REAL-TIME INVESTIGATION OF INDIVIDUAL SILICON NANOSTRUCTURED ELECTRODES FOR LITHIUM-ION BATTERIES
Karki, Khim Bahadur
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Silicon-based anode materials are an attractive candidate to replace today's widely-utilized graphitic electrodes for lithium-ion batteries because of their high gravimetric energy density (3572 mAh/g vs. 372 mAh/g for carbon) and relatively low working potential (~ 0.5V vs. Li/Li+). However, their commercial realization is still far away because of the structural instabilities associated with huge volume changes of ~300% during charge-discharge cycles. Recently, it has been proposed that silicon nanowires and other related one-dimensional nanostructures could be used as lithium storage materials with greatly enhanced storage capacities over that for graphite in the next generation of lithium-ion batteries. However, the studies to date have shown that the nanomaterials, while better, are still not good enough to withstand a large number of lithiation cycles, and moreover, there is little fundamental insight into the science of the improvements or the steps remaining before widespread adoption. This dissertation seeks to understand the basic structural properties and reaction kinetics of one dimensional silicon nanomaterials, including Si-C heterostructures during electrochemical lithiation/delithiation using in-situ transmission electron microscopy (TEM). I present my work in three parts. In part I, I lay out the importance of lithium-ion batteries and silicon-based anodes, followed by experimental techniques using in-situ TEM. In part II, I present results studied on three different nanostructures: Si nanowires (SiNWs), Si-C heterostructures and Si nanotubes (SiNTs). In SiNWs, we report an unexpected two-phase transformation and anisotropic volume expansion during lithiation. We also report an electrochemically-induced weld of ~200 MPa at the Si-Si interface. Next, studies on CNT@α-Si heterostructures with uniform and beaded-string structures with chemically tailored carbon-silicon interfaces are presented. In-situ TEM studies reveal that beaded-string CNT@ α-Si structures can accommodate massive volume changes during lithiation and delithiation without appreciable mechanical failure. Finally, results on lithiation-induced volume clamping effect of SiNTs with and without functional Ni coatings are discussed. In Part III, a conclusion and a brief outlook of the future work are outlined. The findings presented in this dissertation can thus provide important new insights in the design of high performance Si electrodes, laying a foundation for next-generation lithium ion batteries.