Iron oxychloride as a catalyst or electrode material has been attracted attention in recent years. In this paper, the growth and agglomeration of FeOCl nanoparticles, which were synthesized by thermal decomposition method, have been studies systematically. The crystallized FeOCl was formed and agglomerated at a low temperature (140 ℃) under reduced pressure and rotation. The FeOCl crystal grew preferentially along the (010) crystal plane during a sustained thermal decomposition process, thus FeOCl nanosheets with a strip-shaped morphology were formed. With increasing reaction temperature, the size of the synthetized nanoparticles decreased but the agglomeration phenomenon became more prominent. Benefiting from the small nanosheet structure, the FeOCl-220-1 h cathode shows a maximum discharge capacity of 182 mAh g−1. However, after 15 cycles, a dramatic capacity decay was observed due to the pulverization of the agglomerated particles.