Altering the structural properties of A2B12H12 compounds via cation and anion modifications

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Highlights

  • Potential application of newly modified closo-dodecaborates as electrolytes in solid-state batteries.

  • DSC of Na2B12X12 (X = Cl and I) shows increasing phase transition temperatures with larger anionic size.

  • LiyNa1−yB12H12 phases crystallize in Pa3¯ space group from PXRD and neutron vibrational spectra.

  • DSC of LiyNa1−yB12H12 shows intermediate Ttrans between the limits of pure Li2B12H12 and Na2B12H12.

Abstract

The recent discovery of unusually high cationic conductivity in Na2B12H12 above its entropy-driven, order–disorder phase transition near 529 K and the expected similar conductivity behavior in Li2B12H12 above its transition near 615 K have led us to investigate modifications of these two materials in an effort to reduce their transition temperatures and thus extend their high conductivities to more technologically favorable values. Differential scanning calorimetry measurements of perhalogenated Na2B12X12 (X = Cl and I), which are larger anion relatives of Na2B12H12, suggest unfavorably higher transition temperatures near 730 K and 816 K, respectively. New mixed-cation LiyNa2−yB12H12 phases show intermediate transition temperatures between those of Li2B12H12 and Na2B12H12. X-ray diffraction measurements and neutron vibrational spectra corroborate low-temperature ordered structures (for y = 0.67, 1, and 1.33) similar to Li2B12H12, with Li+ and Na+ disordered among the near-trigonal cation sites.

Introduction

The continuous search for stable solid-state electrolytes applicable towards Li-ion and Na-ion batteries has fueled research on desirable new fast-ion conductors based on complex hydrides [1], [2], [3]. For example, earlier electrical conductivity studies of LiBH4 performed by Matsuo et al. showed a threefold increase in Li+ conductivity to values exceeding 10−3 S cm−1 following the orthorhombic (Pnma) to hexagonal (P63mc) phase transition near 390 K [4], [5]. More recently, Udovic et al. investigated the structural and conductive properties of sodium dodecahydro-closo-dodecaborate (Na2B12H12) [6]. This compound displayed superionic conductivity of more than 0.1 S cm−1 above 540 K, upon transforming from the room-temperature ordered monoclinic (P21/n) to the high-temperature disordered cubic phases (Pm3¯n and Im3¯m) near 529 K [6], [7], [8], [9], [10]. The sudden enhancement in ionic conductivity within this borohydride material coincides with the appearance of a cation-vacancy-rich sublattice within spacious interstitial corridors formed by the unusually large and orientationally mobile icosahedral B12H122− anions [7], [8]. Such less restrictive pathways enable more facile cation diffusional jumps [9], [10]. Li2B12H12 was also observed to undergo an order–disorder phase transition near 615 K from a pseudo-fcc (Pa3¯) lattice to an expanded fcc lattice with extensive cation and anion disorder [8], [11], [12], but slow compound degradation once in the disordered phase hindered the measurement of probable superionic behavior.

Since there is a correlation between the solid-state phase transition temperature and conductivity in these systems, the onset of superionicity can be potentially lowered by minor alterations in the cations and anions. From a practical viewpoint, it is desirable that potential solid electrolytes incorporated into future battery technologies should possess fast-ion conductive behavior at or below room temperature. Herein we explore the effect of anionic or cationic modifications of Na2B12H12 on its phase behavior. The B12H122− anion was first modified by substitution of hydrogen with a halogen to form perhalogenated dodecaborate compounds, Na2B12X12, where X = Cl and I. Alternatively, partial substitution of the Na+ by Li+ cations was performed to produce mixed solid-solution LiyNa2−yB12H12 phases.

Section snippets

Materials and methods

Na2B12H12, Na2B12Cl12, Na2B12I12 and Li2B12H12 are commercial products obtained from Katchem [13]. Na2B12X12 (X = Cl and I) samples were used without further purification. LiyNa2−yB12H12 (y = 0.67, 1, and 1.33) samples were synthesized by dissolving stoichiometric amounts of anhydrous Li2B12H12 and Na2B12H12 in 10 ml of deionized water. A two-step drying process was then performed: (1) the excess water was removed by pumping the mixture under dynamic vacuum (ca. 10−2 Pa) at room temperature for two

Anionic modifications

The DSC scans of the Na2B12X12 (X = Cl and I) samples are compared with that for Na2B12H12 in Fig. 1. A reversible phase transition is observed for both compounds, with endothermic peaks on heating and exothermic peaks on cooling. However, the changes in both compounds occur at higher temperatures as compared to Na2B12H12 [8]. There appears to be a correlation between the anion radius and the transition temperatures on both heating and cooling, with the larger anions resulting in higher

Conclusion

Na2B12X12 (X = Cl and I) perhalogenated closo-dodecaborate phases were characterized by DSC, showing reversible solid-state phase transitions, with temperatures shifted to higher values for larger anions. From these results, we conclude that any modifications involving larger anion substitutions push the transition temperature in an undesirable direction, and future efforts will focus on substituting smaller anions into the structure. Structural characterizations are currently underway to

Acknowledgements

This work was partially supported by the DOE EERE under Grant No. DE-EE0002978. The authors thank Dr. J.J. Rush for helpful discussions concerning this work.

References (24)

  • N. Verdal et al.

    J. Solid State Chem.

    (2014)
  • J. Rodriguez-Carvajal

    Physica B

    (1993)
  • T.J. Udovic et al.

    Nucl. Instrum. Methods A

    (2008)
  • N. Verdal et al.

    J. Solid State Chem.

    (2011)
  • N. Verdal et al.

    J. Alloys Comp.

    (2011)
  • J.B. Goodenough et al.

    J. Am. Chem. Soc.

    (2013)
  • A. Unemoto et al.

    Adv. Funct. Mater.

    (2014)
  • K.B. Hueso et al.

    Energy Environ. Sci.

    (2013)
  • M. Matsuo et al.

    Appl. Phys. Lett.

    (2007)
  • A. Remhof et al.

    Phys. Rev. B

    (2010)
  • T.J. Udovic et al.

    Chem. Commun.

    (2014)
  • J.-H. Her et al.

    J. Phys. Chem. C

    (2009)
  • Cited by (0)

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