Hydroxylated closo-Dodecaborates M2B12(OH)12 (M = Li, Na, K, and Cs); Structural Analysis, Thermal Properties, and Solid-State Ionic Conductivity

Closo-borates and derivatives thereof have shown great potential as electrolyte materials for all-solid-state batteries owing to their exceptional ionic conductivity and high thermal and chemical stability. However, because of the myriad of possible chemical modifications of the large, complex anion, only a fraction of closo-borate derivatives has so far been investigated as electrolyte materials. Here, the crystal structures, thermal properties, and ionic conductivities of M2B12(OH)12 (M = Li, Na, K, and Cs) are investigated with a focus on their possible utilization as new solid-state ion conductors for solid-state batteries. The compounds generally show rich thermal polymorphism, with eight identified polymorphs among the four dehydrated compounds. Both Li2B12(OH)12 and Na2B12(OH)12 undergo a first-order transition, in which the cation sublattices become disordered, resulting in an order of magnitude jump in ionic conductivity for Na2B12(OH)12. K2B12(OH)12 undergoes a second-order polymorphic transition driven by a change in the anion–cation interaction, with no evidence of dynamic disorder. The ionic conductivities of M2B12(OH)12 range from 1.60 × 10–8 to 5.97 × 10–5 S cm–1 at 250 °C for M = Cs and Li, respectively, showing decreasing conductivity with increasing cation size. Compared with the analogous M2B12H12 compounds, such relatively low conductivities are suggested to be a consequence of strong and directional anion–cation interactions, resulting in a more static anion framework.