Hierarchical MnO<sub>2</sub>/NiS–MnS with Rich Electro-Microstructural Physiognomies for Highly Efficient All-Solid-State Hybrid Supercapacitors
Yogesh Kumar Sonia, Sumanta Kumar Meher
Abstract
To revolutionize the charge storage efficiency of electrode materials for their utilizations in high Ragone efficient electrochemical energy storage devices, herein, a slow-precipitation-induced material growth approach has been innovated to design a hetero oxide–sulfide [MnO 2 /NiS–MnS (MnO 2 /Ni–Mn–S)] material with smaller crystallite size, ultrathin assembled-sheet-like microstructure, and perceptible phase physiognomies (α-MnO 2, MnS, and α-NiS). The electroredox assessment of MnO 2 /Ni–Mn–S illustrates high pseudocapacitive energy storage efficiency, significant redox reversibility, lowly constrained bulk accessibility of the OH – ions at higher rate electrochemical reaction conditions, dominance of semi-infinite diffusion-controlled electrochemical processes, and extremely low charge-transfer resistance (∼1.45 Ω), total series resistance (∼0.51 Ω) and diffusion (Warburg) resistance. A fabricated 1.8 V MnO 2 /Ni–Mn–S||nitrogen-doped reduced graphene oxide (N-rGO) all-solid-state hybrid supercapacitor (ASSHSC) device with N-rGO as the negative electrode material delivers high area and mass specific capacitance/capacity, ∼100% Columbic efficiency at high-rate operating conditions, and very low charge-transfer and Warburg resistance. The MnO 2 /Ni–Mn–S||N-rGO ASSHSC device also delivers excellent Ragone efficiency ( E D = 31.5 W h kg –1 at P D = 937.5 W kg –1 and E D = 15.5 W h kg –1 at P D = 2767.5 W kg –1 ) and ∼97.6% retention of charge storage after 11,000 uninterrupted charge–discharge cycles. The significantly improved supercapacitive charge storage efficacy of MnO 2 /Ni–Mn–S is ascribed to the cohesive redox activity of Ni 2+, Ni 3+, Mn 2+, and Mn 3+ and nonstoichiometric Ni 2±δ, Ni 3±δ, Mn 2±δ, and Mn 3±δ ions, rich ion-disseminating bulk, S 2– vacancy-induced electronic conductivity, and suitable electro-microstructural physiognomies for the electrochemical processes.