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Development of lithium-ion batteries for automotive applications such as electric vehicles (EVs) has been focused on enhancement of energy density, power, safety, and life. In particular, all-solid-state batteries have been extensively studied in order to respond to these demands. However, the solid electrolytes such as sulfide or oxide materials have important subjects for practical uses. Sulfide solid electrolytes have low chemical stability in air and low mechanical strength for thin separator. Oxide solid electrolytes have low ionic conductivity and large resistance at interfaces between active material and solid electrolyte. We proposed a thin hybrid electrolyte system instead of solid electrolytes [1]. The thin hybrid electrolyte consisting of lithium-ion conducting solid particles of a cubic garnet-type Li7La3Zr2O12 (LLZ) and a gel polymer electrolyte exhibits smaller activation energy for ionic conductivity than a gel polymer electrolyte without LLZ. The fabricated cells using the hybrid electrolyte show high-rate charge-discharge performance [1, 2]. We have also investigated electrochemical properties of other hybrid electrolytes consisting of NASICON-type Li-Al-Ti-P-O (LATP) particles wetted with liquid electrolytes of LiPF6-PC/DEC. Figure 1 shows fast-charge performance of 1 Ah TiNb2O7/LiNi0.5Co0.2Mn0.3O2 cells using the hybrid and the liquid electrolyte at 10 C rate. The charge performance of the cell using the hybrid electrolyte was significantly superior to that of the cell using the liquid electrolyte, which indicates the existence of LATP particles contributing to enhancement of migration of lithium ions in the hybrid electrolyte. Figure 2 shows high-rate discharge properties of the cells using the hybrid and the liquid electrolytes containing different LiPF6 concentration. The voltage decline of the cells using the hybrid electrolytes were more gradual than that of the cell using the liquid electrolytes with LiPF6 concentration of 0.5 M and 1 M. The difference between hybrid electrolytes and liquid electrolytes was clearly observed in the cell with LiPF6 concentration of 0.5 M. In the case of a high LiPF6 concentration of 2 M, the voltage decline of the cells using the hybrid and the liquid electrolytes showed almost no difference. Electrochemical kinetics on the discharge were more effectively enhanced by the existence of LATP particles wetted with the liquid electrolyte containing smaller amount of LiPF6. From these results, we considered that lithium-ionic conducting solid particles in hybrid electrolyte have effects to enhance lithium-ion conductivity by promoting lithium-ion migration on the surface of the solid particles and the amount of lithium-ions for charge transfer on the electrodes. However the detail of this mechanism for effects has not been cleared yet. In order to understand the phenomenon, it is determined that properties of lithium-ion conducting and electrochemical performances of hybrid electrolytes with different ratio of solid particles and liquid electrolyte and so on. In this presentation, we will discuss the role of lithium-ion conducting solid particles in hybrid electrolytes. References [1] K. Yoshima, Y. Harada and N. Takami, J. Power Sources, 302,283-290(2016). [2] N. Takami, K. Yoshima and Y. Harada, J. Electrochem. Soc., 164,A6254(2017). Figure 1

Introduction For improving operational life and safety of lithium-ion battery, 1.5 V vs. Li/Li+ materials such as spinel Li4Ti5O12 (LTO) have been extensively studied as good alternatives to graphite as the anode material. Several kinds of isostructural materials such as Li2AB6O14 (A=M(II), B=M(IV)) have been investigated as a titanium based anode [1-2]. These compounds have a three-dimensional framework of edge and corner shared BO6 octahedra and lithium can be inserted to the tetrahedral vacancy-site. A reversible capacity of approximately 150 mAh/g was obtained in Li2+x SrTi6O14 [2], which is close to the LTO capacity. Although, the charge-discharge curve of this material shows a step-like plateau, the crystal structural change during the charge-discharge process has not been explained in detail. In this work, a new family of Li2(Sr1 -x Na x )Ti6- x Nb x O14 (0≤x≤1) was prepared, and the crystal structure at different state of charge and the electrochemical performance were investigated. Experimental Li2(Sr1 -x Na x )Ti6- x Nb x O14 (0≤x≤1) were synthesized by a conventional solid-state reaction method. A mixture of stoichiometric quantities of raw materials such as Li2CO3, Na2CO3, SrCO3, Nb2O5 and TiO2 was heated initially at 650℃ for 2 hours, then sintered at 1100℃ for 48 hours with an intermittent grinding. The products were characterized by X-ray powder diffraction (XRD). Electrodes of these compounds were prepared as follows; a mixed slurry of the product powder, conductive agent, binder and solvent, was coated on a sheet of Al foil using a doctor blade and then dried at 80℃ under vacuum condition for 12 hours. Electrochemical performance was investigated using a three-electrode glass beaker cell which was assembled using the prepared electrode and a lithium metal as a counter and a reference electrode. The structural changes of the compound during lithium insertion process were characterized using ex-situ X-ray powder diffraction (XRD) at BL19B2 beam line in the SPring-8 synchrotron radiation research facility and ex-situ neutron powder diffraction (NPD) at iMATERIA in the Japan proton accelerator research complex. Results and Discussion The charge-discharge curves of Li2(Sr1 -x Na x )Ti6- x Nb x O14 (x=0 and 0.75) electrodes are shown in Figure 1. The reversible capacity was improved by the substitution of Na and Nb in the potential range between 1.0 V and 2.0 V vs. Li/Li+. Moreover, the step-like plateau was shown in x=0, which was flattened with increasing substitution amount. A maximum capacity of 133.4 mAh/g was obtained at x=0.75. The reversible capacity for x=0 was restricted to 106.8 mAh/g in this potential range, while approximately 150 mAh/g has been reported at a lower potential range such as 0.5 V-2.0 V vs. Li/Li+ owing to an existence of the step-like plateau [2]. Therefore, we consider that the improvement of the reversible capacity is mainly attributed to the lifted-up redox potential area below 1.2 V to around 1.4 V vs. Li/Li+ as shown in Figure 1, namely this results in an enhancement of effective capacity in this potential range. Then, the crystal structure parameters for x=0 and 0.75 were investigated by XRD and NPD measurements to clarify differences among the lithium host sites in the structure. Table 1 shows proposed lithium host sites for Li2(Sr1 -x Na x )Ti6- x Nb x O14 and bond valence sum (BVS) obtained by the Rietveld analysis using neutron diffraction patterns for x=0 and 0.75. The BVS values of 4a and 8c were close to 1.0 for x=0, implying that 4a and 8c sites are mainly used as a lithium host site, and 4b site was getting closer to 1.0 for x=0.75. This result suggests that an effective lithium host site can be added by the substitution. From these findings, we believe that the step-like plateau was flattened due to structural modification on lithium host sites in Li2(Sr1 -x Na x )Ti6- x Nb x O14. The detail of electrochemical performance and structural analysis will be discussed in our presentation. References I. Koseva, J.-P. Chaminade, P. Gravereau, S. Pechev, P. Peshev, J. Etourneau, J. Alloys and Compounds, 389, 47 (2005). I. Beharouak, K. Amine, Electrochemistry Communications, 5, 435 (2003). Figure 1

Introduction Lithium-ion batteries using Li4Ti5O12 anode realize excellent performance of power, fast-charge, life, and safety [1]. However, the theoretical capacity of LTO is low (175 mAh/g) compared to that of graphite anode. We have been developing TiNb2O7 (TNO) as high-capacity anode material in order to replace the LTO anode [2]. The theoretical capacity of TNO is 387.6 mAh/g according to 5 Li insertion per formula unit, regarding electron transfer about Nb5+/Nb3+, Ti4+/Ti3+redox couples. Only a few researchers have reported on a large storage of more than 4 Li insertion (310 mAh/g). Hence, we report the lithium storage mechanism and electrode performance of hydrothermally synthesized TNO particles with the large storage of more than 4 Li insertion. Experimental TNO particle samples were synthesized by hydrothermal method. Niobium (V) chloride was dissolved in ethanol, and become mixed with Titanium (IV) oxysulfate dissolved in dilute sulfuric acid. Ammonia solution was slowly dropped to mixed solution with stirring until pH values were changed to 8. The mixture was transferred to autoclave, and gave a thermal treatment for 5 hours at 160℃. After that, the mixture was washed with deionized water and was frozen and dried in vacuum. The precursor samples were heated from 700℃ to 1000℃ for 30min, and furnace cooled. To compare with electrode performance of TNO, we also prepared TNO particle sample by solid state reaction using conventional method. Electrochemical properties were carried out using a three-electrode cell with synthesized TNO anodes as working electrodes, and Li foils as reference and counter electrodes. The structural changes of TNO particles as lithium insertion proceeded were characterized using ex-situ XRD, and XAFS measurements at the BL16B2 beam line in the SPring-8 synchrotron radiation research facility. Results and discussion Fig.1 shows charge (Li insertion) - discharge (Li extraction) curves of TNO electrodes at the first cycle. The TNO particles prepared by solid state reaction had a reversible capacity of 306 mAh/g, which was correspond to 3.95 of Li insertion amount per formula unit. On the other hand, hydrothermally synthesized TNO particles heated at 1000℃ had a large reversible capacity of 341 mAh/g (4.40 of Li/ formula unit) by the increase in capacity below 1.0 V (vs.Li+/Li). The reversible capacity per volume calculated using the true density of TNO was 1481 mAh/cm3, which is twice higher than that of graphite anode. Fig.2 shows the dQ/dV-V plots at the first discharge curves. The hydrothermally synthesized TNO anode had an additional peak around 1.0 V (vs. Li+/Li), which was significantly different from solid state reaction particles. Primary particle size (ca.100 nm) of hydrothermally synthesized TNO was much smaller than that (ca. 1 μm) of TNO synthesized by solid state reaction as shown in Fig.3. We consider that a short diffusion length of lithium ions, a large surface area, and highly crystallization of thermally synthesized TNO particle would make it possible to insert into a storage site of lithium ions above 4.0 per formula unit. We will give a description about the Li insertion and extraction mechanism of TNO at below 1.0 V (vs. Li+/Li) by using electrochemical analysis, ex-situ XRD, and XAFS measurements. References [1] N. Takami, H. Inagaki, Y. Harada, Y. Fujita, K. Hoshina, J. Electrochem. Soc. 156, A128, 156 (2009). [2] Y. Harada, N. Takami, H. Inagaki, Y. Yoshida, JP Patent No 5230713 filed on Oct. 29, 2010. Figure 1

Lithium-ion batteries using Li4Ti5O12 (LTO) anode realizes high power, long life, and safety for automotive and stationary power applications because the LTO electrode has the advantages of nano-size particle, no lithium plating at quick-charging and low temperature conditions, outstanding safety on internal short-circuit, and thermal stability under high-temperature conditions[1,2]. Recently, LTO-based lithium-ion batteries have been investigated for low-voltage systems such as Start & Stop vehicle systems and the replacement of Pb-acid battery for stationary power supply because the LTO-based batteries have good voltage harmonization with the Pb-acid battery. We have been developing five series-connected LTO-based cells for 12 V low-voltage systems. LiMn1-xFexPO4 (LMFP) is a 4 V-class cathode with high thermal stability compared to such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4. Lithium-ion cell with a combination of the LMFP cathode and the LTO anode shows a voltage plateau of 2.5 V for Mn3+/Mn2+ redox. We have been preliminarily studying a thin hybrid electrolyte based on a cubic garnet-type Li7La3Zr2O12(LLZ) as lithium-ion conducting ceramics for 12 V-class bipolar LTO/LMFP battery with liquid-free and separator-free [3]. The bipolar battery has advantages of size reduction and low-cost. In addition, LMFP and LTO can share the same aluminum current collector, which makes the bipolar structure simple. In this study, we report the electrochemical characteristics, the performance of LTO/LMFP cells, and the development of 12 V-class bipolar LTO/LMFP battery using the LLZ-based hybrid electrolyte. LiMn0.85Fe0.15PO4 and LiMn0.85Fe0.1Mg0.05PO4 as the LMFP cathode were hydrothermally synthesized to obtain a small particle size of about 80 nm and no impurity phase. The discharge capacity of cathode increased from 140 to 160 mAh/g by using the LiMn0.85Fe0.1Mg0.05PO4 cathode. We fabricated 1 Ah and 25 Ah-class LTO/LMFP cells in order to investigate the cell performance. Discharge rate capability of the LTO/LMFP cell was improved to be comparable with that of the LTO/LMO cell. High-temperature cycle test exhibited that the LTO/LMFP cell was significantly superior to that of the LTO/LMO cell. The dissolution amount of Mn from the LMFP cathode at 55°C was two order of magnitude smaller than that from the LMO cathode. Fig.1 shows close-circuit voltage (CCV) at 0.2 C rate and open-circuit voltage (OCV) of the five series LTO/LMFP cells and 12 V Pb-acid battery during charge-discharge cycle. Five series LTO/LMFP cells exhibited good voltage harmonization with the 12 V Pb-acid battery in the voltage range between 10 V and 14 V and a smaller polarization compared with the Pb-acid battery. The LTO/LMPF cells are expected to be a replacement of Pb-acid battery in automotive and stationary power storage applications. Therefore, we fabricated the bipolar batteries constructed from five LTO/LMFP cells stacked together, which contained four bipolar electrodes and two monopolar electrodes as endplates for output voltage of 12 V. Cathodes and anodes in bipolar batteries are disposed in contact respectively with opposite sides of a common current collecting element forming a unitary bipolar structure. LTO and LMFP were coated with the LLZ hybrid electrolyte containing 4wt% polyacrylonitrile (PAN) gel polymer [3]. Fig.2 shows the cross section SEM image of LTO/LLZ/LMFP and typical charge-discharge curve of the bipolar battery. A few micrometers thickness of LLZ hybrid electrolyte layer was thin enough to reduce the internal resistance. This LLZ substrate did not only work for the lithium-ion conductor but also retain sufficient strength and electronic insulation even for a little amount of the PAN gel polymer and separator-free in the hybrid electrolyte layer. The bipolar battery exhibited the voltage plateau of 12.5 V, which is matched with the voltage of Pb-acid battery. Fig.3 shows discharge rate performance of the bipolar battery at various discharge rates. The discharge capacity retention at 5 and 20 C rates was 94 and 75%, respectively. The polarization of the bipolar battery was relatively small even at a high discharge rate. It was demonstrated that the bipolar battery had good performance in terms of discharge rate, low temperature, and cycle life. The performance of the 12 V-class bipolar battery was comparable with that of conventional lithium-ion batteries with a liquid electrolyte, indicating that 12 V-class bipolar LTO/LMFP battery using the LLZ-based hybrid electrolyte has enough performance for practical uses. We are now trying to develop larger bipolar batteries for low-voltage system applications. References [1]N.Takami, H.Inagaki, T.Kishi, Y.Harada, Y.Fujita, K.Hoshina, J.Electrochem.Soc.,156, A128(2009) [2]N.Takami, H.Inagaki, Y.Tatebayashi, H.Saruwatari, K.Honda, S.Egusa, J.Power Sources, 244,469 (2013) [3]K.Yoshima, Y.Harada, N.Takami, J. Power Sources, 302,283(2016) Figure 1

Lithium batteries are capable of a high energy density, high power, and long cycle performance. Demand for lithium-ion batteries has increased for applications ranging from portable devices to large machines such as electric vehicles and energy storage for load leveling. Especially, a high voltage of the battery system is required for these large-scaled applications. However, we can use only around 3.7 V from a conventional monopolar-type lithium-ion battery, and are required to connect in series for the high-voltage systems. To overcome these problems, a bipolar-type cell [1] is considered to be one of the suggestions for improvement the cell voltage. Using solid-state electrolyte makes it possible to fabrication the bipolar batteries without a porous film separator and partitions to avoid liquid junction. The solid-electrolyte, for example, Li2S-P2S5 glass-ceramics[2], LiSICON[3], perovskites[4] and garnets have been investigated to improve an ionic conductivity at RT. The cubic garnet-type with a nominal composition of Li7La3Zr2O12 (LLZ) [5] has high ionic conductivities (2.4×10-4 Scm-1 at RT) and a wide electrochemical window. On the other hand, difficulty of forming an effective electrode-electrolyte interface is known as problems of all-solid-state lithium-ion batteries. Forming a favorable electrode-electrolyte interface by interfacial modification has effectively decreased the interfacial resistance. In this study, we tried to use a lithium-ion conductive binder as buffer layer on active materials and LLZ for improving Li+ ion diffusion in electrode-electrolyte interface. Olivine LiMn1-yFeyPO4 (LMFP) has been studied as a potential 4V-class cathode with high thermal stability compared to such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4. Spinel Li4Ti5O12 (LTO) has advantages of a zero strain property, the suppression of internal short-circuit reaction[6], high power, and long-life performance[7] for the anode in the bipolar battery with the solid-state electrolyte. Li-ion cell with a combination of the LMFP cathode and the LTO anode shows a voltage plateau of 2.5V for Mn3+/Mn2+ redox[8]. 5 series LMFP/LTO cell has a good harmonization with 12V battery system. Current collectors for the LMFP cathode and the LTO anode can be used as the same aluminum foil, which becomes easy to make bipolar electrodes. We thus fabricated 12V-class bipolar LiMn0.8Fe0.2PO4/ LTO battery with the LLZ solid-electrolyte layer, which based on a composite electrode composed of active materials and LLZ coated with a lithium-ion conductive binder, and this single-layered electrode cell takes out 2.5 V. The bipolar batteries were constructed from five cells stacked together, which contained four bipolar electrodes and two monopolar electrodes as endplates to obtain output voltage of 12 V. Cathodes and anodes in bipolar batteries are disposed in contact respectively with opposite sides of a common current collecting element forming a unitary bipolar structure. The LLZ powders were synthesized by solid phase process and synthesized particles were crushed into smaller sizes by bead milling. After mixing LLZ powders and lithium-ion conductive binder, the mixture sprayed and deposited on each electrode. The layer of LLZ powders between the anode and the cathode is working as not only the electrolyte but also as an ionic conductive separator After LLZ powders deposited, cross-sectional SEM image of LLZ/LTO is shown in Fig. 1. Particle diameter of LLZ powders is < 1mm by bead milling process. Electrode- electrolyte interface has effectively pasted up using binder and peeling was not observed. Thickness of LLZ layer, LMP layer, and LTO layer were 20, 60 and 60 mm, respectively. Total thickness of the bipolar battery was 570 mm. Fig. 2 shows fcharge and discharge curves of the bipolar LMFP/Li4Ti5O12 battery at the first cycle and 25°C. The bipolar battery shows the discharge curve has a high voltage slope at 12V corresponding to 2.4V for a single cell and has a capacity of 5mAh. The result of other electrochemical tests of bipolar LMFP/Li4Ti5O12 batteries will be also reported. References [1] D. H. Shen, S. Surampudi, G. Halpert, IEEE 35th International Power Source Symposium, (1992) 281. [2] A. Sakuda, A. Hayashi, T. Ohtomo, S. Hama, M. Tatsumisago, J. Power Source 196 (2011) 6735-6741. [3] P. Knauth, Solid State ion, 180 (2009) 991. [4] H. Kawai, J. Kuwano, J. Electrochem. Soc., 141 (1994) L78-L79. [5] G. Larraz, A. Orera, M. L. Sanjuan, J. Mater. Chem. A, 1 (2013) 11419-11428. [6]N.Takami,H.Inagaki,T.Kishi,Y.Harada,Y.Fujita,and K.Hoshina, J.Electrochem.Soc.,156,A128(2009) [7]N.Takami, H.Inagaki, Y.Tatebayashi, H.Saruwatari, K. Honda, and S.Egusa, J.Power Sources, 244,469(2013) [8]V.Borgel, G.Gershinsky,T.Hu, M.G.Theivanyagam, and D.Aurbach, J.Electrochem.Soc.,160,A650(2013) Figure 1

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