(680d) Formation of a Stable Fluorophosphate-Rich SEI in Li-Metal Batteries Using a Semi-Solid Composite Electrolyte

An Organic-Inorganic Semi-Solid Composite Electrolyte has been designed and fabricated to enable the formation of stable Solid-Electrolyte Interphase (SEI) in high-capacity Lithium Metal Batteries (LMBs). The Electrolyte is based on the conventional lithium hexafluorophosphate carbonate chemistry (LiPF₆ in EC/DEC (1:1)) that has been turned into an ionic gel (physical gelation). The gel contains locally-high concentration of lithium cation and can be easily applied to the electrode’s surface with a desired thickness. The previously reported (by the same group) beneficial anionic species of OF₂P−O−PF₅^- has been detected using Nuclear magnetic resonance (NMR) measurements and the quantification of ¹⁹F spectra showed a significant increase in the content of this species that helps with creating a Fluorophosphate-rich SEI on Li-metal anodes. Electrochemical performance of the gel electrolyte has been investigated using Li||Li and Li||LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) button cells. Transition from whisker-shape to smooth Li deposition has been observed using Scanning Electron Microscope (SEM) images of Li-metal electrode’s surface. The ionic conductivity of the gel, acquired using Electrochemical impedance spectroscopy (EIS), is as high as liquid electrolytes (1-3 × 10^-3 S cm^-1). The Li||NMC622 cell with a 3.1 mAh cm^-2 cathode and 3 g Ah^-1 Electrolyte/Capacity ratio has been galvanostatically cycled by 0.1C (charge) and 0.33C (discharge) rates. After 100 cycles, the cell has maintained a high areal capacity (~3.1 mAh cm^2-) with a 99% retention. An Alumina-based ceramic separator (as an alternative for conventional soft polymer separators) is also being synthesized using the Inverse Opal Infiltration technique to complement the gel electrolyte by imposing more pressure on the Li metal anode help controlling the volume change during charge and discharge.