Structural transformation of LiVOPO4 to Li3V2(PO4)3 with enhanced capacity

Alkali-Ion-Assisted Activation of ε-VOPO4 as a Cathode Material for Mg-Ion Batteries

Rechargeable multivalent-ion batteries are attractive alternatives to Li-ion batteries to mitigate their issues with metal resources and metal anodes. However, many challenges remain before they can be practically used due to the low solid-state mobility of multivalent ions. In this study, a promising material identified by high-throughput computational screening is investigated, ε-VOPO4, as a Mg cathode. The experimental and computational evaluation of ε-VOPO4 suggests that it may provide an energy density of >200 Wh kg-1 based on the average voltage of a complete cycle, significantly more than that of well-known Chevrel compounds. Furthermore, this study finds that Mg-ion diffusion can be enhanced by co-intercalation of Li or Na, pointing at interesting correlation dynamics of slow and fast ions.

Oxygen-Loss-Induced Structural Degradation in ε-LiVOPO4

The ε-LiVOPO₄ cathode for Li-ion batteries has attracted wide attention with its multivalent electronic states and improved discharge capacity of over 300 mAh/g. Oxygen loss stands as a potential cause for structural degradations of the ε-LiVOPO₄ cathode and its derivatives but has been barely studied. Through in situ environmental transmission electron microscopy, we probe lattice oxygen loss and the associated structural degradations by spatially and temporally resolving the atomic-scale structural dynamics and phase transformation pathways in ε-LiVOPO₄. We demonstrate that the mild oxygen loss at 400 °C induces a topotactic phase transformation of ε-LiVOPO₄ → α-Li₃V₂(PO₄)₃ in the particle surface via a nucleation and growth mechanism, leading to the formation of a core-shell configuration. The phase transformation can be reversed by switching to an oxidizing environment, in which the α-Li₃V₂(PO₄)₃ is reoxidized to ε-LiVOPO₄. By contrast, oxygen loss at higher temperatures of 500 and 600 °C results in a high concentration of oxygen vacancies that subsequently induces irreversible structural damages including lattice amorphization and formation of nanocavities. This work illustrates the fundamental mechanisms governing the structural failure of oxide cathodes and underlines possible strategies to overcome such issues by exploiting environmental constraints.

Pyro-synthesis of a high rate nano-Li3V2(PO4)3/C cathode with mixed morphology for advanced Li-ion batteries

A monoclinic Li3V2(PO4)3/C (LVP/C) cathode for lithium battery applications was synthesized by a polyol-assisted pyro-synthesis. The polyol in the present synthesis acts not only as a solvent, reducing agent and a carbon source but also as a low-cost fuel that facilitates a combustion process combined with the release of ultrahigh exothermic energy useful for nucleation process. Subsequent annealing of the amorphous particles at 800°C for 5 h is sufficient to produce highly crystalline LVP/C nanoparticles. A combined analysis of X-ray diffraction (XRD) and neutron powder diffraction (NPD) patterns was used to determine the unit cell parameters of the prepared LVP/C. Electron microscopic studies revealed rod-type particles of length ranging from nanometer to micrometers dispersed among spherical particles with average particle-sizes in the range of 20-30 nm. When tested for Li-insertion properties in the potential windows of 3-4.3 and 3-4.8 V, the LVP/C cathode demonstrated initial discharge capacities of 131 and 196 mAh/g (~100% theoretical capacities) at 0.15 and 0.1 C current densities respectively with impressive capacity retentions for 50 cycles. Interestingly, the LVP/C cathode delivered average specific capacities of 125 and 90 mAh/g at current densities of 9.6 C and 15 C respectively within the lower potential window.

Reinvestigation of trilithium divanadium(III) tris-(orthophosphate), Li(3)V(2)(PO(4))(3), based on single-crystal X-ray data

The structure of Li₃V₂(PO₄)₃ has been reinvestigated from single-crystal X-ray data. Although the results of the previous studies (all based on powder diffraction data) are comparable with our redetermination, all atoms were refined with anisotropic displacement parameters in the current study, and the resulting bond lengths are more accurate than those determined from powder diffraction data. The title compound adopts the Li₃Fe₂(PO₄)₃ structure type. The structure is composed of VO₆ octa­hedra and PO₄ tetra­hedra by sharing O atoms to form the three-dimensional anionic framework _∞³[V₂(PO₄)₃]³⁻. The positions of the Li⁺ ions in the empty channels can vary depending on the synthetic conditions. Bond-valence-sum calculations showed structures that are similar to the results of the present study seem to be more stable compared with others. The classical charge balance of the title compound can be represented as [Li⁺]₃[V³⁺]₂[P⁵⁺]₃[O²⁻]₁₂.