Structural Transformation of Li [ Ni0.5 − x Co2x Mn0.5 − x ] O2 ( 2x ≤ 0.1 ) Charged in High-Voltage Range ( 4.5 V )

In order to evaluate the effect of Co on the structural stability of Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 (x ≤ 0.05), Co-doped Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 samples were synthesized by coprecipitation method. The structural instability, electrochemical properties, reactivity with electrolyte, and thermal stability of the Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 (x ≤ 0.05) were investigated. Co-substituted Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 (x = 0, 0.025, 0.05) showed increase of the specific discharge capacity and improved rate capability compared to Li[Ni 0.5 Mn 0.5 ]O 2 . However, X-ray diffraction study of Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 (charged at 4.5 V) showed that Co substitution increases the lattice parameter in the c axis dramatically, compared to the Co undoped electrode (Li[Ni 0.5 Mn 0.5 ]O 2 ). Electron diffraction analysis confirmed that cobalt substitution induced significant structural transformation during cycling in the high-potential range (4.5 V). Furthermore, increase of Co amount in the Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 (x ≤ 0.05) accelerated Co dissolution at high-temperature storage. The shift of onset temperature of exothermic reaction on differential scanning calorimetric behaviors of the Co-substituted Li[Ni 0.5-x Co 2x Mn 0.5-x ]O 2 (x = 0.025 and x = 0.05) was observed.