Preparación de materiales grafíticosaplicación como ánodos en baterías de ión-litio

Resumen

Lithium-ion batteries are versatile, high performance and low cost energy storage systems. These batteries have turned into fundamental components for most of the portable electronic devices. The inherent attractive for lithium-ion batteries is the use of intercalation electrodes which can insert lithium ions in a reversible way inside their structure. Lithium-ion batteries use carbon materials, mainly, synthetic graphite as anode. The Final Objective of this Thesis is to use graphitic materials prepared from different precursors (anthracites, unburned carbon concentrates from coal combustion fly ashes and carbon nanofibers obtained in the catalytic decomposition of methane to produce hydrogen) as anodes in lithium-ion batteries. The graphitic materials were prepared by heating the precursors in the temperature interval 1800-2900 ºC. The materials were then characterized by determining their structural (degree of structural order and orientation of the graphitic domains), electrical (conductivity) and textural (surface area and porosity) properties. Subsequently, the performance of these graphitic materials as anodes in the lithium-ion batteries was evaluated by means the electrochemical parameters (reversible and irreversible capacity, ciclability and efficiency). Moreover, the relation between these parameters and the materials properties was also studied. Finally, a comparative study of electrochemical properties of the graphitic materials prepared in this work and those of synthetic graphites which are currently been employed as anodic materials for commercial lithium-ion batteries was carried out. In terms of reversible capacity, ciclability, irreversible capacity and efficiency of cycle, the battery performance by using as working electrodes the graphitic materials obtained from the unburned carbon concentrates and the carbon nanofibers, and the synthetic graphites of reference are absolutely comparable. Therefore, the application of these graphitic materials as anodes in lithium-ion batteries appears feasible. An increase of the battery reversible capacity with the structural order of the materials was observed, thus being an important factor to optimize these energy storage systems. However, for materials with a high degree of crystallinity, other no structural factors such as morphology and particle size, were also found to influence on the reversible capacity finally provided by the battery. Generally, the battery cyclability with the graphitic materials prepared is excellent; in addition, it was found to be independent of the materials structural order. After the SEI (Solid Electrolyte Interface) formation, lithium ions intercalation/deintercalation into the graphene layers occurs in almost a reversible way whenever the material porosity was below a specific value. An increase of the degree of structural order of the material lead to an improvement of the battery cycling efficiency, thus increasing the initial efficiency value and lowering the cycle number at which the efficiency reaches ~ 100 %. This effect is directly related with the decreasing of the material porosity.