Li2S Content Research Articles

Structural study of Li2S-GeS2 glasses: Ge[sbnd]S network and local environment of Li

The static structures of (Li2S)x(GeS2)100-x glasses (x = 0, 20, 30, 40 and 50) were investigated by using a combination of diffraction measurements and simulation techniques. The X-ray diffraction results show that the host GeS network is formed by linkages of GeS4 tetrahedra. The three-dimensional structure models of GeS network were generated by using the X-ray diffraction data and reverse Monte Carlo (RMC) simulation. The RMC models show that the number of edge-sharing GeS4 tetrahedra decreases as Li2S content but its ratio to the number of connections between GeS4 tetrahedra remains substantially constant irrespective of the composition. The Li ion sites in the GeS network structures were determined by molecular dynamics (MD) simulation with reference to the previously reported neutron diffraction data. The MD results show that each Li ion is located in a deformed octahedral site consisting of bridging and non-bridging sulphur atoms.

Fast Li-Ion Dynamics in Stoichiometric Li2S–Ga2Se3–GeSe2 Glasses

The temperature dependence of Li-ion transport is explored in novel stoichiometric chalcogenide glasses in the ternary system Li2S–Ga2Se3–GeSe2 using electrochemical impedance spectroscopy (EIS) and 7Li nuclear magnetic resonance (NMR) spectroscopy. The dc conductivity in these glasses monotonically increases with Li2S content with the highest room-temperature conductivity being ∼10–4 S/cm in the glass of composition: 50% Li2S–10% Ga2Se3–40% GeSe2 with an activation energy of 0.37 eV. Analysis of the 7Li NMR central transition line shape reveals that Li ions are randomly distributed in the structure of these glasses and their rapid hopping transport is responsible for the electrical conductivity. The excellent agreement between the experimentally measured dc conductivity and that calculated from the Li-ion hopping frequency obtained from EIS and NMR implies that the ionic transference number in all glasses is close to unity and these glasses behave as strong electrolytes.

(Invited) High Lithium Sulfide-Loading Electrode for Practical Li/S Cells

The development of the Li-ion battery (LIB) has led to a portable electronics revolution during last couple of decades, however, the traditional LIB seems to be insufficient for practical application in some emerging markets due to the limited specific energy of the cell. For mass market conversion to electric vehicles, high specific energy (> 300 Wh kg-1) rechargeable battery systems of low-cost for energy storage are required. Among all alternatives, the lithium/sulfur cell has been considered most promising due to its low cost, high abundance, ultrahigh theoretical specific energy (2,566 Wh kg-1) and theoretical energy density (4,260 Wh L-1) which far exceed those of conventional lithium-ion cells. Nevertheless, after extensive study of both sulfur cathodes and lithium sulfide cathodes, neither of these technologies has been commercialized due to poor long-term cycle life and low demonstrated specific energy of lithium/sulfur cells. Practical applications require high sulfur loading electrodes together with the lowest “dead weight” possible in the cell while maintaining good performance. This is quite challenging because commercial current collectors have mechanical limitations in electrode loading and sulfur utiliation. Due to the insulating nature of S and Li2S, the addition of a significant amount of a conductive additive into the high loading electrodes is required and introduces more surface area, and thus more electrolyte is needed which further leads to more dead weight. Recently, lithium sulfide was proposed to be a more desirable electrode material because it permits the cell to be assembled in discharged state and avoids problems associated with volume expansion of sulfur during discharge. Disadvantages of lithium sulfide include challenging issues due to its sensitivity toward moisture and solubility in many organic solvents. In order to achieve high specific energy, we have developed a novel Li2S/KB@Cf electrode (Li2S content: 74 %,) using a robust 3-dimensional (3-D) current collector with a high Li2S loading up to 11.29 mg cm-2 (S loading: 7.85 mg cm-2), which can deliver an areal capacity of 7 mAh cm-2 after 50 cycles with an Electrolyte/Sulfur ratio of 5.5 (see Fig. 1). We will present our recent progress in high S loading cells and more detailed information about active material development, electrode composition and modifications of the electrolyte and cell configuration. Figure 1

Conductivity Enhancement of Li2S Positive Electrode for All-Solid-State Rechargeable Lithium Batteries

All-solid-state Li/S batteries with inorganic solid electrolytes have been studied because of their advantages of large energy density, good reversibility, and low cost. Preparation of sulfur composite electrodes using solid electrolyte and carbon additives is important for enhancing the utilization of sulfur and Li2S as an active material [1,2]. However, increasing sulfur or Li2S content in the composite electrodes tends to decrease reversible capacity because of their insulating nature. A melt diffusion process of sulfur into carbon pores is widely used for forming good sulfur-carbon contacts and inhibiting dissolution of polysulfide moieties into liquid electrolytes. We have prepared sulfur composite electrodes including 50 wt% sulfur by mechanical milling at 155oC, which is over the melting point of sulfur, and all-solid-state Li/S cells with the composite electrode showed the reversible capacity of over 1000 mAh g-1 at room temperature [3]. Alternative strategy to increase utilization of active materials is enhancing their lithium ion conductivity. Partial replacement of sulfide anions (S2-) in crystalline Li2S with iodide anions (I-) introduces lithium defects to Li2S and increases its lattice constant, and the structural modification will bring about conductivity enhancement. In this study, we have prepared Li2S-LiI solid solutions by mechanical milling [4]. The starting materials of crystalline Li2S and LiI were milled using a planetary ball mill apparatus. X-ray diffraction (XRD) for the (100-x)Li2S∙xLiI (mol%, 0 ≤ x ≤ 20) milled samples indicated that XRD peaks attributable to crystalline LiI disappeared and those attributable to Li2S shifted to the lower angle side with an increase in the LiI content. The lattice constant of Li2S increased with an increase in the LiI content, suggesting that solid solutions in the system Li2S-LiI were prepared by mechanical milling. The solid solution at the composition of x = 20 showed the highest Li+ ion conductivity of 2.2×10-6 S cm-1, which is two orders of magnitude higher than that of Li2S. An all-solid-state cell (Li-In/Li3PS4/Li2S-LiI) operated as a rechargeable battery at room temperature. The cell with the x=20 solid solution as an active material exhibited a stable reversible capacity of 930 mAh per gram of Li2S (80% utilization) for 50 cycles at 0.07C. Further conductivity enhancement was achieved in the ternary system Li2S-LiI-LiBr, and the highest conductivity was over 10-5 S cm-1at the composition with 40 mol% lithium halides. The concept of increasing ionic conductivity of sulfur-based active materials is effective in increasing their utilization and reversible capacity for Li/S cells. Acknowledgement This research was financially supported by ALCA-SPRING project.

Fast lithium ion conducting solid state thin-film electrolytes based on lithium thio-germanate materials

In this study, lithium thio-germanate thin-film electrolytes for lithium rechargeable batteries have been successfully prepared by radio-frequency (RF) magnetron sputtering deposition in Ar gas atmospheres. The targets for RF sputtering were prepared by melting, milling and pressing the appropriate amounts of the starting materials in the nLi2S+GeS2, n=1–4, binary system. 2mm wide×18mm long Au electrodes with a parallel configuration of 2mm spacing were sputtered to ∼100nm thick at a sputtering rate at ∼5nmmin−1 on Al2O3 single crystal substrates. Thin-film electrolytes were grown on this electrode assembly at 50W power and 25mtorr in Ar gas pressure. The ionic conductivities of the thin-film electrolytes were measured from −25°C to 100°C with 25°C increments over the frequency range 0.1Hz–1MHz. The d.c. ionic conductivities determined from complex plane plots of the impedance of the Li2GeS3, Li4GeS4, Li6GeS5, and Li8GeS6 amorphous thin films at 25°C were found to be 1.1×10−4Scm−1, 7.5×10−4Scm−1, 1.7×10−3Scm−1, and 7.0×10−5Scm−1, respectively. As the Li2S content in the thin film increases, the ionic conductivities of the thin films increase from n=1 to n=3. However, for the n=4 Li8GeS6 thin film, the ionic conductivity decreased and the activation energy increased. The maximum in the conductivity for the n=3 film is among the highest ever reported for an amorphous solid state Li+ ion conductor.

Conductivity studies on LiX–Li2S–Sb2S3–P2S5 (X = LiI or Li3PO4) glassy system

New sulfide glasses in the Li2S–Sb2S3–P2S5 system have been prepared by classical quenching technique where glassy domain remains up to 50% molar addition of Li2S and electrical conductivities have been determined by impedance spectroscopy. Room temperature DC conductivity vs Li2S content exhibits two regions implying different conductivity mechanisms. The compositions of low lithium content presented low electronic conductivities close to 0.01 μS/cm at room temperature (due to Sb2S3 semiconducting properties). The compositions of medium lithium content could result to mixed ionic–electronic conductors with predominant ionic conductivity with a maximum close to 1 μS/cm; Arrhenius behavior is found between 25 °C and T g for all glasses, but activation energy is found to be somehow above most similar systems. A comparative study with glasses belonging to the other chalcogenide systems has been undertaken and values of the decoupling index are reported, and in order to validate conductivity data, a circuit equivalent circuit was proposed and fitted parameters were calculated with good agreement.

リチウムイオン伝導性Ｌｉ２Ｓ‐ＧｅＳ２系非晶質材料のメカニカルミリング法による合成

Lithium-ion-conducting amorphous materials in the system Li2S-GeS2 were prepared by use of a high-energy ball-milling process. The amorphous materials were obtained over the wide composition range up to 66.7mol%Li2S content. The composition range in which the amorphous materials were obtained by the ball-milling process was much wider than the glass-forming region by conventional melt-quenching methods. The lithium-ion conductivities of the obtained amorphous materials increased with an increase in the Li2S content. The activation energies for conduction correspondingly decreased with increasing the Li2S content. The 66.7Li2S⋅33.3GeS2(mol%) sample showed the maximum conductivity 1.1×10-4Scm-1 at 298 K.DC polarization measurements suggested that a lithiumion transport number was almost unity for the obtained amorphous materials.

A New Amorphous Lithium-ion Conductor in the System Li2S–P2S3

Abstract Amorphous solid electrolytes in the system Li2S–P2S3 were obtained in a narrow composition range at around 66.7 mol % Li2S content by a high-energy ball-milling process. The 66.7Li2S·33.3P2S3 (mol %) sample showed the highest ion conductivity of 1.1 × 10−4 S·cm−1 at room temperature among the obtained samples. Cyclic voltammetry revealed that the sample had a wide electrochemical window of about 5 V.

Formation and ionic conductivity of Li 2S–GeS 2–Ga 2S 3 glasses and thin films

Li 2S–GeS 2–Ga 2S 3 glasses and thin films were prepared and their conductivities were compared. Bulk glasses were melted in Si-coated silica tubes. The glass-forming region was observed at cation ratios of 0 or 40–70% Li and less than 50% Ga. Thin films with a wider composition range were formed by RF sputtering. Conductivities were almost identical in bulk glasses and films of similar composition and increased along with increasing Li 2S content. Conductivity and glass transition temperature ( T g) increased slightly with addition of a moderate amount of Ga 2S 3. These results are due to the mixed-former effect.

Preparation and thermal properties of novel Li 2S–Sb 2S 3 glassy system

There is great interest in sulfide glasses because of their high lithium ion conductivity. A new Li 2S–Sb 2S 3 glassy system has been prepared by a classical quenching technique. Thermal characterization has been carried out and it has been observed that the T g decreases with an increase in the concentration of Li 2S. However, this tendency stops around 17% Li 2S content. Results have been discussed on the basis of the modification that would occur in the base Sb 2S 3 glass network by the addition of Li 2S.

Ionic conduction in oxysulfide glasses for the Li 2SB 2O 3 (LiBr) systems

Oxysulfide glasses in systems Li 2SB 2O 3(LiBr) have been prepared and studied by differential scanning calorimetry and electrical measurement. In particular, the effects of Li 2S as a modifier and LiBr as a dopant on glass transition temperature and conductivity of the glasses are studied. The results demonstrate that for the glasses of Li 2SB 2O 3 system when the Li 2S content increases, both the glass transition temperature and activation energy of conduction increase, and reach a maximum in the range 30−33 mol% Li 2S. The addition of LiBr decreases the glass transition temperature, but enhances pronouncedly the ionic conductivity of glasses. These results are discussed in reference to variation of the structural entities with the fraction of 4-coordinated borons in the glass.

Synthesis and characterization of the B 2S 3Li 2S, the P 2S 5Li 2S and the B 2S 3P 2S 5Li 2S glass systems

A glass processing method that employs a carbon-coated quartz container was used to study the B 2S 3 containing glasses i.e. (1− x) B 2S 3− xLi 2S and 0.33[(1− y)B 2S 3− yP 2S 5]−0.67Li 2S, which are extremely reactive towards quartz at temperatures above 400°C. This technique resulted in transparent glasses with a high Li 2S content (0.5⩽ x⩽0.75) for the (1− x)B 2S 3− xLi 2S system. The glass forming region for the (1− x) P 2S 5− xLi 2S glass system was expanded from 0.66⩽ x⩽0.68 in previous studies to 0.5⩽ x⩽0.7. The thermal behavior and the Li + ionic conductivity of these glasses were analyzed. An exothermal peak was found during the glass transition for some B 2S 3 containing glasses, and the peak could be converted into a second glass transition by annealing at the peak temperature. The coformer sulfide glasses of the 0.33[(1− y)B 2S 3− yP 2S 5]−0.67Li 2S system generally exhibited higher Li + ionic conductivity than that of the single sulfide network former glasses, the room temperature conductivity of the glass with y=0.3 reached the value of 0.141 mS/cm.