Phase separation in lithium intercalated anatase: A theory

TL;DR

This paper presents a theoretical model describing phase separation and ordering phenomena in lithium intercalated anatase, relevant for Li-ion battery materials, incorporating symmetry analysis, Landau theory, and a microscopic model.

Contribution

It introduces a novel combined microscopic model based on Mitsui and Blume-Emery-Griffiths models, tailored for lithiated anatase, including symmetry considerations and phase diagram analysis.

Findings

Phase separation into Li-rich and Li-poor phases over a wide temperature range.

Possible ferro- or antiferroelectric ordering of Li ions not yet observed experimentally.

Theoretical phase diagrams closely resemble experimental phase coexistence.

Abstract

Lithium intercalated anatase used in Li-ion batteries has some special features: coexistence of Li-rich and Li-poor phases as well as two possible positions for Li ions in the oxygen tetrahedron. A theoretical description of the compound considering those peculiarities is presented. As shown by the performed symmetry analysis, the intercalation induced lattice deformation can be accompanied by the ordering of antiferroelectric type (internal piezoeffect). In the following step, a qualitative illustration of the phase separation in the lithiated anatase is given within the Landau expansion at the proper choice of coefficients. A microscopic model for description of the compound is also proposed which combines features of the Mitsui and Blume-Emery-Griffits models and utilizes the symmetry analysis results. Various ground state and temperature-dependent phase diagrams of the model are studied to find a set of model parameters corresponding to the lithiated anatase. A phase separation into the empty and half-filled phases in a wide temperature range has been found closely resembling the phase coexistence in the intercalated crystal. In the framework of the model, the two-position Li subsystem could have the ordering of ferro- or antiferroelectric types which, however, has not been yet observed by the experiment.