Full cells consisting of nanocrystalline Li 3 V 2 (PO 4) 3 (LVP) positive and standard commercial Li 4 Ti 5 O 12 (LTO) negative electrodes demonstrated outstanding cyclability: capacity retention of 77% over 10,000 cycles. We achieved this stable cycle performance by electrochemical preconditioning of LTO with Li prior to full-cell cycling. The strategy of Li preconditioning not only allows adjustment of the state of charge (SOC) between negative and positive electrodes, but also gives rise to the formation of a protective covering layer on the LTO surface. As we show, this covering layer plays an important role in preventing a key performance-limiting phenomenon-namely, the deposition of vanadium eluted from LVP onto LTO, which degrades the coulombic efficiency of Li + intercalation/ deintercalation into LTO crystals-yielding minimal SOC shifts and stable full-cell cycling.
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 Li 3 V 2 (PO 4) 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 Li 3 V 1.5 Al 0.5 (PO 4) 3 (LVAP) indicates that transition metal doping (25 at.% of Al 3+) 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 Al 3+ , possibly due to an enhancement of Li + diffusivity in the LVP crystals.
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