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A ternary lithium nitride, Li3-x-yMxN (M = Ni and Cu) is one of the fastest lithium conductors at room temperature with a room temperature conductivity of 10-3 S cm-1. However, it has a narrow voltage stability window and is sensitive to mechanical stress, due to its low elastic modulus of only about 10 GPa. This limits its practical applications, such as in lithium metal batteries.

Unlike BN monolayers, which are semiconducting, the bandgap of stacked BN nanosheets decreases with increasing layer thickness. Quantum chemical calculations indicate that the insulating behaviour persists up to about 20 layers, whereas after that the sheet becomes semi-metallic in graphite-like fashion43.

The atomic structure of the composite anode, as revealed by a high-resolution electron microscopy (EM) image, matches that of hexagonal a-Li3N and shows no traces of lithium nitride oxide, which is confirmed by elemental mapping images (Supplementary Figure 1H). In addition, the average Young’s modulus of the composite anode reaches 86.0 GPa, compared with only 10.2 GPa for the bare Li metal foil. This remarkable mechanical strength can be ascribed to the ultrahigh hardness of metal nitrides combined with the carbon shell, which is able to tolerate the stresses caused by the plating and stripping process.

The morphology of the composite anode, as shown by transmission electron microscopy (TEM), can be tailored to yield different types of nanofibres, with diameters in excess of 2 mm and lengths up to 10 mm. Detailed reaction conditions used for the synthesis of type II Li3N nanofibres are summarised in Supplementary Table 2. X-ray powder diffraction data for the fine powder sample matches that for the hexagonal a-Li3N crystal structure (P6/mmm; a = 3.656(2) A, c = 3.868(4) A). For all TEM analyses, samples were prepared dry on 3 mm holey carbon film copper grids in an N2-filled glovebox and transferred to the instrument using an air-tight holder to minimise exposure to air during transfer.

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