Rethinking the Role of Formerly Sub-Sufficient Industrial/Synthesized SEI Additive Compounds - a New Perspective
Authors: Ghaur Adjmal , Pfeiffer Felix , Diddens Diddo , Peschel Christoph , Dienwiebel Iris , Du Leilei , Profanter Laurin , Weiling Matthias , Winter Martin , Placke Tobias , Nowak Sascha , Baghernejad Masoud 
Published: September 2023

In order to improve the performance of lithium-ion batteries (LIBs), novel electrolytes are of primary importance. Recently, fluorinated cyclic phosphazene derivatives in combination with fluoroethylene carbonate (FEC) are mentioned in the literature as a promising electrolyte additive combination, which can decompose to form a dense, uniform, and thin protective layer on the surface of the anode and cathode electrode.[1,2] Additionally, suppressing further electrolyte decomposition and electrode corrosion, thus protecting the structural destruction of the electrodes, are mentioned within this electrolyte composition.[1–3] Furthermore, galvanostatic charge and discharge experiments with different cell composition materials demonstrate that fluorinated cyclic phosphazene compounds as additional additive material tend to improve cycling stability.[1,3,4] Although the electrochemical aspects of cyclic fluorinated phosphazene compounds combined with FEC are briefly introduced, it is still not fully clear how these two compound classes interact constructively during operation mode. Thus, the positive synergistic effect of FEC/Hexafluorocyclotriphosphazene (HFPN)-derivatives on the electrochemical performance during cell operation is not enlightened. The focus of this study is to investigate the complementary effect of FEC and ethoxy(pentafluoro)cyclotriphosphazene (EtPFPN) as additive compounds in an aprotic organic electrolyte in LiNi0.5Co0.2Mn0.3O (NCM523) SiOx/C full cells. Furthermore, the formation mechanism of lithium ethyl methyl carbonate (LEMC)-EtPFPN interfacial products and the reaction mechanism of lithium alkoxide with EtPFPN are proposed and supported with DFT measurements. Additionally, a new effect of FEC regarding the SEI formation will be introduced. The EtPFPN decomposition compounds in the electrolyte after the SEI formation have been investigated via gas chromatography-mass spectrometry (GC-MS) and gas chromatography-high resolution mass spectrometry (GC-HRMS). The electrode electrolyte interface investigation of the SEI has been performed via
in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and scanning electron microscopy (SEM). Constant current cycling is conducted, and in-situ Raman measurements characterize the deposition of electrolyte components and LEMC-EtPFPN traces on the SiOx/C anode material during the SEI formation. Finally, the interplay between EC, EMC, Li-alkoxide, LEMC, FEC, and EtPFPN has been visualized schematically via a reaction mechanism postulated based on analytical data of the electrolyte.
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2023, 2203503.
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