Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.
Proc Natl Acad Sci U S A. 2012 Mar 13;109(11):4080-5. doi: 10.1073/pnas.1201088109. Epub 2012 Feb 27.
From surface hardening of steels to doping of semiconductors, atom insertion in solids plays an important role in modifying chemical, physical, and electronic properties of materials for a variety of applications. High densities of atomic insertion in a solid can result in dramatic structural transformations and associated changes in mechanical behavior: This is particularly evident during electrochemical cycling of novel battery electrodes, such as alloying anodes, conversion oxides, and sulfur and oxygen cathodes. Silicon, which undergoes 400% volume expansion when alloying with lithium, is an extreme case and represents an excellent model system for study. Here, we show that fracture locations are highly anisotropic for lithiation of crystalline Si nanopillars and that fracture is strongly correlated with previously discovered anisotropic expansion. Contrary to earlier theoretical models based on diffusion-induced stresses where fracture is predicted to occur in the core of the pillars during lithiation, the observed cracks are present only in the amorphous lithiated shell. We also show that the critical fracture size is between about 240 and 360 nm and that it depends on the electrochemical reaction rate.
从钢的表面硬化到半导体的掺杂，原子在固体中的插入在改变材料的化学、物理和电子性质方面发挥着重要作用，适用于各种应用。在固体中高密度的原子插入可导致显著的结构转变和相关的机械行为变化：这在新型电池电极（如合金阳极、转化氧化物以及硫和氧阴极）的电化学循环过程中尤为明显。当与锂合金化时，硅的体积膨胀达到 400%，是一个极端情况，是研究的理想模型系统。在这里，我们表明，在结晶硅纳米柱的锂化过程中，断裂位置具有高度各向异性，并且断裂与先前发现的各向异性膨胀密切相关。与基于扩散诱导应力的早期理论模型相反，在锂化过程中预测断裂会发生在柱子的核心，观察到的裂缝仅存在于非晶态锂化壳中。我们还表明，临界断裂尺寸在大约 240 到 360nm 之间，并且取决于电化学反应速率。
