Study on modification and failure of precast solid electrolyte interface film on Li metal anodes

Lithium (Li) metal anodes can be inevitably corroded and destructed due to the direct contact with the liquid electrolyte. Here, a precast solid electrolyte interface (SEI) film is constructed based on poly(vinylidene-co-hexafluoropropylene) (PVDF-HFP) matrix on a copper current collector to avoid side reactions between Li and electrolytes and further prevent dendrites growth. Lithium bis(oxalato)borate (LiBOB), SiC, and polyethylene oxide (PEO) are rationally hybridized, and their effects on the morphology, composition, strength, and other properties of the precast SEI film are studied through both physical and electrochemical characterizations. Due to the addition of LiBOB, SiC, and PEO, the structure and properties of the PVDF–HFP matrix have undergone a distinct change, which affects the cycle stability of the substrate. As a result, the cycling lifetime of the SEI film is increased from 61 to 295 cycles after adding an appropriate amount of LiBOB to the PVDF–HFP. Furthermore, the Li metal anode delivers a significantly improved life-span of 452 cycles at 1.0 mA·cm⁻², 1.0 mAh·cm⁻² by virtue of the robust SEI with SiC and PEO added. Meantime, the Coulombic efficiency of the cell has also increased from 95.0% to more than 99.0%. The morphology and composition characterization results show that the SEI structure gradually collapses during cycling, which are found to arise from the formation of loosely connected Li₂CO₃, LiF, and dilapidated PVDF–HFP mixture. The deteriorative artificial SEI also induces the disconnected electrical contact, and as such the impedance significantly increases, resulting in eventual failure of the Li metal anode.