Structure and Properties of Na2S-SiS2-P2S5-NaPO3 Glassy Solid Electrolytes.

In the development of sodium all-solid-state batteries (ASSBs), research efforts have focused on synthesizing highly conducting and electrochemically stable solid-state electrolytes. Glassy solid electrolytes (GSEs) have been considered very promising due to their tunable chemistry and resistance to dendrite growth. For these reasons, we focus here on the atomic-level structures and properties of GSEs in the compositional series (0.6-0.08y)Na2S + (0.4 + 0.08y)[(1 - y)[(1 - x)SiS2 + xPS5/2] + yNaPO3] (NaPSiSO). The mechanical moduli, glass transition temperatures, and temperature-dependent conductivity were determined and related to their short-range order structures that were determined using Raman, Fourier transform infrared, and 31P and 29Si magic angle spinning nuclear magnetic resonance spectroscopies. In addition, the conductivity activation energies were modeled using the Christensen-Martin-Anderson-Stuart model. These GSEs appear to be highly crystallization-resistant in the supercooled liquid region where no measurable crystallization below 450 °C could be observed in differential scanning calorimetry studies. Additionally, these GSEs were found to be highly conducting, with conductivities on the order of 10-5 (Ω cm)-1 at room temperature, and processable in the supercooled state without crystallization. For all these reasons, these NaPSiSO GSEs are considered to be highly competitive and easily processable candidate GSEs for enabling sodium ASSBs.

Insights into Lithium Sulfide Glass Electrolyte Structures and Ionic Conductivity via Machine Learning Force Field Simulations.

Sulfide-based solid electrolytes (SEs) are important for advancing all-solid-state batteries (ASSBs), primarily due to their high ionic conductivities and robust mechanical stability. Glassy SEs (GSEs) comprising mixed Si and P glass formers are particularly promising for their synthesis process and their ability to prevent lithium dendrite growth. However, to date, the complexity of their glassy structures hinders a complete understanding of the relationships between their structures and properties. This study introduces a new machine learning force field (ML-FF) tailored for lithium sulfide-based GSEs, enabling the exploration of their structural characteristics, mechanical properties, and lithium ionic conductivities. Using molecular dynamic (MD) simulations with this ML-FF, we explore the glass structures in varying compositions, including binary Li2S-SiS2 and Li2S-P2S5 as well as ternary Li2S-SiS2-P2S5. Our simulations yielded consistent results in terms of density, elastic modulus, radial distribution functions, and neutron structure factors compared to DFT and experimental work. Our findings reveal distinct local environments for Si and P within these glasses, with most Si atoms in edge-sharing configurations in Li2S-SiS2 and a mix of corner- and edge-sharing tetrahedra in the ternary Li2S-SiS2-P2S5 composition. For lithium ionic conductivity at 300 K, the 50Li2S-50SiS2 glass displayed the lowest conductivity at 2.1 mS/cm, while the 75Li2S-25P2S5 composition exhibited the highest conductivity at 3.6 mS/cm. The ternary glass showed a conductivity of 2.6 mS/cm, sitting between the two. Moreover, an in-depth analysis of lithium ion diffusion over the MD trajectory in the ternary glass demonstrated a significant correlation between diffusion pathways and the rotational dynamics of nearby SiS4 or PS4 tetrahedra. The ML-FF developed in this study provides an important tool for exploring a broad spectrum of solid-state and mixed former sulfide-based electrolytes.

Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy.

High-field magic angle spinning (MAS) dynamic nuclear polarization (DNP) is becoming a common technique for improving the sensitivity of solid-state nuclear magnetic resonance (SSNMR) by the hyperpolarization of nuclear spins. Recently, we have shown that gamma irradiation is capable of creating long-lived free radicals that are amenable to MAS DNP in quartz and a variety of organic solids. Here, we demonstrate that ball milling is able to generate millimolar concentrations of stable radical species in diverse materials such as polystyrene, cellulose, borosilicate glass, and fused quartz. High-field electron paramagnetic resonance (EPR) was used to obtain further insight into the nature of the radicals formed in ball milled quartz and borosilicate glass. We further show that radicals generated in quartz by ball milling can be used for solid-effect DNP. We obtained 29Si DNP enhancements of approximately 114 and 33 at 110 K and room temperature, respectively, from a sample of ball milled quartz.

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 (Tg) and crystallization temperature (Tc) of MOSN GSEs. The MOSN GSE series 58.3Li2S + 31.7SiS2 + 10[(1 - x)Li0.67PO2.83 + x LiPO2.53N0.314], 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 Tg and Tc 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.

NaPON Doping of Na4P2S7 Glass and Its Effects on the Structure and Properties of Mixed Oxy-Sulfide-Nitride Phosphate Glass.

The preparation, properties, and short-range order (SRO) structures of glasses in the series (1-x)[2/3Na2S + 1/3P2S5] + x[1/3Na2S + 2/3NaPO2.31N0.46] = Na4P2S7-6xO4.62xN0.92x, where 0 ≤ x ≤ 0.5 (NaPSON), are reported on. In this study, these mixed oxy-sulfide-nitride (MOSN) glasses were prepared by adding the nitrided material NaPO3-(3/2)yNy; y = 0.46 = NaPO2.31N0.46 (NaPON) to the base sulfide glass Na4P2S7. For comparison purposes, additions of the unitrided material, y = 0, NaPO3, were also studied (NaPSO). Accordingly, large batches of bubble-free glass could be prepared making this route of nitrogen doping amendable toward scaling-up the glass melting process; though, only small amounts of nitrogen could be incorporated in this manner. Nitrogen and sulfur compositional analysis were combined with XPS, Raman, FT-IR, and 31P MAS NMR spectroscopies to determine the amount of retained nitrogen in the glass after melting and quenching and to determine the effect of the added nitrogen and oxygen on the structure of the base pure sulfide glass Na4P2S7, x = 0.0. The nitrogen content increased linearly with the addition of NaPON, but was found, through quantitative 31P MAS NMR analysis, to be approximately half that expected at each value of x. Despite the small amount of nitrogen retained in these glasses, profound increases in the glass transition (Tg) and crystallization temperatures (Tc) were found with increasing x. For the intermediate values of x, 0.2 and 0.3, no crystallization of the supercooled melt was observed even 250 °C above the Tg. It was found that the addition of NaPON to the series caused a disproportionation reaction, where the oxide and oxy-nitride SRO species preferentially formed covalent, networking phosphate chains, forcing the sodium modifier to ionic sulfide units with large fractions of nonbridging sulfurs (NBSs). This disproportionation reaction was also observed in the NaPO3 doped series of glasses, but to a smaller extent. Oxygen was found in both bridging oxygen (BO) and nonbridging oxygen (NBOs) structures while the sulfur was predominantly found in nonbridging sulfur (NBS) structures. N 1s XPS and 31P NMR studies provided insight into the nitrogen bearing phosphorus units and the wt % of nitrogen that was retained in the quenched glasses. It was found that trigonally coordinated nitrogen (Nt) was preferentially retained in the melt, whereas it is proposed that the linearly coordinated (doubly bonded) nitrogen (Nd) accounts for the lost nitrogen in the glasses.

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 (Na3PS4-xOx, 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 Na3PS4-xOx 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.

Synthesis, Short-Range Order Structure, and Thermal Properties of Mixed Oxy-sulfide Nitride (MOSN) Glasses.

Nitrogen doping has been shown to greatly improve the stability of solid electrolyte (SE) materials at the anode and cathode interfaces in all solid-state batteries (ASSBs) as widely demonstrated by the LiPON family of compositions. In an effort to expand the use of nitrogen in SEs, in this study, mixed oxy-sulfide nitride (MOSN) glasses were prepared by direct ammonolysis of the sodium oxy-sulfide phosphate Na4P2S7-xOx (NaPSO) glass series to understand the combined effects that oxygen and sulfur have on the incorporation of nitrogen. The short-range order (SRO) structures of the Na4P2S(7-x)-3/2yzOx-3/2y(1-z)Ny (NaPSON) glasses were investigated with Raman and infrared (IR) spectroscopies to understand the effect that nitrogen has in the glass structure. The N content of the glasses was quantified by elemental analysis and confirmed through weight change measurements. By combining this information, it was further possible to determine the anion exchange ratio, z, for the N substitution of O and S as a function of the base NaPSO glass chemistry, x. The composition-dependent glass transition temperature, Tg(x), measured with differential scanning calorimetry (DSC), was found to correlate well with the measured N/P ratio, y, in the NaPSON glasses.

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.

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)PS5/2], 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.

Liquid Metal-Elastomer Soft Composites with Independently Controllable and Highly Tunable Droplet Size and Volume Loading.

Soft composites are critical for soft and flexible materials in energy harvesting, actuators, and multifunctional devices. One emerging approach to create multifunctional composites is through the incorporation of liquid metal (LM) droplets such as eutectic gallium indium (EGaIn) in highly deformable elastomers. The microstructure of such systems is critical to their performance; however, current materials lack control of particle size at diverse volume loadings. Here, we present a fabrication approach to create liquid metal-elastomer composites with independently controllable and highly tunable droplet size (100 nm ≤ D ≤ 80 µm) and volume loading (0 ≤ ϕ ≤ 80%). This is achieved through a combination of shear mixing and sonication of concentrated LM/elastomer emulsions to control droplet size and subsequent dilution and homogenization to tune LM volume loading. These materials are characterized utilizing dielectric spectroscopy supported by analytical modeling, which shows a high relative permittivity of 60 (16× the unfilled elastomer) in a composite with ϕ = 80%, a low tan δ of 0.02, and a significant dependence on ϕ and minor dependence on droplet size. Temperature response and stability are determined using dielectric spectroscopy through temperature and frequency sweeps with DSC. These results demonstrate a wide temperature stability of the liquid metal phase (crystallizing at <-85 °C for D < 20 µm). Additionally, all composites are electrically insulating across wide frequency (0.1 Hz-10 MHz) and temperature (-70 to 100 °C) ranges even up to ϕ = 80%. We highlight the benefit of LM microstructure control by creating all-soft-matter stretchable capacitive sensors with tunable sensitivity. These sensors are further integrated into a wearable sensing glove where we identify different objects during grasping motions. This work enables programmable LM composites for soft robotics and stretchable electronics where flexibility and tunable functional response are critical.

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, Tg, 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 Tg and molar volumes of two ternary glass forming series, 0.5Na2S + 0.5[ xSiS2 + (1 - x)PS5/2], the 0.50 NSP series, and 0.67Na2S + 0.33[ xSiS2 + (1 - x)PS5/2], 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 Tg and a weaker MGFE in the molar volume. The 0.67 NSP series of glasses exhibited weak negative and strong positive MGFEs in the Tg 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 Tg 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 Tg 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.

Glasses are promising candidate materials for all-solid-state electrolytes for rechargeable batteries due to their outstanding mechanical stability, wide electrochemical stability range, and open structure for potentially high conductivity. Mechanical stiffness and ionic conductivity are two key parameters for solid-state electrolytes. In this study, we investigate two mixed-network former glass systems, sodium borosilicate 0.2Na2O + 0.8[xBO1.5 + (1 - x)SiO2] and sodium borogermanate 0.2Na2O + 0.8[xBO1.5 + (1 - x)GeO2] glasses. With mixed-network formers, the structure of the network changes while the network modifier mole fraction is kept constant, i.e., x = 0.2, which allows us to analyze the effect of the network structure on various properties, including ionic conductivity and elastic properties. Besides the non-linear, non-additive mixed glass former effect, we find that the longitudinal, shear and Young's moduli depend on the combined number density of tetrahedrally and octahedrally coordinated network former elements. These units provide connectivity in three dimensions, which is required for the networks to exhibit restoring forces in response to isotropic and shear deformations. Moreover, the activation energy for modifier cation, Na+, migration is strongly correlated with the bulk modulus, suggesting that the elastic strain energy associated with the passageway dilation for the sodium ions is governed by the bulk modulus of the glass. The detailed analysis provided here gives an estimate for the number of atoms in the vicinity of the migrating cation that are affected by elastic deformation during the activated process. The larger this number and the more compliant the glass network, the lower is the activation energy for the cation jump.

Structural Characterization of the Short-Range Order in High Alkali Content Sodium Thiosilicophosphate Glasses.

The composition dependence of the short-range order (SRO) structure in highly modified mixed glass former sodium thiosilicophosphate glasses, yNa2S + (1 - y)[xSiS2 + (1 - x)PS5/2], were investigated using infrared (IR), Raman, and 29Si and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies. Both the y = 0.5 and 0.67 glasses undergo disproportionation reactions among the Si and P SRO structures, which lead to various and complex SRO structural units for the Si and P, as shown via the spectra used to characterize the glasses. In the y = 0.5 series, the compositionally expected and experimentally observed SRO units are the P1 and Si2 units in the two binary end-member glasses, where the superscript is the number of bridging sulfur atoms on the P or Si units. However, in the ternary mixed glasses, 0 < x < 1, these units were found to react to form P0 (more highly modified, y = 0.60) and Si3 (less highly modified, y = 0.33) units, indicating preferential association of Na+ ions with the P SRO structures. The Raman spectra were used to resolve the heretofore incompletely studied Si3 SRO unit, which was otherwise difficult to elucidate using 29Si MAS NMR alone. In the y = 0.67 series glasses, the expected P0 and Si0 SRO units were observed for the end-member binary glasses. Like in the y = 0.5 series, the 29Si MAS NMR showed that edge-sharing Si2 (ESi2, y = 0.5) structures were also present in these highly modified glasses, which meant Na2S was not completely incorporated in the network. Evidence of this was shown in the Raman spectra in the form of polysulfide structures Na2Sx (x = 2, 4).

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

Elastic properties of alkali containing glasses are of great interest not only because they provide information about overall structural integrity but also they are related to other properties such as thermal conductivity and ion mobility. In this study, we investigate two mixed-network former glass systems, sodium borosilicate 0.2Na2O + 0.8[xBO1.5 + (1 - x)SiO2] and sodium borogermanate 0.2Na2O + 0.8[xBO1.5 + (1 - x)GeO2] glasses. By mixing network formers, the network topology can be changed while keeping the network modifier concentration constant, which allows for the effect of network structure on elastic properties to be analyzed over a wide parametric range. In addition to non-linear, non-additive mixed-glass former effects, maxima are observed in longitudinal, shear and Young's moduli with increasing atomic number density. By combining results from NMR spectroscopy and Brillouin light scattering with a newly developed statistical thermodynamic reaction equilibrium model, it is possible to determine the relative proportions of all network structural units. This new analysis reveals that the structural characteristic predominantly responsible for effective mechanical load transmission in these glasses is a high density of network cations coordinated by four or more bridging oxygens, as it provides for establishing a network of covalent bonds among these cations with connectivity in three dimensions.

Positive and Negative Mixed Glass Former Effects in Sodium Borosilicate and Borophosphate Glasses Studied by (23)Na NMR.

Glasses with varying compositions of constituent network formers but constant mobile ion content can display minima or maxima in their ion transport which are known as the negative or the positive mixed glass former effect, MGFE, respectively. Various nuclear magnetic resonance (NMR) techniques are used to probe the ion hopping dynamics via the (23)Na nucleus on the microscopic level, and the results are compared with those from conductivity spectroscopy, which are more sensitive to the macroscopic charge carrier mobility. In this way, the current work examines two series of sodium borosilicate and sodium borophosphate glasses that display positive and negative MGFEs, respectively, in the composition dependence of their Na(+) ion conductivities at intermediate compositions of boron oxide substitution for silicon oxide and phosphorus oxide, respectively. A coherent theoretical analysis is performed for these glasses which jointly captures the results from measurements of spin relaxation and central-transition line shapes. On this basis and including new information from (11)B magic-angle spinning NMR regarding the speciation in the sodium borosilicate glasses, a comparison is carried out with predictions from theoretical approaches, notably from the network unit trap model. This comparison yields detailed insights into how a variation of the boron oxide content and thus of either the population of silicon or phosphorus containing network-forming units with different charge-trapping capabilities leads to nonlinear changes of the microscopic transport properties.

Composition Dependence of the Na(+) Ion Conductivity in 0.5Na2S + 0.5[xGeS2 + (1 - x)PS5/2] Mixed Glass Former Glasses: A Structural Interpretation of a Negative Mixed Glass Former Effect.

A negative mixed glass former effect (MGFE) in the Na(+) ion conductivity of glass has been found in 0.5Na2S + 0.5[xGeS2 + (1 - x)PS5/2] glasses where the Na(+) ion conductivity is significantly smaller for all of the ternary glasses than either of the binary end-member glasses. The minimum conductivity of ∼0.4 × 10(-6) (Ω cm)(-1) at 25 °C occurs for the x = 0.7 glass. Prior to this observation, the alkali ion conductivity of sulfide glasses at constant alkali concentration, but variable ratio of one glass former for another (x) ternary mixed glass former (MGF) glasses, has always produced a positive MGFE in the alkali ion conductivity; that is, the ternary glasses have always had higher ion conductivities that either of the end-member binary glasses. While the Na(+) ion conductivity exhibits a single global minimum value, the conductivity activation energy exhibits a bimodal double maximum at x ≈ 0.4 and x ≈ 0.7. The modified Christensen-Martin-Anderson-Stuart (CMAS) model of the activation energies reveals the origin of the negative MGFE to be due to an increase in the dielectric stiffness (a decrease in relative dielectric permittivity) of these glasses. When coupled with an increase in the average Na(+) ion jump distance and a slight increase in the mechanical stiffness of the glass, this causes the activation energy to go through maximum values and thereby produce the negative MGFE. The double maximum in the conductivity activation energy is coincident with double maximums in CMAS calculated strain, ΔES, and Coulombic, ΔEC, activation energies. In these ternary glasses, the increase in the dielectric stiffness of the glass arises from a negative deviation of the limiting high frequency dielectric permittivity as compared to the binary end-member glasses. While the CMAS calculated total activation energies ΔEact = ΔES + ΔEC are found to reproduce the overall shape of the composition dependence of the measured ΔEact values, they are consistently smaller than the measured values for all compositions x. The new concept of an effective Madelung constant for the Na(+) ions in glass is introduced, MD(Na(+)), to account for the difference. Calculated MD(Na(+)) values necessary to bring the CMAS and experimental ΔEact values into agreement are in excellent agreement with nominal values for typical oxide crystals containing Na(+). New MD simulations of oxide glasses were performed and were used to calculate MD(Na(+)) values for Na2O + SiO2 glasses for the first time and were found to agree quite well with the values for the sulfide glasses studied here. Insights from the current study have been used to predict and design new MGF systems that may lead to a positive MGFE in the ionic conductivity.

Lithium ion dynamics in Li2S+GeS2+GeO2 glasses studied using (7)Li NMR field-cycling relaxometry and line-shape analysis.

We use (7)Li NMR to study the ionic jump motion in ternary 0.5Li2S+0.5[(1-x)GeS2+xGeO2] glassy lithium ion conductors. Exploring the "mixed glass former effect" in this system led to the assumption of a homogeneous and random variation of diffusion barriers in this system. We exploit that combining traditional line-shape analysis with novel field-cycling relaxometry, it is possible to measure the spectral density of the ionic jump motion in broad frequency and temperature ranges and, thus, to determine the distribution of activation energies. Two models are employed to parameterize the (7)Li NMR data, namely, the multi-exponential autocorrelation function model and the power-law waiting times model. Careful evaluation of both of these models indicates a broadly inhomogeneous energy landscape for both the single (x=0.0) and the mixed (x=0.1) network former glasses. The multi-exponential autocorrelation function model can be well described by a Gaussian distribution of activation barriers. Applicability of the methods used and their sensitivity to microscopic details of ionic motion are discussed.

Short range structural models of the glass transition temperatures and densities of 0.5Na2S + 0.5[xGeS2 + (1 - x)PS5/2] mixed glass former glasses.

The 0.5Na2S + 0.5[xGeS2 + (1 - x)PS5/2] mixed glass former (MGF) glass system exhibits a nonlinear and nonadditive negative change in the Na(+) ion conductivity as one glass former, PS5/2, is exchanged for the other, GeS2. This behavior, known as the mixed glass former effect (MGFE), is also manifest in a negative deviation from the linear interpolation of the glass transition temperatures (T(g)) of the binary end-member glasses, x = 0 and x = 1. Interestingly, the composition dependence of the densities of these ternary MGF glasses reveals a slightly positive MGFE deviation from a linear interpolation of the densities of the binary end-member glasses, x = 0 and x = 1. From our previous studies of the structures of these glasses using IR, Raman, and NMR spectroscopies, we find that a disproportionation reaction occurs between PS7/2(4-) and GeS3(2-) units into PS4(3-) and GeS5/2(1-) units. This disproportionation combined with the formation of Ge4S10(4-) anions from GeS5/2(1-) groups leads to the negative MGFE in T(g). A best-fit model of the T(g)s of these glasses was developed to quantify the amount of GeS5/2(1-) units that form Ge4S10(4-) molecular anions in the ternary glasses (∼ 5-10%). This refined structural model was used to develop a short-range structural model of the molar volumes, which shows that the slight densification of the ternary glasses is due to the improved packing efficiency of the germanium sulfide species.

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.

A nonlinear and nonadditive composition-dependent change of the ionic conductivity in mixed glass-former (MGF) glasses when one glass former, such as PS(5/2), is replaced by a second glass former, such as GeS2, at constant alkali modifier concentrations, such as Na2S, is known as the mixed glass-former effect (MGFE). Alkali ion conducting glasses are of particular interest for use as solid electrolytes in alkali-based all-solid-state batteries because sulfide amorphous materials have significantly higher alkali ion conductivities than their oxide glass counterparts. In this study of the ternary MGF system Na2S + GeS2 + PS(5/2), we report the careful structural characterization of these glasses using a combination of vibrational, infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectroscopies. Our measurements of the 0.5Na2S + 0.5[xGeS2 + (1-x)PS(5/2)] MGF system show that this glass system exhibits a strongly negative MGFE and non-Arrhenius ionic conductivities. While this negative MGFE in the Na(+) ion conductivity makes these glasses less attractive for use in solid-state Na batteries, the structural origin of this effect is important to better understand the mechanisms of ion conduction in the glassy state. For these reasons, we have examined the structures of ternary 0.5Na2S + 0.5[xGeS2 + (1-x)PS(5/2)] glasses using Raman, IR, and (31)P MAS NMR spectroscopies. In these studies, it is found that the substitution of PS(5/2) by GeS2, that is, increasing x, leads to unequal sharing of the Na(+) in these glasses. Thus, in all MGF compositions, phosphorus groups are associated with a disproportionately larger fraction, f(Na(P)) > 0.5(1 - x), of the Na(+) ions while the germanium groups are found to be Na(+)-deficient relative to the total amount of Na(+) present in the glass, that is, f(Na(Ge)) < 0.5x. From the spectroscopic study of these glasses, a short-range order (SRO) structural model is developed for these glasses and is based on the germanium and phosphorus SRO groups in these glasses as a first step in understanding the unique negative MGFE and non-Arrhenius behavior in the Na(+) ion conductivity in these glasses.

Ionic conductivity of mixed glass former 0.35Na(2)O + 0.65[xB(2)O(3) + (1 - x)P(2)O(5)] glasses.

The mixed glass former effect (MGFE) is defined as a nonlinear and nonadditive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass forming compositions. In this study, mixed glass former (MGF) sodium borophosphate glasses, 0.35Na2O + 0.65[xB2O3 + (1 - x)P2O5], 0 ≤ x ≤ 1, have been prepared, and their sodium ionic conductivity has been studied. The ionic conductivity exhibits a strong, positive MGFE that is caused by a corresponding strongly negative nonlinear, nonadditive change in the conductivity activation energy with changing glass former content, x. We describe a successful model of the MGFE in the conductivity activation energy terms of the underlying short-range order (SRO) phosphate and borate glass former structures present in these glasses. To do this, we have developed a modified Anderson-Stuart (A-S) model to explain the decrease in the activation energy in terms of the atomic level composition dependence (x) of the borate and phosphate SRO structural groups, the Na(+) ion concentration, and the Na(+) mobility. In our revision of the A-S model, we carefully improve the treatment of the cation jump distance and incorporate an effective Madelung constant to account for many body coulomb potential effects. Using our model, we are able to accurately reproduce the composition dependence of the activation energy with a single adjustable parameter, the effective Madelung constant, that changes systematically with composition, x, and varies by no more than 10% from values typical of oxide ceramics. Our model suggests that the decreasing columbic binding energies that govern the concentration of the mobile cations are sufficiently strong in these glasses to overcome the increasing volumetric strain energies (mobility) caused by strongly increasing glass-transition temperatures combined with strongly decreasing molar volumes of these glasses. The dependence of the columbic binding energy term on the relative high-frequency dielectric permittivity suggests that the increased polarizability of the bridging oxygens connecting SRO tetrahedral boron units to phosphorus units causes further charge delocalization away from the negatively charged tetrahedral boron units, leading to a lowering of the charge density, and is the underlying cause of the MGFE.