Fig. 4

Ion-blocking strategy to improve stability in thermoelectric ion conductors. a Schematic for limiting the ion movement by including thin electron-conducting and ion-blocking interfaces; either grain boundaries (red areas) or a secondary phase (yellow areas). b Schematic of ion-blocking electrically conducting interfaces that allow the concentration profile to be reset at each interface so that the ion concentration does not ever reach the upper limit. c Relative resistance variation (R/R₀) as a function of current density for different Cu_1.97S samples at a constant temperature of 750 K without a temperature difference. d Relative Seebeck coefficient variation (S/S₀) as a function of current density for different Cu_1.97S samples under the condition of temperature difference (T_anode = 673 K and T_cathode = 300 K). The insets in (c) and (d) show the optical images of the measured Cu_1.97S samples. The critical current density was measured across the segment in the middle