Redox and Anionic Covalent Organic Frameworks (COFs) as Solid Electrolyte Interphase for Lithium Metal Batteries (LMBs)
Lithium (Li)-metal batteries (LMBs) possess the highest theoretical energy density among current battery designs and thus have enormous potential for use in energy storage. However, the development of LMBs has been severely hindered by safety concerns arising from dendrite growth and unstable interphases on the Li anode. Covalent organic frameworks (COFs) incorporating either redox-active or anionic moieties on their backbones have high Li-ion (Li+) conductivities and mechanical/chemical stabilities, so are promising for solid electrolyte interphases (SEIs) in LMBs. Here, we synthesized anthraquinone-based silicate COFs (AQ-Si-COFs) that contained both redox-active and anionic sites via condensation of tetrahydroxyanthraquinone with silicon dioxide. The nine Li+-mediated charge/discharge processes enabled the AQ-Si-COF to demonstrate an ionic conductivity of 9.8 mS cm–1 at room temperature and a single-ion-conductive transference number of 0.92. Computational studies also supported the nine Li+ mechanism. We used AQ-Si-COF as the solid electrolyte interphase on the Li anode. The LMB cells with a LiCoO2 cathode attained a maximum reversible capacity of 188 mAh g–1 at 0.25 C during high-voltage operation. Moreover, this LMB cell demonstrated suppressed dendrite growth and stable cyclability, with its capacity decreasing by less than 3% up to 100 cycles. These findings demonstrate the effectiveness of our redox-active and anionic COFs and their practical utility as SEI in LMB.
J. Am. Chem. Soc. 145, 45, 24603 (2023)
High-Performance All-Solid-State Lithium Metal Batteries Enabled by Ionic Covalent Organic Framework Composites
Ionic covalent organic frameworks (iCOFs) are crystalline materials with stable porous structures. They hold great potential for ion transport, particularly as solid-state electrolytes (SSEs) for all-solid-state Lithium metal batteries (ASSLMBs). However, achieving an ionic conductivity of over 10−3 S cm−1 at room temperature using pure-iCOF-based SSEs, even adding additives such as lithium salts, is challenging as the voids work as strong resistances. Thus, highly conductive iCOFs typically require quasi-solid-state configurations with organic solvents or plasticizers. In this study, we prepared composites comprising iCOFs and poly(ionic liquid) (PIL) to make all-solid-state iCOFs electrolytes with an exceptional ionic conductivity up to 1.50 × 10−3 S cm−1 and a high Li+ transference number of > 0.80 at room temperature. Combined experimental and computational studies showed that the co-coordination and competitive coordination mechanism established between the PIL, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and iCOFs enabled rapid Li+ transport while restricting TFSI− movement. ASSLMB cells, made of composite SSEs and LiFePO4 composite cathode, demonstrated an initial discharge capacity of 141.5 mAh g−1 at 1 C and r.t., with an impressive capacity retention of 87% up to 800 cycles. Overall, this work presents a breakthrough approach for developing advanced SSEs for next-generation high-energy-density ASSLMBs.
Adv. Energy Mater. 2400762 (2024)
Imidazolate COFs with Ether Chains as Solid Electrolytes for LMBs
Poly(ethylene oxide) (PEO)-based electrolytes are often used for Li+ conduction as they can dissociate the Li salts efficiently. However, high entanglement of the chains and lack of pathways for rapid ion diffusion limit their applications in advanced batteries. Recent developments in ionic covalent organic frameworks (iCOFs) showed that their highly ordered structures provide efficient pathways for Li+ transport, solving the limitations of traditional PEO-based electrolytes. Here, we present imidazolate COFs, PI-TMEFB-COFs, having methoxyethoxy chains, synthesized by Debus–Radziszewski multicomponent reactions and their ionized form, Li+@PI-TMEFB-COFs, showing a high Li+ conductivity of 8.81 mS cm−1 and a transference number of 0.974. The mechanism for such excellent electrochemical properties is that methoxyethoxy chains dissociate LiClO4, making free Li+, then those Li+ are transported through the imidazolate COFs’ pores. The synthesized Li+@PI-TMEFB-COFs formed a stable interface with Li metal. Thus, employing Li+@PI-TMEFB-COFs as the solid electrolyte to assemble LiFePO4 batteries showed an initial discharge capacity of 119.2 mAh g−1 at 0.5 C, and 82.0 % capacity and 99.9 % Coulombic efficiency were maintained after 400 cycles. These results show that iCOFs with ether chains synthesized via multicomponent reactions can create a new chapter for making solid electrolytes for advanced rechargeable batteries.
Angew. Chem. Int. Ed. e202402202 (2024)
Phosphonium COFs with Efficient OH− Conduction for Zinc–Air Batteries
Owing to their well-defined crystalline pore structures and ordered functional ionic groups along the skeleton, ionic covalent organic frameworks (iCOFs) exhibit excellent performance and have significant potential for use in energy storage and conversion devices. Herein, we for the first time developed cationic phosphonium COFs with high hydroxide conduction even with low ion exchange capacity (IEC). Specifically, we synthesized COFs containing quaternary phosphonium groups as excellent ion transport moieties. Then, we fabricated freestanding phosphonium membranes through a vapor-assisted method, which exhibited high hydroxide conductivity of 126 mS cm–1 at 80 °C from a minimal IEC of 1.17 mmol g–1. The resulting film was successfully applied to zinc–air batteries, demonstrating energy density of 96.1 mW cm–2, specific capacity of 95.0 mAh cm–2, and stable operation over 2,300 min. Overall, in addition to investigating a novel cationic functional group, we demonstrated a freestanding film formation method of COF-based materials. The findings can provide a solid foundation for advancing the field of iCOFs to ion transport and promoting electrochemical applications.
Angew. Chem. Int. Ed. (2024)
Anion Exchange Membranes for Fuel Cells
The lack of anion exchange membranes (AEMs) that possess both high hydroxide conductivity and stable mechanical and chemical properties poses a major challenge to the development of high-performance fuel cells. Improving one side of the balance between conductivity and stability usually means sacrificing the other. Herein, we used facile, high-yield chemical reactions to design and synthesize a piperidinium polymer with a polyethylene backbone for AEM fuel cell applications. To improve the performance, we introduced ionic crosslinking into high-cationic-ratio AEMs to suppress high water uptake and swelling while further improving the hydroxide conductivity. Remarkably, PEP80-20PS achieved a hydroxide conductivity of 354.3 mS cm−1 at 80 °C while remaining mechanically stable. Compared with the base polymer PEP80, the water uptake of PEP80-20PS decreased by 69 % from 813 % to 350 %, and the swelling decreased substantially by 85 % from 350.0 % to 50.2 % at 80 °C. PEP80-20PS also showed excellent alkaline stability, 84.7 % remained after 35 days of treatment with an aqueous KOH solution. The chemical design in this study represents a significant advancement toward the development of simultaneously highly stable and conductive AEMs for fuel cell applications.
Angew. Chem. Int. Ed. e202307690 (2023)
Highly Conductivity COFs for Gas Storage, Conversion, and Sensing
Chemically inert organic networks exhibiting electrical conductivity comparable to metals can advance organic electronics, catalysis, and energy storage systems. Covalent–organic frameworks (COFs) have emerged as promising materials for those applications due to their high crystallinity, porosity, and tunable functionality. However, their low conductivity has limited their practical utilization. In this study, copper-coordinated-fluorinated-phthalocyanine and 2,3,6,7-tetrahydroxy-9,10-anthraquinone-based COF (CuPc-AQ-COF) films with ultrahigh conductivity are developed. The COF films exhibit an electrical conductivity of 1.53 × 103 S m−1 and a Hall mobility of 6.02 × 102 cm2 V−1 s−1 at 298 K, reaching the level of metals. The films are constructed by linking phthalocyanines and anthraquinones through vapor-assisted synthesis. The high conductivity properties of the films are attributed to the molecular design of the CuPc-AQ-COFs and the generation of high-quality crystals via the vapor-assisted method. Density functional theory analysis reveals that an efficient donor–acceptor system between the copper-coordinated phthalocyanines and anthraquinones significantly promotes charge transfer. Overall, the CuPc-AQ-COF films set new records of COF conductivity and mobility and represent a significant step forward in the development of COFs for electronic, catalytic, and electrochemical applications.
Small 2306634 (2023)