Iodine-Substituted Lithium/Sodium closo-Decaborates: Syntheses, Characterization, and Solid-State Ionic Conductivity

Abstract

Solid-state electrolytes based on closo-decaborates have caught increasing interest owing to the impressive room-temperature ionic conductivity, remarkable thermal/chemical stability, and excellent deformability. In order to develop new solid-state ion conductors, we investigated the influence of iodine substitution on the thermal, structural, and ionic conduction properties of closo-decaborates. A series of iodinated closo-decaborates, M2[B10H10–n I n ] (M = Li, Na; n = 1, 2, 10), were synthesized and characterized by thermal analysis, powder X-ray diffraction, and electrochemical impedance spectroscopy; the stability and ionic conductivity of these compounds were studied. It was found that with the increase of iodine substitution on the closo-decaborate anion cage, the thermal decomposition temperature increases. All M2[B10H10–n I n ] exhibit an amorphous structure. The ionic conductivity of Li2[B10H10–n I n ] is higher than that of the Li2[B10H10] parent compound. An ionic conductivity of 2.96 × 10–2 S cm–1 with an activation energy of 0.23 eV was observed for Li2[B10I10] at 300 °C, implying that iodine substitution can improve the ionic conductivity. However, the ionic conductivity of Na2[B10H10–n I n ] is lower than that of Na2[B10H10] and increases with the increase of iodine substitution, which could be associated with the increase of the electrostatic potential, mass, and volume of the iodinated anions. Moreover, Li2[B10I10] offers a Li-ion transference number of 0.999, an electrochemical stability window of 3.3 V and good compatibility with the Li anode, demonstrating its potential for application in high-temperature batteries.

Polymorphism of Calcium Decahydrido-closo-decaborate and Characterization of Its Hydrates

“…The large and highly symmetrical borate cages allow for thermally induced orientational disorder that is key to high ionic conductivities, as the transition to a sublattice of rotationally fluidic anions facilitates liquid-like interstitial cation diffusion . A range of novel metal hydrides have recently demonstrated exceptionally high ionic conductivity and large electrochemical stabilities also toward metallic anodes, such as Li, Na, Mg, or Ca, which inspire development of batteries far beyond the commercial lithium battery. − …”

Influence of Solvent System on the Electrochemical Properties of a closo-Borate Electrolyte Salt

“…A series of NMR, QENS, and MD calculations have demonstrated that the cation translational mobility is correlated with the reorientation rate of the LBRA consistent with a "paddle-wheel" mechanism [10,25,27,28]. This cation translational motion and anion reorientation rate can be tuned via temperature (phase transitions), cation vacancies, the type of atom attached to the boron cage (i.e., H, Cl, F, Br), and LBRA size [7,12,13,[29][30][31][32][33].…”

Recent Achievements on Functionalization within closo‐Decahydrodecaborate [B10H10]2− Clusters

“…Halogenation of the closo‐decaborate anion has been extensively studied due to the importance and versatility of halo‐decaborate derivatives though most of which found it exceedingly difficult to get a selective reaction that leads only to a mono‐substituted derivative [21b,46] . In fact, direct halogenation of closo‐decaborate with dihalogens (chlorine, bromine, iodine) and recently with HCl yields a mixture of derivatives with uncontrolled degrees of substitution [B 10 H 10‐y X y ] 2− [21c,47] .…”