In situ investigations of a Li-rich Mn–Ni layered oxide for Li-ion batteries

Abstract

A Li-rich layered oxide with the formula Li[Li_0.2Mn_0.61Ni_0.18Mg_0.01]O₂ was successfully synthesised and characterised using several in situ characterisation techniques. The electronic state and structural evolution of the material upon cycling were investigated using in situ XRD, EXAFS and XANES measurements. XANES and SQUID magnetic measurements showed that the initial material contains a certain amount of Mn³⁺ in a low spin configuration (average Mn oxidation state: +3.75). In situ measurements showed that the first part of the charge (up to 4.4 V vs. Li⁺/Li) corresponds to oxidation of the Mn³⁺ fraction, and that the oxidation of nickel occurs only later, on the main charge plateau at 4.5 V. Electrochemical and structural results tend to show that the main first-charge plateau is a two-phase process where a new phase is created. This new phase is structurally very close to the starting one, and could be an oxygen-deficient spinel with a = 8.25 Å. This process is non-reversible, and further cycling occurs in the new phase formed in situ.