Fast ion transport in mechanochemically synthesized SnF2 based solid electrolyte, NH4Sn2F5

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

Fluoride-ion batteries (FIBs) offer better theoretical energy densities and temperature stability, making them suitable alternatives to expensive Li-ion batteries. Major studies on FIBs operating at room temperature focus mainly on MSnF4 (M: Ba and Pb) solid electrolytes due to their favourable ionic conductivity values. PbSnF4 is the best fluoride ionic conductor known to date. However, it exhibits poor electrochemical stability. The present work demonstrates the development of TlSn2F5 through a single-step mechanical milling method. TlSn2F5 exhibits a better ionic conductivity value compared to the earlier reported various solid electrolytes, such as BaSnF4, KSn2F5, and La0.9Ba0.1F2.9, commonly considered for FIBs. Ionic transport number measurement using the dc polarization method indicates that TlSn2F5 is an ionic conductor. Furthermore, 19F NMR spectra measured at various temperatures demonstrate that the rise in conductivity with temperature is attributed to the rapid transport of fluoride ions. The present study indicates that TlSn2F5 can be utilized as a potential solid electrolyte for fabricating FIBs.

Nd3+ doped BaSnF4 solid electrolyte for advanced room-temperature solid-state fluoride ion batteries

Solid-state lithium batteries have attracted great attention owing to their potential advantages in safety and energy density. Among various solid electrolytes, solid polymer electrolyte is promising due to its good viscoelasticity, lightweight, and low-cost processing. However, key issues of solid polymer electrolyte include poor ionic conductivity and low Li+ transference number, which limit its practical application. Herein, a new-type of ultraviolet cross-linked composite solid electrolyte (C-CSE), composed of ZIF-based ionic conductor (named ZIL) and polymer, is designed with enhanced ion transport. The ZIL is composed of ZIF-8 and ionic liquid, which can provide C-CSE with fast ion transport paths. Moreover, the proper pore size of ZIF-8 can restrict the migration of embedded ionic liquid and thus construct a solid-liquid transport interface between polymer chains and ZIF-8, which could achieve fast ion transport. In addition, ultraviolet irradiation can decrease the crystallization of C-CSE and thus increase the amorphous region. Consequently, the C-CSE show excellent electrochemical performance including high ionic conductivity of 0.426 mS cm-1 at 30 °C, high Li+ transference number of 0.67, and good Li|Li compatibility cycle over 1040 h. Experimental and computational results indicate that diffusion energy barrier of Li+ through ZIF-8 is smaller than that of polymer chains, which reveals a new Li+ transport mechanism between polymer chains and ZIL, from "chain-chain-chain" to "chain-ZIL-chain". This work demonstrates rational design of ion transport paths at the interface of solid electrolyte could facilitate the development of solid-state lithium batteries as a promising novel strategy.

Fluoride-ion batteries are promising “next-generation” electrochemical energy storage devices, and thus, the room-temperature rechargeable fluoride-ion batteries (FIBs) have attracted tremendous attention due to their high theoretical volume energy density and high safety. However, a series of problems including high interface impedance and poor ionic conductivity at room temperature prevent further development and commercial application of FIBs. Herein, rare-earth element Eu3+-doped BaSnF4 solid solutions [Ba1–xEuxSnF4+x (0 ≤ x ≤ 0.06)] are designed and prepared to improve the performance of BaSnF4 solid electrolytes for room-temperature FIBs. It has been found that the as-prepared Ba0.98Eu0.02SnF4.02 solid-state electrolyte can achieve a better ionic conductivity of 3.8 × 10–4 S cm–1 at room temperature after a calcination process at 300 °C for 2 h, which is the improvement of an order of magnitude in comparison with the original samples. In addition, the FIBs based on Ba1–xEuxSnF4+x (0 ≤ x ≤ 0.04) solid-state electrolytes (Sn/Ba0.98Eu0.02SnF4.02/BiF3) show a discharge capacity of 106 mAh g–1 at 1st cycle and 72 mAh g–1 at 20th cycle. Moreover, the Sn/Ba1–xEuxSnF4+x/BiF3 (0 ≤ x ≤ 0.04) batteries also exhibit good cycling stability and rate performance. Therefore, the addition of Eu3+ can better improve the ionic conductivity of the original solid electrolyte material, which provides a new strategy for the preparation and modification of fluoride-ion electrolytes in FIBs chracterization chracterization.

Solid-state batteries are hindered from practical applications, largely due to the retardant ionic transportation kinetics in solid electrolytes (SEs) and across electrode/electrolyte interfaces. Taking advantage of nanostructured UIO/Li-IL SEs, fast lithium ion transportation is achieved in the bulk and across the electrode/electrolyte interfaces; in UIO/Li-IL SEs, Li-containing ionic liquid (Li-IL) is absorbed in Uio-66 metal-organic frameworks (MOFs). The ionic conductivity of the UIO/Li-IL (15/16) SE reaches 3.2 × 10-4 S cm-1 at 25 °C. Owing to the high surface tension of nanostructured UIO/Li-IL SEs, the contact between electrodes and the SE is excellent; consequently, the interfacial resistances of Li/SE and LiFePO4 /SE at 60 °C are about 44 and 206 Ω cm2 , respectively. Moreover, a stable solid conductive layer is formed at the Li/SE interface, making the Li plating/stripping stable. Solid-state batteries from the UIO/Li-IL SEs show high discharge capacities and excellent retentions (≈130 mA h g-1 with a retention of 100% after 100 cycles at 0.2 C; 119 mA h g-1 with a retention of 94% after 380 cycles at 1 C). This new type of nanostructured UIO/Li-IL SEs is very promising for solid-state batteries, and will open up an avenue toward safe and long lifespan energy storage systems.

For next-generation all-solid-state metal batteries, the computation can lead to the discovery of new solid electrolytes with increased ionic conductivity and excellent safety. Based on computational predictions, a new proposed solid electrolyte with a flat energy landscape and fast ion migration is synthesized using traditional synthesis methods. Despite the promise of the predicted solid electrolyte candidates, conventional synthetic methods are frequently hampered by extensive optimization procedures and overpriced raw materials. It is impossible to rationally develop novel superionic conductors without a comprehensive understanding of ion migration mechanisms. In this review, we cover ion migration mechanisms and all emerging computational approaches that can be applied to explore ion conduction in inorganic materials. The general illustrations of sulfide and oxide electrolyte structures as well as their fundamental features, including ion migration paths, dimensionalities, defects, and ion occupancies, are systematically discussed. The major challenges to designing the solid electrolyte and their solving strategies are highlighted, such as lattice softness, polarizability, and structural disorder. In addition to an overview of recent findings, we propose a computational and experimental approach for designing high-performance solid electrolytes. This review article will contribute to a practical understanding of ion conduction, designing, rapid optimization, and screening of advanced solid electrolytes in order to eliminate liquid electrolytes.

Fluoride ion batteries (FIBs) have recently gained much attention as a next generation battery system because of their potential to surpass lithium-ion batteries (LIBs) in many aspects such as high energy density and low cost. FIBs with solid or liquid electrolyte have been studied intensively, amongst liquid-based FIBs has an advantage of good interfacial contact between the electrolyte and electrode, and low-temperature operation over all-solid-state FIBs. However, stability has been a major concern for liquid electrolytes. The instability caused by the strong basicity of F− ion attacking most of the commonly used organic solvents leading to side reactions.1 So far bis(2,2,2-trifluoroethyl) ether (BTFE), which has good base resistance, has been proposed as an effective organic solvent for electrolyte.2 However, the boiling point of BTFE is relatively low (64 °C). While looking beyond organic solvents, ionic liquids (ILs) can be a choice as an electrolyte for FIBs owing to their good properties such as nonvolatility and wide electrochemical window.3 It is commonly known that quaternary ammonium cations undergo Hofmann elimination, during which β-hydrogen is withdrawn by strong bases such as OH− and F− ions.4,5 Therefore, to use ILs as an electrolyte for FIB, it is necessary to improve their stability against F− ion. One way to weaken the basicity of F- ion is to solvate them by a complexing agent. Hagiwara et al. reported that imidazolium-based ILs containing ethylene glycol are stable despite the presence of F− ion.6 Bond formation between F− ion and hydrogen in the hydroxy groups of ethylene glycol and imidazolium-based cation prevents the F− ion from attacking the β-hydrogen of the cation's alkyl chain. However, there have been no reports of FIBs using ILs containing solvated F− ion as electrolytes. In this study, we investigated the thermal stability of choline bis(trifluoromethanesulfonyl)amide (N111(2OH) TFSA), an IL with hydrogen bond donor functional group, against F− ions. Furthermore, we performed charge-discharge tests on liquid-based FIBs using N111(2OH) TFSA containing fluoride salt as an electrolyte.N111(2OH) TFSA (Fig. 1 (a)) was synthesized by ion exchange of N111(2OH) Cl with K TFSA. The melting point of the synthesized N111(2OH) TFSA was estimated to be 38 °C from DSC measurement and it was solid at room temperature. 0.4 mol kg−1 tetramethylammonium fluoride (TMAF) in N111(2OH) TFSA was found to be stable up to about 150 °C (Fig. 1(c)). TGA measurements of N,N,N-trimethyl-N-propylammonium (N1113 TFSA) (Fig. 1(b)) without hydroxy groups were also performed under the same conditions for comparison. A solution of 0.4 mol kg−1 TMAF in N1113 TFSA showed a gradual weight loss right after the measurement was started, with a significant weight loss at about 120 °C (Fig. 1(c)). These results suggest that N111(2OH) TFSA with the hydroxy group exhibits higher thermal stability toward F− ion. This was attributed to the hydrogen bonding of the hydroxy group to F− ion, which weakened the basicity of F− ion. Finally, charge-discharge tests were performed in a three-electrode cell using BiF3 working electrode, Pb counter electrode, and Ag/Ag+ reference electrode. Figure 1 (d) shows charge-discharge curves of BiF3 electrode with 0.4 mol kg−1 TMAF in N111(2OH) TFSA as an electrolyte at 60 °C. The first discharge capacity of the BiF3 electrode was 300 mAh g−1, which represents 99 % of the theoretical capacity of BiF3 (302 mAh g−1) with a columbic efficiency of 85%. This result suggests that the BiF3 was reversibly defluorinated/fluorinated with 0.4 mol kg−1 TMAF in N111(2OH) TFSA.AcknowledgmentThis presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).References(1) V. K. Davis, et al., Mater. Chem. Front., 2019, 3, 2721-2727.(2) V. K. Davis, et al., Science, 2018, 362, 1144–1148.(3) K. Okazaki, et al., ACS Energy Lett., 2017, 2, 1460–1464(4) S. Raiguel, et al., Green Chem., 2020, 22, 5225-5252.(5) H. Sun, and S. G. Dimagno, J. Am. Chem. Soc., 2005, 127, 2050-2051.(6) Z. Chen, et al., J. Phys. Chem. Lett., 2018, 9, 6662−6667 Figure 1

Developing high-performance solid electrolytes that are operable at room temperature is one of the toughest challenges related to all-solid-state fluoride-ion batteries (FIBs). In this study, tetragonal β-Pb0.78Sn1.22F4, a promising solid electrolyte material for mild-temperature applications, was modified through annealing under various atmospheres using thin-film models. The annealed samples exhibited preferential growth and enhanced ionic conductivities. The rate-determining factor for electrode/electrolyte interface reactions in all-solid-state FIBs was also investigated by comparing β-Pb0.78Sn1.22F4 with representative fluoride-ion- and lithium-ion-conductive materials, namely, LaF3, CeF3, and Li7La3Zr2O12. The overall rate constant of the interfacial reaction, k0, which included both mass and charge transfers, was determined using chronoamperometric measurements and Allen-Hickling simulations. Arrhenius-type correlations between k0 and temperature indicated that activation energies calculated from k0 and ionic conductivities (σion) were highly consistent. The results indicated that the mass transfer (electrolyte-side fluoride-ion conduction) should be the rate-determining process at the electrode/electrolyte interface. β-Pb0.78Sn1.22F4, with a large σion value, had a larger k0 value than Li7La3Zr2O12. Therefore, it is hoped that the development of high-conductivity solid electrolytes can lead to all-solid-state FIBs with superior rate capabilities similar to those of all-solid-state Li-ion batteries.

Solid-state synthesis and ion transport characteristics of the β-KSbF4 for all-solid-state fluoride-ion batteries

Compared to conventional lithium-ion batteries (LIBs), all-solid-state fluoride-ion batteries (FIBs) offer superior theoretical energy density and enhanced safety features1. The notable advantage of FIBs is their ability to facilitate multi-electron transfer reactions between metals and metal fluorides, potentially unlocking volumetric energy densities of up to 5000 Wh L− 1. Despite these promising properties, the development of FIBs is still at an early stage. A major challenge is their low ionic conductivity at room temperature compared to cation conductors. As a result, FIBs typically require operation at elevated temperatures, in excess of 140 °C, to achieve desirable performance levels.To overcome this problem, improving the ionic conductivity of electrolytes for FIBs is critical and involves reducing the activation energy for ion migration. High entropy materials have emerged as a potential solution to this challenge, offering a novel way to effectively reduce the activation energy2. Recent advances in this field have led to the development of a lithium superionic conductor, which demonstrates the potential of high-entropy materials to revolutionise ion conductivity3.In our study, we focused on the synthesis of a high-entropy material, (LaCeNdSmGd)F3, using a solid-state reaction method. Extensive analysis using Rietveld refinement and scanning electron microscopy (SEM) confirmed that the synthesised (LaCeNdSmGd)F3 retains the crystal structure characteristic of LaF3. Using electrochemical impedance spectroscopy, we found that (LaCeNdSmGd)F3 exhibits an ionic conductivity of 1.3×10-7 S cm− 1 at 373 K. Furthermore, Arrhenius plots revealed that (LaCeNdSmGd)F3 has a significantly lower activation energy compared to LaF3, highlighting the effectiveness of high-entropy materials in reducing activation energy.The introduction of a wide range of cations into the high entropy material (LaCeNdSmGd)F3 leads to structural disorder and site energy overlap. This complexity within the crystal lattice effectively lowers the energy barriers associated with ion migration, thereby enhancing the ionic conductivity of the fluoride solid electrolyte.

One aspect of the interaction between fast ions and magnetohydrodynamic (MHD) instabilities is the fast ion transport. Coupled kink and tearing MHD instabilities have also been reported to cause fast ion transport. Recently, the ‘kick’ model has been developed to compute the evolution of the fast ion distribution from neutral beam injection using instabilities as phase-space resonance sources. The goal of this paper is to utilize the kick model to understand the physics of fast ion transport caused by the coupled kink and tearing modes. Soft x-ray diagnostics are used to identify the mode parameters in the National Spherical Torus Experiment. The comparison of neutron rates measured and computed from time-dependent TRANSP simulation with the kick model shows that the coupling of kink and tearing mode is important in determination of the fast ion transport. The numerical scan of the mode parameters shows that the relative phase of the kink and tearing modes and the overlapping of kink and tearing mode resonances in the phase space can affect the fast ion transport, suggesting that the synergy of the coupled modes may be causing the fast ion transport.

An all-solid-state fluoride-ion battery is one of the promising candidates for next-generation secondary batteries owing to its high energy density compared to a commercial lithium-ion battery [1]. While the practical capacities of cathodes have recently shown improvements in discharge capacities (up to 200 – 500 mAh g–1) [2], the capacities of anodes are still one order of magnitude smaller than that of cathodes, and moreover, their cycle stability are also very low [3]. It is therefore important to improve the electrochemical performance of the anodes in fluoride-ion batteries. Aluminum (Al) is one of the promising metals owing to the high theoretical capacity (2980 mAh g–1) and the low redox potential (–1.7 V vs. Pb/PbF2). However, there is no report on the practical use of Al because of the significantly low F– ion conductivity of AlF3 (10–11 S cm–1 at room temperature). To secure the low F– ion conductivity of AlF3, we explore the intermetallic alloy between Al and La because LaF3 shows a higher F– ion conductivity (10–8 S cm–1) than that of AlF3. Here, we show that LaAl3 reversibly delivers the high capacity with relatively small capacity fade [4].LaAl3 intermetallic alloy was prepared from La with a purity of 99.9% and Al with a purity of 99.999% by arc melting, and then annealed at 950℃ for 336 h. The LaAl3 phase was confirmed by powder X-ray diffraction. To prepare the anode composites, the LaAl3 powders were mixed with La0.9Ba0.1F2.4 (solid electrolytes) and acetylene black (conductive aids) powders in the weight ratio of 3:6:1, using planetary ball mills with ZrO2 balls under a rotation speed of 100 rpm for 20 h. Using galvanostatic cycling, we performed charge/discharge tests of the all-solid-state fluoride-ion battery, where LaAl3 and PbF2 were used as anode and cathode active materials, respectively. The initial discharge and charge capacities are 445 mAh g–1 and 202 mAh g–1, respectively. While the 1st discharge curve shows a monotonous potential increment, the subsequent charge and discharge curves show stable potential plateaus at –2.0 V and –1.5 V. To elucidate the charge/discharge mechanism of LaAl3, we performed structural and chemical analysis of the 1st discharged and charged LaAl3, using scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS). We found that LaAl3 is decomposed into LaF3 and AlF3 nanocrystals in the initial discharge process. Detailed discussion will be given in this presentation. Acknowledgements This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

The reversed-field pinch (RFP) can spontaneously transition from an axisymmetric magnetic topology to a 3D-helical geometry. Investigations on fast ion transport associated with energetic particle driven Alfvén instabilities, tearing mode induced stochasticity, and neoclassical effects have been performed on the Madison Symmetric Torus. STELLGAP produced shear-Alfvén continua seeded with V3FIT 3D-equilibrium reconstructions describe the response of Alfvénic bursting activity as a direct consequence of the equilibrium change on the fast ion resonance. Far infrared interferometry resolved electron density perturbations associated with the bursts provide a spatial measurement of the mode structure and support the reconstructions. The bursts produce no global resonant fast ion transport; however, their disappearance at a high core-resonant amplitude implies other transport mechanisms at play. Neutral particle analysis and neutron signals suggest fast ion losses at sufficient core tearing mode strength, supporting the lack of Alfvénic activity. The guiding-center code ORBIT corroborates rapid fast ion loss times in the helical state largely as a consequence of remnant tearing modes. Additionally, ORBIT simulations demonstrate little neoclassical enhancement of particle transport. While superbanana orbits may exist, the growth in the core-resonant fast ion island and the associated secondary mode overlap govern the largest transport process, leading to robust fast ion losses in the 3D-RFP.

Recently, fluoride ion batteries have been attracting attentions because of their potential for large capacity far beyond that of conventional lithium ion batteries. However, fluoride ion batteries are still not into practical use. One of the problems with fluoride ion batteries is low ionic conductivity in the electrolyte. At this moment, no solid-state fluoride ion conductors, showing high ionic conductivity at room temperature together with a wide electrochemical window, have been developed. Among fluoride ion conductors, Tysonite-type (La,Ba)F3 (LBF) is known to have a wide electrochemical window and exhibit relatively high conductivity near room temperature. However, its conductivity is still needed to be improved. It was reported that the conductivity of a single crystalline LBF was approximately ten times higher than that of the polycrystalline ones. This indicated that microstructural factors in the polycrystalline LBF, such as the density and/or the grain boundary, deteriorate the ionic conduction. Thus, it is important to understand influences of the microstructures on the ionic conductivity for improving the ionic conductivity of LBF. From the above backgrounds, in this study, dense LBF samples with different grain sizes were prepared by using the spark plasma sintering (SPS) method, and their bulk and grain boundary conductivities were evaluated in order to clarify influences of the microstructures on the ionic conductivity. We first evaluated the relation between the microstructures and the ionic conductivity in Tysonite-type La0.93Ba0.07F2.93 by using the samples sintered at 700 to 1000 ºC. The dense samples, having the relative density higher than 95%, could be obtained by the SPS method when the sintering temperature was 800 ºC or higher. The average grain size was about 0.4, 0.5, and 1 μm in the samples sintered at 800, 900, and 1000 ºC, respectively. The conductivity increased with increasing the sintering temperature. From the AC impedance spectroscopy measurements, it was found that the bulk conductivity was almost independent of the sintering temperature, whereas the apparent grain boundary conductivity increased with increasing the sintering temperature. This indicated that the grain growth due to the high sintering temperature decreased the number of grain boundary, thus the apparent grain boundary resistance. Based on above results, we sintered the samples at 1100 and 1200 ºC, aiming further grain growth. The average grain size of the sintered samples became larger, about 20 and 60 μm by sintering at 1100 and 1200 ºC, respectively. However, the conductivity unexpectedly decreased with increasing the sintering temperature. From AC impedance spectroscopy measurements, a significant decrease in the grain boundary conductivity was observed with increasing the sintering temperature, while the bulk conductivity was almost the same regardless of the sintering temperature. In SEM observation, many pores were observed at the grain boundaries. These pores were considered as a main cause for the deterioration of the grain boundary conductivity. In order to suppress the pore formation at the grain boundaries, the sintering condition was re-examined. By decreasing the rate of rising temperature during the sintering process, the pore formation could be suppressed even for the sintering at 1200 ºC. The conductivity of the sample sintered at 1200 ºC was improved by decreasing the rate of raising temperature. For instance, the conductivity of the sample sintered at 1200 ºC with the slow rate, 2 ºC·min-1, was almost comparable with that sintered at 1000 ºC with the fast rate, 50 ºC·min-1. Throughout this work, it was concluded that the densification and the grain growth would be effective for the enhancement of ionic conductivity in Tysonite-type La0.93Ba0.07F2.93. Acknowledgement: This work was partly supported by JST. K.M appreciate Hatano Foundation for the support to his travel.