Impacts of fluorinated phosphate additive on interface stabilization of 4.6 V battery cathode

Elevating the charge cut-off voltage of lithium-ion battery above 4.2 V vs. Li/Li+ can increase the capacity of cathode and energy density of the battery but requires higher anodic stability of electrolyte and higher interface stability between charged cathode and electrolyte than those of the state-of-the-art commercial electrolyte. Utilization of a small fraction of high-voltage electrolyte additive is a promising and economic approach to mitigate the high-voltage stability issue. Fluorinated ethyl phosphate (FEP) is known as a flame-retardant but we examine it as a high-voltage additive of a commercial electrolyte with 1 M LiPF6 in EC:EMC (3:7 vol ratio). Herein, we report FEP-assisted performance improvement of a lithium-ion cell under high charge cut-off voltage of 4.6 V vs. Li/Li+, with reduced impedance rise. Our surface analysis results reveal that FEP additive in a graphite‖LiNi0.5Co0.2Mn0.3O2 full-cell is effective preferably on cathode over anode by forming a surface-passivating organics-rich and fluorine-rich solid electrolyte interphase (SEI) layer. We propose that the anodic reaction of FEP begins by a single electron transfer from the O atom of P − O − C to the cathode surface and forms FEP-derived SEI species. The SEI layer enables the inhibition of metal-dissolution phenomena from the cathode, evidenced by elemental analysis results on the metal species at graphite anode, leading to superior cycling performance to the full-cell with commercial electrolyte only. A basic understanding of interface stabilization pathway of the cathode via FEP additive in the lithium-ion full-cell is clearly demonstrated.