Battery is one of the most attractive energy storage systems for its usefulness in energy conversion in these days. A great attention has been paid to the Li-ion batteries, and a lot of attempts to develop the system exhibiting higher energy density and higher safety have been tried for many years. However, the safety of Li-ion battery has been problems continuously while the energy density has developed largely. The safety issues of Li-ion battery generally come from thermal runaway, mainly derived from oxygen evolution from cathode materials. Phosphate materials including frames of PO₄ polyanions such as LiFePO₄ are regarded as a suitable cathode material in Li-ion battery in terms of the point of view. Lithium vanadium phosphate (LVP) has high expectations as a cathode material because of its higher operating voltage (~3.6V) and theoretical capacity(197mAh/g) compared to LiFePO₄. While monoclinic LVP has a higher energy density, large hysteresis comes from asymmetric charge/discharge profile, and capacity fading are the main problems when three lithium ions are extracted by raising the voltage up to 4.8V. To overcome these limitations, fine studies about the mechanism have been carried out by a few groups. Although the mechanism studies were carried out by L. Nazar group about structural transition [1] and by Lee and Park about the possible pathway of lithium ion in the structure [2], representatively, the reasons for capacity fading on LVP should be studied more.

In this work, we investigated the structural changes of LVP during the first charge and discharge process of voltage window of 3.0V to 4.8V by using Galvanostatic Intermittent Titration Technique (GITT), synchrotron radiation-based X-ray Absorption Spectroscopy (XAS) and in situ X-ray diffraction (XRD). The irreversible structural transitions between charge and discharge process are one of the possible reason for the capacity fading. Also, we conducted comparison study on the electrochemical and structural properties between LVP and Li₃Ti_0.03V_1.97(PO₄)₃/graphene (LVP-Ti) to overcome limitations of LVP.

In situ XRD patterns show four consecutive two-phase transitions on LVP during the charge process. During the reinsertion of lithium ion, the patterns of LVP look like a combined process of single solid solution and two-phase transitions between full charged LVP (FC-LVP) and Li₂V₂(PO₄)₃ (Li₂ phase) without forming Li₁V₂(PO₄)₃. In GITT data, a two phase transition behavior which was previously reported as a solid solution behavior during the Li-ion insertion from full charged LVP to Li₂ phase in the discharge process is observed. From these data, it can be said that transforming behavior from FC-LVP to Li₂ phase is closer to two-phase reaction.

In LVP-Ti case, the charge capacity was 185.84 mAh/g and the discharge capacity was 171.20 mAh/g in the first cycle, which were higher than those of LVP, 178.84 mAh/g and 162.99 mAh/g. And in situ XRD patterns show four consecutive two-phase transitions as in LVP. During the phase transition in charge/discharge, LVP-Ti faces less lattice mismatch than LVP, resulting in better electrochemical performance, and this phenomenon is originated from smaller particle size and enduring strain. The XAS result shows that the XANES peaks of LVP-Ti change gradually which is in line with the XRD result as same as in LVP case. However, the larger peak area of the pre-edge for LVP-Ti suggesting that more electrons were released upon charging. This means that the higher capacities of LVP-Ti is contributed by further utilization of the V redox reaction.

[1] Yin, S-C., Hiltrud Grondey, Pierre Strobel, M. Anne, and Linda F. Nazar. "Electrochemical property: structure relationships in monoclinic Li3-yV2(PO4)3." Journal of the American Chemical Society 125, no. 34 (2003): 10402-10411.

[2] Lee, S., & Park, S. S. (2012). Atomistic simulation study of monoclinic Li3V2(PO4)3 as a cathode material for lithium ion battery: structure, defect chemistry, lithium ion transport pathway, and dynamics. The Journal of Physical Chemistry C, 116(48), 25190-25197.