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Transition metal selenides (TMSs) have been considering as a kind of promising alternative anode materials for the application of sodium ion batteries due to their high electrical conductivity and high capacity. Here, hollow NiCoSe2 microspheres was prepared via an easy hydrothermal method, followed by the dopamine derived N-doped carbon coated which formed the hollow NiCoSe2@C composite. As-prepared hollow NiCoSe2@C composite has been first used as a new anode material for sodium ion batteries (SIBs). Our selenides displays a quite good electrochemical sodium storage performance. For example, the reversible capacity of as-prepared hollow NiCoSe2@C composite electrode can be maintained at 464.7 mAh g−1 at a current density of 100 mA g−1 after 200 cycles. Moreover, the rate performance of the hollow NiCoSe2@C composite is outstanding. The reversible capacities of 425.3, 420.8, 403.2, 394.7, 378.7, 367.8 and 337.5 mAh g−1 can be achieved at the current densities of 100, 200, 500, 1000, 2000, 3000 and 5000 mA g−1, respectively. Meanwhile, hollow NiCoSe2@C composite electrode exhibits a high discharge capacity of 338 mAh g−1 at a relatively high current density of 0.5 A g−1 after 250 cycles. In addition, the kinetic analysis of electrochemical Na + storage properties of the hollow NiCoSe2@C composite demonstrates that the extrinsic pseudo capacitive behaviour contributes significantly to excellent rate performance and good long-term cycling life. This method can be used to modify the morphologies and structures of the other TMSs for the development of new anode materials as anode materials in the application of sodium ion batteries.

The porous micro-/nanostructured Co3O4 microellipsoids are successfully fabricated through a urea-assisted solvothermal route using polyvinylpyrrolidone (PVP) as the capping reagent followed by thermal treatment in air. The porous characterization has been performed to confirm that porous Co3O4 micro-/nanostructures are composed of numerous primary nanocrystallines. The specific surface area and pore size of the Co3O4 microellipsoids are around ⿼24.2 m2 g⿿1 and 13.5 nm, respectively. The obtained porous Co3O4 microellipsoids demonstrate the high initial discharge capacity of ⿼1314 mAh g⿿1 with a Columbic efficiency of 79.2% at a current density of 50 mA g⿿1 in the potential range of 0.01⿿3.0 V. More impressively, a significantly improved reversible capacity of ⿼1192 mAh g⿿1 is retained at 100 mA g⿿1 after 50 discharge-charge cycles. Excellent rate capabilities (⿼920 mAh g⿿1 at 200 mA g⿿1 and ⿼630 mAh g⿿1 at 600 mA g⿿1) are observed for the porous microellipsoid-like structure. It should be noted that the unique porous micro-/nanostructured microellipsoids play an important role in the enhanced electrochemical lithium storage performance. Therefore, the porous micro-/nanostructured Co3O4 microellipsoids should be suitable as an anode material for lithium ion batteries.

Fiber supercapacitors (FSCs) are attractive energy storage devices for wearable and portable electronics. However, it still remains a great challenge to achieve high volumetric specific capacitance without sacrificing the rate capability of FSC. Here, we demonstrate a novel hierarchically nanostructured hybrid fiber with thin polyaniline (PANI) layer anchored on the highly-conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) hydrogel framework as electrodes for FSCs. The hybrid fiber was prepared via hydrothermal assembly of a commercial PEDOT:PSS dispersion followed by acid treatment and in-situ chemical polymerization of aniline. The incorporation of highly-electroactive PANI boosts the whole capacitance of hybrid fiber; the robust PEDOT hydrogel framework provides abundant ions diffusion channels and rapid electron transfer pathways, improving the utilization efficiency and the kinetics of pseudocapacitance reaction of PANI. Attributed to the synergistic effect, the PEDOT/PANI hybrid fiber in FSC exhibits high volumetric/areal specific capacitance (554.1 F cm−3 or 740.0 mF cm−2 at 5 A cm−3), excellent rate performance and good electrochemical stability. The flexible FSC device delivers a high volumetric energy density of 11.9 mWh cm−3 at 981.4 mW cm−3 and areal energy density of 15.8 μWh cm−2 at 1310.5 μW cm−2, outperforming most of the reported counterparts. The well-designed PEDOT/PANI hybrid fiber based FSC hold a great potential for powering the next-generation wearable and portable electronics.

The monodisperse Co 3 O 4 mesoporous microdisks have been successfully prepared through a facile solvothermal synthesis method and subsequent heating treatment. The Co 3 O 4 microdisks are polycrystalline, and have an average diameter of around 2 μm with an thickness of 300 nm. The specific surface area of Co 3 O 4 mesoporous microdisks is about 108.9 m 2 g −1 with a narrow pore size distribution centered at round 9.68 nm. The as-prepared Co 3 O 4 mesoporous microdisks as anode material in lithium ion batteries exhibit a stable specific discharge–charge capacity of 765 and 749 mAh g −1 after 30 cycles at a current density of 100 mA g −1 . The good electrochemical properties could be attributed to the unique mesoporous structure of Co 3 O 4 materials.

In this study, graphene/Co 3 O 4 composites are prepared through a simple process under mild hydrothermal conditions. Sodium carbonate (Na 2 CO 3 ) can produce many OH − and CO 3 2− ions after dissolution and hydrolysis in a water–isopropanol system, thereby making it applicable as both depositing agent and reducing agent in the synthesis of graphene/Co 2 (OH) 2 CO 3 composites. Remarkably, the galvanosta charge–discharge test shows that the specific capacitance value of the as-obtained composites reaches a high value of 443 F g −1 at a high current density of 5 A g −1 , which is significantly improved in relation to Co 3 O 4 , and the capacitance retains 54% of capacitance at 60 A g −1 . The fact that the specific capacitance remains 97.1% after 1000 continuous charge–discharge cycles at 10 A g −1 indicates that the as-prepared material has an excellent cycle life. The effect of the feeding ratios between Co 3 O 4 and graphite oxide on the electrochemical performance has also been studied.