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First principles simulations of the porous layered calcogenides Li2+xSnO3 and Li2+xSnS3

Jason Howard and N.A.W. Holzwarth
accepted for publication in Phys. Rev. B (2016)   Local copy

First principles simulations of the porous layered calcogenide materials Li2SnO3 and Li2SnS3 are used to study their structures, Li ion mobilities, and their interactions with excess Li. The pristine materials are characterized by a regular pattern of voids within the calcogenide layers which are occupied by intralayer Li ions. The energetically most favorable Li ion migration processes for both materials result in a net motion perpendicular to the layers and involve intralayer Li ions and nearby interstitial sites. The ideal lattice has 8 symmetry related stable interstitial sites within the conventional unit cell which, in addition to participating in the Li ion migration processes, are also important for accomodating excess Li during lithiation processes. Consistent with experimental findings, the simulations find that the addition of Li atoms to Li2SnO3 results in a disruption of the calcogen lattice with the breaking of Sn-O bonds. The estimated voltage versus bcc Li for this system is in qualitative agreement with experiment provided that Sn/Li disorder is taken into account. By contrast, the simulations predict that the addition of Li atoms to Li2SnS3 results in a stable metallic material up to a stoichiometry of Li3SnS3. This prediction has not yet been studied experimentally. Simulations of surfaces of these materials find that it is energetical favorable to add a small amount of excess surface Li. However, interfaces of these materials with Li metal are found to be reactive. Some of the findings may be relevant to other materials having the same crystal structure such as Li2MnO3 and Li2TiO3.