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. 2017 Jun 21:8:15893.
doi: 10.1038/ncomms15893.

Origin of fast ion diffusion in super-ionic conductors

Xingfeng He  1 , Yizhou Zhu  1 , Yifei Mo  1   2
Affiliations

Origin of fast ion diffusion in super-ionic conductors

Xingfeng He et al. Nat Commun. .

Abstract

Super-ionic conductor materials have great potential to enable novel technologies in energy storage and conversion. However, it is not yet understood why only a few materials can deliver exceptionally higher ionic conductivity than typical solids or how one can design fast ion conductors following simple principles. Using ab initio modelling, here we show that fast diffusion in super-ionic conductors does not occur through isolated ion hopping as is typical in solids, but instead proceeds through concerted migrations of multiple ions with low energy barriers. Furthermore, we elucidate that the low energy barriers of the concerted ionic diffusion are a result of unique mobile ion configurations and strong mobile ion interactions in super-ionic conductors. Our results provide a general framework and universal strategy to design solid materials with fast ionic diffusion.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Schematic illustration of single-ion migration versus multi-ion concerted migration.
For single-ion migration (upper insets), the migration energy barrier is the same as the barrier of the energy landscape. In contrast, the concerted migration of multiple ions (lower insets) has a lower energy barrier as a result of strong ion-ion interactions and unique mobile ion configuration in super-ionic conductors.
Figure 2
Figure 2. Li ion diffusion in super-ionic conductors.
(a–c) Crystal structures of (a) LGPS, (b) LLZO and (c) LATP marked with Li sites (partially filled green spheres), Li+ diffusion channels (green bars), and polyanion groups (purple and blue polyhedra). (d–f) The probability density of Li+ spatial occupancy during AIMD simulations. The zoom-in subsets show the elongation feature of probability density along the migration channel (Li: green; O/S: yellow). The isosurfaces are 6ρ0, 6ρ0, 2ρ0 for LGPS, LLZO, LATP, respectively, where ρ0 is the mean probability density in each structure and the inner isosurfaces have twice the density of the outer isosurfaces. (g–i) Van Hove correlation functions of Li+ dynamics on distinctive Li ions during AIMD simulations.
Figure 3
Figure 3. Concerted migration and energy landscape in super-ionic conductors.
(a–c) Migration energy barrier in (a) LGPS, (b) LLZO, (c) LATP for concerted migration of multiple Li ions hopping into the next sites along the diffusion channel. Insets show the Li+ path (green spheres) and O/S ions (yellow spheres). (d–f) The energy landscape of single Li+ along the migration channel (shown in insets) across multiple Li sites (partially filled green sphere) and Li+ pathway (red spheres).
Figure 4
Figure 4. Diffusion model for concerted migration.
(a,b) The potential energy of the structural framework with low (a) or high (b) barriers at the high-energy sites. The mobile ion (grey sphere) configurations and the migration paths (arrows) are illustrated. (c) The energy profile for the concerted migration in the energy landscape (a) and (b) at K=3 eV Å. (d) The energy barrier of concerted migration at different Coulomb interaction strength K.
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