B1. placing lithium manganese iron phosphate substrate in a dry coating machine, adding coating agent and mixing, then adding carbon source and mixing to obtain mixture C;

B2. calcining the mixture C in a protective atmosphere, crushing it to obtain the cathode material;

the coating agent comprises at least one of metal oxide and metal salt,

the carbon source comprises a composite carbon source;

the preparation of the composite carbon source comprises the following steps:

(1) mixing sulfuric acid aqueous solution and nitric acid aqueous solution, adding carbon nanotubes, performing ultrasonic dispersion, adding deionized water, conducting temperature-controlled stirring, performing suction filtration, taking solid phase and dry it to obtain acidified carbon nanotubes;

wherein the dosage ratio of the sulfuric acid aqueous solution, the nitric acid aqueous solution, the carbon nanotube, and the deionized water is 300-450 mL: 100-150 mL: 0.8-1.2 g: 500 mL;

(2) adding reed pulp and potassium permanganate to the sulfuric acid aqueous solution, stirring, performing a temperature-controlled reaction, centrifugal washing, ultrasonically crushing, separating, taking solid phase to add into deionized water, stirring for 15-30 minutes, and performing homogenization treatment 3-4 times under 300 bar pressure to obtain nanocellulose gel, which is sealed and stored at 4° C. for later use;

wherein the dosage ratio of the sulfuric acid aqueous solution, the reed pulp, the potassium permanganate, and the deionized water is 50-80 mL: 1-2 g: 8-12.8 g: 200 mL; the temperature-controlled reaction refers to a reaction at 50° C. for 2-3 hours;

(3) mixing the acidified carbon nanotubes, the nanocellulose gel, and the deionized water, and performing ultrasonic dispersion in an ice-water bath to obtain a carbon-containing mixture solution;

wherein the dosage ratio of the acidified carbon nanotubes, the nanocellulose gel, and the deionized water is 4-7 mg: 2-6 mL: 20-40 mL; the ultrasonic dispersion refers to sonication at an amplitude of 650-700 W for 20-30 minutes;

(4) mixing dimethylformamide, graphene oxide, and diethylenetriamine, performing ultrasonic oscillation for 15-20 minutes, conducting a temperature-controlled reaction, filtering, and drying the solid phase to obtain modified graphene oxide;

wherein the mass ratio of the dimethylformamide, the graphene oxide, and the diethylenetriamine is 40-60:1.5-3:50-60;

the temperature-controlled reaction refers to stirring at 60° C. for 12-16 hours;

(5) taking the modified graphene oxide and deionized water, mixing and ultrasonically dispersing for 1-1.5 hours, adding the carbon-containing mixture solution, and ultrasonically dispersing for 1-2 hours to obtain a dispersion, adjusting the pH of the dispersion to 1.0-1.5, adding aniline, stirring in a 0-5° C. water bath for 1-2 hours, adding ammonium persulfate aqueous solution, reacting at 0-5° C. for 6-8 hours, performing suction filtration, taking the solid phase and drying it to obtain the composite carbon source;

wherein the mass ratio of the modified graphene oxide, the deionized water, the carbon-containing mixture solution, the aniline, and the ammonium persulfate aqueous solution is 1-2:10-20:4-6:1-1.5:2.4-3.8.