Elucidating the Shuttle Mechanism of Vinyl Ethylene Carbonate By in-Situ Advanced Raman Spectroscopy
Authors: Pfeiffer Felix , Griggio Angela , Weiling Matthias , Wang Jian-Fen , Peschel Christoph , Baghernejad Masoud 
Published: December 2024

The protective interphases formed on the anode side (solid-electrolyte interphase; SEI) and the cathode side (cathode electrolyte interphase; CEI) are often considered as the most crucial component of lithium ion batteries (LIBs), due to their high influence on the LIBs performance and lifetime. Effective interphases not only account for increased performance of the LIBs, but also enable the implementation of next generation high energy cells with electrode materials, like Ni-rich LiNixMnyCozO2 (NMC) cathodes. As the interphases mostly consist of electrolyte degradation products, the implementation of film-forming electrolyte additives, which decompose prior to the electrolyte components, is seen as a promising way to facilitate the formation of interphases. Even though the use of film-forming additives, like fluoroethylene carbonate (FEC) or vinylene carbonate (VC) is already a standard practice, the composition and the mechanisms leading to the formation of the interphases are not completely understood yet. Nevertheless, a deep understanding of these processes is necessary to enable the design of tailored interphases and allow the choice of suitable electrolyte additives. Thus, urging the development of techniques capable to obtain these information under real working conditions.

Conventional Raman spectroscopy is a powerful tool for the in-situ investigation of the composition and structural properties of different electrolytes and electrode materials, due to its robustness, simplicity, and straight-forward combination with electrochemistry. However, conventional Raman spectroscopy is not suitable for the investigation of nanometric thin interphases, because of intrinsically low signal intensities and trace quantities of the interphase composition products. Advanced Raman spectroscopy techniques like surface enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) rely on nanostructured substrates of plasmonic active metals (e.g. Au or Ag), providing remarkable signal enhancement. In addition, these techniques are very surface sensitive, due to the nature of the enhancement, and therefore, ideal for the in-situ investigation of interphases in LIBs. [1]

In the presented study, different well (FEC and VC) and lesser (chloroethylene carbonate and vinyl ethylene carbonate; VEC) known carbonate-based film-forming electrolyte additives were investigated toward their influence on the cycling performance of high-voltage NMC622||graphite multilayer pouch cells, followed by an in-situ SHINERS investigation of the additive induced SEIs. It was observed that the addition of an optimized concentration of VEC notably improved the performance and cycle-life of the investigated cells, outperforming the other investigated additive-containing electrolyte formulations by a great margin. The subsequent SHINERS investigation of the respective SEIs was capable to capture specific decomposition products for every investigated electrolyte additive, nevertheless the SHINERS measurements did not reveal an explanation for the observed electrochemical performances. Additionally performed GC-MS investigation of the aged electrolyte after 100 charge/discharge cycles captured products of the transesterification reaction of the electrolyte solvents for the VEC-containing electrolyte. This finding surprisingly indicates the reductive decomposition of the electrolyte and therefore, the formation of an ineffective SEI in the presence of VEC.
As VEC-induced SEI formation was not able to explain the observed electrochemical performance, it was suggested that VEC might form a CEI instead. In fact, in-situ SERS and SHINERS investigations of the cathode surface in the presence of VEC captured the formation of an interphase on the electrodes surface. The Raman investigations indicate that carboxylates are making up the majority of the formed CEI, a species typically formed by the reduction of carbonates. This finding suggests the presence of a shuttle mechanism in which VEC is reduced on the anode side. Instead of forming an SEI, the VEC-decomposition products migrate towards the cathode and form a CEI. The formed CEI could suppress electrolyte oxidation and the dissolution of transition metals into the electrolyte, accounting for the improved performance.To investigate the shuttle mechanism, the reductive decomposition product of VEC was simulated using lithium-naphthalenide, resembling the reactivity of lithiated graphite. An approach established by Lucht et al.[2] Comparing the SERS spectrum of the VEC-induced CEI and the VEC decomposition product revealed matching bands, supporting the claim of the VEC shuttle-mechanism.
References
[1] Pfeiffer, F.; Diddens, D.; Weiling, M.; Frankenstein, L.; Kühn, S.; Cekic‐Laskovic, I.; Baghernejad, M. Quadrupled Cycle Life of High‐Voltage Nickel‐Rich Cathodes: Understanding the Effective Thiophene‐Boronic Acid‐Based CEI via Operando SHINERS. Advanced Energy Materials
2023, 13, 2300827. DOI: 10.1002/aenm.202300827.
[2] Nie, M.; Chalasani, D.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L. Lithium Ion Battery Graphite Solid Electrolyte Interphase Revealed by Microscopy and Spectroscopy. J. Phys. Chem. C
2013, 117, 1257–1267. DOI: 10.1021/jp3118055.