The development of high energy density devices produces a dramatic improvement in the cruising range of electric vehicles and significantly contributes to realizing a sustainable society. In recent years, innovative batteries, such as all-solid battery, metal-air battery and sodium-ion battery, etc., which are estimated to provide a superior battery performance better than the latest lithium-ion battery, are now being given significant attention. Meanwhile, a number of breakthroughs are required in order to achieve practical use of these innovative batteries. In this paper, we summarized the current research approaches of these innovative batteries.

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The direct synthesis of an Ni-Al layered double hydroxide (LDH) as the active material in an Ni–metal hydride secondary battery on an electron-conductive substrate was performed using liquid phase deposition method. A reduction in charge-transfer resistance was achieved by coating a Ni–Al LDH film on a Ni foam surface current collector, as a result of improved adhesion between the current collector and the active material.

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Oxide-based electrocatalysts were investigated as non-platinum cathodes for the oxygen reduction reaction (ORR). Recently, we found that titanium oxide-based compounds produced from oxy-titanium tetra-pyrazino-porphyrazine via oxidation under low oxygen partial pressure exhibited high ORR activity [T. Hayashi et al., ECS Trans., 64(3), 247 (2014)]. However, the mechanism by which the ORR proceeds in the presence of titanium oxide-based catalysts has not yet been analyzed in detail. In this study, the ORR mechanism was investigated based on the Damjanovic model by using a rotating ring disk electrode. The titanium oxide-based catalysts showed high reactivity in the four electron reduction of O₂.

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Thick-film electrodes of tin phosphides, Sn₄P₃ and SnP₃, were prepared using a mechanical alloying method followed by a gas-deposition method, and were evaluated as Na-ion battery anodes in an organic electrolyte and ionic liquid electrolytes. The Sn₄P₃ electrode showed a better cycling performance in the organic electrolyte compared with the SnP₃ electrode and an Sn electrode. The performance of the Sn₄P₃ electrode was further improved by using ionic liquid electrolytes because of a uniformly formed surface layer and resulting uniform sodiation/desodiation reactions on the electrode.

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The anode properties of carbon-coated SiO (SiO-C) electrode materials containing a small amount of olivine-type LiMgPO₄ powder as the additive were investigated in a lithium cell. The 2 wt% LiMgPO₄ (LMP)-containing SiO-C anode exhibited a high charge/discharge capacity of 1500 mAh g⁻¹ (versus SiO) and good cycle performance. In addition, the high-rate discharge performance was improved by the addition of LMP. The thermal stability of the electrode material was studied by differential scanning calorimetry (DSC). The thermal stability of the SiO-C anode material was improved by the addition of LMP powder.

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To develop durable cathodes, their durability in acidic media at 80°C, which is close to the operation temperature of present polymer electrolyte fuel cells, must be evaluated. We attempted to use a Ti₄O₇ rod as the substrate, and it was observed to be more suitable than a glassy carbon rod because of its higher electrochemical stability. We evaluated the durability of titanium-niobium oxides mixed with Ti₄O₇ powder (Ti_xNb_yO_z+Ti₄O₇) as a non-precious- and carbon-free cathode in 0.1 mol dm⁻³ H₂SO₄ at 80°C. We successfully demonstrated that the Ti_xNb_yO_z+Ti₄O₇ did not deteriorate during start-stop and load cycle tests when a Ti₄O₇ rod substrate was employed.

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Li_1.2+xMn_0.3Co_0.2Ni_0.3O₂ (x = 0, 0.2, 0.4) with a Li-rich layered rock salt type structure were synthesized by a high-pressure method. Rietveld analysis using neutron diffraction data elucidated that the Li atoms were located at the octahedral sites in the Li and transitional metal (TM) layers. Furthermore, additional Li was detected at the tetrahedral site in the TM layer for the x = 0.4 sample. The tetrahedral Li caused vacancies at the neighboring TM sites and a structural distortion of the (Li,M)O₆ octahedra. The x = 0.4 sample exhibited larger and smaller charge and discharge capacities of 388 and 135 mAh g⁻¹ at the first cycle, respectively, than the x = 0.2 sample (296 and 151 mAh g⁻¹). The structural modification caused by the introduction of the tetragonal Li affects the initial activation process of the Li-rich layered rock salt type electrodes.

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Ionic conductivity and viscosity of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA)-glyme solvate ionic liquids with the mole fraction of LiTFSA from 50.0 to 54.5 mol% were investigated at 298–323 K. The ionic conductivity of the solvate ionic liquids was found to be inversely proportional to the viscosity, as expected from Walden's rule. The ionic conductivity of the solvate ionic liquid decreased with increasing the mole fraction of LiTFSA, probably due to formation of a bulky lithium species, [Li(TFSA)₂]⁻, which forms at the compositions of more than 50 mol% of LiTFSA.

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Ti-doped lithium vanadium(III) phosphate cathode materials, Li_3−2x(V_1−xTi_x)₂(PO₄)₃, were prepared by a solid state reaction. The Ti-doped samples exhibited that the γ-phase is stabilized at room temperature by substituting Ti for V sites at more than x = 0.10. The discharge capacity of the doped samples at a 60 C rate was found to be much higher than that for the non doped sample. This improved performance found for the doped cathodes was attributed to the increase in ionic conductivity given by the transition of α- to γ-phase via Ti substitution.

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The aqueous lithium-air battery is receiving considerable research attention because of its high theoretical energy density; however, discharge products tend to precipitate and inhibit the chemical reaction in the battery. To clarify the precipitation mechanism, the precipitated discharge products in a carbon-based porous air electrode were visualized three-dimensionally by oblique soft X-ray computed tomography. As a result, precipitation of the discharge products was observed clearly. The precipitate did not dissolve entirely and bubbles emerged in the air electrode during the charging process. This indicates that the charge/discharge products degrade the cycling performance of the aqueous lithium-air battery.

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We have successfully prepared the particle-size controlled H₂Ti₁₂O₂₅ by soft chemical synthetic technique using the ball-milled and annealed Na₂Ti₃O₇ as a starting compound. In this synthetic route, we first prepared the particle-size controlled Na₂Ti₃O₇ by ball-milling and subsequent annealing at 873–1073 K. Then, H₂Ti₃O₇ was prepared by Na⁺/H⁺ ion-exchange reaction using HCl solution, and H₂Ti₁₂O₂₅ was synthesized by heating H₂Ti₃O₇ at 533 K. The initial discharge capacity of the H₂Ti₁₂O₂₅ sample thus obtained was achieved to be 255 mAh g⁻¹, which was higher than those previously reported for H₂Ti₁₂O₂₅ samples, while initial irreversible capacity also increased together with the amounts of smaller particles.

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Charge and discharge properties of amorphous Si nano-flake powder (Si Leaf Powder^®, Si-LP) in lithium bis(trifluoromethanesulfonyl)amide-tetraglyme complex electrolyte, [Li(G4)][TFSA], were investigated for use as an anode in silicon-sulfur battery. Though a high reversible capacity (2,300–2,700 mAh g⁻¹) was obtained initially in [Li(G4)][TFSA], the overpotential gradually increased upon cycling. The increase in overpotential was reduced by dilution with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The addition of fluoroethylene carbonate (FEC) further reduced the overpotential and improved cycleability. Electrochemical impedance spectroscopy analysis revealed that the observed improvements in charge and discharge characteristics are due to the formation of solid electrolyte interface layer with a high ionic-conductivity in the presence of FEC.

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Electron density distributions of LiCoO₂ have been determined by Maximum Entropy method (MEM) and multipole modeling from synchrotron x-ray powder diffraction data. The localization of Co-3d electrons was clearly visualized in the deformation MEM density. An electron density by multipole modeling was investigated by the Bader’s topological analysis to reveal bonding characteristics and interactions between the constituted atoms.

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To compensate a large initial irreversible capacity (Q_irr) of silicon electrode, a novel Li pre-doping technique using Li-naphthalene complex/tetrahydrofuran solutions was developed. Li was successfully doped to amorphous silicon nano-flake particles, and the doping level of Li was easily controlled by adjusting naphthalene concentration. A large Q_irr of the pristine electrode (1200 mAh g⁻¹) was almost completely compensated after pre-doped in 0.1 mol dm⁻³ Li-naphthalene solution for 1 h. In addition, no harmful effects of pre-doping on cycleability were observed.

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The deposition and dissolution of Li at a Ni electrode coated with lithium phosphorus oxynitride (LiPON) thin film were investigated in lithium bis(trifluoromethylsulfonyl)amide-glyme solvate ionic liquid. The LiPON thin film was able to suppress the irreversible decomposition of the solvate ionic liquid. The cathodic deposition of Li and anodic dissolution of the deposited Li occurred on the Ni electrode through the LiPON thin film. These results suggest that the LiPON thin film plays a role as an artificial solid electrolyte interphase film between the Ni electrode and the solvate ionic liquid.

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Zinc-based secondary aqueous batteries are promising candidates as innovative batteries for their high energy density and high safety. Their limited cycle life is chiefly due to the morphology changes of the zinc electrode and thus tracking the zinc distribution during electrochemical processes is indispensable for clarifying the deterioration mechanism of the zinc electrode. We here report the zinc distribution near the electrode in a millimeter scale on zinc reduction and oxidation observed by operando X-ray fluorescence imaging with a high time resolution. The zinc dendritic growth and its dissolution behavior is discussed based on the obtained zinc distribution images.

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This paper discusses an SOC estimation system for lithium ion batteries based on the extended Kalman filter. The accuracy of the estimation is strongly dependent on accuracy of the battery model. We have newly formulated the equivalent circuit model that covers temperature and SOC dependencies. As the result, the error rate of the estimation has been improved significantly. The experimental results show that the new SOC estimation system derives very accurate results for wide range of temperature.

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The charge-discharge performance and the energy density of a metal hydride/air secondary battery using A₂B₇ type hydrogen storage alloys were studied. The MH/air secondary battery showed a high energy density more than 750 Wh/L or 100 Wh/kg, which is higher than the theoretical energy density of lithium ion secondary batteries. A high current discharge up to 1100 mA (83 mA cm⁻²) was demonstrated with no plugging of the positive electrode by discharge products. The results indicated that the MH/air secondary battery is one of the most promising candidates for next generation of energy storage devices to a wide range of applications such as electric vehicles and home energy storage systems.

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A new type of carbon black CB1 was developed, by Asahi Carbon Co., Ltd., for usage of a lithium-ion battery (LIB). Compared with commercially available acetylene black, CB1 in which the network structure of carbon is developed has a relatively larger specific surface area. Therefore, CB1 dispersed in LiFePO₄ precursors by planetary ball milling inhibited LiFePO₄ primary particles’ growth during calcination process. Charge/discharge performance was improved by coexistence of CB1 with polystyrene (PS) carbon layers deposited on LiFePO₄ particle surfaces, compared to the performance for the LiFePO₄ sample covered with carbon layers derived from only PS.

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The growth of the crystal surface of an intercalated metal–organic framework (iMOF) electrode material, 2,6-naphtalnene dicarboxylate dilithium, which forms an organic–inorganic layered structure, was controlled by selecting the solvent species during synthesis. The crystals synthesized under an aqueous solution exhibited a small reversible capacity because the crystal plane in the horizontal direction relative to the layered structure, grew dominantly and exhibited poor lithium intercalation kinetics. In contrast, the crystals synthesized under methanol exhibited a large reversible capacity because the crystal planes in the vertical direction relative to the layered structure were dominant and exhibited favorable lithium intercalation kinetics.

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Electrochemical behaviors of Zn in various concentrations of K₂CO₃ and KOH containing 5 M K₂CO₃ (M = mol dm⁻³) aqueous solutions (carbonate-based electrolytes) have been investigated. The electrochemical reactions of Zn in diluted K₂CO₃ and KOH containing 5 M K₂CO₃ were suppressed while they were activated in concentrated K₂CO₃. In the point of view of the ratio of the cathodic charge q_c to the anodic q_a and Zn oxidation current, 0.5 M KOH containing 5 M K₂CO₃ showed the best performance among all examined electrolytes. The dendritic growth of Zn was not observed during redox cycles in the carbonate-based electrolytes.

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Carbon-modified ramsdellite titanium dioxide, TiO₂(R), was prepared without using metallic Ti powder, and its charge and discharge properties was investigated as an anode of lithium ion batteries. Carbon sources (citric acid and multi-walled carbon nanotubes) were added in the calcination process to obtain a LiTi₂O₄ precursor. The carbon addition was indispensable to obtain stoichiometric LiTi₂O₄ as a precursor and the resulting Li⁺-free TiO₂(R). The carbon-modified TiO₂(R) had a round shape and a smaller particle size of 0.5–1.5 µm. The discharge capacity and rate-capability were improved by carbon modification due to an enhancement of the electronic conductivity and a reduction of the lithium-ion diffusion path.

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The crystal growth process of LiNi_1/2Mn_3/2O₄ was investigated and the octahedral shaped LiNi_1/2Mn_3/2O₄ was successfully synthesized by a low temperature synthesis. The morphology change was accelerated by the spinel-rocksalt phase transformation caused by the oxygen loss. After re-oxidation, high crystalline LiNi_1/2Mn_3/2O₄ with octahedral morphology was obtained. High crystalline LiNi_1/2Mn_3/2O₄ with the particle size of 1–3 µm was obtained by the low temperature synthesis controlling the oxygen partial pressure. High crystalline LiNi_1/2Mn_3/2O₄ crystallized at 850°C exhibited an initial charge capacity of 145 mAh g⁻¹ and an initial discharge capacity of 137 mAh g⁻¹ with a plateau at 4.7 V, and 90% of cycle retention after 100 cycles at 60°C. Microparticulation of high crystalline LiNi_1/2Mn_3/2O₄ enhanced the discharge capacity.

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Electrolyte decomposition processes on cathode and anode active materials significantly affect to the cycle life and the calendar life of lithium-ion batteries. Here we designed a new in-operando FTIR spectroscopy cell to investigate the decomposition process of the electrolyte solutions. The cell is compatible with various cathodes and anodes for lithium-ion batteries and the charging-discharging rate can be increased up to 1 C. In the case of the conventional cathode active material LiMn₂O₄, the in-operando spectra shows adsorption and solvation process of the electrolyte solvent molecules, while no clear evidence of the electrolyte oxidation process is observed. On the other hand, the solid electrolyte interphase (SEI) layer formation processes on graphite anodes are clearly observed. Hence, we think the new measurement technique should be a powerful tool for the development of electrolyte additives.

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The average and local crystal structures, and the electronic structure of 0.4Li₂MnO₃-0.6LiMn_1/3Ni_1/3Co_1/3O₂ were investigated using a combination of a pair distribution function (PDF) analysis and first-principles calculations. A Rietveld analysis showed that the sample had a Li₂MnO₃-type structure (space group: C2/m) and that Ni and Co existed at two sites, 4g and 2b, in the transition metal layer. Based on the maximum entropy method using synchrotron X-ray diffraction data, and first-principles calculations, it was determined that the covalencies between 2b and 8j sites in the transition metal layer, and between 2c and 8j sites in the lithium layer, were relatively low. In addition, based on the average length of bonds between transition metal and oxygen ions in the local structure determined by a PDF analysis using synchrotron X-ray and neutron total scattering data, and the ionic radius, the valence of Mn, Ni and Co in this material before charging was tetravalent, between divalent and trivalent, and trivalent, respectively. These results were consistent with the valence obtained from the density of states calculated by the WIEN2k code.

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Iron-based conversion cathode, FeOF is attractive, because of the low cost and the large specific capacity. However, the synthesis is not easy and it cannot be used as cathode against carbonaceous anode. To overcome these drawbacks, we focused on the LiFeOF phase, which has the same chemical composition as the discharged intermediate product of FeOF cathode. LiFeOF can be easily synthesized from LiF and FeO by the dry ball-milling method at room temperature. The reversible capacity was 292 mAh g⁻¹ with an average voltage of 2.5 V and an energy density over 700 Wh kg⁻¹, which is higher than that of LiFePO₄. In addition, we confirmed the feasibility of LiFeOF cathode against Li₄Ti₅O₁₂ anode.

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We have successfully prepared amorphous MoS₃ (a-MoS₃) by ball milling a mixture of Mo metal and sulfur and investigated structural changes during the charge-discharge processes. The XPS measurements revealed that a-MoS₃ should consist of Mo⁴⁺, S₂²⁻, and S²⁻. In the XRD profiles of the a-MoS₃ electrodes after the initial discharge and charge tests, only the halo patterns due to the amorphous state were observed without crystalline diffraction peaks. The HR-TEM observations of a-MoS₃ electrodes during the 1st discharge process revealed that the lattice fringes due to MoS₂ layer structure disappeared gradually. However, the lattice fringes like MoS₂ were also observed after the 1st charge. The HR-TEM image after the 10th charge did not show the lattice fringes of MoS₂. These results of HR-TEM observations suggested that the reversible structural changes of a-MoS₃ occurred.

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Effects of mechanical stress on lithium chemical potential in electrodes and electrolytes for all-solid-state lithium ion batteries were investigated. Dense film electrodes of LiCoO₂ or LiMn₂O₄ were symmetrically deposited on both surfaces of a plate of various solid state lithium ion conductors, such as Li_0.29La_0.57TiO₃, Li₇La₃Zr₂O₁₂ and Li_1+x+yAl_x(Ti,Ge)_2−xSi_yP_3−yO₁₂. Mechanical stress was applied to the specimen by four points bending tests while measuring electromotive force (EMF) between the two electrodes. EMF proportional to the applied stress was observed. EMF was significantly dependent on the electrode material, but was almost independent of the electrolyte material. These results indicated that lithium chemical potential varied under mechanical stress both in the electrode and electrolyte but the influence of mechanical stress appeared more notably in the electrode than the electrolyte. The lithium chemical potential changes in the electrode and the electrolyte under mechanical stress were discussed based on the idea of local equilibrium.

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All-solid-state cells with LiCoPO₄ (LCP) active material with high redox potential of 4.8 V (vs. Li⁺/Li) were fabricated and the cells operated as a lithium secondary battery at room temperature. The composite electrodes of LCP active material and LiTi₂(PO₄)₃ (LTP) electrolyte with large contact area were synthesized in the media of oleic acid and paraffin wax by heat treatment at 700°C. The prepared LCP-LTP-Acetylene black (AB) composites were applied to electrochemical cells with a conventional organic liquid electrolyte and a sulfide solid electrolyte. The all-solid-state cells using the positive electrode composed of the LCP-LTP-AB composite and the sulfide electrolyte showed the initial discharge capacity of 50 mAh g⁻¹ at 4.8 V (vs. Li⁺/Li). The cells using the electrodes without LTP and the sulfide electrolyte were difficult to operate, suggesting that formation of close contacts between LCP and LTP, and additional ion conduction paths to LCP-LTP particles are effective in increasing capacities in all-solid-state cells.

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Improvement of electrochemical property of K₂V₈O₂₁, a stable phase of vanadium bronzes, was attempted by redox inactive metal ion doping. From the XRD pattern and lattice parameter change, it was confirmed that Ti and Nb can be exchanged with vanadium until 2 mol%. By doping 1 mol% Nb⁵⁺ to K₂V₈O₂₁, the discharged capacity was improved to 210 mAh/g from 140 mAh/g for non-dope system. In ex-situ XRD measurement, non-dope K₂V₈O₂₁ became amorphous in the first discharge process. For Nb-doped system, main reflection of K₂V₈O₂₁ observed after both discharge process and first discharge-charge cycle. The lattice parameter of c axis increased while that of a axis decreased after discharge. Both lattice parameters got back to the initial position after charging process. This result suggests that lithium ions are intercalating into the inter layer part along the c axis during the discharge process. Mechanical mixing active materials with acetylene black was attempted for the further improvement in the electrochemical property. The initial capacity of ball milled Nb-doped K₂V₈O₂₁ was improved to 315 mAh/g. This result suggests that the capacity of K₂V₈O₂₁ is determined by the numbers of lithium ion that can be intercalated into the lattice without the destruction of its crystal structure.

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The reconversion reaction of lithium fluoride (LiF)/iron (Fe) nanocomposite thin film cathodes prepared with the co-deposition method was investigated at room temperature. The LiF:Fe in molar ratio of 1:1 composite thin film was able to be discharged with large capacity of 300 mAh g⁻¹ after the reconversion reaction on the first charge at 25°C. After the 2nd cycle, it showed relatively stable reversible capacity more than 200 mAh g⁻¹. The discharge profile with 2 V plateau was similar to that of ferrous fluoride (FeF₂), and the trace of FeF₂ was detected in the fully-charged LiF/Fe nanocomposite thin film by X-ray photoelectron spectroscopy (XPS). These results suggest that the reconversion reaction from LiF/Fe to FeF₂ proceeded successfully on the first charge as we expected.

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The electrochemical properties of a mechanochemically synthesized Li₂₂Sn₅ electrode in a solvate ionic liquid-based electrolyte were investigated. The electrolyte was composed of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), tetraglyme (G4), and a hydrofluoroether solvent (HFE) in a molar ratio of 1:1:6.2, in which Li⁺ and G4 formed a 1:1 complex cation of [Li(G4)]⁺. The Li₂₂Sn₅ electrode exhibited an initial discharge capacity of 500 mA h g⁻¹ in this electrolyte; however, the capacity decreased with increased numbers of charge-discharge cycles. This was attributed to the Li-Sn alloy’s volume change in the electrode during the electrochemical reaction. To examine the behavior of the electrode material in a lithium-sulfur battery, a full cell consisting of a Li₂₂Sn₅ anode, S cathode, and [Li(G4)][TFSA]/HFE electrolyte was fabricated. The cell was discharged and charged stably without severe side reactions. The dissolution of lithium polysulfides, reaction intermediates with the sulfur cathode, was effectively suppressed in the electrolyte, leading to efficient charge-discharge cycling of the Li₂₂Sn₅-S cell.

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A prototype lithium-ion battery with a bis(fluorosulfonyl)imide (FSI)-based ionic liquid electrolyte was developed. The prototype was mounted on a demonstration module of the “Hodoyoshi-3” microsatellite, which was successfully launched on June 20, 2014. Qualification tests for space application, including radiation tolerance and vacuum tests, revealed negligible degradation of the ionic liquid-based lithium-ion battery (IL-LIB) cell. According to the flight data, the IL-LIB cell can exist stably in an ultra-high vacuum environment despite its thin and flexible pouch casing without any rigid anti-vacuum reinforcements. Furthermore, the power unit showed the same charge–discharge performance as that predicted by the charge–discharge behavior of an identical cell on the ground, suggesting that the IL-LIB cell maintains performance in high vacuum a microgravity environment. These results prove that LIB cells with FSI-based ionic liquids can be used as a power source for space applications.

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