Metal-ion capacitors could merit advantages from both batteries and capacitors, but they need to overcome the severe restrictions from their sluggish reaction kinetics of the battery type electrode and low specific capacitance of capacitor type electrode for both high energy and power density. Herein, we use the Kirkendall effect for the first time to synthesize unique tubular hierarchical molybdenum dioxide with encapsulated nitrogen-doped carbon sheets while in situ realizing phosphorus-doping to create rich oxygen vacancies (P-MoO_2-x@NP-C) as a sodium-ion electrode. Experimental and theoretical analysis confirm that the P-doping introduced oxygen defects can partially convert the high-bond-energy Mo–O to low-bond-energy Mo–P, resulting in a low oxidation state of molybdenum for enhanced surface reactivity and rapid reaction kinetics. The as-prepared P-MoO_2-x@NP-C as an ion-battery electrode is further used to pair active N-doped carbon nanosheet (N-C-A) electrode for Na-ion hybrid capacitor, delivering excellent performance with an energy density of 140.3 Wh kg⁻¹, a power density of 188.5 W kg⁻¹ and long stable life in non-aqueous solution, which ranks the best among all reported MoO_x-based hybrid capacitors. P-MoO_2-x@NP-C is also used to fabricate a zinc-ion hybrid capacitor, also accomplishing a remarkable energy density of 43.8 Wh kg⁻¹, a power density of 93.9 W kg⁻¹, and a long stable life@2A g⁻¹ of 32000 cycles in aqueous solutions, solidly verifying its universal significance. This work not only demonstrates an innovative approach to synthesize high-performance metal ion hybrid capacitor materials but also reveals certain scientific insights into electron transfer enhancement mechanisms.

Using porous carbon hosts in cathodes of Li-S cells can disperse S actives and offset their poor electrical conductivity. However, such reservoirs would in turn absorb excess electrolyte solvents to S-unfilled regions, causing the electrolyte overconsumption, specific energy decline, and even safety hazards for battery devices. To build better cathodes, we propose to substitute carbons by In-doped SnO₂ (ITO) nano ceramics that own three-in-one functionalities: 1) using conductive ITO enables minimizing the total carbon content to an extremely low mass ratio (~3%) in cathodes, elevating the electrode tap density and averting the electrolyte overuse; 2) polar ITO nanoclusters can serve as robust anchors toward Li polysulfide (LiPS) by electrostatic adsorption or chemical bond interactions; 3) they offer catalysis centers for liquid–solid phase conversions of S-based actives. Also, such ceramics are intrinsically nonflammable, preventing S cathodes away from thermal runaway or explosion. These merits entail our configured cathodes with high tap density (1.54 g cm⁻³), less electrolyte usage, good security for flame retardance, and decent Li-storage behaviors. With lean and LiNO₃-free electrolyte, packed full cells exhibit excellent redox kinetics, suppressed LiPS shuttling, and excellent cyclability. This may trigger great research enthusiasm in rational design of low-carbon and safer S cathodes.

The environment benignity and battery cost are major concerns for grid-scale energy storage applications. The emerging dendrite-free Fe-ion aqueous batteries are promising due to the rich natural abundance, low cost and non-toxicity for Fe resources. However, serious passivation reactions on Fe anodes and poor long-term cyclability for matched cathodes still stand in the way for their practical usage. To settle above constraints, we herein use NH₄Cl as the electrolyte regulator to elevate the reaction kinetics of passivated Fe anodes, and also propose a special cathode-free design to prolong the cells lifetime over 1,000 cycles. The added NH₄Cl can erode/break inert passivation layers and strengthen the ion conductivity of electrolytes, facilitating the reversible Fe plating/ stripping and Fe²⁺ shuttling. The highly puffed nano carbon foams function as current collectors and actives anchoring hosts, enabling expedite Fe²⁺ adsorption/desorption, Fe^II/Fe^III redox conversions and Fe^III deposition. The configured rocking-chair Fe-ion cells have good environmental benignity and decent energy-storage behaviors, including high reactivity/reversibility, outstanding cyclic stability and far enhanced operation longevity. Such economical, long-cyclic and green cathode-free Fe-ion batteries may hold great potential in near-future energy-storage power stations.