IR, Raman, and NMR studies of the short-range structures of 0.5Na2S + 0.5[xGeS2 + (1-x)PS(5/2)] mixed glass-former glasses

High-Performance Lithium-Sulfur Batteries via Molecular Complexation

Beyond lithium-ion technologies, lithium-sulfur batteries stand out because of their multielectron redox reactions and high theoretical specific energy (2500 Wh kg-1). However, the intrinsic irreversible transformation of soluble lithium polysulfides to solid short-chain sulfur species (Li2S2 and Li2S) and the associated large volume change of electrode materials significantly impair the long-term stability of the battery. Here we present a liquid sulfur electrode consisting of lithium thiophosphate complexes dissolved in organic solvents that enable the bonding and storage of discharge reaction products without precipitation. Insights garnered from coupled spectroscopic and density functional theory studies guide the complex molecular design, complexation mechanism, and associated electrochemical reaction mechanism. With the novel complexes as cathode materials, high specific capacity (1425 mAh g-1 at 0.2 C) and excellent cycling stability (80% retention after 400 cycles at 0.5 C) are achieved at room temperature. Moreover, the highly reversible all-liquid electrochemical conversion enables excellent low-temperature battery operability (>400 mAh g-1 at -40 °C and >200 mAh g-1 at -60 °C). This work opens new avenues to design and tailor the sulfur electrode for enhanced electrochemical performance across a wide operating temperature range.

Effects of LiPON Incorporation on the Structures and Properties of Mixed Oxy-Sulfide-Nitride Glassy Solid Electrolytes

Glassy solid electrolytes (GSEs) are promising solid electrolytes in the development of all solid-state batteries. Mixed oxy-sulfide nitride (MOSN) GSEs combine the high ionic conductivity of sulfide glasses, the excellent chemical stability of oxide glasses, and the electrochemical stability of nitride glasses. However, the reports on the synthesis and characterization of these novel nitrogen containing electrolytes are quite limited. Therefore, the systematic incorporation of LiPON during glass synthesis was used to explore the effects of nitrogen and oxygen additions on the atomic-level structures in the glass transition (T_g) and crystallization temperature (T_c) of MOSN GSEs. The MOSN GSE series 58.3Li₂S + 31.7SiS₂ + 10[(1 - x)Li_0.67PO_2.83 + x LiPO_2.53N_0.314], x = 0.0, 0.06, 0.12, 0.2, 0.27, 0.36, was prepared by melt-quench synthesis. Differential scanning calorimetry was used to determine the T_g and T_c values of these glasses. Fourier transformation-infrared, Raman, and magic angle spinning nuclear magnetic resonance spectroscopies were used to examine the short-range order structures of these materials. X-ray photoelectron spectroscopy was conducted on the glasses to further understand the bonding environments of the doped nitrogen. Finally, N and S elemental analyses were used to confirm the composition of these GSEs. These results are used to elucidate the structure of these glasses and to understand the thermal property impact oxygen and nitrogen doping in these GSEs.

An electrochemically stable homogeneous glassy electrolyte formed at room temperature for all-solid-state sodium batteries

All-solid-state sodium batteries (ASSSBs) are promising candidates for grid-scale energy storage. However, there are no commercialized ASSSBs yet, in part due to the lack of a low-cost, simple-to-fabricate solid electrolyte (SE) with electrochemical stability towards Na metal. In this work, we report a family of oxysulfide glass SEs (Na₃PS_4−xO_x, where 0 < x ≤ 0.60) that not only exhibit the highest critical current density among all Na-ion conducting sulfide-based SEs, but also enable high-performance ambient-temperature sodium-sulfur batteries. By forming bridging oxygen units, the Na₃PS_4−xO_x SEs undergo pressure-induced sintering at room temperature, resulting in a fully homogeneous glass structure with robust mechanical properties. Furthermore, the self-passivating solid electrolyte interphase at the Na|SE interface is critical for interface stabilization and reversible Na plating and stripping. The new structural and compositional design strategies presented here provide a new paradigm in the development of safe, low-cost, energy-dense, and long-lifetime ASSSBs.

Single sodium-ion solid electrolyte that meets the requirements of practical applications is difficult to design. Here, the authors show how kinetic stability via the creation of a self-passivating solid electrolyte interphase allows a homogenous glass solid electrolyte to exhibit remarkable electrochemical stability with sodium metal.

Stable sodium-sulfur electrochemistry enabled by phosphorus-based complexation

A series of sodium phosphorothioate complexes are shown to have electrochemical properties attractive for sodium-sulfur battery applications across a wide operating temperature range. As cathode materials, they resolve a long-standing issue of cyclic liquid-solid phase transition that causes sluggish reaction kinetics and poor cycling stability in conventional, room-temperature sodium-sulfur batteries. The cathode chemistry yields 80% cyclic retention after 400 cycles at room temperature and a superior low-temperature performance down to -60 °C. Coupled experimental characterization and density functional theory calculations revealed the complex structures and electrochemical reaction mechanisms. The desirable electrochemical properties are attributed to the ability of the complexes to prevent the formation of solid precipitates over a fairly wide range of voltage.

Investigation of Glass-Ceramic Lithium Thiophosphate Solid Electrolytes Using NMR and Neutron Scattering

Solid-state Li batteries require solid electrolytes which have high Li⁺ conductivity and good chemical/mechanical compatibility with Li metal anodes and high energy cathodes. Structure/function correlations which relate local bonding to macroscopic properties are needed to guide development of new solid electrolyte materials. This study combines diffraction measurements with solid-state nuclear magnetic resonance spectroscopy (ssNMR) and neutron pair distribution function (nPDF) analysis to probe the short-range vs. long-range structure of glass-ceramic Li₃PS₄-based solid electrolytes. This work demonstrates how different synthesis conditions (e.g., solvent selection and thermal processing) affect the resulting polyanionic network. More specifically, structures with high P coordination numbers (e.g., PS₄ ^3- and P₂S₇ ^4-) correlate with higher Li⁺ mobility compared to other polyanions (e.g., (PS₃)_n ^n- chains and P₂S₆ ^4-). Overall, this work demonstrates how ssNMR and nPDF can be used to draw key structure/function correlations for solid-state superionic conductors.

New Amorphous Oxy-Sulfide Solid Electrolyte Material: Anion Exchange, Electrochemical Properties, and Lithium Dendrite Suppression via In Situ Interfacial Modification

Glassy sulfide materials have been considered as promising candidates for solid-state electrolytes (SSEs) in lithium and sodium metal (LM and SM) batteries. While much of the current research on lithium glassy SSEs (GSSEs) has focused on the pure sulfide binary Li₂S + P₂S₅ system, we have expanded these efforts by examining mixed-glass-former (MGF) compositions which have mixtures of glass formers, such as P and Si, which allow melt-quenching synthesis under ambient pressure and therefore the use of grain-boundary-free SSEs. We have doped these MGF compositions with oxygen to improve the chemical, electrochemical, and thermal properties of these glasses. In this work, we report on the short-range order (SRO), namely atomic-level, structures of Li₂S + SiS₂ + P₂O₅ MGF mixed oxy-sulfide glasses and, for the first time, study the critical current density (CCD) of these Si-doped oxy-sulfide GSSEs in LM symmetric cells. The samples were synthesized by planetary ball milling (PBM), and it was observed that a certain minimum milling time was necessary to achieve a final SRO structure. To address the short-circuiting lithium dendrite (LD) problems that were observed in these GSSEs, we demonstrate a simple and novel strategy for these Si-doped oxy-sulfide GSSEs to engineer the LM-GSSE interface by forming an in situ interlayer via heat treatment. Stable cycling to ∼1200 h at a capacity of 2 mAh·cm^-2 per discharge/charge cycle under a current density of 1 mA·cm^-2 is achieved. These results indicate that these MGF oxy-sulfide GSSEs combined with an optimized interfacial modification may find use in LM, and by extrapolation, SM, batteries.

Structural and Optical Properties of Pure and Sulfur-Doped Silicate-Phosphate Glass

A series of silicate–phosphate glass materials from the SiO₂-P₂O₅-K₂O-MgO system (pure and doped with sulfur ions) were synthesized by melting raw material mixtures that contained activated carbon as a reducer. The bulk composition of glass was confirmed with X-ray fluorescence spectroscopy. The homogeneity of the glass was confirmed through elemental mapping at the microstructural level with scanning electron microscopy combined with an analysis of the microregions with energy-dispersive X-ray spectroscopy. The structural and optical properties of the glass were studied by using spectroscopic techniques. The infrared spectroscopy studies that were conducted showed that the addition of sulfur caused changes in the silicate–phosphate networks, as they became more polymerized, which was likely related to the accumulation of potassium near the sulfur ions. By using irradiation with an optical parametric oscillator (OPO) nanosecond laser system operating at the second harmonic wavelength, the glass samples emitted a wide spectrum of luminescence, peaking at about 700 nm when excited by UV light (210–280 nm). The influence of the glass composition and the laser-processing parameters on the emission characteristics is presented and discussed. This work also referred to the density, molar volume, and theoretical optical basicity of pure and sulfur-doped glass.

NMR Investigations of Crystalline and Glassy Solid Electrolytes for Lithium Batteries: A Brief Review

The widespread use of energy storage for commercial products and services have led to great advancements in the field of lithium-based battery research. In particular, solid state lithium batteries show great promise for future commercial use, as solid electrolytes safely allow for the use of lithium-metal anodes, which can significantly increase the total energy density. Of the solid electrolytes, inorganic glass-ceramics and Li-based garnet electrolytes have received much attention in the past few years due to the high ionic conductivity achieved compared to polymer-based electrolytes. This review covers recent work on novel glassy and crystalline electrolyte materials, with a particular focus on the use of solid-state nuclear magnetic resonance spectroscopy for structural characterization and transport measurements.

Mechanochemical synthesis of amorphous and crystalline Na2P2S6- elucidation of local structural changes by X-ray total scattering and NMR

The development of all-solid-state sodium-ion batteries as an alternative energy storage system to lithium based techniques demands for sodium conducting solid electrolytes and an understanding of the sodium conduction mechanism governed by the local structure of these glass-ceramic materials. Na₂P₂S₆ was synthesized in an amorphous state with subsequent crystallization. The change of the local structure before and after crystallization was analyzed in detail regarding the presence of structural building blocks such as [P₂S₆]^2-, [P₂S₆]^4-, [P₂S₇]^4-, and [PS₄]^3-. The structure of the crystalline phase differs markedly compared to the corresponding amorphous phase.

Lithium Thiosilicophosphate Glassy Solid Electrolytes Synthesized by High-Energy Ball-Milling and Melt-Quenching: Improved Suppression of Lithium Dendrite Growth by Si Doping

Due to the volatility of P₂S₅, the ambient pressure synthesis of Li₂S + P₂S₅ (LPS) has been limited to planetary ball-milling (PBM). To utilize PBM of LPS to generate a solid electrolyte (SE), the as-synthesized powder sample must be pressed into pellets, and as such the presence of as-pressed grain boundaries in the SE cannot be avoided. To eliminate the grain boundaries, LPS doped with SiS₂ has been studied because SiS₂ lowers the vapor pressure of the melt and promotes strong glass formation, which in combination allows for greater ease in synthesis. In this work, we have examined the structures and electrochemical properties of lithium thiosilicophosphate 0.6Li₂S + 0.4[xSiS₂ + 1.5(1 - x)PS_5/2], 0 ≤ x ≤ 1, glassy solid electrolytes (GSEs) prepared by both PBM and melt-quenching (MQ). It is shown that the critical current density improved after incorporating SiS₂, reaching 1.5 mA/cm² for the x = 0.8 composition. However, the interfacial reaction of MQ GSE with lithium metal introduced microcracks, which shows that further research is needed to explore and develop more stable GSE compositions. These fundamental results can help to understand the interface reaction and formation and as such can provide a guide to design improved homogeneous GSEs with SiS₂ as a glass former, which have no grain boundaries and thereby may help suppress lithium dendrite formation.

Composition Dependence of the Glass-Transition Temperature and Molar Volume in Sodium Thiosilicophosphate Glasses: A Structural Interpretation Using a Real Solution Model

The glass-transition temperature, T_g, and molar volume, V̅, are two physical properties known to exhibit the mixed glass former effect (MGFE), a nonlinear nonadditive increase or decrease from a linear ideal mixing behavior, in ternary glass systems, where the two glass forming species are varied, whereas the glass modifier content remains constant across the system. In the next of our continuing studies of the MGFE in ternary glasses, the T_g and molar volumes of two ternary glass forming series, 0.5Na₂S + 0.5[ xSiS₂ + (1 - x)PS_5/2], the 0.50 NSP series, and 0.67Na₂S + 0.33[ xSiS₂ + (1 - x)PS_5/2], the 0.67 NSP series, have been determined across the full glass forming range in both series, 0 ≤ x ≤ 1. The 0.50 NSP glasses were found to have a strongly negative MGFE in the T_g and a weaker MGFE in the molar volume. The 0.67 NSP series of glasses exhibited weak negative and strong positive MGFEs in the T_g and molar volumes, respectively. Using the short-range order (SRO) structure model for each glass series that was previously developed, the number of bridging sulfurs (BS) and nonbridging sulfurs (NBS) was determined and analyzed for each of these two series of glasses. A clear linear correlation was observed between the T_g and both the fraction of BSs, BS/(BS + NBS), and the number of BS per glass former, BS/GF in both series. The molar volumes of both series of glasses were analyzed using both ideal and real solution models of mixing. The molar volumes of the glasses were best fit to the molar volumes of all of the individual molar volumes of the various SRO units. In the ideal solution model, the molar volumes of the SRO units were only fit to molar volumes of the end member glasses, x = 0 and 1. In the real solution model, the molar volumes were best fit to the full composition dependence of the molar volume of all of the glasses. In both cases, the same molar volumes for the SRO units were used to fit both sets of molar volumes of both glass series. It was found that the best-fit molar volumes of both the P and Si SRO units were essentially the same at the same number of NBS/GF. In this study, therefore, it was observed that the MGFE in the T_g of the glass was linearly correlated with the number of BS per glass former, BS/GF, whereas the MGFE in the molar volumes of the glasses was correlated with the number of NBS per glass former, NBS/GF.

A new model linking elastic properties and ionic conductivity of mixed network former glasses

A new statistical thermodynamic model has been developed to describe the activated process of cation hopping in mixed network former glasses based on the systematic comparison between the adiabatic elastic moduli measured using Brillouin light scattering and the ionic conductivity measured using dielectric impedance spectroscopy.

Elastic properties and short-range structural order in mixed network former glasses

A new statistical thermodynamic model has been developed to describe the speciation of network former elements in ternary oxide glasses, which uses data from NMR spectroscopy and the adiabatic elastic moduli measured using Brillouin light scattering as input.

Iron Oxide Nanoparticles Labeled with an Excited-State Intramolecular Proton Transfer Dye

Excited-state intramolecular proton transfer (ESIPT) is a particularly well known reaction that has been very little studied in magnetic environments. In this work, we report on the photophysical behavior of a known ESIPT dye of the benzothiazole class, when in solution with uncoated superparamagnetic iron oxide nanoparticles, and when grafted to silica-coated iron oxide nanoparticles. Uncoated iron oxide nanoparticles promoted the fluorescence quenching of the ESIPT dye, resulting from collisions during the lifetime of the excited state. The assembly of iron oxide nanoparticles with a shell of silica provided recovery of the ESIPT emission, due to the isolation promoted by the silica shell. The silica network gives protection against the fluorescence quenching of the dye, allowing the nanoparticles to act as a bimodal (optical and magnetic) imaging contrast agent with a large Stokes shift.