Unexpected 18-Fold Overlapped Feathery Fermi Pockets in Typical Thermoelectric Bi0.5Sb1.5Te3

${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_{3}$ is the most widely used $p$-type commercial thermoelectric materials over six decades, yet its complex electronic structure remains uncertain especially in band degeneracy and ${k}_{\mathrm{z}}$ dispersions. Here we show an unexpected band structure of 18-fold overlapped feathery Fermi pockets through substantial angle-resolved photoelectron spectroscopy data. Complemented with transport tests, we suggest that the high performance originates in the cooperation of four electronic features---momentum overlap of Fermi pockets, 18-fold band degeneracy, ultrasharp ${k}_{\mathrm{y}}$ dispersions, and heavy ${k}_{\mathrm{z}}$ bands. This cooperation of band features proposes a new paradigm for promising thermoelectrics.