  Cu-based argyrodites have gained much attention as a new class of
thermoelectric materials for energy harvesting. However, the phase transition
occurring in these materials and low energy conversion performance limited
their broad application in thermoelectric converters. In this work, we disclose
a newly discovered highly efficient Cu8SiSxSe6-x argyrodite with stabilized
high-symmetry cubic phase at above 282 K opening the practical potential of
this material for the mid-temperature region applications. The temperature
range broadening of the high-symmetry phase existence was possible due to the
successful substitution of Se with S in Cu8SiSxSe6-x, which enhances the
configurational entropy. The developed argyrodites show excellent
thermoelectric performance thanks to the increased density of states effective
mass and ultralow lattice thermal conductivity. Further tuning of the carrier
concentration through the Cu-deviation improves the thermoelectric performance
significantly. The dimensionless thermoelectric figure of merit ZT and
estimated energy conversion efficiency {\eta} for Cu7.95SiS3Se3 achieve
outstanding values of the 1.45 and 13 %, respectively, offering this argyrodite
as a low-cost and Te-free alternative for the thermoelectric energy conversion
applications.
