High-Capacity Retention of Si Anodes Using a Mixed Lithium/Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolyte

The commercialization of high-capacity Si electrodes for lithium batteries has stalled due to the inability to overcome the mechanical degradation and electrolyte consumption that occur as a result of the inherent volume expansion upon charging. Using an ionic liquid (IL) electrolyte, trimethylisobutylphosphonium bis-(fluorosulfonyl)imide (P1,1,1,i4FSI) containing a high lithium bis(fluorosulfonyl)imide (LiFSI) salt content of 3.2 mol per kg of IL (50 mol %), inexpensive and high-capacity Si electrodes made from a facile and ball-milling process demonstrated outstanding capacity retention of around 3.5 mAh/cm(2) after 300 cycles when cycled at current densities of similar to 1500 mA/g (C/2.5) at room temperature. Moreover, high capacity retention was maintained for 60 cycles at elevated temperatures up to 80 degrees C, where traditional electrolytes are unable to operate. SEM images suggest that the use of this highly concentrated IL electrolyte promotes the formation of a stable surface layer that accommodates the volume expansion of the Si electrode. This benchmark result suggests that tailoring of the electrolyte for advantageous solid electrolyte interphase properties is a very promising route of premium interest.