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Swinburne Research Bank http://researchbank.swinburne.edu.au Author: Barghamadi, Marzieh; Kapoor, Ajay; Wen, Cuie Title: A review on li-s batteries as a high efficiency rechargeable lithium battery Year: 2013 Journal: Journal of The Electrochemical Society Volume: 160 Issue: 8 Pages: A1256-A1263 URL: http://hdl.handle.net/1959.3/351310 Copyright: Copyright © 2013 The Electrochemical Society. The accepted manuscript is reproduced in accordance with the copyright policy of the publisher. This is the author’s version of the work, posted here with the permission of the publisher for your personal use. No further distribution is permitted. You may also be able to access the published version from your library. The definitive version is available at: http://dx.doi.org/10.1149/2.096308jes Powered by TCPDF (www.tcpdf.org) Swinburne University of Technology | CRICOS Provider 00111D | swinburne.edu.au
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Page 1: Swinburne Research Bank

Swinburne Research Bankhttp://researchbank.swinburne.edu.au

Author: Barghamadi, Marzieh; Kapoor, Ajay; Wen, CuieTitle: A review on li-s batteries as a high efficiency

rechargeable lithium batteryYear: 2013Journal: Journal of The Electrochemical SocietyVolume: 160Issue: 8Pages: A1256-A1263URL: http://hdl.handle.net/1959.3/351310

Copyright: Copyright © 2013 The Electrochemical Society.The accepted manuscript is reproduced inaccordance with the copyright policy of thepublisher.

This is the author’s version of the work, posted here with the permission of the publisher for yourpersonal use. No further distribution is permitted. You may also be able to access the publishedversion from your library.

The definitive version is available at: http://dx.doi.org/10.1149/2.096308jes

Powered by TCPDF (www.tcpdf.org)

Swinburne University of Technology | CRICOS Provider 00111D | swinburne.edu.au

Page 2: Swinburne Research Bank

*Corresponding author. Tel.: +61 3 92145651; Fax: +61 3 92145050 E-mail address: [email protected] (C. Wen).

A review on Li-S batteries as a high efficiency rechargeable lithium battery

Marzieh Barghamadi, Ajay Kapoor, Cuie Wen*

Faculty of Engineering and Industrial Sciences, Swinburne University of Technology

Hawthorn, Victoria, 3122, Australia

Abstract

Energy production and storage are critical research domains where the demands for improved

energy devices and the requirement for greener energy resources are increasing. There is

particularly intense interest in Lithium (Li)-ion batteries for all kinds of electrochemical

energy storage. Li-ion batteries are currently the primary energy storage devices in the

communications, transportation and renewable-energy sectors. However, scaling up the Li-

ion battery technology to meet current increasing demands is still problematic and issues such

as safety, costs, and electrode materials with higher performance are under intense

investigation. The Li-sulphur (S) battery is a promising electrochemical system as a high-

energy secondary battery, particularly for large-scale applications, due to its low cost,

theoretically large specific capacity, theoretically high specific energy, and its ecofriendly

footprint. The Li-S battery exhibits excellent potential and has attracted the attention of

battery developers in large scale production in recent years. This review aims to highlight

recent advances in the Li-S battery, providing an overview of the Li-ion battery applications

in energy storage, then detailing the challenges facing Li-S battery and current applied

strategies for improvement in its efficiency.

Keywords: Energy Storage; Li-ion Batteries; Li-S Batteries; Advances and Improvements in

Cathode

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*Corresponding author. Tel.: +61 3 92145651; Fax: +61 3 92145050 E-mail address: [email protected] (C. Wen).

Contents

1. Introduction ............................................................................................................................ 3

2. Current Li-ion batteries .......................................................................................................... 4

3. Li-S battery ............................................................................................................................ 8

3.1. Fundamental aspects of the Li-S battery ......................................................................... 8

3.2. Challenges for the Li-S battery ..................................................................................... 10

3.3. Advances in Li-S battery ............................................................................................... 12

3.3.1. Advances in cathode ............................................................................................... 12

3.3.2. Advances in anode .................................................................................................. 17

3.3.3. Development in electrolyte ..................................................................................... 19

3.4. Application of Li-S battery in electric vehicles ............................................................ 21

4. Conclusions and outlook ...................................................................................................... 22

Acknowledgements ............................................................................................................ 23

References .......................................................................................................................... 24

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1. Introduction

An energy economy based on fossil fuels is at a serious risk due to the continued increase

in demand for oil, the depletion of non-renewable resources, and with the rate of CO2

emissions showing a dramatic increase in the last 30 years. This increase has also resulted in

a rise in global temperature and a series of associated climate changes.1 Energy is, therefore,

a vital global issue and attempts are being made to rectify current problems by utilizing new

energy resources. In this regard, electrical energy storage is recognized as an essential

element for both stationary and mobile power equipment.2, 3

Electrical energy storage systems play a crucial role in managing the gap between energy

generation and demand, especially for electricity generation from renewable and sustainable

sources, such as solar and wind, and in portable electronics such as personal computers,

cordless tools, and electric vehicles. There are several technologies available such as

flywheels and compressed air, but batteries are at the forefront of energy storage systems,

especially for electricity.4

Battery Energy Storage Systems (BESSs) have appeared as promising storage

technologies for power applications, offering a wide range of power system applications. The

batteries are made from cells that convert chemical energy to electrical energy and vice versa.

They are rated in terms of their energy and power capacities. The amount of energy per mass

or volume that a battery can deliver is a function of the cell’s voltage and capacity.

Significant development is going on in the battery technology. Different types of batteries

have been commercially developed, while some are still in the experimental stage.5, 6 Safe,

low-cost, high-energy-density and long-lasting rechargeable batteries are urgently needed to

address important issues in energy generation, such as the increased energy consumption of

portable devices.7 Rechargeable batteries are used in portable electronics, power tools,

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electric vehicles (EVs), and in stationary electrical energy storage (EES) for a grid supplied

by wind, radiant-solar, and nuclear power.8 Among all the types of batteries, lithium batteries

have attracted the most attention because the theoretical energy density (both gravimetric and

volumetric) of lithium metal is the highest for all solid electrodes.4 Lithium-ion batteries have

been under intense scrutiny over the past 20 years because of their advantages, such as high

energy density, high operating voltage and low rate of self-discharge.9-11

2. Current Li-ion batteries

Lithium-ion batteries employ lithium storage compounds as the positive and negative

electrode materials. During the battery’s cycling, lithium ions (Li+) exchange between the

positive and negative electrodes. Li-ion batteries have been discussed as rocking chair

batteries because the lithium ions “rock” back and forth between the positive and negative

electrodes as the cell is charged and discharged.12 The working mechanism of a typical Li-ion

battery is presented in Figure 1.

Sony commercialized the lithium ion battery (LIB) first in the early 1990s. Until now,

lithium-ion batteries have offered the most practical solutions to a wide variety of electrical

energy storage applications, such as mobile phones, lap-top computers, MP3s, due to their

high voltage, high energy density, light weight and good environmental compatibility in

comparison to other kind of batteries.9, 13 Lithium ion batteries have also found applications

in satellites14 and in biomedical device clinical trials, such as Neurostimulator, Ventricular

Assist, Artificial Heart.15-17 However, the maximum energy density of current lithium ion

batteries is too low to satisfy the demands of key markets such as transport and they need to

be improved.18 Some of the major challenges facing this kind of battery are as follows: (i)

The obtainable or usable capacity is inadequate (and much lower than the theoretical limit)

and diminishes with the rate of cycling; (ii) The power density is insufficient for the intended

applications, especially in EVs; (iii) The energy efficiency is too low due to large polarization

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losses for charge and discharge, with the situation worse at higher cycling rates; (iv) The

cycling life is limited due to capacity fading with cycling; and (v) The price is high. Overall,

the existing Li-ion batteries often suffer from deterioration in their microstructure or the

architecture of the electrodes accompanied by volume expansion or contraction, phase

transformation, and morphology changes of the active electrode materials during cycling.4 In

addition, safety of lithium metal oxide cathodes is an issue because of their intrinsic thermal

properties. The thermal runaway is caused by the exothermic reactions between the

electrolyte, anode and cathode, with temperature and pressure increasing in the battery.19 For

example, a fully charged lithium cobalt oxide can release oxygen, which oxidizes the solvent

and causes the battery thermal runaway.

In order to improve the energy and power density of LIB, the use of anode materials with

larger capacity and higher Li diffusion rate is required. Different materials such as nano

carbon, alloys and metal oxides have been developed. Nano carbons like graphite can

effectively store Li and improve storage ability due to their high surface area and

morphology. Li anode can be alloyed with some elements such as Sn, Si, Al to improve the

capacity and safety. But this will result in large volume change which causes cracking of the

electrode and capacity fading. Metal oxides like Fe3O4, SnO2 and CuO have been proposed as

potential materials for LIB anode, but their capacity decrease rapidly because of large volume

changes.20 SnO2-nanofiber carbon composite21 and Fe3O4 /reduced graphene oxide

nanocomposite22 were applied as anode to deal with this issue.

The main LIB cathode materials are lithium metal oxides like LiCoO2 which have high

energy density, but they suffer from some intrinsic and safety issues in addition to high cost.

Although using nanostructured lithium metal oxides can solve some problems like low

conductivity, it causes more safety issues due to higher surface area.23 Better stability has

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been achieved by coating the cathode LiCoO2 with ZnO, which suppresses the cobalt

dissolution.24

Li-ion materials are currently the subject of intense research.25 Several countries,

including Japan, United States and European countries, are supporting R&D programs aimed

at solving these problems in order to develop advanced and efficient LIB.26 Indeed

identifying new materials offering a better performance than those offered by the current

common anode and cathode used in rechargeable Li battery is necessary. In general, the

performance of any device depends on the properties of its materials; this is also true for

lithium batteries. Thus, a new generation of rechargeable lithium batteries can only be

achieved by a breakthrough in electrode and electrolyte materials.1

Introducing positive electrode materials that offer higher capacity and improved safety

properties, as well as negative electrode materials with improved specific energy, energy

density, rate capability, and longevity are the main research and development goals in the

lithium battery area. Adopting new electrolytes, additives and electrode material coatings to

improve both the cycle life and calendar life of lithium batteries are also receiving increased

attention.27

Reaching beyond the horizon of rechargeable lithium batteries requires an exploration of

new chemistry, especially electrochemistry, and new materials. Non-lithiated cathode

materials not only exhibit higher specific capacities than lithiated cathode materials, but also

provide enhanced safety as they cannot be overcharged. Rechargeable Li-S is highly efficient

lithium rechargeable battery. Sulphur has one of the highest theoretical capacities for the

cathode of lithium batteries in comparison with all other cathode materials in this kind of

battery. Based on complete reactions with metal lithium to form Li2S, it has a theoretical

specific capacity of 1675 mA h g-1.28-30 Table I compares the different kinds of lithium

batteries.

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Table I. Comparison of lithium batteries.

a The molecular mass of O2 is not included in these calculations. b Based on the sum of the volumes of Li at the beginning and Li2S at the end of discharge. c Based on the sum of the volumes of Li at the beginning and Li2O2 at the end of discharge. d Assuming the product is anhydrous LiOH and alkaline conditions. e Based on volume of ZnO at the end of discharge.

Battery Cell voltage (V)

Theoretical capacity (mAhg-1)

Theoretical specific energy (Wh kg−1)

Theoretical energy density (Wh l−1)

Overall reaction References

Conventional Li-ion 3.80 155 387 1,015 Li(C) + CoO2 ↔LiCoO2 15, 31

Li-S 2.20 1672 2,567 2,199b 2Li + S ↔ Li2S 31, 32

Li–air (non-aqueous) 3.00 3862 11,248a 3,436c 2Li + O2 ↔ Li2O2 31, 33

Li–air (aqueous) 3.20 1861 5,789a 2,234d 2Li + ½O2 + H2O ↔ 2LiOH 31, 33

Zn–air 1.65 820 1,086 6,091e Zn + ½O2 ↔ ZnO 31, 34

Al-air 2.70 2980 8100 NA 4Al + 3O2 + 6H2O → 4Al(OH)3 34, 35

Mg-air 3.10 2200 6800 NA Mg + ½O2 + H2O → Mg(OH)2 34

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3. Li-S battery

The lithium–sulphur (Li-S) battery, which is composed of a sulphur composite cathode, a

polymer or liquid electrolyte, and a lithium anode, is a promising candidate for high energy

systems. Figure 2 presents a schematic configuration of a Li-S battery. It is based on the

lithium/sulphur redox reaction, given by:

16 Li + S8 → 8Li2S [1]

Assuming complete conversion, it has a high theoretical specific capacity of 1675mA h

g−1 and a high theoretical specific energy of 2600 Wh kg-1.36, 37

3.1. Fundamental aspects of the Li-S battery

Sulphur is a promising positive electrode material for lithium batteries due to its high

theoretical specific capacity of approximately 1675 mA h g-1. The Gibbs energy of the Li/S

reaction is more than five times the theoretical energy of a Li-ion system, ~ 2600 Wh kg-1.

The concept of electrochemical energy conversion and storage utilizing sulphur as the

positive electrode in an alkali metal anode battery dates back to the 1960s.38 During recent

decades, there has been strong incentive to develop a rechargeable Li/S battery.39, 40 Many

articles regarding the electrochemical properties of the Li–S cell, such as discharge

capacity,36, 41, 42 cycling,42-44 and self-discharge45 have been published.

Among the various types of rechargeable batteries, this system is a very attractive

candidate, because of its high theoretical capacity, high theoretical power density and wide

temperature range of operation. Moreover, elemental sulphur benefits from advantages such

as natural abundance, low cost, excellent safety due to its intrinsic protection mechanism

from overcharge, and its non-toxicity.46-48

In the nature sulphur exists in more than 30 allotropes, and ring-structural

cyclooctasulphur (S8) is the most stable form.28 In the discharge process of a fresh Li-S

battery, an S–S covalent bond of S8 is first broken to form a chain-structural polysulphide

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(PS) anion (Sx2−, x = 8), and then it is further reduced into Li2S through multistage

reactions.49, 50

The reaction mechanism of a Li-S battery is different from that of commercial secondary

lithium batteries with an intercalation-deintercalation mechanism in lithium metal oxide and

graphite. Reduction of sulphur in a Li-S battery is a multistep electrochemical process that

can be composed of different intermediate species. In general lithium metal reacts with

sulphur (S8) to produce lithium polysulphides with a formula of Li 2Sn. Long chain

polysulphides are produced first, such as Li2S8 and Li2S6, which are shortened during further

reduction of sulphur. The final product of discharge is lithium sulphide (Li2S) and the overall

reaction is given by Eq. 1.39, 51 In this process, sulphur accepts electrons from an open-circuit

voltage (OCV) to 2.1V, forming lithium polysulphide and then lithium polysulphide is

reduced. From the viewpoint of phase transitions, the discharge can be divided into four

stages, as follows:

I: Reaction of elemental sulphur with Li is given by:

S8+ 2Li+ + 2e− → Li2S8 [2]

II: A reaction between dissolved Li2S8 and lithium is described as:

Li 2S8+ 2Li+ + 2e−→ 2Li2S4 [3]

III: A transition from the dissolved Li2S4 to insoluble Li2S2 or Li2S by the coexistence of Eqs.

4 and 5:

Li 2S4+ 2Li+ + 2e−→ 2Li2S2 [4]

Li 2S4+ 6Li+ + 6e- → 4Li2S [5]

IV: An equilibrium reaction of insoluble Li2S2 and Li2S is described as:

Li 2S2+ 2Li+ + 2e− → 2Li2S [6]

Eq. 3, is the most complicated of the four stages because it is affected by both the

solubility of the polysulphides in the electrolyte and the chemical equilibrium between each

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type of polysulphide in the solution. Therefore, this reaction is affected strongly by the type

of electrolyte solvents. The outcome of stage III depends on the competition of Eqs. 4 and 5.

The final discharge products are mainly a mixture of Li 2S2 and Li2S. As Eq. 5, is the

predominant reaction, the Li-S cell has high capacity with slightly lower discharge voltages

and a shorter stage IV. Stage IV is kinetically slow and suffers from high polarization

because of the non-conductive nature of Li2S2 and Li2S. Figure 3 shows the voltage profile of

the first discharge of a sample of Li-S cell.52, 53

As the current density increases, both the discharge capacity and the plateau voltage

decrease. X-ray diffraction analysis of discharged sulphur electrodes proves that at low

current density, only Li2S peaks are displayed while at high current density, both elemental

sulphur and Li2S peaks can be observed. The discharge capacity of the sulphur electrode

greatly decreases after discharging at high current density due to under-utilization of the

active material.54

3.2. Challenges for the Li-S battery

Although the Li-S battery has considerable advantages, it still suffers from a series of

problems that have hindered its practical application. The discharge products precipitate

during the second discharge step, covering the positive electrode surface and causing poor

electrode rechargeability and capacity limitation. This is mainly linked to the passivation of

the positive electrode. In fact the discharge stops when the surface is fully covered by these

insulating species.39, 55 The long-chain lithium polysulfides dissolve into the electrolyte and

migrate to the anode to form lower-order polysulfides by reacting with lithium, and then they

diffuse back to cathode to be deoxidized to a longer-chain. This causes the so-called internal

“shuttle” effect, which leads to the corrosion of the lithium anode and consequently causes

poor efficiency and a short cycle life in rechargeable Li-S batteries. Additionally, the

continuous reaction of the soluble polysulfide to the Li anode leads to significant self-

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discharge and the deposition of solid Li2S2 and Li2S on the cathode, which results in active

mass loss and capacity fading. Some amount of the insoluble Li2S and Li2S2 accumulate on

the Li anode. Another challenge is the volume change accompanied by morphology change

which occurs in the electrode upon the active sulphur dissolution and the final products

precipitation. These problems contribute to the fast aging of electrodes and a quick fading of

the practical specific charge of the battery.56-58

Therefore, the Li-S battery is unsuitable for a high energy density primary battery that is

required to have a long calendar life and service time. Another problem is the very poor

electronic conductivity of sulphur, which causes poor electrochemical contact of the sulphur

and leads to low utilization of active materials in the cathode.3 Thus, a large quantity of

conductive agents is needed when making the sulphur cathode. Compositing elemental

sulphur with carbon50, 59 and conducting polymers60-62 can significantly improve the electrical

conductivity of a sulphur cathode.

The successful development of a Li-S battery, regarded as a candidate for the next

generation of batteries, requires extensive research on the electrochemical behavior under

various operating conditions.36, 63 A lot of research has been conducted to mitigate the

negative effect of the polysulphide shuttle. Much of this work has focused on either the

protection of the lithium anode42 or on the restriction of the ionic mobility of the polysulphide

anions.64, 65 However, since protection of the lithium anode causes a slow reaction rate at the

anode during the discharge cycle due to passivation of the anode, this leads to a loss of power

density in the battery. Gel electrolytes and solid electrolytes have been reported as a means of

slowing down the polysulphide shuttle by reducing the ionic mobility of the electrolytes.64, 66,

67 It is also necessary to introduce conductive additives and strong adsorbent agents with a

large surface area to the cathode. The preparation of the sulphur-conductive polymer

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composites, or sulphur-carbon composites, has been reported as softening the impact of the

shuttle effects.52

3.3. Advances in Li-S battery

3.3.1. Advances in cathode

Based on studies reported in the literature over the last few years, the ideal structure for a

sulphur electrode requires the following characteristics: a closed structure for efficient

polysulphide containment, a limited surface area for sulphur electrolyte contact, sufficient

space to accommodate sulphur volumetric expansion and the small characteristic dimensions

of a sulphur electrode to avoid pulverization, a short pathway for both electrons and Li ions

to achieve high capacity at a high power rate, a large conductive surface area on which to

deposit the insulating Li2S2 and Li2S in order to preserve the morphology of the electrodes,

and suitable electrolyte additives to passivate the lithium surface and so minimize the shuttle

effect.68

The literature reports on different strategies that have been considered to improve Li-S

battery electrochemical performance, mainly focused on the combination of a conductive

matrix with sulphur to form a highly conductive composite. In recent years, carbon-based

nanomaterials, including 0-D fullerene, 1-D carbon nanotube and 3-D graphite, have attracted

a great deal of interest.69, 70

Several research attempts have focused on the development of carbon/sulphur

nanocomposites, in which sulphur particles were embedded in the nanopores of the

conductive carbon matrix. They can increase both the electrical and ionic conductivity of the

sulphur cathode while at the same time suppressing the polysulphide shuttle phenomenon.40,

55, 71 Using nanostructured sulphur-carbon composite cathodes can considerably improve both

the cyclability of the battery and the utilization of sulphur in the battery cycles. The pores in

this structure not only act as micro-containers for the elemental sulphur that provides

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sufficient contact to the insulating sulphur and promotes the electrical conductivity, but also

facilitates the transport of Li ions during the electrochemical cycling and accommodates the

produced polysulphides and sulphide ions during the electrochemical reactions.3 Preparing a

uniform mixture of carbon and sulphur or obtaining a composite of carbon-encapsulated

sulphur requires multiple processing steps. Some related works will now be discussed.

Researchers have investigated the use of a sulphur cathode containing carbon nanotube

(CNT) in a Li-S battery, and this has demonstrated great cycling stability and coulombic

efficiency.72

Composite cathodes containing a sulphur/acetylene black (AB) composite, in which the

sulphur was embedded inside the nano-pores of the acetylene black, showed a high discharge

capacity and good cycle performance.73

A nano-sized S/PPyA (poly(pyrrole-co-aniline) ) composite delivered a high initial

discharge capacity and acted as a good conductive matrix, a strong adsorbing agent, and as a

firm reaction chamber for the sulphur cathode materials, and it improved both the capacity

and cycling stability of the cathode.60 It has been observed that the cycling life and specific

capacity of the battery are improved when carbon nanofiber (CNF) is added into the sulphur

electrode because CNF provides a good electrical connection and structural stability.68, 74 Ji et

al.75 showed that loading S in to the porous CNF improves the battery performance. It

provides both high conductivity and high surface area for S and reduces the PS solubility.

Also this structure can accommodate Sulphur volume changes. Figure 4 shows the SEM

images for bare Sulphur and CNF-encapsulated sulphur electrodes. Figure 4(a) shows CNF

formed inside the AAO template and Figure 4(b) shows CNF after sulphur infusion and AAO

etching with a weight ratio of 3:1 (sulphur to carbon). The energy-dispersive X-ray

spectroscopy (EDS) images in Figures 4(d) and (e) confirm the presence of carbon and

sulphur in the electrode.

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Zhang et al.76 introduced a novel polymer, polyaniline polysulfide (SPAn), which can

hold an appropriate amount of sulphur. The polymer has polyanilline as the backbone chain

and 2 four-member rings with S-S bonds as the side chains of aniline. An ordered

mesoporous carbon (OMC) sphere with uniform channels is applied as the conductive agent

in the sulphur cathode. Sulphur filled the holes of the OMCs by a co heating method. The S-

OMC composite exhibits excellent cycling performance compared to the bare S cathode.46, 77

Applying spherical OMC-S as a cathode material provides a battery with high initial

discharge capacity and cyclability.78 Wang et al.79 used microporous–mesoporous carbon

which has a high adsorption capacity and conductivity as the sulphur immobilizer to provide

a stable cathode and consequently a battery with good cyclability. Also cycle life of Li-S

battery extends with encapsulated sulphur in mesoporous hollow carbon capsules. It showed

91% capacity retention after 100 cycles.58

Recent reports suggest that a multi-wall carbon nanotube (MWCNT) is a promising

conductive material that could improve the cycling performance of the Li-S battery. It

provides the electrochemical reaction sites with a large interface area, between the MWCNTs

and the lithium polysulphides, on which the electrochemical reaction can take place, and

accommodates Li2S and Li2S2 without clogging the pores in the cathode.80-82 Choi et al.50

reported an improved discharge capacity and cycle performance of a Li-S battery with a

carbon coating on the surface of the sulphur, which enhanced the electrical contact and

adsorption of the lithium polysulphide.

A breakthrough for Li-S batteries is the application of a Graphene-S cathode in the

battery.83-88 Graphene is a material that could not be imagined 70 years ago. Research on

graphene is very new, having explosively started in 2004 following the development of a

simple technique to prepare a single-layer graphene sheet.89

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Graphene is single layer of sp2-hybridized carbon atoms found in graphite, known for its

unusual electronic properties and possible applications.90-92 In recent reports, two-

dimensional graphene has been considered as a potential electrode material for battery

applications, due to its superior electrical conductivity, high surface area, and broad

electrochemical window. In comparison to CNT and CNF, it is typically impurity-free and

much cheaper. They have many applications in a variety of industries and research.93-96 An

early example of an improvement achieved with graphene was the production of sulphur–

graphene nano sheets (S-GNS) by heating a mixture of elemental sulphur and synthesized

graphene nanosheets. The S-GNS composite showed a significantly improved capacity and

cycle life compared to the pristine S cathode.83 The use of graphene-wrapped sulphur

particles as the cathode material in a Li- S battery has also been reported.85

A functionalized graphene sheet-sulphur nanocomposite (FGSS) with sandwich-type

architecture has been synthesized and studied as a possible cathode material for Li-S

batteries. The unique composite structure and good conductivity of graphene contributes to

the observed good cycling stability.97. Ji et al.84 investigated on the graphene oxide-sulphur

(GO-S) nanocomposite cathode that displays good reversibility and excellent capacity

stability. This improved performance may be related to the reduction of GO by incorporation

of S which improves the conductivity of the GO, and diminution of Li polysulfides

dissolution due to mild interaction between GO and S.98

The design and synthesis of sulphur cathode coated with reduced graphene oxide (RGO)

has been reported where the carbon framework serves as a conductive layer and

nanoelectrochemical reaction chamber.99 A layer-structured sulphur-expanded graphite (EG)

composite has been employed as a cathode in Li-S batteries to improve the electrochemical

performance.100 The EG maintains a layered structure similar to natural flake graphite with a

higher surface area that provides sufficient contact to the insulating sulphur and improves the

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conductivity. Each layer of EG could be considered as a micro current collector to provide

sufficient electrons for a reaction between the cathode materials and lithium ions. The

development of graphene materials, and their applications in the electrochemical energy

fields, is still in its infancy and many challenges remain.

A cathode should have uniform combination of active sulphur and conductive materials

for better performance. Among the components in the sulphur cathode, the binder plays an

important role in improving cell performance, especially in regards to the cycle life. A binder

should not only have high adhesion between the electrode materials and the current collector,

but should also facilitate electron transport and lithium ion diffusion because of the ability to

form a good electric network between the active material and conductive carbon.37

Kim et al.101 investigated PTFE +CMC and PTFE + PVA as binder in Li-S battery. These

cathodes have larger specific surface area with more contact area between the cathode

materials and the electrolyte, leading to decreased interfacial resistance and consequently

improved capacity. Jung et al.51 studied the mixed polymer binder system of PVP and PEI in

order to maintain the initial morphology of cathode during charge–discharge cycles to

improve the cycle performance of battery. The cycling property of polyethylene oxide (PEO)

and polyvinylidene fluoride (PVdF) binder with a carbon nanofiber is also investigated and

proved that the capacity increased by applying binders.74 Gelatine has been used as a binder

in the sulphur cathode, which is electrochemically stable and functions as a highly adhesive

which stabilize the structure of the cathode and effective dispersion agent for the cathode

materials.37 Table II lists the discharge capacity of improved Li-S batteries by applying

different cathode materials.

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Table II. Comparison the discharge capacity of cathode’s materials used in Li-S battery.

Cathode materials Discharge current rate

Initial discharge capacity (mAhg-1)

Cycle number

Residual reversible capacity (mAhg-1)

Electrolyte References

Sulphur 0.4 mAcm-2 710 50 230 LiCF3SO3-DME-DOL 102

Sulphur 0.1 mAcm−2 400 50 100 LiTFSI in Tetraglyme 103

Sulphur 160 mAg-1 1094 80 <150 LiPF6- EC-DMC 88

S-MPCa 250 mAg-1 1584.56 30 804.94 LiTFSI-DOL-DME 3

S-Carbon 100 mAg−1 1232.5 50 800 LiClO4-DEGDME–DOX 25

S-PPyb 50 mAg−1 1280 20 800 LiTFSI-PEGDME 104

S-Carbon 50 mA g-1 1300 30 700 LiTFSI-EMITFSI 46

S-PEGc-CMK-3d 168 mAg-1 1320 20 1,100 LiPF6-TEGDME 7

S-PANe 0.3 mAcm−2 893 50 600 PVDF Gel Electrolyte 59

S- MWCNT 168 mAg-1 734.7 100 491.5 LiPF6 -EC/DMC 82

SPAn 0.2 mAcm−2 980 20 403 LiCF3SO3 /DOL-DME 76

S-MWCNT 0.1 mAcm−2 485 50 300 LiTFSI in Tetraglyme 103

S-CNF 100 mAg-1 1191 20 700 LiCF3SO3-TEGDME 74

S/PPy–MWCNT 0.1 mAcm−2 1309 100 725.8 LiCF3SO3-TEGDME 80

S–PPy Nanowire 0.1 mAcm-2 1222 20 570 LiCF3SO3-DOL-DME 61

S/T-PPyf 0.1 mAcm-2 1151.7 80 650 LiCF3SO3-TEGDME 69

S-OMC 168 mAg-1 1138 80 800 LiTFSI-DOL-DME 71

S-AB 40 mAg-1 934.9 50 500 LiPF6/ PC-EC-DEC 73

S-PPyA 0.1 mAcm−2 1285 40 866 LiCF3SO3-DOL-DME 60

S-CNF 335 mAg-1 1200 150 730 LiTFSI-DOL-DME 68

S-OMC 0.1 mAcm−2 1265.5 25 800 PEO18Li(CF3SO2)2N–SiO2 77

S-MWCNT 60 mAg−1 700 60 482 LiPF6-EC-DMC-EMC 81

S-GNS 50 mAg-1 1611 40 700 LiTFSI-PEGDME 83

S-GO 168 mAg-1 1320 50 735 PYR14TFSI-LiTFSI- PEGDME

84

S- EG 280 mAg-1 1210.4 50 957.9 LITFSI- DME/ DOL 86

S-FGS 168 mAg-1 950 50 800 LiTFSI-DME-DOL 97

TGg-S-RGO 200 mAg−1 1290 100 928 NA 99

S- EG 25 mAg-1 1588 50 1200 LiClO4-DME-DOL 100

S- GNS 160 mA g-1

1598 80 670 LiPF6-EC-DMC 88

a Mesoporous Carbon b Polypyrrole c Polyethylene glycol d The most well-known member of the mesoporous carbon family e Polyacrylonitrile f Tubular polypyrrole g Thermally exfoliated graphene nanosheet 3.3.2. Advances in anode

The use of elemental lithium as the anode in Li-S batteries remains a major issue due to

safety concerns arising from the formation of lithium dendrites during cycling, which can

penetrate the separator and lead to thermal runaway. One way to avoid this safety problem in

the Li-S system is to use a high-capacity anode material other than elemental lithium. Some

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researchers have investigated this possibility. A novel lithium metal-free battery consisting of

a silicon nanowire anode and a Li2S/mesoporous carbon composite cathode, with high

theoretical specific energy, has been reported.105 He et al.30 designed a Li-S cell with non-

lithiated electrode materials. They used graphite anodes and a non-lithiated sulphur

composite cathode (Sulphur- acetylene black- PTFE) by incorporating lithium metal foil to

provide lithium and so enhanced the performance of the cells.

Li negative electrodes have also been replaced by a Sn–C–Li alloy which demonstrated

higher chemical stability towards sulphides.106 Since lithium is so reactive, the protected Li

anode was introduced to the Li-S battery to enhance the charge/discharge performance by

reducing the growth of the solid electrolyte interface (SEI) layer and suppressing the reaction

between the Li and soluble polysulphides.

A Li negative electrode with a Li–Al alloy layer can increase the cycle life of a battery.107

The protection layer on the Li anode can be prepared using a UV cured polymerization

method42 It can be seen the protected anode has a smoother and denser surface morphology.

The protected anode can suppress the overcharge during the charge process and form a stable

SEI layer which causes stable discharge capacity up to 100 cycles in battery. Liu et al.108 used

an anode of lithium-rich multiphase Li2.6BMg0.05 alloy foil for Li-S battery. This alloy is

composed of Li5B4, Li, and Li3Mg7 and provides a battery with lower polarization and longer

cycle life than the pure Li anode based battery due to its improved morphology. Figure 5

compares the surface morphology of a pure and alloyed anode. As shown in Figure 5, lots of

dendrites are detected on the pure Li surface after 70 cycles, while Li2.6BMg0.05 anode

inhibits the formation and growth of Li dendrites and shows a more homogeneous

morphology.

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3.3.3. Development in electrolyte

For the successful operation of a Li-S battery, the electrolyte should have high ionic

conductivity and enough PS solubility, electrochemical stability, chemical stability regarding

the lithium, and safety. Also it should stabilize the chemical composition and structure of the

sulphur cathode by suppressing dissolution of polysulphide. The capacity of a sulphur

electrode is insufficient when using conventional organic liquid electrolytes due to the high

solubility of polysulphides during both the charge and discharge processes.109 A number of

strategies have been explored to address the polysulfide solubility issue, including the design

of adjusted organic liquid electrolytes,110 the use of ionic liquid-based electrolytes,111 and the

application of polymer electrolytes.112

Choi et al.113 studied the effect of different liquid electrolyte combinations based on

DME, DEGDME, TEGDME and DIOX on Li-S battery efficiency; and Chang et al.43

researched on a mixed electrolyte of TEGDME and DOXL. It is found that a mixture of

electrolytes is more suitable because of the lower viscosity and the better wetting of the

electrodes which facilitate ion transportation. The electrochemical performance of Li-S

battery has also been investigated using LiClO4 DOL/DME as electrolyte and proved that an

optimum mixture of these solvents led to better cycle performance. DME offers higher PS

solubility and faster PS reaction kinetic, but high content of DME could increase the

resistance of the battery due to the high solubility of polysulfide, whereas DOL could

improve the interfacial contact between the electrodes and electrolyte.114

Polymer electrolytes have attracted a great deal of research interest for use in the Li-S

battery due to higher safety because of both the absence of flammable organic solvents and

the much lower reactivity toward lithium; also they can control the dissolution of

polysulphides. These electrolytes are divided into two groups: (i) Solid polymer electrolyte

(SPE) in which polymer acts as both mechanical matrix and solvent to dissolve lithium salts;

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(ii) Gel polymer electrolyte (GPE) in which a polymer is gelled by conventional electrolyte

solutions. Here polymer only provides dimensional stability. GPE has been more attention-

grabbing due to higher ion conductivity and major manufacturers of Li-ion batteries have

incorporated this electrolyte.115 It has been reported that PEO with ceramic filler and lithium

salts in the Li-S battery possesses good mechanical properties and ionic conductivity. This

polymer electrolyte postpones diffusion of the lithium polysulphides and sulphur dissolution,

leading to decreasing self-discharge, also the polar groups in the polymer chains can dissolve

the ionic salts.116, 117 Scrosati et al.106 improved the overall operation and safety of the battery

by replacing the common liquid organic solutions by a gel-type polymer membrane, with

trapping (EC: DMC/LiPF6) solution saturated with lithium sulfide in a (PEO/LiCF3SO3)

polymer. In another work the discharge process of Li-S with PVdF gel polymer electrolyte

has been investigated.118 The PVdF gel polymer electrolyte was prepared by LiCF3SO3 as

lithium-ion resource, tetraglyme as plasticizer, and PVdF as a gelling agent which shows a

high first discharge capacity of 1268 mAh g-1.

Another approach is using ionic liquid-based electrolytes. An ionic liquid of N-methyl-N-

butyl-piperidinium (PP14) was synthesized as an electrolyte in the Li-S battery and showed

good chemical and electrochemical stability towards lithium and sulphur. The

dischargeability and reversibility of battery were improved because of a stable structure of

sulphur due to suppressed dissolution of polysulfides in the electrolyte.111

Another efficient and economical strategy to modify electrode/electrolyte interface in Li

batteries is using an additive at small concentration in the electrolyte. LiNO3 is mentioned as

an additive in electrolyte for Li-S batteries by Aurbach and his co-workers119. It this case,

solvents, polysulphide and LiNO3 additives reacted with lithium to form a protective surface

film (SEI) on the surface of the Li anode; this layer not only protected the lithium anode from

chemical reaction with the dissolved polysulphide but also prevented PS from

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electrochemical reduction on the Li anode surface and inhibited the loss of active materials.53,

120. Although the cycle life and discharge capacity of the Li-S batteries improved, the safety

was reduced because of the strong oxidation of LiNO3. Thus, a new additive to the electrolyte

for Li-S batteries has been explored, LiBOB (lithium bis(oxalato) borate).121 This is thermally

more stable, and since its hydrolytic decomposition products are less toxic and corrosive, it is

more environmentally friendly. In batteries with LiBOB, a passivating surface film on is

formed on the lithium anode giving a higher discharge capacity, and a better cycle

performance.

3.4. Application of Li-S battery in electric vehicles

Currently, the transportation sector is the main consumer of fossil fuels that are a

contributing factor in global greenhouse gas (GHG) emissions. Over recent decades,

extensive effort have been devoted to the development and introduction of electric drive

vehicles, including both electric and hybrid vehicles (HEV), as a fundamental solution to the

serious emission and pollution problems, to the benefit of society.46, 122 In 1996 General

Motors released the EV-1, the first all-electric car from a major manufacturer.26 EVs

eliminate or reduce toxic exhaust emissions from automobiles, especially in urban areas of

high air pollution and consequently reduce carbon dioxide emissions, addressing concerns

over global climate change. This generation of vehicles is considered to promise a saving of

non-renewable energy sources by reducing dependence on oil and gas for transportation.

Hybrid electric vehicles are now commercially available and growing in market share and

there is increased interest internationally in the development and commercialization of

modern battery-powered electric vehicles. Leading companies in this industry, such as BMW,

Ford, Nissan, Tesla and Daimler Benz, use lithium-ion batteries as the energy supplier for the

EVs they have produced.123, 124 However, in order to develop Li-ion batteries of adequate

energy density for EVs to have an efficient driving range, it will be necessary to go beyond

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present strategies and develop cells with higher energy density and lower costs. Li-S batteries

are good candidates for this.125, 126 Sion Power Corporation127 is working on the application of

the Li-S system for EV applications. They have developed a cell that can demonstrate 350

Wh kg-1 in unmanned aerial vehicle flights. High efficiency Li-S batteries could provide

appropriate energetic and environmental performance. Applying the Li-S battery in EV’s can

reduce the charge time and increase the cycle life of the EV batteries. It is predicted that Li-S

batteries will be commercially available between 2020 and 2025.122

4. Conclusions and outlook

In recent years, significant effort has been made to improve the performance of Li-S

batteries as an electrochemical energy storage device with high power output/input, excellent

cycle life and a low cost, for use in a number of applications ranging from portable

electronics to electric vehicles. Although theoretically the energy density of Li-S cell is high,

there are many challenges to be addressed before theory becomes practice. As highlighted in

this review, such challenges are frequently rooted in materials discovery and optimization

because the efficiency of a battery depends on the electrode and electrolyte performance.

This review presents the current status of the Li-S battery and discusses its challenges

and its electrochemical reaction mechanism. Generally, one of the main problems of the Li-S

battery is its poor cycle life, mainly caused by PS dissolving into the electrolyte. To

overcome this hurdle, all cathodes, anodes and electrolytes should be modified. Research has

demonstrated that applying a sulphur/carbon-based composite as the cathode has opened up a

new way to Li-S batteries with high efficiency. Among of the many carbon-based materials,

graphene provides hope because of its high electrical conductivity, high surface area and low

cost. The Li metal anode can cause some safety problems. However, promising results have

been achieved recently in Li/S battery efficiency by using a non-lithium metal anode such as

silicon, or by applying protected anode technologies. As mentioned in this review, electrolyte

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has a direct effect on cell performance and various electrolytes have been investigated for the

Li-S battery and, among these, polymer electrolytes have been regarded as good candidate for

future researches on Li-S battery because of their safety and easy design and fabrication.

Although there has been promising progress, many aspects of Li-S batteries are not fully

understood, and will require additional investigation. High initial capacities above 1000 mA h

g-1 can be achieved with an improved cell, but maintenance of the initial cathode morphology

is difficult. To overcome these issues, researchers are explore more profound mechanisms

and reasons of the capacity fading in Li-S batteries after cycling. These issues will be solved

when all the problems are identified. In addition, Li-S batteries have to compete with other

energy conversion and storage technologies, such as fuel cells, which are promising energy

devices for the transport, mobile and stationary sectors. Meanwhile, Sion Power Corporation

is focusing on developing the commercial Li-S battery and claims that the Sion Power's Li-S

will be the next rechargeable power source for a wide variety of applications, including

unmanned vehicle systems, military communications and electric vehicles.

The ultimate goal is to develop a low-cost, high-throughput, environmentally friendly

battery. In order to reach this ambitious goal, there must be a better understanding of the

current situation with Li-S batteries. This review may provide some new insights and

opportunities into the challenges in this area, and so help move towards a commercially

available Li-S battery.

Acknowledgements

This research is financially supported by the Australian Research Council (ARC) through

ARC Discovery Project DP110101974.

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Figure Captions

Fig. 1. Schematic illustration of a typical lithium ion battery.

Fig. 2. Schematic configuration of Li-S battery based on graphene- sulphur composite (G-S)

cathode.

Fig. 3. Voltage profile of the first discharge of a Li-S cell. Reproduced from Zhang,53

(written permission has been obtained from the previous publisher).

Fig. 4. SEM characterizations of hollow carbon nanofiber-encapsulated sulphur: (a) anodic

aluminum oxide (AAO) template after carbon coating, (b) hollow carbon nanofiber-

encapsulated sulfur after etching away AAO template, (c) cross-sectional image of hollow

carbon nanofiber/S array, (d) carbon elemental mapping of (c) and (e) sulfur elemental

mapping of (c). Reproduced from Zheng et al.,68 (written permission has been obtained from

the previous publisher).

Fig. 5. SEM images of surface morphologies after 70 cycles: (a) Li anode; (b) Li2.6BMg0.05

anode. Reproduced from Liu et al.,108 (written permission has been obtained from the

previous publisher).

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Figure 1, black and whiteClick here to download high resolution image

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Figure 2, black and whiteClick here to download high resolution image

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Figure 3, black and whiteClick here to download high resolution image

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Figure 4, black and whiteClick here to download high resolution image