Understanding the electrochemistry of silicon anodes in Li-ion batteries at the atomic scale [electronic resource]
Keywords: Materials Science and Engineering.

Silicon is a promising anode material for lithium-ion batteries (LIBs) due to its high specific capacity of 3579 mAh/g (at room temperature). However, the large capacity of Si is accompanied by a large volume expansion (~ 300%) which irreversibly destroys the Si crystallinity and leads to particle cracking, resulting in loss of mechanical/electrical contact and concomitant capacity fading. In addition, capacity is lost due to the consumption of Li in the uncontrolled solid electrolyte interphase (SEI) growth. These issues render the main reasons limiting large scale commercialization of high capacity Si-based batteries. To understand the (de)lithiation mechanism of Si electrodes and the SEI growth at the atomic scale, in situ synchrotron X-ray reflectivity (XRR) was used to investigate the first two (de)lithiation cycles of Si. A model battery system was utilized, consisting of native oxide terminated single crystalline Si (c-Si) (100) wafer as working electrode, Li metal as counter and reference electrode, and typical non-aqueous LIB electrolytes. I designed a novel electrochemistry cell for in situ synchrotron XRR measurements. From the XRR-derived electron density profiles, the nanoscale thickness, electron density, and roughness of the inorganic SEI layer and lithiated silicon layer, as well as other surfaces layers, were obtained. This thesis comprises four major parts. In the first part, we studied how the SEI layer and lithiated Si layer evolve during the 1st lithiation process of c-Si. We propose a three stage lithiation mechanism with a reaction limited, layer-by-layer lithiation of the c-Si at the lithiated Si/Si interface. Next, I show a detailed study on SEI formation with in situ XRR and ex situ X-ray photoelectron spectroscopy (XPS) experiments. The formation of two inorganic SEI layers was observed at different potentials -- a top-SEI layer, mainly consisting of electrolyte decomposition products including LiF, and a bottom-SEI layer formed through the lithiation of the native oxide. In the third part, the first two cycles of Si (de)lithiation are investigated. We propose that the delithiation of lithiated Si and the lithiation of amorphous Silicon (a-Si) are reaction-limited single-phase processes. Additionally, a "breathing" behavior of the inorganic SEI layer was observed. Interestingly, a low-electron-density layer was found between the SEI and lithiated Si during the delithiation process, suggesting kinetically limited Li ion transport within the SEI, which is speculated to be one of the origins of battery's internal resistance. Since the Si electrode becomes amorphous at the end of the first, and all subsequent cycles, the study on the first two cycles provide a more complete picture of the general behavior of Si anodes. In the last part, the influences of the lithiation rate, the surface orientation of Si electrodes, and SEI composition on Si lithiation and SEI growth are discussed. At the end of each chapter, the implications of my results to battery performance are discussed in a broader context. This study presents a detailed mechanistic model of the first two lithiation processes, and sheds light on fundamental difference of Li ion reaction with crystalline and amorphous materials. The results on SEI nucleation and growth, as well as ion transport properties, motivate further experimental and theoretical studies of the Li ion diffusion properties in the SEI. The results provide fresh insights to mitigate the large volume change and loss of crystallinity of Si anodes during lithiation, give us a better understanding of the lithiation mechanism, and will assist in further investigations that are expected to improve the electrochemical performance of Si-based anodes in LIBs.