Charge compensation mechanisms in Li1.16Ni0.15Co0.19Mn0.50O2 positive electrode material for Li-ion batteries analyzed by a combination of hard and soft X-ray absorption near edge structure

The redox reaction of Li1.16Ni0.15Co0.19Mn0.50O2 positive electrode material during the charging and discharging processes was investigated by using spectroscopic methods, i.e. in situ hard X-ray absorption near edge structure (XANES) at transition metal K-edges and ex situ soft XANES at oxygen K- and transition metal L-edges. The spectral changes of constituent elements during the initial charging to 4.5 V vs. Li/Li+ are quite similar to those of conventional layer-structured positive materials, such as LiNi1/3Mn1/3Co1/3O2. Ni2+ and Co3+ ions are fully oxidized to Ni4+ and Co4+, while Mn4+ remains unchanged. Ligand oxygen ions also take part in charge compensation. In the process of charging to 4.8 V via the plateau voltage region, no significant spectral change appears except partial reduction of Ni and Co ions in spite of lithium extraction. By discharging to 2.0 V the spectra of each element return to those of the pristine material.