Owing to their high specific capacity and abundant reserve, Cu$_{x}$S compounds are promising electrode materials for lithium-ion batteries (LIBs). Carbon compositing could stabilize the Cu$_{x}$S structure and repress capacity fading during the electrochemical cycling, but the corresponding Li$^{+}$ storage mechanism and stabilization effect should be further clarified. In this study, nanoscale Cu$_{2}$S was synthesized by CuS co-precipitation and thermal reduction with polyelectrolytes. High-temperature synchrotron radiation diffraction was used to monitor the thermal reduction process. During the first cycle, the conversion mechanism upon lithium storage in the Cu$_{2}$S/carbon was elucidated by operando synchrotron radiation diffraction and in situ X-ray absorption spectroscopy. The N-doped carbon-composited Cu$_{2}$S (Cu$_{2}$S/C) exhibits an initial discharge capacity of 425 mAh g$^{-1}$ at 0.1 A g$^{-1}$, with a higher, long-term capacity of 523 mAh g$^{-1}$ at 0.1 A g$^{-1}$ after 200 cycles; in contrast, the bare CuS electrode exhibits 123 mAh g$^{-1}$ after 200 cycles. Multiple-scan cyclic voltammetry proves that extra Li+ storage can mainly be ascribed to the contribution of the capacitive storage.