Synthesis and Understanding of Layered Li-Rich Nickel Manganese Oxides for High Energy Density Lithium-Ion Batteries

In the search for high capacity cathode materials for Li-ion batteries, layered oxides with the general formula Li(Li_XMn_YM_Z)O₂ (M=Ni, Mn, Mg, ...) either described as xLi₂MnO₃ · yLiMO₂ are receiving much attention. These oxides are characterised by the presence of lithium in the transition metal layers, and high Mn content, leading to high capacities up to 250mAh.g^-1. However, upon cycling, they present complex structural changes that are still misunderstood.

            In this context, our group worked jointly on synthesis development and understanding of the structure evolution occurring into the lithium rich layered cathode material Li_1.2Mn_0.61Ni_0.18Mg_0.01O₂.

The first step was to study one material synthesized using a solid state route. 1^st cycle characterization was investigated by XANES, in situ XRD and SQUID magnetic measurements. We showed that the main first-charge plateau is a twophase process where a new phase is created. This extra phase apparition was confirmed by Electron Energy Loss Spectroscopy (EELS) spectra interpretation. Therefore, Squid magnetic measurements allow a 1st cycle mechanism proposition.

Evolutions after cycling test (50 cycles) have also been investigated using advanced microscopy tools (High Resolution STEM EELS spectrum imaging). The stability of the spinel defect structure is pointed out to explain the material ageing and the electrochemical performances decrease on cycling.

Others synthesis routes were developed, especially solvothermal way enabling a better primary particles size, composition and morphology control. This synthesis route depends on many input parameters such as pH, temperature, duration… We used the methodology of the design of experiments that allows minimizing the number of experiences to optimize the synthesis and understand the effect of the main parameters of the synthesis. Higher performances and cycling behaviour have been obtained and further understanding studies are under progress to match with experimental observations.