Solid Oxide Electrolyte Research Articles

Highly dense and thermally stable BaCe0.5Zr0.3Y0.1A0.05 Zn0.05O3−δ (A = Gd, Sm) electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC)

Highly dense and thermally stable BaCe0.5Zr0.3Y0.1A0.05 Zn0.05O3−δ (A = Gd, Sm) electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC)

Irreversible oxygen uptake and thermal expansion in the solid electrolyte oxygen-deficient perovskite Ca(Ti0.8Fe0.2)O3−δ

The efficient operation of solid oxide fuel cells for renewable energy generation requires solid electrolytes with high ionic conductivity, thermal stability, and chemical durability. Metal oxide perovskites of the form ABO3 are promising candidates, especially when doped with cations that incorporate interstitial oxygen or oxygen vacancies to enhance ionic conductivity. However, doping can lead to complex and poorly understood crystallographic structures. In this study, we investigate the effects of Fe3+ doping on the orthorhombic Pbnm CaTiO3 perovskite, forming Ca(Ti0.8Fe0.2)O3−δ, and demonstrate the complexity of its phase transitions at operational temperatures. Using synchrotron X-ray diffraction and thermogravimetric analysis, we show that this material undergoes significant oxygen uptake at elevated temperatures, leading to an irreversible expansion of the unit cell and changes in polyhedral coordination upon cooling. These structural modifications are attributed to the partial oxidation of Fe3+ to Fe4+, providing insight into the inconsistencies observed in previous studies of oxygen-deficient perovskites. Our findings highlight the critical role of thermal history and oxygen availability in determining the structural stability and long-term performance of perovskite-based solid electrolytes in solid oxide fuel cells.

Plasma-sprayed dense yttria-stabilized zirconia electrolyte for metal-supported solid oxide fuel cells

Plasma-sprayed dense yttria-stabilized zirconia electrolyte for metal-supported solid oxide fuel cells

Hydrothermal Synthesis of Oxide Solid Electrolyte Nanocrystals with Controlled Shapes for Orientated Ceramics

Oxide solid electrolyte for all-solid-state lithium-ion batteries have advantages over sulfide solid electrolytes in terms of thermal and mechanical stability, safety, and ease of handling. However, high sintering temperatures required to fabricate oxide solid electrolyte ceramics are on obstacle to integration with cathodes and anodes. We recently reported synthesis of lithium lanthanum titanate (LLTO)-based solid electrolyte nanoplates by a hydrothermal method[1]. The hydrothermally synthesized nanoplates could be sintered at a lower temperature than particles synthesized by the solid phase method. It was revealed that the lower sintering temperature was due to the face-to-face stacking of the nanoplates formed through uniaxial molding, which indicated that sintering behvior of oxide solid electrolytes could be controlled by the shape and stacking structure of nanocrystals.Here, we have synthesized various shaped LLTO-based nanocrystals by changing a Ti source in the hydrothermal method to investigate dependence of sintering behavior on the shape of the nanocrystals and have investigated Li ion conductivity of the ceramics fabricated from the nanocrystals. For synthesis of LLTO-based nanocrystals, we newly introduced two types of titanium sources: anatase-type and rutile-type TiO2, in addition to titanium(IV) bis(ammonium lactate) dihydroxide (TALH), which was used for the synthesis of the nanoplate in the previous report. Scanning electron microscopy observation showed that anatase-type TiO2 yielded a mixture of nanocubes and nanorods and rutile-type TiO2 yielded nanosheets. Scanning transmission electron microscopy observation and electron energy loss spectroscopy confirmed that the nanocrystals yielded from anatase- and rutile-type TiO2 had the LLTO-based perovskite crystal structure as the nanoplates yielded from TALH. Comparison of the sintering behavior at 1000°C for these different shaped nanocrystals subjected to uniaxial molding revealed that the nanocrystals with high aspect ratio (nanoplate, nanorod, and nanosheet) sintered more easily than the nanocubes and the ceramics of the nanoplates and nanosheets had {100} and {110} orientation, respectively. These oriented ceramics showed ionic conductivity according to the crystallographic orientations. The present result can provide an insight into controlling ionic conductive direction in oxide solid electrolyte ceramics by using shape-controlled nanocrystals. Acknowledgement This study was supported by a project (No. JPNP20005), commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Reference [1] K. Mimura, N. Hamao, H. Itasaka, Z. Liu, K. Hamamoto, J. Sol-Gel Sci. Tech. 104, 599 (2022).

Super Wide Area Exploration of High Conductive Oxide Solid Electrolytes Using Data-Driven Feature Value Map

Currently, over 100,000 data on inorganic compounds was reported in the Inorganic Crystal Structure Database (ICSD). Until now, exploration of functional materials has been experimentally carried out using intuition and experience from such huge amount of materials. This approach needs several trials and errors, resulting in consuming times. Moreover, experimental space is small and only local optimization is possible. Thus, drastic improvement of material performance is difficult. To overcome this issue, we propose super wide area exploration of chemical composition using huge experimental space. In detail, we introduce data-driven exploration map that consists of intrinsic material properties, such as crystal structure, composition, and their physicochemical properties. We call the map as "feature value map", afterwards. Since functional performances are strongly dependent of material's properties, this approach should potentially lead to novel discoveries beyond the subjective perception of the researcher. In this study, we attempted to expand this feature value map to lithium-ion conductive oxide solid electrolytes. Conventional Lithium-Ion Batteries use flammable organic compounds as electrolytes, which can burn in the worst case if the temperature of the battery rises due to a large load on the battery in some way. All-solid-state batteries use a solid electrolyte, which reduces the risk of ignition. Especially oxide solid electrolytes are known as safe and chemically stable. Although safer, oxide solid electrolytes have lower ionic conductivity and plasticity, resulting in lower battery performance. So far, several exploration approach, including elemental doping, has been carried out. But, still difficulty to satisfy 10-1 S/cm at room temperature, which is a target value for actual use. In this presentation, development of ionic conductivity correlated feature value map and super wide space exploration of materials using this map will be reported.AcknowledgementThis work was partly supported by Priority Project in Aichi Knowledge Hub Aichi, JST Green Technologies of Excellence, the Strategic Innovation Program (SIP) of the Cabinet Office, and joint research with a company.

Fabrication of Ruthenium-Based Cathode Material/Solid Electrolyte Composites

Introduction Oxide-based all-solid-state batteries (ASSBs) are considered safe due to their chemical stability and are attracting attention as a power source for electric vehicles (EVs). Because the driving range of EVs is not sufficient compared to conventional gasoline-powered vehicles, ASSBs with higher capacities are required. To overcome the above issue, it is necessary to adopt the electrode material having high energy density such as Li-rich layered oxides, or to increase the amount of active material in the cell. The positive electrode of an ASSB needs to be a composite of electrode material and solid electrolyte to ensure ionic conductive pathways. Therefore, it is necessary to increase the ratio of active material in the composite cathode to improve energy density in the battery. Previously, the ASSB with positive electrode composed of only active material without any conductive additive was reported for Ru containing oxide, Li2Ru0.8S0.2O3.2 [1], having high electronic and ionic conductivity. Therefore, we focused on the Ru containing Li-rich layered oxide, Li2RuO3 and Li2Mn0.4Ru0.6O3. The garnet-type solid electrolyte was selected as an oxide solid electrolyte due to its high ionic conductivity and the reductive tolerance to Li metal having a large capacity of 3860 mAh g–1. The high stiffness of the oxide solid electrolytes leads to a decrease in the contact area and prevents the formation of a superior electrode/electrolyte interface only by mixing and applying pressure, which hinders the operation of ASSBs. To lower the resistance of the interface, the co-firing of cathode material and solid electrolyte is required. However, side reactions associated with co-firing are an issue. Li2TiO3 as a buffer layer was introduced between the cathode material and the solid electrolyte to suppress side reactions. Li2TiO3 has a similar structure to Li2RuO3, which is expected to improve sinterability by a partial substitution. The co-firing of the cathode materials and the garnet-type solid electrolyte and the effect in the suppression of side reactions using buffer layers will be presented. Experimental Li2RuO3 and LMRO were used as cathode materials, Li6.25Ga0.25La3Zr2O12 (LLZ-Ga) as a solid electrolyte, and Li2TiO3 as a buffer layer. The buffer layer was introduced into the cathode surface by solid-phase and liquid-phase methods. LiOH-H2O and TiO2 were mixed and sintered with the cathode material in the solid-phase method. LiOH-H2O and tetra n-butyl titanate monomer were weighed and dissolved in ethanol separately in the liquid-phase method. Each solution and the cathode material were mixed and stirred overnight, then dried at 60 ºC in a rotary evaporator. The resultant powder was pelletized by uniaxial press and then heated at 800 ºC for 5 hours in an oxygen atmosphere. The electrochemical property of the cathode material with the buffer layer was confirmed by a constant-current charge-discharge test using a coin cell. The cathode composite composed of the cathode material (with buffer layer) and the solid electrolyte was fabricated by co-firing at 600-800ºC after the mixing by hand or ball mill. The obtained samples were subjected to phase identification by X-ray diffraction. The reaction process was observed by SEM observation and EDX composition analysis. All-solid-state cells were fabricated by a uniaxial press using the obtained sample as the positive electrode, Li5.5PS4.5Cl1.5 as the solid electrolyte, which has excellent ion conductivity and can easily form a good interface simply by applying pressure, and Li-In alloy as the negative electrode. Constant current charge-discharge tests were performed on the fabricated all-solid-state cells. Results and discussion X-ray diffraction and EDX analysis confirmed that the impurity phase of La2Li(RuO6) formed by co-firing the cathode material and LLZ-Ga. Therefore, the buffer layer was introduced by the solid-phase and liquid-phase methods. In the solid-phase method, Li2TiO3 was not distributed uniformly on the particle surface of the cathode material. In the liquid-phase method, Li2TiO3 covered the cathode surface more uniformly and thinner than that formed in the solid-phase method. Both Li2RuO3 and LMRO retained their original structure, while the lattice parameter of LMRO changed, indicating that LMRO formed the solid solution with Li2TiO3. No inter-diffusion of lanthanum and ruthenium between the cathode material with buffer layer and LLZ-Ga was observed in the EDX analysis. It suggests that the introduction of a buffer layer suppressed the inter-diffusion. The cathode composite co-fired did not exhibit the discharge capacity in the charge-discharge test using the all-solid-state cell. It is attributed to the low conductivity derived from the low sinter ability of the cathode composite.[1] K. Nagao et al., Sci. Adv. 2020; 6 : eaax7236 (2020). Acknowledgements This work was supported by JSPS KAKENHI Grant Number JP22H04615.

Investigation of Redox Potential of Li2so4 Using All-Solid-State Cells

The demand for power sources for electric vehicles and portable devices has increased rapidly. All-solid-state lithium-ion batteries are promising attention due to their safety and energy density. Especially, the development of solid electrolytes is necessary to realize all-solid-state batteries. Solid electrolytes with high ionic conductivity like sulfides, oxides, and chlorides are used for long-distance conduction in the entire all-solid-state batteries. In addition, those with low ionic conductivity like lithium salts are also used as coating materials and sintering aids. Our group reported active materials and electrolytes using Li2SO4 [1, 2].One of the features required for solid electrolytes is wide electrochemical windows. Although various sulfide solid electrolytes have been studied due to their high ionic conductivity and ductility, they have narrow electrochemical windows [3], leading to oxidative and reductive decomposition during cell cycling. Thus, it is important to consider the electrochemical windows of solid electrolytes.Conventionally, the electrochemical windows of solid electrolytes have been investigated using electrochemical techniques like cycling voltammetry and linear sweep voltammetry (LSV). The working electrode is composed of the objective solid electrolytes and conductive carbon. However, for some materials with low ionic conductivity, it is challenging to ensure ionic conduction at long distances in the composite working electrode. Therefore, few reports have shown that the redox potential of a low ionic-conductive material including Li2SO4 is experimentally determined by electrochemical tests, although the electrochemical stability of lithium ion conductors was investigated using first-principles calculation methods [3, 4].In this study, we used LSV measurements using all-solid-state cells to investigate the redox potential of Li2SO4. The composite working electrode comprised Li2SO4, vapor-grown carbon fiber (VGCF), and oxide solid electrolyte. Li2SO4 and VGCF were ball-milled and the obtained Li2SO4-VGCF and oxide electrolyte were mixed in an agate mortar and pestle to prepare composite working electrodes. As oxide electrolytes, 90Li3BO3·10Li2SO4 glass-ceramic (LBSO) [2] was used in cells for an anodic scan, and 67Li2O·33LiI glass (LOI) [5] was used in cells for a cathodic scan because they have better oxidation or reduction tolerance than sulfide electrolytes. In addition, it was found that the decomposition of sulfide electrolytes affected the LSV curves when sulfide electrolytes were used in a composite working electrode or as a separator layer that directly contacted with the composite working electrode. Therefore, the cell configuration was Li-In/Li3PS4/LBSO (or LOI)/Li2SO4-VGCF-LBSO (or LOI). To confirm the decomposition of Li2SO4 after the LSV measurements, X-ray photoelectron spectroscopy (XPS) measurements were performed on the cells, and the redox mechanism of Li2SO4 was discussed.In the anodic scan, a current peak derived from the oxidation of Li2SO4 was observed at approximately 4.3 V vs. Li-In (4.9 V vs. Li+/Li), which was close to the standard electrode potential of oxidation from SO4 2– to S2O8 2– [6]. In addition, in the cathodic scan, a current peak derived from the reduction of Li2SO4 was observed at approximately 1.5 V vs. Li-In (2.1 V vs. Li+/Li), which almost corresponds to the potential of reduction from SO4 2– to SO3 2– [6]. Furthermore, the XPS analyses confirmed that Li2SO4 was finally reduced to Li2S and Li2O.In summary, the LSV measurements were performed with the all-solid-state cell using oxide electrolytes instead of sulfide electrolytes, which eliminated the overlap of a current caused by the decomposition of sulfide electrolytes. The oxidative and reductive decomposition voltages of Li2SO4 were close to each standard electrode potential. We also found a way to experimentally determine the electrochemical potential of any materials by modifying the cell design. This study will help spread the importance of potential windows and the realization of all-solid-state lithium-ion batteries. Reference s : [1] K. Nagao et al., Sci. Adv. 6 (2020) eaax7236.[2] M. Tatsumisago et al., JCS-Japan, 125 (2017) 433.[3] Y. Zhu et al., ACS Appl. Mater. Interfaces, 7 (2015) 23685.z[4] W. D. Richards, et al., Chem. Mater., 28 (2016) 266.[5] Y. Fujita et al., Electrochemistry, 89 (2021) 334.[6] A. J. Bard et al., Standard Potentials in Aqueous Solution, Routledge (2017).

Oxide Solid Electrolytes in Solid‐State Batteries

AbstractSolid‐state electrolytes (SSEs) have re‐emerged as high‐priority materials for enhancing the safety and power density of electrochemical energy storage devices. However, several challenges, including low ionic conductivity, narrow redox windows, and interface issues, hinder the practical deployment of solid‐state batteries (SSBs). In this review, we evaluate recent advances in the design, synthesis, and analysis of oxide SSEs and identify relevant structural and stability factors, as well as dimensional design concepts, for creating oxide SSEs to meet practical application requirements. We provide an overview of the development and characteristics of oxide SSEs, then analyze bulk and ion transport based on different structures. We summarize the progress made in various synthetic approaches to oxide SSEs and discuss issues related to their stability and factors influencing ionic conductivity. Furthermore, we present the main challenges and future development directions of oxide SSBs to pave the way for the practical applications of oxide SSEs.

(Invited) Microstructure Modeling of Computer-Generated Microstructures to Guide Design of Solid-State Electrolyte Architectures

Due to the ever-increasing interest in solid state batteries, there is a significant amount of ongoing research on developing novel fabrication approaches for creating a variety of solid electrolyte microstructures. This is done with the goal of meeting the multiple design requirements and overcoming fundamental challenges associated with solid electrolytes. Microstructure modeling can be a powerful tool to study and analyze electrolyte microstructures and guide design of improved microstructures. There have been a limited number of studies that have developed and utilized microstructure models to analyze solid electrolytes and solid-state batteries [1, 2]. These studies are typically limited to analyzing experimentally fabricated microstructures by using experimental imagery for generating the computational domain. While this is necessary for validating microstructure models against experimental data, this is not useful for discovering improved microstructures. There have been some past studies in the case of both traditional lithium-ion batteries and solid-state batteries where computer-generated microstructures are considered to inform architecture design [3, 4]. However, these studies typically assume highly idealized porous electrode structures that are not representative of the microstructures that can be fabricated using existing fabrication methods. In the present work, we study computer-generated lithium lanthanum zirconium oxide (LLZO) solid electrolyte microstructures informed by the microstructures that have been fabricated by scalable fabrication methods [4,5]. Although computer-generated microstructures being analyzed may lack some of the finer features of the fabricated microstructures, the type of computer-generated microstructures to be studied will be restricted based on their manufacturability using the state-of-the-art fabrication techniques [4,5]. The design parameters of the computer-generated microstructures (e.g., pore size, shape, and surface features) will be varied to explore the solid electrolyte design space to study tradeoffs associated with key design requirements such as gravimetric and volumetric energy density, rate capability, and mechanical properties. In the future, we will be fabricating microstructures informed by this analysis to validate the model predicted performance characteristics.

Reduction of Li2CO3 Surface Species from Garnet-Type Solid-State Electrolytes via Scalable Open-Air Plasma Treatment

Integrating garnet-type lithium lanthanum zirconium oxide (LLZO) solid electrolytes into all-solid-state or hybrid batteries presents numerous challenges. One significant obstacle is the spontaneous formation of lithium carbonate (Li2CO3) when surfaces of LLZO are exposed to moisture and carbon dioxide, which can lead to high interfacial impedance with electrodes and deleterious effects on the mechanical, electrochemical, and safety performance of the materials. Current methods for removing Li2CO3 include mechanical polishing and thermal treatment under inert gas, both of which are time-consuming and difficult to scale up or implement in practical applications.We report on the use of atmospheric pressure (open-air) blown arc discharge plasma to remove Li2CO3 species from the surface of tantalum doped LLZO (LLZTO). Open-air plasma technology offers a unique opportunity for surface treatments and functionalization by driving reactions and changes to surfaces that are not possible with other plasma or modification approaches. The proccess can also be scaled and implemented in continuous, roll-to-roll processing. In this method, compressed N2 gas at room temperature is passed through a high voltage zone (1 kV) at a controllable rate (20 liters per minute in this study), where it is ionized. The gas blows the plasma discharge out of a nozzle, and the LLZTO sample is placed at approximately ~1 mm away from the afterglow. The uniqueness of the open-air plasma system is the combination of energy sources which are generated: electrons and reactive species (radicals, metastables, and photons) are produced in combination with convective heat to rapidly transfer energy to enable ultrafast precursor conversion or post-treatment.The plasma treatment is carried out entirely in ambient conditions (RH ~40%) and is enabled by use of a custom-built metal shroud that is placed around the plasma nozzle to prevent moisture and carbon dioxide from reacting with the sample. After the plasma treatment, compressed gas is introduced through the shroud to mitigate reformation of the Li2CO3 as the sample is cooling. The surface chemistry of the LLZTO is characterized with X-ray photoelectron spectroscopy to quantify the rate at which Li2CO3 forms in ambient on a fresh LLZTO surface, as well as to assess the effect of plasma process parameters on removal of the Li2CO3 surface species. Specifically, we focus on 3 parameters: treatment time (5, 10, 20 or 40 minutes) and shroud gas chemistry (N2 or O2) used after treatment. The most substantial reduction in Li2CO3 contamination on the LLZTO surface is achieved after 20 minutes of exposure to open-air plasma with a N2 shroud gas.To examine the impacts of the plasma treatment on the interfacial resistance, we prepared symmetric cells comprising LLZTO pellets contacted with nonblocking electrodes made from lithium-tin alloy. We find that the open-air plasma treatment can significantly reduce the interface impedance to levels comparable with those measured when using mechanical polishing to remove the Li2CO3. Additionally, we find that the plasma treatment may also be used to modify the LLZTO structure and microstructure properties, and can also be used on other types of garnets. The results show that this platform demonstrates a path towards open-air processed solid-state batteries with improved interfacial and microstructural properties. Acknowledgments This work was supported by the National Science Foundation (NSF) through award TT-2234636. We acknowledge resources and support from the Advanced Electronics and Photonics Core Facility at Arizona State University. We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160.

Development of Barium-Modified Anodes for Anode-Supported Solid Oxide Fuel Cells Fueled with Ammonia

Solid oxide fuel cells (SOFCs) have attracted significant attention due to their high power generation efficiency and low pollutant emissions. Owing to the use of solid oxide electrolytes, SOFCs are usually operated at high temperatures of 600–800 oC, leading to favorable fuel flexibility. Although alternative fuels such as hydrocarbon fuels are superior to pure hydrogen in terms of storage, transportation, and energy density, by-products from the fuels, as exemplified by carbon, will deteriorate the performance and stability of anodes. Ammonia is promising as a carbon-free fuel for SOFCs because of its low cost, high hydrogen content and volumetric energy density, and ease in liquefaction and transportation under mild conditions. In ammonia-fueled SOFCs, the following reactions occur on the anode. 2NH3 → N2 + 3H2 (1) H2 + O2− → H2O + 2e− (2)Ammonia is decomposed to form nitrogen and hydrogen (1) and then the produced hydrogen is electrochemically oxidized (2). Thus, the anode material has to be active for ammonia decomposition as well as electrochemical hydrogen oxidation. The conventional Ni–YSZ electrode, however, is known to have low activity for ammonia decomposition. Therefore, it is necessary to enhance the catalytic activity of Ni–YSZ-based anode. It has been reported that the addition of basic components to Ni-based catalysts improve the ammonia decomposition activity. In this study, then, we modified the anode of a commercial anode-supported SOFC button cell (Elcogen, ASC–400B) with barium components by dropping a solution containing barium salts onto the electrode. The effect of additive component on the cell performance was evaluated. An aqueous solution of 2.0 M Ba(CH3COO)2–0.7 M C2H5NO2 was dropped onto the anode of anode-supported cell, followed by the heat-treatment at 800 oC for 2 h. This series of process was repeated several times so that desired amounts of barium component were loaded. Performances of the as-received cell and the fabricated cells were evaluated under hydrogen- and ammonia-containing fuels at 600–750 oC. For the as-received cell, cell performances were comparable in both fuels at 700 oC, indicating that the ammonia decomposition proceeded over the anode, while the performance were much different between H2 fuel and NH3 fuel at lower temperatures. On the other hand, comparable performances were obtained under both fuels at all the temperatures investigated. This suggested that the barium component promoted the ammonia decomposition sufficiently over the anode even at 600 oC. This work was supported by JST SICORP Program (Grant Number: JPMJSC2122).

Resolving the Impact of Sr and Ni Diffusion on Moderate-Temperature Sintered BaZr0.44Ce0.36Y0.2O3/SrZr0.4Ce0.4Y0.1O3–NiO Half-Cells Interface for PCEC/PCFC

Proton-conducting solid oxide electrolytes are poised to play a transformative role in steam electrolysis (PCEC) and fuel cells (PCFC) application at moderate to low temperatures, given their high ionic conductivity and inherent advantages in the gas flow configuration over traditional solid oxide cells. However, their refractory nature necessitates high sintering temperatures to ensure sufficient densification which also compromises the protonic conductivity of the material. This paper investigates the impact of Sr and Ni segregation at the electrode/electrolyte interface in BaZr0.44Ce0.36Y0.2O3 (BZCY(54)8/92) proton conductinghalf-cells using a combination of high-resolution STEM-EDX, 3-dimensional atom probe tomography (APT), and Raman spectroscopy to understand structural modifications of the half-cells upon sintering at varying temperatures. Our half-cells are fabricated using an industrially established inverse tape-casting route with NiO-SrZr0.5Ce0.4Y0.1O3-δ as the substrate, ensuring minimal warping and no cracks in the end-fired state. After co-sintering at 1300 °C for 5 hours, the tri-layered green tapes achieved dense, gas-tight electrolyte layers with a He leakage rate well within the threshold necessary for cell operation (~5 × 10–5 hPa dm3 (s cm2)–1). Using Ba0.5La0.5CoO3−δ as the air electrode demonstrates remarkable capabilities and endurance within the 450-600°C temperature range, achieving a peak power density of 1.13 W cm-2 at 600 °C in the fuel cell mode and a high current density of 1.5 A cm-2 at 1.3 V in the electrolysis mode. Furthermore, It is apparent from the latter techniques that upon sintering above 1350 °C, the electrolyte material undergoes evident structural changes with new crystal defects that affect the perovskite host. Ni diffuses into the electrolyte layer and gets dissolved in the bulk while some segregate at the grain boundary, below monolayer coverage in either case (GNi 2.0 at%/nm2).Acknowledgment: The authors gratefully acknowledge financial support through JSPS KAKENHI Grant-in-Aid for Scientific Research (C), No. 19K05672, NEDO (International collaboration) JPNP20005, JSPS Core-to-Core (Solid Oxide Interfaces for Faster Ion Transport) and WPI-I2CNER sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

Effects of Metal Interlayers between Li7La3Zr2O12 Solid Electrolyte and Li Anode on Li Dissolution/Deposition Cycles

Garnet-type oxide solid electrolyte, Li7La3Zr2O12 (LLZ), has relatively high ionic conductivity and high reduction stability for metallic Li. It is considered as a promising candidate for use in all-solid-state batteries. However, practical applications of LLZ are currently hindered by large interfacial resistances at LLZ electrolyte/Li electrode interfaces. One of the reasons for this issue is believed to be the poor wettability of metallic Li on the LLZ surface, resulting in a poor contact interface. To reduce the interfacial resistance, intermediate layers have been introduced.[1-3] In this study, we investigated the effects of intermediate layers such as Sn, In, Bi, etc., which alloy with Li, at the LLZ/Li interface on Li dissolution/deposition reactions.Li6.7La3Zr1.7Ta0.3O12 (LLZ, φ10 mm, 1 mmt) pellets, produced by TOSHIMA Manufacturing Co., Ltd., were used as solid electrolytes. The LLZ surfaces were polished with sandpaper in an Ar-filled glovebox. Bi interlayers were deposited on both sides of the LLZ pellets by thermal evaporation with varying thickness. Subsequently, Li electrodes (10 μmt) were deposited to obtain Li symmetric cells (Li|LLZ|Li). Electrochemical impedance measurements and Li dissolution/deposition tests of the Li|LLZ|Li cells were performed at the temperature of 60°C. In the dissolution/deposition test, the current density was set to 0.2 and 0.5 mA/cm2, and the direction of current flow was switched every 0.5 hours. After the Li dissolution/deposition test, cross-sections of the LLZ pellet were observed by scanning electron microscope (SEM) and analyzed by energy dispersive X-ray spectroscopy (EDX).Figure 1 shows Li dissolution/deposition cycle performance in Li|LLZ|Li cells both with and without a Bi interlayer. The Li|LLZ|Li cell without the interlayer exhibited stable cycle operation for 50 cycles at a current density of 0.2 mA/cm². However, when the current density was increased to 0.5 mA/cm², the overvoltage increased rapidly due to the formation of voids, which decreased the contact area at the LLZ/Li interface. On the other hand, when an 80 nm Bi interlayer was introduced, stable cycling was observed even at a current density of 0.5 mA/cm² with no increase in overvoltage during cycling. EDX analysis of the LLZ/Li interface after the dissolution/deposition tests revealed the presence of the Bi layer at the interface, as shown in Fig. 2, which remained even after repeated dissolution/deposition cycles. It is believed that the Bi layer remains at the LLZ/Li interface after dissolution/deposition cycles, thereby maintaining a good contact interface between LLZ and Li, and improving cycle life. Acknowledgments This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 22H01967.

(Invited) Development of Bulk-Type Solid-State Batteries with Oxide-Based Solid Electrolytes

ALCA-SPRING of Japan Science and Technology Agency (JST) was a 10-year project aiming at development of next-generation batteries. About 170 researchers in 70 laboratories all over Japan joined the project, and we had been working on solid-state batteries with oxide-based solid electrolytes.Solid-state batteries are regarded as promising next-generation batteries because they provide high reliability to lithium-ion batteries. Batteries for vehicles and energy storages of renewables are necessary to realizing a low carbon society, where long lifetime is required. High performance has been achieved in solid-state batteries with sulfide-based solid electrolytes, and they are in the final stage toward vehicle applications. However, sulfide-based solid electrolytes have a drawback: they are unstable and hygroscopic materials. Thus, the batteries should be assembled under strictly-controlled atmosphere, which raises the production cost. In addition, sulfide electrolytes generate harmful and corrosive H2S, when they are decomposed in ambient air. Therefore, replacement of the sulfides with stable oxides is strongly required; however, performance of oxide-based solid-state batteries is much lower than that of the sulfide systems.The highest conductivities having reported for oxides are of the order of 10-3 S cm-1, which is lower than that of sulfides by one order of magnitude; however, they are almost comparable to lithium-ion conductivities of organic-solvent liquid electrolytes used in the current lithium-ion batteries: although the conductivity of organic-solvent liquid electrolytes is of the order of 10-2 S cm-1, the transport number for lithium ion is less than 0.5, which suggests that the ionic conductivity is not a dominant reason for the low performance of oxide-based batteries.High resistance of oxide-based solid-state batteries originates from the interfaces between the battery materials. Sulfide-based solid electrolytes are deformable substance, and thus connection between the battery materials can be achieved only by cold pressing in sulfide-based batteries. On the other hand, oxide-based solid electrolytes are hard materials, and sintering process is necessary to connect the electrolyte particles with each other. In addition, oxide-based solid electrolytes, especially those showing conductivities around 10-3 S cm-1, often show high grain boundary resistance even after the sintering. The grain boundary resistance can be lowered by increasing sintering temperature, indeed. However, it does not lead to high performance in the solid-state batteries, because the solid electrolytes are in contact with active materials in the batteries, and the high-temperature sintering induces mutual diffusion between the solid electrolytes and active materials, which forms resistive impurity layers at the interfaces. Therefore, it is necessary to lower sintering temperature of the oxide solid electrolytes for realizing oxide-based solid-state batteries.We employed three approaches to lower the sintering temperature of garnet-type solid electrolytes: reducing particle size of the solid electrolyte, control the electrolyte composition to form molten phases during sintering, and forming a composite electrolyte with a sinterable solid electrolyte. These approaches have lowered the sintering temperature and enabled us to assemble bulk-type solid-state batteries that are operatable at room temperature.

Effects of Polymer Electrolytes on the Interfacial Contact with Positive Electrode Materials for β”-Al2O3 Based All-Solid-State Sodium Batteries

Introduction All-solid-state sodium batteries (ASSSBs) have attracted large attentions as one of the alternatives to lithium-ion batteries from the viewpoint of the relative abundance and low cost of sodium and safety. Oxide-based inorganic solid electrolytes have been proposed as a promising candidate among various solid electrolytes because of their high ionic conductivity and electrochemical stability, but their hardness causes increased charge transfer resistance at the interface between electrode and solid electrolyte. A major key to the practical application of ASSSBs is to enhance the interfacial contact between active materials and electrolytes. In contrast, polymer electrolytes are relatively soft and easy to contact at the interface, although their ionic conductivity is inferior. Therefore, composite electrolytes that appropriately mix inorganic solid electrolytes and polymer electrolytes have been proposed, and many studies have been conducted, but the ionic conduction paths are so complex that there is no clarity regarding the effect of the polymer electrolyte on the resistance at the interface.In this study, we introduced a softer polymer electrolyte than inorganic solid materials between Na3V2(PO4)3 positive electrode / Na-β"-Al2O3 solid electrolyte to reduce the interfacial resistance by the enhancement of the solid | solid contact. This allows a detailed analysis of the resistive components of composite electrolyte systems with complex ionic conduction paths. Experimental Na3V2(PO4)3 (NVP), which has been reported to exhibit sodium-ion insertion/de-insertion at relatively high 3.4 V (vs. Na+/Na) when used with liquid electrolyte and good cycle characteristics, was used as the positive electrode material. [1] Acetylene black was added as conductive material and polyvinylidene fluoride (PVdF) as binder to form composite electrode. The polymer electrolyte was prepared as the dry polymer film by casting an acetonitrile solution containing sodium bis(fluorosulfonyl)amide (NaFSA) and polyethylene oxide (PEO), followed by completely volatilizing the solvent. [2] The polymer electrolyte membrane was introduced between the oxide solid electrolyte Na-β"-Al2O3 and positive electrode, and Na metal was used as a negative electrode to construct a two-electrode cell. Electrochemical impedance spectroscopy (EIS) was performed with an AC amplitude of 10 mV in a frequency range of 1.0 MHz–10 mHz, after constant current charge-discharge measurements (measurement temperature: 50°C, current density: 5.9 mA g−1, voltage range: 2.7~3.7 V) to investigate the effect of the polymer electrolyte membrane on the junction interface between positive electrode and solid electrolyte. Results and discussions The charge-discharge curves without and with PEO dry polymer electrolyte membrane are shown in Fig. 1(a)(b), respectively. The reversible reaction did not occur in the cell without the polymer electrolyte, whereas a reversible capacity of more than 70 mAh g−1 with a coulombic efficiency of more than 99% was obtained in the cell with the polymer electrolyte.The Nyquist plots of the potential dependence are shown in Fig. 1(c)(d). In the cell without polymer electrolyte, the very large semicircle with potential dependence is observed. Since the bulk and grain boundary resistance of Na-β"-Al2O3 and the charge transfer resistance at Na-β"-Al2O3 / Na interface was very small, this semicircular component was attributed to the charge transfer resistance at Na-β"-Al2O3 / NVP interface. This result suggests that the frequency factor is very small because Na-β"-Al2O3 / NVP interface is the solid | solid contact. On the other hand, in the cell with polymer electrolyte, the overall resistance is greatly reduced. The main components of the semicircles observed in the high frequency up to 5 kHz, the medium frequency from 5 kHz to 1 Hz, and the low frequency region from 1 Hz were attributed to the bulk resistance of the polymer electrolyte, the ion transfer resistance at the polymer electrolyte / Na-β"-Al2O3 interface, and the charge transfer resistance at the polymer electrolyte / NVP interface, respectively. The results suggest that the introduction of the polymer electrolyte increases the frequency factor by the enhancement of the solid | solid contact at Na-β"-Al2O3 / NVP composite electrode junction interface. Conclusion The effect of polymer electrolytes on Na3V2(PO4)3 positive electrode / Na-β"-Al2O3 junction interface of ASSSBs was investigated. In a composite electrolyte system with complex ionic conduction paths, the introduction of a polymer electrolyte as a membrane allows us to analyze the effect of the polymer electrolyte on the junction interface.

Combined Impedance and Electron Paramagnetic Resonance Spectroscopy for Investigating the Dynamics of Li/Solid Li‐ion Conductor Interfaces

AbstractElectrochemical impedance spectroscopy (EIS) is a widely used tool for the electrochemical characterization of all‐solid‐state batteries (ASSBs) with Li‐metal anodes. However, an unambiguous interpretation of the observed impedance response often requires additional independent information on the actual interfacial phenomena obtained. The measurement methodology presented in this study allows to conduct electron paramagnetic resonance (EPR) spectroscopy and EIS concurrently. Therefore, the informative power of EIS experiments can be significantly improved via monitoring of structural changes of paramagnetic lithium at the electrochemical interface. As the solid‐electrolyte‐lithium interface is a critical part of all‐solid‐state batteries, this study employs a model oxide solid electrolyte in contact with lithium metal. During the polarization of the cell with thin evaporated lithium electrodes, the ratio between positive and negative peaks (a/b) of the EPR signal momentarily rises, which indicates an accumulation of lithium on one side of the electrolyte. The peak ratio a/b then drops abruptly, accompanied by current irregularities. Both are indicative of a diminishing contact area, and as a result, finer lithium morphologies form. Shortly after that, a contact loss is observed. The change of the EPR signal shape before cell breakdown can hence be associated with the worsening Li‐electrolyte contact, providing a tool for physical in‐situ cell diagnostics.

The effect of the addition of 2-dimensional manganese-oxide to ceria on density and electrical conduction

Gadolinium-doped ceria (GDC), a prominent solid electrolyte for solid oxide fuel cells, is known to have poor sinterability and requires high temperatures for densification. To address this issue, transition-metal oxides have frequently been used as sintering aids to enhance the sinterability of ceria. In this study, a two-dimensional (2D) sintering aid, distinct from conventional three-dimensional structures, was used. Specifically, 2D MnO2 with nanometer-thickness, produced by a chemical exfoliation method, was utilized to improve the sinterability of GDC. GDC with the sintering additive (2D MnO2) was sintered at various temperatures (800–1200 °C). The addition of 2D MnO2 successfully reduced the sintering temperature. As a result, the sample with high relative density demonstrated high oxygen ionic conduction, even though it was sintered at a lower temperature.

Multivalent metal perovskite YbCoO3 as a novel proton-conducting electrolyte for solid oxide fuel cells

For solid oxide fuel cells, an electrolyte with good stability and high ion conductivity is highly desired but difficult to obtain. Recently, a novel hydrogenation mechanism other than water dissociation have been reported in multivalent metal-based oxides, which provide a new route to increase proton concentration as well as proton conductivity. In this computational study, we propose the multivalent metal perovskite YbCoO3 as a promising proton-conducting electrolyte due to its good thermodynamic and chemical stability, semiconductor characteristics, high-concentration proton incorporation, and low proton migration barrier. Our findings also reveal that charge compensation of the multivalent metal is crucial for the high-concentration proton incorporation. On the other hand, both the shortening of the O-O distance and the Co-H repulsion play key roles in determining the energy barrier to proton migration, where local structural deformations are responsible for facilitating intra- and inter-octahedron proton transfer in YbCoO3. Our results might assist in the development of high-performance proton-conducting electrolytes for advanced solid oxide fuel cells.

Revealing the Interplay of Local Environments and Ionic Transport in Perovskite Solid Electrolytes.

Solid-state ionic conduction is significantly influenced by bottleneck sizes, which impede ion diffusion within solid lattices. Using aberration-corrected scanning transmission electron microscopy and multislice electron ptychography, we directly observed that increased La occupancy in the perovskite solid electrolyte Li0.5La0.5TiO3 correlates with reduced bottleneck sizes formed by four oxygen atoms connecting neighboring A-site cages. This correlation was also confirmed in local aperiodic regions, where smaller bottleneck sizes due to increased La occupancies affect the directionality and dimensionality of the Li+ ion conductivity. Furthermore, while prior studies have focused on averaged Li+ ion diffusion across different bottleneck areas or chemical environments, by devising a molecular dynamics (MD)-based methodology, we quantify the diffusivity of Li+ ions through specific bottleneck regions. Atomistic simulations, including nudged elastic band calculations and this MD-based methodology, revealed that larger bottleneck sizes correlate with smaller local migration barriers and higher local diffusivity. This study elucidates the relationship among local chemistry, lattice structure, and Li+ ion transport, providing insights for the design of advanced oxide solid electrolytes.

Effects of Sintering Temperature on the Electrical Performance of Ce0.8Sm0.2O1.9–Pr2NiO4 Composite Electrolyte for SOFCs

Abstract Novel composite electrolyte Sm0.2Ce0.8O1.9–Pr2NiO4 (SDC–PNO) for solid oxide fuel cells (SOFCs) was prepared. The effects of sintering temperature on the performance of SDC–PNO composite electrolyte were studied. X-ray diffraction (XRD) analysis confirms that the composite samples sintering at different temperatures contain two pure phases corresponding to SDC and PNO, respectively. It indicates that there is no interfacial reaction between Sm0.2Ce0.8O1.9 and Pr2NiO4, and the composite electrolyte keeps regular grain boundaries after long-term operation. The results of scanning electron microscopy (SEM) show that a dense structure can be formed after sintering at 1250 °C. The electrical properties were tested in air by electrochemical impedance spectroscopy technique in the temperature range of 400–800 °C. Results show that an appropriate increase of the sintering temperature can effectively increase the conductivity of SDC–PNO composite electrolyte, and the maximum conductivity of 0.102 S/cm can be obtained in the sample sintered at 1250 °C and tested at 800 °C. In summary, the electrical conductivity of Sm0.2Ce0.8O1.9 can be significantly increased by adding Pr2NiO4, and SDC–PNO composite electrolyte is a promising electrolyte for solid oxide fuel cells.