Low-Temperature Fabrication of Bulk-Type All-Solid-State Lithium-Ion Battery Utilizing Nanosized Garnet Solid Electrolytes.

Garnet-type Li7 La3 Zr2 O12 (LLZ) materials are attracting attention as solid electrolytes (SEs) in oxide-based all-solid-state batteries (ASSBs) owing to their high ionic conductivity. Although the electrochemical stability of LLZ against Li metal is demonstrated with possible high energy density, high-temperature sintering above 1000 °C, which is required to achieve high Li-ion conductivity, results in the formation of insulating impurities at the electrode-electrolyte interfaces. Here, nanosized fine-particle samples of Ta-substituted Li6.5 La3 Zr1.5 Ta0.5 O12 (LLZT) are successfully prepared at a remarkably low temperature of 400 °C utilizing an amorphous precursor oxide. The dense LLZT SE sintered by hot pressing at 500 °C shows room-temperature Li-ion conductivity of 1.03 × 10-4 S cm-1 without any additives. In addition, the bulk-type NCM-graphite full battery cell fabricated with the LLZT fine particles through a hot-pressing sintering method at 550 °C exhibits a good charge-discharge performance at room temperature with the bulk-type areal discharge capacity of 0.831 mAh cm-2 . The nanosized garnet SE strategy demonstrated in this study paves the way for the formation of oxide-based ASSBs by low-temperature sintering.

Low-Temperature Sintering of a Garnet-Type Li6.5La3Zr1.5Ta0.5O12 Solid Electrolyte and an All-Solid-State Lithium-Ion Battery.

Garnet-type Ta-substituted Li7La3Zr2O12 materials attract considerable attention as solid electrolytes for use in future oxide-based all-solid-state lithium-ion batteries owing to their superior ionic conductivity and chemical and electrochemical stabilities. However, high-temperature sintering above 1000 °C, which is needed to realize high lithium-ion conductivity, results in the formation of insulating interface impurities at the electrode-electrolyte interface. Herein, the low-temperature sintering of the Li6.5La3Zr1.5Ta0.5O12 (LLZT) solid electrolyte at a remarkably low temperature of 400 °C was demonstrated using the submicrometer-sized garnet-type LLZT fine powder sample prepared at 600 °C through a reaction of Li2O and La2.4Zr1.2Ta0.4O7. The lithium-ion conductivity at 25 °C was 4.54 × 10-5 S cm-1 without any additives through low-temperature sintering at 400 °C. In addition, the preliminary battery performance of the oxide-based all-solid-state LiNi1/3Co1/3Mn1/3O2-Li4Ti5O12 full-battery cell fabricated at 400 °C using the present LLZT fine powder sample as the solid electrolyte was demonstrated.

Discovery of the Li-Sr-La-Zr-O Compound and the Investigation of Its Lithium-Ion Conductivity.

Single crystals of Li3.957Sr1.957La0.043ZrO6, having a new crystal structure, were successfully grown in air using the floating zone crystallization method. The obtained crystals were colorless and had a rectangular shape with a maximum dimension of 8(φ) × 50 mm. The elemental composition of the crystal was determined via energy-dispersive X-ray spectroscopy. Single-crystal X-ray structure analysis revealed that the crystal was monoclinic, in space group P21/n, with lattice parameters of a = 5.7506 (2) Å, b = 6.2968 (3) Å, c = 8.4906 (3) Å, and ß = 97.066 (1) deg. Crystal structure refinement using 652 independent reflections resulted in a confidence factor (R) of 1.78% and a wR factor of 2.57%. The AC-impedance measurement revealed that the lithium-ion conductivity of the Li3.957Sr1.957La0.043ZrO6 single crystal at 298 K was 6.8 × 10-4 S/cm, and the activation energy calculated from the Arrhenius plot was 0.24 eV. The proposed single crystals exhibit significant potential for application as solid electrolytes for lithium-ion batteries at low temperatures.

Excellent Deformable Oxide Glass Electrolytes and Oxide-Type All-Solid-State Li2S-Si Batteries Employing These Electrolytes.

Oxide-type all-solid-state lithium-ion batteries have attracted great attention as a candidate for a next-generation battery with high safety performance. However, batteries based on oxide systems exhibit much lower energy densities and rate performances than liquid-type lithium-ion batteries, owing to the difficulty in preparing the ion- and electron-transfer path between particles. In this study, Li2SO4-Li2CO3-LiX (X = Cl, Br, and I) glass systems are investigated as highly deformable and high-ionic-conductive oxide electrolytes. These electrolytes show excellent deformable properties and better ionic conductivity. The LiI oxide glass system is a suitable electrolyte for the negative electrode because it shows a higher ionic conductivity and is stable up to 2.8 V. The LiCl or LiBr oxide glass systems are suitable electrolytes for the positive electrode and separation layer because they show high ionic conductivity and kinetic stability up to 3.2 V. The Li2S positive and Si negative composite electrodes employing LiBr and LiI oxide glass electrolytes, respectively, show high battery performances because of increased reaction points between active materials and the solid electrolyte and carbon via a mechanical milling process and are capable of forming good interparticle contact. Therefore, it suggests that the excellent deformable electrolytes are suitable for solid electrolytes in composite electrodes because their ionic conductivity does not change by the mechanical milling process. Furthermore, an oxide-type all-solid-state Li2S-Si full-battery cell employing these positive and negative composite electrodes and a LiBr oxide glass electrolyte separation layer is demonstrated. The full-battery cell indicates a relatively high discharge capacity of 740 mA h g-1(Li2S) and an area capacity of 2.8 mA h cm-2 at 0.064 mA cm-2 and 45 °C despite using only safe oxide electrolytes.

Orthorhombic Crystal System for a Garnet-type Lithium-Ion Conductor.

A single-crystal rod with a composition of Li6.95La3Zr1.95Nb0.05O12 was grown using the floating zone method. The single-crystal rod had a diameter of 8 mm and length of 90 mm. Li6.95La3Zr1.95Nb0.05O12 crystallizes in an orthorhombic system, with space group Ibca, and the single crystal with 0.05 substitutions of niobium had the following lattice parameters: a = 13.1280(3) Å, b = 12.6777(3) Å, and c = 13.1226(4) Å. The reliability values obtained were R = 1.77% and wR = 3.27% in Li6.95La3Zr1.95Nb0.05O12 for 857 independent refractions, with a shift/error for 115 parameters of less than 0.001 value in the single-crystal X-ray diffraction data. Four interspace sites were occupied by the Li ions, constructed by the framework structure. The Li1, Li2, and Li3 ions are located in the octahedral 16f site, and Li4 is in the tetrahedral 8d site. The bulk lithium-ion conductivity in Li6.95La3Zr1.95Nb0.05O12 was 3.21 × 10-4 S cm-1 at 298 K.

Nanoarchitectonics of Acicular Nanocrystal Assembly and Nanosheet Assembly for Lithium-Ion Batteries.

Nanoarchitectonics of metal oxide nanocrystal electrodes were developed for lithium-ion batteries. The electrodes included copper nanoparticles and doped fluorine. For the acicular nanocrystals, charge-discharge reactions progressed at 1.8 V over 100 cycles at 100 and 10 µA. A 15-mmdiameter battery containing acicular nanocrystals showed capacity, coulomb efficiency, and specific capacity, respectively of 20 µAh, 98%, and ~242 mAh/g at 100 µA and 40 µAh, 99%, and 484 mAh/g at 10 µA. The TiO2/SnO2 electrode consisted of a SnO2 sheet-assembled structure with surface layers of anatase TiO2. The TiO2/SnO2 battery operated at 1.3 (100 cycles) and 1.2 (50 cycles) V at 100 and 10 µA, respectively; its capacity, coulomb efficiency, and specific capacity, respectively were 50 µAh, 98%, and 161 mAh/g at 100 µA and 200 µAh, 97-98%, and 643 mAh/g at 10 µA. The characteristic microstructure, chemical composition, and crystal faces of both materials contributed to battery performance.

Relationship between Li+ diffusion and ion conduction for single-crystal and powder garnet-type electrolytes studied by 7Li PGSE NMR spectroscopy.

Ion-conducting garnets are important candidates for use in all-solid Li batteries and numerous materials have been synthesized with high ionic conductivities. For understanding ion conduction mechanisms, knowledge on Li+ diffusion behaviour is essential. The proposed nano-scale lithium pathways are composed of tortuous and narrow Li+ channels. The pulsed gradient spin-echo (PGSE) NMR method provides time-dependent 7Li diffusion on the micrometre space. For powder samples, collision-diffraction echo-attenuation plots were observed in a short observation time, which had not been fully explained. The diffraction patterns were reduced or disappeared for single-crystal garnet samples of Li6.5La3Zr1.5Ta0.5O12 (LLZO-Ta) and Li6.5La3Zr1.5Nb0.5O12 (LLZO-Nb). The inner morphology and grain boundaries affect importantly the collision-diffraction behaviours which is inherent to powder samples. The 7Li diffusion observed by PGSE-NMR depends on the observation time (Δ) and the pulsed field gradient (PFG) strength (g) in both powder and single-crystal samples, and the anomalous effects were reduced in the single-crystal samples. The scattered Li diffusion constants converged to a unique value (DLi) with a long Δ and a large g, which is eventually the smallest value. The DLi activation energy was close to that of the ionic conductivity (σ). The DLi values are plotted versus the σ values measured for four powder and two single-crystal garnet samples. Assuming the Nernst-Einstein (NE) relation which was derived for isolated ions in solution, the carrier numbers (NNE) were estimated from the experimental values of DLi and σ. The NNE values of metal-containing garnets were large (<1023 cm-3) and insensitive to temperature. They were larger than Li atomic numbers in cm3 calculated from the density, molecular formula and Avogadro number for LLZOs except for cubic LLZO (Li7La3Zr2O12, NNE∼ 1020 cm-3).

Toward understanding the anomalous Li diffusion in inorganic solid electrolytes by studying a single-crystal garnet of LLZO-Ta by pulsed-gradient spin-echo nuclear magnetic resonance spectroscopy.

Li diffusion was observed by 7Li nuclear magnetic resonance (NMR) spectroscopy in three single-crystal samples of LLZO-Ta (Li6.5La3Zr1.5Ta0.5O12) grown by the floating zone melting method as well as a crushed sample in this study. Previously, the pulsed-gradient spin-echo 7Li NMR method was applied to Li+ diffusion measurements in inorganic solid electrolyte powder samples. Anomalous Li+ diffusion behaviors were observed such as dependence of the observing time (Δ) and pulsed-field-gradient strength (g), and the diffusive-diffraction patterns in short Δ in the echo-attenuation plots. In the powder samples, it is uncertain that the Li ions diffuse in the bulk within grain, across grains, or both. To date, the origins of the anomalous Li+ diffusion have not yet been clearly understood. From models of atomic-level lithium pathways, the micrometer-space diffusion channels are assumed to be narrow with curvatures. In contrast to the powder samples, a single crystal is supposed to be uniform without grain boundaries and the Li ions in single-crystal samples can diffuse in the bulk with negligible effects from the surface. The single-crystal samples are expected to give us proper answers. We found that the 7Li echo-attenuation plots of the single-crystal samples showed anomalous phenomena in dependence on Δ and g with much reduced manners. We found that the phenomena are inherent characteristics of Li+ diffusion in inorganic solid electrolytes. From the aspects of Li+ carrier numbers, the fast divergent Li+ diffusion constants, observed at short Δ with small g, contribute importantly to the electrochemical high ionic conduction measured by impedance spectroscopy.

High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of a Structural Polymorph of Li3BP2O8.

A new structural polymorph of Li3BP2O8 has been successfully synthesized via a solid-state reaction between Li3PO4 and BPO4 at 4 GPa and 600 °C. The high-pressure phase of Li3BP2O8 (HP-Li3BP2O8) was found to crystallize in monoclinic symmetry with the cell parameters of a = 8.57010(4) Å, b = 11.11812(5) Å, c = 5.55380(3) Å, and ß = 97.7269(3)° [space group P21/ a (No. 14)]. HP-Li3BP2O8 has a new crystal structure that has not been reported so far. The total ionic conductivities measured for the polycrystalline sample by alternating-currrent impedance were 3.4 × 10-7 and 2.1 × 10-6 S/cm at 399 and 456 K, respectively. The lithium ionic conductivity of HP-Li3BP2O8 was higher than that of the low-pressure phase Li3BP2O8 in the temperature range of 375-456 K. This is caused by the difference in the dimensions of the lithium arrangements between LP- and HP-Li3BP2O8.

Lithium-ion conducting oxide single crystal as solid electrolyte for advanced lithium battery application.

Today, all-solid-state secondary lithium-ion batteries have attracted attention in research and development all over the world as a next-generation energy storage device. A key material for the all-solid-state lithium batteries is inorganic solid electrolyte, including oxide and sulfide materials. Among the oxide electrolytes, garnet-type oxide exhibits the highest lithium-ion conductivity and a wide electrochemical potential window. However, they have major problems for practical realization. One of the major problems is an internal short-circuit in charging and discharging. In the polycrystalline garnet-type oxide electrolyte, dendrites of lithium metal easily grow through the void or impurity in grain boundaries of the sintered body, which causes serious internal short-circuits in the battery system. To solve these problems, we present an all-solid-state battery system using a single-crystal oxide electrolyte. We are the first to successfully grow centimeter-sized single crystals of garnet-type by the floating zone method. The single-crystal solid electrolyte exhibits an extremely high lithium-ion conductivity of 10-3 S cm-1 at 298 K. The garnet-type single-crystal electrolyte has an advantageous bulk nature to realize the bulk conductivity without grain boundaries such as in a sintered polycrystalline body, and will be a game-changing technology for achieving highly safe advanced battery systems.

Quantitative analysis of cation mixing and local valence states in LiNixMn2-xO4 using concurrent HARECXS and HARECES measurements.

Cation mixing in positive electrode materials for rechargeable lithium ion batteries, LiNixMn2-xO4 (x = 0, 0.2, 0.5) and Li0.21Ni0.7Mn1.64O4-δ (denoted as x = 0.7), is analyzed by high-angular-resolution electron-channeling X-ray/electron spectroscopy (HARECXS/HARECES) techniques, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Mixing between the tetrahedral lithium sites and the octahedral transition metal sites is quantified, and the site-dependent valence states of the transition metals are examined. In the non-doped (x = 0) sample, Mn was found to occupy only octahedral sites as either Mn(3+) or Mn(4+) For x = 0.2-0.7, some of the nickel ions (6-13% depending on x) occupy tetrahedral anti-sites. All the nickel ions are in the divalent state, regardless of the occupation site. For x = 0.2 and 0.7, manganese ions occupy both octahedral and tetrahedral sites; those in the octahedral sites are tetravalent, while the tetrahedral sites contain a mixture of divalent and trivalent ions. For x = 0.5, manganese occupies only the octahedral sites, with all ions determined to be in the tetravalent state (within experimental accuracy). All the samples substantially satisfied the local charge neutrality conditions. This study demonstrates the feasibility of using HARECXS/HARECES for quantitative analysis of the atomic configuration and valence states in lithium manganese oxide spinel materials.

Ion-exchange synthesis, crystal structure, and physical properties of hydrogen titanium oxide H2Ti3O7.

Hydrogen titanium oxide H2Ti3O7 was prepared from Na2Ti3O7 as a parent compound via Na(+)/H(+) ion exchange in acidic solution at 333 K. It crystallizes in the monoclinic system, space group C2/m, and the lattice parameters of a = 16.0380(8) Å, b = 3.7533(1) Å, c = 9.1982(3) Å, and ß = 101.414(3)°. The crystal structure of H2Ti3O7 was refined to the conventional values of Rwp = 2.60% and Rp = 1.97% with a fit indicator of GOF = Rwp/Re = 1.90 by Rietveld analysis using powder neutron diffraction data. The basic (Ti3O7)(2-) framework in H2Ti3O7 was changed from that in the parent Na2Ti3O7. The atomic coordinate of hydrogen atoms were determined by this study for the first time. The hydrogen site in the layer space was refined with a strict H1-O3 distance of 0.80(2) Å and H2-O4 distance of 0.86(2) Å in H2Ti3O7, respectively. The structural stability of H2Ti3O7 was confirmed by bond valence sums. From these results, protons were suggested as the ordered occupation in the crystal structure.