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The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li2S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li2S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li2S2 precipitates from the solution and then is partially converted via solid-state electroreduction to Li2S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells. Understanding lithium sulfide's electrodeposition and stripping is key to developing practical Li-S batteries. Here, the authors demonstrate that the discharge product in Li-S batteries consists of nano-size solid polysulfide particles and nano-crystalline lithium sulfide. [ABSTRACT FROM AUTHOR]

In this work, valence-to-core (VtC) Kβ sulfur X-ray emission spectroscopy (XES) was used to perform quantitative analysis of different sulfur compounds produced in a lithium sulfur (Li–S) battery during discharge. The analysis is based on the theoretical sulfur Kβ XES spectra obtained from ab initio quantum chemical calculations based on density functional theory. The emphasis is given to the Kβ sulfur XES spectra of the polysulfide molecules (Li2Sx, x = 2,⋯,8) produced electrochemically within the Li–S battery. Ab initio molecular dynamics calculations are used further to calculate also the Kβ spectra of Li2Sx dissolved in a model solvent. Calculated spectra were directly compared with the experimental ones collected with a Johansson type tender XES spectrometer on laboratory synthesized Li2Sx reference standards and pre-cycled battery cathodes. These results demonstrate that sulfur VtC XES can be used effectively to quantitatively analyze electrochemical sulfur conversion, also in a smaller laboratory without the need for large scale synchrotron facilities. [ABSTRACT FROM AUTHOR]

Future optimized lithium‐sulfur batteries may promise higher energy densities than the current standard. However, there are many barriers which hinder their commercialization. In this review we describe how ionic liquids (ILs) and their polymers are utilized in different components of the battery to address some of these issues. For example, IL‐based electrolytes have the potential to reduce the solubility of polysulfides compared to conventional organic electrolytes. Polymerizing ILs directly on the surface of the Li‐metal anode is suggested as an approach to protect the surface of this electrode. Finally, using poly(ionic liquids) (PILs) as binders for the cathode active material may increase the performance of the cathode as compared to polyvinylidene difluoride (PVdF) and could inhibit swelling‐induced degradation. These results demonstrate the advantages of ILs and their polymers for improving the performance of Li−S batteries. [ABSTRACT FROM AUTHOR]

Li-S batteries are promising energy storage technology for the future, however there two major problems remained which need to be solved before successful commercialization. Capacity fading due to polysulfide shuttle and corrosion of lithium metal are directly connected with the type and quantity of electrolyte used in the cells. Several recent works show dependence of the electrochemical behavior of Li-S batteries on type of the electrolyte. In this work we compare and discuss a discharge mechanism of sulfur conversion in three different electrolytes based on measurements with sulfur K-edge XAS. The sulfur conversion mechanism in the ether based electrolytes, the most studied type of solvents in the Li-S batteries, which are enabling high solubility of polysulfides are compared with the fluorinated ether based electrolytes with a reduced polysulfide solubility and in carbonate based electrolytes with the sulfur confined into a ultramicroporous carbon. In all three cases the sulfur reduction proceeds through polysulfide intermediate phases with a difference on the type polysulfides detected at different steps of discharge. [ABSTRACT FROM AUTHOR]

In this study, a gel polymer electrolyte (GPE) based on polymer ionic liquid (PIL) is used in a solvent-free and in a hybrid electrolyte configuration for Li-S batteries. Results obtained in the solvent-free configuration show a high discharge capacity in the first cycle and excellent coulombic efficiency during cycling. Capacity fading and polarization increase during cycling are explained based on the XPS and EIS measurements. The results of the present study are indicating that the increase of various internal resistance contributions and capacity fading are related with an accumulation of polysulfides in the GPE-PIL layer or/and on the surface of the lithium anode. Within a hybrid battery configuration, the thickness of the GPE-PIL layer is thinner, and the volume where polysulfides can be trapped is smaller. Such a configuration shows better cycling stability. The hybrid configuration outperforms cycling stability of the conventional configuration with a liquid electrolyte. This is explained by increased internal resistance in the convential configuration while the polarization in the first 100 cycles is constant in the hybrid configuration. Additionally, the hybrid configuration exhibits excellent C-rate performance. [ABSTRACT FROM AUTHOR]

Despite the great efforts made for encapsulating sulfur particles with conducting materials, such as carbon black, in order to limit polysulfide diffusion in lithium-sulfur (Li-S) battery, the rapid degradation of Li-S batteries still occurs. The influence of the surface area and the porosity of the host carbon matrices on the battery performance, the active material utilization and the encapsulation of polysulfides are not fully understood. In this study, by using carbon host matrices with three different surface areas, we show the influence of host matrices on the Li-S battery performance. Analytical techniques, including UV-Visible spectroscopy and 4-Electrode Swagelok cells, were employed to study polysulfide diffusion from the cathode composite. Our studies revealed that the presence of high surface area host carbon can enhance the performance of the battery via the efficient utilization and encapsulation of sulfur and lithium polysulfides although complete encapsulation cannot be achieved. [ABSTRACT FROM AUTHOR]

Application of UV/Vis spectroscopy for the qualitative and quantitative determination of differences in the mechanism of lithium-sulfur battery behavior is presented. With the help of catholytes prepared from chemically synthesized stoichiometric mixtures of lithium and sulfur, calibration curves for two different types of electrolyte can be constructed. First-order derivatives of UV/Vis spectra show five typical derivative peak positions in both electrolytes. In operando measurements show a smooth change in the UV/Vis spectra in the wavelength region between λ=650 and 400 nm. Derivatives are in agreement with derivative peak positions observed with catholytes. Recalculation of normalized reflections of UV/Vis spectra obtained in operando mode enable the formation of polysulfides and their concentrations to be followed. In such a way, it is possible to distinguish differences in the mechanism of polysulfide shuttling between two electrolytes and to correlate differences in capacity fading. [ABSTRACT FROM AUTHOR]

Battery watch: UV/Vis spectrophotometry is demonstrated as a powerful analytical method for the in situ study of polysulfides. Through the interactions that occur between different chain‐length polysulfide molecules and the UV/Vis radiation, quantitative and qualitative determination of the polysulfides formed during Li–S battery operation can be achieved. [ABSTRACT FROM AUTHOR]

Lithium sulfur rechargeable batteries are foreseen to be used in electric vehicles in the near future. To enable their placement to the market we need to improve their reliability and cycling performance. This can be done by selective change of the chemical environment in the Li-S battery, including electrolyte (solvents and salts), different combinations of cathode composites and different additives. With an aim to understand differences between different chemical environments, suitable and reliable analytical techniques that can effectively monitor the changes of interest, should be developed. In this work we present two newly developed analytical techniques which are capable to detect quantitative and qualitative differences between different polysulfide species in the electrolyte. More specifically, both proposed techniques can detect the polysulfides that have diffused away from the cathode composite. With a modified 4-electrode Swagelok cell we can quantitatively determine the amount of such polysulfides, while an in-situ UV-Vis cell gives us information about the composition of these polysulfides. Combining these techniques with a classical galvanostatic cycling method could lead to a better understanding, consequently, a faster tuning of Li-S battery properties. [ABSTRACT FROM AUTHOR]

The Pitfalls and Opportunities of Impedance Spectroscopy of Lithium Sulfur Batteries (Adv. Lithium-sulfur batteries, impedance spectroscopy, transmission line modeling Keywords: lithium-sulfur batteries; impedance spectroscopy; transmission line modeling EN lithium-sulfur batteries impedance spectroscopy transmission line modeling 1 1 1 03/15/22 20220315 NES 220315 B Impedance Spectroscopy of Li-S Batteries b In article number 2101116, Sara Drvaric Talian, Miran Gaberscek, and co-workers review the opportunities and pitfalls of electrochemical impedance spectroscopy, which is frequently used to study Li-S batteries. [Extracted from the article]

Electrolyte choice is an important decision on the quest for higher-energy batteries. Besides general guidelines on the required properties of an electrolyte suitable for use in lithium–sulfur batteries, the influence of more specific physicochemical properties on its characteristics is not well understood. For this purpose, binary mixtures based on the [DEME][TFSI] and dioxolane electrolyte system for lithium–sulfur batteries was investigated in this work. Selected physicochemical properties were determined for different mixtures of solvents and lithium salt concentrations. All the electrolytes prepared were also tested in the lithium–sulfur battery system. The capacity, Coulombic efficiency, overpotentials and impedance spectra were analyzed and a connection between them and the determined electrolyte properties elucidated. We show that the electrolyte's conductivity does not have a direct connection to any of the battery system properties measured. The highest specific capacities were obtained with batteries compromising 1.0 M LiTFSI and the highest ratio of dioxolane in the binary solvent mixture. On the other hand, the best Coulombic efficiencies were obtained with batteries having high ratios of ionic liquid. Resistance and overpotential are connected parameters and are a function of the ionic liquid content. None of the monitored parameters prevail, since the best electrochemical performance in terms of specific capacity and stability was obtained with the 1.0 M LiTFSI in X[DEME][TFSI] = 0.199 electrolyte. [ABSTRACT FROM AUTHOR]

By employing new electrolytes, the polysulfide shuttle phenomenon, one of the main problems of lithium-sulfur (Li-S) batteries, can be significantly reduced. Here we present excellent Coulombic efficiencies as well as adequate performance of high-energy Li-S cells by the use of a fluorinated ether (TFEE) based electrolyte at low electrolyte loading. The observed altered discharge profile was investigated both by electrochemical experiments and an especially tailored COSMO-RS computational approach, while the details of the discharge mechanism were elucidated by two operando techniques: XANES and UV-vis spectroscopy. A significant decrease of polysulfide solubility compared to tetraglyme is due to different Li+ solvation mode. [ABSTRACT FROM AUTHOR]

Electrochemical reactions in the Li S batteries are considered as a multistep reaction process with at least 2–3 equilibrium states. Here we report a possibility of having a conversion of Li 2 S into sulfur without detectible formation of polysulfides. That was confirmed by using a novel material system consisting of carbon coated Li 2 S particles prepared by carbothermal reduction of Li 2 SO 4 . Two independent in operando measurements showed direct oxidation of Li 2 S into sulfur for this system, with almost negligible formation of polysulfides at potentials above 2.5 V vs. Li/Li + . Our results link the diversity of first charge profiles in the literature to the Li 2 S oxidation mechanism and show the importance of ionic wiring within the material. Furthermore, we demonstrate that the Li 2 S oxidation mechanism depends on the relative amount of soluble sulfur in the electrolyte. By controlling the type and the amount of electrolyte within the encapsulating carbon shell, it is thereby possible to control the reaction mechanism of Li 2 S activation. [ABSTRACT FROM AUTHOR]

Discharge/charge process of classical lithium sulphur battery proceeds through intermediate polysulphides which are soluble in classical electrolyte systems. Due to concentration gradient soluble polysulphides easily diffuse/migrate out from cathode composite forming non-uniform distribution of the sulphur within cathode. Eventually polysulphides can be completely reduced on the metallic lithium anode. In this work we compare the sorption properties of manganese modified zeolite silicalite-1 (MnS-1) with a cathode composite containing SBA-15 additive and a cathode composite without additive. Careful analysis using XPS and FIB microscopy equipped with EDX show improved retention of polysulphide species within cathode composite in the case of MnS-1 zeolite as an additive. Interestingly, the amount of sulphur species detected by XPS on the metallic lithium is very similar regardless on cathode composite we use. Finally, similar cycling behaviour can be observed if MnS-1 zeolite is used as an interlayer between composite cathode and separator. [ABSTRACT FROM AUTHOR]

Abstract: Graphene/cellulose composites were prepared and studied as potential host matrixes for sulphur impregnation and use in Li-S batteries. We demonstrate that with the proper design of a relatively low surface area graphene/cellulose composite, a high electrochemical performance along with good cyclability can be achieved. Graphene cellulose composites are built from two constituents: a two-dimensional electronic conductive graphene and cellulose fibres as a structural frame; together they form a laminar type of pore. The graphene sheets that uniformly anchor sulphur molecules provide confinement ability for polysulphides, sufficient space to accommodate sulphur volumetric expansion, a large contact area with the sulphur and a short transport pathway for both electrons and lithium ions. Nano-cellulose prevents the opening of graphene sheets due to the volume expansion caused by dissolved polysulphides during battery operation. This, in turn, prevents the diffusion of lithium polysulphides into the electrolyte, enabling a long cycle life. [Copyright &y& Elsevier]

The HPLC method including derivatisation was used for the determination of polysulfide species concentration during the operation of lithium-sulfur battery cells. A comparison was made between a glyme based electrolyte, which exhibits high polysulfide solubility, and a fluorinated ether based electrolyte, which was proposed to reduce the dissolution and diffusion of polysulfides. A separate analysis was conducted on porous carbon cathodes and separators obtained from cycled battery cells. The determination of the species concentration trends allowed a deeper understanding of the differences in battery cell operating mechanism. We have determined that polysulfide species are formed in similar concentrations in both examined electrolytes, but they remain trapped in the cathode pores in the fluorinated ether based electrolyte lithium-sulfur battery cells. The polysulfide concentration in the separators of the fluorinated ether cells was below the limit of detection, which indicates that the solubilities of polysulfides in the fluorinated electrolyte are below 50 µM. This results in absence of disproportionation or coproportionation reactions between the polysulfide species in solution. This different mechanism influences the change in length and potential of the voltage plateaus of the battery cells during galvanostatic cycling. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The direct reaction of polysulfide species with the Li metal anode causes several issues in the performance of lithium-sulfur batteries, from poor capacity utilization and fast capacity fade to poor Coulombic efficiency. Nevertheless, several reports indicate formation of favorable lithium SEI when allowing for its contact with polysulfides. With cycling tests, impedance spectroscopy measurements and morphology investigation, we have confirmed the beneficial effect of polysulfides on lithium metal stripping and deposition even without LiNO 3 , which is usually added. Most importantly, highly concentrated catholytes, which are closer to the saturated solutions encountered in lean electrolyte-to-sulfur ratios, were tested. In the 1 M Li 2 S 8 catholyte solution, its effect produced Li electrodes without dendritic growth, but rather a wrinkled surface. To circumvent the issue of viscosity in such solutions, pre-treatment in polysulfide solutions was also tested. The performance of such electrodes showed an increase in cycle life and lower overpotentials, although dendritic growth could not be fully avoided. In order to better understand the effect of pre-treatment on transport/reaction mechanism in Li electrode, a detailed analysis of selected impedance spectra is carried out using physics-based transmission line modelling (TLM). Image, graphical abstract [ABSTRACT FROM AUTHOR]

Development of magnesium sulfur battery is accompanied with all known difficulties present in Li–S batteries, however with even more limited choice of electrolytes. In the present work, the influence of current collector on electrochemical mechanism was investigated in light of different reports where improved behavior was ascribed to electrolyte. Notable differences in cycling behavior are reported when Al current collector is replaced by Cu current collector independent of electrolyte. The initial reduction of sulfur follows similar reaction path no mater of current collector, but formation of MgS can be in competition with formation of CuS in the presence of Cu cations. With the subsequent cycling cells prepared from cathodes deposited on Cu current collector show decrease in the voltage and formation of single plateau during cycling. The change corresponds to the involvement of Cu into the reaction and formation of redox couple Mg/CuS as determined by Cu K-edge XANES measurements. Corrosion of Cu foil is identified by SEM and serves as a source of Cu cations for the chemical reaction between Cu and polysulfides. Mg/CuS redox couple shows improved cycling stability, but theoretical energy density is severely reduced due to substitution of S with CuS as cathode active material. Image 1 • Influence of current collectors on electrochemistry of Mg–S battery is shown. • Increased stability and rate capability can be achieved with Cu current collector. • Formation of CuS was confirmed when using Cu current collector. • Severe degradation of Cu current collector was observed in different electrolytes. • Cu current collectors can not be used in combination with sulfur cathode. [ABSTRACT FROM AUTHOR]