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Improved methods for the flux growth of single crystals of the important battery material LiFePO4have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li1–2xFex)FePO4partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO4stoichiometry. The LiFePO4defects are shown to be sarcopside-like (2 Li+→ Fe2++ vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters on defect concentration, and their observed phase behavior (defects only appear in growths from fluxes which are Fe-rich relative to stoichiometric LiFePO4). The distribution of defects has been studied by aberration corrected scanning transmission electron microscopy and was found to be highly inhomogenous, suggesting that defect-containing crystals may consist of endotaxial intergrowths of olivine LiFePO4and sarcopside Fe3(PO4)2in a manner that minimizes the detrimental influence of FeLidefects on the rate of Li-ion transport within crystallites.

Inorganic Na-ion superionic conductors play a vital role in all-solid-state Na batteries that operate at room temperature. Sodium thioantimonate (Na3SbS4), a popular sulfide-based solid electrolyte, has attracted serious attention due to its advantages of high ionic conductivity at room temperature and impressive chemical stability under ambient conditions. Much research detailing Na3SbS4focused on its synthetic approaches and interfacial stability against Na metal, yet, there is limited information elucidating a fundamental understanding of the Na-ion diffusion mechanisms in Na3SbS4with different crystal structures (e.g., tetragonal and cubic). Herein, we combine real-time electrochemical impedance measurements with theoretical simulations based on density functional theory and  in situquasi-elastic neutron scattering to study the Na-ion conductive properties of Na3SbS4during its phase transition from a tetragonal to cubic structure. Although there is a slight change in the lattice parameters, the energy barrier for Na-ion diffusion in the tetragonal structure was determined to be much larger (5–10 times) than that in the cubic structure from both theoretical and experimental perspectives. The high degree of symmetry in cubic Na3SbS4leads to less interatomic correlations between Na and S(Sb) atoms, a shorter jump distance (2.85 Å), and a larger diffusion coefficient. This research provides insight into understanding the Na-ion diffusion in solid electrolytes with phase transitions and provides fundamental guidance for designing novel solid-state Na-ion conductors.

A simple strategy for developing a cost-effective and efficient Ir-based catalyst toward the oxygen evolution reaction (OER) is to construct a core–shell structure with most of the Ir atoms serving as reactive sites on the surface. However, it has been challenging to achieve a precise control over the thickness of the Ir shell from one to several atomic layers and thus optimize the OER performance. Here, we report a facile synthesis of Pd@IrnL(n: the number of Ir atomic layers) core–shell nanocubes with the shell thickness controlled from one to four atomic layers. Their OER activities showed a volcano-type dependence on the number of Ir atomic layers, with a maximum point corresponding to n= 3, which can be attributed to Pd–Ir intermixing, and possible ligand and/or strain effects. Owing to the better passivation for the Pd cores and the formation of a more stable phase during electrolysis, the Pd@IrnLnanocubes with thicker Ir overlayers exhibited greater OER durability. The Pd@Ir3Lnanocubes delivered the best activity and durability toward OER with η as low as 245 mV at 10 mA·cmgeo–2and a mass activity of 3.33 A·mgIr–1at η = 300 mV. Both values were much better than those of commercial Ir/C and represent the best set of data among the Ir-based core–shell OER catalysts in acidic media.

A successful strategy for reducing the content of Pt without compromising the activity of a Pt-based catalyst is to deposit Pt as an ultrathin overlayer on the surface of another metal. Here, we report a facile one-pot synthesis of Pd@Pt1L(1L: one atomic layer) core–shell octahedra using a solution-phase method. The success of this method relies on the use of metal precursors with markedly different reduction kinetics. In a typical synthesis, the ratio between the initial reduction rates of the Pd(II) and Pt(II) precursors differed by almost 100 times, favoring the formation of Pd–Pt bimetallic octahedra with a core–shell structure. The reduction of the Pt(II) precursor at a very slow rate and the use of a high temperature allowed the deposited Pt atoms to spread and cover the entire surface of Pd octahedral seeds formed in the initial stage. More importantly, we were able to scale up this synthesis using continuous-flow reactors without compromising product quality. Compared to a commercial Pt/C catalyst, the Pd@Pt1Lcore–shell octahedra showed major augmentation in terms of catalytic activity and durability for the oxygen reduction reaction (ORR). After 10000 cycles of accelerated durability test, the core–shell octahedra still exhibited a mass activity of 0.45 A mg–1Pt. We rationalized the experimental results using DFT calculations, including the mechanism of synthesis, ORR activities, and possible Pd–Pt atom swapping to enrich the outermost layer with Pd. Specifically, the as-synthesized Pd@Pt1Loctahedra tended to take a slightly mixed surface composition because the deposited Pt atoms were able to substitute into Pd upon deposition on the edges; ORR energetics were more favorable on pure Pt shells as compared to significantly mixed Pd–Pt shells, and the activation energy barriers calculated for the Pd–Pt atom swapping were too prohibitive to significantly alter the surface composition of the as-synthesized Pd@Pt1Loctahedra, helping sustain their activity for prolonged operation.

Correlated quantum materials are expected to provide the foundation for the next generation of information or energy technologies. A key feature of these materials is the proximity of multiple ground states close in energy, which results in the ability to tune properties with small changes in an external parameter such as magnetic field, composition, or temperature. For example, the colossal magnetoresistance exhibited by manganites is related to charge and orbital ordering and results from a metallic ferromagnetic phase in proximity to a paramagnetic insulating phase. The presence of competing ground states, at the heart of the physics and functionality of these materials, often results in nanoscale phase separation. Probing nanoscale phase separation with conventional diffraction techniques alone is not adequate, particularly when the domains are small or nanosized. In the present work we use a scanning transmission electron microscopy image-based technique of picometer precision strain maps (PPSM) to directly visualize the competing nanoscale phases with charge and orbital ordering in a double-layer manganite. This work underscores the role of subtle structural distortions in determining the electron physics in correlated quantum materials and provides insights into designing new functionalities via spatially tuning multiple competing ground states.