二次電池用鉄系正極の材料探索と反応機構解析

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 山田 淳夫, 東京大学教授 山下 晃一, 東京大学教授 堂免 一成, 東京大学教授 大久保 達也, 東京大学准教授 大久保 将史, 東京理科大学教授 駒場 慎一University of Tokyo(東京大学

A 3.8-V earth-abundant sodium battery electrode

Rechargeable lithium batteries have ushered the wireless revolution over last two decades and are now matured to enable green automobiles. However, the growing concern on scarcity and large-scale applications of lithium resources have steered effort to realize sustainable sodium-ion batteries, ​Na and ​Fe being abundant and low-cost charge carrier and redox centre, respectively. However, their performance is limited owing to low operating voltage and sluggish kinetics. Here we report a hitherto-unknown material with entirely new composition and structure with the first alluaudite-type sulphate framework, Na2Fe2(SO4)3, registering the highest-ever Fe3+/Fe2+ redox potential at 3.8 V (versus ​Na, and hence 4.1 V versus ​Li) along with fast rate kinetics. Rare-metal-free Na-ion rechargeable battery system compatible with the present Li-ion battery is now in realistic scope without sacrificing high energy density and high power, and paves way for discovery of new earth-abundant sustainable cathodes for large-scale batteries.UTokyo Research掲載「新物質発見で電池のレアメタル使用ゼロに」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/materials-discovery-for-earth-abundant-battery/UTokyo Research "Materials discovery for earth-abundant battery" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/materials-discovery-for-earth-abundant-battery

Sodium Intercalation Mechanism of 3.8 v Class Alluaudite Sodium Iron Sulfate

Alluaudite sodium iron sulfate Na$_{2+2x}$Fe$_{2−x}$(SO$_4$)$_3$ is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe$^{3+}$/Fe$^{2+}$ redox reactions (3.8 V vs Na/Na$^+$ ), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), $^{23}$Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe$^{3+}$/Fe$^{2+}$ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions.The present work was financially supported from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the “Element Strategy Initiative for Catalysts & Batteries” (ESICB) project. The synchrotron XRD experiments were performed under KEK-PF User Program (No. 2013G670). Crystal structures and the Fourier difference maps were drawn by VESTA.65 G.O. acknowledges financial support from JSPS Research Fellowships under “Materials Education Program for the Future Leaders in Research, Industry, and Technology” (MERIT) project. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 655444 (O.P.). R.P. gratefully acknowledges financial support through the Marie Curie Actions People Program of the EU’s Seventh Frame work Program (FP7/2007-2013), under the grant agreement n.317127, the ‘pNMR project’. K.J.G. gratefully acknowledges funding from The Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. This work made use of the facilities of the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.This is the final version of the article. It first appeared from American Chemical Society via http://dx.doi.org/10.1021/acs.chemmater.6b0109

A 3.8-V earth-abundant sodium battery electrode

Rechargeable lithium batteries have ushered the wireless revolution over last two decades and are now matured to enable green automobiles. However, the growing concern on scarcity and large-scale applications of lithium resources have steered effort to realize sustainable sodium-ion batteries, Na and Fe being abundant and low-cost charge carrier and redox centre, respectively. However, their performance is limited owing to low operating voltage and sluggish kinetics. Here we report a hitherto-unknown material with entirely new composition and structure with the first alluaudite-type sulphate framework, Na2Fe2(SO4)(3), registering the highest-ever Fe3+/ Fe2+ redox potential at 3.8V (versus Na, and hence 4.1V versus Li) along with fast rate kinetics. Rare-metal-free Na-ion rechargeable battery system compatible with the present Li-ion battery is now in realistic scope without sacrificing high energy density and high power, and paves way for discovery of new earth-abundant sustainable cathodes for large-scale batteries

Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batteries

Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batterie

Increased Conductivity in the Metastable Intermediate in Li_<i>x</i>FePO₄ Electrode

With increasing concerns about energy and environmental issues, lithium ion batteries are now penetrating into large-scale applications such as electric vehicles. As an electrode reaction process, it is generally believed that two-phase reaction with structural rearrangement and large lattice mismatch impedes high-rate capability. However, Li_<i>x</i>FePO₄, with its two-phase reaction between LiFePO₄ and FePO₄, exhibits an exceptional high-rate performance. In this article, after confirming the existence of a single-phase reaction even under moderate rates, we demonstrate an approximately 2 orders of magnitude increase of the conductivity for the quenched intermediate Li_0.6FePO₄. In addition to the widely accepted strain relaxation effect at the two-phase interface, the dramatically increased conductivity due to polaron/lithium carrier density increase in the intermediate phase should be highlighted as an important factor to accelerate the electrode reaction of olivine Li_<i>x</i>FePO₄

Magnetic Structure and Properties of the Rechargeable Battery Insertion Compound Na2FePO4F

The magnetic structure and properties of sodium iron fluorophosphate Na2FePO4F (space group Pbcn), a cathode material for rechargeable batteries, were studied using magnetometry and neutron powder diffraction. The material, which can be described as a quasi-layered structure with zigzag Fe-octahedral chains, develops a long-range antiferromagnetic order below similar to 3.4 K. The magnetic structure is rationalized as a super-exchange-driven ferromagnetic ordering of chains running along the a-axis, coupled antiferromagnetically by super-super-exchange via phosphate groups along the c-axis, with ordering along the b-axis likely due to the contribution of dipole dipole interactions

Kinetics of Nucleation and Growth in Two-Phase Electrochemical Reaction of Li_<i>x</i>FePO₄

The kinetics of a two-phase electrochemical reaction in Li_<i>x</i>FePO₄ was investigated by potential-step chronoamperometry under various experimental conditions: amplitude of potential step, direction of potential step, particle size, and thickness of composite electrodes. Only under a small potential step (10 mV) applied to large Li_<i>x</i>FePO₄ particles (203 nm), the chronoamperogram showed a momentary current increase, followed by gradual decline, indicating that the nucleation and growth governed the electrode kinetics. In that condition, the chronoamperogram was analyzed with the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model, which describes the kinetics of phase transition. The obtained Avrami exponent of ca. 1.1 indicates that the phase transition proceeds with a one-dimensional phase-boundary movement, which is consistent with the previously reported mechanism. From the temperature dependence of the obtained rate constant, the activation energy of the phase-boundary movement in Li_<i>x</i>FePO₄ was estimated to be 42 and 40 kJ mol^–1 in cathodic and anodic reactions, respectively