Insight into the Contribution of the Electrolyte Additive LiBF 4 in High-Voltage LiCoO 2 ||SiO/C Pouch Cells.

High-voltage pouch cells using an LiCoO 2 cathode and SiO/C anode are regarded as promising energy storage devices due to their high energy densities. However, their failure is associated with the unstable, high-impedance cathode electrolyte interphase (CEI) film on the cathode and the solid electrolyte interphase (SEI) film on the anode surface, which hinder their practical use. Here, we report a novel approach to ameliorate the above challenges through the rational construction of a stable, low-impedance cathode and anode interface film. Such films are simultaneously formed on both electrodes via the participation of the traditional salt, lithium tetrafluoroborate (LiBF 4 ), as electrolyte additive. The application of 1.0% LiBF 4 enhances the capacity retention of the cell from 26.1 to 82.2% after 150 cycles between 3.0 and 4.4 V at 1 C. Besides, the low-temperature discharge performance is also improved by LiBF 4 application: the discharge capacity of the cell with LiBF 4 is 794 mAh compared with 637 mAh without LiBF 4 at 1 C and -20 °C. The excellent electrochemical performance of pouch cells is ascribed to the contribution of LiBF 4 . Especially, the low binding energy of LiBF 4 with the oxygen on the LiCoO 2 surface leads to the enrichment of LiBF 4 that forms the protective cathode interface, which fills the blanks of previous research.