Effects of Preload Pressure and Buffer Pads on Coupled Electrochemical-Mechanical Aging of LiFePO4 Pouch Cells

Abstract The lifetime of lithium-ion batteries (LiBs) is strongly influenced by mechanical constraints during module assembly, yet the coupled effects of preload pressure and buffer pads on electrochemical–mechanical degradation remain insufficiently understood. Here, we investigate lithium iron phosphate (LFP)/graphite pouch cells subjected to preload levels of 0.1–2.0 MPa and various buffer-pad configurations under fast charging (3C/1C), conventional cycling (1C/1C), and high-temperature calendar storage (60 °C 100% SOC). A custom force-sensing fixture enabled real-time monitoring of expansion force, allowing irreversible mechanical growth to be decoupled into contributions from solid electrolyte interphase (SEI) formation, electrode stiffening, and viscoelastic relaxation. Results show that the impact of preload is strongly dependent on aging mode: under fast charging, low preload maintained the highest capacity retention, whereas excessive preload promoted stress accumulation from lithium plating and rapid fade; at medium preload, buffer pads redistributed stresses and suppressed irreversible force growth, delaying degradation. By contrast, under conventional cycling and calendar storage, preload and buffering exerted only minor influence on capacity retention, though high preload induced significant mechanical relaxation, particularly with soft pads. Across all conditions, a medium-to-low preload combined with buffer pads emerged as the most favorable configuration, balancing cycling stability with storage durability. These findings highlight the synergistic interplay of SEI growth, stress accumulation, and viscoelastic relaxation in governing battery aging, and provide actionable guidance for rational preload and buffer-pad design to achieve safer, longer-lived, and fast-charging-capable LiB systems.