Excellent Cycle Performance of Pouch Cells at a High Rate Investigated By a Combination of Synchrotron Based X-Ray Diffraction and Absorption Spectroscopy

Lithium-ion batteries are widely developed for vehicle applications and energy storage systems owing to their high power density, long cycle life, and lack of memory effect. The cycling stability of a Li-ion battery plays an important role in its practical application, especially for use in electric vehicles, which is generally affected by the types of electrode materials [1], binder properties [2], and electrolyte compositions [3].

Li-ion cells based on LiFePO₄cathodes have been considered as one of the most promising candidates for large-scale application due to its low cost, low toxicity, good electrochemical and excellent thermal stability. However, the tendency for capacity degradation behaviour of LFP cells is still seen as a problem for practical application.

The capacity fading mechanism of a laminated pouch type 1.0 Ah cell consisting of a graphite anode and an olivine LiFePO₄ cathode during a long-term cycling test was studied in this report. The LiFePO₄cathodes have a mole ratio of iron to phosphorus in the range of 0.93 to 0.99. As shown in Fig. 1, the LFP cell with a Fe/P ratio of 0.99 has the best cyclic performance at the charge/discharge rate of 2 C/5 C. The cycle life curves of the pouch cell show the number of cycles increases to 2269 cycles with the capacity retention (C.R.) of 80% as the Fe/P ratio increases from 0.93 to 0.99.

To investigate the aging mechanism of the LiFePO₄ cathode and graphite anode, changes in the crystalline structure through the charge/discharge reactions were analysed by the synchrotron based X-ray diffraction (XRD) experiments. After long-term cycling tests, the appearance of LiOH and LiOH．H₂O phases in the cycled anodes reveals that some Li ion released from the LiFePO₄ phase during the charge process did not come back to the FePO₄ phase, as shown in Fig. 2. Due to fewer lithium ions, some of the cathode material lost electrochemical activity and this brought down the reversible capacity significantly. In addition, the anode with the Fe/P ratio of 0.95 after 506 cycles has relatively stronger peaks than those of the other samples in Fig. 2(b). It indicates that the decrease in the Fe concentration of stoichiometric LiFePO₄resulted in a shortage of lithium ion for the cathode material, and consequently the capacity fading rate accelerated.

In this study, a combination of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and electrochemical testing are used to understand the physical and electrochemical characterizations of the graphite anode and the LiFePO₄cathode with the different Fe/P mole ratios during long-term cycling tests.

References

[1] X. Han, M. Ouyang, L. Lu, J. Li, Y. Zheng, Z. Li, Journal of Power Sources,251 (2014) 38-54.

[2] Z. Zhang, T. Zeng, C. Qu, H. Lu, M. Jia, Y. Lai, J. Li, Electrochimica Acta,80 (2012) 440–444.

[3] S.S. Zhang, Journal of Power Sources, 162 (2006) 1379-1394.