Capacitive deionization (CDI) is an energy efficient desalination technology. In this study, we conducted CDI experiments under different conditions and observed severe performance degradation from 7.3 to 0.5 mg/g after 70 cycles in symmetric 1.2/0 V operation because of anode oxidation under polarization. To eliminate the degradation, different charging and discharging voltages were tested in an asymmetric cell and 0.8/-0.4 V was selected as the optimized voltage window. Under the optimized voltage window, a capacity of 6.3 mg/g was obtained, only somewhat lower than that of the 1.2/0 V experiment, and the stability and charge efficiency were much higher by greatly mitigating the electrode corrosion. In addition, the process had a lower energy consumption. The stable performance in air- and oxygen-saturated conditions indicates that the optimized CDI has superior capacity and better performance than conventional CDI.