The invention provides a lithium ion secondary battery, which solves the problem of poor safety performance of the lithium ion secondary battery in the prior art. The lithium ion secondary battery comprises a battery case and an electrical core sealed in the battery case; the electrical core comprises a positive plate, a diaphragm and a negative plate which are reeled or laminated; the positive plate comprises a positive current collector and a cathode material positioned on the positive current collector; the cathode material comprises a cathode active material, the cathode active material is nano lithium iron phosphate-coated lithium cobalt oxide; in the anode active material, the content of lithium iron phosphate is 0.1-15 wt%, the content of lithium cobalt oxide is 85-99.9 wt%; the positive plate is electrically connected with a positive terminal through a positive tab, the negative plate is electrically connected with a negative terminal through a negative tab; and the negative tab is the nickel-plated copper tab. The lithium ion secondary battery can reach the safety grade of no firing, no explosion and no liquid leakage when short circuit of 3- milliohm internal resistance lead is generated, and the safety performance is good.