Interfacial Control of Lithiation Using Layered Intermetallic Architectures
Next generation lithium battery materials will require a fundamental shift from intercalation materials to elements or compounds that alloy directly with lithium. Intercalation compounds, like graphite and LiCoO2, provide a stable crystal structure with open sites for mobile lithium ions, but are intrinsically limited in their energy density by the weight and volume of the host material. Intermetallics can electrochemically alloy to Li4.4M (M = Si, Ge, Sn, etc), providing order-of-magnitude increases in energy density. However, this process can lead to volume changes (up to 300%) that rapidly degrade the performance of the battery due to delamination between the active material and its underlying current collector.
Using in situ x-ray reflectivity, we have studied the interface of a model bilayer of silicon and chromium that act as an anode material and current collector respectively. Even before lithiation, we find that the amorphous bilayer intermixes immediately following growth and eventually forms a layered CrSix heterostructure. Lithiation of such a structure begins at the interface between each layer and eventually stabilizes into a multilayer consisting of alternating LiSix phases and more chromium-rich CrSix phases.
Inspired by this purely vertical phase separation and the stability of its lithiation, we have recently grown larger scale silicon/chromium multilayers, e.g. repeating this bilayer structure 20-50 times. With higher silicon content, these multilayers reversibly show 3.3-fold expansion and contraction and maintain their layered structure, as seen by in operando x-ray reflectivity. The electrodes also give substantial improvement over pure-phase silicon thin films in both long-term cycling and high power applications. Multilayers using Ge and Ti also provide similar reversibility and performance. Constraining the alloying reaction in such a layered architecture appears to have similar structural behavior to a more traditional intercalation compound and may be useful for higher voltage alloying reactions in metal oxides and sulfides.