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To improve the cycling performance of carbon-coated SiO (SiO–C) as a high-capacity negative electrode material, we have used X-ray absorption fine structure measurements to assess the electronic state of Si and investigate phase changes during charge/discharge cycles. The results suggest that Si 4 + continuously absorbs Li during the charging process, mainly forming Li 4 SiO 4 . Li silicates with a lower valence are also formed. Neutral Si and Si 2 + mainly contribute to Li insertion during the initial charging stage, whereas low-valence oxides like Si 1 + and Si 2 + form during the middle charging stage. The fully charged state contains all Si phase states. These results suggest that during the discharge process, Li is mainly released from composites with Si 2 + -oxidation state, and Li is partially released from Li 4 SiO 4 at a higher voltage forming Si 0 . Si O composites with Si 1 + –Si 3 + reversibly shift to a higher valence state during cycling. We confirmed that Li 4 SiO 4 readily releases Li after long-term cycling, and Si O composites at the end of the cycle change to a higher valence state. The results indicate that disproportionation is a primary factor decreasing cell cycle performance and confirm that it is possible to design LiB cells with improved cycling performance by controlling the disproportionation linked to the structural deterioration of SiO–C.

Lithium ion batteries for electric vehicle (EV) applications require high energy density and high safety. For this reason, layered oxides such as LiNiCoMnO2 (NCM) and LiNiCoAlO2 (NCA) are selected as positive electrode materials because of their large charge / discharge capacity. In recent years, capacity and energy density have been increasing due to an increase in the Ni content. However, an increase in the Ni content tends to cause heat generation and oxygen release from the crystal structure during charge / discharge, which leads to a decrease in battery safety such as the battery being easily ignited. On the other hand, olivine-type cathode materials such as LiFePO4 (LFP) and LiFeMnPO4 (LFMP) are widely known as highly safe cathode materials that generate less heat and do not cause oxygen release from the crystal structure during charge / discharge. However, these olivine-type materials have smaller charge / discharge capacities and powder densities than layered oxides, and thus have low energy densities and are therefore difficult to apply to batteries for electric vehicles. In this study, we investigated the effects of mixing layered oxides and olivine-type materials to compensate for the advantages and disadvantages of layered oxides and olivine-type materials, and to achieve cathode materials with both high energy density and high safety. As the cathode electrode material, NCM532 was used as a layered oxide, and LFP or LFMP produced by a hydrothermal synthesis method were used as an olivine type material. These materials were mixed at a predetermined weight ratio when preparing the electrode paste. The prepared paste was applied to an Al foil and dried to obtain a mixed electrode of NCM / olivine type material. The mixed state of NCM / olivine type material in the electrode was observed by SEM. As a result, a high dispersion state was obtained regardless of the mixing ratio of both materials. For charge / discharge tests, 50 mAh Laminate cells were prepared using the NCM / olivine type material electrode for cathode and natural graphite for anode. As a result, NCM / LFP mixed electrode showed a slight decrease in energy density due to an increase in the capacity of the 3.1 V region derived from the change in Fe valence as the amount of LFP mixed increased. On the other hand, in the NCM / LFMP mixed electrode, the capacity in the 3.9 V region derived from the change in the Mn valence in LFMP increased as the amount of LFMP mixed increased. Since the Fe content of LFMP was small, the capacity in the 3.1 V region was hardly observed due to the increase in the amount of LFMP. For a safety test, a 2 Ah laminate cells were prepared and an overcharge test was performed. Overcharging was performed up to a voltage of 30 V by applying a current of 20 A. As a result of evaluating the cell using the NCM single electrode, the surface temperature rose to 195.2 oC and the cell was flamed. On the other hand, in the cell using the NCM / LFP electrode, the surface temperature during overcharging was about 50 oC, and although the cell swelled, no leakage or flame occurred. In the case of the cell using the NCM / LFMP electrode, the cell surface temperature increased to 155.4 oC and leakage occured at a weight ratio of 80/20. At the weight ratio of 70/30 or less, the cell surface temperature decreased to around 50 oC, and the cells only swelled and did not flame. In conclusion, we were able to obtain a positive electrode material that has both high safety and high energy density by mixing NCM with an olivine type positive electrode material. In the future, we will consider mixing olivine-type cathode materials with higher energy density oxides such as NCA. Figure 1

The Li pre-doping free cells using tin-phosphate glass (GSPO)-silicon composite anodes and lithium-rich layered oxide (LR-NMC) cathode were developed, in which the irreversible capacity of the cathode was utilized in order to compensate that of the anode. The irreversible capacity was equalized to that of the cathode at a Si/(GSPO + Si) mass ratio, r , of 0.8. The cell using the composite anode of r = 0.8 displayed a discharge capacity of 187 mAh g −1 as the cathode capacity at 30 °C and the 0.1C-rate. The long cycle performance of over 100 charge–discharge cycles was achieved. The cells also showed an excellent high- and low-temperature performance and a good rate performance. A stable performance was observed for over 50 cycles at 60 °C and the 0.5C-rate, and a high capacity of ca. 100 mAh g −1 as the cathode capacity even at −20 °C and the 0.2C-rate. The cell could discharge even at the 10C-rate. Furthermore, the Li pre-doping free cell showed an outstanding safety performance during the nail penetration test. When the 1500 mAh cell using the composite anode was penetrated by a nail, no thermal runaway was observed during the test.

Olivine lithium manganese phosphate (LiMnPO 4 ) becomes research focus because of its high energy density and improved thermal stability. However, its application in lithium ion batteries suffers severely from poor electrochemical activity due to low conductivity and structural instability upon the charge and discharge process. By applying a high-energy ball-milling method we succeed in improving the capacity delivery and rate capability. LiMnPO 4 materials ball-milled without or with acetylene black are able to deliver a high capacity of 135 and 127 mAh g −1 , respectively, more than 50% greater than the pristine one. Particularly, the latter also shows an improved discharge plateau and stable cyclability. High-energy synchrotron radiation X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, laser particle analysis, and galvanostatic charge and discharge are employed to understand the effect of ball-milling on the LiMnPO 4 material.

Well-crystallized LiFePO 4 nanoparticles have been directly synthesized in a short time via hydrothermal process in the presence of organic acid, e.g. citric acid or ascorbic acid. These acid-mediated LiFePO 4 products exhibit a phase-pure and nanocrystal nature with size about 50–100 nm. Two critical roles that the organic acid mediator plays in hydrothermal process are recognized and a rational mechanism is explored. After a post carbon-coating treatment at 600 °C for 1 h, these mediated LiFePO 4 materials show a high electrochemical activity in terms of reversible capacity, cycling stability and rate capability. Particularly, LiFePO 4 mediated by ascorbic acid can deliver a capacity of 162 mAh g −1 at 0.1 C, 154 mAh g −1 at 1 C, and 122 mAh g −1 at 5 C. The crystalline structure, particle morphology, and surface microstructure were characterized by high-energy synchrotron X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and Raman spectroscopy, respectively. And the electrochemical properties were thoroughly investigated by galvanostatic test and electrochemical impedance spectroscopy (EIS).

It was revealed that the metastable region, in which liquid phase epitaxy (LPE) of GaN single crystals proceeds without the generation of polycrystals, expands with growth temperature in the Na flux method. The metastable region appears when LPE growth proceeds at temperatures above 1073 K, although generation of polycrystals inevitably occurs on a crucible at temperatures less than 1073 K. The highest growth rate of 14 μm/h in a small experimental setup was achieved at a temperature of 1163 K with a nitrogen pressure of 5.5 MPa due to complete suppression of the growth of polycrystals on a crucible, even though the supersaturation at this condition reached a fairly high level. Also, an LPE crystal with a flat surface could easily be obtained under high-temperature conditions.

We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 (cm-2), The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the + and – c directions, resulting in a Na concentration of 4.2 × 1014 (cm-3) in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.

We found that a carbon additive could suppress the unfavorable generation of polycrystals in a crucible without reduction in the yield of GaN in the Na flux method. The suppression of polycrystals due to the effect of carbon significantly increased the growth rate of liquid phase epitaxy (LPE), which has been the biggest problem of the Na flux LPE, and enabled an increase in the growth rate above 20 μm/h. A 3 mm-thick 2 in GaN crystal was obtained for the first time. In addition, the carbon additive was found to have another effect in that the nonpolar face could be widely developed. SIMS measurements revealed that carbon added into a Ga–Na mixed melt was hardly taken into LPE crystals, although carbon did have some favorable effects.

The detailed phase transformation in the charge-discharge process for the nickel hydroxide electrode have been examined by using the high-energy synchrotron X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD). The structural refinement for the β-Ni(OH)2 and the β-NiOOH have been done successfully on the basis of two phase models of the ideal and fault phases. At 0% of SOC, it was found that the sample consisted of 80% of the ideal β-Ni(OH)2 phases and 20% of the fault ones. At 100% of SOC, the ideal and fault β-Ni(OH)2 phases were transformed to the ideal and fault β-NiOOH phases, respectively. It was found that the fault phases contained a larger amount of intercalated potassium ions and H2O (OH−) than the ideal ones. At 150% of SOC, the only ideal β-NiOOH phases was transformed to the ideal γ-NiOOH phases, accompanied by further intercalation of the potassium ions and H2O (OH−) into the interlayer

We observed the memory effect, the working voltage lowering, which occurred due to the repeated partial charge-discharge cycling of a Ni-capacity-limited alkaline cell, for example 50–70% state of charge, and γ-NiOOH in the Ni electrode was identified as the cause of the memory effect. During this condition, γ-NiOOH is proposed to be formed by partial overcharging due to the heterogeneity of the conductive materials such as cobalt compounds in the Ni electrode. Using nano-sized conductive materials homogeneously dispersed in the electrode leads to a significant effect on the suppression of the memory effect occurring due to the repeated partial charge-discharge cycling.

In the growth of GaN single crystal using the Na flux method, we succeeded in clarifying the role of Na in promoting nitrogen dissolution in the Ga-Na melt system above 900 K. At the gas-liquid interface of high temperature Ga-Na melt, Na functions to ionize the nitrogen gas. The ionization of the nitrogen gas results in drastic increase of nitrogen dissolution in the melt. In consequence, these synthesize GaN single crystals easily. On the other hand, the addition of a minor amount of Ca or Li to the Ga-Na melt system also increases the nitrogen dissolution. However, the additives function to maintain the nitrogen dissolved in the Ga-Na melt, which results in drastic increase in the nitrogen concentration. In the present study, we report the solubility of GaN in the Ga-Na system and the threshold pressure of nitrogen gas to grow GaN. On the basis of these data, we propose a growth mechanism of GaN and the role of additives to maintain nitrogen.

We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 cm-2. The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the "+" and "-" c directions, resulting in a Na concentration of 4.2×1014 cm-3 in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.

We developed a new liquid phase epitaxy (LPE) growth method for growing large GaN single crystals. In this method, the homo-epitaxial growth of the GaN crystal proceeds by nitrogen dissolution against a Na-Ga solution film coating the GaN thin-film substrate. A growth rate of 4 µm/h was achieved though nitrogen pressure was relatively low at 9.5 atm. Furthermore, the GaN film grown by this method showed a flat surface.

Growth of large and transparent GaN single crystals was carried out by applying the liquid phase epitaxy (LPE) method in a Ca-Na mixed flux system. We have previously reported LPE growth of GaN in a Na flux system, and that GaN crystals grown by LPE have extreme low dislocations and show excellent photoluminescence characteristics. In this study, use of a Ca-Na mixed flux system enabled us to grow transparent GaN crystals under low nitrogen pressure and to further improve the photoluminescence (PL) characteristic. The dislocation density of this crystal is very low (2 ×105 cm-2 in highest point).

We found the yield of GaN grown in a Li-Na flux system to be much higher than that grown in Na flux. Nitrogen dissolution seemed to increase with Li in solution, which promotes the growth of GaN crystals. In this paper, we report that the yield of GaN and habit modifications greatly depend on the composition of the Li-Na flux system and that GaN can be grown in a pure Li flux at about 50 atm.

The structural analysis for the cobalt oxyhydroxide has been done by using high-energy synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis. The relationship between the structure and the electrical conductivity for cobalt oxyhydroxide was investigated. The structural refinement for cobalt oxyhydroxide heat-treated in the temperature range of 80-160°C has been done successfully on the basis of two phase models (L and S phase) with large and small c lattice constants. With increasing treatment temperature, the phase abundance for the L phases was increased, whereas the one for the S phases was decreased. By heat-treatment above 100°C, the cobalt ions for the cobalt oxyhydroxide were oxidized to the higher oxidation state over 3. The electrical resistivity was extremely decreased by treatment temperatures above 100°C. The increase in the electrical conductivity for the cobalt oxyhydroxide could be explained by the increase in cobalt oxidation state.

We grew a 12×5×0.8 mm GaN single crystal using the liquid phase epitaxy (LPE) method with Na flux. This GaN single crystal was grown on a 3 µm-thick GaN thin film synthesized on sapphire using the metal organic chemical vapor deposition (MOCVD) method. It grew in a polyhedral form larger than the MOCVD-GaN substrate. Results indicate that a MOCVD-GaN thin film on a sapphire substrate functions as a seed crystal in Na flux. The use of mixed nitrogen gas containing 40% ammonia instead of pure N2 gas also enabled the growth of a 10 µm-thick GaN homo-epitaxial film on an MOCVD-GaN film under 5 atm, the lowest reported pressure for growing GaN in Na flux. In this paper, we describe the liquid phase epitaxy (LPE) technique for growing bulk GaN single crystals, as well as the results of photoluminescence (PL) measurements. We also compare the PL intensity of the bulk GaN obtained in this study and the MOCVD-GaN. PL measurements revealed that the peak intensity of GaN single crystal grown by LPE indicates 40 times larger than MOCVD-GaN film. Also, dislocation density of bulk GaN crystals could be drastically reduced by the LPE growth technique.