Modern technology, from portable electronics to electric vehicles, is becoming increasingly reliant on lithium-ion (Li-ion) batteries for energy storage. This chemistry possesses a desirable combination of high power and high energy densities and is therefore widely used, but safety is still a significant issue. The risk of thermal runaway (TR) is a major roadblock to the widespread use of this technology. TR is a self-sustaining exothermic reaction which can be triggered by mechanical or electrical damage to a cell, overheating, or by latent defects from manufacturing. The volumetric changes within a cell’s electrodes and internal gas generation can be detected by strain measurements on the surface of the casing, which can complement the electrical and thermal data used by battery management systems (BMS) and even provide insight into the state of a battery when electrical contact has been lost. This research project demonstrates the utility of strain measurements to detect abnormal Li-ion cell behavior and precursors to TR. First, a baseline was established to identify the strain response of Li-ion cells under normal operating conditions, accounting for temperature and cycling rate (or C-rate) effects. Then, the cells were cycled under abuse conditions to identify signs of damage and identify signs of TR onset through strain measurements. The final step was to develop a model which used fundamental data and electrochemical input to predict the mechanical behavior of individual electrodes and full 18650 cells. The samples used in this research were commercial 18650 format (18 mm in diameter, 65 mm tall) cylindrical cells with graphite-silicon anodes and nickel cobalt aluminum oxide (NCA) cathodes. Strain data was collected using strain gages bonded to the cell casing and was used to characterize their mechanical behavior during both normal and abuse cycling conditions. During a charge-discharge cycle at normal conditions, the surface strain was found to be nearly reversible – that is, the strain states at the beginning of charge and the end of discharge were almost the same. The strain profile of the cells was analyzed and found to be directly related to electrochemical reactions occurring within the electrodes, as evidenced by dQ/dV and dε/dV plots. The fact that the dε/dV peaks coincide with – and sometimes precede – the peaks in the dQ/dV plots shows that the electrochemical reactions occurring within the electrodes during charge and discharge can be sensed through strain measurements on the surfaces of cell casings. With the baseline established, cells were then subjected to several abuse scenarios. During the first abuse scenario cells were overcharged to failure, which came in the form of current interrupt device (CID) activation. During overcharge (past 4.2V) the cell potential was seen to increase quickly and reached a plateau at approximately 5V, shortly after which the CID activated, and the cell became electrically inaccessible (0V). The cells’ surface strain also increased dramatically during this abuse scenario, reaching a value that was more than double the peak strain during normal cycling. The CID-activated cells were then heated to TR, during which two events were identified from the strain signature as signs/precursors to TR which could be used for prediction and prevention purposes. Cells were also repeatedly overcharged to 105% and 110% nominal capacity, named 5% and 10% overcharge (OC), respectively. Maximum strain, potential, and temperature were seen to increase slowly during the 5% OC experiments, and quickly during 10% OC, during which the CID activated after an average of 11 cycles. Strain at full discharge (referred to as residual strain) reached a progressively higher value after each OC cycle and was found to closely correlate to the pressure needed to activate the CID. Electrochemical impedance spectroscopy, dQ/dV, and dε/dV analyses confirmed that the degradation modes present were mostly caused by loss of lithium inventory processes. The insights gained from stress measurement, including the ability to predict CID activation, are discussed. A finite element analysis modeling approach to predict the mechanical behavior of individual electrodes and full cells was developed. Electrochemistry was solved in COMSOL Multiphysics using a pseudo 4-dimensional (P4D) model to predict the cell potential and the state of charge of the active material within electrodes. Mechanics were coupled to electrochemistry through volumetric changes of the active material and a thermal strain analogy. The effective mechanical properties of the electrodes were calculated using the Mori-Tanaka homogenization scheme, with the development and assumptions explained fully in this work. The homogenized properties were compared to experimental and published results and were found to be in good agreement. Simulations for stress in graphite anode and nickel manganese cobalt oxide cathode were in agreement with published data. Predictions were also made for graphite-silicon anodes and NMC cathode and a geometry representative of an 18650 format battery. The limitations and future improvements for this model are discussed.
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