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A three dimensional vanadium pentoxide/reduced graphene oxide/carbon nanotube (3D V2O5/RGO/CNT) composite is synthesized by microwave-assisted hydrothermal method. The combination of 2D RGO and 1D CNT establishes continuous 3D conductive network, and most notably, the 1D CNT is designed to form hierarchically porous structure by penetrating into V2O5 microsphere assembly constituted of numerous V2O5 nanoparticles. The highly porous V2O5 microsphere enhances electrolyte contact and shortens Li+ diffusion path as a consequence of its developed surface area and mesoporosity. The successive phase transformations of 3D V2O5/RGO/CNT from α-phase to ε-, δ-, γ-, and ω-phase and its structural reversibility upon Li+ intercalation/de-intercalation are investigated by in situ XRD analysis, and the electronic and local structure reversibility around vanadium atom in 3D V2O5/RGO/CNT is observed by in situ XANES analysis. The 3D V2O5/RGO/CNT achieves a high capacity of 220 mAh g−1 at 1 C after 80 cycles and an excellent rate capability of 100 mAh g−1 even at a considerably high rate of 20 C. The porous 3D V2O5/RGO/CNT structure not only provides facile Li+ diffusion into bulk but contributes to surface Li+ storage as well, which enables the design of 3D V2O5/RGO/CNT composite to become a promising cathode architecture for high performance LIBs.

The conventional view of conversion reaction is based on the reversibility, returning to an initial material structure through reverse reaction at each cycle in cycle life, which impedes the complete understanding on a working mechanism upon a progression of cycles in conversion-reaction-based battery electrodes. Herein, a series of tin-doped ferrites (Fe3–x Sn x O4, x = 0–0.36) are prepared and applied to a lithium-ion battery anode. By achieving the ideal reoxidation into SnO2, the Fe2.76Sn0.24O4 composite anchored on reduced graphene oxide shows a high reversible capacity of 1428 mAh g–1 at 200 mA g–1 after 100 cycles, which is the best performance of Sn-based anode materials so far. Significantly, a newly formed γ-FeOOH phase after 100 cycles is identified from topological features through synchrotron X-ray absorption spectroscopy with electronic and atomic structural information, suggesting the phase transformation from magnetite to lepidocrocite upon cycling. Contrary to the conventional view, our work suggests a variable working mechanism in an iron-based composite with the dynamic phases from iron oxide to iron oxyhydroxide in the battery cycle life, based on the reactivity of metal nanoparticles formed during reaction toward the solid electrolyte interface layer.

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The electrochemical properties of Mg-doped LiFe 0.48 Mn 0.48 Mg 0.04 PO 4 and pure LiFe 0.5 Mn 0.5 PO 4 olivine cathodes are examined and the lattice parameters are refined by Rietveld analysis. The calculated atomic parameters from the refinement show that Mg 2+ doping has a significant effect in the olivine LiFeMnPO 4 structure. The unit cell volume is 297.053(2) Å 3 for pure LiFe 0.5 Mn 0.5 PO 4 and is decreased to 296.177(1) Å 3 for Mg-doped LiFe 0.48 Mn 0.48 Mg 0.04 PO 4 sample. The doping of Mg 2+ cation with atomic radius smaller than Mn 2+ and Fe 2+ ion induces longer Li-O bond length in LiO 6 octahedra of the olivine structure. The larger interstitial sites in LiO 6 octahedra facilitate the lithium ion migration and also enhance the diffusion kinetics of olivine cathode material. The LiFe 0.48 Mn 0.48 Mg 0.04 PO 4 sample with larger Li-O bond length delivers higher discharge capacities and also notably increases the rate capability of the electrode.