Li-S Cells Research Articles

(Invited) Lithium Sulfur Batteries: From Fundamental Understanding to Pouch Cell Integration

With the recent advances of Li-S battery research, new knowledge has been gathered which greatly deepens our understanding of this electrochemical couple. However, a cell-level understanding of Li-S chemistry is rarely reported, although it is expected that the dynamic interactions among sulfur, lithium and electrolyte in the enlarged testing format may lead to quite different findings. Herein, this work discusses the fundamentals underneath a Li-S cell when transiting from a coin cell to a realistic high-energy pouch cell. New challenges and opportunities have been discovered which in turn provide feedback for the rational design of materials/electrode at relevant scales. The potential solutions to further balance the energy density and cycling life of Li-S pouch cells is also proposed.

(Plenary) Challenge for Innovation of Li-S Battery System

Policy recommendations of the electrification of the vehicles have been issued all over the world in these years. Energy density is one of the important keywords for realizing such policies. Several novel systems have been investigated, and Li-S system is quite promising among them. Leading research organizations and start-up companies have manufactured the trial prototype Li-S cell with energy density of 300-400 Wh/kg. And Li-S batteries application is not limited to vehicles. We have tried to innovate the conventional Li-S battery using metal polysulfides like a-TiS4 1,2, Li8FeS5 3, 4, VS4 5, and so on in RISING2 (Research and Development Initiative for Scientific Innovation of New Generation Batteries 2) project. Sulfur is connected to metal atom and dissolution of Li polysulfides into the electrolyte was greatly suppressed. Quite recently 8 Ah cell was constructed and energy density of more than 310 Wh/kg was confirmed. Project target is to confirm 500 Wh/kg by prototype cell for innovative batteries in the end of FY2020. In the presentation, RISING2 activity, state of the art of the Li-S and our recent progress will be introduced. Acknowledgement This work was financially supported by RISING2 project by NEDO and METI.

Nanoporous Polymer Films with a High Cation Transference Number Stabilize Lithium Metal Anodes in Light-Weight Batteries for Electrified Transportation.

To suppress dendrite formation in lithium metal batteries, high cation transference number electrolytes that reduce electrode polarization are highly desirable, but rarely available using conventional liquid electrolytes. Here, we show that liquid electrolytes increase their cation transference numbers (e.g., ∼0.2 to >0.70) when confined to a structurally rigid polymer host whose pores are on a similar length scale (0.5-2 nm) as the Debye screening length in the electrolyte, which results in a diffuse electrolyte double layer at the polymer-electrolyte interface that retains counterions and reject co-ions from the electrolyte due to their larger size. Lithium anodes coated with ∼1 μm thick overlayers of the polymer host exhibit both a low area-specific resistance and clear dendrite-suppressing character, as evident from their performance in Li-Li and Li-Cu cells as well as in post-mortem analysis of the anode's morphology after cycling. High areal capacity Li-S cells (4.9 mg cm-2; 8.2 mAh cm-2) implementing these high transference number polymer-hosted liquid electrolytes were remarkably stable, considering ∼24 μm of lithium was electroreversibly deposited in each cycle at a C-rate of 0.2. We further identified a scalable manufacturing path for these polymer-coated lithium electrodes, which are drop-in components for lithium metal battery manufacturing.

Electrospun PVDF/PSSLi ionomer films as a functional separator for lithium-sulfur batteries

A novel polyvinylidene fluoride/poly(4-styrenesulfonic acid) lithium salt (PVDF/PSSLi) composite membrane has been successfully produced by a facile route. This membrane is proven to be effective for depressing polysulfide shuttle in lithium-sulfur battery, leading to higher discharge capacity and better durability in comparison with the commercial PP separator. The Li-S cell with PVDF/PSSLi separator delivers a discharge capacity of 1194 mAh/g with average Coulombic efficiency of 97% at 0.2 C. Especially, the PVDF/PSSLi membrane exhibits excellent cycling performance at 0.5 C, and the corresponding capacity decay is just 0.26% per cycle during 200 cycles, while that of PP separator is 0.43%; moreover, it possesses stable and high Coulombic efficiency of beyond 97% during cycling.

A light-weight free-standing graphene foam-based interlayer towards improved Li-S cells

A light-weight and free-standing graphene foam interlayer placed between sulfur cathode and separator is investigated to improve the electrochemical performance of lithium-sulfur batteries. The highly conductive and light-weight porous graphene foam not only increases the electron pathway of cathode, but also adsorbs the dissolved high-order lithium polysulfides during cycles, thus the loss of active materials is greatly avoided with only minimum mass addition approximately 0.3 mg cm−2 on cathodic side. Additionally, the atomic layer deposition method is applied to deposit the zinc oxide nano-scale coating on graphene foam interlayer in order to chemically trap the polysulfides with minimized deterioration on conductivity of graphene foam. Among all the graphene foam, graphene foam@zinc oxide and graphene foam/graphene foam@zinc oxide interlayers, the graphene foam/graphene foam@zinc oxide exhibits the best electrochemical performance, delivering an initial specific capacity of 1051 mAh g−1 at 0.5 C and retaining a reversible capacity of 672 mAh g−1 after 100 cycles, while the cell without interlayer only shows 346 mAh g−1. These results demonstrate the strategy of including a zinc oxide modified graphene foam interlayer as an effective light-weight interlayer for improving Li-S cell performance.

Electrolyte Concentration Effect on Sulfur Utilization of Li-S Batteries

Electrolyte is the critical component of the Li-S battery. Past studies have shed light on the behavior of the Li-S cells when the electrolyte salt concentration is increased to the solvent-in-salt regime and demonstrated tremendously improved cycle life. However, there is no systematic study on the Li-S cell when the electrolyte salt concentration is reduced from the standard 1.0M condition. This work investigates the lower salt concentration regime by doing a systematic study with the standard LiTFSI in DME:DOL combination, using sulfur electrodes with relatively high loading (>6 mg cm−2). Although reducing the electrolyte salt concentration lowers the ionic conductivity, it is found that the sulfur utilization and rate capability of the Li-S cells benefits from this process. Similar observations are also found when LiTFSI is replaced with LiI and LiBr, which form less conductive electrolytes than LiTFSI at the same concentration levels. Data correlation indicates a stronger correlation between the Li-S cell's rate-capability and the electrolyte conductivity than electrolyte viscosity. It is proposed in this work that free Li+ concentration, which is proportional to the electrolyte conductivity, is the real rate-capability determining parameter. Reducing the electrolyte salt concentration and replacing LiTFSI with less dissociable salts (LiI, LiBr) both will reduce the free Li+ in the electrolyte, which will allow a higher saturation point of Li2S2/Li2S. This probably will delay the insulating layer buildup on the cathode conductive network and improve the dissolved Li-polysulfide to Li2S2/Li2S conversion efficiency. Both impedance and cathode morphology support this theorem.

MnO2-Coated Sulfur-Filled Hollow Carbon Nanosphere-Based Cathode Materials for Enhancing Electrochemical Performance of Li-S Cells

When comes to very high energy density energy storage systems, the prospect of lithium-sulfur battery (LSB) technology is very promising. The major problem that prevents the commercial production of LSBs is their poor cycling life caused by the migration of polysulfide intermediates from the cathode structure to the anode. Confining sulfur and the discharged polysulfide intermediates inside conductive porous carbon matrices is regarded as a promising and effective strategy to overcome this problem. In this study, a new class of MnO2-coated sulfur-filled hollow carbon nanosphere (HCN)-based cathode materials has been prepared to improve the electrochemical performance of Li-S Cells. The cathode materials constructed with silica nanoparticle hard templates displayed a high initial capacity of 834 mAh g−1 with a capacity fading rate of 0.080% per cycle. This material also showed superior redox kinetics, which enabled the Li-S cells to run at a higher charging-discharging current density.

A Rational Balance Design of Hybrid Electrolyte Based on Ionic Liquid and Fluorinated Ether in Lithium Sulfur Batteries

A series of electrolytes based on ionic liquid (IL) N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl) amide (DEMETFSI) and individual or binary co-solvent(s) consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and/or 1,3-dioxolane (DOL) were investigated. The alterative solvent composition results in very different electrolyte properties (conductivity/viscosity, polysulfide solubility and film formation ability) and Li-S cell performances (specific capacity, cycle stability and rate capability). The hybrid system of DEMETFSI/TTE/DOL provides better overall performances compared to the IL with non- or single co-solvent, which is mainly profited from the synergistic function of TTE and DOL, together favorable for the improvement of ionic conduction of the electrolyte and Li surface chemistry. It is further emphasized that the construction of a rational counterbalance between polysulfide dissolution and shuttle suppression is more significant for the advanced Li-S batteries.

Understanding the Impact of a Nonafluorinated Ether-Based Electrolyte on Li-S Battery

The impact of the electrolyte solvation structure on the performance of Li-S battery was investigated using an electrolyte system containing various solvent ratios of DOL and a nonafluorinated ether: 1,1,2,2-tetrafluoroethyl-2,2,3,3,3-pentafluoropropyl ether (TPE). The cell testing results indicate that increasing the TPE ratio led to both a higher discharge capacity and a higher Coulombic efficiency. The Li electrodes from the Li/Li symmetric cells showed that the one in higher TPE-content electrolyte has a smoother and more continuous SEI layer. AIMD simulation revealed the solvation structure of DOL/TPE – lithium polysulfide changes when the TPE concentration increases, which dictates the lithium polysulfide solubility in such electrolyte. These results shed light on the underpinned mechanism of the improved Coulombic efficiency and cycling performance of the Li-S cell.

Influence of morphology of monolithic sulfur-poly(acrylonitrile) composites used as cathode materials in lithium-sulfur batteries on electrochemical performance.

Solvent-induced phase separation (SIPS) and thermally-induced phase separation (TIPS) derived poly(acrylonitrile) (PAN) based monoliths with different morphology and specific surface area were prepared and thermally converted into monolithic sulfur–poly(acrylonitrile) (SPAN) materials for use as active cathode materials in lithium–sulfur batteries. During thermal processing, the macroscopic monolithic structure fully prevailed while significant changes in porosity were observed. Both the monomer content in the precursor PAN-based monoliths and the tortuosity of the final monolithic SPAN materials correlate with the electrochemical performance of the SPAN-based cathodes. Overall, percolation issues predominate. In percolating SPAN-based cathode materials, the specific capacity of the SPAN-based cells increases with decreasing tortuosity. All monolithic SPAN materials provided highly reversible and cycle stable cathodes reaching reversible discharge capacities up to 1330 mA h gsulfur−1 @ 0.25C, 900 mA h gsulfur−1 @ 2C and 420 mA h gsulfur−1 @ 8C.

The Stability of Polymers in Liquid Li-S Battery

Polymers have been widely employed as the binders, separators, separator modifiers and gel polymer electrolytes in lithium-sulfur batteries. Considering the special electrolyte system and discharge/charge mechanism, to verify whether the common polymer is applicable for Li-S battery is extremely necessary but often overlooked. In this paper the stability of some frequently-used polymersin the ether electrolyte(1 M LiTFSI + 0.2 MLiNO3/dioxolane (DOL) and 1,2-dimethoxyethane(DME) (1:1, v/v)) on the cycling and storage performances of Li-S batteryis examined by testing the solubility of the PEO, PVDF and PVDF-HFP particles/membranes in ether electrolyte and detecting the structure and composition changes of PVDF and PVDF-HFP membranes after cycling and storage as the separators of Li-S batteries. The results show that PEO particle and membrane both can dissolve in DOL and DME(1:1). PVDF and PVDF-HFP particles also dissolve in DOL and DME(1:1) but the membranes remain relatively stable, whereas PVDF-HFP react with Li2Sx in liquid Li-S cellto form sulfone, which induce the crystalline phase transformation (from α-phase to γ-phase) and cystallinity decrease. PVDF can also react with Li2Sxin liquid Li-S cell, but the action is less intensive than PVDF-HFP.

Freestanding oxidized poly(acrylonitrile-co-vinylpyrrolidone)/SnCl2 nanofibers as interlayer for Lithium[sbnd]Sulfur batteries

LithiumSulfur batteries are one of the most promising energy storage systems among the next generation batteries due to their high energy density and the natural abundance of sulfur. Nevertheless, some limitations hinder their implementation to the marketplace which are mostly linked to the shuttle effect resulting fast capacity lost and lithium poisoning by dissolved polysulfides. One of the possible solutions is the use of polysulfide adsorptive interlayers between anode and cathode to inhibit the shuttle effect protecting lithium anode. In this work, oxidized poly(acrylonitrile-co-vinylpyrrolidone) nanofibers containing SnCl2 (oPANVP/SnCl2) are used as an interlayer to enhance the performance of LiS cells. Unlike most of the current literature, the electrospun nanofiber mats are oxidized at 200 °C under air, but not further pyrolyzed to benefit from the functional groups and the partial formation of SnOx. 700 mAh g−1 discharge capacity is obtained at C/5 after 100 cycles by using oPANVP/SnCl2, which is higher than the cells with oPANVP and without interlayer. The improved capacity is mostly associated with the complementary adsorption effect of SnCl2 particles, partially formed SnOx and functional groups of oPANVP. Polysulfide adsorption effect of SnCl2, SnOx, and nitrogen- and oxygen-rich functional groups of oPANVP is proven by X-Ray Photoelectron Spectroscopy.

Designing a high-loading sulfur cathode with a mixed ionic-electronic conducting polymer for electrochemically stable lithium-sulfur batteries

As one of the most widely regarded candidates for energy storage, Li-S are attractive as due to their high theoretical energy density. To be considered for practical viability, a high active material loading must be considered. This is limited by the insulating nature of the active material, amplified polysulfide shuttling, and the cracking in the cathode due to the large volume change between charge and discharge products. A copolymer is prepared with poly(styrene 4-sulfonate) (PSS) and polypyrrole (PPy) to enable rational design of high sulfur loading Li-S batteries. The copolymer with a mixed ionic-electronic conductivity (MIEC) facilitates cooperative charge transport and stabilization during cycling, allowing a high sulfur loading of 6.0 mg cm−2. The resulting Li-S cell displays a high initial discharge capacity of 1108 mA h g−1 at a C/10 rate with a capacity retention of 64% after 200 cycles. The copolymer with MIEC offers a promising approach for realizing practical, long-life Li-S batteries.

Tailoring the structure of clew-like carbon skeleton with 2D Co-MOF for advanced Li-S cells

Metal organic frameworks (MOF)-derived carbon materials can realize self-doping of heteroatoms in situ, which can effectively improve the electrocatalytic activity of electrode material, those are therefore considered as ideal cathode materials for Li-S battery. In this context, the clews of polymer nanobelts (CsPNBs) are regarded as sacrificial template materials, and Co-based metal-organic frameworks (Co-MOF) are in-situ growth on the surface of CsPNBs, after carbonization and activation, the Co-doped hybrid materials are obtained. The physicochemical properties of these hybrid materials can be changed by adjusting activation time and by adding some amount of CsPNBs. When evaluated as a cathode for Li-S batteries, the hybrid nano-architecture exhibits high initial capacities of 1300.69 and 1020.6 mAh g−1 at 0.1 and 0.5C, respectively. The enhanced electrochemical performance of the electrode is closely related to the accessible pore structures optimization as well as Co-doping has cobalt and oxygen-containing groups on the surface of the matrix material. With the help of those advantages, the areal mass loading of the electrodes is improved. Overall, a rarely demonstrated study that involves the correlation between structure and performance of the MOF-derived hybrid materials is systematically elucidated in this work. This research study is crucial to the precise tailoring of the growth for the MOF-derived carbon superstructure to meet their demanding challenge as the electrode materials.

Bicomponent electrolyte additive excelling fluoroethylene carbonate for high performance Si-based anodes and lithiated Si-S batteries

Si-based anode materials for advanced high energy Li-ion batteries and Li-S battery as a promising candidate for “beyond LIBs” have been intensively studied in recent years. Although FEC electrolyte additive plays an important role in improving the cycle stability of Si-based electrodes, its high temperature behavior is still unsatisfactory. Herein, a new bicomponent additive (LiPO2F2 + DMTFA) is proposed to replace FEC in LiPF6/carbonate electrolyte. The resulting electrolyte shows high ionic conductivity (10.9 mS cm-1), good anti-oxidation capability (>5 V vs. Li/Li+) and excellent compatibility with Si-based anodes even at elevated temperature. Moreover, this electrolyte is also fit for S@pPAN cathode developed for Li-S batteries. An in-situ lithiated Si-S@pPAN cell containing the proposed electrolyte delivers a high reversible capacity of about 1400 mA h g-1sulfur with retention of 92.1% for 1000 cycles at 1 C and about 1350 mA h g-1sulfur with a capacity retention of 94.8% for 2000 cycles at 2 C. Its rate performance has been also improved greatly. This cell system avoids some fatal flaws of conventional Li-S cells, including the Li dendrite formation and thermal instability. It is promising for the practical application.

Suppressing Li Metal Dendrites Through a Solid Li-Ion Backup Layer.

The growing demand for sustainable and off-grid energy storage is reviving the attempts to use Li metal as the anode in the next generation of batteries. However, the use of Li anodes is hampered due to the growth of Li dendrites upon charging and discharging, which compromises the life and safety of the battery. Here, it is shown that lithiated multiwall carbon nanotubes (Li-MWCNTs) act as a controlled Li diffusion interface that suppresses the growth of Li dendrites by regulating the Li+ ion flux during charge/discharge cycling at current densities between 2 and 4 mA cm-2 . A full Li-S cell is fabricated to showcase the versatility of the protected Li anode with the Li-MWCNT interface, where the full cells could support pulse discharges at high currents and over 450 cycles at different rates with coulombic efficiencies close to 99.9%. This work indicates that carbon materials in lithiated forms can be an effective and simple approach to the stabilization of Li metal anodes.

Improvement of Li-S battery electrochemical performance with 2D TiS2 additive

As a multifunctional cathode additive, TiS2 has been used in Li-S batteries to promote sulfur utilization and improve cell cycle life. The improvement of Li-S cell electrochemical performance in the presence of TiS2 has been attributed to the ability of TiS2 in adsorbing polysulfide. To maximize the interaction between polysulfides and TiS2, the 2-dimensional (2D) TiS2 nano-sheets with highly exposed TiS2 surface area has been successfully synthesized by a two-step sonication synthetic procedure. The 2D TiS2 particles are micron size in the basal plane direction and are nano-sized along the c-axis direction. 2D TiS2 nano-sheets may adsorb polysulfides more effectively than the raw TiS2 powder. Li-S cells with 2D TiS2 nano-sheets additive exhibit better discharge rate capability and significantly reduced capacity-fading after a long term cell cycling. This improvement can be partially attributed to the uniform distribution of 2D TiS2 nano-sheets particles within the sulfur cathode matrix, which induces more uniform sulfur and lithium sulfide deposition during the cell charge and discharge.

Composites of Sulfur-Titania Nanotubes Prepared by a Facile Solution Infiltration Route as Cathode Material in Lithium-Sulfur Battery.

Achieving high energy density has been the focus of research in rechargeable batteries. Lithiumsulfur system is attractive due to its high theoretical energy density (2500 Wh kg-1). The major problem in Li-S system is associated with the dissolution of lithium polysulfides formed at the cathode during discharge. Shuttling of polysulfides between the cathode and anode during cycling reduces the efficiency of cycling. In the present study, TiO2 nanotubes are prepared from nanoparticles by hydrothermal route. Titania-sulfur composite has been prepared by infiltrating sulfur solution into the TiO2 nanotubes and studied as a cathode material in a non-aqueous electrolyte. Cycling behavior of Li-S cells fabricated using pristine sulfur and TiO2 nanoparticle-sulfur composite is also studied for comparison. Cells with TiO2 nanotubes exhibit better discharge capacity and coulombic efficiency than the cells with TiO2 nanoparticles and pristine sulfur.

Enhancing the Performance and Life Cycle of Lithium –Sulfur Batteries by Nanostructured Carbon and Different Additives

Lithium sulfur (Li – S) battery has been considered as one of the most promising rechargeable batteries among various energy storage devices owing to their low cost, high specific capacity and energy density. Over the last decade, lithium sulfur (Li – S) batteries have been extensively studied because of the abundance of sulfur, their environmental benignity, and high gravimetric (2600 W h Kg-1) and (2800 W h L-1) energy densities and are promising candidates for meeting future-energy storage demands. However, the insulation of sulfur and high solubility of lithium poly sulfides, swelling of cathode value and formation of lithium dendrites results in the low utilization and poor cycling performance. Significant efforts have been made to trap poly sulfides via physical strategies using carbon based materials, but interactions between poly sulfides and carbon are so weak that the device performance is limited. Chemical strategies provide the relatively complemented routes for improving the batteries' electrochemical properties by introducing strong interactions between functional groups and (oxygen, nitrogen and boron, etc.) and chemical additives (metal, polymers, etc.) to the carbon structure and how these foreign guests immobilize the dissolved polysulfide. This review focused on recent studies have reported various material such as metal oxides and sulfides that interact strongly with poly sulfides species and can alleviate the dissolution problem by comparing the poly sulfides adsorption capability candidate materials to provide the useful strategy to screen for suitable candidate materials and valuable information for rational design. Overcoming the loss of active mass and stabilizing cell capacity at high rates is pivotal to the realization of practical Li-S cells. In this review, different separate concepts and materials were studied with the aim to improve the Li-S batteries capacity, cycle life and capacity retention.

TiO2 quantum dots decorated multi-walled carbon nanotubes as the multifunctional separator for highly stable lithium sulfur batteries

Lithium sulfur battery is considered one of the most promising rechargeable energy storage devices due to its ultrahigh theoretical energy density and specific capacity. It still cannot be applied because of some key problems. In particular, the shuttle effect of soluble polysulfide compounds leads to the rapid attenuation of battery capacity, short cycle life and serious self-discharge effect. TiO2 quantum dots and polysulfide compounds have strong interactions and can capture soluble polysulfide compounds, thus inhibiting the shuttle effect. Herein, we introduce a battery separator based on TiO2 quantum dots modified multi walled carbon nanotubes to solve the shuttle effect and adapt to the expansion of electrodes during charging and discharging. The interlayer has abundant spacing and excellent conductivity. This strategy can significantly improve the stability of lithium sulfur batteries. As a result, Li-S cells with MWCNTs@TiO2 quantum dots modified separator deliver an initial capacity of 1083 mAh g−1 and keep a cycle capacity of 610 mAh g−1 after 600 cycles at the rate of 838 mA g−1, which maintains an average capacity decay of only 0.072% per cycle. This simple and effective method can greatly improve the application capacity of lithium sulfur batteries.