The rivet effect: a new insight into improving structural stability in Mg-doped Ni-rich single-crystal layered oxide cathodes for Li-ion batteries

Abstract

Doping of the cation sublattice with Mg²⁺ has been demonstrated as an effective strategy for suppressing electrochemical degradation of Ni-rich layered oxide cathodes for Li-ion batteries confined to preventing collapse of the transition metal (TM) layers due to the so-called “pillar effect”. In this study, using the Mg-doped single crystal Li(Ni_0.6Mn_0.2Co_0.2)_1−xMg_xO₂ (x = 0, 0.05, 0.1) layered oxides as model systems, we suggest a new “rivet” effect of Mg-doping that prevents intragranular cracking of layered oxides with moderate Ni content. A detailed comparative crystallographic study with synchrotron X-ray and neutron powder diffraction combined with support from ab initio calculations and atomic-resolution scanning transmission electron microscopy compositional mapping revealed that Mg²⁺ resides at the TM site up to ∼3% saturation level, and then populates the Li site. The Mg cations at the Li site suppress gliding of the close-packed layers, the associated O3-to-O1 transition, accompanying tensile strain and crack initiation thus acting as clamps for the TM layers (the “rivet” effect). Electrochemical impedance spectroscopy, galvanostatic cycling and galvanostatic intermittent titration indicate hindered Li-ion diffusion by Mg population of the Li site that deteriorates the electrochemical capacity with increasing the Mg content. These findings call for the strategy of combining the Mg doping with shortening the Li⁺ diffusion pathways.