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Shingo Ohta

Bio: Shingo Ohta is an academic researcher from Toyota. The author has contributed to research in topics: Electrolyte & Oxide. The author has an hindex of 17, co-authored 38 publications receiving 3498 citations. Previous affiliations of Shingo Ohta include Tokyo Institute of Technology & Rohm.

Papers
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Journal ArticleDOI
TL;DR: The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements and enhances the Seebeck coefficient without reducing the electrical conductivity.
Abstract: Enhancement of the Seebeck coefficient (S ) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT=S2 x sigma x T x kappa-1) exceeding 2, where T is the absolute temperature and kappa is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO(3) yields unusually large |S|, approximately five times larger than that of SrTiO(3) bulks, while maintaining a high sigma2DEG. In the best case, we observe |S|=850 microV K-1 and sigma2DEG=1.4 x 10(3) S cm-1. In addition, by using the kappa of bulk single-crystal SrTiO(3) at room temperature, we estimate ZT approximately 2.4 for the 2DEG, corresponding to ZT approximately 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.

850 citations

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TL;DR: In this paper, the authors measured the thermal properties of heavy La- or Nb-doped SrTiO3 (STO) bulk single crystals at high temperatures to clarify the influence of doping upon the thermoelectric performance of STO.
Abstract: Electron and thermal transport properties, i.e., electrical conductivity, carrier concentration, Hall mobility, Seebeck coefficient, thermal conductivity, of heavily La- or Nb-doped SrTiO3 (STO) bulk single crystals were measured at high temperatures, (300–1050K) to clarify the influence of doping upon the thermoelectric performance of STO. The temperature dependence of Hall mobility and Seebeck coefficient changed at ∼750K in all samples because the dominant mechanism for carrier scattering changed with increasing temperature from coupled scattering by polar optical phonons and acoustic phonons to mere acoustic phonon scattering. The density-of-states effective mass of Nb-doped STO, which was estimated from the carrier concentration and Seebeck coefficient, was larger than that of La-doped STO. Thermal conductivity of the samples, which was similar to that of undoped STO single crystal, decreased proportionally to T−1, indicating that the phonon conduction takes place predominantly and the electronic con...

520 citations

Journal ArticleDOI
TL;DR: In this paper, Li7−XLa3(Zr2−X, NbX)O12 was synthesized by a solid-state reaction, and their lithium ion conductivity was measured using an AC impedance method at temperatures ranging from 25 to 150°C in air.

511 citations

Journal ArticleDOI
Shingo Ohta1, Shogo Komagata1, Juntaro Seki1, Tohru Saeki1, Shinya Morishita1, Takahiko Asaoka1 
TL;DR: In this paper, an all-solid-state lithium ion battery was constructed with a screen-printing process using Nb doped Li7La3Zr2O12 (LLZONb) as the solid electrolyte and Li3BO3 (LBO) as a liquid electrolyte within the cathode layer.

330 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used heavily Nb-doped SrTiO3 epitaxial films, which were grown on insulating (100)-oriented LaAlO3 single-crystalline substrates by a pulsed-laser deposition method.
Abstract: Carrier concentration dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on insulating (100)-oriented LaAlO3 single-crystalline substrates by a pulsed-laser deposition method. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT of Nb-doped SrTiO3 increases with Nb concentration and it reaches ∼0.37 (20% Nb doped), which is the largest value among n-type oxide semiconductors ever reported.

324 citations


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TL;DR: In this article, the authors provide a background overview and discuss the state of the art, ion-transport mechanisms and fundamental properties of solid-state electrolyte materials of interest for energy storage applications.
Abstract: Solid-state electrolytes are attracting increasing interest for electrochemical energy storage technologies. In this Review, we provide a background overview and discuss the state of the art, ion-transport mechanisms and fundamental properties of solid-state electrolyte materials of interest for energy storage applications. We focus on recent advances in various classes of battery chemistries and systems that are enabled by solid electrolytes, including all-solid-state lithium-ion batteries and emerging solid-electrolyte lithium batteries that feature cathodes with liquid or gaseous active materials (for example, lithium–air, lithium–sulfur and lithium–bromine systems). A low-cost, safe, aqueous electrochemical energy storage concept with a ‘mediator-ion’ solid electrolyte is also discussed. Advanced battery systems based on solid electrolytes would revitalize the rechargeable battery field because of their safety, excellent stability, long cycle lives and low cost. However, great effort will be needed to implement solid-electrolyte batteries as viable energy storage systems. In this context, we discuss the main issues that must be addressed, such as achieving acceptable ionic conductivity, electrochemical stability and mechanical properties of the solid electrolytes, as well as a compatible electrolyte/electrode interface. This Review details recent advances in battery chemistries and systems enabled by solid electrolytes, including all-solid-state lithium-ion, lithium–air, lithium–sulfur and lithium–bromine batteries, as well as an aqueous battery concept with a mediator-ion solid electrolyte.

2,749 citations

Journal ArticleDOI
TL;DR: Li9.54Si1.74P1.44S11.7Cl0.6P3S12 as discussed by the authors showed that Li 9.54 Si 1.54P 1.74Si 1.44 S11.3 has high specific power that is superior to that of conventional cells with liquid electrolytes.
Abstract: Compared with lithium-ion batteries with liquid electrolytes, all-solid-state batteries offer an attractive option owing to their potential in improving the safety and achieving both high power and high energy densities. Despite extensive research efforts, the development of all-solid-state batteries still falls short of expectation largely because of the lack of suitable candidate materials for the electrolyte required for practical applications. Here we report lithium superionic conductors with an exceptionally high conductivity (25 mS cm−1 for Li9.54Si1.74P1.44S11.7Cl0.3), as well as high stability ( ∼0 V versus Li metal for Li9.6P3S12). A fabricated all-solid-state cell based on this lithium conductor is found to have very small internal resistance, especially at 100 ∘C. The cell possesses high specific power that is superior to that of conventional cells with liquid electrolytes. Stable cycling with a high current density of 18 C (charging/discharging in just three minutes; where C is the C-rate) is also demonstrated. The development of all-solid-state batteries requires fast lithium conductors. Here, the authors report a lithium compound, Li9.54Si1.74P1.44S11.7Cl0.3, with an exceptionally high conductivity and demonstrate that all-solid-state batteries based on the compound have high power densities.

2,132 citations

Journal ArticleDOI
TL;DR: The development of novel composite cathode materials including sulfur-carbon and sulfur-polymer composites are described, describing the design principles, structure and properties, and electrochemical performances of these new materials.
Abstract: Electrical energy storage is one of the most critical needs of 21st century society. Applications that depend on electrical energy storage include portable electronics, electric vehicles, and devices for renewable energy storage from solar and wind. Lithium-ion (Li-ion) batteries have the highest energy density among the rechargeable battery chemistries. As a result, Li-ion batteries have proven successful in the portable electronics market and will play a significant role in large-scale energy storage. Over the past two decades, Li-ion batteries based on insertion cathodes have reached a cathode capacity of ∼250 mA h g(-1) and an energy density of ∼800 W h kg(-1), which do not meet the requirement of ∼500 km between charges for all-electric vehicles. With a goal of increasing energy density, researchers are pursuing alternative cathode materials such as sulfur and O2 that can offer capacities that exceed those of conventional insertion cathodes, such as LiCoO2 and LiMn2O4, by an order of magnitude (>1500 mA h g(-1)). Sulfur, one of the most abundant elements on earth, is an electrochemically active material that can accept up to two electrons per atom at ∼2.1 V vs Li/Li(+). As a result, sulfur cathode materials have a high theoretical capacity of 1675 mA h g(-1), and lithium-sulfur (Li-S) batteries have a theoretical energy density of ∼2600 W h kg(-1). Unlike conventional insertion cathode materials, sulfur undergoes a series of compositional and structural changes during cycling, which involve soluble polysulfides and insoluble sulfides. As a result, researchers have struggled with the maintenance of a stable electrode structure, full utilization of the active material, and sufficient cycle life with good system efficiency. Although researchers have made significant progress on rechargeable Li-S batteries in the last decade, these cycle life and efficiency problems prevent their use in commercial cells. To overcome these persistent problems, researchers will need new sulfur composite cathodes with favorable properties and performance and new Li-S cell configurations. In this Account, we first focus on the development of novel composite cathode materials including sulfur-carbon and sulfur-polymer composites, describing the design principles, structure and properties, and electrochemical performances of these new materials. We then cover new cell configurations with carbon interlayers and Li/dissolved polysulfide cells, emphasizing the potential of these approaches to advance capacity retention and system efficiency. Finally, we provide a brief survey of efficient electrolytes. The Account summarizes improvements that could bring Li-S technology closer to mass commercialization.

1,888 citations

Journal ArticleDOI
TL;DR: The field of surface science provides a unique approach to understand bulk, surface and interfacial phenomena occurring during TiO2 photocatalysis as mentioned in this paper, including photon absorption, charge transport and trapping, electron transfer dynamics, adsorbed state, mechanisms, poisons and promoters, and phase and form.

1,768 citations

Journal ArticleDOI
TL;DR: This review describes the recent advances in designing high-performance bulk thermoelectric materials and highlights the decoupling of the electron and phonon transport through coherent interface, matrix/precipitate electronic bands alignment, and compositionally alloyed nanostructures.
Abstract: There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of th...

1,469 citations