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Pre-lithiation of Silicon Anodes by Thermal Evaporation of Lithium for Boosting Energy Density of Lithium Ion Cells

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Lithium ion batteries (LIBs) do not only dominate the small format battery market for portable electronic devices, but have also been successfully implemented as the technology of choice for electric vehicles. However, for successful consumer acceptance and broad market penetration of electric vehicles, further improvements of LIBs in terms of energy density and cost along are required. The practically usable energy density of LIB cells is reduced by parasitic side reactions including electrolyte decomposition and formation of the “solid electrolyte interphase” (SEI) at the surface of the anode, as this process is related to the consumption of active lithium. Especially high-capacity silicon-based anodes suffer from ongoing lithium loss and rapid capacity fading.[1]

Pre-lithiation is considered as a highly appealing technique to compensate for active lithium losses and, therefore, to increase the practical energy density. A critical parameter for a pre-lithiation strategy is to establish a technique for achieving lithiation of the active storage material at the most uniform lateral and in-depth distributions possible. Despite extensive exploration of various pre-lithiation techniques, controlling the lithium amount precisely while keeping an even lithium distribution remains challenging.[2]

Here, we report the thermal evaporation of lithium metal as a novel pre-lithiation technique for silicon anodes that allows both, i.e., a precise control of the degree of pre-lithiation and a homogeneous deposition at the electrode surface. Our results show that the homogeneous distribution is critical and should be taken into account for successful application. Furthermore, a highly uniform Li distribution is able to compensate for the typical Li loss occurring during cycling. We discussed important parameters such as Li nucleation and pre-lithiation conditions (dry-state and wet-state, i.e., without and with liquid electrolyte). In the dry-state, the absence of crystalline phases indicated the relatively slow reaction (=lithiation) between Si and the vapor-deposited Li metal. In wet-state, the conditions are much more complex due to pre-SEI formation by electrolyte decomposition, Li dissolution at low voltage, de-lithiation of LixSiy at higher voltage, and the competition of Li consumption. These phenomena are all taking place simultaneously and subsequently. Further, we comprehensively analyzed the impact of the pre-volume expansion and moderate mechanical crack formation of the Si electrode on the electrochemical performance with respect to resting time. Finally, this work opens up a new approach and a further understanding of pre-lithiation studies for high-capacity Si anodes in LIB cells. The pre-SEI formation described here is therefore a promising strategy to achieve an even enhanced lifetime of high-energy LIB cells.

[1] a) T. Placke, G. G. Eshetu, M. Winter, E. Figgemeier, in Lithium-ion Batteries Enabled by Silicon Anodes (Eds: C. Ban, K. Xu), The Institution of Engineering and Technology (IET), United Kingdom 2021, Ch. 11, p. 349; b) J. Guo, D. Dong, J. Wang, D. Liu, X. Yu, Y. Zheng, Z. Wen, W. Lei, Y. Deng, J. Wang, G. Hong, H. Shao, Adv. Funct. Mater. 2021, 31, 2102546.
[2] a) F. Holtstiege, P. Bärmann, R. Nölle, M. Winter, T. Placke, Batteries 2018, 4, 4; b) G. M. Overhoff, R. Nölle, V. Siozios, M. Winter, T. Placke, Batteries Supercaps 2021, 4, 1163.

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