Phase segregation and nanoconfined fluid O in a lithium-rich oxide cathode.

Lithium-rich oxide cathodes lose energy density during cycling due to atomic disordering and nanoscale structural rearrangements, which are both challenging to characterize. Here we resolve the kinetics and thermodynamics of these processes in an exemplar layered Li-rich (LiMnO) cathode using a combined approach of ab initio molecular dynamics and cluster expansion-based Monte Carlo simulations. We identify a kinetically accessible and thermodynamically favourable mechanism to form O molecules in the bulk, involving Mn migration and driven by interlayer oxygen dimerization.

Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF.

Introducing compositional or structural disorder within crystalline solid electrolytes is a common strategy for increasing their ionic conductivity. (M,Sn)F fluorites have previously been proposed to exhibit two forms of disorder within their cationic host frameworks: occupational disorder from randomly distributed M and Sn cations and orientational disorder from Sn(II) stereoactive lone pairs. Here, we characterize the structure and fluoride-ion dynamics of cubic BaSnF, using a combination of experimental and computational techniques.

Anion-polarisation-directed short-range-order in antiperovskite LiFeSO.

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material LiFeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLiFe oxygen coordination with a preference for polar -OLiFe over non-polar -OLiFe configurations.

Transition metal migration and O formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes.

Lithium-rich disordered rocksalt cathodes display high capacities arising from redox chemistry on both transition-metal ions (TM-redox) and oxygen ions (O-redox), making them promising candidates for next-generation lithium-ion batteries. However, the atomic-scale mechanisms governing O-redox behaviour in disordered structures are not fully understood. Here we show that, at high states of charge in the disordered rocksalt LiMnOF, transition metal migration is necessary for the formation of molecular O trapped in the bulk.

Overscreening and Underscreening in Solid-Electrolyte Grain Boundary Space-Charge Layers.

Polycrystalline solids can exhibit material properties that differ significantly from those of equivalent single-crystal samples, in part, because of a spontaneous redistribution of mobile point defects into so-called space-charge regions adjacent to grain boundaries. The general analytical form of these space-charge regions is known only in the dilute limit, where defect-defect correlations can be neglected. Using kinetic Monte Carlo simulations of a three-dimensional Coulomb lattice gas, we show that grain boundary space-charge regions in nondilute solid electrolytes exhibit overscreening-damped oscillatory space-charge profiles-and underscreening-decay lengths that are longer than the corresponding Debye length and that increase with increasing defect-defect interaction strength.

Correlation Length in Concentrated Electrolytes: Insights from All-Atom Molecular Dynamics Simulations.

We study the correlation length of the charge-charge pair correlations in concentrated electrolyte solutions by means of all-atom, explicit-solvent molecular dynamics simulations. We investigate LiCl and NaI in water, which constitute highly soluble, prototypical salts for experiments, as well as two more complex, molecular electrolyte systems of lithium bis(trifluoromethane)sulfonimide (LiTFSI), a salt commonly employed in electrochemical storage systems, in water, and in an organic solvent mixture of dimethoxyethane and dioxolane. Our simulations support the recent experimental observations as well as theoretical predictions of a nonmonotonic behavior of the correlation length with increasing salt concentration.

Interfacial structure and structural forces in mixtures of ionic liquid with a polar solvent.

Many applications of ionic liquids involve their mixtures with neutral molecular solvents. The chemical physics of these high-concentration electrolytes, in particular at interfaces, still holds many challenges. In this contribution we begin to unravel the relationship between measurements of structural ('solvation') forces in mixtures of ionic liquid with polar solvent and the corresponding structure determined by molecular dynamics simulations of the same mixtures.

The nanostructure of a lithium glyme solvate ionic liquid at electrified interfaces.

Solvate ionic liquids are a subclass of ionic liquids that have the potential to be used in a range of electrochemical devices. We present molecular dynamics simulations of the interfacial structure of thin films of one such lithium based solvate ionic liquid, [Li(G4)][TFSI], an equimolar solution of tetraglyme and lithium bistriflimide. This solvate ionic liquid is shown to form a novel interfacial structure at a plane electrode, which differs in a number of ways from the nanostructure observed for a conventional ionic liquid at similar interfaces.

A graphene surface force balance.

We report a method for transferring graphene, grown by chemical vapor deposition, which produces ultraflat graphene surfaces (root-mean-square roughness of 0.19 nm) free from polymer residues over macroscopic areas (>1 cm(2)). The critical step in preparing such surfaces involves the use of an intermediate mica template, which itself is atomically smooth.