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

Abstract

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.

First-principles study of superionic Na9+xSnxM3−xS12 (M = P, Sb)

“…12 we find, in accordance with experimental findings, 50 that the conductivity of Na 11 Sn 2 SbS 12 is slightly lower than that of Na 11 Sn 2 PS 12 at all elevated temperatures and statistically undistinguishable from the rather imprecise room temperature results. Again, the AIMD simulations closely match the earlier study by Zhang et al 52 for their narrower temperature range. Our results suggest that the change in activation energy for ion transport in Na 11 Sn 2 SbS 12 occurs only around 1000 K, making less likely that frozen-in SbS 4 disorder would be relevant for the transport around room temperature.…”

“…While at temperatures far above 500 K the PS 4 reorientation should be dynamic allowing for the paddle-wheel mechanism, 53 where as proposed by Zhang et al 52 it is assumed that the Na + are propelled by the rotating PS 4 , our new finding of the intermediate temperature phase transition means that we have to expect the actual orientational distribution to be locked in at room temperature and thus to depend on the thermal history of the sample. Different states of frozen-in disorder as a consequence of different thermal histories of the samples might (along with undetected stoichiometry variations) thus be the main reason for the significant differences in the reported migration barriers and conductivities for NSPS from different laboratories.…”

“…Zhang et al confirmed the disorder for both their nominally stoichiometric Na 11 Sn 2 PS 12 and Na 11 Sn 2 PSe 12 samples by diffraction studies. 52 While from the structural studies it was not clear whether the rotational disorder is static or dynamic, the same authors conclude from their high temperature AIMD simulations that the disorder is dynamic (analogous to what had been proposed for Li 10 GeP 2 S 12 before 5 ) and constitutes a paddle-wheel mechanism, i.e. a dynamic coupling of cation hops to PS 4 anion rotations.…”

“…a dynamic coupling of cation hops to PS 4 anion rotations. 52,53 Yu et al 54 suggested on the basis of bonding energy models as well as DFT-based phononand ab initio molecular dynamics (AIMD) calculations that the enhanced conductivity in stoichiometric Na 11 Sn 2 PSe 12 is to be attributed to the lower bonding energy of Na + ion with the surrounding anions and a supposed increased disorder relative to NSPS. In the meanwhile our group has synthesized Na 11 Sn 2 PSe 12 also by a mechanochemical method, 55 which should be more scalable than the previously reported syntheses in sealed ampoules, 51,54 and demonstrated its use in the first high power sodium selenium all solid state batteries.…”

Low-temperature paddlewheel effect in glassy solid electrolytes

“…This compound was identified following a series of studies on other closo-boranes and -carboranes, all of which exhibit transition temperatures above ambient temperature 50,[55][56][57] . Recent studies on Li 10 GeP 2 S 12 , Na 11 Sn 2 PS 12 , glass-ceramics, and other systems have also alluded to contributions to ionic conductivity from anion rotations [64][65][66][67][68][69] . In polymeric conductors, segmental motion of the polymer chains is believed to contribute to ion migration 70 .…”