Real-Time Characterization of Micro-Sized Si-Based Anodes Using In Situ Atomic Force Microscopy

Recently, extensive research efforts have been devoted to developing high energy anodes for lithium-ion batteries (LIBs) by using Si-based materials due to their superior energy densities compared with conventional graphite anodes. Compared to Si nano powders, micro-sized Si-based anodes have unique advantages of high tap-density, low manufacturing cost, and good manufacturing safety. However, the practical implementation of the micro-Si anode requires understanding its electro-chemo-mechanical behaviors and its optimal operational conditions at the cell level. Otherwise, the large volume expansion of Si (c.a., 310 %) will lead to the mechanical pulverization of particles and consequent loss of electrical contact. In addition, cracking of Si particles will produce new SEI layers, which not only consume active Li-ions but increase a cell impedance.<br/>To understand the electro-chemo-mechanical coupling effect of the micro-Si anode material during cycling, we characterized the real-time morphological changes using in-situ Atomic Force Microscopy (AFM). To closely replicate the actual battery operation, we designed a unique unit cell by stacking a porous micro-Si electrode with separator and counter-electrode, of which cross-section was exposed to be measured by an AFM tip in its environmental cell. Using this customized in-situ cell, we successfully measured and quantified the volumetric variations of a micro-Si particle depending on state-of-charge (SOC) and investigated its morphology-performance relationship.<br/>From the in-situ AFM measurement, we successfully characterized multiple important electro-chemo-mechanical behaviors of the micro-Si such as volume-expansion behavior, particle crack generation, and SEI growth. We found micro-Si underwent initial particle pulverization and subsequent large volume expansion (180%) during with respect to lithiation. Then Si surface fractures were generated at critical electrochemical conditions (e.g., during delithiation), and new SEI layer (200nm thickness) was observed at the fractured areas. The Si surface fracture and resultant SEI growth significantly increased the cell impedance and degraded the cell performance. Our observations provide direct evidence for that micro-Si degradation can be triggered by reaching critical electrochemical cycling conditions. Finally, we demonstrate a proof-of-concept that micro-Si anode can deliver an improved cycle life and lower cell resistance by applying the optimal cell cycling conditions. The results propose that micro-Si anode can offer a stable performance after further optimization of the cell materials and systems for next-generation high-energy Li-ion batteries.