Communication: Conductivity enhancement in plastic-crystalline solid-state electrolytes

Abstract

Finding new ionic conductors that enable significant advancements in the development of energy-storage devices is a challenging goal of current material science. Aside of material classes as ionic liquids or amorphous ion conductors, the so-called plastic crystals (PCs) have been shown to be good candidates combining high conductivity and favorable mechanical properties. PCs are formed by molecules whose orientational degrees of freedom still fluctuate despite the material exhibits a well-defined crystalline lattice. In the present work, we show that the conductivity of Li(+) ions in succinonitrile, the most prominent molecular PC electrolyte, can be enhanced by several decades when replacing part of the molecules in the crystalline lattice by larger ones. Dielectric spectroscopy reveals that this is accompanied by a stronger coupling of ionic and reorientational motions. These findings, which can be understood in terms of an optimized "revolving door" mechanism, open a new path towards the development of better solid-state electrolytes.

Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures

“…5), 16 the dc conductivity for all mixtures investigated in the present study is significantly higher than for the pure SN doped with the same percentage of Li salt (inverted triangles; this data set was remeasured here up to higher temperatures than previously reported in Ref. 16). At high temperatures in the PC phase,  dc is found to considerably increase with increasing molecule size.…”

“…48 In addition to the right flank of the -relaxation peak, in The occurrence of such secondary relaxation peaks is typical for glass-forming liquids 49,50,51 and also found for certain PCs, 19 including various SN-GN mixtures. 16,25,26 A detailed treatment of this feature is out of the scope of the present work.…”

“…25,26 This enables the observation of glassy freezing of the orientational disorder below about 150 K, leading to a so-called glassy-crystal state (sometimes also termed "orientational glass"). In an earlier work, 16 we have reported dielectric-spectroscopy measurements of SN-GN mixtures with different mixing ratios of up to 80 mol% GN. These experiments revealed successively stronger coupling of molecular reorientation and ionic translation with increasing GN content.…”

“…This finding indicates that the mentioned conductivity enhancement of the mixtures can be explained by a "revolving door" or "paddle wheel" mechanism 10,11.27 which becomes optimized when part of the smaller SN molecules is replaced by the larger GN molecules. 16 Within this scenario, the molecule reorientations are assumed to provide transient free volume within the lattice, thus enhancing the ionic mobility. Quite generally, it is often assumed that the typical reorientational motions of the molecules or ions in PCs facilitate translational ionic hopping, thus explaining the high ionic conductivity of many members of this material class.…”

“…In the present work, we provide a detailed study of the ionic conductivity and reorientational dynamics of binary mixtures of SN with MN, AN, and pimelonitrile [C 5 H 10 (CN) 2 ; PN] using dielectric spectroscopy. In particular, we have investigated SN 0.85 X 0.15 mixtures (with X = MN, AN, or PN) where 1% LiPF 6 was added to enable a direct comparison to the results of our previous report 16 where the same salt admixture was used. This leads to the overall composition (SN 0.85 X 0.15 ) 0.99 (LiPF 6 ) 0.01 It should be noted that the present study does not aim at achieving exceptionally high values of ionic conductivity (here different salts and higher concentrations may be more favorable 2 ) but, instead, intends to contribute to a better understanding of the fundamental aspects of ion conduction in such mixed PC systems.…”