Grafting polyethyleneimine on electrospun nanofiber separator to stabilize lithium metal anode for lithium sulfur batteries

https://doi.org/10.1016/j.cej.2020.124258Get rights and content

Highlights

  • APANF separator is fabricated by general electrospinning process and functional group grafting.

  • Li ion distribution can be well regulated due to the nanofiber structures and the polar groups on APANF.

  • APANF separator can simultaneously suppress Li dendrite formation and polysulfides shuttling.

Abstract

High-energy-density lithium sulfur (Li-S) batteries are suffering several seemingly insurmountable barriers, including lithium dendrite formation and polysulfides shuttling. Functional separator, which bridges anode, electrolyte and cathode together, has the potential to offer a perfect solution to these concerns. Herein, we develop a functional ammoniated polyacrylonitrile (PAN) nanofiber separator (APANF) which can simultaneously inhibit Li dendrite formation and polysulfides shuttling. Branched polyethyleneimine (PEI) are fixed on the electrospun PAN nanofiber mat via a chemical grafting to provide amino groups. Such strongly polar separator can well regulate the uniform Li ions distribution and induce the formation of the Li3N-rich SEI layer, resulting in an interesting 3D spherical and dendrite-free Li deposit pattern. The coulombic efficiency of resulting Li anode can be improved up to 98.8% with a low overpotential of 15 mV. Meanwhile, the separator can also serve as a block for polysulfides shutting due to the strong chemical adsorption capability of PEI, thus facilitating the capacity retention of sulfur cathode. This work provides an easy and scalable alternative to conventional polyolefin separators for solving problems in both anode and cathode of Li-S battery.

Introduction

Development of advanced energy-storage systems for electrical vehicles and grid storage must fulfill the requirements of high energy density and long cycle lifespan [1]. As the traditional Li-ion batteries are approaching the power limits, lithium-sulfur (Li-S) batteries have provoked an animated tide of research and become one of the most potential candidates for the next-generation energy-storage devices [2], [3]. However, both anode and cathode are facing a lot of issues which hinder the practical applications of Li-S batteries.

Lithium metal anode possesses the highest theoretical capacity (3860 mAh g−1) and lowest potential (−3.04 V vs. standard hydrogen electrode) [4]. However, dendrite formation is the most formidable challenge for the lithium metal anode because the risks of internal short circuit and rapid capacity fading. During repetitive plating/stripping of lithium, the uncontrolled growth is unavoidable due to the huge volumetric expansion, ununiform Li ions distribution and unstable solid electrolyte interface (SEI) layer [5]. Therefore, enormous efforts have been developed to mitigate the irregular growth of lithium metal against the possible reasons above. Modifying SEI layer via adjusting the composition of liquid electrolyte [6], [7] or adding functional additives, [8], [9], [10] introducing “artificial SEI” layer [11], [12] or polar interlayer [13], using solid / gel electrolytes [14], [15], [16], constructing composite lithium anode with a host [17], [18], [19], and employing three-dimensional current collectors [20], [21], [22], have alleviated the dilemma of Li dendrite growth in some ways.

Meanwhile, the sulfur cathode, with a high capacity of 1675 mAh g−1, is also plagued by the insulation nature of sulfur and discharge product Li2S, and the “shuttle effect” of the soluble polysulfides intermediates [23], [24]. In order to improve the performance of sulfur cathode, on the one hand, conductive hosts such as carbon [25], [26], [27], [28] or conductive polymers [29] are used to promote the conductivity of the whole cathode. On the other hand, a mass of method including various encapsulation of sulfur [30], [31], chemical adsorption or electrocatalysis of polysulfides [32], [33], [34], or insertion of interlayer [35], [36], [37], selective ion separator [38] and other polar separator [39], [40], [41] as a functional block, have been studied to mitigate the “shuttle effect”. However, major of present researches are only for either anode or cathode, which can hardly simultaneously suppress both dendrite growth and polysulfides shuttling.

Separator which plays a key role in ion transport and influences rate performance, cell life and safety, is given many requirements [42]. Besides the basic demands of mechanical/electrochemical stability and wettability of electrolytes, functional separators are expected to be able to obstruct polysulfides and provide regular pathways for Li ions. The most common strategy is direct surface modification of traditional polyolefin (PP or PE) separators by coating or dipping with polar materials to enhance the properties of the whole separator [43], [44]. However, on account of the strongly non-polar nature of polyolefin materials, the coating layer usually exhibits poor adhesion with the polyolefin matrix of separator, especially against the repetitive charge/discharge. Besides, by simple coating, the polyolefin separators can hardly promote the thermal stability, still suffering from irreversible crispation or deformation at high temperature. Also, some alternative polymer separators, such as polyvinylidene fluoride (PVDF), chitin, poly (methyl methacrylate) (PMMA), polyacrylonitrile (PAN) and etc., are studied [45], [46], [47]. Typically, electrospinning and solvent evaporation are used to fabricate these separator membranes, which contributes to a series of channels or pathways for Li ion transportation. The three-dimensional membranes with abundant polar groups can improve the uptake of electrolyte, forming a reservoir for Li+, which can regulate the Li+ flux.

Herein, we demonstrate a solution to simultaneously suppress lithium dendrite formation and polysulfides shuttle using an ammoniated polyacrylonitrile nanofiber separator (defined as APANF, illustrated in Fig. 1). PAN is believed to be a suitable polymer for battery separator application, due to the easy processability and excellent resistance to oxidative degradation. To give the multifunctionality, polyethyleneimine (PEI) are further fixed on the PAN via a chemical grafting. With the cross-linked nanofiber structure and the polar surface groups, Li ion distribution could be well regulated, leading to more uniform 3D spherical Li deposition pattern. Meanwhile, Li3N-rich solid electrolyte interface (SEI) layer can be realized, further inhibiting the Li dendrite formation. With such a separator, the Li anode could deliver a high of coulombic efficiency of 98.8% and a low overpotential of 15 mV. On the other hand, APANF also has strong chemical adsorption ability towards polysulfides, therefore effectively blocking the shuttling effect. As a result, the bifunctional APANF can assist the Li-S cell achieving better cycling stability and extended life span. Moreover, APANF can also extend to other Li-metal batteries like Li/NCM523 with carbonate-based electrolyte. This work provides new alternative to conventional polyolefin separators for helping meet the greater demands for the development of practical energy storage devices.

Section snippets

Materials

Polyacrylonitrile (PAN) (Mw = 150 000, Sigma Aldrich), polyethyleneimine (PEI) (branched, Mw = 25 000, Sigma Aldrich), dimethyl formamide (DMF) (Adamas), ethylene glycol (Greagent).

Preparation of PANF

PANF separator was fabricated by electrospinning with PAN/DMF solution. To obtain the electrospinning solution, 1.6 g PAN was dissolved into 18.4 g DMF, followed by stirring at 60 °C for 6 h. The electrospinning solution was then loaded into a 20 mL plastic syringe equipped with a 21-gauge stainless steel needle with

Results and discussions

The APANF separator is fabricated via two steps including electrospinning and grafting as Fig. 2a shows. Free-standing PAN mat is firstly obtained via the electrospinning (Fig. S1) and then react with PEI in 140 °C oil bath with refluxing process. Branched PEI (bPEI) as a common macromolecular initiator has reactive end groups on the one of the ends. At 140 °C, the cyano groups (single bondCtriple bondN) on PAN will open, and the reactive end groups on bPEI can trigger free radical reaction on the main chain of PAN

Conclusion

In summary, PEI grafted PAN nanofiber separator is demonstrated to play a role in both enabling dendrite-free lithium plating on anode and suppressing shuttling of polysulfides from sulfur cathode. The strong polar ammonia groups provided by PEI branches improve the affinity with electrolyte and uniform the Li ion distribution. And the free ammonia groups can contribute to the Li3N-rich SEI layer formation, thus leading to a spherical morphology of lithium deposition. As a result, with APANF

Declaration of Competing Interest

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

Acknowledgements

This work was partly supported by National Natural Science Foundation of China (No. 21878091 and No. 21576090), China, and Fundamental Research Funds for the Central Universities (222201718002), China.

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