Synchronous Carbonization and Phase‐Formation via Flash Joule Heating for Uniform Carbon‐Coated Polyanionic Cathode Materials

Abstract Carbon coating is an effective strategy for enhancing the conductivity of polyanionic cathode materials. However, conventional carbon coating processes suffer from loose carbon coatings and excessive unbound carbon, leading to exacerbated side reactions. Here, the Na 3 V 2 (PO 4 ) 3 (NVP) carbon coating process in real‐time is probed, revealing that the kinetic mismatch between carbon source carbonization and cathode material phase‐formation causes coating failure. By introducing flash Joule heating (FJH) technology, ultrafast thermal shock is leveraged to kinetically synchronize carbonization and phase‐formation, fundamentally eliminating sequential reaction stages, thereby constructing a 2‐nm‐thick uniform carbon coating on NVP. The FJH‐engineered NVP delivers 85 mAh g −1 at −40 °C, and retains 84% capacity after 3000 cycles at 30 C. The coating stabilizes interfaces, reduces resistance, enhances thermal stability, and mitigates runaway risks. The effective preparation of various polyanionic cathode materials confirmed the versatility of FJH method to improve the effect of carbon coating. These findings provide an effective strategy for designing high‐safety, long‐lifespan polyanionic cathode materials.