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.