An Experimental Correlation of Degradation with Cell Reversible and Irreversible Expansion Measurement in Pouch Cells

Abstract

Lithium-ion batteries are used widely in portable devices and play an increasingly significant role in transportation and grid storage. These batteries degrade when they undergo charging and discharging and also when they are stored (calendar aging). The rate of this degradation depends on how the batteries are used and under what conditions. As the battery is cycled, its internal impedance and individual electrode capacities change, causing a change in its electrical performance. In addition, changes in the reversible and irreversible expansion of the cell are also observed. Of particular concern for pack design is the continued growth of irreversible battery thickness, thus affecting the stresses in the battery pack which is typically constrained to occupy a fixed volume. Understanding these dimensional changes will play an important role in accommodating this expansion over life for pack design.Due to the aforementioned dimensional changes, the stresses that the battery faces at the beginning of life are different from the stress at the end of life when battery expansion is constrained due to packaging. To understand the impact of external stress/pressure on battery aging, we have designed a specialized spring loaded fixture which allows for operating the battery under relatively constant pressure, over the entire cell life, while simultaneously measuring the cell expansion. Battery thickness changes were measured for 82 identical pouch cells fabricated at the UofM Battery Laboratory using Targray NMC 622 Single Crystal cathode powder and Superior SLC 1520-T anode powder. The cells were loaded into the fixtures with 4 different initial pressures of 5 psi (standard beginning of life pressure), 15 psi, 25 psi (expected end of life pressure) and without any applied pressure. A baseline cycling protocol was established which included a modest amount of fast charging and cells were divided into three different thermal chambers to simulate the impacts of environmental temperature (room 25C, cold 0C, and hot 45C). Finally the impact of depth of discharge was also included in the test matrix. Three cells were assigned to each test condition to account for cell to cell variability. Instrumenting all the cells with laboratory grade LVDT expansion sensors would be prohibitively expensive [1]. To overcome the issue we used low-cost Inductive Displacement Sensors which provide inexpensive and high-resolution measurements of battery expansion under various load conditions. Further details on the sensor design and implementation are given in [2].