Plastic-Crystalline Solid-State Electrolytes

• Applicant: Privatdozent Dr. Peter Lunkenheimer
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Experimental Condensed Matter Physics
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 315230498

• Final Report Year: 2020

Final Report Abstract

Electrolytes, materials with high ionic conductivity, are essential components of energy-storage and - conversion devices as batteries, fuel cells or supercapacitors. Further progress towards a sustainable energy supply requires significant advances of such devices, for which the finding of better electrolytes is a key factor. The latest development in this field, pursued in this project, are so-called plastic crystals (PCs), with small amounts of admixed salts providing the necessary ions. PCs comprise molecules that can freely rotate, which, via a "revolving door"-like mechanism, is believed to be the reason for the surprisingly high ionic conductivity of many of these materials. PCs, being solid electrolytes, have significant advantages compared to the liquid electrolytes used in current battery technology, which often are harmful and flammable. However, further optimisation of their properties, especially an enhancement of their conductivity, is needed to make PCs ready for application. In this project, we aimed at achieving a better understanding of the microscopic mechanisms of ion motions in PCs. Moreover, we wanted to find ways to optimize their conductivity. For this purpose, we investigated numerous examples from this material class, mainly using dielectric spectroscopy. This method is sensitive to both the rotational motions of the molecules and the translational motions of the ions, providing information on the possible coupling of both dynamics. We confirmed that an enhancement of the conductivity can be achieved by mixing two different plastic-crystalline compounds and demonstrated that this enhancement is an astonishingly universal property of very different types of PCs. Surprisingly, we found that it can be driven by quite different mechanisms, depending on the plastic-crystalline material: For most systems the revolving-door mechanism is relevant, becoming more effective for mixtures. However, some systems seem to reveal a defect-related mechanism where the conductivity is enhanced by the additional disorder introduced by the admixing of a different compound. Which of these mechanisms is dominant, essentially depends on the molecular shapes and/or the mixing ratios. The degree of translation-rotation coupling, the molecular rotation rate, the mixing ratio, and the size and shape of the added molecules all can influence the conductivity. Optimizing these four factors is essential for future electrochemical applications. By achieving a better understanding of the ionic motions in PCs and by systematically investigating the influence of the above-mentioned factors on their conductivity, the present project has paved the way for the development of new PC-based electrolytes.

Publications

• Variation of ionic conductivity in a plastic-crystalline mixture, J. Chem. Phys. 147, 104502 (2017)
  D. Reuter, C. Geiß, P. Lunkenheimer, and A. Loidl
  
• Importance of reorientational dynamics for the charge transport in ionic liquids, Phys. Rev. E 98, 052605 (2018)
  P. Sippel, S. Krohns, D. Reuter, P. Lunkenheimer, and A. Loidl
  
• Ionic conductivity of deep eutectic solvents: the role of orientational dynamics and glassy freezing, Phys. Chem. Chem. Phys. 21, 6801 (2019)
  D. Reuter, C. Binder, P. Lunkenheimer, and A. Loidl
  
• Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures, J. Chem. Phys. 150, 244507 (2019)
  D. Reuter, P. Lunkenheimer, and A. Loidl
  
• Charge transport by global protonic conductivity and relaxational dynamics over hydrogen bonds in Fe2+Fe3+3.2(Mn2+,Zn)0.8(PO4)3(OH)4.2(HOH)0.8, Solid State Ionics 347, 115240 (2020)
  M. Winkler, P. Lunkenheimer, A. Loidl, S.-H. Park, B. Röska, and M. Hoelzel
  
• Ionic and dipolar dynamics in glassy electrolytes, PhD thesis, Augsburg, 2020
  D. Reuter
  
• Ionic conductivity and relaxation dynamics in plastic crystals with nearly globular molecules, J. Chem. Phys. 153, 014502 (2020)
  D. Reuter, K. Seitz, P. Lunkenheimer, and A. Loidl