2019 Volume 87 Issue 6 Pages 341-347
This study explains the charge/discharge mechanism transformation from two-phase to a solid-solution reaction at over 4.5 V vs. Li/Li+ for monoclinic Li3V2(PO4)3 (LVP). An electrochemical characterization that combines galvanostatic cycling and intermittent titration technique confirms that this mechanism transformation occurs in LVP not only for the discharge reaction (lithiation) as previously reported but also for the charge reaction (delithiation), starting gradually within the initial 10 cycles. A similar type of electrochemical characterization of Li3V1.5Al0.5(PO4)3 (LVAP) indicates that transition metal doping (25 at.% of Al3+) accelerates such mechanism transformation completed within an initial cycling. Additional cycling operation at a lower potential (below 4.3 V vs. Li/Li+) shows that the transformed state of LVP is metastable, as plateau recovery can be observed within 100 cycles, while LVAP maintains its solid-solution reaction over 1,000 cycles. In situ X-ray absorption spectroscopy (XAS) analysis suggests that such mechanism transformation of LVP occurs through a change in the coordination environment of vanadium (from an octahedron to a distorted, possibly tetrahedral environment) evidenced by changes in pre-edge peaks in the V K-edge spectra. Such a change in the coordination environment is smoothed and stabilized by the introduced Al3+, possibly due to an enhancement of Li+ diffusivity in the LVP crystals.