Research Article
Difunctional NH2-modified MOF supporting plentiful ion channels and stable LiF-rich SEI construction via organocatalysis for all-solid-state lithium metal batteries

https://doi.org/10.1016/j.jmst.2022.07.017Get rights and content

Highlights

  • Difunctional polar-group modified MOFs were firstly synthesized for composite solid electrolytes.

  • PEO-ZIF-NH2 with LiF-rich SEI exhibits enhanced cycling performance, which was 3.8 times longer than that of PEO-ZIF-CH3.

  • The formation mechanism of LiF-rich-SEI was investigated using first-principles calculations.

Abstract

As an essential part of the performance improvement of lithium metal batteries, the acquisition of dense (LiF-rich solid electrolyte interphase (SEI)) has always been an urgent problem to be solved. Herein, we synthesized Zeolitic Imidazolate Frameworks (ZIFs) modified by two different functional groups (–NH2, –CH3) and used them as the fillers of polyethylene oxide (PEO) composite solid electrolytes to explore the catalytic effect of groups on LiF generation at the Li/electrolytes interface. In a LiFePO4||SPE||Li cell test, the PEO-ZIF-NH2 with LiF-rich SEI exhibits enhanced cycling performance, which was 3.8 times longer than that of PEO-ZIF-CH3. The formation mechanism of LiF-rich SEI was investigated using first-principles calculation, revealing that ZIFs-NH2 makes the C–F bond in TFSI longer compared with ZIFs-CH3, which leads to easier breakage of the C–F bond and promoted the formation of LiF. The simple design idea of using organic catalysis to generate more stable SEI provides a new aspect for preparing high-performance lithium metal batteries.

Introduction

Energy storage equipment is a crucial technology of human society. Lithium secondary battery has many advantages such as high working voltage, long cycle life, and small environmental pollution [1]. However, the energy density of lithium secondary batteries (< 200 Wh kg–1) does not well match the actual requirement of electric vehicles [2]. Besides, the lithium dendrite growth and leakage of organic electrolytes have brought a wide range of safety issues. All-solid-sate lithium metal batteries have attracted significant attention due to their high safety and energy density [3], hence, it has been counted on to replace the organic liquid electrolytes with low thermal stability and low flame point [4]. Therefore, superseding traditional organic electrolytes with solid-state-electrolytes (SSEs) is a matter of great urgency to contemporary research [5].

Solid polymer electrolytes (SPEs) exhibit higher flexibility, better machining performance, and better compatibility with lithium compared with inorganic solid electrolytes [6]. However, two major challenges hinder the practical application of metal lithium in solid electrolytes, viz., low ionic conductivity and Li dendrite growth [7]. Significantly, the formation of the LiF-rich solid electrolyte interphase (SEI) is challenging [8]. Goodenough and co-workers [9] reported that the inhomogeneities at the Li/solid electrolyte interface can induce an irregular lithium plating for dendrite formation. In addition, unstable dendrite growth will lead to volume expansion of lithium metal anode, affecting the interface contact of electrode electrolytes and long-cycle stability. Zheng's group [10] prepared a solid electrolyte by adding Li3PS4 into a polyvinylidene difluoride (PVDF)-based polymer to form a protective layer rich in Li2S, which showed an excellent electrochemical performance. Although bulk LiF is an electron and ion insulator (from 10–13 to 10–14 S cm–1), the LiF-rich SEI can produce huge advantages related to Li+ ion transport and improve the electrochemical performance of the cell [11]. Therefore, it is an effective strategy to inhibit the nucleation of lithium dendrites and reduce side reactions by constructing artificial LiF-rich SEI membranes to optimize the composition and improve ionic conductivity [12].

Metal-organic frameworks (MOFs), as organic-inorganic hybrid porous crystalline materials [13], present the designability and regulation of structure and function [14]. Ideally, the functional MOFs can be synthesized by properly designing ligands and selecting secondary building units of metal ions [15]. Different types of MOFs have been prepared in the past few years and have critical applications in hydrogen storage, gas adsorption and separation sensors, drug slow-release catalytic reactions, and other fields [16], [17], [18]. MOFs have inherent characteristics and significant advantages in catalysis research applications [19]. Especially, the abundant C, N, H, and O elements in the ligands are indispensable elements in most organic catalytic systems. In addition, MOF-derived porous materials have high porosity, homogeneous doping of heterogeneous atoms, and adjustable morphology, which may be beneficial to promoting the generation of LiF-rich SEI.

Herein, a simple design idea is proposed to construct a stable LiF-rich SEI. From the organic molecular catalysis, two kinds of ZIFs were synthesized with different groups modified (ZIFs-NH2 and ZIFs-CH3). polyethylene oxide (PEO) composite solid electrolyte was prepared as fillers to realize the formation of LiF-rich SEI. Consequently, the PEO-ZIFs-NH2 exhibits better electrochemical properties with high ionic conductivity and stable cycling performance. Compared with the van der Waals force, the hydrogen-bond interaction between –NH2 and PEO chains is more powerful which could interrupt the ordered arrangement of PEO and reduce the crystallinity of PEO. Moreover, the organic catalysis interphase between the Li metal and the electrolyte by ZIFs-NH2 prevented more side reactions and stabilized the plating/stripping of lithium metal at the electrolyte/Li interface, thus improving the cycling performance of the SPE cells. In brief, the modification of ZIFs opens a new idea to efficiently catalyze the formation of LiF-rich-SEI to improve the electrochemical performance of batteries.

Section snippets

Description of chemicals

Zinc acetate (Zn(CH3COO)2·2H2O, 99%), 2-methylbenzimidazole (98%, Macklin), 2-aminobenzimidazole (97%, Macklin), poly(ethylene oxide) (PEO, Mw ≈ 1 × 106, Aladdin), LiTFSI (99%, Aladdin), N,N-dimethylformamide (DMF, ≥ 99.5%), acetonitrile (> 99%, Aladdin), methanol anhydrous (≥ 99.8%, Heng xing) were adopted. All chemicals were used without further purification.

Sample and SPEs preparation

0.01 mol Zn(CH3COO)2·2H2O dissolved absolutely in 27 mL DMF and 0.04 mol 2-methylbenzimidazole dissolved absolutely in 27 mL DMF were

Results and discussion

Fig. 1(a, b) shows the synthesis routes of ZIFs and their action mechanism with PEO chains. It can be seen clearly from Figs. S1 and S2(a) in the Supplementary Material that the as-synthesized ZIFs-NH2 and ZIFs-CH3 particles are nearly layered nanosheets with a diameter of less than 500 nm. Fig. S1(b–f) represents the SEM and the energy dispersive spectrum (EDS) of the PEO-ZIFs-NH2 membrane. The ZIFs particles disperse uniformly on the flat surface. The distribution is corresponding to the Zn

Conclusion

In summary, we discussed the interaction between fillers and the PEO interface, and the efficient catalytic generation of LiF-rich-SEI by introducing different functional groups on ZIFs. The results show that the –NH2 group could improve the ionic conductivity of SPEs. The strong hydrogen bond interaction between a large number of –NH2 groups and ether oxygen on PEO segments can interrupt the regular arrangement of PEO and reduce the crystallinity of PEO. In the process of cycles, compared with

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 11872054), the Natural Science Foundation of Hunan Province (Nos. 2020JJ5530, 2020JJ2026, and 2021JJ30643), and the Science and Technology Innovation Project of Hunan Province (No. 2018RS3091).

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    These authors contributed equally to this work.

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