Development Of Lithium-sulfur Batteries Research Articles

Techniques for realizing practical application of sulfur cathodes in future Li-ion batteries

The development of lithium-sulfur batteries is associated with many problems. These problems include polysulfide dissolution, the shuttle phenomenon, the low electric and ionic conductivity of S, and the volume change that occurs during charge and discharge. In this review, various elemental techniques for overcoming these problems are summarized from the standpoints of the supporting materials. These techniques include preventing polysulfide dissolution from the cathodes through physical and chemical adsorption on the supporting materials, the use of electrolytes that do not dissolve polysulfides via the coordination of Li+ and solvents, and the use of ion-exchange polymers to permeate Li+ selectively. The following approaches to enable practical applications of S cathodes in future Li-ion batteries are introduced: the utilization of Li-free anode materials, such as C and Si; the use of Li2S cathodes, which are prepared via a pre-lithiation process; and increasing the areal capacity of the S cathode by using a suitable current collector such as Al foam, thus providing a large amount of space for Li+ to migrate and the electron-conductive path. The utilization of an Al foam current collector is one of the promising approaches to creating a cost-effective Li-ion battery owing to the established mass production of Al foam for use in NiMH batteries; such Li-ion battery can achieve an unprecedentedly high areal capacity of 21.9 mAh cm−2. Owing to the resulting areal capacity, the possibility of developing a lithium-sulfur battery with an energy density greater than 200 Wh kg−1 has been demonstrated. Consequently, the combination of these approaches, as introduced in this review, would help create a bright, sustainable society.

Synthesis and Characterization of Complex Framework Material (CFM) Hosts for Li – S Battery

Great demand for improvements in battery technology has recently been spurred by the rapidly growing personal electronics and electric vehicle industries. The search for high energy density batteries have reached a limit in lithium intercalation electrodes which have a theoretical capacity of ~250 mAh/g1. Lithium sulfur batteries are a promising alternative to intercalation chemistry batteries, as a sulfur cathode features a theoretical capacity of 1675 mAh/g2. However, the development of lithium sulfur batteries has been impeded by two fundamental issues which are: the low conductivity of sulfur (~10-15 S/cm)3, and the dissolution of polysulfides formed during cycling4. While the conductivity issue can be mitigated by incorporating conductive carbon into the cathode, polysulfide dissolution has proven more challenging to address5 , 6. The formation and dissolution of polysulfides results in the loss of active material from the cathode which culminates in cell failure. Previous attempts to prevent polysulfide dissolution by embedding sulfur into porous carbon structures have been moderately successful. However, the efficiency of these approaches is limited by a lack of control over porosity. To improve on these carbon structures, sulfur hosts with engineered structures and chemical properties should be developed. In this work, lithium sulfur battery cathodes were created using complex framework materials as sulfur hosts. The cathodes were created by solution phase infiltration of sulfur into complex framework materials synthesized by microwave-assisted hydrothermal synthesis. the XRD spectrum and SEM image of one of the CFMs is shown in Figure 1. The cathodes demonstrated impressive electrochemical performance with an initial discharge capacity of ~1620 mAh/g which stabilized at 1100 mAh/g after 100 cycles. Acknowledgement: The authors acknowledge the financial support of DOE grant DE-EE 0006825, Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM).

Magnesium Sulfur Batteries

Demands in energy storage require a significant improvement in the energy density of battery systems. Therefore innovative systems with a substantially higher specific energy and improved cycle life have to be developed. The combination of Magnesium and Sulfur in a galvanic element addresses several advantages, such as natural abundance, operational safety and a high volumetric capacity. However the research so far was hindered because a stable electrolyte for Mg-S-Batteries was missing [1]. Muldoon et. al. developed a stable and reversibly working non-nucleophilic electrolyte based on a recrystallized Mg2+ salt complex [Mg(µ-Cl)3(THF)6][HMDSAlCl3] (HMDS=hexamethyldisilazid) [2]. This crystalline electrolyte salt though could only be obtained in THF, revealing a disadvantageous overcharging and fast capacity fading [2]. A break-through in Mg/S battery performance was obtained by Fichtner et. al. They at first synthesized a one-step routine between [(HMDS)2Mg]=magnesium bis(hexamethyldisilazid) and AlCl3in different ethers, enhancing solvent choice and possibility of neglecting aforementioned disadvantages [3]. Furthermore a magnesium-anode enhances safety of Mg-S-battery, due to the missing tendency of dendrite formation [4]. Safety as well as capacity and scalability of fabrication of a battery are important factors to assess market opportunities of these secondary cells. Therefore we are focusing on a cell design that is "easy-to-fabricate" with a realistic material loading. Based on the multiannual experience in the development of lithium-sulfur batteries, we extend our activities to the field of magnesium sulfur-battery-development. In a first attempt S-C-cathodes with Sulfur content of 50 wt.% and 70 wt.% were evaluated with a 1.2 M Mg2+-salt electrolyte dissolved in a mixture of 2:1 by volume of bis(2-methoxyethyl)ether and an ionic liquid. The influence of the cathode composition was minimal at this early stage of development. Both types of batteries showed a similar discharge capacity of 600 mAh/g(Sulfur) in the first cycle. Furthermore different approaches for a working Mg-graphite-composite-anode are presented. Such composite-anodes were prepared via die-pressing and could only be obtained in specific composition. The obtained charge-discharge profiles showed remarkably well defined plateaus, which have not been obtained elsewhere, jet (see Figure 1). The preparation routine of the composite-anode was found to have the highest impact on the obtained performance.

Probing the Role of Solvent and Salt in Lithium Sulfur Redox Reactions

Lithium sulfur (Li-S) battery has the potential to provide a 2 to 3 times higher specific energy than conventional lithium ion batteries.1 The development of Li-S batteries is, however, limited by the dissolution, diffusion and side reactions of soluble polysulfides in the electrolyte.1-3 Fundamental understanding of solvent’s influence on the stability of polysulfides and their activities in Li-S redox reactions is crucial for enabling rational design of electrolyte for Li-S batteries. We compare reported Li-S reaction pathways in high-DN solvents (e.g. DMSO, DMA)4-7 and low-DN solvents (DOL:DME8 / TEGDME9). Interestingly, major reactions proposed in high-DN solvents (e.g. DMSO, DMA)4-7 were also proposed in low-DN solvents (e.g. DOL:DME or TEGDME) in various studies,8-10including the electrochemical and chemical reactions of the trisulfur radical. In view of these proposed reaction pathways in the literature, it is unclear what the major difference is in the Li-S reaction mechanism/pathways in high- and low-DN solvents that contribute to the distinct electrochemical behaviors found in high- and low-DN solvents. Therefore, it is critical to understand and differentiate the major differences in Li-S reaction pathways in DMSO and DOL:DME in order to understand origins responsible for different Li-S behaviors. We apply operando UV-Vis spectroscopy4, 9, 11, 12 to investigate the Li-S redox reactions in the commonly used low-DN solvent DOL: DME (v:v=1:1) and a classic high-DN solvent DMSO as model electrolytes.13 We revealed that Li-S redox reactions in DMSO undergo series of electrochemical and chemical reactions involving S8 2-, S6 2-, S4 2- and S3 •- where S3 •- is the most stable and dominant reaction intermediates.13 In DOL: DME, the dominant reaction intermediates of the Li-S redox reactions is found to be S4 2-.14 To investigate the stability of various polysulfides in DMSO and DOL: DME, we perform UV-Vis studies on a series of chemically-synthesized polysulfides14 in both solvents. Results indicate that (1) S3 •- is strongly stabilized in DMSO and can be formed facilely from S8 2-, S6 2-, and S4 2-; (2) S4 2- is strongly stabilized in DOL: DME and can be formed facilely from S8 2-, S6 2-, and S4 2-. These observations support that S3 •- and S4 2- is the main reaction intermediates of the Li-S redox reactions in DMSO and DOL: DME, respectively. Detailed solvent-dependent Li-S reaction pathways, the stability of S3 •- and S4 2-in the solvents and the implications in the Li-S batteries will be discussed. Acknowledgement This work is supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HKSAR), China, under Theme-based Research Scheme through Project No. T23-407/13-N, and by two RGC projects, No. CUHK24200414 and No. CUHK14200615.

Novel Protection Strategies for Sulfur and Silicon in Advanced Lithium-Sulfur Batteries

In our daily life the demand for electrochemical power sources with high energy density is becoming increasingly important. This kind of resources is fundamental for our future as it may represent a viable alternative to fossil fuels and gases offering a solution to environmental issues as well as energy crises. Until now Lithium-ion batteries (LIBs) have been the dominant power sources in the commercial energy storage devices field. Though representing a tremendous improvement with respect to previous systems like Lead-acid batteries, However, LIBs still cannot represent a feasible alternative where gravimetric energy density, lower than 300 Wh.kg-1, is at stake. In the transportation field, f. i., it is not yet possible to buy an electric vehicle having performance comparable to that of an internal combustion vehicle of the same cost. Among various types of new “beyond Lithium-ion” batteries, the Lithium-sulfur battery holds great potential due to its high theoretical specific capacity of 1672 mAh.g-1 and specific energy of 2600 Wh. Kg-1. Nevertheless, the development of Lithium-sulfur battery has met several challenges for its typical low sulfur utilization, low rate capacity and poor cycle life, mainly due to the electronic and ionic insulating nature of sulfur, the solubility of reductive polysulfides in organic solvents based electrolytes and the large expansion of sulfur upon cycling. Therefore different approaches need to be explored to solve these issues. Short lifetime of Lithium-sulfur batteries is linked, upon other factors, to the use of Li metal anode, which is known to cause safety hazard and internal short-circuit due to the formation of lithium dendrites, which may as well result in thermal runaway. Moreover this material is very expensive. One of the promising solutions is to use lithiated Si instead of Li metal. The theoretical capacity (4200 mAh.g-1) of silicon as anode is higher than the one of Li metal (3860 mAh.g-1), making it ideal to couple with the high capacity sulfur cathode in order to maintain a comparable energy density despite the lower discharge voltage obtained with silicon. However this approach raises another issue, silicon undergoes large volume expansion during lithiation risking compromising the anode structure. Therefore a silicon core is enclosed in a silica shell which is then partially removed by acidic treatment, thus creating empty spaces around the core to allow an effective lithiation. In order to make the whole structure conductive, an external shell is then created around it (carbon, metal,…). In a parallel study, again regarding the anode, the synthesis of nanostructured materials such as, Graphene-Silicon composites to make cages that accommodate silicon’s volume changes during charging and discharging, is being carried out. Silicon nanoparticles are treated with acid to achieve fresh Si surface. Then, Si nanoparticles are reacted with amine group terminated long chain alkenes through addition reaction. Si nanoparticles bearing amine functional groups are dispersed with graphene oxide in mild acidic conditions. In this strategy, the interaction between positively charged nanoparticles and graphene oxide allow the uniform wrapping of the particles and long carbon chains will provide the room for large volume and morphological changes. The graphene oxides are further reduced to graphene to provide conductive pathways for electronic and ionic conductivities. Regarding the cathode, a sustainable solution is suggested to prepare conductive carbon matrices from bio-based materials. Template-assisted hydrothermal carbonization of dextrose and/or β-cyclodextrin is carried out to prepare carbon nanospheres or nanosponges to host Sulfur (S). S is incorporated inside carbon nanosponges via solvent impregnation and thermal diffusion. Then, as prepared carbon-sulfur composite is encapsulated with conductive polymers like polyaniline/polypyrrole. This dual protection strategy in conductive matrices provide conductive network for S and effectively help to suppress the polysulfide dissolution.

ChemInform Abstract: Functionalized Graphene‐Based Cathode for Highly Reversible Lithium—Sulfur Batteries

AbstractReview: 70 refs

Development of Lithium Sulfur Batteries with Improved Cycle Life and High-Power Properties

Tremendous effort has been applied to developing viable lithium sulfur batteries and significant progress has been reported. However, lithium sulfur batteries still exhibit significant capacity decay over cycling, probably due to the detachment or dissolution of LixS from the carbon surface during the discharge process. Here, we report the development of a new type of lithium sulfur batteries using functionalized multiwall carbon nanotube (MWCNT). We assembled an electrode consisting of densely packed MWCNT composites, which enabled strong interactions between the nonpolar carbon and the polar LixS clusters. We characterized the electrodes using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared spectroscopy (FTIR). We found that the electrodes had high cycling performance with specific capacities of 1250 mAh/g and 990 mAh/g at the C/5 and C/2 current rates, respectively. Up to 80% capacity was retained over 300 cycles at C/5 and 200 cycles at C/2. The developed battery may be promising for applications in hybrid vehicles and renewable energy.

Development of lithium-sulfur batteries using room temperature ionic liquid-based quasi-solid-state electrolytes

Quasi-solid-state electrolytes (QSEs), consisting of pseudo-ternary system, i.e., room temperature ionic liquid (RTIL)-lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA)-fumed silica nano particles, were prepared for use as electrolytes in bulk-type all-solid-state cell configuration lithium-sulfur rechargeable batteries. Regardless of the high concentration of RTIL-Li-salt mixtures with fumed silica nano particles, i.e., 80-90% volume ratio, the composites were white powders. Further blending with 5wt% polytetrafluoroethylene (PTFE) powder caused the composite powders to become transparent sheets of 30-200μm in thickness. The electrolyte sheets exhibited liquid-like high apparent conductivities regardless of their solid-like appearance. Sulfur utilization ratios were enhanced using conductive additives with high specific surface areas. Cells with notable performance were successfully assembled after the optimization of the RTIL solvent composition, electrolyte thickness, and conductive additive species. The initial discharge capacity was 1100mAh g−1 at 0.05 C. After 45 discharge-charge cycles, the discharge capacity was still 690mAhg−1. This performance is comparable to lithium-sulfur cells that use the RTIL-Li-salt liquid electrolytes.

Functionalized Graphene‐Based Cathode for Highly Reversible Lithium–Sulfur Batteries

In this article, we highlight the salient issues in the development of lithium-sulfur battery (LSB) cathodes, present different points of view in solving them, and argue, why in the future, functionalized graphene or graphene oxide might be the ultimate solution towards LSB commercialization. As shown by previous studies and also in our recent work, functionalized graphene and graphene oxide enhance the reversibility of the charge-discharge process by trapping polysulfides in the oxygen functional groups on the graphene surface, thus minimizing polysulfide dissolution. This will be helpful for the rational design of new cathode structures based on graphene for LSBs with minimal capacity fading, low extra cost, and without the unnecessary weight increase caused by metal/metal oxide additives.