Preparation method of composite diaphragm, composite diaphragm and lithium ion battery

The invention provides a preparation method of a composite diaphragm, which comprises the following steps: S1, mixing a coupling agent with a solvent, then adding an inorganic filler, and coupling at 40-60 DEG C for 4-5 hours; then adding a dispersing agent and mixing to obtain a modified inorganic filler solution; s2, adding a thickening agent and a binder into the modified inorganic filler solution, and stirring to obtain modified inorganic filler water-based slurry; s3, adding PMMA, an emulsifier and acrylic acid into the modified inorganic filler water-based slurry, heating to 60-65 DEG C, and reacting for 3-4 hours to obtain composite particle slurry; and S4, coating at least one surface of a base membrane with the composite particle slurry obtained in the step S3, and drying to obtain the diaphragm. Compared with the prior art, the method not only solves the problem that the secondary coating process of the diaphragm is complicated at present, but also solves the problems that the production process is