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Chem Soc Rev Review Article between the phosphorene layers. In addition, orthorhombic black phosphorous with its layered crystal structure is thermo- dynamically the most stable allotrope.513 The higher bulk conductivity of black phosphorus compared with Red-P is addi- tional advantages for electrochemical activity in Na cells.514,515 Hembram et al. proposed an atomistic mechanism for the sodia- tion of black phosphorus, based on first principles calculations.521 The layered structure of black phosphorus is maintained up to the composition of Na0.25P, with one-dimensional sodiation (an intercalation process) occurring in the interlayer spaces of the black phosphorus, resulting in sliding of the phosphorene layers because one Na atom tends to bind to four P atoms. At Na levels beyond Na0.25P, the intercalation process changes to an alloying process. Although the sodiation mechanism changes from an intercalation process to an alloying process at critical composition of Na0.25P, the volume expansion of black phos- phorus increases linearly with Na concentration. Ramireddy et al. prepared nanocomposites of black (orthorhombic) phos- phorus with graphite carbon via ball milling and applied the electrochemical test at different cut-off voltage windows.516 Within the voltage window of 0.01–2 V Na/Na+, the composite anode exhibited a high initial capacity of 1300 mA h g1, however, the capacity gradually decreased. In contrast, attractive stable cyclic performances over 100 cycles were observed in the voltage windows of 0.33–2.0 V vs. Na/Na+. The post-cycling SEM studies showed that the electrodes gradually disintegrated and delaminated from the current collectors when electrochemical testing was performed within a larger potential window of 0.01–2.0 V vs. Na/Na+. However, this effect was absent for the restricted potential windows of 0.33–2.0 V vs. Na/Na+, leading to stable cyclic performances. To achieve both high capacity and stable cyclability, Sun et al. proposed a nanostructured phosphorene– graphene hybrid with a few phosphorene layers sandwiched between graphene layers512 (Fig. 39f). They also investigated the two-step sodiation mechanism of intercalation and alloying using in situ transmission electron microscopy (TEM) and ex situ X-ray diffraction (XRD) techniques. This nanoarchitecture delivers several advantages: (1) the graphene layers provide an elastic buffer layer to accommodate the anisotropic volumetric expansion during the sodiation process, (2) the phosphorene layers with an increased interlayer distance offer a short diffu- sion length for sodium ions, and (3) the graphene layers function as an electrical highway. As a result, the phosphorene–graphene hybrid nanostructure exhibited an extremely high specific capa- city of 2440 mA h g1 at 0.05 A g1 and 83% capacity retention after 100 cycles in the voltage range of 0–1.5 V vs. Na/Na+. Recently, to provide an in-depth understanding of the reaction mechanism and the solid electrolyte interface (SEI) formation process of black phosphorous anodes in Na cells, Dahbi et al. investigated the structural change upon the sodiation/desodiation process via ex situ XRD analysis and electrode/electrolyte inter- faces via powerful surface characterization techniques such as HAXPES and TOF-SIMS analyses.522 In the as-prepared electrode, black P exists as a crystalline phase with the orthorhombic lattice. In the full reduction state at 0 V in Na cells, orthorhombic black P changes into trisodium phosphide (Na3P) with a hexagonal lattice. anode material, and its light atomic weight can achieve a higher theoretical capacity of 2596 mA h g1 than any other SIB anodes presently available.510–512 Phosphorous exists in three main allotropes of white phosphorus, red phosphorus, and black phosphorus512,513,518 (Fig. 39a). White phosphorus (white-P) is volatile and unstable; it bursts into flames when it is exposed to the natural atmosphere. Red phosphorus (red-P) is usually amorphous in nature and is widely commercially available. Black phosphorus (black-P) is a crystalline phase, thermodynamically stable below 550 1C, and transforms into red (amorphous) phosphorus at higher temperature.512 For this reason, the amor- phous red-P and orthorhombic black-P forms are being widely studied as anodes for SIBs. However, the electrochemical proper- ties of both red-P and black-P are hindered by the enormous volume change (490%) occurring during the electrochemical sodiation/desodiation process.513 Qian et al. reported the improved electrochemical activity of amorphous red-P carbon composites (a-P/C) compared to pure red-P and black-P514 (Fig. 39b). Upon sodiation/desodiation, the pure red P shows a quite large discharge (sodiation) capacity of 897 mA h g1 but gives only a negligible charge (desodiation) capacity of 15 mA h g1, indicating the inactivity of this material for a sodium ion insertion reaction because of its insulating electronic nature (electrical conductivity of red-P: below 1 1014 S cm1). In contrast, a-P/C exhibited greatly enhanced electrochemical perfor- mance with initial charge/discharge capacities of 2015 mA h g1 and 1764 mA h g1, respectively, and a very high initial Coulombic efficiency of 87%, suggesting that the amorphous structure of phosphorus can effectively buffer the strong volumetric expansion during cycling. Over the same period of time, Kim et al. reported on an amorphous red phosphorus/carbon composite anode, which exhibited an appropriate redox potential of ca. 0.4 V vs. Na/Na+ with a reversible capacity of 1890 mA h g1 and good rate capability delivering 1540 mA h g1 at a high current density of 2.86 A g1 511 (Fig. 39c). They also observed the formation of Na3P at the full sodiated state through ex situ XRD analysis (Fig. 39d). Recently, varieties of amorphous phosphorous with nanoarchitec- tures and 2D or 3D carbon matrices having high conductivity were applied to achieve a high capacity and a stable cycle life.511,513–519 Song et al. reported a novel phosphorus/graphene nanosheet hybrid through a facile ball milling process513 (Fig. 39e). The graphene stacks are mechanically exfoliated to nanosheets that chemically bond with the surfaces of phosphorus particles. This chemical bonding facilitates robust and intimate contact between phosphorus and graphene nanosheets. Furthermore, the graphene at the particle surfaces can assist to maintain electrical contact and stabilize the solid electrolyte interphase upon the large volume change of phosphorus during cycling. As a result, this composite anode delivers a high reversible capacity of 2077 mA h g1 with an excellent cycling stability of 1700 mA h g1 after 60 cycles. On the other hand, black phos- phorus is potentially very attractive, as it has a layered structure similar to graphite but a greater interlayer distance.512,520–523 Namely, black phosphors are composed of the two dimensional single layer of phosphorene, which has a large interlayer channel size (3.08 Å), meaning that sodium (1.04 Å) ions can be stored View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3585 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | Sodium-ion batteries present and future
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