齋藤 守弘

Four ethers were compared as solvents of lithium naphthalenide (Li-NTL) solutions to pre-dope Li into Si electrodes. The solvents of the Li-NTL solutions affected the stability and equilibrium potential (V (eq)). X-ray diffraction, thermodynamic characterization and ultraviolet-visible (UV-vis) spectroscopy were used to clarify the effects of the solvation structure, the lowest unoccupied molecular orbital (LUMO) energy of the solvent molecule and the ion pair structure between Li+ ions and naphthalenide radical anions ([NTL](center dot-)) on doping capacity. A Li-NTL solution having a low V (eq) and sufficient stability under potentials as low as that of Li metal was found to provide the highest pre-doping capacity. In particular, a 2-methyltetrahydrofuran (MeTHF) solution exhibiting the lowest V (eq) showed a pre-doping capacity as high as 3250 mAh g(-1) after 24 h. UV-vis spectra and Walden plots indicated that a Li-NTL solution using MeTHF provided less dissociation than a tetrahydrofuran (THF) solution. The doping capacity is evidently determined by the V (eq) of the Li-NTL solution as a consequence of the dissociation equilibrium of the ion pair of the solvated Li+ ion and [NTL](center dot-) radical ions.

The realization of secondary lithium-oxygen batteries (Li-O-2 batteries, LOBs) with large gravimetric energy density requires the development of an innovative electrolyte with high chemical stability that allows the charge-discharge reaction to proceed with low overvoltage. In this study, we evaluated the potential of an electrolyte solvent, N,N-dimethylethanesulfonamide (DMESA) with a sulfonamide functional group, at a current density of 0.4 mA cm(-2) and a capacity of 4 mA h cm(-2). The voltage at which CO2 was generated during charging was substantially higher than that of a tetraglyme (G4)-based electrolyte with redox mediators, which is one of the standard electrolytes used for LOBs. Experiments using a C-13-containing positive electrode revealed that CO2 generated during charging mainly originated from the decomposition of the positive electrode. The analyses of the charging profile in conjunction with differential electrochemical mass spectrometry suggested the formation of highly degradable lithium peroxide (Li2O2) in the DMESA-based electrolyte. The formation of highly degradable Li2O2 enables a reduction of the charging voltage, leading to further suppression of the electrolyte decomposition.

Recently, LiNO3-based electrolytes using tetraglyme (G4) solvent (LiNO3/G4) have attracted increasing attention for non-aqueous rechargeable Li-air (O-2) batteries (LAB) because of the bifunctional effect of NO3- anion as both redox mediator (RM) at air electrode and additive to form Li2O layer on the surface of Li metal negative electrode (NE), which suppresses Li dendrite growth and electrolyte decomposition. However, the dissociation degree of LiNO3 salt was quite low, which causes to low ionic conductivity and the above effects of NO3- would not work effectively in the electrolyte. In this study, we tried to apply dual solvent system to the LiNO3/G4 electrolyte. Namely, acetonitrile and dimethyl sulfoxide (DMSO) with relatively high dielectric constant and low viscosity were mixed with G4 solvent to increase the number per volume and mobility of Li+ and NO3- as carrier ions for reduction of the large overpotential during charge process and enhancement of the power density. The DMSO mixed electrolyte greatly reduced the large charge overpotential and relative stable operation for the LAB (Li vertical bar O-2) cells. Furthermore, the Li2O passivation layer formed by NO3- anion effectively suppressed the electrolyte decomposition at Li metal NE. These effects were enhanced especially at higher rate of discharge/charge operation. (C) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.

To extend the cyclability of a hybrid capacitor using Li pre-doped Si negative electrodes (NEs) (Si-CAP), we applied a polyimide binder and fluoroethylene carbonate (FEC) as additives in the Li pre-doping step. This binder successfully improved the cyclability by fixing Si nanoparticles as the NE active material on the Cu foil current collector to resist the volume change caused by Li-Si alloying. After Li pre-doping, there were no cracks on the Si NE surfaces, which had been observed in previous work using sodium carboxymethyl cellulose binder. Furthermore, FEC addition into the electrolyte at Li pre-doping improved homogeneity of the Li-Si alloying. As a result, a large amount of Li was pre-doped to the Si NE and a lower open-circuit potential was maintained. A Si-CAP cell of [Li alloyed Si I activated carbon] using polyimide binder exhibited relatively stable charge/discharge behavior and cyclability was maintained up to 800 cycles. Stability of the charge/discharge performance of FEC-containing Si-CAP was improved by formation of a LiF-rich solid-state interphase film on the Si NE. X-ray photoelectron spectroscopy revealed suppression of decomposition of the propylene carbonate solvent by electrochemical reduction. (C) The Electrochemical Society of Japan, All rights reserved.

Silicon is electrochemically alloyed and de-alloyed with Li at potentials close to Li+/Li and is widely recognized as the most promising candidate for the anode of LIBs with a high energy density (250-300 Wh kg(-1)) in the next generation. The most serious issue of Si anode is poor capacity retention owing to large volume changes during charging and discharging. We developed Si LeafPowder (R) with a nano-flake structure to overcome the poor capacity retention. However Si anodes still have some problems such as large irreversible capacity, ceaseless electrolyte decomposition, swelling of the electrode, etc. for use in high-energy density LIBs in the next generation. In this article, these problems and the challenges to mitigate them are overviewed based on our resent data obtained by Si nano-flakes (Si LeafPowder (R)). (C) The Electrochemical Society of Japan, All rights reserved.

To clarify the relationship between ion transport behavior and Li deposition/dissolution reaction at Li metal negative electrode (NE) for Li-air batteries, 1.0 M of LiSO3CF3/tetraglyme(G4) and LiN(SO2CF3)(2)/G4 electrolytes were selected, and the Li+ transport were evaluated from two aspects, i.e.i) Li+ supplying rate to the Li metal NE and ii) charge transfer rate through solid state interphase (SEI) films at Li metal NE. For the former aspect, self-diffusion coefficients D of Li+, anion and G4 solvent together with viscosity eta, density rho, ionic conductivity sigma of electrolytes were measured, and the degree of apparent dissociation. alpha(app) of Li salts was also estimated from the D and sigma. On the other hand, the later aspect was examined with Li | Li symmetric cells containing the glyme-based electrolytes. As a result, the LiN(SO2CF3)(2)/G4 having a higher supplying rate, i.e. sigma, owing to its higher. alpha(app) exhibited an excellent Li deposition/dissolution and Li-air cell performance.

To elucidate factors affecting ion transport in capacitor electrolytes, five propylene carbonate (PC) electrolytes were prepared, each of which includes a salt ((C2H5)(4)NBF4, (C2H5)(4)NPF6, (C2H5)(4)NSO3CF3, (C2H5)(3)CH3NBF4 and LiBF4). In addition to conventional bulk parameters such as ionic conductivity (s), viscosity (h) and density (r), self-diffusion coefficients (D) of the cation, anion and PC were measured by pulsed-gradient spin-echo (PGSE) NMR. Interaction energies (Delta E) were calculated by density function theory calculations based on Hard and Soft Acids and Bases (HSAB) theory for cation-anion (salt dissociation) and solvent-cation/anion (solvation). Delta E values are related to the salt dissociation and solvation, which affect ion diffusion radii formed by solvation and/or ion pairs. The calculated solvation Delta E values were small (around 0.30 eV) and salt dissociation energies were also small. For comparison, the Delta E value for PC-Li+ interaction was larger than that for ammonium cations, because of strong Li+ Lewis acidity. Ammonium salts are highly dissociated and each ion forms a weakly solvated structure, which is quite different from Li+ electrolytes. Weak solvation for the cation and anion in the ammonium salts are important in enhancing fast ion transfer and electrode reactions in capacitor devices.

To clarify the relationship between Li+ transport rate in glyme-based electrolytes and Li deposition/dissolution behavior at Li metal negative electrode (NE) in Li-air batteries (LAB) systems, 1.0 M tetraglyme (G4) electrolytes were prepared containing a Li salt of LiSO3CF3, LiN(SO2CF3)(2), or LiN(SO2F)(2). Two aspects of Li+ transfer between the two phases, i.e., G4 electrolyte vertical bar Li metal NE, were evaluated, namely i) Li+ supplying rate and ii) Li+ charge transfer rate through solid electrolyte interphase (SEI) films. The former was investigated by self-diffusion coefficients D of Li+, anions, and G4 solvent together with ionic conductivity sigma, viscosity, density, and apparent dissociation degree alpha(app) of the Li salts estimated by the Nernst-Einstein equation. The latter was evaluated with Li vertical bar Li symmetric and LAB (Li vertical bar O-2) cells containing the electrolytes. The Li deposition/dissolution reaction basically depended on the Li+ supplying rate in the Li vertical bar Li cell; however Li dendrites were formed. Conversely, the LAB cell performance was controlled by Li oxide layers formed on the NE, resulting in similar discharge/charge properties without Li dendrites. The effects of surface-oxidation was also confirmed with Li vertical bar Li cells containing O-2 gas, where both SEI and charge transfer resistances were reduced. (C) The Author(s) 2017. Published by ECS. All rights reserved.

To elucidate the determination factors affecting Li-ion transport in glyme-based electrolytes, six kinds of 1.0 M tetraglyme (G4) electrolytes were prepared containing a Li salt (LiSO3CF3, LiN(SO2CF3)(2), or LiN(SO2F)(2)) or different concentrations (0.5, 2.0, or 2.7 M) of LiN(SO2CF3)(2). In addition to conventional bulk parameters such as ionic conductivity (sigma), viscosity (eta), and density, self-diffusion coefficients of Li+, anions, and G4 were measured by pulsed-gradient spin-echo nuclear magnetic resonance method. Interaction energies (Delta E) were determined by density functional theory calculations based on the supermolecule method for Li+-anion (salt dissociation) and G4-Li+ (Li+ solvation) interactions. The Delta E values corresponded to ion diffusion radii formed by solvation and/or ion pairs. The order of dissociation energies Delta E was LiSO3CF3 > LiN(SO2CF3)(2) > LiN(SO2F)(2), which agreed well with the dissociation degree of these salts in the electrolytes. From the obtained knowledge, we also demonstrated that increasing the mobility and number of carrier ions are effective ways to enhance sigma of glyme-based electrolytes by using 1,2-dimethoxyethane with lower h and similar dielectric constant to those of G4.

To explore the utility of Si negative electrodes (NEs), we fabricated a new hybrid capacitor system using a Li pre-doped Si (Li alloyed Si) NE, denoted Si-CAP [Li alloyed Si vertical bar activated carbon (AC)]. The Si-CAP cell exhibited a relatively high charge/discharge capacity and higher cell voltage than that of a reference electric double-layer capacitor (EDLC) [AC vertical bar AC]. The effects of additives (vinylene carbonate or fluoroethylene carbonate (FEC)) in Li pre-doping on cell performance were also investigated, with the goal of forming suitable solid electrolyte interphase films on the Si NE surface. The discharge capacity per weight of Si, i.e., Si utilization ratio, of the cells was increased by the additives, especially FEC. Vacuum pressure impregnation (VPI) treatment prior to Li pre-doping was also performed to further increase the Si utilization ratio. The combination of both FEC addition and VPI resulted in a discharge capacity of ca. 700 mAh g(Si)(-1), corresponding to a high energy density of 114 Wh kg(NE + positive electrode)(-1). The Li pre-doping method also improved the rate capability; > 60 Wh kg(-1) was attained even at 1.0 mA cm(-2) (ca. 2500 mA g(Si)(-1)), which was three times larger than that of the reference EDLC. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.

The average self-diffusion coefficients of cations and anions increased with increasing the HF-concentration and temperature in (CH3)(3)N center dot mHF, (CH3)(4)NF center dot mHF, (C2H5)(3)N center dot mHF, and (C2H5)(4)NF center dot mHF melts. The behavior of the self-diffusion coefficients of cations and anions in every melt is in good agreement with an increase in ionic conductivity and a decrease in viscosity. The transference number of (FH)nF(-) anions estimated from the values of self-diffusion coefficients for each ion in every melt was almost constant and their values were around 0.6 in the (CH3)(3)N center dot mHF melt and around 0.7 in the (CH3)(4)NF center dot mHF, the (C2H5)(3)N center dot mHF, and the (C2H5)(4)NF center dot mHF melts. The value of theoretical conductivity (sigma(NmR)) obtained from the average self-diffusion coefficients of cations and anions by using Nernst-Einstein equation was higher than the value of experimental ionic conductivity (sigma). Therefore, it is concluded that the diffusion of (FH)nF(-) anions is faster than that of each cation in every melt and that a part of both cation and anion in each melt may associate by interaction between both ions. (C) 2015 Elsevier Ltd. All rights reserved.

In pure NF3 plasma, the etching rates of four kinds of single-crystalline SiC wafer etched at NF3 pressure of 2Pa were the highest and it decreased with an increase in NF3 pressure. On the other hand, they increased with an increase in radio frequency (RF) power and were the highest at RF power of 200W. A smooth surface was obtained on the single-crystalline 4H-SiC after reactive ion etching at NF3/Ar gas pressure of 2Pa and addition of Ar to NF3 plasma increased the smoothness of SiC surface. Scanning electron microscopy observation revealed that the number of pillars decreased with an increase in the Ar-concentration in the NF3/Ar mixture gas. The roughness factor (Ra) values were decreased from 51.5nm to 25.5nm for the As-cut SiC, from 0.25nm to 0.20nm for the Epi-SiC, from 5.0nm to 0.7nm for the Si-face mirror-polished SiC, and from 0.20nm to 0.16nm for the C-face mirror-polished SiC by adding 60% Ar to the NF3 gas. Both the Ra values of the Epi- and the C-face mirror-polished wafer surfaces etched using the NF3/Ar (40:60) plasma were similar to that treated with mirror polishing, so-called the Catalyst-Referred Etching (CARE) method, with which the lowest roughness of surface was obtained among the chemical mirror polishing methods. Etching duration for smoothing the single-crystalline SiC surface using its treatment was one third of that with the CARE method. © 2014 American Vacuum Society.

Single-nanocrystalline SnO2 (2-4 nm phi) particles completely encapsulated within hollow-structured carbon black structures (Ketjen Black (KB), typically 40 nm phi) were prepared using our original in situ ultracentrifugation (UC treatment) materials processing technology. Ultracentrifugation at 75 000g induces an in situ sol-gel reaction that brings about optimized linking between limited-size SnO2 nanocrystals and microcrystalline graphitic carbons of KB. Efficient entanglement and nanonesting have been accomplished by simultaneous nanofabrication and nanohybridization in the UC treatment, specifically at a ratio of SnO2/KB = 45/55. This composite exhibited a reversible capacity of 837 mA h g(-1) per composite, equivalent to 1444 mA h g(-1) (per pure SnO2 after subtracting the capacity attributed to KB in the composite) for remarkably many cycles, over 1200. Such high performance in regard to both capacity and cyclability has never been attained so far for SnO2 anode materials. The reversibility of changes in the Sn valence state (defined as "formal valence state" in the manuscript) from Sn(2.9+) to Sn(4.4-) was demonstrated by in situ XAFS measurements during the lithiation-delithiation process. Peculiar nanodots of typically 2-4 nm that look like single-crystal SnO2/carbon core-shell structures were found for the optimized dose ratio (45/55) in the HRTEM observation. After 10 cycles, all the materials showed complete encapsulation of the same-sized nanoparticles, which were covered and nested within the KB matrix and an electrolyte-derived polymeric film. These results indicate that the initially prepared SnO2/KB composites were transformed into a new species, represented as LixSnO1.45 (x:0-7.3), which shows perfect reversibility and cyclability. This species can exchange a total of 7.3 electrons, including 2.9 electrons for the conversion reaction (1-2 V) and 4.4 electrons for the subsequent alloying process (0-1 V).

Surface fluorination of TiO2(B) powder was conducted by pure F-2 gas at room temperature for 1 h and the effect on the charge/discharge properties was examined as a negative electrode of Li-ion batteries (LIBs). X-ray diffraction (XRD) pattern was not changed before and after the surface fluorination though the peak intensities became weaker than that of the pristine sample, indicating the etching of the surface of SF-TiO2(B) power. This was supported by scanning electron microscopy (SEM) observation. However, X-ray photoelectron spectroscopy (XPS) analysis clearly revealed that F atoms exist on the surface of TiO2(B) particles and probably were covalently bonded with Ti atoms near the surface. From the charge/discharge tests at a C/6 rate, the SF-TiO2(B) exhibited a higher 1st discharge (203 mAh g(-1)) than the pristine sample (181 mAh g(-1)) with a good cycleability. Impedance analysis revealed that both resistances of solid electrolyte interphase (SEI) film and charge transfer at the SET/active material interface were reduced by surface fluorination, implying the improvement of SET film and permeability of the electrolyte solution to the interphase. The rate capability was improved by the surface fluorination up to 1C rate, at which the SF-TiO2(B) exhibited a high discharge capacity of around 150 mAh g(-1).

A comprehensive investigation of the morphological and interfacial changes of Mn3O4 particles at different lithiation stages was performed in order to improve our understanding of the mechanism of the irreversible conversion reaction of Mn3O4. The micronization of Mn3O4 into a Mn-Li2O nanocomposite microstructure and the formation of a solid electrolyte interphase (SEI) on the Mn3O4 surface were carefully observed and characterized by combining high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption fine structure (XAFS) measurements. Accumulation of a thin SEI film of 2-5 nm thickness on the surfaces of the Mn3O4 particles due to their catalytic decomposition was observed at a depth of discharge (DOD) of 0%. As the DOD increases from 25% to 75%, the SEI layer composed of Li2CO3 and LiF continues to grow to 20-30 nm, and Li2O nanoparticles are clearly observed. At 100% DOD, the Mn-Li2O particles with diameters of 2-5 nm become totally encapsulated within a huge organic-inorganic coating structure, while the overall starting shape of the particles remains.

Perovskite La1-xSrxMnO3 (LSxM) nanoparticles supported on Ketjen Black (KB) were synthesized as the cathode of anionexchange membrane fuel cells. To reduce the particle size of LSxM and to improve the dispersivity on KB, a reverse micelle method was applied by using two kinds of non-ionic surfactants, i.e. polyethylene glycol nonyl-phenyl ether and polyethyleneglycol mono-4-nonylphenyl ether. 20 wt% L0.83S0.17M/KB prepared by the latter surfactant consisted of fine nanoparticles of LS0.17M (average diameter: ca. 4.3 nm), and exhibited a higher activity for oxygen reduction reaction than 45 wt% LS0.23M/KB prepared by a co-precipitation method in our previous report. A single cell using the 20 wt% LS0.17M/KB catalyst for cathode could be stably operated and gave the maximum power density of 129 mW cm(-2), which was comparable to that using a conventional 46 wt% Pt/C as the cathode (138 mW cm(-2)).

Carbon-coated Li2FeP2O7 (Li2FeP2O7-C) and Mg-doped Li2FeP2O7-C were successfully synthesized with pitch as a carbon source and Mg(C2O4)center dot 2H(2)O as a Mg source, and the charge/discharge properties were compared with that of bare Li2FeP2O7. A thin carbon layer (ca. 5 nm in thickness) was coated on Li2FeP2O7 powder after heat-treatment. Li2FeP2O7-C and Mg-doped Li2FeP2O7-C exhibited high discharge capacities of 97 and 104 mAh g(-1), respectively, at C/10 rate, implying a great improvement from that of bare Li2FeP2O7 (69 mAh g(-1)). The cycleability and rate-capability were also improved by carbon-coating and Mg doping. Mg-doped Li2FeP2O7-C kept a high discharge capacity of 88 mAh g(-1) even at 1 C rate. AC impedance analysis revealed that Mg-addition suppresses excess electrolyte decomposition on the surface of Li2FeP2O7 and reduces the resistance of SEI.

The mechanism for ammonia oxidation at Ni-Fe alloy/Sm-doped ceria ( SDC) anode cermet was investigated under SOFC operating conditions in the temperature range of 973 to 1173 K. OCV and AC impedance analysis revealed that ammonia is directly oxidized at Ni-Fe/SDC anode, especially at lower temperatures. It was also found that the activity is determined by a balance between ammonia adsorption and nitrogen desorption on the catalyst, and the synergy effects of Ni and Fe improved the activity of Ni-Fe alloy catalysts for ammonia oxidation.

Hollandite-type Mn oxides were synthesized by a co-precipitation (CP) and a hydrothermal (HT) method as the cathode catalysts for anion-exchange membrane fuel cells (AEMFCs), and their oxygen reduction reaction (ORB.) activities and AEMFC single cell performances were evaluated. In this study, we prepared two kinds of hollandite oxides K0.14MnO2 center dot 0.12H(2)O(KMO-CP) and K0.12MnO2 center dot 0.06H(2)O(KMO-HT) and their partially Co-substituted ones K-0.11(Mn-0.88 Co-0.12)O-2 center dot 0.16H(2)O(KMC0.12-CP) and K-0.11(Mn0.88Co0.12)O-2 center dot 0.08H(2)O(KMC0.12-HT), and examined the ORR activities with a rotating ring-disk electrode (RRDE) in 0.1 M KOH at 50 degrees C. All the samples showed high onset potentials of ORB., ca. 0.9 V vs. reversible hydrogen electrode (RHE), and relatively high ORR currents at 0.75 V and high efficiencies for 4-electron reduction (Eff(4)) of around 90% were obtained for the HT samples owing to higher crystallinity and higher specific surface area than those of the CP ones. The AEMFC single cells prepared with KMO-HT and KMC0.12-HT cathode catalysts were operated stably and exhibited the maximum power densities of ca. 49 and 42 mW cm(-2), respectively, which were comparable to that of the single cell using a conventional 50 wt% Ag/C catalyst.

Hollandite-type Mn oxides were synthesized by a co-precipitation (CP) and a hydrothermal (HT) method as the cathode catalysts for anion-exchange membrane fuel cells (AEMFCs), and their oxygen reduction reaction (ORB.) activities and AEMFC single cell performances were evaluated. In this study, we prepared two kinds of hollandite oxides K0.14MnO2 center dot 0.12H(2)O(KMO-CP) and K0.12MnO2 center dot 0.06H(2)O(KMO-HT) and their partially Co-substituted ones K-0.11(Mn-0.88 Co-0.12)O-2 center dot 0.16H(2)O(KMC0.12-CP) and K-0.11(Mn0.88Co0.12)O-2 center dot 0.08H(2)O(KMC0.12-HT), and examined the ORR activities with a rotating ring-disk electrode (RRDE) in 0.1 M KOH at 50 degrees C. All the samples showed high onset potentials of ORB., ca. 0.9 V vs. reversible hydrogen electrode (RHE), and relatively high ORR currents at 0.75 V and high efficiencies for 4-electron reduction (Eff(4)) of around 90% were obtained for the HT samples owing to higher crystallinity and higher specific surface area than those of the CP ones. The AEMFC single cells prepared with KMO-HT and KMC0.12-HT cathode catalysts were operated stably and exhibited the maximum power densities of ca. 49 and 42 mW cm(-2), respectively, which were comparable to that of the single cell using a conventional 50 wt% Ag/C catalyst.

To improve the tap density of TiO2 (B) as a high potential negative electrode for lithium-ion batteries, the particle size and the shape of the TiO2 (B) were controlled by a new synthetic method using large-size Nbdoped rutile TiO2 as a starting material. The Nb-doped TiO2 (B) particles, Ti0.93Nb0.07O2 (B) and Ti0.90Nb0.10O2 (B), were much smaller (diameter: ca. 100 nm, length: ca. 800 nm) than the conventional TiO2 (B) prepared using fine anatase TiO2 particles as a starting material, and were agglomerated to form secondary particles with a diameter of 3-30 mm. The tap densities of Ti0.93Nb0.07O2 (B) and Ti 0.90Nb0.10O2 (B) were successibly high (0.77 and 0.66 g cm-3, respectively), which were ca. 2-fold higher than that of the conventional TiO2 (B) (0.30 g cm-3). As a result, the discharge capacity per electrode volume was significantly improved for both Nb-doped samples without sacrificing the cycleability. Non-doped TiO2 (B) was prepared from large-size rutile TiO2 by a similar method, but it deteriorated upon cycling, accompanied by the formation of the anatase phase. It was shown that Nb-doping not only improves the discharge capacity per electrode volume, but also effectively stabilizes the TiO2 (B) crystal structure of the small particles. © 2013 Elsevier B.V. All rights reserved.

Pure Si thin flakes (Si Leaf Powder (R)(Si-LP)) of different thicknesses (50, 100, 200, 300 and 400 nm) were prepared, and their charge/discharge properties were investigated as negative electrode materials for lithium ion batteries (LIBs). High reversible capacity (2200-2500 mAh g(-1)) and good capacity retention were obtained for thinner Si-LPs (50-200 nm), while thicker samples (300 and 400 nm) exhibited rapid capacity fade upon cycling at C/6. For the thinner flakes, agglomeration and large cracks were confirmed on the composite electrodes, but no pulverization of the flakes was observed. These data suggested that Li atoms diffused easily within the thinner Si-LPs and the uniformity of Li distribution suppressed the localized physical stress that caused by alloying and de-alloying. In addition, the addition of vinylene carbonate (VC) in the electrolyte was found to be quite effective for improving not only the cycleability, but also the rate-capability. The best performance was obtained for Si-LP (100 nm), which exhibited a superior cycleability (ca. 2300 mAh g(-1) at C/6 after 50 cycles) and a high rate capability (ca. 1400 rnAh g(-1) at 12 C rate). (C) 2011 Elsevier B.V. All rights reserved.

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 degrees C, respectively, 14 wt% for each). The irreversible capacity Q(irr) was successfully reduced to about a half (ca. 1100 nnAh g(-1)) of that of the pristine Si-LP (2336 mAhg(-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 degrees 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)). (C) The Electrochemical Society of Japan, All rights reserved.

The shape and size of TiO2(B) particles were controlled by ballmilling K2Ti4O9, H2Ti4O9 and TiO2(B) powders to improve the tap density of TiO2(B). It was found that only K2Ti4O9 endured the ball-milling process and provided a fine TiO2(B) powder after the ion-exchange and dehydration processes. In contrast, ball-milling of H2Ti4O9 and TiO2(B) powders resulted in a formation of a large amount of the rutile and anatase phase, respectively. In addition, a reduction of the spinning rate to 600 rpm suppressed the formation of the minor impurity of the anatase phase in the TiO2(B) sample prepared from the milled K2Ti4O9 powder. The resulting TiO2(B) (M1-600) powder exhibited a high tap density of 0.71 g cm-3 and a high electrode density of 1.25 g cm-3, which were 2.37 and 1.29 times, respectively, higher than those of the pristine TiO2(B). In charge/discharge tests, M1-600 exhibited a discharge of 213.1 mAh g-1, which is higher than that of the pristine TiO2(B), with a good cycleability. It gave a discharge capacity of 80.1 mAh g-1 even at a high rate of 10 C. © The Electrochemical Society.

To improve the reversible capacity of TiO2(B) negative electrode close to the theoretical one, some preparation and experimental conditions such as the precursor (Na2Ti3O7, K2Ti4O9 and Cs2Ti5O11), the co-solvent in ethylene carbonate-based electrolyte solutions, and homogeneity of the composite electrode were optimized. TiO2(B) powder samples were successfully prepared from K2Ti4O9 and Cs2Ti5O11 precursors, but not from Na2Ti3O7. The sample prepared from Cs2Ti5O11 gave a higher discharge capacity (185.5mAh g(-1)) than that prepared from K2Ti4O9 (156.6mAh g(-1)). The reversible capacity of TiO2(B) was significantly influenced by the kind of co-solvent in the electrolyte solutions. The presence of dimethyl carbonate (DMC) as a co-solvent drastically improved the capacity, and the highest discharge capacity of 235.3 mAh g(-1) was obtained in 1 MLiPF6 dissolved EC + DMC (1:2 by volume). Furthermore, improvement of the homogeneity of the composite electrode by premixing the TiO2(B) and Ketjen Black conductor was effective to improve the discharge capacity. The optimized electrode gave an initial discharge capacity of 314.4 mAh g(-1), which reached 93.9% of the theoretical capacity (335 mAh g(-1)). It also exhibited a good cycleability (287.9 mAh g(-1) after 50 cycles) and a high rate capability (118.5 mAh g(-1) at 10C rate). (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.051201jes]

Pure Si thin flakes (Si Leaf Powder® (Si-LP)) of different thicknesses (50, 100, 200, 300 and 400 nm) were prepared, and their charge/discharge properties were investigated as anode materials for lithium ion batteries (LIBs). Thickness of the thin Si-LP (100 nm) changed reversibly during charging and discharging, while the expanded thickness upon charging was kept after fully discharged for the thick one (300 nm), indicating insufficient Li de-alloying at C/6 rate. The slower kinetics for the thicker Si-LPs was also confirmed by examination of the open circuit potential (OCP) and the rate capability. These suggested that Li atoms diffused easily within the thinner Si-LPs and the uniform Li distribution suppressed the physical stress due to the Li alloying and de-alloying, resulting a good cycleability. In addition, addition of vinylene carbonate (VC) in electrolyte reduced charge transfer resistance of the Li alloying/de-alloying reaction and much more improved the cycleability of Si-LPs. ©The Electrochemical Society.

The current efficiency for NF3 formation and the current loss caused by Ni dissolution were investigated in electrolysis of the NH4F center dot 2HF melts with and without alkali metal fluorides such as CsF, KF, and LiF. The addition of CsF to the melt was most effective for increasing the NF3 current efficiency. In contrast, the addition of KF to the melt decreased the current efficiency for NF3 formation and increased the current loss caused by Ni dissolution. The SEM observation and XRD analysis revealed that the oxidized layer formed on nickel in the melt containing LiF or CsF was composed of NiF2 with highly oxidized nickel fluoride. On the other hand, the oxidized layer in the melt containing KF was composed of only KNiF3, and was very brittle. Therefore, it is concluded that KF is detrimental to the nickel anode and highly oxidized nickel fluoride may relate to the NF3 current efficiency.

The self-diffusion coefficients of cations and anions increased with increasing the HF-concentration and temperature in (CH3)(3)N.mHF and (CH3)(4)NF.mHF melts. Behavior of the self-diffusion coefficients of cation and anion in every melt is good agreement with an increase in ionic conductivity and a decrease in viscosity (1). The transference number of (FH)(n)F- anions estimated from the values of self-diffusion coefficients for each ion in every melt was almost constant and their values were around 0.6 and 0.7 in the (CH3)(3)N.mHF and the (CH3)(4)NF.mHF melts, respectively. Therefore, it is concluded that the diffusion of (FH)(n)F- anions is faster than that of each cation in these melts.

To understand the ionic and nonionic species in (CH3)(4)NF center dot mHF, (CH3)(3)N center dot mHF, (C2H5)(4)NF center dot mHF, and (C2H5)(3)N center dot mHF melts, the structures of these melts were investigated by infrared spectroscopy, NMR, and high-energy X-ray diffraction. Infrared spectra revealed that three kinds of fluorohydrogenate anions, (FH)(n)F- (n = 1, 2, and 3), and molecular hydrofluoric acid (HF) are present in every melt. Ionic conductivity and viscosity of these melts were measured and correlated with their cationic structure. The ionic conductivity of the R4N+-systems was higher than that of corresponding R3NH+-systems because a strong N-H center dot center dot center dot F(HF)(n) interaction prevents the motion of R3NH+ cations in the R3N center dot mHF melts. (CH3)(4)N+ and (CH3)(3)NH+ cations gave higher ionic conductivity than (C2H5)(4)N+ and (C2H5)(3)NH+ cations, respectively, because the ionic radii of former cations were smaller than those of latter. It was concluded that these effects on ionic conductivity can be explained by the cationic structure and the concentration of molecular HF in the melts.

Pure Si platelets and Ni or Cu layer-laminated Si platelets with difference thickness were prepared, and their charge/discharge properties were examined in 1 M LiClO(4)/EC + DEC (1:1 by volume) as alternative negative electrode materials to graphite for Li-ion batteries. The shape of thin platelets and lamination with Ni layer are significantly effective to improve the cycleability in Li-Si alloy system by relieving the stress during the alloying/de-alloying processes, reinforcing the mechanical strength and reducing the Li(+) ion diffusion length. Moreover, the first irreversible capacity is minimized by reduction of the amount of Ketjen Black (KB) in the composite electrode because of electrolyte decomposition on the surface of KB. Consequently, the Si/Ni/Si-LP30 (30/30/30 nm) composite electrode with 5 wt% KB also exhibits over 700 mAh g(-1) even after 50 cycles in 1 M LiPF(6)/EC + DEC (1:1). (C) 2011 Elsevier B.V. All rights reserved.