Ethyl 2-butene phosphite as a film-forming additive for high voltage lithium-ion batteries

Abstract

Increasing the charge cutoff voltage can significantly improve the capacity of lithium-ion batteries. However, the structural degradation of Ni-rich cathodes and high reactivity of electrolytes at the high-potential cathodes greatly affect the cycling stability. In this paper, a new type of cathode film-forming additive, ethyl 2-butene phosphite (EBP), is synthesized based on the molecular design of phosphite by increasing the functional group of carbon-carbon double bond and ring structure. Theoretical calculation shows that EBP has a higher HOMO level and can form a cathode electrolyte interphase (CEI) on the cathode electrode surface before the electrolyte in theory. The electrochemical performance of NCM622/Li half-cells is significantly enhanced by incorporating EBP into the electrolyte, achieving 72.52 % capacity retention over 100 cycles at 0.5C. Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive Spectroscopy (EDS) are used to characterize the morphology of the anode and cathode, revealing that EBP forms a dense and complete CEI film on the surface of the LiNi_0.6Co_0.2Mn_0.2O₂ electrode. This film effectively blocks direct contact between the electrolyte and the cathode active material, prevents the dissolution of transition metals, improves interfacial stability, and consequently enhances the high-voltage cycling performance of the battery.