School of Engineering and Applied Sciences, Harvard University , Cambridge, Massachusetts 02138, United States.
Nano Lett. 2013;13(11):5570-7. doi: 10.1021/nl403197m. Epub 2013 Oct 9.
We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration. To this end, we have constructed an electrochemical cell capable of testing multiple thin-film electrodes in parallel. The stress in the electrodes is measured during electrochemical cycling by the substrate curvature technique. The electrodes are disconnected one by one after delithiating to various states of charge, that is, to various concentrations of lithium. The electrodes are then examined by optical microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the maximum value of the stress during delithiation. The fracture energy was determined to be Γ = 8.5 ± 4.3 J/m(2) at small concentrations of lithium (Li0.7Si) and have bounds of Γ = 5.4 ± 2.2 J/m(2) to Γ = 6.9 ± 1.9 J/m(2) at larger concentrations of lithium (Li2.8Si). These values indicate that the fracture energy of lithiated silicon is similar to that of pure silicon and is essentially independent of the concentration of lithium. Thus, lithiated silicon demonstrates a unique ability to flow plastically and fracture in a brittle manner.
我们已经测量了锂化硅薄膜电极的断裂能随锂浓度的变化。为此，我们构建了一个能够同时测试多个薄膜电极的电化学池。通过基底曲率技术，在电化学循环过程中测量电极中的应力。在将电极脱锂到不同的充电状态（即不同的锂浓度）后，逐个将电极断开。然后通过光学显微镜检查电极，以确定何时首次形成裂纹。所有观察到的裂纹均表现出脆性。通过确定裂纹起始的条件，使用断裂力学分析计算断裂能。在同一组实验中，通过确定脱锂过程中最大应力来测量第二充电状态（锂浓度较小）的断裂能。在锂浓度较小（约为 Li0.7Si）时，断裂能确定为 Γ = 8.5 ± 4.3 J/m(2)，在锂浓度较大（约为 Li2.8Si）时，断裂能的范围为 Γ = 5.4 ± 2.2 J/m(2)至 Γ = 6.9 ± 1.9 J/m(2)。这些值表明，锂化硅的断裂能与纯硅相似，并且基本上与锂浓度无关。因此，锂化硅表现出独特的塑性流动和脆性断裂能力。
