Repository of DUTH

Fabrication and characterization of nano-structures to be used as anodes for lithium-ion batteries (Doctoral thesis)

Αργυρόπουλος, Δημήτριος Παναγιώτης/ Argyropoulos, Dimitrios Panagiotis

Silicon nanoparticles are used to enhance the anode specific capacity for the lithium-ion cell technology. Due to silicon’s mechanical deficiencies during lithiation and delithiation, one of the many strategies that have been proposed consists of enwrapping the silicon nanoparticles with graphene and create a void area between them so as to accommodate the large volume changes that occur in the silicon nanoparticle. This work aims to investigate the electrochemical performance and the associated kinetics of various nanostructured silicon particles. To this end, we prepared silicon nanoparticles (nps) encapsulated by a graphene veil through a ball milling process. Moreover, we created silicon nps with a hollow outer layer enwrapped with graphene with two different void volumes by using thermally grown silicon dioxide as a sacrificial layer, ball milling to enwrap silicon particles with graphene and hydro fluorine (HF) to etch the sacrificial SiO2 layer. In addition, in order to offer a wider vision on the electrochemical behavior of the hollow outer shell Si nps, we also prepared all possible in between process stages of nps and corresponding electrodes (i.e. bare Si nps, bare Si nps enwrapped with graphene, Si/SiO2 nps and Si/SiO2 nps enwrapped with graphene). The morphology of all particles revealed the existence of graphene encapsulation, void, and a residual layer of silicon dioxide depending on the process of each nanoparticle. Corresponding electrodes were prepared and studied in half cell configurations by means of galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy. It was observed that nanoparticles encapsulated with graphene demonstrated high specific capacity but limited cycle life. In contrast, nanoparticles with void and/or SiO2 were able to deliver improved cycle life. It is suggested that the existence of void and/or residual SiO2 layer limits the formation of rich LiXSi alloys in the core silicon nanoparticle. providing higher mechanical stability during lithiation and delithiation processes.