Quaternary precursor of sodium-ion battery as well as preparation method and application of quaternary precursor

The invention discloses a quaternary precursor of a sodium-ion battery as well as a preparation method and application of the quaternary precursor. The quaternary precursor of the sodium-ion battery comprises a nickel-iron-manganese-copper core and a carbon shell, a nickel element and a copper element in the nickel-iron-manganese-copper core are distributed in a gradient manner, the content of the nickel element is reduced from inside to outside in a gradient manner, and the content of the copper element is increased from inside to outside in a gradient manner. According to the invention, carbon coating is carried out on the surface of the precursor, so that an external conductive frame is formed, sodium ion diffusion and electron conductivity are realized, and an internal material structure is protected. Moreover, the precursor material presents different nickel-copper concentration gradient distribution along the radial direction, so that the volume energy density of the positive electrode material can be e