Tunable Redox Activity and Stability of Radical Intermediates in Covalent Organic Polymers for Rechargeable Lithium Ion Batteries

用於可再充鋰離子電池的共價有機聚合物中的自由基中間體活性和穩定性的調控

Student thesis: Doctoral Thesis

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Award date2 May 2023

Abstract

Rechargeable batteries are the key technology to store the renewable energy and reduce the carbon emission. Currently, the commercial inorganic electrode materials dominate the market of rechargeable batteries. However, the inorganic electrodes (such as LiFePO4 and LiNixCoyMnzO2) are facing rising concerns about the high cost, low capacity, resource scarcity, and the environmental footprint. Organic electrode materials featured with low cost, tunable structure, abundant source, and low toxicity are promising competitor to replace the inorganic electrode materials and develop sustainable energy storage system. However, the charge/discharge processes of the organic electrodes often involve the formation of organic radical intermediates, which are unstable and result in the inactivation of organic electrode materials, leading to poor cycling stability and low reversible capacity of the organic batteries. Thus, it is of great importance to regulate the redox activity and stability of organic radicals to improve the electrochemical performance of organic electrode materials.

In chapter 1 of this thesis, we review the research background and recent advances in organic radicals for rechargeable batteries. The history, structures, and redox mechanism of various organic radicals are summarized. Moreover, the challenges and strategies to regulate the redox of organic radicals for high-performance rechargeable batteries are also discussed.

In chapter 2, polyimide covalent organic frameworks (COFs) based on different diimide conjugated units are synthesized and used as cathodes for lithium-ion batteries. The results demonstrate that tuning the size of conjugated units could modulate the molecular orbital energies, charge transport capacities, and spin electron densities of the active units. Moreover, increasing the size of the imide conjugated units could contribute to dispersing the radical electrons and improving the cycling stability of the polyimide electrodes.

In chapter 3, a 3D polyimide is developed as a cathode for lithium-ion batteries and undergoes the redox of naphthalenediimide radicals during the charge/discharge processes. The research results demonstrate that the rigidity effect of the 3D conjugated framework helps to keep the stability of the organic radical intermediates for high-performance organic batteries.

In chapter 4, a new polyimide COF is constructed based on N,N,N,N-tetraphenylphenylenediamine (TPPDA) units and displays a unique star-shaped structure. As a cathode for lithium-ion batteries, the COF exhibits a high work voltage up to 3.6 V, surpassing almost all the COF materials. Different from the polyimide COF in chapter 3, the TPPDA-based COF is served as a p-type electrode, which suffers from the redox of TPPDA radical cations and the insertion/extraction of PF6 anions during the redox processes.

In chapter 5, a novel bipolar polyimide COF with n-type and p-type active units is developed for dual-ion organic batteries. During the charge/discharge processes, the redox of anionic imide radicals and cationic nitrogen-center radicals is used to store the Li+ ions and PF6 anions, respectively. Further investigation indicates that the electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) could quench the imide radicals, resulting in poor electrochemical performance of the COF electrodes.

These successive works not only reveal the redox mechanism of several radical intermediates in organic electrodes but also provide some strategies to regulate the redox activity and stability of organic radicals. These works will expand the palette to design new organic electrode materials and facilitate the development of high-performance rechargeable organic batteries.