Key functional groups defining the formation of Si anode solid-electrolyte interphase towards high energy density Li-ion batteries

Renewable and sustainable energy solution, especially in transportation, is increasingly becoming an important global issue. Improving upon lithium-ion battery (LIB) energy density provides one of the most promising solutions. Silicon (Si) is regarded as the immediate next generation anode for the LIB due to its high gravimetric and volumetric capacity (about 10 times as high as the current commercial solution, graphite's). However, full incorporation of Si in the LIB still remains challenging because of two problems; high volume changes that lead to loss of electrical contacts and unsteady decomposition of electrolyte that forms highly insulating solid-electrolyte interphase (SEI) layer on Si surface. The high volume changes can be alleviated by nano-sizing the Si particles. There are a number of review articles outlining the understandings and solutions to address the physical issue. However, exclusive reviews for the SEI are scarce. In this review, fundamental understandings and challenges of the conventional Si SEI and the solutions implemented in the literature are compiled. We point out the key chemical species in engineering the formation of favorable SEI and key chemical functionalities in protecting the SEI. With the comprehensive mechanisms we summarize, we hope to achieve the full implementation of Si anode as a commercial electrode.