Modified lithium iron manganese phosphate positive electrode material, preparation method thereof and lithium ion battery

The invention discloses a preparation method of a modified lithium manganese iron phosphate positive electrode material, which comprises the following steps: a, nano-crystallizing micron-sized lithium manganese iron phosphate and a dispersing agent to obtain nano-sized lithium manganese iron phosphate slurry; performing nanocrystallization on a micron-sized solid electrolyte to obtain a nano-sized solid electrolyte slurry; b, drying the lithium manganese iron phosphate slurry and the solid electrolyte slurry, and uniformly mixing to obtain a composite material; and c, calcining the composite material in an inert atmosphere to obtain the modified lithium iron manganese phosphate positive electrode material. The dispersing agent is one or more of polyvinylpyrrolidone, polyethylene glycol and polyvinyl alcohol, and the adding amount of the dispersing agent is 1wt%-5wt% of the lithium manganese iron phosphate; and the content of the solid electrolyte in the modified lithium manganese iron phosphate positive elect