Mechanism Study on Structural Transition of Lithium Vanadium Phosphate As Cathode for Lithium-Ion Batteries
Authors: Oh Woong , Park Hyunyoung , Lee Wontae , Jin Bong-Soo , Yoon Won-Sub 
Published: February 2020

To apply high energy material to generate devices like energy storage system (ESS) and electric vehicle (EV), a number of properties should be achieved such as energy density, cost, power and especially safety. There are numerous novel materials, but the lithium transition metal phosphates (LiFePO4 [1], LiCoPO4 [2], LiMnPO4 [3], Li3V2(PO4)3 [4]) are the most promising materials due to its structural and thermal safety. Among them, lithium vanadium phosphate brings out sharp relief as cathode material because of high energy density. In case of all three lithium ions are extracted, the theoretical capacity of LVP is 197mAh/g. The investigation of mechanism during the charge/discharge process and reducing irreversible capacity when it extract three lithium ions are the critical issues on the monoclinic Li3V2(PO4)3(LVP) material to solve
In this work, we investigated the structural changes of the LVP material during the first charge and discharge process of voltage window 3.0 to 4.8 V. One of the key problems related to the electrochemical performance of electrode materials for lithium ion batteries was phase transitions and local structure changes during lithium insertion or extraction. Irreversible structural change between charge and discharge process is the possible reason of the capacity fading. In situ XRD study is an effective tool to track the real time phase transitions of the electrode material. Galvanostatic Intermittent Titration Technique (GITT) was carried out not only for the distinguish phase transition but also for calculating diffusivity and polarization.
We followed the four successive two-phase transitions during extraction of lithium ion from Li3V2(PO4)3 to Li2.5V2(PO4)3, Li2V2(PO4)3, Li1V2(PO4)3 and V2(PO4)3. During the lithium ion intercalation process, the sample shows specific property that the phase change directly V2(PO4)3 to Li2V2(PO4)3 without forming Li1V2(PO4)3 as same as traditional mechanism. On the contrary to the report, the phase transition was observed as two phase reaction which was reported as solid solution [5]. In the GITT data, single drop of the diffusivity coefficient was observed where solid solution case should not shows. At the same part of XRD patterns shows the clear evidence that transforming V2(PO4)3 phase to Li2V2(PO4)3 phase of LVP cathode material is two-phase reaction. More details will be discussed in the meeting.
Figure 1. Contour plots of in situ XRD patterns and Galvanostatic Intermittent Titration Technique data of LVP material during discharge

Reference

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