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

Area of Science:

• Materials Science
• Electrochemistry
• Battery Technology

Background:

• Silicon anodes offer high energy density but suffer from poor cycling stability due to volumetric expansion.
• Carbonate-based electrolytes are common but can lead to undesirable side reactions with silicon anodes.
• Improving the solid electrolyte interphase (SEI) layer is crucial for stable silicon anode performance.

Purpose of the Study:

• To investigate the effect of a phosphonium ionic liquid additive on the cycling stability of silicon/graphite (Si/Gr) and silicon (Si) anodes.
• To analyze the impact of the additive on the solid electrolyte interphase (SEI) composition and morphology.
• To evaluate the performance enhancement in various battery cell configurations.

Main Methods:

• Electrolyte formulation with a phosphonium ionic liquid (trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide, P111i4FSI) in carbonate solvents.
• Extended cycling tests of Si/Gr || LFP, Si || LFP, Li-metal || NMC622, and Si/Gr || NMC622 battery cells.
• Analysis using voltage profiles, dQ/dE, SEM, and XPS to characterize SEI formation and failure mechanisms.

Main Results:

• The phosphonium ionic liquid additive significantly improved cycling stability for both Si/Gr and Si anodes.
• SEI analysis revealed the presence of phosphorus and LiF in the HCHE, correlating with reduced side reactions.
• Si/Gr anodes achieved 96% efficiency at 450 mAh/g for 185 cycles, and Si anodes achieved 99.1% efficiency at 1000 mAh/g for 100 cycles.
• A 3-fold improvement in cell cycling stability was observed.

Conclusions:

• The phosphonium ionic liquid additive effectively stabilizes silicon-based anodes by modifying the SEI layer.
• This additive mitigates SEI growth and lithium availability issues in Si/Gr anodes and structural degradation in Si anodes.
• The findings demonstrate a promising strategy for developing next-generation high-energy-density batteries.