Lithium-ion batteries: following the redox reaction of oxygen and transition metals in the Li1.2Mn0.6Ni0.1Co0.1O2 electrode using X-ray absorption spectroscopy

New generation of cathode materials from the Li-rich NMC group are under constant investigation due to their extremely high energy densities resulting from redox reactions involving both transition metals and lattice oxygen. Although a lot of research has been taken so far, the exact mechanism of lithium (de)insertion in those materials, especially the reactions involving redox of lattice oxygen is still elusive. Due to the design of the batteries the observed reactions starts at the surface of the electrode which contacts the electrolyte and as reaction continues, goes deeper into the bulk structure. In order to follow the reactions taking place in Li-rich NMC materials we aimed to exactly distinguish and characterize the phase transitions taking place on the surface and within the bulk of the Li_1.2Mn_0.6Ni_0.1Co_0.1O₂ electrode. To do so we used comprehensive XAS measurements performed at PHOENIX beamline. The use of both TEY (Total Electron Yield) and TFY (Total Fluorescence Yield) modes, probing surface and the bulk respectively, helped us to characterize and spatially distinguish the phases present in the materials at different state of charge. Investigation of the transition metal L-edges (Co, Ni, Mn) and the oxygen K-edge provided us a unique insight into the contribution of each element into the electrochemical reactions taking place in Li-rich NMC type materials. We found that during the battery lifetime (more than 100 cycles), the Li_1.2Mn_0.6Ni_0.1Co_0.1O₂ electrode undergoes several phase transition including a reversible superstructure formation accompanied by the disappearance of the rhombohedral phase and the development of the spinel structure on the surface of the electrode. Thanks to the XAS spectra obtained at the PHOENIX beamline, we were able to shed light on the spatial evolution of the crystal structure of Li_1.2Mn_0.6Ni_0.1Co_0.1O₂ and explain in what manner the transition metals and oxygen atoms take part in the electrochemical reactions.