Computational Study of Effect of Si Morphology on Mechanical Integrity of Si/CNT Hetetostructure Anode for Li Ion Battery

Over the past decade, lithium ion batteries have emerged as the most important and flag ship energy storage devices among other battery technologies such as Ni-Cd, Ni-MH, Lead acid, etc. Further improvement in battery technology is constantly being sought, since the demand for better energy storage devices is rising continuously. Current state of the art Li ion batteries have graphitic anode, which provides capacity of 372 mAh/g.  Silicon has emerged as the most promising candidate to replace graphite due to its high theoretical gravimetric capacity of 4200 mAh/g. However, commercialization of Si based anodes is hindered by mechanical degradation of the Si based anodes resulting from Li intercalation induced stresses. Different approaches have been suggested to improve the mechanical integrity of the Si based anodes and the retention of electrode capacity. Experimental as well as modeling studies reveal that the use of nano-sized silicon (nanoparticles, nanowires, etc.) can significantly improve the anode performance. However, loss of electronic conductivity due to mechanical failure is still a challenge that prevents commercialization of Si based anodes.

Recently, nanostructured heterostructures of Si with carbon nanotube (CNTs) are being explored in literature. CNTs are known to have very good mechanical strength along with excellent electrical and thermal properties. Li intercalation is known to result in reasonable capacity of ~300 mAh/g. Thus, heterostructures comprised of CNTs and nanostructured Si can be expected to give excellent electrochemical performance. Different Si/CNT heterostructures ranging from core-shell CNT/Si structure to Si nano-globules adhered to the CNT with sufficient separation between the adjacent globules can be synthesized. However, Si morphology in the heterostructure is known to significantly alter the electrode performance as shown by Epur et al. [1]. Understanding the role of morphologies and the resulting diffusion-induced stresses (see Figure 1) on the mechanical degradation of Si/CNT heterostructure needs to be explored to understand the plausible mechanisms contributing to capacity fade.

We utilize a thermodynamically consistent theoretical framework employed in a finite element setting [2] to model the Li intercalation induced deformation processes as well as failure occurring in different Si/CNT heterostructure anodes configurations (see Figure 1). CNT/Si interface is modeled using a novel cohesive law [3]. Two-phase lithiation of the amorphous Si is considered. Different Si/CNT heterostructure morphologies are compared in terms of their mechanical durability during electrochemical cycling to identify the most stable Si/CNT electrode configuration. 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 modeling studies and the comparison to experimental reports with resultant validation of the experimental findings will be presented and discussed.

References:

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.

3.         Ortiz, M. and A. Pandolfi, Finite deformation irreversible cohesive elements for three-dimensional crack-propagation analysis. International Journal for Numerical Methods in Engineering, 1999. 44: p. 1267-1282

Acknowledgements:

The authors would like to acknowledge the financial support from the US Department of Energy’s Office of Vehicle Technologies BATT program (Contract DE-AC02- 05CHI1231), sub contract 6151369, and the National Science Foundation (CBET- 0933141). PNK would like to acknowledge the Edward R. Weidlein Chair Professorship Funds for partial support of this work. In addition, PNK and SM would like to thank the Center for Complex Engineered Multifunctional Materials (CCEMM) for providing a graduate fellowship to perform the simulation experiments reported in this work.