Maintaining a stable solid electrolyte interphase (SEI) is crucial for Li-ion battery safety, especially with high-capacity anode containing silicon. Therefore, our study explored long-term cycling of Si electrodes with artificial alucone-based SEI, deposited by molecular layer deposition (MLD) in combination with a fluoroethylene carbonate (FEC) electrolyte additive. MLD of flexible Li-ion permeable artificial SEI coatings onto electrode resulted in improved capacity, enhanced Si electrode cycle life and capacity retention.
Battery performance is fundamentally limited by the electrode active materials of both positive and negative electrodes. To increase the energy density of battery cells, materials with higher specific capacities than graphite must be used without sacrificing too much of the potential window. Silicon (Si) is attractive due to its high theoretical specific capacity (3579 mAh g–1) and relatively low working potential. However, Si-based electrodes suffer from large volume expansion during alloying with Li, causing SEI layer cracking and continuous electrolyte consumption, leading to rapid capacity fading.
Improving the cycling stability of Si anodes often involves using electrolyte additives, like FEC, with higher reduction potential than electrolyte solvents. Another approach is prefabricating an artificial SEI (a-SEI) as an additional thin surface layer that shields electrolyte from electrode components. This can be done at the active material powder level or the level of an electrode. Molecular layer deposition produces uniform, conformal thin films on complex surfaces, with alucones (aluminum-based organic–inorganic hybrids) being notable for their high elasticity. Research on MLD-treated electrodes has focused on one type of electrolyte, often attributing performance improvements solely to coatings. It is necessary to examine the properties of these SEIs separately to understand their contributions to cell performance and any potential synergistic effects.
In our study, we clarified the influence of alucone coatings and FEC additives on the cycle life of Si anodes, both individually and in combination. A range of different aliphatic alcohols were used as precursors for alucone-derived MLD coatings, which were then tested in half-cells with carbonate electrolyte without additives to assess the effect of alcohol chain length on cell performance and suitability for the a-SEI role. Then, the tests were conducted in the presence of FEC to discriminate the individual contributions of the coating and the benefits of the electrolyte additive. To better understand the working mechanism of this synergy and the final beneficial a-SEI composition, an X-ray photoelectron spectroscopy (XPS) surface study of the model electrodes, before and after being exposed to FEC containing electrolyte for both coated and uncoated variants, was performed.
It was found that the alucone coating layer not only serves as confining barrier for Si-particles, but also scavenges HF present in LiPF6-containing electrolytes. This means that alucone coatings can reduce HF concentration in the cells; consequently, this reduction might affect cathode surface stability in full cells, and reduce metal leaching. The effects of the alucone-based a-SEI in full cells need to be still quantified to understand true significance of this finding.
Combining alucone coatings with FEC electrolyte additives reveals a new way for the design strategies of Si-based anodes. This method can extend to other electrode materials and areas with surface reactivity concerns, significantly contributing to the enhanced energy density of future battery generations.
Contact
Dr. Sigita Trabesinger, Dr. Łukasz Kondracki
Head of Battery Electrodes and Cells Group
Paul Scherrer Institute PSI, 5232 Villigen PSI, Switzerland
Tel.:+41 56 310 5775
E-mail: sigita.trabesinger@psi.ch; Lukasz.kondracki@psi.ch
Original Publication
Title: Synergy of Artificial SEI and Electrolyte Additive for Improved Performance of Silicon Electrodes in Li-Ion Batteries
Authors: Łukasz Kondracki, Janne-Petteri Niemelä, Dominika Baster, Mario El Kazzi, Ivo Utke, Sigita Trabesinger
ACS Applied Energy Materials, (2024) 9336.
DOI: https://doi.org/10.1021/acsaem.4c01862
Acknowledgement to Funding Agency
ŁK would like to thank Dr Fabian Jeschull for fruitful discussions on the electrode preparation. JPN acknowledges the EMPAPOSTDOCS-II programme; it has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement (754364).