Caracterización eléctrica y electroquímica de celdas de litio-ion

Abstract

Tesis para optar al título de Magister en Ciencia y Tecnología de Materiales

La presente tesis está dedicada al estudio del comportamiento y desempeño eléctrico y electroquímico de celdas de Li-ion. Principalmente se muestran en forma comparativa los resultados obtenidos en diferentes modelos comerciales, tanto del tipo prismático como cilíndrico. El objetivo principal ha sido contribuir a la elucidación de los procesos que caracterizan las cargas y descargas de las celdas de Li-ion. Una parte importante del trabajo está centrado en el desarrollo de sistemas de medición confiables para ser utilizados en el futuro en estudios más profundos con este tipo de celdas o celdas de otras tecnologías. Los estudios se llevaron a cabo utilizando técnicas eléctricas de tipo estacionarias y de modulación (impedancia compleja), para cuyo fin se desarrollaron nuevos códigos de cálculo que permiten un mejor análisis de los resultados. En la primera parte de este trabajo de tesis se realizó una búsqueda bibliográfica sobre celdas electroquímicas; su clasificación, diferencias y ventajas de cada una, centrada principalmente en la tecnología de Li-ion y de celdas de combustible. Actualmente, éstas son las tecnologías hacia las cuales se orientan los estudios y desarrollos llevados a cabo en nuestro laboratorio. Se planteó un nuevo modelo de circuito equivalente para describir los procesos de transferencia de carga y de transporte dentro de las celdas de Li-ion y se desarrollaron nuevos programas de ajuste de variable compleja más eficientes y robustos. Los resultados obtenidos de la impedancia compleja permiten, de forma rápida y segura, clasificar las celdas según su resistencia interna, demostrándose que la impedancia compleja debe usarse para el estudio del envejecimiento de celdas. Además, se demostró la importancia que tiene el cálculo de la capacidad diferencial, ya que los picos de capacidad diferencial característicos que aparecen en función del potencial, son propios de la tecnología y su variación relativa junto con los aumentos irreversibles de la resistencia interna permitirían dar una idea cuantitativa de la degradación de la capacidad global de la celda.|The present thesis is dedicated to studying the behaviour and electrical performance of Li-ion cells. Results obtained are presented in a way that allows the comparison between different commercial models, both prismatic and cylindrical. The principle aim was to contribute to elucidate the processes that are characteristic of charge and discharge of Li-ion cells. An important part of the work was the development of reliable measurement systems to be used in the future when performing more detailed studies using this type of cell or other technologies cells. Tests were performed making use of two electrical techniques, stationary and modulated (complex impedance). New calculation codes were developed specifically for this work, codes that lead to a better analysis of the results. The first part of this work involved bibliographical research on electrochemical cells, their classification, differences between them, advantages and disadvantages of using each type. The technology focused on Li-ion and fuel cells, since these are the technologies we are heading to in studies and development performed in our laboratory. Great care was taken to design a new model of equivalent circuit that described the processes of charge transfer and transport in the Li-ion cell. New and more robust and efficient software for the fitting of equations using complex variable was also developed. Results obtained from complex impedance allow a quick and reliable classification of the cells according to their internal resistance, which demonstrates that complex impedance is to be used imperatively in the study of aging of cells. What appears to be understood is the particular importance of studying differential capacity. Characteristic peaks that are seen as a function of potential depend on the technology used. It is thought that its relative variation together with the irreversible increments of the internal resistance, could give a quantitative idea of the degradation of the global capacity of the cell.

The present thesis is dedicated to studying the behaviour and electrical performance of Li-ion cells. Results obtained are presented in a way that allows the comparison between different commercial models, both prismatic and cylindrical. The principle aim was to contribute to elucidate the processes that are characteristic of charge and discharge of Li-ion cells. An important part of the work was the development of reliable measurement systems to be used in the future when performing more detailed studies using this type of cell or other technologies cells. Tests were performed making use of two electrical techniques, stationary and modulated (complex impedance). New calculation codes were developed specifically for this work, codes that lead to a better analysis of the results. The first part of this work involved bibliographical research on electrochemical cells, their classification, differences between them, advantages and disadvantages of using each type. The technology focused on Li-ion and fuel cells, since these are the technologies we are heading to in studies and development performed in our laboratory. Great care was taken to design a new model of equivalent circuit that described the processes of charge transfer and transport in the Li-ion cell. New and more robust and efficient software for the fitting of equations using complex variable was also developed. Results obtained from complex impedance allow a quick and reliable classification of the cells according to their internal resistance, which demonstrates that complex impedance is to be used imperatively in the study of aging of cells. What appears to be understood is the particular importance of studying differential capacity. Characteristic peaks that are seen as a function of potential depend on the technology used. It is thought that its relative variation together with the irreversible increments of the internal resistance, could give a quantitative idea of the degradation of the global capacity of the cell.