Sub-stoichiometric germanium sulfide thin-films as a high-rate lithium storage material

Nanocolumnar, sub-stoichiometric germanium sulfide thin-films with compositions of Ge0.9S0.1 and Ge0.95S0.05, deposited by glancing angle deposition, were investigated as lithium storage materials. The materials are amorphous and homogeneous as deposited, but lithiation induces phase separation leading to the formation of poorly-crystallized lithium sulfide inclusions during the first cycle. The presence of these inclusions raises the lithium diffusion coefficient above that of pure germanium and provides superior capacity retention at high rates. While the lithium sulfide is non-cycling, the low weight percentage of sulfur necessary for enhanced lithiation/de-lithiation does not significantly reduce the specific lithium storage capacity of the films relative to that of germanium. In addition to high capacity and superior lithium transport, the sub-stoichiometric germanium sulfide thin-films show excellent cycling stability at high rates, retaining 88% of their initial capacity after 500 cycles at a rate of 20 C.