HIGH POWER LITHIUM ION BATTERY CHARACTERIZATION

Abstract

Lithium-ion batteries used for motive transport applications need to reliably charge at ultra high charge rates below 10 minutes. Charge performance testing data mapped across the usable range of temperature and charge rate is required to make informed decisions in multiple battery system developmental phases including cell selection, battery management system parameterization, thermal management system design, and charge profile selection. Ultra fast full charging requires new cell characterization techniques due to the high heat generation. This thesis develops and demonstrates techniques used for parameterizing high-power lithium-ion battery cells in 21-70 cell format. Molicel P45B cells are dissected and geometries of internal components such as electrode, current collector, and tab geometry are measured to confirm high power cell design. Electrode chemical composition of active materials was analysed using inductively coupled plasma, computed tomography, and scanning electron microscopy techniques. Cells are cycled at high charge rate in traditional air chamber and in isothermal conditions to identify differences in parameterization results. A T-type thermocouple was inserted and sealed into the center of the cell core which provided direct measurement of inner jellyroll wall. Cell core to wall temperature gradients are evaluated in both conditions. An 8 ˚C core to wall temperature difference was measured during 20-minute fast charge. A novel cell holder and high-rate cell cycler are used to cycle cells in a range of isothermal wall temperatures and rates to parameterize properties and quantify operational trade-offs. The affect of applying temperature profiles during cycling is investigated using multiple profiles and showed that elevated constant voltage stage charge temperatures reduced charge time significantly. Relationships quantifying key cell cycle metrics with respect to the temperatures range of 25 to 55 ˚C and charge rates from 1 hour to 5 minutes were experimentally quantified. The techniques developed in this study support future experimental performance comparisons of degradation response to different electrical and thermal conditioning profiles.