Measurements of the Fracture Energy of Lithiated Silicon Electrodes

5 ABSTRACT: We have measured the fracture energy of 6 lithiated silicon thin-film electrodes as a function of lithium 7 concentration. To this end, we have constructed an electro8 chemical cell capable of testing multiple thin-film electrodes in 9 parallel. The stress in the electrodes is measured during 10 electrochemical cycling by the substrate curvature technique. 11 The electrodes are disconnected one by one after delithiating 12 to various states of charge, that is, to various concentrations of 13 lithium. The electrodes are then examined by optical 14 microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the 15 condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of 16 experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the 17 maximum alue of the stress during delithiation. The fracture energy was determined to be Γ = 8.5 ± 4.3 J/m at small 18 concentrations of lithium (∼Li0.7Si) and have bounds of Γ = 5.4 ± 2.2 J/m to Γ = 6.9 ± 1.9 J/m at larger concentrations of 19 lithium (∼Li2.8Si). These values indicate that the fracture energy of lithiated silicon is similar to that of pure silicon and is 20 essentially independent of the concentration of lithium. Thus, lithiated silicon demonstrates a unique ability to flow plastically 21 and fracture in a brittle manner.