Ionic Motion in Materials with Disordered Structures
Nanoscale conductivity spectroscopy on solid electrolytes using an atomic force microscope
Ion-conducting crystals, glasses, and polymers are widely used as solid electrolytes in batteries, fuel cells, and
chemical sensors. A lot of research work is being carried out in order to find materials with improved ionic
conductivities. One method that is becoming more and more technologically relevant is nanostructuring of
materials. It has, for instance, been found that the ionic conductivity of nanocrystalline ionic conductors can be
increased by adding nanocrystalline insulators. In the case of glasses, a conductivity enhancement can be
achieved by the formation of nanocrystallites during partial crystallization. Furthermore, the ionic conductivity
of polymer electrolytes can be improved considerably by incorporating nanoparticles, such as
Al2O3, TiO2, and ZrO2, into the polymer matrix. Up to
now, there
is no general agreement about the origin of these conductivity enhancement effects. A limiting factor hindering a
better theoretical understanding and thus a more systematic preparation of improved materials is the traditional
characterization of the ion dynamics by means of macroscopic techniques, such as conductivity spectroscopy,
tracer diffusion measurements, and NMR relaxation techniques. In nanostructured solid electrolytes, diffusion
pathways in different phases and at interfaces are believed to play an important role for the ion transport.
Therefore, an experimental method capable of probing ion transport on nanometer length scales would be
highly desirable. We
report on the application of electrostatic force spectroscopy for studying ion transport in solid electrolytes and
carried out time-domain electrostatic force spectroscopy on two different ion-conducting glasses using an
atomic force microscope. We compare the electrostatic force spectroscopic data obtained at different
temperatures with macroscopic electrical data of the glasses. The overall consistency of the data shows that
electrostatic force spectroscopy is capable of probing the ion dynamics and transport in nanoscopic
subvolumes of the samples.
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