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We have been conducting research to explore new materials and their use in batteries by pursuing the phenomenon of high ion diffusion in solids. Lithium ion conducting materials have been particularly explored because of their potential application to all-solid-state batteries with high energy density. In addition to an overview of lithium ionic conductors, we will review the material search process for Li10GeP2S12, which exhibits particularly high ionic conductivity. We will also review the research that clarified the formation region of this material, investigated the ionic conduction mechanism, and applied this material to all-solid-state batteries. [ABSTRACT FROM AUTHOR]

• Crystallization of Li 4x Mg 2 (1-x)P 2 O 7 glass shows an X-ray diffraction pattern similar to Li 2 ZnP 2 O 7 at the crystallization onset temperature. • Crystals similar to Li 2 ZnP 2 O 7 are metastable at the crystallization onset temperatures of glass. • The density difference between glass and crystal affects crack formation and ionic conductivity. • Heat treatment at the crystallization onset temperature shows surface crystallization tendency. In the Li 2 O-MgO-ZnO-P 2 O 5 system, glasses based on the LiMgP 2 O 7 composition were synthesized using the melt-quenching method, specifically focusing on two series: Li 4 x Mg 2(1- x) P 2 O 7 and Li 2 Zn y Mg (1- y) P 2 O 7. Li 2 MgP 2 O 7 (33.3mol% Li 2 O, 33.3mol% MgO, 33.3mol% P 2 O 5) showed an X-ray diffraction pattern similar to Li 2 ZnP 2 O 7 (LZPO) when heat-treated near its crystallization onset temperature, as determined by differential thermal analysis (DTA). However, at 700 °C, thermal decomposition into multiple phases, including Li 4 P 2 O 7 , was observed. In the Li 4 x Mg 2(1- x) P 2 O 7 glass series, LZPO-type crystals formed in the range of x = 0.5 to 0.75. The quasi-binary system of Li 2 MgP 2 O 7 and Li 2 ZnP 2 O 7 , represented by Li 2 Zn y Mg (1- y) P 2 O 7 , formed glasses across all compositions. These glasses formed LZPO-type crystals showing lithium-ion conductivity at the crystallization onset temperature. A surface crystallization tendency was confirmed where crystals grew from the surface towards the interior of the glass substrate. [ABSTRACT FROM AUTHOR]

Abstract: Single‐crystal rods of Li6.5La3Zr1.5Ta0.5O12 were grown by floating zone melting. The typical size of the single‐crystal rod was 8 mm in diameter and 70 mm in length. Li6.5La3Zr1.5Ta0.5O12 crystallizes in a cubic structure with an Ia‐3d space group and the lattice parameter a=12.9455 (6) Å. The crystal structure of Li6.5La3Zr1.5Ta0.5O12 was refined to the conventional values of R=3.32 % and wR=4.45 % using single‐crystal X‐ray diffraction data, and to R=8.82 % and wR=7.45 % using single‐crystal neutron diffraction data. Li‐ions in the crystal structure occupied two crystallographic sites: the distorted tetrahedral 96 h site and the distorted octahedral 96 h site. Analyzing the results of AC impedance measurements, we estimated the total Li‐ion conductivity of member in Li6.5La3Zr1.5Ta0.5O12 to be 1.27×10−3 S cm−1 at 298 K. This value is the highest in the reported Li6.5La3Zr1.5Ta0.5O12 members. Using NMR spectroscopy, we determined the Li diffusion coefficient to be 1.57×10−13 m2 s−1 at 298 K and 7.96×10−13 m2 s−1 at 333 K. These values related to Li‐ion migration are higher than those reported for polycrystalline samples. [ABSTRACT FROM AUTHOR]

This study investigated the conductivity of lithium ions in new pyrochlore materials. A two-step synthesis method, comprising a simple solid-state reaction followed by a high-pressure solid-state reaction was used to synthesize single-phase undoped LiSrB2O6F (where B = Nb5+ or Ta5+). The samples were doped with the lower valent cation, Zr4+, which replaced Nb5+ or Ta5+ at B-sites in LiSrB2O6F and similarly, A-site point defects were introduced by reducing the amount of Sr in the sample. The crystal structure was determined by Rietveld analysis of the X-ray synchrotron diffraction data. Ionic conductivities of the samples were evaluated using electrochemical impedance spectroscopy. Sr2+ deficient samples showed better conductivity (8.21 × 10−6 S cm−1 for Nb-containing samples and 6.38 × 10−6 S cm−1 for Ta-containing samples at 300°C) than that of Zr4+-doped samples (5.58 × 10−6 S cm−1 for Nb-containing samples and 3.58 × 10−6 S cm−1 for Ta-containing samples at 300°C). [ABSTRACT FROM AUTHOR]

Advanced techniques for overcoming problems encountered during in situ electron holography experiments in which a voltage is applied to an ionic conductor are reported. The three major problems encountered were 1) electric-field leakage from the specimen and its effect on phase images, 2) high electron conductivity of damage layers formed by the focused ion beam method, and 3) chemical reaction of the specimen with air. The first problem was overcome by comparing experimental phase distributions with simulated images in which three-dimensional leakage fields were taken into account, the second by removing the damage layers using a low-energy narrow Ar ion beam, and the third by developing an air-tight biasing specimen holder. [ABSTRACT FROM AUTHOR]

New high lithium ion conductor glass–ceramics with NASICON-type structure (Li 1 + x + y Al x Cr y Ge 2 − x − y (PO 4 ) 3 , x + y = 0.5) were synthesized using melt-quenching method and converted to glass–ceramics through heat treatment. Influence of addition of different concentrations of aluminum and chromium in LiGe 2 (PO 4 ) 3 glass-ceramic was investigated. Substitution of Ge 4 + ions by Al 3 + and Cr 3 + ions induced more Li + ions in A 2 vacant sites and also changed the unit cell parameters and resulted in ionic conductivity improvement of glass–ceramics. The glass–ceramics were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray spectroscopy (EDX), Differential Scanning Calorimetry (DSC), Complex Impedance Spectroscopy (CIS) and cyclic voltammetry (CV). The highest lithium ion conductivity of 6.65 × 10 − 3 S/cm was obtained for x = 0.4 and y = 0.1 (Li 1.5 Al 0.4 Cr 0.1 Ge 1.5 (PO 4 ) 3 ) crystallized at 850 °C for 8 h with minimum activation energy of 0.292 eV. [ABSTRACT FROM AUTHOR]

Novel lithium-ion (Li + )-conducting oxysulfides of the Li 10 GeP 2 S 12 family were found in the ternary Li 2 S–P 2 S 5 –P 2 O 5 system; Li 10 GeP 2 S 12 -type solid solutions with compositions Li 3 + 5 x P 1 − x S 4 − z O z ( x = 0.03–0.08, z = 0.4–0.8) were confirmed. The solid solutions showed ionic conductivities from 4.14 × 10 − 5 to 2.64 × 10 − 4 S cm − 1 at 298 K and a wide electrochemical window of 0–5.0 V ( vs. Li/Li + ). Among the solid solutions synthesized, the purest phase had the composition Li 3.35 P 0.93 S 3.5 O 0.5 ( x = 0.07, z = 0.5). Structural analysis revealed that the novel oxysulfides are isostructural to the original Li 10 GeP 2 S 12 structure and the (P/□)(S/O) 4 tetrahedra, which indicates the presence of cation defects with oxygen substitution in the crystal structure. Electrochemical stability of the oxysulfides was confirmed in the voltage range of 0–5.0 V. The solid-electrolyte interphase (SEI) resistivity and its cycle-dependence evaluation for new materials demonstrated that the oxysulfides had lower resistivities and furnished well-contacted SEI layers during the charge–discharge process. [ABSTRACT FROM AUTHOR]

A novel molecular crystal, Li{N(SO 2 CF 3 ) 2 }(C 6 H 4 (OCH 3 ) 2 ) ( 1 ), was synthesized by heating LiTFSA with DMB in 1:1 molar ratio. Single crystal x-ray diffraction study of 1 revealed that the compound possesses a dinuclear structure with five-coordinate lithium centers bridged by TFSA anions. AC impedance measurements showed that ionic conduction through the crystallites of 1 takes place via a hopping mechanism at ambient temperature. [ABSTRACT FROM AUTHOR]

Solid solutions of the lithium ion conductor LaLiO 2 were synthesized using a solid-state reaction method. For the strontium-doped system, Sr 2 + was partially doped at the La sites with the composition La 1 − x Sr x Li 1 + x O 2 ( x = 0.1, 0.2), which introduced excess lithium into the structure, while for the magnesium-doped system, Mg 2 + was partially doped at the Li + site with LaLi 1 − 2 y Mg y O 2 ( y = 0.1, 0.2) which introduced Li + vacancies. The magnesium-doped sample with y = 0.1 in LaLi 1 − 2 y Mg y O 2 showed higher ionic conductivity of 9.22 × 10 − 5 S cm − 1 than that of the respective Sr doped sample of x = 0.1 in La 1 − x Sr x Li 1 + x O 2 (4.64 × 10 − 5 S cm − 1 ) at 300 °C. Higher ionic conductivity in the Mg doped system indicates that vacancies led to high lithium ion conduction in LaLiO 2 . The doped LaLiO 2 systems are a novel lithium conductive oxides family. [ABSTRACT FROM AUTHOR]

Phase stability of the Garnet-structured lithium ion conductor Li7La3Zr2O12 (LLZ) was investigated using high-temperature X-ray diffraction (HT-XRD). The tetragonal LLZ containing CO2 transformed to the cubic phase around 150°C. However, once the tetragonal LLZ heated up to 1000°C, the cubic phase transformed back to the tetragonal phase again, because of the CO2 loss during the heating process. After the CO2 loss, the phase transformation temperature shifted to 650°C. The HT-XRD study for Al-doping process was also performed. The addition of γ-Al2O3 to the tetragonal LLZ eliminated the formation of impurity phase such as La2Zr2O7 and La2O3, resulting in the formation of a pure cubic phase. A comparison of the HT-XRD results suggested that the Li loss is necessary to stabilize the cubic phase with Al. [ABSTRACT FROM AUTHOR]

Abstract: Al2O3 was doped (0.5, 1.0. and 2.0wt.%) into the garnet-type lithium ion-conducting solid electrolyte with a nominal composition of Li7La3Zr2O12 (LLZ) by solid state reaction at 1180°C. The effect of the Al2O3 content in LLZ on the interface behavior with lithium metal was investigated. The lowest interface resistance was observed for 0.5wt.% Al2O3-doped LLZ, which had the highest relative density of 93.7%. The Li/Al2O3-doped LLZ/Li cell showed a short circuit after a limited polarization period. Short circuiting is considered to be due to lithium dendrite formation. The longest period until short circuit was 1000s of polarization at 0.5mAcm−2 and room temperature for the 0.5wt.% Al2O3 doped LLZ. [Copyright &y& Elsevier]

A lithium ion conductor with the garnet-type structure, Li7La3Zr2O12 (LLZ), showing a high Li+ conductivity of 0.49mScm−1 at 25°C, is synthesized via milling (500rpm, 12h) and following sintering at 1125°C for 20h. The improved Li+ conductivity is attributed to the decrease of the particle size, the uniform distribution of particles before sintering, the high concentration of Li+ in the octahedral site (96h), and low occupancies of Li+ in the tetrahedral site (24d). Also, it is found that the cubic phase of LLZ is stabilized by a contamination of Al3+ ion into the LLZ lattice. The structural position of Al3+ is verified to be in the tetrahedral site (24d) by analyzing SEM images, MAS NMR, Pawley refinement using X-ray powder diffraction, and structural refinement using neutron powder diffraction data. [ABSTRACT FROM AUTHOR]

Abstract: Li3SO3N has been prepared by the mechanochemical reaction between lithium sulfamate (LiSO3NH2) and lithium amide or lithium hydride. This is the first example of an anhydrous compound containing the rare [SO3N]3− polyanion. Li3SO3N adopts the low-temperature β structure of isoelectronic Li3PO4. Structural refinement from neutron powder diffraction data supports disordering of nitrogen over two of the three anion sites, although a fully disordered model cannot be conclusively refuted. While the compound is a poor ionic conductor, it is electrochemically stable to at least 5 V vs. Li+/Li0. This simple mechanochemical pathway should be suitable for preparing a host of other alkali-rich nitrogen-containing polyanionic compounds. [Copyright &y& Elsevier]

Abstract: A Li1.4Al0.4Ge1.6(PO4)3 (LAGP) sheet was prepared using a tape casting method with a fine LAGP power prepared from a sol–gel precursor. The addition of nanosize TiO2 as a sintering additive was effective to enhance the ionic conductivity of the tape-cast sheet. The highest electrical conductivity of 8.37 × 10−4 S cm−1 was observed for the tape-cast LAGP sheet with 5 wt.% TiO2 at 25 °C. A hybrid sheet of the tape-cast LAGP and epoxy resin was water impermeable. The electrical conductivity of the water impermeable hybrid sheet with ca. 4% epoxy resin was 4.19 × 10−4 S cm−1 at 25 °C. The three-point bending strength of the LAGP–5 wt.% TiO2–epoxy resin hybrid sheet was 111 N mm−2, which is comparable with a Li1+x+y Al x Ti2−x P3−y Si y O glass ceramic plate. The Li/PEO18(CF3SO2)2N/LAGP–5 wt.% TiO2–epoxy sheet was successfully operated as a lithium electrode in saturated LiCl aqueous solution. [Copyright &y& Elsevier]

A new lithium-ion conducting oxide, hexagonal Li 4 GeO 4 , with a space group of P 6 3 /mmc , was prepared by mechanochemical treatment and subsequent heat treatment at 250 °C. The crystal structure was found in sulfide Li 4 SnS 4 , and this is the first study to report an oxide system. In addition, (100- x)Li 4 GeO 4 · x Li 2 MO 4 (M = S or W; x = 10, 20, 30, and 40 mol%) glass-ceramic electrolytes with the hexagonal phase were prepared by crystallization of their mother glasses. The formability of the electrolytes improved as the quantity of Li 2 SO 4 increased, and the 90Li 4 GeO 4 ·10Li 2 SO 4 glass-ceramic electrolyte showed the highest ionic conductivity of 2.9 × 10−6 S cm−1 at 25 °C. Compared with the experimental results, the bond valence sum (BVS) analysis results indicated that ionic conduction paths are easily connected along the c -axis in the hexagonal phase, suggesting that the relatively small lattices in the a - and b -axes are more advantageous for three-dimensional ionic conduction. • Li 4 GeO 4 -based glass electrolytes are prepared via a mechanochemical process. • The addition of Li 2 SO 4 improves the formability of the electrolytes. • A hexagonal metastable phase is formed by crystallization of Li 4 GeO 4 -based glasses. • The 90Li 4 GeO 4 ·10Li 2 SO 4 glass-ceramic has the highest conductivity of 2.9 × 10−6 S cm−1. [ABSTRACT FROM AUTHOR]

Lithium superionic conductors (LISICONs) are promising materials to realize high safety, high energy all-solid-state lithium batteries. LISICONs with a γ-Li 3 PO 4 -type structure were developed for use in the Li 4 GeO 4 −Li 4 SiO 4 –Li 3 VO 4 ternary system, and the relationship among their composition, phase formation, and ionic conductivity was analyzed. Ionic conductivity of 5.8 ​× ​10−5 ​S ​cm−1 ​at 298 ​K, the highest value among the LISICONs reported to date, was obtained for Li 3.55 (Ge 0.45 Si 0.10 V 0.45)O 4 with a low activation energy of 0.37 ​eV. This was achieved by reducing the activation energy and increasing the pre-exponential factor through dual doping. The Ge4+, Si4+, and V5+ cations of Li 3.55 (Ge 0.45 Si 0.10 V 0.45)O 4 occupied the same site; thus, the solid-solution formation was confirmed. Li 3.55 (Ge 0.45 Si 0.10 V 0.45)O 4 functioned as a solid electrolyte in an all-solid-state cell, indicating the features of a pure lithium ionic conductor without electronic conduction. This work verified that the dual substitution strategy is beneficial for compositional and structural optimization, affording enhanced ion-conducting properties. γ-Li 3 PO 4 -type lithium ionic conductors with high lithium ionic conductivity were synthesized and examined as a solid-electrolyte in the Li 4 GeO 4 −Li 4 SiO 4 –Li 3 VO 4 ternary system. Image 1 • Lithium superionic conductors with a γ-Li 3 PO 4 -type structure were developed. • The highest ionic conductivity was observed for Li 3.55 (Ge 0.45 Si 0.10 V 0.45)O 4. • The dual-cation substitution strategy enhanced the ionic conductivity. • Ionic conductive property was enhanced by modifying E a and σ 0. [ABSTRACT FROM AUTHOR]

Numerous studies have reported that sulfide-based materials have potential for applications in all-solid-state lithium batteries. However, the limitations of the synthesis methods of these materials have impeded their progress. In this study, a series of lithium superionic conductors in Li 10+δ Ge 1+δ P 2-δ S 12 (Li 4- x Ge 1- x P x S 4) are synthesized using the Ar gas flow process. This novel process is performed in three steps through heating in the Al 2 O 3 Tamman tube under Ar gas flow; this process is shorter than the conventional five-step vacuum process. The obtained powders show the formation of a Li 10 GeP 2 S 12 -type solid solution with a space group of P 4 2 / nmc in the sequence 0.05 ≤ δ ≤ 0.50 (0.5 ≤ x ≤ 0.65). All the pressed samples show high ionic conductivities over 2.5 mS cm−1. Additionally, 5 g of product per batch was achieved, which is five times more than that obtained using the conventional synthesis process. Establishing a simple synthesis process with a larger product yield could promote the development and evaluation of all-solid-state lithium batteries with large-scale cells using lithium superionic conductors. Image 1 • A Li 10 GeP 2 S 12 -type superionic conductor was prepared via a novel three-step method. • Five grams of Li 10 GeP 2 S 12 per batch were achieved using this novel method. • High conductive properties and reversible battery operation were demonstrated. [ABSTRACT FROM AUTHOR]

Lithium-ion solid electrolyte disc with high conductivity even at low temperature for electrochemical devices were designed using Li 1+x+y Al x Ga y Ti 2-x-y (PO 4) 3 (x + y = 0–0.5) with NASICON-type structure. The solid-electrolyte discs were synthesized by a solution method at 900–1000 °C. Effects of Al and/or Ga dopants and crystal structures on ionic conductivity of the solid electrolytes were investigated. Almost single phase of Li 1+x+y Al x Ga y Ti 2-x-y (PO 4) 3 (x + y = 0–0.5) systems could be synthesized at sintering temperatures above 900 °C up to x = 0.5, y = 0.25. An ionic conductivity as high as 9.3 × 10−4 S/cm at 30 °C and activation energy of 0.29 eV were obtained with the trivalent-cation double-doped Li 1.5 Al 0.25 Ga 0.25 Ti 1.5 (PO 4) 3 disc sintered at 900 °C. Unlabelled Image • A new trivalent-cation double-doped lithium ion conductive Li 1+x+y Al x Ga y Ti 2-x-y (PO 4) 3 (x + y = 0 ~ 0.5) system was synthesized by a wet-chemical route, and Li 1.5 Al 0.25 Ga 0.25 Ti 1.5 (PO 4) 3 disc gave an ionic conductivity as high as 9.3 x 10-4 S/cm at 30 °C. • It could be seen that the ionic conductivity of these system has a good relationship to the crystal structures, such as the a/c lattice ratio of the crystals. [ABSTRACT FROM AUTHOR]

High lithium ion conductivity solid electrolytes of Li1.4FexAl0.4-xTi1.6(PO4)3 (x=0-0.4) with the NASICON-type structure were synthesized by using a sol-gel precursor. The highest electrical conductivity was obtained for Li1.4Fe0.25Al0.15Ti1.6(PO4)3 sintered at 1040°C for 7h in air. The total, grain boundary, and bulk conductivities of the pellet were 1.01 x 10-3, 2.17 x 10-3 and 1.81 x 10-3 Scm-1 at 25°C, respectively. The contribution of electron (or hole) conductivity to the total conductivity was negligibly small. Li1.4Fe0.25Al0.15Ti1.6(PO4)3 was unstable in water, but stable in saturated LiOH and LiCl aqueous solutions. [ABSTRACT FROM AUTHOR]