Secondary battery and preparation method thereof

A method for preparing a secondary battery, comprising:

• mixing a lithium source, an iron source, a phosphorus source, a transition metal salt source, a bismuth source, and a solvent to obtain an intermediate solution having a pH ranging from 1.0 to 4.0;
• subjecting the intermediate solution to a hydrothermal reaction to obtain a product, and drying the product to obtain a precursor;
• subjecting the precursor to heat treatment to obtain a core, wherein the core comprises lithium iron phosphate and a bismuth salt;
• coating the core with a coating material to obtain a cathode material, wherein the cathode material comprises the core and a conductive polymer coating layer arranged on at least a surface of the core;
• coating a cathode current collector with the cathode material to obtain a cathode plate by rolling;
• coating an anode current collector with an anode material to obtain an anode plate by rolling;
• winding the cathode plate, the anode plate, and a separator to obtain a wound cell;
• assembling the wound cell to obtain a battery module;
• and
• injecting an electrolyte into the battery module to obtain the secondary battery.