Doped poly(4,4'-diaminodiphenyl sulfone) (pDDS) is prepared for use as a cathode-active material of lithium secondary batteries by chemical oxidation method using ammonium persulfate initiator. The synthesized pDDS and doped pDDS are characterized by chemical structure analysis using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Cyclic voltammetry shows that the doped pDDS has a typical pair of redox peaks at 3.75/3.15 V vs. Li/Li+, corresponding to charging/discharging of the lithium ion. The discharge capacity at low current rate (0.05 C-rate) achieves 31.5 and 24.3 mA h g(-1) in the initial and 50th cycles, respectively. The doped pDDS also shows good cycle performance and high-rate capability, making it appropriate as a cathode material of lithium secondary batteries.