This dissertation studies the effects that non-uniform temperature distributions have on the electrochemical behavior of Lithium-ion (Li-ion) batteries. The poor thermal conductivity of Li-ion cells leads to non-uniform internal temperatures. Temperature strongly effects the internal resistance, lifetime, and safety of a Li-ion cell, and therefore the broad question was raised, "What are the effects that non-uniform temperature has on Li-ion cells?" The aim of this dissertation is to bring light to this question. Two experimental systems were developed to apply controlled non-uniform temperature distributions on single and multi-cell groups. First, controlled non-uniform temperatures were applied to a single large-format 10Ah Li-ion pouch cell. Ten-second constant current pulse responses were compared to full-capacity dynamic power discharges. Secondly, five parallel-connected Li-ion cells were tested under non-uniform temperature conditions. The current distribution dynamics, and resulting non-uniform distributions in State-of-Charge, Open-Circuit-Voltage, and energy among the cells was studied. Lastly, a distributed physics-based thermal electrochemical model was developed to numerically predict the distributed behavior in Li-ion cells operated under non-uniform temperature conditions. A model parameterization process is described, the model was validated against the experimental data, and a numerical investigation was performed. Linear temperature distributions effectively reduce the parallel-grouped internal resistance for short-time current pulses, due to the exponential temperature sensitivity of Li-ion cell resistance. However, charge-depleting conditions cause the development of non-uniform State-of-Charge across the cell(s), which serves to reduce the effective energy output. The critical factors dictating the tolerance to non-uniform temperature for Li-ion cells were outlined as (i) the resistance versus temperature sensitivity, and (ii) the gradient of the Open-Circuit-Voltage versus State-of-Charge. Both factors are dependent on the electrochemical characteristics of the cell and this was illustrated by empirically studying two distinctly different Li-ion cell chemistries; LiFePO4/C6 and LiNi0.5Mn0.3Co0.2O2/C6. Finally, the modeling results were analyzed to show how the development of the non-uniform Open-Circuit-Voltage creates an effective resistance component. The effective resistance term minimizes the elevated resistance of the colder regions of the cell(s), but increases the resistance of the warmer regions. This results in minimizing the impact that non-uniform temperature has on the cell performance for the charge-depleting conditions.
|Advisor:||Park, Jae Wan|
|Commitee:||Erickson, Paul A., Moura, Scott J.|
|School:||University of California, Davis|
|Department:||Mechanical and Aeronautical Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 79/08(E), Dissertation Abstracts International|
|Subjects:||Chemical engineering, Mechanical engineering|
|Keywords:||Large format, Lithium-ion cells, Non-uniform current distribution, Non-uniform temperature, Performance effects|
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