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Chem Soc Rev Review Article 0.72 V is related to the conversion reaction of Co3S4 with Na and the formation of a solid electrolyte interphase: NaxCo3S4 + (8 x)Na+ + (8 x)e - 4Na2S + 3Co. The flexible PANI layers coated on both outer and inner surfaces of Co3S4 nanotubes would form ‘‘protective layers’’, thus preventing structure collapse and pulverization of the Co3S4 nanotubes during cycling. 3.2.2.2. Molybdenum sulfides. In the case of molybdenum disulfide (MoS2), Mo and S atoms are covalently bonded to form 2D S–Mo–S trilayers and the adjacent planes are stacked by van der Waals interactions, which facilitate intercalation of the large Na+ ion.411,414 According to previous reports, MoS2 can store the Na+ ions through the intercalation and/or conversion reactions depending on the operation voltage window.413,415–417,419 The electrochemical reaction of MoS2 is interpreted as the following two step reactions: MoS2 + xNa+ + xe - NaxMoS2 (above0.4V),NaxMoS2 +(4x)Na+ +(4x)e -2Na2S+Mo (below 0.4 V).413 Hu et al. prepared MoS2 nanoflowers with expanded interlayer spacing of the (002) plane, which exhibited stable electrochemical performances that followed an interca- lation reaction by controlling the cut-off voltage to 0.4–3.0 V413 (Fig. 31d). As a result, this material delivered a high discharge capacity of 350 mA h g1 at 50 mA g1 and stable cycle retention for over 1500 cycles. On the other hand, the intercalation reaction of MoS2 at a limited voltage above 0.4 V delivered a lower specific capacity in consideration of their theoretical capacity of 668 mA h g1 when 4Na+ ions reacted with MoS2 through the conversion reaction.417,419 However, a conversion type chemical reaction usually brings about a serious volume change to the electrode materials and sluggish kinetics for reconstruction of the original active materials. Recently, to over- come such problems through the electrochemical conversion reactions of MoS2 (below 0.4 V), high conductivity carbon additives and novel nanoarchitecture design were proposed.414–422 Su et al. prepared few-layer MoS2 nanosheets and conducted the electro- chemical test in the voltage range of 0.01–3.0 V.418 The results show that few-layer MoS2 nanosheets exhibited a high capacity of 530 mA h g1 and a high rate capability. Xie et al. prepared a series of sheet-on-sheet structured MoS2/RGO nanocomposites and investigated the effect of heterointerfacial areas on sodium storage performances.419 Computational calculation and experimental results show that the 2D MoS2/rGO heterointerface can increase the conductivity of MoS2 and capture more Na atoms due to maintaining the high diffusion mobility of Na+ ions on the MoS2 surface and high electron transfer efficiency from Na to MoS2, respectively. As a result, this material delivered a high capacity of 352 mA h g1 even at a high current density of 640 mA g1 in the voltage range of 0.01–3.0 V (Fig. 31e). 3.2.2.3. Iron sulfides. Natural and/or synthetic FeS2 materials have been demonstrated to be potential electrode materials among metal sulfides due to their high theoretical capacity of 894 mA h g1 and environmental friendliness. Ahn and co-works reported a Na/synthetic FeS2 battery for the first time423,424 (Fig. 32a). Later, Hu et al. demonstrated room temperature sodium storage performances of the FeS2 microspheres with sulfides have great advantages during the sodiation/desodiation process. The M–S bonds in metal sulfide are weaker than the corresponding M–O bonds in metal oxides, which can be kine- tically favorable for conversion reactions with Na+ ions.410 As a result, transition metal sulfides show improved mechanical stability due to their smaller volume changes and higher initial Coulombic efficiency due to better reversibility of Na2S than that of Na2O during the sodiation–desodiation process.415 Therefore, various metal sulfides have been extensively investigated as high capacity anode materials for LIBs and SIBs such as cobalt sulfides (CoS, CoS2),400–410 molybdenum sulfides (Mo2S, MoS2),411–419 iron sulfides (FeS, FeS2),423–432 tin sulfides (SnS, SnS2),433–445 copper sulfide (CuS),446 manganese sulfide (MnS),447 nickel sulfide (NiS),448,449 titanium sulfide (TiS2),450 tungsten sulfide (WS2),451,452 and zinc sulfide (ZnS).453,454 Depending on the transition metal elements, the Na+ ion storage mechanism of metal sulfide materials can be classified as the conversion reaction and/or combined insertion and the alloying reaction. 3.2.2.1. Cobalt sulfides. Cobalt sulfide is an interesting metal chalcogenide semiconductor material and has a number of applications.400–403 Recently, superior Li-storage properties of cobalt sulfide were achieved by designing a novel nanoarchi- tecture and fabricating hybrid nanocomposites with various carbon additives.404–406 Shadike et al. fabricated CoS2 and CoS2–MWNCT materials and investigated their electrochemical performances with a Na storage mechanism in ether-based electrolytes and commonly used carbonate-based electrolytes. They also investigated the evolution processes of CoS2 and CoS2–MWCNT during cycling via ex situ TEM407 (Fig. 31a). During the sodiation/desodiation process, CoS2 can accommo- date Na+ ions as follows: CoS2 + 4Na+ + 4e 2 Co + 2Na2S. Both CoS2 and CoS2–MWCNT electrodes have a similar sodium storage mechanism, but their electrochemical performances are quite different. Using MWCNTs as carbon additives provides several advantages such as 3D electron conductive networks with a high surface area, which facilitate the fast penetration of sodium ions and diffusion of electrolytes. As a result, the CoS2–MWCNT electrode exhibited a high initial discharge capa- city of 826 mA h g1 with a high Coulombic efficiency of 93%, and a stable cycle life for 100 cycles in an ether-based electrolyte (1 M NaCF3SO3–DGM) (Fig. 31b). Later, Peng et al. proposed the unique hybrid nanocompo- site of cobalt sulfide (CoS) nanoplates anchored onto reduced graphene oxide (rGO) sheets and demonstrated the impressive high specific capacity of 540 mA h g1 at 1 A g1, excellent rate capability, and superior cycle retention of 88% after 1000 cycles in Na cells.409 More recently, Zhou et al. fabricated Co3S4@PANI (polyaniline) nanotubes, in which polyaniline is uniformly coated on both the exterior and inner surfaces of Co3S4 nano- tubes (Fig. 31c).410 Based on the CV results in the potential window of 0.05–2.0 V (vs. Na+/Na), they claimed that Co3S4 electrodes can store Na+ ions through the combined insertion and conversion reaction. In the first scan, a cathodic peak at 0.98 V is commonly assigned to an initial process of the Na+ insertion reaction: Co3S4 + xNa+ + xe - NaxCo3S4. The peak at View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3575 Open Access Article. 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