A composite membrane for a lithium battery, a cathode for a lithium battery, and a lithium battery including the composite membrane. The composite membrane includes a copolymer including a first repeating unit represented by Formula 1 and a second repeating unit represented by Formula 2: wherein Ar1, R1, R2, R3, A, Y−, and m in Formula 1, and R4 to R7, a, and n in Formula 2, are the same as defined in the specification.