Electrical properties of composites based on solid solutions of (Cu1−xAgx)7SiS5I superionic conductors


Purpose. The purpose of this research was to prepare composites, to study frequency and temperature dependences of electrical conductivity, as well as to investigate the concentration behavior of the total conductivity, the ionic and electron conductivity component, and the activation energy of composites based on (Cu1−xAgx)7SiS5I solid solutions.
Methods. Compounds based on Cu7SiS5I, Ag7SiS5I and (Cu1−xAgx)7SiS5I (x = 0, 0.25, 0.5, 0.75, 1) solid solutions were synthesized for preparing the composites. Then, the powders (10-50 μm) were mixed with ethylene-vinyl acetate polymer dissolved in ethyl acetate in the ratio: 90 wt.% of (Cu1−xAgx)7SiS5I – 10 wt.% of EVA (ethylene-vinyl acetate). The stirring was carried out for 10-15 minutes to obtain homogeneous suspensions which were subsequently dried at a temperature of 333 K. The samples were pressed at a pressure of 400 MPa, resulting in samples of composites in the form of discs 8 mm in diameter and thickness of 2.5-4 mm. Investigation of the electrical conductivity of composites based on Cu7SiS5I, Ag7SiS5I compounds and solid solutions of (Cu1−xAgx)7SiS5I (x = 0, 0.25, 0.5, 0.75, 1) was carried out by the method of impedance spectroscopy, in frequency (20 Hz – 2 × 106 Hz) and temperature (292-338K) ranges using the high-precision LCR meter Keysight E4980A. Measurement was carried out by a two-electrode method on blocking gold contacts, which were applied by chemical precipitation from solutions.
Results. The frequency dependences of the total electrical conductivity showed a growth of conductivity with increasing frequency for all composites. It has been established that with increasing Ag content in composites based on solid solutions of (Cu1−xAgx)7SiS5I, the total electrical conductivity decreases non-linearly with the downward-bowing, while the activation energy increases non-linearly with the downward-bowing. Nyquist diagrams were constructed and their detailed analysis was carried out using an electrodes-equivalent circuit. While performing the analysis of composites based on Cu7SiS5I, Ag7SiS5I and solid solutions based on them (Cu1−xAgx)7SiS5I, the electrodes-equivalent circuit was chosen, which is characterized by the presence of the electronic resistance Re, parallelly to which the capacity of the double diffusion layer Cd is included. It is also characterised by the elements responsible for the ionic processes: the resistance Rgb and the capacity of the grain boundaries Cgb, the resistance of the grains Rg with the parallel inclusion of their capacity Cg. Using the Nyquist diagrams and the electrodes-equivalent circuit, the contributions of the ionic and electron components to the total electrical conductivity were separated. It is established that the dependence of the ionic conductivity on the composition is nonlinear with a minimum on the concentration dependence, and it is almost identical for Cu7SiS5I (2.90 × 10−4 S/cm) and Ag7SiS5I (2.96 × 10−4 S/cm). Electronic conductivity with an increase in the content of silver is not necessarily decreasing with the downward-bowing, and its value for Ag7SiS5I is almost two orders smaller than for Cu7SiS5I. The analysis of the compositional dependence of the ratio of the ionic conductivity to the electronic one showed that, due to cation substitution of Cu+ → Ag+, it nonlinearly increases by more than an order of magnitude.
Conclusions. Polymer composites based on solid solutions (Cu1−xAgx)7SiS5I were prepared. Measurements of electrical conductivity of composites by means of impedance spectroscopy in the frequency range from 20 Hz to 2 × 106 Hz and in the temperature range 292-338 K were carried out. Frequency dependences of total electrical conductivity were obtained, Nyquist diagrams were constructed and the dependences analysis were performed. The influence of the cation substitution of Cu+ → Ag+ on the total electrical conductivity and activation energy, on the electronic and ionic components of the conductivity of composites based on (Cu1−xAgx)7SiS5I was studied on the basis of compositional dependences

Keywords: solid solutions, composites, electrical conductivity, activation, compositional dependence