Electrochemical Synergistics between MnS–Co ₃ S ₄ and FeS–FeS ₂ /Nitrogen-Doped Defect-Rich Reduced Graphene Oxide in High-Performance All-Solid-State Asymmetric Pseudocapacitors

To develop high-performance pseudocapacitor devices with superior energy and power output along with excellent cycling durability, we employed kinetically controlled slow precipitation methods to synthesize MnS–Co 3 S 4 and FeS–FeS 2 /N-doped defect-rich reduced graphene oxide (ND-rGO) as the positrode and negatrode materials, respectively. The MnS–Co 3 S 4 exhibits nanocrystallinity, a uniform microstructure, phase uniformity, surface wettability, a large surface area, and a monomodal pore distribution in the mesopore and macropore region. The FeS–FeS 2 /ND-rGO exhibits nanocrystallinity, forms a heterocomposite, exhibits uniform microstructure, phase uniformity, and substantial defect density in ND-rGO. Electrochemical analysis of MnS–Co 3 S 4 reveals fast redox kinetics, high charge storage efficiency, low series resistance (∼0.82 Ω), charge-transfer resistance (∼0.53 Ω), relaxation time (1.59 s), and predominantly diffusion-controlled charge storage. Likewise, FeS–FeS 2 /ND-rGO demonstrates excellent kinetic reversibility and a wide operational window in the negative potential region. The 1.7 V MnS–Co 3 S 4 ||FeS–FeS 2 /ND-rGO ASSAPC device exhibits a hybrid charge storage mechanism combining surface and diffusion-controlled processes, high-rate areal- and mass-specific capacity/capacitance, a high specific energy (23 W h kg –1 ), high power density (3703 W kg –1 ) with robust cyclic charge storage stability (∼97.8% after 13 000 GCD cycles), and ∼100% energy efficiency under a very high-rate condition. This has been attributed to the synergistic interplay of the microstructural porosity, improved redox activity, enhanced conductivity, S 2– ions in the positrode and negatrode, and smooth ion/electron transport in the electrode materials. This study provides key insights into structure–property relationships in sulfide-based hybrid electrodes, underscoring their potential in cost-effective, stable, and high-performance ASSAPC devices for next-generation portable energy storage applications.