High Energy Density Lithium-sulfur Batteries Research Articles

A New Conceptual Cathode and Separator Design Enabling High Energy Density Lithium-Sulfur Batteries

Significant improvement of lithium ion batteries (LIBs) performances has been achieved with a rapid advance of portable electronic devices over the past quarter century. With a growing demand for high performance power sources, world-wide efforts have been devoted to the development of an alternative battery system to replace current LIBs which face performance limitation in terms of capacity and energy density. Recently, lithium-sulfur (Li-S) batteries have received an unprecedented attention owing to their great potentials such as a large theoretical capacity of sulfur, low toxicity and low cost. As cutting-edge nanotechnologies (e.g., C-S composite concept, mesoporous interlayer) have been employed to Li-S batteries, their performances have been dramatically improved for the past two decades. Despite such remarkable progress, there have remained many issues (i.e., limited cycling performance and low rate properties) which should be addressed for successful commercialization of Li-S batteries. In addition, most approaches to solve those problems need so complex and expensive processes which are so hard to implement them to practical mass production. Herein, we report a facile and cost-effective approach for the development of high-performance Li-S batteries. Through combining a functional separator to a high-sulfur-loading cathode, which can mitigate the active material loss and confine polysulfide ions within a specific space ensuring a long cycle life, high-performance Li-S batteries are fabricated and their characteristic features are intensively investigated.

Rational Design of Lithium-Sulfur Battery Cathodes Based on Experimentally Determined Maximum Active Material Thickness.

Rational design of conductive carbon hosts for high energy density lithium-sulfur batteries requires an understanding of the fundamental limitations to insulating active material loading. In this work, we investigate the electrochemistry of lithium sulfide films ranging in thickness from 30 to 3500 nm. We show that films thicker than approximately 40 nm cannot be charged at local charge densities above 1 μA cm-2, and by implication, the maximum useful pore diameter is near 60 nm in a practical cathode. "Activation" overpotentials for Li2S are identified in thicker films, resulting from polysulfide generation, but are shown not to improve the fundamental areal charge limitations. We develop a model for filling of conductive pores with active material to rationally design composites based on local charge density. For low-electrolyte applications, the importance of matching micropore volume to sulfide loading and cycling rate is emphasized.

Highly Stable Lithium Anode Protected with Conformal Aluonce for High Mass-Loading Li-S Batteries

Lithium metal as an ideal anode for next-generation batteries has attracted extensive attention. However, rough electrodeposition, severe parasitic reactions and uncontrolled lithium dendrites cause fast battery fading. Engineering a dendrite-free lithium metal anode for long cycling life is critical for its future development. Here, we reported a conformal, aluminum-based polymer coating with highly crosslinking structure on lithium using molecular layer deposition (MLD) to control undesirable reactions. This hybrid organic/inorganic film functions as an artificial interface to stabilize lithium stripping/deposition and enables minimal side reactions and dendrite-free lithium anodes. Alucone coated Li leaded to several times longer cycle life in a large current rate and achieved steady Coulombic efficiency of 99% at 0.4 mA/cm2. The protected Li also accomplished much improved performance of lithium-sulfur (Li-S) batteries with a high mass-loading of ~5 mg/cm2. Our proposal and findings offer an efficient avenue towards a highly stable lithium anode and the application in high energy density Li-S batteries.

Modeling the Effect of Carbon to Sulfur Ratio in the Cathode on the Electrochemical Performance of a Li-S Cell

Lithium-Sulfur (Li-S) batteries are one of the most promising energy storage systems beyond Li-ion batteries because of the high specific capacity (1675 mAh/g), low cost, natural abundance and non-toxicity of sulfur [1-3]. Although Li-S batteries have many attractive characteristics, they are facing major problems that are required to be solved to improve the cycle life and capacity retention; the precipitation of the insulating solids on the cathode (S, Li2S), infinite charging due to the polysulfide shuttle mechanism and the instability of the lithium anode [1-3]. Carbon to sulfur ratio in the cathode is one of the most critical design parameters in a Li-S battery [1-3]. Because of the insulating nature of both S and Li2S, Li-S cathodes typically require high C:S ratios [1-3]. As this ratio increases, cathode kinetics are improved due to an enhancement in both electronic conductivity and active surface area. However, techno-economic model predictions in a previous study show that too much increase in the C:S ratio results in a significant decrease at the system-level energy density [1]. Therefore, thoughtful selection of the C:S ratio in the cathode is critical to achieve high-energy density Li-S batteries. In this study, the effect of carbon to sulfur ratio in the cathode on the electrochemical performance of a Li-S cell is modeled. For this purpose, a one-dimensional, concentration-independent electrochemical model for an isothermal, constant-current discharge of a Li-S cell is developed based on a previous model [1, 4]. In the model, the current-voltage relationship is predicted using Butler-Volmer kinetics in the anode, Ohm’s Law in the porous separator and the porous electrode model by Newman and Tobias in the cathode [5]. The effect of C:S ratio in the cathode on the electrochemical performance is investigated at 60% depth of discharge by feeding the model with the experimental cell design parameters from the literature (i.e. carbon to sulfur ratio and electrolyte to sulfur ratio in the cathode, anode, separator and cathode thicknesses and current density in references [2, 3]). Consequently, cell voltage at 60% DOD for different C:S ratios are predicted by the model proposed and compared with the experimental data. Cell voltage at 60% DOD predicted by the model as a function of C:S ratio for two different current densities is shown in Figure 1. It can be seen that the cell voltage increases significantly with increasing C:S ratio, especially up to a C:S ratio of 1. Further increase in the carbon content leads to a less apparent increase in the cell voltage for both current densities. This may suggest that the cell is limited by the kinetics at C:S ratios lower than 1 whereas the improvement on the cathode kinetics is less significant at higher ratios. The model predictions are also validated with the experimental data [2] as shown in the figure. It is clear that the model predictions show close agreement with the experimental data. Figure 1. The effect of C:S ratio in the cathode on the cell voltage at 60% DOD predicted by the model for two different Li-S cell designs [2, 3]. The comparison of the model predictions with the experimental data [2] is also presented.

Honeycomb-like Nitrogen and Sulfur Dual-Doped Hierarchical Porous Biomass-Derived Carbon for Lithium-Sulfur Batteries.

Honeycomb-like nitrogen and sulfur dual-doped hierarchical porous biomass-derived carbon/sulfur composites (NSHPC/S) are successfully fabricated for high energy density lithium-sulfur batteries. The effects of nitrogen, sulfur dual-doping on the structures and properties of the NSHPC/S composites are investigated in detail by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and charge/discharge tests. The results show that N, S dual-doping not only introduces strong chemical adsorption and provides more active sites but also significantly enhances the electronic conductivity and hydrophilic properties of hierarchical porous biomass-derived carbon, thereby significantly enhancing the utilization of sulfur and immobilizing the notorious polysulfide shuttle effect. Especially, the as-synthesized NSHPC-7/S exhibits high initial discharge capacity of 1204 mA h g-1 at 1.0 C and large reversible capacity of 952 mA h g-1 after 300 cycles at 0.5 C with an ultralow capacity fading rate of 0.08 % per cycle even at high sulfur content (85 wt %) and high active material areal mass loading (2.8 mg cm-2 ) for the application of high energy density Li-S batteries.

Separator modified with N,S co-doped mesoporous carbon using egg shell as template for high performance lithium-sulfur batteries

The migration of long chain soluble lithium polysulfides through the separator to the anode, the so called “shuttle effect”, is one of the major issues responsible for the capacity degradation of lithium-sulfur (Li-S) batteries. In this work, N and S co-doped mesoporous carbon was prepared by a novel egg shell template method and utilized to modify the separator for high performance Li-S batteries. The discharge capacity of the second cycle retained at 1467mAhg−1 at 1C after 200 cycles and decayed at 0.20% per charge-discharge cycle, and a reversible capacity of 561mAhg−1 was achieved even at 5C rate after 200 cycles. The prepared material showed a porous hollow sphere morphology with larger surface area and N and S co-doped that can provide great electrochemical property as well as surface chemistry for the adsorption of the intermediate polysulfides. After separator modification, XPS analysis revealed that pyrrolic-type N, pyridinic-like N, and thiosulphate were the major factors contributing to the superior electrochemical performance of the CS/S cathode. This work also provided a rational design strategy for the modification of separator to effectively utilize the active sulfur and to retard the migration of dissolved polysulfides, which enhanced the performance of high energy density Li-S batteries with long cycling life.

High Energy Density Lithium-Sulfur Batteries for NASA and DoD Applications

We have been successfully using Li-ion batteries in several NASA missions since 2000, including the Mars Exploration Rovers (Spirit and Opportunity), Mars Science Laboratory (MSL), Phoenix Lander, JUNO, Aquarius, Keppler Rovers and SMAP. Being compact, lightweight and durable, these batteries have contributed to a significant enhancement or even enablement of these missions. However, NASA’s future missions, e.g., Astronaut’s Portable Life Support System (PLSS) for Extra-Vehicular Activities (EVA), small planetary rovers, planetary probes, CubeSats etc., require more advanced battery technologies with higher energy densities. The lithium-sulfur system emerges as the most promising technology, because of its high theoretical specific energy (3-4x) compared to Li-ion cells. However, despite many significant developmental efforts over the last few years, Li-S technology hasn’t matured yet, mainly due to the challenges from the soluble polysulfides forming a redox shuttle and also poisoning the lithium anode. Several attempts are being reported in the literature to develop novel cathode designs, e.g., hierarchical porous carbon structures to sequester sulfur and its reduction products, and also electrolyte solutions to minimize their solubility. 1-5 Good cycle life was reported in some of these cases with nanostructured sulfur cathodes in both organic electrolytes containing suitable additives (LiNO3) and in ionic liquids, but the sulfur loadings are much lower (below 4 mg/cm2), while high loadings of >12 mg/cm2 are essential for high specific energy Li-S cells. In this context, we have developed new sulfur composite cathodes blended with transition metal sulfides (e.g., titanium and molybdenum disulfide), which not only provided adequate electronic and ionic conductivity, but also facile electrochemical activity at sulfur reduction potentials to function as redox mediators.6 With such blended cathodes, we have been able to realize high specific capacities of ≥800 mAh/g at C/3 rates with high material loadings. In this paper, we will describe the performance of these composite sulfur cathodes in different electrolytes, and also in conjunction with a polymer-protected Li anode. Y. Yin, S. Xin, Y. Guo and L. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186 (2013).S. Evers, L. F. Nazar, Acc. Chem. Res., 46, 1135 (2013);X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 8, 500 (2009).A. Manthiram, S.-H. Chung, C. Zu, Adv. Mater. 27, 1980 (2015).S. S. Zhang, Front. Energy Res. 1, 1 (2013).Ratnakumar Bugga, Simon Jones, Jasmina Pasalic, Dan Addison and Ramanathan Thillaiyan, 228th ECS Meeting, Phoenix, AZ, Oct. 11 (2015)

Sparingly Solvating Electrolytes for High Energy Density Lithium–Sulfur Batteries

Moving to lighter and less expensive battery chemistries compared to contemporary lithium-ion requires the control of energy storage mechanisms based on chemical transformations rather than intercalation. Lithium–sulfur (Li/S) has tremendous theoretical specific energy, but contemporary approaches to control this solution-mediated, precipitation–dissolution chemistry require large excesses of electrolyte to fully solubilize the polysulfide intermediates. Achieving reversible electrochemistry under lean electrolyte operation is the most promising path for Li/S to move beyond niche applications to potentially transformational performance. An emerging Li/S research area is the use of sparingly solvating electrolytes and the creation of design rules for discovering new electrolyte systems that fundamentally decouple electrolyte volume from sulfur and polysulfide reaction mechanism. This Perspective presents an outlook for sparingly solvating electrolytes as a key path forward for long-lived, high energy densi...

Lithium-Sulfur Batteries: High Loading Cathode, New Electrolyte and Non-Lithium Metal Anode

Lithium sulfur battery (Li-S) has attracted intensive interest in recent years due to its high specific capacity, low cost, and abundant resources of sulfur. Theoretically, a high specific capacity of 1675 mAh g-1 and a specific energy of 2567 Wh kg-1 are expected for sulfur cathode. Practically, the energy density of Li-S battery is projected to be two to three times higher than that of state-of-the-art Li-ion batteries (180 Wh kg-1). However, the commercial applications of Li-S batteries are still hindered by several challenges.2 One is that with elevated sulfur loading above 3 mg/cm2, which is essential for high energy applications, new problems occur including the cathode cracking, electrode wetting and low sulfur utilization rate. The second and more detrimental issue that limits Li-S cell performance is interfacial stability and cycling reversibility of lithium metal anode, which easily reacts with both electrolyte solvents and the formed polysulfides during battery cycling and leads to unavoidable shuttle and continuous consumption of active sulfur species. These side reactions are significantly exacerbated under conditions of coupling with high-areal loading sulfur cathode and/or cycled at elevated current densities. As a result, thick and porous interface will be quickly built up on lithium anode and leads to quick impedance increase and thus fast capacity decay of the battery.3 The third problem is the amount of electrolyte required for full dissolution of the polysulfides. The dissolved polysulfides cause endless shuttle and continuous Li anode corrosion due to the lack of effective solid electrolyte interface (SEI) on lithium. In addition, the large amount of electrolyte significantly lowers the energy density of the battery, which is undesired for high-efficient portable devices or electric vehicle energy storage applications To overcome the aforementioned hurdles in Li-S batteries, multiple approaches have been proposed and successfully demonstrated by our group, spanning from sulfur cathode, anode and electrolyte. At the meeting, we will present our recent progresses on the Li-S battery system including the rational design of carbon/sulfur composite for large area and high loading sulfur cathode preparation, alternative non-Li anode materials for Li-ion sulfur batteries, and development of new electrolyte.

Hybrid Electrolyte and Bi-Functional Cathode Binder for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are promising energy-storage systems, offering up to five-fold increase in energy density as compared with present state-of-the-art Li-ion batteries thus potentially serving as a means to meet the growing demand for safe, environmentally friendly, high-energy density applications. For commercial batteries, cathodes with a high sulfur (active material) loading (> 2 mg/cm2) will be essential. Most reports on engineered cathode materials for Li-S batteries, however, are based upon low sulfur loadings (typically ~1 mg/cm2), which are impractical and often give misleading results about performance characteristics. It is unknown how such cathodes behave under high sulfur loading conditions. The binder is perhaps the most critical material for achieving a high sulfur loading. We have recently used polyamidoamine (PAMAM) dendrimers with differing surface chemistries as functional binders in Li-S cells with SuperP/S as the cathode material. Without any special efforts to engineer the cathode, we were able to obtain very favorable cycling stability and electrolyte wetting. This was attributed to the high density of the surface functional groups on the dendrimers which results in a larger number of possible binder-S/carbon interactions, and high curvature of the binder and high porosity which allows the sulfur particles to be more extensively exposed to the electrolyte (thus enhancing wetting). In addition, the binding between the large number of oxygen atoms on the dendrimers and the polysulfides, along with the greater number of interior pores, aids in preventing the dissolution of the polysulfides in the organic electrolyte. Dendritic macromolecules are also much more polar than linear conventional polymer-based binder chains and have a uniform distribution of surface groups. This ensures better interfacial interactions between the functional groups and carbon/S particles, as well as better adhesion to the substrate. Alongwith the cathode, the electrolyte is another critical component of Li-S batteries as it regulates the polysulfide shuttle phenomena thus determining the rate capability, cycling stabilities and energy densities of the batteries. The safety of the battery is also governed to a large extent by the electrolyte. Here, we will discuss the fundamental characteristics of dendrimers as aqueous binders for sulfur cathodes and compare their performance with other aqueous, commonly used linear polymeric binders such as styrene butadiene rubber (SBR) and sodium carboxyl methyl cellulose (CMC). Specifically, generation 4 PAMAM dendrimers with hydroxyl, 4-carbomethoxypyrrolidone, and sodium carboxylate surface functional groups served as promising, electrochemically stable binders for high sulfur loadings (~3-4 mg/cm2) with high initial cathode capacities (> 1000 mAh/g). In comparison to the use of a CMC-SBR binder, the electrodes with the dendrimer-based binders showed greatly improved performance. The CMC-SBR binder-based electrodes failed at high C-rates (0.2C) after 40 cycles, whereas the electrodes with the dendrimer-based binders had a capacity retention of >85% for more than 100 cycles. A detailed physicochemical characterization of the electrodes will be presented to substantiate the superior dendrimer-carbon/S interactions. We will also discuss our recently developed lithium aluminum germanium phosphate (LAGP) based solid-polymer-liquid hybrid electrolyte-separator system in conjunction with the above promising cathode materials for Li-S batteries. The hybrid separator showed (i) higher liquid electrolyte uptake, (ii) higher room T ionic conductivity, (iii) lower interfacial resistance with lithium, and (iv) lower cell voltage polarization during lithium cycling at high current density of 1.3 mA/cm2 at room temperature. The enhanced performance is attributed to higher liquid uptake, LAGP-assisted faster ion conduction and dendrite prevention. It is anticipated that by using novel dendrimer chemistries for electrode assembly, and hybrid electrolyte systems, this research will lead to the development and commercialization of high energy density, safe and long-cycle life Li-S batteries. Figure 1

Nanoscale Interfacial Interaction of Polysulfide with Porous Metal Oxides

Sulfur is a promising cathode for high energy density lithium-sulfur battery, but plagued by polysulfide dissolution causing loss of active material and detrimental parasitic reactions. In recent years multiple strategies have been employed to address the parasitic reaction of polysulfide species and increase the life cycle of the Li-S battery. One of the main approaches is employing porous materials as internal reservoirs which can absorb/desorb polysulfides during electrochemical cycling and thereby prevent the shuttling process. Despite intensive efforts to design optimal reservoir materials, the progress in this field is dragged by the knowledge gap in terms of molecular level view about polysulfide interaction with host material. We studied the interaction of polysulfide species including highly reactive trisulfide free radicals with porous silica and alumina metal oxide nano-particles The polysulfide interaction with metal centers of nano-particles are analyzed using combined experimental spectroscopic studies (magnetic resonance and X-ray absorption spectroscopy) and density functional theory based computational methods. This work provides molecular level insight about polysulfide reactivity on metal oxide surfaces which can help in understanding the emergent behaviors of hetrogeneous interfaces.

Pie-like electrode design for high-energy density lithium–sulfur batteries

Owing to the overwhelming advantage in energy density, lithium–sulfur (Li–S) battery is a promising next-generation electrochemical energy storage system. Despite many efforts in pursuing long cycle life, relatively little emphasis has been placed on increasing the areal energy density. Herein, we have designed and developed a ‘pie' structured electrode, which provides an excellent balance between gravimetric and areal energy densities. Combining lotus root-like multichannel carbon nanofibers ‘filling' and amino-functionalized graphene ‘crust', the free-standing paper electrode (S mass loading: 3.6 mg cm−2) delivers high specific capacity of 1,314 mAh g−1 (4.7 mAh cm−2) at 0.1 C (0.6 mA cm−2) accompanied with good cycling stability. Moreover, the areal capacity can be further boosted to more than 8 mAh cm−2 by stacking three layers of paper electrodes with S mass loading of 10.8 mg cm−2.

Efficient Synthesis of Hierarchical Porous Carbons for Application in Li-S Cells

The Li-S battery is a promising candidate to meet the demands on future generation mobile power supplies due to the high theoretical gravimetrical energy density and low price of sulfur. Unfortunately, the active material sulfur exhibits a very low conductivity, which limits the performance. A nanostructured carbon matrix is required. Up to date, large numbers of novel, elaborate host structures have been explored. However, in particular, suitable and effective production processes need to be established. To overcome the existing drawbacks we focus on the development of a scalable and efficient approach for the synthesis of nanostructured porous carbon materials by using of inexpensive, commercially available ZnO nanoparticles as hard template.[1] One major benefit of the process is the in-situ removal of all pore building components through carbothermal reduction. Thus, no toxic/reactive gases are required and purification of the as-prepared porous carbon is not necessary. Adjusting the ZnO-NP template size, porous carbons with internal pore volumes as high as 3.9 cm3 g-1 and tailored pore sizes reaching from 20 nm to 200 nm with ideal template replication are obtained. The carbothermal reduction process, furthermore, facilitates the access of a hierarchical pore system with distinctive microporosity leading to very high specific surface areas even exceeding 3000 m-2 g-1. The synergetic advantage of that bimodal porosity renders them particularly suitable for the application in high energy density lithium-sulfur batteries. A high capacity of > 1200 mAh g-1-sulfur (> 750 mAh g-1-electrode) at a high sulfur loading of ≥ 3 mg cm-2 and good cycle stability was achieved even using a low amount of electrolyte. This allows for further weight reductions and maintenance of high energy density on cell level. [1] Strubel et al., Adv. Funct. Mater. 2015, 25, 287–297. doi: 10.1002/adfm.201402768.

High Energy Density Lithium–Sulfur Batteries: Challenges of Thick Sulfur Cathodes

High energy and cost‐effective lithium sulfur (Li–S) battery technology has been vigorously revisited in recent years due to the urgent need of advanced energy storage technologies for green transportation and large‐scale energy storage applications. However, the market penetration of Li–S batteries has been plagued due to the gap in scientific knowledge between the fundamental research and the real application need. Here, a facile and effective approach to integrate commercial carbon nanoparticles into microsized secondary ones for application in high loading sulfur electrodes is proposed The slurry with the integrated particles is easily cast into electrode laminates with practically usable mass loadings. Uniform and crack‐free coating with high loading of 2–8 mg cm−2 sulfur are successfully achieved. Based on the obtained thick electrodes, the dependence of areal specific capacity on mass loading, factors influencing electrode performance, and measures used to address the existing issues are studied and discussed.

Graphene-wrapped sulfur nanospheres with ultra-high sulfur loading for high energy density lithium–sulfur batteries

Lithium–sulfur (Li–S) battery with high theoretical energy density is one of the most promising energy storage systems for electric vehicles and intermittent renewable energy. However, due to the poor conductivity of the active material, considerable weight of the electrode is occupied by the conductive additives. Here we report a graphene-wrapped sulfur nanospheres composite (S-nanosphere@G) with sulfur content up to 91wt% as the high energy density cathode material for Li–S battery. The sulfur nanospheres with diameter of 400–500nm are synthesized through a solution-based approach with the existence of polyvinylpyrrolidone (PVP). Then the sulfur nanospheres are uniformly wrapped by conductive graphene sheets through the electrostatic interaction between graphene oxide and PVP, followed by reducing of graphene oxide with hydrazine. The design of graphene wrapped sulfur nanoarchitecture provides flexible conductive graphene coating with void space to accommodate the volume expansion of sulfur and to minimize polysulfide dissolution. As a result, the S-nanosphere@G nanocomposite with 91wt% sulfur shows a reversible initial capacity of 970mAhg−1 and an average columbic efficiency>96% over 100 cycles at a rate of 0.2C. Taking the total mass of electrode into account, the S-nanosphere@G composite is a promising cathode material for high energy density Li–S batteries.

Three-dimensional aluminum foam/carbon nanotube scaffolds as long- and short-range electron pathways with improved sulfur loading for high energy density lithium–sulfur batteries

Conductive carbon scaffolds are efficient and effective to build advanced carbon/sulfur composite cathodes for lithium–sulfur (Li–S) batteries. However, the areal sulfur loading is commonly less than 4.0 mg cm−2, which limits the energy density and practical application of Li–S cells. In this contribution, three-dimensional (3D) aluminum foam/carbon nanotube (CNT) scaffolds were applied as the current collectors to build long- and short-range electron pathways and provided enough space for high sulfur loading. The sulfur loading amount on the 3D current collectors ranged from 7.0 to 12.5 mg cm−2. A high initial discharge capacity of 6.02 mAh cm−2 (860 mAh g−1) was achieved on an electrode with an improved sulfur loading of 7.0 mg cm−2. Therefore, the combination of 3D long-range current collectors and short-range CNT conductive scaffold provides an efficient and effective route to make full use of sulfur with a very high sulfur loading amount in a Li–S cell.

Progress on high-energy density lithium-sulfur batteries

Recently, lithium-sulfur batteries as high energy-density energy storage system have attracted more and more attention. In this article, we first introduce lithium-sulfur batteries and then review recent progress in carbon-sulfur cathodes and electrolytes for lithium-sulfur batteries. Furthermore, we analyze and summarize the reported strategies to inhibit lithium polysulfide dissolution. Particularly, we propose a new class of bi-functional Solvent-in-Salt (SIS) electrolyte with high lithium salt concentration for solving the problems resulting from both sulfur and lithium metal electrodes. Finally, we present the outlook for future lithium-sulfur batteries.

Electrolyte materials for high energy density lithium-sulfur secondary battery

Rechargeable Li-S secondary batteries have received ever-increasing attention recently due to their high theoretical specific energy density, wide material sources, low cost and being environmental friendly. Thus, the Li-S battery is becoming the most promising candidate for energy storage devices. But the current research shows that the polysulfides formed as intermediates during the discharge process can dissolve easily into the electrolyte, which will result in shuttle phenomenon. On the one hand, the high solubility of polysulfides leads to the loss of active material; on the other hand, the shuttle mechanism results in reduced columbic efficiency and poor cycle ability. So suppressing the polysulfides dissolution along with inhibiting the occurrence of shuttle phenomenon can effectively improve the whole performance of lithium sulfur battery. In this paper, the electrolytes used effectively in Li-S batteries are reviewed in detail including the organic liquid electrolytes, ionic liquids electrolytes, solid-state electrolytes, gel-like electrolytes, microporous polymer electrolytes, composite-solid polymer electrolytes along with the glass-ceramic electrolytes. Furthermore, the possible development prospects are also discussed.

New Approaches for High Energy Density Lithium–Sulfur Battery Cathodes

The goal of replacing combustion engines or reducing their use presents a daunting problem for society. Current lithium-ion technologies provide a stepping stone for this dramatic but inevitable change. However, the theoretical gravimetric capacity (∼300 mA h g(-1)) is too low to overcome the problems of limited range in electric vehicles, and their cost is too high to sustain the commercial viability of electrified transportation. Sulfur is the one of the most promising next generation cathode materials. Since the 1960s, researchers have studied sulfur as a cathode, but only recently have great strides been made in preparing viable composites that can be used commercially. Sulfur batteries implement inexpensive, earth-abundant elements at the cathode while offering up to a five-fold increase in energy density compared with present Li-ion batteries. Over the past few years, researchers have come closer to solving the challenges associated with the sulfur cathode. Using carbon or conducting polymers, researchers have wired up sulfur, an excellent insulator, successfully. These conductive hosts also function to encapsulate the active sulfur mass upon reduction/oxidation when highly soluble lithium polysulfides are formed. These soluble discharge products remain a crux of the Li-S cell and need to be contained in order to increase cycle life and capacity retention. The use of mesoporous carbons and tailored designs featuring porous carbon hollow spheres have led to highly stable discharge capacities greater than 900 mA h g(-1) over 100 cycles. In an attempt to fully limit polysulfide dissolution, methods that rely on coating carbon/sulfur composites with polymers have led to surprisingly stable capacities (∼90% of initial capacity retained). Additives will also play an important role in sulfur electrode design. For example, small fractions (> 3 wt%) of porous silica or titania effectively act as polysulfide reservoirs, decreasing their concentration in the electrolyte and leading to a higher utilization of sulfur and increased capacities.

Preparation and characterization of sulfur–polypyrrole composites with controlled morphology as high capacity cathode for lithium batteries

Sulfur was highly combined with two types of polypyrrole (PPy) in granular (G-PPy) and tubular(T-PPy) morphology by in-situ oxidation and co-heating methods. The morphology of polypyrrole shows a significant effect on the dispersion status and electrochemical behaviors of the sulfur. A stable capacity close to 500 mAh/g was maintained over 60 cycles for the S/T-PPy composite. Electrochemical measurement results suggest that the S/T-PPy composite is obviously superior to the S/G-PPy composite. It is suggested that the as-proposed tubular PPy could be a promising electric matrix for sulfur active host for a high energy density lithium–sulfur battery.