Insights from Studying the Origins of Reversible and Irreversible Capacities on Silicon Electrodes
Authors: Yu Denis Yau Wai , Lee Pui-kit 
Published: February 2020

Coulombic efficiency (CE) of a lithium-ion battery (LIB) anode is a measure of the charge capacity over discharge capacity for a given cycle. A low CE is not a major issue for half cell tests, where there is plenty of lithium in the counter electrode. However, full cell battery performance can be significantly affected by CE, especially the first cycle efficiency. Most battery research attributes the first cycle irreversible capacity to processes such as solid electrolyte interphase (SEI) formation and electrolyte decomposition [1,2]. This, however, does not adequately give a full picture of what is going on in the electrode. A better understanding of the different contributions leading to the reversible and irreversible capacities is necessary in order to suggest strategies for improvements.
In this study, we use a systematic approach to separate the charge-discharge capacities of silicon electrodes into different contributions, in particularly SEI formation, lithium accommodation in carbon and binder, and lithiation and delithiation of the active material. This is possible because the three different contributions have different characteristics. Firstly, we note that SEI is typically formed during the first discharge above 0.1V before the lithiation of silicon [3]. So by comparing the CE of electrodes discharged to different capacities, SEI contribution can be identified. Secondly, the lithium accommodation in carbon and binder depends on their amount. So by comparing electrodes with different compositions, this contribution can also be distinguished. The remaining capacity is attributed to the lithiation and delithiation of silicon.
The experimental is as follows. Silicon electrodes using commercial silicon particles (Sigma Aldrich), conductive carbon (acetylene black (AB)) and carboxymethyl cellulose (CMC) binder were made with a ratio of 4:3:3, 6:2:2 and 8:1:1. We kept the same ratio between AB and CMC so that the two components can be considered as one. Multiple cells were made, and each of them is initially discharged to different cutoff capacity (250, 500, 1000, 1500, 2000, 2500, 3000 and 3500mAh g-1), and the corresponding charge capacity is recorded. Figure 1a shows a summary of the initial discharge-charge curves of different cells with a composition of 4:3:3. The respective first charge capacity vs. first discharge capacity is summarized in Figure 1b, which shows a linear behavior that is shifted from the origin. The slope of the line corresponds to the intrinsic first cycle efficiency of Si, which is about 90% and is relatively constant regardless of the degree of lithiation. This suggests that no matter how much lithium is initially inserted into the material, 10% of them remains in the lattice and cannot be removed. On the other hand, the x-intercept corresponds to the irreversible capacity of the electrode. About 16 mAh (g Si)-1 can be attributed to SEI formation and electrolyte decomposition in our electrodes.
It is interesting to note that despites the use of the same materials, the behavior of the 8:1:1 electrode deviates from those of the other two electrodes at high discharge capacity. Since the only difference is the electrode composition, the behavior is attributed to insufficient mechanical strength of the electrode, which significantly reduces the reversibility of lithium accommodation. So, our method also enables us to identify the onset of mechanical breakdown in the electrode.
We have developed a method here to distinguish the different contributions to the overall capacity of silicon. The obtained information is valuable for designing electrode to increase its stability. Further works on the analysis of subsequent cycles, effect of particle surface area and binder type are underway and will be presented at the meeting.
Reference:
1) Chen, X., Li, X., Mei, D., Feng, J., Hu, M. Y., Engelhard, M., Zheng, J., Xu, W., Xiao, J., Liu, Jun. & Zhang, J. G. ChemPubSoc. 7, 549-554 (2014).
2) Lin, Y. M., Klavetter, K. C., Abel, P. R., Davy, N. C., Sinder, J. L., Heller, A. & Mullins, B. ChemComm. 48, 7268-7270 (2012).
3) Wang,Z. C., Xu, J., Yao, W. H., Yao, Y. & Yang, W. Y. ECS Trans. 41, 29– 40 (2012).

Figure 1