Physics

Dr. Nikhil Koratkar
Rensselaer Polytechnic Institute in the USA

ABSTRACT:

Silicon (Si) shows enormous potential to replace traditional graphitic anodes in Lithium (Li)-ion batteries due to its exceptionally high theoretical specific capacity of ~4200 mAh/g. After over a decade of intense activity in this field, the community has moved away from bulk Si films to nanoparticles of Si. There are some very good reasons for this- for instance, lithiation of Si films leads to large volume expansion and stress-induced pulverization. The solid electrolyte interface (SEI) for Si films is also notoriously unstable leading to continual capacity fade with charge/discharge cycling. By contrast, nanostructures of Si such as nanowires and nanoparticles are far less prone to fracture compared to bulk Si films and their SEI can be stabilized by carbon based coatings. However, such a strategy significantly reduces volumetric energy density due to the porosity of Si nanoparticle based electrodes. Here we show that contrary to conventional wisdom, Si films can be stabilized by two strategies: (a) anchoring the Si films to a carbon nanotube macrofilm (CNM) current collector and (b) draping the film with a graphene monolayer. The CNM provides an elastic/flexible matrix that accommodates the volume expansion and suppresses the delamination of Si. The graphene drape further toughens the underlying Si film and provides a stable SEI. After electrochemical cycling, the graphene-coated Si films on CNM resembled a tough mud-cracked surface in which the graphene capping layer prevented delamination and pulverization. By contrast, Si films (without graphene) that were deposited on conventional copper foil current collectors were completely eviscerated within few tens of charge/discharge cycles. The graphene-draped Si films on CNM exhibit long cycle life (> 1000 charge/discharge steps) with an average specific capacity of ~806 mAh/g. The volumetric capacity averaged over 1000 cycles of charge/discharge is ~2821 mAh/cm3 which is 2 to 5 times higher than what is reported in the literature for Si nanoparticle based electrodes. The graphene-draped Si anode could also be successfully cycled against commercial cathodes in a full-cell configuration.