Thermal, structural and transport properties of composite solid electrolytes (1-x)(C4H9)4NBF4-xAl2O3

Thermal, structural and electrical properties of composite solid electrolytes (1-x)(C4H9)(4)NBF4-xAl(2)O(3) with nanocrystalline gamma-alumina were investigated by DSC, X-ray diffraction, IR spectroscopy, impedance and electrochemical measurements. It was found that the melting enthalpy of (C4H9)(4)NBF4 in the composites strongly decreases and its value approaches to zero in the composites with x >= 0.9, where x is the molar fraction of alumina, indicating the transformation of (C4H9)(4)NBF4 to an interface-stabilized amorphous state. This effect was quantitatively interpreted in terms of the brick-wall model assuming that a layer of amorphous phase of the ionic salt is formed at salt/oxide interfaces. At the alumina concentration of x = 0.9, corresponding to a volume fraction of alumina f = 0.53, almost all the ionic salt gets into the interface layer the thickness of the amorphous layer is nearly 3 nm. These results agree with the results of X-ray diffraction studies and IR spectroscopy. Introduction of nanocrystalline gamma-alumina into the (C4H9)(4)NBF4 matrix leads to a relative increase in conductivity by more than 2 orders of magnitude, conductivity goes through a maximum of 0.21 mS/cm at 130 degrees C for the composite with x = 0.9. This composite is characterized by a non-Arrhenius temperature dependence, typical for glassy electrolytes. It was shown that the electrochemical voltage for the composite 0.1(C4H9)(4)NBF4-0.9Al(2)O(3) is nearly 4 V.