Tin is an important component of a variety of promising anode and solid-electrolyte chemistries for Li and Na-ion batteries. Here, we report on a first-principles investigation of phase stability and electronic structure in the Li–Sn–S ternary composition space, which hosts several compounds that can either serve as an anode or as a solid electrolyte in Li-ion batteries. Calculations based on density functional theory predict a tendency for charge disproportionation into localized Sn²⁺ and Sn⁴⁺ oxidation states upon addition of Li to SnS₂. Furthermore, the preferred sulfur coordination environment of Sn is found to depend strongly on the Sn oxidation state. As a result, the thermodynamically preferred path of Li-insertion into SnS₂ proceeds according to a conversion reaction involving significant crystallographic rearrangements to accommodate the changes in coordination preferences with Sn oxidation state. Li-insertion according to a metastable topotactic intercalation path is also found to involve charge disproportionation, with the reduction of octahedrally coordinated Sn⁴⁺ to Sn²⁺ accompanied by large volume changes that affect the nature of low-energy Li-vacancy orderings. Although Li₂SnS₃ is the only globally stable ternary compound in the Li–Sn–S composition space, we find several families of metastable candidate solid-electrolyte phases along the Li_4xSn_(1–x)S₂ composition axis that contain three-dimensional channels for rapid Li-diffusion. The results of this study provide fundamental insights about the behavior of Sn in sulfide compounds that can guide the design of more complex electrode and solid-electrolyte chemistries.