Electron-Deficient Property of Tetraborate Ions Maintains the Structural Stability of Layered Hydroxide

Nickel hydroxide is a commonly used anode material for supercapacitors, offering advantages such as high theoretical capacity and abundant raw material availability. However, it also faces challenges, including low electrical conductivity and limited cycling stability. In this work, a heterostructure composed of NixCo1-x(OH)₂ layered material with cobalt-substituted nickel sites and reduced graphene oxide (rGO) was constructed via a one-step hydrothermal approach, and the interlayer anions were further modulated to elevate the entire electrochemical efficacy of the resulting material. Herein, the tetraborate ions (B₄O₇²⁻) with electron-unsaturated characteristics are utilized to bond with the hydroxyl groups within the nickel hydroxide layer, thereby generating a robust pillar effect that enhances the reaction kinetics and improves the stability of the layered structure. Simultaneously, partial substitution of the nickel active site by cobalt facilitates the formation of a more stable layered hydroxide structure, reducing microstructural collapse following proton intercalation and deintercalation. Additionally, the incorporation of rGO provides more active sites for material growth and leads to an enhancement in electron transport efficiency. The electrochemical properties of the as-prepared Ni₉Co-TB/G0.03 composites were markedly enhanced. The specific capacitance reached 1977 F·g⁻¹ under 1 A·g⁻¹, and the capacity retention rate stayed at 88% following 5000 cycles performed at 10 A·g⁻¹. An asymmetric supercapacitor, Ni₉Co-TB/G0.03(+)||AC(-), was constructed with a commercial activated carbon negative electrode, with energy density maximization as the evaluation criterion. The device exhibited a specific capacitance of 164 F·g⁻¹Total at 1 A·g⁻¹Positive. At an elevated current density of 80 A·g⁻¹Positive, 100 F·g⁻¹Total of specific capacitance was retained, equivalent to 61% capacitance retention. This demonstrated excellent rate capability and outstanding overall performance, simultaneously delivering 44 Wh·kg⁻¹ energy density and 16,687 W·kg⁻¹ power density.