A new Mn(II) complex with the formula [Mn2(H2O)(phen)4(seb)2]·4.5H2O (phen: 1,10-phenanthroline; H2seb: sebacic acid), (complex 1), was synthesized using a mild hydrothermal method. The structure of complex 1 was studied using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), single crystal X-ray diffraction (SXRD) analysis, thermogravimetric analysis (TGA), and Fourier infrared spectroscopy (FTIR). Mn2O3 was obtained by calcination of complex 1 at 850 °C in air and its pure phase was confirmed by PXRD analysis. The electrochemical performance of Mn2O3 as a material for the negative electrode used in lithium ion batteries (Lib) shows a specific charge and discharge capacity of 377.7 and 798.6 mAh/g, as was observed at the first cycle. The charge and discharge capacity were approximately 168.1 and 168.4 mAh/g after 100 cycles. The above capacities are repeatable and demonstrate the good structural stability of Mn2O3 under applied electric power.