be a promising candidate for future energy storage systems.