Investigation of FEC effects containing ether electrolyte for fast charging high-nickel NCM|Li cells

• The appropriate FEC has the advantage of high-rate (>5 C-rate) characteristics. • This is due to high ionic conductivity by forming LiF and inorganic chemical species in the depth direction from the interface surface on both the positive and negative electrodes. • We compared to different nickel content LiNiMnCoO 2 cathode material and different loading level. • Our work provides the key importance of improving the trade-off between electrolyte and electrode properties to enhance battery performance, including high-rate and energy-density characteristics. We systematically investigate the effect of fluoroethylene carbonate (FEC) additive in an ether-based electrolyte including lithium bis(fluorosulfonyl)imide (LiFSI) in 1,2-dimethyl ether (DME): 1,1,2,2-tetrafluoroethyl-1H,1H,5H-octafluoropentyl ether (TFOFE) on the high-rate (> 5C-rate) performance in Ni-rich lithium metal batteries. Using cell pairing Li (Ni 0.9 Co 0.05 Mn 0.05 ) O 2 (NCM955) cathodes with Li metal anodes (3.0–4.3 V) varying different FEC amount, we correlate 5C/10C charge–discharge performance to interphase chemistry via X-ray photoemission spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and optical analysis. We find that moderate FEC levels (5 wt%) significantly enhance interfacial conductivity by promoting robust LiF- and phosphate-rich inorganic interphase layers, solid electrolyte interphase (SEI)/ cathode electrolyte interphase (CEI) on both the Li metal and cathode surfaces. This stabilized interphase reduces impedance and enables superior fast-charge/discharge capability, particularly when paired with lower NCM955 loading (half-active material mass NCM) or lower Ni content (Li (Ni 0.6 Mn 0.2 Mn 0.2 ) O 2, NCM622). The insufficient FEC (< 3 wt%) increases cell impedance in thick, Ni-rich electrodes, underscoring the need to balance additive concentration with electrode design. Overall, an optimized ∼5 wt% FEC yields markedly improved high-rate performance and interfacial stability. These findings highlight that tuning additive content in tandem with electrode architecture is critical for enabling fast-charging lithium-metal batteries without sacrificing energy density.