Disclosed is an electrolyte for a battery comprising: (a) an electrolyte salt; (b) an organic solvent; and (c) a functional electrolyte additive. An electrochemical device comprising the electrolyte is also disclosed. The additive used in the electrochemical device effectively controls the surface of a cathode active material, which otherwise causes side reactions with an electrolyte, due to the basic skeleton structure and polar side branches of the additive. Therefore, it is possible to improve the safety of a battery, while not adversely affecting the quality of a battery.