Na⁺/Vacancy Disordered P2-Na_0.67Co_1–<i>x</i>Ti<i>_x</i>O₂: High-Energy and High-Power Cathode Materials for Sodium Ion Batteries

Although sodium ion batteries (NIBs) have gained wide interest, their poor energy density poses a serious challenge for their practical applications. Therefore, high-energy-density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type Na_0.67Co_1–<i>x</i>Ti<i>_x</i>O₂ (<i>x</i> < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g^–1. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na⁺/vacancy ordering. Na_0.67CoO₂ suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization, while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, Na_0.67Co_0.90Ti_0.10O₂ exhibits excellent capacity retention (115 mAh g^–1) even after 100 cycles, whereas Na_0.67CoO₂ exhibits negligible capacity retention (<10 mAh g^–1) at 4.5 V cutoff conditions. Na_0.67Co_0.90Ti_0.10O₂ also exhibits outstanding rate capabilities of 108 mAh g^–1 at a current density of 1000 mA g^–1 (7.4 C). Increased sodium diffusion kinetics from mitigated Na⁺/vacancy ordering, which allows high Na⁺ utilization, are investigated to find in detail the mechanism of the improvement by combining systematic analyses comprising TEM, in situ XRD, and electrochemical methods

Synthesis of TiO₂ Nanoparticle-Embedded SiO₂ Microspheres for UV Protection Applications

Exposure to ultraviolet (UV) radiation induces many serious health issues. Because of serious health concerns, there is an urgent need to develop UV filters with better efficacy and high safety. For this purpose, titanium dioxide (TiO2) nanoparticles are the most desirable materials due to their excellent UV protection abilities. The use of TiO2 as sunscreens has raised some concerns about potential risks due to the formation of TiO2-mediated free radicals. Herein, TiO2 nanoparticles have been successfully embedded in silica (SiO2) microspheres using the emulsion synthesis method. The as-synthesized TiO2 nanoparticles embedded in silica microspheres have shown excellent UV protection ability. TiO2 nanoparticles embedded in silica microspheres suppress the photocatalytic properties compared to bare TiO2 nanoparticles, and these results indicate that TiO2-embedded silica microspheres are promising UV protection materials for sunscreen

Enhancing p‑Type Thermoelectric Performances of Polycrystalline SnSe via Tuning Phase Transition Temperature

SnSe emerges as a new class of thermoelectric materials since the recent discovery of an ultrahigh thermoelectric figure of merit in its single crystals. Achieving such performance in the polycrystalline counterpart is still challenging and requires fundamental understandings of its electrical and thermal transport properties as well as structural chemistry. Here we demonstrate a new strategy of improving conversion efficiency of bulk polycrystalline SnSe thermoelectrics. We show that PbSe alloying decreases the transition temperature between <i>Pnma</i> and <i>Cmcm</i> phases and thereby can serve as a means of controlling its onset temperature. Along with 1% Na doping, delicate control of the alloying fraction markedly enhances electrical conductivity by earlier initiation of bipolar conduction while reducing lattice thermal conductivity by alloy and point defect scattering simultaneously. As a result, a remarkably high peak <i>ZT</i> of ∼1.2 at 773 K as well as average <i>ZT</i> of ∼0.5 from RT to 773 K is achieved for Na_0.01(Sn_1–<i>x</i>Pb_<i>x</i>)_0.99Se. Surprisingly, spherical-aberration corrected scanning transmission electron microscopic studies reveal that Na_<i>y</i>Sn_1–<i>x</i>Pb_<i>x</i>Se (0 < <i>x</i> ≤ 0.2; <i>y</i> = 0, 0.01) alloys spontaneously form nanoscale particles with a typical size of ∼5–10 nm embedded inside the bulk matrix, rather than solid solutions as previously believed. This unexpected feature results in further reduction in their lattice thermal conductivity