Amorphous (a-) Sb2S3 nanoparticle aggregates were synthesized using a facile polyol-mediated process at room temperature and their Na-ion storage capability was evaluated. Owing to the spherical nano-aggregate morphology and amorphous structure, the a-Sb2S3 anode exhibited superior rate capability and cycling stability compared to that of crystalline, microscale Sb2S3 particles. The a-Sb2S3 electrode was also examined as an anode for eco-friendly, Na metal-free seawater flow batteries.