In this manuscript, we report an extensive study of the physico-chemical properties of different samples of O3-NaMnO₂, synthesized by sol–gel and solid state methods. In order to successfully synthesize the materials by sol–gel methods a rigorous control of the synthesis condition has been optimized. The electrochemical performances of the materials as positive electrodes in aprotic sodium-ion batteries have been demonstrated. The effects of different synthesis methods on both structural and electrochemical features of O3-NaMnO${}_2$ have been studied to shed light on the interplay between structure and performance. Noticeably, we obtained a material capable of attaining a reversible capacity exceeding 180 mAhg${}^{-1}$ at 10 mAg${}^{-1}$ with a capacity retention >70% after 20 cycles. The capacity fading mechanism and the structural evolution of O3-NaMnO${}_2$ upon cycling have been extensively studied by performing post-mortem analysis using XRD and Raman spectroscopy. Apparently, the loss of reversible capacity upon cycling originates from irreversible structural degradations.