1:30pm - 2:00pmKeynote
High Energy Density Lithium-Sulfur Batteries with High Sulfur Loading
University of Texas at Austin, United States
Development of next-generation energy-storage systems needs to consider a balance among cost, cycle life, safety, energy, power, and environmental impact. With these considerations, lithium-sulfur (Li-S) batteries have become attractive because of the high charge-storage capacity, natural abundance, and environmental friendliness of sulfur. However, the practical implementation of Li-S batteries is hampered by low electrochemical utilization of sulfur, severe polysulfide diffusion from the cathode to the anode, and degradation of lithium-metal anode, resulting in low practical energy density, poor cycle life, and fast self-discharge. Unfortunately, efforts to overcome these persistent problems result in an incorporation of excessive conductive carbon into the electrode, low sulfur loading per unit area, and a high amount of liquid electrolyte in the cell, defeating the energy-density advantage of sulfur cathodes and the purpose of Li-S cells replacing the current lithium-ion technology. Moreover, the traditional cathode configuration borrowed from the commercial insertion-compound cathodes may not allow the pure sulfur cathode with a conversion reaction to put its unique materials chemistry to good use. Recognizing these challenges, this presentation will focus on unique approaches in engineering the sulfur cathodes, separators, and lithium-metal anode to realize high energy density and long dynamic (cyclability) and static (self-discharge) stabilities. The cathode engineering allows remarkably high sulfur loadings of up to 50 mg/cm2 with good electrochemical utilization. The conventional polymeric separators coated with a thin layer of carbon with optimized pore structures suppress polysulfide diffusion and enhance electron transport. The incorporation of specific inorganic salts to the lithium-metal anode produces a protective layer on lithium metal anode and minimizes the attack by polysulfides. These integrated approaches offer promise to realize high energy density lithium-sulfur batteries with high sulfur loading, long cycle life, and low self-discharge.
2:00pm - 2:30pmInvited
Advanced Lithium-Sulfur Batteries: The Core Contribution of Nanocarbon
Tsinghua University, China
Among various promising candidates with high energy densities, lithium-sulfur (Li-S) batteries with a high theoretical capacity and energy density are highly attractive;[1-2] while the commercial application of Li-S batteries still faces some persistent obstacles, such as the low electrical conductivity of sulfur and lithium sulfide and the dissolution of polysulfides. The introduction of 3D graphene into the field of Li-S batteries sheds a light on the efficient utilization of sulfur by improving the conductivity of the composites and restraining the shuttle of polysulfides. In this presentation, the concept for the rational design of 3D graphene is explained. The advances in the use of 3D graphene in the cathode, separator, and anode is explained.[3-12] New insights on the relationship between the 3D graphene structure and the electrochemical performance are presented.
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2:30pm - 2:45pmOral
Understanding the Performance Bottleneck in Li-S batteries: A Model-informed Approach
1Imperial College London, United Kingdom; 2OXIS Energy Ltd, United Kingdom
Lithium-sulphur (Li-S) batteries could provide the next step-change in battery technology with a promising practical energy density of 500-600 Wh/kg. However, further improvement in the energy density and cycle-life of Li-S cells is arguably held back by a lack of understanding of their complex charge/discharge mechanisms. Acquiring this knowledge requires experimental characterizations in tandem with mathematical modelling.
In this presentation, we demonstrate the key factors determining the cycle-life and rate-capability of Li-S batteries through experimentally validated Li-S models. A zero-dimensional model accounting for degradation due to precipitation as well as polysulfide shuttle was developed to interpret the complex behaviour observed in cycling experiments and to distinguish between reversible and irreversible degradation. This model provides a tool for knowledge based choices between improving performance and sacrificing power or energy, such as through the introduction of recovery cycles. In addition, a one-dimensional model was developed to capture the limited rate-capability of Li-S cells caused by slow ionic transport. The model suggests polysulfide species remain in the separator due to mass transport limitations towards the end of discharge, but the resulting ‘lost’ capacity can be recovered through relaxation, which was then validated experimentally. Finally, we discuss the modelling of precipitation-induced electrode surface coverage, which we show to affect the discharge capacity and the charge characteristics of Li-S cells.
 T. Zhang, R. Purkayastha, G. Minton, M. Marinescu and G. J. Offer, Energy Environ. Sci., 2015, 8, 3477-3494.
 M. Marinescu, T. Zhang, G. J. Offer, Phys. Chem. Chem. Phys., 2016, 18, 584-593.
 T. Zhang, M. Marinescu, M. Wild, L. O’Neill, and G. J. Offer, Phys. Chem. Chem. Phys., 2015, 17, 22581-22586.
 T. Zhang, M. Marinescu, S. Walus, G. J. Offer, Electrochim. Acta, 2016, 219, 502-508.
2:45pm - 3:00pmOral
Poly(3,4-ethylenedioxypyrrole) Coated Carbon Nanotubols Loaded with Sulfur as Cathodes for Long Lasting Li-S Batteries
1Indian Institute of Technology Hyderabad, India; 2Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), India; 3Defence Metallurgical Research Laboratory, Defence Research and Development Organisation, India
Lithium-sulfur (Li-S) batteries are attractive for sulfur is cost effective, and offers high gravimetric capacity and a large energy density. But achieving long term cyclability with moderate capacity loss, and scalability pose formidable challenges. PEDOP is coated onto a composite of sulfur with hydroxyl functionalized multiwalled carbon nanotubes (MWCNTols) by an in-situ polymerization of EDOP monomer with a chemical oxidizing agent. Li-S performances are compared at 0.1 C current-rate, and they reveal that the pristine S based cell with a S-loading of 80% retains a low capacity of 122 mAh gsulfur-1 after 100 cycles, while cells with S/MWCNTols and S/MWCNTols/PEDOP composites with sulfur loadings of 73 and 70% respectively, retain capacities of 384 and 624 mAh gsulfur-1 after 200 charge-discharge cycles, with Coulombic efficiencies of 96 and 98.7% respectively. This performance differential illustrates the conductive polymer coating effectively trap the polysulfides and minimizes the dissolution of polysulfides and inhibit the sulfur loss in cathodes which leads to maximizing the performance of Li-S batteries. The synergistic effect of MWCNTols and PEDOP conductive polymer leads to a good electronic conductivity and alleviates the dissolution of polysulfides into the electrolyte and accommodates the stress and volume expansion during the discharge. The polymer provides electrical interconnects between the insulating sulfur clusters and facilitates Li+ transfer at the interface. The ease of the synthesis, coupled with the remarkable cycling performance delivered by this composite at a high sulfur-loading, demonstrate the promise that this S/CNT/conducting polymer composite has for practical Li-S batteries.
3:00pm - 3:15pmOral
Rational Design of 3D Hierarchical Porous Carbon Nanosheets as Polysulfides Immobilizer to Boost the Performance of Lithium-Sulfur Batteries
1The Hong Kong University of Science & Technology (HKUST), Hong Kong S.A.R. (China); 2Peking University, China
Lithium–sulphur batteries (LSBs) with a high theoretical energy density are being pursued as highly promising next generation large-scale energy storage devices. However, it's launching for practical application is still shackled by poor conductivity, limited sulfur loading and most seriously by polysulfides dissolution in the organic electrolyte. To date, 3D porous carbon nanostructures (3D-PCNs) are attractive candidates for rechargeable batteries because they can integrate multiple advantages of unique collective effects and great potential electrochemical applications. We demonstrated a simple carbonisation method to make novel 3D highly micro-mesoporous, vertically aligned and interconnected carbon nanosheets (3D-VCNs) for solving the hurdles associated with LSBs to bring high performance at low cost. The present 3D nanostructures with the very high surface area of 1750 m2g-1 are highly particular for enhancing the performance of LSBs in the terms of capacity, rate ability, and cycling stability. The developed porous structure is beneficial in facilitating the easy access of electrolyte through the structure of 3D-VCNs infiltered with sulfur (3D-S-VCNs) during the cycling process. As a consequence, the unique 3D-S-VCNs show high initial discharge capacity of 1240 mAh g−1 at the current density of 167 mA g−1 with excellent Coulombic efficiency of ≈100% and presents a long stability up to 300 cycles with high reversible specific capacity of 844 mAh g-1 and capacity retention of ≈80.3% (a capacity decay of only 0.082% per cycle) at the current density of 837 mA g-1. Furthermore, the electrode bears the excellent rate capability and maintains a high reversible capacity of 738 mAh g-1 at the high current density of 3340 mA g-1.
3:15pm - 3:30pmOral
Novel Cathode Materials for Li-S Batteries
Indian Institute of Science, India
There has been a considerable shift in research focus from conventional intercalation compound-based towards element-based electrodes for electrochemical energy storage. Transition from an intercalation compound to an elemental cathode e.g. sulfur theoretically raises the storage capacities by more than one order in magnitude. In this context, sulfur constitutes an important example. Sulfur is both an electron and ion insulator and hence enhancement of its conductivity is absolutely necessary. The conventional procedure is to entrap sulfur in a conducting host e.g. hollow carbon structures. From the electrochemical point of view, the management of the polysulfides formed during the discharge process is a much more challenging and non-trivial problem. The problem aggravates when the polysulfides leach out into the electrolyte and diffuse towards the Li anode, depositing on it as insoluble Li2S and Li2S2. Extensive loss of active material along with polysulfide shuttle causes massive capacity fade. Hence, conversion of sulfur from a non-polar S8 state to polar Sx2- state and vis a vis necessitates its efficient management inside a compatible host. Our work is focused towards development of suitable hosts and methods for hosting sulfur for Li-S batteries. In this presentation, several different types of hosts including their synthesis and structural characteristics will be discussed which can effectively entrap sulfur and various polysulfides for large number of successive discharge and charge cycles at various current densities. The presentation will also include detailed electrochemical and charge transport studies elucidating the chemical design and processing strategies.