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

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.