Large-Scale Nanofiber Manufacturing| Dendrite-Free Li Metal Anode Achieved by Bi-Functional Hostof NHz-Modified Ui0-66 on Zn-Embedded Porous CarbonNanofibers

Beijing University of Chemical Technology Professor Yu Le & Professor Zhou Weidong & Dr. Yang Xue: MOF nanoparticle-coated zinc-containing porous carbon nanofiber bifunctional host for dendrite-free lithium metal anodes

Lithium metal is considered the most promising ultimate anode due to its high theoretical specific capacity, low redox potential and low mass density. However, disadvantages such as dendrite growth and infinite volume expansion severely limit the commercialization of lithium metal anodes. Additionally, side reactions between lithium metal and electrolytes exacerbate electrolyte consumption and irreversible loss of active lithium, ultimately leading to low Coulombic efficiency, rapid capacity degradation and even safety incidents like explosions in battery systems.

Recently, Professor Yu Le, Professor Zhou Weidong and Dr. Yang Xue from Beijing University of Chemical Technology published their latest research titled "Dendrite-Free Li Metal Anode Achieved by Bi-Functional Host of NH2-Modified UiO-66 on Zn-Embedded Porous Carbon Nanofibers" in Advanced Functional Materials, with Dr. Chen Chen from BUCT's School of Chemical Engineering as first author. The authors proposed a dual metal-organic framework (MOF) synthesis strategy to construct a bifunctional mixed ion/electron conductor as a dendrite-free lithium metal anode host, specifically zinc-containing porous carbon nanofibers coated with NH2-modified zirconium-based MOF (UiO-66) nanoparticles (Zn/CF@NH2-UiO-66).

Specifically, two MOFs (ZIF-8 and UiO-66) play different roles in synthesizing the bifunctional host. ZIF-8 particles serve as precursors for subsequent lithiophilic zinc sites and pore-forming agents, forming ZIF-8@PAN precursor fibers through electrospinning with polyacrylonitrile. After high-temperature heat treatment, the derived zinc-containing porous carbon fibers provide numerous lithiophilic sites and sufficient space for lithium metal nucleation and growth. The subsequently introduced NH2-UiO-66 as the fiber outer layer accelerates desolvation of solvated Li+ ions and promotes ion diffusion through in situ synthesis. As expected, symmetric cells assembled using Li-Zn/CF@NH2-UiO-66 composite electrodes exhibit long-term cycling stability, and Li-Zn/CF@NH2-UiO-66||LiFePO4 (LFP) full cells show stable cycling for 1700 cycles at 2C with high capacity retention of 93.4%.

As shown in Figure 1a, the Zn/CF@NH2-UiO-66 fiber bifunctional host was successfully prepared through a novel dual-MOF participation strategy for use as a dendrite-free lithium metal anode. Among them, ZIF-8 nanoparticles and polyacrylonitrile (PAN) were formed into smooth precursor nanofibers (ZIF-8@PAN) through high electrostatic voltage stretching in an electrostatic field and pre-oxidation treatment. The ZIF-8 nanoparticles were tightly arranged and uniformly dispersed in the PAN fibers, resulting in fibers with a smooth surface and a diameter of approximately 500 nm (Figure 1b). Subsequently, under the protection of an inert atmosphere, the ZIF-8@PAN precursor nanofibers were converted into Zn/CF with an average diameter of about 300 nm through a two-stage high-temperature annealing process (Figures 1c-e). Interestingly, the ZIF-8 particles not only served as a precursor template for electrospinning to provide lithiophilic zinc sites but also acted as pore-forming agents, giving Zn/CF an interconnected hierarchical cavity structure that provided sufficient space for the controlled deposition of lithium metal. HAADF-STEM images and corresponding elemental mapping showed the uniform distribution of C, N, Zn, and Zr species, confirming the successful synthesis of the NH2-UiO-66 layer (Figures 1f-h). Additionally, the deconvoluted peaks in the FTIR spectrum of Zn/CF@NH2-UiO-66 at 1654.6, 1261.2, and 765.4 cm−1 corresponded to the bending vibration of the N-H bond in aromatic amines, the stretching vibration of the C-N bond, and the wagging vibration of the N-H bond, respectively, matching the peak positions of NH2-UiO-66 nanoparticles. In contrast, these peaks were absent in the control sample Zn/CF, further confirming the successful modification of the NH2 functional group (Figure 1i).

To clarify the role of the NH2-UiO-66 coating, chronoamperometry combined with electrochemical impedance spectroscopy (EIS) was first performed. As shown in Figure 2a, the Li+ transference number (tLi+) of the Li-Zn/CF@NH2-UiO-66 symmetric cell was 0.42, significantly higher than that of Li-Zn/CF, indicating more uniform Li+ ion flow distribution and faster diffusion kinetics. Moreover, within the temperature range of 30–70°C, the calculated activation energy (Ea) for the Li-Zn/CF@NH2-UiO-66 symmetric cell was lower at 64.68 kJ mol−1, demonstrating that the NH2-UiO-66 coating promoted the desolvation of solvated Li+ ions and accelerated diffusion kinetics (Figure 2b). Furthermore, compared with pure lithium salt, the NMR spectrum of LiTFSI-NH2-UiO-66 showed a clear left shift and reduced solvent binding (Figure 2c). This change in the local coordination environment of Li+ ions was further verified by DFT calculations. As shown in Figures 2d-f, the binding energies of NH2-UiO-66, UiO-66, COOH-UiO-66, and Li with DOL and DME were calculated. Among them, NH2-UiO-66 exhibited the highest binding energies with DOL (−1.26 eV) and DME (−1.14 eV), indicating its tendency to preferentially bind with DOL and DME, thereby promoting the desolvation of solvated Li+ ions, accelerating Li+ nucleation and deposition inside the zinc-embedded porous fibers, alleviating lithium dendrite growth, and improving lithium metal utilization. Figures 2g-i show the migration pathways and energy barriers of Li+ ions in NH2-UiO-66, UiO-66, COOH-UiO-66, and CF models. Benefiting from the desolvation capability of NH2-UiO-66, its migration barrier was significantly lower at 0.085 eV compared to UiO-66 (0.117 eV), COOH-UiO-66 (0.154 eV), and CF (0.213 eV), quantitatively proving that the NH2-UiO-66 coating facilitated Li+ ion migration and improved ion diffusion kinetics.

As shown in Figures 3a-c, due to the sufficient internal space of Zn/CF@NH2-UiO-66 porous carbon nanofibers, uniformly dispersed lithiophilic zinc sites, and rapid ion transport kinetics, lithium metal could deposit gradually from the inside out as the deposition capacity increased. When the lithium deposition capacity reached 12 mAh cm−2, the interconnected spaces between the fibers were completely filled with lithium metal, and no dendritic lithium was observed. In contrast, under the same testing conditions, as shown in Figures 3d-f, the surface of Zn/CF without NH2-UiO-66 coating, despite having lithiophilic sites and a porous structure, exhibited uneven lithium blocks and tiny protrusions. As the deposition capacity increased, localized dendritic lithium formed, and many fragmented fibers caused by volume expansion were observed throughout the deposition process. To further compare the advantages of the porous structure design and lithiophilic nucleation sites, we prepared solid carbon nanofibers coated with NH2-UiO-66 (CF@NH2-UiO-66) and used finite element simulations to reveal the role of ZIF-8 as a morphology/composition regulator during synthesis. As shown in Figures 3g-h, under the same conditions, the Li+ ion concentration distribution and current density distribution of CF@NH2-UiO-66 clearly showed lithium dendrites. In summary, the three strategies—constructing a 3D porous fiber structure, embedding lithiophilic zinc nucleation sites, and applying a coating to enhance Li+ ion diffusion kinetics—worked synergistically and were indispensable.

Subsequently, various electrochemical performance tests were conducted on different hosts. First, as shown in Figures 4a-b, at a high current density of 3 mA cm−2, the Coulombic efficiency (CE) curve of the Zn/CF@NH2-UiO-66 host remained stable for 300 cycles, far outperforming Zn/CF, Zn/CF@UiO-66, and Zn/CF@COOH-UiO-66. Moreover, its charge/discharge curves exhibited stable and overlapping plateaus. Next, under a current density of 1 mA cm−2 and a deposition capacity of 4 mAh cm−2, galvanostatic intermittent titration technique (GITT) was used to compare the reaction kinetics of Li-Zn/CF@NH2-UiO-66 and Li-Zn/CF symmetric cells (Figure 4c). The Li-Zn/CF@NH2-UiO-66 electrode exhibited faster mass transfer kinetics and lower overpotential, whereas Li-Zn/CF showed higher voltage polarization due to the accumulation of residual lithium on the surface after cycling, hindering ion transport. Additionally, at a current density of 1 mA cm−2 and a capacity of 1 mAh cm−2, the Li-Zn/CF@NH2-UiO-66 electrode demonstrated stable cycling for 2400 hours with a voltage hysteresis of only 26.7 mV (Figure 4d). It also exhibited excellent rate performance within the current density range of 1–6 mA cm−2, with voltage hysteresis still significantly better than that of Li-Zn/CF (Figure 4e).

To explore the practical feasibility of Zn/CF and Zn/CF@NH2-UiO-66 hosts, full cells were assembled using LiFePO4 (LFP) as the cathode, and their electrochemical performance was tested. As shown in Figure 5a, at current densities of 0.5, 1, 2, 3, 5, and 10 C, the discharge capacities of the Li-Zn/CF@NH2-UiO-66||LFP full cell were 149, 141, 130, 122, 110, and 92 mAh g−1, respectively. Notably, when the current was restored to the initial 0.5 C, the cell recovered to a high discharge capacity with minimal capacity loss. Long-term cycling tests showed that the Li-Zn/CF@NH2-UiO-66||LFP cell maintained stable cycling for 1700 cycles at 2 C with a capacity retention rate of 93.4% (Figure 5b). Remarkably, even at a high current density of 10 C, the Li-Zn/CF@NH2-UiO-66||LFP cell exhibited stable cycling for 700 cycles with high reversible capacity.

Therefore, thanks to the construction of a 3D internal lithiophilic void structure and the coating of external NH2-UiO-66 nanoparticles, the Zn/CF@NH2-UiO-66 host promotes favorable Li+ desolvation behavior, accelerates ion diffusion kinetics, and guides stable and uniform lithium metal deposition/stripping. This synergistic regulation strategy for ion transport kinetics and lithium deposition behavior provides new insights for the further development of lithium metal anodes.