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

Abstract 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 ZnCl 2 electrolyte and an oxygen‐enriched carbon electrode, an effective (SCN) 2 /SCN − electrochemistry is activated. On the one hand, intensive ion coordination decreases free H 2 O 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.