Modelling, Aging and Optimal Operation of Lithium-ion Batteries

Energy storage has a big role to play in power systems across the world in order to integrate increasing amounts of intermittent renewable sources of energy. Among the different storage technologies, lithium-ion batteries exhibit favourable characteristics that make them suitable for power system applications. However, commercial success of lithium-ion battery based storage is limited not only for grid applications but also for electric vehicles. This is due to three inter-related factors - safety, price-performance ratio and lifetime, which largely offset the advantages that these batteries offer and impede their adoption for potential applications. Any improvement in these factors is tied to better understanding of the functioning and of the limits of these batteries. This work is an attempt to further this understanding using modelling and experimental means, such that the behaviour of these batteries can be predicted over their lifetime, and their operation can be optimized. The contributions of this thesis are three-fold, as described in the following paragraphs. A battery model that is not only able to accurately estimate the electrochemical but also the thermal behaviour of a lithium ion battery is important in order to keep track of performance and safety indices. To this end, a physics based pseudo 2D electrochemical-thermal model of a lithium iron phosphate battery is developed. Parameters for this model are determined through primary information from manufacturer, literature studies and experimental data analysis. The developed model accurately predicts the electrochemical and thermal behaviour of the battery for both charging and discharging conditions for a wide range of current rates. Heat generation in the cell is investigated using the validated model and the important role of reversible heat and the dominant role of graphite electrode is highlighted. The model is extended to determine thermal behaviour of module, pack and study different thermal management systems. Given that battery performance degrades over time, long-term accelerated aging tests are used to quantify calendar and cycle aging in commercial lithium nickel manganese cobalt oxide batteries. Capacity and impedance measurements, electrochemical impedance spectroscopy as well as post-mortem analysis are used to study aging. Calendar aging is analysed as a function of temperature and storage state of charge. In general, low temperatures and low states of charge cause less degradation in the battery. Considerable influence of the periodic characterization process on the calendar aging results is noticed. Cycle aging is analysed as a function of temperature, current rate, depth of discharge and state of charge. In general, fast aging in batteries is observed when they are operated at low temperatures, high current rates and around high states of charge. The effects of local potentials at the two electrodes and staging behaviour of graphite in causing capacity fade and increase in the resistances of the cell are elucidated. Finally, the integration of storage in power systems is investigated. Technically, lithium-ion batteries are found to be suitable for a variety of applications in power systems both at utility scale as well as for home storage. Their economic feasibility is however debatable and dependent on local market conditions. To optimally use batteries in power system applications, an accurate degradation model that takes into account the complex, non-linear dependence of battery aging on operating parameters is developed. A mixed integer linear program is formulated, which produces an optimal charge-discharge schedule for the energy storage when trading in electricity markets. This program optimizes the operation of battery systems considering the twin objectives of maximizing revenue from market transactions and minimizing degradation. Such a multi-objective approach yields a Pareto-front of feasible operating strategies putting onus on a decision maker to choose a desirable operational strategy for implementation.