Polymorphic Effects on Electrochemical Performance of Conversion-Based MnO2 Anode Materials for Next-Generation Li Batteries

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Electrochemical, Photophysical, Morphological and DFT Study of Polymorphic Sn(IV)-Porphyrins Containing Fluorinated Axial Ligand

In this study, we report the polymorphism of six coordinated Sn(IV)- tetrabromophenyl porphyrins axially armed with fluorine-substituted phenolate ligands (structural formula [Sn(TBrPP)2+(A-)2], where A is the axial ligand = 3,5-difluoro phenol, compound 1). One form stabilizes in triclinic system (namely, 1α), and the other stabilizes in monoclinic system (namely, 1β). The two 1α and 1β polymorphs display distinct photophysical and morphological properties in the solid state. X-ray diffraction study reveals that these polymorphs 1α and 1β significantly differ in their supramolecular architecture, different axial phenolate conformations, and noncovalent interactions, which are responsible for their distinct solid-state properties. The crystal packing of these polymorphs dominates by intermolecular C-H···F, C-H···π and C-Br···F interhalogen interactions. Furthermore, the solid-state emission spectra of 1α showed red-shifted emission bands with respect to 1β, in addition the redox behavior of 1α is slightly different in comparison to 1β. Complementary theoretical studies with Hirshfeld surface analysis show the definite role of Br···F interhalogen interactions in the overall stability. Mapping the electrostatic potential isosurfaces with the aid of density functional theory in compound 1 clearly shows the presence of σ-hole, a requisite feature to show halogen interactions in the crystalline state. In addition, lattice energy and single point energy calculation shows that 1α was found to be energetically more favorable and thermodynamically more stable compare to 1β.

Core-Shell Structured C@SiO2 Hollow Spheres Decorated with Nickel Nanoparticles as Anode Materials for Lithium-Ion Batteries

Silicon oxide is regarded as a promising anode material for lithium-ion batteries owing to high theoretical capacity, abundant reserve, and environmental friendliness. Large volumetric variations during the discharging/charging and intrinsically poor electrical conductivity, however, severely hinder its application. Herein, a core-shell structured composite is constructed by hollow carbon spheres and SiO₂ nanosheets decorated with nickel nanoparticles (Ni-SiO₂ /C HS). Hollow carbon spheres, as mesoporous cores, not only significantly facilitate the electron transfer but also prominently enhance the mechanical robustness of anode materials, which separately improves the rate performance and the cyclic durability. Besides, ultrathin SiO₂ nanosheets, as hierarchical shells, provide abundant electrochemical active surface for capacity increment. Moreover, nickel nanoparticles boost the transport capacity of electrons in SiO₂ nanosheets. Such a unique architecture of Ni-SiO₂ /C HS guarantees an enhanced discharge capacity (712 mAh g^-1 at 0.1 A g^-1 ) and prolonged cyclic durability (352 mAh g^-1 at 1.0 A g^-1 after 500 cycles). The present work offers a possibility for silica-based anode materials in the application of next-generation lithium-ion batteries.

Investigation into the Phase-Activity Relationship of MnO2 Nanomaterials toward Ozone-Assisted Catalytic Oxidation of Toluene

Manganese dioxide (MnO₂ ), with naturally abundant crystal phases, is one of the most active candidates for toluene degradation. However, it remains ambiguous and controversial of the phase-activity relationship and the origin of the catalytic activity of these multiphase MnO₂ . In this study, six types of MnO₂ with crystal phases corresponding to α-, β-, γ-, ε-, λ-, and δ-MnO₂ are prepared, and their catalytic activity toward ozone-assisted catalytic oxidation of toluene at room temperature are studied, which follow the order of δ-MnO₂  > α-MnO₂  > ε-MnO₂  > γ-MnO₂  > λ-MnO₂  > β-MnO₂ . Further investigation of the specific oxygen species with the toluene oxidation activity indicates that high catalytic activity of MnO₂ is originated from the rich oxygen vacancy and the strong mobility of oxygen species. This work illustrates the important role of crystal phase in determining the oxygen vacancies' density and the mobility of oxygen species, thus influencing the catalytic activity of MnO₂ catalysts, which sheds light on strategies of rational design and synthesis of multiphase MnO₂ catalysts for volatile organic pollutants' (VOCs) degradation.

Atomistic Insights of Irreversible Li+ Intercalation in MnO2 Electrode

Tunnel-structured MnO₂ represents open-framed electrode materials for reversible energy storage. Its wide application is limited by its poor cycling stability, whose structural origin is unclear. We tracked the structure evolution of β-MnO₂ upon Li⁺ ion insertion/extraction by combining advanced in situ diagnostic tools at both electrode level (synchrotron X-ray scattering) and single-particle level (transmission electron microscopy). The instability is found to originate from a partially reversible phase transition between β-MnO₂ and orthorhombic LiMnO₂ upon lithiation, causing cycling capacity decay. Moreover, the MnO₂ /LiMnO₂ interface exhibits multiple arrow-headed disordered regions, which severely chop into the host and undermine its structural integrity. Our findings could account for the cycling instability of tunnel-structured materials, based on which future strategies should focus on tuning the charge transport kinetics toward performance enhancement.