(777f) Computational Study On Stability of Hybrid Nanostructures of Silicon and Multiwalled Carbon Nanotubes As Lithium-Ion Battery Anode

Lithium-ion batteries are forerunners in energy storage systems and have found use in various applications ranging from mobile phones to electric vehicles. Traditional Li-ion batteries are comprised of graphite based anodes which exhibit Li intercalation capacity of 372 mAh/g. Continuous efforts are being made to explore alternate materials with higher Li intercalation capacity to achieve better battery performance. Research in silicon based anodes has gained importance as it can provide ~4200 mAh/g Li alloying capacity corresponding to the highly lithiated phase of Li_4.4Si. However, the huge volumetric expansion (~280%) associated with Li alloying in Si causes microcracking and loss of electrical contact leading to failure of electrode. 

Due to the ability of nanostructures to accommodate large strains, various anode nanostructured configurations have been studied in the recent past. These include thin films, core-shell nanotubes, nanowires, nanospheres, etc. Carbon nanotube based configurations are favorable for use due to the associated short diffusion lengths, better electrical conductivity (1-D electronic transport), mechanical strength, and an increased ability to alleviate stress. Si deposited by chemical vapor deposition (CVD) on multi-walled carbon nanotubes (MWCNT’s) has been explored earlier as a high capacity anode electrode material for Li ion batteries.  However, during electrochemical cycling, there is a gradual loss of electrical contact between the MWCNTs and Si leading to capacity fade. Since the loss of electrical contact occurs at the interface between the MWCNT and the Si coating, a properly engineered interface between these two components can maintain the integrity of the nanostructured configuration. Hence, it is paramount to understand the mechanism and properties of the bulk materials and interface that affects the disintegration of the nanostructured configuration.

In this work, we utilize a thermodynamically consistent theoretical framework to model the Li alloying induced deformation as well as failure of the anode configuration. A novel cohesive law is used to model the debonding. Transport of Li is coupled with the mechanical equilibrium in a finite deformation setting. Finite element based computational procedure is utilized to simulate the coupled equations. We also investigate the effect of critical parameters that govern the mechanical stability of the interface between the MWCNT and Si coating. Based on the present investigation, critical parameters are identified to design better anode configurations which can provide prolonged electrochemical cycling of the anode without capacity fade.