•Dense garnet-type solid electrolytes are prepared by Spark Plasma Sintering (SPS).•Residual stress of 281 MPa is caused by SPS.•Stress/strain causes little change in bulk ionic conductivity.•Stress/strain increases grain boundary resistance.All-solid-state batteries (ASSBs) have various problems associated with their usage that are normally not encountered in conventional lithium-ion batteries. Stress on interfaces between solid electrolytes and active materials is one of the key issues because the active materials change their volume during charging/discharging. In this work, first, we reveal that garnet-type solid electrolytes, Li6.5La3Zr1.5Ta0.5O12 (LLZT), prepared by the spark plasma sintering (SPS) technique, exhibit a residual tensile stress of more than 100 MPa in the direction of the SPS pressure. Then, the influence of the strain on ionic conduction is investigated in detail. It is demonstrated that the strain causes no change in the bulk resistance, while the grain boundary resistance increases in both the pre-exponential factor and the activation energy. The results suggest the importance of the strength of grain boundaries (including interfaces) for the practical application of ASSBs.

•Grain boundaries of garnet-type solid electrolytes are modified.•Microstructure of LLZT is studied by SEM, STEM and EIS techniques.•Influence of microstructure on short-circuit prevention is investigated.•Li2CO3 and LiOH fill voids and facilitate sintering on the grain boundaries.•Grain boundary modification suppresses short circuit up to 0.6 mA cm−2.Garnet-type solid electrolytes are one of key materials to enable practical usage of lithium metal anode for high-energy-density batteries. However, it suffers from lithium growth in pellets on charging, which causes short circuit. In this study, grain boundaries of Li6.5La3Zr1.5Ta0.5O12 (LLZT) pellets are modified with Li2CO3 and LiOH to investigate the influence of the microstructure of grain boundaries on lithium growth and to study the mechanism of the lithium growth. In spite of similar properties (relative density of ca. 96% and total ionic conductivity of 7 × 10−4 S cm−1 at 25 °C), the obtained pellets exhibit different tolerance on the short circuit. The LLZT pellets prepared from LiOH-modified LLZT powders exhibit rather high critical current density of 0.6 mA cm−2, at which short circuit occurs. On the other hand, the LLZT pellets without grain boundary modification short-circuited at 0.15 mA cm−2. Microstructural analyses by means of SEM, STEM and EIS suggest that lithium grows through interconnected open voids, and reveal that surface layers such as Li2CO3 and LiOH are not only plug voids but also facilitate the sintering of LLZT to suppress the lithium growth. The results indicate a strategy towards short-circuit-free lithium metal batteries.Download high-res image (245KB)Download full-size image

•Surface structure and composition of solid electrolyte sheet were investigated.•At the surface, lattice expansion with Li accumulation was confirmed.•Under the surface, lattice and Li composition sharply dropped.In this study, three different surface-sensitive techniques, grazing incidence X-ray diffraction (GIXD), attenuated total reflectance Fourier transform infrared (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS), were employed to analyze the local structure and composition around the surface region of a lithium ion conducting solid electrolyte sheet. It was revealed that the local structure and composition changed depending on depth from the top surface of the sheet. At the very surface, there was a layer with expanded lattice and high Li composition. And with increasing the depth from the surface, lattice shrank sharply, and then expanded again gradually. The change in the lattice seemed to be accompanied by Li and Al composition. It was supposed that the change in the Li composition and structure is induced by combination of chemical reaction, segregation, and defect distribution.

•Influence of Br2 formation in lithium–bromine batteries (LBBs) is investigated.•Br2 increases interfacial resistance on the solid electrolyte.•Addition of quaternary ammonium bromide suppresses the interfacial resistance.•The reduced interfacial resistance improves cycleability of LBBs.Electrochemical performances of a prototype lithium–bromine battery (LBB) employing a solid electrolyte is investigated. The discharge capacity decreases with repeating charge/discharge cycles. Electrochemical impedance analysis reveals that the capacity fading is mainly due to increase in the interfacial resistance between an aqueous active material solution and a solid electrolyte. Based on the results of symmetric cells and structural analysis of the surface of the solid electrolyte immersed in Br2 solutions, it is suggested that a Li+-depletion layer is formed on the surface of the solid electrolyte as a result of contact with bromine. Addition of tetraethylammonium bromide (TEABr) depresses the interfacial resistance, which results in improved cycleability. LBB with 1.0 M LiBr and 0.25 M TEABr shows discharge capacity of 139 mAh/g-LiBr and Coulombic efficiency of 99.6% at 5th cycle.

Interfacial resistance is one of the severe problems in composite electrodes of all solid state batteries (ASSBs), especially oxide-type ASSBs. Conflicts between poor sinterability and possible unfavorable reaction with active materials limit applicable materials and processes. In this report, a novel approach is proposed to decrease grain boundary resistance among nonsintered solid electrolyte particles. The concept is successfully demonstrated, and the nonsintered grain boundary resistance of a highly conducting solid electrolyte (Li1.3Al0.3Ti1.7(PO4)3) was suppressed by being coated with poorly conducting solid electrolyte (Li2SiO3). Increased total conductivity and variation of apparent activation energy are well explained from the viewpoint of defect chemistry.

The local structure of an electrolyte ion, AsF6–, in a nanoporous carbon electrode was investigated by in situ X-ray absorption spectroscopy (XAS), which was conducted at the As K-edge with a transmission method and a novel sample current method. In the transmission spectra, the absorption decreased with decreasing electrode potential, indicating the contribution of AsF6– ions in and out of the nanopores. Differential transmission spectra demonstrated that AsF6– ions in the nanopores exhibit the white line at lower photon energy compared to free ions, which was clearly demonstrated by the sample current spectra. The change in the absorption energy was explained by the deformation of ions in the nanopores with an assistance of FEFF calculation.

AgI confined in mesopores of Al2O3 exhibits high ionic conductivity for a very wide temperature range. The mechanism of this attractive feature was revealed by detailed investigation of local structure using 109Ag nuclear magnetic resonance (NMR) spectroscopy and X-ray absorption spectroscopy (XAS). All data of NMR and XAS data as well as X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and a.c. impedance spectroscopy were carefully analyzed to reach the most plausible model. It was revealed that the local structure of the super ionic conducting phase of AgI in mesopores is amorphous, which is similar to α-AgI structure, and was stabilized for a very wide temperature range from 30 K to melting point around 850 K.

Nanoporous carbons were prepared by using colloidal crystal as a template. Nitrogen adsorption/desorption isotherms and transmission electron microscope images revealed that the porous carbons exhibit hierarchical porous structures with meso/macropores and micropores. Electric double layer capacitor performance of the porous carbons was investigated in an organic electrolyte of 1 M LiClO4 in propylene carbonate and dimethoxy ethane. The hierarchical porous carbons exhibited large specific double layer capacitance of ca. 120 F g−1 due to their large surface areas. In addition, the large capacitance was still obtained at a large current density up to 10 A g−1, which satisfies demands from the high power application such as hybrid electric vehicles. Capacitance analysis of the hierarchical porous structures revealed the contribution of meso/macropores and micropore to the electric double layer capacitance to be 8.4 and 8.1 μF cm−2, respectively. The results indicated electric double layer is formed even when solvated ions are larger than pore diameters.

Nano-porous graphitized carbons were successfully prepared by using mono-dispersed SiO2 colloidal crystal as a template and mesophase pitch as a carbon source with final heat treatment temperatures (HTT) of 1000–2500 °C. Rate capability of lithium intercalation/de-intercalation of the nano-porous graphitized carbons was investigated. 35–60% of capacities were retained when the current density was increased from 37.2 mA g− 1 to 372 mA g− 1. Electrochemical impedance spectra indicated that formation of SEI layers caused increased polarization.

A highly active Pt/carbon catalyst for polymer electrolyte membrane fuel cells and direct methanol fuel cells was prepared by using porous carbon that was synthesized by a colloidal-crystal templating method. The microstructure of the Pt/porous carbon composites was studied with N2 ad-/desorption isotherms, X-ray diffraction and transmission electron microscopy. In contrast to conventional carbon supports, the porous carbon exhibited an attractive microstructure as a catalyst support, i.e. a large surface area with mono-dispersed three-dimensionally interconnected mesopores (45 nm). A large mesopore surface area prompted dispersion of Pt particles, which resulted in a large effective surface area of Pt with a high activity for the oxygen reduction reaction. The porous structure facilitated smooth mass transportation to give rise to high limiting currents.