Anodes for Lithium-Ion Batteries Based on Type I Silicon Clathrate Ba₈Al₁₆Si₃₀ - Role of Processing on Surface Properties and Electrochemical Behavior

Type I silicon clathrates based on Ba₈Al_ySi_46-y (8 < y < 12) have been studied as potential anodes for lithium-ion batteries and display electrochemical properties that are distinct from those found in conventional silicon anodes. Processing steps such as ball-milling (typically used to reduce the particle size) and acid/base treatment (used to remove nonclathrate impurities) may modify the clathrate surface structure or introduce defects, which could affect the observed electrochemical properties. In this work, we perform a systematic investigation of Ba₈Al_ySi_46-y clathrates with y ≈ 16, i.e, having a composition near Ba₈Al₁₆Si₃₀, which perfectly satisfies the Zintl condition. The roles of ball-milling and acid/base treatment were investigated using electrochemical, X-ray diffraction, electron microscopy, X-ray photoelectron and Raman spectroscopy analysis. The results showed that acid/base treatment removed impurities from the synthesis, but also led to formation of a surface oxide layer that inhibited lithiation. Ball-milling could remove the surface oxide and result in the formation of an amorphous surface layer, with the observed charge storage capacity correlated with the thickness of this amorphous layer. According to the XRD and electrochemical analysis, all lithiation/delithiation processes are proposed to occur in single phase reactions at the surface with no discernible changes to the crystal structure in the bulk. Electrochemical impedance spectroscopy results suggest that the mechanism of lithiation is through surface-dominated, Faradaic processes. This suggests that for off-stoichiometric clathrates, as we studied in our previous work, Li⁺ insertion at defects or vacancies on the framework may be the origin of reversible Li cycling. However, for clathrates Ba₈Al_ySi_46-y with y ≈ 16, Li insertion in the structure is unfavorable and low capacities are observed unless amorphous surface layers are introduced by ball-milling.