Superionic Conduction of Sodium and Lithium in Anion‐Mixed Hydroborates Na₃BH₄B₁₂H₁₂ and (Li_0.7Na_0.3)₃BH₄B₁₂H₁₂

Abstract

invested to stabilize superionic conduction in such materials at room temperature (RT).We have previously reported stable Cs-and Rb-borohydrides containing the closo -borane anion [B 12 H 12 ] 2− on the A -site of an inversed perovskite structure, [ 11 ] whose RT polymorph had already been reported for Cs [ 12 ] and forming during the decomposition of bi-and trimetallic borohydrides. [ 11 ] We suspected that the incorporation of both complex anions in the same structure could provide means of stabilizing compounds with high symmetry, favoring cationic mobility at RT. Herein, we apply this concept to anion-mixed metal boranes containing mobile species of relevance and present a novel class of solid electrolytes with the compounds Na 3 BH 4 B 12 H 12 and (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 , capable of conducting either Na + or both Na + and Li + ions. Unlike order-disorder-governed dodeca-and decaboranes the cationic mobility is not entropically activated in Na 3 BH 4 B 12 H 12 and (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 , thus requiring no HT phase transition. The conduction pathways in Na 3 BH 4 B 12 H 12 are 2D where Na 2 B 12 H 12 -like slabs are in fact nonconducting, reaching RT conductivity values close to the order of 10 −3 S cm −1 and hence superior to many sulfi de glasses, NASICON and rivaling even β-alumina. [ 13,14 ] (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 , on the other hand, forms 1D channels for mixed cation conduction. The chemical reaction forming the compound at 500 K is accompanied by a dramatic increase in ionic conductivity attaining values higher than 10 −1 S cm −1 in the thermal stability region. Conductivity decreases upon cooling owed to full reversibility of the reaction. Such temperatures are nevertheless within reasonable limits for large-scale facilities, which operate at over 573 K in the case of Na-S power grids, for instance.The Figure S1). Ball milling is also attracting attention in battery research due to its fl exibility and the ease with which it is brought to the industrial scale. It has recently been used to prepare promising Na-based materials. [ 13,15 ] We also recently employed it to scan a wide variety of perovskite-type metal-borohydride systems. [ 12,16 ] Here it is used to "activate" the powders, whereby no reaction takes place even at high energies. (Li 0.7 Na 0.3 ) 3 BH 4 B 12 H 12 forms at 498 K (reaction onset) and dissociates to the precursors upon cooling. Meanwhile, the Na-rich Na 3 BH 4 B 12 H 12 forms above 638 K, and once formed it maintains the same structure in a wide temperature range of 100-653 K. Differential scanning calorimetry was used to follow the thermal events for the evolution of Na 3 BH 4 B 12 H 12 It has been recognized that battery grids are one of the more realistic contenders for future large-scale energy storage. [ 1 ] A paradigm shift is taking place in battery research that is focusing on inexpensive and abundant chemical species as alternative to compensate for the dwindling resources of lithium in terms of charge carrier (electrolyte) [ 2 ] as we...

Halogenated Sodium-closo-Dodecaboranes as Solid-State Ion Conductors

“…For instance, Na 3 BH 4 B 12 H 12 and Na 2 (CB 9 H 10 )(CB 11 H 12 ) show significantly improved RT ion conductivities compared to their constituents. 19,46 The cause of the high transition temperature for Na 2 B 12 X 12 , in contrast to Na 2 B 12 H 12 , could be related to directional and more covalent cation-anion bonding, and the large anion size and anion mass, which restrict anion dynamics at low temperatures. Improving knowledge of these aspects may form the basis for a new tool to design fast ion conducting solids for novel types of batteries.…”

“…Ion conductivity of Na 2 B 12 X 12 (X = Cl, Br, I) and related Na-borane compounds and Na-electrolytes as a function of inverse temperature. 15,19,[43][44][45][46] Na-Na jump distances are shortest in Na 2 B 12 Cl 12 , which shows the highest ion conductivity. The ion conductivity decreases above the transition temperature, which is due to slow decomposition.…”

Halide Free M(BH4)2 (M = Sr, Ba, and Eu) Synthesis, Structure, and Decomposition

“…3,4 A second field of metal borohydride application which developed recently is situated in batteries where they play the role of solid state electrolytes due to the high mobility of cations like Li + or Na + in certain borohydrides 5,6 or other mixed-anion hydroborates. 7 Rare earth containing borohydrides have also interesting luminescence and magnetic properties. 8,9 Monometallic borohydrides may be used as precursors for synthesizing bi-or trimetallic borohydrides.…”

A highly stable sodium solid-state electrolyte based on a dodeca/deca-borate equimolar mixture

“…[16][17][18][19] Strategies to stabilize these superionic phases at room temperature and below, including mechanical milling and cation or anion substitutions, have been implemented. [20][21][22][23] Yet, studies on closo-borates have focused almost exclusively on improving conductivity, while only preliminary work has been performed on the assessment of their electrochemical stability window and of their suitability for integration into a battery. Here, we report a new sodium superionic conductor Na 2 (B 12 H 12 ) 0.5 (B 10 H 10 ) 0.5 with a conductivity close to 10 À3 S cm…”