Two-Dimensional Anode Materials for Non-Lithium Metal-Ion Batteries

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Two-Dimensional Anode Materials for Non-Lithium Metal-Ion Batteries Santanu Mukherjee, and Gurpreet Singh ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00843 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Applied Energy Materials

Two-Dimensional Anode Materials for Non-lithium Metal-Ion Batteries

Santanu Mukherjeea,b, Gurpreet Singha,b

aDepartment

of Mechanical and Nuclear Engineering, Kansas State University, KS, USA 66506

bCorresponding

author:

[email protected] [email protected] Tel: 785 532 7085

1 ACS Paragon Plus Environment

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Abstract: Non-Li metal ion rechargeable battery systems e.g. Na, K, Mg, Ca ion systems are at the brink of playing a major role for sustainable energy and grid storage, in part owing to their significant availability as compared to Li-ion rechargeable systems. However, non-Li-based systems pose their own unique set of challenges; the large ionic size of the respective ions especially for Na and K systems, weak kinetics and low voltage window of Mg ion systems etc., which prevents efficient reversibility. Developing efficient electrode materials with novel morphologies is one of the main ways to harness the potential on the non-Li ion systems. It is here that twodimensional (2D) layered materials which have excellent structural, electrochemical, and mechanical properties can be considered to be prime candidates for negative electrode materials in non-Li-based energy storage systems. Therefore, research in the various aspects of 2D materials encompassing their fabrication techniques, tailoring their morphology, and application as anodes in non-Li systems have significantly increased in recent years, with more expected increase in the future. With this perspective in mind, here we provide an exhaustive review of the structure, properties of various 2D materials (graphene, phosphorene, and transition metal dichalcogenides), their performances as anode materials in emerging non-Li-based energy storage systems, and the obstacles that must be overcome at each stage. Keywords: Two dimensional materials, transition metal dichalcogenides, graphene, exfoliation, phosphorene, anodes, van der Waal’s bond.


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Introduction Lithium ion battery storage and current issues Fossil fuels, which have been the predominant energy source worldwide since the Industrial Revolution, are increasingly being criticized for their toxic effluents and negative effects on the environment 1. As a result, concentrated efforts to transition away from these fuel systems are underway

2-3.

Although the application of

renewable sources for energy generation has been growing steadily in recent years, some renewable energy sources (e.g., solar and wind) are intermittent in their availability and therefore require corresponding superior storage systems to fully realize their potential 4. Ideally, these storage systems should entail decreased manufacturing costs, scalability portability, safety, and minimal environmental impact 5-6. Rechargeable metal-ion batteries demonstrate most of these advantages, making them a preferred energy storage system. Figure 1 provides a schematic outline of the type of energy storage systems that are currently employed in the U.S., demonstrating that the overwhelming majority of it is pumped hydro storage (95 %). Battery technology occupies a significant portion (25 %) and thereby lies the importance of robust electrochemical rechargeable systems

7.

The primary aims of battery technology remains in

providing a clean and efficient energy storage and being carbon neutral at the same time.



Page 4 of 64

Figure 1: Energy storage systems in the U.S. and the effectiveness of electrochemical energy storage. (a) denotes the various types of energy storage systems deployed in the U.S. with an overwhelming dependence on pumped hydro, which is about 95 %. (b) provides an interconnected pathway for some of the major goals for modern day non-Li ion electrochemical energy storage, the primary being grid storage

7-8.

100 GW of pumped hydro storage has been deployed worldwide with 32 GW being in Europe and 21 GW in Japan 9. Compressed air-based storage systems are not as popular and there are only 2 systems currently operational worldwide, in Germany and in the U.S 9. The German system is able to generate 290 MW for 2 hours and has a 99 % starting reliability 9. High speed flywheel systems are able to produce a maximum specific energy up to 100 Wh kg-1 10. A steel flywheel in Japan is used to supply power to a nuclear fusion furnace at the Japanese Atomic Energy Center 11.

10-

Batteries comprise a wide class of storage devices, from the Ni-Cd and Ni-Metal

Hydride (Ni-MH) battery for small scale applications to the Pb acid batteries used in 4 ACS Paragon Plus Environment

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automobiles to LIBs used extensively in portable electronics market

for

energy

storage

by

electrochemical

8, 12-13.

techniques

The global

(batteries

and

supercapacitors) is expected to reach US $ 26 billion by 2026 with grid storage systems projected to reach 23.4 GW by then

14.

Out of these, among the rechargeable battery systems, Li ion batteries (LIBs) are the most predominant and are currently used in a variety of portable and consumer electronics, such as laptops and mobile phones

2, 4.

Since their advent in the 1970s,

LIBs have increased in popularity and revolutionized the portable energy storage market due to their high specific energies (120 Wh kg-1 or more) and volumetric energy density (300 Wh l-1 or more). Although the nickel metal hydride (Ni-MH) battery is comparable in terms of volumetric energy densities, the gravimetric energy density of LIB is about 1.5 times more than Ni-MH systems

15.

Also, LIB’s

working high working potential of 3.7 V is approximately three times more than those of Ni-MH systems which usually work at ∼1.2 V

15

Apart from this, the low

reduction potential of Li (-3.04 V vs SHE), consistent performance over long cycle lives ( ~ 1000 cycles), relatively small ionic size and the relative ease of intercalation 2, 16-17.

Despite the said benefits, however, LIBs have significant limitations. Availability and expenditure is one of the primary issues facing LIB systems today. Li is not a very abundant element in the earth’s crust (only 20 ppm) and its relative rarity makes its resources to be expensive

18.

Also, the Li resources are unevenly geographically

distributed which results in increased overhead and transportation expenses. Therefore, with Li resources at a premium, their application in large scale grid storage becomes very difficult. Safety, both environmental as well operational are a major concern for LIB systems. Especially when graphite is used as an anode material in LIBs, the solid-electrolyte interfacial (SEI) layer formation and consequent lithium plating is an important safety issue

19.

In addition, certain toxic cathode materials (e.g. Co in LiCoO2, Ni in



LiNiO2

etc.),

flammable

electrolytes

in

LIBs

Page 6 of 64

create

potential

health

environmental hazards and increase the difficulty of recycling spent cells

and

20.

Choice of electrode materials is another important issue facing LIBs. Spinel materials e.g. LiMn2O4 provide good electronic and ionic conductivity, however they suffer from significant capacity decay at elevated temperatures

19.

These materials also

undergo structural distortion due to the Jahn-Teller effect and so cannot be discharge to low voltages 21. Alloy based anodes e.g. Li3Sb, Li4.4Sn have been studied and these have shown significantly high initial capacities (993 mAh g-1 for Li4.4Sn) along with the ability to overcome the problem of plating 22-23. However, these alloys undergo large volume changes and subsequently large capacity fade over progressive cycling

22-23.

Nanostructured metal oxides having the general formula

MOx have been studied as electrode materials in LIBs and have demonstrated high capacities (∼1000 mAh g-1), however they also exhibit large differences between the charge and discharge voltage values which is also a cause of concern

24.

Undesirable side reactions, pulverization and clumping together of electrodes etc. which occur during cell cycling leads to lowered performance and need to be rectified 12, 25.

Essentially, the type and chemistry of the electrode-electrolyte interface

determine battery longevity, and this interface in LIBs are poorly understood and require significantly more research

16.

Poor performance in aqueous electrolytes is another problem that needs to be addressed for LIBs

15.

Therefore, Na ion batteries (NIBs) and other metal ion (e.g., K, Mg, Ca, etc.) batteries are being considered for niche utilities and applications, as detailed in the following sections. Vaalma et al. have shown that for a battery rated 7 kW, the net production expense of a LIB having LMO cathode, graphitic anode and Cu current collector is $1022 per unit. For a NIB of the same power rating, using a Al current collector instead of Cu, the net production price per unit is $894, indicating approximately a 14 % saving26. Using an NMC (Nickel-Manganese-Cobalt) based cathode drives the cost down for a NIB to $ 841 per unit26. 6 ACS Paragon Plus Environment

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Non-LIBs and rationale for 2D electrode development Application of non-LIBs are primarily to overcome the above-mentioned limitations of existing LIB systems. Among monovalent metal-ion non-LIB systems that have garnered attention, NIB systems are most notable due to the abundance of Na in the Earth’s crust, thereby appreciably decreasing NIB expenses

27.

Also, the low

working voltages of NIBs (-2.71 V vs SHE) permit the use of inexpensive electrolytes, consequently reducing manufacturing costs and downtimes required to fabricate the cells in a protected environment

27.

Also, since LIB electrodes have

been extensively investigated, these materials serve as important starting points and templates for NIB systems

28.

As the intercalation chemistry of Na ions closely

resemble Li ion systems, an existing body of knowledge provides an immense advantage

28.

Also, NIBs have exhibited more stability at room temperature and

better performance in aqueous electrolytes than LIB systems

29.

A close relative of

the NIB, the K ion system (KIB), also advantageously utilizes the abundance of potassium in the Earth’s crust and a low redox K/K+ potential vs SHE (-2.92 V, which is lower than the Na/Na+ system)

28, 30-31.

Also, unlike Na ion systems, K+ ions tend

to intercalate reversibly into graphite similar to LIB systems

32.

However, the large

size of Na+/K+ ions coupled with low reversibilities mean that negative electrodes must be tailored accordingly

27, 33.

On the other hand, these Na+/K+ ion systems can

be advantageously applied in medium to large-scale grid storage systems, which do not require frequent expensive maintenance and high energy densities

34.

Reliability

and able to manage peak loads are another important necessity of grid storage system also, at a lower cost. Since Na/K/ etc. are available abundantly at a cheaper price, they answer to one of the most important demands of a reliable grid storage system

35.

Multivalent ionic systems (i.e., Ca2+, Mg2+ etc.) have also attracted research attention because they demonstrate high energy densities (by virtue of their larger charge) and are comparatively less expensive than LIBs

36-40.

The primary aim is to

utilize the low reduction potentials and multiple ions these systems supply to positively influence energy densities

41-43.

Calcium ion systems are considered



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especially interesting in this regard as its standard reduction potential is only about 170 mV above Li and has faster reaction kinetics than Mg2+ ion systems

44.

Ca2+ and

Mg2+ systems also provide the possibility whereby metallic anodes can be used as they suffer far less plating than LIB systems

45.

Additionally, these non-LIB systems

can be used in aqueous systems, thereby eliminating environmental concerns associated with LIB systems. The development of new electrode materials, especially large open structures e.g. hexacyanoferrates have also provided for enhanced storage of these multivalent ions and improved capacities

46.

However, multivalent ionic systems also have inherent drawbacks. For example, Ca2+ has a significantly large ionic radius that hinders smooth intercalation into the host lattice; whereas Mg2+ and Al3+ ions have a low diffusion coefficient due to their high charge densities

41-42.

Also, a passivating film that forms on Mg in organic

electrolyte environment acts as a barrier to the effective transport of Mg2+ ions and renders the Mg anode effectively inactive

47.

Consequently, practicable anodic

current values are obtained only at potentials significantly far from the Mg/Mg2+ equilibrium potential which make the system unstable

48.

Overall, however, when

large-scale grid storage requires energy to be stored at the levels of several MWh to GWh, non-LIBs are most suitable due to their ready availability and inexpensive procurement

49-50.

A comparative tabular and schematic analysis of these metal ion couples and their important performance parameters is provided in Table 1 and Figure 2 respectively. Table 1. Comparison of essential parameters of primary metal elements in rechargeable battery systems. These important parameters are the most important deciding factors in the choice of the rechargeable-ion battery and the important trade-off is usually between theoretical capacity, expenditure, and safety.


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Important parameters

Li+

Na+

K+

Ca2+

Mg2+

Ref.

Ionic (Shannon) radii (Å)

0.76

1.02

1.38

1.00

0.72

41

Relative atomic mass

6.94

23.00

39.10

40.07

24.31

51

0.0017

2.6

2.4

3.4

1.9

51-52

3861

1166

685

1340

2205

51-52

1378

1193

1059

987

1085

51

E0 vs SHE (V)

-3.04

-2.71

2.931

-3.8

-2.70

51

Crystal configuration

bcc

bcc

Bcc

Fcc

hcp

51-52

Price of ore ($/ton)

5000

150

216

275

2060

51-52

Abundance in earth’s crust (% by weight) Theoretical capacity (mAh g-1) Theoretical capacity (mAh cm-3)

Figure 2: The case for non-Li ion rechargeable batteries from an economic and scale perspective. (a) Graphical representation of the relative abundance of various elements in the earth’s crust (in ppm) clearly demonstrating the availability and consequent advantage in accessing the non-Li alkali metals. (b) is the graphical representation

demonstrating

the

increased

research


in

non-Li

metal-ion


Page 10 of 64

rechargeable batteries in the past decade, thereby indicating a potential moving away from Li-ion systems. (c) A schematic that shows where Na and other non-Li metal-ion systems can fit in the niche for energy storage i.e. small-to-medium scale grid storage. Overall, large size non-Li metal-ion battery systems can be used to store correspondingly large amounts of energy, in spite of their lower energy density, due to the massive advantages that come from their abundance and consequently inexpensive availability

27, 35, 53.

A discussion of non-LIBs and their advantages underscores the need to develop optimal anodes that provide reasonable longevity and ionic reversibility. Various classes of materials have been tested as negative electrode materials, but the results have been inconclusive. Graphite does not work well in NIBs due to lack of sufficient interstitial space, and potassium ion battery (KIB) systems suffer from large thermal runaways with most anodic systems, which make them unsafe

54-55.

Metal oxides, specifically mixed and transition metal oxides, occasionally undergo significant volume changes with conversion reactions or have complicated intercalation mechanisms

27, 56.

Mg ion batteries (MIBs) generally contain pure Mg

metal, but they form an electrochemically inactive layer on the anodic surface that reduces battery efficiency

57.

Two-dimensional (2D) materials offer a promising alternative to these abovementioned problems because they provide expansive, active surface areas in layers of a few or single sheets and interlayer spacing to accommodate bulky alkali ions Well-known

2D

materials,

their

structural

aspects,

and

mechanical

58.

and

electrochemical properties are discussed in subsequent sections. 2D materials Ever since graphene was discovered in 2004, 2D materials have been the subject of extensive research

59.

These materials have recently been utilized in widespread

applications for electronics, photonics, and electrochemical energy storage


59-61.

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These materials demonstrate several advantages, especially over bulk, 0D and 1D materials which are enumerated in the following paragraph. Their size restriction in one dimension gives 2D materials a set of unique properties not otherwise found in their bulk counterparts, and their optical and electronic properties are especially influenced since electronic motion is restricted to two dimensions

61.

2D materials, by virtue of their high surface area-to-volume ratio

provides a greater access to electroactive sites as compared to their bulk, 0D and 1D counterparts thereby potentially improving reaction kinetics

62.

Also, it has been

reported that 2D nanostructures tend to exhibit greater structural stability than bulk and other forms63. The absence of terminating groups and dangling bonds in 2D materials results in lesser defects and consequently lesser carrier scattering than bulk materials

64.

Lastly, the negligible thickness of 2D materials (as compared to

bulk) results in lower effective mass and volumes that tends to improve the stability of devices based on 2-D materials64. Especially in electrochemical energy storage devices, 2D materials have demonstrated higher initial Coulombic efficiency and cycling stability at elevated C-rates as compared to their 0D and bulk counterparts 63.

2D materials, due to their suitable characteristics, have attracted a large amount

of attention in research, especially for electrochemical storage, electronics, device and biomedical applications 65-66. This has been more so especially in the last decade with the fine-tuning of more sophisticated techniques to fabricate and characterize them and greater applications of nanotechnology in general

67-68.

Figure 3 provides

an insight to the significant amount of research interest given to 2D materials, especially in the last decade and some of their most prominent applications.



Page 12 of 64

Figure 3: Graphical and statistical representation in the research growth and applications of 2D materials. (a) Graphical representation demonstrating the significant increase in graphene and graphene related materials in the last decade thereby underscoring its importance in materials engineering. (b) Pie chart indicating the ubiquitous applications of graphene in various sectors, with a stress on the electrochemical energy storage and generation occupying about 27 %. (c) Trend showing the noticeable increase in research activity in other 2D materials e.g. TMDs (MoS2) and phosphorene with MXene in the inset during the same time indicating the growth of 2D materials other than graphene as well, and (d) a break12 ACS Paragon Plus Environment

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up of the applications of 2D layered TMDs

69.

Overall, 2D materials have had

considerable research activity in the last decade due to their attractive functional properties

68, 70.

Graphene or few layer graphene, comprised of a few layers of bulk graphite, demonstrates highly enhanced electrical and mechanical properties than bulk graphite and is one of the strongest known materials

71.

In addition, the sheet like

nature allows it to attach species on either side, resulting in an increased number of metal ions that can be intercalated and stored and consequently high theoretical capacities

71-72.

Phosphorene, another 2D material that has drawn research attention, is comprised of a single layer of black phosphorus which is the most stable allotrope of all forms of phosphorus

73.

Phosphorene resembles graphene with hexagonally arranged

atoms and a puckered structure

73-74.

Although it has been analyzed extensively as

an anode material for LIBs, recent work has implemented phosphorene as an anode in NIBs

75-76..

Transition metal dichalcogenides (TMDs) (e.g., MoS2, MoSe2, WS2, WSe2, etc.) are also promising 2D materials for electrochemical energy storage

62.

This is primarily

because they provide enhanced charge transfer kinetics, extensive surface areas for electrochemical reactions, and high theoretical capacity

77-78.

MXenes (2D metal carbides and nitrides) are another class of important 2D materials that have also shown potential for a variety of applications

79.

excellent

properties.

structural,

conductive,

and

electrochemical

This is due to their However,

application of MXenes in electrochemical energy storage has been mostly limited to LIBs and supercapacitors, which is not the scope of present work. First principle evaluations and reviews by Kumar et al. and Anasori et al. respectively provides a comprehensive understanding of the salient features of MXenes

80-81.

In general, the type and nature of the crystal structure and bonding within the 2D material are important because they determine the electronic and ionic conductivity,



Page 14 of 64

interlayer spacing, and mechanical properties that impart structural stability against chemo-mechanical deformation. The following sections discuss those properties in detail. Properties of 2D materials relevant to rechargeable batteries Structure, Mechanical and Electronic properties are the direct result of the type of structure and bonding exhibited by the materials. These properties are discussed in the following sections. Structure and electronic properties Graphene After extensive trials beginning in the 1960s, Geim et al. isolated graphene sheets in 2004

67, 71.

Graphene is structurally comprised of carbon atoms joined in a

hexagonal lattice

82.

One of the most consequential properties of graphene is its

purity and inherent homogeneity

83.

Because graphene is fabricated from graphite,

impurity atoms are not usually present in the graphene structure

84.

Each carbon atom in a triangular graphene sub-lattice has three nearest neighbors with which it forms σ bonds with a separation of 1.42 Ẵ between them

85.

Considering a van der Waal’s radius of the C to be approximately 1.10 Ẵ, the pore size of graphene is estimated to be 0.64 Ẵ, which is much smaller than the van der Waal’s radii of even hydrogen (2.8 Ẵ) and helium (3.14 Ẵ)

86.

This

electronic/structural configuration of graphene contributes to its interesting properties

85.

Also, because graphene is 2D, the electrons are always restricted in

the z-axis during conduction

85.

Covalent and non-covalent methods are typically used for functionalization of graphene sheets

87.

Covalent modification advantageously introduces a charge

variation (positive and negative) to the graphene that can be manipulated to control the degree of functionalization

87.

Phosphorene As mentioned, phosphorene is comprised of single layers of black phosphorus, like the way graphene is related to bulk graphite. Monolayer black phosphorus, or phosphorene,

demonstrates

a

puckered

honeycomb


structure

with

every

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phosphorus atom linked to three nearest neighbors via covalent bonds

88.

If

phosphorene occurs as a double layer, then the stacking is typically in the form of “AB.” Stacks of three or four layers of phosphorene are usually in the form of “ABA” or “ABAB,” respectively

88.

Reported lattice parameters for phosphorene are 3.35 Ẵ and 4.62 Ẵ

73.

The

semiconducting nature of phosphorene distinguishes it from graphene: the band gap of graphene is negligible, but phosphorene exhibits a finite band gap, causing the primary expectation that it is a p-type semiconductor

89.

Another important

structural aspect of phosphorene is its ability to demonstrate anisotropy related to electronic and phononic dispersions as a direct function of its puckered honeycomb structure

89.

If phosphorene sheets are in the form of nanoribbons, then increasing

narrowness of these ribbons relates to a proportional increase in the magnitude of the band gap

89-90.

This can be attributed to quantum confinement because

phosphorene is a 2D material

89, 91.

TMDs Many TMDs crystallize in a graphite-like layered structure that results in significant anisotropy in their electrical, chemical, mechanical, and thermal properties. Groups IV–VII TMDs typically exhibit a layered structure, whereas groups VIII–X TMDs are commonly found in non-layered morphologies

92.

Each layer in layered structures

typically have a thickness of 6~7 Å, consisting of a hexagonally packed layer of metal atoms sandwiched between two layers of chalcogen atoms

92.

Intralayer M–X

bonds are generally covalent, whereas sandwiched layers are joined by weak van der Waals forces

93.

Metal atoms provide four electrons to fill TMD bonding states so that oxidation states of metal (M) and chalcogen (X) atoms are +4 and –2, respectively. M–M bond length varies between 3.15 Å and 4.03 Å depending on the sizes of the metal and chalcogen ions

94.

The metal coordination of layered TMDs can be trigonal prismatic or

octahedral (typically distorted and sometimes referred to as trigonal-antiprismatic). Depending on the combination of metal and chalcogen elements, one of the two 15 ACS Paragon Plus Environment


Page 16 of 64

coordination modes is thermodynamically preferred. TMDs with different chalcogens on either side of a sandwiched Mo layer (“Janus structure”) have been explored by Zhang et al, the monolayer thick SMoSe sample fabricated by them have indicated the potential for catalyzing hydrogen evolution reaction (HER)

95.

Figure 4 provides schematic as well as structural illustration of the layered structure of the different types of 2D materials being discussed, including MXene for reference purposes.

Figure 4: Structural representation of the layered 2D materials. (a, i), (b, i), (c, i) and (d, i) show the ball and stick atomic model of the various layered materials with their important lattice parameters (and interlayer spacings) labeled. The characteristic layered pattern of each can clearly be recognized from these 16 ACS Paragon Plus Environment

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atomic models. (a, ii), (b, ii), (c, ii) and (d, ii) are the lattice structures studied in more detail, basically showing the directionality of the crystal as well as the order in which the atoms arrange themselves e.g. armchair and the zigzag orientations for graphene and the phosphorene.

(a, iii), (b, iii), (c, iii) and (d, iii) high

resolution TEM (HRTEM) images of lattices of four materials and the typical atomic orientation has been marked on them to match with their corresponding preferred crystallographic arrangements. (a, iv), (b, iv), (c, iv) and (d, iv) SEM images which show the layered structure of all the different types of the 2D materials being discussed. Images a(ii-iii) reproduced with permission from ref 96. Copyright 2013 American Chemical Society. Images a(i, iv) reproduced with permission from ref 58. Copyright 2017 Elsevier publications. Images b(i-ii) reproduced with permission from ref 97. Copyright 2015 American Chemical Society. Image b(iii) reproduced with permission from ref 98. Copyright 2016 IOP Publishing. Image b(iv) reproduced with permission from ref 99. Copyright 2017 Elsevier publications. Image c(i) reproduced with permission from ref 100. Copyright 2015 Elsevier publications. Image c(ii, iii) reproduced with permission from ref 101. Copyright 2013 American Chemical Society. Image c(iv) reproduced with permission from ref 102. Copyright 2015 Springer Nature Publications. Image d(i-iii) reproduced with permission from ref 103. Copyright 2017 Nature publications. Image d(iv) reproduced with permission from ref 104. Copyright 2017 Science publications.

Mechanical properties Mechanical properties are essential to the study of anode materials because various types of electrochemical reactions (i.e., ionic intercalation/deintercalation, alloying,



Page 18 of 64

etc.) produce change in lattice volume. Therefore, to maintain structural fidelity of the system, the electrodes must demonstrate mechanical durability. Graphene typically exists as monolayers attached to a substrate or as stand-alone sheets called “free-standing graphene”

105.

Lee et al. demonstrated that pristine

graphene sheets exhibit a mixture of non-linear elastic performance and brittle characteristics

106.

Graphene also demonstrated a relatively large Young’s modulus

(approximately 1.0 TPa), which approaches values generally exhibited by pristine CNTs

106-107.

However, structural defects, either inherent or deliberately created

during fabrication, sometimes provide desirable variations. Mechanical stress in free-standing single-layer graphene sheets has been shown to cause crack growth and propagation along a straight line, as also corroborated by simulation studies 109.

108-

Crystal defects such as dislocations and grain boundaries (GBs) significantly

affect the final mechanical properties. Dislocations in graphene are usually observed as point defects and occur in directions corresponding to plastic flow

110-111.

GBs

(armchair and zig-zag), which usually manifest as line defects, drastically weaken the graphene sheet, especially if it is polycrystalline graphene

112.

The effect of

mechanical strain on the electronic properties are important as the electrode is expected to undergo considerable wear and tear with alkali ion intercalation. Abinitio studies have indicated that there is no significant energy gap increase with the increasing strain in the uniaxial direction

113.

Phosphorene has demonstrated anisotropy and much weaker mechanical properties than graphene: phosphorene’s Young’s modulus was found to be 20.9 GPa in the armchair direction and 90.5 GPa in the zig-zag direction compared to graphene’s 1.0 TPa

114.

This lowered Young’s modulus of phosphorene can be primarily

attributed to the weaker P-P bonds in phosphorene

88.

Consequently, phosphorene

has higher strength in the zigzag direction (18 GPa) than the armchair direction (8 GPa)

88.

Phosphorene’s mechanical and shear properties are strongly orientation

dependent, which consequently causes anisotropy in the phononic propagation


115.

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Crack propagation in phosphorene occurs when atomic bonds break or cracks develop along crystallographic defects in the armchair and zig-zag directions

116.

TMDs demonstrate high mechanical flexibility and elevated Young’s modulus values 117.

Single-layer MoS2 exhibit Young’s modulus of 270 ±100 GPa, which is higher

than the Young’s modulus of stainless steel

118.The

superior mechanical property, in

this case, is due to lack of defects and stacking faults in the MoS2 lattice

117-118.

In

addition, 2D TMDs are advantageously resistant to strain up to 10 % linear deformation, preventing the TMD from exaggerated volume changes due to interaction with the bulky alkali ion

119.

However, it has been observed that beyond

that regime, especially the semiconducting TMDs are quite responsive to the stress levels with increasing stress reducing the band gap

120.

Fabrication of 2D materials relevant to battery applications The fabrication of 2D materials can potentially introduce stresses and defects into the system that ultimately influence their ionic transport and electrochemical properties materials

121.

Several publications have specifically addressed the fabrication of 2D

122-123.

For the scope of this manuscript, however, a short summary of

important fabrication techniques is provided, especially those which are used for electrochemical storage applications. Fabrication of 2D materials are generally classified as “top down” and “bottom up” 58,

124.

Top-down approaches typically derive layered material from its bulk

precursor, and bottom-up processes grow thin films or deposit atomically thin layers 124-125.

Top-down approaches are relatively straightforward and inexpensive.

Mechanical exfoliation, a simple technique applied to all 2D materials, cleaves individual layers from bulk material using adhesive tape. For graphene, the precursor is highly ordered pyrolytic graphite (HOPG)

126.

Similarly, black

phosphorus and corresponding bulk TMD powder are the precursors for fabricating phosphorene and layered TMD powder, respectively

73, 127.

However, this method

only isolates a few layers of graphene, and the yield is insufficient for large-scale



applications

73, 126, 128-129.

Page 20 of 64

In liquid phase exfoliation, another popular technique to

develop 2D materials, the bulk precursor is sonicated in a liquid medium, which helps to overcome the weak van der Waal’s forces between constituent sheets of the precursor, and few layered materials are obtained

130-131.

Solvents for this

technique include aliphatic pyrrolidines, and ketones. Ultrasound exfoliation, a variation of liquid phase exfoliation, utilizes a Li intercalated bulk precursor to obtain high quality 2D sheets that have been successfully applied for large-scale production 132-133.

For example, Joensen et al. fabricated single-layer MoS2 using n-butyl lithium

to intercalate Li+ ions

134.

These liquid exfoliation-based processes are facile,

scalable, and applicable for bulk production. Top-down techniques, especially liquid phase exfoliation, provide necessary bulk material to fabricate electrodes in electrochemical systems. Another advantage of these top-down approaches are that they increase the surface area necessary for storage of the alkali ions during intercalation, by making the individual sheets surfaces more accessible for ionic adsorption

135.

A novel strategy, which takes the concept of increasing surface area

one step further, is to develop porous or “holey” 2D nanomaterials which ease the tortuosity of the ionic pathways even more

136.

Top down approaches are demonstrated in Figure 5.


Page 21 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Compressive stress

Top down fabrication techniques

Liquid exfoliation

Wedge effect

Shear effect

Shear stress Exfoliated sheets

M echanical exfoliation Intercalant assisted exfoliation

Tensile force Exfoliation under shear stress

Weak Van der Waal’s forces

H2O, NH3, Li+, Na+ etc. Ultrasonication

Intercalated precursor

Exfoliated nanosheets

Figure 5: Top-down fabrication techniques of layered 2D materials for application as electrodes in energy storage are indicated here. From top to bottom: The top figure demonstrates liquid exfoliation whereby the energy in the ultrasonication process helps in shearing away the layers of the 2D material giving rise to individual flakes. The energy to shear apart the weak van der Waal’s bonds is provided by the sonication. The schematic in the middle indicates the simplest, mechanical peeling which can produce thin layers of very 2D materials. This technique is energy efficient and produces high quality crystals but unreliable and inefficient for scaling-up production quantities. Bottom schematic indicates electrochemical exfoliation. This is generally achieved by forcing guest species such as ions (e.g. Li+, Na+) electrochemically which then shears away the crystal into individual sheets upon sonication. Large quantities of 2D materials may be obtained by liquid exfoliation techniques which makes them promising for use in electrochemical energy storage applications. Reproduced with permission from refs 137-138. Copyright 2015 and 2016 Royal Society and Elsevier respectively.



Page 22 of 64

Bottom-up approaches, “build up” thin multiple sheets atomically layer by layer to produce material that can be used for thin film applications in transistors and electronic devices

139.

However, bottom-up approaches have limited applications for

large-scale electrochemical energy storage. Chemical vapor deposition (CVD) is a foremost bottom-up technique used for thin film fabrication

140.

For graphene, CVD

techniques break down a carbon-based precursor at high temperatures whereby the atoms rearrange to form graphene, usually in the presence of a transition metal oxide catalyst

141.

CVD grown graphene, however, tends to introduce localized

defects in the graphene sheets 142-143. Likewise, TMDs have also been fabricated by CVD using appreciably volatile precursors, such as halogens of the metals and thiols or dialkyl sulfides as the source of sulfur used to fabricate highly pure MoS2

145.

144.

Solid vapor deposition has also been

A more fundamental understanding of the

molecular mechanisms involved in bottom-up fabrication approach of 2D materials have been provided by Ye et al

146.

Electrochemistry As mentioned in previous sections, electrochemical properties are a direct function and consequence of the chemical structure and bonding of the material. Graphene Because graphene is a 2D material, electron transport is more pronounced along the edges of a graphene sheet than along the plane, and very fast electron conduction occurs along its edges, resulting in high electrical conductivity

147-149.

A

fundamental aspect of graphene’s electrochemistry is the flexibility of its constituent sheets which help improve flexibility of the overall electrode structure, as compared to bulk carbon or graphite electrodes that are inherently brittle and experience rapid degradation

147, 150.

The average surface area of graphene is approximately 2600

m2g-1; comparatively, CNTs surface areas are approximately 1300 m2g-1

151.

Also,

graphene contains more electrochemical isotropy than its bulk precursors i.e., electrochemical active sites are more evenly spread out throughout its surface than 22 ACS Paragon Plus Environment

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graphite or carbon black 87. This beneficial property enhances electronic conductivity during electrode reactions

87.

Oxygen-containing groups at the edges of graphene sheets strongly influence graphene electrochemistry

152-153.

However, results have been mixed regarding

whether these groups enhance or suppress electrochemical effects. Chou et al. showed that terminal oxygen-containing groups (with carboxylic group addition) tend to enhance electrochemical properties by improving the rate of heterogenous electron transfer

154.

Heterogenous electron transfer in graphene is also related to defects and impurities in the graphene structure formed or obtained during its fabrication

155.

Because the

electrochemical behavior of graphene is similar to the behavior of bulk graphite, graphene obtained by mechanical exfoliation usually improves electron transport at the edge of the plane rather than in the basal direction

155-156.

Doping also strongly influences resultant electrochemical properties of graphene. Nitrogen, boron, sulfur, and hydrogen are common non-metallic dopants used to tailor electrochemical properties

155, 157.

For example, electron-donating dopants

such as nitrogen enhance heterogenous electron transfer, especially towards a ferro/ferricyanide redox system, and strongly catalyze the reduction of H2O2 on the graphene surface

155, 158-159.

However, doping with electron-withdrawing agents

tends to lower the activity and electron transfer of graphene towards the ferro/ferricyanide system

160.

Metallic impurities (e.g., Fe, Ni, and Co), which are

often remnants from fabrication processes, have been shown to significantly enhance electrocatalytic properties of the parent graphene material

161-162.

Intercalation of alkali metal-ion essentially involves adsorption on the surface or in between the adjoining layers, which thereby undergo expansion and consequent mechanical strain

163.

This electrochemical working principle of graphene is

demonstrated in the form of a schematic Figure 6. This expansion/strain can be corelated to the galvanostatic discharge (GCD) curve and during (in-situ) structural characterization.



An important thing to note here is the first cycle irreversibility that occurs in graphene. This can be attributed to the solid-electrolyte interface that is formed on its surface (as a result of its large surface area) and also because of irreversible products as a result of side reactions164. Na C

a

i

iii

ii

iv

b

(004)

(005)

(006)

(005)

1.4

iv (004)

(003)

c

iii

ii

i

1.2 1.0

3.80Å

0.8

4.50Å

0.6 3.86Å

0.4

30 m 180 s 120 s

Intensity(a.u.)

Potential vs Na/ Na+ (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 64

90 s 75 s 60 s 45 s 30 s 15 s 10 s 5s 0s

0.2 0.0 0.0

40 60 20 80 Intercalation percentage(NaxC)

100

22

i

23

24

25 28 29 Degree (2θ)

30

31

ii

Figure 6: Electrochemical energy storage in graphene and its structural relationship. (a) (i) indicates the generic intercalation of alkali metal ions in the graphene sheets demonstrating how the individual sheets coupled with high-surface area can adsorb a larger number of alkali ions on its exposed surfaces (ii), (iii) and 24 ACS Paragon Plus Environment

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(iv) are the thermodynamically preferred and equivalent sites at where the metal ions are usually adsorbed and/or intercalated on the graphene crystal lattice; usually the interstices or the “bridge” locations. (b) (i), (ii), (iii) and (iv) show high quality high resolution in-situ images of the few-layered graphene film undergoing progressive mechanical strain with continual sodiation which corresponds to a color change from dark to vibrant bright. Reproduced with permission from ref 165. Copyright 2015 American Chemical Society. (c) (i) represents the corresponding GCD discharge curve with the schematics indicating increase in interlayered spacing. With greater Na ion intercalation, the progressive increase in lattice parameter is shown alongside, thereby corroborating the observation in figure (a). Reproduced with permission from ref 166. Copyright 2015 Royal Society of Chemistry. (ii) represents the shift in the XRD peaks as well as peak broadening due to progressive intercalation, which has enlarged the lattice as shown in the previous figures. Reproduced with permission from ref 167. Copyright 2015 Royal Society of Chemistry.

Review of graphene anodes for beyond LIB Graphene and its derivatives have attracted significant attention as anodes in nonLi rechargeable metal-ion batteries. Some of the more important results have been summarized in this section. and a more exhaustive list of graphene’s applications as anode in non-Li systems (Na, K, Mg etc.) is provided in Table 2. Pristine graphene as anode has been investigated by Ramos and co-workers

168.

For

a sodium ion system, capacities of 15.9 μAh cm-2 has been observed after 80 cycles of operation

168.

Graphene-like 2D nanosheet frameworks of carbon as anodes for

NIB have been studied by Ding et al

169.

This nanosheet morphology was prepared

by carbonization of organic peat moss (at inert environments at various temperatures of 600, 900, 1100 ⁰C etc.) followed by pyrolysis. The best performance was obtained for the sample carbonized at 1100 ⁰C, with 100 % coulombic efficiency and a specific capacity of 255 mAh g-1 after 210 cycles of operation


169.

Wen et al.


studied expanded graphite as anode materials in NIBs

Page 26 of 64

170.

The expanded graphite

sheets were prepared by a process of oxidation and partial reduction 170. A reversible specific capacity of 284 mAh g-1 was obtained at a current density of 20 mA g-1 with a capacity retention of 73.92 % after 2000 cycles of operation

170.

Likewise, for a

potassium ion battery, pure mechanically exfoliated graphene sheets as anodes have yielded a maximum of 230 mAhg-1 and a capacity retention of 66 %

171.

Doped graphene as anode material for NIBs has been studied by several groups 173.

172-

Xu et al. used nitrogen-doped graphene as a novel anode material for their NIB

system and proposed that the addition of N atoms improves the overall electrochemical performance of the cell

172.

A maximum reversible capacity of 836.2

mAhg-1 was obtained for the first cycle in the cycling window of 0.02 V – 3.0 V and capacity drop in capacity was attributed to progressive formation of a solid electrolyte interface (SEI) layer

172.

Zhang et al. studied an amorphous phosphorus-

based N-doped graphene electrode for applications in NIBs

173.

They have shown

extraordinary stability of the system due to the three electrode components preventing bulk volume expansion and keeping the structure intact over the cycling process, with Coulombic efficiencies as high as 98 % from the second cycle on

173.

Wang et al. studied N-doped porous 2D carbon nanosheets as anode materials for NIBs

174.

They used a polymerization technique to prepare the 2D nanosheets using

graphene oxide and pyrrole as the precursors. N doping was conducted in a KOH environment at elevated temperatures (800 ⁰C)

174.

They demonstrated a reversible

specific capacity of 155.2 mAhg-1, even after 260 cycles of operation, which they attributed the superior properties obtained from N-doping

174.

Ling and Mizuno

performed a first-principles analysis of boron-doped graphene as anode materials in NIBs

175

Their results indicated that the intercalation of sodium tends to preserve

the structure of graphene as well as a sodiation potential of 0.44 V

175.

A theoretical

maximum capacity of 762 mAhg-1 was hypothesized, and intercalation of sodium ions was shown to occur best for boron-doped graphene of stoichiometry BC3


175.

Page 27 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Few-layered graphene as an anode for KIBs has been studied by Share et al.

176.

They demonstrated that the doping of nitrogen between graphene sheets results in a maximum theoretical specific capacity of 350 mAhg-1

176.

Defective graphene as an anode material in NIB and Ca-ion battery systems via abinitio techniques has been analyzed by Datta et al

177.

The authors considered the

Stone-Wales (SW) defect and the divacancy (DV) defect, and they comprehensively demonstrated that pristine graphene does not allow substantial adsorption; however, the presence of these defects enhanced intercalation

177.

Specific

capacities of 1071 mAhg-1 and 2142 mAhg-1 were obtained for the NIB and Ca-ion systems, respectively. The authors attributed the enhanced capacities of the defective graphene systems to improved charge transfers that are not possible in pristine cases

177.

A similar first principles study of defective graphene anodes in

Mg-ion batteries was performed by Er et al

178.

They have demonstrated that the

adsorption of Mg atoms on the graphene surface concentrated at regions of high defect density

178.

A theoretical specific capacity of 1042 mAhg-1 has been achieved

for a defect density of 25 %

178.

Reduced graphene oxide (rGO) is another form of graphene which has found applications as standalone anodes and part of composites to enhance the properties of the final anode system. Standalone rGO anodes have been studied by Wang et al and have been able to demonstrate a reasonable specific capacity of 141 mAh g-1 over 1000 cycles at 0.2 C rate179. Also, rGO has been used as a component, part of a composite to improve the interlayer spacing of MoS2 sheets and have provided 575 mAh g-1 at a current density of 100 mA g-1

180.

A graphene/phosphorus hybrid has been as an anode in NIBs have been analyzed by Song et al181. The graphene helps to stabilize the solid electrolyte interface and constraints the large volume change that occurs in when Na+ ions interact with phosphorus. An initial reversible capacity of 2077 mAh g-1 is obtained with 1700 mAh g-1 being retained after 60 cycles at 98 % coulombic efficiency


181.


Figure 7 demonstrates the three prominent results of graphene anodes in Na, K

b

1.6

c

500

Capacity (mAh g-1)

a

Potential vs.K/ K+ (V)

and Mg ion systems. 1.2 0.8 0.4 0.0 0

50

100

150

200

250

100 80

400

60 300

40

200

20

100

0 0

Speciﬁc capacity (mAh g-1)

e

80

100

tion

2.0 2.0

De s

1.0 1.0

odia

1.5 1.5

0.5 0.5 0.0 0.0 00

Capacity (mAh g-1)

400

300rGO 500rGO 700rGO 900rGO

Sodiati o

n

200

300 200

100 100

100 100 200 200 300 300 400 400 500 500 Specific Capacity (mAhg -1)

0 0

2.5 2.0 1.5 1.0

20

40

60

80

100 120

100

200

300

400

500

600

700

Cycle number

800

900

1000

100

Capacity (mAh g-1)

Potential vs.Mg/ Mg2+ (V)

i

3.0

0.5 0

Capacity (mAh cm-2)

h

60

Cycle number

300

2.5 2.5


g

40

f + Potential Na+ (V) Voltage vs.Na/ (|V| Vs Na/Na )

d

20

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 64

80

60

40

140


0

5

10

15

Cycle number

20

25

30

Figure 7: Electrochemical performance of graphene anodes in K, Na and Mg ion systems. (a) TEM image of the few layered graphene (FLG) sample showing the layered structure at a scale bar of 5 nm. (b) GCD curves for the FLG and the NFLG sample at a current density of 100 mA/g demonstrating maximum capacity being obtained at low voltages of ~0.2 V. (c) capacity retention plot showing the N-FLG samples perform better with both showing almost 100 % coulombic efficiency. Reproduced with permission from ref 176. Copyright 2016 American Chemical Society. (d) HRTEM images graphene sheets showing their 2D layered morphology (e) 1st cycle GCD curves for the rGO samples annealed at 300 ⁰C, 500 ⁰C, 700 ⁰C and 900 ⁰C in Ar environment demonstrating a 1st cycle charge capacity of 115 mAh g-1 at a current density of 100 mA g-1. (f) Charge capacity retention results of the rGO sample annealed at 500 ⁰C at 100 mA g-1. The inset shows the magnified results when the cell was cycled at current densities of 20, 30, 28 ACS Paragon Plus Environment

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50, 80, 100 and 20 mA g-1 for every two cycles. Reproduced with permission from ref 182. Copyright 2014 American Chemical Society. (g) TEM micrograph of the graphene showing the layered morphology. (h) GCD curves for the sample at varying current densities with maximum specific capacity (110 mAh g-1) being obtained at 10 mA g-1. (i) High coulombic efficiency (∼ 100%) with modest specific capacity (80 mAh g-1) but almost 100 % retention indicating electrode stability. Reproduced with permission from ref 183. Copyright 2015 Wiley. Table 2. Performance of graphene anode, fabricated by top down processes, in various non-Li metal ion rechargeable systems. Anode morphology

Fabrication process

Electroly te chemistr y

Voltage range

Performa ncea

Ref .

Na-ion Single layered graphene

CVD on Cu foil substrate

1M NaPF6 in EC:DEC (1:1)

0.003 V – 2.80 V

15.9/20/5b

168

Porous multilayered graphene

Template based technique with ethanol decomposition

1M NaClO4 in PC:DEC:F EC (1:1:0.05 )

0.00 V – 3.00 V

392/100/1 00

184

Crumpled graphene paper

Treatment in water followed by vacuum filtration

1M NaClO4 in PEC

0.005 V – 2.50 V

125/500/1 000

185

Exfoliation followed by milling

1M NaClO4 in EC:DEC:F EC (1:1:0.10 )

0.00 V – 2.00 V

1706/60/2 60

181

Graphene/P stacks



Page 30 of 64

Graphene/SnS2 stacks

Exfoliation in Li media followed by hydrothermal treatment

1M NaClO4 in EC:DEC (1:1)

0.01 V – 2.50 V

618.9/100 /200

186

Graphene nanosheets/Fe2O3

Sonication followed by heat treatment

1M NaPF6 in EC:DMC (1:1)

0.05 V – 3.00 V

400/200/1 00

187

Graphene/Co0.85S e nanosheets

Vacuum filtration method

1M NaCF3SO3 in diglyme

0.01 V – 2.50 V

180.7/100 /500

188

Porous graphene/SbOx

2 step wet chemical method

1M NaClO4 in EC:DMC (3:7)

0.01 V – 3.00 V

350/100/5 0

189

Graphene/TiNb2O7

Surfactanta ssisted solution based technique

0.01 V – 3.00 V

200/70/20 0

190

N doped graphene sheets

Chemical polymerization

0.01 V – 3.00 V

155.2/260 /50

174

N rich graphene

Sacrificial template technique

0.01 V – 3.00 V

250/250/5 0

191

N and S doped graphene sheets

Sol-gel technique followed by controlled sintering

0.01 V – 3.00 V

289/100/1 00

192

N doped graphene/NaTi2(P O4)3

Reaction in presence of Ti precursor, NH4H2PO4 and CH3COONa in

1.50 V – 3.00 V

75/200/20 Cc

193

1M NaPF6 in EC:DMC (1:1) 1M NaPF6 in EC:DMC (1:1) 1M NaClO4 in EC:PC:FE C (1:1:0.05 ) 1M NaClO4 in EC:PC:FE C (1:1:0.02 ) 1M NaClO4 in EC:DEC (1:1) 30

ACS Paragon Plus Environment

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water followed by calcination

B doped graphene sheets Defective graphene

First principles study First principles study

N.A.

N.A.

Average sodiation voltage of 0.44 V

762d

175

N.A.

1450 for 25 % defects in Nad

177

K-ion Expanded graphite/graphene sheets

As-obtained

1M KFSI in EC:DEC (1:1)

0.01 V – 3.00 V

235/200/1 0

194

Graphene sheets

Coated pencil trace on filter paper

0.8M KPF6 in EC:DEC (1:1)

0.00 V – 2.00 V

200/350/4 00

171

0.8M KPF6 in EC:DMC (1:1)

0.01 V – 3.00 V

190/500/5 00

195

0.8 M KPF6 in EC:DEC (1:1)

0.01 V – 3.00 V

800/50/10 0

196

N.A.

2900 for 25 % defects in Cad

177

N doped few layer graphene (N-FLG)

F doped graphene

Defective graphene

Mechanical milling followed by sintering of coal tar and dicyandiamide precursors High temperature solid state synthesis First principles study

N.A.

Mg/ Graphene/FeVO4.0 .9H2O

Wet chemistry technique

1M MgSO4

-1.00 V – 0.00 V

100/50/10 0

197

Graphene/MoS2

Filtration and exfoliation

0.4M (PhMgCl)2

0.00 V – 2.20 V

80/50/20

198



Page 32 of 64

-

/AlCl3/THF Few layered graphene

First principles study

N.A.

N.A.

116d

199

Defective graphene

First principles

N.A.

N.A.

1042d

178

aThe

cycling data is in the form Specific capacity (mAhg-1)/cycle number/current density (mAg-1). bCapacity cCurrent dFirst

is in μAhcm-2.

density is in the form of C rate.

principles study, value indicates capacity in mAhg-1.

Phosphorene The electrochemistry of phosphorene has been studied less extensively than its counterpart, graphene. Black phosphorus contains multiple layers that are held together by van der Waal’s forces of attraction

200.

Although phosphorene has

demonstrated a high theoretical specific capacity of 2596 mAhg-1, especially for Li ion intercalation, it undergoes a large structural change, resulting in loss in structural fidelity and capacity drop

201.

Li and coworkers have employed a

graphene-BP-graphene sandwich anode morphology (fabricated by a solvothermal approach) to overcome this problem, also in Li-ion systems, and have been able to report a reversible capacity of 1401 mAhg-1 after 200 cycles at a current density of 100 mAg-1

202.

A density functional theory-based first-principle study on sodium and

magnesium intercalation/deintercalation in phosphorene was performed

203-204.

The

authors demonstrated that for an arbitrary Na+ ion concentration in the environment, adsorption of Na+ ions on both sides of the phosphorene sheet (double sided) is more favorable than on one side (single sided) because the repulsion between adjacent Na+ ions can be reduced

203.

The authors also proposed, based on

their theoretical study, that the diffusion of Na+ ions in the phosphorene structure is mostly anisotropic in nature due to the puckered structure of phosphorene sheets 203.

Na+ ions can more readily diffuse into the phosphorene sheets than Li+ because 32 ACS Paragon Plus Environment

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the energy barrier for Na+ ion diffusion is approximately half that of Li+ despite its larger size

204.

Chowdhury et al. demonstrated theoretically that the addition of

hexagonal boron nitride (h-BN) as a capping agent maintains the expansion of the out-of-plane lattice constant to as low as 2 %, thereby helping maintain structural integrity

205.

Two Na diffusion sites are typically preferred in phosphorene: the H

site and the B site. The H site typically demonstrates the most thermodynamic stability

203.

The intermediate phases that are formed due to progressive Na+ ion

intercalation range from NaP8 to a 100 % intercalated NaP2. These phase changes occur continually during the galvanostatic charge-discharge process and thereby affect the trajectory of the discharge curve at every stage

76, 206.

It is to be noted

that the work by phosphorene too demonstrates a first cycle irreversibility, like graphene. This loss is almost inevitable and has been attributed to the solidelectrolyte interphase that forms. However, once the electrode stabilizes, fairly high coulombic efficiencies (∼94 – 95 %) and good capacity retention is observed in the subsequent cycles207-208. Figure

8

denotes

a

schematic

that

represents

essentially

the

entire

electrochemistry of phosphorene, with the preferred intercalation sites, intermediate phases formed, and its consequence as seen in the GCD curve as well as observed during structural characterization.



a

Na

Phosphorenelayers

H site

T site

B site

iv

iii

ii

i NaP8

NaP2

NaP4

v

vi

vii

b

1.2

K Na

1.0 0.9 0.6 0.3 0.0 0

PL2/ 3 edge

Counts(a.u.)

Voltage(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 64

150 300 450 600 750 Specific capacity (mAh g-1)

900

i

150

300 Na K1 edge

1050

1200 Energy loss(eV)

1350

ii

Figure 8: Structure-property relationship of phosphorene electrochemistry. (a) (i) is the schematic representation of generic alkali ion (e.g. Na+) intercalation in phosphorene sheets. (ii), (iii) and (iv) are the thermodynamically equivalent preferred sites for ion storage, namely the H, B and T sites respectively i.e. the alkali ions can migrate to any of these sites without any added energy expenditure. Reproduced with permission from ref 209. Copyright 2015 Royal Society of Chemistry. (v), (vi) and (vii) represent intermediate phases possible for Na intercalated phosphorene: NaP8, NaP4 and NaP2 due to continual Na+ ion intercalation into the phosphorene lattice. Reproduced with permission from ref 210. 34 ACS Paragon Plus Environment

Page 35 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Copyright 2015 Royal Society of Chemistry.

(b)(i) is the characteristic

galvanostatic discharge curve for the phosphorene sample with the phase changes corresponding to progressive alkali metal-ion (Na, K) intercalation. It is seen here that the most stable regime is around ~0.28 V for Na+ ions and ~0.58 V for K+ ions. Reproduced with permission from ref 211. Copyright 2018 American Chemical Society. (ii) is a representative EELS spectra of the intercalated phosphorene with the Na K1 edge visible at approximately 1100 eV. Reproduced with permission from ref 207. Copyright 2016 American Chemical Society. Review of phosphorene anodes for beyond LIB The number of experimental works on phosphorene as anodes in non-LIBs has been relatively few. Studies have primarily focused on NIBs, a few of which are briefly summarized below. First principles study by Kulish and coworkers have demonstrated that the maximum amount of Na ion adsorption results in a stoichiometry of NaP with a theoretical capacity of 865 mAhg-1, another intermediate stoichiometry of NaP2 demonstrates a theoretical specific capacity of 433 mAhg-1

210.

Experimentally, pristine phosphorene’s superior electrochemical performance has been studied by Nie et al. They produced a maximum first-cycle discharge and charge capacities of 2631 mAhg-1 and 2025 mAhg-1, respectively, and a Coulombic efficiency of approximately 77 %

207.

Their cell showed no apparent capacity fading

for the first 30 cycles, even at high current densities of 100 mAg-1 and rate capability studies have shown specific capacities as high as 850 mAhg-1 at current densities of 2500 mAg-1 207. Sun et al. studied phosphorene coupled with graphene as a high capacity anode material

212.

It is noted that alloying results in the formation an intermediate Na3P

component which provides the bulk of the specific capacity

212.

Results indicated

that sodiation occurs via a two-step process involving intercalation and alloying mechanisms and a high reversible specific capacity of 2440 mAhg-1 at a current density of 50 mAg-1 and a specific capacity retention of 85% after 100 cycles of 35 ACS Paragon Plus Environment


operation was obtained

212.

Page 36 of 64

Similarly, 4-nitrobenzene diazonium (4-NBD) assisted

black phosphorus covalently functionalized on to graphene has been studied by Liu et al as anodes for sodium batteries

213.

A specific capacity of 1472 mAhg-1 has been

realized after 50 cycles at a current density of 0.1 Ag-1 with coulombic efficiencies around 100 %

213.

Black phosphorus/carbon composite anodes in KIB systems have been studied by different groups. A black phosphorus-carbon (BP:C) nanocomposite provided a capacity of initial capacity of 1300 mAh g-1 for a sodium half-cell. It was observed that Na3P was an intermediate during the cycling214. Significant capacity fading has been reported though, and the final stable capacity range for the sodium half-cell was approximately 400 mAh g-1 at a voltage range of 0.33 V – 2 V Na/Na+214. Along similar lines, a first cycle capacity of 617 mAh g-1 has been obtained with a retention of 270 mAh g-1 after 50 cycles of operation for a composite with the BP:C weight ratio 1:1208. Figure 9 shows the results of some prominent works using phosphorene/black phosphorus anodes. Also, along with these, a list of more results employing phosphorene as the anode in non-Li metal ion rechargeable batteries, both theoretical and experimental is provided in Table 3.


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Figure 9: Performance of phosphorene/black phosphorus as anode material in Na and K ion systems. (a) TEM micrograph of monolayer and bilayer phosphorene demonstrating the with the scale bar representing 2 μm. (b) GCD curves of the P/G hybrid anode material with the 1st, 2nd and 50th cycles shown with a smooth discharge around ~ 0.75 V for the 1st cycle which then drops to ~ 0.5 V from the 2nd cycle onwards. (c) Capacity retention with varying C:P ratio indicating maximum retention obtained at high C:P ratios. It is also noticed that a C:P ratio higher than 2.78:1 has no real advantage over capacity retention. Reproduced with permission from ref 74. Copyright 2015 Nature Publishing Group. 37 ACS Paragon Plus Environment

(d) TEM


Page 38 of 64

micrograph of black phosphorus (BP):C at 1:1 weight ratio. (e) GCD curves for the 1st, 2nd and 25th cycle for the BP:C 1:1 ratio sample demonstrating maximum capacity being delivered around ~1.1 V – 0.1 V. (f) Capacity retention plots for the various BP:C samples, best retention being obtained for the 1:1 ratio sample. Reproduced with permission from ref 208. Copyright 2017 Royal Society of Chemistry. (g) HRTEM image of black phosphorus (BP)/C composite showing its crystallinity. (h) GCD curves showing the cell cycling for the 1st, 2nd and 100th cycle and maximum capacities being obtained at low working voltages around ~ 0.4 V. (i) Plot demonstrating very high capacity retention and an excellent Coulombic efficiency of around 97.2 %. Reproduced with permission from ref 215. Copyright 2017 Wiley. Table 3. Performance of phosphorene anode, prepared by top-down techniques, in various non-Li metal ion rechargeable systems. Anode morphology

Fabrication process

Electrolyt e chemistr y

Voltage range

Performan cea

Ref .

0.00 V – 2.00 V

1500/25/12 5

216

0.00 V – 1.50 V

2400/100/0 .02Cb

74

0.005 V – 1.5 V

1500/100/1 00*

215

Na-ion

Layered Black phosphorus

P/G hybrid

P/C composite layers

High pressure and temperature treatment on red P Liquid exfoliation followed by vacuum evaporation Mechanical milling

1M NaPF6 in EC:DEC (1:1) 1M NaPF6 in EC:DEC:F EC (1:1:0.1) 1M NaClO4 in EC:PEC:FE C (1:1:0.1)


Page 39 of 64 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P/G hybrid layers

Phosphorene

First principle study

N.A.


Sodiation between 0.29 V – 0.70 V

372c

217

N.A.

433 for NaP2 stoichiometr yc

210

0.01 V – 2.00 V

400/50/50

208

218

76

N.A.

K/Mg/Ca-ions Layered black phosphorus

Mechanical milling

0.75M KPF6 in EC:DEC (1:1)

Phosphorene


N.A.

N.A.

410 for 50 % Mg intercalatio nc

Phosphorene


N.A.

N.A.

310.71 for Mg0.5Pc

aThe

cycling data is in the form Specific capacity (mAhg-1)/cycle number/current density (mAg-1). bCurrent cFirst

density is in the form of C rate.

principles study, value indicates capacity in mAhg-1.

* Specific capacity calculation based on net weight of P/G composite. TMDs TMDs in bulk form have been studied as energy storage materials in LIBs since the 1970s, but their application as 2D anode materials has increased only recently with the advent of superior fabrication techniques and dimensionality manipulations 220.

219-

TMDs exhibit a range of properties that make them ideal candidates for

electrochemical energy storage: MoS2 and WS2 are semiconducting, HfS2 exhibits insulating properties, VS2 and TiS2 are semi-metals, and TaSe2 and NbSe2 are superconductors221-222. 2D TMDs are more desirable energy storage materials than



Page 40 of 64

their bulk counterparts because improved intercalation/ionic adsorption can be achieved on the loosely bonded sheets in the 2D form223. MoS2 has been the most studied 2D TMD, and its advantageous electrochemistry is a result of its electrochemically responsive edge plane and basal plane

62.

The

intercalation of a metal ion (Na+ or K+) in the MoS2 structure results in a phase change from 2H to 1T form at approximately 1.1 V, resulting in a discharge plateau in that vicinity and revealing a metallic nature. However, the H phase has been shown to be stable for VS2, NbS2, and MoS2, whereas the T phase provided more stability in TiS2 and NiTe2

221.

Possible adsorption sites for Na ions are typically

hollow face centered cubic sites in the TMD lattice. Figure 9 shows the mechanism of ionic intercalation in TMDs, the van der Waal’s interaction between corresponding layers, and the preferred location of insertion of intercalating ions in the TMD lattice 224-225.

With progressive alkali metal-ion (e.g. Na+) intercalation in the MoS2 lattice,

an initial phase change occurs from 2H to 1T, followed by intermediate 1T phases e.g. Na0.75MoS2 and Na1.0MoS2. Finally, at a stoichiometry of Na1.75MoS2, the structure undergoes a breakdown with the breakage of the MoS2 bonds 224. The GCD curve follows accordingly and the experimental results match up well with the simulated data. A graphical/schematic representation of this entire process is illustrated in Figure 10.


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Figure 10: Structure-electrochemistry relation of layered TMDs. (a) (i) Intercalating sodium ions into layered MoS2 (representative 2D TMD) structure. Reproduced with permission from ref 224. Copyright 2017 Elsevier. (ii) and (iii) represent the conversion of the 2H MoS2 to its 1T form due to a complex of thermodynamic requirements and mechanical stress due to atomic sliding motions. (iv) shows a HRTEM image with the 2H and 1T variants due to sodiation, the two phases being clearly distinct along with the grain boundary. Reproduced with permission from ref 226. Copyright 2014 American Chemical Society. (v), (vi), 41 ACS Paragon Plus Environment


Page 42 of 64

(vii) represent the intermediate intercalated variants of the MoS2 structure with increasing amounts of sodium causing greater lattice distortion. Some of the intermediate phases that are possible are the Na0.375MoS2 (2H), Na0.75MoS2 (1T) and Na1.0MoS2 (1T) (viii) with large amounts of Na insertion i.e. for the Na1.75MoS2 phase, the bonds break and the lattice ruptures and the Na ions “clump together”. (b)(i) represents these changes in a simulated and an experimental GCD curve, with the phase changes and the intermediate phases labeled along the way. It is seen that the phase change from 2H – 1T happens pretty early on with alkali ion intercalation, indicating the intercalation provides the energetics necessary for the phase change. Reproduced with permission from ref 224. Copyright 2017 Elsevier. (ii) represents the XRD characteristic spectra of the 2H and the 1T phase with the (002) peak having disappeared in the latter. Reproduced with permission from ref 227. Copyright 2017 Royal Society of Chemistry.

Review of 2D TMD anodes for beyond LIB Application of 2D TMDs as anode materials in non-LIBs must overcome the same drawbacks faced by anodes in LIBs (i.e., large volume expansion, structural degradation, etc.) modify

228-229.

morphology,

Therefore, strategies to engineer the electrode so as to

fabrication

techniques,

or

composition

to

optimize

performance. Out of the different TMDs, MoS2 has been the most widely studied 2D TMD, especially in NIBs in non-LIB systems. Exfoliated TMDs are one of the most promising 2D materials for stability, as noted in preceding sections. Su et al. applied this technique to obtain few-layered MoS2 nanosheets, demonstrating a reversible capacity of 386 mAhg-1 after 100 cycles of operation at a relatively high current density of 40 mAg-1 and indicating the superior performance of the exfoliated sheets

230.

A sodiation capacity of 165 mAhg-1 after

50 cycles at a current rate of 20 mAg-1 was obtained when N-Methyl pyrrolidone (NMP) was as used an exfoliation medium to obtain 2D MoS2


231.

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Composites of TMDs provide multiple advantages. David et al. used a composite of MoS2 and rGO prepared by an acid-based exfoliation technique to demonstrate a sodiation capacity of 230 mAhg-1 at a current rate of 25 mAg-1 after 20 cycles of operation

232.

However, their performance was limited due to poor interaction

between the MoS2 sheets and rGO matrix

232.

Lu et al. studied MoS2/C nanosphere

composites using a facile spray pyrolysis technique

233.

They obtained 390 mAhg-1

after 2500 cycles of operation at high current rates of 1 Ag-1 which has been attributed to the large interlayer spacing (approximately 0.64 nm) of the composite 233.

Li

et

al.

analyzed

a

similar

MoS2/C

composite,

prepared

polymethacrylate-based resin exchange technique for anodes in NIB

234.

using

a

First-cycle

discharge capacities of 784.3 mAhg-1 were obtained at current densities of 50 mAg-1 and Coulombic efficiencies of approximately 75 %. Non-carbonaceous materials were also tried as composites with a uniform TiO2 coating

234.

235.

Ryu et al. fabricated vine-like MoS2 nanoflakes

They demonstrated that although the non-coated

nanoflower samples provided a higher starting discharge capacity of 1168 mAhg-1 than the coated sample (940 mAhg- 1), the latter provided more capacity retention over 30 cycles of operation (64 % compared to 30 % retention for the non-coated sample). The authors attributed superior performance of the coated sample to the uniform coating that prevented sulfur dissolution 235. Ahmed et al. succesfully coated a thin, amorphous, non-interfering HfO2 layer on MoS2 using an atomic layer deposition (ALD) technique

236.

An excellent 91 % capacity retention after 50 cycles

of operation was obtained with a specific capacity as high as 636 mAhg-1

236.

Thin films, monolayers, and nanosheets of MoS2 have been studied as anode materials in NIBs. Zhang et al. fabricated ultrathin MoS2 nanosheets on CNTs using a hydrothermal technique

237.

A maximum specific capacity of 495.9 mAhg-1 was

obtained, and the anode retained approximately 85 % of the initial capacity after 80 cycles of operation at current rates of 200 mAg-1

237.

Li et al. used a CVD technique

to coat carbon on MoS2 nanosheets for application as anodes in NIBs 238. The anodes demonstrated superior stabilities and rate performances: 500 mAhg-1 at 0.1 Ag-1 for



Page 44 of 64

100 cycles of operation, decreasing to only 351.6 mAhg-1 at 1 Ag-1 currents

238.

Wang et al. used a simple hydrothermal reaction to study MoS2 nanosheets with Ni3S2 for anodes in NIBs

239.

A specific capacity of 568 mAhg-1 was obtained at a

high current rate of 200 mAg-1 with good stability. Xie et al. studied vertically aligned MoS2 nanosheets grown on carbon paper, obtaining specific capacities of 286 mAhg-1 at current rates of 80 Ag-1 after 100 cycles of operation

240.

Other rechargeable battery systems have been the focus of relatively few studies with 2D TMDs as anodes with respect to NIB systems. Xie et al. studied vertical MoS2 “nanorose” patterns grown on graphene in a K ion battery

241.

High capacities

of 679 mAhg-1 at a current rate of 20 mAg-1 were obtained with significant retention over 100 cycles of operation. The authors attributed the good performance to the increase in interlayer spacing of the MoS2 (due to its roselike pattern) and its composite formation with graphene

241.

Pereira and Miranda performed theoretical

studies for MoS2 and WS2 nanotubes as anodes for MIBs, demonstrating that the binding energy and voltage profiles closely mimic LIB systems

242.

The authors

pointed out that ionic migration channels were also very similar to their Li counterparts. Working voltages of 0.63 V and 0.15 V were obtained for bulk MoS2 and WS2, respectively, for the Mg ion systems

242.

Prominent results for other 2D TMDs in NIBs are provided in Table 4, and Figure 11 shows results of TMD anode performances in NIB, KIB, and MIB systems.


b

c

3.0

Capacity (mAh g-1)

a

2.0

1.0 1st 0.5 0

200

400

2nd 600

800


e

900 400 200 0

1000

f

1.5 1.2 0.9 0.6

0

20

40

60

80

Cycle number

100

100 100

Capacity (mAh g-1)

1.8

Potential vs.K/ K+ (V)

d

1000

80

80

60 60

40 20

40 0

0

20

30

40

50

60


70

2.5

i

2.0 1.5 1.0 0.5 0.0 0

0

20

40

60

80


50

100

Cycle number

0 200

150

80

Capacity (mAh g-1)

h

Potential vs.Mg/ Mg2+ (V)

g

10

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Potential vs.Na/ Na+ (V)

Page 45 of 64

60 40 20 0 0

10

20

30

Cycle number

40

Figure 11: Electrochemical performance of layered TMD (MoS2) anodes. (a) TEM micrograph showing the lattice fringes and the layered structure of the MoS2 nanosheets. (b) GCD curves for the 1st, 2nd, 3rd, 5th, 20th and 100th cycles for the MoS2/Na cell indicating most of the specific capacity being obtained between 1.0 V – 0.5 V. (c) capacity retention curve indicating the exfoliated MoS2 nanosheets having better capacity retention than bulk MoS2. Reproduced with permission from ref 230. Copyright 2014 Wiley. (d) SEM micrograph of the MoS2 sample demonstrating its flake-like nature. (e) Ex-situ XRD showing the phase evolution under charged and discharged conditions. (f) Rate performance of the KIB demonstrating high specific capacities even at high C-rates. Reproduced with 45 ACS Paragon Plus Environment


Page 46 of 64

permission from ref 243. Copyright 2017 Springer. (g) HRTEM image of the MoS2/C composite demonstrating the lattice fringes and layered morphology. (h) 1st and 2nd cycle GDC curves of the MoS2/Mg cell. (i) Capacity retention plots of different morphologies with S3 (nanosheet) sample providing the best results. Reproduced with permission from ref 244. Copyright 2015 American Chemical Society. Table 4. Performance of TMD anodes, prepared by top-down approach, in various non-Li metal ion rechargeable systems. Anode morphology

Fabrication process

Electrolyte chemistry

Voltage Performancea range

Ref.

Na ion MoS2 nanosheets

Ultrasonic exfoliation

1M NaClO4 in EC:PC (1:1)

0.01 V – 3.00 V

386/100/40

245

MoS2 nanosheets

Liquid exfoliation

1M NaClO4 in FEC:PC (1:1)

0.4 V – 2.60 V

161/50/0.02Cb

246

1M NaClO4 in DMC:EC (1:1)

0.1 V – 2.25 V

218/20/25

232

1M NaClO4 in EC:DEC:PC (1:1:1)

0.01 V – 2.90 V

280/300/1Cb

247

Exfoliation and restacking

1M NaCF3SO3 in diethylene glycol

0.4 V – 3.00 V

150/70/0.05Cb

244

Pyrolysis

1M NaClO4 in PC

0.1 V – 3.00 V

369/50/0.1C

248

MoS2/graphene sheets

MoS2/C nanosheets

MoS2_PEO nanocomposites MoSe2 nanoplates

Exfoliation in superacid followed by sonication and vacuum filtration Solid state process followed by template removal


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WSe2

WSe2/C

As-obtained

1M NaPF6 in PC:EC:DEC (1:1:0.05)

0.10 V – 2.50 V

117/30/0.1Cb

249

Solid-state

1M NaPF6 in EC:DMC:FEC (1:1:0.05)

0.01 V – 3.00 V

270/50/0.2Cb

250

0.5M KPF6 in PC:EC (1:1)

0.50 V 2.00 V

65/200/50

243

AlCl3-PhMgCl

0.01 V – 2.10 V

80/50/20

251

K/Mg ions

Layered MoS2

Sonication

Layered MoS2/rGO

Hydrothermal synthesis

aThe

cycling data is in the form Specific capacity (mAhg-1)/cycle number/current density (mAg-1). bThe

current density are in C rates which are specified.

Future perspective of emerging battery systems Monovalent systems such as the NIBs and KIBs pose similar advantages and challenges. The necessity for large grid scale storage, long cycle lives (especially in aqueous conditions) and the abundance of these materials have rendered highly promising alternative electrochemical storage systems

252.

One of the foremost

advantages that these systems bring are the enormous cost reduction benefits visà-vis the LIB systems

252.

However, the choice of the negative electrode remains

one of the primary challenges in these systems

252-253.

Hard carbon has been

proposed by Dahn et al as negative electrodes, however their performance is limited by the formation of deposits of Na on the carbon surface leading to safety issues. Graphite, however, acts as a suitable host for K+ ions with intermediates such as KC24, KC36 etc. being reported254. However, graphite anodes for KIB systems suffer from structural degradation due to large volume expansion and does not provide satisfactory long-term performance

255.



Page 48 of 64

Alloying anodes e.g. alloys of P and Sn are another class of negative electrodes that have been tried due to their high initial capacities, however large volume expansions experienced due to the inherent alloying process results in capacity degradation 257.

256-

Low coulombic efficiencies in the first cycle of these types of anodes are another

problem which need further analysis

253.

Pure Sn foils have been studied as anode

materials for KIB systems as well, here too the problem of large volume expansion and anodic degradation persists and have been reported to be worse than NIBs and LIBs258. Oxides, especially those of transition metals have been studied by several groups. Low coulombic efficiency and rapid decline in capacity values have been the major impediments259. Therefore, for both NIBs and KIBs, the perspective ahead lies in transitioning to anode systems with enhanced electrochemical activity and stability which will also be able to provide a reasonable energy density and at the same time in improving reversibility. Multivalent systems like CIBs and MIBs have also attracted attention owing to their ability to carry greater charge. In fact, CIB systems have been studied in more detail than MIB systems

46.

CIB systems also demonstrate improved kinetics as compared

to MIB systems. As for MIB systems, they have been theoretically predicted to exhibit energy densities greater than 100 Wh kg-1 which is much higher than those of Pb-acid systems currently in use

260.

However, both these systems have

considerable drawbacks. Low reversibility and overall poor kinetics limit their applicability. Especially for MIB systems, the reaction kinetics are of significant concern and the choice of electrode and electrolyte is of utmost concern

261.

It is expected that 2D materials with their superior characteristics; high stability, enhanced surface area and improved electrochemical kinetics, reversibility etc. will greatly improve the performance of these emerging metal ion systems. Also, with greater research in 2D materials, scalable and economic fabrication techniques could


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be developed which may further enhance suitability of 2D materials in CIB and MIB for energy storage. Conclusion The ever-growing global energy demand requires the development of safe, reliable energy

storage

systems

comprised

of

readily

available,

inexpensive

and

environemntally beingn materials. Despite their current flaws, LIBs offer high energy densities that make them indispensable for applications in portable electronic devices. On the other hand, non-LIBs are cost effective for large-scale grid storage. Na+, K+, Mg2+ and Ca2+ ion systems all have unique advantages and disadvantages. Conventional anodes have not allowed these systems to realize their full potential in part due to the relatively large size of these metal-ions that cause reduced ionic reversibility and structural damage and because of poor ion-conductivities. As a result of their typical layered structure and appreciable interstitial spaces, 2D materials can host the large mono and multivalent metal-ions. In addition to their many advantages, however, 2D materials also present their own set of challenges, some of which have been discussed in this paper. Insufficient capacity, lowered kinetics, and high polarizability are current obstacles that must be addressed before 2D materials can experience widespread application. Tailoring the microstructure, engineering the electrode design, and manipulating the electrode chemistry are techniques that have been adapted to overcome these issues. If these problems are resolved and the full potential of 2D materials are realized, then an opportunity for robust growth of the fledgling non-LIB industry will emerge, making electrochemical energy storage safer, more reliable, less expensive, and more environmentally friendly. Funding: This work was supported by the National Science Foundation Grant number 1454151.



Conflict of interest: The authors have no competing conflicts of interest.


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Graphical Abstract


ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

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Two-Dimensional Anode Materials for Non-Lithium Metal-Ion Batteries

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