Charge compensation mechanisms in Li_1.16Ni_0.15Co _0.19Mn_0.50O₂ 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 Li_1.16Ni_0.15Co _0.19Mn_0.50O₂ 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 LiNi _1/3Mn_1/3Co_1/3O₂. 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.