Coupled Cation-Anion Dynamics Enhances Cation Mobility in Room Temperature Superionic Solid-State Electrolytes

Nuclear density plot of [Na11Sn2]PS12 from neutron powder diffraction through the (100) plane, x = 0 at 450 K (left panel); The trajectories of S ligands of [PS4] polyanions in [Na11Sn2]PS12 between 21 and 22 ps from AIMD simulation at 1050 K (middle panel); The trajectories of Na in the vicinity of the [PS4] polyanion during the same duration (right panel); The correlated dynamics of the cations and anions are indicated by the arrows, with red arrows showing the rotation of the S atoms and green arrows indicating Na+-ion diffusion

Scientific Achievement

Our work reveals facile [PX4]3- anion rotation in superionic Na11Sn2PS12 and Na11Sn2PSe12, and greatly hindered [SbS4]3- rotational dynamics in their less conductive analogue, Na11Sn2SbS12. Along with introducing dynamic frustration in the energy landscape, the fluctuation caused by [PX4]3- anion rotation is firmly proved to couple to, and facilitate long range Na+-cation mobility, by transiently widening the bottlenecks for Na+-ion diffusion.

Significance and Impact

The combined analysis developed in our work resolves the role of the long-debated paddle-wheel mechanism, and is the first direct evidence that anion rotation significantly enhances cation migration in room temperature rotor phases. These findings deliver important insights into the fundamentals of ion transport in solid electrolytes, and may be particularly important to the design of divalent ion conductors where static anion frameworks rarely support good ion transport.

Research Details

  • Helmholtz free energy surfaces of the S/Se ligands of [PnX4] show that S/Se ligands of [PS4]3- and [PSe4]3- exhibit a very shallow free energy landscape and low rotation barrier of 0.12-0.24 eV, while [SbS4]3- shows a deeper free energy landscape and a higher barrier (0.6-0.86 eV) for the polyanion to rotate.
  • The time-joint correlation analysis developed in our work confirms the dynamical coupling between anion rotation and cation migration, providing clear evidence of the synchronization of the velocity of the cations and polyanions, while showing that hindered rotor anions can retard cation migration.

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DOI: 10.1021/jacs.9b09343

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