Doping Li-rich cathode material Li$_2$MnO$_3$: Interplay between lattice site preference, electronic structure, and delithiation mechanism

TL;DR

This study uses first-principles calculations to explore how various dopants affect the electronic structure, site preference, and delithiation mechanism in Li$_2$MnO$_3$, revealing pathways to enhance battery cathode performance.

Contribution

It provides detailed insights into dopant site preferences and their impact on the oxidation mechanisms in Li$_2$MnO$_3$, guiding the design of high-capacity cathode materials.

Findings

Ni, Mo, Ru dopants favor transition-metal oxidation before oxygen during delithiation.

Dopants provide charge compensation and electronic conduction, enabling oxygen oxidation.

Dopant site preference can be tuned by synthesis conditions.

Abstract

We report a detailed first-principles study of doping in Li$_2$MnO$_3$, in both the dilute doping limit and heavy doping, using hybrid density-functional calculations. We find that Al, Fe, Mo, and Ru impurities are energetically most favorable when incorporated into Li$_2$MnO$_3$ at the Mn site, whereas Mg is most favorable when doped at the Li sites. Ni, on the other hand, can be incorporated at the Li site and/or the Mn site, and the distribution of Ni over the lattice sites can be tuned by tuning the materials preparation conditions. There is a strong interplay between the lattice site preference and charge and spin states of the dopant, the electronic structure of the doped material, and the delithiation mechanism. The calculated electronic structure and voltage profile indicate that, in Ni-, Mo-, or Ru-doped Li$_2$MnO$_3$, oxidation occurs on the electrochemically active transition-metal ion(s) before it does on oxygen during the delithiation process. The role of the dopants is to provide charge-compensation and bulk electronic conduction mechanisms in the initial stages of delithiation, hence enabling the oxidation of the lattice oxygen in the later stages. This work thus illustrates how the oxygen-oxidation mechanism can be used in combination with the conventional mechanism involving transition-metal cations in design of high-capacity battery cathode materials.