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Spontaneous defect formation as the origin of superionic transitions in antifluorite-type structures

Year: 2026
Type: preprint
Abstract: Solid-state electrolytes offer the possibility of safer and higher energy density batteries, yet only a limited number of materials achieve ionic conductivities suitable for practical applications. Type-II superionic conductors, which undergo continuous temperature-driven transitions between low and high ionic conductivity regimes are strong candidates for high-performance solid state electrolytes, but the transition mechanism is not sufficiently understood. In this study, we investigate the archetypical superionic conductors Li2O and Li2S with the antifluorite structure using long-time-scale molecular dynamics simulations with an Allegro machine-learning force field (MLFF) trained on density functional theory data selected using an on-the-fly active learning procedure. To this end, superionic transitions are identified around 1450 K for Li2O and around 1100 K for Li2S through continuous peaks in the heat capacity. Diffusion coefficients and concentration of Frenkel defects are found to exhibit strong non-Arrhenius behavior coinciding with the superionic transition where both diffusion activation energy and defect formation energy decrease significantly across the transition. This suggests a self-enhanced spontaneous defect formation mechanism, which drives the superionic transition and promotes increased ionic diffusion. These results identifies the microscopic origin of type-II superionic behavior as a result of correlated defect formation and support the design of improved solid-state electrolytes.
Source: ChemRxiv
DOI: 10.26434/chemrxiv.15010965/v1
Institution: Aalborg University
Language: English
Open Access status: green
OpenAlex ID: W7223269177
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