Revealing the Proton Slingshot Transport Mechanism in Solid Acid Electrolytes with Machine Learning Molecular Dynamics

Rotation-assisted diffusion has been proposed as a prevalent mechanism in various superionic Li-ion and proton conductors. While polyanion rotation (PR) has long be recognized to correlate with high ionic mobility, its precise role in proton conduction, its coupling with the surrounding environment, and its dynamics at nanosecond scale have remained elusive. We investigate the superprotonic phases of solid acid compounds CsH₂PO₄ and CsHSO₄, elucidating the detailed proton conduction mechanism using nanosecond molecular dynamics simulations driven by equivariant neural network force fields. Our results demonstrate that PR alone does not carry proton across sites. Instead, the combination of PR and active O-H bond reorientation creates a proton slingshot mechanism that governs long-range proton motion. In addition, PR exhibits dynamics on multiple time scales and activation energies, which have not been captured from previous sub-nanosecond simulations. A stronger correlation between PR and surrounding proton bonding is observed in CsH₂PO₄ compared to CsHSO₄, with distinctive rotational dynamics beyond mere rate differences. Our findings provide a detailed understanding of the proton conduction mechanism and PR dynamics in solid-acid compounds, offering new insights into the enhanced ionic mobility facilitated by PR.