The invention provides a preparation method of a lithium battery and the lithium battery. The preparation method comprises a step of adding black phosphorus to the cathode of a lithium battery, wherein the step of adding the black phosphorus to the cathode of the lithium battery comprises the steps of: 1, putting black phosphorus with a preset quantity in a container, and adding a dispersing agenttherein to obtain black phosphorus mixed solution; and pouring the black phosphorus mixed solution into an ultrasonic mixer, and carrying out mixing operation on the black phosphorus mixed solution for a first preset time length by the ultrasonic mixer according to a first mixing parameter so as to obtain black phosphorus solution, thereby effectively improving the electrochemical performance ofthe lithium ion battery. Under same conditions, when being compared with the operation of taking graphite as a cathode material of lithium ion batteries, the operation of taking black phosphorus as acathode material of lithium ion batteries has the advantage that more lithium ions can be embedded into the batteries, thereby effectively improving the energy density of the batteries and the stability of the electrolyte, improving the cycle performance of the batteries and being suitable for large-scale popularization and application.