Polymer-induced solid–electrolyte interphase on hard carbon enabling 5C fast-charging practical sodium-ion pouch cell

Achieving <15 min fast-charging technology for long-life sodium-ion batteries (SIBs) remains a formidable challenge, primarily due to parasitic reactions and unstable solid-electrolyte interphase (SEI) at the hard carbon (HC) interface. Here we develop a universal polymer-induced SEI strategy that enables an Ah-level SIB pouch cell to achieve <10 min fast-charging capability. We design a <4.0 nm functionalized polymer molecular layer, polyethylenesulfonyl fluoride (PESF), coated on the HC surface (PolyHC) to minimize electrolyte decomposition. The PESF with the -SO₂F group attached has a powerful polar feature, which simultaneously induces an anion enriched at the PolyHC interface and tailors extra F atoms, contributing to the architecture of a ∼5.0 nm stable SEI that hybridizes polymer and NaF. This SEI with a resilient polymer skeleton permanently holds the generated inorganic component, enabling long-term structural stability during fast charging. The assembled 1.2 Ah pouch cell, paired with NaNi_1/3Fe_1/3Mn_1/3O₂ cathode and PolyHC anode, displays exceptional fast-charging capability and durability. This method is compatible with various HCs, offering a novel perspective for modulating the HC interface chemistry.