Tuning the electrical and cycling performance of nickel manganese oxide hexagonal-shaped particles via preparation routes for lithium-ion batteries

Abstract

In this study, simple and effective solid-state and sol-gel routes are attempted to synthesize nickel manganese oxide, NiMn2O4. Structural, morphological, electrical, and electrochemical properties are investigated with calcination temperatures. X-ray diffraction (XRD) results confirm the highly crystalline cubic spinel structure with zero impurities for all samples, except NMOS_700, which indicates the presence of a slight NiMnO3 phase. Scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs confirm the formation of hexagonal shape particles of size less than <0.5 μm. At low calcination temperatures, grouped and uneven-shaped particles are observed with increased particle size. Electrical measurements depict the strong dependence of conductivities (σ ac and σ dc) on grain size, grain boundary, and operating temperature. All the samples exhibit conductivities between 10−8 and 10−4 S cm−1 with the varied calcination temperature. Electrochemical performances are explored via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD) profiles. Sample NMOB_700 and NMOB_800 exhibit the initial discharging capacity of 1104 mAh g-1 and 1188 mAh g−1 at 100 mA g−1 current density. All the samples exhibit above 98% columbic efficiency after two initial cycles and show the reversible nature of NMO and excellent cyclability. The electrochemical results confirm that preparation methods and calcination temperature have a great impact on the grain properties of materials. Multiple oxidation states of Mn and Ni is also confirmed through the XPS study.