7Li NMR Study on Irreversible Capacity of LiNi0.8-xCo0.15Al0.05MgxO2 Electrode in a Lithium-Ion Battery

Abstract

Effects of magnesium and the charge/discharge process on the local structure of LiNi0.8-xCo0.15Al0.05MgxO2 (x = 0, 0.05, 0.10) were examined by 7Li MAS NMR. Two distinct 7Li signals were observed at ca. 660 ppm and at ca. 570 ppm for the x = 0 material by the combined use of ultra-fast MAS (100 kHz) and high magnetic field (14 T). We attributed these two signals to two Ni-rich domains for the x = 0 material; the former Li signal is assigned to the Li site, whose local structure is expressed formally as LiNiO2 and the latter to LiNi0.83(Co/Al)0.17O2. The intensities of these Ni-rich domains reduce significantly upon slight addition of magnesium. This leads us to conclude that magnesium preferably replaces nickel in the Ni-rich domains. After the first charge/discharge process, the signal intensities at 500∼750 ppm assigned to the Ni-rich domains in the x = 0 material decrease appreciably. From this, we conclude that the lithium ions responsible for the irreversible capacity after the first cycle belong to the Ni-rich domains, and the reduction of the amount of the Ni-rich domains due to magnesium substitution is responsible for the improved cyclic performance of the x > 0 material as a positive electrode material.