Electron-donating/withdrawing groups functionalized porphyrin complex as high performance organic lithium batteries

Organic electrode materials exhibit wide application prospects in the electrochemical energy storage devices, owing to their adjustable structure, low cost and environmental friendliness. Though organic electrode materials delivered high theoretical energy density, but suffer from low electronic conductivity and high solubility. In order to decrease the solubility and regulate redox potential of the porphyrin molecule, electron-donating group and electron-withdrawing group were functionalized on bipolar porphyrin complex. Herein, we report two novel cathodes of [5,15-bis(ethynyl)-10,20-bis(5-methylthienyl) porphinato] copper(II) (CuDETMP) and [5,15-bis(ethynyl)-10,20-bis(5-chlorothienyl)porphinato]copper(II) (CuDETCP). Strong electron withdrawing effect of chlorine functionalized groups enables higher reversible capacity (≈125 mAh/g vs 100 mAh/g at 1 A/g), while with electron donating group of CuDETMP showed stable cycling capability. Extremely cycling stability up to 2000 cycles with capacity retention more than 80% was achieved in both electrodes. Compared to CuDETMP, higher capacity of CuDETCP benefits from reaction between chlorine atoms and Li cations, which can be a partially reversible process lead to degraded capacity. Nevertheless, CuDETMP and CuDETCP can deliver reversible capacity and provide multiple-redox sites enabling high energy density in respective proper voltage ranges. Through comparison towards physical and electrochemical properties of two molecules, mechanism between substituent and electrochemical performance was clarified. This work not only elaborates the significant correlation between different substituents and electrochemical performance, but also provides a novel strategy to tunable molecular design of organic electrode materials for high-energy rechargeable organic lithium batteries.