Na-deficient O3-type materials Na0.8Ni0.4-xCo2xTi0.6-xO2 (x = 0, 0.05, 0.1, and 0.15) were synthesized by a simple solid-state reaction and investigated as cathode materials for room-temperature sodium-ion batteries. Among these materials, O3-Na0.8Ni0.3Co0.2Ti0.5O2 exhibits excellent cycle stability and good rate capability. It delivers an initial capacity of 84 mAh/g in the voltage range of 2.5-4.2V at a current rate of 0.1 degrees C. Approximately 95% of the capacity is maintained after 90 cycles, and the Coulombic efficiency reaches nearly 99% after the initial cycles. At a current rate of 1 degrees C, the capacity retention is 81% of the initial capacity. These results indicate that Co-substitution can improve the cycle stability during the course of sodium extraction and insertion. (C) 2015 Elsevier Ltd. All rights reserved.