Abstract

Extending the cycle-life of state-of-the-art lithium-ion batteries (LiBs) is, amongst other aspects, predicated on optimizing the properties of the solid electrolyte interphase (SEI), such as its stability and resistance. [1] It is formed at the negative electrode during the first cycles by electrolyte decomposition and acts as a passivating layer between the electrode surface and the electrolyte. [2,3] Since the SEI formation is accompanied by an irreversible loss of cyclable lithium, its stability during cycling is crucial for improving the charge/discharge efficiency of LiBs. Furthermore, the arising resistance associated with the SEI formation, predominantly depending on the SEI thickness, its chemical composition, and its lithium-ion conductivity, all impact the rate performance of the anode. [4] Two of the most crucial factors impacting SEI formation are temperature and electrolyte composition ( i.e. , using electrolyte additives). [5] Additives are added to the electrolyte to inhibit the predominant reduction of other electrolyte constituents and thereby form an SEI with improved stability during cycling. [1,6,7] One of the most prominent additives for LiBs is fluoroethylene carbonate (FEC), which is preferentially reduced compared to common carbonate solvents ( e.g ., ethylene carbonate, ethyl methyl carbonate) due to its higher reduction potential. [7] In this study, we investigated the influence of the formation temperature between 10 and 60 °C, as well as the concentration of FEC (2 and 20 % wt in EC:EMC 3:7 wt/wt with 1 M LiPF 6 , Gotion , USA) on the characteristics of a synthetic graphite (SMG-A5, Resonac, Japan) anode, both in SMG/Li half-cells and in Ni-rich NCM/SMG full cells. We quantified the first-cycle irreversible capacity loss (ICL) in SMG/Li half cells with a lithium reference electrode after two 0.1 C formation cycles and compared the FEC containing electrolytes to the baseline- (LP-57) and an LP-572 (1 M LiPF 6 in EC:EMC 3:7 wt/wt + 2 % wt vinylene carbonate (VC), Gotion , USA) electrolyte. Employing electrochemical impedance spectroscopy in an SMG/Li half-cell setup with a µ-reference electrode and a free-standing graphite electrode, [8,9] we could further quantify the intercalation resistance ( R Int , representing the sum of the charge-transfer and the SEI resistance) after an identical formation protocol at 40 % SOC. Both quantities, the ICL and R Int , increase with increasing formation temperature and higher FEC content. The impact of an increasing R Int on the rate performance was tested in Ni-rich NCM/SMG full cells with a lithium reference electrode, revealing a decreased rate capability and higher susceptibility for lithium plating for cells that were formed at higher temperatures and contained 20 % wt FEC. Additionally, the cycling stability of Ni-rich NCM/SMG full cells at 25 °C was assessed in coin cells. Despite an increased ICL and R Int after formation, cells that were formed at elevated temperatures and contained 20 % wt FEC showed enhanced cycling stability compared to those that were formed at a lower temperature and contained less FEC. Finally, on-line electrochemical mass spectrometry (OEMS) was employed to quantify the evolved gases during the formation of a Ni-rich NCM/SMG full cell containing either of the FEC-containing electrolytes at different temperatures. In accordance with an increased ICL and R Int , a higher formation temperature, as well as a higher FEC content, gave rise to a more pronounced gas evolution during formation. References: [1] D. Y. Wang, N. N. Sinha, J. C. Burns, C. P. Aiken, R. Petibon, J. R. Dahn, J. Electrochem. Soc . 2014 , 161 , A467–A472. [2] E. Peled, J. Electrochem. Soc. 1979 , 126 , 2047–2051. [3] E. Peled, S. Menkin, J. Electrochem. Soc. 2017 , 164 , 1703–1719. [4] S. Solchenbach, X. Huang, D. Pritzl, J. Landesfeind, H. A. Gasteiger, J. Electrochem. Soc. 2021 , 168 , 110503. [5] J. Asenbauer, T. Eisenmann, M. Kuenzel, A. Kazzazi, Z. Chen, D. Bresser, Sustain. Energy Fuels 2020 , 4 , 5387–5416. [6] T. Taskovic, L. Thompson, A. Eldesoky, M. Lumsden, J. R. Dahn, J. Electrochem. Soc. 2021 , 168 , 010514. [7] K. U. Schwenke, S. Solchenbach, J. Demeaux, B. L. Lucht, H. A. Gasteiger, J. Electrochem. Soc. 2019 , 166 , A2035–A2047. [8] S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, H. A. Gasteiger, J. Electrochem. Soc. 2016 , 163 , A2265–A2272. [9] R. Morasch, B. Suthar, H. A. Gasteiger, J. Electrochem. Soc. 2020 , 167 , 100540. Acknowledgements: This work is financially supported by BMW AG. The authors thank BMW AG for their financial support.