Electrolyte for rechargeable lithium battery and rechargeable lithium battery including the same

An electrolyte for a rechargeable lithium battery, comprising

• a non-aqueous organic solvent,
• a lithium salt, and
• an additive,
• wherein the additive includes a composition including a first compound and a second compound, and
• the first compound is a cesium salt compound and the second compound is represented by Chemical Formula 1A or Chemical Formula 1B:
• wherein, in Chemical Formula 1A and Chemical Formula 1B,
• R1 to R8 are each independently a substituted or unsubstituted C1 to C10 alkyl 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, a substituted or unsubstituted C2 to C10 alkynyl group, a substituted or unsubstituted C3 to C10 cycloalkynyl group or a substituted or unsubstituted C6 to C20 aryl group,
• R1 to R8 are each independently present, or
• at least one selected from a group comprising R1 and R2;
• R3 and R4;
• R5 and R6;
• and R7 and R8 are linked to form a substituted or unsubstituted monocyclic or polycyclic aliphatic heterocycle or a substituted or unsubstituted monocyclic or polycyclic aromatic heterocycle, and
• L1 to L4 are each independently a substituted or unsubstituted C1 to C20 alkylene group.