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

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.