Revealing the fundamental proton transport mechanism in solid acid compounds through machine learning molecular dynamics

Rotation-assisted diffusion has been proposed as a prevalent mechanism for enhancing ionic mobility in solid electrolyte. In solid acid compounds CsH₂PO₄ and CsHSO₄, fast proton conduction has long been explained by a two-step process: local proton hopping followed by the polyanion rotation that carries the proton across sites. However, the precise role of polyanion rotation in assisting proton transport remains unanswered, with debate over whether it acts as a paddlewheel or facilitates the Grotthuss mechanism. From nanosecond molecular dynamics simulations driven by equivariant neural network force fields, we propose a proton slingshot mechanism, in which the proton is initially carried by polyanion rotation, followed by O-H bond reorientation, enabling long-range proton jumps toward a neighbor polyanion. Due to limited spatial and time scales in simulations, previous studies of polyanion rotation dynamics examined its characteristics at short times and failed to fully capture the rotational dynamics critical to proton transport. Our work identifies two distinct types of rotational motions in CsH₂PO₄, of which only one, operating on a timescale of hundreds of picoseconds, controls and enables long-range proton transport. The strong coupling between polyanion rotation and transient covalent proton bonding in CsH₂PO₄ leads to the appearance of two rotational time scales with activation energies of 0.16(1) eV and 0.35(1) eV compared to a single activation energy of 0.16(1) eV in CsHSO₄. Additionally, the polyanion rotation in CsH₂PO₄ exhibits frustrated behavior and shows less directional preference than in CsHSO₄ due to binding effect from high proton coordination. Our findings offer deeper insights into the proton conduction mechanism and rotational dynamics in solid-acid compounds, revealing how rotational motion enhances ionic mobility.