Revealing the proton slingshot mechanism in solid acid electrolytes through machine learning molecular dynamics

In solid acid solid electrolytes CsH₂PO₄ and CsHSO₄, mechanisms of fast proton conduction have long been debated and attributed to either local proton hopping or polyanion rotation. However, the precise role of polyanion rotation and its interplay with proton hopping remained unclear. Nanosecond-scale molecular dynamics simulations, driven by equivariant neural network force fields, reveal a nuanced proton slingshot mechanism: protons are initially carried by rotating polyanions, followed by O$-$H bond reorientation, and the combined motion enables long-range jumps. This challenges the conventional revolving paddlewheel model and reveals significant independent proton motion that is assisted by limited rotations. Despite structural similarities, we identify qualitative differences in transport mechanisms between CsH₂PO₄ and CsHSO₄, caused by different proton concentrations. CsH₂PO₄ exhibits two distinct rates of rotational motions with different activation energies, contrasting with CsHSO₄'s single-rate behavior. The higher proton concentration in CsH₂PO₄ correlates with frustrated PO₄ polyanion orientations and slower rotations compared to SO₄ in CsHSO₄. Additionally, we reveal a correlation between O-sharing and proton transport in CsH₂PO₄, a unique feature due to extra proton per polyanion compared to CsHSO₄. Our findings suggest that reducing proton concentration could accelerate rotations and enhance conductivity. This work provides a unified framework for understanding and optimizing ionic mobility in solid-acid compounds, offering new insights into the interplay between proton hopping and disordered dynamics in polyanion rotation.