Operando EQCM-D with Simultaneous in Situ EIS: New Insights into Interphase Formation in Li Ion Batteries

An operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) with simultaneous in situ electrochemical impedance spectroscopy (EIS) has been developed and applied to study the solid electrolyte interphase (SEI) formation on copper current collectors in Li ion batteries. The findings are backed by EIS simulations and complementary analytical techniques, such as online electrochemical mass spectrometry (OEMS) and X-ray photoelectron spectroscopy (XPS). The evolution of mass and the mechanical properties of the SEI are directly correlated to the electrode impedance. Electrolyte reduction at the anode carbon active material initiates dissolution, diffusion, and deposition of reaction side products throughout the cell and increases electrolyte viscosity and the ohmic cell resistance as a result. On Cu the reduction of CuO _x and HF occurs at >1.5 V and forms an initial LiF-rich interphase while electrolyte solvent reduction at <0.8 V vs Li⁺/Li adds a second, less rigid layer on top. Both the shear storage modulus and viscosity of the SEI generally increase upon cycling but-along with the SEI Li⁺ diffusion coefficient-also respond reversibly to electrode potential, likely as a result of Li⁺/EC interfacial concentration changes. Combined EIS-EQCM-D provides unique prospects for further studies of the highly dynamic structure-function relationships of electrode interphases in Li ion batteries.