Mg Intercalation in Layered and Spinel Host Crystal Structures for Mg Batteries

We investigate electrochemical properties of Mg in layered and spinel intercalation compounds from first-principles using TiS₂ as a model system. Our calculations predict that Mg_<i>x</i>TiS₂ in both the layered and spinel crystal structures exhibits sloping voltage profiles with steps at stoichiometric compositions due to Mg-vacancy ordering. Mg ions are predicted to occupy the octahedral sites in both layered and spinel TiS₂ with diffusion mediated by hops between octahedral sites that pass through adjacent tetrahedral sites. Predicted migration barriers are substantially higher than typical Li-migration barriers in intercalation compounds. The migration barriers are shown to be very sensitive to lattice parameters of the host crystal structure. We also discuss the possible role of rehybridization between the transition metal and the anion in affecting migration barriers

Phase Stability and Transport Mechanisms in Antiperovskite Li₃OCl and Li₃OBr Superionic Conductors

We investigate phase stability and ionic transport mechanisms in two recently discovered superionic conductors, Li₃OX (X = Cl, Br), from first principles. These compounds, which have an antiperovskite crystal structure, have potential applications as solid electrolytes in Li-ion batteries. We identify a low-barrier three-atom hop mechanism involving Li interstitial dumbbells. This hop mechanism is facile within the (001) crystallographic planes of the perovskite crystal structure and is evidence for the occurrence of concerted motion, similar to ionic transport in other solid electrolytes. Our first-principles analysis of phase stability predicts that antiperovskite Li₃OCl (Li₃OBr) is metastable relative to Li₂O and LiCl (LiBr) at room temperature. We also find that although the band gap of Li₃OCl exceeds 5 eV, the metastable antiperovskite becomes susceptible to decomposition into Li₂O₂, LiCl and LiClO₄ above an applied voltage of 2.5 V, suggesting that these compounds are most suited for low-voltage Li batteries provided the formation of Li₂O can be suppressed

Ab Initio Structure Search and in Situ ⁷Li NMR Studies of Discharge Products in the Li–S Battery System

The high theoretical gravimetric capacity of the Li–S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li–S phase diagram using computational techniques and complement this with an in situ ⁷Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li₂S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li⁺-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li₂S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li₂S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition