Hoop-strong nanotubes for battery electrodes

The engineering of hollow nanostructures is a promising approach to addressing instabilities in silicon-based electrodes for lithium-ion batteries. Previous studies showed that a SiO_x coating on silicon nanotubes (SiNTs) could function as a constraining layer and enhance capacity retention in electrodes with low mass loading, but we show here that similarly produced electrodes having negligible SiO_x coating and significantly higher mass loading show relatively low capacity retention, fading quickly within the early cycles. We find that the SiNT performance can still be enhanced, even in electrodes with high mass loading, by the use of Ni functional coatings on the outer surface, leading to greatly enhanced capacity retention in a manner that could scale better to industrially relevant battery capacities. In situ transmission electron microscopy studies reveal that the Ni coatings suppress the Si wall from expanding outward, instead carrying the large hoop stress and forcing the Si to expand inward toward the hollow inner core. Evidence shows that these controlled volume changes in Ni-coated SiNTs, accompanied by the electrochemically inert nature of Ni coatings, unlike SiO_x, may enhance the stability of the electrolyte at the outer surface against forming a thick solid electrolyte interphase (SEI) layer. These results provide useful guidelines for designing nanostructured silicon electrodes for viable lithium-ion battery applications.