Investigação teórica do efeito de impurezas substitucionais no bulk de silício na presença de lítio

Abstract

In the context of the search for improving the performance, capacity, and efficiency of lithium-ion batteries (LIBs), the replacement of the graphite anode presents itself as a promising alternative. Silicon (Si) has shown results that indicate that it is an excellent candidate to replace graphite as the anodic material, mainly due to its specific theoretical charge storage capacity, which is about ten times the reported value for graphite. However, Si suffers from a significant volume expansion of about 300% under stress induced by lithiation. This undesirable volume change causes the presence of an unstable and cumulative solid electrolyte interface (SEI), breakage, and cracking of the electrodes, resulting in a battery with low capacity and cyclability. A second limiting factor for the use of Si is associated with its low electronic conductivity. The volume change is considered the main limiting factor for the use of Si as an anodic material and is directly linked to the emergence of Si-Li alloys in the electrode. The presence of lithium (Li) in silicon-based structures results in the breaking of Si-Si bonds and the emergence of Si-Li alloys that are weaker than crystalline Si in terms of yield strength, causing the amorphous Li-Si alloy to move away from the biphasic crystalline/amorphous interface as it starts to expand. In this work, we seek to evaluate, through first-principles calculations based on Density Functional Theory (DFT), the effects on rigidity, stability, and electronic conductivity with the introduction of substitutional impurities in the bulk Si structure in conjunction with substitutional lithium atoms. Rigidity, stability, and electronic conduction were analyzed through the information on bulk modulus, cohesive energy, band structure, and density of states, respectively. Initially, we chose substitutional sites present in the diamond bulk silicon structure and performed the single-atomic inclusion of impurities with boron (B), carbon (C), nitrogen (N), aluminum (Al), and phosphorus (P) atoms. Subsequently, we propose the insertion of pairs of the same impurity in first and second neighbor configurations. We verified that the existence of pairs of substitutional impurities, as well as their position within the structure, alters rigidity and electronic conductivity. This alteration was verified through calculations of bulk modulus and density of states. The results indicated that the inclusion of boron, carbon, and nitrogen was able to significantly enhance the bulk modulus and cohesive energy values, thereby increasing the stability and rigidity of the Si structure, unlike aluminum and phosphorus. All the impurities under analysis (B, C, N, Al, and P) were capable of improving the electronic conduction of silicon by altering its semiconductor character to a metallic character. The second step focused on the study of the combination of lithium atoms and impurities from the chosen substitutional sites. Based on bond length analyses, we proposed a relationship between the stability and rigidity of the structures according to the value of the bond length between the impurity and Li. The analyses indicated that the impurity with the highest value of bond length with Li causes a greater reduction of the bulk modulus and cohesive energy, suggesting a more significant structural change of the compound. Seeking information to corroborate the idea of structural change, we evaluated the value of the angular coefficient of the line of the graphs of cohesive energy and bulk modulus associated with the Si bulk structures with the impurities and the Li atoms in the substitutional sites through a linear fit. We believe that the structures with the angular coefficient values of the cohesive energy and bulk modulus closest to zero can more easily accommodate lithium atoms, resulting in fewer structural problems. From this perspective, the inclusion of nitrogen and phosphorus in the silicon structure proved to be favorable, as these chemical elements indicated, in our results, that they are capable of reducing the structural damage caused by Li.