Progress and perspective on two-dimensional unilamellar metal oxide nanosheets and tailored nanostructures from them for electrochemical energy storage
Introduction
2D nanomaterials have been extensively explored over the past decade, inspired by the experimental discovery of graphene [1]. Graphene is a unilamellar carbon sheet with single-atom thickness and a lateral size of several micrometers or more and can be obtained from graphite, its three-dimensional (3D) layered parent material, via exfoliation. Owing to its ultimate 2D nature, graphene exhibits various unprecedented mechanical, electronic, and optical properties and has been widely explored in various applications, ranging from energy storage and conversion systems to next-generation electronic and optical devices [2], [3]. However, graphene is a simple material composed of only one element, carbon. To expand the versatility and tunability of the composition, polymorphs and functionality, a variety of other unilamellar nanosheets that possess similar molecularly thin 2D features have also been derived from their layered parent compounds, such as transition metal dichalcogenides (TMDs; e.g., MoS2, WS2, MoSe2, WSe2, etc.) [4], [5], [6], graphitic carbon nitride (g-C3N4) [7], layered metal oxides and double hydroxides (LDHs) [8], [9], [10], and transition metal carbides (MAX phases) [11], [12]. In addition, inspired by the attractive properties of 2D nanosheets, many types of non-layered materials, such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), polymers, metals, and non-layered metal oxides [13], have also been explored in 2D forms or, more properly, as lamellar nanostructures with typical thicknesses of several to tens of nanometers.
These ultrathin 2D materials show great promise in energy storage applications. The incorporation of graphene [14], TMDs [6], [15], LDHs [16], [17] and MXenes [11] in energy-related systems is at the center of energy material research. As another emerging class of 2D materials, metal oxide nanosheets have also been studied intensively for energy-related applications [18], [19], [20], [21], [22], [23]. In addition to their 2D features, transition metal oxides exhibit rich redox activities due to their variable valence states, which endow them with high energy storage capability suitable for electrode materials for supercapacitors and rechargeable batteries. There are some excellent reviews on ultrathin metal oxides fabricated by various synthetic methods, including “top-down” and “bottom-up” approaches and their well-designed nanoarchitectures [24], [25]. These oxides are generally represented as a type of material with ultrathin 2D morphology. Although, in some extreme cases, the thickness can reach a few nanometers, such kinds of oxides essentially have a crystallographic structure with 3D ordering, which are different from genuine unilamellar nanosheets having atomic or molecular thicknesses. In this review, we focus on 2D genuine unilamellar metal oxide nanosheets for energy storage applications. Compared with general 2D metal oxide nanostructures, 2D genuine unilamellar metal oxide nanosheets are expected to provide ultimately exposed active sites, shortened diffusion lengths and reduced volume changes to realize notably enhanced electrochemical performance, in terms of high specific capacity, ultrafast rate capability and long cycling stability. Besides, in order to retain the single-layer morphology from restacking/aggregation as well as overcome the poorly conductive nature of metal oxides, in this review, we introduce a heteroassembly of unilamellar metal oxide and conductive nanomaterials, such as graphene nanosheets, into a superlattice-like structure for electrochemical energy storage. The surface charge of one of them turned positive via modification with polycation and subsequent mixing of resulting two suspensions of nanosheets can promote their alternate stacking, which is different from conventional mixing of suspensions of metal oxide and graphene nanosheets, where random or self-restacking is dominating. As a result, all metal oxide nanosheets not only maintain the unilamellar aspect but can also be intimately hybridized with conductive graphene.
Here, we describe “top-down” exfoliation methods, especially soft chemical exfoliation, for the synthesis of 2D genuine unilamellar metal oxide nanosheets. Then, the design and fabrication methods of a new nanoarchitecture – a superlattice using the 2D unilamellar metal oxide nanosheets as building blocks – are summarized. Different from conventional randomly stacked nanosheet-based hybrids, the superlattice structures are molecular-scale integrated hybrids with intimate interactions, which maximize the charge storage capabilities of all of the 2D building blocks. In the next section, we review the utilization of 2D genuine unilamellar metal oxide nanosheets and their superlattices for a range of energy storage systems, including supercapacitors, lithium/sodium-ion batteries, and lithium-sulfur batteries. Finally, a brief summary and outlook for future research directions are given. The understanding, rationale, and methodologies of 2D unilamellar metal oxide nanosheets in this article could also be applicable for fundamental studies and practical applications of other 2D nanosheets.
Section snippets
Synthesis of unilamellar metal oxide nanosheets
Generally, there are two main strategies for the synthesis of 2D nanosheets: top-down and bottom-up synthesis [21]. Bottom-up synthesis is applicable for the synthesis of 2D ultrathin nanosheets of both layered and non-layered metal oxides. However, this method involves strict requirements and difficulties in controlling the anisotropic growth into atomically thin nanosheets. As for 2D genuine unilamellar nanosheets, the top-down method is the preferred method for cost-effective and scalable
Fabrication of nanostructured materials based on unilamellar metal oxide nanosheets
A variety of stable colloidal suspensions of 2D unilamellar metal oxide nanosheets have been successfully obtained via soft chemical exfoliation processes [8], [9], [38], [70]. They have widths in the range of submicrometers to micrometers and thicknesses up to a few nanometers that are inherent to the materials. As mentioned above, these nanosheets are useful in applications for electrochemical energy storage. However, one drawback is that these nanosheets are easy to restack during the
Applications in energy storage
Metal oxide nanosheets have been intensively investigated for energy-related applications. In addition to the 2D features, they have variable valence states and rich redox activities, which endow them with high energy storage capability as electrode materials for supercapacitors and rechargeable batteries. Combination of redox-active nanosheets (metal oxide, hydroxide, etc.) with 2D conductive nanomaterials (graphene, MXene, etc.) is a useful strategy for electrochemical energy storage [22].
Summary and outlook
Recent researches on the synthesis and assembly of 2D genuine unilamellar metal oxide nanosheets for electrochemical energy storage have been reviewed. The performance of unilamellar metal oxide nanosheets and their derived nanostructures for energy storage applications are summarized in Table 2. Compared to conventional thin oxides, 2D genuine unilamellar metal oxide nanosheets are more promising for energy storage because of the abundant active sites on their highly exposed surfaces and
Acknowledgements
This work is partly supported by the World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA), MEXT, Japan.
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