Topochemical Path in High Lithiation of MoS2

Caputo R., Tekin A., Nesper R.

ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, vol.645, no.3, pp.309-316, 2019 (SCI-Expanded) identifier identifier


Lithiation of MoS2/RGO (reduced graphite oxide) electrodes repeatedly reached experimental capacities larger than 1000 mAg(-1), corresponding to at least 6 lithium equivalents per gram of MoS2. At our best knowledge, a convincing explanation is still missing in literature. In most cases, phase separation into Li2S and elemental Mo was assumed to occur. However, this can only explain capacities up to 669 mAg(-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 Li2S formation were found, indeed experimental capacities were below 700 mAhg(-1). Here we present a topochemical mechanism, which sustains multiple charge/discharge cycles at 1000 mAhg(-1), corresponding to an exchange of at least 6 Li per formula unit MoS2. This topochemical reaction route prevents decomposition into binary phases and thus avoids segregation of the components of MoS2. 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 MoS2, all central sulfur atoms are hosted in mutual Mo-S layers such that formal S2- and Mo2- 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 MoS2/RGO nano composites