This invention relates to a core structure of a round secondary Li cell set in a cell tank and including a central rod, a pole roll and two conductive connecting pieces, in which, a connecting piece is set between the conductive connecting pieces and the positive or negative lead-out terminal on the cell tank to connect them as conductive connection, even if error exists between sizes of the coreand the tank, set of cells can be finished smoothly by the buffer of the piece since it is a flexible component of a molded bending structure.