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.