Studies on fluoride ion conductivity of the mechanochemically synthesized β-KSbF4 for all-solid-state fluoride-ion batteries

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

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.

Fluoride ion conductors are developed for use as solid-state electrolytes in fluoride ion batteries which are one of promising candidates for next-generation storage batteries. Ba-doped LaF3 (La0.9Ba0.1F2.9: LBF) is mainly used as a solid-state electrolyte in fluoride ion batteries. However, room temperature conductivity of LBF is considerably low, on the order of 10−6 S cm−1 and it is still unclear the optimal elements to be doped to LaF3. In this study, we have explored La0.9Sr x Ba0.1−x F2.9 system (x = 0, 0.01, 0.025, 0.05, 0.1), in which Ba in LBF is substituted for Sr and investigated the composition dependence of ionic conductivity. We elucidate that the higher concentration of Sr without Ba can significantly improve the ionic conductivity, and the maximum ionic conductivity of La0.9Sr0.1F2.9 is 1.5 × 10−5 S cm−1 at room temperature, which is one order of magnitude larger than that of LBF. The higher ionic conductivity of LSF is due to the larger grain size and higher sintering density of LSF compared to LBF, which results in lower grain boundary resistance. The LSF total ionic conductivity of 10−4 S cm−1 can be achieved at 350 K, which significantly lowers operating temperature of fluoride ion batteries down to 350 K.

Solid-state fluoride-ion batteries (FIBs) attract significant attention worldwide because of their high theoretical volume, energy density, and high safety. However, the large interfacial resistance caused by the point-point contact between the electrolyte and the electrode seriously impedes their further development. Using liquid-phase therapy to construct a conformal interface is a good choice to eliminate the influence of inadequate contact between the electrode and the electrolyte. In this study, a β-PbSnF4 solid-state electrolyte with high room-temperature ionic conductivity is prepared, and a trace amount of the liquid electrolyte (LE) between the electrode and the electrolyte is introduced in order to minimize the interfacial resistance and enhance the cycle life. The Allen-Hickling simulations show that the introduction of an interfacial wetting agent (LE) can significantly reduce the energy barrier of charge transfer and mass transfer processes at the interface and reciprocate FIBs an enhanced interfacial reaction kinetics. As a result, the initial discharge capacity of the fabricated FIBs is 210.5 mAh g-1 and the capacity retention rate is 82.6% after 50 cycles at room temperature, while the initial discharge capacity of the unmodified battery is only 170.9 mAh g-1 and the capacity retention rate is 22.1% after 50 cycles. Therefore, interfacial modification with a trace amount of LE provides a significant exploration for the improvement of FIB performances.

Influence of La doping on structure, AC conductivity and impedance spectroscopy of Ba2SnO4 Ruddlesden Popper oxide

Fluorine is highly electronegative (+2.84 V vs. SHE) element and possess low weight. Fluoride is monovalent, and it has high charge density. Consequently, it enables high ionic conductivity similar to Li+ in certain solids. However, fluorine is a corrosive and toxic gas, which forbids it as one of the electrodes. Nevertheless, fluoride ions can be transported between a metal and a metal fluoride through a fluoride transporting electrolyte, and fluoride-ion batteries (FIB) can be realized. FIB offers high specific energy, energy density, thermal stability, and safety compared to state of the art lithium-ion batteries (LIBs). Primary batteries based on fluoride transport were demonstrated earlier [1]. Recently we have demonstrated first rechargeable FIB that was operable at 150 °C [2]. Several challenges need to be addressed to realize its full potential of FIBs. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries [3-5]. Recently, for the first time, we have demonstrated room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte [6]. BaSnF4 shows high fluoride conductivity of 3.5x10–4 S cm–1, but limited electrochemical stability window. To be compatible with BaSnF4, we have utilized low electropositive Sn, Zn as anode materials to demonstrate RT FIB. However, to enable cells with high operating potentials, the electrolyte should be compatible with highly electropositive metals (e.g., Mg, Ce). Toward this, we developed a new interlayer electrolyte that is compatible with highly electropositive metals like Ce and Mg. Further, utilizing the interlayer electrolyte, we demonstrate the functioning of first high potential RT FIB.

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

Due to the ultrahigh theoretical energy density, no dendrite issue, and abundant resources, fluoride ion batteries (FIBs) have received a lot of attention. Regarding the issue of dissolving inorganic salt CsF in aprotic organic solvents, some anion acceptor (AA) strategies have been proposed. However, the strong binding force greatly constrains F- ion desolvation, resulting in short lifespan, low specific capacity, and poor reversibility of FIBs. Herein, we propose a concept of steric hindrance-driven closed-loop acceptor to address these problems, by using tetraphenylphosphonium chloride (Ph4PCl) with appropriate Lewis acidity to dissolve CsF and prepare a dynamic fluoride ion electrolyte based on the F-Cl exchange reaction, with a high ionic conductivity of 4.1 mS/cm at room temperature. The steric hindrance effects of chlorine and phenyl can accelerate the desolvation kinetics of F- ions. The excellent kinetics of the Ph4PCl-based electrolyte endows FIBs with long-term cycling stability, and the Sn@SnF2 symmetric cells can cycle for 500 h at 100 μA/cm2 and tolerate a critical current density as high as 1250 μA/cm2. Due to the potential dissociation ability of five-coordinated acceptor central phosphorus for fluorides and the closed-loop conversion effect of chlorine, the fluorination and defluorination reaction proceeds in a dissolution-deposition mode. The CuF2//Sn@SnF2 cell (under a high cathode loading of 4.2 mg/cm2) exhibits the highest reversible capacity up to 717.7 mAh/g and remains 316 mAh/g after 65 cycles with a small voltage polarization of only 11 mV. This work points out the novel design concept of AAs for developing high capacity and long lifespan FIBs.

Fluoride ion batteries (FIBs) are among interesting electrochemical energy storage systems that are being considered as alternatives to lithium-ion batteries (LIBs). FIB offers high specific energy and energy density, thermal stability, and safety. Despite the advantages posed by the FIBs, several challenges need to be addressed to realize its full potential. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries. So far rechargeable FIBs have been demonstrated only at an elevated temperature like 150 °C and above. Here, for the first time, we demonstrate room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte. The high ionic conductivity of tetragonal BaSnF4 (3.5 × 10–4 S cm–1) enables the building of RT FIB. We built fluoride ion batteries using Sn and Zn as anodes and BiF3 as a cathode. We have investigated the electrochemical properties of two different electrochemical cells, Sn/BaSn...

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.

Introduction The popularization of electrified vehicles connected to the smart grid community systems with stationary energy storage has attracted much attention as one of the solutions to global environmental problems. The critical technologies for such systems are rechargeable batteries with energy densities higher than that of conventional Li-ion batteries (LIBs). Among various candidates, all-solid-state fluoride-ion batteries (FIBs) operated by fluoride-ion transport [1] are promising owing to an ultra-high theoretical energy density over 5000 Wh/L [2].The FIBs currently we refer to as Fluoride Shuttle Batteries (FSBs) [3] were previously reported as a primary battery [4]. For the development of rechargeable FSBs, both electrolytes with high fluoride-ion conductivity and active materials that undergo reversible fluorination/de-fluorination reactions are crucial. Fluorite- and tysonite-type fluorides have been investigated as potential solid electrolytes since around the 1970s [ 5 , 6]. Recently, our group reported a series of PbxSn2-xF4 (x = 0.96~1.52) compounds [7] with a unique structure consisting of two alternating layers; a double Pb layer and a triple-layer, each flanked by a single Sn layer. The highest conductivity of 1.1 × 10-1 S/cm at 140 °C was achieved for fluorite-type Pb1.2Sn0.8F4 (x = 1.2). As for tysonite-type fluoride, Reddy et al. reported a possible rechargeable FIB using a La0.9Ba0.1F2.9 solid electrolyte [1] in combination with the various metal fluoride active materials such as CuF2, BiF3, and KBiF4. Among many metal fluorides, CuF2/Cu is a fascinating redox couple as the cathode active material owing to its large theoretical capacity (843 mAh/g) and high electro-motive-force (3.5 V vs. Li/Li+) [8]. However, full utilization of their theoretical capacity has not yet been demonstrated. In this study, we investigated the possible rate-determining steps in the CuF2/Cu cathode reaction using Pb1.2Sn0.8F4 (x = 1.2) with a high fluoride-ion conductivity. Experimental The Pb1.2Sn0.8F4 electrolyte was prepared by solid-state reaction using a mixture of PbF2 and SnF2 powders [7]. The fluoride-ion conductivity was measured by electrochemical impedance spectroscopy. The electrochemical cells were prepared by pressing into a 10 mm diameter pellet. Here, the cathode material was assembled using Cu, Pb1.2Sn0.8F4, and C in a 25:70:5 wt% ratio to facilitate both electronic and ionic conduction. Charge/discharge measurements were performed at 140 °C. Structural and electrochemical characterizations were conducted using TEM/SAED and operando XAFS. Results and Discussion The initial charge/discharge curves at 140 °C for the Cu cathode versus the PbF2 anode are shown in Fig. 1. The Cu cathode with Pb1.2Sn0.8F4 electrolyte exhibited a large charge (discharge) capacity of 522 (342) mAh/g. This capacity corresponds to 62 (41) % of the theoretical specific capacity even though the ionic conductivity of Pb1.2Sn0.8F4 was approximately 1000 times higher than that of La0.9Ba0.1F2.9 [1]. The Cu K-edge XAFS measurements during the charging process revealed the oxidation of Cu from Cu0 to Cu2+, and vice versa during the discharging process. The isosbestic points observed for all the spectra indicated that the conversion reaction involved the two phases, in agreement with the results of our DFT calculations [9]. In the TEM/SAED patterns of the cathode after charge, the distinct diffraction spots related to the CuF2 phase were observed on the surface of the metallic Cu particle. The fluoride-ion conductivity of CuF2 was 10-11 S/cm at 140 °C, which was very low compared to Pb1.2Sn0.8F4 and La0.9Ba0.1F2.9. These results suggest that the solid electrolyte may not be a dominant factor in the reaction on the cathode side, and alternatively smooth fluoride-ion diffusion inside the Cu active material plays a key role for redox behavior. As a next step, it may be effective to make the Cu active material a nano-composite with other fluorides, which is currently underway. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan under the RISING and RISING2 projects.

An all-solid-state fluoride-ion battery (FIB) is one of the promising candidates for the next-generation battery owing to its high energy density and high safety. For the practical application of FIBs, it is an urgent task to operate FIBs at lower temperatures. However, there are still two major difficulties in conventional conversion-type pure metal cathodes: low F- ion conductivities and poor cycle stabilities. Here, the conversion-type Sn-based intermetallic alloy is proposed as a new cathode that can overcome the above issues. The present CoSn2 cathode retains the discharge capacity of 229 mAh g-1 after 250 cycles, even at 60°C. CoSn2 is decomposed into CoF2 and SnF2 nanocrystals in the charging process, and the nanoscale network structure of SnF2 provides the fast F- ion conduction path throughout the cathode, facilitating the battery operation at lower temperatures. Moreover, the formed CoF2 and SnF2 phases are merged into the original CoSn2 phase in the discharging process, leading to a highly reversible redox reaction and the high cycle stability of CoSn2. These findings should pave the way to enhance the performance of all-solid-state FIBs at lower temperatures.

The ac conductivity of bismuth zinc vanadate glasses with compositions 50V2O5. xBi2O3. (50-x) ZnO has been studied in the frequency range 10−1 Hz to 2 MHz and in temperature range 333.16 K to 533.16 K. The temperature and frequency dependent conductivity is found to obey Jonscher's universal power law for all the compositions of bismuth zinc vanadate glass system. The dc conductivity (σdc), crossover frequency (ωH), and frequency exponent (s) have been estimated from the fitting of experimental data of ac conductivity with Jonscher's universal power law. Enthalpy to dissociate the cation from its original site next to a charge compensating center (Hf) and enthalpy of migration (Hm) have also been estimated. It has been observed that mobility of charge carriers and ac conductivity in case of zinc vanadate glass system increases with increase in Bi2O3 content. In order to determine the conduction mechanism, the ac conductivity and its frequency exponent have been analyzed in the frame work of various theoretical models based on classical hopping over barriers and quantum mechanical tunneling. The ac conduction takes place via tunneling of overlapping large polarons in all the compositions of presently studied vanadate glasses. The fitting of experimental data of ac conductivity with overlapping large polarons tunneling model has also been done. The parameters; density of states at Fermi level (N(EF)), activation energy associated with charge transfer between the overlapping sites (WHO), inverse localization length (α) and polaron radius (rp) obtained from fitting of this model with experimental data are reasonable.

Conductivity and modulus formulation in lithium modified bismuth zinc borate glasses

Electronic transport and relaxation studies in bismuth modified zinc boro-tellurite glasses