High Concentration Hybrid Electrolytes Using Phosphonium Ionic Liquid Additives for Stable High-Energy Density Li-Ion Anodes

This study explores a phosphonium ionic liquid additive in carbonate-based electrolytes that can improve the cycling stability of silicon/graphite (Si/Gr) and silicon (Si) anodes. The electrolytes consist of a carbonate solvent mixture (EC/EMC/DEC in a 1:1:1 weight ratio) combined with a 10% weight ratio of the ionic liquid additive, trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P_111i4FSI), in LiFSI concentrations of 1 M and 4 M. Si/Gr || LiFePO₄ (LFP), Si || LFP, Li-metal || LiNi_0.6Mn_0.2Co_0.2O₂ (NMC₆₂₂), and Si/Gr || NMC₆₂₂ cells were evaluated through extended cycling tests. Voltage profiles, d<i>Q</i>/d<i>E</i> analyses, and scanning electron microscopy (SEM) demonstrate distinct failure mechanisms for the two anodes: solid electrolyte interphase (SEI) growth and limited lithium availability in Si/Gr anodes and severe structural cracking and pulverization in Si anodes due to volumetric expansion. X-ray photoelectron spectroscopy (XPS) revealed distinct SEI layer compositions. In the base electrolyte, higher lithium concentrations were detected at the surface and deeper SEI layers, attributed to higher Li₂CO₃ content. Conversely, in the SEI layers formed in the high concentration hybrid electrolyte (HCHE), the presence of phosphorus (from the ionic liquid's cation) and a higher amount of LiF within the SEI are commensurate with reduced side reactions and efficient lithium-ion transport. Therefore, a 3-fold improvement in the cell cycling stability was obtained, with Si/Gr anodes cycled at 450 mAh/g and 96% efficiency up to 185 cycles and Si anodes cycled at 1000 mAh/g and 99.1% efficiency up to 100 cycles.