This critical review presents the state of the art research progress, proposes strategies to improve the conductivity of solid electrolytes, discusses the chemical and electrochemical stabilities, and uncovers future perspectives for solid state batteries.

Elucidation of the structure of a new sodium superionic conductor, Na11Sn2PS12via single crystal XRD and AIMD simulations reveal isotropic 3D Na+-ion conduction pathways.

Na super ion conductor (NaSICON), Na 1+n Zr 2 Si n P 3-n O 12 is considered one of the most promising solid electrolytes; however, the underlying mechanism governing ion transport is still not fully understood. Here, the existence of a previously unreported Na5 site in monoclinic Na 3 Zr 2 Si 2 PO 12 is unveiled. It is revealed that Na + -ions tend to migrate in a correlated mechanism, as suggested by a much lower energy barrier compared to the single-ion migration barrier. Furthermore, computational work uncovers the origin of the improved conductivity in the NaSICON structure, that is, the enhanced correlated migration induced by increasing the Na + -ion concentration. Systematic impedance studies on doped NaSICON materials bolster this finding. Significant improvements in both the bulk and total ion conductivity (e.g., σ bulk = 4.0 mS cm −1 , σ total = 2.4 mS cm −1 at 25 °C) are achieved by increasing the Na content from 3.0 to 3.30-3.55 mol formula unit −1 . These improvements stem from the enhanced correlated migration invoked by the increased Coulombic repulsions when more Na + -ions populate the structure rather than solely from the increased mobile ion carrier concentration. The studies also verify a strategy to enhance ion conductivity, namely, pushing the cations into high energy sites to therefore lower the energy barrier for cation migration.

Single-ion conducting solid electrolytes are gaining tremendous attention as essential materials for solidstate batteries, but a comprehensive understanding of the factors that dictate high ion mobility remains elusive. Here, for the first time, we use a combination of the Maximum Entropy Method analysis of room-temperature neutron powder diffraction data, ab initio molecular dynamics, and joint-time correlation analysis to demonstrate that the dynamic response of the anion framework plays a significant role in the new class of fast ion conductors, Na 11 Sn 2 PnX 12 (Pn = P, Sb; X = S, Se). Facile [PX 4 ] 3− anion rotation exists in superionic Na 11 Sn 2 PS 12 and Na 11 Sn 2 PSe 12 , but greatly hindered [SbS 4 ] 3− rotational dynamics are observed in their less conductive analogue, Na 11 Sn 2 SbS 12 . Along with introducing dynamic frustration in the energy landscape, the fluctuation caused by [PX 4 ] 3− anion rotation is firmly proved to couple to and facilitate long-range cation mobility, by transiently widening the bottlenecks for Na + -ion diffusion. The combined analysis described here resolves the role of the long-debated paddle-wheel mechanism, and is the first direct evidence that anion rotation significantly enhances cation migration in rotor phases. The joint-time correlation analysis developed in our work can be broadly applied to analyze coupled cation−anion interplay where traditional transition state theory does not apply. These findings deliver important insights into the fundamentals of ion transport in solid electrolytes. Invoking anion rotational dynamics provides a vital strategy to enhance cation conductivity and serves as an additional and universal design principle for fast ion conductors.

Experimental details, refinement data, Arrhenius plot, the Nyquist impedance plot and bond lengths (PDF) Data for Na 11.08 S 12 SbSn 2 (CIF) Data for Na 11.10 PS 12 Sn 2 (CIF)

Na-O layer provides Na+ diffusion pathway and storage site, whereas benzene layer provides e−conduction pathway and redox center.

A combination of the maximum entropy method and AIMD simulations demonstrates that polyanion [PS 4 ] 3À rotation is facile in the fast ion conductors b-Li 3 PS 4 and its Si-substituted analog, Li 3.25 Si 0.25 P 0.75 S 4 , but absent in the nonconductive phase, g-Li 3 PS 4 . The increased entropy upon the substitution of Si for P stabilizes the high-temperature rotor phase (b-Li 3 PS 4 ) at room temperature. Jointtime correlation analysis and AIMD simulations show that [PS 4 ]/[SiS 4 ] anion rotational dynamics are coupled to and greatly enhance cation diffusion by widening the bottleneck for Li + -ion transport.

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