Effect of the synthesis procedure on the local cationic distribution in layered LiNi1/2Mn1/2O2

The effect of the synthesis procedure on the local cationic distribution in layered LiNi1/2Mn1/2O2 oxides was examined. LiNi1/2Mn1/2O2 oxides were synthesized by solid-state reaction between lithium hydroxide and mixed Ni,Mn oxides. Two types of mixed Ni,Mn oxides were used: an ilmenite-type oxide obtained from co-precipitated Ni,Mn carbonates and a spinel-type oxide obtained from freeze-dried Ni,Mn citrates. The interaction of NiMnO3 ilmenite with LiOH starts at lower temperature as compared to that of Ni1.5Mn1.5O4 spinel. A single LiNi1/2Mn1/2O2 phase with layered crystal structure is formed between 800 and 950 °C. The extent of Li+,Ni2+ disorder between the layers of LiNi1/2Mn1/2O2 was determined by Rietveld refinement of the X-ray powder diffraction patterns. The local Mn4+ arrangement in the layers was determined by X-band electron paramagnetic resonance spectroscopy (EPR). The EPR spectrum of LiNi1/2Mn1/2O2 consists of a single Lorentzian line due to Mn4+ ions only. The dependence of the EPR line width on the local Mn4+ arrangement was used to determine the mean Ni-to-Mn ratio in the first coordination sphere of Mn4+ for LiNi1/2Mn1/2O2. It was found that the extent of Li/Ni mixing between the layers slightly decreased with the increase in the synthesis temperature. At a high synthesis temperature (950 °C) the Ni2+ ions from lithium and the transition metal layers were ordered forming large Ni2+–O–Ni2+ magnetic clusters with a 180°-configuration. The extent of Li/Ni mixing remained insensitive towards the type of the precursor used. The cation distribution in the transition metal layer of LiNi1/2Mn1/2O2 was consistent with α,β-type cationic order with some extent of disordering. Contrary to the Li/Ni mixing, the local Ni,Mn distribution in the transition metal layers was found to depend mainly on the precursor used. The low extent of disordering is observed for LiNi1/2Mn1/2O2 obtained from Ni1.5Mn1.5O4 spinel.