Two‐Dimensional Metal Oxide Nanomaterials for Next...

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Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries

Jun Mei, Ting Liao, Liangzhi Kou, and Ziqi Sun*

J. Mei, Dr. T. Liao, Dr. L. Kou, Dr. Z. SunSchool of ChemistryPhysics and Mechanical EngineeringQueensland University of Technology2 George Street, Brisbane, QLD 4001, AustraliaE-mail: [email protected]. T. LiaoInstitute of Superconducting and Electronic MaterialsUniversity of WollongongNorth Wollongong, NSW 2500, Australia

DOI: 10.1002/adma.201700176

nanomaterials have been discovered and extensively studied, such as metal oxides,[9] transition-metal dichalcogenides (TMDs),[10–12] layered double hydroxides (LDHs),[13,14] 2D covalent organic frame-works (COFs),[15,16] 2D metal–organic frameworks (MOFs),[17,18] silicene,[19] ger-manene,[20,21] black phosphorene,[22–24] arsenene,[25–28] stanene,[29] borophene,[30,31] carbon nitride,[32,33] hexagonal boron nitride,[34–36] and Mxene,[37–39] etc. As an emerging class of nanoscale materials, 2D nanomaterials show unprecedented prop-erties that are unparalleled compared to their corresponding bulk materials, which gives these materials promising applica-tions in a wide range of areas, including high-speed electronic and optical devices, actuators and sensors, energy conversion and storage devices, DNA sequencing, and so on.[40–50] Among these applications, energy-storage devices, such as batteries and capacitors, are crucial for conquering the intrinsic and unavoidable shortcoming of the intermittent power supply from some renewable energy sources, such as solar, wind, and wave energy, by storing

the electricity produced in abundant periods to be used in scarce periods.[51–53] There is no doubt that the development and application of high-performance energy-storage devices represents an effective and convenient alternative method to reduce the consumption of traditional fossil fuels and maintain a sustainable environment and energy supply.[54]

Rechargeable batteries, a class of various promising energy-storage devices, store chemical energy with the ability to deliver electrical energy by repeated charging–discharging processes. Traditional secondary batteries, such as lead acid and nickel–cadmium batteries, have a low energy capacity and a short life-time, and these, combined with heavy weight, limit their appli-cations in long-distance electric vehicles.[55] The introduction of Li-ion batteries (LIBs), Na-ion batteries (NIBs), and other types of rechargeable batteries as next-generation rechargeable batteries, however, make an uninterrupted long-lasting power supply a possibility.[56,57] Owing to their superior electrochem-ical performance and flexibility, next-generation rechargeable batteries are undergoing an explosive growth in further devel-opment as portable rechargeable batteries for smart portable devices and as power supplies for electric vehicles. In terms of the components, the new types of rechargeable batteries are still composed of two core parts, i.e., the electrodes (anode

The exponential increase in research focused on two-dimensional (2D) metal oxides has offered an unprecedented opportunity for their use in energy conversion and storage devices, especially for promising next-generation rechargeable batteries, such as lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs), as well as some post-lithium batteries, including lithium–sulfur batteries, lithium–air batteries, etc. The introduction of well-designed 2D metal oxide nanomaterials into next-generation rechargeable batteries has significantly enhanced the performance of these energy-storage devices by providing higher chemically active interfaces, shortened ion-diffusion lengths, and improved in-plane carrier-/charge-transport kinetics, which have greatly promoted the development of nanotechnology and the practical application of rechargeable batteries. Here, the recent progress in the application of 2D metal oxide nanomaterials in a series of rechargeable LIBs, NIBs, and other post lithium-ion batteries is reviewed relatively comprehensively. Current opportunities and future challenges for the application of 2D nano materials in energy-storage devices to achieve high energy density, high power density, stable cyclability, etc. are summarized and outlined. It is believed that the integration of 2D metal oxide nanomaterials in these clean energy devices offers great opportunities to address challenges driven by increasing global energy demands.

2D Metal Oxides

1. Introduction

The discovery of graphene in 2004 has brought numerous exciting breakthroughs in the fields of physics, chemistry, nanotechnology, and even biological science.[1–4] It is unques-tionable that graphene has brought the family of two-dimen-sional (2D) nanomaterials into the limelight, which features atomic-level thickness, superhigh surface-to-volume ratio, tunable electronic properties, intriguing chemical activity, and extraordinary mechanical strength, as well as some other superior properties.[5–8] To date, a growing number of 2D

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and cathode) and the electrolyte, the same as for traditional batteries. The number of different electrode materials, which govern the performance of batteries, is being continuously expanded, and has been one of the focuses of research in the field of batteries in recent years.[58] It is an objective to find and apply alternative low-cost electrode materials to achieve highly reversible ion-storage capacity, high energy density, long cycle life, and importantly, enhanced safety.[59–70]

Metal oxides are a big family for the development of func-tional nanomaterials, owing to the fact that oxides are the lowest-free-energy states for most metals in the periodic table in the oxidative atmosphere of the Earth, and demonstrate appli-cations ranging from insulators to semiconductors and even superconductors. The family of oxides, due to their unique combination of redox chemistry, rapid ionic-transport chan-nels, short-distance interactions between charge carriers, as well as between carriers and ions, and their earth-abundance, has played a key role in the successful implementation of recharge-able batteries. Among the promising 2D electrode materials for rechargeable batteries, TMDs are a chemically diverse class of compounds having bandgaps from 0 to ≈2 eV and remark-able electrochemical properties. The 2D feature of TMDs pro-vides a convenient environment to accommodate Li+ ions, and the relatively low operation voltage and energy density render their potential as anode materials in LIBs. The wide application of TMDs as anodes for LIBs, however, is still far from reality because of their obvious capacity fading during cycling, caused by the shuttle effect of polysulfide anions against their gradient, and the structural instability during the lithium-intercalation process. LDHs, another class of typical 2D nanomaterials, have been extensively applied in catalysts, ion-exchange hosts, fire-retardant additives, polymer/LDH composites, supercapaci-tors, and hydrogels. The electrochemical application of LDHs in rechargeable batteries, however, has been rarely reported, because of their inferior electrical conductivity, chemical insta-bility, and unexpected side reactions, which result in a low power performance and poor cycle life with limited redox kinetics arising from the relatively sluggish mass-diffusion rates and charge-transfer properties. Moreover, the synthesis of LDHs is relatively difficult to achieve due to their special configura-tion with positively charged metal hydroxide layers, negatively charged ions in between the layers, and high charge density, making the whole route more complex and less cost-effective.[71] Compared with TMDs and LDHs, chemically stable and envi-ronmentally friendly metal oxides are much easier to fabricate via the “top-down” and “bottom-up” routes, and furthermore, as electrode materials for rechargeable batteries, they can exhibit a higher theoretical lithium-storage capacity, depending on the various storage mechanisms, including alloying, insertion, or conversion reactions. Their large-scale application is hindered, however, by the critical issue related to their voltage hysteresis, resulting from the enormous volume changes during cycling, leading to rapid and obvious capacity decay during charging/discharging cycles. The appearance of 2D metal oxide nanoma-terials with tunable size, morphology, and redox reactions has ushered in a new period of popularity as high-performance elec-trode materials for next-generation rechargeable batteries.[72–75]

Many 2D metal oxide nanomaterials, such as V2O5, Cr2O3, MnO2, Co3O4, etc., are fabricated by a variety of synthesis

methods, including “top-down” and “bottom-up” approaches, into rationally designed nanoarchitectures. These well-designed 2D metal oxide nanostructures usually have improved inter-action interfaces with electrolytes and improved interplane ion transport and in-plane carrier-transport kinetics than the corresponding bulk materials, and thus greatly enhance the storage behavior of Li+, Na+, and other ions. Moreover, the 2D nanostructure provides a large specific surface area, which is beneficial to creating an increased number of active sites, achieving a better charge distribution, and speeding up the insertion–extraction or redox reaction rates. Thirdly, the special 2D features of the metal oxide nanosheets are effective for sup-pressing or moderating the volume expansion that arises in the charging–discharging cycles.

Here, we first briefly summarize the recent progress in the synthesis strategies for the fabrication of 2D metal oxide nano-materials and provide an outline of the energy-storage mecha-nisms of next-generation rechargeable batteries, including LIBs, NIBs, and other post Li-ion batteries. Then, a comprehen-sive review is given on the recent advances in 2D metal oxides for application as electrodes for different types of batteries, especially for LIBs and NIBs. Finally, the current issues, oppor-tunities, and future challenges for the fabrication of 2D metal

Ziqi Sun received his Ph.D. degree from the Institute of Metal Research, Chinese Academy of Sciences, in 2009. After finishing his NIMS postdoctoral fellow-ship (Japan) that he held for one year on solid oxide fuel cells, he joined the University of Wollongong (UOW). He is currently a Senior Lecturer at Queensland University of

Technology. His major research interests include metal oxide nanomaterials and bio-inspired inorganic nanoma-terials for sustainable energy harvesting, conversion, and storage.

Jun Mei received his Bachelor’s degree (2013) and Master’s degree (2016) in chemical engineering and technology from Shandong Normal University and from Changchun University of Technology (China), respec-tively. He is currently pur-suing his Ph.D. degree at the School of Chemistry, Physics and Mechanical Engineering

of Queensland University of Technology (QUT) under the supervision of Dr. Ziqi Sun. His major research interests include the design and synthesis of novel nanomaterials for energy conversion and storage devices.

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oxide nanomaterials and their applications in rechargeable bat-teries are discussed. We hope that the integration of 2D metal oxide nanomaterials in next-generation clean-energy devices will offer great opportunities to address the challenges driven by the increasing global energy demands and the pressing envi-ronmental crisis.

2. Fabrication of 2D Metal Oxide Nanomaterials

Due to the large number of members in the family of 2D metal oxide nanomaterials, it is very difficult to find a universal and cost-effective routine to synthesize all types of 2D nanomate-rials by a one-pot approach, even though we never stop trying. In general, the fabrication of 2D metal oxide nanomaterials can be classified into two approaches: “top-down” synthesis and “bottom-up” synthesis. The “top-down” synthesis refers to processes to whittle down the size and dimensions of materials from the bulk, thin films, or other high-dimensional forms by mechanical milling, exfoliation, etc., while “bottom-up” syn-thesis is the opposite process, in which the nanomaterials grow or are assembled from atoms, molecules, or sometimes pro-teins, by techniques like molecular self-assembly, layer-by-layer assembly, vapor deposition, etc.[76] Besides the mainly used “top-down” and “bottom-up” approaches, some other methods combining these two routines have also been employed for the fabrication of, for example, hybrid 2D nanomaterials, in which exfoliated graphene is used to guide the wet-chemistry growth of 2D nanomaterials.[77,78]

2.1. “Top-Down” Synthesis of 2D Metal Oxide Nanosheets

Following the successful fabrication of graphene by exfoliating layered graphite, many types of 2D metal oxide nanosheets have been fabricated via the “top-down” strategy. As in the pros and cons summarized in Figure 1, “top-down” synthesis is a very simple and effective technique that does not need complex facilities and equipment. Moreover, this method is easy to scale up for relatively large-scale production. 2D nanomaterials fab-ricated via the “top-down” approach inherit part of the crystal structure of the host crystals and thus usually still maintain a high crystallinity, which is also a critical advantage for high-speed carrier-charge transport. It has been reported that gra-phene can be produced on a scale of hundreds of kilograms by using a liquid-exfoliation method, opening a door to the com-mercialization of this material.[79] The “top-down” approach, however, also has some disadvantages. The main issue is that suitable layered host crystals, in which 2D platelets are weakly stacked via weak atomic bonds or van de Waals forces, are nec-essary. 2D materials without the corresponding layered host crystals cannot be produced via this “top-down” approach. Besides this requirement regarding the host crystals, the weak quality distribution of the obtained products is another una-voidable challenge. The thickness of the 2D nanomaterials fabricated via the “top-down” technique, such as by liquid exfo-liation, is distributed over a wide range, from a few layers to even thousands of layers. “Top-down” fabrication can be further classified into two subcategories: mechanical/thermal cleavage and liquid exfoliation. Mechanical cleavage was the earliest

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Figure 1. Strategies for the synthesis of 2D metal oxide nanosheets. Top) Schematic illustration of a typical top-down routine together with micros-copy images of the synthesized 2D Cr2O3, ZrO2, Al2O3, and Y2O3 nanosheets; Bottom) schematic illustration of a typical bottom-up synthesis routine together with microscopy images of 2D TiO2, ZnO, Co3O4, and WO3. Top) Reproduced with permission.[85] Copyright 2016, Nature Publishing Group. Bottom) Reproduced with permission.[9] Copyright 2014, Nature Publishing Group.



method to obtain graphene from graphite by using scotch tape, and this method has been modified for synthesizing other 2D nanomaterials by introducing mechanical energy, such as ball milling or ultrasonication.[80,81] Liquid exfoliation is a process that is conducted in solution with the help of organic solvents (e.g., N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), or N,N-dimethylformamide (DMF), etc.) and/or associated with chemical intercalation reactions or ion-exchange reactions.[82,83] In the case of liquid exfoliation, the discovery and application of more-effective, inexpensive, and non-harmful solvents are the barriers to the commercially targeted fabrication of 2D nanomaterials.

With the efforts of researchers all over the world, many sig-nificant achievements have been made reagrding “top-down” synthesis of 2D metal oxide nanomaterials. Recently, Huang et al. reported a modified mechanical exfoliation method for fabricating large-area, high-quality 2D flakes of graphene and bismuth strontium calcium copper oxide (Bi2Sr2CaCu2Ox, BSCCO) from layered crystals.[84] Compared to the previously reported mechanical exfoliation, the biggest changes introduced in this work were that the ambient adsorbates on the substrates were removed by oxygen-plasma cleaning prior to the exfolia-tion and additional heat treatment was carried out to maximize the uniform contact area between the layered crystals and their substrates. Via this method, graphene or BSCCO with sizes 20–60 times larger than for standard exfoliation were fabricated.

The second example is a thermal-exfoliation method designed for the mass production of 2D metal oxides that was developed by Zhao et al.[85] In this case, a series of 2D oxide nanosheets, including Cr2O3, ZrO2, Al2O3, and Y2O3 was obtained by heating the corresponding hydrous-chloride salts. During the rapid heating of the hydrous chloride or oxychlo-ride precursors, water in their crystal lattice was removed and the precursors were hydrolyzed with the release of H2O and HCl gases, which caused large pressures to drive the effec-tive exfoliation of the layered solid MClx (M = metal) and the spontaneous formation of M2Ox nanosheets. The typical mor-phology of the nanosheets is shown in Figure 1. The thickness of these nanosheets ranged from 1.2 nm to several micro-meters. As an example, the electrochemical performance of Cr2O3 nanosheets was evaluated by preparing graphene/Cr2O3 nanosheet electrodes for LIBs. The Cr2O3 (2D) electrodes could achieve an initial delithiation capacity of 974 mA h g−1 at a current rate of 0.2 A g−1 and a capacity retention of 986 mA h g−1 after 297 cycles, which is much higher than that of reference graphene/Cr2O3 particle electrodes. Notably, a lower initial capacity and an increasing capacity over cycling are common for many metal oxide nanomaterial electrodes in previous reports.[86–95] The former could be possibly attributed to the destruction of metal oxides and the formation of a solid electrolyte interphase (SEI), while the latter could be due to an existing activation process or the growth of a gel-like polymeric layer.

2.2. “Bottom-Up” Synthesis of 2D Metal Oxide Nanosheets

“Bottom-up” synthesis methods, such as vapor deposition and wet-chemistry self-assembly, have been widely employed

to fabricate 2D metal oxide nanosheets.[96,97] One typical method for bottom-up growth of 2D metal oxide nanosheets is the physical/chemical vapor deposition (PVD/CVD) tech-nique, where solid or gaseous precursors are evaporated, usu-ally under heating, and deposited on a cooler substrate. PVD/CVD can produce high-area 2D nanosheets on substrates with precisely controlled thickness, excellent crystallinity, and very high charge mobility, and has been widely used for fabricating 2D electronic materials. This method, however, is limited by its low productivity, which handicaps its application in energy-storage devices. Wet-chemistry synthesis is a good choice for the fabrication of 2D nanomaterials, and partially overcomes the conflict between quality and productivity that exists in the exfoliation and the vapor deposition methods. The advantages of wet-chemistry synthesis of 2D nanomaterials include uni-form size, morphology, and thickness of the product, relatively high yield, and suitability for scalable synthesis. The disadvan-tages of this method are its complex steps, strict requirements on reaction-solution preparation, and relatively expensive precursors.

Recently, Sun et al. developed a novel molecular self-assembly synthesis route for the preparation of ultrathin 2D transition-metal oxide nanosheets, such as TiO2, ZnO, Co3O4, WO3, Fe3O4, MnO2, etc. by rationally employing lamellar reverse micelles (Figure 1).[9] This generalized bottom-up method provides a pathway for the synthesis of other 2D metal oxide nanomaterials in large quantities. In the synthesis, inverse lamellar micelles of Pluronic P123 polymer with a co-surfactant were used to confine the growth of the metal oxide along the thickness dimension and resulted in an atomic-level thickness of the obtained nanosheets. Via this approach, TiO2, ZnO, Co3O4, and WO3 nanosheets with widths up to a few tens of micrometers and thicknesses of a few nanometers, corresponding to 2–7 stacking layers of the monolayer, were fabricated. In addition, the specific surface area of TiO2, ZnO, Co3O4, and WO3 nanosheets reached as high as 298, 265, 246, and 157 m2 g−1, respectively. Further studies from this group have demonstrated that these nanomaterials are very promising for application in energy harvesting, conversion, and storage devices, such as solar cells, fuel cells, LIBs, and NIBs.[98–102]

Later, Xiao et al. developed another strategy to synthesize 2D metal oxide nanosheets from solution by using the surfaces of water-soluble salt crystals as growth substrates or templates.[103] After dissolving the salt-template in water, the obtained 2D nanosheets could be reassembled into a binder- and additive-free film by filtration. This low-cost approach with inexpensive salts is applicable for both layered compounds and various binary transition-metal oxides, including hexagonal-MoO3 (h-MoO3), MoO2, MnO, and hexagonal-WO3 (h-WO3). The results showed that the as-synthesized nanosheets had a thick-ness of less than 2 nm and a lateral size exceeding 400 µm2. This unique 2D morphology gives the materials superior electrochemical performance. The 2D h-MoO3 nanosheets exhibit high pseudocapacitive performance of 300 F cm−3 in an Al2(SO4)3 electrolyte, and the capacities of LiClO4 nanosheets in ethylene carbonate/dimethyl carbonate organic electrolytes reach 996 C g−1 and 1100 C cm−3, respectively, at a sweep rate of 2 mV s−1.

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3. Next-Generation Rechargeable Batteries

Here, the application of 2D nanomaterials in LIBs, NIBs, and lithium–sulfur (Li–S) and lithium–air/O2 (Li–air/O2) batteries will be a particular focus of attention. Several other recharge-able ion batteries with larger ionic radii, such as Mg-ion bat-teries, K-ion batteries, Al-ion batteries, and Ca-ion batteries, rarely use 2D metal oxides as electrode materials, and hence, they will not be discussed here. In this section, we intend to give a brief introduction to the energy-storage mechanism of these batteries for a better understanding of the role of the 2D nanomaterials in these devices.

As shown in Figure 2,[104] in a typical lithium-ion battery, the Li+ ions originate from the cathode materials, such as lithium metal oxides or lithium iron phosphate.[105–108] These ions move to the negative electrode (anode) and insert them-selves into the anode materials (e.g., graphite, carbon nano-tubes, metal oxides, nanoalloys, etc.) during discharge. During charging, they then move back across the electrolyte between the electrodes, which contains lithium salts (e.g., LiPF6, LiCF3SO3, etc.) in an organic carbonate (e.g., ethylene, pro-pylene, dimethyl carbonate, etc.) or solid polymer (e.g., poly-ethylene oxide, polyacrylonitrile, etc.).[109–111] Meanwhile, free electrons are initially generated near the cathode and then subsequently transferred to the anode for insertion reactions via the external circuit. LIBs possess the advantages of high energy density, high output voltage (≈3.6 V), and long cycle life within a wide range of working temperature (−30 to +45 °C). LIBs are now used to supply power to a wide variety of devices,

ranging from hybrid electric vehicles, such as bicycles and cars, to portable electronic devices, such as smart phones, fashion-able watches, and high-resolution digital cameras. Despite the impressive growth in the market for LIBs worldwide, the bat-tery technology is still suffering from slow advances regarding its specific capacity, rate capability, cycling life, stability, etc., and alternative electrode materials are urgently required to fur-ther improve the performance of LIBs.

Li–O2 batteries, or Li–air batteries, which usually consist of a metallic lithium anode, a separator or conducting electrolyte, and a porous air or oxygen cathode, use oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow, and the lithium ions move between the anode and the cathode across the electrolyte. Li–O2/air batteries have been considered as a promising technology for next-genera-tion energy supplies because this family of batteries delivers a high energy output in proportion to its weight. This type of rechargeable battery has two basic systems, aqueous and non-aqueous batteries. In the aqueous one, a polymer or inorganic solid-state electrolyte is often employed to separate the lithium anode from the cathode, and in the discharging process, the formation of LiOH occurs based on the reaction ½O2 + 2Li+ + H2O + 2e− ↔ 2LiOH, whereas in non-aqueous Li–O2 batteries, O2 from outside is reduced at active sites on the cathode and reacts with Li+ ions migrating from the anode (Li ↔ Li+ + e−) to form Li2O2 (O2 + 2 Li ↔ Li2O2).[112–114] During charge, when a greater external potential is applied than the standard potential for the discharge reaction, Li ions move back to the anode and are reduced to metal with the release of oxygen at the cathode.

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Figure 2. Schematic representations of Li-ion, Na-ion, Li–O2, and Li–S batteries. Reproduced with permission.[104] Copyright 2012, Nature Publishing Group.



During the design of Li–O2 batteries, the optimization of the air electrode and the discovery of efficient catalysts for the decomposition of Li2O2 or Li2O at a low charge potential play an important role in the improvement of the electrochemical performances.[114] In other words, the catalytic activities of the catalysts (e.g., Co3O4[115,116] or RuO2[117]) toward the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER) have a crucial effect on the round-trip efficiency and the electrochemical performance of the battery. If the ambient air is directly used for lithium–air batteries, some other chal-lenges, such as the degradation and poisoning of the cathode as a consequence of the humidity and CO2 in the air will lower the performance of the battery. Therefore, effective and low-cost electrode materials with stable performance are still urgently demanded for the further development of Li–O2/air bat-teries.[118–121] Following studies of Li–air batteries, other metal–air batteries based on high-theoretical-capacity anode materials (e.g., 3815 mA h g−1 for zinc and 2965 mA h g−1 for aluminum) have also gained great attention.[122–125]

With the purpose of pursuing a higher energy density in rechargeable batteries, lithium–sulfur (Li–S) batteries have been considered as an alternative to Li-ion batteries, which have an electrochemical mechanism that is different from the intercalation of Li ions in the electrodes, and thus can give high energy density. Similar to other rechargeable batteries, Li–S batteries consist of a positive electrode (cathode) composed of a composite containing the elemental sulfur, and a nega-tive electrode (anode) composed of lithium metal, aling with a porous separator.[126] Li–S batteries have the following overall electrochemical reaction: S8 + 16Li ↔ 8Li2S. Based on this reac-tion, the capacities of the anode and cathode of Li–S batteries are as high as 3860 and 1672 mA h g−1, respectively, giving a theoretical energy density of ≈2600 W h kg−1, which is much higher than that of LIBs (387 W h kg−1).[127–130] The practical applications of Li–S batteries, however, have been handicapped by the intrinsic lack of suitable electrode materials. The cur-rently used electrode materials suffer from low electrical and ionic conductivity, a great volume expansion of the sulfur ele-ment, and actual complex and multiple-step electrochemical reactions in the electrodes, which diminish the battery per-formance. To address these issues and achieve high-capacity retention, trapping polysulfides within the cathode structure is an effective solution. As efficient sulfur hosts, 2D metal oxide nanosheets with hydrophilic surfaces can offer sufficient active sites for binding polysulfides. This combination can prohibit the dissolution of long-chain Li2Sx into electrolytes and facili-tate the deposition of short-chain Li2Sx. In addition, metal oxide nanosheets can also promote stable redox activities during repeated charging–discharging cycles. Therefore, the rational design and construction of suitable electrodes for Li–S batteries are necessary for their practical applications.

As lithium resources continue to decline worldwide and the demand for inexpensive and effective energy-storage devices is increasing rapidly, next-generation rechargeable batteries pow-ered by metal ions other than lithium ions are attracting great interest at present. NIBs appear to be a promising alternative technology, because sodium has the fourth highest abundance of all elements on Earth and it is cheap and nontoxic.[131–135] Cur-rently, the biggest drawback is the slow charging–discharging

process in NIBs, which hinders their capacity to supply enough power in high-power applications.[136] The main research target at this moment is to find suitable electrode materials to achieve rapid and reversible sodium intercalation. Some high-performance electrode materials have been developed, such as expanded graphite, MoS2, Li4Ti5O12, Na2FePO4F, and some metal oxides (e.g., TiO2, SnO2, or Fe2O3).[137–142]

4. 2D Metal Oxide Nanomaterials for Batteries

In this section, we will give a detailed summary of some typical 2D metal oxide nanomaterials, including V2O5, Co3O4, SnO2, NiO, MnO2, and Fe2O3 nanosheets, for application in recharge-able LIBs, NIBs, and Li–air/O2 batteries. The key structural parameters for 2D structures, such as thickness, specific surface area, and configuration, and their impact on electrochemical performance are carefully reviewed according to the types of metal oxides. The pros and cons, as well as possible further improvements, are also discussed for each 2D nanomaterial.

4.1. Vanadium Oxide Nanosheets

During the past several decades, vanadium oxides have been studied for use in high-performance energy-related devices due to their unique electron–electron-correlation-dependent electronic structures.[143] Among the various vanadium oxides, V2O5 has been extensively studied for energy-storage appli-cations because of its high specific capacity/energy and its abundance in the Earth’s crust. On the other hand, VO2 is an exciting phase for smart windows, owing to its reversible semi-conductor-to-metal transition.

V2O5 acts as a typical intercalation compound as a cathode material for LIBs, and it has a good reversibility for the inser-tion and removal of Li+ ions (V2O5 + xLi+ + xe− ↔ LixV2O5). The theoretical capacity of V2O5 anode is 294 mA h g−1, which is calculated based on two Li+ ions per unit formula for the inter-calation–deintercalation in LIBs. Three main issues for V2O5, namely, low electrical conductivity, slow lithium-ion diffusion, and irreversible phase transitions during deep discharge, are obstructing its use in practical LIB applications.

The synthesis of V2O5 nanosheets for high-capacity and high-rate lithium storage was first reported in 2012.[144] In the syn-thesis, ammonium persulfate was used to react with bulk V2O5 to form (NH4)2V6O16 nanosheets, and then V2O5 nanosheets were obtained after heating the (NH4)2V6O16 nanosheets at 350 °C (Figure 3A). The synthesized V2O5 nanosheets as a cathode material exhibited enhanced Li-storage performance, including high reversible capacity (290 mA h g−1 at 100 mA g−1), and good cycling and rate performance (144 mA h g−1 at 3 A g−1 and 95 mA h g−1 at 6 A g−1, as shown in Figure 3B), delivering a power density as high as 15.6 kW kg−1 and an energy density of 260 W h kg−1. The enhanced lithium-storage performance of the 2D V2O5 nanosheets was attributed to the high interfacial con-tact area between the electrode and the electrolyte, and short-ened lithium-ion-diffusion and electronic-transport distances.

In 2013, Rui et al. fabricated few-layer V2O5 nanosheets with a thickness of 2.1–3.8 nm via a simple and scalable

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liquid-exfoliation technique by intercalation of formamide mol-ecules into the interlayer spaces of bulk V2O5 (Figure 3C).[145] The products exhibited a 2D sheet-like morphology with lateral dimensions in the range of 100 and 400 nm (Figure 3D). As a promising cathode material for LIBs, these nanosheets dis-play large reversible capacity, with the first discharge capacity reaching 292 mA h g−1 at a current density of 59 mA g−1, which is much higher than the gravimetric capacity of the pre-sent cathode materials (normally less than 200 mA h g−1). The nanosheet cathode also presented high Coulombic efficiency (93.8% after 50 cycles under a rate of 0.2 C) and stable cycla-bility (117 mA h g−1 after 200 cycles at a rate of 50 C), while the reference bulk V2O5 suffered from an abrupt capacity fading, with capacity retention of 46% after cycling 50 times. Remark-ably, as shown in Figure 3E, the V2O5 nanosheet electrode exhibits a gravimetric energy density of 158 W h kg−1 with a power density of 20 kW kg−1, indicating that V2O5 nanosheets can be used as superior electrochemical energy-storage devices with both high power and high energy densities.

Hydrated V2O5 nanosheets with a thickness of 1.5–2.6 nm were also synthesized by Rui et al. in a bottom-up approach, where V2O5·0.76H2O nanosheets were obtained via a control-lable sol–gel synthesis by using vanadium oxytripropoxide as a precursor.[146] They confined the crystal growth within the ab plane to form 2D nanosheets. The specific surface area of the

obtained product was 164.4 m2 g−1. When evaluated as superca-pacitor electrodes, the V2O5 nanosheets achieved a high energy density of 122 W h kg−1 at a power density of 1.1 kW kg−1.[146]

In the same year, Li et al. fabricated 2D leaf-like V2O5 nanosheets by reacting V2O5 powder with H2O2 under ultra-sonic treatment and then diluting, freeze-drying, and further heating the product at 450 °C in air.[147] The thickness of the obtained nanosheets was 60–80 nm, with a specific surface area of 28 m2 g−1. When used as a cathode material for LIBs, it delivered a high discharge capacity (104 mA h g−1) at a cur-rent density of 5000 mA g−1 in the voltage window between 2.0 and 4.0 V and low capacity fading (around 0.22%) per cycle at a current density of 500 mA g−1.[147] Later, ultralarge V2O5 nanosheets were fabricated by the same group by calcination of ultrathin VO2(B) nanosheets (3–8 layers with a thickness of 2–5 nm and a lateral size of over 100 µm) at 350 °C in air, after synthesis by a bottom-up solvothermal procedure.[148] The obtained ultralarge V2O5 nanosheets for LIBs exhibited specific discharge capacities of 141 mA h g−1 and 103 mA h g−1 in the voltage range of 2.5–4.0 V (vs Li/Li+) at current densities of 100 mA g−1 and 5000 mA g−1, respectively. More importantly, 92.6% of the initial capacity after 500 cycles could be retained, corresponding to an average capacity-fading rate of 0.015% per cycle. Additionally, electrochemical impedance spectros-copy (EIS) measurements confirmed that the charge-transfer

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Figure 3. A) Schematic illustration of the preparation of V2O5 nanosheets by thermal exfoliation of (NH4)2V6O16 nanosheets. B) Rate performance of V2O5 nanosheets at different current densities from 0.1 A g−1 to 6 A g−1. A,B) Reproduced with permission.[144] Copyright 2012, Wiley-VCH. C) Sche-matic illustration of liquid exfoliation of layered bulk V2O5 into few-layer nanosheets by intercalation of formamide molecules into the interlayer space of bulk V2O5. D) TEM image and the corresponding colloidal acetone dispersion (inset) of the liquid-exfoliated V2O5 nanosheets. E) Ragone plot of V2O5 nanosheets in comparison with some advanced energy storage and conversion devices. C–E) Reproduced with permission.[145] Copyright 2013, The Royal Society of Chemistry. F) Schematic illustration of the fabrication of isolated single-layers by an intercalation–deintercalation strategy. G) TEM image of a VO2(B) single layer fabricated via the intercalation–deintercalation strategy, and the corresponding Tyndall phenomenon of the dispersion (inset). F,G) Reproduced with permission.[153] Copyright 2012, Wiley-VCH.



resistance (82.14 Ω) was relatively low, demonstrating that the application of 2D V2O5 nanosheets can facilitate lithium-ion insertion–deinsertion reactions during the cycling process.[148]

Although 2D V2O5 nanosheets have shown enough advan-tages to be a promising electrode material for LIBs, the ionic and electronic conductivities of the nanosheets still need fur-ther improvement to achieve the expected high power or energy density. Peng et al. produced oxygen-deficient 2D V2O5 (H-V2O5) nanosheets by hydrogenation at 200 °C, where the oxygen vacancies at the O(II) sites obtained by removing the OH group formed when hydrogen atoms were adsorbed at the oxygen sites.[149] Owing to the formation of oxygen vacancies and V4+/V5+ pairs in the H-V2O5 nanosheets, both the elec-trical conductivity and the Li-ion diffusion rate of the H-V2O5 were significantly improved, enabling a high initial discharge capacity (259 mA h g−1) and a capacity decay of less than 0.05% per cycle.[149] Besides the strategy of enhancing the conductivity by hydrogenation, Liu et al. proposed a novel approach to incor-porate graphene sheets into V2O5 nanoribbons via a sol–gel pro-cess.[150] The resultant graphene-modified nanostructured V2O5 had a specific capacity of 438 mA h g−1 at 0.05 C, corresponding to an energy density of 1034 W h kg−1. It is noteworthy that the concentration of graphene in the hybrids, which was sand-wiched between the nanostructured V2O5 layers, was only 2 wt%. This V2O5-graphene hybrid showed excellent reversible capacity, long cyclability, and significantly enhanced rate capa-bility, providing a new avenue to create high-performance V2O5 electrodes for advanced battery applications.

Among the several polymorphic forms of VO2, including monoclinically distorted rutile VO2(M), rutile VO2(R), tetrag-onal VO2(A), and monoclinic VO2(B), layered VO2(B) is a promising candidate electrode material for LIBs, due to its stable structure, increased edge sharing, and consequent resist-ance to lattice shearing.[151] VO2 has a theoretical capacity of 161 mA h g−1 based on a reversible reaction: VO2 + 0.5Li+ + 0.5e− ↔ Li0.5VO2.[152] Liu et al. proposed a simple and facile intercalation–deintercalation strategy to obtain 2D ultrathin nanosheets, including VO2(B) nanosheets, at room tempera-ture.[153] As demonstrated in Figure 3F, the successive insertion of Li+ and large molecules, such as H2O, can enlarge the inter-planar distance and weaken the strong covalent interactions, so that isolated and free-standing VO2(B) ultrathin nanosheets with an atomic-level thickness of ≈0.62 nm are obtained when dein-tercalation occurs. The transmission electron microscopy (TEM) images in Figure 3G reveal that the nanosheets had an approxi-mate lateral size between 200 and 500 nm. Wang et al. studied the electrochemical performance of VO2(B) nanosheets as a cathode material for LIBs.[154] They demonstrated that VO2(B) nanosheets several nanometers in thickness and tens of nanometers in width present a higher initial discharge capacity (172 mA h g−1 at 300 mA g−1) and slower capacity fading (129 mA h g−1 after 45 cycles), compared with VO2(B) bulk materials (initial dis-charge capacities of 128 mA h g−1 at 300 mA g−1 and 58 mA h g−1 after 45 cycles). Further enhancement of the electrochemical performance with VO2(B) nanosheets was realized by growing VO2(B) nanosheets on N-doped graphene to build a three-dimensional (3D) flower-like hybrid via a hydrothermal reaction of ammonium vanadate with a colloidal dispersion of graphene oxide.[155] This VO2(B) nanosheet/graphene hybrid exhibited

a large capacity (initial discharge capacity of 418 mA h g−1 at 50 mA g−1), high rate capability (199 mA h g−1 at 1000 mA g−1), and excellent cycling stability (60% capacity retention after 50 cycles), resulting from the excellent electronic conductivity provided by the N-doped graphene, the short Li-ion transporta-tion length related to the ultrathin nanosheets, and the improved charge transfer from the anchoring of VO2(B) nanosheets on the N-doped graphene.

Besides LIBs, single-crystalline VO2(B) nanosheets were also fabricated as cathode materials for NIBs by Wang et al.[156] They obtained VO2(B) nanosheets with an average thickness of around 50–60 nm and an average lateral size of up to sev-eral hundred nanometers by a hydrothermal approach. They inferred that the charge and discharge processes taking place in the VO2(B) cathode relied on a transformation between Na0.33VO2 and NaVO2, on the basis of which, the theoretical capacity of VO2 was estimated to be 323 mA h g−1. Electro-chemical tests confirmed that the cathode materials delivered outstanding stability with a discharge capacity of 108 mA h g−1, even achieved at a high current density of 500 mA g−1. The out-standing electrochemical performances of both the V2O5 and the VO2 nanosheets demonstrated that this family of materials is a very promising source of cathode materials for LIBs and NIBs.

For layered vanadium oxide nanosheets, both the “top-down” and “bottom-up” approaches have been explored to fabricate high-purity and single-phase crystals. Controllable synthesis, however, is still a challenge, due to the tendency toward oxi-dization or reduction during the preparation processes. The practical reversible capacity of most V2O5 nanosheets is below 200 mA h g−1, which is lower than their theoretical capacity. Compared to other common cathode materials, such as LiCoO2 (140 mA h g−1) and LiFePO4 (170 mA h g−1), the advantages of vanadium oxides as a cathode material are not so exciting, and more development is needed to meet the requirements of high capacity and stability for practical applications of this family of nanosheets.

4.2. Cobalt Oxide Nanosheets

Co3O4, having a spinel crystal structure with mixed-valence Co atoms in the compound as [CoIICoIII2O4], is an important p-type semiconductor. Due to their unique electronic and mag-netic properties, Co3O4 nanomaterials have been widely used in applications involving catalysis,[157–159] magnetic materials,[160] electroreduction,[161] sensors,[162–165] and capacitors.[166–170] Moreover, the Co3O4 nanosheets have great potential as an alter-native anode material for LIBs, because of their high theoretical capacity of 890 mA h g−1.[171] Figure 4 presents typical fabrica-tion and energy-storage applications of 2D Co3O4 nanosheets. The storage mechanism of Li ions in Co3O4 is primarily based on a typical conversion-type reaction, in which Co3O4 will be converted into a mixture of Li2O and metallic cobalt during the charging process (Co3O4 + 8 Li+ + 8 e− ↔ 3 Co + 4 Li2O). Nev-ertheless, the poor conductivity and obvious volume changes in Co3O4 during the charging/discharging cycles reduce the reaction efficiency and result in inferior rate capability. To address these challenges, one of the most efficient solutions is

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to make the size of the Co3O4 material smaller (micrometer or nanometer level) and the specific surface area higher.[172] The emerging 2D Co3O4 nanosheets provide both ultrathin thick-ness and high specific surface area, as well as some other important factors having direct impact on the lithium-storage performance, such as the crystallinity, morphology, controllable size, etc., for them to serve as high-performance LIB electrode materials.[173,174] To date, a series of 2D Co3O4 nanosheets have been explored as electrode materials for LIBs.

An in situ dealloying and oxidation route was applied to syn-thesize 2D Co3O4 nanosheets by Xu and co-workers.[175,176] In their synthesis strategy, an Al/Co alloy was immersed in NaOH solution to leach the more-active Al metal away from the alloy, and then the remaining Co metallic layers were oxidized to form nuclei, which continued to grow into Co3O4 nanosheets at

the metal/electrolyte interface. This strategy was conducted at room temperature, and the morphology and structure are easily controllable by this method. The thickness of the 2D Co3O4 nanosheets was adjustable in the range from 6 nm to 500 nm by controlling the Co content in the CoAl alloy and the corro-sion time. The synthesized Co3O4 nanosheets have been fur-ther applied as the anode materials for LIBs.[177] It was found that the Co3O4 nanosheets with a thickness of 6 nm showed a high initial capacity of 1259.5 mA h g−1 at 200 mA g−1 and an excellent rate capability. The high first capacity delivered by the Co3O4 nanosheets is likely to be related to the formation of SEI films and the reversible formation and decomposition of polymeric gel-like films on the surfaces of the nanosheets. Even though a capacity loss of about 32.1% after 50 cycles was observed for the 6 nm nanosheet electrode, keeping the capacity

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Figure 4. A) Schematic illustration of the fabrication of porous Co3O4 nanofoils by using graphene as template. B,C) TEM image (B), and rate capa-bilities (C) at different current densities of the graphene-template-fabricated Co3O4 nanofoils. A–C) Reproduced with permission.[183] Copyright 2016, Wiley-VCH. D) Schematic illustration of molecular self-assembly of mesoporous Co3O4 nanosheets with atomic-level thickness from reverse micelles. D) Reproduced with permission.[100] Copyright 2016, Elsevier. E) Scanning electron microscopy (SEM) image of atomically thin self-assembled Co3O4 nanosheets. E) Reproduced with permission.[101] Copyright 2017, IOP Publishing. F) TEM image of atomically thin Co3O4/graphene hybrid nanomate-rial. G) Schematic illustration of the fabrication of atomic-level Co3O4 nanosheets/graphene hybrid nanomaterial in solution. H,I) Cycling performance (H) and comparisons of specific capacity and cycling performances (I) of the atomic-level Co3O4 nanosheets/graphene hybrid nanomaterial with previous reports for LIB application. F–I) Reproduced with permission.[99] Copyright 2016, Wiley-VCH. J) Schematic illustration of the application of self-assembled graphene-like holey Co3O4 nanosheets for the oxygen evolution reaction. J) Reproduced with permission.[100] Copyright 2016, Elsevier. K) Schematic illustration of the application of atomically thin Co3O4 nanosheets coated on a stainless-steel mesh for use in sodium-ion batteries. K) Reproduced with permission.[101] Copyright 2017, IOP Publishing.



around 880 mA h g−1, it was still concluded that the thinner the thickness is the better and more stable is the capacity that the Co3O4 nanosheets exhibit.

Another example of the facile fabrication of 2D Co3O4 nanosheets for high-capacity LIB cathode application was reported by Wang et al., where an ammonia-assisted hydro-thermal method followed by calcination at 450 °C was employed.[178] During the hydrothermal process, ammonia played the roles of both ligand and precipitating and structure directing agents for the controllable formation of the Co(OH)2 nanosheets, which were further transformed into snowflake-shaped Co3O4 nanosheets by heat treatment. The product showed unique micro-/nanostructures with a width of 10 µm and a thickness of 100 nm. Electrochemical testing revealed that the snowflake-shaped Co3O4 electrode had a remarkable capacity (1044 mA h g−1 at 500 mA g−1) with enhanced retention (86–98% at 500–1000 mA g−1) after 100 cycles and superior rate performance (977 mA h g−1 at 3000 mA g−1) at various current densities, due to the greatly facilitated lithium-ion diffusion and electron transport contributed by the unique morphology.

Besides 2D Co3O4 nanosheets fabricated by either deal-loying, the ammonia-assisted hydrothermal method, or the recently developed direct molecular self-assembly from reverse micelles,[9] Co3O4 nanosheets have also been synthesized by using graphene as a sacrificial template. In a typical pro-tocol, metal ions are adsorbed on the surface of the graphene or graphene oxide (GO) to form a metal oxide/graphene (or GO) nanocomposite, and then metal oxide nanosheets were obtained by the removal of the graphene or GO template by pyrolysis in air. Via this method, several types of 2D metal oxide nanosheets, including SnO2, Fe2O3, TiO2, ZrO2, Nb2O5, Ta2O5, NiO, Co3O4, Mn2O3, and NiFe2O4, have been synthe-sized.[179–183] Li et al. synthesized 2D porous Co3O4 nanosheets successfully by calcining the reduced GO (rGO)/Co(OH)2 precursor at 600 °C for 2 h. The obtained Co3O4 nanosheets had a wrinkled-sheet-like morphology with a thickness of 4–8 nm.[182] When used as the anode materials for LIBs, the Co3O4 nanosheets showed an excellent reversible capacity of 1380 mA h g−1, even after 240 cycles at a current density of 500 mA g−1, and retained a capacity as high as 606 mA h g−1 at 10 A g−1.[182] Similar work was carried out by Eom et al.,[183] who synthesized a series of porous metal oxide nanosheets by annealing metal-ion-adsorbed hybridized rGO nanosheets. As shown in Figure 4A,B, porous Co3O4 nanofoils consisting of nanocrystals with a diameter of about 10–20 nm were obtained by growing a Co(OH)2/Co3O4/rGO hybrid and then annealing it in air. The obtained graphene-like, porous 2D Co3O4 nano-foils exhibited a high reversible capacity of 1279.2 mA h g−1, even after 50 cycles and a good rate capability after 100 cycles, even at a high rate of 5 C (Figure 4C). Eom et al. also pointed out that the higher capacity exceeded the theoretical value cal-culated based on the conversion mechanism, which is likely to have resulted from both the conversion reactions between Co3O4 and lithium and the reversible formation of the SEI layer on the surfaces of the Co3O4 nanofoils.

Although the 2D Co3O4 nanosheets could provide a large number of active sites on the surface and cause a smaller volume expansion than their bulk counterpart, they still suffer from poor stability with a high reversible capacity under rapid

and repeated insertion–deinsertion cycles, which is a result of the intrinsic defects in the Co3O4 nanomaterials. To overcome these intrinsic defects that arise from the material itself, some strategies have been proposed to increase electron/ion conduc-tion and to suppress the volume changes during charging/dis-charging, and thus to enhance the rate performance and cycling stability.[184–186] Among these strategies, the incorporation of graphene or GO with Co3O4 nanosheets to form 2D hybrid lay-ered structures has proved to be an efficient approach.[187]

Chen and Wang reported the synthesis of a Co3O4–graphene sheet-on-sheet nanocomposite.[188] In this synthesis, Co(OH)2 nanosheets were grown on graphene oxide (GO) templates via a microwave irradiation method, and then the composite was heat-treated at 300 °C in N2 and air to convert the mate-rial into the nanocomposite. It was demonstrated that graphene is helpful for improving the overall electronic conductivity and inhibiting the volume expansion of Co3O4 nanosheets during cycling processes, while the Co3O4 nanosheets con-tribute by improving the capacity and alleviating the self-agglomeration of the graphene flakes. The overall electro-chemical activity and cycling stability of the nanocomposites were greatly improved compared to the application of only Co3O4. The Co3O4–graphene nanocomposite showed a high initial capacity of 1235 mA h g−1, which was the highest value reached at that time, owing to the lithium storage that occurred in the abundant nanocavities or defects of both the graphene and the porous Co3O4 nanosheets. A high charge capacity of 1065 mA h g−1 at 89 mA g−1 was observed after 30 cycles, cor-responding to decreases of 13.7% and 1.9% compared to the 1st and 3rd cycles, and a capacity of 931 mA h g−1 was obtained at a high current rate of 4450 mA g−1.[188]

Dou et al. prepared atomically thin mesoporous Co3O4-nanosheet–graphene hybrid nanocomposites (ATMCNs-GE) as an anode material for LIBs (Figure 4F,G).[99] They first synthesized ultrathin 2D Co(OH)2 nanosheets with a thick-ness of 1.5 nm via a self-assembly approach and then trans-formed them to mesoporous nanosheets with a thickness of 2 nm by heating at 400 °C. The well-crystallized atomi-cally thin mesoporous Co3O4 nanosheets (with a pore depth of ≈2.0 nm and a specific surface area of ≈157 m2 g−1) were then mixed with graphene nanosheets (3–4 layers) to form Co3O4–graphene hybrid nanosheets. These layer-by-layer hybrid nanosheets showed high discharge capacities and rate capabilities (2014.7 mA h g−1 after 88 cycles at 0.11 C, and 851.5 mA h g−1 after 2000 cycles at 2.25 C) in the voltage range of 0.01–3.0 V, which are far beyond the performance of bare Co3O4 nanosheets and the previously reported Co3O4/C com-posites, as shown in Figure 4H,I.[99] The outstanding electro-chemical performance of the hybrid nanosheets is attributed to the atomic thickness and mesoporous structure of the Co3O4 nanosheets, which facilitate adequate contact between the electrode and electrolyte, rapid diffusion of the electrolyte in the electrode, shortened transport paths for the lithium ions, and the highly electrically conductive and flexible graphene nanosheets, which enhance the electronic/ionic conductivity and improve the electrode stability.

Besides the application of 2D Co3O4 nanosheets for an LIB anode, it was also found that the 2D Co3O4 nanomaterials pos-sess favorable catalytic activity toward both the ORR and the

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OER, and can be used as catalysts for Li–O2 batteries. The 2D Co3O4 nanosheets have the features of high adsorption capacity, large surface area, and, more importantly, excellent surface redox reactivity, which are highly desirable for catalytic appli-cations.[189–191] Dou et al. studied the catalytic performance of mesoporous 2D Co3O4 nanosheets in the oxygen evolution reaction (Figure 4D,J).[100] The graphene-like Co3O4 nanosheets synthesized via a molecular self-assembly approach had a thick-ness of 1.6 nm, an average pore size of 5.8 nm, and a specific surface area of 115 m2 g−1. Owing to the presence of a large amount of low-coordinated atoms in the surface and the highly accessible surface area contributed by the atomic-level thick-ness and mesoporous structure, as well as the low energy bar-rier for catalytic reactions arising from the structural distortion, the graphene-like holey Co3O4 nanosheets exhibited excellent activity toward the OER, with low onset potential, high current density, and long-term cycling stability, indicating that these holey Co3O4 nanosheets are a promising candidate for applica-tions like Li–air/O2 batteries.

Wu et al. synthesized hierarchical mesoporous/macro porous Co3O4 nanosheets as free-standing catalysts for recharge-able Li–O2 batteries.[192] The mesoporous/macroporous Co3O4 nanosheets were obtained by calcining a Co(OH)2CO3 pre-cursor grown on a nickel foam substrate, and had a thickness of around 5 nm, a specific surface area of 62.72 m2 g−1, and a typical mesoporous structure with a main pore size of about 3 nm. When serving as catalysts for Li–O2 batteries, the Co3O4-nanosheet cathode could be discharged and charged between 2 and 4.5 V for 80 cycles at 200 mA g−1 with a capacity limited to 500 mA h g−1 without significant changes in the discharge/charge curves. Moreover, the charge potential was significantly reduced to about 3.7 V and below 4.3 V after 80 cycles. Mean-while, the Co3O4 nanosheet cathode could deliver discharge capacities of 11 882 mA h g−1 and even 2000 mA h g−1 at cur-rent densities of 100 mA g−1 and 500 mA g−1, respectively.[192]

Another shape-controlled hydrothermal synthesis method, for Co3O4 nanoarrays on nickel foam substrates, for Li–O2 batteries was reported by He et al.[193] Two types of Co3O4 nanosheets, hexagonal and rectangular nanosheets, were fab-ricated in this work. The hexagonal Co3O4 nanosheets were thicker and larger in size than the interconnected rectan-gular nanosheets. The specific surface area of the rectangular nanosheets (233.502 m2 g−1), however, was far higher than that of the hexagonal ones (75.295 m2 g−1). Compared with the hexagonal nanosheets, the rectangular nanosheets exhibited superior electrocatalytic performance in non-aqueous Li–O2 batteries, with a higher specific capacity (1380 mA h g−1 at a current density of 50 mA g−1 and 1134 mA h g−1 at a current density of 100 mA g−1) and a better cycling stability over 54 cycles at 100 mA g−1.

Recently, Zhou et al. developed a facile and generalized route to synthesize metal oxide nanosheets, including TiO2, Fe2O3, Co3O4, ZnO, and WO3, by using glucose, urea, and the corre-sponding metal salts as raw materials,[194] in which the glucose was melted into syrup and used to form bubbles together with the urea. By this strategy, a porous carbon foam was obtained with the metal salts in the walls, and a series of 2D metal oxide nanosheets were obtained after annealing the foam. The thicknesses of the obtained nanosheets were ≈15 nm for ZnO,

≈50 nm for TiO2, ≈25 nm for Fe2O3, ≈10 nm for Co3O4, and 30 nm for WO3. It was demonstrated that Co3O4 nanosheets, as the typical example for lithium storage, displayed a capacity of 688 mA h g−1 at a high current density of 1000 mA g−1 and retained 69% of their capacity after 80 cycles at 200 mA g−1.

Quite recently, Dou et al. studied the sodium-storage perfor-mance of atomically thin Co3O4 nanosheets grown on a stain-less-steel mesh for high-performance NIBs (Figure 4E,K).[101] The mesoporous atomically thin Co3O4 nanosheets fabri-cated for this application had a thickness of ≈1.5 nm, an average pore size of ≈6.8 nm, and a specific surface area of 156 m2 g−1. This novel anode delivered a high discharge capacity of 509.2 mA h g−1 for the initial 20 cycles at 50 mA g−1, and 427.0 mA h g−1 at 500 mA g−1, indicating its high capacity, excellent rate performance, and high cycling stability, which is much better than for other Co3O4 nanomaterials for use in NIBs. The superior Na+-storage performance was mainly attrib-uted to the atomic thickness and the unique anode structure.

Overall, Co3O4 nanosheets are very promising electrode materials for LIBs and NIBs, and superior catalysts for Li–O2 batteries. The capacity values of Co3O4 nanosheets in many pre-vious studies are close to or even higher than their theoretical capacity. By combining them with conductive carbon materials, the cycling stability can be greatly enhanced.

4.3. Tin Oxide Nanosheets

SnO2 has a tetragonal rutile structure and is a typical n-type semiconductor oxide with wide bandgap energy (3.6 eV). Com-pared with other Group IV oxides, SnO2 features both excellent electrical conductivity and superior optical properties (superior transparency), owing to a reversible transformation from the stoichiometric Sn4+ surface to a reduced Sn2+ surface due to the variation of the oxygen chemical potential of the system.[195] Owing to their salient properties, SnO2 nanomaterials have been studied extensively for a wide range of energy-related applications, including solar cells[196–199] and LIBs.[200–206] SnO2 presents a theoretical lithium capacity of 790 mA h g−1. The storage mechanism of lithium is likely to include both conversion (SnO2 + 4Li → Sn + 2Li2O) and alloying reactions (Sn + xLi+ + xe− ↔ LixSn).[200] Obviously, the former is an irre-versible reduction reaction, from SnO2 to a mixture of inactive Li2O and metallic tin, which leads to significant capacity loss after several cycles. The latter is a reversible alloying–dealloying reaction, which contributes to high capacity and cycling stability during operation. The biggest issue for SnO2 in practical LIB applications is the drastic volume expansion during charging/discharging, which causes serious pulverization of the nano-particles.[201] To overcome this problem, 2D SnO2 nanosheets with optimized morphology, crystallinity, and composition have been used to ease the rapid capacity loss and improve the reten-tion capability.

In 2010, Wang and co-workers reported a hydrothermal approach to the large-scale synthesis of SnO2 nanosheets with a minimum thickness of 1.5–3.0 nm, as shown in Figure 5A.[202,203] In a typical hydrothermal process, SnCl2·2H2O was added into a basic mixture of ethanol and water, and then yellowish SnO2 nanosheets were obtained after

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a hydrothermal treatment at 120 °C for 6 h. The specific surface area of the SnO2 nanosheets reached 180.3 m2 g−1. In lithium-storage testing as an anode material for LIBs, it was found that the discharge capacity could reach 559 mA h g−1 after 20 cycles with 57% retention.[202] With further optimization of the syn-thesis parameters, the well-defined SnO2 nanosheets with a thickness of ≈2.1 nm (Figure 5B) demonstrated a significantly improved reversible capacity (534 mA h g−1) compared to that of SnO2 nanoparticles (177 mA h g−1) and SnO2 hollow spheres (355 mA h g−1) after 50 cycles at a current density of 156 mA g−1 with a voltage window of 0.01–2 V.[203]

With a similar hydrothermal approach, 3D hierarchical SnO2 structures assembled from 2D nanosheets were developed by Wu et al. (Figure 5C).[204] It was revealed that a high initial dis-charge capacity of 1600 mA h g−1, and 516 mA h g−1 at a cur-rent density of 400 mA h g−1 after 50 cycles, could be achieved by this hierarchical structure, which are much higher than for commercial SnO2 nanoparticles (286 mA h g−1) (Figure 5D). The same group also synthesized SnO2 nanosheet hollow spheres by growing SnO2 nanosheets in a sulfonated gel matrix of polystyrene hollow spheres (sPSHS) used as templates, fol-lowed by calcination (Figure 5E).[205] These hollow spheres retained a higher reversible capacity of 519 mA h g−1 than SnO2 flowers (391 mA h g−1) and SnO2 nanoparticles (269 mA h g−1)

after over 50 cycles at 160 mA g−1, within a voltage window of 0.01–1.2 V (Figure 5F). These hierarchical structures and hollow spherical structures were helpful in facilitating the insertion or deinsertion of Li+ ions, and thus enhancing the electrochemical performance, on account of the large contact area with the elec-trolyte and the strong bonding between the lithium ions and the building blocks.

Besides hydrothermal synthesis, 2D SnO2 nanosheets have also been synthesized by some other methods. Zhu et al. reported the rapid microwave-assisted, gram-scale synthesis of ultrathin 2D SnO2 nanosheets with a thickness of 2–3 nm and a specific surface area of 135.7 m2 g−1.[206] In the experiment, SnCl2·2H2O, hexadecyltrimethylammonium bromide (CTAB), and hexamethylenetetramine (HMT) were mixed in water and then treated in a 700 W microwave reactor for 60 min. Via this method, a product yield of up to 1.63 g per synthesis batch was reached. Electrochemical evaluation showed that the reversible capacity of the high-yield 2D nanosheets could be maintained at 757.6 mA h g−1 at a current density of 200 mA g−1 for up to 40 cycles with a capacity retention of 65.6% in the potential window of 0–2.0 V (vs Li/Li+).

SnO2 nanomaterials have also attracted increasing attention as the anode materials for NIBs due to their high theoretical sodium capacity of 667 mA h g−1 (4Sn + 15Na+ + 15e− ↔ Na15Sn4,

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Figure 5. A,B) Schematic illustration of the preparation of SnO2 nanosheets assembled by oriented attachment (A) and the corresponding atomic force microscopy (AFM) image of a SnO2 nanosheet (B). A,B) Reproduced with permission.[203] Copyright 2012, American Chemical Society. C) SEM image of a SnO2 hierarchical structure assembled from 2D SnO2 nanosheets. D) Cycling performance of the SnO2 hierarchical structure (I) and com-mercial SnO2 nanoparticles (II) at a current rate of 400 mA g−1. C,D) Reproduced with permission.[204] Copyright 2011, American Chemical Society. E,F) TEM image of hollow spheres composed of SnO2 nanosheets (E), and the cycling performance of the SnO2 nanosheet spheres, nanoflowers, and nanoparticles at 160 mA g−1 (F). E,F) Reproduced with permission.[205] Copyright 2011, The Royal Society of Chemistry. G) SEM image of SnO nanosheets. G) Reproduced with permission.[211] Copyright 2010, Wiley-VCH. Inset: higher magnification. H–J) SEM image of flower-like SnO nanosheets (H);. cycle life of SnO, SnO2, and SnO2/C electrodes (I), and rate capabilities of the SnO and SnO2/C electrodes (J). H–J) Reproduced with permission.[212] Copyright 2015, Elsevier.



except for SnO2 + 4Na → Sn + 2Na2O). Park et al. explored the electrochemical properties of SnO2 nanomaterials for NIBs, where the Na/SnO2 cell delivered initial discharge and charge capacities of 747 and 150 mA h g−1, respectively, although the capacity faded to 50 mA h g−1 after 10 cycles.[207] The irrevers-ible formation of sodium oxides (NaxO) and the large volume expansion (≈520%) during reversible alloying–dealloying pro-cesses are the primary reasons for the large fading rate and weak cycling stability in NIB applications.

SnO, another type of tin oxide, also has potential appli-cations in LIBs (theoretical capacity of 875 mA h g−1) and NIBs.[208–210] The selective fabrication of 2D SnO nanosheets is more difficult, however, than that of SnO2, due to the easy oxidization from divalent to the tetravalent state. Sakaushi et al. reported the synthesis of ultrathin SnO nanosheets on dif-ferent substrates (e.g., silica-glass and indium tin oxide) by a one-step solution-based reaction between SnF2 and urea.[211] The nanosheets had a thickness of about 5 nm (Figure 5G) and a width of 500 nm. Then, they prepared SnO nanosheets on the surfaces of carbon nanofibers (CNFs) to form SnO/CNF composites as anode materials for LIBs. The composites showed a higher specific surface area (42.5 m2 g−1) than com-mercial SnO microcrystals (



(gravimetric: 2700 W kg−1, and volumetric: 14 000 W L−1), a high energy density (980 W h kg−1), a good cycling stability (over 40 cycles), and a fast charge rate (10 times faster than the rate of discharge) (Figure 6F).

Sun et al. synthesized crumpled ultrathin NiO nanosheets with a thickness of about 4–5 nm via a facile solvothermal method followed by annealing in air at 300 °C for 2 h (Figure 6G,H).[239] In sodium-storage applications, the crum-pled NiO nanosheets delivered high reversible specific capaci-ties of 299 and 154 mA h g−1, respectively, at current densities of 1 and 10 A g−1. More attractively, the capacity was maintained at as high as 266 mA h g−1 after 100 cycles at 1 A g−1. In lithium storage, the nanosheets presented a high reversible specific capacity of 1242 mA h g−1 at a current density of 0.2 A g−1 and retained 250 mA h g−1 at 15 A g−1. Moreover, the capacity was still 851 mA h g−1 after 170 cycles at 2 A g−1.

Tong et al. investigated single-crystalline mesoporous NiO nanosheets as an oxygen electrode catalyst for a non-aqueous Li–O2 battery.[240] The porous NiO nanosheets were synthe-sized via a facile alcohol–thermal method, and they exhibited an average width of 200 nm (Figure 6I) and a specific surface area of 188 m2 g−1. When assembled as the oxygen electrode for the Li–O2 battery, the nanosheets exhibited excellent catalytic activity toward the decomposition of Li2O2 and Li2CO3 formed during the discharge process, and a more negative recharge pla-teau (≈3.95 V, ≈200 mV) than 4.0 V vs Li/Li+ (Figure 6J). Further-more, the cycling performance of the system was impressive,

without any obvious decay after 40 continuous cycles, when the capacity was limited to 500 mA h g−1 (Figure 6K).

NiO nanosheets could be formed by transformation from various sheet-like precursors, including the common Ni(OH)2. As another promising anode material for LIBs, NiO nanosheets delivered the highest reversible lithium storage in the range from 700 to 1200 mA h g−1. The reversible capacity of NiO nanosheets for NIBs, however, is less than 300 mA h g−1. More-over, compared with Co3O4 nanosheets, the electrochemical performance of NiO nanosheets for Li–O2 batteries is infe-rior. Further research on improving the performance of NiO nanosheets for the applications in NIBs and Li–O2 batteries therefore needs to be carried out.

4.5. Manganese Oxide Nanosheets

Compared to other metal oxides, MnO2 is an environmen-tally friendly material that is abundant in the Earth’s crust. There are several polymorphs for MnO2, including α, β, γ, and δ phases, which are composed of [MnO6] octahedral basic units. To date, various shapes of MnO2 nanomaterials (e.g., rods, sheets, spheres, wires, and tubes) have been syn-thesized successfully.[241–243] Among them, 2D nanosheets have attracted extensive interest for a number of applications, such as catalysis,[244–246] biomedical science,[247–251] and energy storage and conversion devices, especially supercapacitors and

Adv. Mater. 2017, 1700176

Figure 6. A) Schematic illustration of the synthesis of NiO nanosheets. B) SEM image of NiO nanosheets. A,B) Reproduced with permission.[236] Copyright 2014, Nature Publishing Group. C) Cycling performance of NiO nanosheets and nanospheres. C) Reproduced with permission.[237] Copyright 2014, The Royal Society of Chemistry. D) Schematic illustration of the fabrication mechanism for NiO/Ni(OH)2 mesoporous spheres. E,F) TEM image of the NiO/Ni(OH)2 mesoporous spheres (E), and the corresponding power density profile (F). D–F) Reproduced with permission.[238] Copyright 2016, American Chemical Society. G) TEM image of NiO nanosheets obtained from Ni(OH)2 synthesized at 140 °C for 4 h. H) AFM image of NiO nanosheets. G,H) Reproduced with permission.[239] Copyright 2015, Elsevier. I) TEM image of hexagonal NiO nanosheets. J) Potential cut-off galva-nostatic discharge/recharge voltage curves for the first cycle of acetylene black (AB), β-Ni(OH)2/AB, and NiO/AB as the O2 electrodes at a current density of 100 mA g−1. K) Cycling performance with a cut-off capacity of 500 mA h g−1 of a non-aqueous Li–O2 battery with NiO as the oxygen cathodic material. I-K) Reproduced with permission.[240] Copyright 2015, The Royal Society of Chemistry.



batteries.[252–256] MnO2 possesses a high theoretical capacity of ≈1370 F g−1 for capacitors and ≈1232 mA h g−1 as an anode for LIBs, and allows a fast charge/discharge rate over a wide potential window. Generally, γ-MnO2 is the most suitable phase for LIBs, because the layered crystal structure facili-tates the diffusion of Li+, while β-MnO2 is a difficult material for ion transport, due to the very narrow channels formed by 1 × 1 Mn octahedra. Because 2D MnO2 nanosheets or ultrathin flakes offer enough active sites for anchoring ions and chan-nels for transferring ions, the synthesis of 2D MnO2 nanoma-terials is of significance for energy-storage-device applications. Figure 7 presents the typical synthesis and battery application of 2D MnO2 nanomaterials.

In 2000, Liu et al. reported the exfoliation of layered birnes-site-type MnO2 into flask-like nanosheets by the intercalation of tetramethylammonim (TMA) ions, as shown in Figure 7A.[257] The structural-characterization results indicated that the resultant product had a textured polycrystalline structure, and the oxidation state of the manganese exhibited no change.

Afterward, Omomo et al. carried out further intensive study of the intercalation, swelling, and exfoliation behavior of protonic manganese oxide (H0.13MnO2·0.7H2O, as shown in Figure 7B) in tetrabutylammonium (TBA) hydroxide solution for the for-mation of lamellar 2D MnO2 crystals.[258] The results revealed that the exfoliated MnO2 nanosheets had a lateral size in the sub-micrometer range and a thickness of about 0.8 nm. Overall, the exfoliation of 2D MnO2 nanosheets is relatively complex, expensive, and time-consuming. Moreover, with this top-down method, it is still difficult to achieve complete exfoliation into single-layer 2D MnO2 nanosheets.

Later, Oaki and Imai reported a one-pot bottom-up syn-thesis of 2D MnO2 nanosheets and their oriented thin films in aqueous solution under ambient conditions with neither hydrothermal treatment nor special equipment used in the process.[259] Interestingly, the MnO2 nanosheets in the form of films collected from the substrates were around 10 nm in thickness and 2–5 mm in width. In 2008, Kai et al. reported the preparation of single-layer MnO2 nanosheets comprising

Adv. Mater. 2017, 1700176

Figure 7. A) Schematic illustration of the exfoliation of MnO2 nanosheets from layered birnessite-type MnO2. B) SEM image of H0.13MnO2·0.7H2O. B) Reproduced with permission.[258] Copyright 2003, American Chemical Society. C) SEM image of space-confined ultrafine MnO2 nanosheets using porous SiO2 as solid template. C) Reproduced with permission.[269] Copyright 2015, The Royal Society of Chemistry. D) TEM image of δ-MnO2 micro-flower consisting of nanosheets fabricated by a microwave-assisted hydrothermal routine. E,F) Recorded voltage as a function of time during the initial cycles (E) and variation of the discharge capacity with the cycle number of microwave-assisted, hydrothermally synthesized δ-MnO2 nanosheets (F). D–F) Reproduced with permission.[278] Copyright 2012, American Chemical Society. G,H) Schematic illustration of the fabrication of S/MnO2 compos-ites in the interior of carbon nanofibers (G), and the corresponding TEM image of the hollow carbon nanofibers filled with MnO2 nanosheets (MnO2@HCF) (H). H) Reproduced with permission.[281] Copyright 2015, Wiley-VCH.



edge-sharing MnO6 octahedra via a single-step approach, by the chemical oxidation of Mn2+ ions in an aqueous solution of TMA+ within a day at room temperature.[260] The lateral dimen-sions of the obtained MnO2 nanosheets were in the range of 50–500 nm, and the thickness was around 0.9 nm.

With the successful synthesis of a series of 2D MnO2 nanosheets, their applications in energy storage have been extensively studied. Table 1 summarizes the detailed synthesis methods, the morphologies, and the electrochemical perfor-mances of some typical 2D MnO2 nanosheets for applications in capacitors, LIBs, and NIBs. According to the summarized data of the 2D MnO2 nanosheets in energy storage, it is clear that the unique morphology has led to materials with supe-rior electrochemical performance. For example, mesoporous MnO2 nanosheet arrays could exhibit a reversible capacity as high as 1690 mA h g−1, far beyond the theoretical capacity, after 100 cycles at a current density of 100 mA g−1, and there was even still capacity retention of 900 mA h g−1 over 200 cycles.[267]

Besides the abovementioned nanosheets for energy-storage applications, Weng et al. reported a new synthesis strategy to synthesize space-confined ultrafine (UF) MnO2 with porous SiO2 as a solid template and evaluated it as a conversion anode for NIBs, where the resultant MnO2 powder had a flake-like morphology with widths extending up to a few hundred nanom-eters (Figure 7C).[269] When tested as electrodes for NIBs, the ultrafine (UF) MnO2 flakes showed a high reversible sodiation capacity of 567 mA h g−1 and nearly 70% of the initial capacity was retained at a current density of 150 mA g−1 after 500 cycles. Operando synchrotron X-ray absorption near-edge spectroscopy (XANES) analysis revealed that these electrochemical reactions included non-Mn-centered redox reactions (the formation of a polymer-like film, surface space-charge layer, etc.), and a two-phase conversion reaction between Mn(III) and Mn(II) oxides (Mn(III)-O1.5 + Na+ + e− ↔ 1/2Na2O + Mn(II)-O).

2D MnO2 nanomaterials have also been explored as an efficient catalyst for the ORR and OER of lithium–air

Adv. Mater. 2017, 1700176

Table 1. Summary of the fabrication and energy storage application of typical 2D MnO2 nanosheets.

Parametersa) Morphologyb) Preparation Electrochemical performance Ref.

T ≈ 5–10; S ≈ 300–500; SSA ≈ 116

Chemical reaction at 298 K in water for

1–21 days with (NH4)2S2O8 and MnO213 and 180 F g−1 in 1.0 m Li2SO4 and

Na2SO4 solution, respectively

[261]

T ≈ 10 Hydrothermal reaction at 140 °C for 12 h with MnCl2·4H2O, H2O2 and TMAOH

385 F g−1 (0.5 A g−1) in 1.0 m Na2SO4 solution;

93% retained over 5000 cycles (1.25 A g−1)

[262]

T ≈ 5; S ≈ 1500; SSA ≈ 160 Hydrothermal reaction at 160 °C for 30 min with MnSO4·H2O and K2S2O8


94% retained over 2000 cycles (1.0 A g −1)

[263]

2D interwoven network Hydrothermal reaction at 220 °C for 6 h with (NH4)3PO4 and Mn(NO3)2


97% retained over 1800 cycles (2.5 A g−1)

[264]

T ≈ 0.95; S ≈ 200 Chemical reaction at 95 °C for 60 min in water with SDS-H2SO4 solution and KMnO4

868 F g−1 (3 A g−1) in 1.0 m Na2SO4 solution;

91% retained over 10000 cycles (3 A g−1)

[265]

T ≈ 2; S ≈ 50; SSA ≈ 191.3 Soft template route with AOT-isooctane solution and KMnO4

774 F g−1 (0.1 A g−1) in 2.0 m Ca(NO3)2 solution;

97% retained over 10000 cycles

[266]

T ≈ 20–30 Electro-deposition on Ni foam and thermal annealing at 170 °C for 5 h

1690 mA h g−1 (0.1 A g−1) over 100 cycles for LIBs;

retained 900 mA h g−1 over 200 cycles (1 A g−1)

[267]

Cation intercalation (Na+)

in MnO2

Bottom-up strategy with H2O2, TMAOH and

Mn(NO3)2 at room temperature overnight155 mA h g−1 (30 mA g−1) and 145 mA h g−1 (30 mA g−1)

for LIBs and NIBs; 74 and 73% retained over 500 cycles

(80 mA g−1), respectively

[268]

a)T (nm), S (nm) and SSA (m2·g−1) represent thickness; lateral size, diameter or width; and the largest specific surface area based on the BET method, respectively; b)Image in row 1: reproduced with permission.[261] Copyright 2011, ACS; image in row 2: reproduced with permission.[262] Copyright 2012, Elsevier; image in row 3: reproduced with permission.[263] Copyright 2013, Elsevier; image in row 4: reproduced with permission.[264] Copyright 2013, Macmillan Publishers Ltd.; image in row 5: reproduced with permission.[265] Copyright 2015, Wiley-VCH; image in row 6: reproduced with permission.[266] Copyright 2013, Macmillan Publisher Ltd.; image in row 7: reproduced with permission.[267] Copyright 2013, Royal Society of Chemistry; image in row 8: reproduced with permission.[268] Copyright 2016, Wiley-VCH.



batteries.[270–277] Truong et al. developed a microwave-assisted hydrothermal routine for the synthesis of δ-MnO2 nanosheets and α-MnO2 nanotubes for lithium–air batteries, using potas-sium permanganate as a precursor.[278] By controlling the reac-tion time, δ-MnO2 nanosheets (Figure 7D) and α-MnO2 nanow-ires and nanotubes were obtained. As presented in Figure 7E,F, their electrochemical performance in batteries demonstrated that the δ-MnO2 nanosheets with low crystallinity and small size domains had the lowest catalytic activities compared to single-crystal α-MnO2 nanowires and nanotubes, indicating that the catalytic activity of MnO2 depended strongly on the crystallinity.[279]

Ultrathin MnO2 nanosheets are also regarded as one of most promising host materials for sulfur in lithium–sulfur batteries. The 2D feature of MnO2 nanosheets increases the active area significantly, shortens ion-migration paths, and facilitates improved reaction kinetics. Liang et al. synthesized a S/MnO2 composite, where sulfur was entrapped in the host MnO2 nanosheets to inhibit the polysulfides from dis-solving into the electrolyte.[280] A polythionate complex on the surface as a transfer mediator with a high sulfur loading of 75 wt% was formed by concatenating soluble higher poly-sulfides (Li2Sx, x ≥ 4) with thiosulfate groups, which were cre-ated in situ by oxidation of the initially formed soluble lithium polysulfide species. At the same time, the higher polysulfides were converted into insoluble lower ones (Li2S2 or Li2S). Electro-chemical tests confirmed that the composite exhibited a revers-ible capacity of 1300 mA h g−1 at C/20 and a capacity decay rate as low as 0.036% per cycle over 2000 cycles at 2 C. In another case, Li et al. synthesized a complex one-dimensional nano-composite, namely, hollow carbon nanofibers (HCF) filled with MnO2 nanosheets (MnO2@HCF), as the host for sulfur.[281] In the interior of the carbon nanofibers, MnO2 nanosheets were transformed from MnO2 nanowires, and then the S/MnO2 nanosheets were obtained by sulfur loading (Figure 7G,H). Spe-cifically, by coating SiO2 and resorcinol formaldehyde (RF) resin on MnO2 nanowires, followed by carbonizing in N2 atmosphere at 700 °C for 3 h, and then a removal of the SiO2 interlayer in NaOH aqueous solution, the MnO2@HCF composite with an ultrahigh specific surface area of 460 m2 g−1 was fabricated, after which sulfur was introduced by melt-diffusion. It is inter-esting that the sulfur could be encapsulated homogeneously and completely within the hollow carbon fibers, and the content of sulfur was up to 71 wt% in the MnO2@HCF/S composite, corresponding to 3.5 mg cm−2 of sulfur mass loading per unit area in the electrode. When assessed for lithium–sulfur bat-teries, it displayed a high specific capacity of 1161 mA h g−1 (or 4.1 mA h cm−2) at 0.05 C, which decreased to 662 mA h g−1 (or 2.3 mA h cm−2) after 300 cycles, indicating excellent cycling stability. This strategy for the rational design of Li–S battery electrodes is of great significance, because it achieves not only high active mass retention, but also high sulfur utilization at high current densities.

MnO2 is the most widely studied metal oxide for various rechargeable batteries, and has some special features, such as ultrahigh theoretical lithium storage, adjustable valence states of the manganese, and relatively low cost. Remarkably, for LIBs, the reversible capacity of MnO2 nanosheets could reach as high as 1690 mA h g−1. The low conductivity is the biggest

challenge for its commercialization, and a new strategy for the synthesis is still urgently needed.

4.6. Iron Oxide Nanosheets

There is potential to replace graphite with the low-cost metal oxide α-Fe2O3 as the dominant anode material for LIBs. The theoretical capacity of α-Fe2O3 is as high as 1006 mA h g−1, based on the redox conversion reaction from Fe2O3 to form a mixture of Li2O and metallic iron (Fe2O3 + 6Li+ + 6e− ↔ 3Li2O + 2Fe).[282,283] In practice, unfortunately, the capacity retention of α-Fe2O3 for batteries is poor, and the main reason is attrib-uted to its significant volume changes (≈90%) resulting from the Li+ insertion/extraction process during cycling, which leads to serious anode pulverization and the loss of electrical con-nectivity.[284] The emergence of 2D nanosheets, which have the maximum surface area and display quantum-confine-ment effects, offers a new idea to inhibit volume expansion over cycling and to resolve this intrinsic problem of α-Fe2O3. Figure 8 presents the typical synthesis of 2D Fe2O3 nanosheets, as well as their energy storage applications.

Cheng et al. synthesized atomically thin α-Fe2O3 nanosheets with only a half-unit-cell (two Fe ion layers) thickness (



been synthesized via some bottom-up approaches. Zhong et al. obtained three types of iron oxides (α-Fe2O3, γ-Fe2O3, and Fe3O4) by using FeCl3·6H2O, urea, ethylene glycol (EG), and tetrabutylammonium bromide (TBAB) as precursors and refluxing at 195 °C for 30 min.[287] Many uniform flower-like microspheres with a diameter of around 4 µm were obtained, which were each constructed from several dozens of petal-like nanosheets, each approximately 70 nm in thickness. Similar α-Fe2O3 flower-like structures were synthesized by Han et al., by heat-treatment of an iron(III)-oxyhydroxide precursor obtained from FeCl3 and NaClO solution.[288] This product had a diameter of 7–10 µm and a surface area of 115 m2 g−1, with the constituent nanosheets having a thickness of about 20 nm. When the flower-like microspheres were used as the electrode for LIBs, the first discharge capacity was as high as 1820 mA h g−1, but it decreased to 1250 mA h g−1 at the second cycle, which was then maintained at 929 mA h g−1 after 10 cycles.[288]

A porous structure was introduced to further utilize the elec-trochemical activity of α-Fe2O3 nanosheets. Zhu et al. reported a simple synthesis of α-Fe2O3 hierarchical spheres, consisting of ultrathin nanosheets with hollow interiors, by using CuO monodisperse spheres as sacrificial templates.[289] The thickness of the constituent nanosheet building blocks was about 3.5 nm and the specific surface area of the hollow spheres was 139.5 m2 g−1. A reversible discharge capacity as high as 815 mA h g−1 after 200 cycles at 0.5 A g−1 was achieved when the α-Fe2O3 hollow spheres were used as the anode for LIBs. Li et al. grew uniform and well-defined porous α-Fe2O3 nanosheets with a thickness

of around 25 nm on various metallic substrates, such as Ti foil, Cu foil, or stainless steel, by a combination of a solvothermal reaction with the post-annealing method.[290] When the α-Fe2O3 nanosheet films grown on Ti foil were applied as the negative electrodes for LIBs, enhanced lithium-storage properties in terms of high initial capacity (discharge of 1376 mA h g−1 and charge of 1009 mA h g−1 at 100 mA g−1), superior cycling sta-bility (908 mA h g−1 afte

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