Covalent organic frameworks (COF) are a class of crystalline porous organic polymers formed by a linking reaction of organic monomers leading to an extended framework with periodic structures and ordered pores in two or three-dimensional morphologies. The various advantages make COF potential candidates in various fields including electrochemical energy storage. This presentation discusses COF applications as electrodes in battery chemistries beyond the Li-ion viz. non-aqueous liquid electrolyte-based Li/Na-S and aqueous Zn-ion batteries. We have designed COF nanosheets over end-opened multi-walled carbon nanotubes (CNT-CON) and explored their role as sulfur host in metal sulfur batteries. The CNTs provide the conductivity and confine the elemental sulfur, whereas the CONs layer acts as a porous (chemical) trap to prevent polysulfide dissolution electrolyte. Numerous experimental methods investigated the crystallinity, porosity, and framework composition of the cathode material. The molecularly designed covalent organic framework viz. the CNT-CON/S when employed as the cathode in Li-S and Na-S battery exhibited a significant improvement in electrochemical performance (specific capacity and cycling efficiency) as compared to CNT/S and COF/S cathode. All these studies convincingly show that COFs grafted on to conducting carbon structures can serve as alternative (organic) electrodes for high-performance metal-sulfur battery systems (Ruth Gomes et al. ACS Sustainable Chem. Eng. 2020). The second part of the lecture will discuss a redox-active two-dimensional COF and its graphene oxide composite (COF-GOPH) as a cathode in an aqueous rechargeable Zn-ion battery (ZiB). Zinc metal anode and varying concentrations of Zn2+-ion and Li+-ion electrolytes were used in the cell with COF-GOPH cathode. The best battery performance was obtained when the composition of the Zn and Li-based electrolytes are 1:1. Combination and X-ray photoelectron spectroscopy and conductivity measurements reveal that optimum concentration of Li+ ions in the mixed-ion electrolyte was beneficial for better facilitation of Zn+2 diffusion into the electrode, providing superior electrochemical performance. The COF-Zn aqueous mixed-ion system offers a better alternative to conventional aqueous organic batteries with a single type of ions. It ensures excellent electrochemical performance in cycling stability reversibility and capacity retention (Akshatha Venkatesh et al. submitted, 2021).

A few examples of covalent organic frameworks for rechargeable batteries beyond the Li-ion chemistry

ACS Spring Meeting 2021

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