Topochemical Path in High Lithiation of MoS ₂

Lithiation of MoS ₂ /RGO (reduced graphite oxide) electrodes repeatedly reached experimental capacities larger than 1000 mA·g ^–1 , corresponding to at least 6 lithium equivalents per gram of MoS ₂ . At our best knowledge, a convincing explanation is still missing in literature. In most cases, phase separation into Li ₂ S and elemental Mo was assumed to occur. However, this can only explain capacities up to 669 mA·g ^–1 , corresponding to an exchange of four Li. Formation of LiMo alloys could resolve the problem but the Li/Mo system does not contain any binary phases. If signs for Li ₂ S formation were found, indeed experimental capacities were below 700 mAh·g ^–1 . Here we present a topochemical mechanism, which sustains multiple charge/discharge cycles at 1000 mAh·g ^–1 , corresponding to an exchange of at least 6 Li per formula unit MoS ₂ . This topochemical reaction route prevents decomposition into binary phases and thus avoids segregation of the components of MoS ₂ . Throughout the whole lithiation/delithiation process, distinct layers of Mo are preserved but extended or shrunk by slight movements and reshuffling of sulfur and lithium atoms. On addition of 6 Li per formula unit to MoS ₂ , all central sulfur atoms are hosted in mutual Mo–S layers such that formal S ^2– and Mo ^2– anions appear coordinated by lithium cations. Indeed, similar structures are known in the field of Zintl phases. Our first-principles crystal structure prediction study describes this topological path through conversion reactions during the lithiation/delithiation processes. All optimized phases along the topological path exhibit a distinct Mo layering giving rise to a series of dominant scattering into pseudo 001 reflections perpendicular to these Mo planes. The mechanism we present here explains why such high capacities can be reached reversibly for MoS ₂ /RGO nano composites.