Phosphonium-Based Ionic Liquid Additives for Improved Cyclability of Gr/Si Composite Anodes in Lithium-Ion Batteries

This study investigates electrolytes comprising a blend of carbonate solvents [ethylene carbonate (EC)/ ethyl methyl carbonate (EMC)/ diethyl carbonate (DEC) 1:1:1 wt] and vinylene carbonate (VC) additive, alongside two phosphonium-based ionic liquids (ILs), trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P111i4FSI), and tri-n-butyl(decyl)phosphonium bis(fluorosulfonyl)imide (P44410FSI). Electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were utilized to analyze failure mechanisms. EIS findings indicate that the bulk resistance slightly changed across all electrolytes throughout cycling, while the hybrid electrolyte (VC 5% + P111i4FSI 10 wt%) displayed the lowest overall cell resistance. XRD and SEM results show minimal alterations in the graphite (Gr) and silicon (Si) structure postcycling. XPS analysis reveals distinct solid electrolyte interphase (SEI) layer compositions for the base and hybrid electrolytes. In the base electrolyte, higher lithium concentrations were detected at deeper SEI layers, likely due to inorganic lithium species such as LiF or Li2CO3. Higher Li2CO3 content may have hindered lithium-ion transport, reflected in higher impedance. Conversely, in the SEI layer of the hybrid electrolyte, the presence of LiF, poly(VC), and phosphorus (from the ionic liquid cation) led to enhanced anode surface passivation, mitigating side reactions and continuous Li and electrolyte consumption. This improvement extended the cell’s cycle life from 107 to 350 cycles.