Short-chain Polysulfides Research Articles

Thiol-Based Electrolyte Additives for High-Performance Lithium-Sulfur Batteries

Here, we propose thiol-based electrolyte additives (biphenyl-4,4’-dithiol (BPD)) to enhance the capacity retention of lithium-sulfur batteries. In-situ Raman and UV-Vis spectroscopy are used to investigate the effect of the additive on the sulfur reduction mechanism. Raman spectroscopy shows that long chain polysulfides (S8 2-) are formed via S8 ring opening in the first reduction process at ~2.4 V vs Li/Li+ and short chain polysulfides such as S4 2-, S4 -, S3 ․ - and S2O4 2- are observed with continued discharge at ~2.0 V vs Li/Li+ in the second reduction process. In the Kinetic study, rate constants obtained for the appearance and disappearance polysulfide species show that short chain polysulfides are directly formed from S8 decomposition. The polysulfide oxidation and reduction is quasi-reversible. With the BPD additive, an additional reduction process is observed at ~2.1 V vs Li/Li+. This reduction is associated with the formation of BPD-polysulfide complexes. The BPD-polysulfide complexes form at ~2.1 V followed by the formation of short chain polysulfides upon further discharge. The reduction of these complexes is reversible during the charge process. The BPD additive inhibits the formation rate of short chain polysulfides, suggesting that there is a strong interaction between BPD and short chain polysulfides. In-situ UV-Vis spectroscopy shows that the polysulfide solubility decreases in the presence of the BPD additive and forms thiol-polysulfide complexes during the cycling. The (-)ESI-MS shows the formation of BPD-S and BPD-S2 and BPD-S3, suggesting that BPD interacts with short chain polysulfides. The decomposition of the thiol-based additive was found during the battery cycling and resulted in a dense and smooth SEI film on the Li anode.

Evaluating silicene as a potential cathode host to immobilize polysulfides in lithium–sulfur batteries

The internal shuttle effect caused by polysulfides dissolution and migration negatively impacts lithium–sulfur battery performance. In this work, a mesoscale simulation strategy, which involves atomistic calculation and coarse-grained molecular modeling, is employed to evaluate silicene as a potential cathode host material to immobilize polysulfides. Adsorption energies of insoluble polysulfides (Li2Sx with x = 1, 2) and soluble polysulfide Li2S4 on pristine and doped silicene sheets are calculated. Results show that the adsorption is thermodynamically favorable and N-doped silicene is helpful in trapping intermediate discharge products, Li2S2 and Li2S4. The dissociation and reduction of long-chain polysulfides to short-chain polysulfides are observed. Electronic structure analysis shows that Li2Sx molecules interact with silicene via strong chemical bonds. The atomistic structure evolution of Li2S layer formation on silicene is also investigated in this study. It is found that Li2S (110) layer forms first, and then, it is converted to Li2S (111) layer by introducing more Li2S molecules to the substrate. Li2S (111)/silicene interfacial structure is thermodynamically stable, and the interaction is dominated by Li–Si bonds. A coarse-grained model is developed to study and compare the growth of Li2S on silicene and graphene. Li2S-induced surface coverage is faster on silicene than on graphene, which indicates that a silicene-based cathode host will experience more acute surface passivation, which will adversely affect cathode performance.

Mechanistic Understanding of Physical Processes in Lithium-Sulfur Batteries through Experiments and Modeling

Lithium-Sulfur (Li-S) batteries are very promising as next-generation high-energy and low-cost energy storage devices for electric vehicle and stationary applications. However, Li-S batteries presently suffer from low cyclability, low sulfur utilization and high self-discharge. A better understanding of physical processes in Li-S batteries is critical and necessary to improving performance. Here, we have combined experimental and modeling efforts to gain better insight into Li-S batteries. Figure 1 shows first-cycle discharge curves from Li-S coin cells at different C-rates (1C=1680 mAh/g). Comparison of the different discharge curves shows that the difference in the capacity between C-rates stems from the lower plateau rather than higher one. This suggests that the limiting process during discharge at these C-rates is the reduction of short-chain polysulfide (PS) to final solid product. Accordingly, we have performed electrochemical impedance spectroscopy (EIS) to probe the dynamic process corresponding to the lower plateaus of the discharge curves. As shown in Figure 2, the radius of the semi-circle in the EIS grows dramatically at the end of discharge, compared to that at the beginning of discharge. The EIS experiment was repeated in the same solution after cleaning the electrode surface. The EIS spectrum acquired from the cleaned electrode was very close to that at the beginning of discharge, indicating that the effect of change in the Li electrode and the electrolyte, if any, on the EIS spectra is minimal. This also suggests that the discharge process is not limited by chemical reaction, because cleaning the electrode surface should not have any effect on the reaction kinetics. Instead, this suggests that the increasing resistance arises from the formation of passivating film on the electrode surface. Furthermore, we discharged cells containing different concentrations of PS solution, using one of two types of electrodes with different reaction surfaces. Figure 3 shows that the specific capacity obtained using non-porous glassy carbon was much lower than that obtained using porous carbon at all concentrations, due to the limited surface area of glassy carbon. On the other hand, for both types of electrodes, the capacity (proportional to the product of concentration and specific capacity) obtained when using a high concentration solution was higher than that obtained with a low concentration solution. This suggests that transport of redox species through the film could be limiting. A mathematical model was developed based on the assumption that the transport of PS species through the film is indeed limiting. It was assumed that the film porosity is constant, and that ionic diffusion through the film follows Fick’s law: ∂C/∂t=Deff(∂2C/∂x2) Deff=Dεbrugg where D is bulk diffusion coefficient, and brugg is Bruggeman exponent. Good agreement between model and experimental results can be obtained, only if the Bruggeman exponent is dependent on the thickness of the film. This suggests that the film growing process is more complicated than a simple increase in film thickness. Figure captions: Figure 1. Rate performance of Li/S batteries. Figure 2. EIS spectra obtained from Au/Li/Li cell. The EIS spectra were recorded at the beginning of discharge, at the end of discharge (1.5 V), and after cleaning of the discharged electrode surface Figure 3.Specific capacity as a function of concentration for carbon paper (porous) and glassy carbon (non-porous) Figure 1

Analytical Detection of Polysulfides in the Presence of Adsorption Additives by Operando X-ray Absorption Spectroscopy

A mechanism for Li–S battery operation with a composite electrode and an adsorption additive obtained by using operando ultraviolet/visible (UV/vis) spectroscopy and X-ray absorption spectroscopy confirms the role of the adsorption additive and reflects the conversion mechanism of sulfur into Li2S. Operando UV/vis spectroscopy shows a reversible appearance of the long-chain polysulfides in the separator in the fifth cycle, whereas the appearance of mid- and short-chain polysulfides suggests a polysulfide shuttle mechanism. By using a nonsulfur-containing electrolyte, a high-precision analysis of sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra is possible. The XANES analysis shows that polysulfides reach the maximum concentration at the end of the high-voltage plateau, and the low-voltage plateau is characteristic of the polysulfides/Li2S equilibrium. The relative amount of Li2S increases linearly until the end of discharge and reaches a relative amount of 75%. This is confirmed by sulfur K-edge EXAFS analysis. Additionally, a quantitative analysis of EXAFS spectra measured during discharge evidences a decrease of the average S–S coordination number. This can be interpreted as a decrease of the chain length of polysulfides. EXAFS analysis showed that there are no specific interactions of the polysulfide species with the matrix or with other species in the electrolyte.

In-Situ Raman Spectroscopy of Sulfur Speciation in Lithium-Sulfur Batteries

In this talk we report on our use of in-situ Raman spectroscopy and cyclic voltammetry to investigate the mechanism of sulfur reduction in lithium-sulfur battery slurry cathodes. Raman spectroscopy shows that long chain polysulfides (S8 2-) were formed via S8 ring opening in the first reduction process at ~2.4 V vs Li/Li+ and short chain polysulfides such as S4 2-, S4 -, S3 ․ - and S2O4 2- were observed with continued discharge at ~2.3 V vs Li/Li+in the second reduction process. Elemental sulfur can be reformed in the end of the charge process. (1-4) Rate constants obtained for the appearance and disappearance polysulfide species shows that short chain polysulfides are directly formed from S8 decomposition. The rate constants for S8 reappearance and polysulfide disappearance on charge were likewise similar, suggesting that polysulfide oxidation and reduction is quasi-reversible. The formation of polysulfide mixtures at partial discharge was found to be quite stable. We also examined the effect of CS2 addition on the sulfur reduction mechanism. Electrochemical and Raman results show that CS2 stabilizes the S8 2-product and inhibits further discharge of the cell

Direct Observation of the Redistribution of Sulfur and Polysufides in Li–S Batteries During the First Cycle by In Situ X‐Ray Fluorescence Microscopy

The demands on low cost and high energy density rechargeable batteries for both transportation and large-scale stationary energy storage are stimulating more and more research toward new battery systems. Since sulfur is an earth-abundant material with low cost, research on the high energy density Li–S batteries (2600 W h kg⁻¹) are getting more and more attention. The reactions between sulfur and lithium during charge–discharge cycling are quite complicated, going through multiple electron transfer process associated with chemical and electrochemical equilibrium between long- and short-chain polysulfide Li₂Sx intermediates (1 < x ≤ 8). It is reported that the long-chain polysulfides can be dissolved into electrolyte with aprotic organic solvents and migrated to the Li anode side. This so-called “shuttle effect” is believed to be the main reason for capacity loss and low columbic efficiency of the Li–S batteries. In the past few years, a great deal of efforts have been made on how to overcome the problem of polysulfide dissolution through new sulfur electrode construction and cell designs, as well as the modification of the electrolyte. Although it has been reported by several publications that some Li–S cells can sustain more than a thousand cycles based on the thin film electrodemore » configurations, the long-term cycling stability is still one of the major barriers for the real application of Li–S batteries. More in-depth studies on the fundamental understanding of the sulfur reaction mechanism and interactions among the different polysulfide species, the electrolyte and the electrodes are still greatly needed. Various in situ techniques have been developed and applied to study the mechanism of the sulfur chemistry in Li–S batteries during electrochemical cycling, such as transmission X-ray microscopy (TXM), X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), UV–visible spectroscopy, and electron paramagnetic resonance (EPR). The applications of these characterization techniques have demonstrated their power in probing the structure changes, morphology evolutions, and coordination of sulfur and polysulfides with the electrolyte in Li–S cells, providing complementary information to each other thus enhancing the understanding in Li–S battery systems. In this communication, in situ X-ray fluorescence (XRF) microscopy was combined with XAS to directly probe the morphology changes of Li–S batteries during first cycle. The morphology changes of the sulfur electrode and the redistribution of sulfur and polysulfides were monitored in real time through the XRF images, while the changes of the sulfur containing compounds were characterized through the XAS spectra simultaneously. In contrast to other studies using ex situ or single characterization technique as reported in the literatures, the in situ technique used in this work has the unique feature of probing the Li–S cell under operating conditions, as well as the combination of XRF imaging with spectroscopy data. By doing this, the morphology evolution and redistribution of specific sulfur particles during cycling can be tracked and identified at certain locations in a real time. In addition, this technique allows us to select the field-of-view (FOV) area from micrometer to centimeter size, providing the capability to study the Li–S reactions not just at the material level, but also at the electrode level. This is very important for both understanding Li–S chemistry and designing effective strategies for Li–S batteries.« less

In situ Raman spectroscopy of sulfur speciation in lithium-sulfur batteries.

In situ Raman spectroscopy and cyclic voltammetry were used to investigate the mechanism of sulfur reduction in lithium-sulfur battery slurry cathodes with 1 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (1/1, v/v). Raman spectroscopy shows that long-chain polysulfides (S8(2-)) were formed via S8 ring opening in the first reduction process at ∼2.4 V vs Li/Li(+) and short-chain polysulfides such as S4(2-), S4(-), S3(•-), and S2O4(2-) were observed with continued discharge at ∼2.3 V vs Li/Li(+) in the second reduction process. Elemental sulfur can be reformed in the end of the charge process. Rate constants obtained for the appearance and disappearance polysulfide species shows that short-chain polysulfides are directly formed from S8 decomposition. The rate constants for S8 reappearance and polysulfide disappearance on charge were likewise similar. The formation of polysulfide mixtures at partial discharge was found to be quite stable. The CS2 additive was found to inhibit the sulfur reduction mechanism allowing the formation of long-chain polysulfides during discharge only and stabilizing the S8(2-) product.

Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries.

In a room-temperature sodium-sulfur (RT Na-S) battery, the complicated reduction reaction of the sulfur cathode generally involves two main steps: (i) transformation of elemental sulfur into long-chain soluble sodium polysulfides (Na2Sn 4 ≤ n ≤ 8) and (ii) conversion of the long-chain sodium polysulfides into solid-state short-chain polysulfide Na2S2 or disulfide Na2S. It is found that the slow kinetics of the second step limits the efficiency of discharge and induces irreversible capacity loss during cycling. Accordingly, we present here a RT Na-S cell operated with the sulfur/long-chain sodium polysulfide redox couple to avoid the capacity fade. An advanced cathode structure has been developed by inserting a carbon nanofoam interlayer between the sulfur cathode and the separator to localize the soluble polysulfide species and prevent its migration to the anode. The highly reversible sulfur/long-chain sodium polysulfide cell presented here can provide a stable output energy density of 450 Wh kg(-1) at an extremely low energy cost of ∼$10 kWh(-1) (based on the active material of anode and cathode).

Electrochemical properties of the soluble reduction products in rechargeable Li/S battery

In this paper, the electrochemical behavior of the reduction products in solution for Li/S cell is studied by UV–visual spectroscopy and electrochemical impedance spectroscopy (EIS). The results tell that the redox process of the polysulfide intermediate contains five charge-transfer steps in the practical Li/S cell. The formation of final reduction product of Li 2S and the final re-oxidation product of S 8 is completely irreversible. The transform between polysulfide and Li 2S 2 is electrochemical sluggish. The peaks corresponding to transformation Li 2S x ↔ Li 2S y (2 < x < y ≤ 6) are still symmetrical in spite of an increasing polarization with the proceeding of CV scan. While the redox process corresponding to Li 2S m ↔ Li 2S n (4 < m < n ≤ 8) is reversible. The dissolution long-chain polysulfide and deposition of short-chain polysulfide contribute mostly to the electrode deterioration even electrode blockage. Therefore, homogeneous mixing element sulfur with conductive components and alleviating the polysulfide dissolution are equally important to improving the active material utilization and rechargeability for rechargeable Li/S battery.

The evolution of surface layers formed during chalcopyrite leaching

Chalcopyrite (CuFeS 2) leaching in perchloric acid (HClO 4) at an initial pH of one and a temperature of 85 °C has been examined. The rate of leaching of Cu and Fe increased progressively over the duration of the experiment. The Cu leach rate was initially greater (up to 24 h) but thereafter the leach rates for Cu and Fe were approximately equal. After 313 h 81% Cu release was achieved at which time the leach experiment was terminated. Only 25% of the available S was released into solution during the leaching process. Surface speciation over the duration of the leach was examined using X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM). As a result, a three-step reaction pathway is proposed. The first oxidation step involves the release of Cu and Fe into solution and the polymerisation of monosulfide (S 2−) to polysulfide S n 2 - . The subsequent reduction step does not result in the release of cations to solution but does result in the reformation of surface S 2− and other short chain polysulfides, which then on further oxidation restructure to form crystalline elemental sulfur (S 0). This final oxidation step is accompanied by further cation release.