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HIGHLIGHTS • A novel N-doped strategy of C 2 N 3 − in situ trimerization between the 2D MXene interlayers was first proposed. • The ultra-fast pseudocapacitive behavior of Ti 3 C 2 T x /Na 3 TCM anode was managed and verified. • The as-fabricated sodium-ion capacitor delivers excellent electrochemical performance by anode/cathode mass matching. ABSTRACT 2D MXenes are attractive for energy storage applications because of their high electronic conductivity. However, it is still highly challenging for improving the sluggish sodium (Na)-ion transport kinetics within the MXenes interlayers. Herein, a novel nitrogen-doped Ti 3 C 2 T x MXene was synthesized by introducing the in situ polymeric sodium dicyanamide (Na-dca) to tune the complex terminations and then utilized as intercalation-type pseudocapacitive anode of Na-ion capacitors (NICs). The Na-dca can intercalate into the interlayers of Ti 3 C 2 T x nanosheets and simultaneously form sodium tricyanomelaminate (Na 3 TCM) by the catalyst-free trimerization. The as-prepared Ti 3 C 2 T x /Na 3 TCM exhibits a high N-doping of 5.6 at.% in the form of strong TiN bonding and stabilized triazine ring structure. Consequently, coupling Ti 3 C 2 T x /Na 3 TCM anode with different mass of activated carbon cathodes, the asymmetric MXene//carbon NICs are assembled. It is able to deliver high energy density (97.6 Wh kg −1), high power output (16.5 kW kg −1), and excellent cycling stability (≈ 82.6% capacitance retention after 8000 cycles).

A series of multiheteroatom porous carbon frameworks (MPCFs) is prepared successfully from the trimerization of cyano groups of our designed and synthesized 4,4'-(4-oxophthalazine-1,3(4H)-diyl)dibenzonitrile monomers and subsequent ionothermal synthesis. Benefiting from the molecular engineering strategy, the obtained MPCFs framework show a homogeneous distribution of nitrogen and oxygen heteroatoms at the atomic level, confirmed by the transmission electron microscopy mapping intuitively, thereby ensuring the stability of electrical properties. The supercapacitor with the obtained MPCFs@700 as the electrode exhibits a high energy density of 65 Wh kg at 0.1 A g, with excellent long cycle life and cycle stability (98% capacitance retention for 10 000 cycles in 1-butyl-3-methylimidazolium tetrafluoroborate). Another two electrolyte systems employed also demonstrate the delightful results, showing a 112% capacitance retention for 30 000 cycles in 1 M HSO and a 95% capacitance retention for 30 000 cycles in tetraethylammonium tetrafluoroborate in an acetonitrile solution. Moreover, the successful preparation of MPCFs provides new insights into the fabrication of electrode materials intrinsically containing nitrogen and oxygen in the frameworks for readily available components through a facile routine.

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Lithium−sulfur (Li−S) batteries have attracted a great deal of attention for the next-generation energy storage devices due to their inherently high theoretical energy density, high natural abundance, and low cost. However, the dissolution of polysulfides in electrolytes and their undesirable shuttle behavior lead to poor cycling performance, which obstructs practical application. Herein, we report a dual-sulfur-fixing mechanism of epoxy/allyl compound/sulfur system to prepare poly(sulfur-random-4-vinyl-1,2-epoxycyclohexane) (SVE) copolymers as powerful cathode materials. Benefiting from the stable C−S bond and a uniform distribution of ultrafine Li 2 S/S 8 in the SVE-based polymer matrix, the SVE electrodes exerted an embedding effect to reduce polysulfides migration. The thiosulfate/polythionate protective layer derived from the terminal hydroxyl group of SVE also ensured the cycle stability of SVE electrodes during cycling. As a result, optimized SVE electrodes deliver a high reversible specific capacity of 1248 mA h g −1 at rates of 0.1 C, together with a stable cycling performance of no capacity decay per cycle over more than 400 cycles. This work provides an effective strategy for the practical application of organosulfur polymers Li−S batteries and inspires the exploration of the reaction mechanism of epoxy/allyl compound/sulfur system.

Graphene oxide has become an attractive electrode-material candidate for supercapacitors thanks to its higher specific capacitance compared to graphene. The quantum capacitance makes relative contributions to the specific capacitance, which is considered as the major limitation of graphene electrodes, while the quantum capacitance of graphene oxide is rarely concerned. This study explores the quantum capacitance of graphene oxide, which bears epoxy and hydroxyl groups on its basal plane, by employing density functional theory (DFT) calculations. The results demonstrate that the total density of states near the Fermi level is significantly enhanced by introducing oxygen-containing groups, which is beneficial for the improvement of the quantum capacitance. Moreover, the quantum capacitances of the graphene oxide with different concentrations of these two oxygen-containing groups are compared, revealing that more epoxy and hydroxyl groups result in a higher quantum capacitance. Notably, the hydroxyl concentration has a considerable effect on the capacitive behavior.

N,O-Containing micropore-dominated materials have been developed successfully via temperature-dependent cross-linking of 4,4'-(dioxo-diphenyl-2,3,6,7-tetraazaanthracenediyl)dibenzonitrile (DPDN) monomers. By employing a molecular engineering strategy, we have designed and synthesized a series of porous heteroatom-containing carbon frameworks (PHCFs), in which nitrogen and oxygen heteroatoms are distributed homogeneously throughout the whole framework at the atomic level, which can ensure the stability of its electrical properties. The as-made PHCFs@550 exhibits a high specific capacitance of 378 F g, with an excellent long cycling life, including excellent cycling stability (capacitance retention of ca. 120% over 20 000 cycles). Moreover, the successful preparation of PHCFs provides new insights for the fabrication of nitrogen and oxygen-containing electrode materials from readily available components via a facile route.

N, S-co-doped hierarchical porous carbon with stable sodium storage were prepared by designing the precursors and changing the reaction temperature.