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Copper sulfide is a multifunctional material. Copper sulfides are known to be used in photo-electric converters, in plasmonics, in active electrodes of batteries and supercapacitors, etc. The excellent thermoelectric properties of copper sulfide are well-known too. The purpose of this study is to investigate the thermoelectric performance of nanocomposite copper sulfide contained monoclinic Cu2S and tetragonal Cu1.96S phase. According to scanning electron microscopy, the average particle size of the synthesized powder was about 373 nm. The Li0.11Cu1.89S sample showed an electronic conductivity of 50–180 S/cm, a Seebeck coefficient of 0.04–0.31 mV/K in the range of 300–700 K, and high power factor 5.8 μW K−2cm−1 at 672 K. Total thermal conductivity decreases with increasing temperature from 0.61 to 0.22 W∙K−1m−1 in the range of 300–700 K. Such low thermal conductivity and high power factor made it possible to achieve an extremely high dimensionless thermoelectric figure of merit ZT = 1.76 at 672 K, which allows to consider the Li0.11Cu1.89S alloy as a promising thermoelectric material.