Synthesis and Comparative Investigation of the Electrochemical Characteristics of CsAg4Br2.5I2.5 and RbAg4I5 Solid Electrolytes

A new method for the solid-phase synthesis of the superionic conductor CsAg 4 Br 2.5 I 2.5 is proposed, which facilitates the preparation of a single-phase product. The thermal behavior of CsAg 4 Br 2.5 I 2.5 in the temperature range from –160 to +190°С was studied by differential scanning calorimetry, and the absence of polymorphous transitions was confirmed; the only first-order phase transition is observed near 177°C and corresponds to the incongruent melting of the solid electrolyte. It is shown that the dense ceramics can be obtained from CsAg 4 Br 2.5 I 2.5 powder by pressing at room temperature; the optimal value of pressure is determined. Studies of the electrical transport characteristics of CsAg 4 Br 2.5 I 2.5 included measurements of the ionic conductivity by the four-probe method in the range of –60…+120°C and an assessment of the electronic component of the conductivity by the Hebb–Wagner method. The contribution of electron transfer is shown to be negligibly small (~10 –9 S cm –1 ), and the ionic conductivity is close to that of the well-known superionic conductor RbAg 4 I 5 and is characterized by a low activation energy (10.3 kJ mol –1 ). The oxidation potential determined by the stepwise polarization method is 0.78 V, which is noticeably higher than that of RbAg 4 I 5 . The absence of first-order phase transitions at temperatures below the melting point, combined with high ionic conductivity, makes the CsAg 4 Br 2.5 I 2.5 compound more attractive for low-temperature applications, and the increased electrochemical stability makes it more attractive for use in chemical power sources as compared to RbAg 4 I 5 .