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

TL;DR

This study uses advanced density functional theory to analyze defect chemistry and transport mechanisms in layered LiCoO2 and LiNiO2 cathode materials, revealing how synthesis conditions influence defect types and conduction pathways.

Contribution

It provides a comprehensive first-principles analysis of defect thermodynamics and transport in layered transition-metal oxides, highlighting the impact of synthesis conditions on defect chemistry and conduction.

Findings

LiCoO2 defect chemistry varies with synthesis conditions.

LiNiO2 exhibits charge disproportionation and high nickel antisite concentration.

LiMO2 cannot be doped n- or p-type; conduction occurs via small polarons and lithium vacancies.

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.