Influence of Water Contamination on the SEI Formation in Li-Ion Cells: An Operando EQCM-D Study

The interphase formation on carbon (C) anodes in LiPF6/EC + DEC Li-ion battery electrolyte is analyzed by combining operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) with in situ online electrochemical mass spectrometry (OEMS). EQCM-D enables unique insights into the anode solid electrolyte interphase (SEI) mass/thickness, its viscoelastic properties, and changes of electrolyte viscosity during the initial formation cycles. The interphase in the pure electrolyte is relatively soft (G′SEI ≈ 0.2 MPa, ηSEI ≈ 10 mPa s) and changes its viscoelastic properties dynamically as a function of the electrode potential. With increasing electrolyte water content, the SEI becomes thicker and much more rigid. Doubly labeled D218O is added to the electrolyte in order to precisely track the reaction pathway of water at the anode by OEMS. In the first cycle between 2.6 and 1.7 V versus Li+/Li, water is reduced, and hydroxide ions initiate an autocatalytic hydrolysis of EC. With large amounts of water initially present in the electrolyte, most of the formed CO2 gas is scavenged by reactions with hydroxide and alkoxide ions, forming a thick, rigid, and Li2CO3-rich early interphase on the C anode. This layer alleviates the following electrolyte decomposition processes and slows the reduction of EC < 1 V versus Li+/Li.