TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: In the step (i), the baking temperature is 80-100 degrees C, the material paving thickness is 1-50 mm, the baking time is 5 hours, taking pendulum type pulverizer or ball mill to grind the converter sewage into powder. Before step (ii) of acid solution treatment, using fluorescent diffraction and X ray diffraction for component analysis and phase analysis of reaction raw material. In the step (ii), iron phosphate preparation specifically includes (a) weighing the reaction raw material, determining the elemental content and physical form of converter sludge by X-ray fluorescence and X-rays, calculating the required amount of sulfuric acid, preparing 1-3 mol/l sulfuric acid solution, where the calculated content of sulfuric acid is 100-103%, raising water bath temperature to 70-90 degrees C, adding the reaction raw material into the sulfuric acid solution, evaporating to maintain maintain the liquid level is not changed, reacting for 0.5-2 hours, after stabilizing the pH value, adding deionized water, (b) filtering, washing filter cake for 2 to 3 times, and (c) reducing water bath temperature to 60-70 degrees C, edge-to-edge stirring filtered liquid, adding oxidant and phosphate solution, using alkaline regulator to regulate pH value to 2-5, at 60-70 degrees C, stirring in a water bath for 0.5-1 hour, quickly filtering, washing the obtained precipitate, drying the precipitate for 3-4 times at 80-120 degrees C in a drying oven. The synthesis step of carbon-coated multi-element doped lithium iron phosphate comprises (i) drying amorphous hydrated iron phosphate at 80-120 degrees C, using thermogravimetric analysis-differential thermal analysis (TG-DTA) to calculate water structure number of hydrated iron phosphate, according to chemical reaction equation, adding lithium salt, adding reaction raw material, placing in a nylon tank, adding ethanol, methanol or isopropyl alcohol as dispersant, using planetary ball mill for ball-milling, taking out and drying, and (ii) adding carbon source, uniformly mixing and adding in a crucible, placing in a reaction furnace for reaction, where the reaction furnace heating speed is 5-10 degrees C/minutes, reaction temperature is controlled between 680-750 degrees C, carrying out low temperature thermal insulation for 8-10 hours, obtaining carbon-coated multi-element doped lithium iron phosphate. Preferred Components: The oxidant is 30% hydrogen peroxide, nitric acid, or perchloric acid. Phosphate solution is concentrated phosphoric acid, or soluble phosphate of tripotassium phosphate, trisodium phosphate or ammonium phosphate. The alkaline regulating agent is 30% concentrated ammonia hydroxide or concentrated sodium hydroxide, or concentrated potassium hydroxide mixed solution. Lithium salt is lithium hydroxide monohydrate, lithium carbonate or lithium acetate. Carbon source is polypropylene, sucrose or glucose.