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 andelectrochemical properties of LiCoO2was investigated for a 4%doping amount, of practical interest for industrial application.Characterization of materials with low doping amount and precisecontrol of the overall stoichiometry and homogeneity is challengingand could be performed in this study by combining7Li,27Al,59Conuclear magnetic resonance (NMR) and X-ray diffraction (XRD).27Al magic angle spinning (MAS) NMR and XRD studies revealedthat Al doping is homogeneous, and7Li MAS NMR indicates thatits stoichiometry (Li/M = 1.00) is ideal. The electrochemical testshave shown that Al doping improves the cycling stability at 4.6 Vand strongly impacts the voltage curve profile up to 5 V. By an ex situ XRD study of electrochemically deintercalatedLixCo0.96Al0.04O2and LixCoO2materials controlling the state of charge or the voltage, we showed that Al doping delays theformation of the H1-3 and O1 phases. Our7Li MAS NMR and transmission electron microscopy (TEM) results revealed theformation of a lithiated spinel-type phase at the surface of the Al-doped electrode material, together with the formation of a stablecathode electrolyte interphase (CEI) layer unlike for LiCoO2. The different nature of the electrode/electrolyte interphases maytherefore explain better the ionic/electronic conductivities of the Al-doped electrode and may hinder Co dissolution at a highvoltage