First Principles Study of Grain Boundaries in Lithium Lanthanum Titanate

Kathleen Alexander, Massachusetts Institute of Technology

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The movement toward energy independence and greater utilization of renewable energy sources has placed increasing demands on battery technology in terms of both required longevity and scale. Due to safety concerns limiting the application of Li-ion batteries with liquid electrolyte, there is motivation to develop Li-ion batteries that use Li-ion-conducting solid electrolyte, such as Li<sub>3x</sub>La<sub>2/3-x</sub>TiO<sub>3</sub> (LLTO). The most significant challenge associated with the use of these materials is the high resistance to Li-ion conduction at grain boundaries (GBs). Recent efforts have begun characterizing the complex structure of LLTO and the mechanisms of Li-ion transport both in the bulk and at GBs using integrated experimental and computational techniques. From these efforts, it has been determined that a binary Ti–O compound, which blocks Li-ion conduction, is the primary species at most GBs. However, it has also been observed that Li-ion depletion is not present in a few specific GBs. Because the population of these non-depleted GBs is so low, the question as to whether these GBs offer pathways for Li-ion conduction or contribute to GB resistance cannot be answered using experimental techniques, which can only measure the bulk response of a specimen, not the individual contribution of a given GB. Thus, in this work, we use <em>ab initio</em> methods to investigate LLTO GBs. In particular, we consider the thermodynamic driving forces associated with Li and La depletion at the majority of grain boundaries compared to those that do not suffer from this depletion. Additionally, we use quantum molecular dynamics simulations to study Li-ion mobility at both depleted and non-depleted GBs.

Abstract Author(s): Kathleen C. Alexander, Bobby G. Sumpter