Local spin-state tuning enables high-efficiency nickel sulfide cathode for stable alkaline Zn batteries

Nickel sulfide (Ni3S2) has shown great promise as advanced cathode in alkaline nickel-zinc batteries (ANZBs) for its good electrochemical reversibility and electronic conductivity. However, due to the unfavorable OH− adsorption capability and low intrinsic activity of Ni sites originated from the unsuitable d-electron spin state, the overall performances of Ni3S2 are yet unsatisfactory for practical ANZBs. Herein, we demonstrate the engineering of local spit state of Ni sites via a facile electrochemical activation in Ni3S2 to significantly boost its comprehensive energy storage capability. After electrochemical treatment, the spit state of fractional Ni2+ (eg4t2g4) sites of Ni3S2 is successfully transformed into high-valence Ni3+ (eg4t2g3), which considerably enhances the OH− adsorption capability and redox reactivity. The optimal Ni3S2 electrode exhibits a high capacity of 1.11 mAh cm−2 at 1 mA cm−2 and no capacity decay after 3000 cycles, which are far greater than the pristine Ni3S2 (0.39 mAh cm−2) electrode and also comparable to the best ever-reported alkaline cathodes. Furthermore, the assembled full ANZB can afford a peak energy density of 1.59 mWh cm−2 and power density of 33.92 mW cm−2. This work affords valuable insights into local electronic structure modulation of electrode materials for aqueous energy storage devices.