The discovery of low-cost, less toxic, and earth-abundant thermoelectric materials is a great challenge. Herein, with the aid of a unique and safe boronchalcogen method, we discover the new tetragonal alpha-CsCu5Se3, featuring a previously unrecognized structure in the ternary family of Cs/Cu/Se. The structure is constructed by a Chinese-knot-like Cu8Se8 building unit that is further linked into a 3D network. alpha-CsCu(5)S(e)3 exhibits thermal stability that is superior to that of the recently established thermoelectric materials Cu2-xSe and CsAg5Te3 suffering unfavorable phase transitions. Distinct from the liquidlike migration in Cu2-xSe, alpha-CsCu5Se3 obeys a typical crystalline solid thermal transport behavior dominated by Umklapp scattering. In compariosn to the isostructural CsAg5Te3, alpha-CsCu5Se3 shows a 30% volume decrease that leads to stronger orbital overlapping that markedly decreases the band effective mass (m*). With a smaller m* and a softer Cu-Se bond, alpha-CsCu5Se3 eventually realizes a 200% increase in the power factor (8.17 mu W/ (cm K-2), the highest among the copper-rich alkali-metal chalcogenides) and a figure of merit (ZT) of 1.03 at 980 K. Further, the doping in alpha-Cs(Cu0.96Sb0.04)(5)Se-3 boosts the lattice anharmonicity by the lone pairs that, via intensifying the Umklapp scattering and slowing the phonon velocity, ensures a low lattice thermal conductivity (0.40 W/(m K)), and finally leads to a ZTmax value of 1.30 at 980 K. Our discovery represents a step toward low-cost, earth-abundant, and high-performance chalcogenide materials that will shed useful light on future exploration in the related fields.