A novel perspective on the degradation of Si-based anodes

The ongoing electrification of society demands enhancements in energy densities, cycling stability and endurance for future Li-ion batteries. Therefore, the main objective is to develop superior electrode materials, to drive next-generation Li-ion batteries. The integration of Silicon (Si) with high weight percentage into lithium-ion batteries (LIBs) but also its further utilization on solid state batteries (SSBs) provides high potential to revolutionize the energy storage landscape.
Despite considerable advancements in understanding the electrochemical aspects of Si-based anodes, the material science perspective on the electrochemically induced volume swing and the impact of the associated mechanical loading on the silicon´s crystalline structure remains insufficiently explored. This represents a critical knowledge gap crucial for advancing the development of the next generation of anode materials. 
Usually, the degradation of Si-based anodes is mainly attributed to the reformation of the silicon electrolyte interphase (SEI).   Due to the implementation of Si in the anode, recurrent volume expansion and shrinkage during the insertion and extraction of Li-ions occurs. When the Si expands, the SEI layer may fracture, exposing pristine silicon surfaces to the electrolyte. As a result, repeated SEI growth leads to the formation of a rather thick amorphous silicon electrolyte composite (SEC) at the interface of the active silicon material1 .
However, in general as shown in the past in other fields silicon can also undergo transitions from crystalline to amorphous phases when subjected to mechanically loading conditions e.g. by compression, surface scratching, nanoindentation etc. In this case amorphization may kick in, by the formation of shear bands arising from a high density of crystallographic defects, destabilizing the crystalline lattice.
In our recently published work, we answer in this context the following essential questions: 

Is the formation of shear bands, leading to the transformation from a crystalline to an amorphous phase in Si-based anodes, feasible?
Is the stress exerted on the crystalline silicon core during electrochemical cycling sufficient?
Can insights derived from smaller-scale investigations effectively elucidate the failure mechanisms in realistic, large-scale electrodes utilizing Si particles?
Furthermore, could this triggered phase transition contribute to an accelerated capacity loss, or does it potentially have a positive effect on the overall lifetime of high-energy-density Li-ion batteries?

A Team including the Materials Center Leoben Forschung GmbH, Montanuniversität Leoben, Austrian Academy of Science, Varta Innovation GmbH and ESRF – The European Synchrotron was able to answer those questions and could shed light on the continuous capacity loss observed in LIBs employing silicon-rich anodes throughout extended cycling.
Advanced imaging techniques on various length scales such as 4D scanning transmission electron microscopy (STEM), field emission scanning electron microscopy (FESEM), synchrotron X-ray nano-tomography as well as machine learning (ML)-based microstructure analysis were utilized.

a BF- STEM image of the anode after 300 cycles showing pronounced dendrite growth around a silicon particle. b EDS mappings of ROI 1 for the elements: silicon, carbon, fluorine and oxygen clearly highlight the dendrite’s chemical composition. Notably, no F, C or O is detected within the Si particle. c 4D-STEM analysis of ROI 2 showing crystalline Si (red) and amorphous bands (blue) within the Si particle as well as amorphous dendritic phases (green) at the Si interface.  

We could uncover the complexity of degradation mechanisms in silicon-based anodes, including the SEC growth and phase-dependent lithiation speed. Both generate non-hydrostatic strain in the silicon core, which triggers dislocation accumulation. Further, we provide novel insights that this leads to deformation induced amorphization within the silicon core domain.
The shear band formation boosts non-uniform growth of the SEC on the silicon interface which is contributing to the overall stress configuration within the anode, eventually affecting battery cycling behavior.
The gained material science-based mechanistic insights underscore the importance of exploring beyond surface chemical processes, such as the SEI formation. The work delivers a novel perspective understanding the complexities affecting the entire battery system.
In addition, we argue that while the deformation triggered phase transition impacts the lithiation and SEC formation process, over prolonged cycles, it could potentially yield beneficial effects by inhibiting fracture initiation and silicon particle pulverization slowing down the total battery failure as can be witnessed by the changing incline of the capacity at prolonged cycles.  
We point out, that based on our findings and considering the structure-property relationship, it is essential to control the interface kinetics during lithiation. Novel functional designs of the silicon material architecture will revolutionize energy storage applications.
Publication: Amorphous shear band formation in crystalline Si-anodes governs lithiation and capacity fading in Li-ion batteries | Communications Materials (nature.com)

References:

Impact of solid-electrolyte interphase reformation on capacity loss in silicon-based lithium-ion batteries | Communications Materials (nature.com)