NOVELTY - Non-aqueous electrolyte additive comprises dicyanoacetylene or a cyano group-containing unsaturated compound (I), where (I) includes an unsaturated bond and two or more cyano groups linked in cis- or trans-positions of carbons of the unsaturated bond.    USE - The electrolyte additive is useful in non-aqueous electrolyte for the lithium secondary battery (claimed), which is useful in a portable electronic and communication device (e.g. a video camera, a cellular phone and a laptop), where the lithium secondary batteries are lithium ion batteries, lithium ion polymer batteries and lithium polymer batteries.    ADVANTAGE - The additive improves the performance of the lithium secondary battery, which may be smaller and lighter, and has high-capacity charge and discharge and high energy density. The additive enhances the stability, preferably high temperature stability of the electrolyte and simultaneously minimizes a decomposition reaction of the electrolyte. The electrolyte exhibits excellent cycle characteristic and storage characteristics at a high temperature.    DETAILED DESCRIPTION - Non-aqueous electrolyte additive comprises dicyanoacetylene or a cyano group-containing unsaturated compound of formula (R1-C(-CN)=C(-CN)-R2) (I), where (I) includes an unsaturated bond and two or more cyano groups linked in cis- or trans-positions of carbons of the unsaturated bond.    Either R1, R2 = 1-15C alkyl (optionally substituted), H, amine, CN, thiolate, -OR3, -SiR4R5R6, -OSiR7R8R9, -SR10, -SOR11, -BR12R13 or -OBR14R15; or    R1R2 = 3-15C cycloalkyl, 3-15C heterocycloalkyl or 2-20C heteroaryl (all optionally substituted); and    R3-R15 = 1-12C alkyl, 3-15C cycloalkyl, 3-15C heterocycloalkyl, 6-20C aryl or 2-20C heteroaryl.    INDEPENDENT CLAIMS are included for:    (1) a non-aqueous electrolyte (130) for a lithium secondary battery, comprising a base electrolyte, where the base electrolyte includes a non-aqueous organic solvent and a lithium salt dissolved in the non-aqueous organic solvent, and the non-aqueous electrolyte additive; and    (2) a lithium secondary battery comprising a positive electrode (100) including a positive active material layer (102) capable of intercalating and deintercalating lithium, a negative electrode (110) including a negative active material layer, a separator (120) between the positive electrode and the negative electrode, and the non-aqueous electrolyte, or the additive for linking to a surface of the positive electrode or for forming a chelate with a metal ion eluted from the positive active material.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the lithium secondary battery.    Positive electrode (100)    Positive active material layer (102)    Negative electrode (110)    Separator (120)    Non-aqueous electrolyte. (130)