Estudo de propriedades estruturais e mecânicas de cristais de silício dopados com carbonos via simulações atomísticas

Abstract

Over the last few years, silicon has been considered as a possible replacement for the graphite present in the anodes of lithium-ion batteries, demonstrating excellent capacity and energy efficiency. However, its implementation becomes a difficult task due to the expansion in the crystal volume caused by lithiation in the battery charging and discharging process. This process generates high mechanical stress on the structure, preventing its reuse, making it unsustainable for a battery. In a recent study, it was proposed to dope silicon crystals with aluminum, boron, carbon, phosphorus and nitrogen atoms in a substitutional way in order to increase the mechanical resistance of the silicon crystal. To this end, the behavior of cohesion energy and bulk modulus was studied through simulations with the DFT method. Within this study, doping with carbon and nitrogen atoms were responsible for the best results. In another study, it was observed that the [110] direction of the silicon crystal was responsible for the rapid diffusion of lithium ions into the structure, leading to its volumetric expansion. Therefore, in addition to studying the resistance to volume variation, careful observation is necessary regarding the linear expansion of crystals with substitutional dopants. Our work aims to expand the study of silicon crystals with substitutional carbon dopants, using a methodology that allows us to include a greater number of particles, without generating a high computational cost. To this end, we validate the use of the DFTB method through comparisons with DFT and experimental results present in the literature for the silicon cubic unit cell with and without carbon substitutional dopants. The comparison was made in relation to structural and mechanical properties such as cohesion energy and bulk modulus. With the increase in particle number, we developed and characterized new rectangular unit cells to study the mechanical strength of silicon crystal in non-trivial directions. We show the validity of its use by comparing its mechanical properties with the results obtained for the silicon cubic unit cell. Finally, using the new unit cells, we calculated the Young’s modulus for different directions of the pure silicon crystal and with doping carbons.