Promoting effect of magnesium introduced in Li/Ni sites of LiNiO2 for lithium-ion batteries

Abstract

The high cost of Co strongly drives the increase of Ni fraction and the decline of Co ingredient in commercial Ni-rich cathodes, generating convergence towards LiNiO2 (LNO). However, pure LNO suffers from poor cycling and rate performance due to severe Li+/Ni2+ cation mixing and structural degradation. Herein, a series of Mg-doped LNO are synthesized to elucidate the effect of the Mg dopant. It is demonstrated that around 2.21% and 2.79% of Mg2+ ions occupy the Li and Ni sites, respectively. The presence of Mg in the Li sites reduces Li+/Ni2+ cation mixing, enlarges the c-axis lattice, and acts as pillar ions, which facilitates Li+ diffusion and significantly improves the rate performance of Mg-doped LNO. Additionally, the introduction of Mg in the Li/Ni sites disturbs Li/vacancy ordering in the Li layers and electronic rearrangements in the Ni layers, which inhibits the two-phase separation transition at the end of charge. Furthermore, the pillaring effect of Mg2+ ions in Li sites and the stabilizing effect of Mg2+ ions in Ni sites at the end of delithiation alleviate the drastic c-axis shrinkage. Therefore, the optimized Mg-doped LNO exhibits reversible structure evolution, less microcracks, stable impedance growth, and excellent electrochemical performance during long-term cycling.