Path-Dependent Degradation of Lithium Iron Phosphate Batteries via Multi-Method Analysis

Abstract This study investigates pathway-dependent degradation of commercial lithium iron phosphate/graphite batteries under seven operating conditions with different charge cutoff voltages, charging modes, current rates, and depths of discharge. Capacity fading, incremental-capacity characteristics, electrochemical impedance, relaxation-time distributions, and low-rate voltage responses were jointly analyzed to characterize the evolution of battery degradation under different operating pathways. The results show that differences among operating conditions become progressively more pronounced with ageing. Under otherwise similar cycling conditions, lowering the charge cutoff voltage from 3.65 to 3.55 V did not provide the expected lifetime benefit, indicating that degradation depends on the overall state-of-charge window and charging pathway rather than on the upper cutoff voltage alone. Incremental-capacity analysis revealed pronounced attenuation and redistribution of high-voltage differential-capacity features during ageing. The ohmic resistance exhibited a stronger dependence on ageing state than on state of charge, whereas polarization resistance showed a pronounced state-of-charge dependence. From 100% to 70% state of health, the mean pseudo-open-circuit-voltage shift ranged from approximately −19.8 to −12.4 mV across the operating conditions, while the curve-deformation index ranged from 28.1 to 43.8 mV, demonstrating that similar overall voltage shifts can coexist with substantially different state-of-charge-dependent deformation. These results demonstrate that low-rate voltage features provide complementary indicators of pathway-dependent ageing and can support practical health assessment and operating-strategy optimization using limited voltage information in large-scale battery energy storage systems.