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

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.