Additive, electrolyte for rechargeable lithium battery and rechargeable lithium battery including the same

An additive represented by Chemical Formula 1:

• wherein, in Chemical Formula 1,
• X1 and X2 are each independently O, S, CRaRb, or NRc, at least one of X1 and X2 being O or S,
• R1 and R2 are each independently a substituted or unsubstituted C1 to C10 alkyl group, a substituted or unsubstituted C1 to C10 alkoxy group, a substituted or unsubstituted C2 to C10 alkenyl group, a substituted or unsubstituted C3 to C10 cycloalkyl group, a substituted or unsubstituted C3 to C10 cycloalkenyl group, or a substituted or unsubstituted C6 to C20 aryl group,
• Ra, Rb, Rc, and R3 to R6 are each independently hydrogen, a substituted or unsubstituted C1 to C5 alkyl group, a substituted or unsubstituted C2 to C5 alkenyl group, or a combination thereof, and
• n is an integer selected from 1 to 3.