Investigating interphases modification via ethylene carbonate derivatives in high voltage NMC622||graphite lithium-ion batteries by operando SHINERS

Solid Electrolyte Interphase (SEI) has a crucial role in lithium-ion battery performance. In batteries with NMC positive electrode with excess Nickel as LiNi0.6Mn0.2Co 0.2O 2 (NMC622), especially when operated at high voltage (> 4.3 V), the dissolution of transition metals and cross-talk to the negative electrode can lead to SEI damage, rendering the formation of an effective SEI even more important to extend battery lifetime. The incorporation of additive molecules in the electrolyte can favor the formation of stable and effective interphases. In this work the electrochemical performance of four ethylene carbonate-derivative additives, namely fluoro-, chloro-, vinyl-, ethylene carbonate and vinylene carbonate (FEC, ClEC, VEC and VC, respectively) is studied in NMC622||graphite pouch cells operated to a cut-off voltage of 4.5 V, in tandem with morphology analysis of the aged electrode and gas chromatography-mass spectrometry (GC-MS) study of the electrolyte. Improved cycle life and reduced impedance were found for VEC-containing cells, even though products of electrolyte degradation were identified by gas chromatography-mass spectrometry. Additionally, for ClEC, which shows the best capacity performance, but shorter cycle life and reduced Coulombic efficiency, multiple degradation products are shown by GC-MS. Moreover, SEI understanding is still hindered by its reactivity towards the air, its nanometric thickness, and the possible mechanical damage following battery disassembly. Therefore, to gain real-time information about the SEI, operando Shell-isolated nanoparticles enhanced Raman spectroscopy (SHINERS) was performed.