The significance of the stable Rhombohedral structure in Li-rich cathodes for lithium-ion batteries

The lithium-rich layered oxides offer higher capacity, but suffer from severe capacity fade and voltage decay. Aluminum doping can improve the stability of the structure, but the fundamental mechanism has not been fully revealed. In this work, cathode materials with different Al-doping contents are investigated. To characterize the structural evolutions upon cycling, ex situ XRD and ex situ TEM are performed. It is demonstrated that the voltage decay is attributed to the decrease of Rhombohedral phase, while capacity fade is possibly associated with the increase of surface impedance. Therefore, continuous voltage decay and capacity fade are observed for undoped Li-rich cathode. In comparison, the Al-doped cathodes have stable LiNi1/3Co1/3Mn1/3O2 and surface impedance, leading to superior capacity retention and a stable output voltage.