Lithium sulfur (Li-S) batteries are well known for their high theoretical specific capacities, but are plagued with scientific obstacles that make practical implementation of the technology impossible. The success of Li-S batteries will likely necessitate the use of thick sulfur cathodes that enable high specific energy densities. However, little is known about the fundamental reaction mechanisms and chemical processes that take place in thick cathodes, as most research has focused on studying thinner cathodes that enable high performance. In this work, in situ X-ray absorption spectroscopy at the sulfur K-edge is used to examine the back of a 115 μm thick Li-S cathode during discharge. Our results show that in such systems, where electrochemical reactions between sulfur and lithium are likely to proceed preferentially toward the front of the cathode, lithium polysulfide dianions formed in this region diffuse to the back of the cathode during discharge. We show that high conversion of elemental sulfur is achieved by chemical reactions between elemental sulfur and polysulfide dianions of intermediate chain length (Li2Sx, 4 ≤ x ≤ 6). Our work suggests that controlling the formation and diffusion of intermediate chain length polysulfide dianions is crucial for insuring full utilization of thick sulfur cathodes.

Lithium-sulfur batteries are attractive due to their high theoretical specific energy, but the dissolution of lithium polysulfide intermediate species formed during discharge results in capacity fade and limited cycle life. In this study we present the first measurements of ionic conductivity of the polysulfides in a nanostructured block copolymer. The morphology, thermal properties, and the conductivities of polystyrene-b-poly(ethylene oxide) (SEO) containing lithium polysulfides, Li2Sx (x = 4, 8), were studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and ac impedance spectroscopy. We also measured conductivities of mixtures of poly(ethylene oxide) (PEO) and Li2Sx. X-ray absorption spectroscopy was used to confirm the nature of dissolved polysulfides. SAXS measurements on SEO/Li2Sx mixtures indicated that all samples had a lamellar morphology. DSC measurements indicated that SEO/Li2S8 interactions were more favorable than SEO/Li2S4 interactions. The effect of nanostructure on transport of Li2Sx was quantified by calculating a normalized conductivity, which is proportional to the ratio of the conductivity of SEO/Li2Sx to that of the PEO/Li2Sx. The normalized conductivities of both polysulfides peaked at intermediate concentrations. The efficacy of block copolymer electrolytes in Li-S batteries was evaluated by comparing ionic conductivities of polymer electrolytes containing Li2Sx with those containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a common salt used in PEO-based battery electrolytes. The transport of Li2Sx species in SEO is suppressed by factors ranging from 0.4 to 0.04 relative to LiTFSI, depending on x and salt concentration. To our knowledge, this study represents the first systematic investigation of the effect of molecular structure of polymer electrolytes on polysulfide migration.

An understanding of the complex solution phase chemistry of dissolved lithium polysulfides is critical to approaches aimed at improving the cyclability and commercial viability of lithium sulfur batteries. Experimental measurements are frustrated by the versatile sulfur-sulfur bond, with spontaneous disproportionation and interconversion leading to unknown equilibrium distributions of polysulfides with varying lengths and charge states. Here, the solubility of isolated lithium polysulfides is calculated from first-principles molecular dynamics simulations. We explore the associated changes in the dissolution free energy, enthalpy and entropy in two regimes: liquid-phase monodentate solvation in dimethylformamide (DMF) and polymer-like chelation in bis(2-methoxyethyl) ether (diglyme). In both of these technologically relevant solvents, we show that the competition between enthalpy and entropy, related to specific interfacial atomic interactions, conspires to increase the relative stability of long chain dianionic species, which exist as Li⁺-LiS_x^- contact-ion-pairs. Further, we propose a mechanism of radical polysulfide stabilization in simple solvents through the reorientation of the 1^st shell solvent molecules to screen electrostatic fields emanating from the solute and explain nonmonotonicity of the dissolution entropy with polysulfide length in terms of a three-shell solvation model. Our analysis provides statistical dynamics insights into polylsulfide stability, useful to understand or predict the relevant chemical species present in the solvent at low concentrations.

Lithium sulfur batteries have a theoretical specific energy 5 times greater than current lithium ion battery standards, but suffer from the issue of lithium polysulfide dissolution. The reaction mechanisms that underlie the formation of lithium polysulfide reaction intermediates have been studied for over four decades, yet still elude researchers. Polysulfide radical anions formed during the redox processes have become a focal point of fundamental Li-S battery research. The formation of radical species has even been shown to be advantageous to the electrochemical pathways. However, whether polysulfide radical anions can form and be stabilized in common Li-S battery electrolytes that are ether-based is a point of contention in Li-S battery research. The goal of this work was to examine the presence of radical polysulfide species in ether-based solvents. Lithium polysulfide solutions in tetraethylene glycol dimethyl ether and poly(ethylene oxide) are probed using a combination of ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectroscopy. EPR results confirm the presence of radical species in ether-based electrolytes. Comparison of the UV-vis spectra to EPR spectra establishes that the UV-vis absorbance signature for radical species in ether-based solvents occurs at a wavelength of 617 nm, which is consistent with what is observed for high electron pair donor solvents such as dimethylformamide and dimethyl sulfoxide.

The presence and role of polysulfide radicals in the electrochemical processes of lithium sulfur (Li-S) batteries is currently being debated. Here, first-principles interpretations of measured X-ray absorption spectra (XAS) of Li-S cells are leveraged with an ether-based electrolyte. Unambiguous evidence is found for significant quantities of polysulfide radical species (LiS3, LiS4, and LiS5), including the trisulfur radical anion S3-, present after initial discharge to the first discharge plateau, as evidenced by a low energy shoulder in the S K-edge XAS below 2469 eV. This feature is not present in the XAS of cells at increased depth of discharge, which, by our analysis, exhibit increasing concentrations of progressively shorter polysulfide dianions. Through a combination of first-principles molecular dynamics and associated interpretation of in situ XAS of Li-S cells, atomic level insights into the chemistries are provided that underlie the operation and stability of these batteries. Calculated and measured sulfur K-edge X-ray absorption spectra (XAS) and schematic of a lithium-sulfur (Li-S) cell with ether-based electrolyte are reported. The XAS is obtained through the lithium anode probing the electrolyte beyond it. The signature of polysulfide radicals is found as a low-energy shoulder below 2469 eV after discharging to 2.25 V. This feature disappears at greater depths of discharge.

Despite high ionic conductivities, current inorganic solid electrolytes cannot be used in lithium batteries because of a lack of compliance and adhesion to active particles in battery electrodes as they are discharged and charged. We have successfully developed a compliant, nonflammable, hybrid single ion-conducting electrolyte comprising inorganic sulfide glass particles covalently bonded to a perfluoropolyether polymer. The hybrid with 23 wt% perfluoropolyether exhibits low shear modulus relative to neat glass electrolytes, ionic conductivity of 10(-4) S/cm at room temperature, a cation transference number close to unity, and an electrochemical stability window up to 5 V relative to Li(+)/Li. X-ray absorption spectroscopy indicates that the hybrid electrolyte limits lithium polysulfide dissolution and is, thus, ideally suited for Li-S cells. Our work opens a previously unidentified route for developing compliant solid electrolytes that will address the challenges of lithium batteries.

Impregnation of porous carbon matrices with liquid sulfur has been exploited to fabricate composite cathodes for lithium-sulfur batteries, aimed at confining soluble sulfur species near conducting carbon to prevent both loss of active material into the electrolyte and parasitic reactions at the lithium metal anode. Here, through extensive computer simulations, we uncover the strongly favorable interfacial free energy between liquid sulfur and graphitic surfaces that underlies this phenomenon. Previously unexplored curvature-dependent enhancements are shown to favor the filling of smaller pores first and effect a quasi-liquid sulfur phase in microporous domains (diameters <2 nm) that persists ∼30° below the expected freezing point. Evidence of interfacial sulfur on carbon is shown to be a 0.3 eV red shift in the simulated and measured interfacial X-ray absorption spectra. Our results elucidate the critical morphology and thermodynamic properties necessary for future cathode design and highlight the importance of molecular-scale details in defining emergent properties of functional nanoscale interfaces.

Lithium-sulfur batteries have a theoretical specific energy that is a factor of five greater than that of current lithium-ion batteries, but suffer from consequences of the solubility of lithium polysulfide reaction intermediates that form as the batteries are charged and discharged. These species can react with each other and diffuse out of the cathode, causing battery capacity to fade and ultimately, cell failure. In spite of work that has spanned four decades, fingerprints of polysulfides have not yet been established, precluding a systematic study of lithium-sulfur chemistry. Herein we demonstrate the use of principal component analysis of X-ray absorption spectroscopy (XAS) to obtain fingerprints of lithium polysulfides. This approach enables interpretation of spectral data without any assumptions regarding the origin of the observed spectral features or knowledge of the stability of the polysulfide species of interest.We show that in poly(ethylene oxide)-based solid electrolytes containing polysulfides made by chemically reacting Li2S and elemental sulfur, Li2S2 and Li2S6 spontaneously disproportionate to give binary Li2S/Li2S4 and Li2S4/Li 2S8 mixtures, respectively, while Li2S 4 and Li2S8 exist as single molecular species. XAS fingerprints of Li2S4 and Li2S8 are thus presented. © 2014 The Electrochemical Society.

The X-ray absorption spectra (XAS) of lithium polysulfides (Li2Sx) of various chain lengths (x) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (x - 2) internal atoms in the chain, except in the case of Li2S2, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS.

The reduction of sulfur during discharge in a lithium-sulfur (Li-S) cell is known to occur in a series of reaction steps that involve lithium polysulfide intermediates. We present an operando study of the discharge of a solid-state Li-S cell using X-ray absorption spectroscopy (XAS). In theory, the average chain length of the polysulfides, xavg,cell, at a given depth of discharge is determined by the number of electrons delivered to the sulfur cathode. The dependence of xavg,cell measured by XAS on the depth of discharge is in excellent agreement with theoretical predictions. XAS is also used to track the formation of Li2S, the final discharge product, as a function of depth of discharge. The XAS measurements were used to estimate rate constants of a series of simple reactions commonly accepted in literature.