Disentangling cation-polyanion coupling reveals which anion motion dominates cation transport in solid electrolytes

Polyanion dynamics, notably rotation, have long been linked to fast cation transport in solid electrolytes, yet their contribution to ionic conductivity remains unquantified, and even rotation's role, disputed. Here, we quantify the contributions of polyanion translation, rotation, and vibration to conductivity in plastic crystals by combining a constraint algorithm and machine learning molecular dynamics. Contrary to prior belief that rotation primarily facilitates transport, we reveal that each motion mode can dominate: rotation when its frequency matches the cation hopping frequency, translation at higher rotation frequencies and vibration at lower rotation frequencies. We find that rotation and translation each can enhance conductivity by over two orders of magnitude by inducing cation disorder; rotation can further enhance cooperative cation transport; and polyanion rotation, translation, and vibration all modulate the migration barrier and prefactor. These findings demonstrate that polyanion dynamics are crucial to superionicity and offer design principles for superionic conductors.