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Materials 2020, 13, 3453 29 of 58 template method and a solid state sulfuration method exhibited high specific capacity (812 mA·h·g−1 at 0.1 A·g−1), long cycling life (77.2% capacity retention after 350 cycles at 1 A·g−1) and excellent rate capability (400 mA·h·g−1 5 A·g−1) [325]. Liu et al. fabricated flexible Fe1-xS-filled porous carbon nanowires/reduced graphene oxide (Fe1-xS@PCNWs/rGO) hybrid paper in which the PCNWs encapsulated with in situ formed Fe1-xS nanoparticles (NPs) were evenly dispersed between the rGO nanosheets [326]. As an anode, this composite delivered a capacity of 573–89 mA·h·g−1 over 100 consecutive cycles at 0.1 A·g−1 with areal mass loadings of 0.9–11.2 mg·cm−2 and high volumetric capacities of 424–180 mA·h·cm−3 in the current density range of 0.2–5 A·g−1. This is a remarkable result, since the mass loading is a very important parameter for commercialization, and is usually small for SIBs (< 0.5 mA·h·cm−2) [327], compared with that of lithium-ion batteries today (2–3 mA·h·cm−2). Another iron sulfide of interest as an active element of anode SIBs is Fe7S8. Its theoretical capacity is 662 mA·h·g−1 according the reaction [328]: Fe7S8 + 16Na+ + 16e− → 7Fe + 8Na2S. (9) Moreover, its charge-discharge peaks of cyclic voltammetry after the first cycle (reduction peak at 0.92 V corresponding to the reaction between Na+ and Na2-xFeS2; oxidation peak at 1.38 V) [329] is lower than that of FeS2 (cathodic peaks at 1.2, 1.6 and 2.1 V, anodic peaks at 1.5, 2.0 and 2.5 V [330], which is beneficial for the energy density of full cells. Another advantage comes from the fact that Fe7S8 is a semi-metal. Despite the higher conductivity, the association with graphite is beneficial to the electrochemical performance, because it makes possible to increase the loading. The flexible anode 3D carbon-networks/Fe7S8/graphene of Chen et al. [330] used a flexible anode with areal mass loading of 3 mg cm−2 demonstrated a high areal capacity (2.12 mA·h·cm−2 at 0.25 mA·cm−2) and excellent cycle stability of 5000 cycles (0.0095% capacity decay per cycle). Copper sulfides have less been investigated. Nevertheless, promising results were obtained with CuS-RGO composite obtained by microwave-assisted reduction [331]. SnS nanoparticles anchored on three-dimensional N-doped graphene [332]. Oriented SnS nanoflakes were bound on S-doped N-rich carbon nanosheets by a hydrothermal method demonstrated high-rate capability (250.7 mA·h·g−1 at 20 A·g−1) and stable capacity retention (∼98% after 100 cycles at 1 A·g−1) as a SIB anode, with a dominating supercapacitance contribution [333]. Free-standing SnS/C nanofibers prepared by electrospinning used as an anode for SIBs retained a capacity of 481 mA·h·g−1 after 100 cycles at 50 mA·g−1, and 349 mA·h·g−1 at 200 mA·g−1 after 500 cycles [334]. Hollow ZnS-SnS@C nanoboxes encapsulated by graphene delivered a stable capacity of 302 mA·h·g−1 after 500 cycles at 500 mA·g−1 [335]. SnS2 embedded in nitrogen and sulfur dual-doped carbon nanofibers were synthesized using a facile electrospinning technique by Xia et al. [336]. The capacity of the corresponding anode remains at 380 mA·h·g−1 at 500 mA·g−1 after 200 cycles. At high current density of 4 A·g−1, the capacity was still 310 mA·h·g−1. The intercalation of Ni into the van der Waals gap of SnS2 exhibited an initial high reversible capacity of 795 mA·h·g−1 at 0.1 A·g−1, with a stable capacity retention of 666 mA·h·g−1 after 100 cycles. At a current density of 1 A·g−1, the capacity was 437 mA·h·g−1 [337]. To improve the coulombic efficiency caused by the partial irreversible conversion reaction of SnS2, Ou et al. fabricated heterostructured SnS2/Mn2SnS4/carbon nanoboxes by a facial wet-chemical method. Utilized as an anode, this composite delivered an initial capacity of 841 mA·h·g−1 with high ICE of 90.8%, excellent rate capability (488 mA·h·g−1 at 10 A·g−1) and delivered a capacity of 522 mA·h·g−1 at 5 A·g−1 after 500 cycles [338]. The SnS2/Mn2SnS4 heterojunctions were thus efficient to stabilize the reaction products Sn and Na2S. Wang et al. [339] synthesized SnS2 nanosheet arrays on a carbon paper, with a preferential (001) edge orientation, which facilitates rapid electrochemical reaction kinetics with preferential edge orientation. At current density of 50 mA·g−1, this binder-free anode delivered discharge and charge capacities of 1056 and 647 mA·h·g−1, respectively. After this irreversible loss of capacity associated to the formation of the SEI, the coulombic efficiency was better than 98%, and a capacity of 631 mA·h·g−1 was retained after 150 cycles.PDF Image | Electrode Materials for Sodium-Ion Batteries
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