Revealing the Proton Slingshot Mechanism in Solid Acid Electrolytes through Machine-Learning Molecular Dynamics

In solid acid electrolytes CsH2PO4 and CsHSO4, the origin of fast proton conduction has long been debated, attributed either to local proton hopping or to polyanion rotation. However, the precise role of polyanion rotation and its interplay with proton hopping remains unresolved. Nanosecond-scale molecular dynamics simulations, driven by equivariant neural-network force fields, reveal a nuanced proton slingshot mechanism: a rotating polyanion first carries the proton, O-H bond reorientation then extends its displacement, and the combined motion enables long-range jumps. This mechanism refines the conventional Grotthuss-type picture, revealing that long-range transport arises from significant O-H bond reorientation assisted by only limited polyanion rotation. Despite their structural similarity, CsH2PO4 and CsHSO4 exhibit qualitatively different transport behavior associated with their distinct proton concentrations. In CsH2PO4, two rotation rates with different activation energies emerge, while CsHSO4 displays a single-rate process with faster productive polyanion dynamics. The higher proton concentration in CsH2PO4 is associated with greater rotational frustration and slower dynamics of PO4 compared with SO4 in CsHSO4. In CsH2PO4, we further identify a unique correlation between oxygen sharing and proton transport, arising from the extra proton per polyanion. Together, our findings establish a unified framework linking polyanion rotation, proton coordination, and long-range transport, and suggest that tuning proton concentration could accelerate rotation and enhance conductivity.