Yanbin Shen

A P2-NaxCo0.7Mn0.3O2 (x ≈ 1.0) cathode material for Na-ion batteries with superior rate and cycle capability

The development of sodium ion batteries is of key importance in future energy technology, but so far no materials are known with sufficient rate and cycle capability for real applications. Here we report on a P2-NaxCo0.7Mn0.3O2 (x ≈ 1.0) cathode material with outstanding properties composed of layered hexagonal nanosheets synthesized using a simple and scalable co-precipitation and solid state reaction method. The material has excellent cycle capability with 84% capacity retention after 225 cycles at 1C rate, and an extraordinary rate capability with ∼75% of the 1C capacity maintained even at a very high charge/discharge rate of 30C. The unique properties may be attributed to a combination of the layered structure and the nanosheet morphology that mitigates the stress caused by Na ion diffusion during cycling. Moreover, the nanosheets are highly (001) oriented, leading to a large active surface of (010)/(100) planes, which could facilitate easy Na ion extraction/intercalation.

Alkali Metal Ion Templated Transition Metal Formate Framework Materials: Synthesis, Crystal Structures, Ion Migration, and Magnetism

Four transition metal formate coordination polymers with anionic frameworks, namely, Na[Mn(HCOO)3], K[Mn(HCOO)3], Na2[Cu3(HCOO)8], and K2[Cu5(HCOO)12], were synthesized using a mild solution chemistry approach. Multitemperature single-crystal (100-300 K) and powder X-ray diffraction studies of the compounds reveal structures of large diversity ranging from cubic chiral Na-Mn formate to triclinic Na-Cu formate. The structural variety is caused by the nature of the transition metals, the alkali metal ion templation, and the versatility of the formate group, which offers metal-metal coordination through three different O-C-O bridging modes (syn-syn, syn-anti, anti-anti) in addition to metal-metal bridging via a single oxygen atom. The two manganese(II) compounds contain mononuclear, octahedrally coordinated moieties, but the three-dimensional connectivity between the manganese octahedra is very different in the two structures. The two copper frameworks, in contrast, consist of binuclear and mononuclear moieties (Na-Cu formate) and trinuclear and mononuclear moieties (K-Cu formate), respectively. Procrystal electron density analysis of the compounds indicates one-dimensional K(+)-ion conductivity in K-Mn and K-Cu, and the nature of the proposed potassium ion migration is compared with results from similar analysis on known Na(+) and K(+) ion conductors. K-Mn and Na-Mn were tested as cathode materials, but this resulted in poor reversibility due to low conductivity or structural collapse. The magnetic properties of the compounds were studied by vibrating sample magnetometric measurements, and their thermal stabilities were determined by thermogravimetric analysis and differential thermal analysis. Despite structural differences, the metal formates that contain the same transition metal have similar magnetic properties and thermal decomposition pathways, that is, the nature of the transition metal controls the compound properties.

An electrochemical cell for in operando studies of lithium/sodium batteries using a conventional x-ray powder diffractometer

An electrochemical cell has been designed for powder X-ray diffraction studies of lithium ion batteries (LIB) and sodium ion batteries (SIB) in operando with high time resolution using a conventional powder X-ray diffractometer. The cell allows for studies of both anode and cathode electrode materials in reflection mode. The cell design closely mimics that of standard battery testing coin cells and allows obtaining powder X-ray diffraction patterns under representative electrochemical conditions. In addition, the cell uses graphite as the X-ray window instead of beryllium, and it is easy to operate and maintain. Test examples on lithium insertion/extraction in two spinel-type LIB electrode materials (Li4Ti5O12 anode and LiMn2O4 cathode) are presented as well as first results on sodium extraction from a layered SIB cathode material (Na0.84Fe0.56Mn0.44O2).

Light-tuned selective photosynthesis of azo- and azoxy-aromatics using graphitic C3N4

Solar-driven photocatalysis has attracted significant attention in water splitting, CO2 reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C3N4 is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.The synthesis of azo- and azoxy-aromatic dyes via photoreduction of nitroaromatics is hindered by high costs and low catalytic efficiencies and selectivities. Here the authors demonstrate the facile synthesis of these important dyes from their corresponding nitroaromatic precursors by using an inexpensive graphitic C3N4 photocatalyst.