Key Takeaways & Executive Findings
- •• Ethidium cations act as 'anion capturers' to immobilize TFSI−, promoting the formation of a LiF/Li3N-rich solid electrolyte interphase. • Ion–dipole interactions between ethidium groups and solvent molecules boost Li+ desolvation, enhancing ion transport kinetics. • Sulfonate groups exhibit an ion-sieving effect that selectively attracts Li+ while excluding TFSI−, facilitating LiTFSI dissociation and accelerating Li+ migration. • The Z-COF protective layer enables stable Li plating/stripping for over 6300 hours and high-capacity retention in full cells, demonstrating practical potential for high-performance lithium metal batteries.
Abstract
The sluggish Li+ migration kinetics and unstable electrode/electrolyte interface severely hinder the commercial application of high-performance lithium metal batteries (LMBs). Herein, an artificial protective layer is constructed using zwitterionic covalent organic framework (Z-COF) simultaneously containing sulfonate and ethidium groups, aiming to facilitate rapid, uniform Li+ transport and stabilize anode interface. The sulfonate groups with high lithiophilicity provide abundant hopping sites for fast Li+ diffusion. The ethidium cations immobilize TFSI− and solvent molecules by ion–dipole interactions, which accelerate the dissociation of LiTFSI and Li+ desolvation. Moreover, the monodispersed zwitterionic units coupling with ordered micropore structures in Z-COF create exclusive Li+ migration channels, modulate homogeneous space charge distribution, kinetically facilitating uniform Li+ deposition. Experiments and theoretical calculations indicate that C–F and S–N bonds of TFSI− exhibit enhanced cleavage susceptibility driven by electrostatic attraction, realizing a LiF/Li3N-rich electrolyte/electrode interface. The designed Z-COF protection layer enables Li|Li symmetrical cells stable cycling over 6300 h at 2 mA cm−2/2 mAh cm−2. The Z-COF@Li|LiFePO4 (LFP) full cells deliver high-capacity retention of 85.2% after 1000 cycles at 8 C. The assembled Z-COF@Li|LFP pouch cells demonstrate a lifespan of more than 240 cycles. This work provides fresh insights into the practical application of zwitterionic COF in next-generation LMBs.
1. Introduction
Commercial energy storage devices cannot satisfy ever-rising demands for high-end communication terminals and electric vehicles because of their theoretical energy density limits [1–5]. The development of high-energy-density rechargeable batteries has become an urgent problem. Lithium metal batteries (LMBs) are considered promising competitors in this pursuit, owing to ultrahigh theoretical specific capacity of 3860 mAh g−1 and the lowest electrochemical potential (−3.04 V vs. standard hydrogen electrode) [6–9]. However, uncontrollable Li dendrite growth induces poor Coulombic efficiency (CE), irreversible capacity loss and severe safety hazard, dragging LMBs out of practical applications [10, 11].
According to the Sand’s equation, interfacial Li+ migration significantly influences Li electrodeposition process [12–14]. The sluggish Li+ diffusion can lead to the concentration polarization between bulk electrolyte and anode surface, fostering uneven Li plating/stripping [15–18]. In addition, the nonuniform accumulation of free anions leads to an uneven charge distribution near the electrode surface, exacerbating the growth of Li dendrites [19, 20]. Hence, it is imperative to develop a collaborative strategy which simultaneously modulates the ion interface kinetics transfer and local charge distribution to induce rapid and uniform Li deposition for realizing high-performance LMBs.
As an emerging crystalline porous material, ionic covalent organic framework (iCOF) can greatly present both crystal and ionization characteristics, garnering widespread attention in Li+ transportation [21–25]. The ordered open nanochannels of iCOF can serve as fast Li+ conduction pathway [26–29]. The permanent charged ion units on pore walls capable of interacting with Li+ or anions grant iCOF outstanding Li+ selectivity [30–33]. Recently, various reports declared that single anionic COF could effectively inhibit free anions (TFSI− and PF6−) transport while accelerating Li+ migration owing to surface-negative charge-induced ion screening effect [34, 35]. Besides, the single cationic COF could alleviate the degree of Li+ solvation by the intermolecular interaction between the positive charge groups and the solvent molecules, thereby regulating Li+ desolvation process [36–38]. However, single iCOF-modified strategy focused solely on Li+ diffusion or desolvation behaviors, addressing only a port
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Liya Rong, Yifeng Han, Chi Zhang, Hongling Yao, Zhaojun He, Xianbao Wang, Zaiping Guo, Tao Mei (2026). Regulating Li+ Transport and Interfacial Stability with Zwitterionic COF Protective Layer Towards High-Performance Lithium Metal Batteries. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-02017-3
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Frequently Asked Questions
What is the main challenge in lithium metal batteries that this study addresses?
The main challenges are sluggish Li+ migration kinetics and unstable electrode/electrolyte interface, which lead to dendrite growth, poor Coulombic efficiency, and safety hazards.
How does the zwitterionic COF protective layer improve Li+ transport?
The sulfonate groups provide hopping sites for fast Li+ diffusion, while ethidium cations immobilize TFSI− and solvent molecules via ion–dipole interactions, accelerating LiTFSI dissociation and Li+ desolvation. The ordered micropores create exclusive Li+ migration channels.
What are the key performance metrics of the Z-COF protected batteries?
The Li|Li symmetrical cells cycle stably for over 6300 hours at 2 mA cm−2/2 mAh cm−2. Full cells with LiFePO4 cathode retain 85.2% capacity after 1000 cycles at 8 C, and pouch cells last more than 240 cycles.
What is the significance of the LiF/Li3N-rich interface?
The LiF/Li3N-rich solid electrolyte interphase enhances interfacial stability and promotes uniform Li deposition, which is crucial for long-term cycling performance.
How does the zwitterionic COF differ from single ionic COFs?
Unlike single anionic or cationic COFs that address only one aspect, the zwitterionic COF simultaneously regulates Li+ diffusion, desolvation, and charge distribution, offering a collaborative strategy for high-performance LMBs.
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