Effect of substitution (Ta, Al, Ga) on the conductivity of Li7La3Zr2O12

: Recently, there has been a renewed interest in the development of high energy Li-Air batteries. One configuration involves the use of a Li anode in a non-aqueous electrolyte, which is separated from an aqueous electrolyte containing the air cathode by a solid state Li-ion conducting membrane. Several solid state polycrystalline Li-ion conductors, based on pervoskite (1,2), garnet (3,4) and NASICON (Na super ion conductor) (5 through 12) structures, are under consideration as possible membrane materials. One of most widely investigated crystalline Liion conducting membrane materials, based on the NASICON structure, is LiTi2(PO4)3 (5 through 10). In order to sinter crystalline LiTi2(PO4)3 to the high relative densities required for use as a membrane and increase Li-ion conductivity, two approaches have been undertaken (5 through 10). The first it to use a doped material, LiMxTi2 x(PO4)3 (where M=Al, Sc, Y and La) (5,6,9,10). The second is to use LiTi2(PO4)3 containing a small amount of Li2O or Li3PO4 or Li3BO3 (5,7,8). Previous investigations have suggested the total Li-ion conductivity, based on analysis of ac impedance data, of M-doped LiTi2(PO4)3 (where M=Al, Sc, Y and La) and LiTi2(PO4)3 containing a small amount of Li2O or Li3PO4 or Li3BO3 was controlled by Li-ion grain boundary conductivity, which is about 1 to 2 orders of magnitude lower compared to Li-ion bulk conductivity (5 through 10). It has been suggested that for both polycrystalline M-doped LiTi2(PO4)3 (where M=Al, Sc, Y and La) and LiTi2(PO4)3 containing a small amount of Li2O or Li3PO4 or Li3BO3, that both approaches lead to the formation of a continous amorphous film around the grains (5 through 10). It is the transport of Li-ions through this amorphous film which controls the sintering rate (i.e., densification) and grain boundary Li-ion conductivity, and hence, total Li-ion conductivity of the material (5 through 10).

Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system are assured by the use of oxide as an electrolyte instead of the highly-reactive non-aqueous liquid. To develop an oxide-based all-solid-state lithium-ion battery (ASS-LIB) for applications such as electric vehicles, the thicknesses of stacked electrode and electrolyte layers need to be curtailed to several ten to hundred μm order. This must be achieved through powder technology. In oxide-based ASS-LIB, the good contact interfaces should be prepared by a simple powder sintering process, and also produces little ion-blocking impurities. Well-known lithium-ion conductive oxides based on the garnet, perovskite, and NASICON-type structures possess high bulk conductivities of over 10-4 S cm-1; however, these oxides required sintering at high temperature beyond 1273 K to facilitate good contact between the electrode and electrolyte layers. In addition, most positive electrode materials produce impurities after sintering via, for example, their decomposition. To address these issues, the ASS-LIBs were assembled by the use of lithium-ion conductive oxides with low melting point and/or the use of spark plasma sintering (SPS), which could be processed at low temperature with relatively short time.[1] Although the Li-ion conductivity of Li2CO3 was extremely low (about 10-12 S cm-1 at room temperature), the ion-blocking impurities were hardly formed at the electrode/Li2CO3 electrolyte interface since Li2CO3 could be sintered at lower temperatures below 973 K. Li2+x C1-x B x O3, which is isostructural with Li2CO3, is also a Li-ion conductive oxide. According to the literature, the Li-ion conductivity was enhanced dramatically from 10-7 to 10-3 S cm-1 at 473 K as x in Li2+x C1-x B x O3 changes from 0 to 0.25. [2] We used Li2.2C0.8B0.2O3 as electrolyte material and assembled ASS-LIB by SPS method for reducing impurities under sintering. Li2.2C0.8B0.2O3 was synthesized using Li2CO3, LiOH·H2O, and H3BO3 as the starting materials via a conventional solid-state reaction. The required amount of starting materials was ground and heated at 873 K for 10 h in air. The obtained powder was cold-pressed into a pellet and re-heated at 923 K for 10 h in air twice. A single phase of Li2.2C0.8B0.2O3 was identified by a powder XRD pattern of the synthesized product. For ion conductivity measurements, the prepared Li2.2C0.8B0.2O3 was firstly put in a 80 mL ZrO2 cap with five of 5 mm ZrO2 disks, and was crushed by disk mill. Samples for AC impedance measurement were prepared by two sintering methods; conventional furnace sintering (CFS) of uniaxial pressing powder and SPS of Au/ Li2.2C0.8B0.2O3 powder/Au. (Both sides were polished and coated with Au after the CFS method.) The sintering temperature and time at CFS and SPS were 923 K, 2h and 723 K, 1 min, respectively, for densification over 90%. Total Li-ion conductivities for Li2.2C0.8B0.2O3 SPS pellet was 2.1 x 10-6 S cm-1at 303 K, which was higher than that of CFS pellet: 6.7 x 10-7 S cm-1. The growth of Li2.2C0.8B0.2O3 grain was less observed at SEM image of the SPS pellet. The total conductivity was enhanced about three times by suppressing the grain growth via the SPS method, due probably to reducing the high-resistive layer near the grain boundary, which would be formed by long-term duration via the CFS method. Composite electrode powder was prepared from a mixture of 70 wt% LiCoO2 and 30 wt% Li2.2C0.8B0.2O3 electrolyte. Au/composite electrode powder/Li2.2C0.8B0.2O3 powder was assembled by SPS method under the same condition for the sample of AC impedance measurement. Lithium foil was used as a reference/counter electrode. A poly(ethylene oxide)-based polymer electrolyte film was inserted between the lithium foil and the Li2.2C0.8B0.2O3 electrolyte as a separator to reduce the interfacial resistance with adhesion as possible. Electrochemical charge-discharge test was performed at a constant current of 10 μA cm-2 at 333 K between 4.2 and 2.0 V vs. Li/Li+. The ASS-LIB showed an initial charge-discharge profile which is similar to the liquid electrolyte case, and the discharge capacity was 118 mAh g-1. No impurity peak was observed in the powder XRD pattern of the LiCoO2-Li2.2C0.8B0.2O3 composite electrode after SPS method. Thus, we have successfully prepared the oxide-based ASS-LIB, working with demonstrating the comparable discharge capacity to the LIB using liquid electrolyte, by the combination of the low-melting Li2.2C0.8B0.2O3 and the rapid sintering method.

Due to various and indefinite Li-ion distributions within the cuboctahedron surrounded by eight TiO6and local subtle distortions, perovskite-type solid electrolyte Li[Formula: see text]La[Formula: see text]TiO3(LLTO) is suitable to be used as a model system for studying the structure–conductivity relationship. This review is focused on structural characteristics, Li-ion diffusion behavior and conductivity in LLTO. Li-ion concentration, cooling rate of heat treatment and heating temperature are shown to be three main factors influencing the space group structure of LLTO, involving the distributions of Li, La ions and vacancies as well as the distortion of TiO6octahedron. In rhombohedral and some orthorhombic phases, Li ions partially occupy O4windows of cuboctahedrons, whose diffusion could be described by the bond percolation model, whereas in other phases with Li ions inside the cuboctahedrons, the site percolation model is applicable to Li-ion conduction. Li-ion conductivity versus temperature curve exhibits non-Arrhenius behavior, which is divided into three sections with different activation energies, although its first-derivative is continuous. The activation energy is correlated closely with the tilting angle of TiO6octahedron, both of which decrease with the increase of temperature. Experimental studies and theoretical results are discussed in parallel to provide insight into the detailed structure-conductivity relationship.

Na superionic conductor-type LiZr2(PO4)3 (LZP)-related materials are considered promising solid electrolytes that can assist in realizing rechargeable all-solid-state Li-ion batteries with high Li-ion conductivity and electrochemical stability. However, the grain boundary (GB) resistance considerably reduces the total Li-ion conductivity of the sintered polycrystalline body, which is observed in LZP and several other Li-ion conductive oxides. In this regard, the rational design of solid–solid interfaces is known to improve the ionic conductivity. Therefore, examining the ion conduction mechanism at GBs is important from the viewpoints of practical usability and elucidation of the fundamental knowledge on dynamics in crystalline solids. In this study, 32 GB models were constructed, consisting of various Miller indices and terminations, and the corresponding GB Li-ion conductivities were evaluated using molecular dynamics simulations with density functional theory-derived force-field parameters. A few of the GB models exhibited improved Li-ion conductivities compared to the bulk ionic conductivity. Machine learning analysis using descriptors derived from interfacial structure characteristics suggested that the size of cavities around the original Li 6b sites significantly affected the GB ionic conductivity, which could enable the rational design of GB structures.

Ga-doped Li7La3Zr2O12 (Ga-LLZO) is a promising solid electrolyte because it shows higher Li-ion conductivity than LLZO doped with other cations. In this work, Ga-LLZO was prepared by a conventional solid-state reaction and sintered in ZrO2 crucibles to avoid introducing Al into the samples. The particle size distribution, phase structure, morphology, ionic conductivity, and bulk density of the samples were characterized by laser diffraction particle size analyzer, X-ray diffraction (XRD), scanning electron microscope (SEM), AC impedance, and Archimedes method, respectively. The effects of sintering temperature and dopant content on Li-ion conductivity and density were investigated. As for Al-free Li6.4Ga0.2La3Zr2O12, the Li-ion conductivity and bulk density are improved with the sintering temperature rise. But when the sintering temperature is 1250 °C, the Li-ion conductivity and bulk density decline. As for Al-free Li7-3xGaxLa3Zr2O12, the tetragonal phase disappears completely when x ≥ 0.15. The Li-ion conductivity and bulk density of Al-free Li7-3xGaxLa3Zr2O12 are improved with an increase of x (x ≤ 0.2). The ionic conductivity of Al-free Li7-3xGaxLa3Zr2O12 sintered in the air by liquid-phase sintering is close to Ga-LLZO sintered in dry O2 atmosphere. Li6.4Ga0.2La3Zr2O12 sintered at 1200 °C shows the highest Li-ion conductivity of 1.5 × 10−3 S/cm at 30 °C, and the activation energy is about 0.28 eV.

Synthesis and high Li-ion conductivity of Ga-stabilized cubic Li7La3Zr2O12

Lithium hydroxide halide antiperovskite Li-ion conductors are ideal model systems for the systematic investigation of the effect of grain, grain boundary and interfacial resistance on the total Li-ion conductivity in solid-state batteries. Their low melting point (<300°C) empowers the use of melting and solidification to prepare pellets with high relative density without additional sintering steps and with control over grain size. The tunability of the halogen anion site enables control over grain conductivity and interfacial chemistry, with minimal structural perturbation. In this study, we conduct a comprehensive investigation of Li-ion conduction in Li2OHCl(1-x)Brx antiperovskites. We identify Li2OHCl0.9Br0.1 as the composition with the highest Li-ion conductivity of 2.52 E-3 mS/cm at room temperature. We highlight how the thermal expansion coefficient can serve as an indicator for the presence of structural defects hard to probe directly with X-ray techniques and essential in improving bulk Li-ion conduction. The detrimental effect of grain boundaries on ionic conductivity is demonstrated by atomistic calculations and validated experimentally by electrochemical impedance spectroscopy on pellets with controlled grain size. In-situ X-ray photoelectron spectroscopy experiments of Li2OHCl0.9Br0.1 demonstrate its chemical stability in contact with metallic lithium at room temperature. These insights provide design principles to improve Li-ion conductivity of lithium hydroxide halide antiperovskites.

Solid electrolytes are generating considerable interest for all-solid-state Li-ion batteries to address safety and performance issues. Grain boundaries have a significant influence on solid electrolytes and are key hurdles that must be overcome for their successful application. However, grain boundary effects on ionic transport are not fully understood, especially at the atomic scale. The Li-rich anti-perovskite Li3OCl is a promising solid electrolyte, although there is debate concerning the precise Li-ion migration barriers and conductivity. Using Li3OCl as a model polycrystalline electrolyte, we apply large-scale molecular dynamics simulations to analyze the ionic transport at stable grain boundaries. Our results predict high concentrations of grain boundaries and clearly show that Li-ion conductivity is severely hindered through the grain boundaries. The activation energies for Li-ion conduction traversing the grain boundaries are consistently higher than that of the bulk crystal, confirming the high grain boundary resistance in this material. Using our results, we propose a polycrystalline model to quantify the impact of grain boundaries on conductivity as a function of grain size. Such insights provide valuable fundamental understanding of the role of grain boundaries and how tailoring the microstructure can lead to the optimization of new high-performance solid electrolytes.

The application of Li-ion conducting garnet electrolytes is challenged by their large interfacial resistance with the metallic lithium anode and the relative small critical current density at which the lithium dendrites short-circuit the battery. Both of these challenges are closely related to the morphology and the structure of the garnet membranes. Here, we prepared four polycrystalline garnet Li6.4La3Zr1.4Ta0.6O12 (LLZTO) pellets with different particle sizes (nano/micro) and grain boundary additive (with/without Al2O3) to investigate the influence of grain size, the composition of the grain boundary, and the mechanical strength of the pellet on the total Li-ion conduction of the pellet, Li/garnet interfacial transfer, and lithium dendrite growth in all-solid-state Li-metal cells. The results showed that the garnet pellets prepared with nanoparticles and LiAlO2-related grain boundary phase had decreased total Li-ion conductivity because of the increased resistance of the grain boundary; however, these pellets showed higher mechanical strength and improved capability to suppress lithium dendrite growth at high current densities. By controlling the grain size and optimizing the grain boundary with Al2O3 sintering additive, the hot-pressing sintered LLZTO solid electrolytes can reach up to 1.01 × 10-3 S cm-1 in Li+ conductivity and 0.29 eV in activation energy. LLZTO with nanosized grain and LiAlO2-modified grain boundary showed the highest critical current density, which is 0.6 mA cm-2 at room temperature and 1.7 mA cm-2 at 60 °C. This study offers a useful guideline for preparing a high-performance LLZTO solid electrolyte.

High Li-ion conductive composite polymer electrolytes for all-solid-state Li-metal batteries

Fast Li-ion conduction at grain boundaries in (La,Li)NbO3 polycrystals

Multi-spray pyrolysis for combinatorial synthesis of materials libraries and their high-throughput screening: Application to Li-ion conducting electrolytes

The ion-transport phenomenon, determined by the interaction of strain and electrostatic energy, is one of the most important examples that confirms the effects of the polymorphism and atomic morphology. We investigated the correlation between the structural morphology and Li-ion conduction characteristics in α-Li3PS4, a high-temperature phase of the Li3PS4, using ab initio molecular dynamics (AIMD) calculations. We successfully reproduced the thermal disorder and partial occupancy observed at high temperatures by AIMD and confirmed the Li-ion sites and its migration pathways. The activation energy and Li-ion conductivity of α-Li3PS4 at room temperature were predicted to be about 0.18 eV and 80 mS cm-1, respectively, indicating that α-Li3PS4 is one of the fastest Li-ion conductors known so far. The fast Li-ion conduction in α-Li3PS4 is mainly caused by the BCC S-sublattice and tetrahedron-tetrahedron pathway with fully occupied Li-ion sites. Therefore, α-Li3PS4 having a BCC S-sublattice offers a promising structural morphology for effective Li-ion conduction.

The liquid solvents used as electrolytes in conventional Li ion batteries limit the operating temperature range and cause safety problems due to insufficient electrochemical stability. With purpose to avoid these disadvantages solid electrolytes, like lithium conducting oxides, sulfides or phosphates can be used. A promising oxide is the garnet-like Li7La3Zr2O12 that shows one of the highest total Li ion conductivities (about 10-4 S cm-1 at room temperature) in this class of materials. It also enables a wide application range due to its thermal stability and chemical resistance against possible electrode materials, e.g. metallic Lithium.In order to compensate the lower Li ion conductivity compared to liquid electrolytes current work is focused on thin electrolyte layers. Hence, main research aspect of the presented work is the thin film processing of Li7La3Zr2O12 with physical vapor deposition methods, especially RF magnetron sputtering. The growth conditions are optimized with regard to synthesize stoichiometric, crack-free and smooth thin films. Deposition is followed by thermal treatment of the as-grown samples to improve the crystallinity which should in turn improve the Li ion conductivity. The composition, structure and the electrochemical behavior of the resulting thin films are analyzed in order to deposit all-solid-state thin film batteries.

Lithium lanthanum titanate (LLTO) is one of the promising solid-state Li-ion electrolytes for an all-solid-state Li-ion battery system. Although LLTO shows a significantly high Li-ion conductivity of 2.2 × 10−3 S cm−1 in the bulk, the Li-ion conductivity at the grain boundary is largely reduced to 4.2 × 10−5 S cm−1, which prevents the practical application of solid-state Li-ion electrolytes. To solve this problem, the origin of such a low Li-ion conductivity at the grain boundary should be clarified. In this study, we investigated the relationship between the Li-ion conductivity and the geometric structure of the grain boundary in the (Li0.33La0.56)TiO3 polycrystal by using electrochemical strain microscopy in atomic force microscopy combined with electron backscatter diffraction in scanning electron microscopy. The experimental data suggest that the Li-ion conductivity is significantly reduced at the random grain boundaries but not at the coincidence-site-lattice (CSL) grain boundaries. Such a small reduction of the Li-ion conductivity at the CSL grain boundaries may originate from the smaller increment of the activation energy, owing to less composition deviations and less structural distortions at the CSL grain boundaries. These results suggest that it is effective to control the geometries of the grain boundaries for further improvement of the Li-ion conductivity in LLTO.