[en] Highlights: • A facile aqueous synthesis of Cu₇S₄ nanofibers and spherical nanoparticles. • Experiments and calculations reveal the importance of Cl⁻ and Br⁻ in the formation of nanofibers. • Slow reaction rate also leads to the formation of nanofibers. Copper sulfide (Cu₇S₄) nanoparticles were synthesized by adding ethanolic elemental sulfur solution to an aqueous solution containing Cu(II) precursor, branched polyethyleneimine (BPEI), and ascorbic acid. By varying the Cu(II) precursor used, we could control the morphology of the nanoparticles produced to be either quasi spherical (CuF₂, Cu(NO₃)₂, and CuSO₄) or vine like nanofibers (CuCl₂ and CuBr₂). By comparing experimental results and conducting calculations we found that selective adsorption of Br⁻ and Cl⁻ onto specific crystal facets and slowing of the reaction rate owing to formation of Cu(I)–anion–BPEI complexes explain the observed change in morphology from spherical particles to vine like nanofibers. We also found that the differences in morphology of Cu₇S₄ nanoparticles affects the electrochemical performance of Li batteries including the nanoparticles as electrode materials.