Experimental and Theoretical Investigation of the Ion Conduction Mechanism of Tris(adiponitrile)perchloratosodium, a Self-Binding, Melt-Castable Crystalline Sodium Electrolyte

Sodium
perchlorate (NaClO4) crystallizes with adiponitrile
(ADN) as a 1:3 solvate to produce (ADN)3NaClO4, a solid electrolyte for sodium ion conduction. The solid possesses
high thermal stability (up to 150 °C) and the ability to be melt-cast
(Tm = 81 °C). The pressed solid has
a high ionic conductivity of 2.2 × 10–4 S cm–1 at room temperature with a low activation barrier
for ion conduction of 22 kJ mol–1. The high conductivity
is the result of low-affinity ion-conduction channels in the bulk
based on the X-ray crystal structure, and by low grain-boundary resistance
and possibly a grain-boundary percolating network due to a fluidlike
nanoliquid layer between the grains, observable by scanning electron
microscopy and differential scanning calorimetry. When the liquid
nanolayer is rinsed away or removed by excessive drying, the bulk
room temperature ionic conductivity is 4 × 10–5 S cm–1, activation energy for ionic conduction
for an organic solid is 37 kJ mol–1, and the sodium
ion transference number is 0.71. Scanning electron microscopy and
classical molecular dynamics simulations suggest that these cocrystals
form a fluid layer of ADN at the surface, which facilitates the Na+ ion migration between the grains. Density functional theory
calculations are consistent with the possibility of ion conduction
via a solvent–anion coordinated transition state through vacancy
defects in the three symmetry-equivalent ion channels along separate
directions, suggesting the possibility of ionic conductivity in three
dimensions.