Phosphorus-based materials for high-performance rechargeable batteries

Xinyu Qin , Bingyi Yan , Jia Yu , Jie Jin , Yao Tao , Chao Mu , Sicong Wang , Huaiguo Xue and Huan Pang *
College of Chemistry and Chemical Engineering, Institute for Innovative Materials and Energy, Yangzhou University, Yangzhou, 225009, Jiangsu, P. R. China. E-mail:;; Web:

Received 5th April 2017 , Accepted 25th June 2017

First published on 12th July 2017

Lithium/sodium ion secondary batteries are an ideal power source for electric vehicles, portable electronic devices and energy storage devices, and recent studies have found that they are more environmentally friendly than other batteries. Innovative research on new electrode materials is the foundation for the development of neoteric high-performance batteries. Phosphorus offers a high theoretical specific capacity and is naturally abundant, thus making it utilizable in electrode materials. At present, however, our understanding of phosphorous materials is deficient, which hinders its widespread development and application, especially in the area of energy storage. To address this issue, the properties of P allotropes have been reviewed in this work. We introduce the recent development of P as an electrode material for energy storage, including the preparation of composite materials and the influence of the structure of the material on its electrochemical properties, among others. Furthermore, this review highlights that distinct P structures modulate the electrochemical properties of the material. Finally, we present a vision of the future development of phosphorous materials in the energy storage field.

image file: c7qi00184c-p1.tif

Xinyu Qin

Xinyu Qin is currently a student under the supervision of Professor Huan Pang at Yangzhou University of Chemistry and Chemical Engineering, China. Her research mainly focuses on micro/nanocoordination derived materials for electrochemical energy devices.

image file: c7qi00184c-p2.tif

Bingyi Yan

Bingyi Yan received his bachelor's degree in Applied Chemistry from Yangzhou University of Chemistry and Chemical Engineering, Institute for Innovative Materials and Energy, China. He has just been accepted into the Graduate School of Convergence Science and Technology, Seoul National University. His research mainly focuses on electrochemical energy storage nanomaterials and their applications.

image file: c7qi00184c-p3.tif

Huan Pang

Huan Pang received his Ph.D. degree from Nanjing University in 2011. He then founded his research group in Anyang Normal University where he was appointed a distinguished professor in 2013. He has now joined Yangzhou University as a University Distinguished Professor. He has published more than 120 papers in peer-reviewed journals including Chemical Society Reviews, Advanced Materials, and Energy and Environmental Science, with 3900 citations (H-index = 32). His research interests include the development of inorganic nanostructures and their application in flexible electronics with a focus on energy devices.

1. Introduction

Economic development depends on energy from the natural resources coal and oil. For this reason, the widespread depletion of these energy sources is a complex problem that requires close attention.1,2 Renewable but intermittent energy sources such as tidal and solar power have been successfully developed, but these sources require more reliable and efficient electrical energy storage. Accordingly, new energy storage devices are currently being investigated.3

Rechargeable batteries such as Li-ion batteries (LIBs)4–6 and Na-ion batteries (NIBs)6–8 are among the most promising electric power resources. Vast amounts of research have focused on fabricating and improving the quality of these batteries. Of all the factors that have been analyzed, electrode materials play an important role in improving metal-ion battery performance. The electrode materials of these batteries have evolved from metallic lithium (in LIBs) to carbon materials and from alloy materials to composite materials.9–13 Interest in Li has been growing since the early 1970s because it exhibited the highest specific capacity (3860 mA h g−1) and the most negative electrode potential (−3.045 V relative to the standard hydrogen electrode) of the materials known at that time.9,10,14–16 However, researchers have found that lithium secondary batteries based on Li metal electrodes tend to overheat or even explode during the charging process because the electrodes easily form dendrites.3,17–19 Graphite carbon materials are one of the Li-ion battery materials that have been applied to commercialization so far.20–22 However, not only is battery safety compromised23 but also the performance of the charging rate and the battery life are negatively influenced because of graphite's low theoretical specific capacity (specific discharge capacity is 372 mA h g−1, specific volumetric capacity is 818 mA h ml−1) and because of its intercalation potential being too close to the metallic lithium potential.24 Alloy materials generally exhibit a high theoretical specific capacity and good compatibility with the electrolyte, which are prominent advantages relative to carbon electrode materials.25 However, when alloys are used directly as a Li battery electrode, large volume changes and SEI effects26,27 are likely to occur in the Li embedded in/out process. This situation may cause the electrode to gradually pulverize and will eventually reduce the cycling stability of the battery.28–30 Nanoalloys with metal nanoparticles in a matrix of carbon materials are less likely to deteriorate, which can effectively improve the utilization rate and cycling stability of the material.31–33 However, fabricating these alloys greatly reduces the specific energy of the material while increasing the manufacturing costs of the battery, which diminishes the applicability of nanoalloy/carbon composite electrode materials.34–36 Si-based composite materials have adopted material structural designs similar to those of alloy composites due to the high theoretical lithium storage capacity of Si (even if the Si content in the electrode material is only 5%).37 This material is also of considerable value for applications. Therefore, mixed or composite electrodes with low silicon content are currently the best choice for designing high-energy-density battery electrodes.

Theoretically, the lithium storage capacity of pure P ranks only second to that of Si (close to 2600 mA h g−1) and Li.38,39 Additionally, although Si is the most promising anode material for LIBs, Si-based materials cannot be used as anodes in NIBs because Na ions are not electrochemically inserted in Si, despite the existence of Na–Si alloys.40 Thus, P materials constitute a new and more attractive type of practical Li/Na battery electrode material.41–44 Because pure P exists as several allotropes with different properties, it remains unclear which form is more appropriate for use in energy storage electrode materials.45

LIBs have been studied extensively to achieve higher energy storage efficiencies. The common electrode materials used in LIBs are inorganic compounds of LiCoO2, LiFePO4, LiMn2O4 and Li4Ti5O12, which are prepared from limited mineral resources containing Li. Thus, the demand is high for the development of Li as a secondary energy source. Indeed, because of the relatively high price of Li, NIBs can readily replace LIBs because Na resources are more accessible and affordable.46,47 Although NIBs show promise both theoretically and commercially, their energy density is lower than that of LIBs, which is crucial because it affects the electrochemical performance of the material, including its cyclicity and rate capability. Therefore, new electrode composite materials with a high specific capacity and appropriately low redox potentials should be introduced.

Globally, phosphorus rock resources are mainly distributed in Africa, North America, South America, Asia and the Middle East, of which more than 80% of the resources are in Morocco, the Western Sahara, South Africa, Russia, the United States, China, and Jordan.48 Therefore, these widely distributed energy storage materials have received increased attention, from research and development to industrialization.

Herein, we review some of the recent scientific advances that use pure P or P composites to fabricate electrode materials for rechargeable batteries, especially nano-structured materials. First, several allotropes of P are introduced, including red phosphorus (RP) and black phosphorus (BP). Then, some of the key issues concerning the performance and application of these materials are presented. More importantly, a variety of synthetic methods for P electrode materials and nanomaterials are reviewed in detail. Finally, existing problems and future trends are discussed from the perspective of interdisciplinary collaboration.

2. The characteristics of phosphorus allotropes

P allotropes demonstrate distinct properties (Table 1).49 White and red phosphorus are two of the most common allotropes, black and violet phosphorus are relatively rare, and diphosphorus is the rarest type of P allotrope.50
Table 1 Properties of some phosphorus allotropes
Form White (α) White (β) Red Violet Black
Symmetry Body-centered cubic Triclinic Monoclinic Orthorhombic
Pearson symbol aP24 mp84 oS8
Space group I[4 with combining macron]3m P[1 with combining macron] no. 2 P2/c no. 13 Cmca no. 64
Density (g cm−3) 1.828 1.88 2.2–2.34 2.36 2.69
Bandgap (eV) 2.1 1.5 0.34
Refractive 1.8244 2.6 2.4

2.1 White phosphorus (WP)

WP is one of the most important forms of P. It is also the most unstable and most reactive allotrope, with the lowest density and most facile volatilization. WP is composed of tetrahedron P4 molecules in which each atom is bonded to three other atoms by a single bond (Fig. 1a). The tetrahedron P4 molecular structure can be considered to be a combination of six separate P–P bonds, which give rise to ring tension and its inherent instability. At present, two different crystalline forms of WP are known: an α form and a β form. The α form has a body-centered cubic structure and is stable under standard conditions. At 195 K, this form transforms into the β crystalline form with a hexagonal crystal structure.51
image file: c7qi00184c-f1.tif
Fig. 1 Crystal and crystal structure of (a) WP and (b) RP. (c) VP ore and its crystal structure. (d) Crystal structure of BP and a photograph of its crystal. (e) Diphosphorus molecule.

The gradual change from WP to RP occurs more quickly in the presence of light and heat. Accordingly, there is always a certain amount of RP in WP samples, which makes the mixture yellow. Consequently, aged and/or impure WP is also known as yellow phosphorus. When WP is exposed to air in the dark, a faint blue-green color can be observed. WP is easily flammable and toxic in air. Additionally, it is soluble in benzene, oil, and carbon disulfide. Because WP is only slightly miscible in water, it is usually stored in this medium to prevent its spontaneous combustion.52

In industry, phosphate rock can be calcined in an electric furnace or sintered with silicon and carbon in a fuel furnace. WP is prepared after the release of P vapor and condensation with phosphoric acid. Eqn (1) shows the reaction for preparing WP via the carbon thermal reduction of calcium phosphate.53

3Ca3(PO4)2 + 8C = P4 + 8CO2 + 6Ca(1)

2.2 Red phosphorus (RP)

RP is a polymer that can be regarded as a derivative of P4 molecules; one of the P–P bonds ruptures and forms a new bond with an adjacent tetrahedron, thus forming a chain structure (Fig. 1b). WP can form RP when heated to 250 °C or exposed to sunlight. If it undergoes further heating, amorphous RP will start to crystallize. From this perspective, RP is not an allotrope but an intermediate phase between WP and VP, and most of the properties of RP show this type of intermediate behavior. For example, freshly prepared RP with a bright color demonstrates high activity, though it is more stable than WP (ignited at 30 °C); indeed, it can be ignited at 300 °C.54 After a long heating or storage time, the color of RP deepens, and it becomes more stable, less easily igniting in air.55 RP sublimates when heated to 416 °C and returns to WP if the vapor is cooled. RP is neither soluble in water nor in carbon disulphide, ethanol or other organic solvents, which severely restricts its use.

2.3 Violet phosphorus (VP)

In 1865, John Williams Hittorf placed RP in a sealed tube and calcined it under 550 °C for a long time to obtain a new phosphorus allotrope, violet phosphorus. He also found that WP dissolved in molten lead at a high temperature forms a red/violet allotrope after cooling to recrystallization; thus, VP is also known as Hittorf phosphorus.56,57 The unit cell of VP has 84 atoms, and it adopts the monoclinic crystal space group P2/c. This structure is based on two units that are similar to the P8 and P9 groups of As4S4.58 These units link through two P atoms to form an infinite tubular structure with a pentagon interface. The cavity of the structure bends down into the P8 group (Fig. 1c). This type of connection forms two surfaces, of which a P zigzag chain forms both ends of the tubular structure.59,60

VP cannot be ignited in air unless it is heated to 300 °C, and it does not dissolve in any solvent. VP does not react with any base, but it can react with halogens at a low rate. VP also oxidizes in phosphoric acid in the presence of nitric acid. If VP is heated under an inert gas atmosphere (e.g., nitrogen or carbon dioxide), the steam condenses to WP after sublimating. However, if VP is heated to sublimation and is rapidly condensed under vacuum conditions, it will return to the VP state. From this perspective, we regard VP as a polymer with a relatively high molecular mass that can resolve into P2 molecules if heated, polymerize to molecular P4 (WP) under condensation, and re-bond to form VP under vacuum.

2.4 Black phosphorus (BP)

BP has an orthogonal structure and is the least reactive of the P allotropes. Its lattice is comprised of a linked six-member ring, wherein each atom is linked with three other atoms (Fig. 1d).61 BP is a thermodynamically stable P allotrope under normal pressure and temperature (NPT), and thus, BP is typically prepared by heating WP under high-pressure conditions (e.g., 12[thin space (1/6-em)]000 atm). BP shares many similarities with graphite, including its black appearance, electrical conductivity and layered structure; its linked atoms also appear as folded sheets. The phonons, photons and electrons in the laminar structure of BP tend to be highly anisotropic. BP has great potential for application in electronic films and infrared optoelectronic technologies.62 Light absorption in BP behaves very sensitively to polarization, film thickness and doping.63,64 Further, BP phototransistors demonstrate high spectrum detection in the infrared and visible light regions.65,66 Similarities between BP and graphite include the possibility of stripping, which forms phosphorus (a graphite material with good electron transfer properties).67 Spalling BP will be oxidized when exposed to air or water68 and will sublimate when heated to 400 °C under vacuum.69 This type of high-quality BP nanometre sheet with relatively few layers can be prepared by the liquid phase stripping method.70,71

Of all the phosphorus allotropes, BP is the only one with a layered structure, similar to that of graphene, and is one of only two known monotypic van der Waals crystals, in which each layer is held together by weak van der Waals forces. The atoms of BP covalently bond to three neighboring atoms. Unlike graphene, however, BP forms a puckered structure with out-of-the-plane ridges.72 Steven et al.73 investigated BP crystals via atomic force microscopy (AFM) and Raman spectroscopy in the back-scattering configuration, as shown in Fig. 2a and b, respectively. The A2g, B2g, and A1g peaks are clearly observed at wave numbers of 465.9, 438.3, and 359.6 cm−1, which are consistent with previous results. When the incident light is parallel to the b axis (i.e., perpendicular to the covalently bonded plane of the P atoms) of BP, the B1g and B13g peaks are not present in the Raman backscattering.

image file: c7qi00184c-f2.tif
Fig. 2 (a) AFM image and (b) Raman spectrum of exfoliated black phosphorus. Reproduced with permission.73 Copyright 2014, AIP Publishing LLC.

2.5 Diphosphorus (2P)

2P is a gaseous P allotrope that can usually only be prepared under extreme conditions (e.g., from RP at 1100 K), undergoing its thermodynamics steady state between 1200 °C and 2000 °C (Fig. 1e). P4 begins to resolve at rather low temperatures; the percentage of 2P at 800 °C is approximately 1%. 2P molecules begin to decompose into P atoms above 2000 °C.

Clearly, not every type of P allotrope is appropriate for use as an energy storage electrode material; indeed, 2P exists only under extreme conditions. WP molecules are highly symmetrical and possess weak inter-molecular forces, and WP is so unstable that spontaneous combustion easily occurs. VP tends to decompose to WP, making WP and VP unsuitable for electrode materials. In comparison, RP and BP are much more suitable for this application, but each allotrope has its problems; for instance, RP is electrically insulative, whereas BP is too stable to prepare.

Aiming at addressing these two critical problems facing RP and BP, researchers have devised various methods to make these materials more appropriate for electrodes. Compositing RP with carbon, which has good electrical conductivity, is the main approach for solving the electrical insulation issue of RP. Two methods used for the preparation include vapor deposition and ball-milling. Wang et al.38 used a vapor deposition method to obtain nano P/C materials with a P content of approximately 30%. Furthermore, Park et al.74 obtained a composite material with a phosphorus and carbon mass ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]3 using high-energy ball-milling. Sun et al.75 used RP and WP as raw materials to successfully make BP by the HPHT anvil-heating method (i.e., high temperature and high pressure). These approaches effectively improved the electrical conductivity of P, making it more suitable for use as an energy storage electrode for LIBs/NIBs.

The preparation of BP is more difficult. Typically, researchers use high-energy ball-milling to transform RP into BP, but the BP crystal structure prepared by this method is not much perfect because of uncontrollable temperatures and pressures. Nilges et al.76 transferred RP, AuSn and SnI4 to a silica ampoule, which was evacuated to less than 10−3 mbar. The sealed ampoule was then put into a muffle furnace, heated to 673 K within 1 h and held at that temperature for 2 h. The temperature was then raised to 873 K (1 h) and kept at this temperature for 23 h. The final step was to reduce the temperature at a rate of 40 K h−1 to room temperature over a 4 h period. This procedure could synthesize BP crystals larger than 1 cm. After a reaction time of nearly 32.5 h, the formation of BP was achieved with crystal sizes up to 1.5 cm in diameter via a transport reaction (Fig. 3).

image file: c7qi00184c-f3.tif
Fig. 3 Silica ampoule after the reaction procedure. 1, 2 and 3 represent the bulk residue, violet phosphorus and the main black phosphorus product, respectively. Reproduced with permission.76 Copyright 2008, Elsevier Inc.

Stan et al.,77 Köpf et al.78 and Jiang et al.79 optimized the method mentioned above and achieved high-quality BP. Thereafter, studies on how to make high-quality BP and its nanolayers were increasingly reported in the literature,71,80 which solves the problem of preparing BP under certain conditions. Meanwhile, the growing application of RP and BP for energy storage is an emerging field that is promoting excellence in the study of P-based materials.

3. Pure P as an anode material

3.1 BP used in LIBs

Investigations on P-based electrode materials began with BP, which possesses the most stable thermodynamic properties among all the P allotropes, making it difficult to implement via the ball-milling method, though it is the most commonly used approach to prepare BP. Only Nagao et al.81 have investigated technology related to these applications. Sun et al.75 used RP and WP as raw materials to successfully make BP by the HPHT anvil-heating method (Fig. 4). This group simultaneously analyzed the electrochemical activity of pure BP under different temperature and pressure systems. They fabricated BP from WP at 4 GPa and 400 °C to obtain the highest first discharge and charge capacities of 2505 mA h g−1 and 1354 mA h g−1, respectively. When using RP at 4.5 GPa and 800 °C to obtain BP, the highest first discharge and charge capacities were 2649 mA g h−1 and 1425 mA h g−1, respectively (calculated according to the mass ratio of P). Additionally, Sun et al. analyzed why BP is able to intercalate three Li ions in the fully discharged state (Li3P) to yield a high theoretical specific capacity of 2596 mA h g−1.
image file: c7qi00184c-f4.tif
Fig. 4 (a) Schematic diagram of the experimental setup used to produce BP. (b) The crystal structure of BP, transport channels of Li ions in BP and projections along its a-axis and c-axis.75 Reproduced with permission.75 Copyright 2012, American Chemical Society.

Along the (0k0) plane of BP, there is a puckered double-layer structure (Fig. 4b(1)), which makes room for Li ions to insert/extract through the 2D interstitial space (Fig. 4b(2)). The BP structure has the largest space for Li ions to intercalate along the a-axis in the (020) crystal plane. Moreover, there exists an optimal space for Li ion intercalation along either the a-axis or c-axis in the (040) crystal plane, as shown in the projection of the spaces on the yz and xy planes between three neighboring layers (Fig. 4b(3)).

In 2016, Zhang et al.82 were the first to fabricate 2D holey P-based nanosheets using a sublimation-induced strategy. They successfully assembled P nano-domains from the bottom up using ethanol in a wet-chemical solvothermal reaction (Fig. 5a) and obtained a very thin thickness (<5 nm). Because of the unavoidable oxidation during the preparation procedure, these P-based nanosheets not only consist of amorphous BP but also P oxides. LIBs made of these nanosheets possessed a sizable reversible capacity of approximately 1600 mA h g−1 at 0.2 A g−1 and demonstrated a very small capacity decrease after 100 charge and discharge cycles. At a high current density of 20 A g−1, the electrode materials obtained a high capacity of 630 mA h g−1, which is almost equal to the efficiency of the diffusion and electron transfer of Li ions in the nanosheet structure. The pattern shown in Fig. 5b reveals that its X-ray diffraction (XRD) is a medium-range ordered structure with a primary diffraction peak at 2θ = 16°, which bears a striking resemblance to the amorphous state of RP materials. The high-resolution TEM image in Fig. 5c suggests that the layers are composed of massive nano-domains with different crystallographic orientations. The lattice fringes are 0.27 and 0.23 nm, which correspond to the (040) and (002) planes of orthorhombic BP, respectively. To further probe these 2D nanosheet dimensions, AFM strategies (Fig. 5d and e) were used. These measurements revealed a thickness of approximately 0.5–4 nm and a lateral size of up to 1.0 μm, which corresponds to a 1–8 layer thick P nanosheet structure. Additionally, few-layer phosphorus nanosheets were also successfully fabricated. Subsequently, the electrochemical performance of these nanosheets was evaluated (Fig. 5f–i). The first discharge capacity was approximately 880 mA h g−1 and dropped sharply to 100 mA h g−1 after 10 charge/discharge cycles when bulk RP electrodes were used (Fig. 5f and g) because of mechanical cracking/crumbling during large volume changes. However, the material exhibited very high first cycle charge and discharge capacities of approximately 2696 and 1969 mA h g−1, respectively. The nanosheets show an entirely different continuous slope profile with a modest higher working potential (Fig. 5h and i). That is, a continuous reduction potential region under 1.0 V (with the oxidation peak at 0.1, 1.1 V) and a stabilized reduction peak at 1.8 V (with the oxidation peak at 2.3 V) are observed.

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Fig. 5 (a) Schematic illustration of the preparation of P nano-domains. (b) XRD patterns of RP- and P-based nanosheets. The marked peaks belong to the orthorhombic phase of BP. The inset shows the diluted P composite nanosheets in ethanol. (c) HRTEM image of P-based nanosheets showing amorphous regions with some polycrystallinity. (d) AFM image and (e) the corresponding heights of the nanosheets marked in panel (d). (f) Comparison of the cycle performances of P composite nanosheets and RP materials at a current density of 0.2 A g−1. Voltage profiles of (g) RP and (h) P composite nanosheets at 0.2 A g−1. (i) The associated derivative −dQ/dV plot from (g). In (h), a continuous reduction potential is observed below 1.0 V (with the oxidation peak at 0.1, 1.1 V) and a stable reduction peak at 1.8 V (with the oxidation peak at 2.3 V) is observed. Reproduced with permission.82 Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.2 Theoretical calculations of BP as an anode material

Inspired by the unique structure of BP and its high theoretical capacity, an increasing number of reports have explored the full capability of this novel material. Overall, the consensus is that Li3P and Na3P are formed in the discharge process, which has a positive effect and reduces the rupture of the electrode due to a smaller volume change;83 however, the syntheses of other phases during the lithiation/sodiation process remain unclear, and the use of BP in NIBs is in its infancy. Research on rechargeable Mg-ion batteries (MIBs)84,85 has also recently emerged, though there is little research on how to use BP in MIBs. First principles calculations including an ab initio study (AIS) and density functional theory (DFT) calculations of BP in LIBs,86–88 NIBs,87–89 and MIBs88,90,91 can address this issue at the atomic level. Theoretical calculations can highlight the atomization mechanism of BP during lithiation/sodiation/magnesiation, which may then guide other researchers to determine the optimal approach for using BP when fabricating electrodes.

4. P composites as an anode material

Based on the investigations mentioned above, BP is the only allotrope that can be used to make electrode materials. Although RP is a very attractive material as it possesses a theoretical specific capacity of approximately 2596 mA h g−1, its experimental capacity is far below the theoretical value after just a few cycles, probably because of its poor electronic conductivity.92 In 2007, Park and co-workers reported combining BP with carbon into composites, contributing to the advancement of both capacity release and stable cycling.74 Subsequently, many researchers were inspired to prepare phosphorus composites.93–95 Although fabricating nano-structured P composites is essential for improving the electrochemical performance of P, this strategy is not straightforward. Exploiting the potentialities of P composite electrodes is crucial to the advancement of this field.

4.1 RP composites used in LIBs

In 2012, Wang et al.38 were the first to use porous carbon powder and RP powder to prepare RP/C composite anodes using the vaporization/adsorption method. The two substances were placed separately in a vessel, which was later sealed and filled with pure Ar gas (Fig. 6a). The vessel was then heated until its temperature was just above RP's sublimation temperature (i.e., 450 °C) and held at that temperature for 3 h. Then, the RP sublimate diffused into the pores, mainly because of capillary forces and pressure differences, and became adsorbed and deposited by the porous carbon on its internal surface. This vaporization/adsorption strategy is much less inexpensive and complicated than high-energy ball-milling. After the RP adsorption process, the surface area and porous volume dramatically decreased from 917 m−2 g−1 and 0.186 cm−3 g−1 to 2.9 m−2 g−1 and 0.025 cm−3 g−1, respectively.38 Pores in the carbon appeared to be nearly filled by the deposited RP. The TEM image in Fig. 6b clearly shows 5–10 nm particles in the composite. Additionally, the corresponding scanning electron microscopy (SEM) image shows that a small number of RP particles are on the external surface of the composite (Fig. 6c). Furthermore, Fig. 6d displays the cycle performance of the phosphorus composite. The surface area of this newly made nano-structured phosphorus composite was comparatively low, which contributed to its good performance in anode materials with high Coulombic efficiencies. This composite demonstrated highly reversible charge/discharge processes, highlighting its cycling stability and high lithium-storage capacity above 750 mA h g−1. The utilization of RP in the composite reached 92%, which corresponds to a capacity of 2413 mA h g−1 based on RP.
image file: c7qi00184c-f6.tif
Fig. 6 (a) Apparatus for the preparation of the RP/C composite. (b) TEM image of the RP/C composite; scale bar: 50 nm. (c) SEM image. (d) Cycle performance of the RP/C composite. The capacity is referenced to the effective composite material. Reproduced with permission.38 Copyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

No major breakthroughs were reported in this field until 2014 when Li et al.96 encapsulated porous carbon nanofibers (PCNFs) with crystalline RP as free-standing anodes for LIBs. This group took advantage of these flexible freestanding electrodes because all the materials in the electrodes could play a role in charge storage, which is the source of the high volumetric energy and power density of LIBs. The PCNFs with a 3D interconnected structure were fabricated by electrospinning polyacrylonitrile/poly (methyl methacrylate) (PAN/PMMA) followed by a thermal carbonization process. Subsequently, the group used the vaporization/adsorption method to obtain crystalline RP incorporated with PCNFs using the same method as that used by Wang et al.38 The PCNFs were used as a matrix to load crystalline RP. WP generated during the adsorption process of RP was later removed from the composites by CS2. The films were then dried in a vacuum to generate the P-PCNFs films.

The SEM images of the PCNFs and P-PCNFs are shown in Fig. 7a and b, respectively. The PCNFs (Fig. 7a) demonstrate a continuous and interconnected structure with a diameter of approximately 200 nm. After the vaporization/adsorption process, the obtained P-PCNFs retained a fibrous morphology and the interconnected network remained in good condition. Furthermore, no obvious RP particles were detected in the PCNF network, indicating that all the RP are embedded in the PCNFs. The PCNF matrix not only enabled ions to move unimpeded but also gave the composite electrode good conductivity and excellent mechanical flexibility. Fig. 7c exhibits the extraordinary cycle performance of the P-PCNF composite at 0.1 C. The P-PCNF composite electrode delivered an extraordinary reversible capacity of approximately 2030 mA h g−1, based on the mass ratio of the RP percentage. Furthermore, an average Coulombic efficiency of approximately 99.9% over 100 cycles was achieved. SEM examination of the P-PCNFs after battery testing revealed no obvious distortion (Fig. 7d). The surface of the fibers became rough after 100 cycles, owing to the formation of a solid electrolyte interphase (SEI) and some residual electrolyte. The unique structural design ensures the stability and cyclability of this composite electrode.

image file: c7qi00184c-f7.tif
Fig. 7 The FESEM micrographs of (a) PCNFs and (b) P-PCNFs. The insets show the corresponding high-magnification images. (c) Capacity and Coulombic efficiency–cycle number curves of the P-PCNFs electrode at a cycling rate of 0.1 C. (d) FESEM micrographs of the P-PCNFs after 100 cycles between 0.001 V and 2.5 V vs. Li+/Li at a cycling rate of 0.1 C. The inset shows the corresponding high-magnification image. Reproduced with permission.96 Copyright 2014, Elsevier Ltd.

In the second half of 2014, Xiao et al.97 were inspired by strengthened concrete structures to fabricate an RP built-in amorphous TiO2 (A-TiO2) composite. TiO2 is one of the most promising anode materials because of its high safety and outstanding stability. However, it possesses a low theoretical capacity, which has limited its applications. Nevertheless, TiO2 has the ability to form a composite with RP that possesses a high theoretical specific capacity but a rapid capacity decay. In their experiments, Xiao et al.97 used A-TiO2 as the concrete to prevent RP from escaping from the electrode, and the RP acted as steel to enhance the electrochemical capacity of the RP/A-TiO2 composite. A schematic illustration of the preparation of RP/A-TiO2 can be seen in Fig. 8a. First, they used a high-energy ball-milling method to pulverize bulk RP into a powder, which was then dissolved in polyvinyl pyrrolidone (PVP) and placed in air for 48 hours. Subsequently, a nano-sized RP-PVP aqueous solution was obtained and an isopropyl titanate–ethanol solution was then added. The hydrolyzed titanium dioxide attached to the suspended RP in solution. Finally, the obtained precipitate was centrifuged, washed several times and dried to successfully fabricate the RP/A-TiO2 composites. Fig. 8b and c display the electrochemical performance of this novel composite. The first charge and discharge capacities of the RP/A-TiO2 sample are 450 and 770 mA h g−1, respectively, yielding a high irreversible capacity loss of 320 mA h g−1. This loss is slightly higher than that of A-TiO2 (260 mA h g−1), which may be caused by the formation of a SEI layer in RP. Fig. 8c shows the cycling performance of the RP/A-TiO2 composite anode; as expected, excellent cycling stability was achieved. The Coulombic efficiency of this anode stably decreased to approximately 99% as the number of cycles decreased. The RP-TiO2 composite anode revealed a cycling capacity of 369 mA h g−1 over 100 charge/discharge cycles and a logical rate capacity of 202 mA h g−1 at a current density of 1 A g−1. Fig. 8d shows how RP/A-TiO2 functions within the concrete-like structure during the charge/discharge process. The outer TiO2 cluster prevents the inner RP from escaping by buffering during lithiation. Owing to this unique structure, 10 wt% RP in this composite is able to supply over 200 mA h g−1 capacity.

image file: c7qi00184c-f8.tif
Fig. 8 (a) Schematic illustration of the preparation of RP (red)/A-TiO2 (green). (b) Charge/discharge profiles of A-TiO2 and RP/A-TiO2 with a current density of 100 mA g−1. (c) Cycling performances of A-TiO2 and RP/A-TiO2 with a current density of 100 mA g−1. (d) Schematic of the lithiation process in bare RP and RP/A-TiO2. Reproduced with permission.97 Copyright 2014, The Royal Society of Chemistry.

Compared with its results, this method is very novel and worth highlighting, which uses metal oxide to form composites with P. Although it may not help in the present situation, it may give significant guidance for the future.

Because of the above results, more attention was drawn to this area of research. In early 2015, Wang et al.98 fabricated a RP/active carbon (RP/AC) composite with a fairly high RP content of 60.0 wt% via the vaporization/adsorption strategy noted above. Notably, AC replaces the porous carbon with good conductivity and excellent absorbability. The AC was later used as the matrix to load RP. This RP/AC composite delivered a high capacity of 1550 mA h g−1, based on the composite weight. Meanwhile, the RP/AC composite showed a good capacity retention ratio of 83.6% after 50 cycles. Moreover, in all the cycles but the first, the RP/AC composite possessed a Coulombic efficiency of more than 97.5%. The initial discharge capacity is 1971 mA h g−1, which is much higher than the initial charge capacity of 1502 mA h g−1, which contributed to the low initial columbic efficiency value of 76.1%.

In the same year, Bai et al.99 explored the fabrication of graphite/RP@C composite anodes by mixing graphite and RP/porous carbon material. The RP@C was prepared via the vaporization/adsorption strategy. They then triturated a series of graphite/RP@C mixtures with RP@C contents of 10 wt%, 20 wt%, 28.6 wt%, 41.8 wt% and 100 wt%, followed by adding different solvents to form slurries. The as-prepared slurries were then cast onto copper foil and cut into discs to assemble the cells. Fig. 9a reveals the charging/delithiation curves of the graphite/RP@C composite anodes. The voltage platform for the graphite/RP@C composite anodes with different RP@C contents are similar, indicating that the inside graphite and RP@C materials are mixed only physically. As the RP@C content increased from 0 wt% to 28.6 wt%, the capacity of the composite increased gradually. When the RP@C content reaches 41.8 wt%, a decrease can be observed in both graphite and RP@C. A decrease in the electrolyte uptake and electronic conductivity will occur under the influence of too much RP@, which is also the cause of the poor electrochemical performance of the composite anode. When the RP@C content is 28.6 wt%, the composite anode demonstrated a reversible capacity of 500 mA h g−1. This paper illustrates the effects on the electrode capacity brought about by the mass ratio in detail, which can be considered as an instructive strategy. Fig. 9b shows the electrolyte holding ability and electronic conductivity of the electrodes with different RP@C contents. As the RP@C content increased, the electrolyte holdup decreased, whereas the surface resistance increased. Fig. 9c and d exhibit the Nyquist plots before and after the first cycle of electrochemical impedance spectroscopy (EIS) of the Li│graphite/RP@C composite half-cell.

image file: c7qi00184c-f9.tif
Fig. 9 (a) Delithiation curves of the graphite/RP@C composite in the half-cell with different RP@C contents. (b) Electrolyte holdup and surface resistance of the graphite/RP@C composite anode with different RP@C contents. The electrolyte holdup and surface resistance were taken to be those of 100% graphite. Nyquist plots of the half-cells: (c) before the first cycle; (d) after the first cycle. Reproduced with permission.99 Copyright 2015, Elsevier B.V.

In 2015, Li et al.100 used slit-shaped porous carbon to encapsulate RP and used the composite to prepare LIB anodes. Slit-shaped carbon was synthesized by adding formaldehyde dimethyl acetal to FeCl3 in 1,2-dichloroethane. The mixture was then degassed by nitrogen bubbling and stirred for 5 h at 45 °C to form the preliminary polymeric network. The cross-linking was then completed by heating at 80 °C for 19 h. After washing several times with methanol and HCl, as well as Soxhlet extraction with methanol for 1 d, the composite was dried under reduced pressure at 60 °C. Finally, the slit-shaped carbon was prepared via carbonization, and the RP/slit-shaped porous carbon composite was prepared by the vaporization/adsorption strategy. This composite demonstrated a reversible capacity of 1359 mA h g−1 at a current density of 100 mA g−1, and it exhibited an extraordinary capacity retention of 99% from the 2nd cycle to the 50th cycle, as well as a high Coulombic efficiency of approximately 99%. There is a big loss of capacity during the first cycle probably due to the irreversible reaction of P and the contribution of the first discharge capacity of carbon.

In 2016, Wang et al.101 published the most recent report in this field, which described the synthesis of a sandwich-like composite of a reduced graphene oxide (GO) nanosheet and nanosized RP particles (RP@GONs) via a facile high-pressure-assisted spraying strategy. The RP was first wet ball-milled under an argon atmosphere with water and then freeze-dried. GO powder was prepared via a modified Hummers’ method102 followed by ultrasonic and freeze-drying treatments. The RP@GON composite was fabricated by a high-pressure assisted spraying strategy, in which the RP and GO precursors were dissolved in acetone at a mass ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]3. The materials were placed in a spray gun with a high-purity nitrogen gas to supply a high pressure. Finally, a ground collector was used to gather the sprayed hybrids at 100 °C. The as-prepared composite anode demonstrated good cycling performance with a reversible capacity of approximately 990 mA h g−1 after 50 cycles. Meanwhile, the Coulombic efficiency was above 98%.

4.2 The RP composites used in NIBs

NIBs is a promising alternative to LIBs due to the extensive sources and suitable redox potentials (only 0.3 V above that of Li) of Na. However, the crystalline RP cannot reversibly react with Na ions, which is quite different from LIBs, in which the reverse reaction could happen via the electronic improvement of RP by vaporization and condensation on mesoporous carbon.93 Below are mentioned some research studies using some matrix to mix with RP to form a strong network structure which can bear the large volume change during the charge/discharge process and enhance the conductivity between every RP particle in NIBs.103 Such a network structure can provide pathways to transport electrons and make it possible to undergo counterreactions.

In 2013, Kim et al.40 first fabricated an amorphous RP/C composite via ball-milling for 20 h to prepare NIB anodes. The corresponding XRD pattern is shown in Fig. 10a. The RP structure was retained in the composite, which is further supported by the Raman spectra in Fig. 10b. Additionally, the excellent cycle performance of this RP/C composite possessed a low capacity fading of less than 7% over 30 cycles and showed a reversible capacity of 1890 mA h g−1 (Fig. 10c). Moreover, the rate performance of this composite delivered approximately 1540 mA h g−1 at 2.86 A g−1, which is nearly 80% of the reversible capacity delivered at 143 mA h g−1 (Fig. 10d).

image file: c7qi00184c-f10.tif
Fig. 10 (a) XRD patterns of bare RP (dark gray line) and the amorphous RP/C composite (black line). (b) Raman spectra of bare RP (dark gray line), ball-milled bare RP (light gray line), and the amorphous RP/C composite (black line). (c) Cycle performance and (d) rate capability of the amorphous RP/C composite. Reproduced with permission.40 Copyright 2013, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Almost simultaneously, Qian et al.80 used nearly the same method (milling for 4 h longer) to fabricate an anode composite material as Kim40. However, the performance was slightly inferior to theirs, yet the reason is still not clear.

Li et al.103 successfully fabricated RP/CNT composite materials by manually grinding RP and CNTs to the micron scale. The mesh structure formed by the CNTs could increase the buffer space; at the same time, they could effectively cushion expansion pressure and decrease the capacity fading caused by crushing RP (Fig. 11a). Consequently, this type of composite material demonstrated a high specific capacity of 1675 mA h g−1. Meanwhile, it retained a capacity of 76.6% after 10 cycles (Fig. 11b). The simple preparation method and high irreversible capacity show that the electrochemical reaction of Na possesses very good reversibility when the RP particles are on the micron scale.

image file: c7qi00184c-f11.tif
Fig. 11 (a) Proposed function of CNTs during the volume expansion. (b) Charge/discharge curves of RP and the RP/CNT composite. Reproduced with permission.103 Copyright 2013, American Chemical Society.

In 2014, Song et al.104 fabricated novel RP/graphene nanosheet composite nanostructured anodes for NIBs via a ball-milling process involving graphene stacks and RP. The composite anode delivered a high reversible capacity of 2077 mA h g−1 and an excellent cycling stability of 1700 mA h g−1 after 60 cycles. It also demonstrated a high Coulombic efficiency of over 98%. The graphene nanosheets exhibited a large surface area and flexibility, which enabled them to load RP particles during the milling process. Moreover, they could enhance the overall conductivity and tolerance caused by the large volume change of RP during cycling because the nanosheets formed a robust conductive matrix able to maintain electrical contact. Additionally, the graphene nanosheets could form chemical bonds with RP and facilitate intimate contacts between them while also stabilizing the SEI.

In early 2015, Pei et al.105 encapsulated RP nanoparticles in graphene scrolls to prepare a composite as anodes for NIBs. The RP/GO mixture was quickly frozen with liquid N2 to form a rolled-up graphene that could encapsulate the RP nanoparticles (Fig. 12a). This facile method effectively restrained volume changes in the structure. Pei et al.105 also found that the RP/G composite with a RP content of 52.2% delivered the highest capacity of 2355 mA h g−1 and had an excellent cycling stability, which maintained a capacity of 92.3% relative to the second cycle. Even at a high current density of 4 A g−1, the material still achieved a capacity of 1084 mA h g−1. The TEM image and elemental mapping images (Fig. 12b–d) confirm the presence of RP particles in the graphene scrolls.

image file: c7qi00184c-f12.tif
Fig. 12 (a) Schematic illustration of the preparation of RP/graphene nanosheet composites. (b) TEM image of the RP/graphene nanosheet composite with a phosphorus content of 52.2% and elemental mapping images of (c) P and (d) carbon. Reproduced with permission.105 Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Around the same time, Walter et al.106 used an entirely different matrix material (colloidal antimony nanocrystals, Sb NCs) with RP and copper nanowires (Cu NWs) to form composite electrodes. Sb was shown to have excellent cycling stability, in contrast with RP. Furthermore, adding Cu NWs provided the composite with more mechanical stability, which enhanced its electrochemical performance. This RP/Sb/Cu composite exhibited a high rate capacity of over 900 mA h g−1. It also demonstrated a good cycling stability of over 1100 mA h g−1 after 50 cycles at a current density of 125 mA g−1. For comparison, Fig. 13a also shows the data of Li et al.103 for the RP/CNT composite. Fig. 13b reveals the stabilizing effect seen in the RP/Sb/Cu electrodes. The Sb NCs and Cu NWs were considered to impart superior mechanical stability and greatly improve the electronic connectivity. Moreover, they could prevent the RP particles from escaping so as to minimize the loss of electrical connections.

image file: c7qi00184c-f13.tif
Fig. 13 (a) Schematic depiction of the possible stabilizing effect of Cu NWs (with RP in red, CB in black, Sb NCs in grey and Cu NWs in yellow). (b) Cycling stability of the RP/Sb/Cu composite electrodes relative to Sb NCs and RP at a current density of 125 mA g−1. Reproduced with permission.106 Copyright 2016, Macmillan Publishers Limited, part of Springer Nature.

Shortly after the work of Walter et al.106 was published, Zhu et al.107 fabricated a RP/single-walled carbon nanotube (RP/SWCNT) composite via an improved vaporization/adsorption strategy, in which a higher temperature and vacuum conditions are used. Fig. 14a shows the fabrication process of this RP/SWCNT composite. The as-prepared RP/SWCNT composite exhibited an overall Na storage capacity of approximately 700 mA h g−1 at 50 mA g−1 and a fast rate capability of approximately 300 mA h g−1 at 2000 mA g−1. Meanwhile, the composite possessed stable cycling with 80% capacity retention after 2000 cycles. As shown in Fig. 14b and c, Zhu et al.107 compared the cycling performances and Coulombic efficiencies of the RP/SWCNT composite, the RP/SWCNT mixture (hand-milled for 15 minutes), RP and SWCNTs.

image file: c7qi00184c-f14.tif
Fig. 14 (a) Schematic illustration of the synthesis process of the RP/SWCNT composite. (b) Cycling performances and (c) Coulombic efficiencies of the RP/SWCNT composite, the RP/SWCNT mixture hand-milled for 15 min, RP, and SWCNTs at a current density of 50 mA g−1. Reproduced with permission.107 Copyright 2015, American Chemical Society.

Subsequently, Ruan et al.108 fabricated a RP/N-doped carbon nanofiber composite (RP/NCF) using a vaporization/adsorption strategy (Fig. 15a). The NCF was prepared from polypyrrole (PPy) by carbonization. This composite delivered a reversible capacity of 731 mA h g−1 in NIBs and a capacity retention of 57.3% after more than 55 cycles. The SEM images are shown in Fig. 15b–d. The image of PPy (Fig. 15b) exhibits a cross-linked structure that includes nanofibers with small diameters. After carbonization, the composite still maintained its interconnected structure (Fig. 15c). The diameter of the nanofibers in the RP/NCF composite was much larger than those of the other nanofibers (Fig. 15d), indicating that the RP particles were successfully loaded into the NCF.

image file: c7qi00184c-f15.tif
Fig. 15 (a) Schematic illustration of the preparation of RP/NCF. SEM images of the samples: (b) PPy, (d) N-doped carbon and (d) RP/NCF. Reproduced with permission.108 Copyright 2016, The Royal Society of Chemistry.

Wu et al.83 improved the electrochemical properties of the RP anode in NIBs via the space confinement of carbon nanopores. The RP@C nanocomposite was obtained by a simple ball milling strategy followed by vaporization with Ketjen black and multiwalled carbon nanotubes, which effectively encapsulate RP into the HPC substrate. The as-prepared RP@C nanocomposite anode shows a relatively high Na storage capacity of 1290 mA h g−1 and a highly stable cycling of more than 88% after 200 cycles which far exceeds that of the previously reported P@C composite in NIBs.

Song et al.109 synthesized a RP/carbon nanotube and a cross-linked polymer binder composite through multicomponent chemical bonding. The RP/CNT composite was fabricated via the ball-milling method, by which the P–O–C bond formed and helped to maintain the connection between the two components. In addition, the cross-linked carboxymethyl cellulose/citric acid binder stabilized the material. This composite delivered a stable cycling capacity of 1586.2 mA h g−1 after 100 cycles and a high Coulombic efficiency of approximately 99%. Fig. 16a and b shows the chemical bonding between these components. Fig. 16c illustrates the structure with the 3D-cross-linked network of this composite and its chemical bonding, which helped to maintain its stability.

image file: c7qi00184c-f16.tif
Fig. 16 (a) Cross-linked polymer binder formed by the thermally induced condensation of carboxymethyl cellulose and citric acid (c-NaCMC-CA). (b) Interaction between the RP/CNT hybrid and the c-NaCMC-CA binder. (c) Schematic illustration of the structural evolution of the P-based anode during cycling. Reproduced with permission.109 Copyright 2015, American Chemical Society.

In early 2016, Gao et al.110 prepared a 3D integrated carbon/RP/graphene aerogel composite (C/RP/GA) via an improved vaporization/adsorption strategy to load RP nanoparticles into this 3D graphene-based frame. In contrast with the traditional vaporization/adsorption strategy, the method proposed by Gao et al. could encapsulate RP nanoparticles into the structure with a well-defined porosity before the high-temperature vaporization/adsorption process via an in situ self-assembly approach and vapor-phase polymerization of PPy. Finally, a localized vaporization/adsorption process was used, and uniformly deposited RP nanoparticles were obtained, which makes up for C/RP/GA. This structure effectively reduces the mobility of the RP nanoparticles on the matrix in NIBs during electrochemical cycling (Fig. 17a). The C/RP/GA anode delivered a capacity of approximately 1867 mA h g−1 after 100 cycles at 0.1 C and 1095.5 mA h g−1 at 1 C after 200 cycles (Fig. 17b).

image file: c7qi00184c-f17.tif
Fig. 17 (a) Schematic illustration of the C/RP/GA electrode in the NIB system. (b) Cycling performance of the C/RP/GA composite at 1 C (1 C = 2600 mA g−1). Reproduced with permission.110 Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

In 2016, Zhang et al.111 fabricated a flexible paper anode for NIBs that was made of amorphous P and nitrogen-doped graphene (GN), by using an innovative method called ‘phase-transformation’, in which very thin amorphous RP layers are formed within flexible and conductive N-doped graphene frameworks. Bulk RP was heated to form P4 vapors in a small sealed ampule, which were absorbed and deposited within the interlayers of GN to fabricate “butter-bread-like” doped P–C structures. A small number of P–C bonds possibly form, between amorphous P and GN, that would tightly anchor those layers. By spreading amorphous P to form a thin P layer on the doped C, instead of crystalline P nano-particles, the P@GN anode shows ultra-stable efficiency (0.002% decay per cycle from the 2nd to the 350th cycle), and excellent rate capability (809 mA h g−1 at 1500 mA g−1). In situ HRTEM experiments on a prototype P@GN-based nano-battery device were used to verify that the ultra-stable performance and high capacity of P@GN are entirely sustained during the process. Moreover, DFT calculations further revealed that doping sites can enhance the interactions between the Na and C surfaces, leading to ultrafast sodium energy storage.

4.3 BP composites used in LIBs/NIBs

Herein, many types of RP composite electrodes have been reviewed. Usually, the matrices are carbon materials such as porous carbon, carbon nanotubes, graphene and some metallic oxides. In the literature that we reviewed, BP composites were relatively rare, probably because pure BP has the ability to form electrodes on its own. Owing to its layered structure similar to graphite, BP possesses high electron conductivity which is completely different from RP. However, a pure BP anode usually experiences a quick decrease in the charge/discharge process because of the large volume change. In the previous section we reviewed a RP composite anode, which used matrix materials to conduct electrons and maintain the structure. What is different is that BP itself has the ability to conduct electricity. Therefore, solving the problem of volume expansion becomes a novel orientation of this field.112 Here, we introduce BP composites that can act as one component of the electrode composite.

Park et al. (2007) first reported the use of the BP composite in LIBs.74 Originally, stable orthorhombic BP was prepared from WP and RP, which is not sufficiently stable to be an electrochemical material.54 RP can be turned into BP via the ball-milling method; the corresponding X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images of BP are shown in Fig. 18a. Likewise, Super P carbon black materials have been prepared and applied in LIBs. The first discharge and charge capacities are 2010 mA h g−1 and 1814 mA h g−1, respectively, and the first cycle efficiency is up to 90%. In contrast with the irreversible capacity of carbon (at approximately 150 mA h g−1), the electrochemical reaction between BP and Li3P is almost reversible. In addition, ex situ XRD analyses were performed at several potentials selected from the differential capacity plot, as shown in Fig. 18b. The corresponding reaction is as follows:

BP LixP LiP Li2P Li3P.(2)

image file: c7qi00184c-f18.tif
Fig. 18 (a) X-ray diffraction patterns and TEM images of BP. (b) Different capacity plots and X-ray diffraction patterns (numbers correspond to the voltage point) of BP/C composites for the first cycle. Reproduced with permission.74 Copyright 2007, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

A mole of LiP can be observed when the potential reaches 0.78 V (Fig. 18b(2)). Meanwhile, Li2P and Li3P appear at potentials of 0.63 V and 0 V, respectively. The electrode demonstrated an optimal cycle performance between 0.78 V and 2.0 V.

In 2014, Sun et al.112 fabricated a BP nanoparticle/graphite (BP-G) composite via a high-energy ball-milling strategy. The corresponding cross-sectional HRTEM image is shown in Fig. 19a. This group then analyzed the electrochemical performance of this anode (Fig. 19b) and used ab initio density functional theory to explain the chemical interactions between BP and carbon. The average diameter of a BP-G particle is around 300 nm, which is smaller than the average size of particles after the ball-milling. The secondary particles linked with each other to form a continuous structure, which functioned as an electrical pathway and a mechanical backbone so that all nanoparticles were electrochemically active. Overall, the calculations showed that the structure of this as-prepared composite anode plays an important role in the formation of C–P bonds.

image file: c7qi00184c-f19.tif
Fig. 19 (a) The HRTEM image and schematic of the BP-G composite. (b) The charge/discharge profiles of red P, BP/G, and BP-G electrodes at the first cycle between 0.01–2.0 V with a current density of 0.2 C. Reproduced with permission.112 Copyright 2014, American Chemical Society.

In early 2015, Ramireddy et al.113 prepared a BP/C composite via ball-milling to evaluate BP in NIBs for the first time. However, the electronic performance was not satisfactory. They analyzed this inferior performance and determined that the BP nanoparticles did not transform completely, resulting in electrode disintegration and delamination. Unfortunately, they did not provide a solution to this failure.

Sun et al. (2015)114 fabricated a sandwiched BP-G hybrid material after their discovery of BP-G bonds in 2014, the sandwich structure of which was achieved by mixing N-methyl-2-pyrrolidone (NMP) dispersions of BP and graphene, continued by self-assembly after NMP evaporation in an argon-filled glove box. The material shows a specific capacity of 2440 mA h g−1 at a current density of 0.05 A g−1 and an 83% capacity retention after 100 cycles between 0 and 1.5 V. The BP-G sandwiched structure has very good cycling capability due to its enhanced electrical conductivity, and BP nanosheets with increased interlayer distance offer a short and effective diffusion distance for Na ions; thus the corresponding electrodes possess a high reversible capacity.

In 2016, Xu et al.115 reported a high performance nano-structured BP/Ketjen black-MWCNTs (BPC) composite anode material for room-temperature NIBs that is fabricated by the high energy ball milling method. MWCNTs are introduced to form a dual conductive network and increase the structural stability of the composite. With this strategy, the BPC composite with a high phosphorus content (70 wt%) could deliver a very high initial Coulombic efficiency, (>90%), and a high specific capacity with excellent cycle-rate at a high rate of charge/discharge (∼1700 mA h g−1 after 100 cycles at 1.3 A g−1 based on the mass of P). Nano-crystalline BP was transformed into crystalline Na3P through an amorphous NaP intermediate during the hydroxide process and then converted back to amorphous phosphorus with a small amount of amorphous NaP remaining when the hydroxide process was reversed. They used several methods such as EIS, XRD, S/WAXS, TEM and NMR in order to understand its superior sodium storage performance. They found that both the nano-sized phosphorus particles and the nano-carbon matrix were successfully assembled into micron-sized secondary particles in order to maximize the loading efficiency of active materials in the electrode laminate while maintaining excellent capacity retention for long-term cycling.

A few months later, Dahbi et al.26 studied the effects of electrolyte additives on the electrochemical performance of black BP/PANa (sodium acrylate) composite electrodes in NIBs. They examined the influence with two different additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the SEI formation, and the origin of irreversible capacities. BP as an active material is prepared from RP under high-pressure and high temperature conditions. During electrochemical testing, the BP/PANa electrode showed high initial charge and discharge capacities of 2050 and 1610 mA h g−1, respectively, with an average Columbic efficiency of 97% from the second cycle. This is the first time that such high capacities and cycle-rates have been reported for BP electrode materials with the VC additive for Na cells. Moreover, they found that the FEC and VC are both efficient electrolyte additives for the NIBs by forming SEI with different surface stabilization mechanisms. The SEI surface layers modified by the FEC and VC will attain longer cycle life and higher reversibility of the hydroxide process of the black P electrode.

5. Summary and outlook

In conclusion, P materials are a new type of energy storage anodes with high energy density. Herein, we have reviewed the preparation methods for P-based materials and their related electrodes. Overall, the materials have a high reversible specific capacity and good cycling stability, which guarantees their suitability for high capacity secondary batteries. The voltage platform level of P composite materials is fairly clear, and their charged state can be accurately monitored via battery voltage when they are used in batteries. P composite materials have been the focus of recent research, highlighting their potential for widespread applications and favorable market outlook. Although significant progress has been made in understanding P composite materials, several significantly severe problems persist that must be addressed. For instance, the optimization of the matrix material structure is needed to improve the performance of P materials. Although some matrix materials have been reported to improve the performance of P materials to some extent, the structure of an optimal matrix material, which would completely solve the two key problems above, remains unclear. It is important to combine the excellent performance with the necessary improved method to solve the problems we now face. High energy density means high safety risks, and it is the responsibility of the scientists in this field to meet all the safety requirements and efficiently store the sustainable ‘green’ power sources.

Moreover, the electrochemistry community should strive to develop optimal power sources with excellent performance and the necessary safety features. Currently, such outstanding issues are continually being addressed to determine an optimal matrix material and guide future directions of practical P anode materials. In addition to materials synthesis, P-based material electrodes and batteries, including their preparation and characteristics, will become an essential and exciting research topic for further study. Related nanoscale composite materials may have higher specific capacitance and other electrochemical performances which have the potential to be used in supercapacitors and other energy storage devices. Composite nanostructures not only compensate for the disadvantages of the separate components but also incorporate the advantages of all the constituents.


This work was supported by the Program for New Century Excellent Talents of the University in China (NCET-13-0645), National Natural Science Foundation of China (NSFC-21201010, 21671170 and 21673203), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (164200510018), Program for Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN004), Six Talent Plan (2015-XCL-030), and Qinglan Project. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received from the Testing Center of Yangzhou University.


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