High power li-Ion battery performancea mechanistic analysis of aging

Resumo

Nowadays, lithium ion batteries (LIBs) are omnipresent on a worldwide level. As a result of their high energy density, high power density and increasing cycling life, LIBs are a well-founded technology for its use on portable electronic devices, electric vehicles (EVs) and electrical energy storage (EES) systems. The extensive use of LIBs accentuates the fact that these power sources are currently playing a crucial role in the commercial and industrial applications. As a result of the rapid growth and substantial interest devoted to LIBs, an increasing number of opportunities in the industrial and research LIB fields are emerging. Fast charging is gaining momentum in order to meet the demands of high power LIB systems. The ability of a LIB to perform fast charging depends on various factors, including cell active materials, and the design of a reliable fast charging protocol. To date, efficient fast charging protocol and procedures to achieve rapid charges remain lacking. In addition, fast charging/high power duty cycling may affect the battery¿s performance due to accelerated aging. The effects of battery aging are detrimental to LIB systems, reducing its performance and even causing safety issues. In fact, the aging phenomenon is a key issue in durable applications such as EVs and EES, where long-term cycling and long service life are demanded. Hence, it becomes essential to evaluate the performance and the aging mechanisms ongoing on battery systems. This thesis is focused towards the key field of high power applications of commercial LIBs and the study of the performance and identification of battery aging mechanisms through in situ techniques. A broad analysis of the performance and the degradation mechanisms that various cycling schemes (i.e. standard, fast charging and dynamic stress) cause on commercial high power LiFePO 4 batteries is reported in this work. In addition, an efficient, multistage fast charging protocol is developed and analyzed. In order to identify both qualitatively and quantitatively the aging mechanisms ongoing on the tested batteries, we use in situ analyses, including incremental capacity (IC), peak area (PA), and mechanistic model simulations using the `Alawa toolbox. An innovative approach to attain accurate simulation results is carried out by feeding the toolbox with harvested half-cell data from the actual cells. The results show that the developed fast-charge protocol is efficient, completes full charges within ~22 min and does not cause additional degradation in comparison with standard cycling. The end-of-life both for standard and fast charge cycling schemes surpassed 4500 cycles. The quantitative degradation measured for both cycling schemes is equal, caused by a linear loss of lithium inventory (LLI), coupled with a less degree of linear loss of active material (LAM) on the negative electrode (NE). The dynamic stress cycling caused rapid cell degradation, reaching 1100 cycles at end-of-life. The degradation is caused by large LAM on the NE, and in less degree, LLI. Moreover, the cell exhibited Li plating, which was identified and quantitatively measured using the in situtechniques. To finish the study, prognosis analyses on the fast charging scheme are carried out and we found that the use of fast charging protocol does not risk Li plating after all. In total, this thesis investigates the performance and aging mechanisms in commercial high power LiFePO4 batteries. The in situ, online techniques used in this work provide a systematic methodology to attain battery degradation identification, both qualitatively and quantitatively. The use of these techniques could be quite enlightening for the development of diagnosis and prognosis models for battery management system (BMS) applications to effectively manage and control LIB systems. Finally, the combination of proper cell chemistry and architecture, coupled with the use of an efficient fast charging protocol is key to achieving optimal fast charging in high power LIB applications.