Plane wave density functional theory calculations were used to study the lithiation mechanisms of RuO2, SnO2, and SnS2 in the context of lithium ion battery anode materials. Simulated first discharge curves were compared to available experimental data, showing qualitative agreement.  The computations investigated three different mechanisms for lithiation including intercalation, conversion, and alloying, and provided molecular level insights into these processes. Intercalation leads to little disruption in the crystal lattice while conversion involves the reaction of the MX2 moiety with Li leading to Li2X + M. The conversion is stoichiometrically complete at a 4:1 ratio of Li:M. Alloying involves the mixing of M and Li in various proportions. RuO2 was found to operate by two mechanisms up to a 4:1 Li:Ru  ratio: intercalation in the initial stages  followed by conversion up to a 4:1 ratio.  High-resolution TEM images have shown that conversion leads to Ru islands in a Li2O matrix. Experimentally, Li capacities far in excess of the 4:1 ratio have been recorded but the mechanism has been unclear. Computational studies of island structures have found evidence in support of the conjectured storage of lithium at the interface of the metallic Ru and Li2O phases. SnO2, which has a similar crystal structure, is found to operate by similar mechanisms at low Li content, possibly switching to alloying as Li content increases. SnS2, a layered material, is found to undergo conversion even at very low Li content, and the Sn layers are found to undergo alloying with Li beyond the 4:1 ratio of Li:Sn.  Comparing the RuO2 lithiation to SnS2 mechanisms showed other differences.  For instance, with higher lithium content, the distance between Ru atoms in the metallic island showed contraction, while the opposite was found for Sn atoms, which showed expansion.  This promoted more free volume in the Sn phase with higher lithium content for SnS2, while more free volume was found at the interface between Ru metal and Li2O for RuO2.  This shows that the response of the metals to their reduction promotes the different mechanisms for lithiation beyond a 4:1 Li:metal ratio.<br/>

Density functional theory study of lithium ion battery anode materials: Ruthenium (IV) oxide, tin (IV) oxide, and tin (IV) sulfide

249th National Meeting (2015)

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