Recent Progress on Two-Dimensional Nanoflake Ensembles for Energy Storage Applications
Keywords2D nanoflakes Ensembles 3D architectures Supercapacitors Lithium-ion batteries Sodium-ion batteries
In this review, we emphasize the recent developments on two-dimensional nanoflake ensembles and their applications for enhanced electrochemical performance in supercapacitors, lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, and zinc-ion batteries.
An overview of recent advances in three-dimensional hierarchical structures from two-dimensional nanoflake ensembles with controllable shape and compositions is provided.
Enhanced electrochemical energy storage performance based on assemblies of these two-dimensional nanoflake ensembles is discussed in detail.
Although this article summarizes many of the historically significant studies on 2D nanoflakes, the most recent developments are emphasized. Notably, this article aims to describe the different processes used for the preparation of 2D nanoflake-based 3D architectures and the different areas of application in which the unique structures of these materials can be harnessed to achieve greater efficacy as compared to their 2D analogues, and even provide avenues for different applications. Electrochemical systems for energy storage, including supercapacitors, lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, and zinc-ion batteries for advanced energy storage devices, are also discussed. Finally, the conclusions and prospective for these emerging composites are discussed.
Three-dimensional architectures composed of 2D nanoflakes generally possess a large surface area. Likewise, these unique 3D structures can better provide electronic transport and ion transfer channels and increase the contact area between the electrolyte and the active material because of the larger specific surface area. These features are beneficial for achieving improved electrochemical reaction kinetics. Moreover, the 3D structures assembled from these 2D nanoflakes exhibit good electrochemical properties. Thus, the characteristics of the 2D units can be retained in the 3D structure, leading to more extensive applications. Moreover, the deficiencies of the 2D nanoflake materials can be offset in these 3D materials, where the 2D nanoflake materials are connected to the 3D mesh material, thereby improving the scope of application.
3 Synthesis of 2D Nanoflake Ensemble-Based 3D Nanostructures
Two-dimensional nanoflakes, compared to spherical/bulk structures of the same volume, could contribute much more to achieving higher viscosity in their stretched state, and the channeled network generated by these 2D nanoflakes can also form ensembles to furnish 3D hierarchical nanostructures . Two-dimensional nanoflake-based 3D hierarchical frameworks have been prepared by self-assembly into ensembles. Although the first step is to prepare 2D nanoflakes, all the current methods can be classified into two categories: top-down approaches and bottom-up approaches, including mechanical cleavage, liquid exfoliation, ion intercalation and exfoliation, selective etching and exfoliation, chemical vapor deposition (CVD), and wet-chemical synthesis [22, 23, 24, 25]. In the assembly of 2D nanoflakes into 3D hierarchical structures, the unique characteristics of the individual 2D nanoflakes can be largely maintained, and these structures have thus recently attracted great interest for fundamental investigations and practical applications in diverse technologies.
Gong et al.  constructed 3D architectures from 2D nanoflake building blocks such as MoS2 and graphene oxide nanoflakes by exploiting their controllable assembly character. The MoS2 nanoflakes were first fabricated via liquid exfoliation, and graphene oxide was produced by the modified Hummer’s method. The prepared MoS2 and graphene oxide nanoflakes were then employed as building blocks for assembly via hydrothermal processing. Finally, the graphene oxide in the resulting samples was chemically reduced to graphene, giving rise to MoS2-graphene architectures with different MoS2 contents. The as-prepared MoS2-graphene architectures possess a 3D structure with interconnected pores ranging from several nanometers to several micrometers. The sectional overlapping or connection of the flexible nanoflakes might be the result of cross-linking of the functional groups in the graphene nanoflakes, similar to those of pure graphene oxide and graphene hydrogels.
In addition to the aforementioned 2D nanomaterials, there are many other kinds of 2D nanostructured materials, such as MXenes, covalent organic frameworks (COFs), 2D polymers, metal organic frameworks (MOFs), and 2D supramolecular organic nanostructures. MXenes, a novel family of 2D metal carbides, have demonstrated potential as electrode materials for energy storage devices with a volumetric capacitance exceeding that of all-carbon materials [27, 28, 29, 30, 31]. Liu et al.  reported simple ensembles of transition metal oxide (TMO) nanostructures (including TiO2 nanorods and SnO2 nanowires) on MXene (Ti3C2) nanoflakes, assembled through van der Waals interactions. The MXene nanoflakes, acting as the underlying substrate, not only enabled reversible transport of electrons and ions at the interface, but also prevented aggregation of the TMO nanostructures during lithiation/delithiation. Specifically, in the SnO2 nanowires, which are notorious for severe volume expansion, the MXene nanoflakes could alleviate pulverization by providing excellent mechanical flexibility. More importantly, TMOs can confer extraordinary electrochemical properties to composites, and their nanoscale size offers short lithium diffusion pathways and additional active sites. Two-dimensional COF nanoflakes, in which the molecular building blocks form robust microporous networks, are accommodative of Li salts and are considered as potential candidates for solid-state fast Li+ conductors [33, 34, 35, 36, 37, 38, 39]. Chen et al.  demonstrated the first example of cationic moieties incorporated into the skeleton of a COF material that could indeed split the ion pair of the Li salt, increase the concentration of free mobile Li+, and thus improve the Li+ conductivity in COF-based Li+ conductors. Two-dimensional COF nanoflakes were selected as the COF matrix for this study due to their high surface area and unique 2D framework, both of which are beneficial for exposing the ionic moieties to the Li salts.
4 2D Nanoflake Ensemble-Based Materials for Supercapacitors
Supercapacitors have received widespread attention due to their long cycle life, high-power density, environmental friendliness, and rapid charge and discharge ability [41, 42, 43]. Three-dimensional structures composed of 2D nanoflakes have been used as electrode materials for electric double-layer capacitors and pseudo-capacitors [44, 45, 46]. The capacity stems from surface ion adsorption, surface redox reactions, and fast ion intercalation without phase change [47, 48, 49]. Due to their unique structure and excellent electrochemical properties, these 3D structures composed of 2D nanoflakes have received much attention. For example, 2D graphene nanoflakes easily agglomerate under an external force, which negatively affects the inherent properties of the nanoflakes, such as by decreasing the specific surface area, which eventually leads to a decrease in the energy storage performance [50, 51, 52]. In the process of assembling a 3D structure from 2D nanoflakes, the advantages of the original 2D nanoflakes can be well preserved, which has fueled a great deal of basic scientific research and interest in numerous nanotechnologies. Here, we summarize some reports on three-dimensional structures comprising two-dimensional nanoflakes and their application to supercapacitor devices. The composites not only exhibit the characteristics of the 2D nanoflake materials, but also exhibit unique characteristics of the 3D structure, thereby furnishing high specific surface area, rapid ion transport and electron conduction, and excellent stability. In supercapacitor applications, these materials give rise to excellent rate performance, super-high capacitance, and remarkable cyclic stability.
4.1 Carbon Nanoflakes
4.2 Transition Metal-Based Nanoflakes
Comparison of electrochemical performance of 2D nanoflake ensemble-based materials for supercapacitors
530.4 F g−1 at 0.5 A g−1
180 F g−1 after 2000 cycles at 2 A g−1
3D carbon superstructures
364 F g−1 at 0.6 A g−1
200 F g−1 after 10,000 cycles at 1 A g−1
99.5 F g−1 at 0.2 A g−1
39.9 F g−1 after 5000 cycles at 2 A g−1
Na0.5MnO2 nanoflakes-assembled nanowall arrays
366 F g−1 at 1 A g−1
300 F g−1 after 10,000 cycles at 4 A g−1
112 F g−1 at 0.08 A g−1
51.6 F g−1 after 17,000 cycles at 1.26 A g−1
5920 mF cm−2 at 5 mA cm−2
4012.2 mF cm−2 after 3000 cycles at 30 mA cm−2
Hierarchical MnMoO4·H2O@MnO2 core–shell nanoflakes
3560.2 F g−1 at 1 A g−1
2994.2 F g−1 after 10,000 cycles at 1 A g−1
757 F g−1 at 0.5 A g−1
445.2 F g−1 after 2000 cycles at 5 A g−1
5 2D Nanoflake Ensemble-Based Materials for Lithium-Ion Batteries
Lithium-ion batteries (LIBs) are currently the greenest power source with chargeability and are widely used in daily life [73, 74, 75, 76]. Similar to 2D nanoflakes (e.g., transition metal dichalcogenides and metal oxides), 3D architectures have been used as active electrode materials in LIBs [77, 78, 79]. In order to meet the ever-increasing energy demand, the preparation of new synthetic electrode materials is imperative. Due to the huge volume change of electrode materials during the lithiation process, it is still a challenge to find suitable electrode materials.
5.1 Transition Metal-Based Nanoflakes
Molybdenum sulfide (MoS2) has a high theoretical specific capacity (670 mAh g−1), excellent rate performance, and satisfactory cycle stability . Because MoS2 is formed by weak van der Waals forces, the volume effect in the process of ion intercalation is smaller, and the integrity of the electrode material structure is better ensured . However, the further application of MoS2 is hindered by its poor electrical conductivity and unsatisfactory rate performance. Thus, in attempts to solve these problems, recent remarkable advances have involved the engineering of novel MoS2/N-doped graphene nanoflakes with highly improved capacity, rate capability, and stability for LIBs by the invention/introduction of a MoS2/N-doped graphene interface [96, 97]. The strategy of doping carbon nanoflakes can adequately enhance electron transfer and ion diffusion. However, this improvement is limited by the limited contact sites of the carbon nanoflakes and MoS2.
5.2 Transition Metal-Based Nanoflakes on 1D Skeleton
Transition metal-based nanoflake electrodes still face great challenges for practical application, such as fast capacity fading caused by the drastic volume change upon cycling and inferior rate performance due to the poor conductivity. The novel structure composed of ultrathin SnO2 nanoflakes with smaller plate-like units can shorten the distance between ions and electrons, increase the reaction kinetics, and increase the area of contact between the active material and the electrolyte, leading to optimized rate capability, and the low-dimensional tubular hollow structure can alleviate the volume change caused by ion insertion and structural collapse, thereby furnishing improved lithium storage . For instance, Xia et al. demonstrated an unexpected result in which a 1D Cu(OH)2 nanorod/2D SnO2 nanoflake core/shell heterostructure covered with a graphene layer with internal void spaces could be facilely prepared by an in situ growth strategy . It was found that the open space provided a larger reaction surface area and allowed for diffusion of the electrolyte into the inner region of the electrode; thus, the nanoflakes could provide enough sites for insertion and extraction of lithium ions and could alleviate the problems of agglomeration and crushing of the active materials due to volume changes. The unique 3D structure comprising 2D nanoflakes was effective for resolving these problems.
Moreover, the hierarchical tubular structures constructed of nanosized subunits can effectively alleviate the volume stress related to the electrochemical reactions. The rate capability of the TiO2@NC@MoS2 electrode was investigated through galvanostatic measurements at various current densities. The average specific discharge capacities were ~ 925, 855, 756, 670, and 612 mAh g−1 at the current densities of 0.1, 0.2, 0.5, 1.0, and 2.0 A g−1, respectively. When the current density was again reduced to 0.1 A g−1, the capacity of the TiO2@NC@MoS2 electrode quickly returned to 955 mAh g−1, revealing the good reversibility of the electrode materials. The cycling performance of the TiO2@NC@MoS2 electrode was evaluated at a high current density of 1.0 A g−1. From the second cycle onward, the discharge capacity quickly stabilized at around 680 mAh g−1. After relatively slow fading, a high reversible capacity of 590 mAh g−1 was retained after 200 cycles. Importantly, the Coulombic efficiency was nearly 100%, except in the first few cycles.
5.3 Transition Metal-Based Nanoflakes on 2D Skeleton
5.4 Transition Metal-Based Nanoflakes on 3D Skeleton
Comparison of electrochemical performance of 2D nanoflake ensemble-based materials for lithium-ion batteries
3D hierarchical MoS2/C nanoflowers
975 mAh g−1 at 0.1 A g−1
888.1 mAh g−1 after 50 cycles at 0.1 A g−1
MoS2 nanoflakes assembling superstructure
1000 mAh g−1 at 0.05 A g−1
650 mAh g−1 after 500 cycles at 1 A g−1
630 mAh g−1 at 3 A g−1
971 mAh g−1 after 100 cycles at 0.3 A g−1
Three-layered TiO2@carbon@MoS2 hierarchical nanotubes
925 mAh g−1 at 0.1 A g−1
590 mAh g−1 after 200 cycles at 1 A g−1
3D hierarchical dual Fe3O4/MoS2 nanoflakes
1355 mAh g−1 at 0.1 A g−1
650 mAh g−1 after 1000 cycles at 5 A g−1
Porous LiCoO2 nanoflake arrays
104.6 mAh g−1 at 10 C
110 mAh g−1 after 1000 cycles at 0.1 C
1378.6 mAh g−1 at 0.1 A g−1
543 mAh g−1 after 1000 cycles at 1 A g−1
Few-layered MoS2 nanoflakes on rGO
1142 mAh g−1 at 0.1 A g−1
753 mAh g−1 after 1000 cycles at 2 A g−1
As the most promising substitute for LIBs, sodium-ion batteries have attracted increasing attention because of the abundance of sodium reserves. More importantly, because the unique design of the 3D structure is the same for sodium-ion batteries, the synthesis and surface modification of novel 2D nanoflakes applied to lithium-ion batteries can be extended to sodium-ion batteries, potassium-ion batteries, and zinc-ion batteries.
6 2D Nanoflake Ensemble-Based Materials for Sodium-Ion Batteries
Comparison of electrochemical performance of 2D nanoflake ensemble-based materials for sodium-ion batteries
Hierarchical VS2 nanoflakes assemblies
790 mAh g−1 at 0.1 A g−1
700 mAh g−1 after 100 cycles at 0.1 A g−1
2D MoSe2/graphene nanocomposites
432 mAh g−1 at 0.1 A g−1
324 mAh g−1 after 1500 cycles at 3.2 A g−1
MoS2 nanoflakes aligned vertically on carbon paper
348 mAh g−1 at 0.04 A g−1
230 mAh g−1 after 100 cycles at 0.08 A g−1
Hierarchical flower-like VS2 nanoflakes
600 mAh g−1 at 0.1 A g−1
352 mAh g−1 after 700 cycles at 2 A g−1
Self-supported VG/MoSe2/N–C sandwiched arrays
540 mAh g−1 at 0.2 A g−1
298 mAh g−1 after 1000 cycles at 2 A g−1
Carbon-coated hierarchical SnS nanotubes
520 mAh g−1 at 0.05 A g−1
440.3 mAh g−1 after 100 cycles at 0.2 A g−1
201 mAh g−1 at 50 A g−1
441 mAh g−1 after 250 cycles at 0.3 A g−1
Metallic 1T MoS2 sandwich grown on graphene tube
241 mAh g−1 at 0.5 A g−1
313 mAh g−1 after 200 cycles at 0.05 A g−1
6.1 Transition Metal-Based Nanoflakes
6.2 Transition Metal-Based Nanoflakes on 1D Skeleton
Besides VS2 nanoflakes, many other 2D TMD nanoflakes, such as MoS2 and MoSe2, also exhibit superior high-rate performance. A number of 2D TMD nanoflakes containing MoS2 have been developed as electrode materials for SIBs. Similar to the case of lithium-ion batteries, 2D TMDs with larger interlayer spacings are key sodium-ion battery materials, where these materials facilitate rapid insertion and removal of sodium ions. For instance, MoS2 nanoflakes with different interlayer distances ranging from 0.62 to 0.78 nm were prepared for SIBs [103, 137]. The resultant electrode based on MoS2 nanoflakes with the largest interlayer distance exhibited the highest capacity and best rate capability. Moreover, the as-prepared few-layer MoS2 anchored on the N-doped carbon ribbon electrode also showed good cycling stability for 300 cycles, with reversible capacities of 495 and 302 mAh g−1 at 0.05 and 2 A g−1, respectively. Interestingly, the capacity continued to increase during continuous charge/discharge cycles for the MoS2 electrodes when cycled at different current densities, and this increase was especially pronounced at 0.5 A g−1.
6.3 Transition Metal-Based Nanoflakes on 2D Skeleton
Two-dimensional layered metal sulfides with structures analogous to that of graphite, such as MoS2, WS2, and VS2, have been proposed as a promising family of anode materials for SIBs. Sun and co-workers reported the preparation of a new type of hybrid material containing MoS2/graphene nanoflakes prepared by ball milling and exfoliation of commercial bulk MoS2 and graphite . The newly prepared MoS2/graphene (G) hybrids demonstrated extraordinary rate capability at a high current density of 50 A g−1, with a stable reversible capacity of 201 mAh g−1. The composite also showed outstanding cycling stability with 95% capacity retention at 0.3 A g−1 after 250 cycles.
MoS2-rGO/HCS exhibits attractive electrochemical performance for sodium storage, which can be primarily attributed to the favorable architecture and properties as follows: (1) the ultrathin MoS2 nanoflakes tightly overlay the interconnecting rGO/HCS conductive networks, which not only effectively hinders aggregation of the MoS2 nanoflakes and restacking of the graphene nanoflakes, but also can cushion the volume change caused by charging and discharging, and is conducive to improving the stability and recyclability of the structure. (2) The 3D porous scaffolds of rGO/HCS are in intimate face-to-face contact; in this way, the few-layer MoS2 nanoflakes can greatly enhance the electrical conductivity and facilitate electrolyte/ion transport, resulting in satisfactory electrochemical reaction kinetics, large capacity, and optimal rate performance. (3) The larger interlayer spacing may alleviate the stress of sodium-ion insertion and removal, thereby enhancing the speed of insertion of sodium ions and providing more insertion sites. (4) This unique 3D honeycomb-like network structure can increase the contact area between the active material and the electrolyte, thereby increasing the number of sodium-ion insertion sites and shortening the sodium-ion diffusion pathway. At a current density of 5 A g−1, the MoS2-rGO/HCS electrode delivered an average discharge capacity of 364 mAh g−1.
6.4 Transition Metal-Based Nanoflakes on 3D Skeleton
7 Other Applications
In addition to the aforementioned applications, other kinds of potential applications of these 3D architectures have also been demonstrated, such as potassium-ion batteries and zinc-ion batteries.
7.1 2D Nanoflake Ensemble-Based Materials for Potassium-Ion Batteries
Along with the rapid development of LIBs and SIBs, potassium-ion batteries (KIBs) have also become an attractive alternative to LIBs or SIBs, owing to the abundant natural resources and similar chemical/physical properties of K to Li or Na [138, 139, 140, 141, 142]. More importantly, K has a lower standard redox potential than Na (and even Li) in non-aqueous electrolytes, which can be translated into a potentially higher cell voltage of KIBs compared with those of SIBs and LIBs [143, 144, 145, 146, 147]. Okoshi et al.  showed that K electrolytes exhibit higher conductivity than Li and Na electrolytes. Given these advantages, KIBs have rapidly attracted considerable interest, and various materials have been developed and evaluated as potential KIB electrodes.
7.2 2D Nanoflake Ensemble-Based Materials for Zinc-Ion Batteries
8 Conclusions and Outlook
Presently, the development of advanced science and technology is considered the best choice for solving the problems of the energy environment in the twenty-first century. Due to their large surface area and numerous active sites, two-dimensional nanoflake ensemble-based architectures display unprecedented performance in the aforementioned fields. In this review, we outlined recently developed, unique two-dimensional nanoflake ensemble-based architectures for improving energy storage in devices such as supercapacitors, lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, and zinc-ion batteries. The three-dimensional structure composed of two-dimensional nanoflakes can provide more channels for electrons and ions and can also provide better electrode materials for new energy sources.
Irrespective of the potential application, the first target is controlled synthesis of these composites, which requires precise control of the 3D nanostructures, including controlled growth of the layers, pore size, and porosity. Most of these reported 3D architectures have a wide pore size distribution, ranging from one hundred to several hundreds of micrometers. Large pore sizes decrease the mechanical performance of the resulting materials. Thus, single-layered 3D architectures are hardly ever prepared due to their fragile mechanical features and large pore sizes. The preparation of 3D architectures with uniform meso- or micropores and controlled layers is thus a more effective approach. Achieving control of the layer growth and size of the 2D nanoflakes in the assembly process is a key challenge. The second challenge is to further enhance the mechanical and electrical performance of the 3D architectures. Increasing and strengthening the cross-linking between the nanoflakes by enhancing the surface functional moieties and adding cross-linkers will enhance the internanoflake binding and the mechanical and electrical performance. Thus, continued innovative research and development is required to further enhance the performance and applications of 3D architectures.
For energy storage applications, an advanced hybrid nanostructure should generally meet the requirements of having a large specific surface area for reaction and ion exchange, and conductive networks for charge transport, as well as an interface/heterojunction formed by two components for the effective channeling or separation of charge carriers. Among these requirements, much effort has been devoted to increasing the effective surface area of these hybrids. As such, the surface area of 2D nanoflake ensemble structures is generally much larger than the theoretical value for seamless 2D materials, and the former simultaneously supply plenty of active edge sites for various reactions.
In addition, because the material engineering of 3D architectures leads to combined, composite, or hybrid nanomaterials, the original techniques can provide access to a large number of 2D nanoflake-based 3D nanohybrids that integrate the performance of the individual 2D nanomaterials, and can also furnish new collective and synchronic functions. Furthermore, improving the properties of 2D nanomaterials on the nanoscale is a worthwhile undertaking owing to the synergetic chemical coupling effects that can be derived. On this basis, the importance of fundamental understanding of the principles of these synergistic coupling effects should be emphasized. Furthermore, more theoretical investigations on the electronic properties and crystal and surface structures of different 3D architectures, as well as the synergetic effects of modified 2D nanoflake ensemble-based nanomaterials, should be performed based on first-principle calculations, which, in combination with smart experimental strategies, will greatly expedite the development of extremely efficient 2D nanoflake ensemble-based 3D nanocomposites for energy conversion and storage and other applications.
In short, the large-scale production problems must be resolved. Nanomaterial science is a cross-discipline field. The two-dimensional nanoflakes prepared in different fields have different applications, and assembling two-dimensional nanoflakes into a three-dimensional structure undoubtedly produces unexpected effects. The success of nanotechnology depends on the close cooperation between researchers in different disciplines in order to understand current and future needs. It is necessary to choose suitable methods to prepare two-dimensional nanoflakes and then assemble them or compound them with other materials. The development of new two-dimensional nanoplates for assembly is still in the ongoing trial phase, and it is believed that groundbreaking progress in these technologies will continue to be achieved with the efforts of researchers. Practically, however, basic research and real-life application may take decades to establish.
This work was financially supported by the National Natural Science Foundation of China (21571157, U1604123, and 2187051489), Outstanding Young Talent Research Fund of Zhengzhou University (No. 1521320001), the Young Outstanding Teachers of University in Henan Province (2016-130), and Creative talents in the Education Department of Henan Province (19HASTIT039), the Open Project Foundation of Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) (2017-29), Nankai University, and Open Project Foundation of State Key Laboratory of Inorganic Synthesis and Preparation of Jilin University.
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