Periodic Reporting for period 1 - ELECTROCOFS (Molecular Design of Electrically Conductive Covalent Organic Frameworks as Efficient Electrodes for Lithium-Ion Batteries)
Reporting period: 2022-07-01 to 2024-12-31
A major breakthrough in chemistry and materials science has been the development of Lithium-Ion Batteries (LIBs), which show great potential for storing energy from renewable sources and as the power sources for electric cars. However, commercially available LIBs are based on transition metal oxide cathodes, presenting limited energy density and raising relevant environmental concerns. Organic electrode materials were proposed as promising alternatives to transition metal oxide cathodes for a long time because they are composed of abundant elements and their electrochemical performance may be finely modulated by chemical design. In particular, Covalent Organic Frameworks (COFs) have emerged in the past few years as promising organic electrode materials due to their high stability, tunable porosity, and outstanding chemical and structural versatility. Low electronic conductivity remains the main bottleneck for real applications of COFs as electrode materials, usually addressed by adding in large amounts of conductive carbon additives that decrease the energy density of the battery. The overarching objective of this project is to design and synthesize new conductive redox-active COFs as cathode materials to enhance LIBs electrochemical performance.
The specific goals are:
a) To design a new family of redox-active COFs built from unexplored building blocks to achieve an optimal balance between specific capacity, voltage, electrical conductivity and stability.
b) To unravel the fundamental mechanisms of charge transport in COFs.
c) To manufacture and test lithium (and multivalent) batteries using conductive COFs cathode materials, assessing the influence of the processing techniques on the electrochemical performance.
The specific goals are:
a) To design a new family of redox-active COFs built from unexplored building blocks to achieve an optimal balance between specific capacity, voltage, electrical conductivity and stability.
b) To unravel the fundamental mechanisms of charge transport in COFs.
c) To manufacture and test lithium (and multivalent) batteries using conductive COFs cathode materials, assessing the influence of the processing techniques on the electrochemical performance.
In the first 30 months of the project, the following objectives have been achieved:
1) We have developed a new approach to improve the electrochemical performance of n-type anthraquinone-based COF (DAAQ-TFP-COF) cathode material in metal anode (Li, Mg)-based batteries through proper selection of the electrolyte and binder without the need for any additional processing (e.g. exfoliation). Our results demonstrated that the best electrochemical performance (high utilization efficiency of the redox sites and specific capacities close to the theoretical value) was obtained by combining LiTFSI in tetraethylene glycol dimethyl ether (TEGDME) as electrolyte and poly(tetrafluoroethylene) (PTFE) as binder. Using such electrolyte and binder, cyclable symmetric cells consisting of two DAAQ-TFP-COF organic electrodes showed ~80% capacity retention after 2000 cycles at a high current density. The high reversibility and stability of the COF electrode material upon cycling were also confirmed by ex situ IR spectroscopy. In addition, DAAQ-TFP-COF was explored as a cathode in magnesium cells using two different Mg electrolytes; one based on MgCl2 and one containing weakly coordinating anions. This is the first systematic study on the influence of electrolyte and binder on the electrochemical performance of COF cathodes in Li and Mg batteries, being very relevant for use in energy storage devices (O. Luzanin, R. Dantas, R. Dominko, J. Bitenc, M. Souto, J. Mater. Chem. A 2023, 11, 21553-21560).
2) We have synthesized a series of two-dimensional tetrathiafulvalene (TTF)-based COFs incorporating different organic linkers to study their influence on their electrochemical performance. These COFs were investigated as high-voltage organic cathode materials for lithium batteries. The results revealed high average discharge potentials (~3.6 V vs. Li/Li+) and consistent cycling stability (80% capacity retention after 400 cycles at 2C) for the three COF electrodes. In addition, the specific capacity, rate capability, and kinetics varied depending on the structure of the framework.
3) We have reported a proton–electron dual-conductive MOF based on tetrathiafulvalene(TTF)-phosphonate linkers and lanthanum ions. The formation of regular, partially oxidized TTF stacks with short S···S interactions facilitates electron transport via a hopping mechanism, reporting a room-temperature conductivity of 7.2 × 10–6 S cm–1. Additionally, the material exhibits a proton conductivity of 4.9 × 10–5 S cm–1 at 95% relative humidity conditions due to the presence of free −POH groups, enabling efficient proton transport pathways (C. Ribeiro et al. J. Am. Chem. Soc. 2025, 147, 63–68).
4) We have demonstrated that it is possible to synthesize DT-TTF (dithiophene-tetrathiafulvalene) derivatives functionalized with various substituents for the formation of extended networks towards the synthesis of DT-TTF-based COFs. In particular, various mono- and tetraarylated derivatives functionalized with different groups (electron-donating and electron-withdrawing groups) were obtained and the self-assembly of DT-TTF-tetrabenzoic acid derivative was studied by using scanning tunnelling microscopy (STM), which revealed the formation of ordered, densely packed 2D hydrogen-bonded networks at the graphite/liquid interface (C. Ribeiro et al. Chem. Eur. J. 2023, 29, e2023005, selected as Front Cover and Cover Profile).
5) We have published a comprehensive review on the use of COFs as electrodes in lithium-ion batteries highlighting some of the challenges and prospects that are directly related to the ELECTROCOFS ERC-StG project (R. Dantas, C. Ribeiro, M. Souto. Chem. Commun. 2024, 60, 138-149. Part of the themed collection “2023 Emerging Investigators”).
6) With some collaborators, we have contributed to the study of the ferroelectric behavior of Na+ electrolyte which could be used as a solid electrolyte combined with COFs as electrodes (M. C. Baptista, H. Khalifa, A. Araújo, B. A. Maia, M. Souto, M. H. Braga. Adv. Funct. Mater. 2023, 33, 2212344) which is one of the goals of WP3.
7) In parallel, we have also studied the physical (redox and optical) properties of some electroactive metal-organic frameworks that show great potential to be used as electrodes in metal-ion batteries (G. Valente et al. Inorg. Chem. 2023, 62, 7834-7842; S. De, G. Mouchaham et al. J. Mater. Chem. A 2023, 11, 25465-25483; G. Valente et al. Chem. Mater. 2024, 26, 1333-1341; M. Oggianu et al. Chem. Mater. 2024, 36, 3452-3463).
1) We have developed a new approach to improve the electrochemical performance of n-type anthraquinone-based COF (DAAQ-TFP-COF) cathode material in metal anode (Li, Mg)-based batteries through proper selection of the electrolyte and binder without the need for any additional processing (e.g. exfoliation). Our results demonstrated that the best electrochemical performance (high utilization efficiency of the redox sites and specific capacities close to the theoretical value) was obtained by combining LiTFSI in tetraethylene glycol dimethyl ether (TEGDME) as electrolyte and poly(tetrafluoroethylene) (PTFE) as binder. Using such electrolyte and binder, cyclable symmetric cells consisting of two DAAQ-TFP-COF organic electrodes showed ~80% capacity retention after 2000 cycles at a high current density. The high reversibility and stability of the COF electrode material upon cycling were also confirmed by ex situ IR spectroscopy. In addition, DAAQ-TFP-COF was explored as a cathode in magnesium cells using two different Mg electrolytes; one based on MgCl2 and one containing weakly coordinating anions. This is the first systematic study on the influence of electrolyte and binder on the electrochemical performance of COF cathodes in Li and Mg batteries, being very relevant for use in energy storage devices (O. Luzanin, R. Dantas, R. Dominko, J. Bitenc, M. Souto, J. Mater. Chem. A 2023, 11, 21553-21560).
2) We have synthesized a series of two-dimensional tetrathiafulvalene (TTF)-based COFs incorporating different organic linkers to study their influence on their electrochemical performance. These COFs were investigated as high-voltage organic cathode materials for lithium batteries. The results revealed high average discharge potentials (~3.6 V vs. Li/Li+) and consistent cycling stability (80% capacity retention after 400 cycles at 2C) for the three COF electrodes. In addition, the specific capacity, rate capability, and kinetics varied depending on the structure of the framework.
3) We have reported a proton–electron dual-conductive MOF based on tetrathiafulvalene(TTF)-phosphonate linkers and lanthanum ions. The formation of regular, partially oxidized TTF stacks with short S···S interactions facilitates electron transport via a hopping mechanism, reporting a room-temperature conductivity of 7.2 × 10–6 S cm–1. Additionally, the material exhibits a proton conductivity of 4.9 × 10–5 S cm–1 at 95% relative humidity conditions due to the presence of free −POH groups, enabling efficient proton transport pathways (C. Ribeiro et al. J. Am. Chem. Soc. 2025, 147, 63–68).
4) We have demonstrated that it is possible to synthesize DT-TTF (dithiophene-tetrathiafulvalene) derivatives functionalized with various substituents for the formation of extended networks towards the synthesis of DT-TTF-based COFs. In particular, various mono- and tetraarylated derivatives functionalized with different groups (electron-donating and electron-withdrawing groups) were obtained and the self-assembly of DT-TTF-tetrabenzoic acid derivative was studied by using scanning tunnelling microscopy (STM), which revealed the formation of ordered, densely packed 2D hydrogen-bonded networks at the graphite/liquid interface (C. Ribeiro et al. Chem. Eur. J. 2023, 29, e2023005, selected as Front Cover and Cover Profile).
5) We have published a comprehensive review on the use of COFs as electrodes in lithium-ion batteries highlighting some of the challenges and prospects that are directly related to the ELECTROCOFS ERC-StG project (R. Dantas, C. Ribeiro, M. Souto. Chem. Commun. 2024, 60, 138-149. Part of the themed collection “2023 Emerging Investigators”).
6) With some collaborators, we have contributed to the study of the ferroelectric behavior of Na+ electrolyte which could be used as a solid electrolyte combined with COFs as electrodes (M. C. Baptista, H. Khalifa, A. Araújo, B. A. Maia, M. Souto, M. H. Braga. Adv. Funct. Mater. 2023, 33, 2212344) which is one of the goals of WP3.
7) In parallel, we have also studied the physical (redox and optical) properties of some electroactive metal-organic frameworks that show great potential to be used as electrodes in metal-ion batteries (G. Valente et al. Inorg. Chem. 2023, 62, 7834-7842; S. De, G. Mouchaham et al. J. Mater. Chem. A 2023, 11, 25465-25483; G. Valente et al. Chem. Mater. 2024, 26, 1333-1341; M. Oggianu et al. Chem. Mater. 2024, 36, 3452-3463).
One of the results considered beyond the state-of-the-art is the use of COFs as cathodes for multivalent batteries (e.g. magnesium batteries) as they have been relatively unexplored as they are expected to be one of the emerging post-lithium technologies. Among alternative rechargeable battery technologies, magnesium batteries have attracted a lot of interest because Mg is an earth-abundant element, inexpensive, safe under ambient atmosphere, and has a high volumetric capacity. However, Mg2+ insertion in most cathode materials (especially in the inorganic ones) presents several problems because of the strong electrostatic interaction between the divalent cations and the host materials, difficult desolvation and slow Mg2+ diffusion. Organic materials have been proposed as a versatile and environmentally friendly alternative cathodes that can improve the kinetics and cycling stability in Mg batteries. We demonstrated that our COF exhibited good stability in Mg cells using two different electrolytes, which is a consequence of its robust structure. This stands in stark contrast to linear anthraquinone-based polymers reported in Mg electrolytes.