Innovation in rechargeable batteries is urgently required to meet demands for application such as electric vehicles, renewable energy storage, and emergency backups. Next-generation rechargeable batteries include novel rocking-chair batteries, metallic lithium/sodium batteries, metal-air batteries, and flow cell systems. In order to improve battery performances and to bring innovation in rechargeable batteries, detailed knowledge on the battery reaction is indispensable. For clarifying the reaction mechanisms in details and proposing a guideline for the improvement, it is required to understand the heterogeneity (inhomogeneity), non-equilibrium and dynamic phenomena inside batteries under operating conditions. As an example, phenomena at electrode/electrolyte interface are elucidated by using surface-sensitive X-ray absorption fine structure analysis, X-ray photoelectron spectroscopy and nuclear magnetic resonance analysis. These analytical methods are applicable not only for the advanced LIB systems but also for totally innovative systems. Elucidating and understanding the reasons for the inactivity, slow kinetics and deterioration of LIB will be the key to open post LIB technology.

View full abstract

Considerations of energy density, power, and calendar life are critical to effectively develop advanced secondary systems. For next generation battery applications requiring multiple features including long life, large cycle count, high energy density and high power, new strategies are needed for the rational design of electroactive materials and electrodes. This article discusses several conceptual approaches under exploration with examples from our research group. The first approach is the systematic synthesis of materials with structures facilitating ion insertion and deinsertion at high voltage and energy density, where we control materials properties such as surface area, particle size and in particular crystallite size. A second approach is the investigation of novel electrode structures and substrates to increase energy density and capacity retention under cycling, where we have developed strategies for minimizing passive components. A third approach is investigation of catalysts for metal air batteries where the cathode active material is drawn from the air rather than carried in the battery.

View full abstract

This article summarizes our research on aqueous lithium-air rechargeable batteries. Lithium-air batteries have a far higher energy density and lower material cost than lithium-ion batteries, so that they are now attracting growing attention as possible power sources for electric vehicles. Presently, two types of rechargeable lithium-air batteries have been developed; non-aqueous and aqueous types. The aqueous type has a lower specific energy density than the non-aqueous system, but overcomes some severe problems that must still be addressed for the non-aqueous type, such as lithium metal corrosion by water from air and the high polarization of electrode reactions. The key component of the aqueous lithium-air battery is a water-stable lithium metal electrode (WSLE). The WSLE developed in our laboratory consists of lithium metal covered with a lithium conducting polymer electrolyte and a lithium conducting water-stable solid electrolyte, which was successfully operated in a saturated LiOH and LiCl aqueous solution.

View full abstract

Single phase, well-crystallized O3-type NaFeO₂ (alpha NaFeO₂) is prepared by a solid-state method. Electrode performance of O3-type NaFeO₂ is examined as positive electrode materials for rechargeable sodium batteries. O3-type NaFeO₂ can deliver 80–100 mAh g⁻¹ of reversible capacity with a nearly flat voltage profile at approximately 3.3 V vs. Na metal. The electrode performance is significantly deteriorated by oxidation beyond x > 0.5 in Na_1−xFeO₂. X-ray diffraction study reveals that loss of electrode reversibility originates from irreversible structural change, possibly accompanied by iron ion migration in layered host structures. The sodium ion insertion into the host structures would be disturbed by the irreversible structural change when charged beyond x > 0.5 in Na_1−xFeO₂. Acceptable cyclability is, therefore, achieved for O3-type NaFeO₂ as the positive electrode materials in the limited composition of x = 0–0.45 in Na_1−xFeO₂.

View full abstract

To reduce the high irreversible capacity (Q_irr) of Si thin flake (Si-LP) negative electrode, carbon-coated Si-LPs were prepared using citric acid as a precursor and their charge/discharge properties were investigated as negative electrodes in lithium-ion batteries. The carbon-coated powder was homogeneously coated with a thin carbon layer (8–10 and 6–8 nm in thickness for Si-LPs heat-treated at 600 and 700°C, respectively, 14 wt% for each). The irreversible capacity Q_irr was successfully reduced to about a half (ca. 1100 mAh g⁻¹) of that of the pristine Si-LP (2336 mAh g⁻¹), though the cycleability was slightly deteriorated. The cycleability of Si-LP@Cs was significantly improved by the addition of 10 wt% VC in the electrolyte solution. Si-LP@C(700°C) kept high discharge capacities over 2000 mAh g⁻¹ even after 50 cycles with a reduced Q_irr of ca. 1300 mAh g⁻¹ compared with the pristine Si-LP (ca. 2450 mAh g⁻¹).

View full abstract

Zinc oxide electrodes modified with an anion-exchange ionomer (AEI) are fabricated to suppress the dendrite formation of metallic zinc deposition. Charge-discharge measurements show that the discharge capacities and efficiencies of AEI-modified ZnO electrodes are superior to those of bare ZnO electrodes. These improvements are explained by the selective ion permeation through the AEI films. In addition, we demonstrate that the AEI-modified ZnO electrodes work well as a rechargeable negative electrode in an alkaline solution which contains no ZnO additive.

View full abstract

In order to improve oxygen reduction performances of an air electrode using perovskite type La_0.7Ca_0.3CoO₃ as an electrocatalyst, a layered double hydroxide (LDH) containing magnesium and aluminum was examined as an ion-conducting additive. Although the addition of LDH impeded gas permeation of the catalyst layer of the electrode, it was effective in reducing charge transfer resistance of the electrode, and as a result, the oxygen reduction reaction of the air electrode was remarkably enhanced.

View full abstract

Mesoporous cobalt oxide (Co₃O₄) was studied for air electrode of Li-air rechargeable battery. In this study, mesoporous Co₃O₄ with an average pore radius of 3.09 nm and BET surface area of 99 m²/g was successfully prepared by using mesoporous SiO₂ for template. Prepared mesoporous Co₃O₄ was applied for air electrode of Li-air battery after mixing with Pd and it was found that the cell showed a reasonably large discharge capacity of 481 mAh/g_-cat. at 0.1 mA/cm² at the initial cycle. Energy efficiency for charge and discharge was estimated to be ca. 75%. Raman spectroscopy suggests that the main product during discharge was Li₂O₂ and formation of Li₂CO₃ was hardly observed.

View full abstract

To enhance a cyclability and a rate capability in bulk-type solid-state cells using a lithium metal electrode, an indium thin layer prepared by vacuum-evaporation was inserted at the interface between a lithium electrode and an Li₂S-P₂S₅ solid electrolyte layer. The Li/Li₄Ti₅O₁₂ cells using indium thin film were charged and discharged for 120 cycles reversibly and worked at the high current density of 1.3 mA cm⁻². Inserting lithium-alloy thin layer at the interface between the lithium electrode and the solid electrolyte layer brought about a good cyclability and a high rate capability for the all-solid-state lithium metal cells because of the formation and retention of intimate contacts at the interface. The obtained results are useful for achieving bulk-type solid-state lithium batteries with high power and energy densities.

View full abstract

To improve the deviation of cycling performances as Li-ion battery anode, we controlled the morphology of Ni-P layers deposited on Si particles in thick-film electrodes prepared by a gas-deposition method. The spotty Ni-P layers were uniformly coated on Si particles by an electroless deposition in a neutral bath. The resulting electrodes using Ni-P-coated Si exhibited excellent cycling performances and their good reproducibility. The reason for the improvement of performances is probably that the Ni-P layers can effectively release a stress generated by alloying/dealloying reactions of Li with Si.

View full abstract

Charge transfer kinetics at the Li metal electrode/electrolyte interface for three inorganic solid electrolytes and an organic liquid electrolyte were elucidated with two parameters, exchange current density (i₀) for Li/Li⁺ couple reactions and ionic conductivity in electrolyte. The former was evaluated by potential step method with a microelectrode, while the latter was done by electrochemical impedance spectroscopy. Both i₀ and ionic conductivity showed Arrhenius type dependence, and activation energies (E_a) for the charge transfer reactions and ionic conduction were evaluated. In the case of the organic liquid electrolyte, E_a for the Li/Li⁺ couple reactions was higher than that for ionic conduction because of the solvation/desolvation of Li⁺ ion. However, for the Li₂S-P₂S₅ solid solid electrolytes, the E_a for the Li/Li⁺ couple reactions was quite close to that for ionic conduction due to the lack of the solvation/desolvation.

View full abstract

Tetravalent Hf⁴⁺ ion conducting solid electrolytes, Hf_1−x/4(Nb_1−yW_y)_5/(5+y)P_3−xW_xO₁₂, were developed by partially replacing both the Nb⁵⁺ and P⁵⁺ sites in the NASICON-type HfNb(PO₄)₃ solid simultaneously with higher valence W⁶⁺ ions. Among the samples prepared, the highest conductivity was obtained for the Hf_3.85/4(Nb_0.8W_0.2)_5/5.2P_2.85W_0.15O₁₂ (y = 0.2; x = 0.15) solid, which contains a high amount of W⁶⁺ ions and holds the optimum lattice volume for Hf⁴⁺ ion conduction, enabling a significant reduction of the electrostatic interaction between the Hf⁴⁺ cation and the O²⁻ anions in the solid. The conducting species in the Hf_3.85/4(Nb_0.8W_0.2)_5/5.2P_2.85W_0.15O₁₂ solid was directly demonstrated to be the Hf⁴⁺ ion via DC electrolysis.

View full abstract

We have used 2-fluoroethyl propionate (2FEP), ethyl 2-fluoropropionate (E2FP), and ethyl 3-fluoropropionate (E3FP) as organic solvents for lithium secondary batteries. We describe the effect of position isomerism on the physical and electrochemical properties of monofluorinated ethyl propionates (EPs). The relative permittivities of E2FP and E3FP were higher than that of 2FEP, whereas the viscosity of E2FP was lower than those of 2FEP and E3FP. The ionic conductivities of 1 mol dm⁻³ LiPF₆ solutions in E2FP and E3FP were higher than those in 2FEP and EP above room temperature. The use of 2FEP or E2FP as a co-solvent greatly improved discharge capacities of Li | LiCoO₂ coin cells.

View full abstract

A solid electrolyte, Li₁₀GeP₂S₁₂, exhibits a high lithium ionic conductivity of 12 mS/cm at room temperature. Because of its high ionic conductivity, high charge-discharge performance would be expected for the all-solid-state batteries using the Li₁₀GeP₂S₁₂ electrolytes. In this study, all-solid-state batteries using Li₁₀GeP₂S₁₂, were constructed and their battery performances were examined. The batteries using the Li₁₀GeP₂S₁₂ electrolyte showed higher discharge capacities than those with glass electrolyte, 75Li₂S·25P₂S₅, particularly under the high-rate current discharge.

View full abstract

The double-layer capacitors and their degradation after aging process for nano-fibrous carbon electrodes with various graphitic structures have been compared. The nano-fibrous carbon with amorphous structure has successfully been prepared from biomass precursor. The nano-fibrous carbons with graphitic and amorphous structures exhibit similar specific surface area and double-layer capacitance. However, the change of the capacitances of the cells consisting of both carbons by the aging process under high temperature and high voltage condition are different. The amorphous nano-fibrous carbon electrode provides capacitance decrease by the deposit on electrode, while the graphitic nano-fiber provides capacitance increased likely due to the gas evolution.

View full abstract

The electrochemical insertion and de-insertion of sodium ion in hard carbon has been monitored in electrolytes consisting of sodium perchlorate (NaClO₄), propylene carbonate (PC), and an ionic liquid N,N-diethyl-N-methoxyethyl ammonium bis(trifluoromethane sulfonyl)imide (DEMETFSI). Voltammetric observation revealed that the reversible sodium insertion is inhibited by the content of DEMETFSI in the electrolyte. The reversible signs of sodium insertion become obvious when the volumetric content of DEMETFSI was 70%. The inhibition effect for sodium insertion by DEMETFSI is somewhat in relation to the change in coordination with sodium ion in electrolyte.

View full abstract

We have investigated the preparation and electrochemical characteristics of a lithium excess Li_4.3Ti₅O₁₂ carbon composite (Li_4.3Ti₅O₁₂/C) with different carbon sources (polyacryl acid, polyvinyl alcohol, and carboxy methyl cellulose sodium). We have prepared the lithium excess Li_4.3Ti₅O₁₂/C by a spray-drying method followed by calcining at 800°C in a nitrogen atmosphere. The TEM observations indicated that the surface of the Li_4.3Ti₅O₁₂/C particle was clearly covered with an amorphous carbon layer. The Li_4.3Ti₅O₁₂/C materials employing polyvinyl alcohol as the carbon source showed the best rate performance with the highest discharge capacity of 117 mAh (g-Li_4.3Ti₅O₁₂/C)⁻¹ at the 10C rate.

View full abstract

We were able to derive the dQ/dE vs. E curve by carrying out chronopotentiometric measurements using a lithium ion secondary cell incorporating a reference electrode. These in-situ measurements were found to be comparable to the ex-situ measurements, which were obtained using a graphite single particle method. The dQ/dE vs. E curves for the anode of the same type of cells, which contain storage deterioration and cyclic test deterioration data, were then obtained. As a result, an over potential increase in the charge and discharge peaks, as well as a decrease in the charge coulombs was observed. As a result of the dQ/dE vs. E curves of the anode of degraded lithium ion secondary cells incorporating a reference electrode, we were able to evaluate the graphite degradation under conditions close to real environmental conditions.

View full abstract

A quasi-solid-state composite electrolyte, consisting of N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide (DEME-TFSA)-lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA)-fumed silica nanoparticles, has been prepared for use in the electrolyte of a lithium-sulfur battery. Regardless of the solid-state like appearance, the quasi-solid-state electrolyte exhibited a high liquid-like apparent conductivity of 1.2 × 10⁻⁴ S cm⁻¹ at 308 K when the volume ratio of DEME-TFSA-Li-TFSA is 75% in the quasi-solid-state composite. By using the quasi-solid-state electrolyte, a quasi-solid-state lithium-sulfur battery was developed and the cell performance was evaluated. The cell has successfully exhibited initial discharge capacities of 1370 mAh g⁻¹ at 308 K with a 0.05 C via the conversion reaction of sulfur and lithium. During 10 charge-discharge cycles, the discharge capacity decreased to 600 mAh g⁻¹ due to the lower utilization ratio of sulfur by agglomeration of Li₂S at the electrode/electrolyte interface.

View full abstract

A trifluoromethyl group (CF₃-) is a strong electron-withdrawing substituent. We have synthesized 1-(2,2,2-trifluoroethoxy)-2-(2-methoxyethoxy)ethane (TFEMEE). The relative permittivity, viscosity, and anodic stability of TFEMEE were higher than those of 1-ethoxy-2-(2-methoxyethoxy)ethane (EMEE). The presence of the trifluomethyl group can weaken the attractive forces between molecules. The kinematic viscosity of TFEMEE was as low as that of EMEE especially at high temperatures. The use of TFEMEE as a co-solvent greatly improved cycling efficiency of a lithium anode.

View full abstract

Methyl propyl carbonate (MPC) shows a liquid temperature range that is wider than that of diethyl carbonate (DEC). Monofluorinated organic solvents exert the strong polar effect on the physical and the electrochemical properties. 2-Fluoropropyl methyl carbonate (2FPMC) and ethyl 2-fluoroethyl carbonate (E2FEC) are isomeric with each other. The relative permittivity and viscosity of 2FPMC were higher than those of E2FEC, MPC, and DEC. The ionic conductivity of 1 mol dm⁻³ LiPF₆ solution in 2FPMC was lower than those in E2FEC, MPC, and DEC. The use of 2FPMC as a co-solvent greatly improved cycling efficiency of a lithium anode.

View full abstract

Electrochemical deposition and dissolution of magnesium have been investigated in organic solutions consisting of alkylmagnesiumbromides/tetrahydrofuran (RMgBr/THF) having different alkyl-chains mixed with an ionic liquid. Coulombic efficiency for cathodic deposition and anodic dissolution of magnesium, obtained from voltammetric responses, decreased with the increase in the alkyl-chain length of the magnesium complex. The highest current response was observed in the electrolyte system consisting of CH₃MgBr/THF with a quaternary ammonium salt-based ionic liquid.

View full abstract

The behavior of lithium (Li) electrodeposition in room temperature ionic liquids (RTILs) containing aliphatic quaternary ammonium cation was investigated by in-situ optical microscope observation. As a result, round shape Li deposits were obtained after deposition in all cases with vinylene carbonate (VC) as an additive, while most deposits were dendritic without VC. AC impedance spectroscopic measurements indicated that the dendrite growth was suppressed when a surface film with large resistance was generated. The dendrite suppression effect by VC addition was confirmed in the Py14[TFSA]-based and TMHA[TFSA]-based electrolytes as well as in the PP13[TFSA]-based electrolyte.

View full abstract

Sydnone is one of the mesoionic heterocyclic aromatic compounds and is represented by the resonance hybrid of different polarized ionic structures. We have newly synthesized 3-propyl-4-propylsydnone (PPSD) and investigated its physical, chemical, and electrochemical properties. The electron-pair donability for PPSD was higher than that for propylene carbonate (PC), but the electron-pair acceptability for PPSD was lower. The addition of PPSD to an ethylene carbonate (EC)-diethyl carbonate (DEC) equimolar binary mixture at the molar ratio of 1:1:0.05 was effective for an increase in cycling efficiency of a lithium anode.

View full abstract

Influence of carbon surface on the discharge voltage of non-aqueous Li-O₂ battery was investigated. The discharge voltage definitely decreased with the graphitization of as-prepared carbon in spite of almost the same chemical information of carbon surface. Graphitization of carbon led to the dramatic morphological change from granular-type to angulated-type, and as a consequence, the number of defect part toward basal part was decreased. The defect parts of the carbon surface were suggested to promote the Li⁺ containing oxygen reduction reaction (Li⁺-ORR) related with the discharge voltage. Cyclic voltammetry indicated that the ORR potential in the Li⁺ containing media was positively shifted on the edge oriented carbon model plane, compared with those of the graphitized ones. It was, thus, concluded that the carbon surface including defects activated the Li⁺-ORR process on a cathode, resulting in the increase of discharge voltage.

View full abstract

A new synthetic route for TiO₂(B) via lepidocrocite type K_xLi_yTi_2−(x+y)/4O₄ (KLTO) was established. Pure layered titanic acids were obtained by proton exchange of KLTO in 3 mol dm⁻³ HNO₃ and HCl solution. The obtained titanic acid transformed into TiO₂(B) phase at 350°C and it was stable up to 450°C. Plate-like morphology particles was formed during proton exchange and transformation process to TiO₂(B). TiO₂(B) samples with high tapping density (0.9–1.1 g cm⁻³) was prepared and they delivered discharge capacities of more than 100 mAh g⁻¹ even at 2 C.

View full abstract

We prepared two 0.5Li₂MnO₃-0.5LiMn_xNi_xCo_(1−2x)O₂ (x = 1/3, 5/12) samples using the solution method, followed by heat treatment under vacuum reducing conditions. ICP and average-valence analyses clarified that the amounts of lithium and oxygen were decreased by reductive heat treatments. Results of a cycle performance test showed that the heat-treated samples exhibited a high discharge capacity, although the voltage regions contributing to the improvements depended on the metal composition, i.e., 3.3 and 3.8 V for x = 1/3 and x = 5/12, respectively. In order to discuss the crystal structure, a Rietveld analysis by neutron diffraction was carried out. A localized model in which Mn occupied the 4g site and Li occupied the 2b site in the Li₂MnO₃-type structure (S.G.; C2/m) resulted in good fitting.

View full abstract

Electrochemical properties and structure changes of nano-sized Li₄Ti₅O₁₂ during lithium (de)intercalation were investigated using a two-dimensional thin film electrode. Li₄Ti₅O₁₂ thin films were deposited on a Nb:SrTiO₃(110) substrate by a pulsed laser deposition technique. X-ray diffraction and reflectometry measurements confirmed the epitaxial growth of 27.6-nm-thick Li₄Ti₅O₁₂(110) films. Galvanostatic charge-discharge curves showed a large discharge capacity of 217 mAh g⁻¹ at the initial discharge cycle, although the reversible capacity decreased in subsequent cycles. In situ X-ray diffraction measurements clarified the drastic structural changes of the Li₄Ti₅O₁₂ film upon soaking in the electrolyte and during the first intercalation and deintercalation processes. The surface region of Li₄Ti₅O₁₂ had a different structure from the bulk during electrochemical cycling and could cause the nano-sized Li₄Ti₅O₁₂ electrodes to have high capacities and poor stabilities.

View full abstract

We inserted Li into or extracted Li from Li-inserted γ-Fe₂O₃, and analyzed the structure at relaxation time. For multi-stage Li insertion sample, it was indicated that Fe moved from 8a site to 16c site during Li insertion process and Fe moved from 16c site to 8a site at relaxation process after the Li insertion, and it was suggested that Li is inserted at 8a site with Li insertion process and Li moves from 8a site to 16c site after the Li insertion. For extraction sample, it was indicated that Fe moved from 16c site to 8a site during Li extraction process and it was suggested that Li is extracted from 8a site and Fe at 16c site moves into 8a site. It was considered that Li prefer 8a site to occupy kinetically and prefer 16c site thermodynamically, and that 8a site take a role as a diffusion path for both Li insertion and Li extraction. From the first principle calculation, 16c site preference to 8a site of Li was indicated.

View full abstract

We evaluated the charge-discharge behavior of a LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) electrode in a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImFSI) ionic liquid. At 163 mAh g⁻¹ after 50 cycles, the discharge capacity of the NMC cathode in LiTFSI/EMImFSI (TFSI⁻ = bis(trifluoromethylsulfonyl)imide) at 1.0 C in the voltage range 3.0–4.5 V (vs. Li/Li⁺) is comparable to that in the conventional electrolyte, LiPF₆/EC+DMC. Moreover, the rate capability of the cell with LiTFSI/EMImFSI exceeded that with LiPF₆/EC+DMC, especially at high rates, probably owing to the low resistance at the electrode/electrolyte interface. These results suggest that EMImFSI is a suitable electrolyte for lithium-ion batteries utilizing an NMC cathode.

View full abstract

The anode properties of SiO-C electrode materials [SiO-C:α-Fe₂O₃ = x:100 − x (wt%)] containing α-Fe₂O₃ fine powder (spherical particles, ca. 0.16 µm) as the additive were investigated in a lithium cell using lithium metal as the counter electrode. It was observed by scanning electron microscopy (SEM) that spherical α-Fe₂O₃ particles are present between the SiO-C particles. The SiO-C alloying-reaction electrode exhibited a high capacity of ca. 1330 mAh g⁻¹. However, the electrode caused a gradual decrease in cycle capacity at around 30 cycles. On the other hand, the α-Fe₂O₃ conversion-reaction electrode showed a high discharge capacity of ca. 800 mAh g⁻¹, and a good cycle performance over 50 cycles. In addition, charge and discharge were possible even for an α-Fe₂O₃ electrode without the addition of an electronic conductive material such as carbon. SiO-C electrodes containing a small amount of α-Fe₂O₃ fine powder exhibited higher capacities and improved cycle performances compared with the SiO-C electrode. The electrode with x = 50 exhibited a discharge capacity of ca. 1000 mAh g⁻¹ and a good cycle performance over 70 cycles. The α-Fe₂O₃ fine powder works as a conductive path between the SiO-C particles, and prevents the disintegration of the electrode.

View full abstract

Two 7.5 Ah lithium-ion secondary cells were fabricated with the same design of electrodes and separator. One cell had the mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as an electrolyte solution containing vinylene carbonate as an additive to control the solid electrolyte interphace, while the other cell used a conventional electrolyte solution. These cells were used in cycle-life and storage tests and the increases in their impedances were monitored. The activation energy calculated from the temperature dependence of the impedance remained constant after 1,000 h. It was found that the additive like vinylene carbonate do not influence to the activation energy as a function of time for the electrochemical reaction inside the cell.

View full abstract

SnO₂ and CuSnO₂ were successfully sprayed onto a Ni substrate using electrostatic spray deposition to produce negative electrode films with highly porous morphologies for use in lithium batteries. The spray-deposited SnCl₂ decomposed to Sn during heat treatment at 230°C under inert atmosphere and then was oxidized to SnO₂ being exposed to the air. The resultant SnO₂ film, which did not contain any additives such as binders or conductive additives, was used as a negative electrode. At the beginning of some charge-discharge cycles, the SnO₂ film achieved a capacity of 780 mAh g⁻¹, which was close to its theoretical capacity. However, its capacity subsequently decreased because of crystal growth during the Li alloying-dealloying process. The capacity retention of the film was improved by adding Cu to the precursor solution. Although the initial capacity of the CuSnO₂ film was lower than that of the SnO₂ film, the CuSnO₂ film displayed better cycle performance; i.e., it possessed more than double the capacity of the SnO₂ film after 100 cycles.

View full abstract

The present LiFePO₄/C cathode material is synthesized via carbothermal reduction by a high-frequency induction heating method in extremely short heating time (within a few minutes). The electric conductivity of LiFePO₄/C is improved to 1.9 × 10⁻² S cm⁻¹ by a short-time annealing process following a sintering process at 900°C. An increase in the particle size and formation of Fe₂P as impurity are suppressed by annealing at a relatively low temperature: 700°C. The cathode containing annealed LiFePO₄/C shows discharge capacities of 156.0, 136.3, and 100.1 mAh g⁻¹ at 1/10, 1, and 10 C-rates (1 C = 170 mA g⁻¹), respectively, and its polarization between charge and discharge is smaller than that of non-annealed LiFePO₄/C. Furthermore, the annealed LiFePO₄/C cathode shows better charge and discharge rate performance than the non-annealed LiFePO₄/C cathode.

View full abstract

An order-disorder behavior of Li and Ni in LiNi_0.5Mn_0.5O₂ was investigated by means of the ion-exchange preparation and the high-temperature X-ray diffraction measurement using synchrotron radiation. Rietveld refinements at room temperature showed that the occupancy of Ni at the Li site prepared by ion-exchange is less than those by co-precipitation and it increases with an increase in temperature and time for the ion-exchange reaction. In addition, it was found that by high-temperature XRD measurements an order-disorder transition occurs at around 700 K for the sample prepared by the ion-exchange. Further, we have confirmed that an optimum ion distribution in LiNi_0.5Mn_0.5O₂ exists to show superior electrochemical performance as a positive electrode for Li-ion batteries. These results demonstrate a close relation between crystal structure and electrochemical properties for LiNi_0.5Mn_0.5O₂ which is a member of Li₂MnO₃-based materials. It is the first report that presents an order-disorder transition of LiNi_0.5Mn_0.5O₂ at elevated temperatures, on the basis of the experimental results.

View full abstract

The oxygen evolution and reduction properties of La_0.8Sr_0.2CoO₃ are characterized using two-dimensional model electrodes with different reaction planes, synthesized on SrTiO₃ single crystal substrates by pulsed laser deposition. Thin-film X-ray diffraction and reflectivity measurements confirm the epitaxial growth of 29-nm-thick La_0.8Sr_0.2CoO₃ (001), (110), and (111) films on SrTiO₃ (001), (110), and (111) substrates, respectively. Cyclic voltammetry curves in 1-M KOH aqueous solution indicate that the (110) film has the highest activity for oxygen evolution and reduction reactions. An expansion of the La_0.8Sr_0.2CoO₃ lattice is observed after the oxygen reduction process, indicating the formation of oxygen defects, with the highest number of defects being produced in the (110) film. X-ray reflectivity analysis demonstrates the formation of a surface layer on the La_0.8Sr_0.2CoO₃ films during electrochemical cycling due to the decomposition of La_0.8Sr_0.2CoO₃. The surface structure constructed at the electrode/electrolyte interface is a crucial factor influencing oxygen evolution and reduction activity of La_0.8Sr_0.2CoO₃.

View full abstract

Bulk-type all-solid-state batteries currently use a compressed powder pellet with the thickness of several hundred micrometers as a solid electrolyte separator. In this study, all-solid-state lithium secondary batteries (In/LiCoO₂) with a thin Li₂S-P₂S₅ solid electrolyte separator were constructed; their charge-discharge performance was investigated. The all-solid-state batteries using the Li₂S-P₂S₅ separator with a thickness of 3 µm were charged and discharged with the capacity of 93 mAh g⁻¹. This result suggests that bulk-type all-solid-state batteries were successfully prepared with the solid electrolyte separator with several micrometer thicknesses.

View full abstract