A large flat-conjugated copolymer poly[ ethylene-N, N'-bis(2-ethylhexyl)-1,4,5,8-naphthalene diimide] (PENDI), was synthesized through a simple synthetic method, which is preparation via 2,6-dibromo-N, N'-bis(2-ethylhexyl)-1,4,5,8-naphthalene diimide, and (E)-1,2-bis(tributylstannyl) ethene via Stille coupling reaction for use as a cathode material for lithium ion batteries. The PENDI copolymer was characterized by IR, UV-vis spectra, H-1 NMR, and TGA. The electrochemical performance results demonstrated that the reversible capacity of PENDI can be retained at 85% of its initial theoretical capacity with a steady discharge voltage plateau of 2.0-2.5 V after 100 cycles at 0.1 C, and the coulomb efficiency remained above 98%. This can be attributed to the low solubility of the copolymer PENDI in electrolyte and the excellent structural stability of the copolymer PENDI during the charge-discharge tests. The synthesis of polymers with high conjugated systems and more stable structures will become a major breakthrough for the exploitation and optimization of cathode materials. (C) 2016 The Electrochemical Society. All rights reserved.