Abstract: The effect of Na-excess content in the precursor on the structural and electrochemical performances of sodium nickel manganese oxide (NNMO) prepared by sol–gel and electrospinning methods is investigated in this paper. X-ray diffraction results of the prepared NNMO without adding Na-excess content indicate sodium loss, while the mixed phase of P2/O′3-type layered NNMO presented after adding Na-excess content. Compared with the sol–gel method, the secondary phase of NiO is more suppressed by using the electrospinning method, which is further confirmed by field emission scanning electron microscope images. N₂ adsorption–desorption isotherms show no remarkably difference in specific surface areas between different preparation methods and Na-excess contents. The analysis of X-ray absorption near edge structure indicates that the oxidation states of Ni and Mn are +2 and +4, respectively. For the electrochemical properties, superior electrochemical performance is observed in the NNMO electrode with a low Na-excess content of 5wt%. The highest specific capacitance is 36.07 F·g⁻¹ at 0.1 A·g⁻¹ in the NNMO electrode prepared by using the sol–gel method. By contrast, the NNMO electrode prepared using the electrospinning method with decreased Na-excess content shows excellent cycling stability of 100% after charge–discharge measurements for 300 cycles. Therefore, controlling the Na excess in the precursor together with the preparation method is important for improving the electrochemical performance of Na-based electrode materials in supercapacitors.