Influence of Mn(III) content on physical properties and electrochemical behaviour of Li–Mn–Cr spinel-type oxides

Abstract

A series of new oxides Li(4−x)/3Mn(5−2x)/3CrxO4 (0.1 ≤ x ≤ 0.9) has been synthesized by solid state reactions and characterized by thermal analysis and x-ray and neutron diffraction. These phases show a cubic spinel-type structure, space group , in which Li+ cations mainly occupy the tetrahedral 8a sites and transition metal cations occupy the octahedral 16d ones. The presence of some Mn3+ (deduced from thermogravimetric experiments) together with the expected Mn4+ is associated with oxygen non-stoichiometry. This fact has a noticeable influence on the electrical and electrochemical behaviour. The reversible electrochemical extraction/insertion of lithium at 4 V is related to the oxidation/reduction processes of manganese. At high voltage a second process appears that is associated with the Cr3+ oxidation. The mechanism of the lithium motion from/to the tetrahedral 8a sites has been elucidated by in situ x-ray diffraction.