Impact of Overlithiation and Al doping on the battery performance of Li-rich layered oxide materials

Lithium rich layered oxides (LRLOs) are one of the best alternatives for the next generation positive electrodes materials for Li-ion batteries. However, LRLOs suffer a remarkable voltage decay upon cycling that prevents stable and prolonged electrochemical performances and contains large quantities of cobalt in the transition metal blend. Here, we demonstrate the performance in batteries of a series of innovative materials with general formula Li1.2+xMn0.54Ni0.13Co0.13-x-yAlyO2 (where 0.03 <= x <= 0.08 and 0.03 <= y <= 0.05) capable to supply large reversible specific capacities (around 200 mAh g-1), stable cycling performance with reduced voltage decay. This communication sheds light on the interplay between structural and electronic disorder induced by Al/Li codoping in LRLO and the corresponding functional properties in batteries. The impact of substitution of cobalt by Li and Al co-doping on the structural and morphological properties has been examined by Synchrotron X-ray diffraction (XRD), microwave plasma atomic emission spectrometer (MP-AES) and scanning electron microscopy (SEM). Furthermore, to better understand the structure-function interplay of these over-lithiated materials, ex situ analyses are here reported coupling Synchrotron X-Ray diffraction and Raman Spectroscopy. Based on the obtained results, we proved that coupling the over-lithiation with the aluminum doping, is an effective strategy to reduce the cobalt content into the LRLOs structure maintaining high electrochemical performance. Beside the values of specific capacities obtained in lithium cells, these materials exhibit excellent capacity retention and voltage retention, in particular for the material with the smallest content of cobalt, i.e. Li1.28Mn0.54Ni0.13Co0.02Al0.03O2. Furthermore, thanks to the ex-situ analysis of the latter, we explained the structural evolution of the sample upon cycling that showed the formation of a secondary trigonal phase and the occurrence of limited local distortions of layered structure.