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Abstract The polymeric gel electrolytes are attractive owing to their higher ionic conductivities than those of dry polymer electrolytes and lowered water activity for enlarged potential window. However, the ionic conductivity and mechanical strength of the Na‐ion conducting polymeric gel electrolytes are limited by below 20 mS cm−1 and 2.2 MPa. Herein, we demonstrate Na‐ion conducting and flexible polymeric hydrogel electrolytes of the chemically coupled poly(diallyldimethylammonium chloride)‐dextrin‐N,N′‐methylene‐bis‐acrylamide film immersed in NaClO4 solution (ex‐DDA‐Dex + NaClO4) for flexible sodium‐ion hybrid capacitors (f‐NIHC). In particular, the anion exchange reaction and synergistic interaction of ex‐DDA‐Dex with the optimum ClO4− enable to greatly improve the ionic conductivity up to 27.63 mS cm−1 at 25°C and electrochemical stability window up to 2.6 V, whereas the double networking structure leads to achieve both the mechanical strength (7.48 MPa) and softness of hydrogel electrolytes. Therefore, the f‐NIHCs with the ex‐DDA‐Dex + NaClO4 achieved high specific and high‐rate capacities of 192.04 F g−1 at 500 mA g−1 and 116.06 F g−1 at 10 000 mA g−1, respectively, delivering a large energy density of 120.03 W h kg−1 at 906 W kg−1 and long cyclability of 70% over 500 cycles as well as demonstrating functional operation under mechanical stresses.

Abstract Magnesium‐ion batteries (MIBs) have promising applications because of their high theoretical capacity and the natural abundance of magnesium Mg. However, the kinetic performance and cyclic stability of cathode materials are limited by the strong interactions between Mg ions and the crystal lattice. Here, we demonstrate the unique Mg2+‐ion storage mechanism of a hierarchical accordion‐like vanadium oxide/carbon heterointerface (V2O3@C), where the V2O3 crystalline structure is reconstructed into a MgV3O7∙H2O phase through an anodic hydration reaction upon first cycle, for the improved kinetic and cyclic performances. As verified by in situ/ex situ spectroscopic and electrochemical analyses, the fast charge transfer kinetics of the V2O3@C cathode were due to the crystal‐reconstruction and chemically coupled heterointerface. The V2O3@C demonstrated an ultrahigh rate capacity of 130.4 mAh g−1 at 50 000 mA g−1 and 1000 cycles, achieving a Coulombic efficiency of 99.6%. The high capacity of 381.0 mA h g−1 can be attributed to the reversible Mg2+‐ion intercalation mechanism observed in the MgV3O7∙H2O phase using a 0.3 M Mg(TFSI)2/ACN(H2O) electrolyte. Additionally, within the voltage range of 2.25 V versus Mg/Mg2+, the V2O3@C exhibited a capacity of 245.1 mAh g−1 when evaluated with magnesium metal in a 0.3 M Mg(TFSI)2 + 0.25 M MgCl2/DME electrolyte. These research findings have important implications for understanding the relationship between the Mg‐ion storage mechanism and reconstructed crystal phase of vanadium oxides as well as the heterointerface reconstruction for the rational design of MIB cathode materials.