Abstract
The applications of lithium-ion batteries (LIBs) have rapidly embedded into almost every aspect of our life and society. In the LIB industry, a well-controlled and lengthy process is proceeded before the LIBs are brought into markets. The aim of this process is to form a "good" solid electrolyte interphase (SEI) on the graphite anode in a LIB to prevent continuous electrolyte decomposition and graphite exfoliation, which greatly influence the performance and cycle life of a LIB. The SEI formation process usually takes one or two weeks, is the major bottleneck and the most expensive component for mass production of LIBs. Efforts have been devoted to accelerating the SEI formation process by increasing charging rates or skipping the high state-of-charge region. However, the shortened SEI formation time usually comes with a large decrease in capacity retention of a LIB.[1-3] Improving our understanding on the structure and property evolution of SEI during the SEI formation process is urgently required for the development of faster formation protocols without sacrificing the high quality SEIs.[4-7]
Aiming at better understanding of the initial stage of SEI formation process in LIB production, in-situ electrochemical atomic force microscopy (EC-AFM) has been employed to monitor the morphological evolution of the electrode materials in nano-scale levels. While different from most of the previous works, this work applied Galvanic constant current (CC) control to closely mimic the industrial SEI formation processes of different formation rates. For comparison, linear potential sweep (LS) controlled SEI formation of various potential sweeping rates, which is commonly applied in fundamental studies of SEI formation, was also performed in parallel. For the first time, it is revealed that the onset potentials for co-intercalation and electrochemical reduction of solvated lithium-ion complexes, the two competitive and/or sequential processes during SEI formation, vary as a function of potential sweeping rates. The onset potential for co-intercalation shifts positively as the potential changing rate increases, which is in high contrast to the onset potential of their electrochemical reduction. It has been well established that the onset potential of their electrochemical reduction is largely determined by the lowest unoccupied molecular orbital (LUMO) level of the solvent molecules in the solvated lithium ion complexes,[6] so that it may remain largely constant or shift negatively, due to electrode polarization.[2] As a result, the onset potential for co-intercalation becomes more positive than that of the electrochemical reduction as the potential sweeping rate increases. Our experimental results demonstrate that this relationship applies to both CC and LS-controlled SEI formation processes. However, the difference between these two onset potentials is much greater in CC than that of LS controlled processes, due to the associated faster potential changing rates at the beginning of CC controlled lithiation. The difference becomes even more prominent when high charging current is applied to shorten the SEI formation time and lower LIB production cost. This understanding combined with the clear visible evidence of different structural evolution under CC and LS controls should provide more practical guidance to develop protocols for faster formation of SEI with higher quality, which is intensively pursued in industry.
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[2] Bhattacharya, S.; Alpas, A. T., Micromechanisms of solid electrolyte interphase formation on electrochemically cycled graphite electrodes in lithium-ion cells. Carbon 2012, 50 (15), 5359-5371.
[3] An, S. J.; Li, J. L.; Du, Z. J.; Daniel, C.; Wood, D. L., Fast formation cycling for lithium ion batteries. Journal of Power Sources 2017, 342, 846-852.
[4] Wang, L. N.; Menakath, A.; Han, F. D.; Wang, Y.; Zavalij, P. Y.; Gaskell, K. J.; Borodino, O.; Iuga, D.; Brown, S. P.; Wang, C. S.; Xu, K.; Eichhorn, B. W., Identifying the components of the solid-electrolyte interphase in Li-ion batteries. Nature Chemistry 2019, 11 (9), 789-796.
[5] Cai, W. L.; Yao, Y. X.; Zhu, G. L.; Yan, C.; Jiang, L. L.; He, C. X.; Huang, J. Q.; Zhang, Q., A review on energy chemistry of fast-charging anodes. Chemical Society Reviews 2020, 49 (12), 3806-3833.
[6] Xu, K., Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chemical Reviews 2004, 104 (10), 4303-4417.
[7] Xu, K., Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chemical Reviews 2014, 114 (23), 11503-11618.
Figure 1