Tuning of defect-mediated AgInS₂-based chalcogenides for highly efficient electroactive energy storage electrodes

Metal chalcogenides with enhanced electrochemical and electrical properties are promising next-generation supercapacitor electrodes. Cu-, Mn-, and Cu-Mn-doped and pristine AgInS₂ nanoparticles have been synthesized through a simple and low-cost hydrothermal route. Herein, we demonstrate for the first time a direct correlation between dual-dopant synergy and the defect landscape and charge-transfer pathways of an AgInS₂ lattice (I-III-VI₂) that tolerates cation substitution without structural degradation. The structural analysis shows that both pristine and doped samples form a mixed orthorhombic/tetragonal AgInS₂ framework with orthorhombic phase dominance, whereas doping reduces the crystallite size from ∼25 nm to ∼14, ∼9, and ∼18 nm for Cu-, Mn-, and Cu-Mn-doped samples, consistent with the SEM particle-size reduction from 40-55 nm in pristine AgInS₂. Raman and X-ray photoelectron spectroscopy (XPS) verify the Ag-S and S-S vibrational modes and the Ag⁺, In³⁺ and S^2- chemical states of the host lattice, with Cu and Mn incorporated as Cu⁺ and Mn²⁺. UV-vis and photoluminescence (PL) spectroscopy indicate that doping modifies the electronic structure and defect landscape: the optical band gap varies from 3.52 eV to 3.50, 3.55 and 3.40 eV for the Cu-, Mn- and Cu-Mn-doped samples, while broad defect-related emission over 500-900 nm confirms intrinsic and dopant-induced defect states opening multiple charge-transfer pathways. The specific capacitance increases from 72.3 to 96.7, 129.8 and 189.27 F g^-1 for the pristine and Cu-, Mn- and Cu-Mn-doped electrodes at 5 mV s^-1, the capacitive contribution increases from 0.41% to 10.41%, 13.54% and 19.85%, and the charge-transfer resistance decreases from 1.652 Ω to 1.3, 1.22, and 0.693 Ω, with b-values of 0.5-0.69 indicating a hybrid diffusion and capacitive-controlled charge-storage mechanism. The improved performance of the co-doped electrode is ascribed to the synergistic effect of Cu and Mn, which enhances the accessibility of electroactive sites, interfacial charge transfer, and ion transportation, indicating that Cu-Mn co-doping is a promising strategy for defect and composition engineering of high-performance AgInS₂-based supercapacitor electrodes.