The important role of lithiation-induced pressure change on apparent aging of lithium-ion batteries during accelerated cyclic aging tests

In assessing the lifespan of lithium-ion batteries in laboratory conditions, the measured capacity declines during accelerated cyclic aging tests but increases when the cells are left idle. This recoverable capacity loss, known in literature as apparent aging, results from an inhomogeneous lithium distribution in the anode appearing in dV/dQ and capacity difference analysis. This study explores the origin of the apparent aging by correlating lithium distribution in the anode and recoverable capacity loss, with the lithiation induced pressure change and generated electrolyte motion during cycling. To this end, we compressed five commercial pouch lithium-ion cells (NMC/Gr) with holder stiffnesses ranging from 0.9 to 15.4 MPa/mm and offset pressure from 289 to 681 kPa and subjected them to continuous cyclic aging tests. While offset pressure had no significant impact, we observed, that holder stiffnesses over 3.3 MPa/mm induces an apparent capacity loss, which is recoverable by 35 % during a resting phase. The root cause is the so-called electrolyte motion induced salt inhomogeneity (EMSI), driven by continuous high current cycling and electrolyte motion, which is intensified by higher holder stiffness but not by offset pressure. These insights are crucial for improving the accuracy of accelerated aging tests and enhancing battery lifespan predictions. • Capacity loss in compressed Li-ion cells during accelerated cycling is largely recoverable after rest. • Recoverable capacity arises from inhomogeneous lithium distribution in the anode in addition to anode overhang effect. • Lithium distribution inhomogeneity is driven by electrolyte pumping during charge/discharge in compressed cells. • Electrolyte pumping intensifies with graphite volume changes and housing stiffness. • Reducing holder stiffness mitigates apparent aging in accelerated cycling tests.