In Situ TEM Reveals the Hard Carbon Sodium Storage Mechanism and Discovers the Sodium Carbide

ABSTRACT Hard carbon anodes for sodium‐ion batteries rely on Na + insertion into graphite‐like layers below 0.1 V, where closed‐pore structures are critical for high energy density. However, controlling hard carbon microstructures remains challenging. Here, the industrial lignin precursor was simply pre‐pyrolyzed and carbonized to synthesize high‐performance hard carbon with curved graphite‐like layers forming topological cavities. Through GITT and in situ TEM, we visualized Na + (de)intercalation and demonstrated that closed pores (∼1 nm) enhance sodium storage. Even the FFT transform revealed the appearance of a new material lattice (C 32 Na). This work provides insights for tuning hard carbon microstructures and biomass bulk processing and value‐added utilization.