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/cm2after 300 cycles when cycled at current densities of 1500 mA/g (C/2.5) at room temperature. Moreover, high-capacity retention was maintained for 60 cycles at elevated temperatures up to 80 °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.