Sb-doped Bi2Te3 is known since the 1950s as the best thermoelectric material for near-room temperature operation. Improvements in material performance are expected from nanostructuring procedures. We present a straightforward and fast method to synthesize already nanostructured pellets that show an enhanced ZT due to a remarkably low thermal conductivity and unusually high Seebeck coefficient for a nominal composition optimized for arc-melting: Bi0.35Sb1.65Te3. We provide a detailed structural analysis of the Bi2-xSbxTe3 series (0 <= x <= 2) based on neutron powder diffraction as a function of composition and temperature that reveals the important role played by atomic vibrations. Arc-melting produces layered platelets with less than 50 nm-thick sheets. The low thermal conductivity is attributed to the phonon scattering at the grain boundaries of the nanosheets. This is a fast and cost-effective production method of highly efficient thermoelectric materials.