2D Materials Beyond Graphene for High-Performance Energy Storage Applications

TLDR: In this article, the authors provide an overview of current research and important advances on the development of 2D materials beyond graphene for supercapacitors and batteries, and highlight the major challenges to be tackled, and more generally the future directions in the field.

Abstract: Energy crisis is one of the most urgent and critical issues in our modern society. Currently, there is an increasing demand for efficient, low-cost, light-weight, flexible and environmentally benign, small-, medium-, and large-scale energy storage devices, which can be used to power smart grids, portable electronic devices, and electric vehicles. Novel electrode materials, with a high energy density at high power are urgently needed for realizing high-performance energy storage devices. The recent development in the field of 2D materials, including both graphene and other layered systems, has shown promise for a wide range of applications. In particular, graphene analogues, due to their remarkable electrochemical properties, have shown great potential in energy-related applications. This review aims at providing an overview of current research and important advances on the development of 2D materials beyond graphene for supercapacitors and batteries. The major challenges to be tackled, and more generally the future directions in the field, are also highlighted.

Figures

Figure 11. Electrochemical measurement of CNFs@MoS2 for LIBs. a) Galvanostatic discharge–charge (GDC) profiles. b). Cycling performance of CNFs@MoS2 at 0.1 A g–1. c) GDC profiles and d) cycling performance of CNFs@MoS2 at different current densities. e) Long life cycling performance of CNFs@MoS2 at 1 A g–1. Reproduced with permission from reference 169 (Copyright 2014 Wiley).

Figure 6. Electrochemical characterization of PPy/Ti3C2Tx composites for supercapacitors: a) CV curves of PPy/Ti3C2Tx (1:2) film at various scan rates. b) Effect of pyrrole loading on volumetric (left side y-axis) and gravemetric (right side y-axis) capacitance measured at 5 mV s−1. c) Rate performance of tested compositions and comparisons of their capacitances with previously reported Ti3C2Tx electrodes. d) Galvanostatic charge/discharge cycles of a PPy/Ti3C2Tx (1:2) film at various current densities. e) Impedance spectra of pristine Ti3C2Tx and PPy/Ti3C2Tx (1:2) films. Inset: zoom-in of the high-frequency region. f) Cycle life performance showing high capacitance retention of the PPy/Ti3C2Tx (1:2) film after 25000 cycles at 100 mV s−1. Inset shows that the shape of the CV was retained after cycling, confirming the high electrochemical stability of the PPy confined between the MXene layers. Reproduced with permission from reference 82 (Copyright 2016 Wiley).

Figure 7. Cellulose paper (CP)-based asymmetrical, flexible thin film supercapacitors. a) Scheme of the assembled device using graphite/Ni/Co2NiO4 sheets-CP as positive electrode and graphite/Ni/active carbon-CP as negative electrode. b) CV curves of the device under various bending states at 50 mV s–1. c) Cycle performance of the device at different bending states for 20 000 cycles at 100 mV s–1 and 10 mA cm–2. Reproduced with permission from reference 101 (Copyright 2014 Wiley).

Figure 2. Schematic representation of top-down and bottom-up production methods of 2D materials. a) Schematic illustration of experimental setup for electrochemical liquid-phase exfoliation (ELPE) of bulk MoS2 crystal (left); Photograph of a bulk MoS2 crystal held by a Pt clamp before exfoliation (center); Exfoliated MoS2 flakes suspended in a Na2SO4 electrolyte (right). Reproduced with permission from reference 49 (Copyright 2014 American Chemical Society). b) Scheme of ultrasound-induced liquid-phase exfoliation (UILPE) of MoS2 crystal into individual sheets. c) CVD based process relying on the introduction of sulphur atoms on the Mo thin film that was pre-deposited on the SiO2 substrate. Reproduced with permission from reference 56 (Copyright 2012 Wiley). d) Schematic shows the concept of molecular assembly of ultrathin 2D metal oxide sheets from liquid solutions, where metal oxide precursor oligomers are self-assembled into 2D metal oxide sheets with atomic thickness. Reproduced with permission from reference 57 (Copyright 2014 Nature Publishing Group).

Figure 10. Electrochemical and microscopic characterization of MoS2/S-doped RGO electrodes for LIBs. a) CV plots of a representative MoS2/S-doped RGO composite electrode at a sweep rate of 0.1 mV s−1 between 3.0 and 0.005 V. The two peaks at 0.99 and 0.45 V for the initial cycle correspond to the Li+ intercalation process forming LixMoS2, and the further conversion to Mo embedded in Li2S matrix, respectively. b) Galvanostatic charge/discharge curves of a composite electrode at a current densities of 0.1 A g−1 between 3.0 and 0.005 V. The low open circuit voltage (OCV) might be due to self-discharging. c) Comparison of cycling stability of MoS2/S-doped RGO composites with pure MoS2 and physically mixed MoS2 and S-doped RGO at a current density of 0.1 A g−1. d) Long-term cycling performance of MoS2/S-doped RGO composites at a high current density of 10 A g−1 (inset: the Nyquist plot of the composite electrode at frequencies from 100 kHz to 0.01 Hz at different cycling status). e) high-magnification high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of MoS2/S-doped RGO composite after cycling test and the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping of the elements C, Mo, and S. f) High-resolution TEM images of as-synthesized MoS2/S-doped RGO composites after cycling test. Reproduced with permission from reference 152 (Copyright 2015 Wiley).

Figure 1. Typical 2D graphene analogues for energy storage and their structures.

Full text

1
DOI: 10.1002/ ((please add manuscript number))
Article type: (Review)
2D materials beyond graphene for high-performance energy storage applications
Xiaoyan Zhang, Lili Hou, Artur Ciesielski and Paolo Samorì*
ISIS & icFRC, Université de Strasbourg & CNRS, 8 allée Gaspard Monge, 67000 Strasbourg,
France
Email: samori@unistra.fr
Keywords: layered materials, energy storage, supercapacitors, batteries
Energy crisis is one of the most urgent and critical issues in our modern society. Currently,
there is an increasing demand for efficient, low-cost, light-weight, flexible and
environmentally benign, small-, medium-, and large-scale energy storage devices, which can
be used to power smart grids, portable electronic devices, and electric vehicles. Novel
electrode materials, with a high energy density at high power are urgently needed for realizing
high-performance energy storage devices. The recent development in the field of 2D materials,
including both graphene and other layered systems, has shown promise for a wide range of
applications. In particular, graphene analogues, due to their remarkable electrochemical
properties, have shown great potential in energy-related applications. This review aims at
providing an overview of current research and important advances on the development of 2D
materials beyond graphene for supercapacitors and batteries. The major challenges to be
tackled, and more generally the future directions in the field, are also highlighted.

2
1. Introduction
The exhaustion of fossils fuels and climate change are among the greatest problems faced by
our modern society. To counteract the growing energy consumption demand, there is an
urgent need to design sustainable, efficient and low-cost devices for energy production and
storage. Being confronted with the colossal energy requirements against the backdrop of
global warming and the looming energy crisis, the development of clean and renewable
energy materials as well as their devices is highly desirable. Harnessing renewable energy
sources such as sunlight or wind is a first consideration for sustainable energy production.
However, they are diffuse and intermittent, owing to the unreliability of nature. Conversely,
tidal power and wave energy rely on a constant flow thus are more predictable and abundant,
yet, likewise geothermal energy, they can only be produced at selected sites; the collection of
the generated energy and its transmission is unfortunately a big hurdle towards these
technologies. These drawbacks have stimulated the research on efficient energy storage
devices featuring high-energy capacity and excellent cycle performance. Energy storage
devices such as supercapacitors and batteries, with high power/energy densities, are expected
to play essential roles in our daily life as the dominant power sources for portable consumer
electronics (e.g., smartphones, tablets, notebook PCs and camcorders), hybrid electric/plug-in-
hybrid vehicles and smart grids.
[1-6]
The recently increased research efforts on 2D materials, i.e., graphene and its analogues, is to
a great extent the result of the promise that they hold for technological applications including
electronic devices, sensors, catalysts, energy conversion and storage devices, etc., by taking
full advantage of their outstanding electrical, optical, chemical, and thermal properties.
[7-14]
Beyond graphene, other layered materials possessing various elemental compositions and
different crystallographic structures, offer a broad portfolio of material’s solutions with
tunable chemical and physical properties for application as high-performance active
components, which can operate as electrode materials for high-performance electrochemical

3
energy storage devices.
[4, 15, 16]
Although graphene-based nanomaterials have demonstrated
outstanding performance as electrodes in energy storage devices, new alternative
nanomaterials should also be developed in order to further improve the electrochemical
performance. Other 2D materials as graphene analogues (GAs) are expected to have broad
implications in next generation of clean, efficient, and renewable energy systems. Layered
materials of GAs refer to layered materials having similar structure as graphene, with planar
topology and ultrathin thickness (single to few atomic layers). Typical GAs for energy storage
include transition metal dichalcogenides (TMDs), transition metal oxides (TMOs)/hydroxides
(TMHs), metal sulfides, phosphorenes, MXenes, silicences, etc (Figure 1).
[17]
Insert Figure 1 here
Due to their thickness on the atomic scale, their inherent properties differ from those of their
bulk lamellar systems. In particular, the quantum confinement of electrons in the 2D plane
imparts them with unprecedented electrical and electronic characteristics (Table 1).
[18-26]
Moreover, it is well known that the delivered specific capacity of electrode materials is
closely related with the reaction kinetics during the charging/discharging process.
[3]
In view of
their high surface-to-volume ratio, GAs offer high specific surface areas (Table 1) to enable
full utilization of all available sites of active electrode materials.
[27-30]
As a result, the exposed
contact area is significantly enhanced between the electrodes and electrolytes, and also the
paths for transport of charges are largely shortened. Last but not least, GAs also exhibit
excellent electrochemical properties.
[31]
All of these characteristics make them potential
candidates for energy storage devices.
Insert Table 1 here
In this contribution, after a brief introduction on the preparation methods of GAs, we review
the most enlightening recent progresses of GAs in energy storage devices, providing outlooks
and perspectives of this topical area of science and technology.

4
2. Preparation methods of GAs
Since there are already several excellent review articles discussing the unique properties of
layered materials beyond graphene,
[32-37]
in the present review we will not focus on their
properties. Prior to discussing applications in supercapacitors and batteries, we briefly
introduce the methods for the preparation of GAs. The reliable production of high-quality
atomically thin 2D systems and the fine-tuning of their various properties through scalable
approaches is a crucial first step for realizing devices for energy storage. Hitherto, two
approaches have been pursued to obtain monolayer-thick GAs sheets, i.e., the top-down and
bottom-up strategies (Figure 2). The former relies on the chemical
[38]
or mechanical
[39-41]
exfoliation of bulk crystals into individual sheets; the latter allows the generation of GAs from
atoms or suitably designed molecular building blocks, which upon chemical reactions form
covalently linked GAs sheets.
[42]
Insert Figure 2 here
2.1 Top-down approaches
The top-down approaches enable the production of micrometer- and/or nanometer-sized
sheets from bulk crystals. On the one hand, it is generally assumed that the GAs sheets
produced via mechanical cleavage (scotch tape method) possess highest quality and purity,
which makes them suitable for fundamental research, and in particular for realization of
proof-of-concept devices.
[43]
Yet, mechanical cleavage is unsuitable for mass production due
to the low yield and lack of control over the number of layers in the exfoliated samples. On
the other hand, large quantities of mono- and few-layer thick GAs sheets, characterized by a
low content of structural defects, can be obtained by exploring top-down methods such as
ultrasound-induced liquid-phase exfoliation (UILPE)
[44-47]
or electrochemical liquid-phase
exfoliation (ELPE),
[48-52]
which are extremely versatile and can be carried out in a variety of
environments. Other methods, which combine intercalation of bulk crystals with chemical
inserts and its subsequent exfoliation, are also being developed.
[53]

5
2.2 Bottom-up approaches
The bottom-up production of large-area GAs with specific number of layers can be achieved
by making use of chemical vapor deposition (CVD) techniques.
[54]
Among the CVD methods,
sulfurization (or selenization) of metal (or metal oxide) thin films is being extensively
explored.
[55, 56]
Another useful method is to synthesize 2D materials using molecular
precursors via a wet chemical hydrothermal/solvothermal treatment. For instance, GAs like
TMOs can be produced via self-assembly, where amphiphilic block copolymers and short-
chain alcohol co-surfactants are employed as structure-directing agents to confine the stacking
and growth of the metal oxide precursor oligomers along the chosen direction.
[57]
As for energy storage, UILPE
[39, 44-46]
and wet chemical synthesis
[57]
are the most commonly
used approaches for the preparation of layered materials of GAs, owing to the advantages
discussed above as well as of easy and large-scale preparation.
3. Supercapacitors
Supercapacitors, also named as electrochemical capacitors, have attracted tremendous
research interest during the past decades, primarily due to their high power density, rapid
charging/discharging, and excellent cycle stability.
[2]
Undoubtedly, supercapacitors provide a
promising approach to resolve the current energy demand by allowing fast storage of
intermittent renewable energy. Based on the underlying energy storage mechanism,
supercapacitors can be divided into two types: electrical double-layer capacitors and
pseudocapacitors.
[58]
The performance of supercapacitors can be evaluated by using the
following parameters: i) mass/volume capacitance, ii) energy/power density, and iii) cycle
lifetime. The unique characteristics that make a material suitable as an active electrode
include good electrical conductivity, high specific surface area, optimized pore size

Frequently asked questions

Q1. What are the contributions in this paper?

In this paper, the authors review the most enlightening recent progresses of GAs in energy technology, providing a brief introduction on the preparation methods and providing a topical area of this area.