The role of surface reactions and solid electrolyte interphase in silicon electrodes for lithium-ion batteries

In order to utilize renewable energy sources to avoid adverse climate change caused by fossil fuel use, economical, efficient, and long-cycling energy storage means are needed for grid power applications and electric vehicles. Lithium-ion batteries (LIBs) are promising electrochemical energy storage devices for these applications, but capacity, cycle life, and device energy density need to be improved to meet these challenges. Silicon, as a lithium alloy, promises high gravimetric and volumetric charge capacities as a negative electrode in the next generation of LIBs. However silicon has a lithiation potential outside the window of stability of common non-aqueous liquid electrolytes (e.g., lithium hexafluorophosphate in ethylene carbonate and diethyl carbonate mixtures). Consequently, parasitic side reactions occur during continued lithiation and delithiation (cycling) of silicon. However, these side reactions (including electro-reduction and thermal decomposition) form insoluble products that make a solid electrolyte interphase (SEI), passivating an electrode’s surface. Cycling silicon electrodes can entail incomplete passivation (via unstable SEI species) and newly exposed surfaces (due to mechanical wear) and thus continued side reactions that lead to thermal runaway, capacity loss, and cell failure. By understanding interfacial electrode chemistry, it is hoped that novel design suggestions for addressing these problems will be uncovered. Model silicon electrodes studied by X-ray Photoelectron Spectroscopy (XPS), and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) were used to explore the effects of surface layer conductivity and electrolyte additives on SEI composition and structure. Anhydrous and anoxic techniques showed better reproducibility and accuracy in characterizing the SEI over previous studies of composite electrodes exposed to ambient conditions. By comparing silicon oxide and etched silicon surfaces, electrode conductivity was studied as well as how the co-solvent additive fluoroethylene carbonate (FEC) affects the SEI. Both the etched silicon surface and FEC produced SEI species like lithium fluoride that improved stability by resisting further electro-reduction. However, questions about the oxidative stability of some SEI species were raised (namely lithium oxide), suggesting a more stable artificial SEI could be manufactured compared to those formed during naive device operation.