Precise control of morphology of ultrafine LiMn2O4 nanorods as a supercapacitor electrode via a two-step hydrothermal method

Abstract

We report three different synthesis routes, which maintain similar reaction conditions, to find an effective way to precisely control the growth of ultrafine one-dimensional (1D) LiMn₂O₄ in the form of nanorods. We developed a novel method of mixing the precursors through a hydrothermal technique, yielding low dimensional precursors for an effective solid state reaction to synthesize the nanorods. However, to achieve these, highly uniform β-MnO₂ nanorods were initially grown as one of the main precursors. The uniformity observed in the as grown β-MnO₂ nanorods using the hydrothermal technique helps to attract minute LiOH particles upon mixing over their highly confined nano-regime surfaces. This facilitated the solid state reaction between MnO₂ and LiOH to develop one of the finest LiMn₂O₄ nanorods with diameters of 10–80 nm, possessing a high surface area of 88.294 m² g⁻¹. We find superior charge storage behaviour for these finely ordered 1D nanostructures as supercapacitor electrodes in KOH with K₃Fe(CN)₆ as an electrolyte, in contrast to Li₂SO₄. A high pseudo-capacitance of 653.5 F g⁻¹ at 15 A g⁻¹ is observed using a galvanostatic discharge time with a high retention capacity of 93% after 4000 cycles. The enhanced charge storage property may arise from the redox couple [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ and K⁺ ions of the electrolyte. To the best of our knowledge, we demonstrate for the first time the effectiveness of a two-step hydrothermal method in tuning the supercapacitive behaviour of 1D LiMn₂O₄ in a redox additive electrolyte.