Tracing Low Amounts of Mg in the Doped Cathode Active Material LiNiO₂

Abstract LiNiO 2 (LNO) is a high energy density cathode material, yet it has been plagued by poor cycling performance and thermal instability. Adding dopants, such as Mg 2+ , is a common strategy to balance cycling performance and energy density. So far, the influence of Mg 2+ on the crystal structure remained ambiguous, especially when small industrially relevant amounts are used. Here we present a systematic study of LiNi 1-y Mg y O 2 (nominal 0 ≤ y ≤ 0.05, using Mg(OH) 2 nanoparticles) investigated by high-resolution synchrotron X-ray diffraction (XRD) combined with electron microscopy and electrochemical testing. We find that Mg 2+ at first occupies the Ni site until saturating at ~1.7%, then the Li site becomes preferred. This trend influences the lattice parameters and Ni-O bond distances. Doping also modifies the electrochemical behavior of the material, stabilizing the capacity retention but sacrificing specific discharge capacity. Operando XRD reveals that the improved performances are due to the suppression of the H2-H3 phase transition and interlayer distance collapse already at 3% Mg. The combination of structural and electrochemical characterization provides useful insights into the structural chemistry of the Mg 2+ dopant and can serve as a starting point to understand Mg as a component in multi-dopant strategies for cathodes design and application.