Scholarly article on topic 'Efficient Electrolytes for Lithium–Sulfur Batteries'

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Academic research paper on topic "Efficient Electrolytes for Lithium–Sulfur Batteries"

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ENERGY RESEARCH

Energy Storage

Efficient Electrolytes for Lithium-Sulfur Batteries

Natarajan Angulakshmi and Arul Manuel Stephan

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Frontiers in Energy Research

2296-598X

Review Article

15 Jul 2014

26 Mar 2015

26 Mar 2015

www.frontiersin.org

Angulakshmi N and Stephan AM(2015) Efficient Electrolytes for Lithium-Sulfur Batteries. Front. Energy Res. 3:17. doi:10.3389/fenrg.2015.00017

© 2015 Angulakshmi and Stephan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution and reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Efficient Electrolytes for Lithium-Sulfur Batteries

N. Angulakshmi1, A. Manuel Stephan2*

department of Materials Science and Engineering, Politecnico de Torino,

Torino, Italy

2Central Electrochemical Research Institute (CSIR-CECRI) Karaikudi 630 006, India

Abstract

This review article mainly encompasses on the state-of-the-art electrolytes for lithium-sulfur batteries. Different strategies have been employed to address the issues of lithium-sulfur batteries across the world. One among them is identification of electrolytes and optimization of their properties for the applications in lithium-sulfur batteries. The electrolytes for lithium-sulfur batteries are broadly classified as (i) non-aqueous liquid electrolytes, (ii) ionic liquids, (iii) solid polymer and (iv) glass-ceramic electrolytes. This article presents the properties, advantages and limitations of each type of electrolytes. Also the importance of electrolyte additives on the electrochemical performance of Li-S cells is discussed.

Key words: Lithium-Sulfur batteries; poly sulfides; polymer electrolytes; charge-discharge studies; Interfacial properties; ionic liquids; super ionic conductors; electrolyte additives.

e-mail:arulmanuel@gmail.com; amstephan@cecri.res.in

Fax:+91 4565 227779 Phone: +91 4565 241 426

Dr. Arul Manuel Stephan is presently a senior researcher at the CSIR- Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi, India. He has (co) authored for more than 75 International publications, an International and an Indian patent. He is a recepient of JSPS fellow (Prof. Yuria Saito, ONRI, JAPAN), Post-doctoral fellow (Prof. D. Teeters, University of Tulsa, USA), Brain-pool visiting Scientist (Prof. K.S. Nahm, Chonbuk National University, S.Korea) and CSIR-Raman Research Fellow (Prof. R. Bongiovanni, Politecnico de Torino, Italy). His area of research interests are development of innovative and efficient materials for lithium-ion and lithium-sulfur batteries.

Dr. Natarajan Angulakshmi is currently working as a research assistant at the Politecnico de Torino, Italy. She has also served as a Senior Research Fellow at the Central Electrochemical Research Institute (CSIR-CECRI) Karaikudi, India and subsequently as Assistant Professor (Research) at Chonbuk National University, S. Korea. She was also a visiting researcher at University of Mara, Malaysia. She is a co-author for more than 20 publications in International Journals and an Indian Patent. Her research interests are development of electrolytes for lithium-ion and lithium-sulfur batteries.

Introduction

The global warming and depletion of fossil fuel resources have accelerated immense research on

energy storage devices unquestionably (Tarascon, 2001) . In response to the modern society it is

now essential to develop new, low-cost and environmental friendly energy conversions and

storage systems with new advanced materials (Arico et al., 2005; Bruce et al., 2008;

Goodenough and Kim, 2010). Undoubtedly, lithium-ion battery is one of the great successes of

modern electrochemistry due to its appealing properties such as high single cell voltage, no-

memory effect, long cycle life and high energy density. The fundamental aspects of lithium-ion

batteries, working principles and their limitations can be understood from numerous review

articles (Whittingham, 2004) and dedicated books (Schalkwijk and Scrosati, 2002;). The state-

of-the-art lithium-ion batteries are composed of a carbonaceous anode and lithium transition

metal oxide cathode separated by a polyolfine porous separator soaked in a non- aqueous liquid

electrolyte (Manuel Stephan, 2006). The lithium-ion battery has become an inevitable power

source not only for portable electronic devices such as laptop computers, cellular phones, MP3

players, but also find applications in satellites (Santoni et al., 2002) and in medical equipments

(Bock et al., 2012). Nevertheless, with the existing insertion cathode materials (e.g. LiCoO2,

LiFePO4 etc.), lithium-ion batteries have attained a maximum discharge capacity of

approximately 250 mAh g-1 (with a theoretical energy density of 800 Wh kg-1) which is not

sufficient to meet out the demand of key markets such as transport and power grid applications

(Lee et al., 2011). Obviously, intense research has been accelerated to find alternative

electrochemical lithium based- power systems across the world. Among the systems known

today, both Li-S and Li-O2 are expected to fulfill the requirements of mankind with enhanced

capacity and energy density. Nevertheless, so many technological and scientific problems remain unsolved in Li2O systems (Bruce et al., 2012). The Li-S batteries have inspired many researchers recently, because sulfur is electrochemically active and can accept up to two electrons per atom approximately at 2.1 V vs Li/Li+. It has a high theoretical capacity of 1675 mA h g-1, which corresponds to an energy density of 2600 W h kg-1 or 2800 Wh l-1 based on weight or volume respectively. However, their practical applications are impeded by several major issues (Cakan et al., 2013).

Unfortunately, sulfur undergoes a series of compositional and structural changes during cycling, which involves soluble polysulfides and insoluble sulfides (Mikhaylik et al., 2004; Chenon et al., 2003). The typical Li-S batteries are composed of a lithium metal anode, an organic liquid electrolyte, and a sulfur composite cathode as depicted in Figure 1.

Although the concept of elemental sulfur as positive electrode material was introduced almost

five decades ago by Herbert and Ulam (Herbert and Ulam,1962) its technological importance

was only recognized in the beginning of year 2000. In the recent years, many review articles are

available in the literature describing the working principles, advantages and limitations of

lithium/sulfur batteries (Bresser et al., 2013; Barghamadi et al., 2013; Nazar and Evers, 2013;

Mnathiram et al., 2013; Ji and Nazar, 2010). Also numerous articles appear on the fundamental

chemistry (Zhang,2013a; Akridge et al., 2004), performance of modified cathode materials (Fu et

al.,(2012); Ji et al., 2009; Yang et al.,2011, Liang el al., 2009), their fading mechanism (Diao et

al.,2012), AFM and Raman characterizations (Hagen et al., (2013); Yeon et al., 2012; Elazari et

al., 2010) in- situ XRD (Canas et al., 2013a) and impedance analysis (Canas et al., 2013b; Yuan

et.al., 2009). The preparation and characterization of cathode and anode materials for Li-S

batteries are beyond the scope of this article. This article mainly encompasses the basic

properties of electrolytes, their types, electrolyte additives, their advantages and limitations for the applications in Li-S batteries. The electrolytes for Li-S batteries can be classified as (i) nonaqueous liquid (ii) ionic liquids (iii) solid polymer electrolytes and (iv) superionic conductors.

Fig.1 Schematic diagram of a typical Li-S cell. [Adopted from Manthiram et al., 2013]

Lithium-ion reacts with elemental sulfur (Ss) and produces lithium polysulfides of general formula Li2Sn e.g. (Li2Ss, Li2S6). Upon discharge the length of the polysulfide is shortened as the sulfur is being further reduced. The overall reaction is

16Li++ 16e- + Ss —► 8Li2S

The elemental sulfur is a promising electrode material which is available abundantly, cheap, non-toxic and environmentally benign. Despite all these advantages sulfur suffers from poor electronic conductivity.

Figure 2 shows typical discharge-charge profile of a Li-S cell. The discharge process is generally divided into four reduction regions depending upon the phase changes of sulfur species28.

According to Zhang (Zhang 2013a) the first upper voltage plateau between 2.2 -2.3 V corresponds to a solid-liquid two phase reduction from elemental sulfur to Li2S8. This process results in formation of huge voids in cathode due to the dissolution of Li2S8 into liquid electrolyte.

S8 + 2Li+ + 2e- ^ Li2S8------------------------------------(ii)

In the second region, the cell voltage suddenly reduced with an increase in viscosity of electrolyte solution. The solution's viscosity reaches a maximum value at the end of the discharge region which corresponds to the equation;

Li2S8 + 2 Li+ + 2e- ^ Li2S8-n + Li2Sn ---------------------(iii)

The low voltage plateau which occurs between 1.9 -2.1 V offers majority of the toral capacity of Li-S cell. In this third region a liquid-solvent two-phase reduction takes place in which dissolved low-order polysulfide (PS) to insoluble Li2S2 or Li2S.

2Li2Sn + (2n-4)Li ^ nLi2S2 ---------------------------(iv)

Li2Sn + (2n-2)Li ^ nLi2S ---------------------------(v)

In the fourth region a solid-solid reduction from insoluble Li2S2 to Li2S process takes place which is kinetically slow and suffers from high polarization due to the non-conductive and insoluble natures of Li2S2 and Li2S.

Li2S2+ 2Li+ + 2e- ^ 2Li2S -------------------------(vi)

According to Mikhaylik and Ackridge (Mikhaylik and Ackridge, 2004), during the first charge polysulfides do not transform into elemental sulfur in a Li-S cell. Instead, in the second and subsequent charges, the higher-order polysulfides, which are generated at the sulfur electrode diffuse to lithium electrode and form lower-order polysulfides due to parasitic reaction. These species diffuse back again to the sulfur electrode as polysulfide and where polysulfide, of higher order are formed as a result of shuttle mechanism.

The sulfur and lithium sulfides which are strong insulators will easily dissolve in common organic liquid electrolytes. Spontaneously, they diffuse through the liquid electrolyte and subsequently leads to self-discharge and thereby increasing the viscosity of the electrolyte solution21. These drawbacks lead the lithium/sulfur batteries to poor cycle life, low specific capacity and lower efficiency.

Non-aqueous liquid electrolytes

In order to improve the electrochemical performance of Li-S batteries, different strategies have

been adopted which also include the optimization of the electrolyte compositions (Barchasz et

al., 2013). The dissolution of polysulfides, LiSx, leads to an unfavorable side reactions and

adversely deteriorates the performance of the Li-S cells. It is well known that the carbonate-

based solvents such as ethylene carbonate (EC) and diethyl carbonate (DEC) react with

polysulfides irreversibly. In the beginning, dimethyl sulfoxide (DMSO) was explored as a possible electrolyte solvent for lithium-sulfur batteries; however, the sulfur can be reduced only to S42- with an efficiency of 25 % (Brummer et al., 1979). On the other hand, a 50% efficiency was achieved when dimethyl acetamide (DMAc) was used with a corresponding reduction of S22-(Abraham et al., 1978). Although, tetrahydrofuran (THF) exhibited about 100% efficiency at 50 °C, unfortunately, it is found to be thermodynamically unstable with lithium metal (Yamin and Peled, 1983; Kock and Young, 1977).

In a pioneering work, Rauh et al (Rauh et al., 1979) reported electrolytes with high basicity can dissolve huge amount of lithium polysulfides. Also the sulfur solubility can go up to 10 M in dimethyl sulfoxide or ethers like tetrahydrofuran. Based on the electrochemical and spectroscopic studies of poly sulfides in non-aqueous solutions, the same group (Rauh et al., 1977) concluded that dynamic equilibrium, redox chemistry, kinetics are strongly affected by solvent complexation. Unquestionably, the selection of solvents which is stable against nucleophilic attack of polysulfides is mandatory. 1,3-dioxolane has a good polysulfide solubility and high stability in contact with lithium metal forms a passive solid-electrolyte interphase. TEGDME has a glyme structure with high donor number (DN=18.6) and with low dielectric constant of Sr = 7.9 which can dissociate both lithium salt and sulfur active material. The ether based electrolytes have high donor number and co-ordinate with Lewis acidic cation of Li+. 1,2-dimethoxyethane (DME) also has high lithium polysulfide dissolution ability and can provide sufficient amount of active materials (Tachikawa et al., 2011)

Generally, both 1,3-dioxolane and glyme solvents exhibit higher sulfur solubility (Peled et al., 1989; Chu et al., 2000; Shim et al., 2000). The impedance spectroscopic studies on Li-S cells containing 1 M solution of LiClÛ4 in sulfonane have been reported by Kolosnitsyn et al (Kolosnitsyn et al., 2011). It is suggested that the electrochemical processes in Li-S cells were controlled by diffusion in the surface layer on the sulfur electrode at high degrees of chargedischarge and by the transport properties of the electrolyte system at moderate degrees of charging. More importantly, the rate of shuttling of sulfur is determined by the solubility of lithium polysulfides in electrolytes, the transport properties of electrolyte systems and the dissolution rates on the electrodes. The structure of the positive electrode also significantly affects the transfer rate (Kolosnitsyn et al., 2008). The shuttling of polysulfide, although protects the Li-S batteries from overcharging, it adversely induces the self-discharge. Moreover, in Li-S batteries, the dentritic formation is much lower than other type of lithium batteries with metallic lithium (Chang et al.,2002). Due to the instability of ether solvents at higher potential values they are not widely used in lithium batteries, however, they find applications in Li-S system because of 3 V vs Li+/Li (Choi et al., 2008; Mikhaylik et al., 2010). The importance of solvation ability and additives has been illustrated by (Barchasz et al Barchasz et al., 2013) by using different combinations of tetraethylene glycol dimethylether (TEGDME)/1,3-dioxolane (DIOX) binary electrolytes. The authors have also demonstrated that the lithium salt concentration also plays a vital role for the formation of good passivation and helps to reduce the shuttle mechanism and capacity loss. The solvation ability is also a key parameter for better electrochemical performance of Li-S cells.

By employing electrochemical and in-situ X-ray absorption spectroscopy, Gao et al (Gao et al., 2011) demonstrated the influence of electrolytes on the charge-discharge studies of Li-S cells. A discharge capacity of about 1000 mAh/g was delivered by the Li-S cell when TEGDME and DOL/DME were used as electrolytes. While using PC/EC/DEC and EMS/DEC as electrolyte the cell delivered almost no capacity. On the basis of the electrochemical study, DOL/ DME and TEGDME have been suggested as appropriate and promising solvents for the electrolytes of Li-S batteries. Although the discharge products of sulfur in DOL/DME and TEGDME are similar, the authors found a faster self-discharge and a more complete reduction of solution-phase sulfur species upon discharge for the combination of electrolytes, DOL/DME. While comparing the effect of lithium salts, LiPF6, LiCF3SO3 and LiClO4 a relatively stable capacity is seen for LiClO4; however poor safety concerns hamper it from practical application. The electrochemical performance of LiClO4/DOL/DME as electrolyte was reported by Wang and co-workers (Wang et al., 2010). Impedance analysis indicated that the formation of impermeable layer on the surface of cathode considerably increased the interfacial resistance of the battery. They also suggested the combination of electrolyte DME: DOL of 2:1 v/v is capable of delivering a discharge capacity of 1200 mAh g-1 during its initial stage and retained a discharge capacity of 800 mAh g-1 even after 20 cycles.

d Li2SE/Li;S

■ i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i ■ i i i i i i i i i i i i i

0 200 «0 600 eoo 1000 1200 1400 1600 1000 2000

Specific Capacity (mAh/g sulfur)

Fig.2. First discharge and charge profiles of a Li-S cell between 1.5 and 2.8 V vs. Li+/Li.

The cycling behavior of Li/TEGDME/S cells was examined at low temperature by Rhu et al (Rhu et al., 2006). The cell delivered a discharge capacity of 1030 and 357 mAh g-1 at 20 and -10° C respectively. Upon addition of 1,3-dioxalane and methyl acetate to the TEGDME the discharge capacity was enhanced to 1342 and 994 mAh g-1 respectively. The better performance of the cell has been attributed to the reduced viscosity of the electrolyte at lower temperature. The electrochemical properties of ether-based electrolytes have also been reported by Barchasz and co-workers (Barchasz et al., 2013). The same group (Barchasz et al., 2012) has added polysulfide (0.1M Li2S6) as co-salt in order to provide additional capacity and compensate the active mass loss. Unfortunately, it adversely reduced the capacity and cycle life of the cell. On contrary, to the above work Chen and co-workers [Chen et al., 2013] achieved a dramatic increase in capacity, cyclability and rate capability by optimizing the concentration of polysulfide species and the amount of electrolyte in the cell. As sulfur is a conversion type-cathode material, it does not require Li+-ionic electrolyte and the discharge can be performed

[Adopted from Barghamadi et al., 2013]

even with an ammonium solution. In an unconventional way of thinking, Zhang (Zhang, 2013 c) found that the introduction of quarternary ammonium into liquid electrolyte could effectively suppress the disproportion of polysulfide intermediate which increased the Li-S cells capacity retention; however, it is not able to stop the redox shuttle of polysulfide. Very recently, Agostini et al., (Agostini et al., 2014) exploited the use of non-flammable TEGDME -based electrolyte containing Li2S8 electrolytes. The electrochemical properties of the cell were effectively enhanced by the addition of LiNO3 which provided a stable and protective solid electrolyte interface on a lithium surface. An excess amount of electrolyte and low loading of sulfur have significantly enhanced the cycle life and capacity retention of Li-S cells (Bruckner et al., 2014). However, the energy density of the cell was reduced due to the dead weight of the liquid electrolyte. The performance of non-aqueous liquid electrolytes for lithium-sulfur batteries has been displayed as Table 1. Ionic liquids as electrolytes

Ionic liquids are identified as an electrolyte system for lithium-ion batteries due to their unique properties such as non-flammability, wide electrochemical stability, non-volatility, high ionic conductivity, and environmental friendliness (Galinski et al., 2006). However, the reports on ionic liquids as electrolytes for Li-S batteries are very scanty. Very recently, Watanabe and coworkers reviewed the ionic liquids as electrolytes for lithium-sulfur batteries (Park et al., 2013). The same group also demonstrated the anionic effect as solvate ionic liquid electrolytes in rechargeable Li-S batteries (Ueno et al., 2013). Wang et al., (Wang et al., 2008) compared the cycling profile of Li/S cells consisting of 1-ethyl-3-methylimidazolium bis(trifluoro methanesulfonyl) imide and lithium bistrifluoro methanesulfonylimide with a sulfur coated

carbon cathode. The cell delivered higher discharge capacity with ionic liquid than the

conventional organic solvent electrolyte. The physical and chemical properties of a ternary mixture comprising tetra(ethylene glycol) dimethyl ether (TEGDME) as a polymer solvent in mixed electrolytes composed of JV-methyl -n-butyl pyrrolidinium

bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and LiTFSI were reported by Shin and Cairns (Shin and Cairns, 2008). The ternary compound was employed as electrolyte for Li-S cells. The cell delivered a discharge capacity of 887 and 440 mAh g-1at ambient temperature and at 0 °C respectively. With N-butyl-N-methyl-piperidinium-TFSI as electrolyte solvent and sulfur cathode, Yuan et al (Yuan et al., 2006) reported the cycling behavior of Li-S cells. Although, the cells delivered an initial discharge capacity of 1055 mAh g-1 in its first cycle an abrupt decrease in discharge capacity (about 770 mAh g-1) was found after a few cycles. The effect of imidazolium cation on the cycle life of Li-S batteries have been analyzed by Kim and co-workers (Kim et al., 2007). Wang and Byon (Wang and Byon, 2013) designed a new ionic liquid based organic electrolyte which swaps solubility and diffusion rate of polysulfides by a combination of 1,2-dimethoxyethane and highly viscous N-methyl-N-propylpiperidinium bis(trifluoromethane sulfonyl) imide (PP13-TFSI). This engineered electrolyte offered high Coulombic efficiency, capacity retention and suppressed internal shuttling of polysulfides70. Table 2 illustrates properties of ionic liquids as electrolytes for lithium-sulfur batteries. Polymer Electrolytes

In the last three decades, extensive research has been devoted on the development of polymer

electrolytes as they find applications not only in lithium batteries but also other electrochemical

devices such as supercapacitors, fuel cells and electrochromic devices etc. (Manuel

Stephan,2006;). The advantages of polymer electrolytes, their types, advantages and limitations

have already been reported elsewhere (Manuel Stephan et al., 2009; Quartarone and Mustarelli,

2011). In lithium sulfur batteries polymer electrolytes play a vital role in suppressing the dissolution of poly sulfides in lithium/sulfur cells resulting in good cyclability and enhanced capacity retention (Hassoun and Scrosati, 2010a). Hassoun and Scrosati (Hassoun and Scrosati, 2010b) demonstrated an all-solid-state lithium/sulfur cells composed of a Li2S/C cathode, lithium metal anode and a nanocomposite polymer as electrolyte. The cell was capable of delivering a discharge capacity of 300 mAh g-1 at 70 °C. The added -ZrO2 filler in a PEO-LiCFsSOs filler significantly promoted the ionic conductivity of the composite polymeric membrane. Efforts have also been made to replace the highly reactive anode material with alloy anodes with same type of nanocomposite polymer electrolytes (Jeng et al., 2007). Jeddi et al (Jeddi et al., 2013 a,b) proposed a novel polymer electrolyte by blending poly(vinylidene fluoride-co-hexafluoropropene) (PVdF-HFP) with monofunctional poly(methyl methacrylate) (PMMA) containing inorganic trimethoxysilone domains. The gel polymer electrolyte (GPE) was found to be capable of upholding the electrolyte solution and preventing polysulfide diffusion which improved the cycling performance of the sulfur-based electrode material.

Marmorstein and co-workers (Marmorstein et al., 2000) reported the charging -discharging characteristics of Li/S cells with three different polymer electrolytes at various operating temperatures ranging from ambient temperature to 100 °C. Among the systems studied the cell operated between 90 and 100 °C with poly ethylene oxide as electrolyte delivered highest discharge capacity. Unfortunately these systems suffer from poor rate capability. According to Zhang and Tran (Zhang and Tran, 2013d ) the selection of electrolyte materials for lithium/sulfur batteries is very limited. Being a very strong nucleophilic agent, the polysulfides are can react with many electrolyte solvents such as esters and carbonates and lithium salts such

as LiPF6, LiBF4, LiB(C2O4)2, and LiBF2C2O4. Further, elemental sulfur and polysulfides can replace fluorine from fluorinated polymers such as (PVDF-HFP). Eventually, this leads the formation of thiols and vulcanization of unsaturated polymers. In view of the chemical stability, the lithium salts such as LiSO3CF3, LiTFSI, linear or cyclic ethers such as 1,2-dimethyl ether (DME), 1,3-dioxolane (DIOX) as solvents and poly (ethylene oxide), PEO as the ultimate polymer host for polymer electrolyte researchers. The authors also illustrated the influence of gel polymer electrolytes on the cycling performance of Li-S cells. The authors prepared a composite gel polymer electrolyte (CGPE) composed of PEO and SiO2. Results showed that the incorporation of SiO2 in the polymeric matrix has significantly trapped the polysulfide species in the separator and reduced the utilization of sulfur active materials. The authors also concluded the benefits of gel polymer electrolytes and composite gel polymer electrolytes are adversely compensated by sulfur specific capacity. Zhang (Zhang, 2013d) reported the electrochemical properties lithium/sulfur cell with a flexible and free-standing composite gel polymer electrolyte membrane composed of 50%PEO-50%SiO2.

Lecuyer et al. (analyzed the structural evolution of Li/S cells based on PEO-based dry polymer electrolytes by employing SEM and EDX. Diffusion of sulfide species resulted in volume changes of both electrode and electrolyte. Attempts have been to enhance the mechanical property of cathode material by incorporation of PVdF in the composite cathode. Although incorporation of PVdF significantly enhanced the mechanical integrity, it could not solve the problems associate with the cyclability.

The cycling profile of Li-S cells with PEO-based gel electrolytes with LiClO4 and TEGDME. The cell delivered higher discharge capacity than the subsequent cycles and a flat discharge has

been observed at 2.0 V. The capacity fading has been attributed to low utilization of sulfur which arises to the aggregation of sulfur (polysulfide) upon cycling (Jeon et al., 2002). The electrochemical and interfacial properties TiO2-laden PEO-UCF3SO3 complexes have been reported by Shin and co-workers (Shin et al., 2002). The addition of TiO2 into PEO-UCF3SO3 complexes has substantially promoted not only the ionic conductivity, but also interfacial resistance between composite polymer electrolyte and lithium metal anode. The remarkable decrease in the interfacial resistance is attributed to the lowering contact area between lithium and electrolyte. The increased discharge capacity of Li-S cells is attributed to the better interfacial property of composite polymer electrolyte and lithium metal anode.

Superionic conductors

The appealing properties such as high safety, non-flammability, high thermal stability, reliability, and prevention of polysulfide formation and migration qualify the ionically conducting solids as potential electrolytes for lithium sulfur batteries. Based on their properties they are classified as (i) sulfides (ii) oxides and (iii) phosphates (Fergus, 2010). Among the three types, oxides are more stable in air than sulfides (Machinda and Shigematsu, 2004). The search for glass ceramic materials with high stability in contact with lithium metal combined with a high ionic conductivity has been intensified for the fabrication of all-solid-state lithium batteries in order to ascertain the safety aspects. The major challenge for all solid-state batteries depends mainly on the electrode/electrolyte interfaces (Kim et al., 2006).

Nagao et al (Nagao et al., 2011) fabricated an all solid-state lithium sulfur cell comprising sulfur-carbon electrode with Li2S-P2S5 glass-ceramic electrolytes and examined their electrochemical performances. The authors performed the charge-discharge analysis between -

20 and 80 °C. The cells retained a reversible capacity higher than 850 mAh g-1 for 200 cycles at 25 °C. The enhanced discharge capacity of the cell was attributed to the reduced crystallinity of sulfur and particle size of the electrode materials achieved by milling which facilitated better contacts between the particles.

Hayashi and co-workers have (Hayashi et al., 2003) successfully introduced a novel all-solidstate Li/S batteries with Li2S-P2S5 as a highly conductive glass-ceramic electrolytes. The cathode materials consisting of sulfur and CuS were synthesized by mechanical milling using sulfur and copper crystals as starting materials. The cell was capable of delivering over 650 mA h g_1 for 20 cycles. The time of ball milling was found to influence the electrochemical properties of Li-S cell.

In a similar way, Kobayashi and co-workers reported the cycling performance of an all-solidstate-lithium sulfur battery with sulfur electrode and thio-LISICON electrolyte (Kobayashi et al, 2008). The same group also reported the cycling behavior for mesoporous electrode with thio-LISICON electrolyte (Kobayashi et al., 2013). The performance of solid electrolytes for lithium-sulfur batteries is shown in Table 4. Electrolyte additives

Electrolyte additives play an extraordinary role in lithium -ion batteries. Although the amount of additive in the electrolyte is around 5% either by volume or by weight it plays a significant role in the electrochemical performance of lithium-ion batteries. It significantly improves the solid electrolyte interface on the surface of graphite, reduces irreversible capacity, enhances thermal stability, promotes physical properties of the electrolyte such as ionic conductivity, viscosity, wettability of the polyolifine membrane etc., (Zhang, 2006). It also protects battery safety by

lowering flammability of organic electrolytes, increases tolerance and stops battery operations in abuse conditions.

In a similar way, the electrolyte additives play a vital role in lithium/sulfur batteries. The polysulfides involve reactions with cathode and anode during cycling and therefore protection of lithium anode from chemical reactions is mandatory. This will significantly eliminate the shuttling of polysulfide and effectively promotes the columbic efficiency and cycling performance91. Lithium nitrate (LiNO3) reacts with metallic lithium and forms a rigid passivation layer that prevents the chemical reactions of polysulfides with metallic lithium and is attributed to the decomposition of LiNO3 in the non-aqueous liquid electrolyte (Zhang et al., 2012) . Very recently, Lin et al ( Lin et al., 2013) illustrated that the addition of P2S5 as additive promoted the dissolution of Li2S and alleviates the loss of capacity caused by the precipitation of Li2S and also eliminated the shuttling of polysulfides by passivating metallic lithium. Summary and Future outlook

Li-S batteries are considered as one of the ultimate power source by virtue of its appealing properties such as low cost, non-toxic, abundance and above all higher discharge capacity (1640 mAh g-1) and energy density (2400 Wh kg-1) than lithium-ion batteries. Regarding the non-aqueous liquid electrolytes for lithium-sulfur batteries, no one system is found to be optimal in all aspects. In this area, more work is certainly needed. Although, extensive research work has been pursued on the development of cathode materials for lithium/sulfur batteries, reports on solid polymer electrolytes and superionic conductors are very scanty. A few reports illustrate the glass ceramic electrolytes with ionic conductivity exceeding 10-3 Scm-1 at ambient temperature. However, attention should be focused on the major drawbacks such as fragility at low thickness, high cost and poor interfacial property at electrode/electrolyte interfaces. Prevention of shuttling

of polysulfides by suitable polymeric/solid electrolytes will be appreciable and exotic. Ionic liquids in conjunction with polymer electrolytes and sulfurized carbon electrodes may offer a safe, reliable and polysulfide shuttle-free lithium-sulfur batteries.

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Abbreviations

PC - Propylene Carbonate

EC - Ethylene Carbonate

DEC - Diethyle Carbonate

DIOX - 1,3-dioxolane

DME - Dimethyl ether

EMS - Ethyl methane sulfonate

THF - Tetrahydrofuran

DMSO - Dimethylsulfoxide

DMAc - Dimethyl acetamide

TEGDME - Tetra (ethylene glycol) dimethyl

TEOS - Triethoxysilane

PEO - Poly(ethylene oxide)

PVDF - Poly(vinylidene fluoride)

PVDF-HFP - Poly(vinylidene fluoride-co-hexafluoropropene)

PMMA - Polymethylmethacrylate

LiClO4 - Lithium perchlorate

LiBF4 - Lithium tetrafluoroborate

LiPF6 - Lithium hexafluorophosphate

LiCF3SO3 - Lithium trifluoromethylsulfonate

LiTFSI - Lithium bis(trifluoromethylsulfonyl)imide

LiB(C2O4)2 - Lithium bis(oxalate)borate (LiBOB)

LiBF2C2O4 - Lithium bid(difluoro oxalate)borate (LiDFOB)

PYR14TFSI - N-methyl-n-butyl-pyrrolidinium-bis(trifluoromethylsulfonyl)imide

GPE - Gel Polymer Electrolyte

CGPE - Composite Gel Polymer Electrolyte

XRD - X-Ray Diffraction

AFM - Atomic Force Microscope

SEM - Scanning Electron Microscope

EDX - Energy Dispersive X-ray

Li2S - Lithium sulfide

- Phosphorous pentasulfide