Understanding the Redox Reaction Mechanism of Li2CoPO4f Cathode Material for Achieving Two Lithium Intercalation

Lithium ion batteries have attracted considerable attention as important energy storage and conversion systems for applications including electric vehicles (EVs) and hybrid electric vehicles (HEVs), owing to their high energy and power densities, as well as a long cycle life. Recently, lithium transition metal phosphates, such as LiFePO₄, LiMnPO₄, and Li₃V₂(PO₄)₃, have been considered as potential cathode materials for lithium ion batteries. It is well known that the phosphates display much better electrochemical and thermal stability compared to conventional lithium metal oxides [1]. However, recently, fluorophosphates are being developed as advanced cathode materials display higher operating potential, corresponding to the redox reaction of transition metal, when compared to the respective metal oxides and phosphates [2]. In addition, the highly electronegative fluoride ion helps to improve the cycle stability, which fluorophosphates make attractive materials for high energy batteries.

    Our current work is focused on maximizing the deliverable discharge capacity of Li₂CoPO₄F cathode material and to reach theoretical capacity by achieving more than one electron intercalation. The redox couple Co^2+/3+ and Co^3+/4+ was closely followed during galvanostatic charge-discharge test by x-ray photoemission spectroscopy and x-ray diffraction (XRD) analysis. It was found that the incomplete reduction of cobalt ions during discharge initiated the irreversible capacity loss rather than the electrolyte decomposition being solely responsible. A novel approach was carried out to activate the transition metal ions which resulted in a discharge capacity as high as 230 mAh g^-1 at a current rate of 20 mA g^-1 for Li/Li₂CoPO₄F cell. A long plateau at 4.8 V has been observed in the charge cycle with an additional voltage plateau at ~5.0 V, which can be attributed to the two lithium intercalation reaction and has been validated using cyclic voltammetry studies. The detrimental cycle stability observed [3] in Li₂CoPO₄F has been replaced with a stable cycle performance of > 90 % capacity retention as shown in figure 1. A quasi single phase reaction was confirmed using gravimetric intermittent titration technique in addition to the results from ex-situXRD studies. A discussion based on the obtained results will be presented in detail.

References:

[1] S.K. Martha, J. Grinblat, O. Haik, E. Zinigrad, T. Drezen, J.H. Miners, I. Exnar, A. Kay, B. Markovsky, D. Aurbach, Angew. Chem. Int. Ed.48 (2009) 8559–8563.

[2] A. Kraytsberg, Y. E. Eli, Adv. Energy Mater.2 (2012) 922–939.

[3] X. Wu, Z. Gong, S. Tan, Y. Yang, J. Power Sources 220 (2012) 122–129.