We investigated a structural stabilizing effect of large x values on enhancing cyclability for the SiOx electrode with large areal capacity. Electrodes composed of a-SiOx film on roughened Cu substrate with the same areal capacity (2 mAh cm−2) were prepared, so that changes in volume of the lithiated SiOx per unit electrode area were equal. Cycle tests were performed for three x values (0.17, 0.68, 1.02) using half-cell and the morphology of electrodes were analyzed by SEM. Higher x values were found to result in larger inactive phase contents and demonstrated superior cyclability. The SiO1.02 electrode contained 11 times more inactive phase than the SiO0.17 and showed a capacity retention of 98% after 30 cycles. For the SiO0.17 electrode, structural changes such as the pulverization of the particles, fracturing of the electrodeposited Cu tips caused electrical isolation of Li–Si. For the SiO1.02 electrode, the structure was extremely stable. These results reveal that even in electrodes with large areal capacity, the inactive phase exhibited the great buffering effect of the change in volume of Li–Si.