Sub‐nanoscale Engineering of MoO2 Clusters for Enhanced Sodium Storage

Smart construction of battery‐type anodes with high rate and good mechanical properties is significant for advanced sodium ion capacitors (SICs). Herein, a flexible film consisting of MoO 2 subnanoclusters encapsulated in nitrogen‐doped carbon nanofibers (MoO 2 SCs@N‐CNFs) is designed and synthesized via electrospinning toward SICs as anodes. The strong N‐Mo interaction guarantees the stable yet uniform dispersion of high loading MoO 2 SCs (≈40 wt.%) in the flexible carbonaceous substrate. The sub‐nanoscale effect of SCs restrains electrode pulverization and improves the Na + diffusion kinetics, rendering better pseudocapacitance‐dominated Na + ‐storage properties than the nanocrystal counterpart. The MoO 2 SCs@N‐CNFs paper with mass loadings of 2.2–10.1 mg cm −2 can be directly used as free‐standing anode for SICs, which exhibit high reversible gravimetric/areal capacities both in liquid and quasi‐solid‐state electrolytes. The assembled flexible SICs competitively exhibit exceptional energy density and cycling stability. More significantly, the sub‐nanoscale engineering strategy here is promisingly generalized to future electrode design for other electrochemical energy‐related applications and beyond.