SEI layer and impact on Si-anodes for Li-ion batteries

Silicon (Si) with a theoretical specific capacity of ~ 4200 mAh g− 1 is poised to partially replace graphite of specific capacity ~ 372 mAh g− 1 as an anode in lithium-ion (Li+) batteries (LIBs) thanks to gain in gravimetric as well as volumetric energy density. However, the most significant impediment for successful implementation of Si anodes is volumetric swelling (~ 300%) of commercially available submicron sized Si particles (≥ 0.15 μm) during lithiation leading to capacity fade, limiting both cycle and calendar life, and decomposition of liquid organic electrolyte (mainly carbonate-based solvents and inorganic lithium salt) with subsequent formation of solid electrolyte interphase (SEI) at electrode/electrolyte interface. Volumetric swelling and the associated capacity fade can be alleviated within tolerance limit by engineering electrodes design, nanostructuring, nanoparticle chemistry, coating strategy, and the combination thereof, or by using amorphous Si. However, SEI formation at electrode/electrolyte interphase is a natural as well as a complex phenomenon that occurs during Li-ion transport in the very first cycle and cannot be avoided since the LixSiy phase(s) formation falls outside the thermodynamic salt and solvents stability potential versus Li+/Li. Therefore, understanding the interplay between solvents and salt components of the electrolyte as well as their decomposition products towards SEI formation is crucial; also, continuous consumption of lithium and entrapment of desolvated Li+ in SEI leads to depletion of lithium source remains a critical bottleneck that must be addressed. The present chapter deals with a comprehensive understanding of SEI formation on Si electrodes and elucidates the role of carbonate solvents, electrolyte additives, polymeric binders, coating strategies, etc. It also explains SEI formation on Si, and native oxide-terminated Si (Si/SiOx) electrodes, their growth structure, and failure mechanisms. The objective of the chapter is to provide a broad perspective of SEI formation and associated challenges to enable readers to develop novel strategies for fabricating a stable, elastic, mechanically durable all-weather SEI on Si which will withstand thousands of cycles without causing any deleterious effect on cell performance.