Abstract
In laboratory charge-discharge tests of lithium-ion battery electrodes is often used half-cell with a lithium metal anode, which can supply enough lithium ions. On the other hand, practical lithium-ion batteries do not contain lithium metal and use full-cell configuration with almost similar capacities between cathode and anode. Limited number of lithium ions supplied by the cathode contributes as carrier ions, which determines the actual charge-discharge capacity. Since the degradation behavior observed in full-cell is different from in half-cell [1, 2], it is important to understand the problems that occur during charge-discharge in full-cell to maximize battery performance. In this study, we investigated the irreversibility in full-cell with lithium iron phosphate (LiFePO4) cathode and lithium titanate (Li4Ti5O12) anode as an unnatural case, where both active materials show two-phase reaction in almost all composition. The potential changes of cathode and anode were tracked using the three-electrode full-cell with a lithium metal reference electrode. In addition to the electrochemical measurements, the charge-discharge state was analyzed from the X-ray absorption spectroscopy (XAS), which provides the valence change information.
Half-cell charge-discharge tests of LiFePO4 and Li4Ti5O12 at 0.1C result in reversible charge-discharge profile. However, the distinct irreversible capacity is observed in the LiFePO4 / Li4Ti5O12 full-cell configuration. The initial charge and discharge capacity are 162 nd 126 mAh g-1, respectively and this irreversibility is continuously observed in the subsequent cycle. The three-electrode test indicates that the potential of Li4Ti5O12 anode sharply increases despite in the middle of the discharge. Fe K-edge XAS shows similar edge energy for the half-charged and half-discharged LiFePO4 cathode. However, Ti K-edge XAS of half-charged Li4Ti5O12 anode shows the different edge energy from that of the half-discharged. The full-discharged LiFePO4 cathode does not reach the lithiated state even after discharging, which agrees with the results from the three-electrode test. At the end of the charge, little edge-energy shift in the Li4Ti5O12 anode is observed, indicating that the insertion reaction of lithium ions does not progress. Therefore, the irreversible capacity is caused by a side reaction at the Li4Ti5O12 anode in end of the charge.
[1] Bjorklund, D. Brandell, M. Hahlin, K. Edstrom, R. Younesi, J. Electrochem. Soc., 164 (2017) A3054-A3059.
[2] H. Kim, N.P.W. Pieczonka, Z.C. Li, Y. Wu, S. Harris, B.R. Powell, Electrochim. Acta. , 90 (2013) 556-562.