Cathode Bulk Diffusion Dictates Low‐Temperature Performance in Li‐Ion Batteries: Beyond Electrolyte Engineering

ABSTRACT The discharge performance limitations of lithium‐ion batteries (LIBs) at low temperatures (LT) remain widely debated, with competing attributions to interfacial charge‐transfer kinetics and bulk Li + diffusion. Extensive electrolyte engineering has been pursued, yet the intrinsic structural constraints of cathodes remain largely overlooked. Here, we apply a systematic half‐cell decoupling strategy across 36 electrolyte formulations to identify the precise rate‐determining step (RDS) in graphite (Gr), layered NCM811, and olivine LFP electrodes. Our results show that LT anodic delithiation maintains near‐theoretical lithium utilization (≈100%), confirming that full‐cell failure originates predominantly from the cathode. Notably, even with highly optimized electrolytes, cathodes exhibit a persistent capacity retention ceiling of 82%–85% for NCM811 and only 38%–48% for LFP. Distribution of capacitive times (DCT) analysis reveals that cathode performance is predominantly dominated by severely hindered bulk Li + diffusion: the solid‐state diffusion time in LFP surges from 16.2 min at RT to 104 h at −30°C. These findings conclusively establish sluggish bulk Li + diffusion as the primary kinetic bottleneck, urging a paradigm shift from electrolyte‐centric strategies toward intrinsic cathode structural engineering.