A new one-pot hydrothermal synthesis and electrochemical characterization of Li1+xMn2−yO4 spinel structured compounds

Abstract

A simple one step hydrothermal route to Li₁₊xMn_2−yO₄ spinel compounds (x = 0.01–0.06, y = 0.02–0.09) via reduction of commercially available potassium permanganate with common organic reductants (alcohols, acetone, hex-1-ene, and isobutyraldehyde) in lithium hydroxide aqueous solutions is developed. The cubic spinel phase with no other impurities can be isolated after short reaction times (∼5 h) at the relatively low temperature of 180 °C. Scanning electron microscopy imaging reveals that the crystalline products have a distribution of sizes; the majority of the sample is composed of smaller particles between 10 and 30 nm. However, there are noticeably larger 100–300 nm interspersed particles—more prevalent in reactions with acetone and isobutyraldehyde. In corresponding benchtop test reactions, UV-Vis spectroscopy shows that the disappearance of MnO₄⁻ occurs more rapidly when acetone and isobutyraldehyde are used as reducing agents. Cyclic voltammetry performed on our spinels prepared via hydrothermal synthesis shows three reversible redox processes: a wave with E_1/2 of ∼2.9 V (vs. Li/Li⁺) and two close waves between 4.05 and 4.15 V. Galvanostatic cycling of a cell composed of Li_1.02Mn_1.96O₄ prepared from the oxidation of acetone between 3.5 and 4.4 V demonstrates a specific capacity of 104 mA h g⁻¹ on first discharge, with ∼87% capacity retention (90 mA h g⁻¹) after 100 cycles. The specific capacity of all samples correlates with the rate of disappearance of MnO₄⁻ observed in our benchtop reactions, providing a facile way to control particle size and electrochemical behavior.