Herein, we describe a new simple method to improve the irreversible performance of a SiO anode during the initial cycle. The solid state-reacted SiO material was synthesized from bare SiO and lithium powders (three samples with weight ratios of 7:1, 8:1, and 9:1 were prepared) using a heat treatment process at 600 °C. Irreversible phases such as lithium silicates (Li4SiO4 and Li2SiO3) and Li2O were formed upon the solid state reaction with SiO. Electrochemical tests using half-cells were performed to confirm the effects of the solid state-reacted SiO material. The initial Coulombic efficiencies of the three samples were 82.12% (7:1), 79.81% (8:1), and 78.95% (9:1), which were far higher than that of a bare SiO cell (58.52%). Furthermore, the electrochemical performance of a full cell using a 7:1 wt% SiO anode and a lithium cobalt oxide cathode was evaluated. The full cell exhibited an initial Coulombic efficiency of 93.62% and a capacity retention of 74.70% after 15 cycles, which were also far higher than those of a bare SiO cell (66.4% and 55.72%, respectively). A comparison of the solid state-reacted and bare SiO electrodes demonstrated that the pre-formed irreversible phases prevented the consumption of lithium ions during the 1st cycle.