Hierarchical NiMoS and NiFeS Nanosheets with Ultrahigh Energy Density for Flexible All Solid‐State Supercapacitors

Highly flexible supercapacitors (SCs) have great potential in modern electronics such as wearable and portable devices. However, ultralow specific capacity and low operating potential window limit their practical applications. Herein, a new strategy for the fabrication of free‐standing NiMoS and NiFeS nanosheets (NSs) for high‐performance flexible asymmetric SC (ASC) through hydrothermal and subsequent sulfurization technique is reported. The effect of Ni2+ is optimized to attain hierarchical NiMoS and NiFeS NS architectures with high electrical conductivity, large surface area, and exclusive porous networks. Electrochemical properties of NiMoS and NiFeS NS electrodes exhibit that both have ultrahigh specific capacities (≈312 and 246 mAh g−1 at 1 mA cm−2), exceptional rate capabilities (78.85% and 78.46% capacity retention even at 50 mA cm−2, respectively), and superior cycling stabilities. Most importantly, a flexible NiMoS NS//NiFeS NS ASC delivers a high volumetric capacity of ≈1.9 mAh cm−3, excellent energy density of ≈82.13 Wh kg−1 at 0.561 kW kg−1, exceptional power density (≈13.103 kW kg−1 at 61.51 Wh kg−1) and an outstanding cycling stability, retaining ≈95.86% of initial capacity after 10 000 cycles. This study emphasizes the potential importance of compositional tunability of the NS architecture as a novel strategy for enhancing the charge storage properties of active electrodes. The present investigation demonstrates a novel strategy for the fabrication of free‐standing NiMoS and NiFeS nanosheets (NS) through a cost‐effective hydrothermal technique with subsequent sulfurization. Flexible NiMoS NS//NiFeS NS asymmetric supercapacitor delivers an ultrahigh energy density of ≈82.13 Wh kg−1 at 0.561 kW kg−1, and outstanding cycling stability, retaining ≈95.86% of the initial capacity after 10 000 cycles.