(286a) Understanding the Configuration-Mechanical Stability Relationships for Si-CNT Heterostructured Anodes for Li-Ion Battery: A Computational Study

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. [2]. 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 [2]. 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) 1/8^th of Si nano-ring
adhered to CNT. Dimensions of the heterostructure components are specified in
nm.

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.