Rapid Solution Synthesis of Argyrodite-Type Li6PS5X (X = Cl, Br, and I) Solid Electrolytes Using Excess Sulfur.

All-solid-state lithium-ion batteries (ASSLBs) have the potential to be the next-generation energy storage systems because of their high safety features. However, one of the major challenges to the commercialization of ASSLBs is the development of well-established large-scale manufacturing techniques for solid electrolytes (SEs). Herein, we synthesize Li6PS5X (X = Cl, Br, and I) SEs in a total of 4 h by a rapid solution synthesis method using excess elemental sulfur as a solubilizer and reasonable organic solvents. In the system, trisulfur radical anions stabilized by a highly polar solvent increase the solubility and reactivity of the precursor. Raman and UV-vis spectroscopies reveal the solvation behavior of halide ions in the precursor. This result demonstrates that the solvation structure modified by the halide ions determines the chemical stability, solubility, and reactivity of chemical species in the precursor. The prepared Li6PS5X (X = Cl, Br, and I) SEs show ionic conductivities of 2.1 × 10-3, 1.0 × 10-3, and 3.8 × 10-6 S cm-1 at 30 °C, respectively. Our study provides a rapid synthesis of argyrodite-type SEs with high ionic conductivity.

Reaction Mechanism of Li2MnO3 Electrodes in an All-Solid-State Thin-Film Battery Analyzed by Operando Hard X-ray Photoelectron Spectroscopy.

Li2MnO3 is a promising cathode candidate for Li-ion batteries because of its high discharge capacity; however, its reaction mechanism during cycling has not been sufficiently explicated. Observations of Mn and O binding energy shifts in operando hard X-ray photoelectron spectroscopy measurements enabled us to determine the charge-compensation mechanism of Li2MnO3. The O 1s peak splits at an early stage during the first charge, and the concentration of lower-valence O changes reversibly with cycling, indicating the formation of a low-valence O species that intrinsically participates in the redox reaction. The O 1s peak-splitting behavior, which indicates the number of valences of O in Li2MnO3, is supported by the computational results for an O3 to O1 structural transition. This is in agreement with the results of our previous study, wherein we confirmed this O3 to O1 transition based on in situ surface X-ray diffraction analysis, X-ray photoelectron spectroscopy, and first-principles formation energy calculations.

Bottom-up synthesis of ion-pair-bridged metal-organic framework for H+ conduction.

As a novel conceptual synthesis of a metal-organic framework (MOF)-based proton conductor, UiO-66 based on a pyridinedicarboxylic acid phosphate (PyDC-PA) ion pair linker has been developed, in which the phosphoric acid is fixed to the N donor moiety of pyridine via an ionic bond.

Electrochemical and microstructural analysis of LiNi1/3Mn1/3Co1/3O2 cathode composites prepared using the SEED method.

Cathode composites were fabricated using the nuclear growth (SEED) method. Compared to mortar mixing, the SEED method demonstrated higher cycle stability, with a 90LiNi1/3Mn1/3Co1/3O2-10Li7P2S8I composite retaining 99.7% discharge capacity after six cycles compared to 66.1%. Cross-sectional SEM-EDX images suggest that the solid electrolyte was more uniformly distributed in the cathode composite prepared using the SEED method. This study opens up the potential for higher cathode-active material loading ratios.

Transition-Metal Sulfides for High-Performance Lithium Sulfide Cathodes in All-Solid-State Lithium-Sulfur Batteries.

All-solid-state lithium-sulfur batteries (ASLSBs) have been attracting attention as next-generation batteries because of their high theoretical energy density, which exceeds that of traditional lithium-ion batteries. However, the performance of ASLSBs is limited by the sluggish redox reaction kinetics of lithium sulfide (Li2S) and S8 cathodes and the electrochemical degradation of cathode materials and solid electrolytes during cycling. Herein, we report a cathode design consisting of Li2S and transition-metal sulfides. This cathode design enhances the redox reaction kinetics of the cathode and suppresses interfacial degradation between the cathodes and solid electrolytes in the composite cathodes. The interface design uses titanium disulfide, molybdenum sulfide (MoS2), and tungsten sulfide to facilitate redox reaction kinetics, which improves the practical performance of ASLSBs. Among the composite cathodes examined in this work, the Li2S-MoS2 composite cathode exhibited the highest discharge capacity of 661 mA h g-1 (2.09 mA h cm-2) after 100 cycles. Electrochemical impedance analysis demonstrated that transition-metal sulfides, particularly MoS2, suppressed the increase in resistance through cycling of the composite cathodes. This finding suggests that transition-metal sulfides in Li2S composite cathodes multifunction as redox mediators and buffer layers, improving practical battery performance. Therefore, the electrode design offered in this study enhances the electrochemical utilization and long-term stability of ASLSBs.

Li10GeP2S12 solid electrolytes synthesised via liquid-phase methods.

Li10GeP2S12 solid electrolytes for all-solid-state batteries were synthesised through liquid-phase shaking, which is a suspension synthesis method, in 1 d. Additionally, Li10GeP2S12 solid electrolytes were prepared in 7.5 h, which is the fastest synthesis time, through solution synthesis using excess sulfur and a mixed solvent of acetonitrile, tetrahydrofuran, and ethanol. These Li10GeP2S12 solid electrolytes exhibited an ionic conductivity of 1.6 × 10-3 S cm-1, which is the highest ionic conductivity of previous studies of liquid phase synthesis. Therefore, Li10GeP2S12 solid electrolytes with high ionic conductivity can be rapidly synthesised via liquid-phase methods.

Ternary Triazole-Based Organic-Inorganic Proton-Conducting Hybrids Based on Computational Models for HT-PEMFC Application.

We reported a new ternary hybrid anhydrous proton-conducting material based on triazole (Tz), wherein it interacted with TiO2 and cesium hydrogen sulfate (CHS) constructed based on the acid-base interaction. It exhibited high proton conductivity derived by the two acid-base interactions: between CHS and Tz and between Tz and TiO2. As a starting point of discussion, we attempted to theoretically predict the high/low proton conductivity using the push-pull protonated atomic distance (PAD) law, which makes it possible to predict the proton conductivity in the acid-base part based on density functional theory. The calculations indicate the possibility of achieving higher proton conductivity in the ternary composites (CHS·Tz-TiO2) involving two acid-base interactions than in binary hybrids, such as CHS·Tz and TiO2-Tz composites, suggesting the positive effect of two simultaneous acid-base interactions for achieving high proton conductivity. This result is supported by the experimental result with respect to synthesized materials obtained using the mechanochemical method. Adding TiO2 to the CHS·Tz system causes a change in the CHS·Tz interaction and promotes proton dissociation, producing a new and fast proton-conducting layer through the formation of Tz-TiO2 interaction. Applying CHS·Tz-TiO2 to high-temperature proton exchange membrane fuel cells results in improved membrane conductivity and power-generation properties at 150 °C under anhydrous conditions.

Fabrication and electrochemical properties of electrode composites for oxide-type all-solid-state batteries through electrostatic integrated assembly.

All-solid-state batteries, which use flame-resistant solid electrolytes, are regarded as safer alternatives to conventional lithium-ion batteries for various applications including electric vehicles. Herein, we report the fabrication of cathode composites for oxide-type all-solid-state batteries through an electrostatic assembly method. A polyelectrolyte is used to adjust the surface charge of the matrix particles to positive/negative, and the aggregation resulting from electrostatic interactions is utilized. Composites consisting of cathode active material particles (LiNi1/3Mn1/3Co1/3O2 (NMC) or LiNi0.5Mn1.5O4 (LNMO)), solid electrolyte particles Li1.3Al0.3Ti1.7(PO4)3 (LATP), and electron conductive one-dimensional carbon nanotubes (CNT) are formed via an electrostatic integrated assembly of colloidal suspensions. Electrostatic integration increases the electronic conductivity by two orders of magnitude in the NMC-LATP-CNT composite (6.5 × 10-3 S cm-1/3.2 × 10-5 S cm-1) and by six orders of magnitude in the LNMO-LATP-CNT composite (6.4 × 10-3 S cm-1/2.3 × 10-9 S cm-1). The dispersion of CNTs in the cathode composite is enhanced, resulting in percolation of e- path even at 1 wt% (approximately 2.5 vol%) CNT. This study indicates that an integrated cathode composite can be fabricated with particles uniformly mixed by electrostatic interaction for oxide-type all-solid-state batteries.

High Ionic Conductivity with Improved Lithium Stability of CaS- and CaI2-Doped Li7P3S11 Solid Electrolytes Synthesized by Liquid-Phase Synthesis.

Li7P3S11 solid electrolytes (SEs) subjected to liquid-phase synthesis with CaS or CaI2 doping were investigated in terms of their ionic conductivity and stability toward lithium anodes. No peak shifts were observed in the XRD patterns of CaS- or CaI2-doped Li7P3S11, indicating that the doping element remained at the grain boundary. CaS- or CaI2-doped Li7P3S11 showed no internal short circuit, and the cycling continued, indicating that not only CaI2 including I- but also CaS could help increase the lithium stability. These results provide insights for the development of sulfide SEs for use in all-solid-state batteries in terms of their ionic conductivity and stability toward lithium anodes.

Li4SiO4 Doped-Li7P2S8I solid electrolytes with high lithium stability synthesised using liquid-phase shaking.

In this study, mechanical milling and liquid-phase shaking are used to synthesise 3Li2S·P2S5 LiI·xLi4SiO4 (Li7P2S8I·xLi4SiO4) solid electrolytes. When mechanical milling is used, the electrolyte samples doped with 10 mol% of Li4SiO4 (Li7P2S8I·10Li4SiO4) have the highest ionic conductivity at ∼25-130 °C. When liquid-phase shaking is used, they exhibit a relatively high conductivity of 0.85 mS cm-1 at ∼20 °C, and low activation energy for conduction of 17 kJ mol-1. A cyclic voltammogram shows that there are no redox peaks between -0.3 and +10 V, other than the main peaks near 0 V (v.s. Li/Li+), indicating a wide electrochemical window. The galvanostatic cycling test results demonstrate that the Li7P2S8I·10Li4SiO4 has excellent long-term cycling stability in excess of 680 cycles (1370 h), indicating that it is highly compatible with Li. Thus, Li7P2S8I solid electrolytes doped with Li4SiO4 are synthesised using the liquid-phase shaking method for the first time and achieve a high ionic conductivity of 0.85 mS cm-1 at 25 °C. This work demonstrates the effects of Li4SiO4 doping, which can be used to improve the ionic conductivity and stability against Li anodes with Li7P2S8I solid electrolytes.

Operando analysis of electronic band structure in an all-solid-state thin-film battery.

Material characterization that informs research and development of batteries is generally based on well-established ex situ and in situ experimental methods that do not consider the band structure. This is because experimental extraction of structural information for liquid-electrolyte batteries is extremely challenging. However, this hole in the available experimental data negatively affects the development of new battery systems. Herein, we determined the entire band structure of a model thin-film solid-state battery with respect to an absolute potential using operando hard X-ray photoelectron spectroscopy by treating the battery as a semiconductor device. We confirmed drastic changes in the band structure during charging, such as interfacial band bending, and determined the electrolyte potential window and overpotential location at high voltage. This enabled us to identify possible interfacial side reactions, for example, the formation of the decomposition layer and the space charge layer. Notably, this information can only be obtained by evaluating the battery band structure during operation. The obtained insights deepen our understanding of battery reactions and provide a novel protocol for battery design.

Reactions of the Li2MnO3 Cathode in an All-Solid-State Thin-Film Battery during Cycling.

We evaluated the structural change of the cathode material Li2MnO3 that was deposited as an epitaxial film with an (001) orientation in an all-solid-state battery. We developed an in situ surface X-ray diffraction (XRD) technique, where X-rays are incident at a very low grazing angle of 0.1°. An X-ray with wavelength of 0.82518 Å penetrated an ∼2 µm-thick amorphous Li3PO4 solid-state electrolyte and ∼1 µm-thick metal Li anode on the Li2MnO3 cathode. Experiments revealed a structural change to a high-capacity (activated) phase that proceeded gradually and continuously with cycling. The activated phase barely showed any capacity fading. First-principles calculations suggested that the activated phase has O1 stacking, which is attained by first delithiating to an intermediate phase with O3 stacking and tetrahedral Li. This intermediate phase has a low Li migration barrier path in the [001] direction, but further delithiation causes an energetically favorable and irreversible transition to the O1 phase. We propose a mechanism of structural change with cycling: charging to a high voltage at a sufficiently low Li concentration typically induces irreversible transition to a phase detrimental to cycling that could, but not necessarily, be accompanied by the dissolution of Mn and/or the release of O into the electrolyte, while a gradual irreversible transition to an activated phase happens at a similar Li concentration under a lower voltage.

High ionic conductivity of multivalent cation doped Li6PS5Cl solid electrolytes synthesized by mechanical milling.

The performances of next generation all-solid-state batteries might be improved by using multi-valent cation doped Li6PS5Cl solid electrolytes. This study provided solid electrolytes at room temperature using planetary ball milling without heat treatment. Li6PS5Cl was doped with a variety of multivalent cations, where an electrolyte comprising 98% Li6PS5Cl with 2% YCl3 doping exhibited an ionic conductivity (13 mS cm-1) five times higher than pure Li6PS5Cl (2.6 mS cm-1) at 50 °C. However, this difference in ionic conductivity at room temperature was slight. No peak shifts were observed, including in the synchrotron XRD measurements, and the electron diffraction patterns of the nano-crystallites (ca. 10-30 nm) detected using TEM exhibited neither peak shifts nor new peaks. The doping element remained at the grain boundary, likely lowering the grain boundary resistance. These results are expected to offer insights for the development of other lithium-ion conductors for use in all-solid-state batteries.