Differentiating the effects of pressure in NMC-graphite lithium-ion batteries on cell and system level

This study investigates the influence of external pressure on the aging behavior of automotive lithium-ion pouch cells with a reference capacity of 76.6 Ah and NMC- graphite chemistry, and its transferability to the module level. Over a period of 1.5 years, four long-term ageing tests with more than 2000 cycles were conducted under constant-force conditions ranging from 0.14 to 0.44 MPa, until the cells reached a capacity-based state of health (SOHc) of 60% These cell-level studies were complemented by five module tests with up to 2500 cycles, as well as mechanical compression experiments to characterize stiffness and pressure-dependent behavior. Electrochemical diagnostics, including capacity retention, internal resistance, and Differential Voltage Analysis (DVA), were combined with Differential Strain Analysis (DSA) to provide mechanical insight. Post-mortem investigations by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed to identify local degradation mechanisms. The results demonstrate a strong dependence of aging on mechanical boundary conditions. At pressures above 0.16 MPa, resistance growth was significantly reduced compared to low-pressure conditions. Moreover, a partially reversible aging component was identified: increasing pressure from 0.16 to 0.28 MPa decreased internal resistance by approximately 40%, attributed to the displacement of gas and restoration of particle contacts. Compression tests revealed a critical transition around 0.23 MPa from a gas-cushioned to a gas-displaced mechanical regime, consistent with the observed electrical reversibility. DSA proved to be a sensitive diagnostic tool for distinguishing reversible and irreversible aging effects, whereas DVA showed only minor dependence on pressure. Post-mortem analysis confirmed gas-induced degradation as a key mechanism, including SEI decomposition, particle exfoliation, separator deformation, and location-dependent damage concentrated in the central regions of the cells. These findings underline that pressure not only acts globally but also induces local stress distributions relevant for lifetime behavior. In summary, an optimal pressure window between 0.16 and 0.28 MPa was identified, which mitigates degradation at both cell and module level. The study highlights the dual role of pressure as both a stressor and a design parameter, offering a practical pathway to improve the durability and safety of future battery systems.