Reliability study of lithium ion cell and battery pack
Date of Issue2016
School of Electrical and Electronic Engineering
Lithium ion battery (LiB) is becoming the work horse for many electronics gadgets especially for portable electronics products. It is also an enabling technology for electrified transportation that can help in environmental sustainability significantly. The reliability of Lithium ion cell is of increasing concern in present-day energy storage devices where the failures of LiB not only result in severe inconvenience and enormous replacement/repair cost for the battery model/pack, but also risk catastrophic consequences such as fire hazard and possible explosion due to overheat or internal short circuiting. In order to study the reliability of LiB and prevent the failures then extend its lifetime, the modelling approach is widely employed as it is an in situ non-destructive technique (NDT) that is necessary for some applications like Battery Management System (BMS). In this work, an electrochemistry-based electrical model (ECBE) is developed for LiB cells based the first principle of electrochemistry from its discharge curve (i.e. the change in terminal voltage over time within a discharge cycle), and then convert the principle into circuit model. This model is able to compute the internal parameters of a cell, including its maximum initial capacity at the beginning of each discharge cycle. With the internal parameters computed, it can also produce its Nyquist plot and the plot is found to agree well with its experimental electrochemical impedance spectroscopy (EIS) spectra. Moreover, ECBE allows the performance of each component inside LiB be determined real time through its discharging curve non-destructively (i.e., terminal voltage vs. time during discharge), making it is capable of comprehensively identify the various aging causes through in-situ real time characterization. As ECBE is derived based on the first principles, it can be applied to other cell systems. To obtain the parameters values of the circuit elements using a non-linear regression method, Levenberg-Marquart fitting Algorithm (LMA) cum Simulated Annealing (SA). SA is employed to approximate global minima, and LMA is used to delivers rapid and accurate estimates of the local minima due to gradient-descent based algorithm. With these algorithms, the fitting can agree well with the experimental results in around 0.3 seconds computed by i7 8-core CPU. Through this ECBE model, the status of a LiB cell and its maximum charge capacity can be determined in real time. With the computed maximum initial capacity at the beginning of each discharge cycle, State of charge (SoC) have been estimated online without the periodic discharge of the cell fully which can introduce damage to the cell and shorten its lifespan. Furthermore, its value can also be used as an indication of state-of-health (SoH) of a battery. The study of Quality Decision for Overcharged Li Ion Battery from reliability and safety perspective has been conducted. The results show both positive electrode and the total of electrodes resistance and electrode/electrolyte resistance are degraded when COV is excessive. However, the performance of the graphite electrode is insignificant against excessive COV. Although we enjoy the seemingly longer runtime with higher COV, the cell reliability is degrading. Therefore, this method can evaluate if the damage made in the cell by the excessive COV is rendering the cell from further safe usage or it is still acceptable with minor degradation in reliability and safety, thus providing a basis for quality consideration of the cell. Moreover, it also enables battery manufacturers to identify the internal components for their cells that are most vulnerable to the excessive COV so that quality improvement of their batteries can be designed and produced. Furthermore, it also alerts electric vehicles user on the hidden safety issues of their battery pack, and enables BMS to perform reliability balancing, a new patented technique to ensure the safe and reliable operation of battery pack. The effects of temperature on the electrochemistry in LiB and aging rate of LiB have been carried out. The findings allow us to have a better understanding of the effect of temperature and it also reveals phase transformation of the anode when LiB is operating beyond 45 oC. The increasing degradation rate of the maximum charge storage of LiB during cycling at elevated temperature is found to relate mainly to the degradations at the electrodes, and that the degradation of LCO cathode is larger than graphite anode at elevated temperature. In particular, the formation and modification of the surface films on the electrodes as well as structural/phase changes of the LCO electrode, as reported in the literatures, are found to be the main contributors to the increasing degradation rate of the maximum charge storage of LiB with temperature for specific operating temperature range. Larger increases in the Warburg element and cell impedance are also found with cycling at higher temperature, but they do not seriously affect the state of health (SoH) of LiB as shown in this work. A Reliability-based Design Concept for Lithium-ion Battery Pack in Electric Vehicles has been presented. In this work, a method on the design and analysis of lithium-ion (Li-ion) battery pack from the reliability perspective is proposed. The analysis is based on the degradation of the battery pack, which is related to the cells configuration in the battery pack and the state of health (SoH) of all the Lithium ion cells in the pack. Universal Generating Function (UGF) technique is used for reliability analysis. As adding new battery cells to the battery pack in the production process can improve its reliability but it also increases cost, tradeoff between the numbers of the redundant battery cells, the configuration of the redundant cells and their reliability is investigated in this work. From the simulation, we conclude that the reliability could be improved by adding redundant cells as expected, and the configuration of the redundant cells has significant effect on its reliability. The proposed design concept provides a way to select the best redundant cells configuration for good pack reliability, while considering the total cost through the optimal number of the redundant cells. The thesis details the development of the proposed ECBE model and a method on the design and analysis of lithium-ion (Li-ion) battery pack from the reliability perspective, the Quality Decision for Overcharged Li Ion Battery from reliability and safety perspective, the effect of temperature on the electrochemistry in LiB, the effect of temperature on aging rate of LiB.