Nobuyuki Imanishi’s research while affiliated with Mie University and other places

This special issue of Electrochemistry features selected papers from the 65th Battery Symposium in Japan, held in November 2024, which marked a full return to in-person meetings after the COVID-19 pandemic. The symposium saw record participation, highlighting strong interest in lithium-ion batteries, all-solid-state batteries, and emerging chemistries such as sodium-ion and lithium–sulfur systems. Progress in electrolyte design, interfacial science, and data-driven approaches was also emphasized. Additionally, several contributions focused on fuel cell research, reflecting the expanding scope of electrochemical energy technologies. We hope this collection stimulates further innovation in energy storage and conversion.

PEO-based solid polymer electrolytes (SPEs) exhibit a narrow potential window and undergo decomposition when in contact with 4V-class cathode active materials. The objective of this study was to investigate the stability of the contact interface between the PEO electrolyte and the positive electrode material, coated with various inorganic compounds on the surface. The time variation for the impedance of the interface between the PEO-based SPE and the LiNi0.6Mn0.2Co0.2O2 (NMC) coated with various inorganic compounds was measured. The following coated materials were tested: the ionic crystal LiF, the covalent oxide Al2O3, the ferroelectrics BaTiO3 and LiNbO3, the lithium ion conductor La0.45Li0.45TiO3 (LLTO), and the ferromagnetic and highly electron conductive La0.7Sr0.3MnO3 (LSMO). They were coated on the surface of NMC electrode using the sputtering method. The impedance of the Li/SPE/coated NMC cell was measured while maintaining a potential of 4.2 V. In the non-coated NMC, the SPE/NMC interface resistance exhibited a notable increase over time. In all the coated NMCs, the increase in the interface resistance was suppressed, indicating an improvement in the interface stability. In particular, the interface resistance of LiF and Al2O3 remained unchanged for 40 h at 4.2 V, thereby demonstrating the formation of a highly stable SPE/NMC interface. The charge/discharge measurements of the Li/SPE/coated NMC cell revealed that the capacity retention rate of NMC coated with LiF and Al2O3 was significantly enhanced in comparison to that of NMC without coating.

The development of high-energy-density batteries with lithium (de-)intercalation reaction is one of the most promising solutions to realize electric vehicles and power storage system applications with long-term stability. The cathode materials exhibit smaller capacity than the anodes, which limits the energy density of the battery cells [1]. Consequently, the development of new cathode materials with higher capacity than previously reported is required. In the lithium 3d transition metal system, Li-rich materials based on tetrahedrally-coordinated structure, ex . Li 5 FeO 4 and Li 6 CoO 4 , exhibited a reversible two lithium (de-)intercalation reaction over 350 mAh/g with mainly anionic redox [2,3]. The capacity was much higher than the practical cathode materials. Therefore, these Li-rich transition metal oxides with tetrahedrally coordinated structure have potential as high capacity cathode materials. In this study, Li-rich cathode materials based on tetrahedrally-coordinated structure were synthesized, and electrochemical properties in the lithium battery cell were characterized by electrochemical and structural investigations. One of the Li-rich transition metal oxides; Li 5+ x Fe 1- x Mn x O 4 was synthesized by solid state reaction [4]. Phase identification by X-ray diffraction (XRD) measurement was conducted using a diffractometer with Cu Kα radiation. Observation of particle morphology and elemental distribution was performed using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). The prepared active material samples were mixed with Ketjen black as conductor and polytetrafluoroethylene as a binding agent using an agate mortar to prepare a composite electrode. A 2032-type coin cell was prepared using Li metal as the anode and 1 mol/dm ³ LiPF 6 /ethylene carbonate:diethylene carbonate = 50:50 vol% as the electrolyte. Charge-discharge measurements were conducted in the voltage range of 1.7−3.9 V at 50 °C. The ball-milling process with acetylene black (AB) was applied before electrode fabrication to decrease the particle size of the prepared samples and achieve homogeneous electrical conduction in the composite electrode. The active material and AB were mixed with ZrO 2 balls in a ZrO 2 pot. A similar electrode fabrication process was conducted using a mixture of active material and AB. An antifluorite structured Li 6 MnO 4 -type phase with the small starting materials was obtained in the range between 0.6 and 1.0 composition. The amount of residual reagents decreased with increasing excess lithium content in the starting mixture. A single phase with an Li 6 MnO 4 -type structure was obtained for an excess lithium composition of 10 mol%. The particle size of the synthesized samples was about 50 µm, which is quite large compared with conventional cathode materials for lithium batteries. The as-synthesized Li 5.6 Fe 0.4 Mn 0.6 O 4 exhibited first charge and discharge capacities of 680 and 300 mAh/g, respectively, with a large irreversible capacity of 380 mAh/g (coulombic efficiency = 44%). This indicates that the new iron-manganese composition with ordered antifluorite-type structure is lithium insertion/extraction-active although a large irreversible electrode reaction was observed. The ball milling process with AB provided that the particle size of the sample was decreased from ca. 50 µm to 1 μm, and a uniform distribution of Mn, Fe and C signals was realized for the mixture without any local signals. This suggests that a fine and homogeneous composite material composed of the active material and AB was obtained by the ball-milling process. The first discharge capacity of the Li 5.6 Fe 0.4 Mn 0.6 O 4 electrode treated by ball-milling was 450 mAh/g. The coulombic efficiency was 60%. A reversible reaction continued to proceed with ca. 200 mAh/g after the following cycles. The differential capacity plots for the ball-milled Li 5.6 Fe 0.4 Mn 0.6 O 4 ( x = 0.6) at the second, third, and tenth cycles exhibited two pair of redox peaks at around 2.5 V and 3.2 V, which reveals that the lithium (de-)intercalation reaction successfully occurred after the 2nd cycle. A new iron-manganese based ordered antifluorite-type compound has potential as a high-capacity oxide cathode material and thus further investigation is required. Phase formation and electrochemical properties of the Li-rich materials based on tetrahedrally-coordinated structure including the other cation will also be discussed in the presentation. Acknowledgement: This work was financially supported by Tokuyama Science Foundation, Nippon Sheet Glass Foundation for Materials Science and Engineering, and JSPS KAKENHI Grant Number 24K01582. References: M. Tarascon, M. Armand, Nature 2001 , 414, 359-367. Hobayashi, et al., Adv. Energy Mater. 2023 , 13, 2203441. Kobayashi, et al., ACS Appl. Mater. Interfaces 2020 , 12, 43605-43613. R. Goto, et al., Chem. Lett. 2024 , 53(4), upae046.

Rechargeable lithium-air batteries are a promising high energy alternative to lithium-ion batteries. Two main types of lithium-air batteries are under development: non-aqueous lithium-air batteries and aqueous lithium-air batteries. The non-aqueous lithium-air batteries consist of a lithium anode and a carbon-based air electrode in a non-aqueous electrolyte, and offer high theoretical energy density of 3505 Wh kg ⁻¹ and 3436 Wh L ⁻¹ . Over the past two decades, many researchers have developed non-aqueous lithium-air batteries. However, no technological breakthrough has been found to achieve these high energy densities. Aqueous lithium-air batteries consist of a lithium metal anode, a non-aqueous electrolyte, a lithium-ion conductive solid electrolyte that is stable in water, an aqueous electrolyte, and a carbon-air electrode. The cell reaction of this system is as follows ¹ ; 4 Li + O 2 + H 2 O ⇌ 4 (LiOH‧H 2 O) (1) The theoretical energy densities of this system based on equation (1) are 1910 Wh kg ⁻¹ and 2004 Wh L ⁻¹ . Several problems in non-aqueous systems, such as electrolyte decomposition by reactive oxygen species and ingress of atmospheric moisture, do not occur in aqueous systems. In addition, LiOH, the discharge product of the air electrode, dissolves in the catholyte and does not interfere with the electrode reaction. NASICON-type lithium ion conductor of Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) has been mainly used for lithium ion conductive solid electrolyte separators, which is the key material for aqueous lithium-air batteries. Its ionic conductivity ranges from 10 ⁻³ to 10 ⁻⁴ S cm ⁻¹ at room temperature. The heavy weight of the solid electrolyte reduces the gravimetric energy density, but the degree of reduction is determined by the amount of electricity generated. ² The weight ratio of the LATP (100 μm thick) to the total cell weight (excluding the container) decreases from 0.55 at 10 mAh cm ⁻² to 0.42 at 20 mAh cm ⁻² and 0.25 at 50 mAh cm ⁻² . The calculated energy densities are 551, 830, and 1190 Wh kg ⁻¹ for capacities of 10, 20, and 50 mAh cm ⁻² , respectively. The area capacities reported in the literature are usually less than 10 mAh cm ⁻² . ³ Therefore, to improve the energy density of aqueous lithium-air batteries, the weight of the solid electrolyte must be reduced. However, LATP thin films less than 50 μm thick are not mechanically tough, making it difficult to use them as separators for these batteries. Recently, Toray developed a water-impermeable lithium-ion conductive polymer film. ⁴ The film is less than 10 μm thick and the ionic conductivity is 3×10 ⁻⁵ S cm ⁻¹ at 25 °C. Using an H-type cell, pure water was placed in one chamber and a LiCl solution in the other, with this film placed in between. Since no chloride ions were observed in the pure water chamber for 200 hours, no water permeation was considered to have occurred. The test cell system consisted of a lithium metal anode, 4.5 M LiN(CF 3 SO 2 ) 2 in 1,2-diethoxyethane (DEE) anode, this separator, 1M LiOH-10 M LiCl, and a carbon air electrode. All experiments were conducted at 25 °C. A stable open circuit voltage (OCV) of 3.05 V was observed. This OCV is comparable to calculated values and those reported for aqueous lithium-air batteries with LATP separators. ⁵ The total cell resistance was 350 Ω and the cell area was 1 cm ² . The battery was successfully operated for 100 cycles at 0.2 mA cm ⁻² and 0.2 mAh cm ⁻² . References N. Imanishi, Y. Takeda, O. Yamamoto, Electrochemistry , 80 , 706 (2012) M.S. Park, S.B. Ma, D.J. Lee, D. Im, S.-G. Doo, O. Yamamoto, Scientific Reports , 4 , Article number: 3815 (2014) N. Imanishi, O. Yamamoto, Materials Today Advances , 4 , Article number: 100031 (2019) https://www.toray.co.jp/news/details/20220530165632.html S. Sunahiro, M. Matsui, Y. Takeda, O. Yamamoto, N. Imanishi, J. Power Sources , 262 , 338 (2014) Acknowledgments This work was supported by the program to Develop and Promote the Commercialization of Energy Conservation Technologies to Realize a Decarbonized Society of the New Energy and Industrial Technology Development Organization (NEDO; Japan) Grant Number JPNP 21005.

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, Li 2 Ru 0.8 S 0.2 O 3.2 [1] , having high electronic and ionic conductivity. Therefore, we focused on the Ru containing Li-rich layered oxide, Li 2 RuO 3 and Li 2 Mn 0.4 Ru 0.6 O 3 . 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. Li 2 TiO 3 as a buffer layer was introduced between the cathode material and the solid electrolyte to suppress side reactions. Li 2 TiO 3 has a similar structure to Li 2 RuO 3 , 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 Li 2 RuO 3 and LMRO were used as cathode materials, Li 6.25 Ga 0.25 La 3 Zr 2 O 12 (LLZ-Ga) as a solid electrolyte, and Li 2 TiO 3 as a buffer layer. The buffer layer was introduced into the cathode surface by solid-phase and liquid-phase methods. LiOH-H 2 O and TiO 2 were mixed and sintered with the cathode material in the solid-phase method. LiOH-H 2 O 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, Li 5.5 PS 4.5 Cl 1.5 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 La 2 Li(RuO 6 ) 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, Li 2 TiO 3 was not distributed uniformly on the particle surface of the cathode material. In the liquid-phase method, Li 2 TiO 3 covered the cathode surface more uniformly and thinner than that formed in the solid-phase method. Both Li 2 RuO 3 and LMRO retained their original structure, while the lattice parameter of LMRO changed, indicating that LMRO formed the solid solution with Li 2 TiO 3 . 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.

Introduction In recent years, xEVs have been spreading rapidly. In order to extend the driving range, automotive batteries with high energy density and high safety exceeding lithium-ion batteries are required. Solid-state fluoride batteries (SSFBs), in particular, are currently attracting attention as one of the innovative storage batteries with a high theoretical energy density due to multi-electron reactions. Since organic electrolytes are not used, SSFBs have no risk of leakage and firing and a high level of safety. The fluoride ion is expected to be suitable for ion diffusion at a high rate because it is monovalent and has a small atomic mass. However, the operating temperature of SSFBs is still high at about 150 ˚C. The challenges of SSFBs to lower the operating temperature are especially in electrolytes. Currently, PbSnF 4 compounds and tysonite-type fluorides are representative examples of fluoride ionic conductors. However, these electrolytes have issues such as a narrow electrochemical potential window or insufficient ionic conductivity. New fluoride ionic conductors with high ionic conductivity and wide electrochemical potential windows are required. In this study, we focused on K 2 BiF 5 -type fluoride with one-dimensional chains of edge-shared BiF 7 polyhedra acting as an ionic conductive pathway. Previously we demonstrated that the ionic conductivity of K 2- x Rb x BiF 5 (0.0 ≤ x ≤ 0.4) increases with increasing Rb content and the activation energy decreases. K 2- x Rb x BiF 5 with x = 0.4 exhibited the ionic conductivity of 1.0 × 10 ⁻⁵ S-cm ⁻¹ at 150°C and the activation energy of 0.68 eV. The expansion of bottle neck size by the substitution of Rb with larger ionic radius than that of K is effective to enhance fluoride ion conductivity. To further improve the ionic conductivity we synthesized Sn-substituted K 2 BiF 5 to introduce fluorine defects and evaluated their ionic conductivity. Experimental K 2 Bi 1-x Sn x F 5-x with x = 0, 0.05 and 0.10 were synthesized using potassium fluoride KF, bismuth fluoride BiF 3 , and tin fluoride SnF 2 as starting materials. The mechano-chemical treatment was carried out at 600 rpm for 3 - 24 h using a FRITSCH planetary mill with a mill jar and balls made of zirconia. The initial loading of the mixture was 1.5 g. K 2 Bi 1- x Sn x F 5 with x = 0, 0.05, and 0.10 were synthesized by mechano-chemical treatment and subsequent heating. The ionic conductivity was evaluated by the AC impedance measurement using Au sputted electrode. Results and discussion The XRD measurements demonstrated that the target phase was obtained after ball-milling for the sample with x = 0 and 0.05, while a small amount of unknown phase was contained. After heating, a single phase of K 2 Bi 1- x Sn x F 5 was obtained. For K 2 Bi 1- x Sn x F 5 with x = 0.10 K 2 BiF 5 phase did not appear after ball-milling for 3-12 h but appeared after heating at 200 ˚C for 10 h twice. The sample contained a large amount of impurity phase. Considering the lattice parameter of K 2 Bi 1- x Sn x F 5 , since the Sn ²⁺ ion has a smaller ionic radius than that of the Bi ³⁺ ion, the lattice is expected to shrink with Sn substitution. However, the lattice parameters of a -, b - and c -axes and the lattice volume did not change systematically with Sn content. It would be due to compositional deviation caused by the formation of impurities. The ionic conductivity was evaluated by the AC impedance measurement. With increasing Sn content in the K 2 Bi 1- x Sn x F 5 , the ionic conductivity was increased and the activation energy was decreased. K 2 Bi 1- x Sn x F 5 with x =0.10 showed the ionic conductivity of 3.59× 10 ⁻⁸ S-cm ⁻¹ at 25°C and 2.05 × 10 ⁻⁵ S-cm ⁻¹ at 150°C. The activation energy was 0.55 eV. They indicate that aliovalent Sn substitution is effective in improving the ionic conductivity of K 2 BiF 5 . Acknowledgements This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

The lithium metal anode is the best candidate for high energy density batteries because of its high specific capacity and low negative potential. Rechargeable lithium metal batteries (RLMB) have not yet been commercialized. The key factors that limit the practical use of RLMB are the formation and growth of lithium dendrites during the lithium deposition process and the reaction of the lithium anode with the organic solvent of the electrolyte, quantified by the Columbic efficiency (CE). To suppress the lithium dendrite formation and to improve CE, many approaches such as the formation of a protective layer on the lithium electrode and the use of additives to the electrolyte have been proposed. In this study, the effect of a thin cellulose film to improve CE of lithium deposition and stripping on the lithium electrode was examined. The cycle performance of a Li/Li symmetrical cell with a cellulose and polyethylene composite separator was examined for a carbonate electrolyte and an ether electrolyte. The improvements of CE were observed for both electrolytes with the cellulose film separator. The improvement could be explained by the good wettability of the cellulose film separator with the electrolyte.

Garnet-type solid electrolytes stand out as promising Li-ion conductors for the next-generation batteries. It has been demonstrated that the inherent properties of garnets can be tailored by introducing various dopants into their crystal structures. Recently, there has been a growing interest in the concept of high entropy stabilization for materials design. In this study, we synthesized high-entropy garnets denoted as Li6La3Zr0.7Ti0.3Ta0.5Sb0.5O12 (LLZTTSO), wherein Ti, Sb, and Ta occupy the Zr site. The formation of the cubic garnet phase in LLZTTSO was confirmed through X-ray diffraction (XRD), and the resulting lattice parameter agreed with predictions made using computational methods. Despite the substantial porosity (relative density 80.6%) attributed to the low sintering temperature, LLZTTSO exhibits a bulk ionic conductivity of 0.099 mS cm⁻¹ at 25°C, and a total ionic conductivity of 0.088 mS cm⁻¹, accompanied by an activation energy of 0.497 eV. Furthermore, LLZTTSO demonstrates a critical current density of 0.275 mA cm⁻² at 25°C, showcasing its potential even without any interfacial modification.

This study explores multi-component garnet-based materials as solid electrolytes for all-solid-state lithium batteries. Through a combination of computational and experimental approaches, we investigate the thermodynamic and structural properties of lithium lanthanum zirconium oxide garnets doped with various elements. Applying density functional theory, the influence of dopants on the thermodynamic stability of these garnets was studied. Probable atomic configurations and their impact on materials’ properties were investigated with the focus on understanding the influence of these configurations on structural stability, phase preference, and ionic conductivity. In addition to the computational study, series of cubic-phase garnet compounds were synthesized and their electrochemical performance was evaluated experimentally. Our findings reveal that the stability of cubic phase in doped Li-garnets is primarily governed by enthalpy, with configurational entropy playing a secondary role. Moreover, we establish that the increased number of doping elements significantly enhances the cubic phase’s stability. This in-depth understanding of materials’ properties at atomic level establishes the basis for optimizing high-entropy ceramics, contributing significantly to the advancement of solid-state lithium batteries and other applications requiring innovative material solutions.