Comparative analysis of the changes in local Ni/Mn environment in lithium–nickel–manganese oxides with layered and spinel structure during electrochemical extraction and reinsertion of lithium

Abstract

Electron paramagnetic resonance spectroscopy was used to analyse the changes in local Ni, Mn environment in layered LiNi 1/2Mn 1/2O 2 and spinel LiNi 1/2Mn 3/2O 4 after electrochemical extraction and reinsertion of lithium. For layered LiNi 1/2Mn 1/2O 2, the EPR signal from Mn 4+ is only detected, while residual antiferromagnetic correlations between Ni 2+ and Mn 4+ ions gives rise to strong resonance absorption for LiNi 1/2Mn 3/2O 4 spinel. The first charge process of layered LiNi 1/2Mn 1/2O 2 leads to oxidation of Ni 2+ ions located both in the transition metal sites and in the Li sites. The reverse process of reduction of these nickel ions was suggested to proceed between 4.4–3.0 V and 2.5–1.4 V, respectively. Lithium extraction from LiNi 1/2Mn 3/2O 4 spinel leads to oxidation of paramagnetic Ni 2+ to diamagnetic Ni 4+ without significant changes in the local environment of Mn 4+. For both fully delithiated compositions, Li 1− x Ni 1/2Mn 1/2O 2 and Li 1− x Ni 1/2Mn 3/2O 4, an EPR spectrum from localized Mn 4+ ions is observed, indicating an exhaustion of paramagnetic Ni 2+ ions in the vicinity of Mn 4+ ions. Furthermore, it has been found that the Mn 4+ environment including paramagnetic Mn 4+ and Ni 2+ neighbours is restored after the first cycle of charge/discharge.