Electrical properties of composites based on solid solutions of (Cu_1-xAg_x)₇SiS₅I superionic conductors

Abstract

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 (Cu_1-хAg_x)₇SiS₅I solid solutions.

 Methods. Compounds based on Cu₇SiS₅I, Ag₇SiS₅I and (Cu_1-xAg_x)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 (Cu_1-xAg_x)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 (Cu_1-xAg_x)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 (Cu_1-xAg_x)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 (Cu_1-xAg_x)₇SiS₅I, the electrodes-equivalent circuit was chosen, which is characterized by the presence of the electronic resistance R_e, parallelly to which the capacity of the double diffusion layer C_d  is included. It is also characterised by the elements responsible for the ionic processes: the resistance R_gb and the capacity of the grain boundaries C_gb, the resistance of the grains R_g with the parallel inclusion of their capacity C_g. In Nyquist diagrams, two semicircles are observed for the studied composites: the low-frequency semicircle corresponds to the capacity of the double diffusion layer C_d, the resistance R_gb, and the capacity of the C_gb of the grain boundaries, while the high-frequency semicircle corresponds to the resistance R_g and the capacity C_g of the grains. Parallelly to the elements corresponding to the ionic processes, the electrodes-equivalent circuit includes the resistance R_e, which corresponds to the electronic component of conductivity and contributes to both low-frequency and high-frequency semicircles. 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 Ge⁺⁴Si⁺⁴, it nonlinearly increases by more than an order of magnitude.

 Conclusions. Polymer composites based on solid solutions (Cu_1-xAg_x)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 Ge⁺⁴-->Si⁺⁴ on the total electrical conductivity and activation energy, on the electronic and ionic components of the conductivity of composites based on (Cu_1-xAg_x)₇SiS₅I was studied on the basis of compositional dependences.