Understanding the configuration-mechanical stability relationships for Si-CNT heterostructured anodes for Li-ion battery: A computational study
Lithium ion batteries are one of the most sought after energy storage devices due to their relatively higher energy storage density, long cycle life, and an improved rate capability response as compared to other battery technologies such as Ni-Cd, Ni-MH, Lead acid, etc. Current state of the art Li ion batteries still implement graphitic anodes exhibiting a theoretical electrochemical capacity of 372 mAh/g. In the past decade, silicon has evolved as the most promising alternative anode material to graphite for Li ion systems due to its high theoretical gravimetric capacity of 4200 mAh/g. However, colossal volume expansion related mechanical degradation and eventual failure of Si during electrochemical cycling leading to loss in capacity limits the cycle life thus impeding the commercialization pathway.
Experimental as well as modeling studies have revealed that the use of nano-sized amorphous silicon (a-Si) morphologies significantly improves the anode capacity retention over multiple electrochemical cycles. However, a-Si suffers from poor electronic conductivity and charge transport, significant first cycle irreversible loss, inferior performance at higher current rates, and low columbic efficiency. On the other hand, multiwall carbon nanotubes are known to have very good mechanical strength along with excellent electrical and thermal properties. Thus, core-shell heterostructures comprised of carbon nanotube (CNT) core and nanostructured Si shell have emerged as promising candidates for Si based anode for Li-ion batteries. Different Si-CNT heterostructures ranging from uniform silicon thin films coated onto the CNT to Si nano-droplets tethered to the CNT at a specified spacing between the adjacent droplets can be synthesized. However, configuration of the Si component in the heterostructure is known to significantly alter the cycling performance of the electrode as shown by Epur et al. . In this study, we focus on understanding the role of silicon configuration on the mechanical stability of the heterostructure during its electrochemical cycling. Such knowledge will shed light on contributing mechanisms for capacity fade in Si-CNT heterostructured anodes.
We hypothesize that nucleation and growth of voids in silicon during electrochemical cycling can induce its fracture and eventual failure. We utilize a custom developed multi-physics finite element (FE) modeling framework taking into account the Li diffusion induced elasto-plastic deformation of Si . We systematically vary the geometry of Si in the Si-CNT heterostructure and simulate the resulting anode configuration for one complete electrochemical cycle (see Fig. 1). Comparison of the conditions for void growth and nucleation in the Si is done to understand the mechanical stability of the different Si-CNT heterostructured configurations during electrochemical cycling. Qualitative comparison of performed simulation studies with experimental results is made. Results from this study are expected to aid in the fabrication of improved Si/CNT heterostructure anodes. Results of the study will be presented and discussed.
Fig. 1: Schematics of Si-CNT heterostructure geometries studied (a) Continuous Si film coating on CNT (b) Si nano-ring adhered to CNT and (c) 1/8th of Si nano-ring adhered to CNT. Dimensions of the heterostructure components are specified in nm.
1. Epur, R., M.K. Datta, and P.N. Kumta, Nanoscale engineered electrochemically active silicon–CNT heterostructures-novel anodes for Li-ion application. Electrochimica Acta, 2012. 85: p. 680-684.
2. S. Pal, et al., Modeling of lithium segregation induced delamination of a-Si thin film anode in Li-ion batteries. Computational Materials Science, 2013. 79: p. 877-887.
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