Li dendrite formation on graphite anode electrode of Li ion batteries (LIBs) during fast charging processes is one of the significant safety concerns in the application of electric vehicles. Catastrophic failures due to thermal runaway could be triggered by the short circuit due to Li dendrite. In order to understand the mechanism and suppress the dendrite formation, lots of research has been focused on the experimental analysis [1, 2] and nucleation and growth of Li electrodeposit [3]. In addition to the electrode and electrolyte materials, the microstructure inside anode electrode also plays a critical role in determining the dendrite formation because the anode microstructure forms the boundary condition for the Li dendrite formation reaction and other physical and electrochemical processes. Therefore, it is necessary to study the effect of heterogeneous microstructure on the Li dendrite formation during charging process.

This study is to investigate Li dendrite formation according to the electrochemical reaction potential at the interface between the anode active material and electrolyte based on realistic 3D anode electrode microstructures. An Xradia microXCT-400 system (Xradia, Pleasanton, CA) was employed to obtain the computed tomography (CT) images of the anode electrodes. A total of 729 projection images were captured over 182 degrees scan angle. The spatial resolution of the CT had been set as 0.56 μm. A C++ code was developed to rebuild and mesh the realistic microstructure of anode electrode through Micro-CT scanned images. The half-cell LIB model was composed of electrolyte, anode active materials and current collector. By employing 3D finite volume method, another C++ code was developed to simulate the charge processes by solving coupled model equations of charge conservation, mass conservation and electrochemical reaction. The simulation revealed the distribution of physical and electrochemical variables in the electrode such as concentration, voltage, current density, electrochemical reaction rate, overpotential, open circuit potential (OCP) et al. The value of overpotential plus OCP is critical for Li dendrite formation. If the value is lower than zero, Li deposits and then dendrite starts to form. The simulation results revealed that Li dendrite formation doesn’t take place simultaneously and uniformly at the surface of anode active material particles during fast charging processes. The knowledge from the study will help to understand the mechanism of Li dendrite formation in a realistic anode electrode and suppress it in LIBs, which could be useful to improve LIB design and manufacturing.

Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850.

References:

1.   Y, Z., et al., Dendrite-Free Lithium Deposition with Self-Aligned Nanorod Structure. Nano Letters, 2014. 14(12): p. 6889-6896.

2.   Seong, I.W., et al., The effects of current density and amount of discharge on dendrite formation in the lithium powder anode electrode. Journal of Power Sources, 2008. 178: p. 769-773.

3.   Ely, D.R. and R.E. Garcia, Heterogeneous Nucleation and Growth of Lithium Electrodeposits on Negative Electrodes. Journal of The Electrochemical Society, 2013. 160(4): p. A662-A668.