Zinc oxide nanoparticles intercalated with porous carbon as a separator coating for improving the stability of lithium metal anodes

Abstract

Metal lithium negative electrodes are considered the “holy grail” of lithium battery negative electrodes due to their ultra-high energy density and low overpotential. However, the arbitrary growth of lithium dendrites during the cycling process hindered its industrialization process. We prepared porous carbon doped with zinc oxide nanoparticles (ZNC-MOF-5) by high-temperature carbonization of MOF-5, and coated ZNC-MOF-5 on the surface of commercial membranes (ZNC-MOF-5@PP). Used to improve the cycling stability of metal lithium negative electrodes. Zinc oxide nanoparticles in ZNC-MOF-5 have good lithium affinity and can promote Li⁺ deposition. The porous structure with a high specific surface area endows the electrode with high lithium loading capacity, reduces local current density, and obtains a dendrite-free metal lithium negative electrode. The electrochemical cycling performance of Li/Cu batteries indicates that, ZNC-MOF-5@PP. The separator can prevent the growth of dendrites and improve cycling stability, proving that ZNC-MOF-5 can effectively guide the deposition of Li and solve dendrite problems.

Introduction

Lithium metal cathodes are considered the ideal choice for next-generation batteries, such as lithium–air, lithium–sulfur, and all-solid-state batteries, due to their exceptionally high theoretical capacity (3860 mAh/g) and the lowest potential (−3.04 V relative to the standard hydrogen potential).^1,2 However, practical applications of lithium metal cathodes still face several challenges. ³ These challenges include low coulombic efficiency, significant expansion of the lithium cathode during charge and discharge, and the formation of unstable solid–electrolyte interphase (SEI) film on the cathode's surface. The repeated rupture and repair processes associated with the unstable SEI film lead to the continuous consumption of electrolytes and lithium sources. ³ The root cause of these challenges is believed to be the formation and growth of dendritic structures composed of Li⁺ ions during the stripping and deposition process. Therefore, in-depth research on methods to suppress dendrite formation is crucial for the practical application of lithium metal cathodes. Numerous studies have been conducted to enhance the cycling stability of lithium metal cathodes. These studies involve modifications to electrolytes and the addition of additives, the application of functional protective layers on the surface of lithium metal, and the construction of three-dimensional (3D)-structured collectors.^4–6 Among these approaches, 3D-structured fluid collectors have shown promise in maintaining stable electrode structures due to their high specific surface area and mechanical strength. The use of such collectors helps Li⁺ ions to deposit and peel uniformly, reducing the formation of dendritic structures.^7–9 Chi et al. ¹⁰ developed a Li/Ni composite anode material by immersing lithium metal into a nickel foam host and assembling the composite anode into a liquid symmetric cell. Compared to bare lithium, the composite anode exhibited a stable voltage plateau (200 mV) after 100 cycles at a high current density of 5 mA cm⁻², with no growth of lithium dendrites and low interfacial impedance, indicating higher stability. Li et al. ¹⁰ improved the coulombic efficiency by incorporating a 3D copper foam complex into lithium metal to suppress dendrite formation. Additionally, porous carbon, as a 3D-structured material, possesses excellent mechanical properties, electrical conductivity, and a high specific surface area, which promotes the conductivity of Li⁺ ions and electrons. ¹¹ Furthermore, the deposition of these 3D-structured materials on the surface of lithium metal cathodes to form artificial protective films has been shown to enhance the electrochemical performance. ¹²

However, conventional 3D structural host materials such as carbon exhibit poor compatibility with lithium metal and demonstrate high nucleation overpotentials during lithium nucleation. To address this issue, Cui and his research team have developed selective lithium deposition techniques using lithium-friendly metals or metal oxides as “seeds” for lithium deposition. This approach reduces the overpotential and promotes Li⁺ deposition, consequently inhibiting the formation of lithium dendrites. ¹³ Furthermore, ZnO has been shown to play a similar role. Cao et al. initially prepared a 3D layered porous carbon as the primary component of the lithium cathode and utilized lithium-friendly ZnO quantum dots as an inducer to facilitate lithium infiltration into the primary component, effectively suppressing dendrite growth. Similarly, Wang et al. employed ZnO nano-layers to guide lithium deposition within a 3D porous garnet solid-state electrolyte. ¹⁴ MOF-5, a metal–organic framework with a high specific surface area and large pore volume, has been extensively studied.^15–18 High-temperature calcination results in the formation of a 3D porous carbon structure (ZNC-MOF-5) with a uniform distribution of ZnO.^19,20 This structure facilitates the inhibition of lithium metal dendrite formation.

Directly coating the surface of the lithium cathode with ZNC-MOF-5 is the preferred approach for addressing the issue of dendrite growth in lithium metal batteries. However, surface coating or modification of lithium metal cathodes requires a demanding environment and excellent handling techniques. In contrast, coating the separator with ZNC-MOF-5 provides a simple and effective means to inhibit dendrite growth. In both strategies, ZNC-MOF-5 is positioned between the separator and the lithium cathode and exhibits a similar effect to coating ZNC-MOF-5 onto the lithium surface. In this study, ZNC-MOF-5 containing ZnO was prepared and coated onto the surface of a commercial polypropylene (PP) separator to create a modified separator that enhances the performance of the lithium metal anode (ZNC-MOF-5@PP). Guided by the ZnO “seed,” lithium can be uniformly plated on the surface and inside the channels of ZNC-MOF-5. Li–Cu cells employing ZNC-MOF-5@PP as the separator exhibit stable cycling for over 100 weeks with a coulombic efficiency of up to 95%. Li–S cells utilizing ZNC-MOF-5@PP as the separator show a capacity retention of 76% after 400 cycles at a 0.5 C rate. Therefore, a simple and effective method to inhibit dendrite growth in lithium metal batteries is developed to improve the performance and cycle life of lithium metal batteries.

Preparation of materials

Preparation of M

MOF-5 preparation

A total of 1.662 g of Zn(NO₃)₂·6H₂O and 0.356 g of terephthalic acid (H₂BDC) were accurately measured and placed in a beaker. Subsequently, 40 mL of N,N-dimethylformamide (DMF) solution was added to the beaker, and the mixture was stirred at a rate of 300 r/min until it turned transparent. The resulting mixture was transferred to a polytetrafluoroethylene reactor, which was then sealed and subjected to vacuum drying at 130°C for 4 h. After the reaction was completed, the product was allowed to cool naturally to room temperature. White crystals were obtained by filtration, and they were washed with a DMF solution before being filtered again. The crystals were then dried in a vacuum oven at 60 °C for 72 h.

ZNC-MOF-5 preparation

The dried MOF-5 powder was subjected to sintering in a tube furnace according to the following conditions: (i) the temperature was increased from 50°C to 800°C at a rate of 5°C/min; (ii) the sample was held at 800°C for 4 h; (iii) natural cooling to room temperature was allowed while continuously introducing nitrogen gas to protect the resulting black solid powder, ZNC-MOF-5.

Preparation of C-MOF-5

The synthesized ZNC-MOF-5 was rinsed with an abundant 1 mol/L hydrochloric acid solution to remove the ZnO nanoparticles present in the ZNC-MOF-5, serving as a reference for comparison.

ZNC-MOF-5@PP and C-MOF-5@PP separator preparation

The ZNC-MOF-5 and polyvinylidene fluoride are thoroughly mixed in a mortar, maintaining a mass ratio of 8:2. During the mixing process, an adequate amount of N-methyl-2-pyrrolidone is added until a uniform slurry is formed. The resulting slurry is then dried, and the dried separators are subsequently cut into 16 mm diameter discs using a sheet punch.

Experimental results and discussion

Material characterization analysis

Figure 1(a) illustrates the X-ray diffraction (XRD) pattern of MOF-5, displaying characteristic peaks at 2θ = 6.8°, 9.7°, 13.7°, and 15.4°. These peaks confirm the successful synthesis of MOF-5, consistent with the standard spectrum reported in the literature. Figure 1(b) shows the XRD pattern of ZNC-MOF-5 after high-temperature calcination. Notably, sharp diffraction peaks are observed at 31.7°, 34.3°, 36.2°, and 47.5°. A comparison with the standard spectrum reveals a perfect match with the characteristic peak of ZnO (PDF no. 76-0704), indicating the presence of ZnO in ZNC-MOF-5. Previous studies have demonstrated that ZnO exhibits good lithophilicity and can guide the deposition of Li⁺, offering a potential pathway to achieve dendrite-free lithium metal anodes. After washing with hydrochloric acid, the original ZnO diffraction peaks vanish, indicating the complete removal of ZnO from ZNC-MOF-5. The ZnO nanoparticle content is estimated to be ∼14%. The specific surface area and pore size distribution of ZNC-MOF-5 were evaluated using nitrogen adsorption (Brunauer–Emmett–Teller), as depicted in Figure 2. The analysis reveals a specific surface area of 926.95 m²/g and a pore size distribution of 0.6 nm. The significant surface area facilitates a reduction in the local current density during lithium deposition, promoting the uniform deposition of lithium metal and providing effective protection for the lithium metal anode.

Figure 1.

(a) X-ray diffraction (XRD) pattern of MOF-5; (b) XRD patterns of C and ZNC-MOF-5; thermogravimetric graphs of C-MOF-5 (c) and ZNC-MOF-5 (d).

Figure 2.

Shows the specific surface area of ZNC-MOF-5.

The surface morphology of the samples was examined using field emission scanning electron microscopy (SEM). The crystal morphology of ZNC-MOF-5 was investigated, and Figure 3(a) and (b) reveals an ortho-octahedral structure with a diameter of ∼50 nm. Figure 3(c) displays the SEM image of the pristine separator, demonstrating the presence of numerous holes that serve as pathways for lithium ion transport. Upon coating with ZNC-MOF-5 (Figure 3(d)), the original porous structure of the separator becomes obscured, and a significant amount of ZNC-MOF-5 adheres to the surface of the PP separator. The thickness of the ZNC-MOF-5 coating is estimated to be around 10 µm (Figure 3(e)). Transmission electron microscopy (TEM) image confirms that zinc oxide agglomerates were inside the carbonized MOF-5 (Figure 3(f)).

Figure 3.

(a, b) SEM of ZNC-MOF-5; (c) SEM of the original separator; (d) SEM of coated ZNC-MOF-5; (e) cross-sectional view of ZNC-MOF-5 coating; (f) TEM of ZNC-MOF-5.

SEM: scanning electron microscopy; TEM: transmission electron microscopy.

Analysis of electrochemical properties of the composite materials

To investigate the impact of ZNC-MOF-5 on Li⁺ deposition, we constructed Li–Cu batteries using 1 M lithium bis(trifluoromethanesulfonyl)imide/dimethoxyethane as the electrolyte. Three types of separators were employed: conventional PP separators, C-MOF-5@PP separators, and ZNC-MOF-5@PP separators. The separators were positioned with their coating facing the Cu side, and the results are depicted in Figure 4(a) to (d). Notably, the coated separators exhibited higher coulombic efficiencies compared to the PP separators. Specifically, at a current density of 0.3 mA/cm², the coulombic efficiencies were 92.6% for PP, 97.6% for C-MOF-5@PP, and 98.9% for ZNC-MOF-5@PP. This improvement can be attributed to the high specific surface area of C-MOF-5 and ZNC-MOF-5, which reduces the effective current density, leading to a more uniform deposition of Li⁺. Moreover, during the initial discharge curve at a current density of 0.5 mA/cm², the incorporation of ZnO significantly diminished the overpotential. This suggests that the addition of lithium-friendly ZnO can guide the deposition of Li⁺ and prevent dendrite formation in the anode. To further demonstrate the enhanced cycling stability of lithium metal cathodes with ZNC-MOF-5@PP, we conducted cycling tests on Li–Cu cells using the three separators. The current density was set at 0.5 mA/cm², and the capacity was set at 1 mAh/cm². The results are presented in Figure 4(e). We observed that the conventional separator exhibited low Coulomb efficiency, with a short circuit occurring after 25 cycles. However, applying a layer of C-MOF-5 to the PP separator significantly improved cycling stability, extending the cycle time to 70 cycles. This improvement can be attributed to the reduced local current density resulting from the high specific surface area of C-MOF-5 itself. Nevertheless, due to the poor compatibility between carbon materials and lithium metal, a uniform deposition was not achieved, leading to rapid deterioration of the cell after 70 cycles and eventual short-circuiting. In contrast, ZNC-MOF-5 maintains the high specific surface area of C-MOF-5 while also benefiting from the presence of lithium-friendly ZnO. This combination enables the uniform deposition of Li⁺ and ultimately results in good cycle stability and high Coulomb efficiency.

Figure 4.

(a) Electrochemical curves of pure Li–Cu cell at different currents; (b) charge and discharge curves of negative electrode with added C-MOF-5 at different currents; (c) charge and discharge curves of ZNC-MOF-5 at different currents; (d) lithium deposition potential diagram for different negative electrodes; (e) cycle testing of Li–Cu cells with three separators.

To assess the impact of functionalized separators on full-cell performance, Li–S cell tests were conducted. The charge/discharge curves are presented in Figure 5(a), revealing discharge capacities of 868 mAh/g for PP separators, 1130 mAh/g for C-MOF-5@PP separators, and 1158 mAh/g for ZNC-MOF-5@PP separators at a magnification of 0.2C. These high capacities can be attributed to the protective nature of the separator coating, which prevents irreversible reactions of soluble LiPSs with the lithium metal cathode. The cyclic voltammetry (CV) curves of Li–S cells using ZNC-MOF-5@PP separators are depicted in Figure 5(b). During the reduction scan, two peaks emerge at 2.29 and 2.03 V, corresponding to the conversion of S₈ to soluble LiPSs and further reduction of the LiPSs to insoluble Li₂S₂/Li₂S. In the subsequent oxidation scan, two adjacent peaks are observed at 2.37 and 2.43 V, corresponding to the oxidation of Li₂S₂/Li₂S to monomeric sulfur. Importantly, the CV curves exhibit excellent overlap from the second cycle onwards, indicating the outstanding cycling stability of cells utilizing the ZNC-MOF-5@PP separator. Figure 5(c) presents the rate capability of the ZNC-MOF-5@PP separator cell. The reversible specific capacities of the ZNC-MOF-5@PP cells are 1158, 924.3, 830, and 728 mAh/g at magnifications of 0.2, 0.5, 1.0, and 2.0 C, respectively. The corresponding curves are illustrated in Figure 5(d). After 400 cycles, the capacity retention rate for the ZNC-MOF-5@PP separator reaches 76%, which is significantly higher than the retention rates of the C-MOF-5@PP separator (67%) and the PP separator (41%). In the SEM image (Figure 6), it can be observed that the ZNC-MOF-5 modified diaphragm presents smooth characteristics on the negative surface of the Li–S battery, and there is no obvious dendrite phenomenon. This indicates that ZNC-MOF-5 has the ability to inhibit the shuttle effect, which is conducive to maintaining the stability of the interface layer, thereby extending the cycle life of the battery. These results clearly demonstrate the ability of the ZNC-MOF-5@PP separator to effectively protect the lithium metal anode and achieve excellent cycle stability in Li–S batteries.

Figure 5.

(a) Charge and discharge curves for polypropylene (PP), C-MOF-5@PP, and ZNC-MOF-5@PP separators; (b) CV curves of Li–S cells using ZNC-MOF-5@PP separator; (c) rate capability of ZNC-MOF-5@PP separator cells; and (d) cycle curves for three different separator cells.

PP: polypropylene; CV: cyclic voltammetry.

Figure 6.

(a) SEM of the anodes after cycling for PP, (b) C-MOF-5@PP, and (c) ZNC-MOF-5@PP separators.

SEM: scanning electron microscopy; PP: polypropylene.

Conclusion

In conclusion, this study involved the preparation of porous carbon with zinc oxide nanoparticles (ZNC-MOF-5) by utilizing the classical organometallic complex MOF-5 as a precursor system. The resulting ZNC-MOF-5 was coated onto conventional PP membranes to create functional ZNC-MOF-5@PP membranes. The 3D porous structure with a high specific surface area enables the ZNC-MOF-5 to serve as a lithium guide and lithium storage site, effectively preventing the formation of dendrites during lithium deposition. The Li–Cu battery employing the ZNC-MOF-5@PP separator demonstrated a coulombic efficiency above 95% after 100 cycles, when operated at a current density of 0.5 mA/cm² and a capacity of 1 mAh/cm². Additionally, the Li–S battery utilizing the ZNC-MOF-5@PP separator exhibited an initial discharge capacity of 924.9 mAh/cm² at a 0.5C rate, with a capacity retention rate of 76% after 400 cycles. It is important to highlight that the ZNC-MOF-5@PP separator presents a novel approach to effectively inhibit the formation of dendritic structures in lithium metal anodes.