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

The lifetime of lithium-ion batteries (LiBs) is critically influenced by mechanical constraints imposed during module assembly, yet the coupled roles of preload pressure and buffer pads in electrochemical–mechanical degradation remain insufficiently understood. Here, we systematically investigate how preload pressure (0.1–2.0 MPa) and buffer-pad configurations affect the aging behavior of lithium iron phosphate/graphite pouch cells under three representative conditions: fast charging (3C/1C), conventional cycling (1C/1C), and high-temperature calendar storage (60 °C, 100% state of charge). A custom force-sensing fixture was employed to monitor expansion force in real time, allowing irreversible mechanical growth to be decoupled into contributions from solid electrolyte interphase (SEI) formation, electrode stiffness increase, and viscoelastic relaxation. Results show that the impact of preload is strongly aging-mode dependent. Under fast charging, low preload maintained the highest capacity retention, whereas excessive preload induced severe stress accumulation due to lithium plating and thus rapid fade. At medium preload, buffer pads redistributed stresses and suppressed irreversible force growth, delaying capacity loss. In contrast, under conventional cycling and calendar aging, preload and buffering exerted only a minor influence on capacity retention, though mechanical relaxation became the dominant process at high preload, particularly in the presence of soft pads. Across all conditions, a medium-to-low preload combined with buffer pads emerged as the most favorable configuration, balancing dynamic cycling stability with static storage durability. These findings highlight the synergistic interplay of SEI growth, stress accumulation, and relaxation in governing battery aging. The results provide actionable design guidance for optimizing preload and buffer-pad selection in pouch-cell modules, supporting safer, longer-lived, and fast-charging-capable LiB systems.