Influences and Mechanisms of Water on a Solid Electrolyte Interphase Film for Lithium-Ion Batteries

Trace water, one of the most common impurities, has a fatal influence on the performance of lithium-ion batteries (LIBs). In this work, the influences of water content on the solvation structure of lithium ions and the component of solid electrolyte interphase (SEI) film in the lithium bis(oxalato)borate (LiBOB)–ethylene carbonate (EC)/diethyl carbonate electrolyte (DEC) system are studied. Results of attenuated total reflection–Fourier transform infrared analysis and density functional theory (DFT) calculation show that a great number of solvation structures of Li+(EC)2(H2O)2 and Li+(EC)3(H2O) are formed in the electrolyte system with additional H2O. Besides, the free H2O generated by the desolvation effect will react with the desolvated Li+ ions and the heterogeneous organic components in the outer layer of a SEI film. These parasitic reactions will lead to an overgrown and excessive inorganic component-containing SEI film, which harms the Li+ ion conductive characteristics of the SEI film and the electrochemical performance of LIBs. In particular, insoluble LiB(C2O4)(OH)2 and B(C2O4)OH products are generated when the content of the additional H2O reaches 10,000 ppm because of the hydrolytic reaction. Moreover, it is because of the generation of these poor ion-conductive boron-containing substances that the impedance of the cell increases sharply. These studies can provide the theoretical basis for battery failure analysis.