Dottorato in INGEGNERIA ELETTRICA, DEI MATERIALI E DELLE
NANOTECNOLOGIE

Titolo della tesi: Synthesis and Characterization of Nanostructured Silicon-Based Anodes for Advanced Li-ion Batteries

Abstract This thesis describes different growth technique for the synthesis of Si-based anode for high capacity Li-ion batteries. The performed experimental work is described in individual chapters, organized as research articles, with an introductory summary at the beginning of each. Li-alloying materials such as silicon are an attractive alternative to the conventional graphite-based anode who has the potential to store a larger number of Li+ ions. Si full lithiated Li22Si5 alloy (4.4 Li + every atom of Si) has the highest theoretical specific capacity (≈ 4200 mA h g-1) ten times higher than the commercial graphite (372 mA h g-1). However, the Li+ ions insertion into the Si structure leads to a large volume expansion (≈ 300%) which induce a strong mechanical stress on the structure itself. The cyclic repetition of the charge/discharge process can cause a crack of the structure that reflects to a rapid decline of the performances. Nanostructured material, in particular nanowires has inbuilt mechanical strength tolerate the volume expansion in contrast to the bulk and micrometric materials. Nanowires can be grown successfully by the chemical vapor deposition (CVD) using an alternative and cheaper copper (Cu) catalyst compared to gold (Au) according to the vapor solid solid mechanism. In Chapter 3, the growth of a core-shell type of nanowire consisting in a crystalline silicon core surrounded by an amorphous silicon layer is reported and discussed. The electrochemical tests performed demonstrate a good electrical contact between the nanowires and the current collector along with a high initial capacity (≈ 2500 mAh g-1). However, many issues related to the SEI formation raise due to the high irreversible capacity in the first cycle and the rapid capacity fading. Further in Chapter 4, the electrochemical strategy consists of a slow first cycle (C/40) was investigated in order to form a more stable SEI. The high initial efficiency (≈ 95%) and the subsequent approaching to 100% confirmed a good SEI formation. For reducing the capacity lost due to the weight of the inactive part of the cell, the stainless steel (≈ 400 mg cm-2) substrate was replaced by a lighter carbon paper substrate (≈ 10 mg cm-2). The highly porous surface of the carbon substrate allows to achieve remarkable loadings of active material (2-5 mg/cm2) and consequently, improved capacity densities. The CVD growth parameters can be varied and two different morphologies (a) straight wire and, (b) “coral-like” structure were obtained. The ex-situ microscopy analysis showed the formation during the growth of an amorphous Si layer between the base of the nanowires and the substrate (about 2µm thick) which can be responsible for the capacity fading of the cell within 50th cycles. According to this assumption, three different growth parameters combinations were interrogated in Chapter 5 which leads to the successful reduction of a-Si layer (approx. 0.15 µm). The capacity fading was also lowered and the cells showed a very high specific capacity (≈3500 mAh g-1) stables for 40 cycles. The carbon paper substrate demonstrated ≈ 30% of contribution to the total capacity of the cell. The CVD growth technique found to be suitable for growing different morphologies of Si based nano structure with a high control of the growth parameters. However, the CVD growth system has an intrinsic limitation in terms of scalability, due to the costly equipment and many safety issues related to the hazardous gaseous precursors used. In Chapter 6, a cheap, fast and scalable synthesis of Si NWs solvent-free vapor growth, a solution-based method is discussed and investigated. The growth was performed with two different catalyst (Cu and Sn) without using any solvent or reducing agent. For both the catalysts, a dense silicon nanowire network with a high penetration depth inside the fibers of the carbon paper substrate was obtained. High loading was achieved (up to 1.5 mg cm-2) with a small quantity of precursor (0.2ml of phenilsylane). The preliminary electrochemical tests showed good gravimetric and areal capacity for both the metal seeds.