Na0.44Mn1-xCoxO2 (x=0, 0.01, 0.08, 0.11, 0.22, 0.33, 0.44) powders are synthesized as positive electrodes for sodium-ion batteries via a thermopolymerization method. X-ray diffraction and scanning electron microscopy are used to analyze their crystal structures and particle morphologies. Galvanostatic cell cycling is used to characterize the electrochemical properties. The Co-substitution can change the structure of Na0.44Mn1-xCoxO2 from T-type (x=0) into layered P2-type (x=0.11) or P3-type (x=0.44). For the samples synthesized under our specific conditions, the particle morphology can be used as an indicator of their crystal structures: rod-like (T-type), plate-like (P2-type) and granules (P3-type). P2-type Na0.44Mn0.89Co0.11O2 allows more Na+ intercalation into the inter-layer spacing and results in a high discharge capacity with 220mAhg−1 at 12mAg−1 in initial cycle, 180mAhg−1 after 40 cycles in the voltage range of 2.0-4.2V and a good rate performance of 104mAhg−1 at a rate of 600mAg−1 in the voltage range of 2.0-4.0V.