New mechanistic insights into the widely used LiPO2F2 additive enabled by ultra-high-resolution FTICR-MS

Abstract

Understanding how electrolyte additives influence the formation and evolution of the solid electrolyte interphase (SEI) is essential for improving the durability of lithium‑ion batteries. In this work, we investigate the role of lithium difluorophosphate (LiPO2F2, LiDFP) in 1 M LiPF6‑based electrolytes, with and without vinylene carbonate, using ultra‑high‑resolution Fourier transform ion cyclotron resonance mass spectrometry (FT‑ICR MS), mass spectrometry imaging (MSI), and X‑ray photoelectron spectroscopy (XPS). Electrochemical tests on NCM/graphite pouch cells cycled at 30 °C and 60 °C show that LiDFP improves early‑stage cycling performance despite not being directly reduced at the negative electrode. FT‑ICR MS analyses of aged electrolytes reveal that LiDFP mitigates LiPF6 decomposition, thereby preserving electrolyte molecular integrity. XPS results support this behavior, indicating that the SEI formed in the presence of LiDFP is dominated by solvent‑derived species rather than salt decomposition products. MSI spatial mapping further confirms this mechanism by localizing LiPF6 degradation products on graphite surfaces. Overall, these results demonstrate that LiDFP stabilizes the electrolyte through an indirect pathway by suppressing LiPF6 degradation, steering SEI chemistry toward more stable compositions and enhancing cycling performance.