Size-Dependent Fracture of Silicon Nanoparticles During Lithiation
Size-Dependent Fracture of Silicon Nanoparticles During Lithiation
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DOI：
10.1021/nn204476h
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发表时间：
2012-02-01
期刊：
影响因子：
17.1
通讯作者：
Huang, Jian Yu
中科院分区：
文献类型：
--
作者：
Liu, Xiao Hua;Zhong, Li;Huang, Jian Yu
利用原位透射电子显微镜对单个硅纳米颗粒的锂化过程进行了实时研究。发现断裂具有很强的尺寸依赖性;即存在一个类似于150nm的临界粒径，在此粒径以下，颗粒在首次锂化时既不开裂也不破裂；在此粒径以上，颗粒最初形成表面裂纹，然后由于锂化引起的膨胀而破裂。意想不到的表面开裂是由于大的拉伸环向应力的积累引起的，这扭转了表层的初始压缩。应力逆转归因于晶体Si中独特的锂化机制，发生在原始Si内核和非晶Li-Si合金外壳之间的两相边界的移动。虽然由此产生的环向张力倾向于引发表面裂纹，但小尺寸纳米颗粒仍然避免了断裂。这是因为电化学反应中储存的应变能不足以驱动裂纹扩展，这是由控制断裂的两种长度尺度（即颗粒直径和裂纹尺寸）之间的相互作用决定的。这些结果与先前单相模型的结果截然相反，后者只预测了表面层的压缩环向应力，因此在锂化硅颗粒和金属丝中从中心起裂。我们的工作为小硅纳米颗粒在锂离子电池中的应用提供了机械稳健性的直接证据。
Lithiation of individual silicon nanopartides was studied in real time with in situ transmission electron microscopy. A strong size dependence of fracture was discovered; that Is, there exists a critical particle diameter of similar to 150 nm, below which the particles neither cracked nor fractured upon first lithiation, and above which the particles initially formed surface cracks and then fractured due to lithiation-induced swelling. The unexpected surface cracking arose owing to the buildup of large tensile hoop stress, which reversed the initial compression, in the surface layer. The stress reversal was attributed to the unique mechanism of lithiation in crystalline Si, taking place by movement of a two-phase boundary between the inner core of pristine Si and the outer shell of amorphous Li-Si alloy. While the resulting hoop tension tended to initiate surface cracks, the small-sized nanoparticles nevertheless averted fracture. This is because the stored strain energy from electrochemical reactions was insufficient to drive crack propagation, as dictated by the interplay between the two length scales, that is, particle diameter and crack size, that control the fracture. These results are diametrically opposite to those obtained previously from single-phase modeling, which predicted only compressive hoop stress In the surface layer and thus crack initiation from the center in lithiated Si particles and wires. Our work provides direct evidence of the mechanical robustness of small Si nanoparticles for applications in lithium ion batteries.