Understanding the Role of Al Doping of LiCoO2 on the Mechanisms Upon Cycling up to High Voltages (≥4.6 V vs Li+/Li)

The effect of Al doping on the structure and electrochemical properties of LiCoO2 was investigated for a 4% doping amount, of practical interest for industrial application. Characterization of materials with low doping amount and precise control of the overall stoichiometry and homogeneity is challenging and could be performed in this study by combining 7Li, 27Al, 59Co nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). 27Al magic angle spinning (MAS) NMR and XRD studies revealed that Al doping is homogeneous, and 7Li MAS NMR indicates that its stoichiometry (Li/M = 1.00) is ideal. The electrochemical tests have shown that Al doping improves the cycling stability at 4.6 V and strongly impacts the voltage curve profile up to 5 V. By an ex situ XRD study of electrochemically deintercalated LixCo0.96Al0.04O2 and LixCoO2 materials controlling the state of charge or the voltage, we showed that Al doping delays the formation of the H1-3 and O1 phases. Our 7Li MAS NMR and transmission electron microscopy (TEM) results revealed the formation of a lithiated spinel-type phase at the surface of the Al-doped electrode material, together with the formation of a stable cathode electrolyte interphase (CEI) layer unlike for LiCoO2. The different nature of the electrode/electrolyte interphases may therefore explain better the ionic/electronic conductivities of the Al-doped electrode and may hinder Co dissolution at a high voltage.