The multiple effects of Al-doping on the structure and electrochemical performance of LiNi0.5Mn0.5O2 as cathode material at high voltage

Abstract

The application of LiNi0.5Mn0.5O2 as a high-voltage cathode material for lithium-ion batteries is limited by its poor cycle performance. Therefore, we attempt to improve the cyclability of this material at high voltage by using a doping method and propose a detailed mechanism for the effect of the doping amount on the structure and electrochemical performance. In this work, LiNi0.5-zAlzMn0.5O2 (z = 0.00, 0.03, 0.05, 0.08) electrodes were prepared via a simple co-precipitation followed by a solid-state method. X-ray diffraction and Rietveld refinement revealed that a suitable amount of Al doping into LiNi0.5Mn0.5O2 can stabilize the structure and lower the Li/Ni cation mixing, but an excessive doping would lead to Al-ion doping in the lithium layer, which can block lithium diffusion and affect the rate property. Specifically, LiNi0.47Al0.03Mn0.5O2 shows a much higher capacity retention compared to LiNi0.5Mn0.5O2 both at 25 °C (78.5 vs. 68.8% at 0.2 C) and 60 °C (70.8 vs. 69.0% at 0.2 C). Moreover, Al-doping can retard the voltage drop during the discharge-charge state, with the discharge voltage for LiNi0.5-zAlzMn0.5O2 (z = 0.00, 0.03, 0.05, 0.08) decreasing slowly with increasing Al content.