Dual-Function Electrolyte Additive Enhances Stability of Nickel-Cobalt Hydroxides for High-Performance Supercapacitors

Layered double hydroxides (LDHs) are highly attractive electrode materials due to their tunable interlayer spacing, efficient ion-exchange capability, and high reactivity. In this work, sheet-like NiCo LDH was synthesized via a hydrothermal method by optimizing the alcohol-to-water ratio, which effectively shortened the electron/ion transport distance and enhanced charge transfer kinetics. The as-prepared NiCo LDH delivered a specific capacitance of 3585 F g^-1 at 1 A g^-1, but its capacity retention degraded to only 56.9% after 2000 cycles. To address this issue, 0.05 M anhydrous zinc acetate (Zn(CH₃COO)₂) was introduced as an electrolyte additive in 6 M KOH. The Zn²⁺ ions not only suppress the dissolution of NiCo LDH during cycling but also participate in structural reconstruction through in situ formation of a Zn-Co LDH (Zn₂Co₃(OH)₁₀·2H₂O) phase, as confirmed by postcycling XRD (JCPDS #21-1477). This dual mechanism simultaneously stabilizes the LDH framework and enhances redox kinetics, leading to significantly improved cycling performance (89.5% retention after 2000 cycles). Furthermore, the assembled hybrid supercapacitor (NiCo LDH@NF//AC) achieved a high specific capacitance of 621 F g^-1 at 0.3 A g^-1, with an energy density of 82 Wh kg^-1 at a power density of 2.3 kW kg^-1. Notably, the device maintained over 70% capacity retention after 4000 cycles. This study provides an effective strategy for designing long-cycle-life supercapacitor electrodes through electrolyte engineering.