The modern era of thermoelectricity (TE) is born alongside the explosion of nanotechnology. Although high-dose doping rejuvenates TE research, expectations for low thermal conductivity κ and high electrical transport property remain conflicting. The n-type Bi2Te3 with dilute liquid-like copper chalcogenide realizes the hybridization of electronic and ionic conduction and fulfills the concept of phonon-liquid electron-crystal. The presence of Cu2S nano-clusters modulates the concentration of Cu and Cu ion dynamically. Under such circumstances, the solubility of Cu in n-type Bi2Te3 retains at a maximal level without forming undesired impurity phases. Moreover, the Cu ions play a double role as phonon scatters and high-mobility carriers, yielding the reduced κL with enhanced carrier mobility μH. Meanwhile, a high electrical conductivity σ is retained by pinning the carrier concentration nH in the optimal level of 1019 cm−3 with the incorporation of electron donors. Consequently, our nearly single-phase n-type Bi2Se0.02Te3Cu0.03(Cu2S)0.0125 crystal achieves a large S2σ, a record-breaking zT = 1.6 at 363 K, and an ultrahigh average zT = 1.38 in the temperature range of 300 K–500 K.