Harvard Team develops theoretical model of changes of mechanical properties of silicon under lithiation
Although silicon is a promising high-energy-density material for Li-ion batteries, its lithiation leads to very large deformation and drastic changes in mechanical properties, ranging from brittleness of pure crystalline Si to an amorphous material that can sustain large inelastic deformation in the lithiated form. This behavior—typical during the electrochemical cycling of high-capacity electrodes—leads to challenges in the theoretical treatment of the material properties and in the application of the material in batteries.
A team at Harvard University has now used first-principles calculations of the atomic-scale structural and electronic properties of a model amorphous silicon (a-Si) structure to provide a detailed picture of the origin of the changes in mechanical properties of silicon under lithiation. In a paper in the ACS journal Nano Letters, they report that their theoretical model is in excellent quantitative agreement with experimental measurements of lithiation-induced stress on a Si thin film.
The large amount of absorption of Li by Si results in a large volumetric expansion and severe structural changes. The lithiation-induced stress and fracture often lead to the loss of active materials and rapid decay of capacity, which limit its commercialization. This mode of failure can be mitigated by manipulating the structural optimization and deformation patterns of nanostructured Si anodes. Examples include nanowires, thin films, hollow nanoparticles, carbon−silicon composites, and coated-hollow structures. Recent experiments and theories show evidence that the large deformation of lithiated silicon can be accommodated by inelastic flow, which may avert fracture of nanostructured silicon. To develop feasible nanostructured anodes, it is crucial to understand the lithiation, deformation, and stresses from a fundamental perspective.
Since the chemical interaction between Li and Si is local, first-principles quantum mechanical calculations can capture the microscopic mechanism of the lithiation reaction. The local chemical effects lead to the macroscopic mechanical behavior, such as flow of lithiated silicon. Lithiation and flow are both non-equilibrium processes. We extend the continuum theory of plasticity and formulate a yield condition by placing driving forces for lithiation and flow on the same footing. In situ experimental measurements of the stress evolution in an a-Si thin film during a lithiation and delithiation cycle serve both as the motivation for the theoretical work as well as a detailed quantitative test of the theory.—Zhao et al.
In their paper, they note that the lithiation of silicon provides a useful model system to study the interplay of local chemical reactions and macroscopic mechanical deformation. The essence of their results is that the chemical reaction promotes mechanical flow, enabling the material to flow at a lower level of stress.
Kejie Zhao, Georgios A. Tritsaris, Matt Pharr, Wei L. Wang, Onyekwelu Okeke, Zhigang Suo, Joost J. Vlassak, and Efthimios Kaxiras (2012) Reactive Flow in Silicon Electrodes Assisted by the Insertion of Lithium. Nano Letters doi: 10.1021/nl302261w