Tip-Enhanced Raman Spectroscopy to Illuminate the Chemical and Structural Integrity of Borate Glass Coatings on Electrochemically Active Material

Coating of electrochemically active material with a thin protective layer is an invaluable strategy to increase the material's performance and protect it from structural degradation during electrochemical cycling. Carbon and various inorganic materials including a borate glass-based coating have been tested in this regard. We have studied the chemical nature of the borate-based coated layer using state-of-the-art tip-enhanced Raman spectroscopy (TERS), which provides insight into the structural and chemical heterogeneity at the nanoscale. We found that the synthesized borate layer is highly disordered and doped with mobile alkali ions which leached out from the active material. The dopant cations break the boroxol structure present in the vitreous borate and lead to the formation of structural polymorphs such as pyroborate and diborate units. TERS allowed to elucidate that the degree of dopant present in the thin layer is in-between 40% and 60% molar weight with respect to boron oxide moieties.