Variation of ionic conductivity in a plastic-crystalline mixture

Abstract

Ionically conducting plastic crystals (PCs) are possible candidates for solid-state electrolytes in energy-storage devices. Interestingly, the admixture of larger molecules to the most prominent molecular PC electrolyte, succinonitrile, was shown to drastically enhance its ionic conductivity. Therefore, binary mixtures seem to be a promising way to tune the conductivity of such solid-state electrolytes. However, to elucidate the general mechanisms of ionic charge transport in plastic crystals and the influence of mixing, a much broader database is needed. In the present work, we investigate mixtures of two well-known plastic-crystalline systems, cyclohexanol and cyclooctanol, to which 1 mol. % of Li ions were added. Applying differential scanning calorimetry and dielectric spectroscopy, we present a thorough investigation of the phase behavior and the ionic and dipolar dynamics of this system. All mixtures reveal plastic-crystalline phases with corresponding orientational glass-transitions. Moreover, their conductivity seems to be dominated by the "revolving-door" mechanism, implying a close coupling between the ionic translational and the molecular reorientational dynamics of the surrounding plastic-crystalline matrix. In contrast to succinonitrile-based mixtures, there is no strong variation of this coupling with the mixing ratio.

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

“…Together with our previous results on different SN-GN and cyclohexanol-cyclooctanol mixtures, 16,22 the present investigation reveals the parameters that are relevant for the conductivity enhancement in PC mixtures: the size of the added molecules, the degree of translation-rotational coupling, the concentration ratio of large to small molecules, and the molecular reorientation rate at a given temperature. Optimizing these four factors is essential for the development of mixed PC electrolytes for future electrochemical applications.…”

“…1). As  of the investigated materials strongly differs at low temperatures, in this region, just as in the previously investigated mixtures of cyclohexanol and cyclooctanol, 22 strong effects of  on the conductivity can be expected. Indeed with decreasing temperature, the  dc curves of the materials in Fig.…”

Ionic conductivity and relaxation dynamics in plastic crystals with nearly globular molecules

“…11,15,16,29,37 Interestingly, for several PC electrolytes, including both ionic and molecular PCs, it was recently found that the ionic conductivity and the stability range of the plastic phase can be considerably enhanced by admixing a related molecular species of different size. 18,20,22,30,31,32,33 This was, e.g., demonstrated for succinonitrile [SN: C 2 H 4 (CN) 2 ], the most prominent representative of PC electrolytes with neutral molecules: 13,14 It revealed a strong conductivity enhancement when admixing glutaronitrile [GN: C 3 H 6 (CN) 2 ], 18,30 reaching up to three decades for samples with up to 80 mol% GN and with small amounts of added Li ions. 18 Similar results were also found for other SN-based mixtures.…”

“…The PC electrolytes can be classified into two subgroups: (i) Ionic PCs, 11,12,15,16,17,19,20,22,29 composed of cations and anions, of which at least one is sufficiently complex to allow for orientational degrees of freedom. (ii) Molecular PCs, 13,14,18,30,31,32,33 consisting of neutral molecules and a relatively small amount of admixed salt to provide ionic charge carriers. Obviously, the dynamic rotational disorder in PC electrolytes generates a high-entropy medium, which should be favorable for the translational ion hopping.…”

“…Some works assume a direct coupling of the ionic translation dynamics to the rotational motion of the asymmetric ions or molecules via a "paddle-wheel" or "revolving door" mechanism. 10,12,18,32,33,35 Here the ionic conductivity is assumed to be enhanced by transient free volume generated within the lattice by the molecular reorientations. An alternative approach explains the high ionic mobility in PCs based on the peculiar properties of the plastic-crystalline lattice: The weak 2 intermolecular interactions, leading to the high plasticity and orientational disorder in PCs, make them prone to the diffusion of lattice defects and vacancies and/or plane slips.…”