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Ionic conduction mechanism in the Na2(B12H12)0.5(B10H10)0.5 closoborate solid-state electrolyte: interplay of disorder and ion-ion interactions Léo Duchêne, Sarah Lunghammer, Tatsiana Burankova, Wei-Chih Liao, Jan Peter Embs, Christophe Copéret, H. Martin R. Wilkening, Arndt Remhof, Hans Hagemann, and Corsin Battaglia Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00610 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019
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Ionic conduction mechanism in the Na2(B12H12)0.5(B10H10)0.5 closoborate solid-state electrolyte: interplay of disorder and ion-ion interactions Léo Duchêne1,2, Sarah Lunghammer3, Tatsiana Burankova4, Wei-Chih Liao5, Jan Peter Embs4, Christophe Copéret5, H. Martin R. Wilkening3, Arndt Remhof1,*, Hans Hagemann2, and Corsin Battaglia1 1Empa,
Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland de Chimie-Physique, Université de Genève, 1211 Geneva 4, Switzerland. 3Christian Doppler Laboratory for Lithium Batteries, Institute for Chemistry and Technology of Materials, NAWI Graz, Graz University of Technology, 8010 Graz, Austria 4Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland 5Department of Chemistry and Applied Sciences, ETH Zurich, 8093 Zürich, Switzerland 2Département
ABSTRACT: The conduction mechanism of Na2(B12H12)0.5(B10H10)0.5, a particularly promising solid-state electrolyte for sodiumion batteries, is elucidated. We find from electrochemical impedance spectroscopy that the temperature dependent conductivity is characterized by three distinct regimes of conductivity. In the first regime, at temperatures below -50 °C, conductivity remains low before a glass-like transition identified by X-ray diffraction and calorimetry causes a faster increase of sodium conductivity through site disordering. This second regime of faster diffusion above -50 °C is characterized by an apparent activation energy of 0.6 eV, higher than expected from the local microscopic barrier of 0.35 eV observed by, e.g., 23Na nuclear magnetic resonance spin-lattice relaxation. This mechanism of so-called correlated ion diffusion originates from the coupling of the cation and anion motion due to short range ion-ion interactions combined with background energy fluctuations, which we can associate through quasi-elastic neutron scattering experiment to fast librations of the anions. In the third regime, at temperatures above 70 °C, the thermal energy increases above the background energy fluctuations and the activation energy decreases to 0.34 eV reflecting the local energy barrier for non-correlated ion diffusion. We discuss the link between this behavior and the different frustrations responsible for the high conductivity of closo-borate electrolytes and show that our interpretation can also explain the complex conductivity behavior observed in related compounds.
INTRODUCTION: All-solid-state batteries (ASSBs) are intensively investigated as a next-generation energy-storage solution to overcome the limitations of current battery technology i.e. to enable increased energy and power density while improving safety.1 Ion dynamics, electrochemical stability, and interfacial properties of the solid-state electrolyte to be used in such devices are instrumental to the high performance of such ASSBs.2 In particular, high ionic conductivity is an absolute requirement for the battery to be charged and discharged at a reasonable rate. Commercial lithium-ion batteries rely on organic liquid electrolytes to do so, but cannot be operated neither at high temperature due to the electrolyte volatility and thermal instability nor at too low temperatures because of a large increase of cell impedance.3 These problems may be mitigated by the use of a solid-state electrolyte4 if such an electrolyte exhibits not only high ionic conductivity at room temperature but also low activation energy to maintain this high ionic conductivity over an extended temperature range. To fulfil these criteria the community has explored a large scope of materials in four main classes of compounds, i.e.,
oxide ceramics, sulfides and thiophosphates, polymers, and more recently complex-boron hydrides.5–14 Among the latter, we recently reported an alternative solidstate electrolyte for ASSBs, namely Na2(B12H12)0.5(B10H10)0.5, that combines high ionic conductivity of 0.9 mS cm-1 at 20 °C and a large electrochemical stability of 3 V including stability towards sodium metal. Na2(B12H12)0.5(B10H10)0.5 allowed for the fabrication and successful cycling of the first 3 V sodiumion all-solid-state battery using this type of material.15,16 Temperature dependent conductivity measurements on Na2(B12H12)0.5(B10H10)0.5 previously indicated that contrarily to common thermally activated ionic conductivity with a single activation energy, Na2(B12H12)0.5(B10H10)0.5 exhibits a complex conductivity behavior characterized by a gradually decreasing activation energy with increasing temperature. Here we discuss the origin of this behavior from the study of structural and dynamical properties of the material. We show that it arises from the combined effects of ion-ion interactions and disorder leading to correlated ion motion. This behavior is inherent to the structural, chemical, and
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dynamical frustrations associated with high conductivity in closo-borates17 and can also explain the non-Arrhenius conductivity behavior observed in related phases such as Li/Na2(CB11H12)(CB9H10) and Na3(BH4)(B12H12).18,19 EXPERIMENTAL METHODS: Sample Preparation. Na2B12H12, Na2B10H10, NaCB11H12, and NaCB9H10 were purchased from Katchem. Na2B12H12 was used as received whereas Na2B10H10, NaCB11H12 and NaCB9H10 were first dried at 160 °C under vacuum (
