Genki Kobayashi

Air Exposure Effect on LiFePO4

The impact of ambient air exposure on LiFePO 4 /C nanocomposites has been investigated. A pristine sample was prepared without any exposure to ambient air through the whole process of synthesis and characterization and compared to the exposed samples. A small amount of lithium deintercalates from the olivine structure during exposure, a majority of which can be electrochemically reintercalated. This phenomenon changes the initial surface and bulk properties and should be taken into account when diminishing the particle size of LiFePO 4 . Keeping nanocomposites away from oxidative moisture atmosphere could be a solution to minimize these side reactions.

Bacterial nanometric amorphous Fe-based oxide: A potential lithium-ion battery anode material

Amorphous Fe(3+)-based oxide nanoparticles produced by Leptothrix ochracea, aquatic bacteria living worldwide, show a potential as an Fe(3+)/Fe(0) conversion anode material for lithium-ion batteries. The presence of minor components, Si and P, in the original nanoparticles leads to a specific electrode architecture with Fe-based electrochemical centers embedded in a Si, P-based amorphous matrix.

Pure H– conduction in oxyhydrides

Transporting the hydrogen anion Hydrogen cation (H+) transport is common in both biological systems and engineered ones such as fuel cells. In contrast, the transport of hydrogen anions (H−) is far less common and is usually coupled with or compromised by the parallel transport of electrons. Kobayashi et al. examined the transport of H− in a series of rare-earth lithium oxyhydrides (see the Perspective by Yamaguchi). They prevented electronic conduction by using Li+ as a countercation. In an electrochemical cell, the oxyhydride material acted as a solid-state electrolyte for H−, which suggests an alternative avenue for developing energy storage devices. Science, this issue p. 1314; see also p. 1262 A rare earth lithium oxyhydride shows transport of hydride anions. [Also see Perspective by Yamaguchi] A variety of proton (H+)–conducting oxides are known, including those used in electrochemical devices such as fuel cells. In contrast, pure H– conduction, not mixed with electron conduction, has not been demonstrated for oxide-based materials. Considering that hydride ions have an ionic size appropriate for fast transport and also a strong reducing ability suitable for high-energy storage and conversion devices, we prepared a series of K2NiF4-type oxyhydrides, La2-x-ySrx+yLiH1-x+yO3-y, in the hope of observing such H– conductors. The performance of an all-solid-state TiH2/o-La2LiHO3 (x = y = 0, o: orthorhombic)/Ti cell provided conclusive evidence of pure H– conduction.

Synthesis and Structures of Novel Solid-State Electrolytes

Two classes of new materials possessing ion conductivity have been developed: a lithium ion conductor and a hydride ion conductor. Conventional perovskite and ordered rock-salt structures were adopted as frameworks for lithium migration, and electrochemically stable elements such as Al, Ga, Ta, and Sc were used in the materials to facilitate their use as low-potential negative electrodes. New compositions of (Li0.25Sr0.625V(Li,Sr)0.125)(Ga0.25Ta0.75)O3, and Li0.9Sc0.9Zr0.1O2 were found to be novel oxide-based lithium ion conductors. Oxyhydrides with K2NiF4-type structures were synthesized via a high-pressure synthesis method and their use in pure hydride ion conduction was demonstrated. The La2–x–y Sr x+y LiH1–x+y O3–y oxyhydrides showed wide composition ranges of solid solution formation and the conductivity increased with anion vacancies or the introduction of interstitial hydride ions. The performance of an all-solid-state TiH2/o-La2LiHO3 (x = y = 0, o: orthorhombic)/Ti cell provided conclusive evidence of pure H– conduction.

Structural, magnetic, and diffusive nature of olivine-type NaxFePO4

In order to investigate the microscopic magnetic nature of a sodium phospho- olivine, we have measured μ+SR spectra for NaxFePO4 using a powder sample in the temperature range between 2 and 500 K. ZF-μ+ SR measurements below 200 K at TRIUMF showed the appearance of static magnetic order below TN ~ 61 K, where the susceptibility-vs.- temperature curve exhibits a sharp maximum due to an antiferromagnetic (AF) transition. The wide distribution found for the internal magnetic field (Hint) was explained by the formation of AF order with the spread in Hint due to random distribution of Na vacancies. At higher temperatures, the ZF-spectrum for Na0.7FePO4, obtained at J-PARC, is found to change from a low-T static behavior to a high-T dynamic behavior above ~ 300 K. This is consistent with the fact that Na+ ions are reversibly extracted from and intercalated into the NaxFePO4 lattice, while there is no NMR study on Na-diffusion in NaxFePO4 due to large Fe moments.