Flexible electrochemical energy storage devices have attracted extensive attention as promising power sources for the ever-growing field of flexible and wearable electronic products. However, the rational design of a novel electrode structure with a good flexibility, high capacity, fast charge–discharge rate and long cycling lifetimes remains a long-standing challenge for developing next-generation flexible energy-storage materials. Herein, we develop a facile and general approach to three-dimensional (3D) interconnected porous nitrogen-doped graphene foam with encapsulated Ge quantum dot/nitrogen-doped graphene yolk-shell nano architecture for high specific reversible capacity (1,220 mAh g−1), long cycling capability (over 96% reversible capacity retention from the second to 1,000 cycles) and ultra-high rate performance (over 800 mAh g−1 at 40 C). This work paves a way to develop the 3D interconnected graphene-based high-capacity electrode material systems, particularly those that suffer from huge volume expansion, for the future development of high-performance flexible energy storage systems.

Graphene-Co3O4 composite with a unique sandwich-architecture was successfully synthesized and applied as an efficient electrocatalyst for oxygen evolution reaction. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses confirmed that Co3O4 nanocrystals were homogeneously distributed on both sides of graphene nanosheets. The obtained composite shows enhanced catalytic activities in both alkaline and neutral electrolytes. The onset potential towards the oxygen evolution reaction is 0.406 V (vs. Ag/AgCl) in 1 M KOH solution, and 0.858 V (vs. Ag/AgCl) in neutral phosphate buffer solution (PBS), respectively. The current density of 10 mA/cm2 has been achieved at the overpotential of 313 mV in 1 M KOH and 498 mV in PBS. The graphene-Co3O4 composite also exhibited an excellent stability in both alkaline and neutral electrolytes. In particular, no obvious current density decay was observed after 10 hours testing in alkaline solution and the morphology of the material was well maintained, which could be ascribed to the synergistic effect of combining Co3O4 and graphene.

No abstract

The sluggish redox kinetics of polysulfides and difficult oxidation process of Li2S severely hinder practical application of Li–S batteries under high sulfur loading and a low electrolyte dosage. To address these issues, we develop a bifunctional catalyst by manipulation of anion N doping in CoSe2 (N-CoSe2). Theoretical simulation results uncover that an introduced N element into CoSe2 could form a shorter Co–N bond, create a higher charge number of the Co central atom, bring new defect levels, and induce the Co 3d band closer to Fermi level. Further atomic level analysis revealed that N-CoSe2 could form shorter Co–S bonds with sulfur species and simultaneously weaken the S–S bridged bond of Li2S4 and Li–S bond of Li2S, which eventually facilitate the polysulfide conversion reaction in the discharge process and the Li2S oxidation in the charge process. With N-CoSe2 as a bifunctional catalyst, the battery exhibited a high areal capacity of 9.26 mAh g–1 under the low E/S (electrolyte/sulfur) ratio of 4.4 μL mg–1. Understanding the design concept of a bifunctional catalyst with anion doping would provide a new vision for realizing a high-performance Li–S battery.

by their high cost and insufficient lithium resources. [2] As sodium is abundant and shows similar physicochemical properties to lithium, sodium ion batteries (SIBs) have been recognized as a cost effective alternative for applications in ESSs; [3] thus, extensive research studies have been devoted to develop high-performance electrode materials for SIBs. To date, ample potential cathode materials for SIBs have been proposed. [4] As for anode, carbonaceous materials, [5] phosphorus, [6] alloy, [7] and metal oxides [8] have been exploited as potential SIB anode materials. Carbon anodes are considered as the most promising choice for commercialization, owing to their low cost and environmentally benign nature. However, slow ion diffusion kinetics and significant volume changes, resulting from larger ionic radius of sodium than that of lithium, lead to poor rate and cycling performance and hinder the practical application of most carbon anode materials. [9] Considering the intermittence and fluctuation nature of renewable energy and the economic viability for ESSs, developing suitable anode materials with fast sodium storage properties and long-term cycle stability is a crucial issue for SIBs. [10] To enhance the electrochemical performance of carbonaceous anodes, a number of strategies have been proposed. Nanostructure construction has been considered as an effective method, and hollow carbon spheres have been demonstrated to be an attractive morphology for carbon materials due to their high surface area and short sodium ion diffusion path, which are kinetically favorable for ion and electron transports. [11] Besides, doping heteroatoms into carbon has been recognized as a mean to modify carbon structure, as well as to enhance electronic conductivity and wettability of carbon, thus improving electrochemical performance. [12] Maier and co-workers have first prepared hollow carbon spheres, which exhibited 160 mA h g −1 under 100 mA g −1 over 100 cycles, and 50 mA h g −1 under 10 A g −1 .[11a] Ye et al. have prepared hollow carbon spheres with sulfur and nitrogen codoping, which exhibited 169 mA h g −1 under 500 mA g −1 over 2000 cycles and 110 mA h g −1 under 10 A g −1 .[11b] However, further research on carbon-based SIB anodes with ultralong cycle life under high current density, which are essential for the grid electricity storage, is still needed. In addition, previous research studies mainly focused on perfect spheres without large pores on the surface, which would limit the electrolyte diffusion into the Carbon materials have attracted significant attention as anode materials for sodium ion batteries (SIBs). Developing a carbon anode with long-term cycling stability under ultrahigh rate is essential for practical application of SIBs in energy storage systems. Herein, sulfur and nitrogen codoped mesoporous hollow carbon spheres are developed, exhibiting high rate performance of 144 mA h g −1 at 20 A g −1 , and excellent cycling durability under ultrahigh current density. Interestingly, during 7000 cyc...