Composite cathode material for lithium ion battery and preparation method thereof

The invention provides a composite cathode material for a lithium ion battery and a preparation method thereof. The composite cathode material of the invention comprises nano Si and graphitized needle coke; the average grain diameter is 5-15 microns; and the mass content of the nano Si in the composite cathode material is 5-30 percent. The composite cathode material of the invention not only improves the lithium embedding capacity of needle coke by 2.5-5 times but also overcomes the defects of low conductivity and easy volume expansion of silicon with high lithium embedding capacity. The preparation method of the invention comprises the following steps of: grinding the needle coke; carrying out graphitization treatment at argon atmosphere; mixing with nano silicon; depositing carbon on the particle surface of Si/needle coke; and then carrying out surface treatment on the composite cathode material. On one hand, the preparation method of the composite cathode material solves the problem of lower lithium embedding capacity of a single needle coke cathode material, on the other hand, the preparation method of the composite cathode material overcomes the defects existing in the nano Si used as the cathode material.