Defect chemistry in layered transition-metal oxides from screened hybrid density functional calculations

TLDR

The authors conduct a comprehensive first‑principles investigation of intrinsic point‑defect thermodynamics and transport in layered oxide cathodes LiMO₂ (M = Co, Ni) using DFT with the HSE screened hybrid functional. They model lithium over‑stoichiometric LiCoO₂ by introducing negatively charged lithium antisites and positively charged small hole polarons, and analyze defect energetics with HSE‑DFT calculations. The study reveals that LiCoO₂ exhibits a complex defect chemistry controllable by Li‑excess synthesis to suppress Co antisites, while LiNiO₂ shows persistent Ni³⁺ charge disproportionation and high Ni occupancy in Li layers that cannot be eliminated by synthesis; moreover, LiMO₂ is intrinsically non‑dopable, with electronic transport via small‑polaron hopping and ionic transport through lithium vacancy migration (monovacancy or divacancy).

Abstract

We report a comprehensive first-principles study of the thermodynamics and transport of intrinsic point defects in layered oxide cathode materials LiMO$_2$ (M=Co, Ni), using density-functional theory and the Heyd-Scuseria-Ernzerhof screened hybrid functional. We find that LiCoO$_2$ has a complex defect chemistry; different electronic and ionic defects can exist under different synthesis conditions, and LiCoO$_2$ samples free of cobalt antisite defects can be made under Li-excess (Co-deficient) environments. A defect model for lithium over-stoichiometric LiCoO$_2$ is also proposed, which involves negatively charged lithium antisites and positively charged small (hole) polarons. In LiNiO$_2$, a certain amount of Ni$^{3+}$ ions undergo charge disproportionation and the concentration of nickel ions in the lithium layers is high. Tuning the synthesis conditions may reduce the nickel antisites but would not remove the charge disproportionation. In addition, we find that LiMO$_2$ cannot be doped $n$- or $p$-type; the electronic conduction occurs via hopping of small polarons and the ionic conduction occurs via migration of lithium vacancies, either through a monovacancy or divacancy mechanism, depending on the vacancy concentration.