A novel radical monomer containing triphenylamine and the 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) radical has been synthesized. The corresponding linear homopolymer of 4-carboxy-N,N-diphenylaniline-2,2,6,6-tetramethylpiperidin-1-yloxy (PTPA-TEMPO) was then prepared by chemical oxidative polymerization. The chemical structure and electrochemical properties of the prepared polymers were characterized by Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, scanning electron microscopy, cyclic voltammetry, and galvanostatic charge–discharge testing by the simulated lithium-ion half-cell method. The results demonstrated that the as-synthesized functional polymers exhibited an initial discharge capacity of up to 140 mAh g⁻¹ with two well-defined plateaus at the potential of 3.8 and 2.7 V versus Li/Li⁺. Furthermore, the PTPA-TEMPO electrode showed superior cycling and rate performances. The improved electrochemical performances were attributed to the construction of the novel linear radical molecular structure with PTPA as the conductive polymer backbone, which improved the long-range charge-carrier transportation and facilitated the Li⁺-ion insertion–extraction process in the aggregated polymer bulk during the charge–discharge process.