First-principles analysis of the interplay between electronic structure and volume change in colquiriite compounds during Li intercalation

TL;DR

This study uses first-principles calculations to analyze volume changes in colquiriite-type fluorides during lithium intercalation, identifying LiCaMnF6 as a promising zero-strain cathode material due to minimal volume change and favorable electrochemical properties.

Contribution

The paper provides a systematic first-principles investigation of volume changes in various Li$_x$CaMF$_6$ compounds, revealing structural mechanisms behind zero-strain behavior and identifying LiCaMnF$_6$ as an optimal cathode candidate.

Findings

Li$_x$CaFeF$_6$ shows about 10% volume change from x=1 to 2.

Li$_x$CaMnF$_6$ exhibits minimal volume change (~10%) with high voltage.

LiCaMnF$_6$ is a promising zero-strain cathode material.

Abstract

A main source of capacity fading in lithium-ion batteries is the degradation of the active cathode materials caused by the series of volume changes during charge and discharge cycles. The quaternary colquiriite-type fluorides Li$_x$CaFeF$\mathrm{_6}$ and Li$_x$CaCoF$\mathrm{_6}$ were reported to have negligible volume changes in specific Li concentration ranges, making the underlying colquiriite structure a promising candidate for so-called zero-strain behavior. Using first-principles electronic structure calculations based on density functional theory with a Hubbard-$U$ correlation correction on the transition-metal ions, we systematically investigate the equilibrium volumes of the colquiriite-type fluorides Li$_x$CaMF$\mathrm{_6}$ with M =Ti, V, Cr, Mn, Fe, Co, and Ni at the Li concentrations $x$=0, 1, and 2. We elucidate the connection between the total volume of the structures and the local volumes of fluorine coordinated octahedra around the cations, and we find trends along the series of the 3d transition-metal elements. In the lithiation step from $x$=1 to $x$=2 we find volume changes of about 10 %, and we discuss the discrepancy to the experimentally reported smaller value for Li$_x$CaFeF$\mathrm{_6}$. The suitability as cathode material was furhter investigated by calculating the theoretical voltages and capacities. From $x$=0 to $x$=1 we describe the compensating structural mechanisms that lead to an exceptionally small volume change of Li$_x$CaMnF$\mathrm{_6}$, which posseses a high theoretical voltage and moderate capacity. This compound is therefore a particularly promising zero-strain cathode material.