NOVELTY - Electrolyte for a lithium battery comprises a non-aqueous organic solvent, a lithium salt, and an additive comprising a sulfone-based compound and a 3-30C organic peroxide or an azo-based compound.    USE - The electrolyte is used for a lithium battery (claimed), useful as power source in portable electronic instruments or digital devices, e.g. cellular telephones, notebook computers, or camcorders.    ADVANTAGE - The lithium battery comprising the inventive electrolyte has improved charge-discharge and cycle life characteristics, recovery capacity ratio at high temperature, and swelling inhibition properties.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (a) a method for preparing an electrolyte for a lithium battery, which comprises adding a sulfone-based compound, a 3-30C organic peroxide or azo-based compound, and a poly(ester)(meth)acrylate or its polymer to a lithium salt-containing organic solution to prepare a composition for a polymer electrolyte, where the poly(ester)(meth)acrylate is a polymer derived from a (polyester)polyol with at least three hydroxyl (-OH) groups, a portion or all of which are substituted with a (meth)acrylic ester and any remaining hydroxyl groups that are not substituted with the (meth)acrylic ester are substituted with a group having no radical reactivity; and impregnating or coating the composition for a polymer electrolyte on a current collector to prepare a solid polymer electrolyte; and    (b) a lithium battery comprising a positive electrode (2) including a material that is capable of reversible intercalation/deintercalation of lithium ions or a material that is capable of reversibly forming a lithium-containing compound as a positive active material; a negative electrode (4) including a lithium metal, a lithium-containing alloy, or a material that is capable of reversible intercalation/deintercalation of lithium ions; and an electrolyte (26).    DESCRIPTION OF DRAWING(S) - The figure is a cross-sectional view of a lithium secondary battery cell.    Positive electrode (2)    Negative electrode (4)    Electrode assembly (8)    Insulators (22, 24)    Electrolyte (26)