Even though Ag9GaSe6 shows competitive thermoelectric performance comparing with commercial thermoelectric materials, the thermal stability of this argyrodite-type superionic conductor limits its practical application. The present work investigates the electrical transport behavior of Ag9GaSe6 and validates Ag ions segregation as the instable source. Due to the temperature gradient during the measured process, Ag ions migrate, and thus resulting in a graded distribution of Ag ions in the sample. Therefore, the electrical transport properties vary essentially for the first and second measurements. However, the graded distribution of Ag ions can be maintained once it is constructed at a given temperature gradient, indicating that Ag9GaSe6 can be used as a functionally graded thermoelectric material. In addition, the thermoelectric performance of Ag9GaSe6 is further optimized by creating Ag vacancies and alloying Ag with Cu. As a result, a maximum zT value of similar to 1.6 has been achieved in the Cu-doped sample. The samples show also good stability and repeatability, which can be considered as a good n-type candidate for matching p-type thermoelectrics.