The structure and strength of glass fibers of different chemical composition

PROGRESS REPORT Sodium-Ion Batteries

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From Crystalline to Amorphous: An Effective Avenue to Engineer High-Performance Electrode Materials for Sodium-Ion Batteries Zhixuan Wei, Dongxue Wang, Xu Yang, Chunzhong Wang, Gang Chen, and Fei Du* global concerns about the sustainability of LIBs, due to the rarity and uneven distribution of lithium on earth.[3] With this in mind, batteries employing earth-abundant elements as charge carriers with similar working principles, such as sodiumion batteries (SIBs) and potassium-ion batteries (PIBs), are promising as realistic alternatives for grid-level stationary applications.[4] As the critical component for rechargeable batteries, electrode materials play an important role in improving the energy density of the whole system, which can be realized by increasing the output voltage or capacity.[5] In recent years, since the emerging sodium and potassiumbased batteries have the potential to meet large-scale grid energy storage, intensive research have been devoted to this field, accompanied by significant progress. However, there is still considerable room for improvement regarding the electrode-active materials for SIBs and PIBs. This is because the accommodation of sodium or potassium in host materials is much more difficult than lithium ions, owing to their much larger ionic radii.[6] Their magnitude may cause severe pulverization of the electrode resulting from the distortions in the host lattice after repeated (de)intercalation and subsequently, a loss of contact between the active material and the current collector, resulting in the failure of the cell. During the past few decades, crystalline materials, as the primary electro-active species of choice in the battery field, have been thoroughly investigated. However, their synthesis can be time consuming, energy intensive, and costly. Besides, the storage capacity in crystalline materials is critically dependent on several factors including orientation of the crystallites, exposure of electrochemically active facets, phase transitions, and structural stability.[7] Generally, the charge storage mechanism of these kinds of material is based on guest ion insertion/extraction during the charge/discharge process. However, these host materials have limited ion channels for guest ions. As their counterpart, amorphous materials are intrinsically different in the arrangement of atomic clusters in the manner of short-range ordering, while these clusters are randomly linked with each other.[8] Hence, the amorphous state can be easily identified by conventional characterization technique such as no obvious diffraction peaks in powder X-ray diffraction (XRD). Owing to such long-range disordered and short-range ordered

Room-temperature rechargeable sodium-ion batteries appear to be promising alternatives for grid and other storage applications to lithium-ion batteries because of the natural abundance, low cost, and environmental benignity of sodium. In response to the ever-increasing development for these technologies, an intensive exploration for appropriate electrode materials with high energy density is still underway. This Progress Report highlights the recent research in the investigation of amorphous materials, whose isotropic physical and chemical properties can provide multiple pathways for ions and facilitate ion diffusion. This Progress Report focuses on sodium-ion batteries, but it is hoped that the engineering strategies may provide guidance on further research and design of functional materials for both sodium and potassium-ion batteries.

1. Introduction The continuous consumption of traditional fossil energy supplies, and their increasing environmental impact, has spurred a global desire for renewable clean energy sources such as solar, tide, and wind. However, due to the intermittent nature of the renewable energy, developing efficient storage and mobile systems are critical to integrate the energy into constant and controllable power delivery.[1] Rechargeable batteries, able to directly and reversibly convert between chemical and electrical energy, are ideal power sources for portable devices, automobiles, and backup power supplies. In particular, since lithiumion batteries (LIBs) were first commercialized in 1991, they have revolutionized the market of portable electronic devices including cell phones, laptops, digital cameras, etc. They are also promising power sources for electric vehicles and hybrid electric vehicles.[2] The increasing demand, however, raises

Z.-X. Wei, D.-X. Wang, Dr. X. Yang, Prof. C.-Z. Wang, Prof. G. Chen, Prof. F. Du Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education) College of Physics Jilin University Changchun 130012, P. R. China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201800639.

DOI: 10.1002/admi.201800639

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structure, they possess unique merits related to their amorphous structure, such as isotropic property, enhanced kinetics property, large surface area, and free volume, ultimately beneficial to the accommodation of lattice distortion or expansion, and improvement of specific capacity and cycle stability. As is known, amorphous materials are common in nature and it has been demonstrated that a wide range of materials are characteristics as amorphous, such as rubber, glass, plastic, and asphalt.[9] As potential energy storage materials, amorphous materials have already demonstrated their superior electrochemical properties over bulk crystalline materials in LIBs.[10] Apart from time, cost, and energy savings, the main merits of amorphous materials over their crystalline counterpart are listed below. i) Enhancing the cell voltage. Maier and coworkers used a specially designed electrochemical cell to quantitatively compare the amorphous with the crystalline phases. The results show that the enhanced Gibbs free energy of amorphous phases plays a beneficial role in achieving a higher potential in Li batteries.[11] ii) Increasing ion storage sites and diffusion channels. In the highly crystalline materials, ion storage sites are confined only to the limited ionaccessible crystallographic sites. Introducing structural defects (vacancies and void spaces) by amorphization can provide more hosts for both ions (ion storage sites) and electrons (redox centers).[10g] iii) Facilitating solid-state Li+ diffusion. The short-range ordered property of amorphous materials decreases the entropic energy associated with the formation of crystalline structure and creates fruitful open framework to promote ionic transfer. Besides, the increased interfacial areas of amorphous materials could also facilitate fast ion transfer via percolation effect. The formation of the homogeneously amorphous structure also facilitates ion diffusion by enhancing the atomic/ionic mobility within the matrix.[12] iv) Tolerating severe volume change. Amorphous materials can effectively accommodate stress during the ion (de)intercalation process, as they can undergo reversible shape and volume changes due to their homogeneous expansion and contraction. v) Improving reaction activity. The kinetics of the reaction can be correlated to the overpotential. Guo et al. used Galvanostatic intermittent titration technique (GITT) to investigate the overpotential change of a conversion reaction during the lithium (de)intercalating toward amorphous and crystalline structure, respectively.[10e] Their result exhibited a decreasing trend of overpotential in the amorphous material during the first lithiation. Such a phenomenon may result from the decreased resistance caused by the defects generated from the volume change in the first lithiation process. The low overpotential resulted in low charge/discharge hysteresis. Given the underlying potential of amorphous materials, in this report, we highlight the recent progress in this promising field. We thoroughly summarize several state-of-the-art amorphous materials that have been successfully introduced into energy storage systems, specifically in SIBs. Some other aspects of material synthesis, Na intercalation chemistry, and how the amorphous structures enhance the electrochemical performance of various electrode materials are also discussed, with the purpose of providing insights into viable structural design for functional materials.

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Zhixuan Wei received her bachelor’s degree from Jilin University in China. From 2014, she works as a graduate student in Key Laboratory of Physics and Technology for Advanced Batteries for her Ph.D. degree. Her research interests mainly focus on the design and synthesis of functional materials for rechargeable batteries. Dongxue Wang received her bachelor’s degree from Jilin University in 2014. She is currently studying for her doctor’s degree in college of physics at Jilin University. Her research interests focus on the design and synthesis of NASICON-type compounds as the electrode materials for room-temperature sodium-ion batteries and potassium-ion batteries. Fei Du received his Ph.D. degree from Jilin University in 2008. After that, he has done the postdoctoral research at State Key Lab of Superhard Materials and University of Tokyo. He is currently a professor and the vice dean of College of Physics at Jilin University. His current research interests focus on the design of new battery electrode materials and systems.

2. Amorphous Cathode Materials 2.1. Iron-Based Materials Since being reported by Padhi et al.,[13] olivine LiFePO4, which is environmentally friendly and cost-effective, has been attracting extensive attention as one of the most promising candidates of cathode material for LIBs due to their thermal stability and high voltage, and has been successfully commercialized.[14] Inspired by this, the analog, NaFePO4, shows great potential in the exploration of suitable electrode materials for SIBs. As one of the mostly investigated iron phosphate compounds, NaFePO4 has two phases: the maricite and olivine phases.[15] Maricite NaFePO4 is a thermodynamically stable phase, which can be synthesized via solid state reaction, while it appears to

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Figure 1.  A) Systematic diagram for the nanocomposite of SWNT–amorphous porous FePO4 nanoparticles. Reproduced with permission.[20] Copyright 2012, American Chemical Society. B) An illustration of the formation mechanism of the mesoporous FePO4 nanospheres. C) Electrochemical characterization of the FePO4 nanospheres: (a) CV curve conducted at a scan rate of 0.1 mV s−1 (voltage window 1.5–3.8 V); (b,c) Galvanostatic discharging/ charging profiles performed at a current density of 20 mA g−1 and the corresponding cycling performance; and (d) rate capability. Reproduced with permission.[21] Copyright 2014, American Chemical Society. D) Electrochemical performances of the FeF3/C nanocomposites: (a) cyclic voltammograms between 1.25 and 4.5 V at a scanning rate of 0.2 mV s−1; (b) charge/discharge rate performance in the voltage range of 1.5–4.5 V; (c) charge/discharge voltage profile of the first, second, and third cycles. Reproduced with permission.[27] Copyright 2017, Royal Society of Chemistry. E) Electrochemical characterization of the mesoporous amorphous FeOF nanococoons: (a) galvanostatic discharging/charging profiles performed at a current density of 0.2 A g−1; and (b) rate capability. Reproduced with permission.[30] Copyright 2015, Royal Society of Chemistry.

be impossible to remove sodium ion from its orthorhombic structure. Consequently, its frustrated cationic channels in the structure make it electrochemically inactive.[16] In contrast, olivine NaFePO4 is electrochemically active for sodium ion storage due to the very similar channel structure to its Li counterpart, while it is metastable at room temperature and thus can only be produced by a cation-exchange method from olivine LiFePO4.[17] Taking these facts into account, crystalline NaFePO4 seems to be a less appealing substitute for SIBs. To address the issue, it is of great significance to develop noncrystalline iron phosphates. Amorphous FePO4 can be prepared through direct chemical synthesis at low temperature and has been successfully applied in lithium-ion storage.[18] Considered as a conceptually defect-free phase, amorphous FePO4 provides reliable continuous pathways for Na+ during the charge–discharge process. The theoretical capacity of this nanostructured material is as high as 178 mA h g−1 for the anhydrous form.[19] Moreover, the flat potential change of amorphous FePO4 resulting from the single phase-change Na ion insertion/extraction reaction makes it facile to control the charge/discharge process in practical. Meanwhile, to overcome the poor conductivity, there have been many investigations into the combination of conductive carbon-derived composites with FePO4. Liu et al. first reported a facile hydrothermal process to produce a networked

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nanocomposite of single-wall carbon nanotubes (SWNTs)/ amorphous porous FePO4 nanoparticles for SIBs (Figure 1A).[20] The amorphous FePO4 nanoparticles provide easy diffusion pathways for Na ions benefiting from the periodic, porous structures at the nanometer scale, while electrons can quickly move through the interconnecting SWNTs. The obtained nanocomposite exhibits electrochemical performance with specific capacity of 66 mA h g−1, as well as good cycle life over 300 cycles. Fang et al. used a simple chemically induced precipitation method to successfully prepare mesoporous amorphous FePO4/C nanospheres (Figure 1B),[21] exhibiting a high initial discharging capacity of 151 mA h g−1 at 20 mA g−1 and stable cyclability of 94% capacity retention over 160 cycles (Figure 1C). In addition, biofacilitated energy-efficient synthesis methods[22] and microemulsion techniques[23] have also been used to prepare amorphous FePO4/C nanocomposites with different morphologies, with encouraging electrochemical performance. Based on the promising results of iron phosphates, efforts have been devoted to exploring cathode materials with higher energy density (higher voltage or theoretical capacity). Since the molecular weight of F is much smaller than that of polyanions, fluorides promise a larger theoretical capacity. Furthermore, once a conversion reaction is involved, specifically the fluoridebased materials, extremely high capacities are expected, enabled by fluorides’ ability to transfer multiple electrons per formula

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Figure 2.  A) First four charge–discharge cycles of bilayered V2O5 and orthorhombic V2O5 electrodes. Both cells were cycled at 20 mA g−1, within the potential window of 3.8–1.5 V (vs Na/Na+) from 1 m NaClO4 in PC. B) Small and wide angle X-ray scattering (SAXS and WAXS) spectra for bilayered V2O5: electrochemically deposited vacuum annealed sample (blue); after discharging with the current of 630 µA (black), 120 µA(gray), 20 µA (light gray); and after cycling at 120 µA in charge state (red). Model structures and critical interlayer spacing depicting transformations occurring upon Na+ intercalation and deintercalation are also shown. Reproduced with permission.[32] Copyright 2012, American Chemical Society. C) Electrochemical characterization of the as-prepared amorphous and nanocrystalline V2O5: (a) charge–discharge curves for the (a) crystalline, and (b) amorphous V2O5 against Na/Na+ at a current density of 23.6 mA g−1 (0.1 C); the rate capability of the (c) crystalline and (d) amorphous V2O5 when discharged at current densities ranging from 23.6 (0.1 C) to 1170 mA g−1 (5 C). Reproduced with permission.[12] Copyright 2014, Royal Society of Chemistry.

unit.[24] Iron trifluoride, with its high theoretical specific capacity (≈712 mA h g−1) and high operation voltage (≈2.74 vs Na/Na+), resulting from the higher ionicity of FeF bonds, has attracted considerable interest in cathode materials for SIBs.[25] In the LIBs, fluoride disordering or amorphization, along with defect generation or strain relief, is an effective way to improve their ion storage performance. This improvement may result from a new structure being reconstructed from the amorphous structure, one more in favor of ion diffusion in the electrode materials.[26] For Na ion storage, Zhang et al. prepared amorphized iron fluoride densified from its open parent phase,[27] carbonized Fe-MOF, via a vapor-solid fluoridation reaction and dehydration reaction, which can deliver 302 mA h g−1 discharge capacity at current density of 15 mA g−1 (Figure 1D). Furthermore, previous studies suggest that substitution of fluoride sites with oxygen can increase the electronic conductivity mainly owing to the weakened boding strength between cations and anions.[28] Taking this into account, iron oxyfluoride (FeOxF2-x), offering the combined advantages of fluorides and oxides, including high reaction voltage, improved capacity, conductivity, and cycle stability, is a promising cathode candidate for SIBs.[29] However, it should be noted that the existing synthesis strategy suffers from several limitations, such as high reaction temperature and high pressure, usage of corrosive hydrofluoric acid, or toxic F2. Fu et al. synthesize amorphous FeOF with mesoporous structure via a solution plasma processing

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method.[30] The as-prepared material provides fruitful open pores beneficial to the electrolyte penetration, increase in the conductivity and reduce in the diffusion pathway, leading to a better electrochemical performance than the crystalline FeOF (Figure 1E). As the result of these merits, the amorphous species delivered a stable capacity of 190 mA h g−1 at a high current rate (20 A g−1).

2.2. Vanadium-Based Oxides Vanadium oxide is known for its multioxidation states and diverse crystalline structures and has been intensively studied for energy conversion in the recent years.[31] V2O5, characteristics as layered structure, has become a promising cathode material for SIBs because of the high theoretical capacity of 236 mA h g−1. Tepavcevic et al. took advantage of the shortrange order in V2O5 (Figure 2A).[32] They used electrochemical deposition to prepare the bilayered V2O5 electrodes as cathode material for SIBs. The electrostatic attraction of the as-designed oxide layers provided a strong driving force for the diffusion of ions into the framework, leading to ordering of the overall structure with both short-range order (within the layers) and longrange order (between the layers). The deintercalation of sodium ion from bilayered V2O5 electrodes accompanied with the loss of long-range order, while short-range order was preserved

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(Figure 2B). Benefiting from the unique structure, the obtained bilayered V2O5 electrodes exhibited high capacity with high redox potentials, giving an energy density of ≈760 Wh kg−1. Uchaker et al. prepared amorphous and nanocrystalline V2O5 through a combination of sol–gel processing paired with electrochemical deposition and then compared their electrochemical performance.[12] Specifically, they used a diluted vanadium pentoxide solution as the precursor. The hydrous vanadium oxide was deposited through the electrochemical deposition of VO2 and the catalyzed gelation of V4+. After the electrochemical deposition on Ni-foil, the electrodes were processed under different thermal treatment conditions to obtain the amorphous and crystalline samples. The generated open channels of amorphous structure reduce the diffusion barrier for sodium ions, leading to a high discharge capacity (241 mA h g−1) and high rate capability (78 mA h g−1 at a current density of 1180 mA g−1), whereas its crystalline counterpart only had a capacity of 120 mA h g−1 (Figure 2C). The different diffusion behavior accounts for their different properties, for amorphous V2O5, the diffusion occurs through isotropic percolation while the sodium pathway in the crystalline V2O5 is confined along a preferential way. Also, they claimed that the difference also arises from the fast Faradaic reactions that occur in amorphous phase, stemming from the percolated diffusion network. Additionally, the coulombic efficiency of the amorphous phase is much higher and stable compared to the nanocrystalline counterpart.

3. Amorphous Anode Materials 3.1. Transition Metal Oxides Transition metal oxides (TMOs) are attractive as anodes for SIBs due to their multiple valence state, low cost, natural abundance, and environmental friendliness. Based on the different intrinsic properties of the transition metal, their sodium storage relies on two different mechanisms, namely, intercalation type (e.g., TiO2) and conversion reaction with Na (e.g., Fe2O3). As presented in the following text, amorphous TMOs are also being investigated in hope of finding appropriate electrode material for SIBs.

3.1.1. Titanium Oxides It has been claimed that highly crystalline and/or micronized TiO2 cannot easily support Na+ insertion because of the ionic size of Na+ and a much higher sodium diffusion barrier compared to Li. However, several breakthroughs have shown the possibilities of the sodium storage capability of TiO2 recently. Xiong et al. reported the feasibility of electrochemically grown titanium dioxide nanotubes as anodes for SIBs.[33] They used electrochemical anodization of Ti foil to synthesize densely packed, vertically oriented amorphous TiO2 nanotubes that are electronically connected to the current collector (Figure 3A).

Figure 3.  A) Scanning electron microscopy (SEM) top-view images of TiO2NT electrodes: amorphous TiO2NT a) before and b) after cycling in Na system. B) Charge/discharge galvanostatic curves of amorphous 80 nm I.D. TiO2NT in Na half cell (red for discharge and black for charge) cycled between 2.5 and 0.9 V versus Na/Na+ at 0.05 A g−1 (C/3, discharge the electrode in 3 h). C) Specific capacities as a function of cycle number measured at a current density of 0.05 A g−1 in Na system: red solid squares represent discharge and black open squares represent charge; and at 3 A g−1 in Li system: red solid circles represent discharge and black open circles represent charge. Reproduced with permission.[33] Copyright 2011, American Chemical Society.

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Figure 4.  A) Ex-situ XRD patterns of (a) NaVO3, (b) discharged phase Na2.2VO3, and (c) charged phase Na1.5VO3 versus Na+/Na. B) Charge– discharge curves of NaVO3 in the potential window between 0.8 and 2.6 V versus Na+/Na with a C/50 rate and insets of the initial discharge derivative curve and the cycling performance of NaVO3 versus Na+/Na with a C/50 rate. Reproduced with permission.[37] Copyright 2013, Elsevier B.V. C) Electrochemical characterization of the as-prepared amorphous (Fe2O3@GNS) and nanocrystalline (Fe2O3@GNS-500) Fe2O3: Initial three discharge–charge curves for Fe2O3@GNS (a) and Fe2O3@GNS-500 (b) at 100 mA g−1. (c) Cycling performance at 100 mA g−1 and coulombic efficiency of Fe2O3@GNS and Fe2O3@GNS-500. (d) Rate capability of Fe2O3@GNS and Fe2O3@GNS-500. (e) Cycling performance for Fe2O3@GNS and pure graphene nanosheets cycled at a current of 2 A g−1, and coulombic efficiency of the Fe2O3@GNS sample. (f) Electrochemical impedance plots and Randles equivalent circuit of Fe2O3@GNS and Fe2O3@GNS-500 after three cycles at 100 mA g−1. (g) Schematic illustration of transport paths of Na+ ions into the amorphous Fe2O3 and crystalline Fe2O3, and the conversion reaction of sodium-ion batteries. Reproduced with permission.[38] Copyright 2016, American Chemical Society.

They found that the sodium ion storage properties depend on the size of the nanotubes. Nanotubes with about 80 nm in inner diameter and above 15 nm in wall thickness show an increase in the specific capacity upon cycling, as shown in Figure 3B, ranging from 60 mA h g−1 in the initial cycle to 140 mA h g−1 after 50 cycles. It was proposed that sodium ions could not intercalate into the nanotubes with smaller diameter less than 45 nm. This can be understood that the dissolved Na+ ions in the electrolyte not reaching a critical concentration to effectively intercalate into the TiO2 walls. In addition, TiO2 on Ti foam[34] and carbon nanotube network/carbon fiber paper substrates[35] also exhibit promising sodium ion storage capability. Bella et al. used density functional theory calculations to provide an insight into the difference of the mechanism of Na insertion in the amorphous and anatase phase of TiO2 to propose the explanation for their different electrochemical behavior. According to their results, the oxide lattice in the flexible structure of amorphous TiO2 can easily relax around the Na ion, and the amorphous phase is believed to contain several high favorable insertion sites for accommodating Na ions, these sites can also act as traps for the Na ions. In contrast, the rigid crystal structure of anatase only allows a minimal distortion of the lattice.[36] Their computation results provide a way to deeply understand the fundamental difference between crystalline and amorphous materials.

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3.1.2. Vanadium Oxides Venkatesh et al. reported an amorphous vanadate Na1.5+xVO3 achieved by an electrochemical reaction from crystallized NaVO3.[37] The crystallized monoclinic NaVO3 was synthesized by the conventional solid-state method. During the initial discharge reaction, the monoclinic NaVO3 phase is destroyed, leading to the novel vanadium-based oxide (Figure 4A). The resultant amorphous oxide exhibits reversible electrochemical sodium intercalation/de-intercalation properties through a solid-solution-like process, for 0 < y < 0.7, with redox cycling at 1.8 V versus Na+/Na and a capacity of 150 mA h g−1 (Figure 4B). The result opens a route to the investigation of amorphous matrices involving transition metal oxides.

3.1.3. Iron Oxides Li et al. used a facile ice-assisted chemical reduction method to uniformly anchor amorphous Fe2O3 nanoparticles on the surfaces of graphene nanosheets.[38] In comparison with its crystalline counterpart, amorphous Fe2O3 provides more accommodation vacancies and transfer channels for Na+ ions to be inserted in and extracted from the host. Moreover, the strong interfacial interaction via oxygen bridge bonds is good for the electron transfer and buffering the volume

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Figure 5.  A) Schematics of WP, RP, and BP. Reproduced with permission.[41] Copyright 2014, American Chemical Society. B) Initial charge/discharge curves of three phases of phosphorus: red phosphorus, black phosphorus, and a-P/C nanocomposite. All the electrodes were tested at a constant current density of 250 mA g−1 in the voltage range of 0.01–2 V versus Na+/Na. C) Charge–discharge profiles at a constant rate of 250 mA g−1; the inset shows the change in the reversible capacities with cycle number. Reproduced with permission.[46] D) Voltage profiles (black) of amorphous red P/C composite and the corresponding electrode thickness change (gray) during sodiation and desodiation. E) Ex situ XRD patterns of amorphous red P/C composite electrodes collected at various points as indicated in (D). Reproduced with permission.[47]

change. The amorphous materials delivered a high capacity of 440 mA h g−1 at the current density of 100 mA h g−1. Even at the high current density of 2 A g−1, it exhibits a specific capacity of 219 mA h g−1, which is much better than its crystalline counterpart (Figure 4C).

3.2. Alloy-Reaction Compounds Alloy reactions of elements such as Si, Sn, Sb, Ge, or P with Li have been extensively studied as negative electrodes for LIBs because of their high theoretical specific charge.[39] In light of successful lithium alloy anodes, attempts have recently been made to develop alloy anodes for sodium ion storage.[40] However, the volume change and pulverization of the active materials during the sodiation/desodiation is much more severe due to the larger ionic radius. To tackle this problem, amorphizing the electrode at the very beginning may be an effective strategy to mitigate the severe volume change.

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3.2.1. Phosphorus Compounds Phosphorus is a nonmetallic element, which is known to have three common allotropes—white, red, and black (Figure 5A).[41] White phosphorous is volatile and unstable because it always exists as tetrahedral structured P4 molecules, whose PP bonds are very weak (≈200 kJ mol−1), leading to a low activation barrier to oxidation.[42] It burns when exposed to natural atmosphere. Red phosphorus can be easily changed from white P by means of heat, light or X-rays. It has a wide variety of structures and is usually amorphous in nature and is widely commercially available. The covalent bonds in red P are strong; hence, it has higher chemical stability and less reactivity, making it attractive for analysis and application. Black phosphorus can also be obtained from white P under high pressure and high temperature. It is a crystalline phase with layered material in which individual atomic layers are stacked together by van der Waals interactions. It is thermodynami­cally stable below 550 °C and can transform to red phosphorus at higher temperatures.[43] Hence, the red and black forms of phosphorus have been considered as candidates for anode materials.

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The complete reaction of Na with P can form sodium phosphide (Na3P). The three-electron-transfer reaction provides an extremely high theoretical capacity of 2595 mA h g−1.[44] The key challenge associated with the phosphorous anode is the dramatically structural collapse caused by its large volume change (>490%) during sodium insertion/extraction. To address the issue, efforts have been devoted to design and construct novel microstructured/nanostructured P-based materials to accommodate the volume change.[41,45] Among the various approaches, the construction of an amorphous P composite is one of the most effective ways to minimize the mechanical stresses induced by volume change. Qian et al. reported an amorphous red phosphorus and carbon composite obtained by facile high-energy ball-milling, which realized the reversible sodium storage property of P.[46] The nanocomposite delivered a high specific capacity of 1764 mA h g−1 and good cycle stability over 100 cycles, better than both pure red-P and black-P (Figure 5B,C). Red-P delivered a high discharge capacity of 897 mA h g−1, whereas it can only give a small charge capacity of 15 mA h g−1, indicative of bad reactivity of red-P due to its insulating nature. In contrast, black-P shows a better sodium storage performance with charge capacity of 2035 mA h g−1; however, the reversibility should be further improved. The smaller reversibility of crystalline phosphorus emphasizes the importance of the amorphous phase in releasing the strain arising from intercalation. Meanwhile, the carbon matrix in the composite can offer electrochemically favorable and kinetically facile nanodomains for the reaction, which enhances the electron transport. Simultaneously and independently, Kim et al. prepared an amorphous P/carbon nanocomposite via ball-milling method, which demonstrated superior sodium storage performances, including a high reversible capacity (1890 mA h g−1), superior rate capability with capacity of 1540 mA h g−1 at 2.86 A g−1.[47] Furthermore, Na3P was formed as the final product when being discharged to 0.01 V confirmed by ex situ XRD measurement (Figure 5D,E). Subsequently, Song et al. reported a phosphorus/graphene nanosheet hybrid through ball milling of red phosphorus and graphene stacks.[48] Their result shows that the graphene stacks are mechanically exfoliated to nanosheets that chemically bond with the surfaces of phosphorus particles, acting as a robust conductive matrix to maintain electrical contact (Figure 6A). Benefiting from the unique structure, the amorphous hybrid yields a high reversible capacity of 2077 mA h g−1 with excellent cycling stability (1700 mA h g−1 after 60 cycles, Figure 6B). In addition, many different forms of carbon, including N-doped graphene papers (Figure 6C),[49] highly ordered mesoporous carbon (Figure 6D),[50] MOFderived N-doped microporous carbon (Figure 6E)[51] have been introduced into the amorphous P/C based anode to buffer the severe volume change and achieve high reversible capacity and long cycle life.

naturally raised interest of employing Si-based alloys as anode materials for SIBs. From the Na-Si binary phase diagram studied by the thermal synthesis of Na-Si materials, it is known that the most Na-rich phase for Na-Si binary compounds is Na1Si, which means that a theoretical capacity of 954 mA h g−1 would be achieved based on the alloying reaction of Si with Na.[53] However, although electrochemical sodiation of crystalline Si has been explored in experiments, reversible sodium insertion is hard to achieve due to the poor sodium diffusion kinetics, and positive binding energy during sodiation.[54] Encouragingly, recent reports predict that the insertion energetics of single atoms into amorphous Si are thermodynamically more favorable than that in crystalline Si.[55] Regarding Na, it is reported that a single Na atom has a high insertion cost into crystalline Si (1.51 eV vs Na in bulk), but not in amorphous Si (0.09 eV vs Na in bulk). Hence, using amorphous Si material can be a promising method to implement high capacity sodiation. Xu et al. first reported that reversible electrochemical Na ion uptake in Si was experimentally achieved for a significant capacity.[56] They produce Si nanoparticles, containing both crystalline and a large fraction of amorphous Si (Figure 7A,B) from expanding thermal plasma chemical vapor deposition of silane. The nanoparticles demonstrated a reversible capacity of 279 mA h g−1 for Si and a capacity retention of 248 mA g−1 after 100 cycles at 20 mA g−1 (Figure 7C). Through XRD analysis after sodiation, they concluded that the Na insertion into the amorphized Si on the surface of crystalline phase as well as the amorphous Si particles in the sample contributes to the formation of Na1Si. In addition, Lim et al. reported that the sodiation capability of amorphous Si is strongly influenced by the synthesis method.[57] They compared the electrochemical sodiation properties of amorphous silicon particles prepared using three synthetic techniques, including (1) chemical delithiation, where Si was first lithiated and then delithiated either in an aqueous or organic solution; (2) the mechanical method through high-energy ball milling; and (3) mechanical fusion where a Si-Sn composite containing amorphous silicon was synthesized through ball milling. Among the three samples, the mechanically fused Si-Sn particles that contained partially amorphized Si-rich interior cores surrounded by a Sn shell demonstrated the most stable specific capacity of 230 mA h g−1 (Figure 7D). The Sn shell in this case can protect the amorphized Si from oxidation and can considerably reduce the first-cycle irreversible capacity as well as charge-transfer resistance upon the first sodiation at the same time. Very recently, Han et al. prepared sponge-like amorphous silicon by reacting silicon tetrachloride with commercial magnesium powder (Figure 7E).[58] Taking advantage of the synergistic effect of the amorphous nature and porous structure, the Si electrode was successfully applied in SIBs. When cycled at 100 mA g−1, the electrode exhibits a reversible capacity of 176 mA h g−1 after 100 cycles, and a reversible capacity of 142 mA h g−1 can be maintained over 400 cycles.

3.2.2. Silicon Compounds

3.2.3. Sn-Based Alloy

Silicon, possessing high theoretical lithiation capacity up to Li4.4Si and relatively low discharge potential, has been extensively investigated for LIBs.[39b,52] The attractive specific capacity

Tin is a well-known active element for sodium storage, which can reversibly alloy with sodium to achieve a high theoretical capacity of 847 mA h g−1 (Na15Sn4). However, the Sn anodes

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Figure 6. A) Schematic illustration of the synthesis of phosphorus/graphene nanosheets (P/G) hybrid and B) their electrochemical properties: (a) The cyclic voltammograms of the P/G hybrid anode at a scanning rate of 0.1 mV s−1; (b) typical discharge–charge voltage profiles of the P/G hybrid anode; (c) cycle stability and coulombic efficiency of the P/G hybrid anode at a current density of 260 mA g−1; (d) rate capability of the P/G hybrid anode. The specific capacity is calculated based on the mass of the phosphorus. Reproduced with permission.[48] Copyright 2014, American Chemical Society. C): (a) Illustrative scheme of the designed novel “butter-bread”-like anode structure consisting of amorphous P layer@N-doped graphene frameworks. There may be PC bonds formed between P and graphene layer, which provide stable contact after cycling. (b) SEM image of the cross section of a P@GN paper, the inset shows its paper-like appearance. (c) High-resolution transmission electron microscope (HRTEM) image and the corresponding fast Fourier transform (FFT) pattern of the P@GN portion, confirming its amorphous structure. Reproduced with permission.[49] Copyright 2016, American Chemical Society. D): (a) Schematic illustration of the preparation process for the P@CMK-3 material. (b) Schematic illustration of the lithiation/sodiation process of red P particles, red P particle-carbon core–shell composite, and nanostructured red P confined in the channels of CMK-3 in LIBs and SIBs. Reproduced with permission.[50] Copyright 2016, American Chemical Society. E) Schematic illustration of the preparation process for (a) P@N-MPC and (b) sodiation process of P@N-MPC. Reproduced with permission.[51]

face the same volume change problem as other alloy/dealloytype anodes. It undergoes two-step sodiation to form amorphous NaSn2 in the first step and sequentially to several Na-rich amorphous phases and crystalline Na15Sn4 phase in the second step, which leads to a total volumetric expansion of about 420%.[59] Since tin-based anodes will be eventually amorphized during the electrochemical cycling, attempts to form amorphous phases at the very beginning can be an effective way to solve the problem of degradation of the cycle life performance. Among various well-investigated tin-based compounds, Sn-based composite oxides (tin composite oxides (TCO)) are reported as promising candidates because of their low cost, high theoretical capacity, and nontoxic nature. More importantly, most TCOs are in amorphous form. In 1994, Fuji Photo Film Co. Ltd, Japan, filed a patent for nonaqueous LIBs in which TCOs were used as the active anode material.[60] Since then, much attention has been given to the potential of substituting this class of compounds for carbon negative electrodes in LIBs. The TCO active material has a basic formula represented

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by SnMxOy, where M is a group of glass-forming metallic elements whose total stoichiometric number is equal to or more than that of tin and is typically comprised of a mixture of B(III), P(V), and Al(III).[61] Honma et al. proposed that the tin-phosphate glass works effectively as anode active materials for rechargeable sodium ion batteries for the first time.[62] The glass composition was prepared by melting a batch of raw materials in a nitrogen atmosphere using a platinum crucible followed by a quenching process. A good reversible capacity over 320 mA h g−1 was observed in the obtained electrodes, showing the promising possibility as the sodium ion storage material. The sodiation process due to the reduction of Sn2+ to Sn0 state led to both the precipitation of nanoscale tin alloy particles in a phosphate glass matrix and the structural modification of phosphate glass matrix. Very recently, our group reported the characterization of amorphous Sn2P2O7/reduced graphene oxides nanocomposite synthesized by the facile ball-milling process as an anode material for SIBs.[63] The composite delivers a high specific capacity

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Figure 7.  A) X-ray diffraction patterns on Si nanoparticles. B) Raman spectroscopy of Si nanoparticles showing the coexistence of amorphous-Si and crystalline-Si. Electrochemical properties of Si nanoparticles: C) charge/discharge voltage profiles at 20 mA g−1 and voltage response of discharge/ charge at different current rates. Reproduced with permission.[56] D) Charge–discharge voltage curves of the mechanical fusion synthesized a-SiSN electrode. Reproduced with permission.[57] Copyright 2016, Elsevier Ltd. E) Electrochemical performances of the sponge-like amorphous silicon anodes for sodium ion batteries: Typical galvanostatic discharge–charge curves for the first five cycles of the Si sample in the potential range of 0.01–3.0 V versus Na/Na+ at 20 mA g−1. Reproduced with permission.[58] Copyright 2018, Royal Society of Chemistry.

of 480 mA h g−1 with a capacity retention of 80% after 100 cycles, thanks to the improved kinetic properties (Figure 8A). According to the ex situ XRD results, after being charged to 0.01 V, the electrode shows typical amorphous states, indicating an ultrasmall initial particle size of the Na-Sn alloy (Figure 8B). It also demonstrates an excellent rate capability compared with the crystalline counterpart, which can be attributed to the synergistic effect of short diffusion length and the conductive framework of reduced graphene oxide (rGO).

3.2.4. Sb-Based Compounds Attention has been paid to the application of Sb-based anodes in sodium ion batteries since Qian et al. successfully demonstrated

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the sodium storage capability of Sb.[64] The mechanically mixed Sb-C nanocomposite offers a high capacity of 610 mA h g−1, close to the theoretical one (660 mA h g−1), corresponding to the insertion of three Na ions. Encouragingly, Darwiche et al. reported that even commercial micrometric Sb demonstrated outstanding cycling performances versus Na at a high rate, largely superior to the better-known ones versus Li.[65] In their report, in situ XRD investigation shows that the reaction of Sb versus Li goes through an alloying/dealloying process, whereas the process in Na/Sb includes amorphous intermediate phases during the cycling, which may act as a buffer to relieve strain, accounting for the improved cycling seen in the system. Hence, the outstanding properties of Sb in SIB enable the development of new negative electrode chemistry for SIBs, which is not viable in the case of the LIBs. On the other hand, some metal compounds can store

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Figure 8.  A) GITT curves of a-Sn2P2O7/rGO. B) Ex situ XRD pattern of a-Sn2P2O7/rGO at various charge states. Reproduced with permission.[63]

Na ions through a combined electrochemical conversion and alloy/dealloying mechanism to give superior theoretical capacities when serving as anode materials for SIBs, among which the sulfides are most widely explored owing to the more favorable kinetic properties for conversion reactions given by the weaker MS bonds. Antimony sulfides (Sb2S3) have a theoretical capacity of 946 mA h g−1 based on the conversion reaction (Sb2S3  + 6Na+  +6e−  → 2Sb + 3Na2S) and subsequent alloying reaction (2Sb + 6Na+ +6e− → 2Na3Sb).[66] Owing to the relative reversibility of the formation of sodium sulfides compared to the formation of sodium oxides, along with the fast reaction kinetics of Sb with Na ions, Sb2S3-based anodes show great promise for sodium ion storage. Hence, several researchers reported amorphous Sb2S3 to explore high energy density host materials. Zhao and Manthiram reported amorphous Sb2S3 embedded in graphite layers using the high energy mechanical milling method.[66a] Such a structure not only enhances sodium ion

diffusion and electron transport in the electrode, but also effectively alleviates the pulverization of the active material caused by volume change during sodiation/desodiation. Compared with the crystalline electrode, the as-prepared electrode exhibited a high initial Coulombic efficiency of 84%, superior rate capability, and remarkably stable cycling performance. Specifically, stable reversible capacities of 656, 495, and 315 mA h g−1 were delivered after 100 cycles at 1, 10, and 15 A g−1, respectively (Figure 9A,B). Hwang et al. fabricated amorphous Sb2S3 nanoparticles via a polyol-mediated room-temperature synthetic protocol (Figure 9C).[66b] By dissolving the SbCl3 precursor in ethylene glycol, which acted as a chelating agent and dispersion medium, a Sb-glycolate complex was formed. The reaction with thioacetamide (offering S2− ions in the dispersion medium) promoted gradual transformation of the Sb-glycolates into SbSx nuclei, the growth (Ostwald ripening) of which led to aggregation of the spherical nanoparticles to form the amorphous structure.

Figure 9.  A) (a and b) Cyclic voltammetry (CV) scans of the amorphous Sb2S3-graphite electrode using sodium as a counter electrode/reference electrode at a scan rate of 0.05 mV s−1; (c) galvanostatic intermittent titration technique (GITT) curves of the Sb2S3-graphite electrode; the current pulse of C/30 lasted for 1 h and the relaxation time was for 1 h. B) Cycle performance and potential profiles of amorphous Sb2S3-graphite (a) and its crystalline counterpart (b) under galvanostatic conditions at the current density of 1 A g−1. Reproduced with permission.[66a] Copyright 2015, Royal Society of Chemistry. C) Galvanostatic charge–discharge voltage profiles measured at a current rate of 50 mA g−1 over the first five cycles (a), and cycle performance (b) of the amorphous Sb2S3 nanoparticle anode. Reproduced with permission.[66b] Copyright 2016, Royal Society of Chemistry.

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The obtained amorphous Sb2S3 nanoparticles displayed better cycling and rate performance compared to the commercial, crystalline Sb2S3 electrode.

4. Characterization Techniques for Amorphous Materials As we mentioned before, the long-range disordered nature of amorphous materials is a double-edged sword, it provides amorphous materials several merits for energy storage, but makes them hard to be analyzed through common characterization techniques such as XRD and high-resolution transmission electron microscopy (HRTEM), which are used to study the long-range ordered crystalline structure. However, efforts are being devoted to this field to figure out the structural and chemical changes occurred during the electrochemical reaction. Here we summarized several techniques that allow us to explore the amorphous electrodes. Note that the techniques here are not restricted to SIBs field, with the purpose to provide a comprehensive introduction to benefit the future studies for amorphous materials for energy storage.

4.1. Fourier-Transform Infrared Spectroscopy Fourier-transform infrared spectroscopy (FTIR) can provide information about local environment of cations in the lattice. Yang et al. performed FTIR studies on Sn2P2O7 before and after amorphization.[63] The preservation of the vibration modes indicates the maintenance of the short-range structural ordering. Venkatesh et al. compared the FTIR spectrum of sodium vanadate NaVO3 and its amorphous derivant upon electrochemical sodium insertion (Na1.5+yVO3), which were very similar.[37] This indicates that the VO4 tetrahedra in the parent structure are maintained while forming the amorphous phase.

4.2. Raman Spectroscopy Raman spectroscopy is used to observe low-frequency modes in a system, which is useful in battery studies. For amorphous material, it can also quantitatively tell the portion of amorphous phase in the structure. When Xu et al. first reported the reversible Na-ion uptake in Si nanoparticles, they observed the coexistence of amorphous and crystal phase, and the amount of amorphous Si is calculated to be significant.[56] This method is of high value to study the materials that are mixture of amorphous and crystalline.

4.3. X-Ray Absorption Spectroscopy X-ray absorption spectroscopy (XAS) is commonly used to study the local atomic or electronic structure of amorphous materials.[10a,d,g,67] There are mainly two regions in the XAS spectrum, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). They can provide metal valence and local environment of metal ions in amorphous structure, respectively. Xiong et al. observed the change

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of the oxidation state of Ti ions in amorphous TiO2 through XANES.[33] Upon discharging in the presence of Na transporting ions, Ti atoms in host matrix were partially reduced. Also, with the intercalation of bulky Na ions, the chemical disorder in the structure was enhanced. This together with the imbalance of the sizes between host Ti and intercalated Na ions gave the explanation of the limited capacity. Chae et al. used K-edge EXAFS to give an evidence of the absence of long-range ordering and a clear short-range ordering in amorphous V2O5.[10b] After a lithiation/delithiation cycle, they observed that the FT peaks for V-O and V-V are very close to those in the pristine sample in terms of intensity and position, which is distinctively different from the crystalline counterpart. This is because of the absence of the irreversible phase transitions during the lithium insertion/removal.

5. Summary and Outlook In this minireview, we summarized several types of amorphous materials as electrodes for SIBs. Benefiting from their isotropic physical and chemical properties, they can provide multiple pathways for ions and facilitate ion diffusion. In general, the amorphous phase can be easily achieved by the facile high-energy ball milled. Furthermore, these materials have larger specific surface area than their crystalline counterparts and therefore can show better performance and be applicable in more fields. They can also provide more candidates for sodium ion hosts, such as the amorphous Si. More importantly, the amorphous states have less structural confinement when inserting/exchanging large-sized Na ions, which can be introduced to the emerging potassium ion batteries for the exploration of host materials for K ions. However, at present, the understanding of amorphous material is much less than crystal ones. The amorphous state can be more easily identified than analyzed. The internal structures of amorphous materials are so complicated that many concepts about the structure, dynamics, and thermodynamics of amorphous nanomaterials still need to be established. Hence, it is hard to understand the relationship between their structure and properties to optimize the amorphous materials. Systematically modeling by theoretical calculations may be indispensable for obtaining deeper insight. Fortunately, the amorphization of materials provides an effective avenue for the exploration of electrodes for large-size ion storage and will thereby inspire the development of energy storage systems.

Acknowledgements Z.-X.W. and D.-X.W. contributed equally to this work. This work was financially supported by funding from “973” project (Grant No. 2015CB251103), the Joint Project between Jilin Province and Jilin University (Grant No. SXGJQY2017-10), and the Science and Technology Development Project, Jilin Province (Grant Nos. 20180101211JC, 20170101168JC, and 20180414004GH).

Conflict of Interest The authors declare no conflict of interest.

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Keywords amorphous, electrode materials, energy storage, sodium-ion batteries Received: April 26, 2018 Revised: June 3, 2018 Published online:

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