The emergence of a robust lithium gallium oxide surface layer on gallium-doped LiNiO2 cathodes enables extended cycling stability

Abstract

LiNiO₂ is a promising cobalt-free cathode for lithium-ion batteries due to its high theoretical capacity and low cost. Although intensely studied, the occurrence of several phase transformations and particle pulverization causing capacity fading in cobalt-free LiNiO₂ have yet to be effectively resolved. Herein, a sol–gel synthesis process is utilized for gallium (Ga) doping of LiNiO₂ at 2% (solution-doping) and 5% (excess-doping) molar ratios. Transmission electron microscopy and X-ray diffraction Rietveld refinement reveal the opportune formation of an α-LiGaO₂ shell at 5% doping beyond the solubility limit of 2%. Alongside solution-doping at the Ni and Li crystallographic sites, the emergence of this α-LiGaO₂, isostructural and lattice-matched to the Rm LiNiO₂, is shown to improve capacity retention by a factor of 2.45 after 100 cycles at C/3. Particles with the LiGaO₂ shell experience significantly less pulverization during extended cycling. In contrast, the solution-doped LiNiO₂ with 2% Ga experiences extensive particle fracturing similar to the baseline undoped LiNiO₂. In turn, no significant electrochemical performance difference is found between the solution-doped and baseline LiNiO₂. The evidence garnered suggests that a surface gallium oxide phase achievable with excess Ga is key to enabling extended cycling using Ga doping.