Chemistry of Sputter-Deposited Lithium Sulfide Films

Lithium sulfide (Li₂S) is under intense study because it is the insulating discharge product of ultrahigh energy density lithium-sulfur (Li-S) batteries and a candidate cathode material for lithium-metal-free Li-S cells. In this work, we report the fabrication of an apparatus for sputter deposition of Li₂S films ranging in thickness from a few nanometers to several micrometers at rates over 2 nm min^-1. High-temperature annealing of the films is shown to produce crystalline films of high chemical purity Li₂S for the first time. We provide evidence via complementary X-ray photoelectron and Raman spectroscopy that sputter deposition produces a unique sulfide structure composed of polymer-like chains of Li₂S units. The electrochemistry of these films is markedly different from annealed crystalline Li₂S, which is shown to be electrochemically inactive. The full theoretical capacity of the as-deposited sulfide structure could be realized during galvanostatic charge, suggesting a facile, solid-state charge process. Finally, we explore trends in the plasma chemistry that lead to nonstoichiometry and depth inhomogeneity during Li₂S sputter deposition.