Modified Anion Packing of Na₂B₁₂H₁₂ in Close to Room Temperature Superionic Conductors

Abstract

Three different types of anion packing, i.e., hexagonal close packed (hcp), cubic close packed (ccp), and body centered cubic (bcc), are investigated experimentally and with ab initio calculations in the model system NaBH. Solvent free and water assisted mechanical grinding provide polycrystalline samples for temperature-dependent synchrotron radiation X-ray powder diffraction and electrochemical impedance spectroscopy. It is shown that among the common close packed lattices, the hcp anionic backbone creates very favorable conditions for three-dimensional ionic conduction pathways, comprised of O-O, T-T, and T-O-T (O for octahedral, T for tetrahedral) cation hops. The hcp lattice is stable with respect to ccp and bcc lattices only at higher volumes per formula unit, which is achieved either by cationic substitution with larger cations or partial substitution of hydrogen by iodine on the closo-anion. It is found that the partial cationic substitution of sodium with lithium, potassium, or cesium does not lead to enhanced conductivity due to the obstruction of the conduction pathway by the larger cation located on the octahedral site. Substitution on the closo-anion itself shows remarkable positive effects, the ionic conductivity of NaBHI reaching values of close to 10 S cm at a rather low temperature of 360 K. While the absolute value of σ is comparable to that of NaCBH, the temperature at which it is attained is approximately 20 K lower. The activation energy of 140 meV is determined from the Arrhenius relation and among the lowest ever reported for a Na-conducting solid.

Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid-state electrolyte

“…7 shows the patterns from the HT-PXD experiments. The result is consistent with the reports made by Verdal et al (2014) and Sadikin et al (2017). Na 2 B 12 H 12 undergoes to two successive order-disorder phase transitions starting at 511 K, where the P2 1 /c phase transforms into the cubic Pm " 3 3n phase and both phases coexist until 540 K. The primitive cubic phase is transformed into the more disordered Im " 3 3m phase at 587 K. Simultaneously, another minor transition into the -phase Fm " 3 3m takes place (see Fig.…”

“…Temperature-induced phase transitions in Na 2 B 12 H 12 have already been studied (Verdal et al, 2014;Sadikin et al, 2017). Nevertheless, no CTE for the low-and high-temperature (-polymorph Pm " 3 3n and -polymorph Im " 3 3m) phases of Na 2 B 12 H 12 was determined.…”

Sodium‐Ion Battery Electrolytes: Modeling and Simulations

“…In an extensive study of the Na 2 B 12 H 12 compounds, DFT and MD simulations were used as complementary techniques to the experiments . The hcp packed anionic structure was shown to provide fast Na transport in all three dimensions, with the ions jumping in a T d –O h –T d fashion with very low activation energies.…”

Ethanol- and Methanol-Coordinated and Solvent-Free Dodecahydro closo-Dodecaborates of 3d Transition Metals and of Magnesium

“…Our interest in TM closo-borates started with studies of closo-borates as solid electrolytes [13][14][15][16][17][18][19]. The TM closo-borates were considered as materials for electrodes in Li-and Na-ion solid-state batteries thus providing chemical compatibility of electrode/electrolyteinterface.…”