Investigation of constant stack pressure on lithium-ion battery performance

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 and cell lifetime (Mussa et al., 2018, Zhou et al., 2020, Müller et al., 2019, Li et al., 2022, and Cannarella and Arnold, 2014). In this work, a fixture was designed that applies constant pressure to the cell independent of displacement. The fixture uses pneumatics to apply a constant stack pressure independent of elastic and plastic swelling. Cells constrained by the constant pressure fixture and a conventional displacement based fixture were evaluated using a Hybrid Pulse Power Characterisation (HPPC) test to measure internal resistance and maximum deliverable power. Multiple stack pressures were applied to investigate the variance in pressure over operational conditions and performance between constant pressure and constant displacement based methods. All tests were further compared to a control case with no applied stack pressure. The constant pressure based method reduced pressure variation during charging and discharging, reduced the discharge impedance and improved discharged power, but did not improve charge performance. Discharge performance benefits from constant pressure could influence pack design to improve vehicle performance.