Elsevier

Carbon

Volume 201, 5 January 2023, Pages 285-294
Carbon

Magnetron sputtering engineering of typha-like carbon nanofiber interlayer integrating brush filter and chemical adsorption for Li–S batteries

https://doi.org/10.1016/j.carbon.2022.09.020Get rights and content

Highlights

  • Typha-like interlayer serves as both membrane and brush filters.

  • A 7.9-fold exposure of active sites ensure efficient anchoring-conversion effect.

  • An ultralight flexible interlayer was constructed by a scalable strategy.

Abstract

Enhancing the adsorption and catalytic conversion of polysulfides is critical in improving the cycling stability of lithium–sulfur (Li–S) batteries. Herein, an ultralight typha-like carbon nanofiber (CNF) based interlayer (MoS2/Al2O3@CNF) is constructed by scalable electrospinning and eco-friendly magnetron sputtering, to act as a “police constable” between the cathode and separator. The co-sputtering of MoS2 and Al2O3 endows the interlayer with both chemical adsorption and catalytic conversion active sites. Besides, the typha-like nanofibrous interlayer serves as both membrane and brush filters, leading to more efficient polysulfide blocking. The structural superiority of this interlayer compared to the CNF membrane results in a 693.8% higher specific surface area of 254 m2 g−1. In addition to these benefits, the unique structure allows for a 6.9-fold increase in exposure of active sites. The cell with such an interlayer delivers stable long-term cycling performance with an ultralow capacity decay rate of 0.035% per cycle over 1000 cycles at 0.5C. The long-term cycling performance can be attribute to the structural stability of the typha-like nanofibrous membrane, as evidenced by the postmortem SEM images. This work provides a valuable strategy for designing functional interlayers toward the long-term cycling stable Li–S batteries.

Introduction

With the continual development of electronic equipment and electric vehicles, several studies have proposed energy-storage equipment with high energy densities [[1], [2], [3]]. Lithium–sulfur (Li–S) batteries have a high theoretical energy density of 2600 Wh kg−1 and a theoretical specific capacity of 1675 mAh g−1, which is approximately 3–5 times that of a lithium-ion battery [[4], [5], [6]]. In addition, sulfur cathode material has the advantages of being derived from abundant natural resources while being inexpensive and having low toxic characteristics [[7], [8], [9]]. Li–S batteries have received considerable attention as candidates for next-generation high-energy-density batteries. However, some drawbacks limit their further development, including the insulation property and sluggish conversion of sulfur [10], the volumetric expansion of the cathode material during cycling [11], and the shuttle effect of the soluble polysulfides [12,13]. All these result in a rapid decay of the battery capacity and low utilization of the active material [14].

Many strategies have been developed to address these issues [15,16]. Most studies have focused on the design of electrode materials toward creating 3D structural conductive porous hosts for active materials, which could enhance the electrochemical performance by morphology control and structural engineering [17,18]. Or combining sulfur with carbon materials such as carbon spheres [19], carbon nanotubes [20,21], and graphene [22] or conductive polymers such as PANI [23] and PPY [24], which can compensate for the poor electrical conductivity of sulfur. However, the lower relative content of encapsulated sulfur in the cathode greatly offsets the advantages of Li–S batteries in terms of high energy density. Constructing a functional interlayer between the cathode and separator to suppress the shuttle effect is a feasible strategy for Li–S batteries [25].

Carbon nanofiber membranes as lightweight freestanding interlayers can be used as physical barriers to inhibit the shuttle effect of soluble polysulfides [26]. Although nanofiber membranes have good filtration applications, brush filters appear to be more promising than membrane filters in terms of filtration efficiency [27,28]. However, non-polar carbon nanofiber membranes with mechanical (steric) filtration can not achieve effective blocking of short-chain polysulfides. Chemical adsorption is another strategy for anchoring polysulfides efficiently. Designing a brush filtration interlayer with chemical adsorption sites is promising for inhibiting the shuttle effect.

Previous studies have reported that metal oxides (Al2O3, TiO2) exhibit a strong adsorption ability toward polysulfides [29,30]. However, the lack of catalytic active sites in metal oxides prevents the fast conversion of sulfur species, and the accumulation of polysulfides inevitably causes sluggish reaction kinetics, resulting in polarization. Metal sulfides (SnS2, MoS2) with efficient catalytic performance have been regarded as promising polar materials for accelerating the redox kinetics of polysulfides [31,32]. Co-modification of metal oxides and metal sulfides may be an effective strategy for functionalizing the interlayer to achieve effective inhibition and fast conversion of polysulfides [33]. However, common separator–modifying methods that include a large amount of binder can obstruct membrane pores, resulting in low ionic conductivity and creating an obvious dilemma in terms of application [34,35].

As an efficient physical vapor deposition (PVD) technique, magnetron sputtering has been widely used to prepare metal, alloy, and semiconductor materials due to its advantages of high efficiency, low temperature, good compactness, and excellent adhesion to substrates [36]. In addition, magnetron sputtering provides the possibility of constructing an interlayer with membrane and brush filtration functions by structural control.

In this study, scalable electrospinning was conducted to fabricate the precursor of a carbon nanofiber membrane. Following calcination, a simple green technique of magnetron sputtering was employed to decorate Al2O3 and MoS2 particles on the CNF surface for designing a typha-like nanofiber membrane (MoS2/Al2O3@CNF). Co-deposition of Al2O3 with chemical adsorbability and MoS2 with catalytic property can mitigate the shuttle effect of polysulfides and facilitate the conversion of sulfur species. Besides, the typha-like structural interlayer provides an effective membrane and brush filtration dual-effect. Due to the ultrahigh specific surface area, more adsorption sites in Al2O3 and catalysis sites in MoS2 could be exposed. When the brush-membrane filtration is combined with strong chemical interactions, the effective capture and conversion of polysulfides is performed by a “police constable.” In addition, an interlayer with a unique structure can serve as an electrolyte stockroom to promote ion transport. More importantly, the interlayer modifying method overcomes the issues of increased membrane thickness and particle agglomeration. Together, these functionalized nanofibrous interlayers with typha-like structures show great potential for enhancing Li–S battery cycling stability.

Section snippets

Preparation of the CNF membrane

First, a CNF membrane was fabricated using scalable electrospinning (Fig. 1). Here, 12 wt% PAN (Mw = 150000, Aldrich) was dissolved in N, N-dimethylformamide (DMF, CP, Sinopharm Chemical Reagent Co., Ltd.) and stirred over night at room temperature. The PAN solution was extruded and then drawn into fibers under a voltage of 20 kV. The distance between the needle and the collector was approximately 20 cm. Finally, the as-spun nanofiber membrane (approximately 22.9 μm) obtained at the collector

Results and discussion

A schematic for fabricating a typha-like MoS2/Al2O3@CNF interlayer and an interlayer used in the Li–S battery are shown in Fig. 1. As previously illustrated, a scalable electrospun PAN nanofiber membrane was calcined to prepare the CNF membrane. Magnetron sputtering was conducted to construct a typha-like nanofiber membrane by co-sputtering MoS2 and Al2O3 on the CNF surface. The interlayer was inserted between the cathode and separator to act as a “police constable,” trapping polysulfides and

Conclusions

In this study, a flexible conductive carbon nanofiber interlayer decorated with MoS2/Al2O3 was engineered by magnetron sputtering. The interlayer with good compatibility between the separator and cathode acted as a “police constable” to trap polysulfides. Due to the typha-like structure, abundant exposed adsorption and catalytic sites in the MoS2/Al2O3@CNF ensured effective adsorption and fast conversion function. When membrane and brush filtration were combined, efficient inhibition of the

CRediT authorship contribution statement

Shuanglin Wu: Methodology, Investigation, Visualization, Writing – review & editing, Writing – original draft. Xiaolin Nie: Methodology, Investigation. Zhihui Wang: Visualization. Zhifeng Yu: Writing – review & editing. Fenglin Huang: Conceptualization, Writing – review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no competing financial interests or personal relationships that could influence the work reported in this paper.

Acknowledgements

We are grateful to the Jiangsu Key R&D Program (BE2017060, BE2016707), the China Postdoctoral Science Foundation (169483), the 111 Project (B17021), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX22_2331).

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