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

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/Li(2)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.