Effect of the high pressure on the structure and intercalation properties of lithium–nickel–manganese oxides

Abstract

Lithium–nickel–manganese oxides (Li 1+ x (Ni 1/2Mn 1/2) 1− x O 2, x=0 and 0.2), having different cationic distributions and an oxidation state of Ni varying from 2+ to 3+, were formed under a high-pressure (3 GPa). The structure and cationic distribution in these oxides were examined by powder X-ray diffraction, infrared (IR) and electron paramagnetic resonance (EPR) in X-band (9.23 GHz) and at higher frequencies (95 and 285 GHz). Under a high pressure, a solid-state reaction between NiMnO 3 and Li 2O yields LiNi 0.5Mn 0.5O 2 with a disordered rock-salt type structure. The paramagnetic ions stabilized in this oxide are mainly Ni 2+ and Mn 4+ together with Mn 3+ (about 10%). The replacement of Li 2O by Li 2O 2 permits increasing the oxidation state of Ni ions in lithium–nickel–manganese oxides. The higher oxidation state of Ni ions favours the stabilization of the layered modification, where the Ni-to-Mn ratio is preserved: Li(Li 0.2Ni 0.4Mn 0.4)O 2. The paramagnetic ions stabilized in the layered oxide are mainly Ni 3+ and Mn 4+ ions. The disordered and ordered phases display different intercalation properties in respect of lithium. The changes in local Ni,Mn-environment during the electrochemical reaction are discussed on the basis of EPR and IR spectroscopy.