Fast‐Charging Hard Carbons: A Fully Organic SEI Enables Low‐Coordination Interfacial Environments and Fast Na ⁺ Desolvation

Abstract Fast‐charging capability becomes a critical bottleneck for the practical deployment of sodium‐ion batteries (SIBs), particularly due to sluggish Na + desolvation and interfacial transport at hard carbon (HC) anodes. Herein, we present a comprehensive study on Na + desolvation and transport kinetics across solid electrolyte interphases (SEIs) with diverse chemical natures. Although inorganic‐rich SEIs are generally regarded as favorable for Na + transport, our results reveal that certain organic‐rich SEIs can deliver comparable or even superior kinetic performance. Guided by these insights, we construct a Poly(MMA)‐based artificial SEI on commercial HC ( Type‐1 ), which reorganizes the Na + –DME solvation shell at the inner Helmholtz plane into a Na + –DME/Poly(MMA) coordination environment. This interfacial reconstruction markedly enhances Na + desolvation and interphase transport, enabling exceptional rate performance (236 mA h g −1 at 5 C) and long‐term cycling stability (99% capacity retention over 1000 cycles) for the commercial Type‐1 HC. The effectiveness of the Poly(MMA)‐derived interphase is further validated in both coin‐type and pouch‐type full sodium‐ion chemistries, as well as in lithium‐ion batteries. This work unveils the pivotal role of interfacial solvation structure, beyond the organic/inorganic ratios of SEI, in governing Na + kinetics, offering a new design paradigm for next‐generation fast‐charging SIBs.