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 M 2 B 12 (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 Li 2 B 12 (OH) 12 and Na 2 B 12 (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 Na 2 B 12 (OH) 12 . K 2 B 12 (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 M 2 B 12 (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 M 2 B 12 H 12 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.