Boosting the rate capability and cycling stability of binder-free NiCo2S4 nanoarray electrodes via manganese doping

Abstract

Developing binder-free electrodes with superior rate capability and cycling stability is critical for supercapacitors, yet remains challenging. Although NiCo₂S₄ (NCS) nanoarrays exhibit high capacity and conductivity, their practical application is limited by poor rate performance and structural degradation. Herein, we synthesize Mn-doped NiCo₂S₄ (Mn-NCS) hollow nanoneedle arrays directly on Ni foam via hydrothermal methods. Optimized Mn_0.2-NCS achieves an exceptional specific capacitance of 8.92 F cm⁻² (1749 F g⁻¹) at 1 mA cm⁻² and retains 73.4% capacitance at 15 mA cm⁻², far exceeding undoped NCS (45.4%). The electrode also maintains 90.6% capacity retention after 5000 cycles at 15 mA cm⁻², demonstrating unparalleled stability. Experimental characterization reveals Mn doping reduces ion diffusion resistance (12.1 vs. 20.8 Ω s⁻¹ for NCS) and promotes surface-dominated charge storage (61.6% capacitive contribution at 1 mV s⁻¹). Density functional theory calculations confirm enhanced structural stability, increased electronic states near the Fermi level and significantly strengthened OH⁻ adsorption energy. This work establishes Mn doping as an effective strategy to engineer high-performance ternary sulfide electrodes.