  The mixed ionic-electronic conductor $\beta$-Zn$_4$Sb$_3$ is a cheap and high
performing thermoelectric material, but under operating conditions with a
temperature gradient and a running current, the material decomposes as Zn
readily migrates in the structure. Here, we report an improved stability of
$\beta$-Zn$_4$Sb$_3$ by introducing ion-blocking interfaces of stainless steel
to segment the sample, produced by a rapid one-step Spark Plasma Sintering
synthesis. The stability of the samples is tested under temperature gradients
and electric currents, which reveals significantly improved stability of the
segmented samples compared to unsegmented samples. The segmented samples are
stable under temperature gradient from 250{\deg}C to room temperature with no
external current, whereas the unsegmented sample decomposes into ZnSb and Zn
under the same conditions. The thermoelectric figure of merit, zT, of the
segmented sample is slightly reduced, mainly due to the increased thermal
conductivity. In conclusion, a rapid one-step synthesis of segmented
$\beta$-Zn$_4$Sb$_3$ is developed, which successfully improves the long-term
operational stability by blocking the Zn ion migration.
