Structural, Electrochemical, and Thermal Aspects of Li [ ( Ni0.5Mn0.5 ) 1 − x Co x ] O2 ( 0 ≤ x ≤ 0.2 ) for High-Voltage Application of Lithium-Ion Secondary Batteries

We investigated structural, electrochemical, and thermal properties of layered Li[(Ni 0.5 Mn 0.5 ) 1-x Co x ]O 2 (0 ≤ x ≤ 0.2) oxides synthesized via coprecipitation. The prepared materials had a well-ordered 03 type α-NaFeO 2 layer structure. The occupation of divalent Ni in the Li layer decreased monotonously with increasing Co amount in Li[(Ni 0.5 Mn 0.5 ) 1-x Co x ]O 2 . Because of the improved structural integrity and electrical conductivity, the Co substitution for Ni and Mn gave rise to the increment on the initial discharge capacity. However, the replacement brought about severe capacity fading during extensive cycling in a Li-ion cell. To elucidate the possible reasons for the capacity fading, electrochemically and chemically delithiated Li 1-δ [(Ni 0.5 Mn 0.5 ) 1-x Co x ]O 2 powders were examined through the storage at 60°C for 300 h in the electrolyte. With increasing Co content, the amount of dissolved Ni, Co, and Mn greatly increased. Furthermore, the original 03 (R3m) layer structure was completely transformed to O1 (P3ml) phase for the Li 0.1 [(Ni 0.5 Mn 0.5 ) 0.8 Co 0.2 ]O 2 , being accompanied by a severe particle degradation. However, Li 0.1 [Ni 0.5 Mn 0.5 ]O 2 maintained its original structure with uniform surface morphology, which would be mainly attributed to the presence of divalent Ni in the Li layer. A high-temperature X-ray diffraction study with a combination of thermal gravimetric analysis also confirmed that the 03 phase was stable to 200-250°C without significant weight loss in that region for the Li 0.1 [Ni 0.5 Mn 0.5 ]O 2 . Whereas the Li 0.1 [(Ni 0.5 Mn 0.5 ) 0.8 Co 0.2 ]O 2 having O1 layer structure showed a gradual weight loss at the temperature, which would result from the oxygen loss from the oxide. The Li[Ni 0.5 Mn 0.5 ]O 2 has a large amount of Ni 2+ in the Li layer, which provided significant structural, electrochemical, and thermal stabilities at a highly delithiated state, compared to the Li[(Ni 0.5 Mn 0.5 ) 0.8 Co 0.2 ]O 2 .