Date Published: February 23, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Shuxing Wu, Kwan San Hui, Kwun Nam Hui.
Black phosphorus (BP) is rediscovered as a 2D layered material. Since its first isolation in 2014, 2D BP has triggered tremendous interest in the fields of condensed matter physics, chemistry, and materials science. Given its unique puckered monolayer geometry, 2D BP displays many unprecedented properties and is being explored for use in numerous applications. The flexibility, large surface area, and good electric conductivity of 2D BP make it a promising electrode material for electrochemical energy storage devices (EESDs). Here, the experimental and theoretical progress of 2D BP is presented on the basis of its preparation methods. The structural and physiochemical properties, air instability, passivation, and EESD applications of 2D BP are discussed systemically. Specifically, the latest research findings on utilizing 2D BP in EESDs, such as lithium‐ion batteries, supercapacitors, and emerging technologies (lithium–sulfur batteries, magnesium‐ion batteries, and sodium‐ion batteries), are summarized. On the basis of the current progress, a few personal perspectives on the existing challenges and future research directions in this developing field are provided.
Since Novoselov et al. exfoliated graphene from graphite via the mechanical cleavage method in 2004, 2D materials have attracted intensive interest.1, 2, 3 Graphene is a 2D single layer of sp2‐bonded carbon atoms that are densely packed in a honeycomb crystal lattice that has a series of unexpected chemical and physical features, such as remarkably high electron mobility at room temperature (15 000 cm2 V−1 s−1),4 strong mechanical strength (≈1 TPa),5 excellent optical transparency (≈97.7%),6 intriguing thermal conductivity (4.84 × 103–5.30 × 103 W m−1 K−1),7 and large theoretical specific surface area (SSA ≈ 2630 m2 g−1).8 Numerous laboratory results demonstrate the potential of graphene in transforming the landscape of current electrochemical energy storage devices (EESDs).9, 10 The unprecedented properties of graphene have led to massive research efforts on other 2D materials for this application, such as transition‐metal oxides/hydroxides,11, 12, 13, 14, 15 transition‐metal dichalcogenides (TMDs),2, 16, 17 hexagonal boron nitride (h‐BN),18, 19 and transition‐metal carbides and nitrides (MXenes).20, 21, 22, 23 In a 2D material, the atomic organization and bond strength along the two dimensions are analogous and significantly stronger than those in a third dimension.23, 24 Its physicochemical characteristics are different from those of the bulk counterpart, and two well‐established allotropes, namely, single layer and few layers, are involved.16, 25 The following striking properties make the 2D material a predominantly promising material for EESDs: (1) its large lateral size and ultrathin characteristic endow it with ultrahigh SSA and high ratios of exposed surface atoms, thereby making it an ideal platform for energy storage;26 (2) its “all‐surface” nature offers an opportunity to engineer properties tailored by surface treatments;16 (3) it can intercalate ions and store energy in the 2D channels among nanosheets through the rapid ion adsorption mechanism;11, 27 (4) it can serve as a building block for various hybrid and hierarchical nanostructures from zero dimension to three dimensions, because no single material can perfectly fulfill the rigorous requirements of EESDs;28 and (5) its atomic thickness offers maximum mechanical flexibility and high packing density, thereby making it promising for developing highly flexible EESDs.26, 29
The reliable production of 2D BP with uniform size is significant for exploring its structural and physiochemical properties and its potential applications. Driven by the interesting properties and promising applications of 2D BP, concerted research efforts have been dedicated to developing various synthetic strategies for fabricating 2D BP. Reliable preparation methods, such as mechanical cleavage, liquid exfoliation, and chemical synthesis, have been explored to produce 2D BP for fundamental and applied research. All methods can generally be divided into top‐down and bottom‐up approaches. The top‐down method typically uses mechanical force or chemical intercalation to break the weak van der Waals bonding among stacked layers to obtain mono‐ or few‐layer nanosheets from bulk BP. The bottom‐up approach relies on the direct synthesis of 2D BP from different molecule precursors via chemical reactions. In this review, we summarize the current methods used for fabrication along with highlights of their advantages and disadvantages (Figure6).
Unlike most 2D materials studied to date (graphene, TMDs, and h‐BN), which are stable under ambient conditions, BP exhibits air instability.3, 16 Bulk BP is stable at atmospheric conditions for a few months, but exfoliated BP shows a relatively high reactivity and air instability.86 Small bumps can be seen on the surface of the BP flakes (five layers) shortly after exfoliation in ambient conditions (Figure12a). After a few days, the BP flakes degraded, and large droplets were observed (Figure 12b). The degradation of BP can be quantified by Raman spectroscopy. The Raman peak intensity decreased gradually after continuous exposure in air (Figure 12c).97 Similarly, the degradation of BP can be revealed by atomic force microscopy (AFM). Figure 12d shows the AFM images of the mechanically exfoliated BP samples (thicker than 150 nm) after exposure in ambient conditions at different times. No bubbles were observed for the BP samples shortly after sample fabrication. After exposure to ambient conditions for 1 d, bubbles appeared on the BP surface. After 7 d, the bubbles on the samples coarsened to form large bubbles. A theoretical study showed that BP has a strong dipolar moment out of plane, which endows it with a strongly hydrophilic characteristic.144 Researchers have attributed the presence of droplets on the surface of the exfoliated BP to adsorbed water.145 Island and co‐workers146 studied the water condensation on the surface of exfoliated BP (initial thickness from 8 nm at its thinnest part to 30 nm at its thickest part). Water droplets already formed on the surface after 3 h, and water completely covered the flake after 5 d, which in turn led to a large convex meniscus (Figure 12e). The height across the flake is more than doubled over the test period (Figure 12f). The volume increased at a rate of ≈7 µm3 min−1 in the first 15 h and then increased at a rate of ≈2 µm3 min−1 after 60 h (Figure 12g). As a result of water absorption, a volume increase of more than 200% was observed after 5 d. The results suggest that the thinner flakes absorbed water faster than the thicker ones, and the appearance of a significant amount of oxygen atom was preferentially localized in the thinnest parts of the BP flake. Marcus–Gerischer theory explains the thickness dependence of exfoliated BP reactivity in terms of electronic confinement.147 When a thin sample is synthesized, the band gap shifts toward high energies and close to the energy levels of oxygen acceptor states, thereby strongly enhancing the rate of charge transfer and hence the oxidation rate.97 Abellán et al.148 also demonstrated that BP degradation is enhanced with the decreasing thickness of the flakes. They also found that lateral dimensions could influence the environmental instability of 2D BP. For example, the environmental degradation of a 2 µm2 flake proceeds twice as fast as that of a 7 µm2 flake. Absorbed moisture has two adverse effects on 2D BP: (i) physical changes, such as volume expansion and uneven surfaces, and (ii) chemical changes toward a liquid phase, which eventually disappears from the surface.149
Although the exact degradation mechanism of BP has not been explained, the rapid and universal degradation upon exposure to ambient conditions is an invariable issue encountered in 2D BP manipulation. At this point, effective methods should be developed to reduce or eliminate degradation. To date, four strategies, namely, encapsulation, functionalization, liquid‐phase surface passivation, and doping, were actively investigated to passivate the exfoliated 2D BP.
The fundamental air stability properties of 2D BP make it a strong candidate for numerous applications. Nevertheless, 2D BP is considered an electrochemically active material for the energy storage mechanism (Figure15), ranging from hosting ions (such as Li+, Mg+, or Na+ in metal‐ion batteries) to accumulating electrostatic charges on the surface (as in electric double‐layer capacitors (EDLCs)), as shown in Table3.
As a new member of the 2D layered material family, monolayer BP and FLP provide many opportunities for investigating fundamental phenomena and practical applications. In this review, we discussed the preparation, structure, and fundamental properties of 2D BP. The use of 2D BP in batteries and SCs is reviewed. High‐quality 2D BP can be prepared successfully through mechanical cleavage, liquid exfoliation, and electrochemical exfoliation. Extensive experimental and theoretical studies demonstrated the outstanding electronic, mechanical, and transport properties of 2D BP; such properties are highly anisotropic and layer dependent. Thus, 2D BP is a promising high‐performance electrode material for most EESDs due to its unique characteristics. Thus far, many significant improvements have been achieved with 2D BP. Despite significant progress in this field, fundamental studies and energy storage application research on 2D BP is still in its early stages, and considerable effort is needed to address unresolved issues and to investigate new concepts.
The authors declare no conflict of interest.