Role of Dynamically Frustrated Bond Disorder in a Li⁺ Superionic Solid Electrolyte

Abstract

Inorganic lithium solid electrolytes are critical components in next-generation solid-state batteries, yet the fundamental nature of the cation−anion interactions and their relevance for ionic conductivity in these materials remain enigmatic. Here, we employ first-principles molecular dynamics simulations to explore the interplay among chemistry, structure, and functionality of a highly conductive Li + solid electrolyte, Li 3 InBr 6 . Using local-orbital projections to dynamically track the evolution of the electronic charge density, the simulations reveal rapid, correlated fluctuations between cation−anion interactions with different degrees of directional covalent character. These chemical bond dynamics are shown to correlate with Li + mobility and are enabled thermally by intrinsic frustration between the preferred geometries of chemical bonding and lattice symmetry. We suggest that the fluctuating chemical environment from the polarizable anions functions like a solvent, contributing to the superionic behavior of Li 3 InBr 6 by temporarily stabilizing configurations favorable for migrating Li + . The generality of these conclusions for understanding solid electrolytes and key factors governing the superionic phase transition is discussed.

Alloying effects on superionic conductivity in lithium indium halides for all-solid-state batteries

“…Across all compositions and volumes, except Cl 6 , we see the non-Arrhenius behavior, consistent with our previous study on Li 3 InBr 6 . 13 The non-Arrhenius behavior indicates a change in diffusion mechanism and a variable E a . The change in E a does not occur at the same temperature for all compositions but often occurs between 800 K and 900 K (see Fig.…”

“…In contrast, we previously showed that in Li 3 InBr 6 , many Li-Br bonds have polar-covalent character, with a narrower-than expected distribution of angles and shortened bond distances. 13 Polar-covalent bonds were rigorously defined via distance cutoffs and angle cutoffs based on the centers of Maximally Localized Wannier Functions (MLWF). 18 In this work, we adopt the same definition and analysis to classify bonds as containing chiefly polar-covalent or chiefly ionic character.…”

“…13 The octahedral site is too large in Br 6 , so each Br only has 1-2 simultaneous polar-covalent bonds. Since we no longer consider Li or the halides to be purely ionic, we stop including the charge superscripts.…”

“…The distance from Li to the centroid r Lic depends on the cell volume and the Br/Cl composition; on average, r Lic is nonzero because of the oscillatory motion and a strong polar-covalent interaction, as described in our previous work. 13 We compute r Lic by extracting the Li-halide distance, r LiX , from the Li-X pair correlation function and subtracting r Xc . We report r Lic /r Xc in Fig.…”

“…It follows that more Br neighbors lead to increased competition between polar-covalent bonds, which we previously showed to increase the jump rate in Li 3 InBr 6 . 13 At the x = 3 composition, most Li ions have 3 Br neighbors. If we define Π(Br > 3) as the percentage of Li with an excess (>3) of Br neighbors, then the average value of Π ave (Br > 3) = 1.8%.…”