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Review Article Chem Soc Rev known that the main reason for the faster capacity fading of the phosphorous anode is the continuous pulverization during the sodiation–desodiation process.240 One strategy for dealing with this problem is fabricating a binary metal–phosphide form by employing secondary metals (M–P, M = Ni, Fe, Co, Cu and Sn).455–466 This is because if these elements can form an intermediate compound (NaxM or NaxP, x Z 0,) during the charge–discharge process, pulverization can be partially repaired and the accumulation of pulverization can be terminated.460,462 Therefore, a combined two-step reaction consisting of conver- sion and alloying is a very effective way to overcome the huge volume expansion issues. An excellent example of this process is tin–phosphorus compounds of Sn4P3. Kim et al. prepared an intermetallic compound of Sn4P3 by facile high-energy mechanical ball milling and demonstrated its electrochemical performance as an anode material for SIBs.460 This material delivered a reversible capacity of 718 mA h g1 and very stable cycle performance with negligible capacity fading over 100 cycles with an appropriately low redox potential of about 0.3 V vs. Na/Na+. These properties of the Sn4P3 electrode can be ascribed to the fact that the pulverization of Sn and P during the alloy process was partially self-healed by the conversion reaction process462 (Fig. 33a). Recently, Liu et al. proposed uniform yolk–shell Sn4P3@C nanospheres.463 The rationally designed void space in between the shell and nanoparticles allows for the expansion of Sn4P3 without deforming the carbon shell or disrupting the SEI on the outside surface. As a result, yolk–shell Sn4P3@C nanospheres exhibited superior excellent cycling per- formance for over 400 cycles (Fig. 33b). The enhancement in Sn4P3 can be attributed to a reversible reaction of Sn4P3 + 9Na 2 4Sn + 3Na3P, which repairs the cracks, damage, and aggregation of Sn particles that occurred in the alloy process of 4Sn + 15Na 2 Na15Sn4 during cycling and, hence, terminates pulverization. It means that the damage can be healed by itself during cycling465 (Fig. 33c). Similar to Sn4P3 materials, the stable cycling perfor- mances of SnP3/C composites were derived by the self-healing effect of the conversion reaction for the alloying process.462 To date, binary-intermetallic systems such as NiP3,455 (CuP2,456 Cu3P457), FeP458 and CoP,464 FeP4466 exhibit impressive results, however, these binary inter-metallic systems still need to improve in terms of material design and electrode formulation for high performance practical SIBs. 3.3 Alloying reaction materials Na+ insertion materials, such as carbonaceous materials and titanium-based oxide compounds, have been successfully applied as Na storage materials that deliver a reasonable capacity with relatively small volume expansions during the electrochemical insertion/extraction reaction with Na.241,300 However, these materials still suffer from limited capacity utilization due to their intrinsic constraint ascribed to their structures, which lowers the specific energy density of SIBs. Similar to the conversion materials, alloying materials can be suggested as attractive anodes for SIBs because they can store a large number of sodium ions in the host structure with a relatively low operating potential (below 1.0 V).244,245 Multiple reactions below 0.8 V, FeS2 nanocrystals led to the formation of Na2S involving most likely only amorphous phases. This amorphous state can effectively reduce mechanical stress upon expansion and contraction during cycling. The FeS2 nanocrystals delivered a high capacity above 500 mA h g1 for 400 cycles at a current density of 1 A g1. Compared to pyrite (FeS2), a few studies on ferrous sulfide (FeS) are reported due to poor cyclability and rate capability.431,432 Recently, Wei et al. proposed the flexible and self-supported carbon-coated FeS on carbon cloth films, which display high reversible capacity and superior rate capability.431 3.2.2.4. Tin sulfides. Tin-based sulfide (SnS, SnS2) compounds have attracted considerable attention due to their high theoretical capacity with combined conversion and alloying electrochemical reactions.433–437 Wu et al. proposed tin-sulfide nanocomposite (SnS–C) anode materials based on combined conversion and alloying reactions: SnS + 2Na+ + 2e 2 5Na2S + Sn (conversion reaction), Sn + 3.75Na+ + 3.75e 2 Na3.75Sn (alloying reaction)433 (Fig. 32d). The prepared SnS–C composite has a small crystalline size of SnS and good carbon coating, which synergistically facil- itates electrochemical utilization and maintains the structural integrity. As a result, the prepared SnS–C composite exhibited a high Na storage ability delivering a capacity of 568 mA h g1 at 20 mA g1 and an excellent cycling stability of 97.8% after 80 cycles as well as a high-rate capability. Later, Zhu et al. proposed a 3D porous interconnected metal sulfide/carbon nanocomposite by the ESD technique without adding carbonaceous materials such as carbon nanotubes and graphene.436 SnS2 has also been applied as an anode material for SIBs.438–448 SnS2 has a sandwich structure that consists of covalently bonded S–Sn–S trilayers separated by a relatively large van der Waals interaction.438 A large interlayer d-spacing of 5.90 Å can effectively accommo- date Na+ ions.439–441 In addition, intermediate products of amorphous NaS2 suppress the pulverization and aggregation during Na–Sn alloying reactions.440 Liu et al. synthesized exfoliated-SnS2 restacked on graphene that ultrasmall SnS2 nano- plates (with a typical size of 20–50 nm) composed of 2–5 layers are homogeneously decorated on the surface of graphene.442 This unique structure facilitates Na+ ion diffusion and delivers a high capacity of 650 mA h g1 at 200 mA g1 with stable cyclability at B610 mA h g1 without notable capacity fading for 300 cycles (Fig. 32e). SnS2 can store the Na+ ions through following three step processes: (1) intercalation reaction: xNa+ + SnS2 + xe - NaxSnS2, (2) conversion reaction: 4Na+ + SnS2 + 4e - 2Na2S + Sn, (3) alloying reaction: Sn + 3.75Na+ + 3.75e - Na3.75Sn. Such improved electro- chemical performances of conversion materials are attributed to the electro-conducting carbons. The introduction of carbon additives such as graphene and/or carbon nanotubes into active materials is indispensable to have advantages over conversion materials such as effective stress relief, accommodation of large volume expansion/shrinkage, and facilitation of electron and Na+ ion transport. 3.2.3 Transition metal phosphide (TMP). Phosphorus based transition metal phosphide (TMP) compounds have been investigated as promising anode materials for SIBs. It is well View Article Online 3578 | Chem. Soc. Rev., 2017, 46, 3529--3614 This journal is © The Royal Society of Chemistry 2017 Open Access Article. Published on 28 March 2017. 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