Superionic phases
of bulk anhydrous salts based on large cluster-like
polyhedral (carba)borate anions are generally stable only well above
room temperature, rendering them unsuitable as solid-state electrolytes
in energy-storage devices that typically operate at close to room
temperature. To unlock their technological potential, strategies are
needed to stabilize these superionic properties down to subambient
temperatures. One such strategy involves altering the bulk properties
by confinement within nanoporous insulators. In the current study,
the unique structural and ion dynamical properties of an exemplary
salt, NaCB
11
H
12
, nanodispersed within porous,
high-surface-area silica via salt-solution infiltration were studied
by differential scanning calorimetry, X-ray powder diffraction, neutron
vibrational spectroscopy, nuclear magnetic resonance, quasielastic
neutron scattering, and impedance spectroscopy. Combined results hint
at the formation of a nanoconfined phase that is reminiscent of the
high-temperature superionic phase of bulk NaCB
11
H
12
, with dynamically disordered CB
11
H
12
–
anions exhibiting liquid-like reorientational mobilities. However,
in contrast to this high-temperature bulk phase, the nanoconfined
NaCB
11
H
12
phase with rotationally fluid anions
persists down to cryogenic temperatures. Moreover, the high anion
mobilities promoted fast-cation diffusion, yielding Na
+
superionic conductivities of ∼0.3 mS/cm at room temperature,
with higher values likely attainable via future optimization. It is
expected that this successful strategy for conductivity enhancement
could be applied as well to other related polyhedral (carba)borate-based
salts. Thus, these results present a new route to effectively utilize
these types of superionic salts as solid-state electrolytes in future
battery applications.