Energy Focus: Tetrathiafulvalene mediates oxidation, reduces polarization in Li-O 2 energy batteries
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esearchers in pursuit of higher density memories have set their sights on a new generation of spintronic materials, in which both electron charge and spin are used to convey information. The magnetoelectric multiferroic BiFeO3 (BFO), which exhibits direct coupling between ferroelectric and antiferromagnetic order, is particularly interesting for such an application. However, the complex interplay of strain and magnetic response in this system is only poorly understood. Now, Daniel Sando and colleagues at the Unité Mixte de Physique CNRS/ Thales shed light on the fundamental mechanisms governing antiferromagnetism and demonstrate tunable control of this ordering for spintronics devices. As described in the April 28 online edition of Nature Materials (DOI: 10.1038/ nmat3629), the researchers first deposited 70-nm thick BFO films onto differ-
Energy Focus Tetrathiafulvalene mediates oxidation, reduces polarization in Li-O2 energy batteries
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on-aqueous Li-air (O2) batteries promise a significantly higher theoretical energy-storage density than conventional lithium-ion batteries. However, major increases in the lithium-oxygen round-trip charging/discharging efficiency and cycle life are needed before lithium-air batteries become viable. When a Li-O2 battery discharges, electrons, lithium ions, and oxygen gas react to form nanoparticles of Li2O2, and the opposite may be expected when the battery charges. However, transporting charge between the Li2O2 particles and the solid electrode surface is very difficult, and in practice, most of the Li2O2 polarizes, preventing the battery from completely recharging. Reporting online May 12 in Nature Chemistry (DOI: 10.1038/ NCHEM.1646), Y. Chen, S.A. Freunberger, Z. Peng, O. Fontaine, and P.G. Bruce
ent substrates using pulsed laser deposition, thereby imparting different inplane strain states ranging from –2.6% compressive to 1.3% tensile strain. They next probed the hyperfine interactions of 57Fe nuclei in BFO using Mössbauer spectroscopy, which allowed them to access the local magnetic environment around the Fe3+ ions. The results showed that at high strains, the typical helical antiferromagnetic spin cycloid vanishes. To better understand this, the researchers conducted Landau-Ginzburg and effective Hamiltonian theory calculations. These demonstrate that at low strain states one of two spin cycloid orderings is stable, while at higher strains a collinear antiferromagnetic ordering is preferred. The researchers confirmed these results using Raman spectroscopy and tested the effect of these strains on magnetic hysteresis. It is therefore possible to greatly change exchange bias and giant magnetoresistance (GMR) using strain. This understanding suggests that coupling BFO to a piezoelectric material such as PbZrxTi1–xO3
from the University of St. Andrews, Graz University of Technology, Chinese Academy of Sciences, and Université Montpellier may have found a solution to the polarization puzzle in the form of a redox mediator, tetrathiafulvalene (TTF). In a discharging Li-O2
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