This paper reviews electrolyte additives used in Li-ion batteries. According to their functions, the additives can be divided into these categories: (1) solid electrolyte interface (SEI) forming improver, (2) cathode protection agent, (3) LiPF6 salt stabilizer, (4) safety protection agent, (5) Li deposition improver, and (6) other agents such as solvation enhancer, Al corrosion inhibitor, and wetting agent. The function and mechanism of each category additives are generally described and discussed.