Enabling Effective Thiocyanogen/Thiocyanate Couple for the Zinc─Pseudohalogen Battery

Pseudohalogens are neutral molecules with similar chemical properties to halogens that can be utilized in zinc batteries. For instance, thiocyanogen, (SCN)2, features a fairly positive standard redox potential when coupled with SCN− (0.77 V vs SHE, standard hydrogen electrode) and serves as a feasible cathode to pair zinc anode. However, the severe hydrolysis of (SCN)2 and generation of trithiocyanate ion, (SCN)3−, and parathiocyanogen, (SCN)x, seriously undermine the couple reversibility, leading to a poor electrochemical performance. Herein, by selecting a water‐in‐salt ZnCl2 electrolyte and an oxygen‐enriched carbon electrode, an effective (SCN)2/SCN− electrochemistry is activated. On the one hand, intensive ion coordination decreases free H2O and SCN− contents to suppress detrimental hydrolysis of (SCN)2 and generation of (SCN)3− and (SCN)x. On the other hand, high electronegativity of oxygen‐containing functional groups facilitates the adsorption of in situ generated (SCN)2 and reduces side reactions. As a result, Zn─(SCN)2 battery can be charged–discharged for 500 cycles and exhibits high capacities (479 mAh g−1 based on the mass of (SCN)2 or 1.73 mAh cm−2 based on device area) and remarkable energy densities (644 Wh kg−1 or 2.216 mWh cm−2). This first reported Zn─(SCN)2 battery pioneers the zinc─pseudohalogen battery and raises it to a higher level among aqueous batteries.