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Chem Soc Rev Review Article mechanical buffer to accommodate for the volume changes, which can provide a more stable structure and more efficient electronic conduction during cycling.532,535,542 Liu et al. reported highly porous Ni3Sn2 microcages composed of tiny nanoparticles532 (Fig. 40a). According to their report, the Ni–Sn intermetallic anode can act as storage for Na ions via the following sodiation– desodiation mechanisms: Ni3Sn2 + 7.5Na+ 7.5e - 2Na3.75Sn + 3Ni, Na3.75Sn - Sn + 3.75Na+ + 3.75e. After the first sodiation process, the Ni3Sn2 porous microcages are converted into both in situ formed zero-dimensional electroactive Na–Sn particles and three-dimensional conducting Ni in hollow matrix form. The mechanical strain of Sn during charge/discharge processes is effectively suppressed by the hollow core structure and the presence of the Ni matrix in the hollow microcages. Moreover, homogeneously encapsulated Ni converted from the sodiation of Ni3Sn2 is beneficial for the necessary electron transport. As a result, it demonstrated a high reversible capacity of 348 mA h g1 and a stable cycle retention of B91% after 300 cycles at a 1C-rate. Antimony based binary intermetallic compounds, including copper–antimony (Cu2Sb) and iron– antimony (FeSb2) compounds, were also investigated.533,534 During the sodiation process, the reaction starts with the conversion of Cu2Sb and FeSb2 followed by the formation of nanocrystalline Na3Sb and the amorphous phase of Na–Cu–Sb and Fe4Sb, respectively. However, the irreversible formation of intermediate compounds leads to a lower reversible storage capacity than that for a pure Sb electrode. On the other hand, in the case of active alloying elements, Sn–Sb and Sn–P (in Section 3.2.3.1) binary-compounds have received much atten- tion as anode materials for SIBs due to their high reversible capacity and stable capacity retentions. Xiao et al. reported a high capacity with the reversible alloy reaction in SnSb/C nano- composites for SIBs for the first time.499 Based on the CV results, they demonstrated Na+ storage, which is composed of the alloying–dealloying reaction of Na–Sb and Na–Sn as well as Na insertion into super P carbon (Fig. 40b). Further, initial charge–discharge voltage profiles of SnSb/C nanocomposite electrodes revealed two main plateaus, of which the plateau in the higher potential region (around 0.45 V at discharge and 0.58 V at charge) is mainly related to the Na–SnSb alloying– dealloying reaction that produces Na3Sb and metallic Sn. The plateau in the lower potential range (0.05 V at discharge and 0.17 V at charge) is mainly attributed to the Na-ion insertion into super P carbon and the Na–Sn alloying–dealloying process. After initial cycles, however, most of reversible capacity is led by the alloy reactions. According to the above results, the detailed sodium storage mechanism of the SnSb/C nanocomposite electrodes can be described as follows: SnSb + 3Na+ + 3e 2 3Na3Sb + Sn, Na3Sb + Sn + 3.75Na+ + 3.75e 2 3Na3Sb + Na3.75Sn. Later, Ji et al. demonstrated the improved electrochemical per- formance of SnSb binary inter-metallic compounds through employing a porous carbon fiber and controlling the SEI for- mation using FEC additives.539 These porous CNF–SnSb nano- composite electrodes delivered a high reversible capacity of 350 mA h g1 at 0.2C, an excellent capacity retention for more than 200 cycles and an enhanced reversible capacity of more than The reduction product is found to be the same as red P. After forming Na3P, the cell is oxidized to 2.0 V, and reformation of the crystalline phase of black P was not found. This observation shows that black P is a metastable polymorph, and amorphous P would be formed as the oxidation product in the Na cell.522,524 They also observed that the VC-added electrolyte especially improves the reversible capacity and achieves a longer cycle life for black P electrodes with NaPF6 in EC/DEC by forming the stable SEI. On the other hand, Xu et al. proposed a black phosphorous– ketjenblack-multiwalled carbon nanotube composite with a high phosphorous loading of 70% as an anode for SIBs.524 3.3.2.3. Bismuth. Bismuth (Bi), belonging to the same group in the periodic table as phosphorus and antimony, has been recently regarded as a potential anode material for LIBs525,526 and SIBs527–531 due to the unique layered crystal structure with a large interlayer spacing. Ellis et al. reported that the sodiation and desodiation mechanisms reversibly follow the Na–Bi equili- brium phase diagram with the formation of NaBi and Na3Bi.527 Bi reacts with Na to form Na3Bi, giving a theoretical capacity of 385 mA h g1. Later, Sottmann et al. showed that alloying of sodium and bismuth proceeds via two distinct structural mechanisms depending on the crystallite size in the Bi/C anode.529 The transformation of NaBi into c-Na3Bi (c: cubic) requires less disturbance of the crystal structure than NaBi into h-Na3Bi (h: hexagonal) conversion. According to their report, phase fractions of the Na–Bi phases in the charged (2 V) and discharged state (0 V) in the 100th cycle show that c-Na3Bi is favored in the nanocrystalline anode as it forms on the crystallite surfaces. On the other hand, through DFT simulations, Su et al. calculated that Bi could provide facile sites for Na+ ion diffusion and accommodation, based on the intercalation mechanism instead of the alloying process.530 Their ex situ XRD and TEM results consistently showed that bismuth undergoes the Na+ ion intercalation process in Na cells. They also prepared a bismuth– graphene nanocomposite (Bi@graphene) and demonstrated its sodium storage performances in a bismuth crystal structure. The Bi@graphene nanocomposite demonstrated reasonable rate performance ascribed to the unique layered crystal structure of Bi, which has a large interlayer spacing along the c-axis (d(003) = 3.95 Å) to accommodate the Na+ ions. 3.3.3 Binary inter-metallic compounds. Another approach to develop a high-performance anode materials is design of binary intermetallic alloys which is resulting in new physio- chemical properties. Most studies on binary alloys are focused on developing, in particular, Sn–M and Sb–M compounds (M = metal). Among them, secondary element M is classified into two main categories; namely, electrochemical inactive elements532–537 (Ni, Cu, Fe, Zn and Mo) and electrochemical active elements538–541 (Sn, Sb, Bi) through an alloying–dealloying reaction with Na. During the sodiation/desodiation process, these compounds can store Na+ ions through two electro- chemical reaction mechanisms of conversion and alloying. In this binary compound system, the primary beneficial role of the secondary elements M is to improve the cycling performances. And, the two different intermediate phases can work as a View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3587 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. 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