Investigation of Constant Stack Pressure on Lithium-Ion Battery Performance

Abstract

Current research involving applying stack pressure to lithium-pouch cells has shown both performance and lifetime benefits. Fixtures are used to mimic this at the cell level and conventionally prescribe a constant displacement onto the cell. This increases stack pressure but also causes pressure to vary. Despite this, applying an initial stack pressure improves cell conductivity, as well as cell lifetime [1, 2]. In this paper, a fixture was designed that applies constant pressure to the cell independent of displacement. Thefixture uses pneumatics to apply a constant stack pressure independent of elastic andplastic swelling. Cells constrained by the constant pressure fixture (CPF) fixture anda conventional fixture (MBPF) were evaluated using a Hybrid Pulse Power Characterization (HPPC) test, to measure internal resistance and maximum deliverable power. Multiple stack pressures were applied to investigate differences in pressure variation and performance between constant pressure and constant displacement, while all tests were additionally compared to control tests with no applied stack pressure. The CPF reduced pressure variation during charging and discharging. revealing the performance effects of maintaining uniform pressure. Applying constant stack pressure reduced the discharge impedance and improved discharged power of the tested cell but did not improve charge performance. Additionally, all constraining methods reported lower internal resistance with maximum current compared to 50% maximum current. Discharge performance benefits from constant pressure could influence pack design to improve vehicle performance.