【Background】
Lithium metal has become a research hotspot in the field of energy science due to its ultra-high theoretical capacity (3860 mAh g-1) and extremely low reduction potential (-3.04 V). However, severe dendrite growth and unstable solid electrolyte interface (SEI) hinder their practical application. It was found that covalent organic frameworks (CTFs) with triazinyl groups with lithium-philic functional groups have great potential for uniform transport and deposition of lithium ions. However, most of the reported covalent triazine frameworks have low crystallinity, which severely affects the mechanical strength of the solid electrolyte interfacial layer. In addition, multi-component covalent triazine frameworks with adjustable structures and functions and multiple lithium-philic organic functional groups have not been used in the field of lithium metal batteries (LMBs) so far, especially for interface protection.
【Job Introduction】
Recently, Professor Wang Yong's team at Shanghai University designed three types of CTFs by adjusting the molar ratio of the monomer feed and different numbers of linkers and node elements. Among them, the partially fluorinated covalent triazine backbone (4C-TA0.5TF0.5-CTF), which is the main product, is obtained by equimolar amounts of F-substituted and unsubstituted marginal aromatic rings (present in the imine-linked two-component 2C-TA-CTF), which are used for the first time as a protective layer for lithium metal anodes to achieve the above objectives. The results of theoretical calculations show that the donor-donor-π-acceptor interaction of the four-component 4C-TA0.5TF0.5-CTF is significantly enhanced due to the asymmetric substitution of aromatic rings on both sides of the triazine nucleus, which constitutes an ideal lithium migration pathway. On the one hand, the two-dimensional 4C-TA0.5TF0.5-CTF can make full use of a variety of functional groups, such as C-F bonds, triazine nuclei, C=N bonds and aromatic rings, which not only regulates the local electron cloud, but also improves the crystallinity, porosity and mechanical strength. On the other hand, the high electronegativity and Li+ affinity of fluorine can show maximum charge polarization, accelerate the surface diffusion of Li+ and enhance the migration kinetics of Li+. In addition, the C-F bond can improve the chemical and thermal stability of the electrode surface. As a result of these beneficial synergistic effects, the 4C-TA0.5TF0.5-CTF-modified Li anode provides a high average coulombic efficiency (CE) of 98.1% and good stability over 600 cycles, which is far superior to the three-component 3C-TF-CTF and two-component 2C-TA-CTF electrodes. In addition, the whole battery assembled by the 4C-TA0.5TF0.5-CTF modified Li anode and LiFePO4 cathode can provide a capacity of 116.3 mAh g-1 (capacity retention: 86.8%) and significantly improved rate performance after 1000 cycles at 5C. In addition, a series of in-situ/ex situ characterizations were performed to investigate the lithium deposition mechanisms of various functional groups in 4C-TA0.5TF0.5-CTF, such as C-F bonds, triazine nuclei, C=N bonds, and aromatic rings. This study not only broadens the complexity of the covalent organic framework, but also provides a promising strategy for the molecular design and study of the highly stable lithium-metal interface layer. The article was published in the top international journal Angew. Chem. Int. Ed. Ph.D. student Lu Xiaomeng is the first author of this paper.
【Content Description】
The multi-component covalent triazine-based framework was chosen here because it has the following advantages over the traditional covalent triazine framework: (i.) The successful integration of fluorine-substituted and non-substituted units on the edge aromatic ring unit of the triazine core through one node and two linkers not only facilitates the efficient separation of HOMO and LUMO energy levels, but also promotes enhanced donor-donor-π-acceptor interaction and intramolecular charge transfer (push-pull effect). (ii.) F substitution regulates the electron cloud distribution of adjacent carbon atoms, directing a larger dipole moment (a strong built-in electric field) to maximize interlayer interactions. (iii.) The multi-component covalent triazine framework, with its ordered porous structure and fully exposed lipophilic sites (C-F bonds, triazine nuclei, C=N groups, and aromatic rings), contributes to the construction of a sustainable high-speed channel for Li+ transport, allowing Li+ to pass evenly with less resistance.
1. Molecular design and theoretical calculations
图1.（a）不同组分2C-TA-CTF、3C-TF-CTF和4C-TA0.5TF0.5-CTF的分子结构示意图。 （b）4C-TA0.5TF0.5-CTF的分子结构侧视图。 （c）2C-TA-CTF、（d）3C-TF-CTF和（e）4C-TA0.5TF0.5-CTF的静电势和供体受体分布。 （f）引入吸电子F后的新型D1-D2-π-A系统示意图。
First, 2C-TA-CTF, 3C-TF-CTF and 4C-TA0.5TF0.5-CTF were prepared by solvothermal method. Among them, the multi-component 4C-TA0.5TF0.5-CTF is formed by the reaction of fluorine-containing and non-fluorinated monomers with triazine nuclei at a molar ratio of 50/50, thereby adjusting the structure and functional group richness of different covalent triazine frameworks. Then, the relationship between the structure and the Li+ migration was better understood through theoretical calculations. The results show that when both the aldehyde group and the fluorinated aldehyde group are introduced into the structure at the same time, both the aldehyde group and the triazine ring can be used as electron donors (it should be noted that the triazine group is usually an electron acceptor in the absence of fluorine), while the fluorinated aldehyde group is the electron acceptor, and aniline (attached to the triazine ring) acts as a π bridge to achieve rapid electron transfer by establishing a sufficiently strong built-in electric field. Compared to 2C-TA-CTF and 3C-TF-CTF, 4C-TA0.5TF0.5-CTF exhibits the lowest ΔE=1.917 ev by efficient separation of donor and acceptor locations, favoring electron transfer and the establishment of specific Li+ guide channels. Therefore, under the push-pull effect, electrons are more likely to accumulate around the fluorine group due to the enhanced donor-donor-π-acceptor interaction, so that Li+ is uniformly deposited on the surface of lithium metal through the cation channel with less resistance.
2. Material characterization and physicochemical characterization
图2.（a, d）2C-TA-CTF、（b, e）3C-TF-CTF和（c, f）4C-TA0.5TF0.5-CTF的SEM图像。 （g）2C-TA-CTF、（h）3C-TF-CTF和（i）4C-TA0.5TF0.5-CTF的TEM图像。 （j-l）4C-TA0.5TF0.5-CTF的AFM图像以及杨氏模量分布。 （m）2C-TA-CTF、3C-TF-CTF和4C-TA0.5TF0.5-CTF粉末的XRD图。 （n）2C-TA-CTF、3C-TF-CTF和4C-TA0.5TF0.5-CTF的傅立叶变换红外光谱。 （o）4C-TA0.5TF0.5-CTF的氮吸附-解吸等温线和相应的孔径分布曲线（插图）。
Scanning and transmission electron microscopy showed that 2C-TA-CTF, 3C-TF-CTF and 4C-TA0.5TF0.5-CTF showed homogeneous honeycomb, hedgehog spherical and succulent structures, respectively. Atomic force microscopy revealed that the average thickness of 4C-TA0.5TF0.5-CTF was about 1.6 μm, and the average Young's modulus was as high as 10.0 GPa, which was significantly higher than that of spontaneously formed SEI (about 150 MPa), which was conducive to alleviating the volume change of lithium metal and avoiding the generation of lithium dendrites. Powder X-ray diffraction showed that the peaks of 4C-TA0.5TF0.5-CTF in the (110) and (200) crystal planes shifted to a higher direction, which means that the partial fluorination greatly enhanced the π-π interaction, reduced the interlayer stacking distance, and extended the long-range ordering. In addition, FTIR spectroscopy and Raman spectroscopy also confirmed the successful synthesis and chemical composition of 2C-TA-CTF, 3C-TF-CTF and 4C-TA0.5TF0.5-CTF. It is worth mentioning that the adsorption-isothermal curve of 4C-TA0.5TF0.5-CTF is type IV., with a specific surface area of 503.8 m2 g-1, an average pore size distribution of 2.0 nm, and a total pore volume (P/P0=0.99) of 0.25 cm3 g-1.
3. Lithium deposition and stripping behavior
图3.（a）2C-TA-CTF、3C-TF-CTF和4C-TA0.5TF0.5-CTF在0.5 mA cm-2下的库仑效率。 （b）2C-TA-CTF、3C-TF-CTF和4C-TA0.5TF0.5-CTF在不同电流密度下的倍率性能。 （c）对称电池组装示意图。 对称电池在电流密度为（d）1 mA cm-2、（e) 3 mA cm-2和（f）5 mA cm-2 时的恒电流循环曲线。 （g）全电池组装示意图。 （h）全电池的倍率性能。 全电池在（i）3 C、（j）5 C下的长期循环性能。
In order to evaluate the utilization rate of lithium metal anode during repeated deposition/stripping, a half-cell based on 4C-TA0.5TF0.5-CTF was assembled. After 600 cycles at a current density of 0.5 mA cm-2, the average CE value of the half-cell reached 98.1%, which was significantly higher than that of 3C-TF-CTF and 2C-TA-CTF. With the gradual increase of current density, the CE value of 4C-TA0.5TF0.5-CTF is almost not affected by different current densities, and shows good rate performance. In order to further evaluate the effect of 4C-TA0.5TF0.5-CTF on promoting uniform deposition of lithium, a constant current charge-discharge test was carried out on symmetrical batteries. It is worth noting that at an extremely high current density of 5 mA cm-2, [email protected]||[email protected] still enables a stable lithium deposition/stripping process (67 mV, ~900 h), while Li@2C-TA-CTF||Li@2C-TA-CTF fluctuated violently from start to finish (~219 h). In order to study the practicability of the 4C-TA0.5TF0.5-CTF modified anode, the whole battery was assembled by matching the 4C-TA0.5TF0.5-CTF modified electrode and the positive electrode (LiFePO4) for electrochemical performance testing. [email protected]||The LFP whole cell was cycled at a current density of 5 C, and the life of the battery was still significantly extended (1000 cycles), and the capacity retention rate reached 86.8%, which indicates [email protected]||LFP batteries can be compatible with higher current densities.
4. Lithium transport characteristics and deposition mechanisms
图4.（a）2C-TA-CTF、3C-TF-CTF、4C-TA0.5TF0.5-CTF和LiTFSI的电解质之间模拟亲和能的化学配位情况。 （b）2C-TA-CTF和（c）4C-TA0.5TF0.5-CTF的原位傅立叶变换红外透射光谱。 (d-i）4C-TA0.5TF0.5-CTF在不同工作电位下的原位原子力显微镜图像。 （d1-i1）4C-TA0.5TF0.5-CTF的三维原子力显微镜形貌图像。 （j）4C-TA0.5TF0.5-CTF的高度剖面图。
To gain insight into the atomic-scale interphase chemistry of Li+ with three different CTFs and bis(trifluoromethylsulfonyl)imide (LiTFSI) electrolyte components, we performed density functional theory (DFT) calculations. The binding energy between Li+ and 4C-TA0.5TF0.5-CTF was -2.495 eV, indicating that there was a strong interaction between the two, and Li+ was also easier to desoluble from ether-based solution clusters and further bind to 4C-TA0.5TF0.5-CTF. The transfer of TFSI- is hindered by the repulsion of fluorine(F)-derived electronegative sites, which makes the migration of lithium ions easier and the plating/stripping cycleability of lithium ions improved. The in-situ Fourier transform infrared technology was used to dynamically observe the electrolyte transition during charging and discharging, and the ability of the 4C-TA0.5TF0.5-CTF layer to inhibit electrolyte degradation and avoid side reactions was further clarified. In situ AFM experiments were performed to investigate the morphology of the 4C-TA0.5TF0.5-CTF protective layer when discharged under an ether-based electrolyte system at the microscopic level. 4C-TA0.5TF0.5-CTF maintains benign electronic contact throughout the discharge process, which is better reflected in the corresponding three-dimensional (3D) images. In addition, the position-specific height profile of 4C-TA0.5TF0.5-CTF shows an almost negligible height change, suggesting that it has low strain characteristics and is conducive to stabilizing the electrochemical microenvironment of the interface.
图5. [email protected]在不同循环后电极的（a-c）俯视和（d-f）横截面SEM图像。 （g）[email protected]和裸锂上沉积过程的示意图。
图6.（a-d）4C-TA0.5TF0.5-CTF和（e-h）裸锂上锂沉积的原位光学显微镜图像。 （i）原位拉曼等高线图和相应的电压-时间曲线。 （j）4C-TA0.5TF0.5-CTF中的Li+传输示意图。
In-situ optical microscopy observed that the [email protected] electrode showed a dense and uniform lithium deposition surface during the whole deposition process, indicating that the 4C-TA0.5TF0.5-CTF layer could effectively prevent the growth of lithium dendrites and stabilize the electrode/electrolyte interface. In situ Raman testing further elucidated the unique interactions between multiple functional groups in 4C-TA0.5TF0.5-CTF and Li+. Unlike the reversible changes during discharge and charging, there was no further significant change in the intensity of C=N linkage, aromatic ring, and triazine nuclei throughout the lithium deposition process, indicating that the 4C-TA0.5TF0.5-CTF protective layer was quite stable, and the lithium metal was mainly deposited under the protective layer. This is consistent with the findings of in-situ AFM and in situ optical microscopy. Based on the above theory and experimental evidence of in-situ characterization, the origin and mechanism of Li+ migration behavior in the 4C-TA0.5TF0.5-CTF interface layer are proposed in Figure 6j.
【Conclusion】
In conclusion, a multifunctional 4C-TA0.5TF0.5-CTF with partial electronegativity channel and high crystallinity was reasonably designed through a multi-component synthesis strategy, so as to adjust the interfacial stability. Theoretical studies have confirmed that 4C-4C-TA0.5TF0.5-CTF can enhance the π-conjugation of the triazinyl framework. IN ADDITION, THE ELECTRONEGATIVE FLUORINE (F) MODIFICATION PROVIDES LOCAL POLARIZATION, RESULTING IN A LARGER DIPOLE MOMENT AND MORE PRONOUNCED HOMO-LUMO SEPARATION, RESULTING IN A UNIQUE INTRAMOLECULAR DONOR-DONOR-π-ACCEPTOR SYSTEM. Therefore, 4C-TA0.5TF0.5-CTF fully exposed to the lipophilic site can adjust the local charge distribution and promote charge transfer, thereby homogenizing the Li+ flux and promoting Li+ diffusion. The whole battery with 4C-TA0.5TF0.5-CTF as the protective layer and LiFePO4 as the cathode showed excellent capacity (116.3 mAh g-1 at 5 C) and stable cyclability (13.2% capacity attenuation rate in 1000 cycles) while preventing the formation of lithium dendrites. In addition, we used in-situ Raman, in-situ Fourier transform infrared, in-situ atomic force microscopy, in-situ optical microscopy, and theoretical calculations to analyze the lithium deposition mechanism of the 4C-TA0.5TF0.5-CTF electrode, especially multifunctional groups such as C-F bonds, triazine nuclei, C=N linkages, and aromatic rings. Our work lays the foundation for the prospective design of improved COF-based ASEI molecules to achieve highly stable lithium-metal batteries.
Xiao-Meng Lu, Haichao Wang, Yiwen Sun, Yi Xu, Weiwei Sun, Yang Wu, Yifan Zhang, Chao Yang, and Yong Wang，Covalent Triazine Based Frameworks with Donor-Donor-π-Acceptor Structures for Dendrite-Free Lithium Metal Batteries, Angew. Chem. Int. Ed. 2023.
https://doi.org/10.1002/anie.202409436.
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