Shuttle Effect Of Polysulfides Research Articles

Hierarchical MoS2-Carbon Microspheres: A Robust Anode for High Performance Lithium Ion Battery

With higher theoretical lithium ion capacity of 670 mAh g–1 comparing with commercial graphite electrodes, molybdenum disulfide (MoS2) may be a promising alternative for lithium ion batteries (LIBs) because it offers unique layered crystal structure with Mo atoms sandwiched between two layers of closely packed S atoms and the MoS2 layers are linked by weak van der Waals interaction. The large and tunable distance between layers enables the anticipated excellent rate and cycling stability because they can promote the reversible lithium ion intercalation and de-intercalation without huge volume change and consequently prevents the pulverization of active materials during the repeated charge and discharge processes. However, either bulk or nanoscale MoS2 delivers poor conductivity for the electron/ion transfer, thus leading to obvious capacity loss after several cycles. Moreover, the MoS2 based electrodes provide high capacity through similar electrochemical redox reactions with lithium sulfide batteries in essence, which may also lead to their degradation by the polysulfide shuttling effect. To overcome these barriers, numerous efforts have been devoted into the engineering of MoS2 nanostructures with optimized electrochemical performances. One effective strategy is to make the MoS2 into a few expanded layers, which provides a larger surface area, shorter lithium ions diffusion path and thus improved kinetics. On the other hand, combining conductive materials such as carbon or polymer with MoS2 will necessarily enhance the electron transport, cycling stability, structural integrity during the lithium-ion insertion/extraction processes. Herein, we prepared hierarchical MoS2-carbon microspheres via continuous and scalable ultrasonic nebulization route. The structure, composition, electrochemical properties are investigated in detail. The MoS2-carbon microspheres consist of MoS2 nanosheets with a few layers bridged by carbon (15 wt%), which separates the exfoliated MoS2 layers and prevents their aggregation and restacking. The novel architecture offers additional merits such as overall large size, high packing density, which promote their practical applications. The MoS2-C microspheres have been demonstrated to deliver excellent electrochemical performances in terms of low resistance, high capacity even at large current density, stable cycling performances, etc. The electrodes exhibited 800 mAh g–1 at 1000 mA g–1 over 170 cycles. At higher current density of 3200 mA g–1, a capacity of 730 mAh g–1 can be also maintained. The MoS2-C microspheres are practically applicable not only because of the continuous and large scale synthesis via current strategy, but also the robust and integrated architecture which ensures the excellent electrochemical properties.

Nitrogen-Doped Porous Carbon with High Degree of Graphitization Derived from Silk Cocoon for Lithium-Sulfur Batteries

Lithium/sulfur batteries (Li/S) are receiving significant attention due to its high theoretical specific capacity (1675 mA·h/g) and high theoretical specific energy (2600 W·h/kg), and inexpensive sulfur. However, the practical applications of Li/S batteries are still greatly hindered because of the low coulombic efficiency, poor cycle life and rate capability. To enhance the inherent insulating property of sulfur and insoluble discharge products and alleviate the shuttling effect of soluble polysulfides, here, a new type of nitrogen-doped porous carbon (NPC) derived from renewable silk cocoon has been successfully prepared via simultaneously activation and graphitization using metal salts FeCl3 and ZnCl2. Interestingly, the as-prepared NPC exhibits an extremely high degree of graphitization and possesses a high specific surface area of 1058 m2 g-1 and a large pore volume of 0.9026 cm3 g-1. The NPC/S composite with encapsulated sulfur content of 63% was prepared by a facile melt-diffusion method. The NPC/S electrode delivered a high initial discharge capacity of 1499 mAh g- 1 at 80 mA g-1 and coulombic efficiency of over 98%. Especially, the NPC/S composite still show excellent cycle life when the rate recover to 800 mA g-1 after cycling at various rates from 80 to 2560 mA g-1. Taking into account the low-cost of renewable biomass carbon resource and facile strategy, this work is an effective approach to increase the utilization of sulfur and improve the electrochemical performance of Li/S batteries.

Mesoporous TiO2 Nanocrystals/Graphene as an Efficient Sulfur Host Material for High-Performance Lithium-Sulfur Batteries.

Rechargeable lithium-sulfur (Li-S) batteries are promising in high-energy storage due to the large specific energy density of about 2600 W h kg(-1). However, the low conductivity of sulfur and discharge products as well as polysulfide-shuttle effect between the cathode and anode hamper applications of Li-S batteries. Herein, we describe a novel and efficient S host material consisting of mesoporous TiO2 nanocrystals (NCs) fabricated in situ on reduced graphene oxide (rGO) for Li-S batteries. The TiO2@rGO hybrid can be loaded with 72 wt % sulfur. The strong chemisorption ability of the TiO2 NCs toward polysulfide combined with high electrical conductivity of rGO effectively localize the soluble polysulfide species within the cathode and facilitate electron and Li ions transport to/from the cathode materials. The sulfur-incorporated TiO2@rGO hybrid (S/TiO2@rGO) shows large capacities of 1116 and 917 mA h g(-1) at the current densities of 0.2 and 1 C (1 C = 1675 mA g(-1)) after 100 cycles, respectively. When the current density is increased 20 times from 0.2 to 4 C, 60% capacity is retained, thereby demonstrating good cycling stability and rate capability. The synergistic effects of TiO2 NCs toward effective chemisorption of polysulfides and conductive rGO with high electron mobility make a promising application of S/TiO2@rGO hybrid in high-performance Li-S batteries.

Synergistic Ultrathin Functional Polymer-Coated Carbon Nanotube Interlayer for High Performance Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have been intensively investigated as a next-generation rechargeable battery due to their high energy density of 2600 W·h kg(-1) and low cost. However, the systemic issues of Li-S batteries, such as the polysulfide shuttling effect and low Coulombic efficiency, hinder the practical use in commercial rechargeable batteries. The introduction of a conductive interlayer between the sulfur cathode and separator is a promising approach that has shown the dramatic improvements in Li-S batteries. The previous interlayer work mainly focused on the physical confinement of polysulfides within the cathode part, without considering the further entrapment of the dissolved polysulfides. Here, we designed an ultrathin poly(acrylic acid) coated single-walled carbon nanotube (PAA-SWNT) film as a synergic functional interlayer to address the issues mentioned above. The designed interlayer not only lowers the charge transfer resistance by the support of the upper current collector but also localizes the dissolved polysulfides within the cathode part by the aid of a physical blocking and chemical bonding. With the synergic combination of PAA and SWNT, the sulfur cathode with a PAA-SWNT interlayer maintained higher capacity retention over 200 cycles and achieved better rate retention than the sulfur cathode with a SWNT interlayer. The proposed approach of combining a functional polymer and conductive support material can provide an optimiztic strategy to overcome the fundamental challenges underlying in Li-S batteries.

Probing Polysulfide Shuttle Effect in the Li-S Battery

The lithium-sulfur battery is a strong contender among the beyond Li-ion chemistries due to its superior capacity and energy density as compared to the conventional Li-ion cells. However, experimental evidences suggest that even the first discharge capacity is appreciably lower than the theoretical limit. This non-ideality is usually associated with the so-called “polysulfide shuttle effect”. On the cathode side, solid sulfur dissolves in the electrolyte and undergoes successive reduction to different intermediate and reduced-order sulfide ions. Due to the underlying potential and concentration gradients, these negative ions travel toward the anode and chemically oxidize the lithium metal, while being reduced simultaneously. Theoretical capacity is achieved when sulfur undergoes complete electrochemical reduction; hence the above chemical reaction(s) limit the available capacity. Lithium sulfide (Li2S) is the final discharge product which forms when the ionic product exceeds the solubility limit. This can passivate the surface, block the porous structure and may lead to the reacting species starvation in the cathode. The shuttle effect is a resultant manifestation of these coupled phenomena. In this work, a computational study is presented in order to quantify the influence of shuttle effect on the Li-S cell capacity. The model correlates concentration evolution and microstructural changes with cell performance. Based on this understanding, electrode modifications such as, cathode coating and microstructure properties are studied in order to fundamentally understand the deleterious influence of the “shuttle effect” on cell performance. Figure 1

Improvement of Cycling Performance of Lithium-Sulfur Batteries by Using Magnesium Oxide as a Functional Additive for Trapping Lithium Polysulfide.

Trapping lithium polysulfides formed in the sulfur positive electrode of lithium-sulfur batteries is one of the promising approaches to overcome the issues related to polysulfide dissolution. In this work, we demonstrate that intrinsically hydrophilic magnesium oxide (MgO) nanoparticles having surface hydroxyl groups can be used as effective additives to trap lithium polysulfides in the positive electrode. MgO nanoparticles were uniformly distributed on the surface of the active sulfur, and the addition of MgO into the sulfur electrode resulted in an increase in capacity retention of the lithium-sulfur cell compared to a cell with pristine sulfur electrode. The improvement in cycling stability was attributed to the strong chemical interactions between MgO and lithium polysulfide species, which suppressed the shuttling effect of lithium polysulfides and enhanced the utilization of the sulfur active material. To the best of our knowledge, this report is the first demonstration of MgO as an effective functional additive to trap lithium polysulfides in lithium-sulfur cells.

Three-dimensional graphene framework with ultra-high sulfur content for a robust lithium–sulfur battery

Lithium–sulfur batteries can deliver significantly higher specific capacity than standard lithium ion batteries, and represent the next generation of energy storage devices for both electric vehicles and mobile devices. However, the lithium–sulfur technology today is plagued with numerous challenges, including poor sulfur conductivity, large volumetric expansion, severe polysulfide shuttling and low sulfur utilization, which prevent its wide-spread adoption in the energy storage industry. Here we report a freestanding three-dimensional (3D) graphene framework for highly efficient loading of sulfur particles and creating a high capacity sulfur cathode. Using a one-pot synthesis method, we show a mechanically robust graphene–sulfur composite can be prepared with the highest sulfur weight content (90% sulfur) reported to date, and can be directly used as the sulfur cathode without additional binders or conductive additives. The graphene–sulfur composite features a highly interconnected graphene network ensuring excellent conductivity and a 3D porous structure allowing efficient ion transport and accommodating large volume expansion. Additionally, the 3D graphene framework can also function as an effective encapsulation layer to retard the polysulfide shuttling effect, thus enabling a highly robust sulfur cathode. Electrochemical studies show that such composite can deliver a highest capacity of 96 mAh·g–1, a record high number achieved for all sulfur cathodes reported to date when normalized by the total mass of the entire electrode. Our studies demonstrate that the 3D graphene framework represents an attractive scaffold material for a high performance lithium sulfur battery cathode, and could enable exciting opportunities for ultra-high capacity energy storage applications.

Acacia Senegal–Inspired Bifunctional Binder for Longevity of Lithium–Sulfur Batteries

The sulfur cathode in traditional lithium–sulfur batteries suffers from poor cyclability due to polysulfide shuttling effect as well as large volume change during charge/discharge processes. Gum arabic (GA), a low cost, nontoxic, and sustainable natural polymer from Acacia senegal, is adopted as a binder for the sulfur cathode to address these issues. The excellent mechanical properties of GA endow the cathode with high binding strength and suitable ductility to buffer volume change, while the functional groups chemically and physically confine sulfur species within the cathode to inhibit the shuttling effect of polysulfides. Additionally, GA shifts the electrode fabrication process from the organic solvent process to an aqueous process, eliminates the use of toxic organic solvents, and achieves uniformly distributed electrode with lower impedance. A remarkable cycling performance, i.e., 841 mAh g−1 at low current rate of C/5, is achieved throughout 500 cycles due to the bifunctions of the GA binder.

Understanding the Effect of a Fluorinated Ether on the Performance of Lithium–Sulfur Batteries

A high performance Li-S battery with novel fluoroether-based electrolyte was reported. The fluorinated electrolyte prevents the polysulfide shuttling effect and improves the Coulombic efficiency and capacity retention of the Li-S battery. Reversible redox reaction of the sulfur electrode in the presence of fluoroether TTE was systematically investigated. Electrochemical tests and post-test analysis using HPLC, XPS, and SEM/EDS were performed to examine the electrode and the electrolyte after cycling. The results demonstrate that TTE as a cosolvent mitigates polysulfide dissolution and promotes conversion kinetics from polysulfides to Li2S/Li2S2. Furthermore, TTE participates in a redox reaction on both electrodes, forming a solid electrolyte interphase (SEI) which further prevents parasitic reactions and thus improves the utilization of the active material.

Additive Effect on the Electrochemical Performance of Lithium–Sulfur Battery

Lithium difluoro(oxalato) borate (LiDFOB) was investigated as an electrolyte additive for the Li-S battery. This additive was identified to be an efficient electrolyte additive to suppress the polysulfide shuttling effect existing in the conventional Li–S chemistry. To detect the positive impact of the new additive, oligo(ethylene glycol) functionalized silane was employed as the electrolyte solvent due to its high solvation capability with the lithium polysulfides. The electrochemical results and the SEM data of Li–S battery using the new electrolyte confirmed the role of the LiDFOB as a critical component to eliminate the shuttling of the dissolved polysulfides thus enabling a high coulombic efficiency.

Improved Performance of Lithium Sulfur Battery with Fluorinated Electrolyte

Lithium-ion secondary battery is very promising for energy storage in powering electric vehicles (EV's) due to its high energy density and high durability against many charges and discharge cycles. However, currently these batteries cannot offer a suitably long driving range (i.e., >300 km) for pure electric vehicles. To compete in the market with gasoline-based vehicles, new batteries are required for the next-generation EV’s to provide much higher energy density, and reduce cost factors. 1 Lithium-sulfur battery is a promising energy storage system due to its superior specific capacity (1675 mAh per gram of sulfur), its wide range of operating temperature, intrinsic overcharge protection and more importantly low cost. However, this battery technology has not matured to date due to several technological barriers such as rapid capacity fading and low coulombic efficiency, which are believed to be mainly associated with the loss of sulfur active material during repeated charge and discharge process through the dissolution of lithium polysulfides into the electrolyte and the side reactions of dissolved polysulfide species with the electrolyte solvent and the lithium anode. [2-7] To mitigate the severe shuttling effect of polysulfides in the conventional electrolyte, a new electrolyte approach was adapted by incorporating a fluorinated solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) in the electrolyte formulation. Based on the density functional theory, the fluorine substitution in ethyl propyl ether lowers both LUMO and HOMO energy levels, resulting in simultaneously high reduction potential and oxidation stability of the fluorinated ether. The theoretical calculation indicates that the fluorinated electrolyte solvents are thermodynamically more favorable for electrochemical reduction reaction forming a passivation layer on both electrodes than their non-fluorinated counterparts under certain voltage conditions. Our experimental results demonstrated that a binary solvent electrolyte comprising of TTE and 1, 3-dioxolane (DOL) displayed a superior cycling performance in Li-S cell employing a carbon/sulfur nanocomposite electrode with 75% of sulfur content. The new fluorinated electrolyte suppressed the deleterious shuttling effect and therefore improved the capacity retention and coulombic efficiency in cell tests. SEM/EDS analysis confirmed the improved performance was due to the detainment of polysulfides inside the electrode after the formation of a passivation layer. Also, this electrolyte has an amazing effect on the self-discharge of the Li-S cell and to the best of our knowledge; this is the first electrolyte which has shown excellent shelf life at the fully charged stage.Reference:1. Bruno Scrosati, Jürgen Garche, J. Power Sources 195 (2010) 2419–24302. P.G. Bruce, L.J. Hardwick, K.M. Abraham, Mater. Res. Soc. Bull. 36 (2011) 506.3. W. Wang, V.G. Pol, K. Amine, Adv. Mater. 25 (2013) 1608.4. S.S. Zhang, J. Power Sources 231 (2013) 153.5. J. Nelson, S. Misra, Y. Yang, A. Jackson, Y. Liu, H. Wang, H. Dai, J.C. Andrews, Y. Cui, M.F. Toney, J. Am. Chem. Soc. 134 (2012) 6337.6. Y. Yang, G. Zheng, S. Misra, J. Nelson, M.F. Toney, Y. Cui, J. Am. Chem. Soc. 134 (2012) 15387.7. B. Zhang, X. Qin, G.R. Li, X.P. Gao, Energy Environ. Sci. 3 (2010) 1531.