Efficient charge storage by ZnCo2S4 nanoflakes@MgCo2O4 nanorods composite in Mg2+/Zn2+/K+ conducting electrolytes

Abstract

Heretofore unreported, a unique binder free composite cathode of zinc cobalt sulfide nanoflakes (NFs) and magnesium cobaltite nanorods (NRs) (ZnCo2S4@MgCo2O4) is presented and its potential for efficient energy storage is realized in terms of (1) a high electrical conductivity of ~0.7 mS cm−1 imparted by the mixed-valence states of Co2+/Co3+, (2) the mesoporous structure of the MgCo2O4 NRs enabling fast ion-diffusion and (3) the 3D ZnCo2S4 flakes with large exposed facets that allow abundant electrolyte-electroactive site-interactions. These factors cumulatively result in a high specific capacitance (SC) of 609 F g−1 (at 1.25 A g−1), Emax of 217 Wh kg−1 and a Pmax of 8 kW kg−1 over a voltage window of 1.6 V along with a cycle life of 10,000 cycles with 98 % SC retention for the ZnCo2S4@MgCo2O4//AC asymmetric supercapacitor (ASC) with KOH as the electrolyte. These performance metrics are superior to those reported in literature for similar oxides/chalcogenides. The electrolyte's role in controlling the redox-behaviour of the ASC is also assessed by using four different uni-molar electrolytes that obey the following order: KOH > Mg(OH)2 > MgCl2 > ZnCl2, in terms of their ionic conductivities (σ: 189, 84, 83 and 58 mS cm−1), cation transport number (t+: 0.85, 0.83, 0.75 and 0.57) and SCs (609, 315, 208, 100 F g−1 at 1.25 A g−1) for the corresponding ASCs. With the KOH electrolyte, a dominant contribution from diffusional transport (68 %) relative to the capacitive double layer storage (32 %) (at 10 mV s−1) is ascertained which confirms the ZnCo2S4@MgCo2O4 composite's ability to exhibit facile Faradaic reactions for charge storage, thereby aligning with the morphological and spinel structure benefits. A green-LED-illumination with a 3S(series)-configuration encompassing three-fully-charged ASCs, further validates this configuration's applicability for niche applications in consumer-electronic-devices.