Lithium ion battery

The invention belongs to the technical field of batteries, and discloses a lithium ion battery which comprises a positive pole piece and a negative pole piece, an active material of the positive pole piece comprises lithium cobalt oxide single crystals, an active material of the negative pole piece comprises graphite and silicon particles mixed in the graphite, the positive pole piece further comprises a lithium supplement agent, the lithium supplement agent is Li2NiO2, and the lithium supplement agent is lithium cobalt oxide single crystals. Wherein the mass percentages X1 and Y1 of the lithium cobalt oxide and the Li2NiO2 in the positive pole piece material meet the following conditions: X1 + Y1 is greater than or equal to 90%, 0% lt; y1/(X1 + Y1) < = 6%. According to the invention, for a single crystal lithium cobalt oxide positive electrode matched silicon-graphite composite negative electrode battery system, Li2NiO2 is introduced into the lithium cobalt oxide positive electrode as a lithium supplement agent, so that the problem of low initial coulombic efficiency of the negative electrode caused by the silicon-graphite negative electrode can be solved, and the advantage of high volume energy density of the system is fully exerted.