Enhanced lithium storage and chemical diffusion in metal-LiF nanocomposites: Experimental and theoretical results

Abstract

An extra storage of Li has been observed experimentally at low potential in Me/LiF nanocomposites (where Me refers to transition metals such as Cu, Co, etc.), with a pseudocapacitive behavior characterized by a high rate performance. To understand the mechanistic details of the lithium storage anomaly, we have performed comparative ab initio calculations on the atomic and electronic structure of the nonpolar Cu LiF (001) and model Li LiF (001) interfaces. For this aim, we inserted extra Li atoms at several possible sites of the periodic two-dimensional Me/LiF (Me=Cu,Li) interfaces. The energetically most favorable site for extra Li atom is above the surface F- ion with Cu atoms on the other side of the interface, atop the surface Li+ ions. An increase of the inserted Li atom concentration in the Cu LiF interface is accompanied by an increase of the electron charge transfer from extra Li atoms toward the transition metal adlayers, in agreement with a recently proposed mechanism of interfacial charge storage. This is supported by an analysis of the densities of states projected on different atoms including extra Li, as a function of inserted Li concentration. The Cu LiF (001) interface permits an insertion of only one monolayer of extra Li atoms, unlike Li bilayer in the case of Ti Li2 O (111). Diffusion of the excess Li along the interface is found to be accelerated, owing to the splitting of the individual pathways for Li+ and e-, which explains a high rate performance observed experimentally at low potential. We also compare theoretical estimate and experimental capacity results in the Cu LiF nanocomposite. © 2007 The American Physical Society.