Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films.

Perovskite oxides ABO3 continue to be a major focus in materials science. Of particular interest is the interplay between A and B cations as exemplified by intersite charge transfer (ICT), which causes novel phenomena including negative thermal expansion and metal-insulator transition. However, the ICT properties were achieved and optimized by cationic substitution or ordering. Here we demonstrate an anionic approach to induce ICT using an oxyhydride perovskite, EuVO2H, which has alternating layers of EuH and VO2. A bulk EuVO2H behaves as a ferromagnetic insulator with a relatively high transition temperature (TC) of 10 K. However, the application of external pressure to the EuIIVIIIO2H bulk or compressive strain from the substrate in the thin films induces ICT from the EuIIH layer to the VIIIO2 layer due to the extended empty V dxy orbital. The ICT phenomenon causes the VO2 layer to become conductive, leading to an increase in TC that is dependent on the number of carriers in the dxy orbitals (up to a factor of 4 for 10 nm thin films). In addition, a large perpendicular magnetic anisotropy appears with the ICT for the films of <100 nm, which is unprecedented in materials with orbital-free Eu2+, opening new perspectives for applications. The present results provide opportunities for the acquisition of novel functions by alternating transition metal/rare earth layers with heteroanions.

Topochemical Nitridation with Anion Vacancy-Assisted N(3-)/O(2-) Exchange.

We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO3-xHx (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO3-xHx, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N(3-)/H(-)-exchanged product EuTi(4+)O2.82N0.12□0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N(3-)/O(2-) exchange, yielding a fully oxidized Eu(3+)Ti(4+)O2N with the GdFeO3-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO3 proceeded through a 1:1 exchange of N(3-) with O(2-) only above 600 °C and resulted in incomplete nitridation to EuTiO2.25N0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO2.82H0.18 play a crucial role in promoting anion (N(3-)/O(2-)) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.

Battery deactivation with redox shuttles for safe and efficient recycling.

To safely recycle spent lithium-ion batteries (LIBs), their deactivation as a pretreatment is essential. However, the conventional deactivation methods, mainly inducing an external short circuit, cannot be applied to LIBs with disconnected electrical circuits or Li deposited, despite their safety risk. Here, we propose a deactivation method using redox shuttles (RSs). The addition of an RS with redox potentials located between the two electrode potentials into a LIB electrochemically induces an internal short circuit with or without disconnected electrical circuits. A fully charged LIB discharges to approximately 0 V when a deactivation agent containing ferrocene or phenothiazine as an RS is added. Moreover, we demonstrate that RSs introduced into LIB can simultaneously dissolve Li deposited on the negative electrode surface and return it to the positive electrode as mobile ions. These characteristics of our method contribute to the improvement in safety and collection rate of Li in the recycling processes, promoting the sustainability of LIBs.

Heterogeneous intercalated metal-organic framework active materials for fast-charging non-aqueous Li-ion capacitors.

Intercalated metal-organic frameworks (iMOFs) based on aromatic dicarboxylate are appealing negative electrode active materials for Li-based electrochemical energy storage devices. They store Li ions at approximately 0.8 V vs. Li/Li+ and, thus, avoid Li metal plating during cell operation. However, their fast-charging capability is limited. Here, to circumvent this issue, we propose iMOFs with multi-aromatic units selected using machine learning and synthesized via solution spray drying. A naphthalene-based multivariate material with nanometric thickness allows the reversible storage of Li-ions in non-aqueous Li metal cell configuration reaching 85% capacity retention at 400 mA g-1 (i.e., 30 min for full charge) and 20 °C compared to cycling at 20 mA g-1 (i.e., 10 h for full charge). The same material, tested in combination with an activated carbon-based positive electrode, enables a discharge capacity retention of about 91% after 1000 cycles at 0.15 mA cm-2 (i.e., 2 h for full charge) and 20 °C. We elucidate the charge storage mechanism and demonstrate that during Li intercalation, the distorted crystal structure promotes electron delocalization by controlling the frame vibration. As a result, a phase transition suppresses phase separation, thus, benefitting the electrode's fast charging behavior.