Understanding Superionic Conductivity in Lithium and Sodium Salts of Weakly Coordinating Closo-Hexahalocarbaborate Anions

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  • Mathias Jørgensen, Sandia National Laboratories, California
  • ,
  • Patrick T. Shea, Lawrence Livermore National Laboratory, Livermore, CA, USA
  • Anton E. Tomich, University of California, Riverside, USA
  • Joel B. Varley, Lawrence Livermore National Laboratory, Livermore, CA, USA
  • Marnik Bercx, University of Antwerp, Antwerp, Belgien
  • Sergio Lovera, University of California, Riverside, USA
  • Radovan Cerný, Laboratory of Crystallography, University of Geneva, Schweiz
  • Wei Zhou, National Institute of Standards and Technology, Gaithersburg, USA
  • Terrence J. Udovic, National Institute of Standards and Technology, Gaithersburg, University of Maryland
  • ,
  • Vincent Lavallo, University of California, Riverside, USA
  • Torben René Jensen
  • Brandon C. Wood, Lawrence Livermore National Laboratory, Livermore, CA, USA
  • Vitalie Stavila, Sandia National Laboratories, California, USA

Solid-state ion conductors based on closo-polyborate anions combine high ionic conductivity with a rich array of tunable properties. Cation mobility in these systems is intimately related to the strength of the interaction with the neighboring anionic network and the energy for reorganizing the coordination polyhedra. Here, we explore such factors in solid electrolytes with two anions of the weakest coordinating ability, [HCB 11H 5Cl 6] - and [HCB 11H 5Br 6] -, and a total of 11 polymorphs are identified for their lithium and sodium salts. Our approach combines ab initio molecular dynamics, synchrotron X-ray powder diffraction, differential scanning calorimetry, and AC impedance measurements to investigate their structures, phase-transition behavior, anion orientational mobilities, and ionic conductivities. We find that M(HCB 11H 5X 6) (M = Li, Na, X = Cl, Br) compounds exhibit order-disorder polymorphic transitions between 203 and 305 °C and display Li and Na superionic conductivity in the disordered state. Through detailed analysis, we illustrate how cation disordering in these compounds originates from a competitive interplay among the lattice symmetry, the anion reorientational mobility, the geometric and electronic asymmetry of the anion, and the polarizability of the halogen atoms. These factors are compared to other closo-polyborate-based ion conductors to suggest guidelines for optimizing the cation-anion interaction for fast ion mobility. This study expands the known solid-state poly(carba)borate-based materials capable of liquid-like ionic conductivities, unravels the mechanisms responsible for fast ion transport, and provides insights into the development of practical superionic solid electrolytes.

OriginalsprogEngelsk
TidsskriftChemistry of Materials
Vol/bind32
Nummer4
Sider (fra-til)1475-1487
Antal sider13
ISSN0897-4756
DOI
StatusUdgivet - feb. 2020

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