A new redox system, poly(1,4-phenylene-1,2,4-dithiazol-3′,5′-yl) (PPDTA), is presented as a cathode material for high-capacity lithium secondary batteries. This polymer, composed of 1,2,4-dithiazolium ring and benzene, is synthesized by a cyclooxidative reaction of aromatic thioamides with organic oxidizers. The advantages of this system are: (1) high theoretical charge density; (2) fast redox rate; and (3) electrical conductivity. The high capacity is realized by the successive redox reactions of an SS bond and a 7-π-electron unsaturated 1,2,4-dithiazolium ring. The fast redox rate is achieved by the π-conjugation and by the SS bond in a 1,2,4-dithiazolium ring. MO calculation reveals that the SS bond in a 1,2,4-dithiazolium ring of PPDTA causes a gap narrowing and a downshift of HOMO and LUMO levels, being consistent with the electrochemical experiment. A discharge test showed a capacity of 420 mAhg−1, which is about 3–4 times higher than those of inorganic intercalation compounds and ordinary conducting polymers.