Effect of temperature gradients in large-format pouch-type NMC/graphite lithium-ion battery on degradation

One way cell-to-pack architectures achieve higher energy density is through enlarged cell sizes. However, increasing cells’ size or aspect ratio can result in severe temperature gradients due to non-uniform current density, localized heat generation, and uneven heat transfer by thermal management systems. The effects of temperature gradients on cell degradation remain underexplored. In this paper, we develop a three-segment calorimeter to impose in-plane temperature gradients and measure spatially resolved heat generation. Large-format cells (101.8 Ah, 57.9 × 10.5 × 0.89 cm) are cycled 400 times under a controlled 10 °C longitudinal gradient. An electrochemical-thermal-life model incorporating side reactions and lithium plating is validated against measured voltage, heat generation rate, and capacity fade. Temperature gradients induce current redistribution toward the positive tab region, increasing local current density by 15-20% and increasing surface lithium concentration by 18.5%. This concentrated electrochemical stress triggers lithium plating at cycle 350, 50 cycles earlier than under uniform temperature. Combined temperature gradients and fast charging amplify capacity fade 2.24-fold beyond additive effects, reducing capacity retention to 78.2% after 400 cycles. Spatial temperature uniformity, rather than average temperature control, is critical for preventing accelerated degradation in large-format cells. Effective thermal management requires uniformity-focused cooling strategies, such as multi-zone temperature control or optimized coolant flow distribution, to prevent localized degradation.