Microstructural evolution induced by micro-cracking during fast lithiation of single-crystalline silicon

Yong Seok Choi,Matt Pharr,Chan Soon Kang,Seoung-Bum Son,Seul Cham Kim,Kee-Bum Kim,Hyunchul Roh,Se-Hee Lee,Kyu Hwan Oh,Joost J Vlassak

doi:10.1016/j.jpowsour.2014.04.124

Abstract

We report observations of microstructural changes in {100} and {110} oriented silicon wafers during initial lithiation under relatively high current densities. Evolution of the microstructure during lithiation was found to depend on the crystallographic orientation of the silicon wafers. In {110} silicon wafers, the phase boundary between silicon and LixSi remained flat and parallel to the surface. In contrast, lithiation of the {100} oriented substrate resulted in a complex vein-like microstructure of LixSi in a crystalline silicon matrix. A simple calculation demonstrates that the formation of such structures is energetically unfavorable in the absence of defects due to the large hydrostatic stresses that develop. However, TEM observations revealed micro-cracks in the {100} silicon wafer, which can create fast diffusion paths for lithium and contribute to the formation of a complex vein-like LixSi network. This defect-induced microstructure can significantly affect the subsequent delithiation and following cycles, resulting in degradation of the electrode.

Highlights

Because of their high power densities, lithium-ion batteries are employed in applications that are especially sensitive to weight and size, such as portable electronic devices and electric vehicles [1,2,3,4,5,6]
We report on an experimental study in which we applied relatively large lithiation current densities to {100} and {110} oriented silicon wafers to evaluate the effect of crystal orientation on the microstructural evolution of the lithiated phase
We have studied the microstructural evolution of both {100} and {110} silicon wafers during initial lithiation under relatively high current densities

Summary

Introduction

Because of their high power densities, lithium-ion batteries are employed in applications that are especially sensitive to weight and size, such as portable electronic devices and electric vehicles [1,2,3,4,5,6]. Silicon has drawn considerable attention as one of the most promising anode materials due to its huge theoretical capacity of ~4200 mAh/g (Li22Si5) [8,9,10,11] Associated with this large capacity, insertion of lithium into silicon causes large volumetric expansion in the range of 300% to 400% [11,12,13,14]. Defects, geometrical instabilities, and solid-electrolyte interface (SEI) layers can cause spatially inhomogeneous lithiation These inhomogeneities may lead to a non-uniform strain distribution in the lithiated phase, resulting in fracture and the degradation of the capacity of the battery. We observe uniform lithiation of {110} silicon wafers, but a complex microstructure with micro-cracks in {100} silicon wafers Since these micro-cracks only occur in the {100} substrates, we suggest that the formation of crystalline LixSi depends on crystal orientation and plays an important role in micro-crack generation. These observations have important ramifications for mitigation of damage during lithiation of crystalline silicon

Experimental procedures

Results and discussion

Conclusion