Atrial Lead Malposition in a Dual Chamber (DDD,M) Pacemaker

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Metal–Organic Framework-Derived Materials for Sodium Energy Storage Guoqiang Zou, Hongshuai Hou, Peng Ge, Zhaodong Huang, Ganggang Zhao, Dulin Yin, and Xiaobo Ji* Due to their low maintenance cost, relatively high energy conversion efficiency, and convenience, secondary electrochemical batteries are effective devices to incorporate these sporadic electric energies for large-scale energy storage.[2] Lithium-ion batteries (LIBs), one of the most successful secondary electrochemical batteries, have been extensively applied in diverse fields, including portable electronic equipment, hybrid electric vehicles, and electric vehicles.[3] However, an explosion in the demands for LIBs has elevated the price of lithium sources, which may restrict their large-scale application in energy storage due to the scarcity (20 ppm) and extremely uneven distribution of lithium in the Earth’s crust (Table 1).[4] Hence, batteries with abundant sources and low price are urgently desired for large-scale energy storage. Sodium-ion batteries (SIBs), explored in the 1970s around the same as LIBs but largely disregarded after the commercial application of LIBs in the 1990s, have regained broad attention in recent years and have been deemed powerful alternatives to LIBs due to abundance of sodium minerals (23 600 ppm), their much lower cost, and similar electrochemical properties to that of LIBs (Table 1).[1a,4c,e,5] The electrical energy storage/release processes in SIBs involve the insertion/extraction of sodium ions into/out of the negative and positive electrode materials, which is similar to the rocking-chair principle of LIBs.[2a,4c] Furthermore, compared to the Cu current collector used in LIB anode materials, low-cost aluminum foil can be utilized as the current collector for both electrodes in SIBs, which will largely decrease the cost and weight of the Na-ion complete cells. Nevertheless, the ionic radius of the sodium ion is larger than that of the lithium ion (Table 1), and thus, many electrode materials with excellent storage performances in LIBs show poor performances in SIBs, displaying poor rate capabilities resulting from sluggish diffusion kinetics, which simultaneously damages the phase stability, transport properties, and solid electrolyte interface (SEI) formation.[6] Additionally, the molar mass of the sodium ion is heavier than that of the lithium ion, and the Na+/Na reduction potential versus the standard hydrogen electrode (SHE) (−2.71 V) is more positive than that of Li+/Li (−3.04 V), resulting in a low operating voltage and poor energy density. These disadvantages make developing suitable electrode materials for SIBs

Recently, sodium-ion batteries (SIBs) are extensively explored and are regarded as one of the most promising alternatives to lithium-ion batteries for electrochemical energy conversion and storage, owing to the abundant raw material resources, low cost, and similar electrochemical behavior of elemental sodium compared to lithium. Metal–organic frameworks (MOFs) have attracted enormous attention due to their high surface areas, tunable structures, and diverse applications in drug delivery, gas storage, and catalysis. Recently, there has been an escalating interest in exploiting MOF-derived materials as anodes for sodium energy storage due to their fast mass transport resulting from their highly porous structures and relatively simple preparation methods originating from in situ thermal treatment processes. In this Review, the recent progress of the sodium-ion storage performances of MOF-derived materials, including MOF-derived porous carbons, metal oxides, metal oxide/carbon nanocomposites, and other materials (e.g., metal phosphides, metal sulfides, and metal selenides), as SIB anodes is systematically and completely presented and discussed. Moreover, the current challenges and perspectives of MOF-derived materials in electrochemical energy storage are discussed.

1. Introduction Today, fossil fuels, such as coal, oil, and natural gas, are the most widely utilized energy sources around the world. A series of global problems related to the energy crisis, greenhouse effect, and other environmental pollutions derived from nonrenewal of fossil fuels have urged the fast development of the cleaner energy sources, including solar, wind, and wave energy.[1] Nevertheless, integrating intermittent renewable energy sources into the grid and constantly harmonizing electricity generation and consumption are also challenging. Dr. G. Zou, Prof. H. Hou, Prof. X. Ji State Key Laboratory of Powder Metallurgy Central South University Changsha 410083, P. R. China E-mail: [email protected] Dr. G. Zou, Prof. H. Hou, Dr. P. Ge, Z. Huang, G. Zhao, Prof. X. Ji College of Chemistry and Chemical Engineering Central South University Changsha 410083, P. R. China Prof. D. Yin National and Local United Engineering Laboratory for New Petrochemical Materials and Fine Utilization of Resources Hunan Normal University Changsha 410081, P. R. China

DOI: 10.1002/smll.201702648

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difficult and thus delay the development of SIBs. Many materials, including Prussian blue (PB) analogs,[7] transition metal sulfides (MSs),[8] transition metal oxides,[9] organic cathode materials,[10] and polyanionic compounds,[11] have been extensively explored as cathode materials for SIBs. Nevertheless, developing a suitable anode material with a large reversible capacity, high stability, and desirable high energy density for SIBs is also a challenge. Graphite, initially explored as an anode material for LIBs in the 1980s, is the most successful commercial anode material for LIBs.[2a] Unfortunately, graphite does not suitably intercalate sodium ions due to the large radius of Na+. Ge and Fouletier investigated the electrochemical intercalation behavior of sodium in graphite and inferred a theoretical capacity of 35 mA h g−1 (NaC64).[12] For a long time, nearly no progress on the exploration of SIB anodes occurred. However, in 2000, Stevens and Dahn reported that hard carbon possessed a reversible capacity of 300 mA h g−1, almost equal to the lithium insertion capacity in graphite.[13] Compared to the long cycling life of LIB anodes, the cycle performance was insufficient for large-scale application; however, this finding reignited interest in finding a suitable anode material. Subsequently, many materials were developed as SIB anodes, including metals/alloys,[14] metal oxides,[15] MSs,[16] metal phosphides (MPs),[17] phosphorus,[18] and carbonaceous materials.[19] Although metals, alloys, and metal oxides delivered high specific capacities, these materials typically suffered from poor cycling lives caused by large volume expansions during Na+ insertion/extraction. Therefore, ample sources and stable anode materials with good cycling stabilities, high energy densities, and fast rate performances are the key factors for the development of SIBs. Metal–organic frameworks (MOFs), initially named by Yaghi and Li in the 1990s, are highly porous materials composed of transition metal ions and organic linkers through covalent coordination linkages.[20] In the last two decades, MOFs have drawn tremendous attention for their high specific surface areas, ultrahigh porosities, well-defined pore structures, and abundant organic ligands/transition metal ions.[21] In addition, the porosities and structural topologies can be effectively tuned by various combinations of metal ions and organic linkers.[22] Given the advantages of MOFs, more than 20 000 MOFs with diverse crystal structures and morphologies have been explored and applied in many fields, including catalysis,[23] drug delivery,[24] energy storage,[25] gas storage/separation,[26] magnetism,[27] fluorescence and sensing,[28] optoelectronics,[29] and proton conduction.[30] In addition to their controllable pore sizes, large surface areas and highly stable structural materials, MOFs can be prepared with high yields through consuming low-cost raw materials (transition metal salt and organic ligands), making these materials promising candidates for next-generation clean energy storage. Recently, some studies on the direct use of MOFs in supercapacitors,[31] fuel cells,[32] and LIBs/SIBs[33] have been reported. However, reports on the direct utilization of MOFs in SIB anodes are limited, which may be attributed to their poor electrical conductivity and instability in organic electrolyte. To conquer these difficulties, through thermal treatment methods, many nanostructured materials have been exploited utilizing MOFs as sacrificial templates, such as MOF-derived porous carbon,[34] metal oxides,[24d,35] and metal/metal oxide/ carbon nanocomposites.[36] Notably, MOF-derived materials can

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Guoqiang Zou is a Ph.D. candidate at the College of Chemistry and Chemical Engineering, Central South University. His present research interests are MOF-based and carbonbased materials as anodes for sodium- and lithium-ion batteries.

Hongshuai Hou is an Associate Professor at the College of Chemistry and Chemical Engineering, Central South University. He received his Ph.D. from Central South University (2016). His current research interests are electrochemistry and key materials for electrochemical energy storage devices. Xiaobo Ji is a “Shenghua” Professor at Central South University and a Fellow of the Royal Society of Chemistry, specializing in the research and development of batteries and supercapacitor materials and their systems. He received his Ph.D. in Electrochemistry (2007) under the supervision of Prof. Richard Compton at the University of Oxford and undertook postdoctoral work at MIT with Prof. Donald Sadoway. preserve the porous structures and high surface areas of the MOFs with enhanced electrical conductivities and structural stabilities, showing elevated storage performances for SIBs as well as a largely expanded application range. In summary, MOF-derived nanostructures possess many inimitable merits with controlled chemical compositions, tunable porosities, and surface areas, and shortened electron and ion transmission distances. Metal oxides have been investigated as SIB anodes and delivered high theoretical specific capacities; however, their capacity retentions and cycling lives are poor, which may be ascribed to their powdery structures, aggregation tendencies, and poor electrical conductivities.[15l,m] Fortunately, MOF-derived metal oxide/carbon nanocomposites have naturally resolved these problems, in which metal oxide nanoparticles are uniformly distributed in a porous carbon matrix. Thus, this structure

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www.advancedsciencenews.com Table 1.  Comparison of the physicochemical properties of metallic Na and Li. Category

Sodium

Lithium

Abundance [ppm]

23 600

20

Distribution

Universally

70% in South America

Price, carbonate [$ per ton]

≈250–300

≈5800

0.102

0.076

Ionic radius [nm] −1

23

6.9

E0 vs SHE [V]

−2.7

−3.04

Melting point [°C]

97.7

180.5

Molar mass [g mol ]

effectively avoids metal oxide agglomeration and structural collapse, enhances the electronic conductivity, increases electrolyte wettability, and facilitates the formation of an SEI film. Based on the abovementioned merits, MOFs are regarded as near-perfect sacrificial templates for the preparation of highperformance electrode materials for SIBs. In this review, all newly published reports on MOF-derived materials, including MOF-derived porous carbons, metal oxides, metal oxide/carbon nanocomposites, and other nanocomposites (e.g., MPs, MSs, and metal selenides (MSes)), as SIB anodes are summarized to highlight the major challenges and perspectives for MOF-derived materials in sodium energy storage.

2. MOF-Derived Materials as Electrodes for SIBs 2.1. MOF-Derived Nanoporous Carbon Materials Nanoporous carbon materials with tunable and uniformly distributed pore sizes, high specific surface areas, low toxicities and good chemical stabilities have attracted numerous attention from scientific researchers for various applications, including energy storage,[37] catalysis,[38] and gas separation/ storage.[34a] Considering the poor sodium energy storage performance of graphite caused by the insufficient interlayer spacing, nanoporous carbon materials have been deemed as promising alternative anodes to graphite for SIBs due to their abundant micro/mesopores and high surface areas, which can provide channels for fast Na+ transfer and can improve electrolyte infiltration into the electrode materials.[19a,c,39] Generally, the typical template synthetic route for porous carbons can be divided into three steps: (i) preparation of the precursor composed of a carbon source and inorganic template, (ii) high-temperature carbonization, and (iii) elimination of the inorganic template. The conventional chemical template procedures can be difficult, high cost, and time consuming. Although many methods for the synthesis of porous carbon materials have been proposed, the development of a more obtainable and efficient method to fabricate porous carbon materials with uniform and plentiful pores is still a challenge. Due to their relatively high surface areas and abundant welldefined nanoporous structures, MOFs have been utilized as sacrificial templates to obtain nanoporous carbons.[34a,40] The as-resulting materials retain the structural properties of the

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MOFs with satisfactory high surface areas and rich pores, revealing MOFs as a possible perfect parent material for porous carbon. In general, MOF-derived nanoporous carbon can be classified into two categories: (i) carbon acquired by the direct carbonization of a parent MOF in an inert atmosphere and (ii) carbon prepared by the pyrolysis of a precursor composed of an MOF and a polymerizable small molecule (a secondary carbon source) in the cavities of the MOF. In the first category, high-surface-area porous carbon can be directly obtained under a high temperature up to 900 °C for Zn-MOFs (MOF-5, ZIF-8, etc.) without any post-treatment, as the as-obtained zinc is reduced by the organic-linkerderived carbon under high temperature, resulting in evaporation of zinc at 900 °C. In addition, the evaporation of zinc metal would simultaneously produce many pores.[34b] However, for other MOFs synthesized from higher-boiling-point metals (e.g., Cu, Fe, Ni, and Co) the preparation of highsurface-area porous carbon requires the entire removal of the metallic species through using a post-acid washing treatment (HCl, H2SO4, HF, etc.).[41] In the second category, polymerizable small organic molecules, such as furfuryl alcohol,[42] glycerol,[43] and ethylenediamine,[44] are introduced into the meso/micropores of MOF-5/ZIF-8 and further polymerized into polymer compounds at a certain temperature. Through pyrolysis at high temperature, porous carbon materials with enhanced surface areas and modified channel structures can be prepared. In 2011, Xu and co-workers reported an N-doped porous carbon with an extremely high surface area of 3405 m2 g−1 and total pore volume of 2.58 cm3 g−1 prepared by introducing furfuryl alcohol as a secondary carbon source into ZIF-8.[42] Compared to the numerous reports on MOF-derived porous carbon materials as anodes for LIBs, the works discussing MOF-derived porous carbons for sodium energy storage are relatively seldom. Zheng and co-workers[45] first reported N-doped porous carbon (ZIF-C) prepared from ZIF-8 as an anode for SIBs and compared the sodium energy storage performances of CMK-3 and ZIF-C. ZIF-C was prepared from the facile pyrolysis of ZIF-8 in an N2 atmosphere at 930 °C, while CMK-3 was obtained by a typical template method (SBA-15) (Figure 1). The as-obtained N-doped carbon material retained similar physical properties as the parent ZIF-8 and presented a uniform polyhedron morphology with an average size of ≈300 nm and a high surface area of 1251 m2 g−1 (Figure 1c,d). Additionally, elemental analysis revealed a nitrogen content in the acquired carbon of ≈3.1 wt%. When utilized as an anode for SIBs (Figure 1g,h), microporous ZIF-C delivered a capacity of 136 mA h g−1 at a current density of 50 mA g−1 after 50 cycles, outperforming CMK-3 (≈78 mA g−1). In addition, a capacity of 129 mA h g−1 was obtained for ZIF-C upon increasing the current density to 500 mA g−1, revealing an outstanding rate capability. The superior performance may be associated with the highly microporous structure of ZIF-C, in which the particularly small pore size of ZIF-C can largely hinder electrolyte decomposition (Figure 1i). Recently, a cube-shaped porous carbon (CPC) was prepared from the simple carbonization of MOF-5 at 1000 °C for 2 h in an Ar atmosphere with a heating rate of 5 °C min−1 (Figure 2a).[46]

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Figure 1.  Schematic diagram of the preparation of a) ZIF-C and b) CMK-3. c,d) Scanning electron microscopy (SEM) images of ZIF-8 and ZIF-C carbon, respectively. e) SEM image of CMK-3 prepared utilizing SBA-15 as a hard template. f) High-resolution transmission electron microscopy (HRTEM) image of ZIF-C. g) Discharge/charge performances of ZIF-C and CMK-3 anodes at a current density of 50 mA g−1 (the inset shows the related Coulombic efficiencies). h) Rate performances of the as-obtained samples under different current densities. f) Contact situations between the mesopores/ micropores and electrolyte. Reproduced with permission.[45] Copyright 2014, the Royal Society of Chemistry.

The as-obtained material showed abundant micro/mesopores, high mechanical strength, a large surface area (2316 m2 g−1), and good electrical conductivity. Obviously, in contrast to the smooth surface of MOF-5, the surface of the CPC was rough with a highly porous structure (Figure 2b–e). Furthermore, the interlayer distance (d(002)) of the material was 0.38 nm, larger than 0.37 nm, thus benefiting sodium-ion insertion and extraction based on the calculation results reported by Cao et al.[19b] When investigated as an anode material for SIBs (Figure 2f–j), the obtained carbon exhibited a high overall sodium-storage capacity of 240 mA h g−1 at a current density of 100 mA g−1 after 100 cycles. Particularly, a high specific capacity of 100 mA h g−1 was attained after 5000 cycles at a current density of 3200 mA g−1. The excellent electrochemical performance of the CPC anode for SIBs can be ascribed to the subsequent reasons: (1) the larger interlayer distance of the graphitic layers (0.38 nm) benefits the reversible insertion/extraction of sodium ions; (2) the high surface area provides a substantial capacity contribution originating from the surface capacitance behavior; and (3) the abundant pores boost the electrolyte/electrode contact area and largely decrease the sodium-ion diffusion

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distance, thus efficiently accelerating mass transfer and relieving volume changes. Very recently, N-doped carbon nanofibers and tubules with a large fraction of graphitic carbon and large interlayer spacing were prepared by Goodenough and co-workers.[47] A bimetallic MOF was used to prepare N-doped graphitic carbon. The rich N content in the ZIF-8 precursor provided the N source, while Co in ZIF-67 served as a catalyst to graphitize carbon. When used as an anode for SIBs, a high capacity of 346 mA h g−1 at 120 mA g−1 was obtained. Remarkably, no obvious capacity decay was observed after 10 000 cycles. These studies suggest that MOF-derived amorphous porous carbons can act as suitable anode materials for SIBs, offering a potential route for the preparation of carbon-based SIB anodes. However, reports on heteroatom (P, S)-doped porous carbon materials obtained from MOFs for SIBs are rare, and further research is needed. Furthermore, the relatively low initial Coulombic efficiencies derived from the high surface areas and low carbon yields resulting from the high-temperature pyrolysis of the parent MOFs constitute a new challenge for large-scale application.

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Figure 2.  a) Schematic illustration of the preparation of the CPC anode. b) SEM image of MOF-5. c,d) SEM images of the CPC. e) TEM image of CPC. f) Cyclic voltammetry (CV) curves of SIBs employing the CPC as an anode at a scan rate of 0.1 mV s−1. g) Discharge/charge voltage curves at 100 mA g−1. h) Cycling performance at 100 mA g−1. i) Rate capabilities of the CPC anode at various current densities. j) Long cycling performance at 3.2 A g−1. Reproduced with permission.[46] Copyright 2016, Elsevier.

2.2. MOF-Derived Metallic Oxides Metal oxides, including SnO2,[15e] TiO2,[15d] Fe2O3,[15n] Co3O4,[48] and CuO,[15k,49] have been investigated as electrode materials for SIBs due to their high theoretical capacities, environmental friendliness, extensive sources, and high chemical stabilities. Unfortunately, metal oxides possess several fatal problems, such as fast capacity decay with increased cycling numbers and poor rate performances, which may be attributed to their poor intrinsic electrical conductivities and intense powdering processes of the electrode materials caused by large volume

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changes and particle aggregation during repeated sodiumion insertion/extraction. Thus, the design and preparation of various nanostructured metal oxides with controllable particle diameters, shapes, and surface areas for SIBs have received much attention. Various techniques have been developed to prepare metal oxides; however, their practical application is still challenging, as their complex preparation procedures cannot satisfy large-scale production requirements. Based on the abovementioned merits, MOFs have been widely utilized to prepare metal oxides through facile pyrolysis in air, which provides an effective way to synthesize metal oxides with tunable

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physical/channel structures. Since the metal ions are regularly dispersed in the skeleton structures of MOFs, the sizes/shapes of metal oxides can be well manipulated by employing an appropriate heating treatment, which is important to improve the sodium-ion energy storage performances of metal oxides. Overall, metal oxides with various nanostructures and high surface areas can be achieved by the carbonization of resourceful MOFs, delivering enhanced capacities, longer cycling lives, and elevated rate performances in comparison with the corresponding materials with low surface areas and unevenly distributed structures. Copper oxides, containing CuO and Cu2O, are fascinating anode materials due to their high theoretical capacities,

chemical stabilities, and low cost. Pan and co-workers[50] fabricated a porous CuO/Cu2O composite with a hollow octahedron structure (CHO-1) (Figure 3a,b) comprising clustered nanoparticles by simple annealing of a Cu-based MOF template at 300 °C in air. The transmission electron microscopy (TEM) image (Figure 3c) confirmed the hollow structure inside CHO-1. The influences of the temperature on the morphology and structure were also explored (Figure 3). When the calcination temperature was increased to 350 °C, the octahedron porous structure decomposed to form a clustered structure (CHO-2). Finally, the octahedron structure was completely destroyed upon elevating the temperature to 400 °C (CHO-3), forming nonregular agglomerates (Figure 3d). In addition, the

Figure 3.  a,b) SEM images of CHO-1. c) SEM image of CHO-2. d) SEM image of CHO-3. e) TEM image of CHO-1. f) Cycling tests of the prepared materials at 50 mA g−1. g,h) Rate assessments under different current densities and a long cycling test of CHO-1 at high current density, respectively. Reproduced with permission.[50] Copyright 2015, the Royal Society of Chemistry.

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specific surface areas of CHO-1, CHO-2, and CHO-3 changed from 10.65 to 2.39 m2 g−1. When assessed as anode materials for SIBs (Figure 3f–h), CHO-1 displayed a high reversible capacity of 415 mA h g−1 after 50 cycles at 50 mA g−1 as well as an outstanding rate performance. Nevertheless, the CHO-2 and CHO-3 samples delivered reversible capacities of 248 and 185 mA h g−1 after the same number of cycles, respectively, much lower than that of CHO-1. The excellent electrochemical storage capacity of CHO-1 may be attributed to the following reasons: (1) the relatively large surface and porous structure of CHO-1 can improve the electrolyte wettability of the electrode, decrease the Na-ion transfer distance, facilitate Na-ion insertion/extraction into/out of the active materials, and facilitate Na-ion diffusion; (2) the hollow structure might efficiently alleviate volume expansions triggered by Na+ insertion and efficiently maintain structural stability. Titanium dioxides, including anatase, bronze, and rutile, are considered as promising candidates for SIB anodes due to their low cost, excellent structural stabilities, and high safety during repeated charge/discharge processes compared to other electrode materials. Recently, Zhang et al. prepared a porous cake-like TiO2 with an anatase-phase structure (PT-1) from a Ti-based MOF template[51] at 380 °C in a muffle furnace (Figure 4a–d). PT-1 exhibited a capacity of 250 mA h g−1 at a current density of 50 mA g−1 after 50 cycles. Furthermore, the as-obtained TiO2 material displayed reversible capacities of 248.3, 223, 189.6, 174.7, 151.7, and 117.3 mA h g−1 at current densities of 0.05, 0.1, 0.5, 1.0, 2.0, and 4.0 A g−1, respectively. In contrast, PT-2 prepared at a calcination of 500 °C using the same heating rate presented a reversible capacity of 115 mA h g−1 after 50 cycles, much lower than that of PT-1 (Figure 4f–h). The authors concluded that the enhanced capacity, good cycling stability, and excellent rate performance were mainly attributed to the larger surface area (96.28 m2 g−1) (Figure 4e), highly porous structure, and lower charge transfer resistance of PT-1. Recently, bimetallic oxides (AB2O4) have attracted considerable attention due to their improved electrical properties and enhanced electrochemical performances and magnetic properties prompted by the synergistic effects between various metallic species. Nevertheless, the controlled syntheses of these materials are continuously to huge challenges.[52] MOF-derived bimetallic oxides have demonstrated significant electrochemical energy storage activities because of their high surface areas and compositional elasticities. Wang and co-workers[53] prepared hollow MgFe2O4 microboxes by utilizing PB microcubes as selfsacrificial templates. The scanning electron microscopy (SEM) and TEM images (Figure 5a,b) confirmed a uniform and hollow microbox structure with a particle size of 570 nm. Figure 5c presents the two lattice spacings of 0.171 and 0.161 nm, corresponding to the (422) and (511) crystalline planes of MgFe2O4. When utilized as an anode material for SIBs, the as-acquired specimen delivered a high capacity of 406 mA h g−1 at 50 mA g−1 and preserved a reversible capacity of 135 mA h g−1 after 150 cycles. Furthermore, the authors also investigated the impact of different ratios between Super-P (conductive carbon) and the active material on the rate capability (Figure 5d–f). Lu and co-workers[54] obtained porous CoFe2O4 nanocubes (PCFO-NCs) with a surface area of 106.8 m2 g−1 using PB as

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an Fe source and template. The X-ray diffraction (XRD) pattern (Figure 6a) indicated the as-obtained sample possessed a highly pure phase. In contrast to PB’s solid cube structure with a smooth surface, the PCFO-NCs exhibited a fluffy surface with an increased porous structure (Figure 6b,c). A reversible capacity of 360 mA h g−1 was achieved at 50 mA g−1 after 50 cycles for the SIB employing the PCFO-NCs as an anode (Figure 6d). In addition, good rate performance and long cycling life with a capacity retention of 87.3% (152.6 mA h g−1 at 2.5 A g−1 after 500 cycles) were also observed (Figure 6e,f). Notably, a relatively high initial Coulombic efficiency of 68.8% was obtained. In contrast to the above methods, Zhou and co-workers[55] explored a simple scheme to prepare 2D porous bimetallic oxides (Co3O4/ZnO) with a nanosheet structure by the carbonization of hybrid bimetallic MOFs (ZIF-67 and ZIF-8) (Figure 7a–d). Benefiting from the unique properties of the parent hybrid MOFs, the as-obtained specimens possessed ample oxygen vacancies, high surface areas, and rich channel structures, which greatly enhanced their rate performances based on kinetic diffusion and afforded more active sites for electrochemical storage. As expected, the hybrid Co3O4/ZnO materials exhibited greatly improved capacities with a high rate capacity of 242 mA h g−1 at 2 A g−1 and excellent cyclic stabilities with a maximum capacity retention of 91% after 1000 cycles compared to pure ZnO and Co3O4 (Figure 7e–g). The enhanced performance was attributed to the inimitable structure of the samples and the harmonious multistep conversion reaction between Co3O4 and ZnO. MOF-derived metal oxides show markedly improved electrochemical storage performances for SIBs compared to metal oxides prepared from other methods,[15k,56] resulting from the 3D porous nanostructures derived from MOFs. In addition, these results infer that the carbonization temperature is a vital factor in optimizing the sodium-storage performances for MOF-derived metal oxides. More impressively, these research findings suggest that MOFs can be utilized as ideal templates to prepare bimetallic oxides with highly porous structures and large surface areas.

2.3. MOF-Derived Metal Oxide/Carbon Nanocomposites Although metal oxides have delivered high capacities for sodium energy storage, their poor cycling stabilities caused by large volume expansion and structural collapse during repeated charge/discharge processes must be effectively resolved. Many efforts, including nanocrystallization, nanoporous carboncoated and carbon-supported techniques, have been confirmed to be efficient approaches to mitigate the huge volume expansions, retain metal oxide stability, and improve electrochemical performances. However, the particle sizes and porous structures of metal oxide/carbon nanocomposites cannot be effectively tuned by conventional chemical preparation methods, and the agglomeration of particles also seriously limits the performances of the active materials. Fortunately, MOFs can be utilized to directly generate metal oxide/carbon nanocomposites with high surface areas and porous structures by simple annealing treatments in an inert gas atmosphere, producing

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Figure 4.  a,b) SEM images of PT-1 and PT-2, respectively. c,d) TEM images of PT-1. e) Nitrogen adsorption/desorption isotherms of these samples. f) Cycling performances at a current density of 50 mA g−1. g,h) Rate capabilities at different current densities and long cycling life investigation of PT-1 at 1 A g−1, respectively. Reproduced with permission.[51] Copyright 2017, Elsevier.

homogeneously dispersed metal oxide nanoparticles within nanoporous carbonaceous matrices. Initially, Zhu et al.[57] obtained a hierarchical hollow NiO/Ni/ graphene composite from an Ni-based MOF (Ni-MOF) with a unique hierarchical hollow ball-in-ball nanostructure through consecutive pyrolysis and oxidation treatments (Figure 8a). During the pyrolysis of Ni-MOF, graphene-coated Ni nanoparticles were obtained at 450 °C under an N2 atmosphere. Then, the nanomaterial was transformed to an NiO/Ni/graphene composite

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by annealing in air at 200 °C. As displayed in Figure 8b,e, the NiO/Ni/graphene composite exhibited a hierarchical hollow structure, which retained the same structure as the hollow ballin-ball microspherical structure of Ni-MOF, indicating that the Ni-MOF structure was unaltered during the annealing process. Note that a graphene film with a thickness of ≈2 nm was homogeneously covered on the nanoparticle surfaces, and a lattice distance of 0.21 nm corresponding to the (200) interplane spacing of NiO was clearly observed (Figure 8f,g), which is crucial to

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Figure 5.  a) SEM image of MgFe2O4 microboxes. b) TEM and c) HRTEM images of porous and hollow MgFe2O4 microboxes. Electrochemical properties of the MgFe2O4 microbox electrodes at different active material ratios of samples A (7:2:1) and B (8:1:1). d) Discharge/charge curves and e) cyclic performance of sample A at a current density of 50 mA g−1. f) Rate performances of the samples. Reproduced with permission.[53] Copyright 2017, Elsevier.

enhance the electronic conductivity of the NiO/Ni nanoparticles, to relieve volume changes, and to promote the SEI film formation. Figure 8h,i shows the electrochemical performance of the NiO/Ni/graphene composite electrode for SIBs. The material delivered capacities of 385, 295, 248, and 207 mA h g−1 under current densities of 200, 500, 1000, and 2000 mA g−1, respectively. Although the electrochemical performance of the NiO/ Ni/graphene composite electrode for SIBs must be further improved, this work suggests that MOFs with well-defined structures are ideal precursors to prepare nanostructured metal oxide/ carbon nanocomposites for sodium energy storage. Huang and co-workers[58] reported nitrogen-doped carboncoated Co3O4 nanoparticles (Co3O4@NC) derived from a controlled two-step annealing procedure of a Co-based MOF (ZIF-67). The two-step annealing procedure first consisted of a carbonization process at 550 °C for 2 h in Ar and then a subsequent oxidation process at 150 °C for 12 h in air (Figure 9a). As shown in Figure 9b–f, the as-obtained Co3O4@NC polyhedra were constructed of closely packed

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nanoparticles with a rough surface, a core/shell structure, a well-matched lattice spacing, and a very uniform distribution of C, O, Co, and N elements. Co3O4@NC possessed high porosity with a surface area of 101 m2 g−1. When employed as an electrode for SIBs (Figure 9g–k), high reversible capacities of 506, 317, and 263 mA h g−1 were achieved at current densities of 100, 400, and 1000 mA g−1, respectively, and a large capacity retention was obtained after 1100 cycles at 1 A g−1. The excellent sodium-storage performance of this material was attributed to the uniformly nitrogen-doped carbon-coated porous structure, which minimized volume expansion, modified the electronic conductivity, and increased the capacitive behavior. The excellent performance of Co3O4@NC indicates that the preparation of high-performance metal-oxide@NC nanocomposites from MOFs is a feasible and valuable method to fabricate anode materials for SIBs. Recently, Wang and co-workers[59] also prepared a porous hollow Co3O4 material with N-doped carbon-cladding polyhedron structure (Co3O4/N–C) by the calcination of another

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Figure 6.  a) XRD pattern of the PCFO-NCs. b,c) SEM images of the PB precursor and PCFO-NCs. d) Cycling performances at 50 mA g−1. e) Rate performances under different current densities, and f) long cycling stability at a high current density of 2.5 A g−1 for the SIB anode utilizing the PCFO-NCs. Reproduced with permission.[54] Copyright 2017, Elsevier.

cobalt-based MOF ([Co6O(TATB)4]·(H3O+)2·Py) at 700 °C for 3 h in vacuum with a heating rate of 2 °C min−1 (Figure 10a). As shown in Figure 10b–e, the obtained sample was composed of numerous tiny nanoparticles covered with an N-doped carbon film, which reduced the sodium-ion diffusion distance, enhanced the electronic conductivity, and accommodated volume expansions during Na+ insertion. As presented in Figure 10f,g, the obtained Co3O4/N–C material preserved capacities of 368 and 276 mA h g−1 at current densities of 100 and 500 mA g−1 after 50 cycles for sodium energy storage, respectively. Additionally, the Co3O4/N–C anode displayed specific capacities of 673, 615, 520, 432, 385, and 326 mA h g−1 under current densities of 50, 100, 200, 500, 1000, and 2000 mA g−1, respectively.

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Typical CuO/Cu2O cannot fulfill the demands of fast charge/ discharge and long cycling life for large-scale applications due to their poor cycling stabilities caused by large volume expansions. A carbon-based matrix was proposed as an efficient way to modify the storage performances of CuO/Cu2O. Kim et al.[60] described a CuO/Cu2O material in a graphitized porous C matrix (CuO/Cu2O-GPC) derived from a Cu-based MOF. The specimen was successfully fabricated by a carbonization process at 800 °C for 5 h in N2 and a following oxidation reaction at 200 °C for 9 h in air (Figure 11a). As presented in Figure 11b–d, CuO/Cu2O-GPC exhibited an octahedron shape, conserving the shape of Cu-MOF after the two-step annealing process.

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Figure 7.  a) Design scheme for the synthesis of the 2D porous hybrid Co3O4/ZnO nanosheets. b,c) TEM images of the Co3O4/ZnO nanosheets. d) HRTEM image of the Co3O4/ZnO material. e) Cyclic performances of the samples at a current density of 300 mA g−1 (CoZn-O1, CoZn-O2, and CoZn-O3 were separately studied, corresponding to Co3O4/ZnO weight ratios of 3.9, 1.5, and 0.64, respectively, in the bimetal oxides). f) Rate properties and g) cycling performance at 2000 mA g−1 of the CoZn-O2 electrode. Reproduced with permission.[55] Copyright 2017, the Royal Society of Chemistry.

Notably, the CuO/Cu2O-GPC surface was very rough and highly porous. Figure 11e,f exhibits the corresponding TEM, high-resolution TEM (HRTEM), and element mapping images of CuO/Cu2O-GPC, which evidently demonstrate the CuO/

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Cu2O micro-nanoparticles in the porous C matrix and disclosed the uniform distribution of C, Cu, and O. When utilized as an anode for SIBs, average capacities of 318.5, 285.7, 261.3, 227.3, and 209.6 mA h g−1 at current densities of 50, 300, 500, 1000,

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Figure 8.  a) Illustration of the preparation of NiO/Ni/graphene. b,c) SEM images of NiO/Ni/graphene (scale bar: 10 µm for panel (b) and 500 nm for panel (c)). d) TEM image of the prepared specimen (scale bar: 200 nm). e) Selected-area electron diffraction pattern of NiO and Ni (scale bar: 21 nm). f,g) HRTEM images of the prepared sample (scale bar: 5 nm). h) Cycling capacity of the half-cell employing NiO/Ni/graphene as an anode at a specific current of 1 A g−1 (the initial five cycles correspond to the activation process under 200 mA g−1). i) Rate capabilities of the as-obtained material at various current densities. Reproduced with permission.[57] Copyright 2016, American Chemical Society.

2000, and 3000 mA g−1 were obtained, respectively. In addition, a capacity of 302.9 mA h g−1 was delivered at a current density of 50 mA g−1 after 200 cycles. However, the initial Coulombic efficiency of CuO/Cu2O-GPC for SIBs was low (44.8%), which may be attributed to SEI layer formation, thus limiting its application. To further elevate the sodium-ion storage performance of CuO, a porous CuO/reduced graphene oxide composite (CuO/RGO) was developed by Li et al.[61] CuO/RGO was obtained from the pyrolysis of Cu-based MOF/GO templates at 350 °C for 2 h in air with a heating rate of 5 °C min−1 and exhibited a high capacity of 466.6 mA h g−1 after 50 cycles at a current density of 100 mA g−1. In addition, a large capacity of 347.6 mA h g−1 was maintained at 2 A g−1, showing enhanced storage performance for SIBs compared to other previously reported CuO-based electrodes. The authors claimed that the enhanced performance was attributed to the combined effect of CuO and RGO, which further decreased the charge transfer resistance, accommodated volume variations, and prevented the accumulation of CuO nanoparticles.

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A hierarchical hollow Fe2O3@MIL-101(Fe)/C derived from an Fe-based MOF (MIL-101(Fe)) was reported by Li et al.[62] As displayed in Figure 12a,b, Fe2O3@MIL-101(Fe)/C possessed a hollow structure with a visible hollow interior and a shell thickness of ≈150–200 nm. The electrochemical performance results are shown in Figure 12c–f. Bare Fe2O3 and Fe2O3@MIL-101(Fe)/C delivered capacities of 462 and 662 mA h g−1 after 200 cycles at a current density of 200 mA g−1. Fe2O3@MIL-101(Fe)/C exhibited an initial charge/ discharge capacity of ≈916/1052 mA h g−1 at a current density of 200 mA g−1, corresponding to a high Coulombic efficiency of 87.1%. Indeed, at a high current density of 4 A g−1, a high reversible capacity of 421 mA h g−1 was obtained. The superior electrochemical performance of the Fe2O3@MIL-101(Fe)/C material may be attributed to its inherent hollow nanostructure, which can reduce both the electron and ion transport distances, broaden the surface areas of the as-obtained samples, and better accommodate volume expansions during the Na+ insertion process.

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Figure 9.  a) Illustration of the preparation of Co3O4@NC. b,c) SEM images of Co3O4@NC. d,e) TEM images of the as-obtained sample at different magnifications. f) Corresponding elemental mappings for C, N, O, and Co. g–j) Discharge/charge profiles, discharge and charge capacities, and rate capabilities of the prepared samples. k) Long-term cycling stability of the Co3O4@NC anode. Reproduced with permission.[58] Copyright 2016, the Royal Society of Chemistry.

As mentioned above, titanium dioxides are attractive SIB anodes due to their small volume expansions during Na+ insertion compared to other metal oxides; however, their inherently low ion diffusion coefficients and poor electronic conductivity seriously restrict their development. Surface carbon coating and inner defects are two effective methods to improve the storage performances of titanium dioxides. Recently, our group reported a carbon-coated rutile titanium dioxide (CRT) material with rutile TiO2 nanoparticles uniformly distributed in a carbon matrix via a simple carbonization process of a titanium MOF at 900 °C for 2 h.[63] As depicted in Figure 13a–d, CRT consisted of numerous nanoparticles. When used as an anode for sodiumion energy storage, CRT exhibited a reversible sodium-storage capacity of ≈175 mA h g−1 at a C rate of 0.5 C after 200 cycles (1 C = 168 mA g−1) and delivered excellent rate capacities of 177.8, 154.4, 130.6, 105.8, 86.8, and 70.6 mA h g−1 under 0.5, 1, 2, 4, 10, and 20 C rates, respectively. More impressively, the material showed long cycling life and a remarkable rate capacity of ≈70 mA h g−1 after 2000 cycles under a high 20 C rate (Figure 13e,f). Shi et al.[64] prepared an anatase TiO2@C composite with a porous structure by annealing a Ti-MOF (MIL-125(Ti)) at

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600 °C in Ar for 5 h. The as-acquired sample presented a reversible storage capacity of 148 mA h g−1 under a current density of 0.5 A g−1 over 500 cycles and good rate capability with a capacity of 88.9 mA h g−1 at a high current density of 2.5 A g−1. The authors concluded that the excellent storage performance was attributed to the high dispersion of TiO2 nanoparticles in the carbon matrix and the combined action of the carbon matrix and anatase TiO2 nanoparticles. He et al.[65] explored TiO2–б nanocrystals with abundant defects cramped into a mooncake-shaped porous carbon matrix by a simple magnesium reduction of a TiO2/C material originating from the dead-burn of an MIL-125(Ti) precursor in inert gas. The prepared material delivered a greatly enhanced sodium-storage performance with a capacity of 330.2 mA h g−1 at a current density of 50 mA g−1 within the voltage range of 0.001–3.0 V and an ultralong cycling life with only slight decay over 5000 cycles. Significantly, the SIB utilizing this material displayed a superior rate capability with capacities of 330.2, 280.5, 249.4, 217.2, and 172.2 mA h g−1 at current densities of 50, 100, 200, 500, and 1000 mA g−1, respectively. The greatly enhanced storage performance of

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Figure 10.  a) Schematic illustration of the preparation of the Co3O4/N–C composite. b,c) SEM images of the Co3O4/N–C composite. d) TEM image and e) HRTEM image of the as-prepared sample. f) Cycling stability and g) rate performance for the SIB employing the Co3O4/N–C composite as an anode. Reproduced with permission.[59] Copyright 2017, American Chemical Society.

TiO2 for SIBs may be ascribed to the synergistic effect of the abundant defects and regular carbon-coated structure, which boosted electronic conductivity, reduced the sodiumion diffusion distance, and improved the sodium-ion storage kinetics. To integrate the high cycling stability of TiO2 and the high capacity of Fe2O3, Yu et al.[66] developed tiny ilmenite FeTiO3

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nanoparticles surrounding carbon nanotubes (FTO⊂CNTs) by an annealing treatment of Fe-MOF@TiO2 nanorods at 600 °C for 4 h in Ar. The Fe-MOF@TiO2 nanorods were prepared from the in situ growth of TiO2 on the surface of the Fe-MOF nanorods (Figure 14a). As shown in Figure 14b, Fe-MOF presented a regular needle-like nanorod morphology with a uniform length of ≈750 nm, a radius of 75 nm, and smooth surface.

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Figure 11.  a) Schematic diagram of the preparation of the CuO/Cu2O-GPC anode obtained from Cu-MOF. b–d) SEM images of Cu-MOF, Cu-GC, and CuO/Cu2O-GPC at different magnifications. e–h) TEM image, HRTEM image, elemental mapping images, and electrochemical performance of the CuO/Cu2O-GPC electrode material, respectively. Reproduced with permission.[60] Copyright 2016, the Royal Society of Chemistry.

The as-prepared FTO⊂CNTs displayed a homogeneous length of ≈800 nm and a radius of 100 nm, which were larger than those of Fe-MOF and were attributed to the TiO2 shell (Figure 14c). In addition, the solid Fe-MOF nanorods were converted into a hollow nanotube structure after the annealing treatment. As depicted in Figure 14d, the FTO⊂CNTs displayed similar hollow structures with a tube-like shape and a wall thickness of ≈30 nm. Interestingly, the FTO nanoparticles were embedded in the wall of carbon matrix (Figure 14e). When utilized as an anode for SIBs, the as-acquired material delivered outstanding

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cycle properties with a capacity of 358.8 mA h g−1 after 200 cycles at a current density of 100 mA g−1 and extraordinary rate performances with capacities of 445.4, 390.3, 354.9, 307.3, 273.3, 233.2, and 201.8 mA h g−1 at densities of 50, 100, 200, 500, 1000, 2000, and 5000 mA g−1, respectively (Figure 14f,h). Upon recovering the current density to 100 mA g−1 after 130 cycles, a high rever­sible capacity of 386.2 mA h g−1 was expressed with a small capacity decay of 1.1%. Impressively, a capacity of 210 mA h g−1 was obtained at a high current density of 2 A g−1 after 3500 cycles, indicating the high cycling stability of the material. This

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Figure 12.  a) SEM and b) TEM images of as-acquired hollow Fe2O3@MIL-101(Fe)/C. c) Cycling capacities of bare hollow Fe2O3 and Fe2O3@MIL101(Fe)/C at a current density of 200 mA g−1. d) Charge/discharge profiles of the corresponding anode for SIBs under the 1st, 2nd, and 200th cycles at a current density of 200 mA g−1. e) CV curves at a scan rate of 0.05 mV s−1 for the Fe2O3@MIL-101(Fe)/C anode for sodium storage. f) Rate performances of the as-prepared Fe2O3@MIL-101(Fe)/C anode for SIBs under various current densities ranging from 500 to 4000 mA g−1. Reproduced with permission.[62] Copyright 2016, Nature Publishing Group.

report provided a promising way to explore hollow nanomaterials with high sodium-storage performance. Zhang et al.[67] explored porous MnO@C nanorods prepared by simple annealing of an Mn-based MOF. As shown in Figure 15a–d, MnO@C possessed a rod-like morphology with a diameter of ≈200 nm, lengths of 3–10 mm, and a highly rough surface. In detail, the MnO nanoparticles had diameters of 3–5 nm and were uniformly dispersed in the porous carbon matrix. Furthermore, the MnO@C nanocomposite delivered an outstanding electrochemical performance with a high reversible

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sodium-storage capacity of 260 mA h g−1 after 100 cycles at a current density of 50 mA g−1 and a capacity of 140 mA h g−1 at a high current density of 2 A g−1 (Figure 15e,f). Kaneti et al.[68] developed an Ni-doped Co/CoO/N-doped carbon nanocomposite utilizing hybrid bimetallic Ni–Co–ZIF as a template. The as-prepared Ni-doped Co/CoO/NC nanocomposite possessed a highly porous structure with a surface area of 552 m2 g−1. When used as an anode for sodium energy storage, the material delivered a capacity of 218.7 mA h g−1 at a current density of 500 mA g−1 after 100 cycles. In addition,

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Figure 13.  a,b) SEM images of CRT. c,d) TEM images of CRT. e) Cycling stability at a high rate of 20 C. f) Rate performance of the obtained samples under different C rates (1 C = 168 mA g−1). Reproduced with permission.[63] Copyright 2016, Elsevier.

the nanocomposite anode delivered capacities of 307, 261, 218, 189, 156, and 110 mA h g−1 at current densities of 100, 200, 500, 1000, 2000, and 5000 mA g−1, respectively. Notably, a capacity of 261 mA h g−1 was obtained upon recovering the current density to 100 mA g−1, indicating the good rate capability of this hybrid electrode. MOF-derived metal oxide/carbon nanocomposites show largely enhanced electrochemical storage performances compared to the corresponding materials prepared using other

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approaches,[15n,69] especially in terms of the cycling life and rate capability, which can be attributed to the carbon-coated porous structures retaining from the parent MOFs. In addition, the initial Coulombic efficiency is a very important factor for the applications of electrode materials, as Na+ mainly originates from the cathode material in the complete cell. Thus, these works may provide an efficient way to modify the electrochemical performance through constructing MOF-derived bimetallic oxide/carbon hybrid materials.

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Figure 14.  a) Schematic of the preparation of the FTO⊂CNTs anode for SIBs. SEM images of b) Fe-MOF and c) FTO⊂CNTs. d,e) TEM images of FTO⊂CNTs. f–h) Cycling stability at 100 mA g−1, rate performance, and long cycling life test at 2 A g−1 of the obtained specimen. Reproduced with permission.[66] Copyright 2017, American Chemical Society.

2.4. Other MOF-Derived Nanomaterials MPs, MSs, and MSes have been extensively explored as anodes for LIBs and are regarded as some of the most competitive alternative SIB anodes due to their high theoretical capacities, safer operating potentials, and easily controlled morphologies. However, recent reports on MPs, MSs, and MSes for SIBs, for example, FeP,[70] CuP2,[17d] CoS,[71] and CoP,[72] exhibited fast capacity fading with increased cycles caused by substantial volume changes due the sodium-ion insertion/extraction processes, seriously hindering their practical applications. Reasonably designed micro/nanostructures (e.g., nanowires, hollow spheres, and porous structures) and carbon coatings have been considered as effective methods to remit the volume changes, modify the interfacial contact area of the electrode and electrolyte, and resolve the poor cycling stability of MPs, MSs, and MSes. Although some processes have been reported for these materials in sodium energy storage, simpler approaches to prepare highly dispersed MP, MS, and MSe nanoparticles in

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carbon matrices with controllable porous structures and morphologies are urgently needed. Given their plentiful pores and unique nanostructures, MOFs have also been considered as ideal precursors and metal sources to obtain highly dispersed MP, MS, and MSe nanomaterials. Ge et al.[73] reported porous core/shell CoP@C polyhedra attached to RGO networks (CoP@C-RGO-NF) prepared using a low-temperature phosphidation process of the core/shell Co@C polyhedral structures derived from ZIF-67/RGO (Figure 16a). As shown in Figure 16b–d, CoP@C-RGO-NF possessed an even and well-defined polyhedral structure with a particle size of 800 nm, which was similar to the morphology of ZIF-67 and indicated that the phosphidation treatment did not destroy the precursor structure. This obtained material was utilized as a binder-free anode for SIBs and presented a noteworthy storage performance with a specific capacity of 473.1 mA h g−1 at a current density of 100 mA g−1 after 100 cycles. In addition, reversible capacities of 543.4, 405.3, 253.6, and 155 mA h g−1 were observed at current densities of 200, 400,

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Figure 15.  a) SEM image and b–d) TEM images with different magnifications of the MnO@C material. e) Cycling performance of the obtained sample. f) Rate capability of the MnO@C electrode. Reproduced with permission.[67] Copyright 2017, Elsevier.

800, and 1600 mA g−1, respectively (Figure 16e,f). Notably, the capacity reached 540.3 mA h g−1 upon returning the current density to 100 mA g−1, indicating a high rate stability. The authors claimed that the excellent electrochemical performance was ascribed to the combined action of the core/shell CoP@C polyhedra and RGO network, as inimitable core/shell structure and carbon layer decreased the diffusion distance, relieved volume changes, increased the contact area between the electrode and electrolyte, and enhanced the electronic conductivity. Li et al.[74] developed a core/shell-structured CoP/FeP porous microcube bound by educed graphene oxide (RGO@ CoP@FeP) through a low-temperature phosphorization treatment of GO@Co(OH)2@prussion blue (Figure 17a), in which porous FeP obtained from PB served as the core, CoP served as the shell, and RGO nanosheets interconnected the core/shell microcubes. As depicted in Figure 17b,c, RGO@CoP@FeP possessed an RGO-coated and interconnected hierarchical microcube structure with a side length of 400 nm. The TEM image shown in Figure 17d confirmed that the resulting material featured a core/shell structure with a CoP shell and

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a porous FeP microcube core. The energy dispersive X-ray spectroscopy mapping images displayed in Figure 17e evidenced a uniform distribution of Co, Fe, P, and C elements along the core/shell microcubes. As exhibited in Figure 17f, the core/shell-structured RGO@CoP@Fe electrode delivered a capacity of 456.2 mA h g−1 after 200 cycles at a current density of 100 mA g−1, corresponding to an initial Coulombic efficiency of 56.96%, which was superior than those of two individual component materials (i.e., CoP@C-FeP and C-FeP). Furthermore, specific capacities of 480.2, 435.1, 403.3, 374.6, and 341.2 mA h g−1 were attained at current densities of 0.1, 0.2, 0.5, 1, and 2 A g−1, respectively, which were much better than those of the CoP@C-FeP and C-FeP anodes (Figure 17g). Additionally, a capacity of 468.3 mA h g−1 was maintained upon reducing the current density to 0.1 A g−1, showing the excellent electrochemical stability of the RGO@CoP@C-FeP material. The excellent electrochemical performance of the material was attributed to the unique RGO-interconnected core/shell structure, which offered sufficient space to alleviate volume changes during sodium-ion insertion/extraction,

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Figure 16.  a) Schematic diagram of the preparation of CoP@C-RGO-NF. b–d) SEM images of CoP@C-RGO-NF. e) Cycling performance and f) rate capability of the as-obtained sample. Reproduced with permission.[73] Copyright 2017, Elsevier.

increased the structural stability, shortened the diffusion length, and furnished more sodium-ion storage sites. This work provided a new route for the preparation of MOF-derived core/shell metal phosphide materials coated by RGO, which can be expanded to other fields. Recently, nitrogen-doped yolk–shell-structured CoSe/C dodecahedra (CoSe/C) were developed by Zhang et al.[75] The CoSe/C nanocomposite was obtained by a reaction between cobalt species in an MOF and selenium (Se) powder in an inert atmosphere under certain temperatures (Figure 18a).

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Figure 18b–g displays the influence of the temperature on the resulting morphology. The obtained samples possessed dodecahedral shapes with a size distribution of ≈500 nm and a rough surface. CS-700 (Figure 18e) and CS-800 (Figure 18f) were composed of nanoparticles with an average radius of ≈7.5 nm. Additionally, the particles size of CS-900 (Figure 18g) increased with a uniform diameter of ≈40 nm. Figure 18h,i displays the porous dodecahedral structure and well-matched fringe spacing of CoSe. In addition, a carbon layer covering the CoSe nanoparticle surface was clearly observed. The elemental

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Figure 17.  a) Schematic illustration of the preparation of RGO@CoP@C-FeP. b,c) SEM, d) TEM, and e) elemental mapping images of the core/ shell-structured CoP@FeP microcubes. f) Cycling stabilities at 100 mA g−1 and g) rate performances of the as-prepared samples. Reproduced with permission.[74] Copyright 2017, Elsevier.

mapping images shown in Figure 18j demonstrate the uniform distribution of Co, Se, C, and N elements. When utilized as an anode for SIBs, CS-800 exhibited a superior sodium-storage performance with high specific capacities of 594.1, 560.4, 538.2, 513.5, 487.6, 456.6, 422.2, 389.9, and 361.9 mA h g−1 at current densities of 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 9.6, 12.8, and 16 A g−1, respectively (Figure 18k), and a steady specific capacity of 531.6 mA h g−1 after 50 cycles at a current density of 500 mA g−1 (Figure 18l), which are larger than those of most of other anode materials. Zhu et al.[76] developed a hierarchical durian-like nickel sulfide (NiS2) material by a sulfuration reaction of an Ni-based MOF and sublimed sulfur. When used as a sodium energy storage anode, a capacity of 280.6 mA h g−1 was obtained at a current density of 100 mA g−1 after 60 cycles. Moreover, capacities of 397.4, 330.6, 290.5, 252.7, and 209.8 mA h g−1

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were delivered under current densities of 50, 100, 200, 300, and 500 mA g−1, respectively. MOF-derived MPs, MSs, and MSes deliver high cycling capacities and excellent rate performances for SIBs, outperforming those of MPs, MSs, and MSes prepared by other methods,[77] which indicates that the promise of these preparation approaches for MPs, MSs, and MSes from MOFs. The excellent electrochemical performances can be ascribed to the carbon matrix structure containing highly dispersed nanoparticles. Notably, examples of MOF-derived MPs, MSs, and MSes are rare and should be further explored and expanded. The electrochemical storage performances of the reported MOF-derived materials for SIBs are summarized Table 2. The initial Coulombic efficiencies of most MOF-derived materials are less than 60%. This poor initial Coulombic efficiency can

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Figure 18.  a) Schematic illustration of the synthesis of the nitrogen-doped CoSe/C nanocomposites. SEM and TEM images of the CoSe/C materials obtained under different temperatures: b,e) CS-700, c,f) CS-800, d,g) CS-900. h–j) TEM, HRTEM, and elemental mapping images of CS-800. k) Rate performances of the prepared specimens. l) Cycling performances of the as-prepared electrodes at a current density of 500 mA g−1. Reproduced with permission.[75] Copyright 2017, American Chemical Society.

Table 2.  Electrochemical storage performances of reported MOF-derived materials for SIBs. Materials Microporous carbon

First C.E.

Initial discharge/charge capacities [mA h g−1]

Cyclability [mA h g−1]

Rate capability [mA h g−1]

19.3%

851/164 at 50 mA g−1

136 at 50 mA g−1 after 50 cycles

129 at 0.5 A g−1

g−1

g−1

g−1;

Potential window [V]

Year/Ref.

0.01–2.0

2014/[45]

g−1

0.01–2.5

2015/[78]

Nitrogen-doped porous carbon

31.3%

Cube-shaped porous carbon

34.3%

1822.8/626 at 100 mA g−1

240 at 100 mA g−1 after 100 cycles, 100 at 3.2 A g−1 after 5000 cycles

110.2 at 0.8 A g−1; 103.2 at 1.6 A g−1; and 100 at 3.2 A g−1

0.01–3.0

2016/[46]

N-doped carbon hollow tubules

47.1%

735/346 at 120 mA g−1

346 at 0.12 A g−1 after 10 000 cycles

238 at 1.5 A g−1; 147 at 4.5 A g−1; and 128 at 7.0 A g−1

0.0–3.0

2017/[47]

Hollow CuO/Cu2O octahedral composite

45%

Not given

415 at 50 mA g−1 after 50 cycles

273.5 at 0.5 A g−1; 217.2 at 1 A g−1; and 153.8 at 2.5 A g−1

0.005–3.0

2015/[50]

Porous cake-like TiO2

57%

Not given

250 at 50 mA g−1 after 50 cycles; 173 at 1 A g−1 after 2500 cycles;

189.6 at 0.5 A g−1; 151.7 at 2 A g−1;117.3 at 4 A g−1

0.01–3.0

2017/[51]

Hierarchical porous TiO2 nanopills

38%

516.2/196.4 at 100 mA g−1

164.3 at 100 mA g−1 after 50 cycles; 100 at 500 mA g−1 after 3000 cycles

170 at 0.05 A g−1; 110 at 0.5 A g−1

0.01–3.0

2017/[79]

Hollow MgFe2O4 microboxes

51%

406/207 at 50 mA g−1

135 at 50 mA g−1 after 150 cycles

172 at 0.1 A g−1; 112 at 0.5 A g−1; 85 at 1 A g−1

0.01–3.0

2017/[53]

Porous CoFe2O4 nanocubes

68.8%

573/394 at 50 mA g−1

350 at 50 mA g−1 after 50 cycles; 152.6 at 2.5 A g−1 after 500 cycles

329.7 at 0.2 A g−1; 273.4 at 1 A g−1; 224 at 1 A g−1; 175.5 at 2.5 A g−1

0.01–3.0

2017/[54]

2D porous Co3O4/ZnO

≈61%

Not given

310 at 300 mA g−1 after 100 cycles

375 at 0.2 A g−1; 265 at 1 A g−1; 242 at 2 A g−1

0.01–3.0

2017/[55]

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677.2/209 at 50 mA

144.7 at 50 mA

after 200 cycles

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135.5 at 0.2 A

117.6 at 0.4 A

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www.advancedsciencenews.com Table 2. Continued. Initial discharge/charge capacities [mA h g−1]

Cyclability [mA h g−1]

Rate capability [mA h g−1]

Potential window [V]

Year/Ref.

Hollow NiO/Ni/graphene 48.7% composite

992/483 at 200 mA g−1

≈190 at 1 A g−1 after 200 cycles

385 at 0.2 A g−1; 295 at 0.5 A g−1; 248 at 1 A g−1; 207 at 2 A g−1

0.005–3.0

2015/[57]

Nitrogen-doped carbon-coated Co3O4 nanoparticles

63.5%

813/516 at 100 mA g−1

373 at 0.2 A g−1 after 60 cycles; 175 at 1 A g−1 after 1100 cycles

506 at 0.1 A g−1; 317 at 0.4 A g−1; 263 at 1 A g−1

0.01–3.0

2016/[58]

Hollow Co3O4/N–C polyhedra

65.7%

712/468 at 100 mA g−1

368 at 0.1 A g−1 after 50 cycles; 276 at 0.5 A g−1 after 50 cycles

520 at 0.2 A g−1; 432 at 0.5 A g−1; 385 at 1 A g−1; 326 at 2 A g−1

0.01–3.0

2017/[59]

Graphitized porous-Cmatrix-coated CuO/Cu2O

44.8%

Not given

302.9 at 50 mA g−1 after 200 cycles

285.7 at 0.5 A g−1; 227.3 at 2 A g−1; 209.6 at 3 A g−1

0.01–3.0

2016/[60]

CuO/RGO composite

≈63%

Not given

466.6 at 100 mA g−1 after 50 cycles

369.9 at 1 A g−1; 347.6 at 2 A g−1

0.005–3.0

2017/[61]

0.05–3.0

2016/[62]

Materials

First C.E.

g−1

662 at 200 mA g after 200 cycles

42%

324.7/136.4 at 84 mA g−1

175 at 84 mA g−1 after 200 cycles, 70 at 3.36 A g−1 after 2000 cycles

154.4 at 0.168 A g−1; 86.8 at 1.68 A g−1; 70.6 at 3.36 A g−1

0.01–3.0

2016/[63]

Anatase TiO2@C

38.1%

393.3/149.7 at 100 mA g−1

167.4 at 100 mA g−1 after 110 cycles; 148 at 500 A g−1 after 500 cycles

125.5 at 0.5 A g−1; 111.1 at 1 A g−1; 88.9 at 2.5 A g−1

0.01–3.0

2016/[64]

Defect-rich TiO2–б/C composite

57.3%

455.5/261.2 at 200 mA g−1

235 at 200 A g−1 after 600 cycles; 88.5 at 10 A g−1 after 5000 cycles

128.3 at 2 A g−1; 98.1 at 5 A g−1; 88.6 at 10 A g−1

0.01–3.0

2017/[65]

956/534.6 at 100 mA g−1

358.8 at 100 mA g−1 after 200 cycles

307.3 at 0.5 A g−1; 233.2 at 2 A g−1; 201.8 at 5 A g−1

0.01–3.0

2017/[66]

560/233 at 50 mA g−1

260 at 50 mA g−1 after 100 cycles; 140 at 5 A g−1 after 5000 cycles

186.3 at 0.5 A g−1; 138.7 at 2 A g−1; 115.7 at 4 A g−1

0.005–3.0

2017/[67]

2455.6/1163.6 at 100 mA g−1

473.1 at 100 mA g−1 after 100 cycles

405.3 at 0.4 A g−1; 253.6 at 0.8 A g−1; 155 at 1.6 A g−1

0.01–3.0

2017/[73]

Hierarchical hollow Fe2O3@MIL-101(Fe)/C Carbon-coated rutile TiO2

87.1%

FeTiO3/carbon nanotubes 55.9% (FTO⊂CNTs) MnO@C nanorods

41.6%

Porous core/shell CoP@C 47.3% polyhedron/3D RGO network

1052/916.3 at 200 mA

−1

−1

−1

450 at 2 A g ; 421 at 4 A g

Core/shell CoP/FeP porous microcubes/RGO

56.9%

968/551.4 at 100 mA g−1

456.2 at 100 mA g−1 after 200 cycles

403.3 at 0.5 A g−1; 374.6 at 1 A g−1; 341.2 at 2 A g−1

0.01–3.0

2017/[74]

Yolk–shell-structured CoSe/NC dodecahedra

≈65%

Not given

531.6 at 500 mA g−1 after 50 cycles

513.5 at 1.6 A g−1; 456.6 at 6.4 A g−1; 389.9 at 12.8 A g−1; 361.9 at 16 A g−1

0.01–3.0

2017/[75]

Durian-like NiS2

76.2%

Not given

280.6 at 100 mA g−1 after 60 cycles

290.5 at 0.2 A g−1; 252.7 at 0.3 A g−1; 209.8 at 0.5 A g−1

0.01–3.0

2017/[76]

Amorphous carbon nitride

67.2%

851/164 at 83 mA g−1

175 at 1.67 A g−1 after 2000 cycles

430 at 0.083 A g−1; 146 at 8.33 A g−1

0.01–2.5

2015/[80]

54%

632.9/335.4 at 100 mA g−1

218.7 at 500 mA g−1 after 100 cycles

218 at 0.5 A g−1; 156 at 2 A g−1; 110 at 5 A g−1

0.01–3.0

2017/[68]

Ni-doped Co/CoO/Ndoped carbon

be attributed to electrolyte decomposition under high voltage, SEI film formation, and other irreversible reactions between the sodium species and surface functional groups. Notably, the initial Coulombic efficiencies of the N-doped carbon-coated metal oxides and hybrid bimetallic oxides are typically higher than those of other anode materials, which suggests that constructing heteroatom-doped carbon-coated metal oxides and hybrid multimetallic oxides is an efficient method to modify the initial Coulombic efficiencies. The reversible specific capacities of these MOF-derived anodes are primarily in the range of 200–500 mA h g−1, in which the capacities of the Co-based compounds and Fe-based oxides are relatively higher than those of the other materials, and the TiO2-based anodes show relatively preferable cycling stabilities.

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3. Conclusions and Perspectives Currently, there is an urgent need for the exploration of SIBs in large-scale energy storage grids due to dwindling lithium resources. The search for suitable electrode materials for SIBs has been the focus of a recent research, and numerous materials with various nanostructures have been shown to possess Na+ storage activity. In the past decade, MOFs have been extensively explored for various fields, and no other material has shown a broader application field than MOFs. Due to their intrinsic well-defined, highly porous structures with high surface areas, controllable pore sizes, and various compositions, MOFs have been demonstrated as ideal precursors for the preparation of diverse nanostructured materials. These

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MOF-derived nanomaterials exhibit similar structural properties as their corresponding parent MOFs and deliver remarkable electrochemical performances for sodium energy storage. In this review, the recent progress on four types of MOFbased nanomaterials, including porous carbons, metal oxides, metal oxide/carbon nanocomposites, and other MOF-derived hybrid materials, for SIB anodes was summarized. These MOF-derived materials are typically obtained by one- or two-step annealing treatments or heating reactions under certain temperatures in air or under an inert atmosphere. MOFderived porous carbons have shown outstanding cycle stabilities and rate capabilities due to their unique porous structures and good electronic conductivities. In addition, MOF-derived metal oxides have also delivered enhanced specific capacities and stabilities compared to other metal oxides due to their unique structural characteristics inherited from the parent MOFs, which can effectively accommodate volume changes and accelerate charge and mass transfer. To further enhance the electronic conductivities and stabilities of metal oxides, various metal oxide/carbon nanocomposites with ultrafine nanoparticles well distributed in carbon matrices have been obtained from various MOFs, and these nanocomposites show largely elevated storage performances and long cycle lives for SIBs. Furthermore, other MOF-derived hybrid materials, mainly including MPs, MSs, and MSes, have also presented impressive sodium-storage performances, greatly expanding the variety of MOF-derived materials. Witnessing the fast development of MOF-derived materials for SIB electrodes will be very thrilling in the years to come. Although numerous advancements on the exploration of MOF-derived materials for sodium energy storage have been achieved, many obstacles still exist, and the industrialization of SIBs for large-scale energy storage is full of challenges. For MOF-derived anode materials, multiple problems still need to be resolved. First, the initial Coulombic efficiencies of most MOF-derived anodes are under 60% due to their high surface areas, which seriously restrict their practical applications, as the minimum initial Coulombic efficiency for practical applications is 90%. Second, the cycling lives and rate performances must be further improved to satisfy actual demands. Thus, to reduce the irreversible sodium-storage capacity, decrease the Na+ and electron transfer distances, and enhance the electrochemical storage performance of SIBs, more precise control over the multichemical compositions, surface textures, and nanostructures of MOF-derived materials is urgently needed. Third, further understanding of the carbonization processes of parent MOFs and the correlation between the MOF-derived meso/nanostructures and their sodium energy storage performances is required for further advancement. Finally, all these problems can be solved by the efforts of the researchers, and MOF-derived materials will become promising SIB anodes in large-scale energy storage applications.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51622406, 21673298, and 21473258), the National Postdoctoral Program for Innovative Talents (BX201600192), the Innovation-Driven Project of Central South University (Grant Nos.

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2017CX004 and 2016CX020), the Distinguished Young Scientists of Hunan Province (13JJ1004), the Fundamental Research Funds for the Central Universities of Central South University (2017zzts115 and 2017zzts454), and the Fundamental Research Funds for the Central Universities of Central South University (2016zzts022 and 2016zzts238).

Conflict of Interest The authors declare no conflict of interest.

Keywords metal oxides, metal phosphides, metal sulfides, MOF-derived materials, porous carbon Received: August 1, 2017 Revised: October 13, 2017 Published online:

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