Unraveling the Distinct Roles of Mg Occupation on Li or Co Sites on High-Voltage LiCoO2

A high-voltage LiCoO2 cathode material has been increasingly studied due to its high capacity; however, the structural instability of this material results in poor cycling performance at high voltages, restricting its application. We use a controlled Mg doping strategy to modulate the crystal and electronic structure of the material and unravel the different effects of Mg occupation at Li and Co sites on the structure and electrochemical performance of LiCoO2. Mg substitution at Li sites provides much better electrochemical performance than Mg substitution at Co sites. Compared with bare LiCoO2, the substitution of Mg for Li and Co significantly enhances the capacity retention from 0.5% to 58.6% and 85.6% (500 cycles at 5C), respectively. Mg substitution at Li sites provides a “pillar’ to stabilize the layered structure and increases the interlayer spacing (I(LiO2)) to reduce the energy barrier for Li+ migration. The stress and strain on the crystal structure caused by the substantial expansion and contraction during cycling are alleviated, while the stability of oxygen in the Li0.96Mg0.04CoO2 sample is enhanced; additionally, the destruction of the CoO6 octahedron is also significantly inhibited, all of which confirm the increase in stabilization due to the Mg substitution in LiCoO2. This study offers some insights on the distinct effects of the same dopant at different crystal sites, which is instructive to develop a precisely controlled doping strategy.