Determination of the state of health of li-ion batteries: the irreversible entropy production approach
- Autores: Victòria Júlia Ovejas Benedicto
- Directores de la Tesis: Angel Cuadras Tomas (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Olfa Kanoun (presid.), Marcos Quílez Figuerola (secret.), Igor Villarreal Sarria (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Electrónica por la Universidad de las Illes Balears y la Universidad Politécnica de Catalunya
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
In recent years, portable applications have experienced an exponential growth and consequently, the demand of batteries has increased accordingly. It is widely known, though, that the performance of batteries decreases with time and use. This loss of performance is easured by the State-of-Health (SoH) of the cells. However, there is no consensus in defining this parameter. Experimental, theoretical or even heuristic approaches can be found in literature and commercial systems, but usually, they only work for particular conditions and they are not linked to the degradation suffered by the cells themselves.
The aim of this study is to find a parameter directly related to this degradation. For this purpose, we investigate the irreversible entropy production in Li-ion cells because irreversible entropy is related to energy dissipation and thus, to irrversibilities due to system or energy degradation.
In order to evaluate the degradation of the cells and its correspondence to irreversible entropy generation, we studied different Li-ion chemistries (NMC, LFP and LCO). Batteries were cycled at different discharge rates (close to and far from equilibrium) and evaluated at different SoHs. Therefore, capacity fade and impedance rise (the most commonly used techniques in SoH determination) were characterized and related to irreversible entropy generation. In addition, post-mortem analysis was carried out to achieve a deeper knowledge of the causes and effects of degradation.
As a result of this study, we introduced a new parameter for system degradation characterization, the Relative-Entropy-Production (REP), defined as the irreversible entropy generation ratio at actual state and the initial state. In particular, we found irreversible entropy production evaluated at low discharge rates was higher as more degraded were the NMC cells. In the case of LFP cells, irreversible entropy production decreased during initial cycles but then increased towards the EoL. This behavior coincided with a capacity increase during initial cycles. In addition, we found a relationship between irreversible entropy generation and the phase transformations taking place during the discharge processes in all the evaluated cells because the materials undergoing phase transformations expand and contract yielding to cracks and other structural.
Irreversible entropy production is found to be a promising magnitude to characterize battery aging. Even though much research has still to be carried out, the idea is to define, in the future, a threshold in irreversible entropy production that the cells can stand before considering their EoL is reached.
