Structural characteristics and electrochemical performance of layered Li[Mn_0.5-xCr_2xNi_0.5-x]O₂ cathode materials

Li[Mn_0.5-xCr_2xNi_0.5-x]O₂ (0 < 2x <0.2) (Mn/Ni = 1) cathode materials have been synthesized by a solution method. X-ray diffraction patterns of the as-prepared materials were fitted based on a hexagonal unit cell (α-NaFeO₂ layer structure). The extent of Li/Ni intermixing decreased, and layering of the structure increased, with increasing Cr content. Electrochemical cycling of the oxides, at 30 °C in the 3-4.3 V range vs. Li/Li⁺, showed that the first charge capacity increased with increasing Cr content. However, maximum discharge capacity (∼143 mAh g^-1) was observed for 2x = 0.05. X-ray absorption near edge spectroscopic (XANES) measurements on the K-edges of transition metals were carried out on pristine and delithiated oxides to elucidate the charge compensation mechanism during electrochemical charging. The XANES data revealed simultaneous oxidation of both Ni and Cr ions, whereas manganese remains as Mn⁴⁺ throughout, and does not participate in charge compensation during oxide delithiation.