Quantifying the Lithium Loss in the Graphite Anodes of Commercial Batteries

Abstract

Loss of Li-ions, which results in capacity loss, mainly occurs on the negative electrode in the form of Li plating or a surface film. Quantifying the loss of lithium in graphite anode is essential for studies such as waste battery recycling, lithium plating on negative electrodes, and interfacial film composition, which can guide the manufacturing and application chain of commercial lithium-ion batteries.Various methods for quantifying lithium losses have been proposed. You et al.[1] proposed the use of inductively coupled plasma (ICP) method to quantify lithium loss in graphite. This is a low-cost method that is easy to apply on a large scale, but it requires large sample amount and high instrumentation precision. Fang et al. [2] used water titration in conjunction with gas chromatography to quantify lithium loss from lithium metal batteries. This method is more subtle, but the produced trace amounts of gas are often too difficult to collect. In this study, we used water to quantify the lithium loss in graphite based on the method of Fang et al. We then combined pH and X-ray photoelectron spectroscopy (XPS) analysis to quantify the total lithium loss in graphite.To perform the quantify method, we washed the middle part of the negative electrodes of fresh and aged cells with DEC. We then cut out 2 cm² the negative electrodes and placed them in two volumetric flasks filled with 20 mL of deionized water each. After stirring, we sealed the flasks and placed them in a 60 °C oven for a week to ensure that the lithium compounds were fully reacted and dissolved with water. We added BaCl2 to remove CO3 2- and F- ions, which can hydrolyze to produce OH- and affect pH. We then performed a pH test to quantify the LiOH content in solution. As reported, LiOH in solution is mainly from Li and Li2O compounds[2], so we were able to quantify the Li in graphite in the form of Li/Li2O.To analyze the surface composition of graphite materials, we conducted an XPS test. We quantified the total lithium loss based on the percentage of interfacial lithium compounds occupied by Li/Li-O. It should be noted that this quantification method is based on the assumption that the surface material is homogeneous. There are three fitted peaks for the aged negative electrode, which belong to LiF, Li2CO3, and Li/Li–O compounds, corresponding to an amount of 73.66%, 19.79%, and 6.55% Li, respectively. Here, the Li/Li–O compounds correspond to the pH quantified part (1.10%). Therefore, the lithium loss in the aged negative electrode can be quantified as 16.82%.In conclusion, we proposed a water reaction method to quantify the loss of lithium in graphite, which was then combined with pH and XPS analysis to quantify the total lithium loss. This method offers a subtle yet effective way to quantify lithium loss. By quantifying the total lithium loss, we can gain insights into the interfacial film composition, lithium plating on negative electrodes, and waste battery recycling, which can guide the manufacturing and application of commercial lithium-ion batteries.Reference[1] H. You, B. Jiang, J. Zhu, X. Wang, G. Shi, G. Han, X. Wei, H. Dai, Journal of Power Sources 2023, 564, 232892.[2] C. Fang, J. Li, M. Zhang, Y. Zhang, F. Yang, J. Z. Lee, M. H. Lee, J. Alvarado, M. A. Schroeder, Y. Yang, B. Lu, N. Williams, M. Ceja, L. Yang, M. Cai, J. Gu, K. Xu, X. Wang, Y. S. Meng, Nature 2019, 572, 511.[3] J. Guo, S. Jin, X. Sui, X. Huang, Y. Xu, Y. Li, D. W. Peter Kjær Kristensen, D.-I. Stroe, Journal of Materials Chemistry A 2023.