Insight into the Contribution of the Electrolyte Additive LiBF₄ in High-Voltage LiCoO₂||SiO/C Pouch Cells

High-voltage pouch cells using an LiCoO₂ 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 (LiBF₄), as electrolyte additive. The application of 1.0% LiBF₄ 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 LiBF₄ application: the discharge capacity of the cell with LiBF₄ is 794 mAh compared with 637 mAh without LiBF₄ at 1 C and −20 °C. The excellent electrochemical performance of pouch cells is ascribed to the contribution of LiBF₄. Especially, the low binding energy of LiBF₄ with the oxygen on the LiCoO₂ surface leads to the enrichment of LiBF₄ that forms the protective cathode interface, which fills the blanks of previous research.