Masaki Matsui | Kobe University (original) (raw)

Papers by Masaki Matsui

Electrochemistry

In situ/operando techniques for electrochemical systems are useful for understanding the electroc... more In situ/operando techniques for electrochemical systems are useful for understanding the electrochemical reactions, as we presented in Part 1. Here we present a series of in situ/operando techniques for battery applications. Now the in situ/operando techniques presented in this paper has become powerful tools for the development of advanced battery systems such as Li-ion batteries, solid-state batteries, and other beyond Li-ion batteries. In the present paper we introduce the in situ/operando cell design of each measurement technique and discuss how we apply each technique for in the advanced battery materials development.

The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethyl... more The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethylene oxide) (PEO)-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME) was examined as a protective layer between lithium metal and a water-stable lithium ion-conducting glass ceramic of Li 1+x+y (Ti,Ge) 2−x Al x P 3−y Si y O 12 (LTAP). The lithium ion conductivity and salt diffusion coefficient of PEO 18 LiTFSI were dramatically enhanced by the addition of TEGDME. The water-stable lithium electrode with PEO 18 LiTFSI-2TEGDME, as the protective layer, exhibited a low and stable electrode resistance of 85 Ω•cm 2 at 60 °C, after 28 days, and low overpotentials of 0.3 V for lithium plating and 0.4 V for lithium stripping at 4.0 mA•cm −2 and 60 °C. A Li/PEO 18 LiTFSI-2TEGDME/LTAP/saturated LiCl aqueous solution/Pt, air cell showed excellent cyclability up to 100 cycles at 2.0 mAh•cm −2 .

Journal of Power Sources, 2015

A novel aqueous lithium-air rechargeable cell with the configuration of Si/1 M LiClO 4 in ethylen... more A novel aqueous lithium-air rechargeable cell with the configuration of Si/1 M LiClO 4 in ethylene carbonate-diethylene carbonate/Li 1+x+y Al x (Ti,Ge) 2-x P 3-y Si y O 12 /5 M LiCl-1 M LiOH aqueous solution/carbon black, air is proposed. A silicon anode composed of mechanically milled silicon power with an average particle size of ca. 0.5 μm, vapor grown carbon fiber and a polyimide binder was examined. The open-circuit voltage at the charged state was 2.9 V at 25 °C. The discharge capacity of 700 mAh g-silicon-1 was retained for 40 cycles at 0.3 mA cm-2 with cutoff voltages of 3.5 and 1.5 V. Significant capacity fade was observed at deep charge and discharge cycling at 2000 mAh g-silicon-1 .

Frontiers in Chemistry, 2019

Passivation of magnesium metal anode is one of the critical challenges for the development of mag... more Passivation of magnesium metal anode is one of the critical challenges for the development of magnesium batteries. Here we investigated the passivation process of an intermetallic anode: Mg 3 Bi 2 synthesized by solid-state and thin film process. The Mg 3 Bi 2 composite electrode shows excellent reversibility in magnesium bis(trifluoromethansulfonylamide) dissolved in acetonitrile, while Mg 3 Sb 2 , which has same crystal structure and similar chemical properties, is electrochemically inactive. We also fabricated the Mg 3 Bi 2 thin film electrodes, which show reversibility with low overpotential not only in the acetonitrile solution but also glyme-based solutions. Surface layer corresponding to the decomposed TFSA anion is slightly suppressed in the case of the Mg 3 Bi 2 thin film electrode, compared with Mg metal. Comparative study of hydrolysis process of the Mg 3 Bi 2 and the Mg 3 Sb 2 suggests that the both intermetallic anodes are not completely passivated. The bond valence sum mapping of the Mg 3 Bi 2 indicates that the fast Mg 2+ diffusion pathway between 2d tetrahedral sites is formed. The electrochemical properties of the Mg 3 Bi 2 anode is mainly due to the less passivation surface with the fast Mg 2+ diffusion pathways.

Journal of The Electrochemical Society, 2017

A comparative study for electrodeposited magnesium metal was conducted using an organohaloalumina... more A comparative study for electrodeposited magnesium metal was conducted using an organohaloaluminate-based electrolyte solution and a conventional ionic electrolyte solution. The deposition / dissolution process of the magnesium metal in the ionic electrolyte solution shows high overpotential with low coulombic efficiency, compared with the organohaloaluminate-based electrolyte solution. The magnesium metal deposited in the ionic electrolyte solution shows porous morphology and poor crystallinity. An initial electrolytes decomposition process on Pt substrate was analyzed with in situ FTIR. The in situ FTIR spectra show passivation process of the electrode during the 1 st cathodic scan due to the decomposition of the conventional ionic electrolyte solution. The surface layer on magnesium metal was characterized by using XPS. The XPS spectra for the magnesium metal immersed in the ionic electrolyte solution proved that a MgF 2-based passivation layer corresponding to the decomposed TFSA anion is formed.

Solid State Ionics, 2015

High lithium ion conductivity solid electrolytes of Li 1+x Cr x Ti 2-x (PO 4) 3 (x=0-0.5) and Li ... more High lithium ion conductivity solid electrolytes of Li 1+x Cr x Ti 2-x (PO 4) 3 (x=0-0.5) and Li 1.4 Al x Cr 0.4-x Ti 1.6 (PO 4) 3 (x=0-0.4) with the NASICON-type structure were synthesized using a precursor prepared by the sol-gel method. The highest total electrical conductivity of 3.55×10-4 S cm-1 at 25 °C was obtained for x=0.4 in Li 1+x Cr x Ti 2-x (PO 4) 3 (LACTP) sintered at 1120 °C for 6 h in air. Higher electrical conductivity was observed by the partial substitution of Cr 3+ with Al 3+. The highest total electrical conductivity of 1.06×10-3 S cm-1 and bulk conductivity of 1.77×10-3 S cm-1 at 25 °C were obtained for x=0.3 in Li 1.4 Al x Cr 0.4-x Ti 1.6 (PO 4) 3 sintered at 1070 °C for 6 h in air. The contribution of electronic conductivity to the total conductivity was negligibly small. Li 1.4 Al 0.3 Cr 0.1 Ti 1.6 (PO 4) 3 was unstable in water, but stable in saturated LiOH with saturated LiCl aqueous solution.

Nature communications, Apr 25, 2017

Reversible dendrite-free low-areal-capacity lithium metal electrodes have recently been revived, ... more Reversible dendrite-free low-areal-capacity lithium metal electrodes have recently been revived, because of their pivotal role in developing beyond lithium ion batteries. However, there have been no reports of reversible dendrite-free high-areal-capacity lithium metal electrodes. Here we report on a strategy to realize unprecedented stable cycling of lithium electrodeposition/stripping with a highly desirable areal-capacity (12 mAh cm(-2)) and exceptional Coulombic efficiency (>99.98%) at high current densities (>5 mA cm(-2)) and ambient temperature using a diluted solvate ionic liquid. The essence of this strategy, that can drastically improve lithium electrodeposition kinetics by cyclic voltammetry premodulation, lies in the tailoring of the top solid-electrolyte interphase layer in a diluted solvate ionic liquid to facilitate a two-dimensional growth mode. We anticipate that this discovery could pave the way for developing reversible dendrite-free metal anodes for sustainab...

Electrochemistry, 2016

The surface morphology of the electrodeposited lithium metal from electrolyte solutions containin... more The surface morphology of the electrodeposited lithium metal from electrolyte solutions containing electrolyte additives: fluoroethylene carbonate (FEC), vinylene carbonate (VC) and lithium bis(oxalate)borate (LiBOB), were investigated. All the film forming additive improved the surface morphology. The FEC especially shows the most uniform surface morphology compared with the other electrolyte additives and the additive-free electrolyte. The surface analyses of the lithium metal were conducted using X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The analytical study revealed that the FEC suppresses the decomposition of the PF 6 − anion, resulting in the formation of a thin stable SEI layer on the lithium metal.

Solid State Ionics, 2014

The Al-free cubic garnet-like Li 5+2x La 3 Nb 2-x Sc x O 12 series with x = 0-0.75 was successful... more The Al-free cubic garnet-like Li 5+2x La 3 Nb 2-x Sc x O 12 series with x = 0-0.75 was successfully synthesized by solid state reaction. Final sintering at 1150 °C for 12 h in a magnesia crucible with the mother powder method resulted in stoichiometric single phases that corresponded to each nominal composition. The relationship between the lithium content and ionic conductivity was investigated for each composition. The conductivity of the samples showed a positive dependency on the Li content and the optimum Li content for Li conduction was 6.25-6.5 in the formula unit. The highest bulk conductivity of 3.70×10-4 S cm-1 was observed for x = 0.625 at 25 °C and that for x = 0.75 at 50 °C was 6.05×10-4 S cm-1 .

Solid State Ionics, 2015

The phase formation of Li 7-3x Al x La 3 Zr 2 O 12 was investigated by high temperature X-ray dif... more The phase formation of Li 7-3x Al x La 3 Zr 2 O 12 was investigated by high temperature X-ray diffraction measurements. The tetragonal phase was obtained in the range of x = 0 to 0.20 and the cubic phase was formed at x = 0.25 at room temperature. A linear change of the lattice parameters was observed from x = 0 to 0.20, which is in good agreement with the systematic composition change. The solubility limit of Al 3+ was determined to be x = 0.25. The reversible phase transition between the tetragonal phase and the cubic phase was observed at 640 °C for x = 0. The marked increase of ionic conductivity at the phase transition temperature is consistent with the structural change of the ordering/disordering of lithium ions. In addition, the phase transition temperature decreased with increasing x, and the cubic phase was stabilized at x = 0.25 from room temperature to 900 °C. The ionic conductivity increased with x and the highest ionic conductivity of 3.1×10-4 S cm-1 at 25 °C was obtained for x = 0.25. Al 3+ substitution introduces lithium vacancies, which has a strong influence on the phase transition between the tetragonal and cubic phases. The results obtained here suggest that control of the lithium vacancies is the key to obtain a fast lithium-conducting phase.

Electrochemistry, 2014

Degradation processes of carbon negative electrodes for all-solid lithium polymer batteries were ... more Degradation processes of carbon negative electrodes for all-solid lithium polymer batteries were investigated using X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and Scanning Electron Microscope (SEM). The cycling performances of the electrodes are significantly affected by the mixing speed of the electrode slurry. The cross-sectional SEM images of the electrodes containing vapor-grown carbon fiber (VGCF) show that more voids remained in the electrodes, if the slurry was mixed at high speed. The voids in the composite electrode expand and increase during the cycling test of the cell resulting in the capacity fading. On the other hand, the electrodes containing carbon nanotubes (CNT) show an opposite trend, because the high mixing speed improves the dispersibility of the CNT resulting in the formation of the less voids in the electrode. The electrode containing both VGCF and CNT prepared at high mixing speed shows the best cycleability among all the electrodes. In spite of some voids initially form in the electrode, the CNT seems to have prevented further increase and expansion of the voids.

Membranes, Jan 25, 2013

The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethyl... more The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethylene oxide) (PEO)-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME) was examined as a protective layer between lithium metal and a water-stable lithium ion-conducting glass ceramic of Li1+x+y(Ti,Ge)2-xAlxP3-ySiyO12 (LTAP). The lithium ion conductivity and salt diffusion coefficient of PEO18LiTFSI were dramatically enhanced by the addition of TEGDME. The water-stable lithium electrode with PEO18LiTFSI-2TEGDME, as the protective layer, exhibited a low and stable electrode resistance of 85 Ω·cm2 at 60 °C, after 28 days, and low overpotentials of 0.3 V for lithium plating and 0.4 V for lithium stripping at 4.0 mA·cm-2 and 60 °C. A Li/PEO18LiTFSI-2TEGDME/LTAP/saturated LiCl aqueous solution/Pt, air cell showed excellent cyclability up to 100 cycles at 2.0 mAh·cm-2.

Chemical communications (Cambridge, England), Jan 21, 2015

The electrochemical process of an aqueous Li-O2 cell is investigated. Li2O2 is detected as a disc... more The electrochemical process of an aqueous Li-O2 cell is investigated. Li2O2 is detected as a discharge product of an aqueous Li-O2 cell using a catalyst-free carbon-based electrode. The electrolyte solution saturated with lithium hydroxide prevents hydrolysis of the Li2O2. Since the electron transfer process is based on the oxygen-peroxide redox couple, the galvanostatic charging-discharging profile shows stable cycling with an extremely low charging overpotential of <0.1 V at 1.0 mA cm(-2).

Electrochemistry, 2014

This article summarizes our research on solid electrolytes for rechargeable aqueous lithium-air b... more This article summarizes our research on solid electrolytes for rechargeable aqueous lithium-air batteries. Aqueous lithium-air batteries have potential application as a power source for electric vehicles, because of their high specific energy density. A water-stable lithium ion conducting solid electrolyte is the key material for lithium-air batteries to use lithium metal in aqueous circumstance. In this article, two types of lithium ion conducting solid electrolytes, NASICON-type Li 1+x A x Ti 2−x−y Ge y (PO 4) 3 (A = Al, Fe) and garnet-type Li 7−x La 3 Zr 2−x A x O 12 (A = Nb, Ta) are introduced, and the conductivity behavior of these solid electrolytes by elemental substitution, their chemical stabilities in water and electrochemical stabilities with lithium metal are discussed. Lithium ion conductivities of 1.3 × 10 −3 and 5.2 × 10 −4 S cm −1 at 25°C were observed in Li 1.4 Al 0.4 Ti 1.4 Ge 0.2 (PO 4) 3 and Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 , respectively. These solid electrolytes are unstable in water, but stable in saturated LiOH with saturated LiCl aqueous solution. The former solid electrolyte is unstable in contact with lithium metal, while the latter electrolyte shows stability against lithium metal.

Solid State Ionics, 2013

Solid ionic conductors composed of Li 1.4 Al 0.4 Ti 1.6 − x Ge x (PO 4) 3 (x = 0-1.0) with the NA... more Solid ionic conductors composed of Li 1.4 Al 0.4 Ti 1.6 − x Ge x (PO 4) 3 (x = 0-1.0) with the NASICON-type structure were synthesized with a precursor prepared using the sol-gel method. The electrical conductivity was examined as a function of x in Li 1.4 Al 0.4 Ti 1.6 − x Ge x (PO 4) 3 at various sintering temperatures and for various sintering periods. The highest electrical conductivity was obtained for Li 1.4 Al 0.4 Ti 1.4 Ge 0.2 (PO 4) 3 sintered at 900°C for 11 h in air. The total and bulk conductivities of the sintered pellet were 1.29 × 10 −3 and 2.35 × 10 −3 S cm − 1 at 25°C, respectively. The grain boundary resistance of Li 1.4 Al 0.4 Ti 1.4 Ge 0.2 (PO 4) 3 was significantly increased to 552 Ω cm 2 from 12.2 Ω cm 2 by immersion in distilled water at 50°C for one week, whereas the bulk resistance was not increased. However, no significant increase of the bulk and grain boundary resistance was observed after immersion in a saturated aqueous solution of LiOH and LiCl.

Solid State Ionics, 2014

Al 2 O 3 was doped (0.5, 1.0. and 2.0 wt%) into the garnet-type lithium ion-conducting solid elec... more Al 2 O 3 was doped (0.5, 1.0. and 2.0 wt%) into the garnet-type lithium ion-conducting solid electrolyte with a nominal composition of Li 7 La 3 Zr 2 O 12 (LLZ) by solid state reaction at 1,180 °C. The effect of the Al 2 O 3 content in LLZ on the interface behavior with lithium metal was investigated. The lowest interface resistance was observed for 0.5 wt% Al 2 O 3-doped LLZ, which had the highest relative density of 93.7%. The Li/Al 2 O 3-doped LLZ/Li cell showed a short circuit after a limited polarization period. Short circuiting is considered to be due to lithium dendrite formation. The longest period until short circuit was 1,000 s. polarization at 0.5 mA cm-2 and room temperature for the 0.5 wt% Al 2 O 3 doped LLZ.

Journal of Power Sources, 2014

Air electrodes of carbon materials were examined in saturated LiOH with 10 M LiCl. The lowest ove... more Air electrodes of carbon materials were examined in saturated LiOH with 10 M LiCl. The lowest over-potential was observed for Ketjen black. CO was detected after polarization for the oxygen evolution reaction.

Journal of Power Sources, 2013

Si-amorphous carbon (C)-carbon paper (CP) substrate for lithium secondary batteries. Direct depos... more Si-amorphous carbon (C)-carbon paper (CP) substrate for lithium secondary batteries. Direct deposition of Si/C composite on CP by pyrolysis of PVC mixed with Si. Initial Coulombic efficiency of 90% and initial capacity over 1700 mAh g À1. Excellent cycleability with 70% capacity retention after 100 cycles.

Journal of Power Sources, 2014

Carbon coated silicon (Si/C) dispersed in a carbon paper (CP) was examined as the anode for solid... more Carbon coated silicon (Si/C) dispersed in a carbon paper (CP) was examined as the anode for solid polymer lithium-ion batteries. The CP was prepared by pyrolysis of poly(acrylonitrile) fiber and Manila hemp non-woven cloth at 2600 °C under an inert atmosphere. The Si/C composite was formed by pyrolysis of a slurry consisting of Si power and a solution of polyvinylchloride in tetrahydrofuran as the carbon source. Si/C:CP with a weight ratio of 20:100 had a high initial capacity of 980 mAh g-1 of Si/C and a high initial columbic efficiency of 77%, and also exhibited excellent capacity

Journal of Power Sources, 2014

A rechargeable aqueous lithium-air cell with a third auxiliary electrode for the oxygen evolution... more A rechargeable aqueous lithium-air cell with a third auxiliary electrode for the oxygen evolution reaction was developed. The cell consists of a lithium metal anode, a lithium conducting solid electrolyte of Li1+x+yAlx(Ti,Ge)2-xSiyP3-yO12, a carbon black oxygen reduction air electrode, a RuO2 oxygen evolution electrode, and a saturated aqueous solution of LiOH with 10 M LiCl. The cell was successfully operated for several cycles at 0.64 mA cm-2 and 25 °C under air, where the capacity of air electrode was 2000 mAh gcarbon-1. The cell performance was degraded gradually by cycling under open air. The degradation was reduced under CO2-free air and pure oxygen. The specific energy density was calculated to be 810 Wh kg-1 from the weight of water, lithium, oxygen, and carbon in the air electrode

Electrochemistry

In situ/operando techniques for electrochemical systems are useful for understanding the electroc... more In situ/operando techniques for electrochemical systems are useful for understanding the electrochemical reactions, as we presented in Part 1. Here we present a series of in situ/operando techniques for battery applications. Now the in situ/operando techniques presented in this paper has become powerful tools for the development of advanced battery systems such as Li-ion batteries, solid-state batteries, and other beyond Li-ion batteries. In the present paper we introduce the in situ/operando cell design of each measurement technique and discuss how we apply each technique for in the advanced battery materials development.

The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethyl... more The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethylene oxide) (PEO)-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME) was examined as a protective layer between lithium metal and a water-stable lithium ion-conducting glass ceramic of Li 1+x+y (Ti,Ge) 2−x Al x P 3−y Si y O 12 (LTAP). The lithium ion conductivity and salt diffusion coefficient of PEO 18 LiTFSI were dramatically enhanced by the addition of TEGDME. The water-stable lithium electrode with PEO 18 LiTFSI-2TEGDME, as the protective layer, exhibited a low and stable electrode resistance of 85 Ω•cm 2 at 60 °C, after 28 days, and low overpotentials of 0.3 V for lithium plating and 0.4 V for lithium stripping at 4.0 mA•cm −2 and 60 °C. A Li/PEO 18 LiTFSI-2TEGDME/LTAP/saturated LiCl aqueous solution/Pt, air cell showed excellent cyclability up to 100 cycles at 2.0 mAh•cm −2 .

Journal of Power Sources, 2015

A novel aqueous lithium-air rechargeable cell with the configuration of Si/1 M LiClO 4 in ethylen... more A novel aqueous lithium-air rechargeable cell with the configuration of Si/1 M LiClO 4 in ethylene carbonate-diethylene carbonate/Li 1+x+y Al x (Ti,Ge) 2-x P 3-y Si y O 12 /5 M LiCl-1 M LiOH aqueous solution/carbon black, air is proposed. A silicon anode composed of mechanically milled silicon power with an average particle size of ca. 0.5 μm, vapor grown carbon fiber and a polyimide binder was examined. The open-circuit voltage at the charged state was 2.9 V at 25 °C. The discharge capacity of 700 mAh g-silicon-1 was retained for 40 cycles at 0.3 mA cm-2 with cutoff voltages of 3.5 and 1.5 V. Significant capacity fade was observed at deep charge and discharge cycling at 2000 mAh g-silicon-1 .

Frontiers in Chemistry, 2019

Passivation of magnesium metal anode is one of the critical challenges for the development of mag... more Passivation of magnesium metal anode is one of the critical challenges for the development of magnesium batteries. Here we investigated the passivation process of an intermetallic anode: Mg 3 Bi 2 synthesized by solid-state and thin film process. The Mg 3 Bi 2 composite electrode shows excellent reversibility in magnesium bis(trifluoromethansulfonylamide) dissolved in acetonitrile, while Mg 3 Sb 2 , which has same crystal structure and similar chemical properties, is electrochemically inactive. We also fabricated the Mg 3 Bi 2 thin film electrodes, which show reversibility with low overpotential not only in the acetonitrile solution but also glyme-based solutions. Surface layer corresponding to the decomposed TFSA anion is slightly suppressed in the case of the Mg 3 Bi 2 thin film electrode, compared with Mg metal. Comparative study of hydrolysis process of the Mg 3 Bi 2 and the Mg 3 Sb 2 suggests that the both intermetallic anodes are not completely passivated. The bond valence sum mapping of the Mg 3 Bi 2 indicates that the fast Mg 2+ diffusion pathway between 2d tetrahedral sites is formed. The electrochemical properties of the Mg 3 Bi 2 anode is mainly due to the less passivation surface with the fast Mg 2+ diffusion pathways.

Journal of The Electrochemical Society, 2017

A comparative study for electrodeposited magnesium metal was conducted using an organohaloalumina... more A comparative study for electrodeposited magnesium metal was conducted using an organohaloaluminate-based electrolyte solution and a conventional ionic electrolyte solution. The deposition / dissolution process of the magnesium metal in the ionic electrolyte solution shows high overpotential with low coulombic efficiency, compared with the organohaloaluminate-based electrolyte solution. The magnesium metal deposited in the ionic electrolyte solution shows porous morphology and poor crystallinity. An initial electrolytes decomposition process on Pt substrate was analyzed with in situ FTIR. The in situ FTIR spectra show passivation process of the electrode during the 1 st cathodic scan due to the decomposition of the conventional ionic electrolyte solution. The surface layer on magnesium metal was characterized by using XPS. The XPS spectra for the magnesium metal immersed in the ionic electrolyte solution proved that a MgF 2-based passivation layer corresponding to the decomposed TFSA anion is formed.

Solid State Ionics, 2015

High lithium ion conductivity solid electrolytes of Li 1+x Cr x Ti 2-x (PO 4) 3 (x=0-0.5) and Li ... more High lithium ion conductivity solid electrolytes of Li 1+x Cr x Ti 2-x (PO 4) 3 (x=0-0.5) and Li 1.4 Al x Cr 0.4-x Ti 1.6 (PO 4) 3 (x=0-0.4) with the NASICON-type structure were synthesized using a precursor prepared by the sol-gel method. The highest total electrical conductivity of 3.55×10-4 S cm-1 at 25 °C was obtained for x=0.4 in Li 1+x Cr x Ti 2-x (PO 4) 3 (LACTP) sintered at 1120 °C for 6 h in air. Higher electrical conductivity was observed by the partial substitution of Cr 3+ with Al 3+. The highest total electrical conductivity of 1.06×10-3 S cm-1 and bulk conductivity of 1.77×10-3 S cm-1 at 25 °C were obtained for x=0.3 in Li 1.4 Al x Cr 0.4-x Ti 1.6 (PO 4) 3 sintered at 1070 °C for 6 h in air. The contribution of electronic conductivity to the total conductivity was negligibly small. Li 1.4 Al 0.3 Cr 0.1 Ti 1.6 (PO 4) 3 was unstable in water, but stable in saturated LiOH with saturated LiCl aqueous solution.

Nature communications, Apr 25, 2017

Reversible dendrite-free low-areal-capacity lithium metal electrodes have recently been revived, ... more Reversible dendrite-free low-areal-capacity lithium metal electrodes have recently been revived, because of their pivotal role in developing beyond lithium ion batteries. However, there have been no reports of reversible dendrite-free high-areal-capacity lithium metal electrodes. Here we report on a strategy to realize unprecedented stable cycling of lithium electrodeposition/stripping with a highly desirable areal-capacity (12 mAh cm(-2)) and exceptional Coulombic efficiency (>99.98%) at high current densities (>5 mA cm(-2)) and ambient temperature using a diluted solvate ionic liquid. The essence of this strategy, that can drastically improve lithium electrodeposition kinetics by cyclic voltammetry premodulation, lies in the tailoring of the top solid-electrolyte interphase layer in a diluted solvate ionic liquid to facilitate a two-dimensional growth mode. We anticipate that this discovery could pave the way for developing reversible dendrite-free metal anodes for sustainab...

Electrochemistry, 2016

The surface morphology of the electrodeposited lithium metal from electrolyte solutions containin... more The surface morphology of the electrodeposited lithium metal from electrolyte solutions containing electrolyte additives: fluoroethylene carbonate (FEC), vinylene carbonate (VC) and lithium bis(oxalate)borate (LiBOB), were investigated. All the film forming additive improved the surface morphology. The FEC especially shows the most uniform surface morphology compared with the other electrolyte additives and the additive-free electrolyte. The surface analyses of the lithium metal were conducted using X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The analytical study revealed that the FEC suppresses the decomposition of the PF 6 − anion, resulting in the formation of a thin stable SEI layer on the lithium metal.

Solid State Ionics, 2014

The Al-free cubic garnet-like Li 5+2x La 3 Nb 2-x Sc x O 12 series with x = 0-0.75 was successful... more The Al-free cubic garnet-like Li 5+2x La 3 Nb 2-x Sc x O 12 series with x = 0-0.75 was successfully synthesized by solid state reaction. Final sintering at 1150 °C for 12 h in a magnesia crucible with the mother powder method resulted in stoichiometric single phases that corresponded to each nominal composition. The relationship between the lithium content and ionic conductivity was investigated for each composition. The conductivity of the samples showed a positive dependency on the Li content and the optimum Li content for Li conduction was 6.25-6.5 in the formula unit. The highest bulk conductivity of 3.70×10-4 S cm-1 was observed for x = 0.625 at 25 °C and that for x = 0.75 at 50 °C was 6.05×10-4 S cm-1 .

Solid State Ionics, 2015

The phase formation of Li 7-3x Al x La 3 Zr 2 O 12 was investigated by high temperature X-ray dif... more The phase formation of Li 7-3x Al x La 3 Zr 2 O 12 was investigated by high temperature X-ray diffraction measurements. The tetragonal phase was obtained in the range of x = 0 to 0.20 and the cubic phase was formed at x = 0.25 at room temperature. A linear change of the lattice parameters was observed from x = 0 to 0.20, which is in good agreement with the systematic composition change. The solubility limit of Al 3+ was determined to be x = 0.25. The reversible phase transition between the tetragonal phase and the cubic phase was observed at 640 °C for x = 0. The marked increase of ionic conductivity at the phase transition temperature is consistent with the structural change of the ordering/disordering of lithium ions. In addition, the phase transition temperature decreased with increasing x, and the cubic phase was stabilized at x = 0.25 from room temperature to 900 °C. The ionic conductivity increased with x and the highest ionic conductivity of 3.1×10-4 S cm-1 at 25 °C was obtained for x = 0.25. Al 3+ substitution introduces lithium vacancies, which has a strong influence on the phase transition between the tetragonal and cubic phases. The results obtained here suggest that control of the lithium vacancies is the key to obtain a fast lithium-conducting phase.

Electrochemistry, 2014

Degradation processes of carbon negative electrodes for all-solid lithium polymer batteries were ... more Degradation processes of carbon negative electrodes for all-solid lithium polymer batteries were investigated using X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and Scanning Electron Microscope (SEM). The cycling performances of the electrodes are significantly affected by the mixing speed of the electrode slurry. The cross-sectional SEM images of the electrodes containing vapor-grown carbon fiber (VGCF) show that more voids remained in the electrodes, if the slurry was mixed at high speed. The voids in the composite electrode expand and increase during the cycling test of the cell resulting in the capacity fading. On the other hand, the electrodes containing carbon nanotubes (CNT) show an opposite trend, because the high mixing speed improves the dispersibility of the CNT resulting in the formation of the less voids in the electrode. The electrode containing both VGCF and CNT prepared at high mixing speed shows the best cycleability among all the electrodes. In spite of some voids initially form in the electrode, the CNT seems to have prevented further increase and expansion of the voids.

Membranes, Jan 25, 2013

The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethyl... more The electrochemical properties of a composite solid polymer electrolyte, consisting of poly(ethylene oxide) (PEO)-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME) was examined as a protective layer between lithium metal and a water-stable lithium ion-conducting glass ceramic of Li1+x+y(Ti,Ge)2-xAlxP3-ySiyO12 (LTAP). The lithium ion conductivity and salt diffusion coefficient of PEO18LiTFSI were dramatically enhanced by the addition of TEGDME. The water-stable lithium electrode with PEO18LiTFSI-2TEGDME, as the protective layer, exhibited a low and stable electrode resistance of 85 Ω·cm2 at 60 °C, after 28 days, and low overpotentials of 0.3 V for lithium plating and 0.4 V for lithium stripping at 4.0 mA·cm-2 and 60 °C. A Li/PEO18LiTFSI-2TEGDME/LTAP/saturated LiCl aqueous solution/Pt, air cell showed excellent cyclability up to 100 cycles at 2.0 mAh·cm-2.

Chemical communications (Cambridge, England), Jan 21, 2015

The electrochemical process of an aqueous Li-O2 cell is investigated. Li2O2 is detected as a disc... more The electrochemical process of an aqueous Li-O2 cell is investigated. Li2O2 is detected as a discharge product of an aqueous Li-O2 cell using a catalyst-free carbon-based electrode. The electrolyte solution saturated with lithium hydroxide prevents hydrolysis of the Li2O2. Since the electron transfer process is based on the oxygen-peroxide redox couple, the galvanostatic charging-discharging profile shows stable cycling with an extremely low charging overpotential of <0.1 V at 1.0 mA cm(-2).

Electrochemistry, 2014

This article summarizes our research on solid electrolytes for rechargeable aqueous lithium-air b... more This article summarizes our research on solid electrolytes for rechargeable aqueous lithium-air batteries. Aqueous lithium-air batteries have potential application as a power source for electric vehicles, because of their high specific energy density. A water-stable lithium ion conducting solid electrolyte is the key material for lithium-air batteries to use lithium metal in aqueous circumstance. In this article, two types of lithium ion conducting solid electrolytes, NASICON-type Li 1+x A x Ti 2−x−y Ge y (PO 4) 3 (A = Al, Fe) and garnet-type Li 7−x La 3 Zr 2−x A x O 12 (A = Nb, Ta) are introduced, and the conductivity behavior of these solid electrolytes by elemental substitution, their chemical stabilities in water and electrochemical stabilities with lithium metal are discussed. Lithium ion conductivities of 1.3 × 10 −3 and 5.2 × 10 −4 S cm −1 at 25°C were observed in Li 1.4 Al 0.4 Ti 1.4 Ge 0.2 (PO 4) 3 and Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 , respectively. These solid electrolytes are unstable in water, but stable in saturated LiOH with saturated LiCl aqueous solution. The former solid electrolyte is unstable in contact with lithium metal, while the latter electrolyte shows stability against lithium metal.

Solid State Ionics, 2013

Solid ionic conductors composed of Li 1.4 Al 0.4 Ti 1.6 − x Ge x (PO 4) 3 (x = 0-1.0) with the NA... more Solid ionic conductors composed of Li 1.4 Al 0.4 Ti 1.6 − x Ge x (PO 4) 3 (x = 0-1.0) with the NASICON-type structure were synthesized with a precursor prepared using the sol-gel method. The electrical conductivity was examined as a function of x in Li 1.4 Al 0.4 Ti 1.6 − x Ge x (PO 4) 3 at various sintering temperatures and for various sintering periods. The highest electrical conductivity was obtained for Li 1.4 Al 0.4 Ti 1.4 Ge 0.2 (PO 4) 3 sintered at 900°C for 11 h in air. The total and bulk conductivities of the sintered pellet were 1.29 × 10 −3 and 2.35 × 10 −3 S cm − 1 at 25°C, respectively. The grain boundary resistance of Li 1.4 Al 0.4 Ti 1.4 Ge 0.2 (PO 4) 3 was significantly increased to 552 Ω cm 2 from 12.2 Ω cm 2 by immersion in distilled water at 50°C for one week, whereas the bulk resistance was not increased. However, no significant increase of the bulk and grain boundary resistance was observed after immersion in a saturated aqueous solution of LiOH and LiCl.

Solid State Ionics, 2014

Al 2 O 3 was doped (0.5, 1.0. and 2.0 wt%) into the garnet-type lithium ion-conducting solid elec... more Al 2 O 3 was doped (0.5, 1.0. and 2.0 wt%) into the garnet-type lithium ion-conducting solid electrolyte with a nominal composition of Li 7 La 3 Zr 2 O 12 (LLZ) by solid state reaction at 1,180 °C. The effect of the Al 2 O 3 content in LLZ on the interface behavior with lithium metal was investigated. The lowest interface resistance was observed for 0.5 wt% Al 2 O 3-doped LLZ, which had the highest relative density of 93.7%. The Li/Al 2 O 3-doped LLZ/Li cell showed a short circuit after a limited polarization period. Short circuiting is considered to be due to lithium dendrite formation. The longest period until short circuit was 1,000 s. polarization at 0.5 mA cm-2 and room temperature for the 0.5 wt% Al 2 O 3 doped LLZ.

Journal of Power Sources, 2014

Air electrodes of carbon materials were examined in saturated LiOH with 10 M LiCl. The lowest ove... more Air electrodes of carbon materials were examined in saturated LiOH with 10 M LiCl. The lowest over-potential was observed for Ketjen black. CO was detected after polarization for the oxygen evolution reaction.

Journal of Power Sources, 2013

Si-amorphous carbon (C)-carbon paper (CP) substrate for lithium secondary batteries. Direct depos... more Si-amorphous carbon (C)-carbon paper (CP) substrate for lithium secondary batteries. Direct deposition of Si/C composite on CP by pyrolysis of PVC mixed with Si. Initial Coulombic efficiency of 90% and initial capacity over 1700 mAh g À1. Excellent cycleability with 70% capacity retention after 100 cycles.

Journal of Power Sources, 2014

Carbon coated silicon (Si/C) dispersed in a carbon paper (CP) was examined as the anode for solid... more Carbon coated silicon (Si/C) dispersed in a carbon paper (CP) was examined as the anode for solid polymer lithium-ion batteries. The CP was prepared by pyrolysis of poly(acrylonitrile) fiber and Manila hemp non-woven cloth at 2600 °C under an inert atmosphere. The Si/C composite was formed by pyrolysis of a slurry consisting of Si power and a solution of polyvinylchloride in tetrahydrofuran as the carbon source. Si/C:CP with a weight ratio of 20:100 had a high initial capacity of 980 mAh g-1 of Si/C and a high initial columbic efficiency of 77%, and also exhibited excellent capacity

Journal of Power Sources, 2014

A rechargeable aqueous lithium-air cell with a third auxiliary electrode for the oxygen evolution... more A rechargeable aqueous lithium-air cell with a third auxiliary electrode for the oxygen evolution reaction was developed. The cell consists of a lithium metal anode, a lithium conducting solid electrolyte of Li1+x+yAlx(Ti,Ge)2-xSiyP3-yO12, a carbon black oxygen reduction air electrode, a RuO2 oxygen evolution electrode, and a saturated aqueous solution of LiOH with 10 M LiCl. The cell was successfully operated for several cycles at 0.64 mA cm-2 and 25 °C under air, where the capacity of air electrode was 2000 mAh gcarbon-1. The cell performance was degraded gradually by cycling under open air. The degradation was reduced under CO2-free air and pure oxygen. The specific energy density was calculated to be 810 Wh kg-1 from the weight of water, lithium, oxygen, and carbon in the air electrode