Sodium-ion batteries (SIBs) are the most promising alternative to lithium-ion batteries (LIBs) due to their low cost and environmental friendliness; therefore, enhancing the performance of SIBs' components is crucial. Although most of the studies have focused on single-phase cathode electrodes, these materials have difficulty in meeting the requirements in practice. At this point, composite materials show superior performance due to balancing different structures and are offered as an alternative to single-phase cathodes. In this study, we synthesized a Na_0.44MnO₂/Na_0.7MnO_2.05 composite material in a single step with cobalt substitution. Changes in the crystal structure and the physical and electrochemical properties of the composite and bare structures were studied. We report that even if the initial capacity is slightly lower, the rate and cyclic performance of the 1% Co-substituted composite sample (CO10) are superior to the undoped Na_0.44MnO₂ (NMO) and 5% Co-substituted (CO50) samples after 100 cycles. The results show that with the composite cathode phase transformations are suppressed, structural degradation is prevented, and better battery performance is achieved.