Silicon Nanoparticles: Size and Morphology Effects in Lithium Ion Batteries

Silicon in a form of nanoparticles has attracted a significant interest in the field of lithium-ion batteries, due to the enormous capability of lithium intake. While attracting substantial attention for next-generation lithium-ion batteries, the large volume changes due to lithiation/delithiation during charge/discharge significantly restrict wide application of those materials. To counteract the high-volume expansion associated with lithiation, silicon nanoparticles emerged as materials engineered to extend cycle life. However, correlating particle morphology, size and structure with performance in and lifetime of battery is still needed to make use of the diverse approaches toward predictable tunability of nanoparticle properties. For instance, various sources and various pathways for nanoparticles preparation of silicon lead to different battery performances resulting in large discrepancy of the results. Herein, we examine silicon nanoparticles prepared through pyrolysis of silane gas, compare them to conventional crystalline samples and correlate their morphological characteristics with the battery performance and cycle life. We demonstrate that temperature and silane concentration during synthesis influence the size and morphology of the silicon nanoparticles. For instance, relatively low temperatures typically produce a mixture of small and large round particles with smooth surfaces while relatively high temperatures produce particles with rougher surfaces with a narrower size distribution. These differences are correlated to crack formation and propagation in the electrode during formation cycles, as evidenced by post-mortem analysis. Different degradation pathways of the battery anodes resulting in different rates of capacity fading are demonstrated depending on the starting nanoparticle structure. In addition, in the present work we demonstrate the characterization of silicon nanoparticles with small angle neutron scattering (SANS) as a complementary method to conventional approaches for characterization such as microscopy, which allows the analysis of nanoparticles as an ensemble.