Carbonate and Ionic Liquid Mixes as Electrolytes To Modify Interphases and Improve Cell Safety in Silicon-Based Li-Ion Batteries

Among the candidates as negative electrode, silicon is now one of the most attractive alternatives to graphite and has been the subject of many investigations for the past decade. The commercialization of Si electrodes is nevertheless blocked by the inability to overcome the mechanical degradation and electrolyte consumption occurring as a result of the inherent volume expansion upon silicon alloying. The unique combination of their properties renders ionic liquids very attractive and promising candidates to replace the benchmark organic carbonates and could enable an enhanced control of species constituting the solid-electrolyte interface (SEI). In the present study, evolutions of ionic liquid based electrolytes (pure ionic liquid and ionic liquid/carbonate mixes) and the subsequently formed SEI are monitored upon aging and cycling in full Li-ion cells using nonprelithiated silicon electrodes. X-ray photoelectron spectroscopy, typically probing the first few nm of the surface of the sample, allowed monitoring of the evolution and possible degradation of the ionic liquid based electrolytes upon aging and cycling of complete Si/NMC batteries. Magic angle spinning NMR combined with scanning transmission electron microscopy-electron energy loss spectroscopy is more sensitive to changes occurring in the SEI composition. The degradation of ionic liquid components PYR13 and TFSI is evidenced and their influence on the formation of species at the surface of the silicon electrode clearly observed. However, the presence of the ionic liquid components does not prevent the degradation of carbonates in the parasitic reactions that are consuming the cyclable lithium. Therefore, the failure mechanism scenario is similar to that observed for the full cell using benchmark carbonate electrolytes. Hazard level assessments nevertheless reveal that the addition of ionic liquids is in fact able to moderate the intensity of safety relevant events and improve the cell safety.